POLARISED LIGHT 365 



its axes parallel to and at right angles with the plane of the 

 polariser, is by itself totally inoperative, the polarised beam 

 passing through in its own plane unobstructed, and therefore 

 unresolved further. But it is very different if there precede 

 the quarter-wave in the stage a selenite or other polarising 

 preparation, in the usual position, the quarter-wave coming 

 after this latter. The plane-polarised beam, let us suppose 

 polarised in a perpendicular plane, is, by the selenite or other 

 preparation, now already ' resolved ' into two planes at right 

 angles to each other, but at 45 angle with the original 

 plane. Therefore the quarter-wave, whose own axes are per- 

 pendicular and horizontal, being at 45 angle with those of 

 the selenite, now again resolves each of these. The demon- 

 strator should get these different actions of a quarter-wave 

 plate thoroughly understood, and must at least understand 

 them himself clearly. 



It is easily shown that any film whose thickness is an odd 

 number of quarter-waves, gives substantially the same phe- 

 nomena, particularly in homogeneous yellow light. But with 

 increasing thickness the amount of residual colour from all 

 but the yellow waves, increases also. 



Circular polarisation by reflection from silver, or in a 

 Fresnel's rhomb, can be easily projected, in a manner too 

 obvious to need description, but is not a very effective experi- 

 ment, unless in a complete course of lectures. 



212. Eotary Polarisation. But the effect of a quarter- 

 wave plate coming after some other preparation should be 

 worked out further. The axis of the plate is to be vertical, 

 as just now observed, and it resolves again (into perpendicular 

 and horizontal) the rays which passed through the first 

 crystal plate. For simplicity let this be a simple ' even ' 

 film. Each of the first crystal's sets of vibrations, is now 

 resolved again, separately, into horizontal and perpendicular 

 with a quarter-wave difference of phase. So resolved, one 

 compounds into a circular orbit in one direction, and the 



