242 ROTATORY POLARIZATION. 



This difference is extremely small. When is one milli- 

 metre, the angle of rotation, p, corresponding to the rays of 

 mean refrangibility, = 30. But for these rays, A = *0005 of 

 a millimetre ; and therefore ju'-ju = -00008, 



(246) The phenomena hitherto described take place only 

 in the direction of the axis of the crystal. Sir Gr. B. Airy disco- 

 vered that when a plane-polarized ray is transmitted through 

 rock-crystal in any direction inclined to the axis, it is divided 

 into two pencils which are elliptical-polarized ; the elliptical 

 vibrations in the two rays being in opposite directions, and 

 the greater axes of the ellipses coinciding respectively with 

 the principal plane, and with the perpendicular plane. The 

 ratio of the axes in these ellipses varies with the inclination 

 of the ray to the optic axis, being a ratio of equality when 

 the direction of the ray coincides with the axis, and in- 

 creasing indefinitely with its inclination to that line. With 

 respect to the course of the refracted rays, Sir Gr. B. Airy found 

 that it was still determined by the Huygenian law; but 

 that the sphere and spheroid, which determine the velocities 

 and directions of the two rays, do not touch, as in all other 

 known uniaxal crystals the latter surface being contained 

 entirely within the former. This is a necessary consequence 

 of the fact, that the interval of retardation of the two pencils 

 does not vanish, with the inclination of the ray to the optic 

 axis. 



Sir Gr. B. Airy has given an elaborate calculation, founded 

 on these hypotheses, of the forms of the rings, &c., displayed 

 by rock-crystal in plane-polarized and circularly-polarized 

 light ; and he has found a striking agreement between the 

 results of calculation and experiment. Among the most re- 

 markable of the phenomena whose laws are thus developed, 

 is that produced by the superposition of two plates of rock- 



