of the Selenium Cell. 37 
between the two parts of this experiment would indicate that 
a selenium cell, when exposed to light, experiences the same 
changes in resistance whether a current is flowing or not. 

Goal. 
ef! ine 
772 .77@ 
a ee 
In Bidwell’s theory the phenomenon is explained by 
assuming that light facilitates the molecular re-arrangement 
in the surface-layer of a selenide “through which an electric 
current is passing.” The experiments just described show 
that apparently the changes in resistance occur independently 
of the flow of current. Furthermore, according to Bidwell’s 
theory it is most difficult to account for the réle played by 
the large excess of free selenium, whose presence is abse- 
lutely necessary to the development of sensibility in a cell. 
Granting that in a selenium cell most, if not all, of the 
conduction is electrolytic in character, due to the presence of 

