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MOTION PICTURE PHOTOGRAPHY
the standardized distance of three-quarters of an inch apart or sixteen pictures per foot. Each successive picture when thrown upon the screen will be as nearly as possible in perfect register, that is in exactly the same place upon the screen. If this is not done an unpleasant jumpiness or wavering of the picture will result.
In recording, that is in photographing, a motion picture at the rate of sixteen per second, there are several operations in making each frame or picture which must be accomplished in onesixteenth part of a second. It is not possible to utiHze all of this sixteenth part of a second in making the exposure because the film must be drawn down into position for a succeeding exposure before the next sixteenth part of a second. During this very short period of time it is necessary to cut off the light from the lens by means of the shutter, draw the film down accurately just three-fourths of an inch, hold it in place, and expose it to the image from the lens long enough to impress that image upon the sensitive surface, then completely cover the film exposed in the frame aperture before repeating this cycle of operations. All must take place in the sixteenth part of a second.
It will be appreciated that a mechanism which fulfills these conditions must be accurately and substantially constructed and be able to perform this cycle of operations many thousands of times without appreciable wear. It is possible to construct an intermittent mechanism which will draw the film down so rapidly that only a fifth or sixth part of this sixteenth of a second is used in changing the film, but such a mechanism wears out many times more rapidly than one which takes a longer time to pull the film down for the next exposure.
In constructing a camera, therefore, it has been the generally accepted practice to use an intermittent mechanism, comparatively slow in moving the film and to make up for its slowness by increasing the "rapidity" or "speed" of the film. Although these words are not correct, they are often used to indicate the sensitiveness of the photographic emulsion. Sensitiveness of the film is its ability to record the lens image in a given time.
There are many types of camera movement, but the best of these is probably the harmonic cam. This is often called the Lumiere, or the Lumiere-Carpentier movement, as it was first used in a camera of that name. The harmonic cam is a trian
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