EPOCH OF YOUNG AND FRESNEL. 449 



verifications of it, both from the goodness of the 

 observations, and the complexity and beauty of the 

 phenomena. 



We have now to consider the progress of the 

 undulatory theory in another of its departments, 

 according to the division already stated. 



Sect. 3. Explanation of Double Refraction by the 

 Undulatory Theory. 



WE have traced the history of the undulatory theory 

 applied to diffraction, into the period when Young 

 came to have Fresnel for his fellow-labourer. But 

 in the mean time, Young had considered the theory 

 in its reference to other phenomena, and especially 

 to those of double refraction. 



In this case, indeed, Huyghens's explanation of 

 the facts of Iceland spar, by means of spheroidal 

 undulations, was so complete, and had been so fully 

 confirmed by the measurements of Haiiy and Wol- 

 laston, that little remained to be done, except to 

 connect the Huyghenian hypothesis with the me- 

 chanical views belonging to the theory, and to 

 extend his law to other cases. The former part of 

 this task Young executed, by remarking that we 

 may conceive the elasticity of the crystal, on which 

 the velocity of propagation of the luminiferous 

 undulation depends, to be different, in the direction 

 of the crystallographic axis, and in the direction of 

 the planes at right angles to this axis; and from 



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