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THE MOTION PICTURE CAMERA
gular cam with curved sides, working between two guides which it moves up and down as it revolves. As it accomplishes the downward movement of the fingers in a third of a revolution it permits of a larger shutter opening than any other movement in general use.
The Geneva, or Maltese Cross movement has been used in camera construction, and while it gives a quicker downward pull of the film than the harmonic cam, it has several disadvantages which preclude its use. In ihe Geneva movement the downward draw of the film is accomplished in about an eighth of a revolution, but, as this movement has four bearing surfaces which are liable to wear unevenly it has not found much favor as a camera movement. Should one side, for instance, wear a trifle more than the other three sides, every fourth picture in the negative would be slightly out of register with the other three. In addition to this, slight variations in the thickness of the negative film, or its pliability, cause it to ride the intermittent sprocket more or less snugly, causing a variation in the frame line, or an up and down movement of the picture.
The harmonic cam, on the other hand, revolves once for each frame taken. Any small amount of wear, being the same for each successive picture, is not appreciable. This wear may be readily taken up in most constructions by loosening two screws which hold one of the guides between which the cam runs, and the guides may be adjusted firmly against the cam. The shutter opening with the Lumiere movement may be greater than i8o degrees, which is much more than any other movement in common use. The shutter blade could be reduced to 120 degrees were it not for the fact that it must have an additional width sufficient to cover the aperture opening, so that the smallest shutter blade that can be used in any movement is that fraction of a revolution during which the film moves downward plus a segment wide enough to completely cover the aperture opening from corner to corner. The Pathe, Prevost, the Universal, the Gillon, and many other makes of cameras, use the harmonic cam.
Almost all other movements are some variation of the rod and crank principle. That is, a rod, or other connection, fitted to a crank pin on the shutter shaft actuates the up and down movement of the claws. Since the downward movement of the crank is one-half of a revolution, no rod and crank motion can
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