< 237 ') 



CHAPTEE XIY. 



ROTATORY POLARIZATION. 



(241) IN the phenomena hitherto considered the changes 

 in the plane of polarization, which a polarized ray undergoes 

 in reflexion or refraction, are determinate in amount, and are 

 wholly independent of the distances traversed by the ray in 

 either medium. There are certain cases, however, in which 

 the change of the plane of polarization increases with the 

 thickness of the medium traversed ; and this plane is made 

 to revolve, sometimes from left to right (like the hands on 

 the dial-plate of a clock), and sometimes in the opposite 

 direction. This remarkable phenomenon was first observed 

 by Arago. 



When a polarized ray, of any simple colour, traverses a 

 plate of Iceland spar, beryl, or any other uniaxal crystal, in 

 the direction of its axis, it suffers no change of any kind. 

 But when the ray traverses in the same manner a plate 

 of rock-crystal, its plane of polarization is found to be altered 

 at emergence ; and the change increases with the thickness of 

 the plate. In some crystals of this substance, the plane 

 of polarization is turned from left to right, while in others it 

 is turned in an opposite direction ; and the crystals themselves 

 are called right-handed or left-handed, according as they pro- 

 duce one or other of these effects. 



(242) The phenomena of rotatory polarization in rock- 

 crystal were analyzed with great diligence and success by 

 M. Biot, and were reduced by him to the following general 

 laws. 



