On a Fundamental Question in Electro-Optics. 381 



regular and unmistakable. With these limitations, the 

 induction extends to four liquid dielectrics, two positive and 

 two negative : and all the experiments point clearly in one 

 direction. 



General Result.- — It appears that the proper and immediate 

 optical effect of electric strain is a positive or negative retard- 

 ation of the one component light whose plane of polarization 

 is perpendicular to the line of force, the sign of the retarda- 

 tion being, of course, the same as the nominal sign of the 

 dielectric. Therefore, of two vibrations which are (on 

 Fresnel's hypothesis) perpendicular and parallel respectively 

 to the line of force, it is only the latter that is immediately 

 affected by the electric strain, this vibration along the line of 

 force having its velocity of transmission retarded or accelerated 

 according as the dielectric is of the positive class or the 

 negative. 



I venture to regard this result as a General Law of double 

 refraction in Electro-Optics, though the proof extends only 

 to four different dielectrics. As the best proof that I can 

 offer, I will merely give a condensed historical sketch of the 

 experiments. It will be seen in this way how the law was 

 first suggested and then confirmed by the phases of a new 

 electro-optic effect. It will be seen also that the proof of the 

 law is independent of all hypotheses, independent even of 

 everything previously known in electro-optics. 



The Plate Cell is a piece used in all the experiments. 



E&rth .„ 









! 



, — 





L 



T'tw 





■ 



PC 



There is an end-view of it given in the adjacent figure. It 

 consists of five slabs of plate glass, each 10 inches by 7|, 



