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 cireular 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 as 
follows :— | 
On any convenient support, there projects an arm terminating in 
