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BRITISH HIGH-SPEED CINEMATOGRAPHY 503
I. CAMERAS
Senior3 has described a camera using multiple-flash illumination which has been used for studying underwater explosions. This consists of a flashing light source operated by a commutator coupled to a camera capable of transmitting 100 feet of 35-mm film continuously at the rate of about 90 feet per second. The highest picture frequency obtainable is around 1500 per second, but each individual exposure is extremely short, and hence frame-by-frame analysis of the record is relatively easy. For photographing small charges, a mirror is used, placed at 45 degrees under the surface of the water, but for most work the camera and lighting equipment were enclosed in a watertight compartment and completely submerged. The work carried out with this camera is particularly interesting, and has included examination of the explosion bubble and of the "split" of the water caused by the impact of the explosion, the influence of explosions near structures including bulkheads of vessels, a photoelastic study of the shock waves traveling through solid bodies, and stereophotography, using two cameras coupled together. It is claimed that stereo work is particularly easy, as the same flash can be used for the two cameras, and there is thus no difficulty in maintaining picture synchronism. The flash is triggered from one of the cameras, but the film is run through both at the same speed.
Another -interesting camera is that designed by Henry4 for the study of textile fibers during spinning and weaving. This consists of a drum camera, illumination being provided by a series of sparks in air or, alternatively, flashes from a gas-discharge tube. The equipment is capable of taking a series of 120 pictures at a frequency of 1500 per second, and can record an object moving at the rate of 10 meters (32.81 feet) per second, the accuracy of its position at any instant being determinable to within 0.5 mm. Field sizes up to 2 feet in diameter have been covered. Henry designed the equipment to be made easily in his own workshop at a low cost and succeeded in producing it for under 200 pounds (approximately 800 dollars). It has given excellent results, an example of which is shown in Fig. 1, in solving problems in the textile field, for which it was designed.
Brailsford and Shrubb2 have designed a camera for the photography of metallic arc welding, which consists of a drum camera capable of taking 25 inches of 35-mm film (Fig. 2). The optical system for compensating for the film movement consists of a square glass prism rotated between the back of a conventional folding pocket camera and