39* ON LIGHT. 



phenomena so apparently paradoxical as to stand in 

 seeming contradiction with all previous optical ex- 

 perience ; and which any one, antecedent to their verifi- 

 cation by trial, would have pronounced impossible.* 



(173.) One highly important conclusion from this 

 theory must, however, be noticed. The directions within 

 the crystal of the two axes of double refraction or the 

 " optic axes" stand in no abstract geometrical relation to 

 those of the angles and edges of its " primitive form," or 

 to its axes of symmetry. They are resultant lines deter- 

 mined by the law of elasticity of the luminiferous ether 

 within its substance as related to its crystalline form, and 

 to the wave-length of the particular coloured ray transmitted. 

 They are not, therefore, the same for all the coloured 

 rays. In the generality of biaxal crystals, the difference 

 of their situations and of the angle between the two, is 

 but small : but in some, as in the salt called Rochelle salt 

 (tartrate of soda and potash), it is very great, amounting 

 to at least 10, by which the direction within the crystal 

 of either axis for the extreme red rays differs from that 

 for the extreme violet. f In this salt the variation in po- 

 sition of the optic axes progresses pretty uniformly in 

 passing from a red to a violet illumination. In Car- 

 bonate of lead, on the other hand, it varies slowly in 



* This alludes to the phenomena of what is called conical re- 

 fraction, pointed out by the late Sir Win. R. Hamilton, as a neces- 

 sary consequence of FresnePs theory, and demonstrated to exist as 

 a matter of fact, subsequently, by Dr Lloyd. 



f See a paper by the author of these pages in Phil. Trans .^ 1820, 

 " On the action of crystallized bodies on homogeneous light," where 

 the singular phenomena to which this gives rise are fully described. 



