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Tanuary 23, 1915.
MOTOGRAPHY
123
Fundamental Principles
TRANSFORMED BY FEATHERSTONE
THE PHOTOGRAPHIC NEGATIVE.
WE have seen how the "moving" pictures on the screen are simply the much enlarged "projections" of thousands of tiny images on the film which runs through the projecting machine. We will now examine the film itself and then go to the factory and see how it is made. There are two kinds, "positive" and "negative," and it is the positive films which are run through the projectors in the theaters, a piece of such positive film being shown in Figure 6, enlarged to nearly twice actual size, the standard width being l}i inches, and the usual length of a reel 1,000 feet, having 16,000 separate and complete images, or positive photographs.
The body of the film is composed of celluloid, which is almost perfectly transparent, and is prepared for the film manufacturer by coating its surface with what is known as a "sensitive emulsion." This simply means that there are chemicals in the coating which turn black wherever the light strikes, but remain unchanged where the light does not strike. Now suppose we should put such a chemically coated film in a camera, and point the lens toward any object having light and dark parts. The lens would bring all parts to. a focus on the film, and if we looked in the camera we should
Figures 5 and 6 — At the left a negative print of the Mary Pickford film shown at the right as a positive.
see a small reproduction of the object, projected by its own light upon the film.
But since light turns the chemicals dark, we should find upon "developing" the film that wherever the object was light the image on the film is dark, while the shadows are represented in the image as clear or transparent. Now if such an image were projected upon a screen, the enlarged picture would look very strange, as the sky would be black, and black clothes would be white, and the shadows of
each object would be I brighter than the "highlights."
POSITIVES FROM NEGATIVES.
Such a film is illustrated in Figure 5, and is known in photography as a "negative," the reason for its name being, of course, that its light values are reversed, or, exactly opposite to those of the object which was photographed. How then do we get a film which is suitable to use in the projecting machine? Nothing could be simpler. Just put another piece of sensitive film face to face with the one that has the developed images upon it, and let some plain, even light shine through the images onto the sensitive chemicals in the "emulsion," and the white shadows become dark, and the black high-lights come back into^ their own. This is exactly how the positive film shown
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Fig. io. Negative of "normally exposed" outdoor scene. Note that all bright objects (.sky, water, shirt-fronts, collars and dresses) are black. Faces and arms are nearly black, while black suits are white and dark trees and shrubbery nearly so. Exposure approximately according to Fig. 8.
Fig. ii. Positive print, made from the "negative" of Fig. io. Note that the degrees of light and shadow are exactly opposite to those of the negative. If you fold the page so that the positive and negative come face to face and look through, toward a strong light, you will see nothing but black.