822 PHOTOGRAPHY violet ; though under similar circumstances no intermediate altera tion was produced upon the nitrate. . . . Nothing but a method of pre venting 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
- ebeck. call attention to an important observation by Dr Seebeck
of Jena in 1810. In the Farlenlehre 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 con tinued for from fifteen to twenty minutes, whilst a constant position for the spectrum is maintained by any means, I observe the follow ing. In the violet the chloride is a reddish brown (sometimes more violet, sometimes more blue), and this coloration extends well be yond 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 photo graphic 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. , de The first to found a process of photography which gave iepce. 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 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 i.s 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 ^i^th 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 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 lack of the glass. The action of light on most organic matter is apparently one of oxidation. In the case of asphaltum or bitumen of Judiea 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 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 " Memoire," dated " Kew, le 8 D6cembre 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.
