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MANAGERS AND PROJECTIONISTS 161
business it is more than likely also to receive only a portion of the light passing through the aperture, which means an unevenly illuminated image on the screen, and in addition to all this it will receive more or less stray light reflected from the edge of the aperture. Nor is this all the story of the difficulties of the motion picture projection lens, because due to manufacturing limitations, and the further fact that it must work in conjunction with the revolving shutter, its diameter is necessarily limited. Its diameter is also, to an extent, limited by the further fact that too-large diameter makes for lack of depth of focus in the screen image, which latter is a very important item indeed, if the projection lens be considerably above, or to one side of the center of the screen.
Jn the following we shall do our best to convey a clear understanding of practical projection optics, which is, we can assure you, no easy task.
The collector lens element of the condenser (collector lens is the one next the arc) must pick up the light emanating from the light source, which approaches the lens in diverging rays, see Fig. 36H.
THE LAW OF LIGHT.— Light diminishes in intensity as we recede from the source of light. If the luminous source be a point, then the intensity diminishes as the square of the distance increases. If we call the quantity or amount of light received by a certain given area at the distance of one foot 12 C. P., then at a distance of 2 feet its intensity would be J4th of 12, or 3 C. P., and at a distance of 3 feet it would be l/9th of 12, and at a distance of 10 feet it would be l/100th of 12, and so on. This is the true meaning of the law, which reads :
With an open light source, light intensity varies inversely as the square of the distance from its source.
This law has its base in the fact that light is propagated, or travels in straight lines, and naturally those lines are diverging — separate from each other as they go forward. You may prove the law as follows : Place a light source, which may be an incandescent globe, though the law holds strictly true only with point light sources, at a distance of nine feet from a white wall. Hold a cardboard six inches square at a distance of exactly 2.25 feet from the light source, which is one-fourth the distance from light to screen, and you will find the area of the shadow of the cardboard which is cast upon the wall to be sixteen times that of the card*