372 Mr. G.W. White on the 



identical in both cases, and that this difference approaches 

 zero as the selenium slab becomes infinitely thin." 



(2) It was hoped that the results o£ this comparison could 

 be used to obtain an estimate of the " effective depth of 

 penetration of the light in selenium"; that is, supposing 

 there were a uniform specific conductivity throughout the 

 illuminated layer, to find how deep this layer would have to 

 be to account for the observed change in conductivity of a 

 selenium block on illumination. Marc (1906) estimated the 

 value 5 X 10 ~ 5 mm. ; from the investigation of thin selenium 

 films. Grippenberg* showed this value to be much too small. 

 Brown f, assuming the identity of heat and light actions in 

 selenium, calculated the depth of the light-affected layer to 

 be l'4xl0- 2 mm. 



In the present work, the method whereby it was hoped to 

 arrive at the " effective depth " consisted of the examination 

 of a rectangular block of selenium, one face of which was 

 illuminated, the conductivity being measured both in a 

 direction perpendicular to the incident light and parallel to 

 it ; that is, the preparation was to be used as a bridge of 

 both types. The conductivity in the first direction (bridge A) 

 is that of two conductors in parallel, one being the illuminated 

 layer and the other the selenium in the dark. The con- 

 ductivity is therefore a function of the specific conductivity 

 of selenium in the dark, the "effective" or "average con- 

 ductivity" in the light, the "depth of penetration/' and the 

 dimensions of the block. The actual function can be easily 

 deduced from the elementary laws of "conductors in parallel." 

 The conductivity in the second direction (bridge B) is that 

 of two conductors in series, and is another function of the 

 above quantities. Now the specific conductivity of selenium 

 in the dark and the dimensions of the bridge can be 

 measured, and hence from the equations expressing the 

 conductivities of the illuminated block in the two directions, 

 the "effective depth" and "effective specific conductivity in 

 the light " can be calculated. 



Experimental Arrangements and Results. 



The bridge was made by crystallizing selenium in a mould 

 of which one side was a copper plate and the opposite side 

 a semi-transparent film of platinum deposited on glass by 

 "cathode disintegration." A plate A of " bakelite " — an 

 insulator which will withstand continued heating at a high 



* Physik. Zeit. xiii., 1912. 



t Phys. Rev. vol. xxxiv. p. 201 (1912). 



