274 NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



uranyl salts, or indeed whether they constitute the whole of the 

 phosphorescence spectrum. To that end some method permitting 

 of more complete resolution must be devised. The pronounced 

 changes in the color of the phosphorescent light would make it seem 

 probable that we have to do in these observations chiefly with com- 

 ponents of the phosphorescence that are of rapid decay and which, 

 after a few hundredths of a second, disappear leaving behind other 

 components which constitute the phosphorescence of long duration. 

 These, which are probably of relatively insignificant initial bright- 

 ness, doubtless overlap the phosphorescence of short duration but 

 occupy, as a whole, a somewhat difi^erent portion of the spectrum. 

 In that case since one has to do with a different group of bands 

 in observing the initial and the later phases of phosphorescence there 

 would be an actual discontinuity between the processes discussed 

 above and the great change of slope is readily explained. 



Summary. 



1. The regions of selective excitation (the bands of excitation 

 for the Lenard and Klatt sulphides, are shown to coincide in position 

 and extent with absorption bands in the transmission spectrum. 



2. The spectrum of the phosphorescent light, during the first few 

 thousandths of a second after the close of excitation, contains one or 

 more groups of overlapping bands, the crests of each group forming 

 a spectral series having a constant frequency interval. 



3. The decay of phosphorescence during the first three hun- 

 dredths of a second after the close of excitation may be described 

 as consisting of two processes each showing a linear relation between 

 /"^/2 and time. The first and more rapid process lasts for less than 

 .01 second for the three sulphides studied under the intensity of 

 excitation employed. The second process probably persists for .06 

 second or more. 



4. The phosphorescence of long duration of the sulphides under 

 consideration is probably due to another group of bands of com- 

 paratively feeble initial brightness which come under observation 

 only after the phosphorescence of short duration has vanished. 



Cornell University, 



Department of Physics, 

 Marcli, 1917. 



