1892.] 071 the Motion of the Ether near the Earth. 677 



Summary. 



!A real acd apparent change of wave-length. 

 A real but not apparent error in direction. 

 No lag of phase or change of intensity, except that 

 appropriate to altered wave-length. 



I No change of frequency. 

 No error in direction. 

 A real lag of phase, but undetectable without 

 control over the medium. 

 A. cliange of intensity corresponding to diflferent 

 distance, but compensated by change of radiating 

 power. 

 r An apparent change of wave-length. 

 Receiver alone moving J An apparent error in direction. 



produces j No change of phase or of intensity, except that 



^ appropriate to different virtual velocity of light. 



I may say, then, that not a single optical phenomenon is able to 

 show the existence of an ether stream near the earth. All optics go 

 on precisely as if the ether were stagnant with respect to the earth. 



Well then perhaps it is stagnant. The experiments I have quoted 

 do not prove that it is so. They are equally consistent with its 

 perfect freedom and with its absolute stagnation ; though they are 

 not consistent with any intermediate position. Certainly, if the ether 

 were stagnant nothing could be simpler than their explanation. 



The only phenomena then difficult to explain would be those 

 depending on light coming from distant regions through all the layers 

 of more or less dragged ether. The theory of astronomical aber- 

 ration would be seriously complicated ; in its present form it would 

 be upset. But it is never wise to control facts by a theory ; it is 

 better to invent some experiment that will give a different result in 

 stagnant and in free ether. None of those experiments so far de- 

 scribed are really discriminative. They are, as I say, consistent with 

 either hypothesis, though not very obviously so. 



Mr. Michelson, however, of Harvard, tl.S., has invented a plan 

 that will discriminate ; and, what is much more remarkable, he has 

 carried it out. 



That it is an exceptionally difficult experiment you will realise 

 when I say that the experiment will fail altogether unless one part in 

 400 millions can be clearly detected. 



Mr. Michelson reckons that by his latest arrangement he could see 

 1 in 4000 millions if it existed (which is equivalent to detecting an 

 error of ywou ^^ ^^ ^^^^ ^^ ^ length of 40 miles); but he saw 

 nothing. Everything behaved precisely as if the ether was stagnant ; 

 as if the earth carried with it all the ether in its immediate neigh- 

 bourhood. And that is his conclusion. If he can repeat it and get 

 a different result on the top of a mountain, that conclusion may be 

 considered established. At present it must be regarded as tentative. 



I have not time to go into the details of his experiment (it is 

 described in ' Phil. Mag.,' 1887), but I may say that it depends on 



