380 0. U. VONWILLER. 



is approximately proportional to the nth power of the 

 intensity of the light, n being about 0'4, but decreasing with 

 increasing intensities. On this assumption of the relation 

 between the light absorbed and the effect produced, the 

 intensity of the light falls off more rapidly than the effect 

 as the depth penetrated increases. For instance, if w = '4, 

 at a depth at which the intensity of the light is 1/1000 of 

 its value at the surface, the effect produced is (1/1000) 4 or 

 1/16 of its value at the surface, so that the effect is still 

 appreciable at depths where the intensity of the light is 

 extremely small. 



It is not claimed that this theory is correct, some of the 

 assumptions made are certainly wrong — n is not a constant 

 for instance, but it is given in order to show that a selenium 

 cell does not necessarily possess maximum sensitiveness 

 for light of the colour which is most strongly absorbed. 

 Further experiments are in progress which it is hoped will 

 give more information on the subject. 



The experiments described in this paper were carried 

 out in the Physical Laboratory of the University of 

 Sydney. 



