130 M. H. Brongersma on Double Refraction 



prism and mounting the plate movable horizontally and verti- 

 cally, by which different parts could be successively brought 

 into the field, I succeeded in improving his method. It is, 

 however, very possible that the kind of glass used by Kerr 

 was not without influence upon the result. According to my 

 experiments a piece of very hard English glass, of the same 

 thickness (18 millim.) as that employed by Kerr, becomes 

 strongly doubly refracting in consequence of the drilling. 

 With it the described phenomena were only faintly shown. 



Of other glass plates, some, which in consequence of the 

 drilling had become much more strongly doubly refracting 

 than those above mentioned, yielded perceptible but much less 

 distinct results. 



The above is besides in complete accordance with the result 

 derived by Kerr from his experiments — namely, that glass 

 under the action of electrical influence behaves like glass which 

 is compressed in the direction of the lines of force. 



The difficulties in this investigation are not inconsiderable. 

 For it is necessary to make the difference of potential very 

 great ; and then many a glass plate is perforated by sparks. 

 Besides this, it is not easy to insulate sufficiently the conduct- 

 ing connexions. To this must be added that many glass plates 

 become so strongly doubly refracting in consequence of the 

 drilling as to be useless for the investigation. This was the 

 case chiefly with plates whose apertures were drilled with a 

 metallic instead of a diamond drill, presumably in consequence 

 of the great pressure attending the employment of the former. 

 The influence of the nature of the glass I have not yet been 

 able to observe. 



Kerr saw in these phenomena a confirmation of Faraday's 

 theory respecting dielectrics. That great physicist already 

 regarded it as probable that under the influence of electricity 

 an isotropic body passes into the anisotropic state, so that it 

 behaves like a doubly refracting crystal. He did not, how- 

 ever, succeed in confirming this by experiment. Still, in my 

 opinion, it is not sufficiently proved that these phenomena 

 cannot be of a secondary order. The motions of the mole- 

 cules, on their arrangement in a limited portion of the plate, 

 may have for their consequence a development of heat ; and 

 this, again, may be the cause of the observed double refrac- 

 tion, as Werner Siemens has already experimentally demon- 

 strated by the heating of the insulating medium of a condenser 

 accompanying the charge and discharge *. 1 hope that a con- 

 tinued investigation will soon enable me to enunciate a defi- 

 nite view. 



* BerL Monatsber. 1864, p. 614. 





