1920] The Theory of Relativity 23 



line-of-motion system became the shorter thwartwise system, they 

 would see the interference fringes in the interferometer move to the 

 right or left. But nothing of the kind happened, and the inevitable 

 conclusion was that there is no appreciable relative motion between the 

 earth and the ether, — -that is, that the earth drags the ether in its 

 vicinity along with it at full earth-velocity! 



But if so, how can we explain the Airy experiment with the water- 

 filled telescope, where the ether seemed to be dragged with a velocity 

 less than half that of the earth? And especially should be asked, 

 how shall we explain the aberration of light, which seemed to show that 

 the ether isn 't dragged along at all ? Yet undoubtedly the Michelson- 

 Morley experiment seems to indicate that the whole earth is sur- 

 rounded by an envelope of stagnant ether, at rest with respect to the 

 earth. This is absolutely contradictory, and physicists lost no time 

 in making further tests. 



Lodge spun a huge double disk of steel with tremendous velocity, 

 and sent* a split ray of light around in a groove between the two 

 disks, half in one direction, half in the other. If the discs dragged 

 the ether in the narrow groove around with them, the two half rays, 

 one going with the dragged ether, the other against it, ought to inter- 

 fere and produce fringes. Again no effect. Mascart and others de- 

 vised most beautiful and accurate tests, but all of them gave negative 

 results. ■ In short, as Lodge says : ' ' Interference methods all fail to 

 display any trace of relative motion between earth and ether. ' ' Wood 

 sums it up as follows : ' ' Every experiment, with the exception of 

 the one performed by Michelson and Morley, is in accord with the 

 hypothesis of a stationary ether, ' ' — that is, an ether perfectly station- 

 ary in space. 



But how then can we explain the negative result of the Michelson- 

 Morley experiment? Certainly interference methods are the most 

 accurate and sensitive we have in the science of optics, and if they fail 

 to detect relative motion between the moving earth and the fixed 

 ether how can we believe that it exists ? Lorentz and Fitzgerald then 

 came independently to the conclusion that a moving piece of matter 

 contracts in the direction of its motion. While this contraction is 

 small, — less than three inches in the case of the earth's diameter in 

 the direction of its orbital motion, — yet it would account exactly for 

 the negative result of the Michelson-Morley experiment, because the 

 longer path of the light parallel to the line of motion of the earth 



