'248 Proceedings of the Royal Physical Society. 



I. On the Action between the M.aterial Molecules and the Etherial 

 Medum, considered with reference to the Theory of the Refraction 

 of Light in Orystallked or Isotropic Media. By Alfred R. Catton, 

 M.A., F.R.S.E., F.C.P.S., Fellow of St John's College, Cambridge, j 

 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 

 iu definitely 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, ox 

 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 



