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THE MOTION PICTURE CAMERA
sort of spring pressure. A positive in and out movement is one in which the pins are pressed in and withdrawn by a mechanical movement, such as a cam or drunken screw. In the Pathe or Gillon types the in and out movement is accomplished by a drunken screw. A drunken screw is a thread having an irregular pitch, the thread used for the in and out movement being a continuous one with the contours so placed as to force the pins into the perforations at the beginning of the downward throw of the cam and withdraw them at the bottom of the throw. In the Prevost movement the in and out throw of the pins is accomplished by small harmonic cams of the same design as the larger cam which produces the up and down movement. Most of the rod and crank types of movement have a cam working against a spring to push the fingers in and out, the cam pressing the fingers in and the spring pushing them out when released by the cam. It is possible to operate a movement of this type so fast that the spring does not have an opportunity to withdraw the fingers quickly enough, thereby causing creeping and losing of the loop. The Ememann camera has a rod and crank movement with cam and spring for the in and out finger movement.
There are many types of movement beside those mentioned, none of which, however, are enough in general use to justify discussion here. In purchasing a camera, therefore, make sure that the movement is some modification of the harmonic cam with a positive in and out movement of the claws. A second choice is one of the better types of rod and crank movement. The DeBrie camera is one of the highest type of rod and crank movement.
As nearly all parts of a camera movement shift backward and forward i6 times per second they are subjected to a great deal of wear. All of these parts subject to wear must, of course, be kept constantly but lightly lubricated, and should be provided with means for adjustment so that there is the least possible amount of play between bearing surfaces. The finger shuttle, that is a frame bearing the fingers, which moves up and down, is carried in some sort of guides which should be provided with adjustable gibs for taking up wear.
The shutter is the revolving blade which cuts oflF the light from the lens while the film is being moved downward for the next picture, or exposure. The circular revolving shutter is so
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