Second and Third Order Tests of the "JEther Drift." 75 



as great as 1/300000*. This agrees with the analytical 

 theory of a quiescent aether, which shows there should be no 

 effect if second order quantities be neglected. 



Mascart f and Eayleigh's % negative results on the differ- 

 ence in rotation in qnartz 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 aether-drift. (Of course if a negative result were obtained, 

 it might be open to such a hypothetical explanation by savin a 

 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 simul- 

 taneously the light propagated over the identical path in the 

 opposite direction, there should not be a complete eclipse if 

 the aether 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 MichelsonH, of 



* As this experiment was performed in the latter part of November, 

 when the motion of the solar system has to be subtracted from the earth/s 

 motion, this limit is far too high, and the experiment ought therefore to 

 repeated at some other time of the year. 



t Annates de VEcole Normale. torn. i. p. 157. 



X Phil. Mag. Aug. 1902. 



§ Physikal-Zeit. Band v. p. 585. 



f| Michelson, Phil. Mag. Dec. 1904, p. 71G. 



■fl L. c. p. 717. 



