The extinction coefficient of an absorbing medium is given by in the expression T T = 10~e •*•() where I0 is the intensity of the incident light and I that of the emergent light. Schwarzschild* found that when the product of intensity of illumination into time of exposure (It) is constant the density or blackness of a photographi plate was not necessarily constant. His experience is summarised in the equation II ft-2\" r2~U) . for constant density of the finished photographic negative. An

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... ative expression for the density may be used S = hi where k and n are constants. The significance of the constant k has been discussed by Plotnikow,f but this need not detain us here. Schwarzschild found the value 0-86 for whilst subsequent observers have recorded somewhat different values. Thus MalletJ found for Paget plates 0-792 -0-962 (mean of 18 observations, 0-865). Wratten plates 0-798 -0-878 ( " 13 observations, 0-846). Imperial plates 0-765 -0-897 ( " 16 observations, 0-846). Again, Robertson § obtained the value 0-86, and Henri and his colleagues found the value to be 0-90.|| The significance of this Schwarzschild constant is obvious in all measurements of absorptive power by photographic methods, especially when a rotating sector photometer is employed. This instrument involves the use of two light beams, one of which is reduced in intensity by the absorbing medium, whilst in the case of the second beam the time of exposure is reduced by means of an * ' Astrophys.