EFFECT OF LIG-HT ON ELECTRICAL CONDUCTIVITY OF SELENIUM. 367 



effect with the depth penetrated, the whole of the selenium 

 would be affected to the same extent when one side is 

 illuminated and the simultaneous illumination of both 

 sides should produce an effect equal to that obtained when 

 one side alone is subjected to an illumination of double the 



Fig. 2. 



10 



























V$^ 



iS^-"^ 

























8 



10 



intensity. The fact that the effect is greater when one 

 side is illuminated than when the other is can be readily 

 understood, the wires being nearer the surface on one side 

 than on the other. 



The sensitiveness of the cell is seen to be much less for 

 green light than for red as the intensity of the light was of 

 the same order in both cases while the effects produced 

 were very much greater with red. Similar results were 



