CIRCULAR POLARIZATION. 229 
from certain sections of it which Fresnel pointed out. 
The properties of rays circularly polarized also led our 
colleague to new and very curious means of producing 
coloured polarization.* 
* The author must be supposed here to allude to that remarkable 
instance of circular polarization which is produced by transmitting a 
plane polarized ray along the axis of quartz or rock crystal, and which 
depends, as he says, not on the nature of the crystal, but on the section 
of it, that is to say, on the thickness: the effect continually changing 
as slices are cut from the crystal perpendicular to its axis of increas- 
ing thickness. This statement is somewhat remarkable, as he here 
unequivocally ascribes the discovery to Fresnel, which has been 
usually by English writers ascribed to himself. 
The term “rotatory”’ polarization has been since appropriated to 
describe this phenomenon. Yet the student must be careful to dis- 
tinguish the application of this term from that of “ circular’ polar- 
ization. The light is in fact circularly polarized: but the effect 
called “rotation” is quite distinct from the “circularity.” It may 
be desirable to add a brief explanation. Let a ray, R, polarized in a 
R 
F 
c 
¥ 
- Pp 
L- 
, f p” 
N. 
\ ; 
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a 7] 
