INORGANIC RESPONSE 



'55 



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 





FIG. 98. MODIFICATION 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, w r e 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 E' (fig. 98, a). On 



