Double Refraction of Quartz. 123 



There may possibly be a connection of the same kind as that 

 between the change from partial reflexion to total reflexion 

 within glass, and the accompanying change from plane polarized 

 light to elliptically-polarized light. The cases are at least thus 

 far analogous, that the change in the light and the interruption 

 of continuity go together. But we are so much in the dark re- 

 specting the physical constitution of quartz, that we cannot at 

 present go farther. 



It might have been desirable to verify my suppositions by 

 more direct experiments on the separate rays of quartz. I can 

 only plead that the duties of my office have not allowed me 

 the necessary time. They would (under all circumstances) have 

 been much more troublesome than those which I have the honor 

 of laying before the Society: and I do not think that they 

 would have been more satisfactory. The appearances presented 

 by depolarization are admirably adapted to the discovery of the 

 most delicate differences in the nature and course of rays. The 

 same want of time 1 hope will be allowed as an excuse for the 

 want of accurate measures*: without which no theory, however 

 satisfactory in general explanations, can be considered as firmly 

 established. 



G.B. AIRY. 



Observatory, Cambridge, 

 Dec. 30, 1830. 



It is much to be wished that the rings and spirals exhibited by quartz may be 

 accurately measured, their diameters in different directions ascertained, and a comparison 

 with theory instituted, by means of Biot's measures of the doubly refractive energy of quartz. 

 Should the observations and the theory disagree, it would shew, either that there is 

 some latent error in the theory, or that the difference of curvature of the sphere and 

 spheroid near their vertices is not the same as that which is inferred from Huyghen's 

 construction, modified as above 



Q 2 



