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312
ARTHUR C. HARDY
[J. S. M. p. E.
but the illumination of the remaining portion would be unaltered. In other words, placing this mask at the gate would have exactly the same effect as moving in the black border surrounding the screen, a result to be expected from the fact that the two planes are conjugate to each other. On the other hand, if the same mask were placed at the projection lens, the illumination of the entire screen would be reduced as shown by equation (3), but the illumination would still be nearly uniform over the entire picture area. The hole in the mask is then said to operate as an aperture stop as opposed to a field stop. Although the insertion of such a mask or aperture can serve no useful purpose in practice, we shall see later that the source of light or the condenser may produce substantially the same result, and a consideration of the effect of apertures is consequently in order.
Diffusing Glass Gate
Screen
FIG. 2. Effect of the stop positian.
Let us consider the effect of placing a small stop or aperture at some point on the axis of the optical system between the projection lens and the screen, as shown in Fig. 2. As this figure is drawn, this stop is the aperture stop of the system. Although this is obvious from the figure, it is easily proven in any case by substituting the effective area of the stop for the area of the projection lens in equation (3) , and the distance of the stop from the screen for the corresponding distance of the projection lens from the screen. If the resulting illumination is less than without the stop, the latter is the aperture stop of the system. We should normally expect the illumination to decrease toward the edge of the screen as the fourth power of the distance from the center of the stop. In this case, however, the presence of the projection lens in the system causes the illumination to decrease at an even greater rate because the effective area of the stop is reduced. Thus, as Fig. 2 has been drawn, light is received at the edge of the screen from only that portion of the stop through which the projection lens can be seen. If no portion of the projection lens can be seen through