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286 H. E. EDGERTON AND K. J. GERMESHAUSEN [J. S. M. P. E. doors is insufficient to fog the film in a stroboscopic-light camera be- cause the film passes the lens so rapidly. When motion pictures are taken at high speed with any type of camera* the film must move rapidly, and one of the important problems in the design of either type of cameras is to make the film travel at the requisite speed without vibrating, fluttering, or break- ing. The rapidly moving film must be guided properly, but the friction in sliding contacts may generate enough heat to ignite it. Static charges of electricity resulting from the friction must also be avoided, as they cause dendriform exposures on the film. Further than simply traveling smoothly at a high speed, it is important that the film accelerate rapidly so that it will attain the proper speed be- fore much of it has passed through the camera. The acceleration must be uniform as well as rapid, as sudden jerks are likely to break the film. TABLE I Speed of Film in Terms of Frame Height and Rate of Exposure Frame Exposures per Second Height, 500 1000 2000 4000 8000 16,000 32,000 64,000 (inches) Film Velocity (Ft. per Sec.) 0.75° 31.25 62.5 125.0 250 500 1000 2000 4000 0.30 6 12.50 25.0 50.0 100 200 400 800 1600 0.15 6.25 12.5 25.0 50 100 200 400 800 0.075 3.175 6.25 12.5 25 50 100 200 400 a Standard 35-mm. frame height. 6 Standard 16-mm. frame height. The height of the frame as well as the rate of making exposures is a factor influencing the film speed, since it is not necessary for the film to move as rapidly for a small frame as for a large one. Most of the very high-speed pictures are small in size, in some cases so minute as to be of little use in presenting detail even after enlarge- ment. Conversely, if the film speed is increased in order to produce larger pictures, the camera becomes so bulky that it is no longer port- able, and the subjects must be brought to the camera —a serious limitation to its usefulness. Table I shows the speed of the film in feet per second as a function of the height of the frame and the num- ber of exposures per second. * Possible exceptions are those high-speed cameras employing stationary film and a large number of lenses, 11 or a number of spark sources of light (method of Cranz, 8 described by Ende 6 ).