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1949
BRITISH HIGH-SPEED CINEMATOGRAPHY
505
drilling holes in it. The whole box containing the film drum can be detached from the camera for taking to the darkroom. The film is held on to the drum by a phosphor-bronze clip, which was found satisfactory for speeds of revolution up to 3000 per minute. For the photography of arcs, the camera lens was protected by means of a sheet of plate glass so that, if it became sputtered, it could be cleaned or renewed.
The camera which problably exhibits the greatest novelty is that designed by Marley.1 It was primarily intended for the study of explosive detonations. For this reason, such a camera must be suitable for recording rapid changes in a subject of high luminosity for a very short period of time. The picture frequency must be very high,
Fig. 2 — Diagram of Brailsford and Shrubb camera which operates at a rate up to 2500 frames per second.
of the order 100,000 per second; and the exposure time for each picture, of the order of a few microseconds.
The camera (Figs. 3 and 4), consists of a series of 59 lenses, arranged round the periphery of a disk about 12 inches in diameter. A second fixed disk contains a series of slots, each one-thirty-second of an inch wide, forming diaphragms for the lenses, which are 3.5 inches in focal length and thus stopped down to an aperture of about //27. An annular ring containing a similar series of slots can be moved through a small angle to form a shutter to expose all the lenses instantaneously. It is actuated by three springs and its opening can be synchronized with the detonation by an electromagnetically operated release gear.
The only continuously moving part of the camera is a disk of Hiduminium light alloy containing 16 slots mounted on the shaft of an electric motor and rotated continuously at a high speed.