CIRCULAR POLARIZATION. 



227 



other means by which to produce circular polarization ; 

 and, as usual, a remarkable discovery was the reward of 

 these efforts. This discovery may be announced in two 

 words ; there is a particular kind of double refraction 

 which communicates to rays circular polarization, as the 

 double refraction of Iceland spar communicates the com- 



The mechanical conception of two rectilinear vibrations at right 



angles compounded, giving an elliptical or circular vibration, may be 

 illustrated by a very simple contrivance, which may be described a& 

 follows : 



On any convenient support, there projects an arm terminating in 



