Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/846

750 function of their direction of motion through the ether; and accordingly that the length and breadth of stone supporting block were differently affected, in what happened to be, either accidentally or for some unknown reason, a compensatory manner.

26. There is already one experiment, which I have never seen criticised either way, tending in a sense precisely contrary to. observed the polarization produced by a pile of plates, and considered that he had proved that the azimuth of the plane of polarization varied with the direction of orbital motion of the Earth, and hence that the ether was streaming past them. If so, polarization by reflexion is the only phenomenon known which is capable of showing a first-order effect of the general ethereal drift. The experiment seems to me extremely difficult, but to be well worthy of repetition by other observers. [I believe that Lord objection to the experiment as performed by  is that the effect was unseen until an illegitimate or unsafe magnifying device was employed.]

Meanwhile I shall hope to examine the question of ether motion near moving matter in a simpler fashion (§ 33).

Assuming for the present that the ether is not disturbed in a viscous manner by the motion of gross matter through it, we can make the following assertions:—

General Statements Concerning Aberration.

27. A ray is straight whatever the motion of the medium, unless there are eddies, and accordingly no irrotational currents of ether can divert a ray. But, if the observer is moving, the apparent ray will not be the true ray, and accordingly the line of vision will not be the true direction of object.

In a stationary ether, wave-normal and ray coincide, but the line of vision of a moving observer slants across both (§ 20).

In a moving ether, wave-normal and ray enclose an angle, and line of vision depends upon motion of observer. If the observer is stationary his line of vision is the ray; if he moves at the same rate as the ether his line of vision is the wave-normal (§ 13).

The line of vision, in fact, always depends on the motion of the observer, not at all on the motion of the ether so long as it has a velocity-potential. Hence nothing can be simpler than the theory of aberration if this condition is satisfied.

A similar but more general condition (to be obtained in the next section) suffices to secure the straightness of a ray whatever happens, or more generally that whatever the path of a ray may be by reason of reflexion or refraction in a stationary ether, the same it shall be in a moving one; and readily accounts for the absence of all effect on direction due to the general relative drift of the medium, whether in the