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MANAGERS AND PROJECTIONISTS 646
screen while you run the projector very slowly. You will observe that during the time the intermittent sprocket is in motion the master blade of the revolving shutter cuts off all the light from the screen. You will also observe that, according to whether you have a two-wing or a three-wing shutter, all the light is cut off from the screen two or three times during each complete cycle of the intermittent, or during each revolution of the shutter.
In projection, what we therefore have on the screen is a succession of flashes of more or less brilliant light, and a succession of almost equal periods of time when no light from the lens reaches the screen.
After running very slowly, as above directed, gradually increase the speed of the projector and you will find that when you get up to normal projection speed there will be an apparently uninterrupted screen illumination.
WHY MORE THAN ONE BLADE.— What we call "flicker" is the visibility of the period during which the screen is dark. The revolving shutters of all motion picture projectors have more than one blade. The reason for this is that, since light interruptions must be at the rate of 36 or more per second in order to render them invisible, the one-blade shutter would not be practical, because the light interruptions would come too far apart. As already explained, in motion picture projection the screen is alternately more or less brilliantly lighted and totally dark, insofar as light from the lens is concerned.
FLICKER. — The human eye is a peculiar instrument. It will transmit to the brain, as separate impressions, a certain number of impressions per second. Beyond that number, the impressions become merged into each other, so that the effect is that of continuity. This involves what is termed "persistence of vision," which is the peculiarity of the eye which makes the illusion of "moving pictures" possible. If the flashes of light and darkness come too far apart, or if they be disproportionate to one another, then the eye will perceive them. Under this condition persistence of vision operates incompletely, and instead of the illusion of even, steady illumination, the recurring flashes of light and darkness will be perceived in the form of what is termed "flicker." Flicker is a very serious matter indeed, in that it causes eye strain exactly in proportion to its amount. If excessive the strain on the eyes is very great and very highly injurious. Experience has taught that with a low illumination value, such as is repre