110 PHYSICS OF ETHER 



like twice the limit of observation. Lodge estimates from this experi- 

 ment that the disks must have communicated less than the eight- 

 hundredth part of their velocity to the ether. It is to be noted that 

 the masses of these disks were not great, being only some two or three 

 centimeters thick and about one meter in diameter. If we suppose 

 the ether to be set in motion by means of reactions of a viscous 

 nature, the experiment would be conclusive. To this extent, that the 

 ether is not viscous, the test seems to be valid, but as there are other 

 modes conceivable by which such movement might be brought about, 

 it is not conclusive. If now we have to give up the notion of a quies- 

 cent ether, it will be necessary to suppose such motions are engen- 

 dered in some way depending on the mass of the moving system, which 

 we might imagine to be the fact in the case of the earth and the sur- 

 rounding ether (possibly as, Des Coudres suggests, through gravita- 

 tional action). It would be desirable to repeat this experiment, using 

 great masses, and also testing to a much higher degree of sensibility 

 (the third order would be possible) by means of double refraction. 

 Michelson has recently attempted to determine directly whether the 

 velocity of the ether diminished as we recede from the earth, but 

 with negative results. He sent two interfering rays in opposite 

 directions around the four sides of a rectangle of iron piping from 

 which the air had been exhausted, the same being in a vertical east 

 and west plane, the horizontal length of which was 200 feet and the 

 height 50 feet. Assuming an exponential law for the variation in the 

 velocity of the ether as we recede from the earth, he finds that if 

 the earth carries the ether with it, this influence must extend to a dis- 

 tance comparable with the earth's diameter. The negative result in 

 many of the experiments on refraction and interference which differ- 

 ent investigators have obtained and which apparently follow on the 

 assumption of a mobile ether have been usually experiments capable 

 of giving only second order effects instead of the first order effects 

 looked for, which, as mentioned above, are quite as consistent with 

 a quiescent ether, as Stokes and Rayleigh have shown. Among these 

 may be mentioned the experiments of Hoek, Ketteler, Mascart, and 

 others on interference in ponderable media, over opposite paths rela- 

 tively to the earth's motion ; as also those of the two latter with double- 

 refracting media. All of the experiments were first order tests, and 

 hence should give negative results on either theory, since, with a ter- 

 restrial source of light, the phenomena are independent of the orient- 

 ation of the apparatus neglecting second order effects. 



Q 



The positive results of Fizeau and of Angstrom have not been 

 confirmed and should not be seriously considered. In the experiments 

 of the latter, the variation of the position of the Frauenhofer lines, 

 as obtained by a grating when observed in directions with and oppo- 

 site to the earth's orbital motion, has never been noted since, beyond 



