34^ ON LIGHT. 



the substance in question (or of any crystallized body 

 whose primitive form is the acute or obtuse rhomboid, 

 the regular hexagonal prism, and some others, compris- 

 ing all those primitive forms which can be described as 

 syiujueirical to one line and to o?ie only) it is the only direc- 

 tion endued with this property. And on the other hand, 

 the amount of the double refraction or the angular sepa- 

 ration of the two rays into which the incident ray is 

 divided, is greatest when they lie in a plane perpendi- 

 cular to this axis. On account of these properties, the 

 line in question is sometimes called the oJ>tic axis of the 

 crystal. 



(124.) If a crystal of Iceland spar, or any similar 

 body, be cut into the form of a prism, in such a manner 

 as to have its refracting edge parallel to its optic axis, 

 neither of the two refracted rays will emerge parallel to 

 the incident one, or to each other. They will diverge, 

 including an angle between them, greater as the refract- 

 ing angle of the prism is greater, exactly as if the medium 

 had two different refractive indices. And in this parti- 

 cular case both refractions follow the ordinary " law of 

 the sines," and there is no deviation of either ray from 

 the plane of incidence. And what is extremely remark- 

 able, not only the refractive indices, but the dispersive 

 powers of the two refractions differ, in some cases widely, 

 so as to give two spectra (when a sunbeam is refracted) 

 of very different lengths. In the case of Iceland spar, 

 the respective refractive indices for the ordinary and 

 extraordinary ray are 1-654 and i'4^2>- ^^ is inconse- 

 quence of this great difference that the two images of a 



