September 22, 1892] 



NA TURE 



499 



The mirror M reflects light received from Sj to a point s,, just in 

 time to catch the source there ; as it travels steadily to the left. 



Parenthetically I may say that the time taken on the double 

 journey, SiMSo, is not quite the same as the double journey 

 SMS when all is stationary, and that this is the principle of 

 Michelson's great experiment referred to below. 



For the rest of the lecture I am going to call the medium 

 which conveys light, " ether " simply. Every one knows that 

 ether is the light- conveying medium, however little else they 

 know about the properties of that tremendously important 

 material. 



We have arrived at this : that a uniform ether stream all 

 through space causes no aberration, no error in fixing direction. 

 It blows the waves along, but it does not disturb the line of 

 vision. 



Stellar aberration exists, but it depends on motion of obser- 

 ver, and on motion of observer only. Etherial motion has no 

 effect upon it, and when the observer is stationary with respect 

 to object, as he is when using a terrestrial telescope, there is no 

 aberration at all. 



Surveying operations are not rendered the least inaccurate by 

 the existence of a universal etherial drift ; and they therefore 

 afford no means of detecting it. 



But observe that everything depends on the etherial motion 

 being uniform everywhere, inside as well as outside the teles- 

 cope, and along the whole path of the ray. If stationary any- 

 where it must be stationary altogether. There must be no 

 boundary between stationary and moving ether, no plane of 

 slip, no quicker motion even in some regions than in others. 

 For (referring back to the remarks preceding Fig. 4) if the ether 

 in receiver is stagnant while outside it is moving, a wave which 

 has advanced and drifted as far as the telescope will cease to 

 drift as soon as it gets inside, but will advance simply along 

 the wave-normal ; and in general at the boundary of any such 

 change of motion a ray will be bent, and an observer looking 

 along the ray will see the source not in its true position, not 

 €ven in the apparent position appropriate to his own motion, 

 but lagging behind that position. 



Such an aberration as this, a lag or negative aberration, has 

 never yet been observed ; but if there is any slip between layers of 

 ether, if the earth carries any ether with it, or if the ether being 

 in motion at all is not equally in motion everywhere throughout 

 every transparent substance, then such a lag or negative aber- 

 ration must occur : in precise proportion to the amount of the 

 carriage of ether by moving bodie?. 



On the other hand, if the ether behaves as a perfectly friction- 

 less inviscid fluid, or if for any other reason there is no rub 

 between it and moving matter, so that the earth carries no ether 

 with it at all, then all rays will be straight, aberration will have 

 its simple and well-known value, and we shall be living in a 

 virtual ether stream of 19 miles a second, by reason of the 

 orbital motion of the earth. 



It may be difficult to imagine that a great mass like the earth 

 can rush at this tremendous pace through a medium without 

 disturbing it. It is not possible for an ordinary sphere in an 

 ordinary fluid. At the surface of such a sphere there is a viscous 

 drag, and a spinning motion diffuses out thence through the fluid 

 so that the energy of the moving body is gradually dissipated. 

 The persistence of terrestrial and planetary motions shows that 

 etherial viscosity, if existent, is small ; or at least that the 

 amount of energy thus got rid of is a very small fraction of the 

 whole. But there is nothing to show that an appreciable layer 

 of ether may not adhere to the earth and travel with it, even 

 though the force acting on it be but small. 



This, then, is the question before us : — 



Does the earth drag some ether with it ? or does it slip through 

 the ether with perfect freedom ? (never mind the earth's atmo- 

 sphere : the part it plays is not important). 



In other words, is the ether wholly or partially stagnant ^ near 

 the earth, or is it streaming past us with the opposite of the full 

 terrestrial velocity of nineteen miles a second ? Surely if we are 

 living in an ether stream of this rapidity we ought to be able to 

 detect some evidence of its existence. 



It is not so easy a thing to detect as you would imagine. We 

 have seen that it produces no deviation or error in direction. 

 Neither does it cause any change of colour or Doppler effect ; 



I The_ word " stationary " is ambiguous. I propose to use "stagnant," 

 as meaning stationary with respect to the earth, i.e. a$ opposed to stationary 

 in space. 



NO. II95, VOL. 46] 



that is, no shift of lines in spectrum. No steady wind can 

 affect pitch, simply because it cannot blow waves to your ear 

 more quickly than they are emitted. It hurries them along, 

 but it lengthens them in the same proportion, and the result is 

 that they arrive at the proper frequency. The precise effects of 

 motion on pitch are summarized in the following table : — 



Changes of Frequency due to Motion. 



Source approaching shortens waves. 



Receiver approaching alters relative velocity. 



Medium flowing alters both wave-length and velocity in 

 exactly compensatory manner. 



What other phenomena may possibly result from motion? 

 Here is a list : — 



Phenomena resulting from Motion. 

 (i) Change or apparent change in direction ; observed by 

 telescope, and called aberration. 



(2) Change or apparent change in frequency ; observed by 

 spectroscope, and called Doppler effect. 



(3) Change or apparent change in time of journey ; observed 

 by lag of phase or shift of interference fringes. 



(4) Change or apparent change in intensity ; observed by 

 energy received by thermopile. 



Motion of either source or receiver can alter frequency, motion 

 of receiver can alter apparent direction, motion of the medium 

 can do neither ; but surely it can hurry a wave so as to make it 

 arrive out of phase with another wave arriving by a different 

 path, and thus produce or modify interference effects. 



Or again it may carry the waves down stream more plentifully 

 than up stream, and thus act on a pair of thermopiles, arranged 

 fore and aft at equal distances from a source, with unequal 

 intensity. 



And again, perhaps the laws of reflection and refraction in a 

 moving medium are not the same as they are if it be at rest. 

 Then, moreover, there is double refraction, colours of thin plates 

 and thick plates, polarization angle, rotation of the plane of 

 polarization ; all sorts of optical phenomena. 



It may be, perhaps, that in empty space the effect of an ether 

 drift is difficult to detect, but will not the presence of dense 

 matter make it easier? Consider No. 3 of the phenomena 

 tabulated above. 



I expect that everyone here understands interference, but I 

 may just briefly say that two similar sets of waves " interfere " 

 whenever and wherever the crests of one set coincide with and 

 obliterate the troughs of the other set. Light advances in any 

 given direction when crests in that direction are able to remain 

 crests, and troughs to remain troughs. But if we contrive to 

 split a beam of light into two halves, to send them round by 

 different paths, and make them meet again, there is no guarantee 

 that crest will meet crest and trough trough ; it may be just the 

 other way in some places, and wherever that opposition of phase 

 occurs there there will be local obliteration or " interference." 

 Two reunited half-beams of light may thus produce local 

 stripes of darkness, and these stripes are called interference 

 bands. 



If I can I will produce actual interference of light on the 

 screen, but the experiment is a difficult one to make visible at a 

 distance, partly because the stripes or bands of darkness are 

 usually very narrow. I have not seen it attempted before. 

 [Very visible bands were formed on screen by three mirrors, 

 one of them semi-transparent, arranged as in Fig. 7.] 



Now a most interesting and important, and I think now well- 

 known, experiment of Fizeau proves quite simply and definitely 

 that if light be sent along a stream of water, travelling inside 

 the water as a transparent medium, it will go quicker with the 

 current than against it. You may say that is only natural ; a 

 wind helps sound along one way and retards it the opposite way. 

 Yes, but then sound travels in air, and wind is a bodily transfer 

 of air, hence, of course, it gives the sound a ride ; whereas light 

 does not really travel in water, but always in ether. It is by no 

 means obvious whether a stream of water can help or hinder it. 

 Experiment decides, however, and answers in the affirmativf. It 

 helps it along with just about half the speed of the water ; not 

 with the whole speed, which is curious and important, and really 

 means that the moving water has no effect whatever on the ether 

 of space, though it would take too long to make clear how this 

 comes about. Suffice for present purposes the fact that the 

 velocity of light inside moving water, and therefore pre- 



