COLOUES OF CRYSTALLINE PLATES. 145 



pressure. Accordingly, the vibrations of the ray on entering the 

 plate are resolved into two in these rectangular directions, and 

 these are propagated with unequal velocities ; the colour developed is 

 determined by the interval of retardation. These results of theory 

 were experimentally confirmed by Fresnel, by the method of inter- 

 ferences ; and it was found that the velocity with which a ray traversed 

 the glass was greater or less, according as it was polarized parallel 

 or perpendicular to the axis of compression. The bifurcation of the 

 ray at oblique incidences is a necessary consequence of this difference 

 of velocities ; but this was also shown by Fresnel by direct experi- 

 ment. A series of glass prisms were placed together with their 

 refracting angles alternately in opposite directions, and the ends of 

 the alternate prisms powerfully pressed by screws. A ray trans- 

 mitted through the combination was found to be divided into two 

 oppositely polarized.* 



The opposite effects of compression and dilatation may be seen 

 in a thick plate of glass which is bent by an external force. 

 When this body is interposed between the polarizing and analyz- 

 ing plates, so as to form an angle of 45 with the plane of 

 primitive polarization, two sets of coloured bands are seen sepa- 

 rated by a neutral line ; and these vanish altogether when the com- 

 pressing force is withdrawn. By crossing the glass with a plate 

 of mica or sulphate of lime, Sir David Brewster found that the 

 parts towards the convex, or dilated side of the neutral line, had 

 acquired a positive double-refracting structure, and those at the 

 concave, or compressed side, a negative one.f The intimate con- 

 nexion between the double-refracting property, and the internal 

 state of the body as to condensation or rarefaction, is likewise 

 proved by the curious observation of M. Biot, that glass, when 

 in a state of sonorous vibration, possesses the power of depolarizing 

 the light. 



In these cases of induced double refraction, the phenomena are 

 related to the form of the entire mass ; and are essentially different 

 from those produced by regular crystals, in which the law of elas- 

 ticity and of double refraction depends solely on the direction, and 

 is the same in all parts of the substance. Sir David Brewster has 

 lately succeeded in communicating a regular double-refracting 

 structure to a mixture of resin and white wax, by pressing it into 



Annalcs dc Chimit, torn. xx. 1 rhil. Tnms. 1310. 



