Professor Airy on tltc Double Refraction of Quart;.. 



Art. XX. — On the Nature of the Light in the Two Rays 

 produced by the Double Refraction of Quartz. By G. B. 

 Airy, M. A. M. G. S. Late Fellow of Trinity College; 

 Plumian Professor of Astronomy and Experimental Philo- 

 sophy in the University of Cambridge; and Fellow of the 

 Cambridge Philosophical Society. (Abridged from the Cam- 

 bridge Transactions. Head February °.l, 1831.) 



1 propose in this paper to offer 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 distinguished foreigner ; the mode of calcula- 

 tion is original to me, and is, to the best of my knowledge, 

 new. 



1 1 is will known that the rays produced by the double re- 

 fraction of oak 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 

 passing through the ray (or, if a biaxal crystal, in tliL- plane 

 equally inclined to the planes passing through the ray and the 

 two axes) and the other in a plane perpendicular to the for- 

 mer. From the exact agreement of the phenomena of depola- 

 rization with the calculations made on this hypothesis, we are 

 justified in supposing that the law holds true when the rays 

 are so little separated that it is difficult to observe them in the 

 common mode of inspection. Now it has generally been sup- 

 posed that the two rays of quartz are polarized in the same 

 way : differing from those of calc spar only in the magnitude 

 and direction of their separation. It was known, however, al- 

 most as soon as Arago and Biot commenced their observations, 

 that there is some anomaly in the rays passing in the direction of 

 the axis of quartz ; and the latter of these observers establish- 

 ed the difference of right-handed and left-handed quartz. 

 Fresnel by a simple experiment, (see Note A.) (which I have 

 repeated) showed that the light in the direction of the axis of 



