ALTERATIONS IN THE DIELECTRIC 147 



K is not constant, but decreases as E increases, and by an amount 

 proportional to E 2 . This is on the supposition that the speed of 

 the rav with its electric vibrations along the lines of force is alone 

 affected. Some experiments by Kerr * appeared to establish this, 

 but some experiments by Aeckerlein f showed that in nitrobenzene 

 the ray with its electric vibrations along the lines of force was 

 retarded and that with its vibrations perpendicular was advanced 

 and about in the ratio 2:1. It is to be noted that the change in K 

 is exceedingly minute, the change in phase being only a fraction of 

 a period in a path of several centimetres through a field of perhaps 

 100,000 volts per centimetre. The Kerr effect in glass is exceed- 

 ingly small, so small that it cannot always be ceitainly distinguished 

 from the effect due to the elastic stresses present. 



* Phii. May. [5], 37, 1894, p. 380. 

 t Phys Zeit., VII, 190fi, p. 594. 



