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THE PROJECTOR 225
Why a Shutter is Necessary
(63) The human eye has the ability to retain upon its retina the impression of any light incident thereon for a small fraction of a second after the light itself has vanished. This "persistence of vision" makes motion pictures possible.
Darkness is invisible, except as a contrast to light. (64) The light upon a screen surface is the only thing that is visible to or makes any impression upon the eye. Looking at a screen we see certain shades of light thereon, while at the same time certain portions of the screen surface are more or less invisible. It is the pattern formed by various shades of light that forms the screen image.
(65) If we were to remove the rotating shutter and project a reel of film, we would see very little picture. In its stead would be a series of more or less jumbled streaks of light. As the film moves across the aperture the transparent areas (light spots) move with it, and, with the shutter removed, the eye sees them move in the form of streaks of light. Due to persistence of vision, the bright spots in the picture will be impressed upon the eye while the film is at rest, and the eye will also see them while the film is in motion over the aperture, very largely obliterating the shadings that form the picture on the screen.
(66) It is obviously necessary to cut off all light from the screen while the film is in motion over the aperture,* and that is exactly what the master blade of the shutter does. Each time the intermittent sprocket comes to rest, the master blade moves away from in front, "opens the lens," permitting light to pass through.
We have cut the light off the screen while the film is in motion over the aperture, but to the eyes in the audience the fact is not apparent. Why this is so we shall presently explain.
*It has been found that, without affecting the screen image, the lens may be about one-third open when the intermittent sprocket starts to move, and may be about the same distance open when the sprocket finally comes to rest, which of course adds to the total of screen illumination.