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288 RICHARDSON'S BLUEBOOK OF PROJECTION
another lose loop in the film, below and to the right of the intermittent sprocket; beyond this loop the perforations are engaged by the lower or "take-up" sprocket. Below this again the film passes over a "pad roller" — a free running rotating member without sprocket teeth, and then enters the soundhead at the bottom of the illustration.
(13) In the soundhead there are other sprockets moving the film downward, and below the soundhead it enters the lower magazine where it is wound up on another reel. The lower magazine is much like the upper magazine, with one exception. The spindle on which the wire reel is mounted cannot be free-running, it must be forced to rotate so the reel will wind up the film as it moves down from the soundhead. The spindle is made to rotate by a device known as the "take-up."
(14) The take-up, the three sprockets in the projector mechanism, and whatever sprockets there are in the soundhead, all are mechanically linked to, and thus driven by, the projector motor, a portion of which can be seen at the extreme lower right of Fig. 125. The upper and lower sprockets of the illustration, and all the soundhead sprockets, revolve in unison. The intermittent sprocket does not. It revolves intermittently — start-and-stop, start-and-stop — stopping 24 times a second. The mechanical arrangements that secure this result will be explained shortly.
(15) While the intermittent sprocket is in motion a shutter, not clearly shown in this illustration, intercepts the light beam. Otherwise the picture on the screen would be blurred. When the intermittent sprocket stops, the photograph before the aperture at that moment becomes motionless; then the shutter moves out of the way and allows light to shine through that photograph to the screen, projecting one photograph.
(16) When as many as 24 photographs per second are projected in this way, sandwiched by intervals of darkness; and when any object occupies a slightly different position in each successive photograph, an optical illusion is produced whereby the object appears to be moving.