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THE CINEMATOGRAPH CAMERA 47
the action of the cross is as follows: The central pin wheel 'A* is the driving member and, if rotated in the direction indicated, extension 'D' will eventually engage with slot 'E\ cut in cross 'B\ and cause it to rotate until slot 'E' occupies the position *F\ At the same time, and in synchronism with this action, pin 'G' will engage slot 'H\ cut in cross 'C, and rotate it from 'EP to T. Sprocket wheels are attached to the shafts which support both crosses and the film is drawn tightly between them as shown at 'L\
The sprocket wheels will both rotate in an anti-clockwise direction. The section of film between the sprockets is utilised as the 'gate' of the camera and is not held under any more tension than the guiding effect supplied by the sprockets themselves. It may be assumed, however, that some type of 'back-plate' was used to ensure an adequate control over the focal plane.
Although this is a very neat arrangement, and one which would permit the construction to become very small, it has several disadvantages. Firstly, it would require exceptionally accurate workmanship to manufacture two crosses which would both 'mesh' perfectly with a central driving pin, and, supposing this could be accomplished, such an arrangement would only succeed with film which had been perforated to an equal accuracy and remained free from shrinkage. Quite obviously such objections could be overcome by spring loading one sprocket with respect to its driving cross. Unfortunately such a move would tend to destroy the desired results, namely to obtain adequate control over the film without the use of complicated gate and tensioning devices.
The next noteworthy step was made by Arthur S. Newman when Messrs. Blair & Danelo requested him to modify a camera operated by the 'dog' principle, that is, a movement similar to that shown in Figure 10. Details of this modification are not available, but it is known to have included the addition of 'pilot pins' in the film gate. Pilot pins are used in almost every camera these days, and their purpose is to enter the film at the moment it is brought to rest by the intermittent mechanism. This entry is completed before the shutter is opened and the pilot pins remain stationary, controlling the film, during the exposure. The pins move away and out from the perforations after the shutter has closed and just before the intermittent starts to move the film forward once more. The action of these pins, and the accuracy with which they are made, will be discussed when modern cameras and equipment are dealt with. For the present it is worth noting that they were in use, like most essential parts of present day cameras, as far back as 1900. Another unusual feature of this camera was an oscillating shutter,