366 ON LIGHT. 



direction the velocities of propagation of the ordinary 

 and extraordinary rays within the crystal are the same, 

 and that therefore, supposing the two corresponding un- 

 dulations propagated from the same point in its surface 

 to run out internally, the one in the form of a spherical, 

 the other of a spheroidal shell, these shells will have a 

 common axis, viz. : the shorter axis of the spheroid, 

 which will therefore wholly include the sphere, being in 

 contact with it at the poles of the spheroid. Cceteris 

 paribus, too, it is equally obvious that, when we come to 

 consider different sorts of crystals possessing the pro- 

 perty of double refraction, the intensity of this quality, 

 or the amount of angular separation of the two refracted 

 rays at the same incidence, will be determined by the 

 greater or less amount of ellipticity of the spheroid in 

 question. Should this ellipticity be nil, the spheroid will 

 coincide over its whole extent with the sphere, and there 

 will be no double refraction. This is the case with all crys- 

 tallized bodies, whether mineral or artificial salts, whose 

 primitive form is the cube. In some cases (compara- 

 tively rare ones), of which quartz or common rock crystal 

 is an example, the spheroid is of the kind called prolate, 

 or one formed by the revolution of an ellipse round its 

 longest diameter, and is therefore wholly contained within 

 the sphere. In these, then, the velocity of the extraordi- 

 nary ray within the crystal is less than that of the ordi- 

 nary ; and the latter ray, which is the more refracted of 

 the two in Iceland spar, is in such crystals the less so. 

 On comparing different crystals, however, it is not found 

 (which perhaps might have been expected) that the el- 



