RECENT RESEARCHES IX PHOTOGRAPHY, 717 

 EECE]^T KESEAECHES IK PHOTOGEAPHY. 



By E. meld OLA. 



A SUBSTANTIAL contribution has been recently made to our 

 knowledge of the action of light upon silver salts — a contribu- 

 tion 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 Yogel 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 ice 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. Yogel, 

 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 experi- 

 ments, 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. Yogel'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, 

 w^hen 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 Yogel, 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 con- 

 jecture 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. Yogel's anticipa- 

 tions, its sensitiveness for the green rays. The peculiar action of the 

 Wortley dry plates was thus shown to be due to the coating of color- 

 ing-matter, and the next step made by Yogel was to seek some sub- 

 stance which especially absorbed the yellow, and at the same time 

 acted as a sensitizer by fixing the free bromine, liberated by the action 



* Poggendorff'8 Annalen, vol. cl, p. 453. 



