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HANDBOOK OF PROJECTION FUR
board itself. It therefore follows that the amount of light falling upon one square inch of the cardboard would be sixteen times as much as would be the light falling upon a similar area of a screen located four times as far away from the light source. The operation of this law and the practical effect of distance of crater from face of collector lens is
very clearly shown in Fig. 36H, in which F is an ordinary Ay2 inch diameter plano-conv e x collector lens, having a free opening of 4^4 inches, its face located 2y2 inches from the center of the crater floor.
With the crater thus established, it requires no unusual power of discernment to understand that the lens will receive all light rays within the That much may be very
Figure 36H.
space bounded , by lines A — B. readily understood.
Remembering that once the light rays have left the source (crater floor) they travel in absolutely straight lines to infinity, it is readily seen that if we move lens F back to the position occupied by lens C it cannot and will not receive as great an amount of light as it did in the first position. In fact we believe that even the most obtuse will not dispute the proposition that a lens in position C, Fig. 36H, must of necessity have the diameter shown (7*4 inches) in order to receive as great an amount of light as is received by the 4^4 inch lens opening in position F.
The perpendicular dotted lines represent the faces of lenses at different distances from the center of the crater as shown,