588 
Proceedings of the Royal Society 
The discoveries of Sir David Brewster show that the optical pro- 
perties of crystals are connected with the arrangement in space of 
the material molecules of which they are built up. Thus when 
the material molecules are symmetrically arranged with respect to 
three planes at right angles to another (as in the prismatic system), 
or where there is only one plane of symmetry (as in the oblique 
system), or none (as in the anorthic), there are two optic axes. 
But when they are symmetrically arranged about one line as an 
axis, there is only one optic axis which coincides with the axis of 
symmetry of the crystal. In the cubic system, which is symmetrical 
in every direction, every straight line becomes an optic axis. 
Again, in quartz and dextro- and Isevo-tartaric acids (as ob- 
served by Pasteur), the direction of rotation of the plane of polariza- 
tion is to the right or left according as the hemihedral forms which 
occur on crystals of these substances turn to the right or left. 
Here, then, a want of symmetry in the arrangement of the material 
molecules is connected with a want of symmetry (so to speak) in 
optical properties. 
The bearing of the experiments of De Senarmont, Des Cloizeaux, 
and others, is then discussed, and it is shown that, in general, 
whenever and from whatever cause the arrangement of the material 
molecules is changed, the optical properties are also changed. The 
influence of heat and pressure on crystalline refraction is well- 
known. Thus in a rhombohedron of calcite, increase of tempera- 
ture alters the angles between the faces, making them approach 
more nearly to a cube, and at the same time the extraordinary re- 
fractive index is increased. A similar observation has recently 
been made by Fizeau in quartz. From the facts brought forward 
in this paper, it is concluded that the ether within all bodies is of 
the same nature as in vacuo, and that the optical properties of 
crystals are caused entirely by the direct action of the material mole- 
cules on the ether. Of course the action which the ether exerts at 
a given point within a crystal is not, as in vacuo, the same in every 
direction. For in crystals of the prismatic system, the action of 
the material molecules is different in different directions ; in other 
words, it tends to compress the ether more in one direction than 
in another, and in consequence the resistance of the ether to com- 
pression must also be different in different directions. 
