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226 RICHARDSON'S BLUEBOOK OF PROJECTION
Why the Second Blade
(67) As has been shown, it is essential that the light be cut off from the screen while the film is in motion over the aperture. That fact explains the master blade, but what is the purpose of the cut-off blade?
The science of optics has not yet given a completely satisfactory answer. The facts are that the periods of darkness must not be too long and the periods of light and darkness must be correct in length and of approximately equal duration in time. If we used but one shutter blade (the master blade) we would have a terrific flicker. If— when projecting 90 feet of film per minute —we add a second blade (called the cut-off blade) of approximately equal width with the master blade, and located at the opposite diameter from the master blade, so that periods of light and darkness are equally spaced and of approximately equal duration (in practice the light holds just a little longer than the dark intervals), all visible flicker will disappear. Not only that, but the impression of light from one period of illumination will carry over (persistence of vision) through each dark period, so that we shall have the illusion of constant screen illumination. (68) Moreover, the difference in position of the various moving objects in the picture will be so slight that they will blend together into another illusion, namely, that of continuous motion. Put into a different phraseology, each picture "fades" into the next (which as a matter of fact is a precisely correct statement), and thus the slight differences in position of moving objects is merged into continuous motion.
Setting the Shutter
(69) Setting the shutter is a very simple operation, once we remember that the function of the master blade is to cut off all light (cover the light beam) while the film is in motion over the aperture. First loosen the shutter so that it may be rotated on its shaft while the flywheel of the mechanism is held stationary. Then, by means of the flvwheel, move the mechanism in its normal