ON LIGHT. 363 



touching at the same time all the wave surfaces in pro- 

 gress, at the same time, through the crystal, which have 

 originated in one and the same plane wave sweeping 

 over its external surface (just as in the explanation of 

 ordinary refraction given in our first part, in the case of 

 spherical waves, in which case the latter line is perpen- 

 dicular to the wave surface) ; he was enabled to explain 

 every particular of the double refraction in Iceland spar, 

 so far as the direction of the extraordinary ray is con- 

 cerned, including its deviation from the plane of the 

 angle of incidence, and its non-conformity with the 

 ordinary law of the sines except in special cases. The 

 results of his reasoning have been compared with experi- 

 ment, with extreme care, by M. Malus, as already men- 

 tioned, and found exactly in accordance with fact. We 

 cannot, of course, in an essay like the present, give any 

 account of the special conclusions, or of the mathe- 

 matical reasoning on which they are founded; which 

 involve more geometry than the generality of our 

 readers are likely to possess. But we can put into a 

 very few words, and we think make readily intelligible, 

 the main feature of the reasoning, that which determines 

 the deviation of the extraordinary ray from perpendi- 

 cularity to the wave surface, and from the plane of in- 

 cidence. 



(143.) Let A B c D represent a plane wave descending 

 perpendicularly upon the upper surface E H (supposed 

 horizontal) of a crystal of Iceland spar, of which E H M I 

 is the principal section, or that cutting through both the 

 obtuse angles of the rhomboid, and in which its axis lies. 



