

ELECTRICAL DOUBLE REFRACTION. 583 



magnetic axis of a magnetised body, and suggests the resultant 

 moment of momenta as the definite measure of ' magnetic 

 moment.' The explanation of all phenomena of electromagnetic 

 attraction or repulsion, and of electromagnetic induction, is to be 

 looked for simply in the inertia and pressure of the matter of 

 which the motions constitute heat. Whether this matter is or is 

 not electricity whether it is a continuous fluid interpermeating the 

 spaces between molecular nuclei, or is itself molecularly grouped 

 or whether all matter is continuous, and molecular heterogeneous- 

 ness consists in finite vortical or other relative motions of contiguous 

 parts of a body, it is impossible to decide, and perhaps in vain to 

 speculate, in the present state of science." (Reprint of Papers^ 

 p. 419.) 



ELECTRICAL DOUBLE REFRACTION. It is known that whenever 

 a singly refracting transparent body is subjected to a mechanical 

 action, this body acquires for the moment double refracting pro- 

 perties, the axis of double refraction being directed along the line 

 of pressure or of traction. 



Kerr has shown that any singly refracting solid or liquid placed 

 in an electrical field acquires a transient double refraction ; the 

 axis of double refraction coincides with the line of force, and 

 according to the nature of the body, the velocity of the extra- 

 ordinary ray is greater or less than that of the ordinary ray. 



If 8 is the intensity of double refraction that is the difference 

 of path between the ordinary and extraordinary ray for unit thickness 

 of the dielectric and if V is the difference of electrostatic potential 

 between two points at a distance d, the electrical force F in the 



V 



region in question is equal to . Kerr deduces from his experi- 



ments the ratio 



V 2 



k being a constant characteristic of the body, positive or negative 

 according to circumstances. It follows from this that the intensity 

 of electrical double refraction is proportional to the square of the 

 electrical force. 



We have seen (107) that the dielectric may be considered as 

 subjected to a strain in the direction of the lines of force propor- 

 tional to the square of the force. It appears then that the phe- 

 nomenon observed by Mr. Kerr may be considered as an accidental 

 double refraction, due to the electrostatic tension of the medium. 



