72 HISTORY OF OPTICS. 



though less conspicuously, to many other kinds of crystals. ITuy 

 gheus had noticed the same fact in rock-crystal ; 5 and Malus found it 

 to belong to a large list of bodies besides ; for instance, arragonite, 

 sulphate of lime, of baryta, of strontia, of iron ; carbonate of lead ; 

 zircon, corundum, cymophane, emerald, euclase, felspar, mesotype, peri- 

 dote, sulphur, and mellite. Attempts were made, with imperfect suc- 

 cess, to reduce all these to the law which had been established for Ice- 

 land spar. In the first instance, Malus took for granted that the extra- 

 ordinary refraction depended always upon an oblate spheroid ; but M, 

 Biot 6 pointed out a distinction between two classes of crystals in 

 which this spheroid was oblong and oblate respectively, and these he 

 called attractive and repulsive crystals. With this correction, the law 

 could be extended to a considerable number of cases ; but it was after- 

 wards proved by Sir D. Brewster's discoveries, that even in this form, 

 t belonged only to substances of Avhich 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 sphe- 

 roid, which are used in the construction for uniaxal crystals, transform 

 themselves into the two successful convolutions of a sino-le continuous 



o 



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 

 refractions 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 refraction 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. 



I 



CHAPTER VI. 



DISCOVERY CF THE LAWS OF POLARIZATION. 



F the Extraordinary Refraction of Iceland spar had appeared 

 strange, another phenomenon was soon noticed in the same 



' Traite de la Lxmiire, ch. v. Art. 20. 6 Biot, Traite de Phys. iii. 330. 



