The wind therefore causes a positive or negative change of phase in every direction except that whose cosine is , the same direction as that already (§ 13) indicated as possessing a zero Doppler effect.
But the observation 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.
16. 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, by 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 the virtual thickness of a slab immersed in an ether stream, or the time retardation it causes in a beam.
Interference Effects as modified by Ether Motion through Dense Stationary Bodies.
17. The calculation of the lag in phase caused by Fresnel’s ethereal motion is a very simple matter. A dense slab of thickness , which would naturally be traversed with the velocity , is traversed with the velocity ; where is the relative velocity of the ether in its neighbourhood; whence the time of journey through it is
