J96 DISSERTATION SECOND. [paht i. 



It is not, however, in this general view, that the inge- 

 nuity of the theory appears, but in its application to ex- 

 plain the equality of the angles of incidence and reflection ; 

 and, most of all, the constant ratio which subsists between 

 the sines of the angles of incidence and of refraction. Few 

 things are to be met with more simple and beautiful than 

 this last application of tbe theory ; but that which is most 

 remarkable of all is, the use made of it to explain the 

 double refraction of Iceland crystal. This crystal, which 

 is no other than the calcareous spar of mineralogists, has 

 not only the property of refracting light in the usual man- 

 ner of glass, water, and other transparent bodies, but it has 

 also another power of refraction, by which even the rays 

 falling perpendicularly on the surface of the crystal are 

 turned out of their course, so that a double image is form- 

 ed of all objects seen through these crystals. This pro- 

 perty belongs not only to calcareous spar, but, in a greater 

 or less degree, to all substances which are both crystallized 

 and transparent. 



The common refraction is explained by Huygens, on 

 the supposition, that the undulations in the luminous fluid 

 are propagated in the form of spherical waves. The double 

 refraction is explained on the supposition, that the undula- 

 tions of light, in passing through the calcareous spar, as- 

 sume a spheroidal form ; and this hj r pothesis, though it 

 does not apply with the same simplicity as the former, yet 

 admits of such precision, that a proportion of the axes of 

 the spheroids may be assigned, which will account for the 

 precise quantity of the extraordinary refraction, and for all 

 the phenomena dependent on it, which Huygens had stu- 

 died with great care, and had reduced to the smallest num- 

 ber of general facts. That these spheroidal undulations 

 actually exist, he would, after all, be a bold theorist who 

 should affirm ; but that the supposition of their existence is 



