Popular Science Monthly/Volume 5/October 1874/Recent Researches in Photography
By R. MELDOLA.
A SUBSTANTIAL contribution has been recently made to our knowledge of the action of light upon silver salts—a contribution which we cannot but consider as of the highest importance to photography, both as a science and as an art.
In the autumn of last year Dr. Herman Vogel announced, as the result of some experiments that he had been making, that “we are in a position to render bromide of silver sensitive for any color we choose—that is to say, to heighten for particular colors the sensibility it was originally endowed with.” This discovery is such a decided advance that it will be interesting to trace it from the beginning. Dr. Vogel, in the first instance, found to his astonishment that some dry bromide plates, prepared by Colonel Stuart Wortley in this country, were more sensitive to the green than to the blue portions of the spectrum. This result was so totally opposed to the generally-received notions that the subject was submitted to further examination. In the next experiments, a comparison was instituted between dry bromide plates and the same plates when wet from the bath-solution of silver nitrate. The results showed a decided difference in the behavior of the plates. The sensibility of dry bromide plates appears to extend to a greater extent into the least refrangible end of the spectrum than is the case with wet plates. In Dr. Vogel's plates, which received the spectrum formed by the battery of prisms of a direct vision spectroscope from a ray of sunlight reflected from a heliostat, and passing through a slit 0.25 mm. wide, the photographic impression of the spectrum, when developed by an acid developer, extended, in the case of the dry plates, into the orange, but with wet plates not quite into the yellow. The bromide plates prepared by Vogel, moreover, did not exhibit that increased sensitiveness for the green rays which characterized Colonel Stuart Wortley's plates, and this led the German investigator to conjecture that the latter plates contained some substance which absorbed the green to a greater extent than the blue. To test this conclusion, one of the plates was washed in alcohol-and-water in order to remove the yellow coloring-matter with which the plate was coated, and it was then found to have lost, in accordance with Dr. Vogel's anticipations, its sensitiveness for the green rays. The peculiar action of the Wortley dry plates was thus shown to be due to the coating of coloring-matter, and the next step made by Vogel was to seek some substance which especially absorbed the yellow, and at the same time acted as a sensitizer by fixing the free bromine, liberated by the action of light, upon the silver bromide. Both these ends are fulfilled by the coal-tar color known as coralline. A plate dyed with this substance and exposed to the spectrum, exhibited two maxima of photographic action, one the ordinary maximum in the indigo (near G), and the other almost as strong in the yellow, thus affording complete confirmation of Dr. Vogel's views. Aniline green was next tried. This dye is stated to absorb the red rays, and a corresponding increase of sensitiveness for the red rays was observed, the photograph again presenting two maxima of activity, the one in indigo and one in the red, coinciding in position with the absorption band of the dye. Thus, Dr. Vogel's results may be summarized by saying that a dyed film of silver bromide exhibits maxima of sensitiveness in those regions where the coloring-matter exerts its maximum of absorptive power, but the precise conditions under which these results can be obtained must be considered at present as unknown, since many observers, in repeating the experiments, among others Dr. Van Monckhoven, have failed to obtain other than negative results.
In a communication made to the French Academy on the 27th of last month, however, the well-known physicist, M. Edmond Becquerel, stated that some experiments made at his instigation by M. Deshaies at the Conservatoire des Arts et Métiers had been productive of positive effects, and that some of Dr. Vogel's results with coralline and aniline green had been reproduced. M. Becquerel, however, does not confine himself to bromide films; similar results have been obtained by iodized collodion in which coralline was dissolved. A most remarkable action was observed also in the case of chlorophyll when this substance was used as a tinctorial agent. Although the collodion possessed only a faint-green color from the dissolved chlorophyll, the spectral image was of a much greater length than when plain collodion was used. Under these last circumstances the spectrum extended from the ultra-violet to between G and F, with the usual maximum of action near G, while with chlorophyll the region of strongest action extended from the ultra-violet to the line E in the green, and at the same time a weaker but yet distinct impression extended from E to beyond B in the red, with a strong band between C and D. By a close examination of the spectral image a second band of less intensity could be detected on the least refrangible side of the band between C and D, and other still weaker bands appeared in the green. The most striking confirmation of Vogel's results is to be found in the fact, observed by M. Becquerel, that the band between C and D corresponds in position with the characteristic hand of the absorption spectrum of chlorophyll dissolved in collodion. The same results were obtained by M. Becquerel with every plate tried and with collodions containing different quantities of chlorophyll.
It must be admitted, then, that a film exerting selective absorption in intimate contact with a sensitive film of silver bromide or iodide affects the latter in those parts of the spectrum where the selective action is taking place. Here, surely, is a wide field for investigation, and one the importance of which will be at once obvious to the physicist. Practically also, when the precise conditions of action are made known, valuable results may be anticipated from the application of this principle to science and to art. Since the year 1842, when M. Becquerel photographed the whole solar spectrum from the extreme violet to the extreme red, and when Dr. J. W. Draper photographed the violet, blue, and extreme red, no successful attempts have been made to imprint the least refrangible end of the spectrum; and this, when we consider the great importance that the study of the solar spectrum has assumed of late years, and the painful or even dangerous character of prolonged eye-observation, is to us a matter of wonder. M. Becquerel's result, it will be remembered, was obtained by a film of silver iodide, first insolated or exposed to diffused light and then to the action of the spectrum. Here, again, is another question—the precise action of insolation on sensitive plates—demanding explanation at the hands of the physicist. The practical aspect of Dr. Vogel's discovery need not here be discussed at length. Attention may be called to the well-known difficulty of getting reds or yellows to imprint themselves in portraiture, a difficulty which now bids fair to be overcome.
Then, again, in what we must consider as a higher sphere of practical utility, great advantage to the study of solar physics is likely to accrue. In point of fact, the photographic method of comparing spectra described in a recent communication to the Royal Society now becomes available for the whole extent of the solar spectrum, and our knowledge of the true composition of the sun will be thus in course of time recorded permanently on “that retina which never forgets.”
Great results have already been achieved by photography, and greater may be looked for. It must not be forgotten that in this most interesting branch of chemical physics we are in a period either of provisional hypothesis, or, worse still, of no hypothesis at all, so that valuable additions to our knowledge of physical and chemical laws should be forthcoming. The changes wrought by a beam of light on sensitive surfaces are sometimes physical and sometimes chemical. We may appropriately recall here the fact that mechanical pressure upon a sensitized surface of a silver salt acts in the same manner as a ray of light, giving a dark stain under the action of reducing agents. The experiment of Grove also, in which an electric current is set up by the incidence of a beam of light upon a prepared Daguerreotype plate, should not be forgotten. The equivalence between light and the other form of force has not yet been established, and it may not be going too far to conjecture that thermodynamics may possibly in the future have to appeal to the action of light upon a photographic plate. In the mean time we look forward to the promised continuation of Dr. Vogel's researches with no little hope.—Nature.