September 22, 1892^ 



NATURE 



501 



An apparent change of wave-length. 

 „ . , .1 An apparent error in direction. 



Receiver alone moving | ^^ ^^^^^^ ^f ^\,^.^^ ox of intensity, 



produces \ except that appropriate to different 



\ virtual velocity of light. 



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

 to show the exastence of an ether stream near the earth. All 

 optics goes 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. Cer- 

 tainly, 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 aberration 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 experi- 

 ment that will give a different result in stagnant and in free 

 ether. None of those experiments so far described are really 

 discriminative. They are, as I say, consistent with either 

 hypothesis, though not very obviously so. 



Mr. Michelson, however, of the United States, 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 realize 

 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 i in 4000 millions if it existed (which is equivalent to 

 detecting an error oi-^^^^ of an inch in a length of forty 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 neighbourhood. And that is his con- 

 clusion. 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 1 may say that it depends on 

 no doubtful properties of transparent substances, but on the 

 straightforward fundamental principle underlying all such simple 

 facts as that — It takes longer to row a certain distance and back 

 up and down stream than it does to row the same distance in 

 still water ; or that it takes longer to run up and down a hill 

 than to run the same distance laid out flat ; or that it costs 

 more to buy a certain number of oranges at three a penny and 

 an equal number at two a penny than it does to buy the whole 

 lot at five for twopence. 



Hence, although there may be some way of getting round 

 Mr. Michelson's experiment, there is no obvious way ; and I 

 conjecture that if the true conclusion be not that the ether near 

 the earth is stagnant, the experiment will lead to some other 

 important and unknown fact. 



The balance of evidence at this stage seems to incline in the 

 sense that the earth carries the neighbouring ether with it. 



But now put the question another way. Can matter carry 

 neighbouring ether with it when it moves ? Abandon the earth 

 altogether ; its motion is very quick, but too uncontrollable, and 

 it always gives negative results. Take a lump of matter that you 

 can deal with, and see if it pulls any ether along. 



That is the experiment I set myself to perform, and which, in 

 the course of the last year, I have performed. 



I take a steel disk, or rather a couple of steel disks clamped 

 together with a space between. I mount it on a vertical axis and 

 spin it like a teetotum as fast as it will stand without flying to 

 pieces. Then I take a parallel beam of light, split it into two by 

 a semi-transparent mirror (Michelson's method), a piece of glass 

 silvered so thinly that it lets half the light through and reflects 

 the other half ; and I send the two halves of this split beam 

 round and round in opposite directions in the space between the 

 disks. They may thus travel a distance of 20 or 30 or 40 feet. 

 Ultimately they are allowed to meet and enter a telescope. If 

 they have gone quite identical distances they need not interfere, 

 but usually the distances will differ by a hundred-thousandth of an 

 inch or so, which is quite enough to bring about interference. 



The mirrors which reflect the light round and round betweei> 

 the disks are shown in Fig. 10. If they form an accurate square 

 the last two images will coincide, but if the mirrors are the least 

 inclined to one another at any unaliquot part of 360° the 

 last image splits into two, as in the kaleidoscope is well known, 

 and the interference bands may be regarded as resulting from 

 those two sources. The central white band bisects normally the 

 distance between them, and their amount of separation de- 

 termines the width of the bands. There are many interesting 

 optical details here, but I shall not go into them. 



The thing to observe is whether the motion of the disks is 

 able to replace a bright band by a dark one, or vice versd. If 

 it does, it means that one of the half beams, viz. that which is 

 travelling in the same direction as the disks, is helped on a 

 trifle, equivalent to a shortening of journey by some quarter 

 millionth of an inch or so in the whole length of 30 feet ; while 

 the other half beam, viz., that travelling against the motion of 

 the disks, is retarded, or its path virtually lengthened, by the 

 same amount. 



If this acceleration and retardation actually occurs, waves 

 which did not interfere on meeting before the disks 

 moved, will interfere now, for one will arrive at the common 

 goal half a length behind the other. 



Now a gradual change of bright space to dark, and vice 

 versd, shows itself, to an observer looking at the bands, as a 



Fig. io.~Plan of steel disks one yard in diameter, and optical frame ; show- 

 ing the light going round and round, three times each way, between the 

 disks. 



gradual change of position of the bright stripes, or a shift of 

 the bands. A shift of the bands, and especially of the middle 

 white band, which is much more stable than the others, is what 

 we look for. 



At first I saw plenty of shift. In the fiist experiment the 

 bands sailed across the field as the disks got up speed until the 

 crosswire had traversed a band and a half. The conditions 

 were such that had the ether whirled at the full speed of the 

 disks I should have seen a shift of three bands. It looked very 

 much as if the light was helped along at half the speed of the 

 moving matter, just as it is inside water. 



On stopping the disks the bands returned to their old position. 

 On starting them again in the opposite direction, the bands 

 ought to have shifted the other way too ; but they did not ; 

 they went the same way as before. 



The shift was therefore wholly spurious ; it was caused by 

 the centrifugal force of the blast of air thrown off from the 

 moving disks. The mirrors and frame had to be protected 

 from this. Many other small changes had to be made, and 

 gradually the spurious shifts have been reduced and reduced, 

 largely by the skill and patience of my assistant, Mr. Davies, 

 until now there is barely a trace of them. 



But the experiment is not an easy one. Not only does the 

 blast exert pressure, but at high speeds the churning of the air 



NO. I 195, VOL. 46] 



