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548 HIGGONS
instrument away from the intense illumination area. The cell connects to an electrical instrument providing a basic range of 3000 foot-candles which will be found adequate for most outdoor work, except under rare conditions of very bright sunlight. Two carefully calibrated slipon diffusion multipliers with 1/2-mch aluminum aperture plates are supplied to extend the range to 30,000 and 300,000 foot-candles, respectively. An exposure table will be supplied with each instrument giving appropriate camera settings for the various types of cameras and films in common use today.
Thus, a completely reliable exposure determinant is provided for use with presently available illumination sources and with sufficient flexibility which probably will be capable of handling other ranges which may now be under development.
ACKNOWLEDGMENT
The writer wishes to acknowledge the very excellent assistance and co-operation of J. H. Waddell, A. T. Williams, and L. D. Smith, who played an important part in the development of this design.
DISCUSSION
MB. A. J. SHAFER: What sort of allowances apply for reciprocity failure.
MR. E. T. HIGGONS: I believe the reciprocity failure does not enter into it too seriously.
MR. J. H. WADDELL: We might comment that the exposure tables that will be furnished with the meter are going to be established from actually running the picture tests. We know values now, and on the lower speed ranges, where one might be taking from 100 to 500 pictures a second, picture tests are under way for that particular range. We know, for example, that 150 to 170 thousand footcandles will allow pictures to be taken at 5000 a second at//ll.
CHAIRMAN BRIAN O'BRIEN: Perhaps I may add one point on reciprocity failure at very low exposures. Many of you will recall that the Gurney solid-state theory of the photographic reaction predicts that after a certain minimum exposure time is reached, further shortening the exposure and increasing the intensity of the light correspondingly so that the product of time and intensity remains constant, will go from that point up to higher and higher intensities, and there is no new curve of reciprocity failure. The curve of reciprocity failure levels off. At exposure time less than 1 microsecond, a new reciprocity failure seems to come in for all thus far tested of the fast photographic emulsions. Therefore, in the event that any of you have occasion to work below \ microsecond, and particularly below Vio microsecond, this reciprocity failure, apparently not predicted by the Gurney and Mott theory, occurs. This is under investigation hi our laboratory at the moment, and I expect within a few months some numerical data will be forthcoming.