The Structure of the Æther.
In the issue of Nature of June 13 (p. 150) Dr. C. V. Burton raises an objection, raised elsewhere by Prof. Hicks and Sir Oliver Lodge, to the correlation of the magnetic vector with the velocity of the æther, on the ground that the motion of an observer relative to the æther would alter the relative velocity of the æther, and thus produce a change in the magnetic vector in the direction of the change in the observer's motion.
If it were stated definitely that the magnetic force in the free æther was proportional to the velocity of the æther relative to the observer, the objection would be valid; but this is reading into the scheme of the æther more than can legitimately be done. In the discussion the fact has been apparently overlooked that the correlation of the two vectors extends only to their rates of change in space and time, so that if identified with one another at any one point at any one instant, they will be identical at all points at every instant; otherwise they may differ by any constant, corresponding to a uniform but undetermined constant drift of the tether as a whole, or, what is the same thing, to a uniform unknown velocity of the observer through the æther.
But it should be noticed that it is not permissible to speak of the velocity of an observer relative to the æther, as though the æther were a material medium given in advance. Even if it were possible to isolate a fixed frame of reference in such an æther — which appears questionable in an infinite continuum — there is no physical means of determining the velocity of a system relative to it. The æther, as we know it, is defined by its electromagnetic properties, and one property is that a uniform drift of the æther as a whole has no effect on electromagnetic phenomena. It is known that the correlation between a stationary and a moving system as regards the electrodynamic equations is exact, and not only correct to the second order. The objection to Prof. Larmor's scheme of the æther does not apply if that scheme is stated accurately as follows:— "The propagation of electromagnetic effects through space, relative to a given frame of reference, may be illustrated by the propagation of disturbances in a rotationally elastic medium, it being possible for a given frame of reference to construct such a medium, in which the rotational displacement at any point is proportional to the electric force at that point, and the velocity relative to the frame of a point of the medium is proportional to the magnetic force." Since the velocity of a point depends on the frame of reference, it follows that the media constructed for two frames of reference moving relatively to one another with constant velocity will not be identical. The ather is, in fact, not a medium with an objective reality, but a mental image which is only unique under certain limitations (cf. footnote, p. 334, "Æther and Matter"). Two frames of reference imply two æthers; so long as we restrict outselves to a single frame, the objection to Larmor's scheme does not arise. E- Cunningham.
St. John's College, Cambridge, June 28.