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THE NATURE OF LIGHT
photographic plate or upon a film stretched flat or upon a piece of photographic paper, as in a photograph, or upon a screen in a moving picture theater, so that no matter by what means we
Fig. 13.
Let us take the points A and B in these two diagrams. In both the upper and lower diagrams the image of A will formj in the plane a and that of B will form in the plane b. It is in these two planes that the sensitive surfaces should lie to render sharply the images of A or B as the case may be.
We desire to receive both of these images however on the plate at once and utilize the two following means for obtaining the result. First we compromise between the two planes a and b and place our plate in the plane "C." We do this because the circle of confusion at C, is common to both and is the smallest mean between the planes a and b. This compromise prepares us for better results in our 2d procedure. This consists of placing a diaphragm close to the lens. This diaphragm is a piece of black cardboard with a smooth, round hole in it and its function is to diminish the angle on the rays of light that represent the extremes of the oones of light which form the images a and b. This has the desired effect of reducing the size of the circles of confusion at C to an inappreciable size. This size depends on the distance between A and B and on the size of the hole in the diaphragm. A circle not greater than 1/100 inch is permissible in stills but for the cinema film one of 1/400 inch is about the limit
of size.
produce a photographic image it is practically always done upon a flat surface. Let us for the purpose of our analysis, consider any object or any image as being composed of a collection of a
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