238 ROTATORY POLARIZATION. 



I. In different plates of the same crystal, the rotation of 

 the plane of polarization is always proportional to the thick- 

 ness of the plate. The same thing holds, very nearly, in 

 plates of different crystals. 



II. When two plates are superposed, the effect produced 

 is, very nearly, the same as that which would be produced by 

 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. 



III. The rotation of the plane of polarization is different 

 for the different rays of the spectrum, and increases with 

 their refrangibility . For a given plate, the angle of rotation 

 is inversely as the square of the length of the wave. Thus the 

 angle of rotation, produced by a plate of rock-crystal whose 

 thickness is a millimetre, is 17| for the extreme red of the 

 spectrum, 30 for the rays of mean refrangibility, and 44 

 for the extreme violet. 



Since the rays of different colours emerge polarized in 

 different planes, it follows that if a beam of white light be let 

 fall upon the crystal, and be received after emergence upon 

 an analyzing plate, this will reflect a portion of the light in 

 every position of the plane of reflexion ; and this light will 

 be beautifully coloured, the colour varying with the thick- 

 ness of the crystal, and the position of the analyzing plate. 

 For the analyzing plate will reflect the rays of different 

 colours in different proportions, depending on the positions 

 of their planes of polarization with respect to the plane of 

 reflexion ; and the resulting colour will be a compound tint, 

 which can be easily estimated. 



(243) The curious distinction which was found to sub- 

 sist between different specimens of rock-crystal, has been con- 

 nected by Sir John Herschel with a difference of crystalline 

 form. The ordinary form of the crystal of quartz is the 

 six-sided prism terminated by the six-sided pyramid. The 



