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643 HANDBOOK OF PROJECTION FOR
engages with upper sprocket A, Fig. 227, and is clamped thereby to one or more idler rollers, B. The film may pass over the upper sprocket as is shown in Fig. 227, or it may pass under the sprocket. Having been attached to upper sprocket A it is carried down over the aperture, as shown at C, with sufficient slack film to form upper loop D. The film is then engaged with intermittent sprocket E, and is locked thereto by the idler, which may be a roller or may be a shoe, after which the gate F is closed and locked shut. The film is now engaged with lower sprocket G, Fig. 227, sufficient slack film being left to form lower loop H, Fig. 227. The film is locked to lower sprocket G by means of idler I, after which it is carried down through the fire trap into the lower magazine and attached to the take-up reel. Upper and lower loops D and H must have sufficient slack film so that when the intermittent acts the film between the upper sprocket and the top of the gate, and between the intermittent sprocket and the lower sprocket, will not be pulled tight. The reason for these loops is that whereas upper sprocket A runs continuously, pulling the film out of the upper magazine and feeding it to the intermittent, and lower sprocket G runs continuously, taking the film away from the intermittent sprocket and feeding it into the lower magazine, sprocket E only acts intermittently, so that the strip of film between the top of the gate and the intermittent sprocket is standing still about four-fifths of the time, and moving at high speed the rest of the time, its high speed while in movement exactly equalling the total travel of the rest of the film. In other words, while upper sprocket A feeds three-quarters of an inch of film in a given time, which we will assume to be 5/80 of a second, the film between the upper end of the gate and the inmittent sprocket moves exactly three-quarters of an inch in 1/80 of that time, standing still 4/80 of the time. The offices of the upper and lower loops are to enable the constantly running film above and below to join with the intermittently running film over the aperture.
GENERAL INSTRUCTION NO. 21— THREAD IN FRAME. — Modern practice demands that the film be threaded in frame. By this it is meant that when threading the projector the projectionist must so place the film that one whole photograph will register exactly over the aperture, neither the upper or lower frame line being visible. This can be accomplished in several ways. The projectionist may look through the lens while placing the film, or he may hold a