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274 CINEMATOGRAPHIC ANNUAL
from the different objects around us is minus various frequencies, which have been absorbed by the object. Thus, a red rose reflects those frequencies which give us the sensation of red, and absorbs all the others. Similarly, its green leaves reflect the green vibrations, and absorb the others. Thus it is with all colors: black, of course, means an almost complete absorption of all frequencies, while its opposite, white, is a complete reflection of all frequencies. Gray is merely an imperfect white; uniformly absorbed in all frequencies, cutting down the chromatic brilliance of the object, though not necessarily lessening its visual brilliance.
Furthermore, scientists have found that white light may be reduced to three primary colors, which can be combined to form all the others. These three are red, blue, and green: they correspond to the three different units of our optic nervous system. If all three units are excited equally, we get the effect of white; if they are affected unequally, we get the effect of color corresponding to that mixture of these primary colors. Thus it will be apparent that if we can make three photographs of an object, each one so filtered as to just record the proportion of the frequencies of the total reflected light in the picture that one of these three nerve-units would get, and then in some way combine the three, each having been colored its appropriate hue, we should get an exact reproduction of the object in its original color. This is the idea behind all color photography. In actual practice it has been found possible to use only two color-images — those of the red and green — and still get a fairly good color-picture. Of course the loss of the blue means also the loss of absolute fidelity in the color representation; for instance, white is actually rendered as a pale yellow, which we see as white; but it also means such a degree of mechanical simplification that the sacrifice of perfect accuracy seems justified. This is especially so in kinematography, where the mechanical difficulties are already so numerous.
Two Kinds of Color Process
But, whether two or three colors are used is not the chief difference between the various color processes. Regardless of the number of colors used, all color-photographic processes range themselves into two groups; ADDITIVE and SUBTRACTIVE processes. Every system of color photography thus far devised or suggested falls under one of these two heads. Some combine the two. Briefly, in an additive process, the film itself carries no actual color: the colorvalues are latent, and are revealed by appropriate filters placed or moved between the film and the screen. In a subtractive process, the picture is in itself a complete, self-contained, color-record, needing no filters or other special equipment for projection. Each of these systems has its individual advantages and disadvantages. For instance, the additive processes' films are in no way special, and may thus be handled in the ordinary manner: but at the same time, special apparatus is required for both taking and showing. On the other hand, though the subtractive processes