402 HISTORY OF OPTICS. 



The laws of the ordinary and the extraordinary 

 refraction in Iceland spar are related to each other; 

 they are, in fact, similar constructions, made, in the 

 one case, by means of an imaginary sphere, in the 

 other, by means of a spheroid ; the spheroid being 

 of such oblateness as to suit the rhombohedral form 

 of the crystal, and the axis of the spheroid being 

 the axis of symmetry of the crystal. Huyghens 

 followed this general conception into particular 

 positions and conditions; and thus obtained rules, 

 which he compared with observation, for cutting 

 the crystal and transmitting the rays in various 

 manners. "I have examined in detail," says he 3 , 

 "the properties of the extraordinary refraction of 

 this crystal, to see if each phenomenon which is 

 deduced from theory, would agree with what is 

 really observed. And this being so, it is no slight 

 proof of the truth of our suppositions and prin- 

 ciples ; but what I am going to add here confirms 

 them still more wonderfully ; that is, the different 

 modes of cutting this crystal, in which the surfaces 

 produced give rise to refractions exactly such as 

 they ought to be, and as I had foreseen them, 

 according to the preceding theory." 



Statements of this kind, coming from a philoso- 

 pher like Huyghens, were entitled to great con- 

 fidence ; Newton, however, appears not to have no- 

 ticed, or to have disregarded them. In his Opticks, 



3 See Maseres's Tracts on Optics, p. 250 ; or Huyghens, Tr. 

 sur la Lum. ch. v. Art. 43. 



