If we consider the various colours of the model we shall find that they are reproduced satis factorily with their various tones.
The images of a pure yellow object are equally transparent in the transparencies that control the distribution of the red and green lights, but this image is opaque in the transparency placed in the beam of blue light. The sum of the red and green lights, which are the only ones to reach the corresponding points of the screen, constitutes yellow (§ 5), which is brighter accord ing as the original colour was purer. The image of a vermilion is transparent in the transparency projected by red light and opaque in the two others, so that the screen receives only red light at the corresponding points. The image of a yellowish-green object is transparent in the transparency projected by green light, slightly transparent in that passed by red light, and opaque in the other. On the screen this object will be formed by green light tinged with yellow by the addition of a little red light.
The image of a sky blue is completely trans parent in the transparency illuminated with blue light and a little less transparent in the others, so that in the mixture of the three beams the blue will predominate slightly, thus giving a blue with an admixture of much white. The image of a myrtle-green is opaque in the trans parencies distributing the red and blue lights and a little less opaque in that controlling the green light. The image, formed only by green light of much diminished intensity, will therefore appear a dark green.
868. Subtractive Synthesis. Three-colour syn thesis by the actual superposition of coloured positive films, of dyes, or of inks, is up to the present the only one which lends itself to the production of prints on paper and other opaque supports.
The role of the superposed coloured layers is to absorb at each point of the image, which we obviously assume to be lighted by white light, those radiations which the object photo graphed did not emit. Each of the colours must therefore absorb the spectral rays transmitted by the corresponding selection filter of which it is thus the complementary, as pure and as saturated as the dyes or pigments at our disposal permit.
For example, the image printed from the negative made with the green filter must be formed of pink dye or pigment, transmitting or diffusing all the visible rays not transmitted by the green filter—and only those, that is to say, the total of the blue-violet and red rays. For
the various points of the subject have recorded themselves on the negative with a density which is less according as the emission of green rays is less, and in consequence the positive printed from this negative shows most density in the points which did not emit any green rays ; the pigment used to form this image must therefore absorb the green 'rays and only them.
869. Let us assume that three film images have been superposed, obtained, for instance, by dyeing gelatine reliefs (§ 676) in suitable colours. These images (called elementary monochromes) have been printed under the selective negatives previously considered (§§ 866 and 867).
These three films are colourless in the parts corresponding with the pure whites of the subject ; their superposition in register will naturally produce white. A black object will be represented in each of the films by the colour at its deepest intensity. If each of the colours actually absorbs one-third of the spectrum, any rays which pass through two of the films will necessarily be absorbed by the third, and the absence of any transmitted light will give the sensation of black. Images of a neutral-grey must be of the same density in all three films ; if the respective colour intensities are balanced, the absorption due to their super position will be the same for all visible radiations, and a neutral-grey will be the result.
The image of a yellow will be represented by yellow in one of the films, the others being colourless at the corresponding points. The image of vermilion is formed by the superposi tion of yellow and of pink. The resulting colour will be due only to the rays in the spectral region transmitted by one and the other of the two superposed dyes ; it will therefore be a pure red. The image of a yellowish-green object will be due to the superposition of yellow at full intensity with greenish-blue of reduced inten sity. This will transmit all the green rays and a portion of the red rays, incompletely absorbed by the non-saturated greenish-blue, thus repro ducing the yellowish-green of the subject.