III. On the Nature of the Light in the Two Ray a 

 produced by the Double Kef ruction of Quartz. 



By G. B. AIRY, M.A.; M.G.S.; 



LATE FELLOW OF TRINITY COLLEGE; PLUMIAN PROFESSOR OF ASTRONOMY AND 



EXPERIMENTAL PHILOSOPHY IN THE UNIVERSITY OF CAMBRIDGE: 



AND FELLOW OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY. 



[Read February 21, 1831.] 



I propose in this paper to otter some conjectures as to the 

 nature of the light forming the two rays produced by the double 

 refraction of quartz ; to describe the experiments on which they 

 are founded ; and to explain the calculations by which the theory 

 and the experiments are compared. The subject is one to which 

 (I believe) no attention has been paid, except by one distin- 

 guished foreigner ; the mode of calculation is original to me, 

 and is, to the best of my knowledge, new. 



It is Avell known that the rays produced by the double re- 

 fraction of calc spar, (calcareous spar, Iceland spar, or rhom- 

 bohedral carbonate of lime) and most other doubly refracting 

 crystals, are entirely polarized : one in the principal plane pass- 

 ing through the ray (or, if a biaxal crystal, in the plane equally 

 inclined to the planes passing through the ray and the two 

 axes) and the other in a plane perpendicular to the former. 

 From the exact agreement of the phenomena of depolarization 

 with the calculations made on this hypothesis, we are justified in 

 supposing that the law holds true when the rays are so little 



