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MOTION PICTURE PROJECTION diameter is 2.96567 inches, and the sixteenth part of this is .1853 inch. This is the distance from sprocket tooth to sprocket tooth, because there are sixteen teeth on the circumference of the sprocket. These considerations determine the lay out of the sprocket teeth. We now come to the film. A pitch circle of 2.96S67 multiplied by four, as there are four circumferences of the sprocket pitch circle in a so-called foot of film, gives a length of 11.86268 inches. In this length the sixty- four holes or perforations are to be made. Such are the general considerations regulating the perforating of films. The point to be remembered is that it is the diameter of the sprockets which fixes the film length for the sixty-four perforations corresponding to four revolutions of the sprocket, and to approximately the foot of film. Sometimes the shrinkage of the film is taken into consideration also. There is a saving clause. The film does not entirely or even half envelop any sprocket in its passage t 'lie machine. It only covers a small arc of the circle or circumference of the sprocket. It is therefore a matter of less moment to have the film exactly corresponding to the pitch circle, than if it passed com- pletel yor nearly completely around the sprockets. LEADER AND TRAILER OF THE FILM There should be some feet of film without pictures at the be- ginning and another such piece at the end of each film. If there is not, then several inches of film, perhaps a foot or more is wasted in the distance from gate to bottom reel. Again at the top the motion should cease before the end of the film leaves the upper reel. Mere would be a second section of wasted film if the film were printed from end to end. The piece of film pre- ceding the pictures is called the leader; the piece at the end, after the pictures have ceased is called the trailer. The Society of Motion Picture Engineers have recommended that each film shall be provided with a black leader and with a red. green or blue trailer. Why black is stipulated for the leader, while there is the choice of three colors for the trailer is not divulged in the re- port, from which this recommendation was taken. 260