70 STUDIES IN LUMINESCENCE. 



3. Hysteresis. The behavior of the phosphorescent substance with a 

 given excitation depends upon its previous history. Some semi-permanent 

 change is produced by excitation which persists for several hours, or even 

 for several days, after visible phosphorescence has ceased. 



4. Effect of Red and Infra-red Rays. In the case of Sidot blende the 

 semi-permanent condition produced by excitation may be destroyed and 

 the screen restored to a standard state by brief exposure to the red and 

 infra-red rays. 



