DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
733 
is meant by “moving medium” seems necessaiy before we can adequately test the 
question whether electromagnetic waves in it move with it or lag behind. 
I suppose that it must be desirable to examine substances other than water, 
especially those with a much higher refractive index. I hope to do this, though it 
may be noted that the value of n which would make Fresnel’s and Thomson’s 
theories exactly agree, is-1’4142, and that the available range of i-efractive indices of 
liquids and solids affords but a narrow margin for discrimination between the two 
hypotheses. 
The balance of evidence is at present strongly in favour of Fresnel’s hypothesis, 
and T propose ordinarily to assume its truth. I cannot, indeed, understand the 
possibility of Thomson’s theory, though I detect no flaw in his work, for it seems to 
require a distinction between the case of source or receiver moving through a medium, 
and the case of medium flowing past source or receiver ; tliat is, it seems to demand a 
knowledge of absolute velocity. 
Fresnel’s Law. 
7. The statement of Fresnel’s law^ can bo thrown almost into the form of 
hypothesis {b), § 3, and at the same time its apparent licence of language about 
“ free ” and “ bound ” ether can be lessened, by supposing that the “ modification ” 
induced by the encroachment of matter on the ether is really a condensation, in the 
ratio 1 :n^; no motion in the ether other than what is necessarily involved in that act 
being postulated. On this method of statement the ether outside a moving body is 
absolutely stationary, but, as the body advances, ether is continually condensing in 
front, and, as it were, evaporating behind, while inside it is streaming through the 
body in its condensed condition at a pace such that what is equivalent to the normal 
quantity of ether in space may remain absolutely stationary. To this end its speed 
relatively to the body must be v/n'^, and accordingly its speed in space must be 
v (1 — 1/n^). 
Thus, instead of saying that a portion of the ether is moving with the full velocity 
of the body while the rest is stationary, it is probably preferable to say that the whole 
internal ether is moving with a fraction of the velocity of the body. 
One or other form of statement is absolutely involved in the Fresnellian idea 
of increased ethereal density, as may be rigorously shown (vide Lord Rayleigh, 
‘ Nature,’ March, 1892 ; vide also Eisenlohr), thus ;— 
Consider a slab moving forward flatways with velocity v, let its internal ethereal 
density be and let the external ether, of density 1, be stationary. Let the speed 
of the internal ether through space be xv, and consider that the amount of ether 
enclosed between two planes moving with the slab, one outside and one inside, must be 
constant; it follows at once that 
V — rd [u — xv ) 
