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CHAPTER XXIII
CINEMATOGRAPHY IN COLORS. ADDITIVE PROCESSES
The basis of all cinematographic projection is persistence of vision. A picture formed on the retina persists for a brief period of time, and if a succeeding series of pictures is similarly formed with but small intervals of time between the formation of each picture, the result will be a composite, in which fundamental phases of movement are no longer visible or distinguishable.
This applies equally to color. If red is projected, and immediately afterwards green, and before the red has had time to fade from the retina, the green is virtually superimposed, the resultant color in the brain will be yellow. Nor are we conscious that the latter color is actually composed of red and green. Should blue-violet be added in the same way, the result is white. Obviously this process is an additive one, for we start with a black screen and add light to light.
The only disadvantage of this method is that after some time we become conscious of an irritation, which takes the form with some people of an intense frontal headache and eyestrain. From the practical point of view, it is clear that as there must be approximately the same number of pictures projected as in black and white, the consumption of film is threefold, in order to show the same total amount of subject. Further than that, as a rule a special form of projector is required and this obviously limits the use of this process.
Attempts have been nfade to produce each projected picture complete in itself in colors on the screen, and while this method produces less eyestrain, the apparatus necessary for projection is more complicated, and consequently more costly. There is the further disadvantage of having to keep the pictures in register all the time, which entails the watching of the screen by a competent operator, which again increases the cost beyond commercial limits.
Were it possible to obtain each picture in colors on the film itself, one would have the same amount of film used as in black and white, the eyestrain, or color bombardment, as it is called, would be eliminated. Numerous processes have been attempted or patented to this end, mostly by means of a two-color method.
We may, therefore, divide the subject into two main divisions: the additive and subtractive. The former is again divisible into projection by persistence of vision, and simultaneous projection. The former being defined as that in which each picture is projected in a single color. The
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