502 Profs. F. C. Brown and L. P. Sieg on the Seat oj 



light on entering the crystal operates a mechanism which 

 controls by secondary action the conductance of the entire 

 crystal. 



In the last paragraph we noted two possibilities, viz., either 

 that the light penetrates almost without absorption, or that 

 the crystal is made up of a sympathetic mechanism, which 

 may be operated at various points throughout the crystal. 

 The crystal was of acicular form, and slightly greater than 

 0*1 mm. in thickness. Not only was it not transparent to a 

 large degree, but under the microscope no trace of light 

 could be seen through the crystal. This was also the case 

 when the crystal was examined between crossed nicols. 

 Thus the second conclusion seemed unavoidable. But the 

 idea was so novel that it could not be accepted without further 

 confirmation. 



Evidence deduced from the Law of Superposition of Inten- 

 sities. — This confirmation was next sought by illuminating a 

 flat acicular crystal by two twenty-watt tungsten lamps 

 placed in certain combination positions as described below. 

 The point to be established is that illumination from the back 

 side of the crystal acts on the same parts of the crystal, and 

 to the same extent, either directly or indirectly, as those 

 acted on by illumination directed on the front face. For the 

 sake of argument, imagine as before the crystal to be divided 

 lengthwise into two parallel conductors, and further consider 

 that light from one side may, or may not, act on both con- 

 ductors. The same arrangement of apparatus as that repre- 

 sented in fig. 2 was used, with the addition of the two lamps 

 above noted. Let the two lamps, A and B, be turned on 

 separately and then together, while stationed on the front 

 side, and let the corresponding changes of conductance be 

 noted. From these observations we obtain the law of varia- 

 tion of conductivity change with change of intensity of 

 illumination, or the law of action when one intensity of 

 illumination is added to another intensity at the same places 

 on the crystal. In an ordinary selenium cell this action has 

 long been known to vary, except under very special conditions, 

 with the square root of the intensity of illumination, following 

 the relation 



AC = KI- 5 , 



where AO is the change in the conductance in the crystal, 

 K is a constant, and I the intensity, Then if A X C and A 2 

 represent the changes due to intensities \ 1 and I 2 , the change 

 in conductance due to the two acting together should be 



AC = K [(AiC/K^ + CAsC/K) 2 ]' 5 . 



