It would be somewhat difficult to fix a date when what we now know as " photographic action " was first re-corded. 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 opera-tions. We may take it that Scheele, the Swedish chemist, was the first to enter upon a scientific investigation of the darkening action of sunlight on silver chloride. He found by experiment 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 caustic (silver nitrate), caused the precipitation of new silver chloride, and that on applying liquor 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 photo-graphy. 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. About twenty years later than Scheele's experi-ments Count Bumford contributed a paper to the Philo-sophical Transactions of the Eoyal Society (1798) 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 solutions could be brought about by a temperature somewhat less than that of boiling water. Robert Harrup in 1802, however, con-clusively 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 Bitter proved the existence of rays lying beyond the violet limit of the spectrum, and found that they had the power of blackening silver chloride. Such a discovery naturally gave a direction to the investigations of others, and See-beck (between 1802 and 1808) and Berard turned their attention to this particular subject, eliciting information which at the time was of a valuable nature. We need only mention two or three other cases where the influence of light was noticed at the beginning of this century. 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, be it observed, 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. Davy found that puce-coloured oxide of lead, when damp, became red in the red rays, whilst it blackened in the violet rays, and that the green oxide of mercury became red in the red rays,—again an example of the necessity of ab-sorption to effect a molecular or chemical change in a sub-stance. Desmortiens in 1801 observed the change effected in Prussian blue, and Bockman noted the action of the two ends of the spectrum on phosphorus, a research which, it may be mentioned, Draper extended further in America at a later date.

To England belongs the honour of first producing a photograph by the utilization of Scheele's observations on chloride of silver. In June 1802 Wedgwood published in the Journal of the Boyal 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 nitrate of silver 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 then goes on to describe the method of using this prepared paper by throwing shadows on it, and infer-entially by what we now call "contact printing." He states that he has been unable to fix his prints, no wash-ing 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 most 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 altera-tion 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 which are practised at the present time (1884), 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 hyposulphite of soda, discovered by Chaussier in 1799, or three years before Wedgwood published his paper, was used for making the print permanent. Here we must Seebeck. call attention to an important observation by Dr 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, how-ever, 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 prominently forward by Dr J. M. Eder as being undoubtedly the first record we have of photographic action lending itself to production of natural colours, a fact which, in describing the history of photographic phenomena, has been more or less overlooked. We shall see later on that this observation of Seebeck was allowed to lie fallow for many years, until it was again taken up and published as a novelty. In photography perhaps, above all other technical applications of science, there has been a great flood of rediscovery, owing, no doubt, in the first instance to the fact that much published in one country has remained unknown in others, and also to the fact that it is difficult to boil down photographic literature and to ascertain what is really scientifically true and what is merely the result of unscientific use of the imagination. Photography has suffered greatly also from the fact that those who follow it are usually artists rather than scientific men, and fall into mistakes of theory which must of necessity lead to wrong conclusions. N. de The first to found a process of photography which gave Niepce. pictures that were subsequently unaffected by light was Nicephore de NIEPCE (q.v.). 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. In his description 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 o of white petroleum. After this mixture has been allowed to stand two or three days it becomes free from turbidity and is ready to be used. The plate is placed in a dish and covered with the solvent. By degrees the parts un-affected by light dissolve away, and the picture, formed of modified asphaltum, is developed. The plate is then lifted from the dish, as much as possible of the solvent being allowed to drain away. It is next placed on an inclined support and carefully freed from all 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 than did his original agent.
Later still, Daguerre and Niepce used as a solvent the brittle residue obtained from evaporating the essential 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." Berhaps there is no sentence which could be quoted that illustrates more forcibly the advance made in photography from the days when this process was described. The ratio of three hours to ^fth of a second is a fair estimate of the progress made since Niepce. The develop-ment was conducted by means of petroleum-vapour, which dissolved the parts not acted upon by light. As a rale 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 Judœa 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 con-tinued 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 develop-ment 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 does not appear that he was very successful in his first endeavours : it was not till 1827 that he had any success worth recounting. At that date he communi-cated a paper to Dr Bauer of Kew, the secretary of the Boyal 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. There lies before the present writer an authentic MS. copy of Niepce's " Mémoire," dated " Kew, le 8 Décembre 1827," in which he says it will be found 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.

Daguerre and Niepce. Daguerreotype.—We have already 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 shrewd observers as Niepce and Daguerre, who had formed a partnership for prosecut-ing their researches, would not have thus formed iodide of silver without noticing that it changed in colour when exposed to the light. What parts respectively Daguerre ^nd Niepce played in the development of the daguerreotype, which we shall shortly describe, will probably never be known with absolute accuracy, but in a letter from Dr Bauer to Dr Bennett, F.R.S., dated 7th 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 Nicephore, but Mr Isidore observes that for the present that letter might be considered confidential."

process.

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 5th December 1829. At his death some letters from Daguerre and others were left by him in which the use of iodine, sulphur, phosphorus, &c, is mentioned as having been used on the metal plates, and their sensi-tiveness to light, when thus treated, commented upon. We Da- are thus led to believe that a great part of the success in guerreo- 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 Talbot-type process. It will, however, be convenient to continue the history of the daguerreotype, premising that it was published on 6th February 1839, whilst Talbot's process was given to the world on 25th January of the same year.

Daguerreotype pictures were originally taken on silver-plated copper, and even at the present day 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 bymeans, first of finely-powdered pumice and olive oil then of 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 down-wards, resting on supports, and the vapour from the iodine is allowed to form upon it a surface of silver iodide, which is the sensitive compound. It is essential to note the colour of the surface-formed iodide at its several stages, the varying colours being due to interferences caused by the different thicknesses of the minutely thin film of iodide of silver. 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 " solar-ization," but which was in reality due to the oxidation of the iodide, itself altered by prolonged exposure to light. As a matter of history it may be interesting to remark that the development of the image by means of 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 in-volved so long an exposure that he sought some means of reducing it by the application of different reagents. Having 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 of mercury, the vapour of which attached itself to the altered iodide of silver. 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 pro-cess was the resensitizing of the iodized film by bromine vapour. Mr Goddard published his account of the use of bromine in conjunction with iodine in 1840, and M. Claudet. 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 im-provement to be noticed in the process was toning or gilding the image by a solution of gold, a practice introduced by M. 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 Grove and others. These repro-ductions are admirable in every way, and furnish a positive proof, if any were needed, that the daguerrean image is a relief.

Fox-Talbot. Fox-Talbot Process.—In January 1839 Fox Talbot described the first of his processes, photogenic drawing, in process. 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. We must turn, however, to the Philosophical Magazine for the account of the full details of his method, which consisted essentially in soak-ing paper in common salt, brushing one side only of it with about a 12 per cent, 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 sensitive-ness was attained. This is the same in every respect as the method practised by Wedgwood in 1802; but, when Calotype. 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 iodide of silver 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 "gallo-nitrate" is not a chemical compound, but 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 in the dark, develops itself, 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 nitrate of silver was first known to Eev. J. B. Beade. When impressing images in the solar microscope he employed gallic acid and silver in order to render more sensitive the chloride of silver 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 Beade's method had never been legally published. It would be beyond the scope of the present article to give the slight improvements which Talbot afterwards made in the process. In one of his patents he recognizes the value of the proper fixing of his photogenic drawings by the use of 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 iodide of silver was held in suspension to a glass plate, since in this he was undoubtedly preceded by Niepce de St Victor in 1848. Albumen Albumen Process on Glass.—It was a most decided step on°»lass *n a(^vance wnen Niepce de St Victor, a nephew of Nice-8 ' phore 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 M. Blanquarfe Evrard it became more practicable ; but it was carried out in its greatest perfection by M. 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 with an egg-whisk or fork, 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 nitrate of silver 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 nitrate of silver, 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. Baper 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. This procedure is still followed in some meteorological observatories for obtaining transparent magnetograms, barograms, &c. Reflexion will show that in images ob-tained by this process the high lights are represented by metallic silver, whilst the shadows are translucent. Such a print is technically called a " negative." When chloride of silver 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, rendering it easy of execution and putting it into the hands of the comparatively untrained. This was the introduction of collodion, a vehicle which up to the present day holds its own against the more rapid pro-cesses on account of the facility with which the plates are prepared, and also because it is a substance totally un-affected by silver nitrate, which is not the case when any organic substance is employed, and, it may be said, in-organic as well in many instances. Thus albumen forms a definite silver compound, as do gelatin, starch, and gum. The employment of collodion for use in photo-graphy was first suggested by Le Gray, who has been already mentioned in connexion with the albumen process. He does not appear to have gone beyond suggestion, and it remained for Archer of London, closely followed by Fry, to make a really practical use of the discovery. Collodion is a solution of cotton or cellulose in which some atoms of its hydrogen have been replaced by N02 by treatment with a more or less dilute mixture of sulphuric and nitric acids. The action of the sulphuric acid is to take up the molecules of water formed by elimination of the hydrogen from the cotton, which combines with oxygen from the nitric acid, the latter acid supplying the cotton with N02. According to the temperature of the acids and their dilution a tri-nitro or di-nitro cellulose is said to be formed, one of which is the explosive gun-cotton, insoluble in ether and alcohol, whilst the other, though inflammable, is readily soluble in a mixture of these two solvents. 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 per-fectly adapted for holding in situ salts soluble in ether and alcohol. Where such salts 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 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 cotton (pyroxylin) precipitates, but, even if less diluted, it forms a film which is " crapey" and uneven. Such was the material with , which Le Gray proposed to work, and which Archer actu-ally 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 thick-ness, followed suit, though not immediately. When once a fine- 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 pos-sess a delicacy and refinement of detail that certainly eclipsed the finest print obtained from a calotype nega-tive. 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 pictures became possible to hundreds who previously had been excluded from this art-science by force of circumstances. We can Collo- merely give an outline of the procedure, referring the reader dion for further information to the manuals of photography, process. ^ glass plate is carefully cleaned by the application of a detergent such as a cream of tripoli powder or 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 previously freed from grease. A collodion containing soluble iodides and bro-mides is made to flow over the plate, all excess being drained off when it is covered. A good standard formula for the collodion may be taken to be as follows,—55 grains of pyroxylin, 5 oz. of alcohol, 5 oz. of ether; and in this liquid are dissolved 2h grains of ammonium iodide, 2 grains of cadmium iodide, and 2 grains of cadmium bromide. When the collodion is set, i.e., when it is in a gelatinous condition, the plate is immersed in a bath of nitrate of silver—a vertical form being that mostly used in Britain, whilst a horizontal dish is used on the Continent—a good formula for which is 350 grains of silver nitrate with 10 oz. of water. The plate is steadily lowered into this solu-tion without pause, 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 solu-tion of pyrogallic acid in water restrained in its action by the addition of acetic acid. One of the old formulae employed by 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 solu-tion, 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 nitrate of silver are added to the pyrogallic-acid solution and the developing action continued.

In 1844 Hunt introduced another reducing agent, which has continued to be the favourite down to the present time, viz., ferrous sulphate. By its use the time of neces-sary exposure of the plate is reduced, and the image de-velops with great rapidity. A sample of this developing solution is 20 grains of ferrous sulphate, 20 minims of acetic acid, with 1 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. There are other intensifiers 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; but into these it is not necessary to enter here. 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 cyanide of potassium, the latter salt having been first introduced by Martin and Gaudin in 1853 (La Luniiere, 23d April 1853). Twenty-five grains of cyanide of potassium to one. ounce of water is the strength of the solution usually em-ployed. 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, not, as is often considered to be the case, to merely dissolve the silver salt itself. It may be well to remark that the utility of bromides in the collodion process seems to have been recognized in its earliest days, Archer (1852) and Bingham (1850) both mentioning it. We notice this, since as late as the year 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 Mr 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 Mr Fry with ferrous sulphate and ferrous nitrate, whilst Dr Diamond gave effect to the matter in a practical way. Mr Archer used mercuric chloride to whiten the image. To Mr Hunt, however, must be awarded the credit of noticing the action of this salt on the image, in his paper in the Philosophical Transactions of 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, as has been done more recently, the positive pictures may be produced on japanned iron plates (ferrotype plates) or on japanned leather. This process is still practised by some photographers, and from the number of ferrotype plates sold the number of portraits taken by it must be still very large.

Moist Collodion Process.—From what has been stated above it will be 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 May 1854 Messrs Spiller and Crookes 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 since, but it is scarcely necessary to record them here; for details the reader may consult the volumes of the Photographic Journal, 1854-55. Collo- Dry Plates.—It would appear that the first experiments dion dry -with, collodion dry plates were due to M. Gaudin. In plates- La Lumiere of 22d April and 27th May 1854 he describes his researches on the question; whilst in England Mr G. It. Muirhead, on the 4th 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. Dr 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 pyro-gallic acid and silver. In this method we have a double manipulation, which is long in execution, though perfectly effective, as we know from experience. Alkaline A great advance was made in all dry-plate processes devel- "by me introduction of what is known as the "alkaline oper* developer," which is, however, inapplicable to all plates on which silver nitrate is present in the free state. It will be remembered that 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 to the state of sub-salts (e.g., sub-iodide of silver), .these re-duced particles really acted as nuclei for the crystallized metal. It will be evident that in such a method of develop-ment the molecular attraction acts at distances relatively great compared with the diameters of the molecules them-selves. If it were possible to reduce the altered particles 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 mole-cular attraction. Alkaline development accomplished this to a very remarkable extent; but the method is only really practicable when applied to films containing bromide and chloride of silver, as iodide is only slightly amenable to the alkaline body. We have not been able to trace the exact date of the introduction of this developer. It is believed to be of American origin; and it is known that in the year 1862 Major 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 bromide or chloride of silver, after being acted upon by light, as when a plate was exposed to the camera image, 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 Abney investigated this action and was able to demonstrate what actually occurred during the development. One of the experiments will show on what grounds this conclusion was arrived at. A dry plate was prepared by the bath process in the usual manner (to be described below), and exposed in the camera. The exposed film was covered with another film of collodio-bromide emulsion, which of course had not seen the light.





An image was obtained from the double film by means of the developer, which penetrated through the upper unexposed film, and the development was prolonged until an image appeared through the same 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 bromide of silver in close contiguity to the exposed particles was reduced to the metallic state. Hence, from this and similar experi-ments Abney was able to announce that silver bromide could not exist in the presence of freshly precipitated or reduced metallic silver, and that a sub-bromide was immediately formed. Thus Ag2Br2 + Ag2 = 2Ag2Br. From this it will be seen that the deposited silver is well within the sphere of molecular attraction, and that consequently a less ex-posure (i.e., the reduction of fewer molecules of the sensi-tive 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 hydrokinone 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. In 1884, however, Herr Egli and Arnold Spiller brought out hydroxylamin as a reducing agent, which promises to be of great use if it can be prepared cheaply enough.

Another set of developers for dry plates dependent on Other the reduction of the silver bromide and the metallic state dry-plate is founded on the fact that certain organic salts of iron 0p™s" can be utilized. In 1877 Mr Carey Lea of Philadelphia and Mr William Willis announced almost simultaneously that a solution of ferrous oxalate in neutral potassium oxalate was effective as a developer, and from that time it has been universally acknowledged as a useful agent in that capacity; and it is a rare favourite, more especially amongst Continental photographers. In 1881 Abney showed that the addition of a small quantity of sodium hyposulphite very greatly increased its rapidity of action by reducing the time of exposure necessary to get a developable image. In 1882 Dr Eder demonstrated that gelatin chloride of silver 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 gelatino- chloride plates to be readily developed but also collodio-chloride plates. These, it may be said, were undevelopable except by the precipitation method until the advent of the agents last-mentioned; the chloride being as readily reduced as the sub-chloride rendered the development of an image impracticable.

Amongst the components of an alkaline developer we Re-mentioned a restrainer. This factor, generally a bromide Cramer or chloride of an alkali, serves probably to form a com- ^jf^ pound with the silver salt which has not been acted upon veloper. 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 bromide of silver the developer has only to remove one atom of bromine, whereas it has to remove two in the case of sub-bromide of silver. The sub-bromide formed by light and that sub-sequently 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.

Alkaline development was first used by Major Russell in a dry-plate process in which the collodion was merely bromized by means of bromides soluble in alcohol. 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 amen-able to it, the iodide being almost entirely unaffected by the weak developer which was at that time in general use. Dry- One of the most successful, bath dry-plate processes was plate introduced by Mr. R. Manners Gordon and was a really batl1 beautiful process. The plate was given an edging of process. a^i-|Umen an(j £nen coate(j -with ordinary iodized collodion to which one grain per ounce of cadmium bromide had been added. It was kept in the silver-nitrate bath for ten minutes, after which it was washed thoroughly. The following preservative was then applied :—_
C Gum arabic 20 grs.
1. -I Sugar candy 5 „
(.Water 6 dr.
„ ( Gallic acid 3 grs.
_\ Water 2 dr.

These ingredients were mixed just before use and, after filtering, applied for one minute to the plate, which was allowed 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 circum-stances. 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 the following alkaline developer :—

j / Pyrogallic acid 96 grs.
' 1 Alcohol 1 02.
9 J" Potassium bromide 12 grs.
' 1 Water 1 oz.
g t Ammonium carbonate 80 grs.
' \ Water 1 oz.

|The development of the image requires 6 minims of No. 1, J drachm of No. 2, with 3 drachms of No. 3. If properly exposed the image appears rapidly and gradually gains in intensity, and when all action from the developer ceases the plate is washed and further intensified with pyrogallic acid and silver as is a wet plate. The image is finally fixed in sodium hyposulphite.

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 reflexion from the back of the glass should take place at the vertical 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 reflexion 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 for photographers to cover the back of their dry plates with some material which should be in optical contact with it, and which at the same time should absorb all the photographically active rays, and only replace those which were incapable of reducing the silver salt. This was called " backing a plate."

Collodion Emulsion Processes.—In 1864 Bolton and Sayce published the germ of a process which revolutionized photographic manipulations, and by a subsequent substitution of gelatin for collodion gave an impetus to photo-graphy which has carried it to that state of perfection at which it has arrived at the present time (1884). In the ordinary collodion process it will be recollected that a sen-sitive 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 iodide and bromide of silver 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 let-ting this collodion, containing the salts in suspension, flow over the glass plate. Gaudin had attempted to do this with chloride of silver, and later G. W. Simpson had suc-ceeded in perfecting a printing process with collodion con-taining chloride of silver, citric acid, and nitrate of silver; but the chloride until recently has been considered a slow working salt, and nearly incapable of development. Up to the time of Bolton and Sayce's experiments iodide of silver had been considered the staple of a sensitive film ; and, though bromide had been used by Major Bussell 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 iodide of silver in collodion was not thought practicable, and the inventors of the process turned their attention to bromide of silver, 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 result-ing 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 remark-able, 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 ap-plication 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; but for some time it lay dormant. In fact there was at first much Modifi- to discourage trial of it, since the plates often became cations veiled on development. Mr Carey Lea of Philadelphia, in the an(j ^ Cooper, jun., of Reading, may be said to have process. g-ven reai impetus to the method. Mr 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 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. It may be well to mention that the effect of the addition of the mineral acids such as Carey Lea employed is to prevent the forma-tion of (or to destroy when formed) any sub-bromide or oxide of silver, either of which acts as a nucleus on which development can take place. Captain 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 Mr 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 pyro-xylin 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. The brief exposure necessary for a collodion emulsion plate, or indeed any dry plate, had not been recognized till the introduction of this developer. This at once placed in the hands of photographers an instru-ment which by judicious use enabled them to shorten the time of exposure of their plates and to render possible effects which had before been considered out of the question. As an example of the preparation of a collodion emulsion and the developer usually employed with it we give the following,—oz. of alcohol, 5 oz. of ether, 75 grains _of pyroxylin. In 1 oz. of alcohol are dissolved 200 grains oof zinc bromide; 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, 1 oz. of alcohol being added. The silvered alcohol is next poured into the other half of the collodion and the brominized collodion dropped in, care being taken to shake between the operations. An emulsion of bromide of silver is formed in suspension; and it is in every case left for 10 to 20 hours to what is 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 into a flat dish and the solvents allowed to evaporate till the pyroxylin becomes gelatinous. 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 dried or treated with two or three doses of alcohol, and then redissolved in equal parts of alcohol (specific gravity, -805) and ether (specific gravity, "720). In this condition it is a washed emulsion, and a glass plate can be coated with it and the film dried, or it may be washed and a preservative applied. An excellent preservative introduced by Colonel Stuart Wortley is as follows :—

1. Salycin, a saturated solution in water.
, /Tannin 60 grs.
" \ Distilled water 1 oz.
, /Gallic acid 48 grs.
'_ 1 Water 1 oz.

To make the preservative, take 2 oz. of No. 1, 1 oz. of No. 2, \ oz. of No. 3, 40 grains of sugar, and 7 oz. of water. The plates are immersed in this solution and dried. It is often necessary to give the plate a previous coating with very dilute albumen or gelatin in order to make the film of collodion adhere during development, which can be effected by the strong alkaline developer, or by the ferrous oxalate developer, previously noticed.

The type of a useful alkaline developer is as follows :—
/ Pyrogallic acid 96 grs.
' \ Alcohol 1 oz.
/ Potassium bromide 12 grs.
" \ Water distilled 1 oz.
/ Ammonium carbonate 80 grs.
'' 1 Water 1 oz.

To develop the plate 6 minims of No. 1, \ 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 development is conducted in a flat dish, sometimes the solution is poured on the plate. 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—(1) by saturating a saturated solution of neutral potassium oxalate with ferrous oxalate, and adding an equal volume of a solution (10 grains to 1 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 1 volume by measure of a saturated solution of ferrous sul-phate, 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 collodion emulsion plates could be prepared had turned all investigation into this channel, and collodion was not the only vehicle that was tried for holding the sensitive salts in suspension. As early as September 1871 Dr B. L. Maddox had tried emulsifying the silver salt in gelatin, and had produced negatives of rare excellence, as the present writer can testify from personal knowledge. In November 1873 Mr King described a similar process, getting rid of the soluble salts by washing. Efforts had also been made in this direction by Mr Burgess in July 1873. Mr B. 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 Mr 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 the Belgian chemist 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 in truth be said that the starting-point of rapid plates was 1878, and that the full credit of this discovery should be allotted to Mr 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. Dr van Monckhoven of Ghent, in experimenting with this process, hit upon the plan of obtaining the emulsion by splitting up silver car-bonate with hydrobromic acid, leaving 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 bromide of silver 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. Photographers' opinions as to the respective 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 Herschell intro-duced a plan for making an alcoholic gelatin emulsion with the idea of inducing rapid drying of the plates, and in the same year Dr H. Vogel of Berlin brought forward his ideas for 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 method was called " collodio-gelatin emulsion," and apparently was only a shortlived process, which is not sur-prising, since its preparation involved the inhalation of the fumes of acetic'acid.

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° Fahr., 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 Mr Mansfield ' in 1879. Another improvement was effected by Mr 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. It would be impossible to enumer-ate many minor improvements in this process that have from time to time been made; it is sufficient to have stated in historical sequence the different important stages through which it has passed. 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 T-gth second ,,
By this it will be seen what advances have been made in the art of photography during the forty-five years of its existence.

The following is an outline of two representative processes. All Gelatin operations should be conducted in light which can act but very emul-slightly on the sensitive salts employed, and this is more necessary sions. 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, and which is safe, 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 em-ployed, it is as well to have a double thickness of orange paper. The following should be weighed out:—

1. Potassium iodide 5 grs.
2. Potassium bromide 135 „
3. Nelson's No. 1 photographic gelatin 30 „
4. Silver nitrate 175 ,,
_ J Autotype or other hard gelatin 100 „
t Nelson's No. 1 gelatin 100 „

Nos. 3 and 5 are rapidly covered with water or wached for a few seconds under the tap to get rid of any adherent dust. No. 2 is dissolved in 1J oz. of water, and a little tincture of iodine added till it assumes a light sherry colour. No. 1 is dissolved in 60 minims of water. No. 4 is dissolved in ^ oz. of water, and No. 3 is allowed to swell up in 1 oz. of water, and is then dissolved by heat. All the flasks containing these solutions are placed in water at 150° Fahr. 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. 1 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 emul-sion is next placed in boiling water, which is kept in a state of ebullition for about three-quarters of an hour. It is then ready, when the contents of the flask have cooled down to about 100° Fahr., 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 fail 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 dis-solves, 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 1 oz. of water, and strong ammonia of specific gravity '880 added to it till the oxide first precipitated is just redissolved. This ammoniacal solution is then dropped into Nos. 2 and 3 as previously described, and finally No. 1 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° Fahr., 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° Fahr., is poured on them in sufficient quantity 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, by a simple contrivance, 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 be dry in about twelve hours, and they are ready for immediate use. Expo- Exposure.—"With a good emulsion and on a bright day the ex-sure, posure 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 circum-stances, an exposure with a portrait-lens may be from half a second to four or five seconds. Develop- Development of the Plate. —To develop the image either a ferrous ment of oxalate solution or alkaline pyrogallic acid may be used. The plate. former is conveniently prepared as described on p. 826. No chemical restrainer such as bromide of potassium 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 50 grs.
1. < Citric acid 10 „
( Water 1 oz.
0 ( Potassium bromide 10 grs.
I Water 1 oz.
o j Ammonia, -880 1 dr.
*o i Water 9 „

One drachm 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 suffi-cient 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. Intensi- Intensifying the Negative.—Sometimes it is necessary to intensify tying the negative, which can be done in a variety of ways with mercury negative, salts. An excellent plan, introduced by the Platinotype Company, is to use a saturated solution of mercuric chloride in water, and a subsequent addition of 2 grains to the ounce of platinic chloride. This is put in a dish and the metallic solution allowed to act till sufficient density is obtained. With most other methods with mercury the image is apt to become yellow and to fade ; with this apparently it is not. Varnish- Varnishing the Negative.—The negative is usually protected by ing nega- receiving first a film of plain collodion and then a coat of shellac or tive. 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.

Printing Processes. The first printing process may be said to be that of Fox Talbot (see above, p. 824), which has continued to be generally employed to the present day (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 vary with different manufacturers. 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 pro-portions—80 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 made to float on a 10 per cent, 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 Toning and Fixing the Print. —The next operation is to tone and and fix- fix the print. In the earlier days this was accomplished by means ing print, of a bath of sel d'or,—a mixture of hyposulphite of soda and auric chloride. This gilded the darkened parts of the print which light had reduced to the semi-metallic state; and OH 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 sulphide of silver 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 auric chloride with a salt, such as the carbonate or acetate of soda, chloride of lime, borax, &c. 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 combined 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 and subsequent years. MM. Davanne and Girard, a little later, also investigated the matter with fruitful results.
The following may be taken as two typical toning-baths :—
{
Auric chloride 1 part.
Sodium carbonate 10 parts.
Water 5000 „
( Borax 100 „
w \ Water 4000 „
o\ i Auric chloride 1 part.
V \ Water 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 is :—
Auric chloride 2 parts.
Chloride of lime 2 ,,
Chalk 40 „
Water S000 „

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 enable all free silver nitrate to be carried away from the image, 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 in the matter, 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 per cent, solution of hyposulphite of soda, to which it is best to add a few 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 just recommended helps to remove. If it is not removed, there is a sulphur compound left behind, according to 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. Whether this be due to the hygroscopic nature of the hyposulphite and its reaction on the organic salt of silver, or to the destruction of the hyposulphite and sulphurizing of the Mack organic salt, seems at present to be an undetermined question. 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 other 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, as has already been said, 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 sepa-rately, are equally or more efficacious. On drying, the print assumes a darker tone than what it has after leaving the fixing-bath.

Different tones can thus be given to a print by different toning-baths ; 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 chloride emulsion processes we have already stated that Gaudin had 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 Mr George "Wharton Simpson worked out a method which has been more or less successfully employed, and is still one of the best with which we are acquainted. The formula appended is the original one which Mr Simpson published :—

f Silver nitrate 60 parts.
- t Distilled water 00 „
, I Strontium chloride 64 ,,
"\ Alcohol 1000 „
, I Citric acid 64 „
>m\ Alcohol 1000 „

To every 1000 parts of plain collodion 30 parts of No. 1, 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 con-tains 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 bo applied to glass, as before stated, or to paper, and the printing carried on in the usual manner. The toning takes place by means of the chloride-of-lime bath or by ammonium sulpho-cyanide 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. This is a beautiful process, and deserving of more attention than has hitherto been given to it. Gelatino- Gelatino-citro-chloride Emulsion.—A modified emulsion printing citro- process was introduced by Abney in 1881, which consisted in sus-chloride pending silver chloride and silver citrate in gelatin, there being no emul- excess of silver present. The formula of producing it is as follows:—

{Sodium chloride 40 parts.
Potassium citrate 40 ,,
Water 500 „
0 ( Silver nitrate 150 ,,
'_"i Water : 500 „
„ ( Gelatin 300 „
6'\ Water 1700 „

Nos. 2 and 3 ai'e mixed together whilst warm, and No. 1 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. "Printing Printing with Salts of Uranium.—The sensitiveness of the salts with of uranium to light seems to have been discovered by Niepce, and uranium the fact was subsequently applied to photography by Burnett in salts. 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 ; bat it can be developed into a strong one by 6 to 10 per cent, solution of silver nitrate to which a trace of acetic acid has been added, or by a 2 per cent, solution of auric chloride. In both these cases the silver and gold are deposited in the metallic state. Another developer is a 2 per cent, solution of ferro-cyanide of potassium 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. Wothly jyothly Type. —A variation was introduced in the uranium pro-type, cess by Herr "Wothly in 1864, when he employed uranic nitrate with other salts in the collodion, and then coated starched paper with the product. The paper was printed until it assumed a bluish-black image, which was subsequently intensified by means of gold. The most generally used Wothly-type formula, however, consisted of a triple salt of silver nitrate, uranic nitrate, and amnionic nitrate, which were dissolved in collodion. This compound was applied to paper sized with arrowroot, and, after drying, the printing proceeded in the usual manner, the image being subsequently fixed with hyposulphite of soda. The prints produced by this method were very beautiful, but for some reason they found no great favour with the public.





Printing with Chromates.—The first mention of the use of potassium bichromate for printing purposes seems to have been with 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 wash-ing it in water, when a white picture on an orange ground was obtained. In 1840 M. E. Becquerel announced that paper sized with iodide of starch and soaked in bichromate of potash 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 bichromate of potash spread upon a litho-graphic 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 lithography. The same process, with slight modifications, has been used quite recently by Simonet and Toovey of Brussels, and is capable of producing most excellent results. Dixon's method, however, was not published till 1854, when it appeared in the Scientific American, and consequently, as regards priority of publication, it ranks after Fox Talbot's photo-engraving process (see below), which was published in 1852. On 13th Decem-ber 1855 M. Alphonse Poitevin took out a patent in England, in which he vaguely described a method of taking a direct carbon-print by rendering gelatin insoluble through the action of light on bichromate of potash. This idea was taken up by Mr Pouncey of Dorchester, who perhaps was the first to produce veritable carbon - prints, notwithstanding that Testud de Beauregard took Carbon-out a somewhat similar patent to Poitevin's at the end of 1857. prints.

Mr Pouncey published his process on 1st January 1859 ; but, as described by him, it was by no means in a perfect state, half-tones being wanting. The cause of this was first pointed out by Abbe Laborde in 1858, whilst describing a kindred process in a note to the French Photographic Society. He says, " In the sensi-tive 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." Mr 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 unprepared 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 the paper support and could be transferred to glass. Poitevin opposed this patent, and his opposition was successful, 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 (to whom electric lighting owes so much) 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 Carbon-coating paper with a mixture of gelatin, sugar, and colouring tissue, matter, and rendered sensitive to light by means of bichromate of potash or ammonia. 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 re-transferred to its final support of paper. In 1869 J. E. 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, &c., without any adhesive material being applied. This was a most important and fruitful improvement. Johnson also added soap to the gelatin to prevent its excessive brittleness on drying, and made his final support of gelatinized paper, rendered insoluble by chrome alum. In 1874 J. B,. 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 the state of perfection in which we find it to-day.

According to 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 with a mallet 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 twro rollers ; one of wood about 5 inches in diameter is fixed near the top of the room and the other over a trough contain-ing 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 lathes. If it be required to make the tissue sensitive at once, 120 grains of potassium dichromate should be mixed with the ingredients in the above formula. The carbon-tissue wdien prepared should be floated on a sensitizing bath consisting of one part of potassium dichromate in forty parts of water. This is effected by turning up about 1 inch 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 solu-tion, where it is allowed to remain till the gelatin film feels soft. It is 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 Printing slowly dried. As the tissue is coloured, it is not possible to with ascertain by inspection of it whether the printing operation is carbon- sufficiently carried out, and in order to ascertain this it is usual tissue. to place a piece of ordinary silvered paper in an " actinorneter," or "photometer," alongside the carbon-tissue to ascertain the amount of light that has acted on it. There are several devices for ascer-taining this amount, the simplest being an arrangement of a varying number of thicknesses of gold-beater's skin. The value of 1, 2, 3, &c, 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 actinorneter 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 (already described). 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 india-rubber squeezer called a "squeegee" and the two surfaces adhere. About a couple of minutes later they are placed in warm water of about 90° to 100° Fahr., 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 in 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.

Such is a brief outline of carbon-printing as practised at the present day, subject, of course, to various modifications which need not be entered into here. We ought, however, to mention that 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 Printing with Salts of Iran.—Sir John Herschel and Mr Hunt in with salts sundry papers and publications entered into various methods of of iron, printing with salts of iron. At the present time there are two or three which are practised, being used in draughtsmen's offices for copying tracings. When a ferric salt is exposed to light it be-comes reduced to the ferrous state, and when this latter compound is treated with potassium ferri-cyanide a blue compound is formed. If, therefore, a solution of a ferric salt be brushed over a paper, and the latter be dried, and then exposed behind a tracing, the parts of the ferric salt on the paper exposed beneath the white ground are converted into a ferrous salt, and if potassium ferri-cyanide be brushed over the paper, or the paper floated upon it, the tracing shows white lines on a blue ground. Another method is to mix ferri-cyanide of potassium with a ferric salt, and expose it behind a tracing or drawing. Whore the light acts, the mixture is con-verted into a blue compound. The resulting print is the same as the foregoing. Another method of producing blue lines on a white ground is to expose paper coated with gum and a ferric salt to light, and then treat it with potassium ferro-cyanide. This body forms an insoluble blue compound with the ferric salt, whilst the ferrous salt is inactive, or only gives a soluble body. A further development of printing with salts of iron is the beautiful platinotype process. Sized paper is coated with a solution of ferric oxalate and a platinous salt, and exposed behind a negative. It is then floated on a hot solution of neutral potassium oxalate, when the image is formed of platinum black. This process was introduced by Mr W. Willis in 1874. The rationale of it is that a ferrous salt when in solution is capable of reducing a platinum salt to metallic platinum. In this case the ferrous salt is dissolved by the potassium oxalate, and at the moment of solution the platinum salt is reduced and forms the image.

Photo-mechanical Printing Processes.—Allusion has already been made to the invention of Poitevin, who claimed to have discovered that a film of gelatin impregnated with bichromate of potash, 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. Tessie de Motay (in 1865) and Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general method of procedure consisted in coating metallic plates with gelatin impregnated with bichromate or tri-chromate of potash or ammonia and mercuric chloride, then treat-ing with oleate of silver, 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 not long before it was simplified by Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more dur-able 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 un-manageable. 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 of 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 " Lichtdruek " 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 " Lichtdruek " was patented in England by Ernest Edwards under the name of " heliotype." Helio-This consisted in coating a glass plate, the surface of which was type, very finely ground, with bichromated gelatin to which a certain amount of chrome alum had been added. The film itself was much thicker than that of the Albert type, since it had to be detached from the surface of the glass by stripping, which was rendered possible by the previous application of a waxing solution to the plate. After the film was stripped off it was exposed under a negative for the time necessary to give a good image with printing ink, after which the inner side was exposed to light for almost the same length of time. The gelatin sheet was then transferred to a. pewter plate, to which it was cemented by thick india-rubber cement and soaked in water till all the soluble bichromate was extracted. After this it was placed in a type printing-press and inked with a lithographic or gelatin roller, and an impression pulled on paper in the same manner as in printing with type, save that a greater pressure was brought to bear on the surface. This pressure was necessary for two reasons,—the relief of the image would be too great if only a moderate pressure were used, and the entire surface was so large that a heavy pressure was requisite to make the paper bite on the ink. Between each pull the gelatin film was damped, the surface moisture taken off with a dry cloth, and the inking proceeded with. The drawback to this process is undoubtedly the great relief that is given from the film being so thick, but it is a more manageable process in some respects than that of Albert, since the support is unbreakable. We should mention that Edwards also patented the use of two or more inks of different degrees of stiffness. The stiffest, which was generally black, adhered to the most deeply printed parts of the image, the next stiffest to the next most deeply printed parts, and so on. By this means the least deeply printed parts acquired a different tone from that of the deeper printed parts, which was an advantage as regards artistic effect. The same method of inking could be applied to Albert's process with the same results. Since the time of the heliotype patent many im-provements have been made in the minor details of the operations, and various firms now produce prints in greasy ink very little if at all inferior to silver prints. Wood- Woodbury Type.—This process was invented by Mr W. Wood-bury bury about the year 1864, though we believe that Mr J. W. Swan type. 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. M. 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 Mr Woodbury obtained, and for which he was entitled to the fullest credit. Woodbury type is a combination of the principle upon which intaglio printing is based with that upon which a carbon-print is obtained. The general features of the procedure will be understood from the foregoing description of the carbon-process. An image is obtained on bichro-matized gelatin from a negative of the usual kind by exposing a thick layer of gelatin to light and then washing away all its soluble parts from the back of the exposed print. This is the mould which it is necessary to obtain. At first Woodbury made electrotypes from the mould, from which he could obtain prints mechanically. The intaglio was placed on a specially devised printing - press, and the mould filled with gelatin containing colouring matter such as Indian ink. A piece of paper perfectly even in thickness was placed in con-tact with the mould, and a piece of flat glass under pressure brought down upon this. The excess of pigmented gelatin was squeezed out, and, when slightly set, it adhered to the paper and was brought away from the mould. After drying, a perfect picture was obtained in pigment, the image being reversed as regards right and left; but that difficulty was surmounted by using a reversed negative, and also by a modification of the process subsequently introduced by Mr Woodbury. The gelatin relief was made as before, and then by means of very heavy pressure in a hydraulic press the mould was squeezed into soft metal, from which the prints could be after-wards taken off. This is the same principle as that on which nature-printing is conducted, and at first sight it seems strange that material such as gelatin should be able to impress metal. Mr Woodbury found that it made very little if any difference in the sharpness of the image if the relief was reversed and the back of the relief pressed into the mould. This of course made the print correct as regards right and left. He has not, however, been con-tent with his original operations, but has further simplified them, the outcome being what is known as the " stannotype process." In 1880 he read a description of it before the French Photographic Society. The modification consisted in taking a mould in gelatin from a positive on glass. The mould, when hardened by chemical means (as was indeed the case with the original Woodbury-type process), was attached to a sheet of flat glass, and then covered by the foil and passed through a rolling press the cylinders of which were covered with thick india-rubber. This forced the tinfoil into every crevice of the mould, yielding a block impervious to moisture and ready to have gelatin impressions taken from it. At first Mr Woodbury took an electrotype from the relief, covered with tinfoil, obtained from a negative, but he abandoned this for a simpler plan. He took a positive on glass in the ordinary manner adopted by photographers, from which he made a mould in gelatin. This he covered with tinfoil and printed direct from it.

Photo-Lithography. Reference has already been made to the effect of light on geiatin impregnated with bichromate of potash, 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 photo-lithography. In the spring of 1859 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 trans-ferring 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, bear-ing 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. James, B.E., F.E.S., brought forward the Southampton method of photo-litho- South-graphy, which had been carefully worked out by Captain ampton de Courcy Scott, B.E. We give a detailed description of method-it as practised at Southampton.

Preparation of the Paper.—The mixture consists of 3 oz. of Nelson's " fine art" gelatin and 2 oz. of potassium bichromate dissolved in 10 oz. of water and added to the 40 oz. of water with which the gelatin, after proper soak-ing, has been previously mixed. Good and grainless bank post-paper (chosen on account of its toughness) of medium thickness is made to float on this solution (after it has been strained) for three minutes, when it is hung up in the dark to dry. It is again floated on the solution and hung up for desiccation by the corners opposite to those which were previously uppermost, and then passed through a copper-plate or lithographic press to obtain a smooth surface. The paper is next placed upon a negative and printed in the ordinary manner, the negative being very dense in those parts which should print white, and perfectly transparent where the black lines have to be impressed. From about two minutes' exposure in sunshine to an hour in dull light is requisite to give sufficient intensity to the prints, which are next covered with greasy printer's ink, made from lithographic printing ink, pitch, varnish, palm oil, and wax. The inking is best done by covering a lithographic stone with a fine layer by means of a roller, and then passing the paper through the press as if pulling a lithographic print,—an operation which may have to be repeated twice to ensure the whole surface being covered, and yet not too thickly. The inked print is placed face uppermost on water of a temperature of about 90° Fahr., and, when the soluble parts of the gelatin have taken up their full quantity of water, the paper is laid on a sloping glass plate, inked surface uppermost, and a gentle stream of warm water poured over it. This removes the soluble gelatin and the greasy ink lying on it, the removal being-helped by the application of a very soft sponge. When all the gelatin and ink except that forming the image have been removed, the paper is allowed to dry till ready to transfer to stone. The method admits of several variations in detail, such as coating the gelatin with albumen and removing the soluble albumen by cold water, some of them being excellent, especially where the relief of the developed print is small, as relief is an enemy to the production of fine work on a lithographic stone, since the ink, in passing through the press, squeezes out and produces broad lines which should be otherwise fine.

Another method of producing a transfer, called the Papyro-"papyrotype process," was published by Abney in 1870, tyPe in which the ink is put on to a surface of gelatin by means metnod-of a soft roller; and this has the great advantage that the ink can be removed at pleasure if any part is not I satisfactorily inked, without the basis of the print being destroyed. In this process tough paper is coated with a fine layer of gelatin and subsequently treated with alum or chrome alum, afterwards receiving another coating, as in the Southampton method. The printing too is carried out as in the Southampton method, but not so deeply. After withdrawing the prints from the printing-frame they are soaked in cold water, and a roller is passed over them charged with an ink made of 4 parts of best lithographic chalk ink mixed with 1 part of palm oil. A roller coated with velvet is said to be better than the ordinary composi-tion rollers. The ink takes when the work is all clear; the transfer is exposed to light, and is ready to be put down on stone or zinc.

Photo-Engraving and Photo-Reliefs.

This may be divided into two classes, one the production of an engraved plate for printing by the copper-plate press, and the other for the production of cliches for printing with type. Niepce's process is still generally employed for the first when line engravings have to be reproduced. A copper plate is covered with asphaltum, a film negative placed in contact with it, and the necessary exposure given. After development with olive oil and turpentine the lines are shown as bare copper. The plate after being waxed at the back is next plunged into an acid bath and etched as are etched plates. When a half-tone negative has to be reproduced on copper Fox Talbot's method, described in his patents of 1852 and 1858, is still the simplest. A print on gelatin is transferred to a copper plate, and the surface etched by means of different strengths of ferric chloride, which renders the gelatin insoluble and imper-meable ; hence it will be seen that a weak solution of ferric chloride is able to reach the copper through the gelatin more readily than a strong one. In order to be successful it is necessary to give a grain to the plate; this is effected by sprinkling it with powdered resin, which is then warmed.

Relief plates for printing with type are usually made on zinc. If an ordinary photo-lithographic transfer be trans-ferred to zinc and then sprinkled with resin, the zinc may be immersed in weak acid and the uncovered parts eaten away. The regularity of the erosion is much increased by previously immersing the plate in a weak solution of copper sulphate. The particles of metallic copper deposited on the zinc form with it and with dilute acid galvanic couples, which rapidly eat away the zinc. The etching bath should be kept in motion. The depth of the erosion is increased by littering the surface again with powdered resin, which adheres to the lines, and then heating the plate. The warmed resin runs down the eroded lines and protects them from under-cutting when again placed in acid. This process is applicable to line-engravings. Mepce's bitumen process is also applicable, but in that case a posi-tive must be applied to the plate to be etched. There exist several methods by which half-tone negatives may be reproduced for working off in the printing-press. They depend principally on breaking up the whole surface by means of lines. Thus, if, between the surface on which the printing is to take place (and which has been coated with some sensitive medium) and the positive, a film on which a network of lines has been photographed be inter-posed, it is evident that the resulting print will consist of the half-tone subject together with an image of the net-work of lines. This can be etched in the manner described above. Most of these processes are secret, but it is be-lieved that this is the one most generally practised.

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 Seebeck of Jena (1810) describes the impression he obtained on paper impregnated with moist chloride of silver.

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 chlor-ides. 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 means of chloride of lime, and made the surface more sensitive by applying a solution of lead chlor-ide 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. Foitevin obtained coloured images on ordinary chloride of silver paper by preparing it in the usual manner and washing it and exposing it to light. It was afterwards treated with a solution of bichromate of potash 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, vol. lxi. p. 11). In the Bulletin de la Société Française (1874) St Florent describes experiments which he made with the same object. He immerses ordi-nary or albuminized paper in silver nitrate and afterwards plunges it into a solution of uranium nitrate and zinc chloride acidulated with hydrochloric acid ; it is then ex-posed to light till it takes a violet, blue, or lavender tint. Before exposure the paper is floated on a solution of mer-curic 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 chloride of silver 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 pro-duced by the metal and the colour impressed on a plate prepared with a chloride of such a metal. In 1880 (Proc. Roy. Soc.) Abney showed that the production of colour really resulted from the oxidation of the chloride that was coloured by light. Blates immersed in a solution of hydro-xyl 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 mix-ture of two or at most three sizes of molecules. In 1841, during his researches on light, Bobert Hunt published some results of colour-photography by means of fluoride of silver. A paper was washed with nitrate of silver 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 chloride of silver paper, this 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 chloride of silver 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 com-pound 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 sirhply 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. That photography in colours may one day be accomplished is still possible, though the bright tints of nature can never be hoped for, since, as a rule, they are produced by sunshine, whereas on the plate they have to be viewed by diffused light. Reducing Action of Light on Silver Salts.—The action of light on ^.ctn°n °' sensitive bodies has occupied the attention of many experi-° ' mentalists from a very early period of photography. In 1777 Scheele, according to Hunt (Researches in Light), made the following experiments :—

" 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 men-struum dissolved a quantity of luna cornila (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 (nitrib 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 coruua acquires from, the sun's light, and likewise the solution of silver fjoured 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 nitrated 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 re-ducing 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 libe'rate'd from the chloride ; but it ié 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 vacuo, will not discolour in the light, but keeps its ordinary white colour. A pretty experiment is to seal up- in vacuo, at one end of a bent tube, perfectly dry chloride, and at thè other a drop of mercury. The mercury vapour vola-tilizes 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 saltb of silver halogen absorbents were necessary, and it was by following out this law that the present rapidity in obtain-ing camera images has been rendered possible. To put it briefly, then, the 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. There is another action which seems to occur almost simultane-ously 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. An example of this has already been shown in the section on "photographs in natural colours." If a sensitive salt be exposed to light and then treated with an oxidizing substance, such as bichromate of potash, per-manganate of potash, hydroxyl, ozone, an image is not developed, but remains unaltered, showing that a change has been effected in the compound. If such an oxidized salt be treated very cautiously with nascent hydrogen the oxygen is withdrawn, and the image is again capable of development.

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 the chloride of silver with the spectrum whereby he obtained coloured but pounds. Scheele in 1777 allowed a spectrum to fall on the same materialj and found that it blackened much more readily in the violet rays than in any other. Senebier's experi-ments have been already quoted at the beginning of this article. We merely mention these two for their historical interest, and pass on to the study of the action of the spectrum oh different compounds by Sir J. Herschel which is to be found in the Philosophical Transactions for 1840. He there describes many interesting experiments, which became the foundations 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 Becquerel, Draper, Poitevin, H. W. Vogel, Schumann, and Abney. Fig. 1 (see pp. 836-38), which appeared in the Proceedings of the Royal Society for 1882, shows the most recent researches by the last-named experimenter as regards the action of the spectrum on the three principal haloid salts of silver. We may mention that in two instances exception has been taken to these results—(1) by H. W. Vogel, who recognizes a difference of behaviour in the spectrum in chloride and bromide of silver when precipitated in alcoholic and aqueous solutions, and (2) by Schumann to the effect of the spectrum on the double iodide and bromide, and iodide and chloride. The latter experimenter finds that when the two salts are mixed after precipitation the results are correct, but that if the precipitations of the two salts take place together the most refrangible maximum of sensitiveness disappears. The dia-gram (see fig. 1), however, will give a very approximate approach to the truth. Nos. 33 and 34 show the effect of the spectrum on a peculiar modification of silver bromide made by Abney, in which the silver bromide is seen to be sensitive to the infra-red rays. This modification is, and will be, largely used in investigating this part of the spectrum.


Films.—In 1874 Dyes and Dr Vogel of Berlin called attention to this sub- sensitive ject. He found that when films were stained films" with certain aniline and other 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 " chemi-cal sensitizer " were present. The term " optical sensitizer " seems a misnomer, since it is meant to imply that it renders the salts of silver sensi-tive 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 by many, 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 sensi-tive to light; and Dr Amory went so far as to take a spectrum on a combination of silver with eosine, which was one of the dyes experimented upon by Major 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 action of the dye. The subject has been discussed again recently owing to the production of so-called iso-chromatic films, i.e., films which are supposed to be sensitive to all colours, and which are prepared on gelatin or collodion plates by dyeing them with eosine or some similar dye ; and the instructions given indicate that, if a coloured picture or landscape be photographed through yellow glass, the "yellows" will be denser in the negative than will the "blues." Experiment shows if a film after preparation be dipped in a solution of " eoside of silver," made by precipitating eosine with silver nitrate, wash-ing the precipitate, and then dissolving in water faintly alkaline, a negative taken in the usual way will give the "yellows" equally as dense as the " blues." The action of the yellow glass is to cut off the blue rays to which the normal salt is most sensitive, and to leave the yellow rays unaltered; these then expend their energy upon the organic salt of silver. The advantage of rendering the yellows of a picture most intense in a negative is that the resulting print will be more nearly true to nature, since these are the most luminous rays. Further experiment ought surely to show how this can be done without the introduction of the tinted glass.


Action of the Spectrum on Chromic Salts.— The salts most usually employed in photography



(B.e.)
3AgI + AgCl + AgN03 on P.
paper, or paper washed,
hoth dry Agl+AgCl+AgNO., wet, or P.
3AgI+AgCl+KN02 wet
3AgI + AgCl + AgN03, or D. 3AgI + AgCl + KN02 on paper, developed with gal-lic acid or ferrous citro-oxalate

are the bichromates of the alkalis. The result Spec-of spectrum action in connexion with them is tru.m confined to its own most refrangible end, com- chromic11 mencing in the ultra-violet and reaching as far saits_ as in the solar spectrum. The accompanying diagram (fig. 2) shows the relative action of the
I

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.

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. 1 is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of bichro-mate of potash absorbs those rays alone which are effective in altering the bichromate. A reference to pp. 831, 833 will show that the 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 ,trunl a°-the blue rays, however, are the most effective, tl°11°nas-The action of light on this body is to render it less soluble in its usual solvents. Compare this statement with that on p. 822.

Action of the Spectrum on Salts of Iron.— Spec-Many ferric salts have been used from time to trum-time in the production of prints, the most fon common at the present time being the ferric jr0I1-oxalate, by which the beautiful platinotype prints are produced. We give this as a repre-sentation (fig. 3) of the spectra obtained on ferric


No.3.

No.4-

FIG. 3.—Same description as for fig. 2.

salts in general. Here, again, we have an ex-ample of the rigorous 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., ferro-cyanide 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 pro-ducing 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 of Light on Uranium.—The salts of Light uranium are affected by light in the presence of action on organic matter, and they too are only acted upon uraumm-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.
It would be going beyond our province to do more than enumerate the other metallic com-pounds which are amenable to chemical change by the impact of radiation; suffice it to say that some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are all affected, 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. For a more detailed account we must refer the reader to The Chemical Effects of the Spectrum by Dr J. M. Eder (London).

Any article descriptive of photography would be incomplete without a brief notice of the development of the camera. The inventor of the camera obscura was Giambattista della PORTA (q.v), who was born at Naples about 1540. Except as a scientific toy, his apparatus was not of any practical use, though it is the parent of the apparatus which have grown up with photography. The principles which govern photographic lenses have been briefly given under LIGHT (vol. xiv. p. 593 sg.) and OPTICS (vol. xvii. p. 802 sq.), and we need only state here that the finest camera which can be manufactured is useless unless the lens with which it has to be worked gives a flat field and an approximately achromatic image. FIG Daguerre's camera is shown in the ac companying figure (fig. 4), according to Hunt (Photography, 4th ed., p. the idea existed of movin
¡9), by which it will be seen that at first the plate away from the camera. The first camera made in England, as far as is known, was that by Mr Palmer of Newgate Street, London, on the plan of Mr Fry and for him, in 1839. It was a very primitive apparatus, and was furnished with a lens made in the same year. The ordinary form of camera was simply a box, at one end of which was a lens, and at the other a ground glass for focusing, for which could be substituted a dark slide holding a sensitive plate. The adjustment of the focus was made by a rack and pinion motion attached to the lens. The arrangement, however, subsequently introduced for ob-taining a rough approximation to focus was to have a sliding inner box as in Daguerre's camera; and finally to obtain the greatest sharp-ness the rack and pinion motion attached to the lens was used. It is evident that this form of camera has an advantage over the single box, since it allows more than one lens to be used. Ottewill's folding camera was a great improvement, in that, for outdoor work, it enabled a cumbersome article to be folded up into a compact space. Figs. 5 and 6 show it set up for use, and folded. A still more portable form was made by Mr George Edwards of Carlton Colville (Suffolk) in 1853, and for it he obtained the medal of the Society of Arts. Its portability is shown by the fact that for a 7-inch by 5.J-inch plate its weight was only 2 lb 3 oz. Broadly speaking its principle was that of a couple of frames attached by screws to a solid bar, one of

Fig. 5.—Ottewill's Camera, set up for use.

Fig. 6.—Ottewill's Camera, folded.

was the camera. This instrument is still used at the present day. It did not come into general use owing to its complicated arrangement of screws,—for the main point in any camera is that there should he as few loose screws as possible. The next improvement is that known as the bellows form, originally introduced, it is believed, by Captain Fowke, R.E., about 1854. Its introduction maybe said to mark a new era in camera construction, and from that time to the present the bellows is to be found in nearly every improved form. After this invention the square instead of the tapering form of bellows was that most generally adopted. It is unnecessary to trace every improvement that has been introduced, but we give two typical

FIG. 7.—Hare's Camera.

ones (figs. 7 and 8), which are manufactured by Hare and Meagher respectively. It will be noticed that in both these cameras there is an arrangement by which the focusing screens can be made to tilt at an angle with the axis of the ones (figs. 7 and 8), which are manufactured by Hare and Meagher respectively. It will be noticed that in both these cameras there is an arrangement by which the focusing screens can be made to tilt at an angle with the axis of the lens. This is called a swing - back ar- rangement, and is necessary when pho- tographing architec- tural subjects to pre- vent vertical lines converging ill the picture. When the ground glass is in a vertical plane, no matter what tilt is given to the camera, vertical lines will always be shown as parallel in the picture. It will also be noticed that in these cameras there is an arrangement for focusing the lens by means of a rack and pinion motion in fig. 7, and by means of a screw in fig. 8. The gradual motion which can thus be given to the focusing screen is a great advantage, since lenses need not be constructed with rack and pinion mo-tion. Many suggestions have been put forward for adapting lens. This is called a swing-back arrangement, and is necessary when photographing architectural subjects to prevent vertical lines converging ill the
picture. When the ground glass is in a vertical plane, no matter what tilt is given to the camera, vertical lines will always be shown as parallel in the picture. It will also be noticed that in these cameras there is an arrangement for focusing the lens by means of a rack and pinion motion in fig. 7, and by means of a screw in fig. 8. The gradual motion which can thus be given to the focusing screen is a great advantage, since lenses need not be constructed with rack and pinion mo-tion. Many suggestions have been put forward for adapting

FIG. 8.—Meagher's Camera,

FIG. 9.—Marion & Co.'s Camera.

the camera for a developing chamber, and we believe Archer's could be used for this purpose. Mr Newton in 1852 introduced a camera in which wet plates after exposure were developed by dipping in troughs of solutions ; and we might name many others who subsequently worked at the same idea. It met, however, with no very great success. The introduction of dry plates was a great step for the landscape photographer, as it enabled him to carry a supply of plates in the field, and to develop them at home. To economize space and weight, what are known as "double backs" were in-vented. A " double back " is a dark slide in which two plates are placed back to back, being separated by an opaque plate. Each side of the slide can be drawn up or out so as to expose each plate. "What are known as changing boxes answer the same purpose. They hold from one to two dozen plates, and by means of a special arrangement each plate can be conveyed to or removed from the dark slide without exposure to light. There are other plans also by which a certain number of plates can be carried in the camera itself and exposed in succession. The writer's opinion of such instruments is that they possess no striking advantage »nd many disadvantages, unless for very special purposes. Even for a miniature camera for taking instantaneous street views whilst holding the apparatus in the hand the use of double backs is to be preferred. An excellent specimen is a camera made by Marion & Co. of London (see fig. 9): it is entirely of metal, and fitted with a finder and in-stantaneous shutter,—one which should stand any amount of rough usage. The whole apparatus, including a dozen plates, can easily be carried in the pocket. The dark slides are strongly made of metal.

In the preceding sketch, brief though it is, of the successive improvements in cameras, probably enough has been said to show the very remarkable development that has taken place since the days when a cigar-box and spectacle lens were used to obtain an image on a sensitive plate. (W. DE W. A.)


Share this page:


Footnotes

3 For further details the reader is referred to Instruction in Photography, p. 99.



The above article was written by: Captain Wm. de Wiveleslie Abney, R.E., F.R.S., author of Instructions in Photography.