THE ELECTRIC AJs^D LUMINIFEROUS MEDIUM. 
773 
This formula of Fresnel, for the change of the velocity of propagation in a moving 
ponderable medium, was specially constructed so as to insure that the laws of reflexion 
and refraction of the raijs shall be the same as if the media were at rest, a circum¬ 
stance which must be intimately connected Avith the dynamical reason for its validity. 
The laws of reflexion and refraction of rays can be deduced from tlie theory of 
exchanges of radiation, on the single hypothesis that a condition of equilibrium of 
exchanges is possible in an enclosure containing transparent non-radiating bodies. 
One interpretation of Fresnel’s principle is therefore that the exchange of radiation 
between the walls of an enclosure containing transparent bodies is not affected by any 
motion imparted to these bodies, a conclusion which may be connected with the law 
of entropy. 
73. On the present theory, magnetic force or rather magnetic induction consists in 
a permeation or flow of the primordial medium through the vortex-aggregate which 
constitutes the matter; apparently it has not been tried (see howmver § 81) whether 
light-waves are carried on by this motion of the medium and their effective velocity 
is thereby altered, as we would be led to exjrect. It has been shown, however, by 
W 1 LBERFORCE+ that the velocity of light is not sensibly altered by motion along a 
field of electric displacement, so far negativing any theoiy that Avould connect electric 
displacement with considerable bodily velocity of the mther; and it has also been 
verified, by Lord Rayi.eigh, that the transfer of an electric current across an electro¬ 
lyte does not affect the velocity of light in it. 
As motion of the sether represents magnetic force, the fact that the magnetic 
permeability is almost the same in all sensibly non-rnagnetic bodies as in a vacuum 
must be taken to indicate that the eether flows with practically its full velocity in all 
such media, so that there is very little obstruction interposed by the matter; it 
follows that, in the motion of a body through the aether, the outside aether remains 
at rest instead of flowinp- round its sides. Tlie aether we thus assume to be at rest 
O 
in any region, except it be a field of magnetic force, even though masses are moving 
through the region ; so that the coefficient of Fresnel, which is null for free aether 
and very small for but slightly ponderable media, would represent simply a change of 
velocity due to slight unilateral change of effective elasticity somehow produced by 
the motion through the quiescent medium of the vortices constituting the matter. 
74. The notion of illustrating magnetic induction by the permeation of a fluid 
through a porous medium containing obstacles to its motion has been shown by Lord 
Kelvin! to lead to a complete formal representation of the facts of diamagnetism ; 
* A. Fresnel, letter to Arago, Annaies cle Cliimie,’ ix., 1818. 
t L. R. WiLBERFORCE, ‘Trans. Cambridge Phil. Soc.,’ vol. 14, 1887, p. 170. 
X Lord Kelvin (Sir W. Thojison), “ Hydrokinetic Analogy for the magnetic influence of an ideal 
extreme diamagnetic,” ‘ Proc. R.S. Edin.,’ 1870, ‘ Papers on Electrostatics and Magnetism,’ pp. 572-83 : 
“ General hydrokinetic analogy for Induced Magnetism,” ‘ Papers on Electrostatics and Magnetism,' 
1872, pp. 584-92. 
