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THE PROJECTOR 227
the point of starting to move. Be sure to get it exactly
at the point of starting. Next, rotate the shutter on its shaft in its normal direction, meanwhile holding the flywheel motionless, until a point is reached where the forward edge of the master blade is approximately twothird of the way across the projection lens. Tighten the shutter on its shaft and the job is done, though some slight adjustments may be needed after trial.
Before starting to set the shutter of a projector equipped with a shutter self-setter, be sure to place this device in the center of its movement.
Rotating Shutter and Intermittent Speed
(70) Knowing that the shutter must have two blades, and that the flashes of light and darkness on the screen must be of almost equal duration (there may be some variation, but too much will result in flicker), it is evident that the narrower the shutter blades are the greater is the amount of light that will pass through to the screen. What then determines the necessary width of the master blade, and as a consequence the width of the cut-off blade?
You will recall that the master blade must be wide enough to "close the lens" while the film is in motion over the aperture, although, as indicated, a slight opening is permissible at the start and stop of the intermittent sprocket. It is evident, then, that the shorter the time the intermittent movement is in motion the shorter the time the lens must be covered. (71) Put another way, the faster the speed of the intermittent movement and, therefore, the intermittent sprocket action, the less time must be given to covering the lens and therefore more light will pass through to the screen.
The projectionist can do nothing to alter the intermittent speed. Most movements are now what are known as "three-to-one." One complete rotation of the cam carrying actuating pin D, Fig. 62, is one complete cycle of action. Divide that action into four equal parts: the intermittent movement star, and therefore the intermittent sprocket mounted on its shaft, is in action moving