LAWS OF DOUBLE REFRACTION. 405 



the crystallization has relation to a single axis of 

 symmetry, as the rhombohedron, or the square 

 pyramid. In other cases, as the rhombic prism, 

 in which the form, considered with reference to its 

 crystalline symmetry, is biaxal, the law is much 

 more complicated. In that case, the sphere and 

 the spheroid, which are used in the construction for 

 uniaxal crystals, transform themselves into the two 

 successive convolutions of a single continuous curve 

 surface; neither of the two rays follows the law 

 of ordinary refraction; and the formula which 

 determines their position is very complex. It is, 

 however, capable of being tested by measures of 

 the refraction of crystals cut in a peculiar manner 

 for the purpose, and this was done by MM. Fresnel 

 and Arago. But this complex law of double re- 

 fraction was only discovered through the aid of the 

 theory of a luminiferous ether, and therefore we 

 must now return to the other facts which led to 

 such a theory. 



