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102 RICHARDSON'S. BLUEBOOK OF PROJECTION
from the light source and is 3 feet wide by 3 feet high and is therefore 9 square feet in area. A fourth screen located 4 feet from the light source reaching to the same boundary lines would be 4 feet high, and if it were also 4 feet wide would have 16 square feet of area.
(4) Light travels in perfectly straight lines through any transparent medium that is of even density throughout.
(5) It changes its direction only upon encountering a medium of different density at an angle.
Barring a change of direction the rays will travel as indicated by the black lines within the space bounded by the outer lines so that if screen A is removed the rays will cover screen B exactly and if both screens A and B are removed, they will cover screen C, and so on indefinitely.
(6) Considering these facts we discover that, since screen B has four times the area of screen A, doubling the distance quadruples space the rays will cover. The illumination of screen B will be only one-fourth as brilliant as screen A, assuming that both screens have equal powers of reflection. Stated differently, the surface of screen B is four times as great as that of screen A. The light has equal power of illumination at both screens but at screen B it is spread over four times as much surface. Hence the illuminating power per unit area is decreased in the ratio of four to one. If another screen square in shape and reaching to the same diverging boundary lines were located four feet from the light source, it would be 16 square feet in area. The ratio of decrease in illumination would then be 16 to 1. An understanding of this law is especially important in connection with the distance of the light source from the collector lens or mirror.
(7) Projectionists are called upon to handle very difficult optical systems. This is particularly true of the motion picture projector, in which two separate optical systems are combined, one of them highly corrected and one either not corrected at all or only partially so. (8) These systems are so joined that the first picks up