374 0. U. VONWILLER. 



With red light the rise between the third and fifth minute 

 is greatest for the weakest light, or the conductivity is 

 still altering appreciably here when it has attained a prac- 

 tically steady state with the strongest illumination, but 

 with green light the greatest rise between the third and 

 tenth minute of exposure is obtained under the strongest 

 illumination and it is found that the rise ceases to be 

 appreciable at about the same time for all the intensities 

 used in the tests with green light. 



Miss Louise McDowell in a paper on the Electrical Pro- 

 perties of Selenium 1 describes some experiments on the 

 rate of rise and fall of the conductivity on exposure to 

 light of different colours and intensities, obtaining results 

 similar to some of those here described. Miss McDowell 

 suggests that the differences obtained may be due to differ- 

 ences in the powers of penetration of light of different 

 colours. A. H. Pfund 2 found that in the visible spectrum 

 the absorption by conducting selenium increased as the 

 wave length of the light decreased. The results here 

 described seem to show that the effect produced by the 

 light falls off much more rapidly with the depth penetrated 

 when green light is used than when red is, if we may 

 assume that the time taken in the production of the change 

 in conductivity in any small portion of the selenium depends 

 on the magnitude of the change, being less for large changes 

 than for small ones. 



For let A B fig. 6, represent the upper surface of the 

 selenium upon which light is incident, and C D the lower 

 surface ; let AM represent the total change in conductivity 

 in the surface layer, and E F, G H, C N, the changes in 

 layers at depths A F, A G, A C. The change is produced 

 most rapidly at A and is slowest at C Now let the intensity 

 of the light be increased so that AM' E F' G H' and N' 

 represent the change produced at A, E, G, C. Each of 



1 Phys. Rev., July 1909. 2 Phys. Rev., May 1909. 



