158 



Dr. J. Kerr on Measurements 



actions are subject to uniform laws in media of the one class 

 as of the other, in dielectrics as in crystals. I venture there- 

 fore to accept the principal result of my present experiments 

 as a general law, and to enunciate accordingly, although the 

 actual measurements were limited to one dielectric. 



Law. — The intensity of electro-optic action of a given dielec- 

 tric — that is, the quantity of optical effect (or the difference of 

 retardations of the ordinary and extraordinary rays) per unit 

 of thickness of the dielectric — varies directly as the square of the 

 resultant electric force. 



2. To prepare the way for an account of the experiments, I 

 must begin with a description of the cell which contains the 

 dielectric. The cell-block is composed of three thick slabs of 

 plate-glass, which stand on the experimental table, with their 

 plate-faces vertical and in close contact. 



The diagram gives an end view of the block. The two di- 

 mensions exhibited in the diagram are about 10 and 6 inches; 

 the third dimension of the block, or joint thickness of the 

 slabs, is exactly 3j-| inches. The inner rectangle in the dia- 

 gram represents a tunnel, which passes pretty uniformly 

 through the block, leaving wide margins of polished surface 

 on all the faces of the slabs. As in the case of the old plate 

 cell, the tunnel is closed by two panes of thin and carefully 

 selected plate glass, which lie in contact with the ends of the 

 block. Outside of the panes are two thick plates of india- 

 rubber cloth ; and outside of the cloths are two strong planks 

 of mahogany, which are a little wider horizontally than the 

 slabs of glass. The planks are connected by a strong screw- 

 bolt at each end. The tunnel is continued fairly through the 

 cloths and planks. 



