504: Profs. F. 0. Brown and L. P. Sieg on the Seat of 



action is independent of the side which is illuminated, and 

 further that added intensities produce an effect the same in 

 degree whether impinging on the same or on opposite sides 

 of the crystal. The fact that no light goes through the 

 crystal compels us to believe that the crystal conductivity is 

 controlled by sympathetic centres located throughout the 

 crystal. The nature and distribution of these centres will be 

 discussed in a later paper. 



An Apparent Spreading of Light Action. — Since the light 

 can act throughout the depth of the crystal, indirectly it is 

 true but still undiminished by any apparent absorption, why 

 can it not also act to one side of the place struck by the 

 beam of light ? This in fact was indicated by the experiment 

 described above, in which illumination beyond the point of 

 contact produced an effect on the conductance. To check 

 this point further two parts of the same crystal were illumi- 

 nated by two separate beams of light, leaving a small dark 

 space between. The theory is simple. If the action spreads 

 only in the direct path of the beam, and not at all laterally, 

 then the change in the conductance for illumination by two 

 slits should be equal to the sum of the changes when the 

 crystal is illuminated by each of the slits separately. Con- 

 versely, if the effects of the two do spread laterally so as fully 

 to overlap, then the change in the conductance should follow 

 the square root law. The following series of observations is 

 typical of several series that were obtained. 







Table III. 





Conductance of Acicu 



ar Crystal illi 



iminated, and 





in Dark 



as indicated. 





i dark. 



By Slit No 



l. 



By Slit No. 2. 



By Slits IS T os. 1 

 and 2 together. 



23 



29 





28. 



32 



23-2 



29 





28 



31 



23 



28 





27 



29-6 



22-6 



28-2 





27-5 



29-4 



22-6 



27'5 





27 



305 



229 



28-3 





27-5 



30-5 





A 1 0=5-4 



Vo' 



A 2 C=4-6 

 4 2 +4-6 2 =7-2 



AC=7-6 



From Table III. it is evident that the combined effect is 

 not the sum of the two individual effects. Within the limits 

 of error the square root law holds, and the total combined 

 effect of both slits acting at different places on the same side 

 is identical with that when they act on the same place. In 

 other words, the actions of two separate illuminations overlap 



