Talking pictures : how they are made, how to appreciate them (c. 1937)

The Road Ahead The film next to the cube is exposed through its celluloid back to the blue light, and therefore photographs the blue elements of the object. There is a very thin yellow coating on this film, however, which prevents the blue light from passing through to the back film. Since this yellow layer permits the red light to pass, the back film photographs the red elements of the object. These films are developed like ordinary camera negatives, but instead of ordinary prints being made from them, a process somewhat like lithography is used. The prints are really much more like little transparent billboard posters than ordinary photographs. To make these colored prints, an intermediate step is taken, comparable to preparing the lithography stones. From each negative a "matrix' ' is made which will have the desired image raised in relief like a rubber stamp, but not nearly so high. Each of these matrices is then coated with a dye, complementary in color to that with which its negative was photographed. Then it is successively pressed against a clear film so prepared that it absorbs the dye as paper absorbs ink from a rubber stamp. In this way the matrices can be used over and over to make large numbers of prints. The matrices are aligned carefully so that each successive colored image is printed exactly on top of the other color or colors beneath it on the print. Another effective color photographic system which is moving ahead with great rapidity, and which promises startling advances for the future, uses a film with three color sensitive emulsions, one on top of another. The top emulsion is sensitive only to blue light; the mid [ 277 ]