SUBJECTIVE SHARPNESS OF IMAGES 583 



to break up the image of the lamp filament which otherwise would have 

 been formed near the principal planes of the projection lens and would 

 have destroyed the uniformity of illumination within the figures of 

 confusion of the out-of-focus image on the screen. The screen 

 brightness was about 10 foot-lamberts with the projector running 

 without film. 



Projection Machine 



Acme Portable, with two-bladed shutter. There was no provision 

 for reproducing the sound track. The screen imag^, in sharp focus, 

 was said by competent judges to represent very good motion picture 

 projection practice. 



Projection Lens 



Bausch and Lomb Series "0", 6.00-inch focus. There was fitted 

 over the lens barrel a brass ring with an extremely sharp turned edge 

 to serve as an index for the measurement of lens displacement. The 

 lens could be set to the nearest 0.0003 inch by means of the focusing 

 mechanism. The image was put out of focus by moving the lens 

 toward the film. At sharp focus the linear magnification was 12 times. 



Measuring Microscope 



Mounted rigidly on the frame of the projector, and fixed with respect 

 to the film gate. The micrometer scale was focused on the index mark 

 on the barrel of the projection lens. A lens displacement of 0.060 inch 

 caused the index to traverse 50 divisions of the scale. 



External Aperture 



An adjustable black paper mask mounted 1| inches from the prin- 

 cipal planes of the lens, on the screen side. The opening was rec- 

 tangular, with sides horizontal and vertical, of constant area 0.49 square 

 inch. The ratio of height to width could be varied continuously from 

 2.5 to 0.40 without changing the area. The opening was uniformly 

 filled with light under all conditions. 



Viewing Screen 



White Bristol board, 7.2 inches high by 7.6 inches wide (the image 

 size of an available television receiver to be used for comparison). 

 The screen was hung at the back of a black- velvet-lined box 18 inches 

 high, 22 inches wide and 12 inches deep. The viewing distance was 

 always 30 inches. 



The viewing room was completely darkened except for a little stray 

 light from the projection machine. 



