the luminescence; of sidot beende. 49 



While the result obtained in this one case is not sufficient to establish a 

 general law, we are nevertheless of the opinion that the behavior of Sidot 

 blende is typical, and that in no case of phosphorescence is there any change 

 in the form of a band during decadence. In complex cases of phosphores- 

 cence we do not imply by this that the phosphorescence spectrum as a whole 

 remains unchanged in form, but rather that the distribution of intensity 

 in each band is unaltered. If the phosphorescence consists of several 

 bands, it is to be expected that the different bands will decay at different 

 rates. In fact Sidot blende itself furnishes an extreme illustration of this, 

 for the violet bands die out in one or two tenths of a second, while the green 

 band persists for hours. 



Numerous cases in which the color of a phosphorescent substance seems 

 to change as the phosphorescence dies out at first appear to contradict 

 this view. We think, however, that all cases of this kind may be shown 

 to belong to one of the two following classes : 



i . Cases of real color change ; for example, anisic acid at low tempera- 

 tures, where the phosphorescence changes from blue to greenish yellow. 1 

 Such cases are probably due to the presence in the luminescence spectrum 

 of several phosphorescence bands, which decay at different rates. In the 

 case of anisic acid the results would be explained by the presence of a 

 brilliant but rapidly decaying band in the blue, and a persistent band of 

 smaller initial intensity in the yellow. 



2. Cases where the apparent change in color is due to the fact that the 

 color sense in the eye is either weak or entirely absent at low intensities. 

 At very low intensities all colors appear to the eye as gray. However 

 brilliant may be the color of the phosphorescence light initially, it loses this 

 color and changes to a gray or faint white as it becomes fainter. But 

 such a change as this is in the retina, and does not indicate any change 

 in the phosphorescence spectrum. 



It is interesting to note that attention was called to both of these causes 

 of change of color during decay by the elder Becquerel. 2 



The data of Table 1 1 might also be used to study the law of decay of 

 phosphorescence, i. e., the relation between intensity and time. More 

 reliable results are obtained, however, by studying one wave-length at a 

 time, since the data necessary for plotting a decay curve may in this case 

 be obtained more quickly and are, therefore, less liable to error due to 

 fluctuating conditions. Measurements of phosphorescence spectra during 

 decadence, in the case of other substances, are described in Chapters IV 

 and VII. 



Nichols and Merritt, Physical Review, iS, p. 355, 1904. 



-Ed. Becquerel, Comptes Rendus, 49, p. 27,1859; Ann.de Chimie et de Physique, series 4, 62, p. 20, 1 861. 



