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THE PROJECTOR 317
this being the beam of light incident in whole upon the projector aperture. It may be defined as a cone of light bounded by straight lines drawn from the outer, free, diameter of the converging element (mirror or lens) to corresponding points upon the aperture opening.
Projector manufacturers hold this definition to be correct, but declare that the effective beam is bounded by straight lines reaching from the projector aperture opening to a rectangle on the face of the converging element, the same shape as the projector aperture, but so large that each of its corners would touch the converger's free diameter edge.
This statement is incorrecc, for we discover that in some measure a round beam of light can be put through a rectangular opening. This may be proven by painting a converger (mirror or lens) with opaque water color and making a pin hole at its free diameter edge. Upon trial it will be found that a thin cone of light will go forward, some portion of which will always fall upon the aperture opening. It will be found by making other pin holes nearer the center of the converger that more of the spot light made by the cone will reach the aper
FlGURE 140.
ture opening, until finally, near the center, it is likely the entire cone will enter the aperture opening and therefore become available to the projection lens. This pin-hole experiment should be very interesting to projectionists.
The action of each cone of light may be investigated by blowing smoke into it. Tobacco smoke will serve, all film first having been put carefully away. A thin metal