26 The Theory of the Aether 



simultaneously. In this idea he found no difficulty ; as he says : 

 " It is certain that a space occupied by more than one kind of 

 matter may permit the propagation of several kinds of waves, 

 different in velocity; for this actually happens in air mixed 

 with aethereal matter, where sound-waves and light- waves are 

 propagated together." 



Accordingly he supposed that a light-disturbance generated 

 at any spot within a crystal of Iceland spar spreads out in the 

 form of a wave-surface, composed of a sphere and a spheroid 

 having the origin of disturbance as centre. The spherical wave- 

 front corresponds to the ordinary ray, and the spheroid to the 

 extraordinary ray ; and the direction in which the extraordinary 

 ray is refracted may be determined by a geometrical construc- 

 tion, in which the spheroid takes the place which in the 

 ordinary construction is taken by the sphere. 



Thus, let the plane of the figure be at right angles to the 

 intersection of the wave-front with the surface of the crystal ; 

 let AB represent the trace of the incident wave-front ; and 

 suppose that in unit time the disturbance from B reaches the 

 interface at T. In this unit-interval of time the disturbance 

 from A will have spread out within the crystal into a sphere 

 and spheroid : so the wave-front corresponding to the 



ordinary ray will be the tangent-plane to the sphere through 

 the line whose trace is T, while the wave-front corresponding 

 to the extraordinary ray will be the tangent-plane to the 

 spheroid through the same line. The points of contact N 



