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Nov., 1934] SxROBOSCOPIC-LlGHT PICTURES 287 When film speeds higher than about 150 or 200 ft. per sec. are de- sired, the film is usually placed on the periphery of a drum. 3 ' 4 - 7 ' 8 ' 9 The length of film that can be used is limited to the circumference of the drum, but very high film velocities can be attained in this way more easily than with a long strip. There are no acceleration prob- lems, since the film may be brought up to speed as slowly as desired. Cameras of this type require a shutter that remains open during one revolution only, to prevent multiple exposure of the film. In motion picture photography a certain amount of blurring of the images of moving objects is desirable, since the blurring produces the proper impression to the eye when the pictures are projected. If, however, the pictures are to be used to obtain scientific or engineering data from a frame-by-frame study, then maximum sharpness is de- sirable. The exposure time of cameras employing a moving optical system usually is from one-half to two-thirds of the time elapsing between successive frames, although in some cameras the exposures may be longer than the interval between the pictures, as described by Jen- kins. 2 It is possible to design the optical system for short exposure, but this leads to the difficulty of lighting the subject adequately. Light of high intensity is accompanied by heat, which may prove detrimental to the subject. Shadow or silhouette photographs are often employed for high-speed motion pictures, because the amount of light required is much less than that required for taking photo- graphs by reflected light. However, this technic has its limitations, because the arrangement of the subject or the character of the re- sults desired may often prevent its being employed. The exposure time for the stroboscopic-light camera is usually less than Vioo.oooth of a second. It can not be made longer, as the motion of the film during exposure would blur the picture. Pictures taken with the stroboscopic-light camera will never for that reason show a moving object blurred by the motion of the object, except for such very high velocities as are attained by bullets or for close-ups of very rapidly moving subjects. The heat of the stroboscopic light is less than of a continuous light, as the light is extinguished between exposures. A stroboscopic light that has been extensively used is the electric spark in air 7 ' 8 ' 9 produced when an electrical condenser discharges into an open air-gap. The duration of the flash can be made as short as Vi,ooo,oooth of a second, or less. A spark-gap, however, pre-