200 JAMES CLEUK MAXWELL 



Hence, tlio specific inductive capacity of any 

 medium is equal to the square of the electrical refrac- 

 tive index of that medium. 



Since Maxwell's time the mathematical laws of 

 the reflexion and refraction of electric waves have 

 been investigated by various writers, and it has been 

 shewn that they agree exactly with those enunciated 

 by Fresnel for light. 



Hitherto wo have been discussing the propagation 

 of electric waves in an isotropic medium, one which 

 lias identical properties in all directions about a pint. 

 Let us now consider how these laws arc modified if 

 the dielectric be crystalline in structure. 



Maxwell assumes that the crystalline character 

 of the dielectric can be sufficiently represented by 

 supposing the inductive capacity to be different in 

 different directions; experiments have since shewn 

 that this is true for crystals such as Iceland 

 Spar and Aragonite ; he assumes also, and this, too, 

 is justified by experiment, that the magnetic per- 

 meability does not depend on the direction. It 

 follows from these assumptions that a crystal will 

 produce double retraction and polarisation of electric 

 waves which fall upon it, and, further, that the laws 

 of double refraction will be those given by Fresnel 

 for light waves in a doubly refracting medium. 

 There will be two waves in the crystal. The dis- 

 turbance in each of these will be pi anc\ polarised ; 

 their velocity and the position of their piano of 

 polarisation can bo found from the direction in which 

 they are travelling by Frcsnul's construction exactly. 



Maxwell's theory, then, would appear to indicate 



