82 Professor Airy on the 



Should any difference be found, I have no hesitation in fixing 

 on the modification above described as that to which it ought 

 in propriety to be attached. 



I am now able to explain rny conjectures on the nature of 

 the light in the two rays of quartz. 



1. I suppose the ordinary ray to consist of light elliptically 

 polarized, the greater axis of the ellipse being perpendicular to 

 the principal plane ; and the extraordinary ray to consist of light 

 elliptically polarized, the greater axis of the ellipse being in the 

 principal plane. 



2. I suppose that when the ordinary ray is right-elliptically- 

 polarized, the extraordinary ray is left-elliptically-polarized : and 

 vice versa. 



3. I suppose that the proportions of the axes of the two 

 ellipses are the same : each proportion being one of equality 

 when the direction of the ray coincides with the axis, and he- 

 coming more unequal, according to some unknown law, as the 

 direction is more inclined to the axis : the minor axes of the 

 ellipses having sensible magnitudes when the rays are inclined 

 10° to the axis. 



4. I suppose that the course of the rays after refraction can 

 be determined by the construction given by Huyghens for calc 

 spar, with this difference only, that the prolate spheroid for de- 

 termining the course of the extraordinary ray must not be sup- 

 posed to touch the sphere for determining the course of the 

 ordinary ray, but must be entirely contained within it. 



These conjectures were originally suggested by the desire of 

 finding some connecting link between the peculiar double re- 

 fraction in the axis discovered by Fresnel*, and the double 



* It does not appear, I think, that Fresnel had made any distinct supposition as to whether 

 the two rays in the axis should be considered as the ordinary and extraordinary ray in their 



ultimate 



