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Rh as great as 1/300000. This agrees with the analytical theory of a quiescent æther, which shows there should be no effect if second order quantities be neglected.

Mascart and Rayleigh's negative results on the difference in rotation in quartz with and against the drift are not decisive, since the calculated effect, although not of the second order, is the difference between two first-order effects. A reexamination of the problem with greater experimental refinements should give important results.

With the present uncertainty on both the analytical and the experimental sides, decisive results, which will be free from any hypothetical explanation, seem only possible in the direct comparison of the velocities of light with and against the æther-drift. (Of course if a negative result were obtained, it might be open to such a hypothetical explanation by saying that the group velocity, relative to the medium itself, was a function of the absolute motion of this medium.) Thus Wien proposes to use two synchronized Foucault mirrors or two Fizeau toothed wheels. This plan is of course of long standing, but has been recently revived. The mechanical difficulties in the way do not give much encouragement to hope for success; but with present refinements the test is not beyond possibility. The objection which Newcomb and Michelson have raised to this mode of comparison, that the phases of the synchronizing systems would be affected by the earth's motion in the same way as the propagation of the light, does not seem to be well taken. For granting a certain phase difference in the rotating-mirror or wheel systems, this difference in phase of the two systems can still be so changed as to give an eclipse, say, along the drift. If now we observe simultaneously the light propagated over the identical path in the opposite direction, there should not be a complete eclipse if the æther were at rest. Any method, therefore, which allows a comparison of two rays, propagated over the same path in opposite directions, is a valid test of the problem. It remains then to devise a method which will certainly show a difference between these two intervals of time equal to one part in ten thousand. The method proposed by Michelson, of