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68 MOVING PICTURES chap. ride over any variations in the thickness of the films should such exist—a remote possibility in view of the wonderful mathematical accuracy with which the films are made to-day. It is impossible for the film to jump the sprocket wheel, even when the machine is run at a very high speed. The contact roller is so mounted, however, that it offers no resistance to the free passage of the film. The film now passes behind the lens for exposure, travelling through what is known as the "gate," which consists of two parts, one fixed and rigid, the other having a slight swinging movement. By means of the gate the film is guided evenly into position, and held rigidly behind the lens during exposure. The swinging part of the gate is clamped to the fixed section by means of a spring catch, so that when the gate is closed the film is imprisoned between two sheaths which hold it firmly, and keep the surface flat and even during exposure. The end of the focussing tube C presses against the moving part of the gate and tends to hold it rigid. The film is pulled through the gate by an ingenious plan. There are two claws or hooks jF, the points of which engage with a perforation on either side of the film. These two fingers are controlled by an up-and-down cam move- ment attached to the bottom sprocket. As the latter revolves, the hooks are lifted and carried upward. Reach- ing the limit of their upward travel, they drop upon the film, clutching it by the perforations, and as the claw action has a sharp downward movement, the gripped film is pulled down suddenly and smartly through the gate three-quarters of an inch at a time. The extent of the downward travel reached, the claws are lifted so as to disengage with the film, are swung upwards once more, and the same cycle of operations is repeated until the whole length of film has been exposed. After the exposure is made, the film passes downwards and through mechanism which is almost a duplicate of that immediately above the lens. It passes under the rocking contact roller, which keeps the perforations meshed with the teeth on the sprocket wheel, after pass-