INTERFERENCE OF POLARIZED LIGHT. 233 



mum and minimum of elasticity corresponding to the di- 

 rections of greatest and least pressure. Accordingly the 

 vibrations of the ray, on entering the substance, are resolved 

 into two in these directions, and these are propagated with 

 unequal velocities. 



These results of theory were experimentally confirmed by 

 Fresnel ; and he found that the velocity with which a ray 

 traversed the glass was greater or less, according as its plane 

 of polarization coincided with, or was perpendicular to, the line 

 in which the pressure was exerted. The double refraction of 

 the ray is a necessary consequence of this d inference of velo- 

 cities : but this was also established by Fresnel by direct 

 experiment. A series of glass prisms were placed together, 

 with their refracting angles alternately in opposite directions, 

 and the ends of the alternate prisms were powerfully pressed 

 by screws. A ray transmitted through the combination was 

 found to be divided into two oppositely polarized. The 

 compressed prisms, in this arrangement, acquired a double- 

 refracting structure, the axis of pressure being also the axis of 

 double refraction ; and their refracting angles being all turned 

 in the same direction, the divergence of the two rays was in- 

 creased in proportion to their number, and thus rendered 

 sensible. The intermediate prisms served to correct the de- 

 viation, and to render the combination achromatic. 



(239) The effects of unequal density and elasticity may 

 be much more regularly produced by the application of heat. 

 These effects may be studied by applying a bar of hot iron to 

 the edge of a rectangular plate of glass, and placing it in the 

 polarizing apparatus, so that the heated edge may form an 

 angle of 45 with the plane of primitive polarization. At the 

 end of some time, the whole surface of the plate will be 

 observed to be covered with coloured bands, the parts near 

 the opposite edges having acquired a positive double-re- 



