INTERFERENCE OF POLARIZED LIGHT. 235 



(240) As the double-refracting structure is communicated 

 to bodies which do not possess it naturally, by mechanical 

 compression or unequal temperature so, by the same means, 

 that structure may be altered in the bodies in which it already 

 resides. Thus Sir David Brewster and M. Biot found that 

 the double refraction of regular crystals may be altered, and 

 the tints they display made to rise or descend in the scale, by 

 simple pressure. 



But the changes induced by heat are more remarkable. 

 Professor Mitscherlich discovered the important fact, that 

 heat dilates crystals differently in different directions, and so 

 alters their form ; and their double-refracting properties are 

 found to undergo corresponding changes. Thus, Iceland 

 spar is dilated by heat in the direction of its axis ; while it 

 actually contracts, by a small amount, in directions perpen- 

 dicular to it. The angles of the primitive form thus vary, 

 the rhomboid becoming less obtuse, and approaching the 

 form of the cube, in crystals of which form there is no 

 double refraction (93). Professor Mitscherlich accordingly 

 conjectured that the double-refracting energy of the crystal 

 must, in these circumstances, be diminished ; and the conjec- 

 ture was verified by experiment. In fact, the extraordinary 

 index in Iceland spar is found to increase considerably with 

 the temperature, while the ordinary index undergoes little or 

 no change. 



We have seen (217) that the inclination of the optic axes, 

 in biaxal crystals, is a simple function of the three principal 

 elasticities of the vibrating medium, and that the plane of 

 the axes is that of the greatest and least elasticities. IF, 

 then, these elasticities be altered by heat in different propor- 

 tions, the inclination of the axes will likewise vary ; and it 

 may even happen that the plane of the axes will shift to a 

 position at right angles to that which it formerly occupied. 

 All these variations have been actually observed. Professor 



