NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



271 



in our curves is not identical with the first process as observed by 

 the usual long time methods. 



Assuming the second process to continue ; the intensity after i 

 second would be about i/i,ooo of that at .01 sec. or roughly 

 1/20,000 of its initial brightness; whereas as is well known these 

 substances retain an easily visible phosphorescence after many 

 seconds. 



Fig. 7. 



This can only be accounted for by supposing that one or more 

 later processes of successively slower decay follow one another; 

 making up a more complicated curve of decay than has generally 

 been assumed. Carl Zeller," the only previous investigator to 

 determine the earlier stage of this type of phosphorescence, has 



i^Zeller, Physical Review, (i), 31, p. 367; also Carnegie Publications, 

 No. 152, p. 124. 



