ON LIGHT. 391 



ray (which may always 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 

 plate, 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 

 passage 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 their 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, 

 in the direction of the rotation of the ray which travels 

 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- 

 stration, which, however, are very obvious. 



