Helium 



1 minute after exci- 

 tation. Exposed 2 

 days. 



{Hg exciting lines are 

 visible.) 



15 minutes after exci- 

 tation. Exposed 

 10 days. 



Fig. 3. ^Spectrum of zinc sulphide phosphorescence {after Ives and Luckiesh). Photo- 

 graphs were taken by a special device one minute (middle) and fifteen minutes (bottom) 

 after exposure to the light of the mercury arc and compared with a helium spectrum (top). 

 In the middle photograph, the mercury exciting lines are visible. It will be noted that the 

 narrow band of phosphorescent light does not shift its position during decay of phosphorescence. 



