The history of three-color photography (1925)

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656 History of Three-Color Photography the prism under the same respective angles as those of their emergence, an identical compound ray is reconstituted. Whence follows the practical process : — three negatives are taken in the three regions of the simple rays of the spectrum. Positives are made from these negatives, either by transmitted or reflected light. To these three positives are supplied the uniform colors red, yellow and blue as they occur. The three proofs are placed back again in the places where they were obtained. In looking at them through the analysing prism they form one and the same image. The same effect is obtained on projecting the rays which pass through the prism on to a screen. In pursuit of this line of thought, a solution is found still more pure and simple, in which the use of all predetermined artificial colors disappears. This is the result of the following principle : — A ray of white light traverses a prism ; the red, the yellow and the blue emerge at different angles. If one sends in the reverse direction, and at the same angle as that of the emergence of the red, a ray of white light, this ray is decomposed and all the red that it contains will take the direction of the first ray. Also the inverted white ray will give yellow and 3* Fig. 186. Donisthorpe's Prismatic Dispersion System. blue rays in the direction of the direct white pencil. Then the same apparatus, which serves to decompose the subject into three negatives, taken in the red, yellow and blue region of the spectrum, may be used, these negatives having been once obtained, to recompose them. It will be sufficient for this synthesis to replace the three cliches by their positives, noncolored, and to send through each of them the corresponding colored ray. Thus one will have the reproduction of a natural object, either direct in the eye or on a screen. This solution is remarkable in that the result does not depend on an artificially colored product. The colors are thus transformed under purely geometrical conditions and these conditions regenerate in turn the colors. The apparatus only renders in this way that which it receives." Cros' idea seems to have lain dormant for many years, and to have received but little attention from experimenters. Wordsworth Donisthorpe* described a method of prismatic dispersion, which although faulty in detail, is essentially that given by Cros. He suggested the use of one large prism and three smaller ones, as shown in Fig. 186. The light rays were