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645 HANDBOOK OF PROJECTION FOR
is about one-fifth of one-sixteenth of a second, or 1/80 of a second. We may therefore say that at the rate of sixty feet per minute, 4/80 of a second is consumed by the actual projection of the picture, and 1/80 of a second is consumed in the removal of one photograph and the substitution of the next — in the pulling down of the film.
WHY THE LENS MUST BE CLOSED.— If, however, the change of photographs over the aperture be made with the light projected constantly to the screen, there will be streaks of white up and down across its surface. The reason of this is as follows : White makes a greater impression on the eye than do colors of less brilliancy. Suppose we project a moving picture to the screen without any revolving shutter — with the light projected constantly. Suppose in this scene there is a man in evening dress, with a broad expanse of white shirt front. The dark colors of the evening dress make little impression on the eye, but the dazzling white of the shirt front makes a very great impression, and as the film moves and the figure of the man in one photograph is substituted for the figure of the man in the next, as the figure of the man in the first photograph is jerked down out of the way the eye will see and follow the brilliant whiteness of the shirt, though it will not see and follow the darker clothing. Also, as the other figure comes into view the eye will quickly catch the white of the shirt, and not see the dark clothes until the figure comes to rest. We would therefore have a white streak across the screen. These white streaks are technically known as "travel ghost."
Due to this phenomenon, it is necessary that the lens be closed during the time the intermittent movement is in action and the film moving, and this is the function performed by the revolving shutter.
The revolving shutter of a projector (except in the case of the one and one-and-a-half-to-one shutter, the action of which will be explained further on) revolves exactly once to every complete cycle of the intermittent movement. If a certain edge of the master blade of the shutter occupies a certain position with relation to the lens when the intermittent begins to act, it will occupy precisely the same position the next time the intermittent begins to act, having meanwhile made one complete revolution.
Without any film in the projector, open the gate, block the automatic fire shutter up and project the white light to the