368 O. U. VONWILLER. 



obtained with other cells tested, but the ratio of the 

 increase in conductivity produced by a green light of given 

 intensity to that produced by a red light of given intensity 

 was not the same for different cells. Of course in neither 

 case was the light used homogeneous; it was desired simply 

 to find whether there was any essential difference in the 

 behaviour of the selenium when the nature of the light 

 varied, and the screens described afforded a means of 

 obtaining lights which differed considerably in colour. It 

 is intended to carry out further and more systematic 

 investigation on tlie effect when using homogeneous light 

 of various wave lengths. 



A number of observations of the rate at which the change 

 of conductivity is produced show further differences 

 in the behaviour of the cell upon illumination by red and 

 green light. Upon the application of light the change in 

 conductivity is not instantaneous, but a definite time is 

 occupied in the attainment of the maximum conductivity, 

 and similarly when the illumination ceases, the fall to the 

 original value occupies a considerable period of time. The 

 rate of change is greatest at the beginning, decreasing as 

 the final value is approached, and when the colour of the 

 light is the same the greater the intensity the greater is 

 the rate, relatively to the total effect, at which the change 

 occurs, both when the cell is illuminated and when it is 

 darkened. On comparing results obtained when using 

 green and red light it is found that the rate of change at 

 the start is much greater in comparison with the total 

 effect produced for green than for red, but that as the 

 maximum value is approached the rise becomes relatively 

 slower with green than with red. 



In table III are given results obtained with Cell 22 

 when illuminated with the red light at distances of 100 

 cms., 300 cms. and 600 cms., and with the green at 200 cms. 



