Professor Airy on the Double Refraction of Quartz. '525 



quartz is not one ray, but two rays moving in the same direc- 

 tion, and with different velocities. He showed, moreover, that 

 a new kind of light may be produced by causing polarized 

 light to undergo two internal reflexions in a glass rhomb with 

 certain angles, the plane of polarization being inclined to the 

 plane of incidence at an angle of 45° ; and that this light is 

 exactly similar to one or other of the two rays above-mentioned, 

 according as the plane of polarization is on one or the other 

 side of the plane of incidence. And by a mathematical inves- 

 tigation, of which I am unable to supply the deficient steps, 

 he showed that the effect of the internal reflexions is to retard 

 by one quarter of an undulation the undulations perpendicular 

 to the plane of incidence, so that in the light thus modified 

 the particles of ether which were originally in a straight line 

 will at any time be found in the form of a circular helix, and 

 each will revolve uniformly in a circle. (See Note B.) And 

 from the nature of the original experiment it appeared that in 

 right-handed quartz it is necessary to suppose the right-circu- 

 lar polarization transmitted with the greater velocity ; in left- 

 handed quartz the contrary. I have repeated and varied most 

 of Fresnel's experiments relating to this subject, and am per- 

 fectly convinced of the correctness of his views. 



Now if, in the experiment with the glass rhomb, the planes 

 of polarization and incidence be inclined at any other angle 

 than 45°, the magnitudes of the undulations parallel and per- 

 pendicular to the plane of incidence will no longer be equal : 

 but the alteration of their periods will be the same as before. 

 The displacement of the particles of either will still be repre- 

 sented by a helix, but instead of being traced round a circular 

 cylinder, it must be supposed traced round an elliptic cylinder. 

 This modification may properly be called (as Fresnel has called 

 it) elliptical polarization. (See Note C.) This term has since 

 been used by Dr Brewster to express thenatureof thelight(prr> 

 bably indentical with this, or nearly BO) reflected from metallic 

 surfaces. Should any difference be found, I have no hesitation 

 in fixing on the modification above described .is that to which 

 in ought, in propriety to he attached. 



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

 the light in the two rays of quartz. 



