IT would be somewhat difficult to fix a date when -what we now know as “photographic action” was first recorded. No doubt the tanning of the skin by the sun’s rays was what was first noticed, and this is as truly the effect of solar radiation as is the darkening of the sensitive paper which is now in use in photographic printing operations. We may take it that 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 photography. 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 experiments Count Rumford contributed a paper to the Philosophical Transactions of the Royal 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, conclusively showed in Nicholson's Journal that, at all events, salts of mercury were reduced by visible radiation and not by change of temperature. In 1801 we come to the next decided step in the study of photographic action, when Ritter 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 Seebeck (between 1802 and 1808) and Bérard 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 absorption to effect a molecular or chemical change in a substance. Desmortiens in 1801 observed the change effected in Prussian blue, and Böckman 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 Royal Institution the paper—“An account of a method of copying paintings upon glass and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy.” He remarks that white paper or white leather moistened with a solution of 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 inferentially by what we now call “contact printing.” He states that he has been unable to fix his prints, no washing being sufficient to eliminate the traces of the silver salt which occupied the unexposed or shaded portions. Davy in a note states that he has found that, though the images formed by an ordinary camera obscura were too faint to print out in the solar microscope, the images of small objects could easily be copied on such paper.

“In comparing the effects produced by light upon muriate of silver (silver chloride) with those upon the nitrate it seemed evident that the muriate was the 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 alteration was produced upon the nitrate. ... Nothing but a method of preventing the unshaded parts of the delineations from being coloured by exposure to the day is wanting to render this process as useful as it is elegant.”

In this method of preparing the paper lies the germ of the silver-printing processes 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 permanènt. Here we must 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, however, it had a light yellow tint; in the red and beyond the red it took a rose or lilac tint. This image of the spectrum shows beyond the red and the violet a region more or less light and uncoloured. This is how the decomposition of the silver chloride is seen in this region. Beyond the brown band, . . . which was produced in the violet, the silver chloride was coloured a grey-violet for a distance of several inches. In proportion as the distance from the violet increased, the tint became lighter. Beyond the red, on the contrary, the chloride took a feeble red tint for a considerable distance. When moist chloride of silver, having received the action of light for a time, is exposed to the spectrum, the blue and violet behave as above. In the yellow and red regions, on the other hand, it is found that the silver chloride becomes paler; . . . the parts acted upon by the red rays and by those beyond take a light coloration.”

This has been brought prominently forward by Dr J. Μ. 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.

The first to found a process of photography which gave pictures that were subsequently unaffected by light was Nicéphore de Niepce (q.vf. His process, which he called provisionally “héliographie, 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. Tn 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 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 unaffected 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.” Perhaps 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 ^⅛- σ th of a second is a fair estimate of the progress made since Niepce. The development was conducted by means of petroleum-vapour, which dissolved the parts not acted upon by light. As a rule silver plates seem to have been used, and occasionally glass; but it does not appear whether the latter material was chosen because an image would be projected through it or whether simply for the sake of effect. Viewed in the light of present knowledge, a more perfectly developable image in half-tone would be obtained by exposing the film through the bach of the glass. The action of light on most organic matter is apparently one of oxidation. In the case of asphaltum or bitumen of Judaea the oxidation causes a hardening of the material and an insolubility in the usual solvents. Hence that surface of the film is generally hardened first which first feels the influence of light. Where half-tones exist, as in a landscape picture, the film remote from the surface first receiving the image is not acted upon at all, and remains soluble in the solvent. It is thus readily seen that, in the case of half-tone pictures, or even in copying engravings, if the action were not continued sufficiently long when the surface of the film farthest from the glass was first acted upon, the layer next the glass would in some places remain soluble, and on development would be dissolved away, carrying the top layer of hardened resinous matter with it, and thus give rise to imperfect pictures. In carbon-printing development from the back of the exposed film is absolutely essential, since it depends on the same principles as does heliograρhy, 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 communicated a paper to Dr Bauer of Kew, the secretary of the Royal Society of London, with a view to its presentation to that society. Its publication, however, was prevented because the process, of which examples were shown, was a secret one. 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.

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 prosecuting 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 and 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 Nicéphore, but Mr Isidore observes that for the present that letter might be considered confidential.”

Dr Bauer evidently knew more of “poor Nicéphore’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 Nicéphore 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, <⅛c., is mentioned as having been used on the metal [dates, and their sensitiveness to light, when thus treated, commented upon. We are thus led to believe that a great part of the success in producing the daguerreotype is due to the elder Niepce; and indeed it must have been thought so at the time, since, on the publication of the process, life-pensions of 6000 francs and 4000 francs were given to Daguerre and to Isidore Niepce respectively. In point of chronology the publication of the discovery of the daguerreotype process was made subsequently to the 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 by means, 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 downwards, 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 “solarization,” 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 involved 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- ρrinting. The development of the image was effected by placing the exposed plate over a slightly heated (about 7 5° 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 process 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 Μ. Claudet employed a combination of...