INORGANIC RESPONSE 155 
It appears at first very curious that this difference 
of electric potential should . be maintained between 
opposite faces of a very thin and highly conducting 
sheet of metal, the intervening distance between the 
opposed surfaces being so extremely small, and the 
electrical resistance quite infinitesimal. A homogeneous 
sheet of metal has become by the unequal action of 
light, molecularly speaking, heterogeneous. The two 
opposed surfaces are thrown into opposite kinds of 
electric condition, the result of which is as if a certain 



Fic. 98.—MopiricaTion oF THE SENSITIVE CELL 
thickness of the sheet, electrically speaking, were made 
zinc-like, and the rest copper-like. From such un- 
familiar conceptions, we shall now pass easily to others 
to which we are more accustomed. Instead of two 
opposed surfaces, we may obtain a similar response by 
unequally lighting different portions of the same surface. 
Taking a sheet of metal, we may expose one half, say 
A, to light, the other half, B, being screened. Electro- 
lytic contacts are made by plunging the two limbs in 
two vessels which are in connection with the two non- 
polarisable electrodes E and Ef’ (fig. 98, a). On 
