The measurement of absorptive power

The extinction coefficient of an absorbing medium is given bye in the expression I/I0 =10-e where IO 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 photographic plate was not necessarily constant. His experience is summarised in the equation I1 /I2 =(t2 /t1 )n for constant density of the finished photographic negative. An alternative expression for the density may be used S =k Itn wherek andn are constants. The significance of the constantk has been discussed by Plotnikow,f but this need not detain us here. Schwarzschild found the value 0·86 forn whilst subsequent observers have recorded somewhat different values. Thus Mallet 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).