742 
DK. OLIVER LODGE ON ABERRATION PROBLEMS. 
The wind therefore causes a positive or negative change of phase in every direction 
except that whose cosine is -§ u/V, the same direction as that already (§ 13) indicated 
as possessing a zero Doppler effect. 
But the ohservation of the lag of phase thus caused by motion of the entire ethereal 
medium is not so easy as might appear, and, in fact, it has not yet been detected ; for 
the simple reason that it is liable to affect both the interfering rays equally : as we now 
show. 
Devices for Observing the Lag of Phase. 
IG. The possible ways in which change of phase, produced by a moving medium, 
may be looked for, are ;—to split a beam of light into two halves, and then— 
(1) Make the medium flow with one half beam and against the other. 
This is successful, and is the Fizeau experiment; but it entails control over the 
medium, and artificial motion of it; the terrestrial orbital motion cannot be utilized 
in this way. 
(2) Send the two beams, not parallel, but round contours in two different planes; 
or, say one across the line of ether motion, and the other along. 
This is Michelson’s experiment ; but it only attempts an effect whose magnitude 
is the second order of aberration magnitudes ; because, before the beams can be 
brought together again to interfere, a reversal or complete circuit is necessary. 
(3) Make the medium flow at different rates along the two beams : as for 
instance, b 3 ' interposing a dense substance in one of them. 
But, on Fresnel’s hypothesis, this ought to fail; because the free ether, which is 
the only ether in motion, is unaffected by the dense substance. The only way to move 
either more or less than the normal quantity of ether in any given space, is to move 
bodily a dense substance occupying that space. So long as that is stationary, with 
respect to source and receiver, motion of the whole produces no effect. 
To prove that on Fresnel’s law, no dense substance can cause different interference 
effects when moving than it causes when stationary, we can proceed to calculate tlie 
virtual thickness of a slab immersed in an ether stream, or the time retardation it 
causes in a beam. 
Interference Effects as niodifed by Ether Motion through Dense Stationary Bodies. 
1 7. The calculation of the lag in phase caused by Fresnel’s ethereal motion is a 
very simple matter. A dense slab of thickness 2 , which would naturall}" be traversed 
with the velocity V//x, is traversed with the velocity (V/g) cos e + cos 6 ; where v 
is the relative velocity of the ether in its neighbourhood ; whence the time of journey 
through it is 
