Page:Popular Science Monthly Volume 5.djvu/737

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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