On Material Molecules and the Etherial Medium. 240 



arranged, with respect to three planes at right angles to 

 each other (as in the prismatic), or where there is only one 

 plane of symmetry (as in the oblique), or none, as in the 

 anorthic system, there are two optic axes. When the 

 material molecules are symmetrically arranged about one 

 straight line as an axis, there is only one optic axis, which 

 coincides with the axis of symmetry of the crystal ; but if 

 they are symmetrical about three lines at right angles to 

 each other, there is no double refraction ; in other words, 

 every straight line in the crystal becomes an optic axis. 



The optical properties of crystals, therefore, are connected 

 with the arrangement in space of the material molecules of 

 which they are built up. When the latter varies, the former 

 vary also. 



Again, in quartz and dextro- and lgevo-tartaric acids (as 

 observed by Pasteur), the rotation of the plane of polarisa- 

 tion is to the right or the left, according as the hemihedral 

 faces, which occur on crystals of these substances, are turned 

 to the right or left. Here, then, a want of symmetry in the 

 arrangement of the material molecules is coincident with a 

 want of symmetry, so to speak, in optical properties. 



But, in general, if the position of the molecules be 

 changed in any manner, as by pressure, heat, or otherwise, 

 the optical properties are also changed. 



Thus, it has been shown by Sir David Brewster, that if to 

 a crystal of the cubic system, or to a transparent substance 

 like glass, we apply pressure in one direction, so that the 

 mean distance of the material molecules in this direction 

 becomes less than the distance in other directions, the sub- 

 stance becomes doubly-refracting. In reality, when pres- 

 sure is applied in one direction, the crystal, as of fluor-spar 

 for instance, can no longer be said to belong to the cubic 

 system, but becomes changed into a crystal of the pyramidal 

 BTBtem, the axes of which only differ by an extremely small 

 amount, and the direction of the pressure is the direction 

 ■ the principal, and therefore of the optic axis. In this 

 way glass, jelly, crystals of common salt, &c, can be ren- 

 dered doubly refracting. This is also the case ii* the nn an 

 distance of the molecules be increased in sonic directions, 



