Instruments for testing the speed and uniformity of action of focal-plane shutters are an essential part of any laboratory for developing or testing photographic apparatus and some simple device for setting and checking shutter speed should be available in the field. Every such speed tester must contain some form of time counting element—pendulum, tuning fork or clock-work. Elaborate shutter testers, suitable for determining all the characteristics of all types of shutter, have been developed and used in certain of the photographic research laboratories. For the study and setting of focal-plane shutters (whose efficiency need not be measured, as it can be simply calculated from linear dimensions), the following simple kinds of apparatus are adequate: [Illustration: FIG. 23.—Apparatus for testing focal plane shutter speed throughout the travel of the curtain.] _Clock dial type of shutter tester._ This consists essentially of a black clock dial carrying a white pointer which makes its complete revolution in one second or less. If this dial is photographed by the camera under test, the width of the sector traced during the exposure by the moving pointer shows the time interval. If the dial is photographed at several points on the plate—beginning, middle and end of the shutter travel—the complete characteristics of the shutter can be determined. _Interrupted light type of shutter tester._ For the study of uniformity of shutter action alone the apparatus shown in Fig. 23 may be employed. _A_ is a high intensity light source, such as an arc or a gas filled tungsten lamp. _L_ is a convex lens, focussing an image of the light source on a small aperture in the screen _E_. _D_ is a sector disc which, driven by the motor _M_, interrupts the transmitted light with a frequency determined by the number of openings of the sector and by the speed of rotation, which must be measured by a tachometer. The light diverging from the aperture in _E_ falls upon the shutter _S_, which for this test is reduced to a narrow slit of one millimeter or less. Passing through the shutter opening the light falls upon the photographic plate _P_. The principle is simple: If the light is uninterrupted, the plate _P_ is exposed at all points; due to the interruptions, a series of parallel lines of photographic action result, and their distance apart gives a measure of the speed of the shutter at any chosen point in its travel. A performance curve of the French Klopcic shutter is shown in Fig. 24. The variation in speed lies over a range of two to one. So serious is this defect in these shutters that diaframs are sometimes inserted in the French cameras to cut off part of the light from the lens on the most exposed end of the plate. This expedient produces uniformity of photographic action, but does not overcome the movement of the image, which is one of the chief faults of excessive exposure. [Illustration: FIG. 24.—Performance of Klopcic shutter.] [Illustration: