eile REPORT— 1885. 
reflexion and refraction can be made to agree with experiment by the 
simple supposition that for longitudinal and transverse disturbances 
respectively, the ether in a transparent body is loaded differently. This 
same theory of the loading of the ether will not account for double 
refraction if we assume that the vibrations are strictly in the wave front. 
If, however, we admit that in a crystal the vibrations may be normal to 
the ray, instead of in the wave front, Wresnel’s beautifal laws follow at 
once from the equations given by Lord Rayleigh, which are quite con- 
sistent with the theory of reflexion and refraction, but there is a diffi- 
culty in dealing with the pressural wave. Neither of the strict elastic 
solid theories of Green can be accepted as representing the facts of ex- 
periment, and the interesting modification of Green’s theory suggested by 
De St. Venant fails also. In all there are too many constants for the 
requirements of the experimental results, and the theories do not indicate 
the meaning of the arbitrary relations between these constants with 
sufficient clearness and certainty. 
The suggestions of Cauchy and Briot, with the elegant mathematics ot 
Sarrau on the periodic distribution of the ether in a transparent body, 
lead to expressions for the relation between the refractive index and wave 
length which agree well with experiment so Jong as we steer clear of 
substances which present the phenomena of anomalous dispersion, but 
of this they give no account. 
While the formule given by Cauchy and Hisenlohr seem to represent 
the laws of metallic reflexion with considerable exactness, the theory on 
which these formule rest, requiring as it does a negative value for the 
square of the refractive index, is inconsistent with the conditions of 
stability of an elastic solid. 
Nor is it surprising that a simple elastic solid theory should fail. 
The properties we have been considering depend on the presence of 
matter, and we have to deal with two systems of mutually interpenetrating 
particles. It is clearly a very rough approximation to suppose that the 
effect of the matter is merely to alter the rigidity or the density of the 
ether. The motion of the ether will be disturbed by the presence of 
the matter; motion may even be set up in the matter particles. The 
forces to which this gives rise may, so far as they affect the ether, enter 
its equations in such a way as to be equivalent to a change in its density 
or rigidity, but they may, and probably will, in some cases do more than 
this. The matter motion will depend in great measure on the ratio 
which the period of the incident light bears to the free period of the 
matter particles. If this be nearly unity, most of the energy in the 
incident vibration will be absorbed in setting the matter into motion, and. 
the solution will be modified accordingly. 
Parr III, 
THEORIES BASED ON THE MUTUAL REACTION BETWEEN 
THE ETHER AND MATTER. 
Chapter I.—Tue Propacation or WAVES THROUGH TWO MUTUALLY 
INTERPENETRATING MEDIA. 
§ 1. In the optical theories hitherto considered attempts have been 
made to account for the phenomena of reflexion, refraction, and dispersion 
by the hypotheses of modifications produced in the properties of the ether 
