378 0. U. VONWILER. 



light shines on it, its apparent resistance is high on account 

 of the thickness being less than the length of the free path 

 of the conducting electrons in the material, whereas when 

 light of longer wave length is used the absorption being 

 less, the thickness of the layer affected is much greater 

 and though the specific conductivity is less than when 

 illuminated by blue light, the observed increase in conduc- 

 tivity is greater on account of the thickness of the layer 

 affected exceeding the free path of a conducting electron. 

 The values found by Pfund for the absorption of light seem 

 to indicate that it would be almost completely absorbed in 

 an exceeding thin film, but as he himself points out, his 

 readings may be considerably in error on account of his 

 being able to obtain but one film and so not being able to 

 correct for surface losses, and it must be remembered that 

 the electrical properties of conducting selenium depend 

 greatly on the method employed in preparing it, and it is 

 not improbable that the optical properties also depend on 

 this and the numerical values of the coefficient of absorp- 

 tion deduced from Pfund's observations may differ consider- 

 ably from those which hold for the selenium in the cells 

 used. Certainly the experiments described here seem to 

 show that the layer affected is much thicker than is 

 assumed by Pfund while they also show that the absorption 

 of green light is greater than that of red as Pfund found. 



The fact that the sensitiveness of a cell is not a maxi- 

 mum for light of that colour which is most strongly 

 absorbed by the selenium can be explained on another 

 theory. 



Let us suppose that the increase in conductivity in a thin 

 layer of selenium depends on the absorption of energy in 

 that layer, increasing as the amount of energy absorbed 

 increases, and let I be the intensity of the light incident 

 on the surface, and a the coefficient of absorption of the 



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