140 KEPORT ON PHYSICAL OPTICS. 



the plane of primitive polarization. But when a ray passes in the 

 same manner through a plate of rock-crystal, the phenomena are 

 very different. Two images are given in every position of the 

 prism ; and these images are of complementary colours, while the 

 colours change in the most beautiful manner as the prism is turned 

 round in its cell. These phenomena indicate that the plane of 

 polarization has been changed, and differently for the different 

 rays of the spectrum. They were first observed by M. Arago ; 

 and he has given an account of his observations in his memoir on 

 the colours of crystalline plates, read to the Institute in the year 

 1811. 



The subject was then taken up by M. Biot, in a paper published 

 in the Memoires de Vlnstitut in the year 1812 ; and the analysis 

 of the phenomenon was completed in a second memoir read in the 

 year 1818*. When a polarized ray of any simple colour passes 

 through a plate of rock-crystal in the direction of the optic axis, 

 it is still polarized after emergence ; but its plane of polarization 

 is changed. The angle through which the plane is made to re- 

 volve varies with the colour of the light, and with the thickness 

 of the plate, being proportional to that thickness divided by the 

 square of the length of the fit or wave. In some crystals the 

 plane of polarization revolves from left to right, while in others it 

 is turned in an opposite direction ; and the crystals themselves are 

 denominated right-handed or left-handed, according as they produce 

 one or other of these effects. When two plates are superposed, 

 the effect produced is, very nearly, the same as that due to a single 

 plate whose thickness is the sum or difference of the thicknesses of 

 the two plates, according as they are of the same or of opposite 

 denominations. 



This curious distinction between plates cut from different 

 crystals has been connected by Sir John Herschel with a corre- 

 sponding diversity in the crystalline form. The ordinary form of 

 the crystal of quartz is the six-sided prism terminated by the six- 

 sided pyramid. The solid angles formed at the junction of the 

 pyramid and the prism are sometimes replaced by small secondary 

 planes, which in the same crystal lean all in the same direction ; 

 and it is found that when that direction is to the right (the apex of 



" Memoire sur les rotations que certaines substances impriment aux axes de 

 polarisation des rayons iumineux." 



