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PHOTOGRAPHY (Gr. #os, light, and ypa<jxiv, to write), the science and art of producing pictures by the action of light on chemically prepared (sensitized) plates or films.


It would be somewhat difficult to fix a date when what we now know as " photographic action " was first recorded. No doubt the tanning of the skin by the sun's rays was what was first noticed, and this is as truly the effect of solar radiation as is the darkening of the sensitive paper which is now in use in photographic printing operations. We may take it that K. W. Scheele was the first to investigate the darkening action of sunlight on silver chloride. He found that when silver chloride was exposed to the action of light beneath water there was dissolved in the fluid a substance which, on the addition of lunar caustic (silver nitrate), caused the precipitation of new silver chloride, and that on applying a solution of ammonia to the blackened chloride an insoluble residue of metallic silver was left behind. He also noticed that of the rays of the spectrum the violet most readily blackened the silver chloride. In Scheele, then, we have the first who applied combined chemical and spectrum analysis to the science of photography. In 1782 J. Senebier repeated Scheele's experiments, and found that in fifteen seconds the violet rays blackened silver chloride as much as the red rays did in twenty minutes. 1 In 1798 Count Rumford contributed a paper to the Philosophical Transactions entitled " An inquiry concerning the chemical properties that have been attributed to light," in which he tried to demonstrate that all effects produced on metallic solution could be brought about by a temperature somewhat less than that of boiling water. Robert Harrup in 1802, however, conclusively showed in Nicholson's Journal that, at all events, salts of mercury were reduced by visible radiation and not by change of temperature.

In 1801 we come to the next decided step in the study of photographic action, when Johann Wilhelm Ritter (1776-1810) proved the existence of rays lying beyond the violet, and found that they had the power of blackening silver chloride. Such a discovery naturally gave a direction to the investigations of others, and Thomas Johann Seebeck (1770-1831) (between 1802 and 1808) and, in 1812, Jacques Etienne Berard (1780-1869) turned their attention to this particular subject, eliciting valuable information. We need only mention two or three other cases 1 It may here be remarked that had he used a pure spectrum he would have found that the red rays did not blacken the material in the slightest degree.

where the influence of light was noticed at the beginning of the 19th century. William Hyde Wollaston observed the conversion of yellow gum guaiacum into a green tint by the violet rays, and the restoration of the colour by the red rays both of which are the effect of absorption of light, the original yellow colour of the gum absorbing the violet rays, whilst the green colour to which it is changed absorbs the red rays. Sir Humphry Davy found that puce-coloured lead oxide, when damp, became red in the red rays, whilst it blackened in the violet rays, and that the green mercury oxide became red in the red rays again an example of the necessity of absorption to effect a molecular or chemical change in a substance. U. R. T. Le Bouvier Desmorties in 1801 observed the change effected in Prussian blue, and Carl Wilhelm Bockman noted the action of the two ends of the spectrum on phosphorus, a research which John William Draper extended farther in America at a later date.

To England belongs the honour of first producing a photograph by utilizing Scheele's observations on silver chloride. In June 1802 Thomas Wedgwood (1771-1805) published in the Journal of the Royal Institution the paper " An account of a method of copying paintings upon glass and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy." He remarks that white paper or white leather moistened with a solution of silver nitrate undergoes no change when kept in a dark place, but on being exposed to the daylight it speedily changes colour, and, after passing through various shades of grey and brown, becomes at length nearly black. The alteration of colour takes place more speedily in proportion as the light is more intense.

" In the direct beam of the Sun two or three minutes are sufficient to produce the full effect, in the shade several hours are required, and light transmitted through different-coloured glasses acts upon it with different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little action upon it ; yellow and green are more efficacious, but blue and violet light produce the most decided and powerful effects."

Wedgwood goes on to describe the method of using this prepared paper by throwing shadows on it, and inferentially by what we now call " contact printing." He states that he has been unable to fix his prints, no washing being sufficient to eliminate the traces of the silver salt which occupied the unexposed or shaded portions. Davy in a note states that he has found that, though the images formed by an ordinary camera obscura were too faint to print out in the solar microscope, the images of small objects could easily be copied on such paper.

" In comparing the effects produced by light upon muriate of silver (silver chloride) with those upon the nitrate it seemed evident that the muriate was the more susceptible, and both were more readily acted upon when moist than when dry a fact long ago known. Even in the twilight the colour of the moist muriate of silver, spread upon paper, slowly changed from white to faint violet ; though under similar circumstances no intermediate alteration was produced upon the nitrate. . . . Nothing but a method of preventing the unshaded parts of the delineations from being coloured by exposure to the day is wanting to render this process as useful as it is elegant."

In this method of preparing the paper lies the germ of the silver-printing processes of modern times, and it was only by the spread of chemical knowledge that the hiatus which was to render the " process as useful as it is elegant " was filled up when sodium thiosulphate (hyposulphite of soda), discovered by Francois Chaussier in 1799, or three years before Wedgwood published his paper, was used for making the print permanent. Here we must call attention to an important observation by Seebeck of Jena in 1810. In the Farbenlehre of Goethe he says:

" When a spectrum produced by a properly constructed prism is thrown upon moist chloride of silver paper, if the printing be continued for from fifteen to twenty minutes, whilst a constant position for the spectrum is maintained by any means, I observe the following. In the violet the chloride is a reddish brown (sometimes more violet, sometimes more blue), and this coloration extends well beyond the limit of the violet; in the blue the chloride takes a clear blue tint, which fades away, becoming lighter in the green. In the yellow I usually found the chloride unaltered ; sometimes, however, it had a light yellow tint ; in the red and beyond the red it took a rose or lilac tint. This image of the spectrum shows beyond the red .and the violet a region more or less light and uncoloured. This is how the decomposition of the silver chloride is seen in this region. Beyond the brown band, . . . which was produced in the violet, the silver chloride was coloured a grey-violet for a distance of several inches. In proportion as the distance from the violet increased, the tint became lighter. Beyond the red, on the contrary, the chloride took a feeble red tint for a considerable distance. When moist chloride of silver, having received the action of light for a time, is exposed to the spectrum, the blue and violet behave as above. In the yellow and red regions, on the other hand, it is found that the silver chloride becomes paler; . . . the parts acted upon by the red rays and by those beyond take a light coloration."

This has been brought forward by J. M. Eder as being the first record we have of photographic action lending itself to production of natural colours. This observation of Seebeck was allowed to lie fallow for many years, until it was again taken up and published as a novelty.

The first to found a process of photography which gave pictures that were subsequently unaffected by light was Nicephore de Niepce. His process, which he called provisionally " heliographie, dessins, et gravures," consists in coating the surface of a metallic plate with a solution of asphaltum in oil of lavender and exposing it to a camera image. He recommends that the asphaltum be powdered and the oil of lavender dropped upon it in a wine-glass, and that it be then gently heated. A polished plate is covered with this varnish, and, when dried, is ready for employment in the camera. After requisite exposure, which is very long indeed, a very faint image, requiring development, is seen. Development is effected by diluting oil of lavender with ten parts by volume of white petroleum. After this mixture has been allowed to stand two or three days it becomes clear and is ready to be used. The plate is placed in a dish and covered with the solvent. By degrees the parts unaffected by light dissolve away, and the picture, formed of modified asphaltum, is developed. The plate is then lifted from the dish, allowed to drain, and finally freed from the remaining solvents by washing in water. Subsequently, instead of using oil of lavender as the asphaltum solvent, Niepce employed an animal oil, which gave a deeper colour and more tenacity to the surface-film.

Later, Louis Jacques Mande Daguerre (1789-1851) and Niepce used as a solvent the brittle residue obtained from evaporating the oil of lavender dissolved in ether or alcohol a transparent solution of a lemon-yellow colour being formed. This solution was used for covering glass or silver plates, which, when dried, could be used in the camera. The time of exposure varied somewhat in length. Daguerre remarked that " the time required to procure a photographic copy of a landscape is from seven to eight hours, but single monuments, when strongly lighted by the Sun, or which are themselves very bright, can be taken in about three hours." Perhaps there is no sentence that illustrates more forcibly the advance made in photography from the days when this process was described. The ratio of three hours to T^rytb. of a second is a fair estimate of the progress made since Niepce. The development was conducted by means of petroleum-vapour, which dissolved the parts not acted upon by light. As a rule silver plates seem to have been used, and occasionally glass; but it does not appear whether the latter material was chosen because an image would be projected through it or whether simply for the sake of effect. Viewed in the light of present knowledge, a more perfectly developable image in half-tone would be obtained by exposing the film through the back of the glass. The action of light on most organic matter is apparently one of oxidation. In the case of asphaltum or bitumen of Judaea the oxidation causes a hardening of the material and an insolubility in the usual solvents. Hence that surface of the film is generally hardened first which first feels the influence of light. Where half-tones exist, as in a landscape picture, the film remote from the surface first receiving the image is not acted upon at all, and remains soluble in the solvent. It is thus readily seen that, in the case of half-tone pictures, or even in copying engravings, if the action were not continued sufficiently long when the surface of the film farthest from the glass was first acted upon, the layer next the glass would in some places remain soluble, and on development would be dissolved away, carrying the top layer of hardened resinous matter with it, and thus give rise to imperfect pictures. In carbon-printing development from the back of the exposed film is absolutely essential, since it depends on the same principles as does heliography, and in this the same mode of procedure is advisable.

It would appear that Niepce began his researches as early as 1814, but it was not till 1827 that he had any success worth recounting. At that date he communicated a paper to Dr Bauer of Kew, the secretary of the Royal Society of London, with a view to its presentation to that society. Its publication, however, was prevented because the process, of which examples were shown, was a secret one. In an authentic MS. copy of Niepce's " M6moire," dated "Kew, le 8 Decembre 1827," he says that "in his framed drawings made on tin the tone is too feeble, but that by the use of chemical agents the tone may be darkened." This shows that Niepce was familiar with the idea of using some darkening medium even with his photographs taken on tin plates.

Daguerreotype. We have noticed in the joint process of Daguerre and Niepce that polished silver plates were used, and we know from the latter that amongst the chemical agents tried iodine suggested itself. Iodine vapour or solution applied to a silvered plate would cause the formation of silver iodide on those parts not acted upon by light. The removal of the resinous picture would leave an image formed of metallic silver, whilst the black parts of the original would be represented by the darker silver iodide. This was probably the origin of the daguerreotype process. Such observers as Niepce and Daguerre, who had formed a partnership for prosecuting their researches, would not have thus formed silver iodide without noticing that it changed in colour when exposed to the light. What parts respectively Daguerre and Niepce played in the development of the daguerreotype will probably never be known with absolute accuracy, but in a letter from Dr Bauer to Dr J. J. Bennett, F.R.S., dated the 7th of May 1839, the former says:

" I received a very interesting letter from Mons. Isidore Niepce, dated 12th March [about a month after the publication of the daguerreotype process], and that letter fully confirms what I suspected of Daguerre's manoeuvres with poor Nic6phore, but Mr Isidore observes that for the present that letter might be considered confidential."

Dr Bauer evidently knew more of " poor Nicephore's " work than most people, and at that early period he clearly thought that an injustice had been done to Niepce at the hands of Daguerre. It should be remarked that Nicephore de Niepce died in 1833, and a new agreement was entered into between his son Isidore de Niepce and Daguerre to continue the prosecution of their researches. It appears further that Niepce communicated his process to Daguerre on the 5th of December 1829. At his death some letters from Daguerre and others were left by him in which iodine, sulphur, phosphorus, etc., are mentioned as having been used on the metal plates, and their sensitiveness to light, when thus treated, commented upon. We are thus led to believe that a great part of the success in producing the daguerreotype is due to the elder Niepce; and indeed it must have been thought so at the time, since, on the publication of the process, life-pensions of 6000 francs and 4000 francs were given to Daguerre and to Isidore Niepce respectively. In point of chronology the publication of the discovery of the daguerreotype process was made subsequently to the Talbottype process. It will, however, be convenient to continue the history of the daguerreotype, premising that it was published on the 6th of February 1839, whilst Talbot's process was given to the world on the 25th of January of the same year.

Daguerreotype pictures were originally taken on silver-plated copper, and even now the silvered surface thus prepared serves better than electro-deposited silver of any thickness. An outline of the operations is as follows. A brightly-polished silver plate is cleaned by finely-powdered pumice and olive oil, and then by dilute nitric acid, and a soft buff is employed to give it a brilliant polish, the slightest trace of foreign matter or stain being fatal to the production of a perfect picture. The plate, thus prepared, is ready for the iodizing operation. Small fragments of iodine are scattered over a saucer, covered with gauze. Over this the plate is placed, face downwards, resting on supports, and the vapour from the iodine is allowed to form upon it a surface of silver iodide. It is essential to note the colour of the surface-formed iodide at its several stages, the varying colours being due to interference colours caused by the different thicknesses of the minutely thin film of iodide. The stage of maximum sensitiveness is obtained when it is of a golden orange colour. In this state the plate is withdrawn and removed to the dark slide of the camera, ready for exposure. A plan frequently adopted to give an even film of iodide was to saturate a card with iodine and hold the plate a short distance above the card. Long exposures were required, varying in Paris from three to thirty minutes. The length of the exposure was evidently a matter of judgment, more particularly as over-exposure introduced an evil which was called ' splarization," but which was in reality due to the oxidation of the iodide by prolonged exposure to light.

As a matter of history it may be remarked that the development of the image by mercury vapour is said to be due to a chance discovery of Daguerre. It appears that for some time previous to the publication of the daguerreotype method he had been experimenting with iodized silver plates, producing images by what would now be called the "printing out ' process. This operation involved so long an exposure that he sought some means of reducing it by the application of different reagents. Haying on one occasion exposed such a plate to a camera-image, he accidentally placed it in the dark in a cupboard containing various chemicals, and found after the lapse of a night that he had a perfect image developed. By the process of exhaustion he arrived at the fact that it was the mercury vapour, which even at ordinary temperatures volatilizes, that had caused this intensification of the almost invisible camera-image. It was this discovery that enabled the exposures to be very considerably shortened from those which it was found necessary to give in mere camera-printing.

The development of the image was effected by placing the exposed plate over a slightly heated (about 75 C.) cup of mercury. The vapour of mercury condensed on those places where the light had acted in an almost exact ratio to the intensity of its action. This produced a picture in an amalgam, the vapour of which attached itself to the altered silver iodide. Proof that such was the case was subsequently afforded by the fact that the mercurial image could be removed by heat. The developing box was so constructed that it was possible to examine the picture through a yellow glass window whilst the image was being brought out. The next operation was to fix the picture by dipping it in a solution of hyposulphite of soda. The image produced by this method is so delicate that it will not bear the slightest handling, and has to be protected from accidental touching.

The first great improvement in the daguerreotype process was the resensitizing of the iodized film by bromine vapour. John Frederick Goddard published his account of the use of bromine in conjunction with iodine in 1840, and A. F. J. Claudet (1797-1867) employed a combination of iodine and chlorine vapour in 1841. In 1844 Daguerre published his improved method of preparing the plates, which is in reality based on the use of bromine with iodine. That this addition points to additional sensitiveness will be readily understood when we remark that so-called instantaneous pictures of yachts in full sail, and of large size, have been taken on plates so prepared a feat which is utterly impossible with the original process as described by Daguerre. The next improvement in the process was toning or gilding the image by a solution of gold, a practice introduced by H. L. Fizeau. gold chloride is mixed with hyposulphite of soda, and the levelled plate, bearing a sufficient quantity of the fluid, is warmed by a spirit-lamp until the required vigour is given to the image, as a consequence of which it is better seen in most lights. Nearly all the daguerreotypes extant have been treated in this manner, and no doubt their permanence is in a great measure due to this operation. Images of this class can be copied by taking electrotypes from them, as shown by Sir W. R. Grove and others. These reproductions are admirable in every way, and furnish a proof that the daguerrean image is a relief.

Fox-Talbot Process. In January 1839 Fox Talbot described the first of his processes, photogenic drawing, in a paper to the Royal Society. He states that he began experimenting in 1834, and that in the solar microscope he obtained an outline of the object to be depicted in full sunshine in half a second. He published in the Philosophical Magazine full details of his method, which consisted essentially in soaking paper in common salt, brushing one side only of it with about a 1 2 % solution of silver nitrate in water, and drying at the fire. Fox Talbot stated that by repeating the alternate washes of the silver and salt always ending, however, with the former greater sensitiveness was attained. This is the same in every respect as the method practised by Wedgwood in 1802; but, when we come to the next process, which he called " calotype " or " beautiful picture," we have a distinct advance. This process Talbot protected by a patent in 1841.

It may be briefly described as the application of silver iodide to a paper support. Carefully selected paper was brushed over with a solution of silver nitrate (100 grains to the ounce of distilled water), and dried by the fire. It was then dipped into a solution of potassium iodide (500 grains being dissolved in a pint of water), where it was allowed to stay two or three minutes until silver iodide was formed. In this state the iodide is scarcely sensitive to light, but is sensitized by brushing " gallo-nitrate of silver " over the surface to which the silver nitrate had been first applied. This " gallonitrate " is merely a mixture, consisting of 100 grains of silver nitrate dissolved in 2 oz. of water, to which is added one-sixth of its volume of acetic acid, and immediately before applying to the paper an equal bulk of a saturated solution of gallic acid in water. The prepared surface is then ready for exposure in the camera, and, after a short insolation, develops itself in the dark, or the development may be hastened by a fresh application of the " gallo-nitrate of silver." The picture is then fixed by washing it in clean water and drying slightly in blotting paper, after which it is treated with a solution of potassium bromide, and again washed and dried. Here there is no mention made of hyposulphite of soda as a fixing agent, that having been first used by Sir J. Herschel in February 1840.

In a strictly historical notice it ought to be mentioned that development by means of gallic acid and silver nitrate was first known to Rev. J. B. Reade. When impressing images in the solar microscope he employed gallic acid and silver in order to render more sensitive the silver chloride paper that he was using, and he accidentally found that the image could be developed without the aid of light. The priority of the discovery was claimed by Fox Talbot; and his claim was sustained after a lawsuit, apparently on the ground that Reade's method had never been legally published. Talbot afterwards made many slight improvements in the process. In one of his patents he recognizes the value of the proper fixing of his photogenic drawings by hyposulphite of soda, and also the production of positive prints from the calotype negatives. We pass over his application of albumen to porcelain and its subsequent treatment with iodine vapour, as also his application of albumen in which silver iodide was held in suspension to a glass plate, since in this he was preceded by Niepce de St Victor in 1848.

Albumen Process on Glass. It was a decided advance when Niepce de St Victor, a nephew of Nic6phore de Niepce, employed a glass plate and coated it with iodized albumen. The originator of this method did not meet with much success. In the hands of Blanquart fivrard it became more practicable; but it was carried out in its greatest perfection by G. Le Gray.

The outline of the operations is as follows: The whites of five fresh eggs are mixed with about one hundred grains of potassium iodide, about twenty grains of potassium bromide and ten grains of common salt. The mixture is beaten up into a froth and allowed to settle for twenty-four hours, when the clear liquid is decanted off. A circular pool of albumen is poured on a glass plate, and a straight ruler (its ends being wrapped with waxed paper to prevent its edge from touching the plate anywhere except at the margins) is drawn over the plate, sweeping off the excess of albumen, and so leaving an even film. The plate is first allowed to dry spontaneously, a final heating being given to it in an oven or before the fire. The heat hardens the albumen, and it becomes insoluble and ready for the silver nitrate bath. One of the difficulties is to prevent crystallization of the salts held in solution, and this can only be effected by keeping them in defect rather than in excess. The plate is sensitized for five minutes in a bath of silver nitrate, acidified with acetic acid, and exposed whilst still wet, or it may be slightly washed and again dried and exposed whilst in its desiccated state. The image is developed by gallic acid in the usual way.

After the application of albumen many modifications were introduced in the shape of starch, serum of milk, gelatin, all of which were intended to hold iodide in situ on the plate; and the development in every case seems to have been by gallic acid. At one time the waxed-paper process subsequently introduced by Le Gray was a great favourite. Paper that had been made translucent by white wax was immersed in a solution of potassium iodide until impregnated with it, after which it was sensitized in the usual way, development being by gallic acid. In images obtained by this process the high lights are represented by metallic silver, whilst the shadows are translucent. Such a print is called a " negative." When silver chloride paper is darkened by the passage of light through a negative, we get the highest lights represented by white paper and the shadows by darkened chloride. A print of this kind is called a " positive."

Collodion Process. A great impetus was given to photography in 1850, on the introduction of collodion (?..), a very convenient vehicle on account of the facility with which the plates are prepared, and also because it is a substance as a rule totally unaffected by silver nitrate, which is not the case with other organic substances. Thus albumen forms a definite silver compound, as do gelatin, starch and gum. The employment of collodion was first suggested by Le Gray, but it remained for Frederick Scott Archer of London, closely followed by P. W. Fry, to make a really practical use of the discovery. When collodion is poured on a glass plate it leaves on drying a hard transparent film which under the microscope is slightly reticulated. Before drying, the film is gelatinous and perfectly adapted for holding in situ salts soluble in ether and alcohol. Where such saltj are present they crystallize out when the film is dried, hence such a film is only suitable where the plates are ready to be immersed in the silver bath. As a rule, about five grains of the soluble gun-cotton are dissolved in an ounce of a mixture of equal parts of ether and alcohol, both of which must be of low specific gravity, -725 and -805 respectively. If the alcohol or ether be much diluted with water the gun-cotton (pyroxylin) precipitates, but, even if less diluted, it forms a film which is " crapey " and uneven. Such was the material which Le Gray proposed and which Archer brought into practical use. The opaque silver plate with its one impression was abandoned; and the paper support of Talbot, with its inequalities of grain and thickness, followed suit, though not immediately. When once a negative had been obtained with collodion on a glass plate the image showing high lights by almost complete opacity and the shadows by transparency (as was the case, too, in the calotype process) any number of impressions could be obtained by means of the silver-printing process introduced by Fox Talbot, and they were found to possess a delicacy and refinement of detail that certainly eclipsed the finest print obtained from a calotype negative. To any one who had practised the somewhat tedious calotype process, or the waxed-paper process of Le Gray with its still longer preparation and development, the advent of the collodion method must have been extremely welcome, since it effected a saving in time, money and uncertainty. The rapidity of photographic action was much increased, and the production of a different character of pictures thus became possible.

We give an outline of the procedure. A glass plate is carefully cleaned by a detergent such as a cream of tripoli powder and spirits of wine (to which a little ammonia is often added), then wiped with a soft rag, and finally polished with a silk handkerchief or chamois leather. A collodion containing soluble iodides and bromides is made to flow over the plate, all excess being drained off when it is covered. A good standard formula for the collodion is 55 grains of pyroxylin, 5 oz. of alcohol, 5 oz. of ether; and in this liquid are dissolved 2J grains of ammonium iodide, 2 grains of cadmium iodide and 2 grains of cadmium bromide. When the collodion is set the plate is immersed in a bath of silver nitrate a vertical form being that mostly used in England, whilst a horizontal dish is used on the continent of Europe a good formula for which is 350 grains of silver nitrate with 10 oz. of water. The plate is steadily lowered into this solution, and moved in it until all the repellent action between the aqueous solution of the silver and the solvents of the collodion is removed, when it is allowed to rest for a couple of minutes, after which period it is taken out and placed in the dark slide ready for exposure in the camera. After undergoing proper exposure the plate is withdrawn, and in a room lighted with yellow light the developing solution is applied, which originally was a solution of pyrogallic acid in water restrained in its action by the addition of acetic acid. One of the old formulae employed by P. H. Delamotte was 9 grains of pyrogallic acid, 2 drachms of glacial acetic acid and 3 oz. of water. The image gradually appears after the application of this solution, building itself up from the silver nitrate clinging to the film, which is reduced to the metallic state by degrees. Should the density be insufficient a few drops of silver nitrate are added to the pryogallic acid solution and the developing action continued.

In 1844 Robert Hunt introduced another reducing agent, which is still the favourite, viz. ferrous sulphate. By its use the time of necessary exposure of the plate is reduced and the image develops with great rapidity. A sample of this developing solution is 20 grains of ferrous sulphate, 20 minims of acetic acid, with I oz. of water. This often leaves the image thinner than is requisite for the formation of a good print, and it is intensified with pyrogallic acid and silver. Other intensifiers are used to increase the deposit on a plate by means of mercury or uranium, followed by other solutions to still further darken the double salts formed on the film.

Such intensifying agents have to be applied to the image after the plate is fixed, which is done by a concentrated solution of hyposulphite of soda or by potassium cyanide, the latter salt having been first introduced by Martin and Marc Antoine Augustin Gaudin in 1853 (La Lumikre, April 23, 1853). Twenty-five grains of potassium cyanide to one ounce of water is the strength of the solution usually employed. The reaction of both these fixing agents is to form with the sensitive salts of silver double hyposulphites or cyanides, which are soluble in water and salt. The utility of bromides in the collodion process seems to have been recognized in its earliest days, Scott Archer (1852) and R. J. Bingham (1850) both mentioning it. We notice this, since as late as 1866 a patent-right in its use was sought to be enforced in America, the patent being taken out by James Cutting in July 1854.

Positive Pictures by the Collodion Process. In the infancy of the collodion process it was shown by Home that a negative image could be made to assume the appearance of a positive by whitening the metallic silver deposit. This he effected by using with the pyrogallic acid developer a small quantity of nitric acid. A better result was obtained by P. W. Fry with ferrous sulphate and ferrous nitrate, whilst Hugh Diamond gave effect to the matter in a practical way. F. Scott Archer used mercuric chloride to whiten the image. To Robert Hunt, however, must be rewarded the credit of noticing the action of this salt on the image (Phil. Trans., 1843). The whitened picture may be made to stand out against black velvet, or black varnish may be poured over the film to give the necessary black background, or, more recently, the positive pictures may be produced on japanned iron plates (ferrotype plates) or on japanned leather. This process is still occasionally practised by itinerant photographers.

Moist Collodion Process. It is seen that for the successful working of the collodion process it was necessary that the plate should be exposed very shortly after its preparation; this was a drawback, inasmuch as it necessitated taking a heavy equipment into the field. In 1856, Sir William Crookes and J. Spiller published in the Philosophical Magazine a process whereby they were enabled to keep a film moist (so as to prevent crystallization of the silver nitrate) several days, enabling plates to be prepared at home, exposed in the field, and then developed in the dark room. The plate was prepared in the usual way and a solution of zinc nitrate and silver nitrate in water was made to flow over it. The hygroscopic nature of the zinc salt kept sufficient moisture on the plate to attain the desired end. Various modifications in procedure have been made.

Dry Plates. It would appear that the first experiments with collodion dry plates were due to Marc Antoine Augustin Gaudin. In La Lumiere of the 22nd of April and the 27th of May 1854 he describes his researches on the question; whilst in England G. R. Muirhead, on the 4th of August 1854, stated that light acts almost as energetically on a dry surface as on a wet after all the silver has been washed away from the 'former previous to desiccation. J. M. Taupenot, however, seems to have been the first to use a dry-plate process that was really workable. His original plan was to coat a plate with collodion, sensitize it in the ordinary manner, wash it, cause a solution of albumen to flow over the surface, dry it, dip it in a bath of silver nitrate acidified with acetic acid, and wash and dry it again. The plate was then in a condition to be exposed, and was to be developed with pyrogallic acid and silver. In this method we have a double manipulation, which is long in execution, though perfectly effective.

A great advance was made in all dry-plate processes by the introduction of what is known as the " alkaline developer," which is, however, inapplicable to all plates on which silver nitrate is present in the free state. The developers previously described, either for collodion or paper processes, were dependent on the reduction of metallic silver by some such agent as ferrous sulphate, the reduction taking place gradually and the reduced particles aggregating on those portions of the film which had been acted upon by light. The action of light being to reduce the silver iodide, bromide or chloride, these reduced particles really acted as nuclei for the crystallized metal. It will be evident that in such a method of development the molecular attraction acts at distances relatively great compared with the diameters of the molecules themselves. If it were possible to reduce the altered particles of silver salt it was plain that development would be more rapid, and also that the number of molecules reduced by light would be smaller if the metallic silver could be derived from silver compounds within shorter distances of the centres of molecular attraction. Alkaline development accomplished this to a very remarkable extent; but the method is only really practicable when applied to films containing silver bromide and chloride, as silver iodide is only slightly amenable to the alkaline development. The introduction of this developer is believed to be of American origin; and it is known that in the year 1862 Major C. Russell used it with the dry plates he introduced.

An alkaline developer consists of an alkali, a reducing agent and a restraining agent. These bodies, when combined and applied to the solid silver bromide or chloride, after being acted upon by light, were able to reduce the sub-bromide or sub-chloride, and to build up an image upon it, leaving the unaltered bromide intact, except so far as it was used in the building up. In 1877 Sir W. Abney investigated this action. A dry plate was prepared by the bath process m the usual manner (to be described below), and exposed in the camera. The exposed film was covered with another film of collodiobromide emulsion, which of course had not seen the light. An image was obtained from the double film by means of the alkaline developer, which penetrated through the upper unexposed film. The development was prolonged until an image appeared through the unexposed film, when the plate was fixed, washed and dried. A piece of gelatinous paper was cemented on the upper film, and a similar piece on the lower after both had been stripped off the glass. When quite dry the two papers were forcibly separated, a film adhering to each. The upper film, although never exposed to light, showed an image in some cases more intense than the under film. The action of the alkaline developer was here manifest: the silver bromide in close contiguity to the exposed particles was reduced to the metallic state. Hence, from this and similar experiments, Abney concluded that silver bromide could not exist in the presence of a freshly precipitated or reduced metallic silver, and that a sub-bromide was immediately formed. From this it will be seen that the deposited silver is well within the Sphere of molecular attraction, and that consequently a less exposure (i.e. the reduction of fewer molecules of the sensitive salt) would give a developable image.

The alkalis used embraced the alkalis themselves and the mono-carbonates. The sole reducing agent up till recent times was pyrogallic acid. In the year 1880 Abney found that hydroquinone was even more effective than pyrogallic acid, its reducing power being stronger. Various other experimentalists tried other kindred substances, but without adding to the list of really useful agents until recently.

The following are some of the most effective :

Eikonogen Developer.

Eikpnogen 25 parts.

Sodium sulphite .... 5 i> Sodium carbonate .... 50 Potassium bromide .... j ,, Water . .... 1000 _ This is a one-solution developer, and acts energetically.

Metal Developer.

Solution A.

Metol 2 parts.

Sodium sulphite 18 Water 100 Solution B.

Sodium carbonate .... 6 parts.

Potassium bromide ... ID Water 100 For use, take one part of A to from I to 3 parts of B.

Amidol Developer.

Amidol 3 parts.

Sodium sulphite 100 Potassium bromide . . . I to 3 Water 1000 This developer requires no addition of alkali.

Ortol Developer.

Solution A. Ortol ........ 15 parts.

Sodium metabisulphite . . 7 Water 1000 Solution B.

Sodium carbonate .... loo parts. Sodium sulphite .... 125 Potassium bromide ... 3 ,, Water _ 1000 A and B solutions are mixed together in equal proportions. Besides these, there are several more, such as adurol, glycin, pyrocatechin, which have been used with more or less success. They all give a black in lieu of that dark olive-green deposit ;of silver which is so often found with pyrogallol developers. All are alkaline developers, and the image is built up from the sensitive salt within the film. They are applicable to gelatin or collodion plates, but for the latter rather more bromide of an alkali is added, to retard fogging.

Another set of developers for dry plates dependent on the reduction of the silver bromide and the metallic state is founded on the fact that certain organic salts of iron can be utilized. In 1877 M. Carey Lea of Philadelphia and William Willis announced almost simultaneously that a solution of ferrous oxalate in neutral potassium oxalate was effective as a developer, and from that time its use has been acknowledged. In 1882 J. M. Eder demonstrated that gelatino-silver chloride plates could be developed with ferrous citrate, which could not be so readily accomplished with ferrous oxalate. The exposure for chloride plates when developed by the latter was extremely prolonged. In the same year Abney showed that if ferrous oxalate were dissolved in potassium citrate a much more powerful agent was formed, which allowed not only gelatine-chloride plates to be readily developed but also collodio-chloride plates. These plates were undevelopable except by the precipitation method until the advent of the agents last-mentioned owing to the fact that the chloride was as readily reduced as the sub-chloride.

Amongst the components of an alkaline developer we mentioned a restrainer. This factor, generally a bromide or chloride of an alkali, serves probably to form a compound with the silver salt which has not been acted upon by light, and which is less easily reduced than is the silver salt alone the altered particles being left intact. The action of the restrainer is regarded by some as due to its combination with the alkali. But whichever theory is correct the fact remains that the restrainer does make the primitive salt less amenable to reduction. Such restrainers as the bromides of the alkalis act through chemical means; but there are others which act through physical means, an example of which we have in the preparation of a gelatin plate. In this case the gelatin wraps up the particles of the silver compound in a colloidal sheath, as it were, and the developing solution only gets at them in a very gradual manner, for the natural tendency of all such reducing agents is to attack the particles on which least work has to be expended. In the case of silver sub-bromide the developer has only to remove one atom of bromine, whereas it has to remove two in the case of silver bromide. The sub-bromide formed by light and that subsequently produced in the act of development are therefore reduced. A large proportion of gelatin compared with the silver salt in a film enables an alkaline developer to be used without any chemical restrainer; but when the gelatin bears a small proportion to the silver such a restrainer has to be used. With collodion films the particles of bromide are more or less unenveloped, and hence in this case some kind of chemical restrainer is absolutely necessary. We may say that the organic iron developers require less restraining in their action than do the alkaline developers.

In Major Russell's process the plate was prepared by immersion in a strong solution of silver nitrate and then washed and a preservative applied. The last-named agent executes two functions, one being to absorb the halogen liberated by the action of light and the other to preserve the film from atmospheric action. Tannin, which Major Russell employed, if we mistake not, is a good absorbent of the halogens, and acts as a varnish to the film. Other collodion dry-plate processes carried out by means of the silver- nitrate bath were very numerous at one time, many different organic bodies being also employed. In most cases ordinary iodized collodion was made use of, a small percentage of soluble bromide being as a rule added to it. When plates were developed by the alkaline method this extra bromide induced density, since it was the silver bromide alone which was amenable to it, the icdide being almost entirely unaffected by the weak developer which was at that time in general use.

Dry-Plate Bath Process. One of the most successful bat! dry-plate processes was introduced by R. Manners Gordon The plate was given an edging of albumen and then coated wit! ordinary iodized collodion to which one grain per ounce o cadmium bromide had been added. It was kept in the silver nitrate bath for ten minutes, after which it was washec thoroughly. The following preservative was then applied:

( Gum arabic ....... 20 grs.

' < Sugar candy ....... 5 (Water ........ 6 dr.

2. J Gallic acid < Water 3 grs. 2 dr.

These ingredients were mixed just before use and, after filtering, applied for one minute to the plate, which was allowec to drain and set up to dry naturally. Great latitude is admissible in the exposure; it should rarely be less than four times or more than twenty times that which would be required for a wet plate under ordinary circumstances. The image may be developed with ferrous sulphate restrained by a solution of gelatin and glacial acetic acid, to which a solution of silver nitrate is added just before application, or by an alkaline developer.

In photographic processes not only has the chemical condition of the film to be taken into account but also the optical. When light falls on a semi-opaque or translucent film it is scattered by the particles in it and passes through the glass plate to the back. Here the rays are partly transmitted and partly reflected, a very small quantity of them being absorbed by the material of the glass. Theory points out that the strongest reflection from the back of the glass should take place at the " critical " angle. In 1875 Abney investigated the subject and proved that practice agreed with theory in every respect, and that the image of a point of light in development on a plate was surrounded by a ring of reduced silver caused by the reflection of the scattered light from the back surface of the glass, and that this ring was shaded inwards and outwards in such a manner that the shading varied with the intensity of the light reflected at different angles. To avoid " halation," as this phenomenon is called, it was usual to cover the back of dry plates with some material which should be in optical contact with it, and as nearly as possible of the same density as glass, and which at the same time should absorb all the photographically active rays. This was called " backing a plate."

Collodion Emulsion Processes. In 1864 W. B. Bolton and B. J. Sayce published the germ of a process which revolutionized photographic manipulations. In the ordinary collodion process a sensitive film is procured by coating a glass plate with collodion containing the iodide and bromide of some soluble salt, and then, when set, immersing it in a solution of silver nitrate in order to form silver iodide and bromide in the film. The question that presented itself to Bolton and Sayce was whether it might not be possible to get the sensitive salts of silver formed in the collodion whilst liquid, and a sensitive film given to a plate by merely letting this collodion, containing the salts in suspension, flow over the glass plate. Gaudin had attempted to do this with silver chloride, and later G. W. Simpson had succeeded in perfecting a printing process with collodion containing silver chloride, citric acid and silver nitrate; but the chloride until recently has been considered a slow working salt, and nearly incapable of development. Up to the time of W. B. Bolton and B. J. Sayce's experiments silver iodide had been considered the staple of a sensitive film on which to take negatives; and though bromide had been used by Major Russell and others, it had not met with so much favour as to lead to the omission of the iodide. At the date mentioned the suspension of silver iodide in collodion was not thought practicable, and the inventors of the process turned their attention to silver bromide, which they found could be secured in such a fine state of division that it remained suspended for a considerable time in collodion, and even when precipitated could be resuspended by simple agitation. The outline of the method was to dissolve a soluble bromide in plain collodion, and add to it drop by drop an alcoholic solution of silver nitrate, the latter being in excess or defect according to the will of the operator. To prepare a sensitive surface the collodion containing the emulsified sensitive salt was poured over a glass plate, allowed to set, and washed till all the soluble salts resulting from the double decomposition of the soluble bromide and the silver nitrate, together with the unaltered soluble bromide or silver nitrate, were removed, when the film was exposed wet, or allowed to dry and then exposed. The rapidity of these plates was not in any way remarkable, but the process had the great advantage of doing away with the sensitizing nitrate of silver bath, and thus avoiding a tiresome operation. The plates were developed by the alkaline method, and gave images which, if not primarily dense enough, could be intensified by the application of pyrogallic acid and silver nitrate as in the wet collodion process. Such was the crude germ of a method which was destined to effect a complete change in the aspect of photographic negative taking 1 ; but for some time it lay dormant. In fact there was at first much to discourage trial of it, since the plates often became veiled on development.

M. Carey Lea of Philadelphia, and W. Cooper, jun., of Reading, may be said to have given the real impetus to the method. Carey Lea, by introducing an acid into the emulsion, established a practicable collodion emulsion process, which was rapid and at the same time gave negative pictures free from veil. To secure the rapidity Carey Lea employed a fair excess of silver nitrate, and Colonel H. Stuart Wortley gained further rapidity by a still greater increase of it; the free use of acid was the only means by which this could be effected without hopelessly spoiling the emulsion. The addition of the mineral acids such as Carey Lea employed is to prevent the formation of (or to destroy when formed) any silver sub-bromide or oxide, either of which acts as a nucleus on which development can take place. Abney first showed the theoretical effect of acids on the sub-bromide, as also the effect of oxidizing agents on both the above compounds (see below). A more valuable modification was introduced in 1874 by W. B. Bolton, one of the originators of the process, who allowed the ether and the alcohol of the collodion to evaporate, and then washed away all the soluble salts from the gelatinous mass formed of pyroxylin and sensitive salt. After washing for a considerable time, the pellicle was dried naturally or washed with alcohol, and then the pyroxylin redissolved in ether and alcohol, leaving an emulsion of silver bromide, silver chloride or silver iodide, or mixtures of all suspended in collodion. In this state the plate could be coated and dried at once for exposure. Sometimes, in fact generally, preservatives were used, as in the case of dry plates with the bath, in order to prevent the atmosphere from rendering the surface of the film spotty or insensitive on development. This modification had the great advantage of allowing a large quantity of sensitive salt to be prepared of precisely the same value as to rapidity of action and quality of film.

A great advance in the use of the collodion bromide process was made by Colonel Stuart Wortley, who in June 1873 made known the powerful nature of a strongly alkaline developer as opposed to the weak one which up to that time had usually been employed for a collodion emulsion plate, or indeed for any dry plate.

An example of the preparation of a collodion emulsion and the developer is the following: 2i oz. of alcohol, 5 oz. of ether, 75 grains )f pyroxylin. _ In i oz. of alcohol are dissolved 200 grains of zinc jromide 2 ; it is then acidulated with 4 or 5 drops of nitric acid, and added to half the above collodion. In 2 drachms of water are dissolved 330 grains of silver nitrate, i oz. of alcohol being added. The silvered alcohol is next poured into the other half of the collodion ind the brominized collodion dropped in, care being taken to shake jetween the operations. An emulsion of silver bromide is formed n suspension; and it is in every case left for 10 to 20 hours to what s technically called " ripen," or, in other words, to become creamy when poured out upon a glass plate. When the emulsion has ripened it may be used at once or be poured out jnto a flat dish and he solvents allowed to evaporate till the pyroxylin becomes gelatinJus. In this state it is washed in water till all the soluble salts are carried away. After this it may be either spread out on a cloth and Iried or treated with two or three doses of alcohol, and then relissolved in equal parts of alcohol (specific gravity, -805) and ether specific gravity, -720). In this condition it is a washed emulsion, ind a glass plate can be coated with it and the film dried, or it may >e washed and some of the many preservatives, such as albumen, jeer, coffee, gum, etc., applied.

The type of a useful alkaline developer for collodion plates is as ollows :

. 96 grs. I oz.

. 12 grs.

Water distilled i oz.

Ammonium carbonate .... 80 grs.

Water i oz.

!"o develop the plate 6 minims of No. i, J drachm of No. 2, and 3 drachms of No. 3 are mixed together and made to flow over the plate after washing the preservative off under the tap. Sometimes the 1 . J Pyrogallic acid < Alcohol 2 . j Potassium bromide ' I An account of Sayce's process is to be found in the Photographic News of October 1865, or the Photographic Journal of the same date. 1 The advantages of this salt were pointed out by Leon Warnerke n 1875.


3 u CL, development is conducted in a flat dish, sometimes the solution is poured on the plate. 1 The unreduced salts are eliminated by either cyanide of potassium or sodium hyposulphite. Intensity may be given to the image, if requisite, either before or after the " fixing" operation. Where resort is had to ferrous oxalate development, the developer is made in one of two ways (i) by saturating a saturated solution of neutral potassium oxalate with ferrous oxalate, and adding an equal volume of a solution (10 grains to I oz. of water) of potassium bromide to restrain the action, or (2) by mixing, according to Eder's plan, 3 volumes by measure of a saturated solution of the potassium oxalate with I volume by measure of a saturated solution of ferrous sulphate, and adding to the ferrous oxalate solution thus obtained an equal bulk of the above solution of potassium bromide. The development is conducted in precisely the same manner as indicated above, and the image is fixed by one of the same agents.

Gelatin Emulsion Process. The facility with which silver bromide emulsion could be prepared in collodion had turned investigation into substitutes for it. As early as September 1871 Dr R. L. Maddox had tried emulsifying the silver salt in gelatin, and had produced negatives of rare excellence. In November 1873 J. King described a similar process, getting rid of the soluble salts by washing. Efforts had also been made in this direction by J. Burgess in July 1873. R. Kennett in 1874 may be said to have been the first to put forward the gelatin emulsion process in a practical and workable form, as he then published a formula which gave good and quick results. It was not till 1878, however, that the great capabilities of silver bromide when held in suspension by gelatin were fairly known; in March of that year C. Bennett showed that by keeping the gelatin solution liquid at a low temperature for as long as seven days extraordinary rapidity was conferred on the sensitive salt. The molecular condition of the silver bromide seemed to be altered, and to be amenable to a far more powerful developer than had hitherto been dreamt of. In 1874 J. S. Stas had shown that various modifications of silver bromide and chloride were possible, and it seemed that the green molecular condition (one of those noted by Stas) of the bromide was attained by prolonged warming. It may be said that the advent of rapid plates was 1878, and that the full credit of this discovery should be allotted to C. Bennett. Both Kennett and Bennett got rid of the soluble salts from the emulsion by washing; and in order to attain success it was requisite that the bromide should be in excess of that necessary to combine with the silver nitrate used to form the emulsion. In June 1879 Abney showed that a good emulsion might be formed by precipitating a silver bromide by dropping a solution of a soluble bromide into a dilute solution of silver nitrate. The supernatant liquid was decanted, and after two or three washings with water the precipitate was mixed with the proper amount of gelatin. D. B. van Monckhoven of Ghent, in experimenting with this process, hit upon the plan of obtaining the emulsion by acting on silver carbonate with hydrobromic acid, which left no soluble salts to be extracted. He further, in August 1879, announced that he had obtained great rapidity by adding to the bromide emulsion a certain quantity of ammonia. This addition rapidly altered the silver bromide from its ordinary state to the green molecular condition referred to above. At this point we have the branching off of the gelatin emulsion process into two great divisions, viz. that in which rapidity was gained by long-continued heating, and the other in which it was gained by the use of ammonia a subdivision which is maintained to the present day. Opinions as to the merits of the two methods are much divided, some maintaining that the quality of the heated emulsion is better than that produced by alkalinity, and vice versa. We may mention that in 1881 Dr A. Herschel introduced a plan for making an alcoholic gelatin emulsion with the idea of inducing rapid drying of the plates, and in the same year H. W. Vogel of Berlin introduced a method of combining gelatin and pyroxylin together by means of a solvent which acted on the gelatin and allowed the addition of alcohol in order to dissolve the pyroxylin. This " collodio-gelatm emulsion " was only a shortlived process, which is not surprising, since its preparation involved the inhalation of the fumes of acetic acid.

1 For further details the reader is referred to Instruction in Photography, nth ed., p. 362.

The warming process introduced by Bennett was soon superseded. Colonel Stuart Wortley in 1879 announced that, by raising the temperature of the vessel in which the emulsion was stewed to 150 F., instead of days being required to give the desired sensibility only a few hours were necessary. A further advance was made by boiling the emulsion, first practised, we believe, by G. Mansfield in 1879. Another improvement was effected by W. B. Bolton by emulsifying the silver salt in a small quantity of gelatin and then raising the emulsion to boiling point, boiling it for from half an hour to an hour, when extreme rapidity was attained. Many minor improvements in this process have been made from time to time. It may be useful to give an idea of the relative rapidities of the various processes we have described.

Daguerreotype, originally .... half an hour's exposure.

Calotype 2 or 3 minutes' Collodion 10 seconds' Collodion emulsion 15 seconds' ,, Rapid gelatin emulsion .... jVth second TECHNIQUE or PHOTOGRAPHY Gelatin Emulsions.

The following is an outline of two representative processes. All operations should be conducted in light which can act but very slightly on the sensitive salts employed, and this is more necessary with this process than with others on account of the extreme ease with which the equilibrium of the molecules is upset in giving rise to the molecule which is developable. The light to work with is gaslight or candlelight passing through a sheet of Chance's stained red glass backed by orange paper. Stained red glass allows but few chemically effective rays to pass through it, whilst the orange paper diffuses the light. If daylight be employed, it is as well to have a double thickness of orange paper. The following should be weighed out: Potassium iodide 5 B rs - 3- 4- Potassium bromide ...._. Nelson's No. I photographic gelatin Silver nitrate ( Autotype or other hard gelatin ' ' 135 3 175 100 Nelson's No. I gelatin loo Nos. 3 and 5 are rapidly covered with water or washed for a few seconds under the tap to get rid of any dust. No. 2 is dissolved in i J oz. of water, and a little tincture of iodine added till it assumes a light sherry colour. No. i is dissolved in 60 minims of water. No. 4 is dissolved in J oz. of water, and No. 3 is allowed to swell up in I oz. of water, and is then dissolved by heat. All the flasks containing these solutions are placed in water at 150 F. and carried into the " dark room," as the orange-lighted chamber is ordinarily called; Nos. 3 and 4 are then mixed together in a jar or flask, and No. 2 added drop by drop till half its bulk is gone, when No. I is added to the remainder, and the double solution is dropped in as before. When all is added there ought to be formed an emulsion which is very ruddy when examined by gaslight, or orange by daylight. The flask containing the emulsion is .next placed in boiling water, which is kept in a state of ebullition for about threequarters of an hour. It is then ready, when the contents of the flask have cooled down to about 100 F., for the addition of No. 5, which should in the interval be placed in 2 oz. of water to swell and finally be dissolved. The gelatin emulsion thus formed is placed in a cool place to set, after which it is turned into a piece of coarse canvas or mosquito netting made into a bag. By squeezing, threads of gelatin containing the sensitive salt can be made to fall into cold water; by this means the soluble salts are extracted. This is readily done in two or three hours by frequently changing the water, or by allowing running water to flow over the emulsion-threads. The gelatin is next drained by straining canvas over a jar and turning out the threads on to it, after which it is placed in a flask, and warmed till it dissolves, half an ounce of alcohol being added. Finally it is filtered through chamois leather or swansdown calico. In this state it is ready for the plates.

The other method of forming the emulsion is with ammonia. The same quantities as before are weighed out, but the solutions of Nos. 2 and 3 are first mixed together and No. 4 is dissolved in i oz. of water, and strong ammonia of specific gravity -880 added to it till the oxide first precipitated is just redissolved. This solution is then dropped into Nos. 2 and 3 as previously described, and finally No. I is added. In this case no boiling is required; but to secure rapidity it is as well that the emulsion should be kept an hour at a temperature of about 90 F., after which half the total quantity of No. 5 is added. When set the emulsion is washed, drained, and redissolved as before; but in order to give tenacity to the gelatin the remainder of No. 5 is added before the addition of the alcohol, and before filtering.

Coating the Plates. Glass plates are best cleaned with nitric acid, rinsed, and then treated with potash solution, rinsed again, and dried with a clean cloth. They are then ready for receiving the emulsion, which, after being warmed to about 120 F., is poured on them to cover well the surface. This being done, the plates are placed on a level shelf and allowed to stay there till the gelatin is thoroughly set ; they are then put in a drying cupboard, through which a current of warm air is made to pass. It should be remarked that the warmth is only necessary to enable the air to take up the moisture from the plates. They ought to dry in about twelve hours, and they are ready for use.

Exposure, With a good emulsion and on a bright day the exposure of a plate to a landscape, with a lens whose aperture is one-sixteenth that of the focal distance, should not be more than one-half to one-fifth of a second. This time depends, of course, on the nature of the view ; if there be foliage in the immediate foreground it will be longer. In the portrait-studio, under the same circumstances, an exposure with a portrait lens may be from half a second to four or five seconds.

Development of the Plate. To develop the image either a ferrous ux.il.it i- solution or alkaline pyrogallic acid may be used. _No chemical restrainer such as potassium bromide is necessary, since the gelatin itself acts as a physical restrainer. If the alkaline developer be used, the following may be taken as a good standard :

Pyrogallol ......... 5 grs.

Citric acid ......... 10 Water .......... i oz.

Potassium bromide ....... 10 grs.

Water .......... I oz.

Ammonia, -880 ........ i dr.

Water .......... 9 One dram of each of these is taken and the mixture made up to 2 oz. with water. The plate is placed in a dish and the above poured over it without stoppage, whereupon the image gradually appears and, if the exposure has been properly timed, gains sufficient density for printing purposes. It is fixed in a solution of hyposulphite of soda, as in the other processes already described, and then thoroughly washed for two or three hours to eliminate all the soluble salt. This long washing is necessary on account of the nature of the gelatin.

Intensifying the Negative. Sometimes it is necessary to intensify the negative, which can be done in a variety of ways with mercury salts. An excellent plan, introduced by Chapman Jones, is to use a saturated solution of mercuric chloride in water. After thorough washing the negative is treated with ferrous oxalate. This process can be repeated till sufficient density is attained. With most other methods with mercury the image is apt to become yellow and to fade ; with this apparently it is not.

Varnishing the Negative. The negative is often protected by receiving first a film of plain collodion and then a coat of shellac or other photographic varnish. This protects the gelatin from moisture and also from becoming stained with the silver nitrate owing to contact with the sensitive paper used in silver printing. Another varnish is a solution of celloidin in amyl acetate. This is an excellent protection against damp.

Printing Processes.

The first printing process may be said to be that of Fox Talbot (see above), which has continued to be generally employed (with the addition of albumen to give a surface to the print an addition first made, we believe, by Fox Talbot).

Paper for printing is prepared by mixing 150 parts of ammonium chloride with 240 parts of spirits of wine and 2000 parts of water, though the proportions may vary. These ingredients are dissolved, and the whites of fifteen fairly-sized eggs are added and the whole beaten up to a froth. In hot weather it is advisable to add a drop of carbolic acid to prevent decomposition. The albumen is allowed two or three days to settle, when it is filtered through a sponge placed in a funnel, or through two or three thicknesses of fine muslin, and transferred to a flat dish. The paper is cut of convenient size and allowed to float on the solution for about a minute, when it is taken off and dried in a warm room. For dead prints, on which colouring is to take place, plain salted paper is useful. It can be made of the following proportions 90 parts of ammonium chloride, 100 parts of sodium citrate, 10 parts of gelatin, 5000 parts of distilled water. The gelatin is first dissolved in hot water and the remaining components are added. It is next filtered, and the paper allowed to float on it for three minutes, then withdrawn and dried.

Sensitizing Bath. To sensitize the paper it is floated on a 10 % solution of silver nitrate for three minutes. It is then hung up and allowed to dry, after which it is ready for use. To print the image the paper is placed in a printing frame over a negative and exposed to light. It is allowed to print till such time as the image appears rather darker than it should finally appear.

Toning and Fixing the Print. The next operation is to tone and fix the print. In the earlier days this was accomplished by means of a bath of sel d'or a mixture of hyposulphite of soda and gold chloride. This gilded the darkened parts of the print which light had reduced to the semi-metallic state: and on the removal of the chloride by means of hyposulphite an image composed of metallic silver, an organic salt of silver and gold was left behind. There was a suspicion, however, that part of the coloration was due to a combination of sulphur with the silver, not that pure silver sulphide is in any degree fugitive, but the sulphuretted organic salt of silver seems to be liable to change. This gave place to a method of alkaline toning, or rather, we should say, of neutral toning, by employing gold chloride with a salt, such as the carbonate or acetate of soda, chloride of lime, borax, etc. By this means there was no danger of sulphurization during the toning, to which the method by sel d'or was prone owing to the decomposition of the hyposulphite. The substances which can be employed in toning seem to be those in which an alkaline base is combmeu with a weak acid, the latter being readily displaced by a stronger acid, such as nitric acid, which must exist in the paper after printing. This branch of photography owes much to the Rev. T. F. Hardwich, he having carried on extensive researches in connexion with it during 1854 an d subsequent years. A. Davanne and A. Girard, a little later, also investigated the matter with fruitful results.

The following may be taken as two typical toning-baths:

Gold chloride Sodium carbonate Water .... Borax . Water . . gold chloride (ft)

Water i part. 10 parts. 5000 100 4000 i part. 4000 parts In the latter (a) and (/3) are mixed in equal parts immediately before use. Each of these is better used only once. A third bath gold chloride 2 parts.

Chloride of lime 2 Chalk 40 Water 8000 These are mixed together, the water being warmed. When cool the solution is ready for use. In toning prints there is a distinct difference in the modus operandi according to the toning-bath employed. Thus in the first two baths the print must be thoroughly washed in water to remove all free silver nitrate, that salt forming no part in the chemical reactions. On the other hand, where free chlorine is used, the presence of free silver nitrate or some active chlorine absorbent is a necessity. In 1872 Abney showed that with such a toning-bath free silver nitrate might be eliminated, and if the print were immersed in a solution of a salt such as lead nitrate the toning action proceeded rapidly and without causing any fading of the image whilst toning, which was not the case when the free silver nitrate was totally removed and no other chlorine absorbent substituted. This was an important factor, and one which had been overlooked. In the third bath the free silver nitrate should only be partially removed by washing. The print, having been partially washed or thoroughly washed, as the case may be, is immersed in the toning-bath till the image attains a purple or bluish tone, after which it is ready for fixing. The solution used for this purpose is a 20% solution of hyposulphite of soda, to which it is best to add a dew drops of ammonia in order to render it alkaline. About ten minutes suffice to effect the conversion of the chloride into hyposulphite of silver, which is soluble in hyposulphite of soda and can be removed by washing. The organic salts of silver seem, however, to form a different salt, which is partially insoluble, but which the ammonia helps to remove. If it is not removed there is a sulphur compound left behind, according to J. Spiller, which by time and exposure becomes yellow.

The use of potassium cyanide for fixing prints is to be avoided, as this reagent attacks the organic coloured oxide which, if removed, would render the print a ghost. The washing of silver prints should be very complete, since it is said that the least trace of hyposulphite left behind renders the fading of the image a mere matter of time. The stability of a print has been supposed to be increased by immersing it, after washing, in a solution of alum. The alum, like any acid body, decomposes the hyposulphite into sulphur and sulphurous acid. If this be the case, it seems probable that the destruction of the hyposulphite by time is not the occasion of fading, but that its hygroscopic character is. This, however, is a moot point. It is usual to wash the prints some hours in running water. We have found that half a dozen changes of water, and between successive changes the application of a sponge to the back of each print separately, are equally or more efficacious. On drying the print assumes a darker tone than it has after leaving the fixing bath.

Different tones can thus be given to a print by different toningbaths; and the gold itself may be deposited in a ruddy form or in a blue form. The former molecular condition gives the red and sepia tones, and the latter the blue and black tones. The degree of minute subdivision of the gold may be conceived when it is stated that, on a couple of sheets of albuminized paper fully printed, the gold necessary to give a decided tone does not exceed half a grain.

Collodio-chloride Silver Printing Process. In the history of the emulsion processes we stated that Gaudin attempted to use silver chloride suspended in collodion, but it was not till the year 1864 that any practical use was made of the suggestion so far as silver printing is concerned. In the autumn of that year George Wharton Simpson worked out a method which has been more or less successfully employed. The formula appended is Simpson's :

J Sijver nitrate 60 parts.

I Distilled water 60 Strontium chloride 64 Alcohol 1000 .

Citric acid 64 Alcohol 1000 ,, To every 1000 parts of plain collodion 30 parts of No. I, previously mixed with 60 parts of alcohol, are added ; 60 parts of No. 2 are next mixed with the collodion, and finally 30 parts of No. 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it contains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the key to the whole process; for, unless some body were present which on exposure to light was capable of forming a highly-coloured organic oxide of silver, no vigour would be obtained in printing. If pure chloride be used, though an apparently strong image would be obtained, yet on fixing only a feeble trace of it would be left, and the print would be worthless. The collodio-chloride emulsion may be applied to glass, or to paper, and the printing carried on in the usual manner. The toning takes place by means of the chloride of lime or by ammonium sulphocyamde and gold, which is practically a return to the sel d'or bath. The organic salt formed in this procedure does not seem so prone to be decomposed by keeping as does that formed by albumen, and the washing can be more completely carried out. There are in the market several papers which are collodio-chloride.

Gelatino-citro-chloride Emulsion. A modified emulsion printing process was introduced by Abney in 1881, which consisted in suspending silver chloride and silver citrate in gelatin, there being no excess of silver present. The formula of producing it is as follows :

Sodium chloride 40 parts.

Potassium citrate 40 , Water 500 Silver nitrate 150 ,, Water 500 Gelatin 300 ,, Water 1700 Nos. 2 and 3 are mixed together whilst warm, and No. I is then gently added, the gelatin solution being kept in brisk agitation. This produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. They may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given. Most, if not all, of the gelatin papers now extant are made somewhat after this manner.

Printing with Salts of Uranium. The sensitiveness of the salts of uranium to light seems to have been discovered by Niepce, and was subsequently applied to photography by J. E. Burnett in England. One of the original formulae consisted of 20 parts of uranic nitrate with 600 parts of water. Paper, which is better if slightly sized previously with gelatin, is floated on this solution. When dry it is exposed beneath a negative, and a very faint image is produced ; but it can be developed into a strong one by 6 to 10 % solution of silver nitrate to which a trace of acetic acid has been added, or by a 2% solution of gold chloride. In both these cases the silver and gold are deposited in the metallic state. Another developer is a 2% solution of potassium ferrocyanide to which a trace of nitric acid has been added, sufficient to give a red coloration. The development takes place most readily by letting the paper float on these solutions.

Self-toning Papers. There are several self-toning papers based on the chloride emulsion process. These contain the necessary amount of gold to tone the print. The print is produced in the ordinary way and then immersed in salt and water or in some cases potassium sulphocyanide. The print is finished by immersing in weak hyposulphite of soda.

Printing with Chromates: Carbon Prints. The first mention of the use of potassium bichromate for printing purposes seems to have been made by Mungo Ponton in May 1839, when he stated that paper, if saturated with this salt and dried, and then exposed to the sun's rays through a drawing, would produce a yellow picture on an orange ground, nothing more being required to fix it than washing it in water, when a white picture on an orange ground was obtained. In 1840 Edmond Becquerel announced that paper sized with iodide of starch and soakeu in potassium bichromate was, on drying, more sensitive than unsized paper. Joseph Dixon of Massachusetts, in the following year, produced copies of bank-notes by using gum arabic with potassium bichromate spread upon a lithographic stone, and, after exposure of the sensitive surface through a bank-note, by washing away the unaltered gum and inking the stone as in ordinary fithography. The same process, with slight modifications, has been used by Simonau and Toovey of Brussels, and produces excellent results. Dixon's method, however, was published in the Scientific American for 1854, and consequently, as regards priority, it ranks after Fox Talbot's photoengraving process (see below), published in 1852. On the 13th of December 1855 Alphonse Poitevin took out a patent in England, in which he vaguely described a method of taking a direct carbonprint by rendering gelatin insoluble through the action of light on potassium bichromate. This idea was taken up by John Pouncey of Dorchester, who perhaps was the first to produce veritable carbon-prints, notwithstanding that Testud de Beauregard took out a somewhat similar patent to Poitevin's at the end of 1857.

Pouncey published his process on the 1st of January 1859; but, as described by him, it was by no means in a perfect state, halftones being wanting. The cause of this was first pointed out by Abbé Laborde in 1858, whilst describing a kindred process in a note to the French Photographic Society. He says, "In the sensitive film, however thin it may be, two distinct surfaces must be recognized an outer, and an inner which is in contact with the paper. The action of light commences on the outer surface ; in the washing, therefore, the half-tones lose their hold on the paper and are washed away." J. C. Burnett in 1858 was the first to endeavour to get rid of this defect in carbon printing. In a paper to the Photographic Society of London he says, " There are two essential requisites ... (2) that in printing the paper should have its wnprepared side (and not its prepared side, as in ordinary printing) placed in contact with the negative in the pressure-frame, as it is only by printing in this way that we can expect to be able afterwards to remove by washing the unacted-upon portions of the mixture. In a positive of this sort printed from the front or prepared side the attainment of half-tones by washing away more or less depth of the mixture, according to the depth to which it has been hardened, is prevented by the insoluble parts being on the surface and in consequence protecting the soluble part from the action of the water used in washing; so that either nothing is removed, or by steeping very long till the inner soluble part is sufficiently softened the whole depth comes bodily away, leaving the paper white." This method of exposing through the back of the paper was crude and unsatisfactory, and in 1860 Fargier patented a process in which, after exposure to light of the gelatin film which contained pigment, the surface was coated with collodion, and the print placed in warm water, where it separated from tht paper support and could be transferred to glass. Poitevin success, fully opposed this patent, for he had used this means of detaching the films in his powder-carbon process, in which ferric chloride and tartaric acid were used. Fargier at any rate gave an impetus to carbon-printing, and J. W. Swan took up the matter, and in 1864 secured a patent. One of the great features in Swan's innovations was the production of what is now known as " carbon-tissue," made by coating paper with a mixture of gelatin, sugar and colouring matter, and rendered sensitive to fight by means of potassium or ammonium bichromate. After exposure to light Swan placed the printed carbon-tissue on an india-rubber surface, to which it was made to adhere by pressure. The print was immersed in hot water, the paper backing stripped off, and the soluble gelatin containing colouring matter washed away. The picture could then be retransferred to its final support of paper. In 1869 J. R. Johnson of London took out a patent in which he claimed that carbon-tissue which had been soaked in water for a short period, by its tendency to swell further, would adhere to any waterproof surface such as glass, metal, waxed paper, etc., without any adhesive material being applied. This was a most important improvement. Johnson also applied soap to the gelatin to prevent its excessive brittleness on drying, and made its final support of gelatinized paper, rendered insoluble by chrome alum. In 1874 J- R- Sawyer patented a flexible support for developing on ; this was a sized paper coated with gelatin and treated with an ammoniacal solution of shellac in borax, on which wax or resin was rubbed. The advantage of this flexible support is that the dark parts of the picture have no tendency to contract from the lighter parts, which they were apt to do when a metal plate was used, as was the case in Johnson's original process. With this patent, and minor improvements made since, carbon-printing has arrived at its present state of perfection.

According to P. E. Liesegang, the carbon-tissue when prepared on a large scale consists of from 120 to 150 grains of gelatin (a soft kind), 15 grains of soap, 21 grains of sugar and from 4 to 8 grains of dry colouring matter. The last-named may be of various kinds, from lamp-black pigment to soluble colours such as alizarin. The gelatin, sugar and soap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the colours, which have been ground on a slab. The mixture is filtered through fine muslin. In making the tissue in large quantities the two ends of a piece of roll-paper are pasted together and the paper hung on two rollers; one of wood about 5 in. in diameter is fixed near the top of the room and the other over a trough containing the gelatin solution, the paper being brought into contact with the surface of the gelatin by being made to revolve on the rollers. The thickness of the coating is proportional to the rate at which the paper is drawn over the gelatin : the slower the movement, the thicker the coating. The paper is taken off the rollers, cut through, and hung up to dry on wooden laths. If it be required to make the tissue sensitive at once, 120 grains of potassium bichromate should be mixed with the ingredients in the above formula. ^The carbon-tissue when prepared should be floated on a_ sensitizing bath consisting of one part of potassium bichromate in 40 parts of water. This is effected by turning up about I in. from the end of the sheet of tissue (cut to the proper size), making a roll of it, and letting it unroll along the surface of the sensitizing solution, where it is allowed to remain till the gelatin film feels soft. It js then taken off and hung up to dry in a dark room through which a current of dry warm air is passing. Tissue dried quickly, though not so sensitive, is more manageable to work than if more slowly dried. As the tissue is coloured, it is not possible to ascertain by inspection whether the printing operation is sufficiently carried out, and in order to ascertain this it is usual to place a piece of ordinary silvered paper in an actinometer, or photometer, alongside the carbon-tissue to ascertain the amount of light that has acted on it. There are several devices for ascertaining this amount, the simplest being an arrangement of a varying number of thicknesses of gold-beater's skin. The value of I, 2, 3, etc., thicknesses of the skin as a screen to the light is ascertained by experiment. Supposing it is judged that a sheet of tissue under some one negative ought to be exposed to light corresponding to a given number of thicknesses, chloride of silver paper is placed alongside the negative beneath the actinometer and allowed to remain there until it takes a visible tint beneath a number of thicknesses equivalent to the strength of the negative. After the tissue is removed from the printing-frame supposing a double transfer is to be made it is placed in a dish of cold water, face downwards, along with a piece of Sawyer's flexible support. When the edges of the tissue begin to curl up, its surface and that of the flexible support are brought together and placed flat. The water is pressed out with an indiarubber squeezer or " squeegee " and the two surfaces adhere. About a couple of minutes later they are placed in warm water of about 90 to 100 F., and the paper of the tissue, loosened by the gelatin solution next it becoming soluble, can be stripped off, leaving the image (reversed as regards right and left) on the flexible support. An application of warm water removes the rest of the soluble gelatin and pigment. When dried the image is transferred to its permanent support. This usually consists of white paper coated with gelatin and made insoluble with chrome alum, though it may be mixed with barium sulphate or other similar pigments. This transfer- paper is made to receive the image by being soaked in hot water till it becomes slimy to the touch; and the surface of the damped print is brought into contact with the surface of the retransfer-paper, in the same manner as was done with the flexible support and the carbon-tissue. When dry the retransfer- paper bearing the gelatin image can be stripped off the flexible support, which may be used again as a temporary support for other pictures. If a reversed negative be used the image may be transferred at once to its final support instead of to the temporary flexible support, which is a point of practical value, since single-transfer are better than double-transfer prints.

Printing with Salts of Iron. Sir John Herschel and Robert Hunt entered into various methods of printing with salts of iron. At the present time two or three are practised, being used in draughtsmen's offices for copying tracings (see SUN-COPYING), Photo-mechanical Printing Processes. Poitevin claimed to have discovered that a film of gelatin impregnated with potassium bichromate, after being acted upon by light and damping, would receive greasy ink on those parts which had been affected by light. But Paul Oreloth seems to have made the discovery previous to 1854, for in his patent of that year he states that his designs were inked with printing ink before being transferred to stone or zinc. C. M.TessiedeMotay (in 1865) and C. R. Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general procedure consisted in coating metallic plates with gelatin impregnated with potassium or ammonium bichromate or tri-chromate and mercuric chloride, then treating with silver oleate, exposing to light through a negative, washing, inking with a lithographic roller, and printing from the plates as for an ordinary lithograph. The half-tints by this process were very good, and illustrations executed by it are to be found in several existing works. The method of producing the plates, however, was most laborious, and it was simplified by A. Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more durable than those of Tessie de Motay. He added gum-resins, alum, tannin and other such matters, which had the property of hardening gelatin; but the difficulty of adding sufficient to the mass in its liquid state before the whole became coagulated rendered these unmanageable. It at last occurred to him that if the hardening action of light were utilized by exposing the surface next the plate to light after or before exposing the front surface to the film and the image, the necessary hardness might be given to the gelatin without adding any chemical hardeners to it. In Tessie de Motay 's process the hardening was almost absent, and the plates were consequently not durable. It is evident that to effect this one of two things had to be done: either the metallic plate used by Tessie de Motay must be abandoned, or else the film must be stripped off the plate and exposed in that manner. Albert adopted the transparent plate, and his success was assured, since instead of less than a hundred impressions being pulled from one plate he was able to take over a thousand. This occurred about 1867, but the formula was not published for two or three years afterwards, when it was divulged by Ohm and Grossman, one of whom had been employed by Albert of Munich, and had endeavoured to introduce a process which resembled Albert's earlier efforts. The name of " Lichtdruck " was given about this time to these surface-printing processes, and Albert may be considered, if not the inventor, at all events the perfecter of the method. Another modification of " Lichtdruck " was patented in England by Ernest Edwards under the name of " heliotype."

Woodbury Type. This process was invented by W. Woodbury about the year 1864, though we believe that J. W. Swan had been working independently in the same direction about the same time. In October 1864 a description of the invention was given in the Photographic News. Marc Antoine A. Gaudin claimed the principle of the process, insisting that it was old, and basing his pretensions on the fact that he had printed with translucent ink from intaglio blocks engraved by hand; but at the same time he remarked that the application of the principle might lead to important results. It was just these results which Woodbury obtained, and for which he was entitled to the fullest credit. Woodbury subsequently introduced certain modifications, the outcome being what is known as the " stannotype process," of which in 1880 he read a description before the French Photographic Society (see PROCESS).

Photo-lithography. Reference has been made to the effect of light on gelatin impregnated with potassium bichromate, whereby the gelatin becomes insoluble, and also incapable of absorbing water where the action of the light has had full play. It is this last phenomenon which occupies such an important place in photolithography. In the spring of 1859 E. J. Asser of Amsterdam produced photographs on a paper basis in printer's ink. Being anxious to produce copies of such prints mechanically, he conceived the idea of transferring the greasy ink impression to stone, and multiplying the impressions by mechanical lithography. Following very closely upon Asser, J. W. Osborne of Melbourne made a similar application; his process is described by himself in the Photographic Journal for April 1860 as follows: "A negative is produced in the usual way, bearing to the original the desired ratio. ... A positive is printed from this negative upon a sheet of (gelatinized) paper, so prepared that the image can be transferred to stone, it having been previously covered with greasy printer's ink. The impression is developed by washing away the soluble matter with hot water, which leaves the ink on the lines of print of the map or engraving." The process of transferring is accomplished in the ordinary way. Early in 1860 Colonel Sir H. Tames, R.E., F.R.S., brought forward the Southampton method of photo- lithography, which had been carefully worked out by Captain de Courcy Scott, R.E. The " papyrotype process " was published by Abney in 1870 (see LITHOGRAPHY and PROCESS).

Photographs in Natural Colours.

The first notice on record of coloured light impressing its own colours on a sensitive surface is in the passage already quoted from the Farbenlehre of Goethe, where T. J. Seebeck of Jena (1810) describes the impression he obtained on paper impregnated with moist silver chloride. In 1839 Sir J. Herschel (Athenaeum, No. 621) gave a somewhat similar description. In 1848 Edmond Becquerel succeeded in reproducing upon a daguerreotype plate not only the colours of the spectrum but also, up to a certain point, the colours of drawings and objects. His method of proceeding was to give the silver plate a thin coating of silver chloride by immersing it in ferric or cupric chlorides. It may also be immersed in chlorine water till it takes a feeble rose tint. Becquerel preferred to chlorinize the plate by immersion in a solution of hydrochloric acid in water, attaching it to the positive pole of a voltaic couple, whilst the other pole he attached to a platinum plate also immersed in the acid solution. After a minute's subjection to the current the plate took successively a grey, a yellow, a violet and a blue tint, which order was again repeated. When the violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it produced the spectrum colours, but it was found better to heat the plate till it assumed a rose tint. At a later date Niepce de St Victor chlorinized by chloride of lime, and made the surface more sensitive by applying a solution of lead chloride in dextrin. G. W. Simpson also obtained coloured images on silver chloride emulsion in collodion, but they were less vivid and satisfactory than those obtained on daguerreotype plates. Poitevin obtained coloured images on ordinary silver chloride paper by preparing it in the usual manner and washing it and exposing it to light. It was afterwards treated with a solution of potassium bichromate and cupric sulphate, and dried in darkness. Sheets so prepared gave coloured images from coloured pictures, which he stated could be fixed by sulphuric acid (Comptes rendus, 1868, 61, p. n). In the Bulletin de la Societi francaise (1874) Colonel St Florent described experiments which he made with the same object. He immersed ordinary or albuminized paper in silver nitrate and afterwards plunged it into a solution of uranium nitrate and zinc chloride acidulated with hydrochloric acid; it was then exposed to light till it took a violet, blue or lavender tint. Before exposure the paper was floated on a solution of mercuric nitrate, its surface dried, and exposed to a coloured image.

It is supposed though it is very doubtful if it be so that the nature of the chloride used to obtain the silver chloride has a great effect on the colours impressed; and Niepce in 1857 made some observations on the relationship which seemed to exist between the coloured flames produced by the metal and the colour impressed on a plate prepared with a chloride of such a metal. In i8&o Abney showed that the production of colour really resulted from the oxidation of the chloride that was coloured by light. Plates immersed in a solution of hydrogen peroxide took the colours of the spectrum much more rapidly than when not immersed, and the size of the molecules seemed to regulate the colour. He further stated that the whole of the spectrum colours might be derived from a mixture of two or at most three sizes of molecules.

In 1841, Robert Hunt published some results of colour-photography by means of silver fluoride. A paper was washed with silver nitrate and with sodium fluoride, and afterwards exposed to the spectrum. The action of the spectrum commenced at the centre of the yellow ray and rapidly proceeded upwards, arriving at its maximum in the blue ray. As far as the indigo the action was uniform, whilst in the violet the paper took a brown tint. When it was previously exposed, however, a yellow space was occupied where the yellow rays had acted, a green band where the green had acted, whilst in the blue and indigo it took an intense blue, and over the violet there was a ruddy brown. In reference to these coloured images on paper it must not be forgotten that pure salts of silver are not being dealt with as a rule. An organic salt of silver is usually mixed with silver chloride paper, the organic salt being due to the sizing of the paper, which towards the red end of the spectrum is usually more sensitive than the chloride. If a piece of ordinary silver chloride paper is exposed to the spectrum till an impression is made, it will usually be found that the blue colour of the darkened chloride is mixed with that due to the coloration of the darkened organic compound of silver in the violet region, whereas in the blue and green this organic compound is alone affected, and is of a different colour from that of the darkened mixed chloride and organic compound. This naturally gives an impression that the different rays yield different tints, whereas this result is simply owing to the different range of sensitiveness of the bodies. In the case of the silver chlorinized plate and of true collodio-chloride, in which no organic salt has been dissolved, we have a true coloration by the spectrum. At present there is no means of permanently fixing the coloured images which have been obtained, the effect of light being to destroy them. If protected from oxygen they last longer than if they have free access to it, as is the case when the surface is exposed to the air.

A method devised by Gabrielle Lippmann, of Paris, by which the natural colours of objects are reproduced by means of interference, may be briefly described as follows: A sensitive plate is placed in contact with a film of mercury, and the exposure to the spectrum, or to the image of coloured objects to be photographed, is made through the back of the plate. On development, the image appears coloured when viewed at one particular angle, the colours being approximately those of the object. The necessary exposure to produce this result was very prolonged in the first experiments in which the spectrum was photographed, and a longer exposure had to be given to the red than was required for the blue. Lippmann at first employed collodion dry plates, prepared, it is believed, with albumen, and it required considerable manipulation to bring out the colours correctly. A. Lumiere used gelatin plates dyed with appropriate dyes (orthochromatic plates); the exposure was much diminished, and very excellent representations were produced of all natural colours. The main point to aim at in the preparation of the plate seems to be to obtain a very sensitive film without any, or, at all events, with the least possible, " grain " in the sensitive salt. A formula published by Lumiere seems to attain this object. Viewed directly, the developed images appear like ordinary negatives, but when held at an angle to the light the colours are vivid. They are not pure monochromatic colours, but have very much the quality of colours obtained by polarized light. It appears that they are produced by what may be termed " nodes " of differentcoloured lights acting within the film. Thus in photographing the spectrum, rays penetrate to the reflecting mercury and are reflected back from it, and these, with the incident waves of light, form nodes where no motion exists, in a somewhat similar way to those obtained in a cord stretched between two points when plucked. In the negative these nodal points are found in the thickness of the silver deposit. When white light is sent through the film after the image has been developed, theoretically only rays of the wavelengths which formed these nodes are reflected to the eye, and thus we get an impression of colour.

Action of Light on Chemical Compounds. Reference has been made above to early investigations on the chemical action of light. In 1777 Karl Wilhelm Scheele (Hunt's Researches in Light) made the following experiments on silver salts:

" I precipitated a solution of silver by sal-ammoniac; then I edulcorated it and dried the precipitate and exposed it to the beams of the Sun for two weeks; after which I stirred the powder, and repeated the same several times. Hereupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstruum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's light, and likewise the solution of silver poured on chalk, is silver by reduction. ... I mixed so much of distilled water with well-edulcorated horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the Sun, and shook it several times each day ; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black ; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver."

This, as far as we know, is the first intimation of the reducing action of light. From this it is evident that Scheele had found that the silver chloride was decomposed by the action of light liberating some form of chlorine. Others have repeated these experiments and found that chlorine is really liberated from the chloride; but it is necessary that some body should be present which would absorb the chlorine, or, at all events, that the chlorine should be free to escape. A tube of dried silver chloride, sealed up in vacua, will not discolour in the light, but keeps its ordinary white colour. A pretty experiment is to seal up in vacua, at one end of a bent tube, perfectly dry chloride, and at the other a drop of mercury. The mercury vapour volatilizes to a certain extent and fills the tube. When exposed to light chlorine is liberated from the chloride, and calomel forms on the sides of the tube. In this case the chloride darkens. Again, dried chloride sealed up in dry hydrogen discolours, owing to the combination of the chlorine with the hydrogen. Poitevin and H. W. Vogel first enunciated the law that for the reduction by light of the haloid salts of silver halogen absorbents were necessary, and it was by following out this law that the present rapidity in obtaining camera images has been rendered possible. To put it briefly, then, the visible action of light is a reducing action, which is aided by or entirely due to the fact that other bodies are present which will absorb the halogens.

In the above we have alluded to the visible results on silver salts. It by no means follows that the exposure of a silver salt to light for such a brief period as to leave no visible effect must be due to the same effect, that is, that any of the molecules are absolutely reduced or split up by the light. That this or some other action takes place is shown by the fact that the silver salt is capable of alkaline development, that is, the particles which have suffered a change in their molecules can be reduced to metallic silver, whilst those which have not been acted upon remain unaltered by the same chemical agency. Two theories have been offered to explain the invisible change which takes place in the salts of silver. One is based on the supposition that the molecules of the salt can rearrange their atoms under the vibrations caused by the ether waves placing them in more unstable positions than they were in before the impact of light took place. This, it is presumed, would allow the developer to separate the atoms of such shaken molecules when it came in contact with them. The other theory is that, as in the case of the visible effects of light, some of the molecules are at once reduced and that the developer finishes the disintegration which the light has begun. In the case of the alkaline development the unaltered molecules next those primarily reduced combine with the reduced silver atom and again form an unstable compound and are in their turn reduced.

The first theory would require some such action as that just mentioned to take place and cause the invisible image formed by the shaking apart of the light-stricken molecules to become visible. It is hard to see why other unacted upon molecules close to those which were made unstable and which have been shaken apart by the developer should themselves be placed in unstable equilibrium and amenable to reduction. In the second theory, called the " chemical theory," the reduction is perfectly easy to understand. Abney adopts the chemical theory as the balance of unsubstantiated evidence is in its favour. There is another action which seems to occur almost simultaneously when exposure takes place in the absence of an active halogen absorbent, as is the case when the exposure is given in the air, that is, an oxidizing action occurs. The molecules of the altered haloid salts take up oxygen and form oxides. If a sensitive salt be briefly exposed to light and then treated with an oxidizing substance, such as potassium bichromate, potassium permanganate, hydrogen peroxide, ozone, an image is not developed, but remains unaltered, showing that a change has been effected in the compound which under ordinary circumstances is developable. If such an oxidized salt be treated very cautiously with nascent hydrogen, the oxygen is withdrawn and the image is again capable of development. 1 Spectrum Effects on Silver Compounds. The next inquiry is as to the effect of the spectrum on the different silver compounds. We have already described Seebeck's (1810) experiments on silver chloride with the spectrum whereby he obtained coloured photographs, but Scheele in 1777 allowed a spectrum to fall on the same material, and found that it blackened much more readily in the violet rays than in any other. Senebier's experiments have been already quoted. We merely mention these HAG F E DCBA Agl+AgNOs on paper P.

AgCl+AgNOs on paper P.

Agl+AgNOa in albumen (l.e.)

Agl prepared in bath, treated with KI, washed, redipped in silver bath, developed with pyrogallic acid.

Grey AgBr in gelatin, developed alka- D. line or ferrous ozalate (I.e.)

Orange AgBr in collodion or gelatin, alkaline ferrous oxalate or acid developer.

Green AgBr in collodion, developed ferrous oxalate ... . .

AgCl in collodion, excess of AgNOs or Nad present, ferrous citrate or acid development.

Agl+AgBr, washed from AgNOa FIG. i. Spectrum Effects on Salts of Silver. [P. = print; D. = developed; I.e. = long exposure].

two for their historical interest, and pass on to the study of the action of the spectrum on different compounds by Sir J. Herschel (Phil. Trans., 1840). He describes many experiments, which 1 See Abney, " Destruction of the Photographic Image," Phil. Mag. (1878), vol. v.; also Proc. Roy. Soc. (1878), vol. xxvii.

have become the foundation of nearly all subsequent researches of the same kind. The effects of the spectrum have been studied by various experimenters since that time, amongst whom we may mention Edmond Becquerel, John William Draper, Alphonse Louis Poitevin, H. W. Vogel, Victor Schumann and W. de W. Abney. Fig. i is compiled from a cut which appeared in the Proc. Roy. Soc. for 1882, and shows the researches made by Abney as regards the action of the spectrum on the three principal haloid salts of silver. No. 7 shows the effect of the spectrum on a peculiar modification of silver bromide made by Abney, which is seen to be sensitive to the infra-red rays.

Effect of -JDyes on Sensitive Films. In 1874 Dr H. W. Vogel of Berlin found that when films were stained with certain dyes and exposed to the spectrum an increased action on development was shown in those parts of the spectrum which the dye absorbed. The dyes which produced this action he called " optical sensitizers," whilst preservatives which absorbed the halogen liberated by light he called " chemical sensitizers." A dye might, according to him, be an optical and a chemical sensitizer. He further claimed that, if a film were prepared in which the haloid soluble salt was in excess and then dyed, no action took place unless some " chemical sensitizer " were present. The term " optical sensitizer " seems a misnomer, since it is meant to imply that it renders the salts of silver sensitive to those regions of the spectrum to which they were previously insensitive, merely by the addition of the dye. The idea of the action of dyes was at first combated, but it was soon recognized that such an action did really exist. Abney showed in 1875 that certain dyes combined with silver and formed true coloured organic salts of silver which were sensitive to light; and Dr Robert Amory went so far as to take a spectrum on a combination of silver with eosin, which was one of the dyes experimented upon by J. Waterhouse, who had closely followed Dr Vogel, and proved that the spectrum acted simply on those parts which were absorbed by the compound. Abney further demonstrated that, in many cases at all events, the dyes were themselves reduced by light, thus acting as nuclei on which the silver could be deposited. He further showed that even when the haloid soluble salt was in excess the same character of spectrum was produced as when the silver nitrate was in excess, though the exposure had to be prolonged. This action he concluded was due to the dye.

Correct Rendering of Colours in Monochrome. In Plate II., fig. 14 the sensitiveness of a plate stained with homocol is shown, and it is evident that as it is sensitive throughout the visible spectrum there must be some means of cutting off by a transparent screen so much of the spectrum luminosity at different parts that every colour having the same luminosity to the eye shall be shown on a negative of equal density. When this is done the relative luminosities of all colours will be shown by the same relative densities or in a print by different depths of greys. Abney devised a sensitometer which should be used to ascertain the colour of the screen that should be employed. By proper means the luminosity of the light of day coming through a red, a green, a blue and an orange glass can be very accurately measured; if |-in. squares of these coloured glasses, together with a white glass of the same area, be placed in a row and cemented on white glass, we have a colour-screen which we can make available for finding the kind of light-filter to be employed. This is readily done by reducing the luminosity of U.e.) the light coming through all the glasses to that of the luminosity of the light coming through the blue glass. If the luminosity of the blue be 5 and that of the white light 100, then the luminosity of the former must be reduced to-jVof its original value, and so with the other glasses. The luminosity of the light coming through each small glass square can be made equal by rotating in front of them a disk in which apertures are cut corresponding to the reduction required. The blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of 18. When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity. Another plan, based on the same principles, is to place segments of annuluses of vermilion, chrome yellow, emerald green, French blue and white on a disk, and to complete the annuluses with black segments, the amount of black depending on the luminosity of the pigments, which can be readily measured. When the disk is rotated, rings of colour, modified in brightness by black, are seen, and each ring will be of the same luminosity. As before, a screen (light-filter) to be used in front of the lens must be found which will cause the developed images of all the rings to appear of equal opacity. It must be remembered that the light hi which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured.

Action of the Spectrum on Chromic Salts. The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of No. I No. 2 FIG. 2. The top letters have reference to the Fraunhofer lines; the bottom letters are the initials of the colours. The relative sensitiveness is shown by the height of the curve above the base-line, the various parts of the spectrum on potassium bichromate. If other bichromates are employed, the action will be found to be tolerably well represented by the figures. No. i is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen.

Action of the Spectrum on Asphaltum. This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents.

Action of the Spectrum on Salts of Iron. The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced. We give this as a representation (fig. 3) of the spectra obtained on ferric salts in general. Here, again, we have an example of the law that exists as to the correlation between absorption and chemical action. One of the most remarkable compounds of iron is that experimented upon by Sir J. Herschel and later by Lord Rayleigh, viz. ferrocyanide of potassium and ferric chloride. If these two be brushed over paper, and the paper be then exposed to a bright solar spectrum, action is exhibited into the infra-red region. This is one of the few instances in which these light-waves of low refrangibility are capable of producing any effect. The colour of this solution is a muddy green, and analysis shows that it cuts off these rays as well as generally absorbs those of higher refrangibility.

Action oj Light on Uranium. The salts of uranium are affected by light in the presence of organic matter, and they too are only acted upon by those rays which they absorb. Thus nitrate of uranium, which shows, too, absorption-bands in the green blue, is affected more where these occur than in any other portion of the spectrum.

Some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are affected by light, but in a less degree than those which we have discussed. In the organic world there are very few substances which do not change by the continuous action of light, and it will be found that as a rule they are affected by the blue end of the spectrum rather than by the red end (see PHOTOCHEMISTRY).

The following table gives the names of the observers of the action of light on different substances, with the date of publication of the several observations. It is nearly identical with one given by Dr Eder in his Geschichte der Photo-Chemie.




Silver. Nitrate solution mixed with chalk, gives in sunshine copies of writing Nitrate solution on paper Nitrate photographically used J. H. Schulze . .

Hellot .... Wedgwood and Davy. Fulhame .

1727 1737 1802 1797 Nitrate with white of egg. Nitrate with lead salts Chloride Rumford . B. Fischer Herschel . J. B. Beccarius 1798 1812 1839 1757 Chloride in the spectrum . Chloride photographically used . Chloride blackened .... Iodide Scheele. . . . Wedgwood Lassaigne . Davy ....

1777 1802 1839 1814 Iodide by action of iodine (on metallic silver). Iodide photographically used Iodide with gallic acid Iodide with ferrous sulphate . Chloride and iodide by chlorine and iodine (on metallic silver). Bromide Bromide by action of bromine (on metallic silver). Sulpho-cyanide Nitrite Daguerre .

Herschel . Talbot .... Hunt . . . . Claudet . . .

Balard . . . Goddard .

Grotthus . Hess . . . .

1839 1840 1841 1844 1840 1826 1840 1818 1828 Oxide with ammonia ....

Mitscherlich . Bergman n .

1827 1779 Vauquelin .

1798 Carbonate Buchholz .

1800 Bergmann .

1779 Benzoate Citrate Trommsdorf . Vauquelin 1793 1798 Henry and Plisson 1829 Rose 1830 Stromeyer 1830 Pelouze and Gay- 1833 Lussac. Hunt .

1844 Hunt . . .

1844 Sulphide by vapour of sulphur (on metallic silver). Phosphide by vapour of phosphorus (on metallic silver). gold. Oxide Niepce. Niepce .


1820 1820 1777 Chloride on paper Hellot . . .

!737 Chloride on silk . PW .

Fulhame .

1794 Chloride in ethereal solution . . Chloride with ferrocyanide and ferricyanide of potassium. Chloride and oxalic acid .

Rumford . Hunt . . .

Dobereiner Hunt . . .

1793 1844 1831 1844 Plate of gold and iodine vapour Goddard . .

1842 Substance.




Gehlen 1804 Chloride with lime .

Herschel .

1840 Iodide Herschel .

1840 Hunt .

1844 Double chloride of platinum and Dobereiner 1828 potassium.


Oxide (mercurous) ....

Gay-Lussac and 1811 Th6nard.

Oxide Davy . . . .

1812 Oxide (mercuric)

Davy ....

1797 Oxide (more accurate observa- ( Abildgaard 1797 Harup not till .

1801 Chloride (mercurous) . . .

K. Neumann pre- 1739 viously to Chloride (mercuric) . . .

Boullay 1803 Chloride with oxalic acid .

Bergmann .

1776 Meyer . . . .

1764 Oxalate (mercuric) . . . .

Bergmann .

1776 Oxalate (mercurous) . . .

Harff . . . .

1836 Sulphate and ammonia (mer- Fourcroy .

1791 curous).

Acetate (mercurous) ....

Garot 1826 Bromide (mercuric) . Iodide (mercurous) . . . | Lowig Torosewicz Artus . . .

1828 1836 1836 Iodide (mercuric)

Field . . .

1836 Citrate (mercuric) ....

Harff . . .

1836 Tartrate and potassium (mer- Carbonell and 1831 curous).

Bravo Carbonate (mercuric).

Davy ....

1812 Nitrate Herschel 1840 Sulphide (mercuric) ....

Vitruvius . ' . .

I B.C.


Sulphate (ferrous)

Chastaing .

1877 Chloride (ferric) and alcohol Bestuscheff 1725 Chloride and ether ....

Klaproth .

1782 Oxalate (ferric)

Dobereiner 1831 Ferrocyanide of potassium .

Heinrich .

I808 Sulphocyanide Grotthus .

1818 Prussian blue Scopoli 1783 Ferric citrate with ammonium . .

Herschel 1840 Ferric tartrate Herschel . . .

1840 Chromate Hunt ....

1844 Copper.

Chloride (cupric dissolved in Gehlen 1804 ether).

Oxalate with sodium ....

A. Vogel . . .

1813 Chromate > Chromate with ammonium .

Carbonate Hunt . . . .

1844 Iodide Sulphate Chloride (cuprous) ....

A. Vogel . . .

1859 Copper plates (iodized)

Kratoch . . . Talbot ....

1841 1841 Manganese.

Sulphate Brandenburg 1815 Oxalate Suckow 1832 Potassium permanganate Peroxide and cyanide of potas- Frommberg Hunt . . . .

1824 1844 sium Chloride Hunt . . . .

1844 Lead.

Oxide Davy ....

1802 Iodide ) Sulphite 5 Schonbein .

1850 Peroxide Gay-Lussac lltl Red lead and cyanide of potas- Hunt . . . .

1844 sium Acetate ...:...

Hunt . . . .

1844 Nickel.

Nitrate ~| Nitrate with ferro-prussiates . r Hunt . . . .

1844 Iodide J Tin.

Purple of cassius Uncertain .

Various Substances.

Cobalt salts Hunt .

I S l l Arsenic sulphide (realgar) Antimony sulphide ....

Sage .... Suckow 1803 1832 Substance.



Bismuth salts 1 Cadmium salts t Hunt . . . .

1844 Rhodium salts J Vanadic salts ..._...

Roscoe 1874 Iridium ammonium chloride .

Dobereiner 1831 Potassium bichromate Mungo Ponton 1838 Potassium with iodide of starch Becquerel .

1840 Metallic chromates .

Hunt . . . .

1843 Chlorine and hydrogen .

Gay-Lussac and 1809 Thenard.

Chlorine (tithonized) ....

Draper 1842 Chlorine and ether ....

Cahours 1810 Chlorine in water Berthollet . . .

1785 Chlorine and ethylene Gay-Lussac and 1809 Thenard Chlorine and carbon-monoxide Davy . . . .

1812 Chlorine and marsh gas . . . .

Henry .

1821 Chlorine and hydrocyanic acid .

Serullas 1827 Bromide and hydrogen .

Balard ....

1832 Iodine and ethylene ....

Faraday 1821 Cyanogen, solution of ...

Pelouze and 1837 Richardson.

Various other methyl compounds Hydrocyanic acid Hypochlorites (calcium and po- Cahours Torosewicz Dobereiner 1846 1836 1813 tassium)

Uranium chloride and ether .

Gehlen 1804 Molybdenate of potassium and Jager . . . .

1800 tin salts.

Crystallization of salts under J influence of light.

Petit . . . . Chaptal . . . Dize . . . .

1722 1788 1789 Phosphorus (in hydrogen, nitro- Bockmann.

1800 gen, etc.)

Phosphuretted hydrogen A. Vogel . . .

1812 Nitric acid Scheele.

1777 Hog's fat Vogel . . . .

1806 Palm oil Fier .

1832 Asphalt Niepce 1814 Resins (mastic, sandarac, gam- Senebier 1782 boge, ammoniacum, etc.).

Guaiacum Hagemann 1782 Bitumens all decomposed, all residues of essential oils.

Daguerre .

1839 Coloured extracts from flowers .

Senebier 1782 Similar colouring matters spread Herschel .

1842 upon paper.

Yellow wax bleached Pliny . . . .


Eudoxia macrembolitissa (purple loth cent.


Other purple dyes j Cole . . . . Reaumur .

1684 1711 Oils generally Senebier 1782 Nitric ether Sencbier 1782 Nicotine Henry & Boutron- A 1 U*, 1836 Charlard.

Santonine Merk .

I88 3 Effect of Hydrogen Peroxide on Sensitive Plates. Dr W. J. Russell made a series of experiments on the effect of exposure of sensitive plates to the action of vapours and gases for long periods. It has long been known that contact of plates with such substances as wood caused a sensitive surface to show " fog " on development. By a somewhat exhaustive series of experiments, Russell showed that the probable cause of this fog is hydrogen peroxide, since substances which favoured its formation produced the same effect. This is somewhat remarkable, as this same substance will completely destroy the effect that light has had on a sensitive plate; indeed, it affords one way of destroying a light image on a sensitive collodion plate. The experiments of Russell give a warning to store exposed plates for brief periods. It appears that negatives wrapped in paraffin paper are secure from this danger.

The Application of Photography to Quantitative Measures. In order to employ photography for the measurement of light it was necessary that some means should be devised by which the opacity of the deposit produced on the development of a plate could be determined. It is believed that in 1874 the first attempt was made by Sir W. Abney to do this. In the Phil. Mag. he showed how density could be measured by means of an instrument, the diaphanometer, he had devised, in which transparent black wedges were used to make matches between the naked light and the same light after passing through the photographic opacity that had to be measured. In 1887, owing to the perfecting of the rotating sectors, which could be made to increase or diminish the apertures at pleasure during its rotation, the measurement of opacities became easy. The Rumford method of comparing the light through the deposit with the naked beam, using the sectors to equalize the illumination, was adopted, the deposit being placed between the light and the screen, the comparison light being a beam reflected from the same light on to the screen.

Owing to the fact that photographic deposit scatters light more or less, the opacities measured by this plan were slightly greater than_was shown when such opacities were to be used for contact printing. The final plan adopted by Abney was to place the part of the plate carrying the deposit to be measured behind a screen constructed as above. C D (fig. 4) is a dull black card with an aperture cut in it which may be of any desired shape. This aperture was covered with transparent paper, as was also a portion B, the same size as A, but pasted on the black card itself. Light thrown from behind A would be matched with light thrown on to B from the front when a FIG. 4.

rod in the path of this last beam was made to prevent this light falling on A. When a portion of a plate bearing a deposit was placed behind and close to A, the light thrown on B had to be diminished by the sector till the two squares appeared equally bright and the aperture of the sector was noted and compared with that required when the deposit was removed.

With this screen accurate measures of printing densities can be made, and it can also be used in the determination of the comparative photographic brightness of the light issuing from different objects. For instance, the relative brightness of the different parts of the corona as seen in a total eclipse can be readily determined if a " time scale " of gradation is impressed on the plate on which it is taken. Both scale and streamer can then be enlarged optically and thrown on the part of the screen A. The measures of the streamer densities can then be directly compared with the densities of the scale and the relative " photographic " brightness of the different parts of the streamer be ascertained by comparison with this scaje also.

The same method of measurement was adopted in ascertaining quantitatively the sensitiveness of the spectrum of ordinary plates and of plates in which dyes are present. The figures on PI. IV show reproductions of plates which were exposed to the spectrum. No. I js a continuous spectrum taken with the electric light ; no. 7 is an impressed continuous spectrum; no. 8 shows the bright lines of metals ; no. 3 the line_ spectrum of volatilized lithium and sodium to indicate the position of the spectrum colours. Nos. 4 and 2 are the absorption and fluorescent spectra of eosin. No. 5 is the graduation scale formed by a bromogelatin "Seed" plate stained with homocol, a cyanine derivative sensitive to the red; no. 6 is a similar scale formed by an unstained plate. The small numbers placed below the different bands show an empiric scale which is made to apply to each of them. The first step is to measure ' Plate \o the Sptctru rater of the etrto-tigttt bright titles of O 1O SO SO 4O Empiric Scale of the spectrum FIG. 5.

the opacity of the gradation scale, next the opacity of the continuous spectrum at the various numbers of the empiric scale, and also the opacity of the other bands at the same scale numbers. The continuous spectrum will give the sensitiveness of the plate to the different parts of the spectrum when the measures of its different opacities are compared with those of the scale of gradation, and a curve of sensitiveness can be plotted from these comparisons. It is evident that the measures of the other two bands will give us information as to the fluorescence and the absorption of the eosin. Fig. 5 shows the curve of opacity of the image of the spectrum at its different parts, and also the curve of sensitiveness of the plate to the different parts of the spectrum. This last is derived from a comparison of the measured densities with those of the gradation scale.

Measurement of the Rapidity of a Plate. The first attempt that was made to ascertain the rapidity of a plate was by Abney (Phil. Mag. 1874), who demonstrated that within limits the transparency of deposit varied as the logarithm of the exposure.

The last formula has been accepted for general use, though it is believed that it is not absolutely correct, though very approximately true and sufficiently near to be of practical value. This belief is based on the further researches described below. 1 In 1888 Sir W. Abney pointed out that the speed of a plate could be determined by the formula T = E-p(logE+C) 2 , where T is the transparency, E is the exposure (or time of exposure X intensity of light acting), and C a constant. If the abscissae (exposures) are plotted as logarithms, the curve takes the same form as that of the law of error, which has a singular point, a tangent through which lies closely along the curve and cuts the axis of Y at a point which has a value of 2/VE. If the total transparency be unity, this ordinate has a value of 1-212, the singular point having a value of 0-606. The ordinate of the zero point of the curve will be where the tangent to the singular point cuts the line drawn at i -2 12. The difference between the measurements of this zero point for two kinds of plates (i.e. C in the formula) from the points in the abscissae marking the same exposure, will give the relative sensitiveness of the two plates in terms of log x 1 . In 1800 Hurter and Driffield (Journ. Soc. Ghent. Ind. Jan. 19, 1891) worked out a less empirical formula connecting the exposure E with the density of deposit, which in an approximate shape had the form D =7log(E/), where D is the density of deposit (or log I/T), i the " inertia " of the plate, T the transparency of the deposit. In the customary way a small portion of a plate was exposed to a constant light at a fixed distance and for a fixed time, and another small portion to the same light for double the time, and so on. By measuring the densities of the various deposits and constructing a curve, a large part of which was approximately a straight line, it was found possible, by the production of the straight portion to meet the axis of X, to give the relative sensitiveness of different plates by the distance of the intersection from the zero point L. (See also Exposure Meters, below, under i, APPARATUS.)

Effect of Temperature on Sensitiveness. In 1876 Abney showed that heat apparently increased, while cold diminished, the sensitiveness of a plate, but the experiments were rather of the qualitative than the quantitative order. In 1893, from fresh experiments, 2 he found that the effect of a difference in temperature of some 40 C. invariably caused a diminution in sensitiveness of the sensitive salt at the lower temperature, a plate often requiring more than double the exposure at a temperature of about 1 8 C. than it did when the temperature was increased to +33 C. The general deduction from the experiments was that increase in temperature involved increase in sensitiveness so long as the constituents of the plate (gelatin, Sic.) were unaltered. Sir James Dewar stated at the Royal Institution in 1896 that at a temperature of 180 C. certain sensitive films were reduced in sensitiveness to less than a quarter of that which they possess at ordinary temperatures. It appears also, from his subsequent inquiry, that when the same films were subjected to the temperature of liquid hydrogen ( 252 C.) the loss in sensitiveness becomes asymptotic as the absolute zero is approached. Presumably, therefore, some degree of sensitiveness would still be preserved even at the absolute zero.

Effect of Small Intensities of Light on a Sensitive Salt? When a plate is exposed for a certain time to a light of given intensity, it is commonly said to have received so much exposure (E). If the time be altered, and the intensity of the light also, so that the exposure (time X intensity) is the same, it was usually accepted that the energy expended in doing chemical work in the film was the same. A series of experiments conducted under differing conditions has shown that such is not the case, and that the more intense the light (within certain limits) the greater is the chemical action, as shown on the development of a plate. Fig. 6 illustrates the results obtained in three cases. The exposure E is the same in all cases. The curves are so drawn that the scale of abscissae * Those applicable to the correction of star magnitudes as determined by photography have been verified and confirmed .by Schwarzchild, Michalke and others.

2 Abney, Proc. Roy. Soc. 1893.

5 Abney, Proc. Roy. Soc. 1893, and Journ. Camera Club, 1893.

is the intensity of the light in powers of 2, and the ordinates show the percentages of chemical action produced. If the chemical action remained the same when the intensity of light was reduced, E remaining the same, each of the curves would be shown as a straight line at the height of 100, which is the transparency of deposit with the unit of light. As it is, they show diminishing percentages as the light intensity is diminished.

o Set It ttVf t Intensities of Light FIG. 6.

Thus, when the intensity of the light is reduced to^V of the original, and the time of exposure is prolonged 64 times, the useful energy expended on a lantern plate is only 50 % of that expended when the light and time of exposure are each unity. In the cases to which the diagram refers, the light used was a standard amyl acetate lamp, and the unit of intensity taken was this light at a distance of 2 ft. from the plate, and the unit of time was 10 seconds. The lamp being moved to 16 ft. from the plate, gave an intensity of ^the unit, and the time of exposure had to be increased to 640 seconds, so that E was the same in both cases. Further, it was found that when the times of exposure on different parts of the plate were successively doubled, light at a fixed distance being used for one series, and altered for a second series, the slopes of the curves of transparency (i.e. the gradation) were parallel to one another. This investigation is of use when camera images are in question, as the picture is formed by different intensities of light, not very different from those of the amyl acetate lamp, the time of exposure being the same for all intensities. The deductions made from the investigation are that with a slow plate the energy expended in chemical action is smaller as the intensity is diminished, while with a quick plate the variation is much less. As a practical deduction, we may say that to obtain proper contrast in a badly lighted picture it is advisable to use a slow plate.

f< i 1 ,?; 8- th 5 ' n / 2 3 4 S 6 Scale of Intensities in Powers of 2 FIG. 7.

Effect of very Intense Light on a Sensitive Salt. Another investigation was made as to the effect of very intense light on sensitive surfaces. In this case a screen of step-by-step graduated opacities was made use of, and plates exposed through it to the action of lights markedly differing in intensity, one being that of the amyl acetate lamp, another that of the arc light, and a third the light emitted from the spark of a Wimshurst machine. The exposures were so made that one of the opacities produced on the plate from exposure to each source of light was approximately the same. The unit of intensity of light is, of course, in each case widely different. The slope of the curve due to the spark light is less steep than that due to the arc light, and the latter, again, is much less steep than that due to the amyl acetate lamp. A further investigation was made of the effect of increasing the time of exposure when the intense light was diminished, and it was found that with all plates the useful chemical energy acting on a plate was least with the most intense light, but increased as the intensity diminished, though the time was correspondingly increased. This is the reverse of what we have recorded as taking place when a comparatively feeble light was employed. Further, it was proved that the variation was greatest in those plates which are ordinarily considered to be the most rapid. It follows, therefore, that there is some intensity of light when the useful chemical energy is at a maximum, and that this intensity varies for each kind of plate.

Intermittent Exposure of a Sensitive Salt. The same investigator has shown that, if a total exposure is made up of intermittent exposures, the chemical action on a sensitive salt is less than it is when the same exposure is not intermittent. It was also proved that the longer the time of rest between the intermittent exposures (within limits) the less was the chemical action. We may quote one case. Exposures were first made to a naked light, and afterwards to the same light for six times longer, as a rotating disk intervened which had 12 apertures of 5 cut in it at equal intervals apart, and 720 intermittent exposures per second were given. The plate was moved to different distances from the light, so that the intensity was altered. The apparent loss of exposure by the intervention of the disk increases as the intensity diminishes, the ratios of the chemical energy usefully employed of the naked light exposure to that of the intermitting exposures being:

For intensity I i to -815 1 I -500 A i ,, -423 .. .. sV i -37 These results appear to be explicable by the theoretical considerations regarding molecular motion.

Effect of Monochromatic Light of Varying Wave-lengths on a Sensitive Salt. It has been a subject of investigation as to whether the gradation on a plate is altered when exposures are made to lights of different colours; that is to say, whether the shades of tone in a negative of a white object illuminated by, say, a red light, would be the same as those in the negative if illuminated by a blue light. Abney J announced that the gradation was different; and, quite independently, Chapman Jones made a general deduction for isochromatic plates that, except with a certain developer, the gradation was steeper (that is, the curve shown graphically would be steeper) the greater the wave-lengths of the light to which the sensitive salt was subjected. For plates made with the ordinary haloid salts of silver Chapman Jones's deduction requires modification. When monochromatic light from the spectrum is employed, it is found that the gradation increases with wave-lengths of light which are less, and also with those which are greater, than the light whose wave-lengths has a maximum effect on the sensitive salt experimented with. Thus with bromo-iodide of silver the maximum effect produced by the spectrum is close to the blue lithium line, and the gradation of the plate illuminated with that light is less steep than when the light is spectrum violet, green, yellow or red. From the red to the yellow the gradation is much the steepest. Whether these results have any practical bearing on ordinary photographic exposures is not settled, but that they must have some decided effect on the accuracy of three-colour work for the production of pictures in approximately natural colours is undoubted, and they may have a direct influence on the determination of star magnitudes by means of photography. Reproduction of Coloured Objects by means of Three Photographic Positives. Ives's Process. A practical plan of producing images in approximately the true colours of nature has been devised by preparing three positives of the same object, one 1 Proc. Roy. Soc., 1900.

illuminated by a red, the other by a green, and the third by a blue light; the images from these three transparencies, when visually combined, will show the colours of the object. This plan was scientifically and practically worked out by F. E. Ives of Philadelphia, though in France and elsewhere it had been formulated, especially by Hauron Du Cros.

The following description may be taken as that of Ives's process: by the trichromatic theory of colour- vision every colour in nature can be accounted for by the mixture of two or three of the three-colour sensations, red, green and blue, to which the eye is supposed to respond. Thus a mixture of a red and green sensation produces the sensation of yellow; of a green and blue, that of a blue-green; of red and blue, that of purple; and of all three, that of white. For the sensations we may substitute those colours which most nearly respond to the theoretical sensations without any material loss of purity in the resulting sensation. We must take the spectrum of white light as the only perfect scale of pure colours. It has been proved that the red sensation in the eye is excited by a large part of the visible spectrum, but with varying intensities. If, then, we can on a photographic plate produce a developed image of the spectrum which exactly corresponds in opacity and position to the amount of red stimulation excited in those regions, we shall, on illuminating a transparent positive taken from such a negative with a pure red light, have a representation of the spectrum such as would be seen by an eye which was only endowed with the sensation of red. Similarly, if negatives could be taken to fulfil the like conditions for the green and for the blue sensations, we should obtain positives from them which, when illuminated by pure green and blue light respectively, would show the spectrum as seen by an eye which was only endowed with a green or a blue sensation. Evidently if by some artifice we can throw the coloured images of these three positives on a screen, superposing them one over the other in their proper relative positions, the spectrum will be reproduced, for the overlapping colours, by their variation in intensity, will form the colours intermediate between those used for the illumination of the positives. For the purpose of producing the three suitable negatives of the spectrum, three light-filters, through which the image has to pass before reaching the photographic plate, have to be found. With all present plates these are compromises. Roughly speaking, the screens used for taking the three negatives are an orange, a bluish-green and a blue. These transmit those parts of the spectrum which answer to the three sensations. When these are obtained an image of a coloured object can be reproduced in its true colours.

Abney devised sensitometers for determining the colours of the screens to be placed before the lens in order to secure the threecolour negatives which should answer these requirements. Their production depends upon the same principles indicated as necessary for the correct rendering in monochrome of a coloured object. When the sensitonjeter takes the form of glasses through which light is transmitted to the plate, the luminosities of the coloured lights transmitted are determined, and also their percentage composition in terms of the red, green, and blue lights, and thence are deduced the luminosities in terms of red, green and blue. For ascertaining what screen should be used to produce the red negative the luminosity transmitted through each glass is so adjusted that the luminosity of the red components in each is made equal by rotating a disk with correct apertures cut out close to the row of glasses. This gives a sensitometer of equal red values. A coloured screen has to be found which, when placed in front of the lens, will cause the opacities of the deposit on the plate, corresponding to each square of glass, to be the same throughout. This is done by trial, the colour being altered till the proper result is obtained. In a similar way the " green " and " blue " screens are determined. Coloured pigments rotating on a disk can also be employed, as indicated in the paragraph on the correct rendering of colour in monochrome.

As to the camera for the amateur, whose plates are not as a rule large, all of the three negatives should be obtained on one plate, since only in this way can they be developed and the densities increased together. (For commercial work the negatives often cannot be taken on the same plate, as it would make the plate too large to manipulate.) The camera may be of an ordinary type, with a repeating back, bringing successively three different portions of the plate opposite the lens. It is convenient to have a slide, in front of which a holder containing the three screens can be fixed, which will then be close to the plate; such a one has been devised by E. Sanger-Shepherd. The light passes through them one by one as the plate is moved into the three positions. The three exposures are given separately, after which the plate is ready for development. The three separate exposures are, however, a source of trouble at times, particularly in the case of landscapes, for the lighting may vary and the sky may have moving clouds, in which case the pictures would show variations which should not exist. Sanger-Shepherd has a " one-exposure " camera by which the three images are thrown side by side on the plate. Thus any movement in the picture affects all three negatives alike. Abney has also introduced a " one-exposure " camera which takes in a larger angle than that of Sanger-Shepherd. The next point is the exposures which should be given through each screen. This can be done by placing in front of the plate and extending its whole length a scale of gradation through which the light coming from a sun-illuminated white card passes, as well as through the screens. In the case of the three-exposure camera the times of exposure are varied till the densities of the image of the gradation appear the same in each of the three images. In the case of the one-exposure camera, the light reaching the plate through the screens is altered by cutting off with a shutter more or less of the lens used. As the plates employed for the purpose of the three-colour negatives must be sensitive to every colour, the ordinary dark-room light should be most cautiously used. If used at all, it should be very feeble and development must be carried out in a dish with a cover to it. The plate is manipulated in the usual way.

Joly's Process. Professor J. Joly, of Dublin, in 180:7 introduced a colour process by which an image in approximately natural colours could be thrown upon a screen by an optical lantern, only one transparency being employed, instead of three, as in the Ives process. A " taking " screen was ruled with alternating orange, blue-green and blue lines -j-J-^ to $$-$ in. apart, touching one another and following one another in the above order. When such a screen was placed in front of a sensitive plate in the camera, and exposure made to the image of a coloured object, there were practically three negatives on the same plate, each being confined to the area occupied by lines of the same colour. The shades of colour and the depth of the colours used in ruling depended on the brand of plate. When a perfect triune negative was obtained, a transparency was made from it, and in contact with this was placed a screen ruled with lines the same distance apart, but of the colours corresponding to the three colour sensations, namely red, green and blue. The red lines were made to fall on the image taken through the orange lines, the green on that of the blue-green, and the blue or violet on that of the blue. On the screen there are practically three differently coloured images shown by one transparency. The eye blends the different colours together and a picture is seen in approximately the correct colours of the original.

Autochrome. A very remarkable process, founded on J. Joly's process, was introduced in 1907 by A. Lumiere et ses Fils of Lyons. Starch grains of very minute size, some of which were dyed with a red stain, a second portion with a green, and a third portion with a blue, are mixed together in such proportions that a fine layer of them appears grey when viewed by transmitted light. Under a magnifying glass the grains are coloured, but owing to the want of focus in the eye the colours blend one with the other. Such a layer is embedded on the surface of a glass plate in a waterproof vehicle, and a film of sensitive emulsion held in situ in some material, the composition of which has not been published, covers this layer. When such a plate is placed in the camera, with the back of the plate next the lens, the light passes through the coloured granules, and again we have three negatives on one plate, but instead of each negative being represented by lines as in the Joly process they are represented by dots of silver deposit. Owing to the way in which the threecoloured film is prepared, it is evident that a positive taken from such a negative could not be backed with granules of the right colour, as the granules are placed at random in the layer. Lumiere, to overcome this difficulty, converted the negative into a positive in a very ingenious way. The plate was developed with pyrogallic and ammonia in the usual way, but instead of fixing it it was plunged into a solution of potassium permanganate and sulphuric acid. This dissolved all the silver that had been deposited during development and left a film of unaltered silver salt. On looking through the plate the colours of the coloured layer coming through the different dots where the silver was at first deposited appeared in view, and the image was the image in colour of the object photographed. The plate after being washed was taken into the light and redeveloped with an alkaline developer, which converted the sensitive salt of silver to the metallic state. The image now consisted of black particles of silver and the coloured image. The plate was next fixed in hyposulphite of soda to remove any unreduced silver salt that might be left, and the picture after washing was complete. The coloured image so obtained is a very close representation of the 'true colours, but as the " taking " screen is the same as the " viewing " screen some little variation must result.

Positives in Three Colours. Ives was the first to show that a transparency displaying approximately all the colours in nature could be produced on the same principles that underlie the threecolour printing. This he effected by printing each of the three negatives, produced for his triple projection process as already described, on gelatine films sensitized by bichromate of potash. Each of the three transparent films was dyed with a colour complementary to the colour of the light which he transmitted through the positives when used for projection. Thus the " red " positive he dyed with a blue-green dye, the " green " positive with a purple dye, and the " blue " positive with a yellow dye. These three films, when superposed, gave the colours of the original object. Sanger-Shepherd has made the process a commercial success (see PROCESS) and produces lantern slides of great beauty, in which all colours are correctly rendered. Instead of using a dye for the " red " transparency, he converts the silver image of a positive image into an iron salt resembling Prussian blue in colour. (W. DE W. A.)

n. PHOTOGRAPHIC APPARATUS Photographic apparatus consists essentially of the camera with lens and stand, lens shutters, exposure meters, prepared plates for the production of negatives or transparencies, sensitive papers and apparatus for producing positive prints, direct or by enlargement. Besides these there are many subsidiary accessories.

Since the introduction of highly sensitive dry plates and their extended use in hand cameras, the art and practice of photography have been revolutionized. Numerous special forms of apparatus have been created suitable for the requirements of the new photography, and their manufacture and sale have become important industries. The value of the exports of photographic materials from the United Kingdom in 1906 was 22,716. The most important improvement has been in the construction of anastigmatic lenses, which, having great covering power, flatness of field, and freedom from astigmatism, can be worked with very much larger apertures than was possible with the earlier forms of rectilinear or aplanatic lenses. The increased rapidity of working thus gained has rendered it easy to photograph objects in very rapid motion with great perfection. This has encouraged the construction of the very light and compact hand cameras now so universally in use, while, again, their use has been greatly simplified by improvements in the manufacture of sensitive plates and films and the introduction of light, flexible, sensitive films which can be changed freely in daylight. The introduction in 1907 of Messrs Lumiere's " Autochrome " process of colour photography has also been a great advance, tending to popularize photographic work by the facility it offers for reproducing objects in the colours of nature.

The Camera.

Historical. The camera obscura (q.v.) was first applied to photographic use by Thomas Wedgwood between 1792 and 1802. No description of his camera is available, but it was probably one of the sketching cameras then in use. In 1812 W. H. Wollaston found that by using a meniscus lens with a concave surface towards the object and the convex towards the screen, a diaphragm being placed in front, the projected image of the camera obscura was greatly improved in sharpness over a larger field. The first photographic lenses made by V. and Ch. L. Chevalier in Paris (1830-1840) were on this principle. The photographic camera in its simplest form is a rectangular box, one end of which is fitted to carry a lens and the opposite one with a recess for holding the focusing screen and plate holders, these ends being connected by a rigid or expanding base-board and body, constructed to keep out all light from the sensitive plate except that passing through the lens. In 1816 Joseph Nic6phore Niepce, of Chalon-sur-Sa6ne, for his photographic experiments made a little camera, or artificial eye, with a box six inches square fitted with an elongated tube carrying a lenticular glass. There are now in the Chalon Museum cameras of his with an iris diaphragm for admitting more or less light to the lens; some with an accordion bellows, others with a double expanding rigid body for adjusting the focus. The iris diaphragm was adopted later by Chevalier for his photographic lenses. In 1835 W. H. Fox Talbot constructed simple box cameras for taking views of his house on sensitive paper, and claimed them as the first photographs of a building (Phil. Mag. 1839, 14, p. 206). Fr. von Kobell and C. A. Steinheil, early in 1839, made a camera with an opera glass lens for taking landscapes on paper. Later in 1839 J. W. Draper successfully used a camera for his daguerreotype experiments made of a spectacle lens, 14 in. focus, fitted into a cigar box. He also used a camera fitted with a concave mirror instead of a lens. Similar cameras were constructed by A. T. Wolcott (1840) and R. Beard (1841) for reversing the image in daguerreotype portraits. They have also been recommended by V. Zenger (1875) and D. Alach (1890) for scientific work.

L. J. M. Daguerre's camera, as made by Chevalier in 1839 for daguerreotype, was of Niepce's rigid double body type, fitted with an achromatic meniscus lens with diaphragm in front on Wollaston 's principle, the back part with the plate moving away from the lens for focusing, and fixed in its place with a thumbscrew. This expanding arrangement enabled lenses of different focal lengths to be used. With modifications cameras of this type were in use for many years afterwards for portrait and studio purposes. For work in the field they were found inconvenient, and many more portable forms were brought out, among them G. Knight's and T. Ottewill's single and double folding cameras (1853), made collapsible with hinges, so as to fold on to the base-board. Cameras with light bodies made of waterproof cloth, etc., also came into use, but these were superseded by cameras with collapsible bellows-body of leather, which, invented by Niepce, were used in France, in 1839, by Baron A. P. de Seguier and others for daguerreotype. The first record of them in England is, apparently, J. Atkinson's portable stereoscopic camera of parallel-side bellows form (Ph. Journ. 1857, 3, p. 261), which was soon followed by C. T. H. Kinnear's lighter conical form, made by Bell of Edinburgh (Ph. Journ. 1858, 4, p. 166). They have since been made in various patterns, conical, oblong and square, by P. Meagher, G. Hare and others, and are still, in modified forms, in general use as studio, field or hand cameras. When wet collodion plates were used many cameras were fitted with arrangements for developing in the field.

Information on these and other early cameras will be found in the photographic journals, in C. Fabre's Traitk encyclopedique de photographic, vol. i., and in J. M. Eder's Ausfuhrliches Handbuch der Photographic, 2nd ed., vol. i., pt. ii.

The distinctive feature of present day photography is the world-wide use of the hand camera. Its convenience, the ease with which it can be carried and worked, and the remarkably low prices at which good, useful cameras of the kind can be supplied, concurrently with improvements in rapid sensitive plates and lenses, have conduced to this result. It has also had a valuable educational influence in quickening artistic perception and scientific inquiry, besides its use in depicting scenes and passing events for historical record. Small portable cameras had been made by B. G. Edwards (1855), T. Scaife (Pistolgraph, 1858), A. Bertsch (1860), T. Ottewill (1861), and others, but it was not until rapid gelatin dry plates were available in 1881 that T. Bolas brought out his " detective " camera (Ph. Journ. 1881, p. 59). It consisted of a double camera (one as finder, the other for taking the picture) enclosed in another box, suitably covered, which also contained the double-plate carriers and had apertures in front of the viewing and taking lenses. In another forir the finder was omitted. A month later A. Loisseau and J. B. Germeuil-Bonnaud patented an opera glass camera. Various forms of portable magazine cameras followed, among them A. Pumphrey's " Repeating Camera " (1881), W. Rouch's " Eureka " (1887), R. Krugener's camera (book form, 1888), and others in collapsible or box forms disguised as books, watches, etc., but they did not come into general use before 1888, when the Eastman Company of Rochester, U.S.A., brought out their very portable roll-film cameras, now known under the trade name of " Kodak." The manufacture of these and other light hand cameras has since become a very important and flourishing industry in Great Britain, Germany, France and the United States. It is noteworthy that the most modern form of hand camera, the reflex, goes back to an early type of portable camera obscura, figured by Johann Zahn in 1686, in which a mirror was used for reflecting the image on to a horizontal focusing screen, at the same time reversing it. The first photographic camera on this principle was T. Sutton's (1860), which has served as a basis for many subsequent developments. A. D. Loman's (1889) and R. Krugener's (1891) were early examples of the hand camera type, but great improvements have since been made.

Modern cameras differ so much in details of improved construction that only a few of the more important requirements can be noticed. A camera should be well and strongly made of seasoned wood or of metal, perfectly rigid when set up, to avoid any shifting of the axis of the lens in respect to the sensitive plate. The front and back of the camera should normally be vertical and parallel, and the axis of the lens perpendicular to the centre of the plate, but arrangements are usually made by vertical and lateral adjustments on the camera front for raising the lens to take in less foreground or vice versa, or for moving it right or left, the latter becoming a vertical movement when the camera has to be turned on its side. In the Adams " Idento " camera the lens and finder can be rotated together on the rising front according as the camera is used horizontally or vertically, the finder showing in either case the identical view projected on the plate. The best modern field cameras are fitted with a swing-back or swing-front and sometimes with both. A swing-back is necessary for bringing back the plate to the vertical position, so as to prevent convergence of vertical lines, when the camera has to be tilted. A rising swing-front, in which the lens is tilted, answers the same purpose, provided the camera is kept level. If further tilting is necessary, when taking high buildings etc., the swing-back and front may both be required, but must be kept vertical and parallel and the effect is that of an abnormal rising front. Many modern cameras are fitted with a double rising front. The vertical and side swings are also useful for equalizing the definition of objects at different distances from the camera, but they alter the perspective. These swing-movements should preferably be round the central horizontal or vertical axis of the back or front, but are frequently effected by simple inclination of the back or lens front on a hinge. When the rising front is used a lens of extended covering power is desirable, and it may be necessary to stop it down to obtain good definition over the extended area of the picture. A slight inclination of the lens may also be useful in readjusting the focus. The camera and plate carriers must be perfectly light-tight and all inner bright surfaces made dead black to prevent reflections from bright spots being thrown on the plate. The black varnish used, preferably of shellac and lampblack in spirit, must have no deleterious effect on the plates. Although the weight and bulk are increased it is convenient to have the camera square and fitted with a reversible back, so that the greatest length of the plate may be horizontal or vertical, as desired. Many cameras are fitted with revolving backs to be used in either position. In some French cameras the back part of the camera with the bellows is reversible, to be used upright or horizontal.

Focusing. -The earlier cameras were focused by drawing out the back and clamping it with a thumb-screw working in a slot in the base-board. When bellows cameras were introduced they were focused by an endless screw, and these are still used for large copying cameras. Most modern cameras are fitted with rack and pinion movements working either in front or at the back of the camera or both. Many hand cameras, requiring to be brought to focus at once, are fitted with studs (infinity catches) which fix the front in focus for distant objects, nearer distances being noted on an engraved scale attached to the base-board. Such scales should be verified by measurement. In hand cameras with fixed infinity focus, the necessary adjustments for distance of near objects are made on the lens mount. The focusing screen may be ruled with parallel cross lines for purposes of measurement, and as a check on the verticality of the camera when photographing buildings or other objects with vertical lines. The distance of the lens from the focusing screen and from the sensitive plate in the dark slide must coincide exactly. This can be tested by measurement or by focusing a bright, well-defined object on the screen and then on a groundglass plate placed in each of the slides to be examined. A level or other means of showing that the camera is level and the plate vertical should be attached to the camera, also a view meter or finder, showing the exact extent of the picture on the focusing glass. In the view meter the picture is viewed directly through a pin-hole mounted at the back of the camera as it appears in a frame with cross wires on the rising front, adjusted to the size of the plate and the focus of the lens. Finders are practically small reflex cameras, and a reduced image is seen reflected from a mirror or prism. A rectangular concave glass mounted on the camera is also a convenient form, it can be combined with a mirror for vertical observation, and in Watson's new form is also arranged as a level and telemeter (B. J. A. p. 724, 1908). The image seen in the finders should correspond exactly with that on the plate. When the rising front is used special arrangements have to be made to ensure the correspondence of the images in the finder and on the groundglass. This is done in the " Adams Identoscope " (1908), which is fitted to the swing front and adjusted by a lever to follow the movement of the lens.

Plate-holders or Dark-slides. The dark-slides or backs, holding sensitive plates, are made either single or double, the former usually for wet plates, the latter for dry plates. The ordinary book-form double dark-slide has been in use since the early days of calotype paper negatives, and contains two plates separated by a blackened metal plate; three of them usually form a set, the shutters being numbered I to 6, the odd numbers on the opening side. Inner frames can be used for smaller plates if desired. The slides should fit easily into the camera and the shutters run smoothly out and jn. They must be perfectly light-tight, the corner joints, the hinges in the shutters, and the openings in the sides and top of the bookform slides are all weak points requiring occasional careful examination or protection by metal plates. The shutters of dark-slides are either jointed or solid and removable; the former is perhaps the more convenient, but both forms may become liable to let in light. Various forms of solid slides, single and double, are now made in wood or metal, or of wood for the frame and metal for the shutters; they are lighter, more compact and less liable to admit light to the plates. In some cases one slide can suffice for the exposure of several plates or stiff films, enclosed in separate envelopes, as in the Wishart-Mackenzie " slide, the " Victrix " and other similar ones, or contained in a single packet, as in the " Premo Filmpack," and similar arrangements which enable twelve thin celluloid films to be placed in the camera, exposed one after the other, and removed again safely in daylight, the pack being replaced, if necessary, by another. The packets of films are made of light cardboard, and effect a great saving of bulk and weight (fig. i). Rollholders are also a convenient way of carrying sensitive celluloid films in lengths of six or twelve exposures, rolled on spools, which can be changed in daylight. Changing boxes for holding a reserve of plates or celluloid films in sheaths, are used with some magazine and other cameras. They are arranged to fit on the camera in place of the dark-slide and the plates are changed automatically so that exposed plates are placed in order successively at the back, a fresh plate going forward for exposure and the number of the exposure being recorded at the same time.

Studio cameras, for portraiture, are usually of the square bellows type, of solid construction, to take large and heavy lenses; adjustable from front and back with rack and pinion movements, to enable long or short focus lenses to be used, with extra extension for copying or enlarging. They are generally fitted with repeating backs, allowing two or more exposures to be made on one plate. The backs are square or reversible, so that the plates can be used upright or lengthways, and are fitted with double swing movements at the back. When single dark slides are used they are best fitted with a flexible shutter to avoid jerking and movement of the camera. For portraiture they are mounted on solid pillar stands, being raised or lowered with an endless screw or rack-work, and the table-top usually has vertical and horizontal angular movements. Large cameras with long extension for copying purposes are made in many forms with special arrangements for the various photo-mechanical processes, and are mounted on substantial table-stands with screw adjustments for obtaining the various motions above noted, and also a rectilinear traversing motion right or left. All these stands should be absolutely rigid and free from tremor. Process cameras are, however, sometimes mounted, FIG. I. Premo Film-pack.

FIG. 2. Sinclair Folding Camera.

together with the copying board, on swinging stands, to avoid the effects of vibration.

Portable and field cameras include cameras of the Hare and Meagher types for outdoor work and general purposes on plates 15 in. X 12 in. to 8J in. X 6J in., and in lighter forms from 6J in. X 4f in. to 4i in. X 3i in. For general purposes they are usually made with square bellows and folding tail-board, rather more substantially than those with conical bellows intended for_ outdoor work. There are many patterns, the principal modern improvements in field cameras being swinging fronts, tripod head and turntable in the base-board, double and sometimes triple extension movements from the back and front for lopg or short focus lenses, and the use of aluminium for some of the metal-work. They are fitted with a focusing screen and are intended for use on a tripod stand, though some of the smaller sizes of the modern light hand or stand cameras can be used as hand cameras with finders. The plates are carried in the usual dark-slides, but the smaller sizes, from half-plate downwards, can be fitted with roll-holders for flexible films, or with film packs or other daylight changing arrangements. Folding and Hand Cameras. Folding cameras form a class of modern portable cameras which have many conveniences for hand or stand work from quarter-plate to 7 in. X 5 in. They may have all the fittings of a stand camera and be made to take glass plates, flat or roll films, but have the advantage of forming when closed a convenient package enclosing camera, lens and shutter, all in position for immediate use when opened out (fig. 2). Most of them are fitted with focusing glass and finders, and may focus by scale in the same way as hand cameras. With an apparatus of this kind on a light stand any class of ordinary indoor or outdoor work can be undertaken within the size of the plate, and the extension of the bellows, which should be quite double the focus of the lens.

The multiplicity of forms and arrangements of hand cameras makes it difficult to classify them into distinct types; but they may be mainly divided into box and folding cameras, and further into (a) cameras with enclosed changing magazines for plates or flat films ; (b) with enclosed roll film on spools ; (c) with separate changing magazines, changing boxes or roll-holders; (d) with single, double or multiple plate carriers or film-packs. Most cameras that will take glass plates in the ordinary plate-holders will take cut films in suitable sheaths or can be fitted with envelope slides, film-packs or roll-holders. The normal size for hand cameras is the quarterplate (4i in. X Z\ in.), or the continental size 9X12 cm. ; 5 in. X 4 in. is also a popular size, and cameras for the post-card size, 5! in. X si in. or 15 X 10 cm. have been largely adopted. Smaller sizes are also made for lantern plates and for the lighter pocket cameras, some in the form of stereoscopes, field-glasses or watches, as in the " Ticka," but the pictures are small and require enlarging. Hand cameras are constructed on the same principles as stand cameras, but, being specially intended for instantaneous work, they are simplified and adapted for rapid focusing and exposing. The fpcusing screen is superseded or supplemented by finders arranged to show the limits of the subject on the plate, the focus being; adjusted by the infinity catches and focusing scales above noticed. Swing-backs and fronts are often dispensed with, but are desirable adjuncts, and a rising and falling front particularly so. Lenses of fairly large aperture, f/6 to f/8, and good covering power, preferably of the anastigmatic type, or a rapid aplanat, should be used, but for very rapid work anastigmats working from //4 to f/6 will be more useful. Hand cameras can also be fitted with telephoto objectives of large aperture. Some cheap hand cameras are fitted with single landscape lenses or aplanats working about //n or lower, but the want of intensity limits their use to well-illuminated subjects. Shutters of the between-lens type are now generally used in hand cameras, and for ordinary purposes should give fairly accurate exposures from ^ to fa of a second or less and also time exposures. Some central shutters are speeded for shorter exposures to rjj of a second, but for these focal plane shutters are preferable, and for the more rapid exposures to j^ of a second and less are necessary. The shutter should be efficient, regular in action, and readily ^released by gentle pressure, pneumatic or otherwise. Mechanism for automatically changing plates or films in hand cameras of the box magazine type must be certain in action, simple and not readily put out of order, special care being taken to avoid rubbing or abrasion of the plates in changing or transport. In changing plates or films the number of plates exposed should be recorded automatically, and duplicate exposures prevented as far as practicable. A circular level placed near the finder is useful.

The choice of a hand camera depends upon the circumstances in which it is to be used, and the purpose for which it is principally required. For general work and with the modern facilities for carrying and changing plates and films in daylight, the numerous folding hand or stand cameras for plates, flat or roll films, with full adjustments, will be found most useful. Box or magazine cameras in which a supply of cut films or plates can be carried, changed mechanically, and exposed rapidly in succession, are convenient, but their use is limited and they are liable to get out of order. A third class are the reflex and other hand cameras with focal plane shutters for specially rapid instantaneous work as noticed below. There are two types of light folding hand or stand cameras, specially adapted for hand camera work those made for taking glass plates and cut films, and the folding pocket Kodak or other roll - film cameras. The former are now made of very light construction with mahogany or metal bodies, wooden or aluminium baseboards, thin metal darkslides (fig. 3). The cameras of the pocket Kodak type are of similar construction, but made to take roll films FIG. 3. Ernemann's Pocket Camera, on spools, or with an attachment for focusing glass and dark-slides for taking plates and cut films. Attached to a sling-strap _the quarter-plate size can be quite conveniently carried in a side-pocket. Watson's " Deft " folding camera is fitted with a focal plane shutter (fig. 4). The " Selfix carbine " camera has a self-erecting front bringing the lens at once into position for use on opening out. Those fitted with lenses of fairly large aperture, double extension, and rising and falling fronts are to be preferred. ' Of box or magazine cameras there is an immense variety. In some the lens is fixed in focus for all objects within a certain distance, in others it is adjusted by a focusing scale on the lens or by an Flr A __ rhf ., rw t Fo i d!ng FIG. 4. The " Deft ' Focal-plane Camera.

extending front. Some have a single magazine, others two or more. Some take only glass plates, others plates or cut films.

All of them are, however, self-contained and ready for immediate exposure. One of the earliest forms of single magazine cameras, still in use, as in the " Eureka " and " Yale," is the " bag," in FIG. 5. Double-magazine Box Camera.

FIG. 6. The Verascope, Richard.

which a supply of plates or films in sheaths, is kept in a magazine behind the camera, ready for exposure, the plates as exposed being lifted with the fingers into a bag or expanding chamber above the magazine and placed behind the rest of the plates at the back, a fresh plate taking its place in front. In some forms the magazines are removable and replaceable by others. The arrangement is simple and effective, but the bag, usually made of soft leather or cloth, is liable to wear and puncture, and may make dust. The cameras with double magazines in which _unexposed plates are kept in one recess and transferred successively after exposure to a second recess are more complicated, and many ingenious devices have been invented for effecting the change (fig. 5). Some forms are effective and popular on account of their compactness and readiness for immediate exposure, but there is always a risk of the mechanism failing, and care has to be taken in charging them to lay the plates truly in their places. The very handy binocular cameras, or photo-jumelles, of which the " yerascope " (fig. 6) is a type, are of this class, and have additional FIG. 7. Beck's Dai-Cornex Daylight-loading Camera, magazines. So also are hand cameras of R. and J. Beck's " Frena " type, specially constructed for using stiff celluloid films. The films are notched on two sides and packed in bundles alternately with cards similarly notched. The pack of films and cards is placed in a magazine at the back of the camera, and by the movement of . a lever, after exposure, the exposed film and its following card are released, and by turning the camera down are dropped into a second receptacle. A " folding Frena " is now made as a folding camera with attached magazine for films, without which it can be used separately for plates. R. and J. Beck's new Dai-Cornex " is a great improvement in this form of camera, being a daylightloading box magazine camera for plates, the plates being packed in a bundle of ridged sheaths, so that they are FIG. 8. Watson s Vnl Camera. quite prote cted from light and can be put into or taken out of the camera in full daylight. In other respects it resembles other magazine cameras (fig. 7). Another useful magazine camera is the " Zambex," carrying either plates or films, held in skeleton frames in envelopes which can be loaded or unloaded in daylight, and are kept ready for use in the back of the camera and exposed consecutively. For work in which speed is of primary importance hand cameras fitted with very rapid lenses and focal plane shutters are necessary, and several forms of portable collapsible cameras of this kind are now available, such as the Goerz-Anschutz, Zeiss's " Palmos," Watson's " Vril " (fig. 8) Adams, " Idento," etc., and are lighter and more portable than the reflex cameras. Hand cameras are generally fitted with screw-bushes for mounting on a tripod stand when time exposures are wanted. The light folding wooden or aluminium stands noted belaw are specially suitable.

Twin-lens and Reflex Cameras. For photographing animals, objects in motion, FIG. o Camera fitted public functions, etc., it is important to with Twin Lenses, section have the means of watching the movement till the critical moment of exposure arrives. For this it is convenient to have a camera fitted with twin lenses working in two separate compartments (fig. 9) or more simply with a mirror throwing a full-sized unreversed image of the object from the lens on to the focusing screen (fig. 10). With the former, which has the advantage that the image is seen before, during, and after exposure, the lenses must be ol exactly equal focus and focused together by the same motion ol the rack-work, the object being viewed on the focusing screen ol the upper compartment, and the plate kept ready in the lower to show working.

A, Hood of finder.

B, Ground glass screen.

C, Mirror.

D, Viewing lens.

E, Working lens.

F, Shutter.

G, Focusing pinion. H, Plate carrier.

i, Plate.

to be exposed when desired. Binocular hand cameras are also made on this principle, one compartment serving for focusing, the other holding lens and plates. Stereoscopic cameras are another form of twin-lens cameras, and are usually made for also taking single panoramic pictures.

In reflex cameras only one lens is necessary, though two are convenient, and can be used somewhat as in fig. 9. They generally consist of a cubical box camera containing a movable mirror facing ;he lens at an angle of 45 and throwing up the image projected from t on to a horizontal focusing screen, on which it is viewed through a lexible hood which folds down in the upper part of the camera when not in use (fig. 10). In order to get the greatest rapidity of exposure a focal-plane shutter is generally fitted, and by a single movement of the release the mirror is smoothly lifted and the plate exposed simultaneously. They should be fitted with anastigmatic lenses working at large apertures for very rapid work. In some forms the lens is fixed, but usually there is a front Dellows extension for long-focus lenses, with rising and falling front, to which swing motion may be given, a swing-back not being generally used with the focal plane shutter. In the " Ernex " camera E. Human has made an arrangement by which the camera back, horizontal viewing screen and reflector are made to swing simultaneously, by a rack and pinion movement. They may also have reversing or revolving backs for quickly changing the position of the plate, s in. X 4 in. and 3J in. X 4i in. are the usual sizes of the plates, but larger and smaller sizes are also available. These cameras require the best workmanship and perfect mechanism "or successful working and freedom from any jarring movement in releasing the shutter or mirror. The focusing screen must also oe in accurate register with the focus of the Tens on the plate. Those forms in which the image can also be viewed at the height of the eye, as in the Graflex (fig. 10), are preferable. Although reflex cameras are rather heavy and bulky as hand cameras, they have many advantages over the ordinary hand camera with finder and focusing scales for the purpose of the press photographer, the naturalist and others, in observing and recording very rapid movements, and have come into very general use for such purposes. They permit the accurate focusing of a fullsized image on the groundglass up to the moment of exposure, especially useful when lenses of long or short focus are required and when the rising or swing front is in use. The aspect of this image on the ground-glass is also a great aid in the selection and FIG. 10. Reflex Camera.

A, Lens.

B, Mirror.

C, Ground-glass.

D, Plate.

E, Supplementary mirror.

placing of the subject and in judging the exposure required for ft. They practically have all the advantages of a stand camera and can be used as such on a stand for subjects requiring prolonged exposure. They are also coming into increasing use in studio work for portraits of children, etc. Their use and adjustments are discussed by G. E. Brown in the British Journal Almanac for 1909. Panoramic Cameras. Many so-called " panoramic " cameras have been introduced from time to time, among them T. Sutton's (1861), and J. R. Johnson's " Pantascopic " (1864), but did not FIG. II. Section of " Al- Vista " Panoramic Camera.

come into general use till the use of curved surfaces of celluloid film enabled such cameras of convenient size and weight to be put on the market. They are on the same principle as one made by F. von Martens in 1845 for curved daguerreotype plates, and coverangle of 150. P. Moessard's " Cylindrographe " of 1889 ing an angle of 150" was the first of the modern type.

It consists of a semi-circular FIG. 12. " Al- Vista " Panoramic Camera, closed.

camera, the front of it formed of light-proof cloth and the back by the curved flexible carriers. The lens is fitted on a vertical axis, so that the nodal point of emergence remains motionless, and is revolved round it by means of a handle worked by hand and carrying a view meter. The illumination of the image is regulated by an adjustable vertical slit in a tube attached to the lens inside the box, and by altering the rate at which the lens is revolved. The pictures taken embrace less than 180. The apparatus folds together and is quite portable; it is fully described in Moessard's Le Cylindroeraphe (Paris, 1889). The " Al-Vista " (1901) and the " Panoram Kodak " (1900) are on the same principle, but arranged as roll-holder hand cameras, in two sizes, carrying film for several exposures, 7 in. X 2i in. or 4 in. X 12 in. They work instantaneously, and by means of a clock-spring the lens rotates rapidly over a half-circle when released. The angle of view is about 120" (figs, ii and 12). The views taken with this kind of camera are sometimes disappointing, on account of the development of cylindrical perspective on a plane surface causing apparent distortion. This distortion is avoided in Carl Zeiss's "Palmos Panoram" camera for plates 6| in. X 3! in., fitted with " Tessar " lens and focal plane shutter, and other similar cameras which can be used for stereoscopic or single pictures. Other more elaborate instruments driven by clockwork have been made for making a complete tour of the horizon. Among them C. Damoizeau's " Cyclographe," which can be used with lenses of different foci and takes the pictures on a roll-film, which is unrolled as the instrument revolves on its axis, the lens also rotating on its nodal point of emergence; and thus the image always remains sharp (Bull. Soc. Franc, d. Phot., 1891, p. 183). Commandant A. Daubresse has improved on Moessard's apparatus, by placing the lens vertically between two right-angled prisms, the upper of which receives the image and projects it through the lens on to the lower prism, from which, by rotation of the system on the vertical axis, it is projected on to a cylindrical film through an angle of 360 (Ibid. 1906, p. 430; E. Jb., 1 97> P- 9')- The " Periphote " and Ernemann's " Rundblick " camera are improved forms (E. Jb., 1908, p. 322).

Many early forms of panoramic cameras are described in B. J. A .

1892, p. 517. Colonel R. W. Stewart's " Panoram " (1893), A. Chevalier's " Photographic Plane Table," J. Bridges Lee's " Photo-Theodolite " (1894), and similar cameras fitted with telescopes, levels and divided circles, are instruments of precision suitable for photographic surveying. Improved instruments for topographical surveying with stereo-photographic apparatus, on the principle worked out by Dr C. Pulfrich, of Messrs Zeiss & Co., in his stereo-comparator (1903), are being practically developed, and much information regarding them will be found in papers by E. Dolezal and others in J. M. Eder's Jahrbiicher, 1903 to 1908; also a paper by Lieut. F. V. Thompson in Geographical Journal, 1908, xxxi. 534.

Cameras for Three-Colour Photography. Many forms of camera have been constructed for making the three negatives required for trichromatic photography. They fall into two types: (i) those with a repeating back fitted with three colour-screens or filters red, green and violet through which the colour impressions are made successively with one lens upon a single colour-sensitive plate, as in the Sanger-Shepherd system. The colour-screens are placed immediately in front of the sensitive plate in the repeating back, which is moved on for each exposure. In a more recent form, by the same FIG. 13. Diagram of Camera maker, the three images are taken for Three-colour Photography, on the sensitive plate with one exposure. The camera is divided into three compartments, and fitted with a special diaphragm which can be regulated for the varying sensitiveness of different batches of plates. The central image is impressed directly on the plate; the other two by reflection from prisms arranged so as to equalize the sizes of the three images on the sensitive plates, the light rays passing in each case through a suitable colour-filterred, green and blueviolet somewhat on the principle of F. E. Ives's camera of 1900 (fig. 13). It is convenient and successful in working. (2) Cameras made on the reflecting principle of L. Ducos du Hauron (1876), elaborated by F. E. Ives (1894) in his photochromoscope, in which three images are taken through three colour-screens on separate plates with one lens, the respective exposures being regulated by reflection of the light coming from the lens by plane mirrors on to the sensitive plates, and its filtration through the colour-screens in front of them. Many variations of this method have been proposed, in which reflecting prisms replace the mirrors. The different systems have been discussed by W. Gamble (Ph. Jour. 1905, xlv. 150), the latter also by E. T. Butler (Ibid. p. 199). Sir W. de W. Abney has described three-colour cameras for landscape work in Ph. Jour. 1904, xliv. 81, and 1908, xlviii. 331.

Enlarging Cameras. These cameras vary in form, according to the nature of the illumination, but ordinarily consist of a double or triple extension bellows camera, with a holder for the negative or transparency at one end, and for the sensitive plate or paper at the other, the lens being placed on a fixed partition between the two. Some recent forms of " daylight enlargers " can be used as an ordinary camera. Other cheaper ones are on the fixed focus principle. Enlargers for use with artificial light are made like a magic lantern, with a condenser, projecting an enlarged image on to a sensitive plate or paper fixed on an easel or screen. A simple arrangement for daylight enlarging is to fix a suitable camera on to a larger one by a sliding front, and mount the two on a studio stand tilted so that the image may be illuminated by the open sky.

Cinematographs. Many special cameras and lenses have been introduced for taking on a long flexible sensitive film an extended series of small photographs of the successive phases of movements, and again projecting them on a screen so as to reproduce the scene, with an illusion of motion, in what are known as " living pictures," biographs, etc. As each photograph requires a certain minimum time for exposure and must be kept in true position in sequence with the rest, some means of regulating the intermittent exposures and keeping the film in position have to be adopted ; and there are many different ways of doing it, either by a continuous or intermittent motion and exposure of the film while it is being unwound from one roller on to another. The films used are similar to the ordinary celluloid films, but in narrow bands from if in. to 2 in. in width, the length varying with the number of exposures required, at the rate of 16 to 20 per second. They are perforated on both sides, so that they may run true and have the necessary intermittent motion, the perforations fitting on to studs on a sprocket wheel in connexion with the driving wheel and crank handle. Special lenses of short focus, from I in. to 3 in., with good covering power and large apertures //4 to f/2, are required both for photographing and projecting; several such are noted below. Absolute rigidity in the camera is essential. Special stands are made for the purpose, but if a tripod stand is used it should be well braced. Special apparatus is required for developing and fixing the exposed films. They are wound on large rollers supported over troughs containing the necessary solutions (see CINEMATOGRAPH). The mechanical arrangements are treated in H. V. Hopwood, Living Pictures (1899); F. P.


A method of cinematography in colour was introduced by G. A. Smith and C. Urban in 1908, the main features of it being the use of a film sensitive to all colour waves to the furthest red; superimposing the colour records by persistence of vision; the use of two-colour records instead of three, in order to reduce the interval between the successive presentations; adaptation to existing cinematograph machinery and films. These conditions are fulfilled by the use, in place of the ordinary revolving sector shutter in front of the lens passing intermittent white light, of a special, more rapidly revolving shutter divided into four sectors, one fitted with orange-red glass, another with bluish-green glass and two intermediate opaque sectors, so that at every revolution of the shutter an exposure is made through the red and green glasses alternately. The former passes white and yellow, and then orange, scarlet to deepest red; whilst the latter also passes white and yellow, green, blue-green, blue, all in proportion according to the red and green sensitiveness of the specially sensitized panchromatic emulsion on the film. The same shutter and colour screens are used for projection, some supplementary blue rarys being added. The results are satisfactory and the method promises to be of great practical value (see Jour. Roy. Soc. Arts, 1908, 57, No. 2926).

Special cameras are made for various branches of scientific research in photo-micrography, photo-spectroscopy, astronomical photography, etc.

Tripod Stands. Field cameras are usually supported on wooden tripod stands, folding in two or more sections, the head being separate or fixed in the base-board of the camera. The legs should be capable of extension to about 5 ft. and adjustable in length for use on uneven ground. A tripod stand may be light, but must be firm and rigid when set up. To prevent slipping, shoes of indiarubber or cork may be fitted to the points of the legs, and in some cases it may be desirable to strengthen the tripod by a folding adjustable brace. W. Butler's " Swincam " camera stand is made to enable the camera to be securely fixed in awkward positions, and has many valuable special features, great extension, swivel points to the feet, etc. For hand cameras the very light, portable metal folding and walking-stick stands are convenient.

Photographic Objectives or Lenses.

The objective is the most important item of photographic apparatus, because upon it depends the perfection with which a correct and well-defined picture is projected upon the plane surface of the sensitive plate of objects in the different planes forming the field of view, which naturally would come to a focus on a series of curved surfaces. This flattened picture must be equally illuminated and sharply defined, within a limit of confusion from riff to . zkv f an inch, over a sufficiently wide angle. A good objective must also pass sufficient light to produce the required effect on the photographic plate with short exposures; the chemical and visual foci must coincide exactly, and it must not distort straight or parallel lines. The fulfilment of these conditions is complicated by the presence of sundry focal displacements or aberrations, (i) Spherical aberration, or non-coincidence of the foci of the central and marginal pencils of rays passing through the lens. It is corrected by varying the curves of the component lenses and by the use of a diaphragm. (2) Coma, or blur, due to lateral spherical aberration of oblique rays, and mostly found in unsymmetrical combinations and single view lenses. It is partly eliminated by the diaphragm. (3) Astigmatism, which accompanies coma in single lenses, and is usually present in symmetrical aplanats, manifests itself by forming two sets of images of points off the axis, lying in two separate curved surfaces, one set focusing tangentially as more or less horizontal lines, the other radially as more or less vertical lines. It increases with the obliquity of the rays and causes want of definition and difference of focus between horizontal and vertical lines away from the centre. (4) Curvature of field, also increasing with the obliquity of the rays. (5) Distortion, outward or inward, according to the nature and construction of the objective. With the single meniscus view lens, used with its concave surface towards the object and a diaphragm in front, a square will appear barrel shaped from inward contraction of the lines towards the centre ; but with the convex surface towards the object and the diaphragm behind, it will appear with concave sides from outward expansion from the centre. It can be corrected by using two such lenses with the convex sides outwards and a central diaphragm, as in periscopic or rectilinear lenses. Lenses of the orthoscopic and telephoto types generally show the latter form of distortion. (6) Chromatic aberration, produced by the dispersion of the white light passing through the lens, and the different coloured rays composing it coming to a focus at different distances from the visual focus in the order of their wave-lengths. It thus affects both the positions and sizes of the image for the different colours. For ordinary photographic work it suffices for the blue-violet and yellow rays to be coincident, but for the new processes of photography in three colours, apochromatic lenses, in which perfect coincidence of the coloured rays is secured, are required to obtain the accurate register of the three images. The corrections are effected by compensating lenses of different refractive powers (see ABERRATION).

In constructing photographic objectives these aberrations and distortions have to be neutralized, by regulating the curves of the different positive and negative component lenses, the refractive and dispersive indices of the glasses from which they are made, and the distances of the refracting surfaces, so as to make the objective as far as possible stigmalic or focusing to a point, giving an image well defined and undistorted. This perfect correction could never be effected in objectives made before 1887, and very few could be effectively used at their full apertures, because although linear distortion could be overcome there were always residual aberrations affecting the oblique rays and necessitating the use of & diaphragm, which by lengthening out the rays caused them to define clearly over a larger surface, at the expense of luminous intensity and rapidity of working. The introduction of rapid gelatin dry plates enabled photographs to be taken with much greater rapidity than before, and led to a demand for greater intensity of illumination and better definition in lenses to meet the requirements of the necessarily very rapid exposures in hand cameras. For studio and copying work quick-acting lenses are also valuable in dull weather or in winter.

The rapidity of a lens with a light of given intensity depends upon the diameter of its aperture, or that of the diaphragm used, relatively to the focal length. In order, therefore, to obtain increased rapidity combined with perfect definition, some means had to be found of constructing photographic objectives with larger effective apertures. This necessity had long been recognized and met by many of the best makers for objectives of the single meniscus and aplanatic types, but with only partial success, because such objectives are dependent upon the diaphragm for the further correction necessary to obtain good definition over an extended field. The difficulty was in the removal of astigmatism and curvature of the field, which, as J. Petzval had shown, was impossible with the old optical flint and crown glasses. In 1886 Messrs E. Abbe and O. Schott, of Jena, introduced several new varieties of optical glasses, among them new crown glasses which, with a lower dispersion than flint glass, have a higher instead of a lower refractive power. It was thus rendered possible to overcome the old difficulties and to revolutionize photographic optics by enabling objectives to be made free from astigmatism, working at their full apertures with great flatness of field independently of the diaphragm, which is now chiefly used to extend the area of definition or angle of view, and the so-called " depth of focus " for objects in different planes.

Photographic objectives may be classed as follows:

1. Single achromatic combinations. "1 2. Unsymmetrical doublets. lni,i .

3. Symmetrical doublets.

4. Triple combinations. J 5. Anastigmatic combinations symmetrical ~| and unsymmetrical. I j 6. Telephotographic objectives.

7. Anachromatic combinations. J They are also sometimes classified according to their rapidity, a expressed by their effective apertures, into " extra rapid," with apertures larger than//6; " rapid," with apertures f rom //6 to//8; " slow," with apertures less than //I I. Another classification is according to the angle of view, " narrow angle " up to 35; " medium angle " up to 60; " wide angle " up to 90, 100, or more. Many lenses are made in series, differing in rapidity and angle of view as well as in length of focus.

i. Single Achromatic Combination or Landscape Lens. This is the earliest form of photographic objective, evolved from W. H. Wollaston's improved single periscopic meniscus camera obscura lens (1812). It was made achromatic by Ch. Chevalier, and so used by L. J. M. Daguerre, though it required correction for chemical focus, as did the object glasses of telescopes or opera classes first used for photography. The single landscape lens usually consists of an achromatic compound meniscus, formed of a biconvex positive crown cemented to a biconcave negative flint to secure achromatism and partially correct the spherical aberration, and may' be taken as the type of the " old photographic achromat " (fig. 14) - 1 It is used with its concave side towards the object and a diaphragm in front, thus producing inward or barrel-shaped distortion, inherent in this type of objective, and rendering it unsuitable for copying or architecture, though not very noticeable in landscape work. The full aperture has to be largely reduced by a diaphragm to improve definition; so it is slow, though many improved forms have been brought out. It has ..^ always been popular for pure landscape work on p ij-S account of the equality of illumination over f T the plate, depth of focus, and the softness and * ' brilliancy of the image owing to its thinness and freedom from reflecting surfaces. In some of its improved and " long focus " forms it is preferred by portraitists for large heads, on account of the general softness it gives when used with large apertures.

The following are some of the" best-known improved objectives of this type: T. Grubb's "Aplanatic" (1857), //is to 7/30 FIG. 16.- -Rapid Landscape Lens. Long Focus.

FIG. 15. Grubb's " Aplanatic " Lens.

(fig. 15) ; J. H. Dallmeyer's " Wide Angle Landscape Lens " (1865), //I5, angle 75. In it distortion was reduced and marginal definition improved. The " Rapid (long focus) Landscape Lens " (1884), //I2, angle 40* (fig. 16), was a modification of it, and at //8 is useful for heads in portraiture. W. Wray's Landscape Lens " (1886), flu, is also useful for portraiture in the larger sizes at //8. Fr. Voigtlander's " WideAngle Landscape Lens " (1888)

FIG. 17. Rectilinear Landscape Lens.

1 In the diagrams of lenses which follow, a uniform system of indicating the nature of the glass employed by means of the shading has been adopted.

Flint glass is indicated thus:

Crown glass of low refractive power thus :

Crown glass of high refractive power thus:

(These two are used indiscriminately in lenses made before the introduction of the new Jena glass.)

Extra light flint glass thus: In most cases the front of the lens is on the right.

f/lS, angle 90, with great covenng power and depth of focus. T R. Dallmeyer's " Rectilinear Landscape Lens " (1888), //I4, angle 60 (fig. 17), was of novel construction, free from distortion, brilliant in working and useful for copying. Messrs Ross's " WideAngle Landscape Lens " (1890), //i6, angle 70, triple cemented and made of Jena glass. Many other excellent objectives of this type have been made by British and foreign makers and are still used, though somewhat superseded by the fully corrected anastigmats specially made to work singly, or as single elements of anastigmatic doublets, as noticed in 5.

2 Unsymmetrical Doublets: Old Types. This class includes objectives with comparatively large apertures formed of two dissimilar combinations, in most cases correcting each other, with a diaphragm between them. In some the single elements may be used independently. All the older " portrait lenses, some of the aplanatic doublets and Fr. von Voigtlander's " Orthoscopic Lens (1857), now disused, are of this type. Even with the present improved conditions, the portraitist working in a studio requires a quick-acting objective of large effective aperture and comparatively short focus, giving a brilliant well-defined image of near objects in different planes over a restricted field of view. The early single lenses were found to be too slow for portraiture by the daguerreotype and talbotype processes, and the efforts of opticians were directed to the problem of obtaining the maximum amount of light, together with good definition and flatness of field, and about 1840 compound lenses were brought out by Andrew Ross and C. Chevalier, consisting of two achromatic compounds, one at each end of a tube. Ross's lens, made for H. Collen, is interesting as the first lens corrected photographically, so that the visual and chemical foci were coincident (fig. 18). Ch. Chevalier also combined lenses of different foci, as is now done for " convertible objectives, used singly or combined. He also fitted them with iris diaphragms. These forms were soon superseded by the compound portrait lens, calculated by J. Petzval and brought out by FIG. 1 8. First English Portrait Lens. FIG. 19. Portrait Lens.

Fr. von Voigtlander in 1841. It consists of two dissimilar achromatic combinations widely separated. At first the diaphragms were in front, but now they are central. The front element is a plano-convex composed of a biconvex crown cemented to a planoconcave flint, while the back element is a double convex, composed of a biconvex crown separated by an air-space from a concavoconvex flint (fig. 19). This form of objective quickly supplanted all other for portraitures, and is still largely used, though it has defects which prevent its use for general purposes and is being superseded for portraiture by some of the rapid anastigmats. In his " Quick Acting Portrait Lens " (1860), fj^, angle 45, J. H. Dallmeyer improved the correction for spherical aberration, and in his " Extra Quick Acting Portrait Lens " (1860), f/2 -2, used for cinematograph work, attained greater rapidity. In the " Patent Portrait Lens " (1866), // 3 , // 4 and //6, angles 5 to 55 (fig. 20), he made great changes in the form and relative positions of the back elements, giving a flatter field ' and freedom from flare spot. By separating the two components of the back element more or less spherical aberration could be introduced to give softer definition FIG. 20. Dallmeyer's Patent Portrait Lens.

and greater depth of focus. In 1875 Dr. A. Steinheil made an Unsymmetrical aplanatic portrait combination of peculiar construction, working at //3- 2. It was an improvement on his similar symmetrical " Portrait-Aplanat," FIG. 21. Portrait Antiplanet. FIG. 22. Group Antiplanet. Form I. of 1874, but was superse_ded in 1881 by the " Portrait Antiplanet," //4 and free from astigmatism over an angle of 14.

It had six reflecting surfaces and nearly approached a triplet (fig. 21). Steinheil's "Group Aplanats " (1879), //6'4, angle 70, were an improvement on the ordinary " Aplanats, but were superseded in 1881 by the " Group Antiplanets," f/$, angle 70, lenses of a distinct type (fig. 22). They were a further advance on the " Aplanats, working at larger apertures and giving better definition. This lens is interesting as the first in which astigmatism was eliminated by combining a " crown-shaped " lens of high refractivity, with a " flint-shaped " of lower refractivity, though made of the old glasses. In his " Rapid Antiplanet" (1893), f/6-5, angle 30, Dr R. Steinheil improved the " Group Antiplanet " as regards astigmatism and covering power by replacing the thick back combination by a triple longfocus negative element consisting of a crown between two flints, with a heavy barium crown in the front element instead of a flint (fig. 23). Voigtlander, who originally constructed the Petzval portrait lens, improved it in 1878 and 1885, and now makes two lenses on the same principle, series I. //3'2, angle 28, for ordinary portraiture and projection, and series la., //2-3, angle 22 (1900) for astrophotography, cinematography, etc., when intense illumination is required over a small field. Both are quite free from coma.

FIG. 23. R. Steinheil's Im- FIG. 24. Ordinary Angle proved Group Antiplanet. Actinic Doublet.

Most of the above are portrait objectives of large aperture, but Unsymmetrical doublets have also been made for landscape work. J. T. Goddard's " Combination Landscape Lens " (1859) was one of the first, and was free from distortion, gave a flat field, and could be used as a convertible lens. In 1864 T. Ross issued his " Actinic Doublets," modified from the Collen lens, in three series "small angle," f/8, angle 40 to 50; " ordinary angle," //id, angle 60 to 75 (fig. 24); " large angle," //i6, angle 80 to 95 . These lenses were similar to the " Globe," but Unsymmetrical and more rapid. The separate elements could be used alone. Some of them were fitted with a shutter near the diaphragm. They were superseded by the " Symmetrical " lenses.

3. Symmetrical Doublets. This class includes objectives formed of two similar combinations of lenses, usually of the convergent meniscus form, with their concave surfaces inwards and a diaphragm between them; consequently they are rectilinear and practically free from marginal distortion. Until the introduction of anastig,matic doublets they were in general use for all purposes under the names " Aplanat," " Rectilinear," " Symmetrical, " Euryscope," etc. They are still largely used and have been improved by the use of Jena glasses in their construction.

The first recorded lens of this type was Dr J. W. Draper's combination used in 1839 for daguerreotype portraits, consisting of two double-convex Tenses 4 in. diameter, with a united focus of 8 in., mounted in a tube with a diaphragm 3$ in. in front. In 1841 T. Davidson made a combination of two single landscape lenses very similar to the later rectilinear doublets. Being slower than the Petzval portrait lens its value as a non-distorting lens for general purposes was not recognized. G. S. Cundell (1844) combined two unconnected meniscus lenses with a diaphragm between them. In 1860 T. Sutton brought out his " Panoramic Lens," which worked on curved plates covering about 100. It was followed FIG. 25. C. A. Steinheil's " Periskop." FIG. 26. A. Steinheil's "Aplanat."

by C. C. Harrison's " Globe Lens " (1862), angle 75, composed of a symmetrical pair of deep compound menisci, the exterior surfaces forming part of a Sphere. Though defective and slow it was popular for a time. C. A. Steinheil's " Periskop " (1865, f 113-5, angle 90, was a symmetrical doublet formed of two plain crown menisci with central diaphragm (fig. 25). It gave a larger field than the " Globe," the lenses being closer together. Being nonachromatic it had to be adjusted for chemical focus. It was quite free from distortion, with a very flat field, and both nodal points together. It is _ considered the best possible combination of two plain lenses, and is still _ used in some of the cheaper hand cameras with fixed focus, the difference of the chemical and visual foci being allowed for in the camera or by adjustable lens mounts. G. Rodenstock's " Bistigmats " are of this class. J. Zentmayer made a similar unsymmetrical lens. In A. Steinheil's " Aplanat " (1866) the same principle was carried out with achromatized lenses, and a great improvement was effected in the construction of non-distorting objectives of fairly large aperture. It consisted of two positive cemented flint menisci, each composed of a dense flint with negative focus outside and a light flint with positive focus inside, its concave surfaces facing the centre (fig. 26). This use of flint glasses alone was peculiar, former achromatic lenses having been made of flint and crown. These lenses were made in three rapidities : " Ordinary," f/6 or//7, angle 60; " Landscape," //I2 to//!5, angle 90, also used in convertible sets; " Wide Angle Landscape," //2O to //25, angle 104; " Wide Angle Reproduction," similar to the last, but with sharper definition. The " Aplanat " had many advantages over previous doublets and the triplet, being more rapid, perfectly symmetrical, so that there was no necessity for turning them when enlarging, and free from distortion or flare. There was no chemical focus. Each component could be used alone for landscape work with double focus, subject to the ordinary defects of single lenses. By the use of Jena glasses in the "Universal Aplanat" (1886) the components of this lens were brought closer together, its intensity increased, and it was made more portable. J. H. Dallmeyer had been working in the same direction simultaneously with Steinheil, and in 1866 brought out his " Wide Angle Rectilinear," //I5, angle 100, made of flint and crown, the front element being larger than the back (fig. 27). It was slow for ordinary purposes and was succeeded in 1867 by the well-known " Rapid Rectilinear," f/8, on the same FIG. 27. Wide-Angle Rectilinear Lens.

FIG. 28. Rapid Rectilinear Lens.

principle as Steinheil's " Aplanat, but made of flint and crown (fig. 28). Ross's " Rapid " and " Portable Symmetrical " lenses, Voigtlander's " Euryscopes," and other similar lenses of British and foreign manufacture are of the same type, and still in use. They are excellent for general purposes and copying, but astigmatism is always present, and although they can be used with larger apertures than the triplets they displaced, they require stopping down to secure good marginal definition over the size of plate they are said to cover. By the use of Jena glasses they have been improved to work at larger apertures, and some are made with triple cemented elements. 4. Triple Combinations: OW Types. This class comprises objectives composed of three separate combinations of glasses widely separated from each other. An early form of this type was made by Andrew Ross (1841) for W. H. Fox Talbot, others by F. S. Archer, J. T. Goddard (1859), T. Sutton (1860), but they never came into general use. J. H. Dallmeyer's " Triple Achromatic Lens (i86l),//io, angle 60, now out of date, was an excellent non-distorting ^~~ lens, very useful for general work and copying (fig. 29). As made by Dallmeyer, the inner surfaces of the front and back FIG. 2 9 .-Triple Achromatic Lens. ^ve^buTi'n Triplets " (1861), //i6, they were flat. The centre lens was an achromatic negative serving to flatten the field.

5. Anastigmatic Combinations, Symmetrical and Unsymmetrical. As already stated, it was found practically impossible to obtain flatness of field, together with freedom from astigmatism, in objectives constructed with the old optical glasses. A. Steinheil attempted it in the " Antiplanets," but with only partial success. The Abbe and Schott Jena glasses, issued in 1886, put a new power into the hands of opticians by largely increasing their choice of glasses with different refractive and dispersive powers. Whereas the old glasses had high refractivity with higher dispersion, in the new ones high refractivity with lower dispersion could be set against lower refractivity with higher dispersion.

Between 1887 and 1889 the first attempts to make anastigmatic objectives with the new glasses were made by M. Mittenzwei of Zwickau, R. D. Gray of New Jersey, E. Hartnach and A. Miethe of Berlin (" Pantoscope "), K. Fritsch of Vienna (" Apochromat ") and Fr. von Voigtlander of Brunswick, with more or less success, but progress was hindered by the instability of some of the early glasses, which was afterwards overcome by sandwiching the soft glasses between two hard ones. In 1888 Dr H. L. H. Schroeder worked out for Messrs Ross the " Concentric Lens " (fig. 30) issued in 1892 (Ph. Jour., id, p. 276). It was a symmetrical doublet of novel construction, each element consisting of a plano-convex crown of high refractivity cemented to a plano-concave flint of lower refractivity, but about equal or higher dispersion. Both the FIG. 30. Concentric Lens.

uncemented surfaces were spherical and concentric. At //1 6 it gave sharp definition and flatness of field with freedom from astigmatism, distortion or flare over an angle of 75. It was an excellent lens, though slow, and has been superseded by the " Homocentric " and other more rapid anastigmats. Dr Paul Rudolph, of Messrs Carl Zeiss& Co., Jena, worked out in 1889 a new and successful method of constructing a photographic objective by which astigmatism of the oblique rays and the want of marginal definition due to it could be FIG. 31. Anastigmat. FIG. 32. Anastigmat.

Series II. //6-3- Series Ilia. Jjg.

eliminated without loss of rapidity, so that a comparatively extended field could be covered with a large aperture. This he did on the principle of the opposite or opposed gradation of the refractive indices in the front and back lenses, by a combination of two dissimilar systems of single lenses cemented together, the positive element of each having in one case a higher and in the other a lower refractive index than that of the negative element with which it was associated. The front system, relied upon for the correction of spherical aberration, was made of the old glasses, a crown positive of low and a flint negative of high refractivity, whilst the back system, relied upon for the anastigmatic flattening of the field, was made of the new glasses, a crown positive of high and a flint negative of low refractivity. Both systems being spherically and chromatically corrected for a large aperture, the field was flattened, the astigmatism of the one being corrected by the opposite astigmatism of the other, without destroying the flatness of the field over a large angle (see E. Jb., 1891 and 1893; M. von Rohr's Geschichte, and O. Cummer, Photographic Optics, for further details). They were issued by Messrs Zeiss and their licencees (in England, Messrs Ross), in 1890, in two different types. The more rapid had five lenses (fig. 31), two of ordinary glasses in the front normal achromat, and three in the back abnormal achromat, two crowns of very high refractive power, with a negative flint of very low refractive power between them.

FIG. 33. Anastigmat. Series VI.

FIG. 34. Satz Anastigmat. Series VIo.

The fifth lens assisted in removing spherical aberrations of higher orders with large apertures. The second type, series Ilia., //9, 1899 (fig. 32), had only two lenses, the functions of which were as above. These combinations could not be used separately as single lenses. They are now issued as " Protars," series Ha., f/8; Ilia., //9; V., //l8. In 1891 Dr Rudolph devoted himself to perfecting the single landscape lens, and constructed on the same principle a single combination of three lenses, the central one having a refractive index between the indices of the two others, and one of its cemented surfaces diverging, while the other was converging. At //I4'5 this lens gave an anastigmatically flat image with freedom from spherical aberration on or off the axis. It was, however, not brought out till 1893, as a convertible lens or " Satz- Anastigmat," series VI.,//I4'5, and VIo.,//7-7 (figs. 33 and 34). In the meantime Dr E. von Hoegh (C. B. Goerz) and Dr A. Steinheil had also been working at the problem and had independently calculated lenses similar to Rudolph's, but, whereas he had devoted himself to perfecting the single lens, they sought more perfect correction by combining two single anastigmatic lenses to form a doublet. Dr Rudolph had had the same idea, but Messrs Goerz secured the priority of patent in 1892, and in 1893 brought out their " Double Anastigmat," .now known as FIG. 35.

Ross-Goerz " Dagor." Series III. Ross-Goerz. Series IV. " Dagor." It was the first symmetrical anastigmat which combined freedom from astigmatism with flatness of field and great covering power at the large aperture of //7>7 (fig. 35). Both these types of Zeiss's " Protars " and Goerz's " Dagor " anastigmats have since been made by Messrs Ross in England. Messrs Steinheil brought out their first " Orthostigmats " in 1893, but, owing to patent difficulties, were unable to manufacture them in Germany, and they were issued later in France and England. They were followed by a second type, which has since been issued in several series by Messrs Steinheil and 'by Messrs Beck in England (fig. 36). According to Dr R. Steinheil (E. Jb., 1897, p. 172) this lens was an application of two principles recognized by Dr A. Steinheil as necessary for the spherical and anastigmatic correction of a lens. He attempted to carry them out in the " Antiplanet," but was prevented by the want of suitable glass. He found that for anastigmatic correction an objective should have the separating surface between two media concave towards the medium of higher refraction (new achromat), and for FIG. 36. Steinheil's FIG. 37-" Collinear."

" Orthostigmat." Series II.

spherical correction the separating surface should be convex towards the higher refracting medium. A fully corrected cemented lens cannot, therefore, be made with less than three glasses, but with uncemented lenses an air-space may form one of the media. In 1895 Dr D. Kaempfer worked out the " Collinear " for^ Messrs Voigtlander, constructed on the same principles as the " Orthostiemat," type II., and similar to it (fig. 37). It is made in three series: II., //5'4 and //6- 3 ; III., //6'8 and fin (convertible); IV., f/12'5, and the apochromatic collinear //8, calculated by Dr H. Halting for three-colour reproduction, etc. (Ph. Jour., 1901, 25, In 1894 Dr Rudolph extended the application of his principle by combining the old achromat and the new achromat into a single quadruple cemented lens (fig. 38), which, according to T. R. Dallmeyer, was the most perfectly corrected single lens that had been Series Vila. //6-3- FIG. 38.

Series VII. //I2-5. evolved up to 1900, Dr Rudolph having succeeded in obtaining freedom from spherical aberration and astigmatism, and also in eliminating coma (Ph. Jour. 1901, 25, p. 68). These lenses were issued in 1895 as series VII. singly and Vila., in combinations now known as " Convertible Protars, and the earlier series VI. and Via. were withdrawn. The single lenses of series VII., //I2'5, angle 85, have great anastigmatic flatness of field and only very slight marginal distortion, a condition not realized before in a single lens. The relative rapidities of the double combinations of series Vila, vary from //6'3 to //8, according to the lenses used. They are excellent lenses for all general purposes.

In their " Convertible Protars," series IV. (1908), //I2'5, angle 60, Messrs Zeiss have simplified and cheapened the construction of these lenses by the use of new Jena glasses, so that they consist of three instead of four lens elements cemented together, while possessing the same high efficiency as series VI I. They are issued as " single " or " double " Protars, //I2'5 and //6;3 or //7, also in sets of three or four objectives of different foci, which are combined to give pictures of different angles of view from the same standpoint. With both series when using the " Protar " lens singly, it should be screwed behind the iris diaphragm of the mount, to avoid curvature of the field, and when two such lenses are combined the one with the greater focal length should be placed in front.

In 1895 Messrs Goerz patented a double anastigmat, f/5'6, with quintuple single lens components as a convertible lens, for which greater sharpness of definition and intensity, with perfect freedom from astigmatism and distortion in the single lens, were claimed. It was issued in 1898, but, like an earlier analogous quintuplet of Messrs Turner & Reich (1895), it has not come into use on account of the cost and difficulty of construction. The latter firm, however, brought out in 1906 a new symmetrical quintuplet at//6'8.

A triple anastigmatic combination containing remarkable new features, constructed and patented by H. D. Taylor, was issued in 1895 by Messrs Taylor, Taylor & Hobson under the name of the " Cooke Lens," and later by Messrs Voigtlander as the "Triple Anastigmat." It consists of three single lenses, two of them positive crossed lenses of crown glass with high refraction and low dispersion, with their most convex sides outwards, and between them, in front of the diaphragm, a single biconcave of light flint (fig. 39). All these lenses are designed to be free from diaphragm corrections, while the focal power of the negative lens is made as closely equal to the combined focal powers of the two positive lenses as may be FIG. 40. "Cooke" Lens. Series III.

FIG. 39. " Cooke " Portrait Lens. Aperture //4~5.

necessary for the flattening of the field and correcting marginal astigmatism. They are not convertible, but arrangements are made for replacing the back lens by a low-power extension lens (Ph. Jour. 1895, 19, p. 64). Series III., //6'5 (fig. 40), and series IV., //5'6, are portrait lenses. In the larger objectives of series II. the back lenses are adjustable for uniform sharp definition or a soft diffusion of focus. In a later series VI. (190?). //5'6, this adjustment for diffusion is given to the front lens and is so arranged for portraiture that the diffusing adjustment and iris diaphragm can be operated from the back of the camera while viewing the focusing screen. A special fully corrected " Process " lens on the same general principle has recently been brought out for three-colour work and fine-line reproduction. Another distinctly new type of anastigmatic objective involving several new principles of construction was patented by H. L. Aldis in 1895, and brought out by Messrs Dallmeyer in three series, under the name of "Stigmatic" (Ph. Jour., 1896, 20, p. 117). It also approaches the triplet construction and depends on the introduction of air-spaces between the component lenses. According to Aldis, three conditions must be observed to obtain a flat field free from marginal astigmatism : (i ) The converging lenses must be of high, the diverging of low, refractive index; (2) the converging and diverging components must be separated by a considerable interval; (3) thick meniscus glasses should be used. The first " Stigmatic " was a portrait lens, series I., 1896, //4- It has been made in two forms, first with a triple front lens, and a back negative system formed of a single thick crown lens of high refractivity with a negative cemented meniscus. In the second form (fig. 41) the front component consists of a cemented positive and negative, and both parts of the back component are cemented lenses. All the converging lenses are of dense baryta crown, while both the diverging lenses in the back component are a light silicate crown. It is fully corrected for spherical and chromatic aberration, free from distortion and nearly so for astigmatism, giving equal illumination over a flat field of 60. Diffusion of focus is obtained by unscrewing the back cell. Series II. (1897) is on the same principle but differs in construction, working at //6 over an angle of 85 as a universal and convertible lens (fig. 42). The front or back component can be used alone, giving the choice of two focal lengths, 1 1 and twice the focal length of the complete lens. The principles of its construction were described by T. R. Dallmeyer in Ph. Jour. 1897, 21, p. 167. Series III., //7'5, will at //i6 give sharp definition over a plate two sizes larger. The single components are not convertible.

FIG. 41. Stigmatic Portrait Lens. Series I.

FIG. 42. Stigmatic Lens. Series II.

In 1897 Messrs Zeiss issued the " Planar," an objective of large aperture based on the principle of the Gauss telescope objective. It is a symmetrical doublet, each element consisting of three lenses, the two inner ones being a double convex and a double concave, of equal refractive but different dispersive power, cemented together and separated by an air-space from the outer convex meniscus (fig. 43). Its special points are its good colour correction, large relative aperture and intensity, varying from //3'6 to //6, with perfectly sharp definition and anastigmatic flatness of field over an angle of view from 62 to 72. It is a very rapid wide-angle lens useful for instantaneous work with the cinematograph and hand cameras, also for portraits and groups, photo-micrography and enlargements or reductions (see E. Jb., 1898, p. 79, Von Rohr, p. 390, and Lummer, p. 81). Apochromatic planars with reduced secondary spectrum were brought out in 1903 for three-colour photography, and are also useful for astrophotography, the circle of diffusion being very small. The " Unar " (1900), f/4'5 in the smaller and//6 - s in the larger sizes, angle 65 and 68, was a further improvement by Dr Rudolph. It 5 11 consists of two unsymmetrical combinations, each formed of two single lenses of very transparent glass, dense baryta crown and light flint, separated by positive and negative air-spaces (fig. 44). The separate halves cannot be used as single lenses, neither being fully corrected for colour. It is well adapted for portraiture, groups or landscapes, especially for rapid hand camera work, on account of its covering power, with freedom from astigmatism and sharp definition with large relative aperture.

FIG. 43. Planar. Series la.

FIG. 44. Zeiss's " Unar."

In 1898 Messrs Goerz patented their " Double Anastigmat Celor," series I6..//4-5 to//5-5- It is a symmetrical doublet, each element consisting of two thin single lenses : a positive of high and a negative of low refractive index, separated by an air-space (fig. 45). It is derived from the triple anastigmats by decreasing the refractive power of the central convex meniscus to the refractive power of air, so that it becomes a convex air-space between a double convex and a double concave lens. Less deeply curved surfaces can be given to the lenses, and the doublet gives anastigmatic flatness of field over an angle of 62 to 66, equal to the best anastigmats with a still larger aperture. Series \c., //6-3, is similar and recommended for hand cameras, the aperture being smaller. Goerz's " Hypergon," (1900) f/22, angle 135, is a FIG. 45. Goerz's " Celor." FIG. 46. Goerz's " Alethar."

symmetrical doublet of remarkable construction, consisting of only two single semi-globular, very thin lenses, with diaphragm at the centre of curvature between them. Astigmatism and curvature have been eliminated, and definition is good over the above wide angle with no distortion. Chromatic aberration is uncorrected, but compensated for by using a small stop. A star mask is fitted in front of the lens to allow for falling off of illumination towards the margin (E. Jb., 1901, p. 103). The " Syntor " (1903), Series Id., //6-8, angle 64 to 70, is on the same principle as the " Celor," but cheaper, for use in hand cameras or telephoto combinations. The " Alethar," series V. (1903), f/l i, is a lens with diminished secondary spectrum, for three-colour reproductions, half-tone process work, and general purposes. It is a symmetrical doublet, each element consisting of a negative and positive separated by an air-space (fig. 46). The negative is composed of three cemented lenses, which correct the spherical and chromatic aberrations more fully than hitherto possible, so that all the colours of the spectrum are focused in the same invariable plane. It gives great crispness of definition at full aperture (W. Zschokke, E. Jb., 1904, p. 165). Goerz's " Pantar," //6-3 (1904), is a convertible 4-lens anastigmat, and an improvement on the " Dagor," in that the single elements are completely corrected for coma, and thus form efficient long-focus lenses for landscape, etc., at an aperture of //I2-5, while the doublets formed by various combinations of the single elements are universal objectives working from //6-3 to //7-7- The single elements are similar to those of the " Dagor," but have an additional negative FIG. 47. Aldis Lens. Series II.

FIG. 48. Aldis Lens. Series III.

lens at the back, so that the outer two of the three cemented surfaces have a collective and the inner one a dispersive action, by which coma is eliminated (E. Jb., 1905, p. 55).

In 1902 H. L. Aldis issued the Aldis Lens," //6, a doublet composed of a cemented meniscus in front and a single double convex back lens. It is a long-focus objective with short back focus, and is made in two forms, series II., //6 (fig. 47), and series III. (i9 3). //7'7 (fig- 4 8 ) In tne latter the back element is very thin, and the front combination of infinite focal length. By discarding the symmetrical form simplicity is secured, while open or reflecting surfaces are avoided. Special attention has been paid to perfect correction of spherical aberration in the centre of the field. It is ighter, smaller and cheaper than series II. The " Duo " lens of the same maker (1907) is intended to replace the front lens and double the focus, but with less rapidity and without any loss of quality. The " Trio " (1908) is similar, but only increases the focus one and a half times and is thus more suitable for cameras of short extension. The Aldis " Oxys " anastigmat, series II. (1908), //5-6s, angle 85, is an improved form. Being an unsymmetrical cemented doublet it is free from the defects incidental to air-spaces and is constructed to give more perfect correction for flatness of field with large aperture and wide angle.

It is generally stated that it is impossible to make a spherically, chromatically and anastigmatically corrected photographic objective with the old optical glasses. K. Martin, of Messrs Busch of Rathenow, has, however, shown (E. Jb., 1902, p. 68) that it is quite possible to do so with a system of separated lenses, and that it is immaterial whether the index of the flint or the crown is the higher. An anastigmat on this principle was issued by Messrs Busch in 1902, as the " Omnar," series III., //y-y (fig. 49). Series II.,//5-5, angle 75, and I., //4'5, have since been issued. It is a symmetrical FIG. 49. " Omnar," Series III.

FIG. 50. Ross's " Homocentric."

doublet, each element consisting of a negative flint meniscus of higher refraction, and a positive crown of Tower refraction with an air-space between them in the form of a negative lens. The back element can be used alone. The " Lumar series, by G. Rodenstock, is similar. In 1902 Messrs Ross brought out the " Homocentric," a symmetrical doublet, each element consisting of a negative and positive meniscus separated by an air-space (fig. 50). It is constructed so that all rays of light emanating from any one point of the object are converged again into one point in the image. It is also quite free from spherical zones, is not altered in focus with different diaphragms, and thus has exquisite defining power. The colour correction is so perfect that the different coloured images are identical in size and position, thus rendering it specially suitable for three-colour and process work. The back lens can be used alone, with diaphragms, as a single lens of about double the focus of the doublet. It is made in several series: II.,//5-6, and III.,//6-3, for rapid and instantaneous work; V., //8, for ordinary purposes; VI., //8, for process work and three-colour reproduction. A later series, IV. (1907), " Compound Homocentric," //6-8, differs from the others in being a symmetrical doublet composed of two triple cemented elements, very close together and separated by a diaphragm. It is FIG. 51. Zeiss's " Tessar."

FIG. 52. Voigtlander's " Heliar."

specially suitable for outdoor work, also for copying and enlarging, having good covering power. Zeiss's " Tessar ' (1902) is a rapid unsymmetrical doublet, formed of two separated uncemented positive and negative lenses in the front element and a cemented meniscus at the back (fig. 51). The two halves cannot be used separately. The glasses used are very transparent, permanent and lessen the secondary spectrum. Three series are made by Messrs Ross, lc., //3'5 for cinematographic work and portraiture, and 7/4-5 for handcamera work and portraiture; 116., //6-3 for general purposes, and VIII., the " Apochromatic Tessar, specially corrected for threecolour work and reproduction. They all give fine definition over a large flat field, free from any zonal aberration. The//3'5 portrait lenses, with double the field and covering power of the Petzval lens, are anastigmatic and free from distortion. Messrs Voigtlander's " Heliar" (1902), //4~5, angle 50, calculated by Dr H. Halting, is an objective of large aperture, suitable for portraits and very rapid instantaneous work, being well corrected for astigmatism, coma and curvature of field, with freedom from flare. It is a triplet consisting of a central negative lens, with cemented double front and back lenses (fig. 52). The negative lenses are of light silicate flint, the two positive of the heaviest baryta crown. Besides being a rapid universal lens, it is specially suitable for half-tone process work, with a large diaphragm (E. Jb., 1903, p. 117). The " Dynar " (1903)1 //6, angle 60 , is of somewhat similar construction, but differs from the " Heliar " in the positive lenses of the cemented pairs being outside instead of towards the central lens. It can only be used as a whole.-. It is made of hard colourless Jena glasses, giving great brilliancy and uniformity of illumination over a large angle, and is specially adapted for very rapid hand-camera work.

Dr R. Steinheil's " Unofocal " (1903), //4'5 is a symmetrical doublet, each element consisting of two single separated lenses of equal refractive power and of equal focus of opposite signs, hence its name. Each half can be used as a single lens with small stops. In its construction a quite new principle was followed, the separation of the lenses fulfilling an important part in the colour correction, as explained by Conrad Beck in Ph. Journ. (1904), 44, p. 177. This plan satisfies the Petzval condition and removes its restrictions, so that a lens of //4>5 can be produced with telescopic central definition, perfect freedom from distortion and flare over a flat field of 60, with great equality of illumination (fig. 53). They are made by Messrs Beck in two series: II., //4'5, for portraiture, rapid handcamera work, telephotography and projection; and I..//6, in which the lenses are closer together, for hand-camera work and general purposes. E. Arbeit's Euryplan " anastigmats (1903), made by Schulze Bros., Potsdam, are apochromatic objectives of quite new construction, giving perfect definition with large apertures over a FIG. 53. Beck-Steinheil " Unofocal."

FIG. 54. Euryplan, wide angle, made in four series: I., //4'5, angle 80; II., f/5-6, angle 90; III., //6-8 to 7-5, angle 82; IV., f/6-5. They are symmetrical doublets, each element consisting of three lenses, a new achromat formed of a biconvex of heavy baryta crown of high refractivity and low dispersion, separated by an air-space from a positive meniscus of the same baryta crown, with its concave side towards the diaphragm. In series I., //4 - 5, the two positives are placed outside (fig. 54), in series II. and III. they are inside. The single elements are fully corrected astigmatically and chromatically, and can be used singly at double the focus (E. Jb. 1904, p. 35).

Beck's Isostigmar " (1907) is a new anastigmat showing a distinct departure from the ordinary principles of construction, in that it does not fulfil the Petzval condition that the sum of the focal powers of its individual lenses multiplied by the reciprocals of their respective refractive indices should be equal to zero, or 2(l/jif) =o. It is a 5-lens combination, two separated thin single lenses in the front element and three in the back (fig- 55)- I? departing from the Petzval condition very low power lenses can be used, thus reducing the initial errors to be corrected; no individual component having a shorter focal length than one-half that of a complete objective. A special feature is the excellent correction of the oblique spherical aberrations and central aberrations, giving a practically flat field without astigmatism over angles from FIG. 55. Beck's stigmar."

Iso- 60 to 90. The half combinations can also be used alone with diaphragms as long focus lenses of different foci (Ph. Journ. 1907, 47, p. 191). It is issued in six series: I. (1908), //4'5, large aperture, series, for reflex press work and portraiture; Ia.,//6-5, angle 6o-65, long focus, for portraiture, etc.; II..//S-8, angle 70, for general use, III.//7-7, angle 65, similar to II. but less rapid ; IV. //6-3, angle 90; wide an^le, giving satisfactory definition at full aperture over an angle trom 80 to 85 . Having such a large reserve of covering power the latter is very useful when an extended use of the rising front is required, either at a wide or ordinary angle. V. (1908), //n, " Process " lenses specially corrected to give a flat field for copying. They can be fitted with suitable reversing prisms. VI. (1908), //5-6, variable portrait lenses, adjustable for sharp or soft definition from the back of the camera while focusing.

The above represent the principal types of anastigmats, but many more objectives of the kind, triple or quadruple, cemented or uncemented, with air-spaces, in many modifications, have been issued by English and foreign makers.

6. Telephotographic Objectives. For some years past special objectives, or attachments, have been constructed for photographing near or distant objects on an enlarged scale with an ordinary camera, the extension required being very much less than would be needed to obtain an image of the same size with an ordinary long-focus lens without enlargement. They consist of a combination of a positive converging with a negative dispersing lens, by which the image is picked up and enlarged to varying degrees, according to the system of lenses used and the extension given to the camera, thus producing the same effect as a positive lens of very much longer focus. Enlarged images of this kind can also be made by a combination of two converging lenses, one of them forming an image of the object, which is received on the other of shorter focus and projected on the sensitive plate, being enlarged more or less according to the optical conditions and relative positions of the lenses and sensitive plate. The photoheliographs at Greenwich and other solar observatories, designed by Warren de la Rue, are on this principle. Portable apparatus of the kind was made in 1869 by MM. Borie and de Tournemire, and later by Jarret, but this system requires much greater extension of the camera, entailing more loss of intensity of the image, and has never come into use.

The modern telephotographic combination is generally looked upon as an application of the principle of the " Barlow " lens, but it really goes back to the Galilean telescope (c. 1610). J. B. Porta mentions the combination of concave and convex lenses for giving enlarged and clearer images of near and distant objects (Magia Naturalis, lib. 17, cap. 10, 1589). J. Kepler showed that by a combination of a convex with a concave lens images of objects could be depicted on paper of a larger size than by the convex lens alone, but reversed (Dioptrice, Prob. cv. 1611). Christopher Scheiner made use of the same principle in his " Helioscope " for solar observations (Rosa Ursina, cap. vii. 1630). F. M. Deschales and P. Z. Traber also dealt with the question, and in J. Zahn's Oculus artificialis Teledioptricus (1686) we find figured a reflecting camera fitted with a compound enlarging lens on this principle. In his Nova Dioptrica (1692), W. Molyneux has given some interesting problems for calculating the position of the compound focus of a convex with a concave lens, also the angles subtended by an object on the focal plane. If for the simple unconnected glasses then used we substitute a system of photographically corrected positive and negative lenses, suitably mounted, and put a sensitiye plate in place of the paper, we have the modern telephotographic arrangement. I. Porro seems to have been the first to use a combination of this kind for photographing an eclipse in 1857, and later for terrestrial objects. It consisted of a small achromatic single lens combined with a concave lens. Many attempts were afterwards made in France, and also in England, to utilize the principle, but special lenses for the purpose were not available. Ad. Steinheil constructed one in 1889 for the Brussels Observatory, and another in 1890 for the Marine Department in Berlin. In 1891, curiously enough, three such combinations were worked out quite independently and patented, by T. R. Dallmeyer in London, A. Miethe in Berlin and A. Duboscq in Paris. Since that time these combinations have been greatly improved by increase in the working apertures and reduction in size and weight, so that they can be used in hand cameras. They are exceedingly valuable for obtaining details of inaccessible objects at a distance, whether architectural or topographical, and for photographing animals without approaching them too closely. Large portraits can also be taken with much better perspective effects and more conveniently than by using long-focus lenses much nearer to the sitter. With the very perfect telephotographic objectives now available the loss of intensity of illumination, which no doubt was the bar to early progress in this direction, has been overcome, and definition has also been improved, so that snapshots can readily be made with combinations of high intensity, while with those of ordinary intensity the exposures are not unduly prolonged, and good definition can be obtained over an extended field.

The optical principle on which these combinations are based is yery simple, and will be understood from fig. 56. It depends mainly on the fact that in order that a real image may be thrown on the screen of an object AB, the rays proceeding from it, which pass through the positive system Lj, must come to a focus at a point / within the secondary focus /" of the negative system L 2 . Falling within this limit, they will be intercepted by L 2 and made less convergent, so that instead of coming to a focus at /, they will continue to converge till they reach the screen at /*, and will there form a proportionally larger image a'b' of AB than the image ab given by the positive lens alone at /; just as stated in Kepler's problem. Moreover, this image a'b' will be of the same size as if it had been produced directly by a positive lens L 3 with a focal length equal to //'", and this distance is the equivalent focal length of the entire system. It can be found from the formula F =/i/2/d, where ft and Ji are the focal lengths of L! and L 2 respectively, and d=fi+fis, s being the distance between the lenses. In many instruments of the kind a scale showing the value of d is engraved on the mount. If the rays from AB come to a focus in front of Lj, on it, or beyond /", no real image can be projected on the screen. There is therefore a certain limit, which is greater in proportion to the length of focus of the negative system, within which the focus of the positive system LI may fall and produce a series of well-defined images on the screen, which can be varied in size by altering the amount of separation of the two systems of lenses within the above limit, and the distance of the screen from LJ. Every change in the position of the screen will involve a corresponding adjustment of the lenses. The greater the extension of the camera and the closer the lenses, the greater the size of the image, and vice versa. The camera extension for a given magnification can be found by multiplying the focal length of the negative system by the number of magnifications, less one. The magnification produced by a given camera extension is found by dividing the latter by the focal length of the negative system, and adding one.

In its usual form (fig-. 57) the telephotographic combination consists of a quick-acting portrait lens, or an anastigmatic doublet of FIG. 57. T. R. Dallmeyer's Compound Telephotographic Lens.

large aperture and relative intensity of suitable focal length, fitted at one end of a tube, in which slides a smaller tube carrying a properly corrected negative system, which may vary in focus, but must be of shorter focus than the positive (usually about half); the shorter the focus the greater the magnifying power for a given extension of camera. The amount of separation of the lenses is limited on the one hand by the position of the focus of the positive system, and on the other by the focus of the negative system, as explained above, and can be adjusted within these limits by a rack and pinion. The tubes are adjusted so that when closed up the two foci may coincide, or nearly so, and d = o, or its minimum value; and when opened to their fullest extent the focus of the positive may fall upon the negative system, or so that d may not exceed the focal length of the negative system. Within these limits the focal length of the combination will be positive, and a real image formed on the screen. Several forms of them have been brought out by various makes, some, as Zeiss's, with a special positive lens, others for use with anastigmats and other lenses of large apertures. The negative lenses are also made of various powers.

Messrs Dallmeyer's " Adon " (1902) is a telephotographic lens, for use with hand cameras, composed of two achromatic combinations adjusted for parallel rays, a front positive lens 4J in. focal length, and a back negative lens of 2\ in. focus. These are mounted to permit of great variation in the separation, so that when the " Adon " is fixed on the front of a suitable lens, near or distant objects may be taken on an enlarged scale without altering the focus of the camera, or the enlargement can be varied with further extension of the camera. Used alone it is a complete telephoto ens of moderate magnifying power, and will cover plates 15 in. X 12 in. In 1903 a special form, the " Junior Adon," was made in three kinds for use with kodaks and similar folding hand cameras, single and double extension, giving a fixed degree of magnification without loss of rapidity, while focusing can be effected by scale. It is intended to replace the front lens of an R.R. or anastigmatic lens and cannot be used independently. Messrs Busch's " BisTelar," //9 (1905), is another compact fixed focus telephoto lens, specially for use with hand cameras. It is a complete lens in itself, requiring no attachments and can be fitted to a central shutter. _ It is made in three sizes magnifying from two to three times. An improved form of this lens (1908), working at the large aperture of f/7, is similar to an old form of " Dialytic " lens worked out by J. Petzval, having a positive front and negative back meniscus, with their concave surfaces facing inwards (fig. 58). As in the old " Orthoscopic " and lenses of that type there is some outward distortion, but it is very slight. These lenses are made in five sizes with . foci from 8 to 22 in., requiring camera extensions from 4i in. to 1 1 J in. They magnify about twice. According to K. Martin, a telephotocombination of the Bis-Telar type can be used in a reversed position as a projecting lens for the lantern, with the advantage of increasing the illumination from a given source of light (E. Jb. 1908, p. 46).

Captain Owen Wheeler proposed in 1907 a high-power telephoto arrangement, made by Messrs Staley, in which the negative attachment consists of three negative lenses, any single one of which can be used separately, giving magnifications of about 6, 9, and 13 diameters with a camera extension of 14 in. By combining the three a magnification of_ 30 diameters is attainable with the same short extension, which is a great advantage in many ways. In 1908 Messrs Zeiss issued their " Special Tele-objectives " in two sizes working at //io, the larger with an aperture of 3-14 in. and 32 in. focal length fitted in a special " tele-camera " for plates 9 X 12 cm. with a monocular field glass magnifying four times as finder. The smaller one, with 18 in. focus, is adapted for hand cameras with 6 in. bellows extension. They consist of specially corrected positive and negative combination with a definite focal length and requiring a definite camera extension, and are specially suitable for balloon photography, instantaneous portraiture, etc. The theory, construction and use of telephoto lenses has been fully described by T. R. Dallmeyer in his Telephotography.

7. Anachromatic Lenses. For large portraiture a certain amoun' of softness and diffusion of the image has long been recognized by artists as desirable, and in 1895 the " Dallmeyer-Bergheim Lens was constructed with this special object. It is composed of a single uncorrected positive meniscus front lens, with a diaphragm in front of it, and an uncorrected negative meniscus back fens, and in the larger sizes it has great range of focal length on the telephotographic principle. The spherical and chromatic aberration produced by the uncorrected single lenses gives the diffusion of focus which produces the peculiarly soft and delicate, effect aimed at. It is most useful for large heads and life-size studies, the great depth of focus conducing to uniformity of definition. There is no distortion, and by stopping down to about one-third perfect definition can be obtained. It works with great brilliancy, both elements being single glasses. It was the first of the anachromatic portrait lenses. Since 1903 Messrs C. Puyo and L. de Pulligny have been experimenting with various combinations of uncorrected lenses for producing the same effect in portrait and landscape photography by the diffusion of focus produced by chromatic aberration, and suitable lenses of this kind have recently been brought out in Paris as Les Objectifs d' artiste. In their construction the principal points to be considered arc spherical aberration, to be minimized in the form and arrangement of the lenses selected ; distortion, corrected by using a symmetrical system ; astigmatism, avoided by using combinations of low power. The lenses used by Puyo have been: (i) a plano-convex crown with convex side in front at//8 or//g, or even //5 for heads; (2) a simple thin concavo-convex meniscus, with concave side in front, is better and suitable for full lengths at//io; (3) a symmetrical system formed of two similar crown menisci, concave sides inwards, is generally useful when worked at //io, or even //5- Arrangements are made in mounting these lenses for automatically making the necessary correction for colour. Another form is the " Adjustable Landscape Lens," formed of an anterior plano-convex crown, 3 cm. diameter, and a posterior plano-concave crown, each of io cm. focus, and the same radii of curvature. In contact they have an infinite focus, but when slightly separated any focus can be obtained up to about io cm. In such a telephotographic system, properly stopped down, anastigmatism, flatness of field, and rectilinearity are secured over a fairly large field. These lenses are fully described in Les Objectifs d'artiste, by L. de Pulligny and C. Puyo (Paris, 1906), and various forms, portrait and landscape, have been made by Messrs Hermagis, Turillon & Morin (see Fabre, T. E. P. Suppl. D. 101).

Diaphragm Apertures. In order to regulate the intensity of the illumination by the lens, to enlarge its field, and, in the case of the older forms of objectives, to extend the area of good marginal definition, diaphragms are used, usually with circular apertures. They are made in different ways: (i) as single metal plates, fitting into a slot in the lens tube (Waterhouse diaphragms) ; (2) Rotatory : a single plate revolving on a central axis and pierced with apertures cut to fit centrically in the opening of the lens; (3) Iris: a form of diaphragm now very generally used, and very convenient, because it can be easily adjusted as required for intermediate apertures. As a rule they are placed at the optical centre between the elements of a compound lens or in front of a single one.

In order to provide a uniform system of diaphragm apertures, the Royal Photographic Society in 1881 drew up some rules, which were revised in 1891 and again in 1901. The former standard unit f/A. and the numerical notation used with it, have been abolished in favour of the unit f/i established at the International Congress in Paris 1900. Intensity ratio is denned as dependent upon the effective aperture of a lens, and not upon the diameter of the diaphragm in relation to the focal length of a lens. The effective aperture of the lens is determined as follows: The lens must be focused for parallel rays. An opaque screen is then placed in the principal focal plane, and a pinhole is made in the centre of the plate (in the axis of the lens) ; an illuminant is placed immediately behind the pinhole itself, when the diameter of the beam emerging from the front surface of the lens may be measured. (It will be found that except in the case of the diaphragm being placed in front of the lens, the diameter of the diaphragm itself is seldom that of the effective aperture.) Every diaphragm is to be marked with its true intensity ratio as above defined^ but the present intensity ratios are retained in their order of sequence: //I.//I-4.//2 J/2-8,//4,//5-6,//8,//u-3, //i6, //22-6, //32,//45-2, //64, etc., each diaphragm requmng double the exposure required by the preceding one. In other cases apertures are to be made in uniformity with the scale, with the exception of the highest intensity, e.g. a lens of f/6-3 would be marked for f/6-3, //8, etc. The corresponding numbers are known as/ numbers, but are only applicable for a lens focused for distance. Other systems of notation are in use, but the above is generally adopted (see Fabre, T.E.P. Suppl. C. 38). Special diaphragms are in use for process work with ruled screens (see N.S. Amstutz, Handbook of Photoengraving, 1907). Standards for the screws of photographic lensflange fittings, and for the screws fitted to cameras for attachment to the stand or for fixing movable parts, have also been laid down (Ph. Journ. 1901, 25, p. 322).

Instantaneous Shutters.

The general use of rapid dry plates and hand cameras has rendered it necessary to have some mechanical means of regulating exposures in small fractions of a second, especially for objects in rapid motion, and this instantaneous shutter has become an essential part of modern photographic equipment in many forms and patterns, but practically three types are preferred the between-lens shutter, the roller-blind shutters, used before or behind the lens, and the focal plane shutter, in front of and close to the plate and forming part of the back of the camera. The usual limit of rapidity of the two former is nominally about rfar of a second, and for ordinary purposes higher speeds are seldom required, while with the latter speeds of i^ot to usta of a second may be attained.

Two important factors in the use of lens shutters are the rapidity or speed, measured by the total duration of exposure from opening to closing, and the efficiency, measured by the ratio of the time during which the shutter is fully open and the time occupied in opening and closing. Both factors are more or less variable, either with differences of construction, of diaphragm opening or of position of the shutter with regard to the plate and lens. In any case the efficient exposure is always less than the actual, and may be considerably so.

The rapidity required of a shutter in photographing moving objects is regulated by the minimum time necessary to produce a well-exposed image upon the plate, with a loss of definition, or blurring, by displacement not exceeding too. or preferably jtu to slo of an inch, if enlargement is extended. This will depend on the state of the light and the illumination of the object, the relative intensity of the lens as measured by its effective aperture and focal length, the sensitiveness of the plate, and the amount of effective light passing through the shutter during the exposure. The amount of displacement to be guarded against depends upon the rate of movement of the object, the direction in which it is moving with reference to the axis of the lens, its distance from the camera, and the focal length of the lens. It will be proportionately less as the distance of the object increases, and as the rate of its motion and the focal length of the lens for a given distance decrease, and vice versa. It will be greatest when the object is moving at right angles to the axis of the lens, and least when the motion is directly towards it ; but in that case there will be some increase in the apparent size of the object as it approaches the camera. For example: An object moving I m. an hour advances 17-6 in. per second. With a lens of 5-in. focus this would represent a displacement on the ground glass, for an object o ft. away, amounting to -146 in. per second, and it would require exposures between ^ and V of a second to give maximum or minimum displacements of the image between T JTJ and tl s of an inch. An object at the same distance moving ten times as fast would require i-io of the above exposures. If, however, the distance be increased, the possible exposure may also be increased in the same proportion, so that the object moving 10 m. an hour at 500 ft. distance would only require the original exposures of fr to ,V of a second. On the other hand, the limits of exposure for an object moving I m. an hour within 10 ft. of the lens would be between j* and T b of a second. This is entirely independent of the sensitiveness of the plate, and only represents the maximum duration of exposure permissible in order to reduce the blurring of the image between certain limits. The sensitiveness of the plate, and the intensity and amount of light acting upon it through the lens and shutter, must be adjusted so as to produce the desired photographic effect within that time. With a lens of 8 in. focal length the displacement would have increased in the first instance to -23 in. per second, and the maximum exposure permissible would be from 8 V to ^ of a second. This shows that there is an advantage in using short-focus lenses for very rapid exposures. In practice, most work of this kind is done upon quarter-plates (4.JX31 in.) with lenses of 4$ to 55 in. focus. As the displacement will be greatest for an object moving at a right angle across the axis of the lens, an exposure sufficient for this case will be sufficient for any other. Sir William Abney has discussed this question practically in his Instantaneous Photography, and it is treated mathematically by W. B. Coventry in his Technics of the Hand Camera, in which will be found formulae and tables for ascertaining the distances and limiting exposures for moving objects, allowing for a blur of T ^ of an inch. In foreign treatises the limit is usually calculated for a displacement of -fa of a millimetre, or about jjp of an inch.

An efficient shutter should fulfil the following conditions: It should be light and compact, simple in construction and action, strongly made, and not liable to get out of order; capable of being set without admitting light into the camera; easily released with a slight pressure of the finger, if a pneumatic release is not fitted, and free from any tendency to shake the camera on release. It should open and close quickly, allowing the largest possible proportion of the exposure to be made with the full aperture, and it must not cut off any of the effective light passing through the lens, but should distribute it evenly all over the plate: though in landscape work it is an advantage to give the foreground more exposure than the sky. It should be adjustable for variable instantaneous and for prolonged or " time " exposures. With a good shutter there is less risk of shaking the camera in short " time exposures, from } to i second, than there is in taking off a cap. Shutters working between the lenses must permit of the use of diaphragms in the lenses, and of alterations of speed while set. Above all, a shutter must be constant in its action, giving short and variable exposures always correctly or relatively so, an important condition which cannot always be fulfilled, and the exposures marked on the indicator should be capable of being repeated with tolerable certainty. Shutters should also be adaptable tor use with different lenses. Three methods of varying the speed of a shutter are in use: (l) by altering the length of the slot; (2) by the retarding action of a pneumatic brake; (3) by varying the tension of a spring. The latter is considered by W. B. Coventry as far the best. They are usually released by the pressure of the finger on the end of a lever holding the moving part in a state of tension; or better, by J. Cadett's system of pneumatic pressure, applied by means of a compressible rubber bulb and tube, which may drive a piston acting on the lever holding the shutter, or inflate a collapsible bulb at the other end of the tube and thus exert the necessary pressure on the lever. With W. Watson's " Antinous " release a flexible wire acts directly on the piston or trigger release of a^cylinder shutter. It is also adapted for roller-blind, focal plane, flap, and various forms of between-lens shutters. It is durable, effective and convenient (see fig. 3). In many cases both methods can be used as desired, the mechanical release being preferable on account of its convenience and freedom from liability to shake the camera.

The following are the principal types of instantaneous shutter: (l) Flap, (2) drop, (3) combined drop and flap, (4) rotary, (5) roller blind, (6) focal plane, (7) moving blade central, (8) iris. They can be applied in four different positions: (a) in front of the lens; (b) centrally, near the diaphragm; (c) behind the lens (d) immediately in front of the sensitive plate. They all, however, come under two main classes: Lateral, including those in which the exposure commences and ends at the circumference of the lens aperture; and Central, those in which the exposure begins and ends at the centre of the aperture. Some of them are " lateral " in their single form and " central " when double. The form and position of the effective aperture of a shutter, relatively to the lens and plate, have a strong influence, either favourable or unfavourable, on the amount of effective light passing through the lens, and its even distribution over the plate. This is especially the case during the incomplete phases of opening and closing the aperture. It seems to be agreed that the best position for lens shutters of the lateral type is behind the objective, and for those of the central type, between the component lenses. In this latter position the whole of the plate is illuminated during the full period of exposure, with a gradually Increasing intensity, until the full opening is reached, and then the illumination gradually falls off until the shutter is closed. The most effective shutter is one in which the first and third phases of incomplete illumination, during the opening and closing, are the shortest compared with the second phase of full opening.

With the focal plane shutters, however, different portions of the plate are exposed in succession, the lens working at its full aperture and efficiency throughout the exposure.

To secure successful results in using instantaneous shutters, the operator should make himself acquainted with the working of his shutter and its efficiency in various circumstances of exposure with the lenses, plates and developer he proposes to use ; ascertaining the actual .value of the various exposures marked on the indicator, and f what is more important, how far they can be depended on for regularity. There are many simple ways in which the actual time of exposure from opening to closing can be ascertained sufficiently closely for practical purposes. They depend upon the measurement of the trace left on a sensitive plate by the passage of a brightly illuminated object revolving at a known speed or falling vertically through a known distance, when photographed with different speeds of the shutter against a dark background. These, and the more elaborate methods for obtaining more accurate determinations of the shutter-exposure periods and of the corresponding effective exposures i.e. showing the actual effect of the shutter through its different phases from opening to closing have been described by Sir William Abney in the work already mentioned, by A. Londe in La Photographic moderne and La Photographie instantanee. An apparatus for testing shutters at the National Physical Laboratory was described by J. de Graaf Hunter in the Optician, 1906.

I. Flap Shutters. The simple flap shutters consisting of a hinged flap opening upwards in front of the lens, though favourites in early days lor landscape work, and still useful for intermittent exposures or as sky-shades for securing cloud effects or increasing foreground exposures, have been almost superseded by quicker and more compact forms. They are used with single and double flaps for portraiture and studio work, for which purpose they are made to act noiselessly and not attract the attention of the sitters. Guerry's (figs. 59 and 60) is a good example of the type. W. Watson's " Silent " FIG. 59. Guerry's Single-flap Shutter.

FIG. 60. Guerry's Double-flap Shutter.

shutter is hemispherical in form and collapsible, the two wings opening out and folding together, when actuated by a special " Antinous " release, and R. & J. Beck's is another form, a single lifting flap with pneumatic release.

2. Drop Shutters. The old simple drop shutter, in which a plate having an opening in it falls in front of the lens aperture, has been superseded by the more compact and quicker-working roller-blind shutters, which act on much the same principle. It had a theoretical interest in connexion with the effect of different forms of aperture circular, square, or elongated used with shutters of the lateral type, but it is now generally recognized that a more or less extended rectangular opening, of at least the full width of the lens aperture, is best for securing the even admission of light from all parts of the image with shutters of the rectilinear lateral type, to which this and similar shutters, in which a single opening passes across the lens aperture, belong. In Busch's " sky shade " shutter (1907), fitting on the front of the lens a single leaf moves vertically upwards and descends again, giving less exposure to the sky.

3. Combined Drop and Flap Shutters. In early dry-plate days several forms of this kind of shutter were brought out, under the names of Phoenix, Phantom, etc., but are now little used. In these shutters, in addition to the drop slide, there was also a lifting flap, which on release opened from below, and, having fully uncovered the aperture, released the drop slide, which fell and closed the shutter. They were useful and effective in the smaller sizes, but heavy and cumbrous in the larger. Speed could only be estimated very roughly by the use ol india-rubber bands for giving tension.

4. Rotary Shutters. These are of the lateral type, and consist of a circular metal disk revolving on an axis eccentric to the axis of the lens, and furnished with a radial sector-shaped opening, which passes laterally in front of the lens aperture when the tension of a spring is released (fig. 61). They are used in various patterns in cheap hand cameras, usually in front of the objective, though they may be placed behind it or between the component lenses. So long as the opening is at least equal to the size of the lens aperture, the illumination is sufficiently even, but the openings are usually elongated so as to give a longer period of full opening. Working by a spring they are more portable and con"*" "" venient than drop shutters. Beck's FIG. 61. -Rotary Shutter. " Celverex " between-lens shutter (1906) is of this type, the disk being revolved by a spring and the variations of exposure obtained by altering the size of the opening passing over the lens aperture, and not the tension of the spring. It is speeded for exposures of A, yj, fa, jg sec.; also "bulb" and "time." It is fairly accurate and consistent in action, but loses efficiency at the highest speeds by the diminution of the opening.

5. Roller-Blind Shutters. For general use the well-known rollerblind shutter of the single lateral type, as made by Thorn ton- Pickard and others, is undoubtedly one of the most popular and efficient. It possesses most of the qualities laid down as essential to a good shutter, gives good illumination, appears to be fairly regular in its action and can be used for time or instantaneous exposures. It consists of a light mahogany or aluminium box, arranged so that it can be fitted in front of or behind the objective. It is made in different sizes, and each size can be adjusted to smaller objectives (fig. (>2 >. It is also made with a disappearing cord, and in an improved pattern, the " Royal," all the fittings are inside the box. By pulling the cord FIG. 62. Thornton-Pickard Roller-Blind Shutter with automatic exposure appliance.

FIG. 63. Mechanism of the ThorntonPickard RollerBlind Shutter.

A, Upper roller.

B, Lower roller, c, Cord.

L>, Black curtain. H, Aperture incurtain. R, Rubber ring adapter.

an opaque black curtain with an elongated rectangular aperture is unrolled from the lower roller on to the upper one, and held by a coiled spring on the lower roller (fig. 63). Pressure on a pneumatic bulb inflates a second smaller bulb, raising a lever which releases the spring, and thus brings the blind down with a rapidity which can be adjusted by turning a handle actuating the spring, the corresponding speed being shown on an indicator. For time exposures, pressure on the bulb opens the shutter, and another pressure closes it, but an arrangement is now made by which time exposures of }, i, J, i, 2, 3 seconds can be given automatically, the pressure of the bulb opening the shutter, which closes of itself at the expiration of the exposure required. The theory of shutters of this type has been very fully discussed by Coventry (op. cit. p. 50), who shows that for any given tension of the spring the actual exposure decreases as the size of the lens aperture diminishes, while the effective exposure remains constant for all apertures. This is peculiar to the lateral shutter. He also shows that with plates of very different rapidities, though the exposure may be the same, the actual exposure effective is less with the rapid plate and a small stop than with the slow plate and a large stop; consequently the blur due to the movement of the object would be proportionately less on the rapid plate than on the slow one. Also that for any given lens the smaller the shutter the more rapid the exposure can be made, though with the same lens a larger shutter is capable of giving a more efficient though less rapid exposure. It is better, therefore, for moderate exposures, to have a larger shutter than the size of the lens requires. Sir William Abney had given diagrams of the action of a shutter of this kind in his book referred to; they show clearly that the centre of the plate gets more exposure than the margins; but practically this is not very noticeable, and the action is very regular.

6. Focal Plane Shutters. These are also roller-blind shutters with mechanism similar to the foregoing, but arranged so that the slit in the curtain may move rapidly closje in front of the sensitive plate, exposing different portions of it in turn, the intensity of the exposure aemg regulated by the width of the slit, whether adjustable or not, and the rapidity with which it is moved by the unwinding of a spring. The advantages of these shutters are now being fully appreciated, :he principal being that they are quite independent of the lens, so that one shutter will serve for different lenses, and any suitable lens nay be used at its full intensity, without the loss of efficiency inlerent in the ordinary forms of lens-shutters. They thus add effectively, if not actually, to the speed of a slow lens, or if a lens be stopped down there is less loss of efficiency, with a gain in increased depth and definition. They are particularly well adapted for the very short exposures required in photographing near and quickly noving objects, racing horses, divers, etc., and many reflex and other land cameras are fitted with them. They are constructed in different forms, either for short exposures with high speeds alone, or for short and prolonged exposures ; with a single slit of fixed or variable width moved at regulated speeds, or with a series of slits or openings varying in width, their speeds being adjusted by the S i6 tensions of the springs. Thus the new Goerz-Anschutz shutter has ten tensions and nine curtain apertures, providing for ninety different speeds or exposures, ranging from f 6 to j-j'oo of a second, besides autobulb exposures from i to 5 seconds and time exposures (fig. 64). Most of these shutters are now provided with a self-capping device for protecting the sensitive plate during the setting of the shutter. As the slit moves progressively over the plate, if it is too narrow or moving too slowly, it may cause distortion of the images of quickly-moving objects, especially if near the camera, but with due care in regulating the width of the slit and the duration of exposure this is practically not often perceptible, especially if the slit is arranged to move in the same direction as the object.

FIG. 64. Goerz-Anschiitz Focal Plane Shutter.

The theory of these shutters is discussed by Coventry (op. cil. p. 69), more fully by Fabre (T. E. P. Suppl., C. p. 128), and their practical use in Focal Plane Photography (" Photo-Miniature Series,' No. 77, 1907).

7. Moving Blade Central Shutters. These shutters, in which two thin metal or ebonite plates or opaque curtains with round or rectangular apertures, or in other cases two curved blades, pass very quickly over each other in opposite directions, are largely used in many patterns fitted between the lenses of a combination or attached to them in front or behind. Formed of two single lateral shutters opening and closing in the centre of the lens aperture, they become central, the exposure taking place during the short period in which the openings are passing each other or the curved blades opening out and closing again. To obtain the greatest efficiency the size of the openings should correspond with the full aperture of the lens. If each plate moves as fast as a drop shutter the combination gives double the speed, corresponding to half the exposure. The sensitive plate will be most evenly and strongly illuminated when the leaves of the shutter work inside the lens near the diaphragm, as in Bausch and Lomb's " Unicum " and other similar between-lens shutters, in general use (fig. 65). This necessitates the fitting of the lens to the shutter, but with adapters FIG. 65. Bausch and Lomb's FIG. 66. Lancaster's " Unicum " Shutter. " See-Saw ". Shutter, it is possible to fit other lenses. Some forms are, however, suitable for use in front of the lens, such as the " Constant " and Lancaster's " See-Saw " (fig. 66), while those of the double roller-blind type can be used either in front of or behind the lens, though this position is not a favourable one. In these the rectangular form of aperture is the best, circular apertures cutting off a good deal of light, as in the case of drop shutters. W. B. Coventry (op. cit. p. 60) has discussed the action of the double roller-blind shutter as typical of the central class of shutters, and shows that while, under similar conditions, with the lateral shutter the effective exposure is constant and the actual exposure variable at all apertures, it is the reverse with the central shutters, and it will not be so easy to calculate exposures with different sized stops. A central shutter, acting as a diaphragm of variable aperture, g^ves a more efficient exposure than a lateral shutter of the same dimensions, as long as the opening is greater than the lens aperture, the coefficient of illumination of the lens varying as long as the shutter opening is smaller than that of the diaphragm used. It is desirable, therefore, to increase the speed and use as large an aperture as possible, so that the diaphragm used may be entirely uncovered during the greater part of the exposure.

FIG. 67. Goerz's "Sector" Shutter.

8. 7m Shutters. These are a further development of the double curved blade central shutters, and constructed on the principle of the " Iris " diaphragm, with several leaves opening out from the centre of the lens and closing again. They are usually fitted between the lenses of double objectives, and can be made very light and compact. Theoretically this central position of the shutter is the best, and the " Iris " is the best form for ensuring the most equal distribution of light over the plate, provided, as before, that the opening is equal to the full aperture of the lens. They are made so that the periods of opening and closing may be as short as possible compared with that of full opening. They require great care in construction and fitting to the lens, and so are expensive. They can, however, be used with convertible sets of lenses of different foci fitting the same mount. Several forms are made by British and foreign makers, with three, four or more leaves. . Goerz's " Sector " shutter (fig. 67) may be taken as a type. Georgen's " Central " shutter is very light and smooth in working, and can be used in front of a lens for telephoto work. Further details regarding the different forms of shutters, theoretical and practical, will be found in the works by Abney, Coventry, Eder, Fabre and Londe.

Exposure Meters.

When gelatin dry plates came into general use, and were made of many different degrees of sensitiveness, the want of a guide to the proper exposure for the various makes of plates under different conditions of lighting began to be felt, and several methods were devised for meeting it. Some of them depend solely upon data derived from observations of the action of the principal factors affecting the result, namely: (i) the speed of the plate; (2) the actinic power of the sun's light for the time of year in a given latitude and its position at the particular time of day; (3) the effective diaphragm aperture of the lens ; (4) the nature of the subject and its illumination as affected by local and atmospheric conditions. With others these data are supplemented by, and practically based upon, actinometric observations of the action of the light upon sensitive paper exposed near the camera or the subject at the time. Both methods are in many cases of undoubted use, but the information given by instruments of this kind can only be considered as approximate, and much is left to the judgment of the operator, whose surest guide will be an intelligent study of the principles on which these instruments are based, together with carefully-recorded observations of the combined working of his lenses, shutters, plates and methods of development under the varying conditions of practical work. Before using any of these instruments it is necessary to know approximately the relative sensitiveness or " speed " of the plate in use. In the early days of gelatin dry plates their rapidities were stated as so many times those of wet plates, or (as they are still) " ordinary," " instantaneous," " rapid ' or " extra-rapid," terms which, though suitable for one make of plate, may not be so for others. This was improved upon by the adoption, in 1878, of Leon Warnerke's " Sensitometer," which was in use as a standard for some years. It consisted of a transparent scale of 25 squares of different intensities, marked with opaque numbers and arranged so thr.t each third number indicated a doubled rapidity. This was placed in a frame in front of the sensitive plate, and exposed for thirty seconds to the constant light emitted by a phosphorescent tablet, supplied with the instrument, which was previously excited by burning one inch of magnesium ribbon in front of it. The exposed plate was then developed and fixed, and the highest number visible indicated the rapidity of the plate. In 1890 F. Hurter and V. C. Driffield introduced an entirely new system of calculating the sensitiveness of plates of different rapidities. They make a series of exposures in seconds on different parts of the plate in geometrical progression with a standard candle at one metre distance. After development for a certain fixed period with a standard developer, fixing, washing and drying, the ' densities " or logarithms of the opacities of the different parts are measured by a special photometer and plotted on a skeleton diagram, producing a curve, one portion of which will practically be a straight line. The position of this line with reference to a scale of exposures given on the diagram decides the rapidity of the plate, while its length indicates the " capacity " of the plate for the truthful rendering of tone. The elaborate investigations by which these results were obtained are of great interest, and were published in the Journal of the Society for Chemical Industry (1890, 1891), and later ones in the Photographic Journal (1898). A complete account of the system by V. C. Driffield was published in 1903, as No. 56 of the " Photo Miniature Series." The sensitiveness shown on the H. & D. scale is directly proportional to the speed number given. The method has been adopted by saveral dry-plate makers in denoting the sensitiveness of their different brands, and is more or less the basis on which the plate-speeds for the modern English dry-plate actinometers and exposure meters are calculated. Several systems of photometry and measurement of the speeds of dry plates have been discussed at the meetings of the Congres International de Photographic, in 1889, 1891, 1900 and 1905, but no definite standard has been finally adopted. In Germany the use of J. Scheiner's sensitometer has been adopted, and appears to be extending. It is based on a system of photographing the graduated tints given by rotating sectors. A full account of the instrument, and of a system of sensitometry based on its use, is given by J. M. Eder in the Photographische Correspondenz (1898) p. 469, and (1900) p. 244. In 1901 Chapman Jones brought out a convenient plate-tester on the same principle as the Warnerke sensitometer, but extended by the addition of a colour sensitometer, which is useful for the comparison of orthochromatic dry plates, colour screens, light filters, etc. It consists of a screen plate, 4iX3i in., containing a series of twenty-five tints of graduated densities; a series of coloured squares, blue, green, yellow and red, and a strip of neutral grey, all five being of approximately equal luminosity; a series of four squares of special pure colours, each representing a definite portion of the spectrum ; also a space of line design, over which is superposed a half-tone negative. To use the instrument, a quarter-plate of the brand to be tested is exposed behind the screen for a few seconds to the light of a standard candle placed at the distance of a foot, developed, fixed and washed. An examination of the plate will show the sensitiveness, range of gradation, possible range of exposure, sensitiveness to colour, size of grain, amount of halation, and the most suitable light for development. It can be used for many other tests, and enables any brand of plates to be readily tested by the user and compared with any standard he may find convenient. In making these and similar tests, a standard developer should be allowed to act for a fixed period and at a uniform temperature (Ph. Journ., 1901, 25, p. 246).

The next important factor is the actinic power of the light. It depends normally on the height of the Sun for the latitude of the Elace at the time when the photograph is taken, and exposures in right sunlight are found to vary approximately as the cosecant of the sun's altitude above the horizon. The light of the sun itself is practically the same at any given time and place year after year, but is liable to more or less local and temporary diminution by the amount of cloud, haze, dust, etc., present in the atmosphere at the time. It is also affected by the time of day, increasing from sunrise to noon, and then decreasing to sunset, the remaining factor is the effective diaphragm aperture of the lens in relation to its focal length. In most cases of ordinary outdoor exposures this can be taken at its normal value, but becomes smaller and increases exposure if the focal length is much increased for photographing near objects. Besides these principal factors, the nature and colour of the objects, their distance, and the amount of light received and reflected by them under various atmospheric conditions, have a great influence on the exposure required. W. B. Coventry has shown (op. cit. p. 75) how the " light coefficient L," for full sunlight, can be found, and has given a table of values of L for the latitude of London for every hour of the day in periods of ten days throughout the year, also the relative coefficients for " diffused light, " cloudy," " dull " and " very dull." Tables of exposures for different subjects under varying conditions of light have been published by W. K. Burton, A. S. Platts, F. W. Mills, Sir D. Salomons and others, and in preparing them Dr J. A. Scott's tables, showing monthly and daily variations of light for countries about N. lat. 53 , are generally used. The more modern tables, such as are published in the printed " exposure notebooks," also take into account the plate speeds, but unfortunately there is no uniform standard of plate speeds, owing to the difficulty of fixing a definite standard of light. The subject is fully treated in the British Journal Almanac (1901), p. 675, the Watkins Manual, H. Boursault's Calcul du temps de pose en plioloaraphie, and similar works by A. de la Baume Pluvinel, G. de C. d'Espinassoux and others.

Based on the same principle as these exposure tables, various portable exposure meters have been brought put, in which scales representing the coefficients for plate-speed, light and diaphragm are arranged as in a slide rule, so that, when properly set, the normal exposure required can be found by inspection, and increased or diminished according to circumstances. In Hurter and Driffield's " Actinograph " the light coefficient is given by a printed card showing the curves for every day in the year and for every hour of the day, the unit being the jfo part of the brightest possible diffused daylight when the altitude of the sun is 90. The lens " scale shows the ratios of aperture to focal length in general use, and is calculated for single, double and triple systems of lenses. The " speed " scale is based on the exposure in seconds which with one actmograph degree of light will produce a perfect negative of an ordinary landscape. An additional scale is given for five different degrees of illumination " very bright," " bright," " mean," " dull," " very dull." A table of factors for " views," " portraiture," " interiors," " copying," is also given, and these regulate the figure to be taken for the exposure. The scales are engraved on boxwood, and there are two sliding pieces (fig. 68).

It is specially adapted for use with plates of speed numbers agreeing with the H. & D. scale, but can be used with any plate of which the relative speed number is known. Convenient exposure meters have been made since 1890 by A. Watkins, of Hereford, in different SPICD OF PLATE HURTCR tDRIFriELO'S ACTINOGRAPH PATENT NO 5545 1888 FIG. 69. Watkins's " Standard " Meter.

FIG. 68. Hurter & Driffield's Actinograph.

forms based upon an actinometrical test of the light at the time of exposure. In the complete " Standard Meter " (1890) scales corresponding to "speed of plate," "diaphragm/ numbers," "light," " subject " and " enlarging," marked P. D. A. S. and E., are arranged on rings adjustable round a cylinder. The plate-speeds are taken from a table and the " light coefficient," or " actinometer number," is ascertained at the time by exposing a piece of sensitive paper in the actinometer at the end of the instrument for the number of seconds required to match a fixed tint as shown by an attached pendulum. Many improvements have been made in it and the latest pattern (1908) is made in magnalium (fig. 69). The " Dial " meter (1901) is a simpler form in a circular metal case with four apertures marked " plate," " stop," " act " and " exp." above the corresponding scales, and an actinometer for testing the light.

The numbers showing the speed of the plate in use, the / value of the diaphragm, and the actinometer exposure in seconds are brought into the respective apertures and the exposure required is read off in the " exposure " aperture.

An " indoor meter " is also made, and a " hand camera calculator " for use with the " Standard " or " Bee " meters. The " Queen Bee " and " Bee " meters (1903)

are later, smaller and more convenient patterns which have superseded the " Dial " meter and have the plate numbers and exposures marked round the case, and the scales of "/ numbers " and " light " on a revolving glass plate. This is revolved till the / number on the right is opposite the speed number of the plate; opposite the " actinometer number " on the left, found as above, will be found the exposure in seconds (fig. 70). The _ "Queen Bee" meter is similar to the FIG - 7- The Watkins s " Bee," but of better construction and ** ee Meter.

fitted with a pendulum.

G. F. Wynne's " Infallible " exposure meter (1893) is also in dial form, but the sensitive paper is exposed directly, no pendulum is used, and the scales are open on the dial. In use, the glass carrying the movable scale is turned untfl the actinometer time in seconds upon the exposure scale is opposite the diaphragm number of the plate, as given in the list of plate speeds; the correct exposure will then be found against each stop given on the scale. There are practically only two scales: the scale of diaphragms representing the diaphragm apertures or / numbers, the speed of plate and the variation of exposure due to subject; and the time scale, representi f i ing the actinometer time and the .-,-. - lnla1 ' exposure (fig. 71). The actino- hble Exposure Meter. is pT ^ ect( ! d ' by a ye i low g i ass screen when not in use. In a smaller form the scales are on the circumference of a locket, and the actinometer at the back. An " Infallible " Printmeter is also made for showing exposures in contact printing on sensitive papers, but can also be used for testing speeds of plates and papers. Beck's " Zambex " Exposure Meter gives the exposure and stop to be used, also the depth of focus to be obtained with different diaphragm apertures. The required exposure is set to the " speed " number on the next scale of the meter. The third scale corresponds to the times of darkening the sensitive paper in the actinometer attached to the meter, and shows the diaphragm aperture suitable for the given exposure. Other scales show the distances that will be in focus with the different stops used, arranged so that the focal depth of four different lenses can be found. Several other exposure meters are made on the principle of the slide rule, with scale corresponding to the factors of " plate speed," " diaphragm number," " light," " subject," " exposure," and the exposure is found by simple inspection without an actinometer. They are designed for use with particular brands of plates, but can be used for others of similar speeds.

Another class of exposure meters comprises those in which the intensity of the light is estimated visually by extinction through a semi-transparent medium of increasing intensity, such as J. Decoudun's (1888), in which the exposure is judged by the disappearance of a series of small clear openings on a graduated scale of densities when laid on the most important part of the image as seen on the ground-glass. Its indications are not very definite, and the paper scale changes in density after a time. A better form is E. Degen's Normal Photometer " (1903), consisting of two sliding violet glass prisms, one adjusted for the diaphragm apertures, the other for the actinic illumination of the object. They are mounted with their outer faces parallel. In use the upper slide with prism is drawn out so that the pointer coincides with the division indicating the diaphragm aperture to be used ; the object to be photographed is then viewed directly through openings at one end of the instrument, and the lower slide is drawn out and pushed back slowly till the object viewed is almost obscured. The attached pointer will then indicate the exposure required, or, reversing the order, the diaphragm aperture for a given exposure can be found. Auxiliary scales are attached for very short or very long exposures. The principle of construction is that the logarithms of the times of exposure are proportional to the thickness of the coloured prisms. " G. Heyde's Actino-Photometer " (1906) is on a somewhat similar principle, and consists of a circular metal box with dark violet glass viewing screens in the centre of both sides, with an obscuring iris inside the case worked by revolving the back of the box. On the front of the instrument exposure tables are given for plates of every rapidity, and for diaphragm apertures from //3 to f/4$. Exposure meters of this type are specially applicable for open-air work where there is sufficient light for ready measurement. Other simple actinometers are in use for carbon and process printing, consisting generally of translucent graduated scales in different densities of paper, coloured gelatin, etc., or of a photographed scale graduated by increasing exposures. The " Burton actinometer," for pigment printing, made on this principle, contains several small negatives of different densities, one of which is selected of equal depth to the one to be printed, and the progress of the printing is estimated by exposing a piece of sensitive paper under it and examining it from time to time.

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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