ON LIGHT. 391 



ray (which may ahvays be considered as compounded of 

 two circularly-polarized ones of opposite characters as 

 already stated, i.e.^ in which the particles of the ether cir- 

 culate in opposite directions) is incident on a quartz 

 jDlate, in this manner ; the crystal operates an analysis of 

 the ray and resolves it into two such rays circularly polar- 

 ized ; which it propagates as such, the one as an ordi- 

 nary, the other as an extraordinary one. On their em- 

 ergence at the opposite face of the plate they recompound 

 a plane-polarized ray ; but, having gained or lost on one 

 another, by reason of their difference of velocity in their 

 j)assage through it, a number of revolutions or parts of a 

 revolution proportional to the thickness of the plate, the 

 two circular rays at the instant of their reunion have no 

 longer a common zero-point as at thefr entry: and from this 

 it may be demonstrated* that the plane of polarization 

 of the recomposed will not be coincident with that of 

 the incident ray, but will have been turned round, 

 /// the directiofi of the rotation of the ray ivhich t?-aveis 

 fastest within the quartz, through an angle also propor- 

 tional to the thickness of the plate. As the angle of dis- 

 placement, moreover, differs for the differently coloured 

 rays of the spectrum ; the effect will be that, when passed 

 through an analyzing tourmaline the different colours 

 will be differently absorbed, and the result will be the 

 production of a compound tint in the beam finally deli- 

 vered into the eye, the colour of which will vary with 

 the rotation of that plate in its own plane, as observed. 



* Our necessary limits forbid us to give the steps of the demon- 

 Etration, whicli, however, are very obvious. 



