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EVOLUTION OB' PROFESSIONAL CAMERA 57
occultations per second of a sufficiently brilliant source of light would not destroy the impression of continuous illumination of a screen upon which the light was projected.
In accordance with this established fact, it would have been necessary to photograph fifty pictures of a moving object each second, and to project them at the same speed at which they were taken.
This, however, involved serious difficulties because of the extremely high speed at which the intermittent movement would have to function and because of the shortness of the exposure time thus available. Inordinate consumption of film was also to be considered with a view to the economic factor involved and the bulk and weight of the film which would have been necessary for a picture of even a short duration. This problem was solved by reducing the number of pictures to be taken in a second to one-third of fifty or, to be more exact, sixteen per second, and to maintain the film stationary in the projecting machine during three periods of the projector's shutter movement. The condition of fifty occultations per second was thus realized for the synthesis of motion, while simplifying the problems involved in its analysis, and maintaining intact the synchronization of movement between the subject and the projected image.
The problems inherent to the intermittent motion of the film in the camera were nevertheless quite difficult to solve in consideration of the necessity of perfect registration and of the ease with which the photographic emulsion can be marred when submitted to friction.
In order to intermittently and positively move the film for an exact distance in its alloted path, Edison in his "Kinetoscope" and Lumiere in his "Cinematographe", conceived the idea of perforating the edges of the film and designing a mechanism which would be supplied either with a sprocket intermittently rotated by a Geneva movement (mostly used in projection machines) , or with two metallic teeth or fingers (for the camera) driven by the action of cams in such manner that they would alternately engage into and disengage from the perforations after completing a stroke corresponding to the length of film to be displaced.
The Edison perforations were rectangular in shape and their number was four for each length of displacement of film. The Lumiere perforations were round and at a distance from each other equal to the length of displacement of film.
There is little doubt that Lumiere, and perhaps Edison, were guided in the adoption of the film perforations by the successful operation of a similar system used in the Jacquard weaving loom. In this, an endless band of perforated paper was used to select the "simples" for each rising and lowering of the warp threads.
No matter what prompted the use of film perforations, this proved to be the most practical means of film control, and it could not be superseded by serious attempts of devising other systems, the most noteworthy of which was, perhaps, the Demeny cam and beater mechanism. This mechanism enjoyed very little success in spite of the perseverance displayed by the constructor, Gaumont, in his attempts to perfect it.