587 
of Edinburg] Session 1865-66. 
cation. The a- symmetrical arrangement of the convolutions in the 
two hemispheres which previous observers have referred to in their 
descriptions, is also well illustrated in these specimens. The higher 
differentiation of the cerebral convolutions in the Chimpanzee over 
that of the lower apes affords room for a greater amount of vari- 
ability of arrangement in it than in them. Hence, in depicting the 
brain of this animal, just as in the representation of its face and 
figure, every drawing should be a portrait, and every description 
whilst embracing the great general outlines in which all the speci- 
mens probably agree, should yet indicate the special modifications 
in construction exhibited by the individual. 
6. On the Theory of the Kefraction and Dispersion of Light. 
Part I. By Alfred K. Catton, M.A., F.E.S.E., Fellow of 
St John’s College, Cambridge, Assistant to the Professor 
of Natural Philosophy in the University of Edinburgh. 
Supposing the phenomena of light to be caused by the inde- 
finitely small vibrations of a highly elastic medium pervading space, 
it is a simple problem to determine the motion of such a medium 
in vacuo ^ or in space, where matter does not exist, as in these cases 
the problem is reduced to the determination of the motion of a 
homogeneous elastic medium. 
On proceeding, however, to investigate the motion of the ethe- 
real medium in crystals, for the purpose of accounting for the phe- 
nomena of crystalline refraction, the question arises, whether there 
is an action between the material molecules and the etherial medium. 
In other words, are the laws of the refraction of the ether within 
crystals, independent of the existence of material molecules, so 
that the ether may be treated as a single elastic medium, or are 
the phenomena of crystalline refraction produced, wholly or partially, 
by a direct action between the material molecules and the ether ? 
It is necessary, therefore, to consider at the outset, whether there 
are any physical facts which throw light on this question. For 
this purpose the observations of Sir David Brewster, De Senarmont, 
Des Cloizeaux, Mitscherlich, and others, are discussed at length 
in the paper. 
