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510 JONES AND EYLES November
designed clock or flashing lamp in the picture has not proved satisfactory for most types of work carried out at frequencies higher than 500 pictures per second. For this reason, several specially designed time bases have been employed. One of these described by Eyles8 was designed in the Kodak Research Laboratory, at Harrow, England, for use with the Kodak high-speed camera, and has been manufactured by a firm of instrument makers and fitted to practically all of these cameras used in Great Britain. This time base consists essentially of an optical system for projecting the image of a condenser, illuminated by a small tungsten-filament light source, on to the edge of the film between the picture and the perforations. The beam of light is interrupted by shutter blades on an electrically maintained 500-cycle tuning fork. The beam is interrupted twice in each cycle and 1000 flashes of light per second therefore fall upon the film, to produce timing marks in the form of a series of short lines. At high camera speeds, these are sufficiently far apart to enable the resulting trace to be interpreted easily, but at lower speeds, to prevent overlapping, a second shutter is provided for controlling the width of the beam and which can be set from outside the time base by means of a dial calibrated in picture frequencies. The whole time base is made up as a flat unit which is bolted on to the base of the camera and rests on the tripod head. The only modification to the camera, apart from fitting the bolts, is the cutting of a hole immediately below the lower sprocket through which the optical system can project an image on to the film. The unit is energized from the same supply as that used to operate the camera.
Several other experimental time bases have been used, most of which depend upon a flashing source of light held close to the film inside the camera, the frequency of flash being controlled by a tuningfork system outside. One of the most compact devices consists of a tiny plastic case containing a small spark gap and lens which throws an image of the spark on to the edge of the film. This is bolted to the inside of the camera cover plate, through which two leads run to an induction coil and a tuning fork controlling the frequency of sparking.
IV. UNUSUAL APPLICATIONS OF HIGH-SPEED CAMERAS
Extensive use of high-speed cinematographic equipment has made it clear that there are many thousands of applications for this technique in industrial and scientific investigations. Many of the applications on record are very similar to those reported in the United States.