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THE CINEMATOGRAPH CAMERA 45
films screened by equally uncertain projectors. As would be expected, the improvements to camera and projector mechanisms were both gradual and inter-dependent. In fact, at the outset, the mechanism used in the camera was often identical with that used in the projector.
Early Mechanisms
One of the earliest camera mechanisms which enjoyed any measure of success was manufactured by Demeny and employed the eccentric pin, or 'dog movement', seen in Figure 10. The mechanism consisted of a spring-loaded gate channel 'F', at the lower end of which was mounted a free guide roller 'B\ From this point the film 'D' was led directly to the continuously rotating sprocket wheel 'C, around which it was guided by rollers 'G'. Between gate roller 'B' and the sprocket 'C a circular flange 'H\ carrying an eccentric pin 'A', was arranged to make one complete revolution for every picture length drawn forward by the sprocket. The mechanism was so adjusted that the pin 'A' would just touch the film as it formed a straight line between roller 'B' and sprocket 'C\ Since the eccentric pin travelled in the opposite direction to the sprocket wheel the film would be pulled through the gate 'F' at a speed and by an amount dependent upon the combination between the action of the eccentric pin and the direct forward pull of the sprocket.
The eccentric pin caused the film to form into a looped path, as indicated at 'E', and then to pass out of contact on reaching position 'J' until it again occupied position 'A'. The loop so formed then supplied a reservoir of film from which the sprocket 'C could draw its supply until the loop again returned to a straight line. By such time the pin would have travelled from position 'J' to position 'A', and the cycle of operations would then be repeated. A considerable tension was applied to the film held between the gate sections at 'F' since this was the only means available to bring the film to rest after the 'dog' had formed the loop.
Although this mechanism certainly provided an intermittent motion of the film at the gate, it suffered from many serious disadvantages. The variations in film shrinkage were, at that time, so great that the pictures did not remain in step with the perforations and consequently appeared to creep either up or down the screen when projected. Attempts were made to overcome this tendency by providing means to adjust the throw of the pin 'A' to suit the various shrinkages encountered. Although the 'dog'