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CHAPTER XXVI
THE PRISMATIC DISPERSION AND ALLIED PROCESSES
The Prismatic Dispersion Process. — In this method all color filters are done away with, and separation of the colors is effected by the dispersion of light by prisms. There are two distinct methods which differ widely from one another ; in the one, the light is dispersed into a single spectrum and the three negatives obtained by the action of the three different regions. Positives from the negatives thus obtained are placed in the path of the spectral rays and recombined into a colored virtual image. In the second process, the light is split up into a great number of individual spectra, which are all formed side by side on a single negative plate ; from this a positive can be made and viewed in the taking apparatus, or projected. One may possibly look upon this method as an optical screen-plate process. Both processes are comparatively simple to work, but the apparatus is costly and they are hardly applicable to general work or exhibition purposes.
The first suggestion as to the former of these processes was made by Chas. Cros1 (p. 677) in 1869. He suggested that the color filters might be replaced by a prism, which should be turned for each exposure in such a way that in the first case, it would send into the camera only red rays ; secondly, only the yellow rays and finally only the blue rays, or by taking simultaneously the three negatives by the three beams, resulting from the decomposition of the light emitted from or reflected by the subject, by the prism. A system of lenses was so placed as to group the rays proceeding from the subject. This compound beam would fall upon the prism, which would decompose it and spread it out into a spectrum, and three elementary lenses would receive respectively the red, yellow and blue rays and form three partial images on the sensitive surfaces. Perhaps it would be necessary to place before each objective a prism, which would compensate for the elongation of the images.
In dealing with the positives Cros is rather more explicit and said : "The synthesis by refraction gives one of the most elegant solutions of the problem. It is based on the following principles : the path of a simple colored ray, which traverses a succession of refractive media, is the same in two directions, that is to say, that the source of the ray and its focus may change places without the path varying. But if a compound ray, containing red, yellow and blue, passes through a prism, each of these rays will be projected at a different place. If then, from each of these places where these rays fall, one transmits rays of the same kind through
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