On Material Molecules and the Etherial Medium. 249 
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 S3^mmetrically 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 Isevo-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 
system, the axes of which only differ by an extremely small 
amount, and the direction of the pressure is the direction 
of 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 if the mean 
distance of the molecules be increased in some directions, 
