250 Proceedings of the Royal Physical Society. 



and not in others, as when a piece of glass is rendered 

 doubly-refracting by bending, the inner strata on the con- 

 cave side of the bend being compressed, and those on the 

 outside dilated. 



The optical properties of doubly-refracting crystals are 

 also changed by pressure, as was also shown by Sir David 

 Brewster, and also by the action of heat. Now crystals, ex- 

 cept those of the cubic system, are, in general, dilated dif- 

 ferently in different directions by the action of heat. Thus, 

 in a rhombohedron of calcite increase of temperature alters 

 the angles between the faces, making them approach more 

 nearly to a cube ; and, at the same time, the extraordinary 

 refractive index is increased. A similar property has been 

 recently observed by Fizeau in quartz, . who finds that the 

 extraordinary refractive index is diminished by rise of tem- 

 perature, the rhombohedron in this case departing further 

 from the form of a cube. In gypsum, to take the case of a 

 biaxal crystal, Mitscherlich found that change of tempera- 

 ture alters in a remarkable manner the angle between the 

 optic axes. More recently, Des Cloizeaux has shown that 

 this is the case in a number of minerals, such as brookite, 

 chrysoberyl, and certain species of felspar. In general, when 

 the heat is removed, and the crystal regains its original 

 temperature, the angle between the optic axes becomes the 

 same as before, the molecules having then regained their 

 original positions. Des Cloizeaux also found that, in certain 

 minerals, the angle between the optic axes became perma- 

 nently changed by exposing them to a high temperature. 

 In these cases we may suppose a permanent change to have 

 taken place in the position of the molecules, as when a solid 

 is distorted beyond the limits of elasticity, or the limits 

 within which Hooke's law holds, in which case a permanent 

 change in the position of the molecules takes place. 



In the above and similar cases, therefore, a change in the 

 position of the material molecules produced by heat, causes a 

 change in optical properties. 



It is well known that we may render a cylinder of glass 

 transiently doubly refracting by immersing it in heated oil, 

 a circular cylinder giving the same rings as a uniaxal, and | 



