Light Action in Certain Crystals of Metallic Selenium. 503 



Recently one of us* has predicted from theoretical con- 

 siderations that the above law should hold also for these 

 crystals. Next remove the lamp B to the position B' behind 

 the crystal, so as to illuminate the other side of the crystal 

 with the same effective intensity as it previously illuminated 

 the front face. Now if the lamp at B' acts only on the 

 back half of the crystal and the lamp A acts only on the 

 front half, or even less than half of the crystal, we should 

 have the change of conductivity by A and B together in the 

 positions A and B' equal to the sum of the changes by A and 

 B separately in these same positions. On the other hand, if 

 the effect of the two lamps is the same whether the lamp B 

 is in front or behind, we can conclude without hesitation 

 that the illumination from the front or back produces the 

 same action in the crystal and acts on the same parts. The 

 following table (No. II.) is a sample set of observations 

 which answers the question beyond doubt. 



Table II. 



Both Lamps in front. Conductance with Crystal 

 Illuminated, and in Dark as indicated. 



-rii id -r a -r> x -d By both A and B 



In dark. By Lamp A. By Lamp B. •> tocrefcheri 



66-8 97 98 1105 



68 101 1015 1105 



67 98 99 110 



68 98-5 100 110 



67-5 986 996 1102 



A 1 C = 31-1 A,C=321 AC=427 



V31-l 3 +32P=44-7 



Same with Lamp A in front, and Lamp B at back. 



66 96 96 109 



67 96-5 98 109 

 67 96-5 97 108 

 67 97-5 97 108 



66-8 964 97 108-5 



A t C=29-9 A a C=302 AC=417 



V 2^+30^ = 4 24 



We note that the two lamps A and B acting together 

 produce almost identically the same effect whether they act 

 on the same side or on opposite sides. For the range tested 

 the change in the conductance varies as the square root of 

 the intensity. Thus the conclusion is unavoidable that light 



* F. C. Brown, Phvs. Rev. he. cit. 



