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. Becquerel 1 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 = a x +bJ 



h 2 = a 2 -\-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+J r 2) 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/(a 1 +M 2 +i/(a 2 +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 phosphorescence in Emanations- 

 pulver, and has been shown by him to give a very satisfactory explanation 

 of the change in the form of the decay curve resulting from changes in 

 temperature. 



If Becquerel's explanation of the form of the decay curve is correct we 

 should expect to find a change in the phosphorescence spectrum during 

 decay. For, referring to the figure, the light given out during the first 

 few seconds is chiefly due to band A, while that emitted during the later 

 stages of decay is almost entirely due to band B. Unless these two bands 

 are coincident some shift in the wave-length of maximum intensity is to 

 be expected. Our own experiments with Sidot blende show that no meas- 

 urable change occurs in the form of the spectrum, or the position of its 

 maximum, during the first 10 seconds, and the later work of Pierce shows 

 that the wave-length of maximum intensity is unchanged for at least 1 



'Becquerel, /. c. 



