EFFECT OF LIGHT ON ELECTRICAL CONDUCTIVITY OF SELENIUM. 375 



these changes is produced more rapidly than the change at 

 the corresponding point under the weaker illumination, so 

 that the final steady value will be reached in a shorter 

 time with the stronger illumination than with the weaker. 



Fig. 6. 



A 



M 



M I 



E 



F/ 



" F>* 





G 



h/ h> 







C 



n/n/ 



D 



B 



Now suppose that the effect falls off much more rapidly as 

 the depth increases as in fig. 7, so that it is negligible before 

 the lower surface is reached. Here if A M, the effect at 

 the upper surface is equal to that in the former case (fig. 6), 

 the change in the surface layer will occur with equal rapidity 

 in the two cases, but in lower layers, as E F, the effect is much 

 less and will occur more slowly in the latter example, so 

 that the total effect produced by the light will be less than 

 before but the initial rate of rise almost as great, and, in 

 comparison with the total change produced, greater than 



