Abstract
THE author deplores the “whole train of lamentable conditions in photography relative to exposure,” and sets himself “to establish a rational scientific foundation for the practice of photography and for the study of light as it is daily observed in nature.” He says that the apertures of lenses, the sensitiveness of emulsions, and the chemical energy of lights lack simple units of measurement, and that therefore photography is not scientifically practised. He adopts &4 as the unit for lens apertures, calling the figures which represent the intensities of the ordinary apertures up to the “cone unit values.” His standard of “actinicity” is “that rate of emission which will produce a least visible tint in one minute (or 64 seconds) when the convergence is (or 4000 cone units).” This is an “actino.” “The speed of an emulsion is defined as the time required for it to suffer an effect which is known as normal exposure, when it is exposed to a surface having an intensity of one actino and through a diaphragm having a convergent value of one cone unit.” He gives full instructions for the use of these standards, and works out a number of problems that will prove of much interest to the curious. His “f/i actinometer” is a box which has an opening equal in diameter to its distance from the opposite side, where a piece of sensitive paper is placed. The first experiment suggested in “unit actinometry” is to measure or estimate the average diameter of a flame, put the sensitive paper at a distance of two diameters (a) from it, and time the period necessary to produce the first visible effect. The calculation as explained gives “the average intensity of the flame in actinos.”
Unit Photography.
By F. M. Steadman. Pp. xi + 160. (London: Constable and Co., Ltd., 1914.) Price 8s. 6d. net.
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Unit Photography . Nature 96, 368 (1915). https://doi.org/10.1038/096368a0
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DOI: https://doi.org/10.1038/096368a0