500 Profs. F. G. Brown and L. P. Sieg on the Seat of 



Table I. 



Acicuh 



ir crystal, 8 X 0' 2 X 0*2 mm. 



Nature of 



Conductance with front 



Conductance with back 



illumination. 



side illuminated. 



side illuminated. 



In dark. 



27, 26-5 



26-5 



On contact No. 1. 



38, 38, 39 



38,39 



0'5 mm. from contact. 



65, 67, 64 



64,64 



i-o „ 



69, 69, 69 



53,52 



1-5 „ 



63, 64, 61 



42,44 



2-0 „ „ 



55, 54, 53 



38, 40 



2'5 „ 



47, 48, 55 



40,42 



3-0 „ 



45, 46, 44 



39,43 



On contact No. 2. 



33,33,31 



33,37 



Lame 



liar crystal, 3 x 1 x 0'2 mm. 



In dark. 



61 



61 



On contact No. 1. 



68,70 



71,71,81,84 



0*25 mm. from contact. 



94,94 



93,94 



0-50 „ 



96,99 



102, 103 



0-75 „ 



98, 100 



99, 100 



l'OO „ 



98,95 



94,95 



1-25 „ 



90,88 



86,86 



150 „ 



85,86 



82,82 



1-75 „ 



86, 87 



86,82 



2-00 „ 



80,76 



86, 85 



On contact No. 2. 



64,64 



76,78 



The above data, which are typical of a large number of 

 observations, call forth the following conclusions : — 



1. The change of resistance by light is a property of the 

 crystal, and not an action taking place at the contacts. 



2. Illumination at different points along the crystal pro- 

 duces approximately the same effect at all places. 



3. The crystal is changed in conductance by approximately 

 the same amount, whether the illumination is on the side of 

 the contacts or on the opposite side. 



From these three we draw the general conclusion that the 

 light acts throughout the crystal, and that the conductivity 

 is almost uniform throughout the crystal. 



Incidentally it should be noted that all faces of these 

 metallic selenium crystals seem to be excellent reflectors of 

 light, perhaps less than 5 per cent, entering the crystal, but 

 no exact measurements of this point have yet been made. 

 On the other hand, we have measured the absorption of light 



