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1949 BRITISH HIGH-SPEED CINEMATOGRAPHY 511
There is, however, one point which does not seem to have been made in the literature on high-speed photography so far. It is that the use of the technique is frequently limited by the fineness of detail which has to be recorded. Engineers requiring to make use of high-speed photography often do not realize the fact that although the object to be photographed is plainly visible to the camera it does not necessarily mean that the movement in which they are interested will be easily discernible from the photographs produced. The amplitude of movement of the subject may be so small that the high-speed camera has great difficulty in resolving it. With modern equipment, the visual resolving power of the optical system expressed in lines per millimeter appears quite high, but the photographic resolving power is reduced by low contrast in the final photograph, which is accentuated by underexposure, and further degraded by imperfect compensation for film movement, and movement of the subject itself during the exposure. Thus, relatively small movements may be difficult even to detect, let alone accurately measure, unless they can be amplified in some way or examined against some fixed point.
The vibration of a mechanical sieve, which appeared to have quite a large amplitude, was extremely difficult to measure from a highspeed film record made of part of the sieve itself. The problem eventually was solved by embedding a bright steel ball in the surface of the sieve and illuminating it with a bright source of light, thus producing a point source of light. By running the high-speed camera at a relatively slow rate, this drew out a trace which could be seen on the screen and variations in its shape, which were quite marked, gave the required information on the changes in the form of vibration of the sieve, which could not be detected at all from a film taken of the sieve directly, and without resort to this optical trick.
Another example concerned the examination of the joints in railroad lines, and the movement of the fishplates which are used to secure the rail ends together. As a locomotive wheel passes over such a joint, there is a movement which appears to the eye to have considerable' amplitude. Nevertheless, a direct record taken with a high-speed camera showed nothing which could be measured accurately. This problem was solved simply by placing a dial gauge, calibrated in thousandths of an inch, between the underside of the top flange of the rail and a large mass of concrete which was laid under the track. The readings of this dial gauge could be ascertained easily from a film taken of it as the wheel passed over the joint, and the movements of