248 Proceedings of the Royal Physical Society. 
I. On the Action between the Material Molecules and the Etherial 
Med itm, considered with reference to the Theory of the Refraction 
of Light in Crystallised or Isotropic Media. By Alfred R. Catton, 
M.A., F.R.S.E., F.C.P.S., Fellow of St John's College, Cambridire, 
Official Assistant to the Professor of Natural Philosophy in the Uni- 
versity of Edinburgh. 
The object of this paper was principally to discuss the 
bearing of a number of well-known facts on certain theo- 
retical views, which are indicated by the title of the paper, 
as to the influence which the material molecules exert on 
the propagation of light in crystallised or isotropic media. 
Supposing the phenomena of light to be caused by the 
indefinitely small vibrations of a highly elastic medium 
pervading space, it is a simple problem to determine the 
motion of such a medium in space where matter does not 
exist, as in this case, the problem is reduced to the deter- 
mination of the motion of a homogeneous elastic medium ; for 
there can be no doubt that if matter were annihilated, the 
ether would be homogeneous throughout the universe. 
On proceeding, however, to investigate the motion of the 
ether in crystals or isotropic media, the question imme- 
diately arises, whether in these cases the ether may be 
treated as a single elastic medium, the action of the ma- 
terial molecules not being considered, as in the theories of 
Fresnel, Cauchy, Neumann, Maccullagh, and Green, or 
whether the phenomena of crystalline refraction and reflec- 
tion are due to a direct action between the material mole- 
cules and the etherial medium. 
It becomes necessary, therefore, to consider, at the outset, 
the physical facts which throw light on this question. 
It was shown by Sir David Brewster, in 1818, that crys- 
tals belonging to the prismatic, oblique, and anorthic sys- 
tems are biaxal, those belonging to the pyramidal and 
rhombohedral systems uniaxal, while crystals of the cubic 
system do not possess double refraction (a fact which had 
been previously stated by Hauy.) 
Hence, when the material molecules are symmetrically 
