. Carnegie Institution of Washington publication. PHENOMENA OK PHOSPHORESCENCE- 203 cence of solids decays in accordance with a more complicated law. As stated above, the curve obtained by plotting values of I~- as ordinates and the corresponding values of / as abscissas is not linear throughout, as the simple theory would lead us to expect. The curve is straight, both for small values of / and for large values of /, but shows a sharp curvature for intermediate values. It has been suggested by H. Becquerel1 that the form of curve observed in the case of solids is to be accounted for by assumin
. Carnegie Institution of Washington publication. PHENOMENA OK PHOSPHORESCENCE- 203 cence of solids decays in accordance with a more complicated law. As stated above, the curve obtained by plotting values of I~- as ordinates and the corresponding values of / as abscissas is not linear throughout, as the simple theory would lead us to expect. The curve is straight, both for small values of / and for large values of /, but shows a sharp curvature for intermediate values. It has been suggested by H. Becquerel1 that the form of curve observed in the case of solids is to be accounted for by assuming that the phosphores- cence spectrum consists of two bands, each of which obeys the simple law, but having different rates of decay. If a decay curve is plotted for each of the two assumed bands the equations of the two curves will be U h = ax+bJ h 2 = a2-\-b2t. Fig. 187. Illustrating Becquerel's explanation of the form of the decay curve in solids. and each of the two curves will be a straight line. But if we plot / against /, where / is the observed total inten- sity (7 = /i+Jr2) the resulting curve will not be straight but will take the form shown by curve C, Fig. 187. In this figure A and B are the decay curves for the two bands taken separately. In the case represented in the figure the two bands are assumed to decay at widely different rates, and the band which decays more rapidly (.4) has initially the greater intensity. It will be seen that under these circumstances the decay curve for total intensity has all the characteristics of the curves determined by experiment. Becquerel cites one substance for which the observed intensity can be represented by the expression /=i/(a1+M2+i/(a2+M)2 with remarkable accuracy. In the case of this particular substance Becquerel also found independent evidence of the existence of two bands in the phosphorescence spectrum. This way of accounting for the form of the decay curve was also pro- posed by Pierce in discussing the decay of pho
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