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Jan. 1910. to this paper, had not the same errors been reproduced by Weinek in an elaborate memoir of recent date. A correct treatment of the question has been given by Schwarzschild on the lines of the present paragraph. But of course the whole question has now entered on a new phase.

11. It is easy to see that the old and the new laws of aberration agree to the first order of small quantities. Beyond the first order, the present accuracy of astronomical observations does not encourage us to look for any means of discriminating between them. Nevertheless it is interesting to ask whether the principle of relativity, as outlined above, has robbed us of the theoretical possibility of detecting the effect of the motion of the solar system through the ether of space. This has been asserted, and we have now to find a satisfactory justification for the assertion. It may be thought that a sufficient argument is contained in the statement italicised in § 6. But this does not appear to cover the case of aberration without a more critical examination. The motion of the observer does affect his observations, even when expressed in terms of the transformed variables, and we cannot dispense with the necessity of explaining in detail how the expected compensation operates when the observations are corrected in a definite, specified manner. And the question is subtle enough to leave ample room for misapprehension. The most obvious instance of this seems worthy of attention. Let us imagine observations made simultaneously by an observer E on the Earth and an observer S on the Sun (or rather the centre of gravity of the solar system, supposed to be moving with uniform velocity through space). Referred in the usual way to a sphere (fig. 4), let S be the apex of the Sun’s ether-velocity V0, and E the apex of the Earth’s ether-velocity V1. Let P represent the true direction of a star, and P0, P1 the positions

which it appears to the observers S and E respectively to occupy. Now we should have complete agreement between the two observers, and total compensation for E, if the correction applied by E had the effect of changing P1 into P0. The observer E must