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290 H. E. EDGERTON AND K. J. GERMESHAUSEN [J. S. M. p. E. ate heat and electrostatic charges. The camera shown in Fig. 1 is constructed in such a manner that during exposure the film lies against a large moving sprocket instead of against a gate. The sprocket must be large enough to prevent the curvature of the film from throwing the image appreciably out of focus, but not so large as to introduce difficulties in acceleration or to make the camera bulky and awkward. A consideration of those and other factors leads to a compromise, resulting in a sprocket of about 5 inches in diameter with twenty 35-mm. frames around its periphery; or, in the case of 16-mm. film, fifty frames. An aluminum roller is pressed by a spring against the sprocket in order to force the film down to the base of the teeth as it comes from the supply reel. A metal plow is located at the bottom of the sprocket to peel off the film from the teeth in case the take-up motor should not exert enough pull. Two square holes cut into the sprockets conveniently permit lining up the camera and adjusting the focus, since a clear view of the image cast by the lens on the film may be viewed through the film from the back. A telescope is mounted through the rear of the camera in such a way as to afford an enlarged view of the center of the frame for finally adjusting the focus critically just before starting the camera. The location of the pictures on the film is determined by the po- sition of the film at the instant the stroboscopic lights flash. For satisfactory projection the pictures must be accurately and definitely located with respect to the sprocket holes ; so, to accomplish this, a commutator is located on the same shaft as the sprocket, having as many contacts as there are frames around the sprocket. In order to eliminate vibration, the commutator must be carefully and ac- curately made, so that the segments are uniform and the surface is smooth. Only a small amount of power is needed to trip the electri- cal circuits, and therefore the brush may be light in construction. The brushes are adjusted when the camera is at rest, as there is no appreciable time-lag in the electrical circuits after the brush makes contact. The source of stroboscopic light that has made this type of camera possible consists of mercury-arc tubes through which elec- trical condensers are discharged. Its important properties are: (1) the light is actinic; (2) the discharge time is short; (3) the timing of the flashes may be accurately controlled by a small amount of power; (4) the tubes are relatively simple; and (5) as many tubes may be operated in parallel as desired.