" 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".
An experiment on the dark rays of Bitter, by Dr. Young, included in his Bakerian Lecture,* is a very important one. Dr. Young, after referring to the experiments of Bitter and Wol-laston, goes on to say : " In order to complete the comparison of their properties (the chemical rays) with those of visible light, I was desirous of examining the effect of their reflection from a thin plate of air capable of producing the well-known rings of colours. For this purpose I formed an image of the rings, by means of the solar microscope, with the apparatus which I have described in the Journals of the Royal Institution; and I threw this image on paper dipped in a solution of nitrate of silver, placed at the distance of about nine inches from the microscope. In the course of an hour, portions of three dark rings were very distinctly visible, much smaller than the brightest rings of the coloured image, and coinciding very nearly, in their dimensions, with the rings of violet light that appeared upon the interposition of violet glass. I thought the dark rings were a little smaller than the violet rings, but the difference was not sufficiently great to be accurately ascertained : it might be as much as 1/30 or 1/40 of the diameters, but not greater. It is the less surprising that the difference should be so small, as the dimensions of the coloured rings do not by any means vary at the violet end of the spectrum so rapidly as at the red end. The experiment in its present state is sufficient to complete the analogy of the invisible with the visible rays, and to show that they are equally liable to the general law, which is the principal subject of this paper :" that is the interference of light.
M. B. G. Sage, in the Journal de Physique, 1802, mentions a fact observed by him, that " the realgar which is sublimated at the Solfaterra under the form of octahedral crystals, known under the name of ruby of arsenic, effloresces by the light; " and that ordinary native realgar from Japan changes to orpi-ment by exposure to sunshine.
In 1806, Vogel exposed fat, carefully protected from the influence of the air, to light, and found that it became in a short time of a yellow colour, and acquired a high degree of rancidity. Vogel subsequently discovered that phosphorus and ammonia exposed to the sun's rays were rapidly converted into phosphu-retted hydrogen, and a black powder, phosphuret of ammonia. He also noticed that the red rays produced no change on a solution of corrosive sublimate (bichloride of mercury) in ether, but that the blue rays rapidly decomposed it. Dr. Davy, much more recently, repeated a similar set of experiments to those of Vogel. He found that corrosive sublimate was not changed by exposure; but that the Liquor Hydrarg. Oxymur. of the old London Pharmacopia quickly underwent decomposition in the sunshine, depositing calomel (chloride of mercury).
* Philosophical Transactions, 1804.
Seebeck, in and subsequently to 1810, made some important additions to our knowledge of the influences of the solar radiations, the most striking of his statements being the pro-duction of colour on chloride of silver ; the violet rays rendering it brown, the blue producing a shade of blue, the yellow preserving it white, and the red constantly giving a red colour to that salt. Sir Henry Englefield, about the same time, was enabled to show that the phosphorescence of Canton's phosphorus was greatly exalted by the blue rays.
Gay-Lussac and Thénard, being engaged in some investigations on chlorine, on which elementary body Davy was at the same time experimenting, observed that hydrogen and chlorine did not combine in the dark, but that they combined with great rapidity, and often with explosion, in the sunshine, and slowly in diffused light. Seebeck collected chlorine over hot water, and, combining it with hydrogen, placed different portions of it in a yellowish-red bell glass and in a blue one. In the blue glass combination took place immediately the mixture was exposed to daylight; but without explosion. The mixture in the red glass was exposed for twenty minutes without any change; but it was found that the chlorine had undergone some alteration, probably a similar one to that subsequently noticed by Dr. Draper, who found that chlorine having been exposed to sunshine would unite with hydrogen in the dark. If the gases were placed in a white glass and exposed to sunshine, they exploded; but if the gas had been previously exposed to the action of the solar radiations in the yellow-red glass, it combined with hydrogen in the white glass in the brightest sunshine without any explosion.
Berzelius noticed some peculiar conditions in the action of the solar rays upon the salts of gold ; and Fischer pursued some researches on the influence of the prismatic rays on horn-silver.
The most important series of researches, however, were those of Bérard in 1812, which were examined and reported on by Berthollet, Chaptal, and Biot. These philosophers write : " He (M. Bérard) found that the chemical intensity was greatest at the violet end of the spectrum, and that it extended, as Bitter and Wollaston had observed, a little beyond that extremity. When he left substances exposed for a certain time to the action of each ray, he observed sensible effects, though with an intensity continually decreasing in the indigo and blue rays. Hence we must consider it as extremely probable, that if he had been able to employ reactions still more sensible, he would have observed analogous effects, but still more feeble, even in the other rays. To show clearly the great disproportion which exists in this respect between the energies of different rays, M. Bérard concentrated, by means of a lens, all that part of the spectrum which extends from the green to the extreme violet ; and he concentrated, by means of another lens, all that portion which extends from the green to the extremity of the red. This last pencil formed a white point so brilliant that the eyes were scarcely able to endure it; yet the muriate of silver remained exposed more than two hours to this brilliant point of light without undergoing any sensible alteration. On the other hand, when exposed to the other pencil, which was much less bright and less hot, it was blackened in less than six minutes." This is the earliest intimation we have of any hypothesis that the luminous and chemical powers may be due to dissimilar agencies. On this, the Commissioners remark : " If we wish to consider solar light as composed of three distinct substances, one of which occasions light, another heat, and the third chemical combinations, it will follow that each of these substances is separable by the prism into an infinity of different modifications, like light itself; since we find by experiment, that each of the three properties, chemical, colorific, and calorific, is spread, though unequally, over a certain extent of the spectrum. Hence we must suppose, on that hypothesis, that there exist three spectrums, one above another ; namely, a calorific, a colorific, and a chemical spectrum. We must likewise admit that each of the substances which compose the three spectrums, and even each molecule of unequal refran-gibility which constitutes these substances, is endowed, like the molecules of visible light, with the property of being polarized by reflection, and of escaping from reflection in the same positions as the luminous molecules, etc".
From the time when the difficulty of fixing the photographs which they obtained, stopped the progress of Davy and Wedgwood, no discoveries were made until 1814, when M. Niepce, of Chalons, on the Soane, appears to have first directed his attention to the production of pictures by light.
It does not seem that his early attempts were very successful; and, after pursuing the subject alone for ten years, he, from an accidental disclosure, became acquainted with M. Daguerre, who bad been for some time endeavouring, by various chemical processes, to fix the images obtained with the camera obscura. In December, 1829, a deed of copartnery was executed between M. Niepce and M. Daguerre. for mutually investigating the subject.
M. Niepce had named his discovery Heliography.* In 1827, he presented a paper to the Royal Society of London, on the subject; but as he kept his process a secret, it could not, agreeably with one of their laws, be received by that body. This memoir was accompanied with several heliographs on metal (plated copper and pewter) and on glass plates ; which were afterwards distributed in the collections of the curious, some of them still existing in the possession of Mr. Robert Brown, of the British Museum. They prove M. Niepce to have been then acquainted with a method of forming pictures, by which the lights, semi-tints, and shadows, were represented as in nature; and he had also succeeded in rendering his Heliographs, when once formed, impervious to the further effects of the solar rays. Some of these specimens appear in a state of advanced etchings ; but this was accomplished by a process similar to that pursued in common etchings.
The ease with which nitric acid could be applied to etch these Heliographic plates will be apparent when the process of obtaining the pictures is understood.
* Sun-drawing: a more appropriate name than Photography, since it remains a problem (1853) of difficult solution, whether Light, or some agent associated with Light, is active in producing the chemical changes we are considering.