506 Profs. F. C. Brown and L. P. Sieg on the Seat of 



about 100 times the distance between the electrodes, and so 

 if the effect of the light had been confined to the neighbour- 

 hood of the tip of the crystal, this great change in the con- 

 ductance could surely not have taken place. We must 

 conclude from this that the conductivity near the electrodes 

 was changed by light acting 6 mm. to one side. In fact 

 there was no discernible diminution in the conductance by 

 removing entirely the projecting 6 mm. of the crystal. This 

 shows quite clearly that the part acted on by light normally 

 contributes nothing directly to the conductance. In another 

 crystal, which was 15 mm. long, we observed a change of 

 conductance by illuminating the end when it was 10 mm. 

 from the electrodes. In this instance the distance from one 

 electrode to the end and back to the other electrode was 170 

 times the distance between the two electrodes. The sensibility 

 by exposing at the end was almost as great as that prevailing 

 by exposure to light close to the contacts. The cause of the 

 irregularity in the sensibility along the crystal is yet un- 

 known. A number of experiments were made to show that 

 diffuse or scattered light could have made only an inappreciable 

 effect. 



One more experiment was performed to make certain that 

 the above action at a distance was not due to a widely vary- 

 ing specific resistance along the different axes of the crystal. 

 The two electrodes were placed on the same side of the 



Ffr. 4. 



crystal about 0*1 mm. apart, and the crystal projected 6 mm. 

 beyond one of the electrodes, as shown in fig. 4. The pro- 

 cedure was to illuminate the crystal at various points to see 



