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STROBOSCOPIC-LIGHT PICTURES 285 machinery. There are, however, many problems that require a far greater speed than is practicable with the intermittent-type of cam- era, and a great deal of thought and effort has been applied to speed up the camera mechanism. Intermittent mechanisms were aban- doned for speeds above about 200 frames per second because of the mechanical difficulties involved in accelerating the film between pic- tures, and cameras that employed film moving at a constant speed were designed. There are two general methods of making exposures on a continu- ously moving film: (1) Those employing a moving optical system, which holds the image sta- tionary with respect to the film during the exposure time. (2) Those employing a source of stroboscopic light, the flashes of which are of sufficiently brief duration to produce a sharp image on a moving film. High-speed motion-picture cameras have been constructed utiliz- ing one or the other of the two methods, or both. Each method has advantages and disadvantages, which must be carefully considered with respect to the particular problem at hand. The first type of camera 2 ' 3 - 4 ' 5 ' 6 is especially adapted to studying subjects that produce their own light or are brightly illuminated, common examples of which are the burning of vapors, the action of explosives, the movements of an electric arc, the reactions in a photoflash lamp, and many others. The stroboscopic-light type of camera is of very limited use in the study of such problems. The principal advantage of the stroboscopic-light type of cam- era 7 ' 8 ' 9 ' 10 over the moving-optical-system type is that the exposure time may be made so short as effectively to stop the motion of rapidly moving objects. The stroboscopic light in its present state of de- velopment makes an exposure of a few millionths of a second, which is considerably shorter than is feasible by the moving-optical-system method; especially because as long an exposure as possible is usually desired in the moving-optical-system method in order to attain suf- ficient density of image on the film. In high-speed cameras employ- ing stroboscopic light, the film is moved past the lens at a constant speed; and each time the film has moved the distance occupied by one frame, the subject is illuminated by a short brilliant pulse of light. The time at which the flash occurs is controlled by a com- mutator rigidly attached to the film-driving mechanism, and the duration of the flash is so short that no appreciable blurring of the picture occurs. Normal illumination such as that encountered in-