148 EEPOET ON PHYSICAL OPTICS. 



index, however, undergoing the smallest proportionate diminu- 

 tion.* 



The inclination of the optic axes, in biaxal crystals, is a simple 

 function of the elasticities of the vibrating medium in the direc- 

 tion of three rectangular axes ; and the plane of the optic axes is 

 that of the greatest and least elasticities. If, then, these three 

 principal elasticities be altered by heat in different proportions, the 

 inclination of the axes will likewise vary ; and if, in the course of 

 this change, the difference between the greatest elasticity and the 

 mean, or between the mean and the least, should vanish and after- 

 wards change sign, the two axes will collapse into one, and finally 

 open out in a plane perpendicular to their former plane. All 

 these variations have been actually observed. Professor Mitscherlich 

 found that, in sulphate of lime, the angle between the axes (which 

 is about 60 at the ordinary temperature) diminishes on the applica- 

 tion of heat ; that, as the temperature increases, these axes approach 

 until they unite ; and that, on a still further augmentation of 

 heat, they again separate and open out in a perpendicular plane. 

 The primitive form of the crystal undergoes a corresponding 

 change, the dilatation being greater in one direction than in another 

 at right angles to it. Sir David Brewster has observed an analo- 

 gous, and even yet more remarkable property, in glauberite. At 

 the freezing temperature this crystal has two axes for all the rays 

 of the spectrum, the inclination of the axes being greatest in red 

 light, and least in violet. As the temperature rises, the two axes 

 approach, and those of different colours unite in succession ; and at 

 the ordinary temperature of the atmosphere, the crystal possesses 

 the singular property of being unlaxal for violet light and biaxal 

 for red. When the heat is further increased, the axes which have 

 united open out in order, and in a plane at right angles to that in 

 which they formerly lay ; and at a temperature much below that 

 of boiling water, the planes of the axes for all colours are perpen- 

 dicular to their first position, f The inclination of the optic axes 

 in topaz, on the other hand, augments with the increase of tem- 

 perature ; and the variation, M. Marx has observed, is much 

 greater in the coloured than in the colourless varieties of this 

 mineral. + 



* Phil. Mag., Third Series, vol. i. 409. 



t Edin. Trans., vol. xi. ; and Phil. Mag., Third Series, vol. i. 417. 



J Jahrb. do- Chanic, vol. ix. 



