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other emulsion layers will render color-separation positives in accordance with their original color sensitivity.
In the further course of such a process, these positive, silver images must now be replaced in the respective complementary order by colored images of blue-green, magenta, and yellow. The black and white positive from the bluesensitive emulsion layer must be colored yellow; the positive image from the green-sensitive emulsion, magenta; and the image from the red-sensitive emulsion, blue-green. If these changes can be made, then the problem is solved as far as basic principles are concerned.
Many years ago Dr. Fischer of the "Neue Photographische Gesellschaft" in Berlin was experimenting with certain substances which could be used to produce a dye in a photographic emulsion during development by a chemical process known as "coupling." Proceeding from earlier observations of Homolka, he added certain colorless chemicals to a developer which formed a dye in the gelatine emulsion during development by "coupling" with oxidation products of the developing agents. The dye was formed only where silver was deposited in the emulsion.
Next, Dr. Fischer found three different substances which, when added to the developer, possessed the properties of producing, in developing the latent image, one of the respective subtractive primary colors, although of course, still quite imperfect. The patents which he took out in 1911 and 1912 to cover this process of dye-coupling during development have since expired, and he was not successful in solving his problem completely for practical use, although he made a fine start toward such a solution.
In principle the color development of the three-emulsion layer film is possible, as has been already mentioned, but the practical solution of this problem offers considerable difficulties. A large number of very exact operations must be performed while processing the film: first, the three emulsion layers must be given a blue-green development; next, the dye in the two upper layers must be removed without affecting the bottom layer; then, the two upper layers must be given a magenta development and the upper layer has to be bleached without affecting the second layer; finally, the top layer must be given a yellow development.
It will be noted that apart from the complexity of the treatment involved, a new developer must be used for each color development and each developer must contain the correct proportion of the substances required to produce the color.
It would thus seem that although this method can render excellent results when undertaken by a skilled staff using special mechanical equipment in processing the film, the amateur and photographic dealer are not in a position to do this work. If it is desired that the method of processing be more practical and simpler, then it is necessary to arrange the process in such a way that the developer used produces three dye images simultaneously, one in each layer of the film. This, however, is only possible in case each of the three different sub
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