80 Professor Airy on the 



separated that it is difficult to observe them in the common 

 mode of inspection. Now it has generally . been supposed 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 almost 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 established the 

 difference of right-handed and left-handed quartz. Fresnel by 

 a simple experiment* (which I have repeated) shewed that the 

 light in the direction of the axis of quartz is not one ray, but 

 two rays moving in the same direction, and with different ve- 

 locities. He shewed moreover that a new kind of light may be 

 produced by causing polarized light to undergo two internal 

 reflections 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 abovementioned according as the plane 

 of polarization is on one or the other side of the plane of in- 

 cidence. And by a mathematical investigation, of which I am 

 unable to supply the deficient steps, he shewed that the effect 

 of the internal reflections is to retard by one quarter of an un- 

 dulation 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 circlef. And from the nature of the original experiment 



* It is not easy to make this experiment in a satisfactory manner. If the axes of the 

 crystals are not precisely adjusted, several images will he seen. I have not succeeded in 

 obtaining two only, though I have made the others much more faint than the two principal. 



t As I cannot appreciate the mathematical evidence for the nature of circular polarization, 

 1 shall mention the experimental evidence on which I receive it. 1". The light when re- 

 ceived 



