1894.] 



Fundamental Question in Electro- Optics. 



253 



effect of electric strain is a positive or negative retardation of the 

 one component light whose plane of polai'isation is perpendicular 

 to the line of force, the sign of the retardation 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 tbe 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 di- 

 electrics. 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. There is an 



end-view of it given in the adjacent figure. It consists of five slabs 

 of plate glass, each 10 in. by 7f in., arranged face to face in one block. 

 The inner rectangle represents a tunnel (5f in. by 3 in.) which passes 

 right through the block. Inside are shown the conductors with sup- 

 porting frame, the shaded pieces being of brass, and the unshaded of 

 plate glass. The lengths of the conductors, at right angles to the 

 plane of the figure (and parallel to the light), are 6| in. and 7^ in., the 

 thickness of the cell being nearly 8 in. By means of wires, which 

 pass through the wall of the cell, the internal conductors are con- 



