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 



p'// 



