REPORT ON PHYSICAL OPTICS. 405 



When a polarized ray traverses a plate of Iceland spar, beryl, 

 or almost any other uniaxal crystal, in the direction of its axis, 

 it suffers no change of any kind ; so that when the emergent 

 ray is analysed by a double-refracting prism, the two pencils 

 into which it is divided are colourless, and one of them vanishes 

 when the principal section of the prism is parallel, or perpendi- 

 cular to 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; these images are of complementary 

 colours ; and 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 pub- 

 lished in the 3Iemoires de I'Institut, 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 revolve, 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, ac- 

 cording 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 

 sLx-sided pyramid. The solid angles formed at th» junction of 

 the pyramid and the prism are sometimes replaced by small 

 secondary planes, which in the same crystal lean all in the 



* " Memoire sur les Rotations que certaines substances impriment aux axes 

 de polarisation des Rayons luuiincux." 



