The history of three-color photography (1925)

214 History of Three-Color Photography one, sensitive to those rays which they themselves absorbed. In connection with this, the author (Vogel) has noticed that many dyes not only increase the sensitiveness for the rays they absorb, but they also decrease the sensitiveness for the more refrangible rays. This is marked with fuchsin. "Here the yellow sensitiveness of silver bromide dyed with it is distinctly greater than the sensitiveness for the blue, which is ordinarily much the greater. The sensitiveness of the dyed plate to the blue is less than that of the undyed plate. With the other dyes the increased sensitiveness for the absorbed rays also goes hand in hand with a decrease in the sensitiveness of the adjacent more refrangible rays. Thus silver bromide, dyed with naphthalen red, is much more sensitive to the blue-green, lying next to it in the spectrum, than is pure silver bromide. A similar result is to be noted with rosanilin picrate and aldehyde green9. Further investigations by the author (Vogel) established the fact that all three halides were themselves slightly sensitive to the less refrangible rays. This had not been noticed before, because people had exposed the plates for much too short a time. With long exposures, however, the sensitiveness of these substances extends into the red of the solar spectrum, with silver bromide even beyond that and into the invisible infra-red, so that the author was able to photograph the infra-red rays.10 Consequently the addition of the dyes really increases the color-sensitiveness of the photographic film instead of causing it." Eder has paid considerable attention to this particular subject11 it being only possible to sum up the main conclusions of his work, and his views may be considered as practically accepted at the present time. The conclusions arrived at are : 1. The dye must stain the silver halide grain; 2. Dyes which vigorously sensitize are all so-called substantive dyes, that is to say, they color substances direct, and probably by molecular attraction. Staining of the silver halide grain is no proof of color-sensitizing; 3. A dye sensitizes for those rays which it absorbs, or more correctly for those rays which the dyed silver halide absorbs ; 4. The maximum of sensitiveness lies about the same position as the maximum absorption of the dye, with a general shift towards the red. More correctly stated the maximum sensitiveness agrees with the maximum absorption of the dyed halide ; 5. Dyes with narrow, intense absorption bands generally give narrow, intense sensitizing bands, and those with broad, ill-defined bands give broad, ill-defined stretches of sensitiveness; 6. The brilliancy of color of the dye has no special influence ; 7. Neither the fugitive character of the dye. nor its fluorescence, have any action on the color-sensitizing properties.