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558 BERKLEY AND MANSBERG November
conditions; another is the grid drive available in connection with these single transients; another one is the total and second-anode accelerating voltages; another is the optical-reduction ratio to be used with the particular camera; another is that of determining how the repetition rate and brightness of the phenomenon will affect the final negative.
According to the so-called reciprocity law, to produce a particular density on the photographic emulsion, the light intensity and the duration of the exposure are reciprocally related. This relation holds quite well for ordinary exposures such as are found in standard amateur or commercial photography, but fails under very short time or intermittent exposures such as are found under our conditions. This failure of the reciprocity law has been investigated by many workers but is subject to so many considerations that it is not possible to evaluate it satisfactorily.2
Also it is not generally possible to evaluate satisfactorily under particular conditions the effects of the writing rate, brightness, drive, accelerating voltage, reduction ratio, or developer. For this reason, the best solution is to refer to the tables of proper exposure for single transients which are given in the instruction books either for the oscillographic equipment in use, or in connection with the camera supplied for oscillographic use. These proper exposures can only be determined well by empirical methods.
UNDESIRED FILM FOG
We have stated that when correctly prepared, an oscillogram should equal or surpass the best draftsman's drawings of a wave shape or cathode-ray-tube image. One of the reasons why this frequently is not accomplished is the presence of undesired cathode-ray-tube screen illumination from a variety of internal and external sources. It is sometimes desirable to photograph some cathode-ray-tube equipment in toto showing both the complete equipment with the control panels, knobs, and accessory personnel, including at the same time, a suitably exposed and illuminated oscillogram on the face of the cathode-ray tube. In the case where the cathode-ray tube is sufficiently bright and where the phenomenon is repetitive and stationary, a constant illumination of the general scene may well be employed. It is usually quite difficult to evaluate the effects of a flashlamp in connection with the exposure coming from the cathode-ray-tube image itself. Where the cathode-ray-tube image is of relatively low intensity, it is usually