346 Dr. J. Kerr on a new Relation between 



to the dielectric of plate glass. It gives evidence of permanent 

 and irregular strain in the neighbourhood of the terminals ; it 

 exerts a pretty strong photogyric action in the polariscope, se- 

 parating the blue and red by a small angle; it is also imper- 

 fectly transparent for very faint light ; but its chief defect is, 

 that it allows a spark-discharge over its surface, a length of 7 

 inches, before the distance of the spark-terminals has much ex- 

 ceeded 2\ inches. With all these deficiencies, the dielectric of 

 resin gives a definite and regular effect ; and the action is con- 

 trary to that of glass. 



All the arrangements are essentially as in the fifth experiment 

 (14) . The block is tied to two fine insulating pillars of glass ; the 

 induction-wires of the dielectric are connected with the knobs of 

 the secondary coil ; and the light is seen through the centre of the 

 block, midway between the terminals. A pretty good initial ex- 

 tinction is obtained in the polariscope between the blue and red ; 

 the light is then well restored by electric action, and the com- 

 pensator is introduced between the dielectric and the second 

 Nicol. By tension of the compensator in a direction parallel 

 to the lines of force, the restored light is constantly and di- 

 stinctly strengthened ; and by compression parallel to the lines 

 of force, it is regularly and greatly weakened, though never 

 quite extinguished. 



To test the value of these indications, I repeated the illus- 

 trative optical experiment (12) with the block of resin put in 

 place of the large block of plate glass ; and the results were satis- 

 factory. I varied the experiment also by manipulating the 

 block of resin itself as a compensator against the small square 

 of compressed glass, the resin being simply pulled or pushed 

 gently at both ends ; and the results were equally distinct. The 

 lines of stress being parallel, compression of glass and com- 

 pression of the resin always reinforced each other, compression 

 of glass and extension of the resin always counteracted each 

 other, but never down to perfect extinction. It is true that 

 these last results might be due more or less to the thin plates 

 of glass which limit the resin. From all the observations, 1 infer 

 that dielectrization of resin is optically equivalent to tension of 

 the resin along the lines of force. 



22. Dielectric of Quartz. — This is a plate perpendicular to the 

 axis, made as for ordinary experiments in the. polariscope, thick- 

 ness 3 millims., length 20. Two fine holes are drilled into the 

 plate as in the dielectric of plate glass, their bottoms flat, and 

 with -j-Q of an inch of clear crystal between them. Wires of 

 copper are inserted in the borings, and are fixed along with the 

 crystal to simple bearings made of glass rods ; and the whole 

 piece is coated very deeply with fused lac, a narrow window 



