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514 RICHARDSON'S BLUEBOOK OF PROJECTION
second, it responds (for reasons made plain below) by creating a thousand-cycle pulsation in the direct current flowing through it. If the opaque and transparent portions of the sound track are more crowded, and succeed each other five thousand times a second, the photo-cell creates a 5,000-cycle ripple in its direct current.
How the Photo-Cell "Sees" Frequency
(5) Consider a high-pitched sound, say of 9,000 cycles (which is 18,000 alternations), photographed on a sound track. The photo-cell must see 9,000 transparent areas and 9,000 dark areas in one second's time if it is to create a 9,000-cycle pulsation in its direct current. The film moves downward at a uniform rate of 18 inches per second. Therefore if 18,000 different areas (9,000 light and 9,000 dark) are to pass through the sound gate in one second's time, each of these light and dark areas must occupy exactly one one-thousandth of an inch along the length of the film. (6) Now if the dark areas are to cut off the light completely, and the transparent areas are to transmit it completely, then the light beam itself must obviously be not more than one one-thousandth of an inch in height.
If 4,500-cycle sound were to be considered satisfactory as the upper limit of reproduction, then the light beam would need to be focussed to only 1 /500th of an inch. But in every case the light must be brought down to some definitely narrow line at the point where it crosses the sound track. If this is not done at all, if a flood of light, say, an inch high, is permitted to pass through the film to the photo-cell, then it is plain that nothing higher than 18-cycle sound can be reproduced.
The function, therefore, of the slit assembly diagrammed in Figure 117 is to reduce the height of the beam of exciting light to the dimensions necessary for highest frequency of sound desired.
The exciting lamp, which co-operates in this work, is unusual in that its filament appears as straight, horizontal line of wire. The light from this straight line of incandescence passes through the condenser lenses of Figure