232 INTERFERENCE OF POLARIZED LIGHT. 



was augmented. Single-refracting crystals, such as muriate 

 of soda, and fluor spar, acquired the property of double 

 refraction by the same means. 



The opposite effects of compression and dilatation may be 

 very well seen, and studied, in a thick plate of glass bent by 

 an external force. The entire mass of the plate is thus thrown 

 into an altered state of density, the parts towards the convex 

 side of the plate being dilated, and those towards the concave 

 side compressed; while, about the middle of the thickness, 

 there is a surface in which the particles are in their natural 

 state. Accordingly, when this body is interposed between 

 the polarizing and analyzing plates, so as to form an angle 

 of 45 with the plane of primitive polarization, two sets of 

 coloured bands are seen, separated by a neutral line ; and 

 these vanish altogether when the compressing force is with- 

 drawn. The parts towards the convex or dilated side of the 

 neutral line are found to have acquired a positive double- 

 refracting structure, and those on the concave, or compressed 

 side, a negative one. 



In these cases of induced double refraction, the pheno- 

 mena are related to the form of the entire mass ; and the 

 axes of double refraction are single lines within the sub- 

 stance, fixed in position, as well as direction. In this respect 

 the phenomena are essentially different from those produced 

 by regular crystals, in which the laws of the double refraction 

 depend solely on the direction^ and are the same in all parts 

 of the substance. 



(238) The phenomena described in the preceding article 

 are in perfect accordance with the wave-theory. Owing to 

 the connexion of the vibrating medium with the solid in 

 which it is contained, its elasticity is rendered unequal in 

 different directions by the effects of compression, the maxi- 



