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1949 BRITISH HIGH-SPEED CINEMATOGRAPHY 513
to a fixed point in space just in front of it. A convex lens attached to a reflecting prism was fastened to the anvil, which, by means of suitable illumination, presented to the camera lens a uniform disk of light of high brightness (Fig. 6a). This was then masked so that only the bottom semidisk was visible. A shutter blade was hung in front of this, suspended on a long piece of elastic which stretched right across the shop, a weight being attached below the point of suspension of the shutter. This provided a pendulum of very long period which was virtually insensitive to the sudden movement of the anvil, and thus provided a fixed point in space with respect to which measurement of the movement of the anvil could be made. A dashpot below the weight assisted in keeping the shutter blade stable by damping its movement, and enabled it to be set so that, from the camera viewpoint, its top edge was just below the bottom edge of the mask on the illuminated disk (Fig. 6b, 6c). The camera thus saw a narrow horizontal slit of light which varied in width according to the form of vibration of the anvil under the shock of the hammer blow. In this way, the amplitude and duration of the vibration of the anvil were determined successfully (Fig. 6d).
In some cases, the use of ultraviolet and infrared illumination has been found of value in ultrahigh-speed photography. For example, in connection with the photography of the rolling of red-hot steel bars, in order to show the surface texture as the bars emerged from the rollers, it was found that this could not be observed in high-speed films taken in a straightforward manner, because the intrinsic brightness of the red-hot surface of the bar prevented any rendering of texture by external lighting. It was realized, however, that hot metal does not emit any appreciable quantity of ultraviolet radiation and thus the bars were successfully photographed by means of ultraviolet lighting using a Wratten Filter No. ISA over the camera lens to cut out all visible light. Ordinary high-speed panchromatic film is sufficiently sensitive in the region just below the visible to enable a good record to be made and, in this region also, the ordinary glass lenses are sufficiently transparent to the near ultraviolet. Thus, the use of special materials and of quartz lenses was avoided. Infrared illumination has also been used for the study of behavior at t{ie electrodes of mercury-vapor arcs which normally are difficult to examine, owing to the high brightness of the arc itself closely adjacent to the electrode. The radiation of infrared light is, however, rather higher from the electrode itself than from the arc, thus enabling a satisfactory record