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FILM TRANSMISSION EQUIPMENT 115 the projector. The film is then pushed into the space between the sprockets and the pad rollers and then into the film gate opening. It is important to ensure that the film is not caught between the pressers and the sprockets and also that it is properly engaged with the teeth on the sprocket wheels. After the film has been centered in the gate (that is, when it is squarely in the center of the light opening), the gate may be closed. If the film is not centered prop- erly, the frame lines will show during projection and this will necessitate racking. This is an adjustment which centers the frame in the mask opening at the gate. It is possible for the picture to get out of adjustment during projection. Then the bottoms of legs and feet are seen at the top of the screen and people appear to be foot- less ! The same racking control is also used in that instance to cor- rect it. Probably the thing which causes the most grief to the beginner in a projection room is the two loops, one above and one below the picture gate. If either of these is lost, trouble results in the form of jerky projection. Quite often it is the lower one which is lost. When this happens, the bottom drive sprocket pulls the film through the gate continuously, snatching it off the intermittent pull-down claw with probable damage to the film perforations and certain ruin for the picture on the screen. Often, to save a show from a stoppage, it is possible to insert a finger between the lower edge of the gate and the sprocket, and by a sharp, steady thrust against the film pull sufficient through from the top loop during the time that the pull- down claw is retracted and the film in the gate is therefore free. However, it should be emphasized that this is dangerous both to the film, which may suffer torn perforations if the claw should happen to emerge just as the film is being pulled through in this way, and to the fingers which could be badly lacerated if they came into contact with the revolving sprocket teeth (these teeth are rather like small, circular saws and can cut through flesh very easily). This method is neither recommended nor advised but used to illustrate an emer- gency cure. When threading the projector it is essential to maintain the proper size loops above and below the gate. The lower one which