502 NICHOLS— PHOSPHORESCENCE OF METALLIC SULPHIDES. 



spectra of these sulphides and enable us not only to record the 

 changes of color but to explain them. 



Many interesting points are still to be determined which are 

 likely to have a bearing upon the theories of luminescence. 



It will be possible for example, having shown that the bands of 

 short duration are really separable by cooling from the components 

 of slow decay, to locate these properly by the use of the spectro- 

 photometer and phosphoroscope. 



We shall then know whether the green band which persists at 

 low temperatures is the same for all the sulphides or, as is more 

 likely, varies in position and character with the compound employed ; 

 in what manner the components of long duration depend on the 

 composition; whether the loss of brilliancy which many of these 

 substances undergo with age, affects both sets of bands, etc. 



Physical Laboratory of Cornell University, 

 April, 1916. 



