FIG. 25.—Optical system of shutter tester for Air Service, U. S. Army.] A more complete apparatus, adapted both to absolute speed determinations and to the study of uniformity of action, is that worked out and used in the United States Air Service (Fig. 25). At _A_ is a high intensity light source, an image of which is focussed by the lens _L__{1} upon a slit _E_, in front of which stands a tuning fork _T_, of period 1024 or 2048 per second. The light diverging from the slit is received by a second lens, _L__{2} which is arranged either to focus the slit image upon the shutter curtain or to render the rays parallel, so that an entire camera may be inserted. In the latter case the camera lens _L__{3} serves to focus the slit image on the curtain _C_. After passing through the curtain aperture the light is focussed by the lens _L__{4} on the rotatable drum _D_, which carries a strip of sensitive film. The operation of testing a shutter consists in focussing the slit image on the portion of the shutter whose performance is required, striking the tuning fork to set it vibrating, rotating the drum rapidly and setting off the shutter. There is thus obtained on the sensitive film an exposed strip resembling in appearance the edge of a saw, the number of teeth showing the time interval in vibrations of the tuning fork. Three exposures usually give all the points necessary for a practical knowledge of the shutter's uniformity of action. A point of some importance, learned from numerous shutter tests, is that a focal-plane shutter should be tested in the position in which it is to be used. Aerial camera shutters should be tested in the horizontal position. =Types of Focal-plane Shutters.=—A variety of means have been utilized for securing the necessary variation in speed in focal-plane shutters. Their success is to be measured by the actual speed range and by the uniformity of speed attained. In aerial cameras at present in use we find _variable tension_ of the curtain spring, the aperture being fixed; _variable opening_ with fixed tension; _multiple curtain openings_ with fixed spring tension; and combinations of two or all of these methods of speed control. The problem of covering the aperture during the operation of winding up or setting the shutter has led to further elaborations of shutter mechanism. These take the form of _lens or shutter flaps_, _auxiliary curtains_, and shutters of the _self-capping_ type. Shutters embodying all these features are briefly described below.