AND MODERN PHYSICS. 199 



produced by the matter shews itself in a change of 

 inductive capacity or of permeability. It is not likely 

 that such a supposition should be the whole truth, 

 and we may, therefore, expect results deduced from it 

 to bo only approximation to the true result 



Now, electro-magnetic experiments show that, 

 excluding magnetic substances, the permeability of 

 all bodies is very nearly the same, and differs very 

 slightly from that of air. The inductive capacity, 

 however, of different bodies is different, and hence 

 the velocity with which electro-magnetic waves travel 

 differs in different bodies. 



But the refraction of waves of light depends on 

 the fact that light travels with different velocities in 

 different media; hence we should expect to have 

 waves of electric displacement reflected and refracted 

 when they pass from one dielectric, such as air, to 

 another, such as glass or gutta-percha; moreover, 

 for light the refractive index of a medium such as 

 glass is the ratio of the velocity in air to the velocity 

 in the glass. 



Thus the electrical refractive index of glass is the 

 ratio of the velocity of electric waves in air to their 

 velocity in glass. 



Now let K be the inductive capacity of air, K I that 

 of glass, taking the permeability of air and glass to be 

 the same, wo have the result that 



Electrical refractive index = \/^i / K . 



But the ratio of the inductive capacity of glass to 

 that of air is known us the specific inductive capacity 

 of glass. 





