32(j Professor Airy on the Double Refraction 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 axis of the two el- 

 lipses are the same : each proportion being one of equality when 

 the direction of the ray coincides with the axis, and becoming 

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

 is more inclined to the axis : the minor axes of the ellipses hav- 

 ing 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 or- 

 dinary 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, (see Note D,) and 

 the double refraction commonly recognized. All the phenome- 

 na of colours which I have observed agree perfectly with the re- 

 sults of my hypotheses. 



I may mention that I have found observations corresponding 

 to many detached parts of the phenomena which I have viewed 

 assembled, in the early memoirs of Arago and Biot. But the 

 method which these philosophers used, (particularly the latter,) 

 of examining a small part only at a time, does not appear to be 

 well adapted to the discovery of the laws of light. In the ex- 

 periments which I am about to describe, every thing depends 

 on the form of the coloured curves ; and to attempt to discover 

 this from observation of detached parts would be perfectly hope- 

 less. These coincidences I have recognized only since I made 

 my own observations. 



It must be observed that all the phenomena mentioned below 



