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HARPER'S LIBRARY of LIVING THOUGHT

THE ETHER

OF

SPACE

BY

SIR OLIVERLODGE, F.R.S

HARPER

BROTHERS

NEOPYORKXLONDON

THE ETHER OFSPACE

BY

SIR OLIVER LODGE, F.R.S.

D.Sc. Land., Hon. D.Sc. Ox on. et Viet.

LL.D. St. Andrew's, Glasgow, and Aberdeen

Vice-President of the Institution of Electrical Engineers

Rumford Medallist of the Royal Society

Ex-President oftlte Physical Society of London

Late Professor of Physics in the University College of Liverpool

Honorary Member of the A merican Philosophical Society of Philadelphia

of the Manchester Philosophical Society ', of the BatavianSociety of Rotterdam; and of the Academy of Sciences of Bologna

Principal of the University of Birmingham


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ILLUSTRATED

NEW YORK AND LONDON

HARPER & BROTHERS

1909

Copyright, 1909, by HARPER & BROTHERS.

All rights reserved.Published May, 1909.

TO THE FOUNDERS OF

UNIVERSITY COLLEGE, LIVERPOOL,

ESPECIALLY TO THOSE BEARING THE NAMES

OF RATHBONE AND OF HOLT

THIS BOOK IS INSCRIBED

PREFACE

INVESTIGATION of the nature and proper-1 ties of the Ether of Space has long been forme the most fascinating branch of Physics, and

I welcome the opportunity of attempting tomake generally known the conclusions to whichI have so far been led on this great and perhapsinexhaustible subject.

OLIVER LODGE.

UNIVERSITY OF BIRMINGHAM,March, 1909.

CONTENTS

CHAPTER PAGE

INTRODUCTION. GENERAL AND HIS-TORICAL xv

I. THE LUMINIFEROUS ETHER AND THE MOD-


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6. Normal reflection in moving medium ... 44

Experiments on Ether drift

7. Interference Kaleidoscope 53

8. Hoek's experiment 56

9. Experiment of Mascart and Jamin ... 57

10. Diagram of Michelson's experiment ... 64

Illustrations of Ether Machine (Lodge)

11. Diagram of course of light 72

12. General view of whirling part of Ether

Machine 7 6

13. General view of optical frame 79

14. Drawing of optical details .... Facing p. 80

15. View of Ether Machine in action . . Frontispiece

1 6. Appearance of interference bands and mi-

crometer wires 80

17. Iron mass for magnetisation 84

18. Appearance of bands 83

19. Arrangement for electrification 85

INTRODUCTION

ETHER or ^Ether (aWrip probably from


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Newton employs the term for the medium

which fills space not only space which appears

to be empty, but space also which appears to be

full ; for the luminiferous ether must undoubtedly

xv

INTRODUCTION

penetrate between the atoms must exist in thepores so to speak of every transparent sub-stance, else light could not travel through it.The following is an extract from Newton'ssurmises concerning this medium:

"Qu. 18. If in two large tall cylindrical

Vessels of Glass inverted, two little Thermo-meters be suspended so as not to touch theVessels, and the Air be drawn out of one ofthese Vessels, and these Vessels thus pre-pared be carried out of a cold place into awarm one; the Thermometer in vacuo willgrow warm as much and almost as soon asthe Thermometer which is not in vacuo.And when the Vessels are carried back intothe cold place, the Thermometer in vacuowill grow cold almost as soon as the otherThermometer. Is not the Heat of the warmRoom conveyed through the Vacuum by the

Vibrations of a much subtiler Medium thanAir, which after the Air was drawn out re-mained in the Vacuum? And is not thisMedium the same with that Medium bywhich Light is [transmitted], and by whosexvi

INTRODUCTION

Vibrations Light communicates Heat toBodies? . . . And do not the Vibrations ofthis Medium in hot Bodies contribute to theintenseness and duration of their Heat?And do not hot Bodies communicate theirHeat to contiguous cold ones by the Vibra-tions of this Medium propagated from theminto the cold ones ? And is not this Mediumexceedingly more rare and subtile than theAir, and exceedingly more elastick and


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active? And doth it not readily pervadeall bodies? And is it not (by its elastickforce) expanded through all the Heavens?""Qu. 22. May not Planets and Comets,and all gross Bodies, perform their motionsmore freely, and with less resistance in this^Ethereal Medium than in any Fluid, whichfills all Space adequately without leavingany Pores, and by consequence is muchdenser than Quick-silver and Gold? Andmay not its resistance be so small, as to beinconsiderable ? For instance ; if this JEiker(for so I will call it) should be supposed700000 times more elastick than our Air,and above 700000 times more rare; itsxvii

INTRODUCTION

resistance would be above 600000000 times

less than that of Water. And so small aresistance would scarce make any sensiblealteration in the Motions of the Planets inten thousand Years."

That the ether, if there be such a thing inspace, can pass readily into or through matter isoften held proven by tilting a mercury barom-eter; when the mercury rises to fill the trans-parent vacuum. Everything points to its uni-versal permeance, if it exist at all.

But these, after all, are antique thoughts.

Electric and Magnetic information has led usbeyond them into a region of greater certaintyand knowledge; so that now I am able to advo-cate a view of the Ether which makes it not onlyuniformly present and all-pervading, but alsomassive and substantial beyond conception. Itis turning out to be by far the most substantialthing perhaps the only substantial thing inthe material universe. Compared to ether thedensest matter, such as lead or gold, is a filmygossamer structure; like a comet's tail or a milkyway, or like a salt in very dilute solution.xviii

INTRODUCTION

To lead up to and justify the idea of the real-ity and substantiality, and vast though as yetlargely unrecognized importance, of the Ether of


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Space, the following chapters have been written.Some of them represent the expanded notes oflectures which have been given in various placeschiefly the Royal Institution; while the firstchapter represents a lecture before the Ashmo-lean Society of the University of Oxford in June,1889. One chapter (viz., Chap. II) has alreadybeen printed as part of an appendix to the thirdedition of Modern Views of Electricity, as well asin the Fortnightly and North American Reviews;but no other chapters have yet been published,though parts appear in more elaborate form inProceedings or Transactions of learned societies.

The problem of the constitution of the Ether,and of the way in which portions of it are modi-fied to form the atoms or other constituent unitsof ordinary matter, has not yet been solved. >Much work has been done in this direction byvarious mathematicians, but much more re-mains to be done. And until it is done, somescepticism is reasonable perhaps laudable.Meanwhile there are few physicists who willxix

INTRODUCTION

dissent from Clerk-Maxwell's penultimate sen-tence in the article "Ether," of which the be-ginning has already been quoted:

"Whatever difficulties we may have informing a consistent idea of the constitution

of the aether, there can be no doubt that theinterplanetary and interstellar spaces arenot empty, but are occupied by a materialsubstance or body, which is certainly thelargest, and probably the most uniformbody of which we have any knowledge."

THE ETHER OF SPACE

THE ETHER OF SPACE

THE LUMINIFEROUS ETHER AND THEMODERN THEORY OF LIGHT

THE oldest and best known function for anether is the conveyance of light, and hencethe name " luminif erous " was applied to it;


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though at the present day many more functionsare known, and more will almost certainly bediscovered.

To begin with, it is best to learn what we canconcerning the properties of the InterstellarEther from the phenomena of Light.

For now well-nigh a century we have hada wave theory of light; and a wave theory oflight is quite certainly true. It is directlydemonstrable that light consists of waves of somekind or other, and that these waves travel at acertain well-known velocity, achieving a distanceequal to seven times the circumference of theearth every second; from New York to London

THE ETHER OF SPACE

and back in the thirtieth part of a second; andtaking only eight minutes on the journey fromthe sun to the earth. This propagation in time

of an undulatory disturbance necessarily in-volves a medium. If waves setting out fromthe sun exist in space eight minutes beforestriking our eyes, there must necessarily be inspace some medium in which they exist andwhich conveys them. Waves we cannot have,unless they be waves in something.

No ordinary matter is competent to transmitwaves at anything like the speed of light: therate at which matter conveys waves is the veloc-ity of sound a speed comparable to one-millionth of the speed of light. Hence the

luminiferous medium must be a special kind ofsubstance; and it is called the ether. Theluminiferous ether it used to be called, becausethe conveyance of light was all it was then knownto be capable of; but now that it is known to doa variety of other things also, the qualifyingadjective may be dropped. But, inasmuch asthe term "ether" is also applied to a familiarorganic compound, we may distinguish the ultra-material luminiferous medium by calling it theEther of Space.

Wave motion in ether, light certainly is; but

what does one mean by the term wave? Thepopular notion is, I suppose, of something heav-ing up and down, or perhaps of something break-

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THEORY OF LIGHT


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ing on a shore. But if you ask a mathematicianwhat he means by a wave, he will probably replythat the most general wave i such a functionof x and y and t as to satisfy the differentialequation

*y_v**.

dt 2 dx 2 'while the simplest wave is

y = a sin (x - vt) .

And he might possibly refuse to give any otheranswer.

And in refusing to give any other answer thanthis, or its equivalent in ordinary words, he isentirely justified; that is what is meant by theterm wave, and nothing less general would beall-inclusive.

Translated into ordinary English, the phrasesignifies, with accuracy and comprehensive com-pleteness, the full details of "a disturbanceperiodic both in space and time." Anything thusdoubly periodic is a wave; and all waveswhether in air as sound waves, or in ether aslight waves, or on the surface of water as oceanwaves can be comprehended in the definition.

What properties are essential to a mediumcapable of transmitting wave motion ? Roughly,we may say two : elasticity and inertia. Elasticityin some form, or some equivalent of it, in order

THE ETHER OF SPACE

to be able to store up energy and effect recoil;inertia, in order to enable the disturbed sub-stance to overshoot the mark and oscillate be-yond its place of equilibrium to and fro. Anymedium possessing these two properties cantransmit waves, and unless a medium possessesthese properties in some form or other, or someequivalent for them, it may be said with

moderate security to be incompetent to transmitwaves. But if we make this latter statement,one must be prepared to extend to the termselasticity and inertia their very largest andbroadest signification, so as to include anypossible kind of restoring force, and any possiblekind of persistence of motion, respectively.

These matters may be illustrated in manyways, but perhaps a simple loaded lath, or


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spring, in a vise will serve well enough. Pull itto one side, and its elasticity tends to make itrecoil ; let it go, and its inertia causes it to over-shoot its normal position. That is what inertiais : power of overshooting a mark, or, moreaccurately, power of moving for a time evenagainst driving force power to rush up hill.Both causes together make it swing to and frotill its energy is exhausted. This is a disturb-ance simply periodic in time. A regular seriesof such springs, set at equ'al intervals and startedvibrating at regular intervals of time one afterthe other, would be periodic in space too; and

THEORY OF LIGHT

so they would, in disconnected fashion, typify

a wave. A series of pendulums will do just aswell, and if set swinging in orderly fashion willfurnish at once an example and an appearance ofwave motion which the most casual observermust recognise as such. The row of springsobviously possesses elasticity and inertia; andany wave-transmitting medium must similarlypossess some form of elasticity and some formof inertia.

But now proceed to ask what is this Etherwhich in the case of light is thus vibrating?What corresponds to the elastic displacement

and recoil of the spring or pendulum? Whatcorresponds to the inertia whereby it overshootsits mark? Do we know these properties in theether in any other way?

The answer, given first by Clerk-Maxwell, andnow reiterated and insisted on by experimentsperformed in every important laboratory in theworld, is:

The elastic displacement corresponds toelectrostatic charge roughly speaking, toelectricity.

The inertia corresponds to magnetism.This is the basis of the modern electromagnetictheory of light.

Let me attempt to illustrate the meaning ofthis statement, by reviewing some fundamentalelectrical facts in the light of these analogies:

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THE ETHER OF SPACE

The old and familiar operation of charging aLeyden jar the storing up of energy in astrained dielectric any electrostatic chargingwhatever is quite analogous to the drawingaside of our flexible spring. It is making use ofthe elasticity of the ether to produce a tendencyto recoil. Letting go the spring is analogous topermitting a discharge of the jar permittingthe strained dielectric to recover itself theelectrostatic disturbance to subside.

In nearly all the experiments of electrostaticsetherial elasticity is manifest.

Next consider inertia. How would one illus-trate the fact that water, for instance, possessesinertia the power of persisting in motionagainst obstacles the power of possessingkinetic energy? The most direct way would

be to take a stream of water and try suddenlyto stop it. Open a water-tap freely and thensuddenly shut it. The impetus or momentumof the stopped water makes itself manifest by aviolent shock to the pipe, with which everybodymust be familiar. This momentum of water isutilised by engineers in the "water-ram."

A precisely analogous experiment in Electricityis what Faraday ca led "the extra current."Send a current through a coil of wire Around apiece of iron, or take any other arrangement fordeveloping powerful magnetism, and then sud-

denly stop the current by breaking the circuit.6

THEORY OF LIGHT

A violent flash occurs if the stoppage is suddenenough a flash which means the bursting ofthe insulating air partition by the accumulatedelectromagnetic momentum. The scientific name

for this electrical inertia is "self-induction."

Briefly we may say that nearly all electro-magnetic experiments illustrate the fact ofetherial inertia.

Now return to consider what happens when acharged conductor (say a Leyden jar) is dis-charged. The recoil of the strained dielectriccauses a current, the inertia of this current causes


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it to overshoot the mark, and for an instant thecharge of the jar is reversed; the current nowflows backward and charges the jar up as atfirst; back again flows the current; and so on,charging and reversing the charge, with rapidoscillations, until the energy is all dissipated intoheat. The operation is precisely analogous tothe release of a strained spring, or to the pluck-ing of a stretched string.

But the discharging body, thus thrown intostrong electrical vibration, is imbedded in the all-pervading ether; and we have just seen that theether possesses the two properties requisite forthe generation and transmission of waves viz.,elasticity, and inertia or density; hence, just asa tuning-fork vibrating in air excites aerial waves,orsound, so a discharging Leyden jar in etherexcites etherial waves, or light.

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THE ETHER OF SPACE

Etherial waves can, therefore, be actually pro-duced by direct electrical means. I dischargehere a jar, and the room is for an instant filledwith light. With light, I say, though you cansee nothing. You can see and hear the spark,indeed ; but that is a mere secondary disturbancewe can for the present ignore I do not meanany secondary disturbance. I mean the true

etherial waves emitted by the electric oscillationgoing on in the neighbourhood of the recoilingdielectric. You pull aside the prong of a tuning-fork and let it go : vibration follows and sound isproduced. You charge a Leyden jar and let itdischarge: vibration follows and light is ex-cited.

It is light, just as good as any other light. Ittravels at the same pace, it is reflected and re-fracted according to the same laws; every ex-periment known to optics can be performed withthis etherial radiation electrically produced

and yet you cannot see it. Why not? For nofault of the light; the fault (if there be a fault)is in the eye. The retina is incompetent torespond to these vibrations they are too slow.The vibrations set up when this large jar is dis-charged are from a hundred thousand to amillion per second, but that is too slow for theretina. It responds only to vibrations between400 billion and 700 billion per second. Thevibrations are too quick for the ear, which re-


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THEORY OF LIGHT

spends only to vibrations between 40 and 40,000per second. Between the highest audible andthe lowest visible vibrations there has beenhitherto a great gap, which these electric oscilla-tions go far to fill up. There has been a greatgap simply because we have no intermediatesense organ to detect rates of vibration between40,000 and 400,000,000,000,000 per second. Itwas therefore an unexplored territory. Waveshave been there all the time in any quantity,but we have not thought about them nor at-tended to them.

It happens that I have myself succeeded ingetting electric oscillations so slow as to beaudible the lowest I had got in 1889 were 125

per second, and for some way above this thesparks emit a musical note; but no one has yetsucceeded in directly making electric oscillationswhich are visible though indirectly everyonedoes it when they light a candle.

It is easy, however, to have an electric os-cillator which vibrates 300 million times asecond, and emits etherial waves a yard long.The whole range of vibrations between musicaltones and some thousand million per second isnow filled up.

With the large condensers and self-inductancesemployed in modern cable telegraphy, it is easyto get a series of beautifully regular and gradu-ally damped electric oscillations, with a period of

THE ETHER OF SPACE

two or three seconds, recorded by an ordinarysignalling instrument or siphon recorder.

These electromagnetic waves in space have

been known on the side of theory ever since1865, but interest in them was immensely quick-ened by the discovery of a receiver or detectorfor them. The great though simple discoveryby Hertz, in 1888, of an "electric eye," as LordKelvin called it, made experiments on thesewaves for the first time easy or even possible.From that time onward we possessed a sortof artificial sense organ for their appreci-ation an electric arrangement which can vir-


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tually "see" these intermediate rates of vibra-tion.

Since then Branly discovered that metallicpowder could be used as an extraordinarily sensi-tive detector; and on the basis of this discovery,the "coherer" was employed by me for distantsignalling by means of electric or etheric waves,until now when many other detectors are avail-able in the various systems of wireless teleg-raphy.

With these Hertzian waves all manner ofoptical experiments can be performed. They canbe reflected by plain sheets of metal, concen-trated by parabolic reflectors, refracted byprisms, and concentrated by lenses. I havemade, for instance, a large lens of pitch, weigh-ing over three hundredweight, for concentrating10

THEORY OF LIGHT

them to a focus. 1 They can be made to showthe phenomenon of interference, and thus havetheir wave-length accurately measured. Theyare stopped by all conductors, and transmittedby all insulators. Metals are opaque; but evenimperfect insulators, such as wood or stone, arestrikingly transparent; and waves may be re-ceived in one room from a source in another,the door between the two being shut.

The real nature of metallic opacity and oftransparency has long been clear in Maxwell'stheory of light, and these electrically producedwaves only illustrate and bring home the well-known facts. The experiments of Hertz are, infact, the apotheosis of Maxwell's theory.

Thus, then, in every way, Clerk-Maxwell's bril-liant perception or mathematical deduction, in1865, of the real nature of light is abundantlyjustified; and for the first time we have a truetheory of light no longer based upon analogywith sound, nor upon the supposed properties of

some hypothetical jelly or elastic solid, butcapable of being treated upon a substantial basisof its own, in alliance with the sciences ofElectricity and of Magnetism.

Light is an electromagnetic disturbance of theether. Optics is a branch of electricity. Out-

1 See Lodge and Howard, Philosophical Magazine forJuly, 1889. See also Phil. Ma%., August, 1888, page 229.


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II

THE ETHER OF SPACE

standing problems in optics are being rap-idly solved, now that we have the means of defi-nitely exciting light with a full perception ofwhat we are doing, and of the precise mode ofits vibration.

It remains to find out how to shorten down thewaves to hurry up the vibration until the lightbecomes visible. Nothing is wanted but quickermodes of vibration. Smaller oscillators must beused very much smaller oscillators not muchbigger than molecules. In all probability onemay almost say certainly ordinary light is theresult of electric oscillation in the molecules oratoms of hot bodies, or sometimes of bodies nothot as in the phenomenon of phosphorescence.

The direct generation of visible light byelectric means, so soon as we have learnt howto attain the necessary frequency of vibration,will have most important practical consequences;and that matter is initially dealt with in a sec-tion on the Manufacture of Light, 149, inChapter XIV of Modern Views of Electricity.But here we abandon further consideration ofthis aspect of our great subject.

II

THE INTERSTELLAR ETHER AS ACONNECTING MEDIUM

SO far I have given a general idea of thepresent condition of the wave theory oflight, both from its theoretical and from itsexperimental sides. The waves of light are not

anything mechanical or material, but are some-thing electrical and magnetic they are, in fact,electrical disturbances periodic in space andtime, and travelling with a known and tremen-dous speed through the ether of space. Theirvery existence depends upon the ether, and theirspeed of propagation is its best known and mostcertain quantitative property.

A statement of this kind does not even initially


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express a tithe of our knowledge on the subject;nor does our knowledge exhaust any large partof the region of discoverable fact; but the state-ment above made may be regarded as certain,although the absence of mechanics or ordinarydynamics about it removes it, or seems to removeit, from the category of the historically soundest

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THE ETHER OF SPACE

and best worked department of Physical Scienceviz., that explored by the Newtonian method.Though in truth there is every reason to supposethat we should have had Newton with us inthese modern developments.

There is, I believe, a general tendency to under-rate the certainty of some of the convictions towhich natural philosophers have gradually, inthe course of their study of nature, been im-

pelled; more especially when those convictionshave reference to something intangible andoccult. The existence of a continuous space-filling medium, for instance, is probably regardedby most educated people as a more or less fancifulhypothesis, a figment of the scientific imagina-tion a mode of collating and welding togethera certain number of observed facts, but not inany physical sense a reality, as water and air arerealities.

I am speaking purely physically. There maybe another point of view from which all material

reality can be denied, but with those questionsphysics proper has nothing to do; it accepts theevidence of the senses, regarding them as thetools or instruments wherewith man may hopeto understand one definite aspect of the uni-verse; and it leaves to philosophers, equippedfrom a different armory, the other aspects whichthe material universe may nay, must possess.

By a physical "explanation" is meant a clear

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A CONNECTING MEDIUM

statement of a fact or law in terms of somethingwith which daily life has made us familiar. Weare all chiefly familiar, from our youth up, withtwo apparently simple things, motion and force.We have a direct sense for both these things. Wedo not understand them in any deep way, prob-


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ably we do not understand them at all, but weare accustomed to them. Motion and force areour primary objects of experience and con-sciousness; and in terms of them all other lessfamiliar occurrences may conceivably be statedand grasped. Whenever a thing can be soclearly and definitely stated, it is said to be ex-plained, or understood; we are said to have "adynamical theory" of it. Anything short ofthis may be a provisional or partial theory, anexplanation of the less known in terms of the moreknown, but Motion and Force are postulated inphysics as the completely known: and no at-tempt is made to press the terms of an explana-tion further than that. A dynamical theory isrecognized as being at once necessary andsufficient.

Now, it must be admitted at once that of veryfew things have we at present such a dynamicalexplanation. We have no such explanation ofmatter, for instance, or of gravitation, or ofelectricity, or ether, or light. It is always con-ceivable that of some such things no purely

dynamical explanation will ever be forthcoming,

THE ETHER OF SPACE

because something more than motion and forcemay perhaps be essentially involved. Still,physics is bound to push the search for an ex-planation to its furthest limits; and so long as itdoes not hoodwink itself by vagueness and merephrases a feebleness against which its leadersare mightily and sometimes cruelly on their

guard, preferring to risk the rejection of worthyideas rather than permit a semi-acceptance ofanything fanciful and obscure so long as itvigorously probes all phenomena within itsreach, seeking to reduce the physical aspect ofthem to terms of motion and force so long itmust be upon a safe track. And, by its failureto deal with certain phenomena, it will learn italready begins to suspect, its leaders must longhave surmised the existence of some third, asyet unknown, category, by incorporating whichthe physics of the future may rise to higherflights and an enlarged scope.

I have said that the things of which we arepermanently conscious are motion and force,but there is a third thing which we have likewisebeen all our lives in contact with, and which weknow even more primarily, though perhaps weare so immersed in it that our knowledge realisesitself later viz., life and mind. I do not nowpretend to define these terms, or to speculate asto whether the things they denote are essentially


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one and not two. They exist, in the sense in16

A CONNECTING MEDIUM

which we permit ourselves to use that word, andthey are not yet incorporated into physics. Tillthey are, they may remain more or less vague;but how or when they can be incorporated, isnot for me even to conjecture.

Still, it is open to a physicist to state how theuniverse appears to him, in its broad characterand physical aspect. If I were to make theattempt, I should find it necessary, for the sakeof clearness, to begin with the simplest and mostfundamental ideas; in order to illustrate, byfacts and notions in universal knowledge, thekind of process which essentially occurs in con-nection with the formation of higher and lessfamiliar conceptions in regions where the com-mon information of the race is so slight as to be

useless.

Primary Acquaintance with the External World.

Beginning with our most fundamental sense, Ishould sketch the matter thus:

We have muscles and can move. I cannotanalyze motion I doubt if the attempt is wiseit is a simple immediate act of perception, adirect sense of free unresisted muscular action.We may indeed move without feeling it, andthat teaches us nothing, but we may move so

as to feel it, and this teaches us much, and leadsto our first scientific inference viz., space; thatis, simply, room to move about. We might

THE ETHER OF SPACE

have had a sense of being jammed into a fullor tight-packed universe; but we have not: wefeel it to be a spacious one.

Of course we do not stop at this baldness of

inference : our educated faculty leads us to realisethe existence of space far beyond the possibilityof direct sensation; and, further, by means of thedirect appreciation of speed in connection withmotion of uniform and variable speed we be-come able to formulate the idea of "time," oruniformity of sequence; and we attain othermore complex notions acceleration, and thelike upon a consideration of which we neednot now enter.


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But our muscular sense is not limited to theperception of free motion: we constantly find itrestricted or forcibly resisted. This "muscularaction impeded" is another direct sense, that of"force"; and attempts to analyze it into any-thing simpler than itself have hitherto resultedonly in confusion. By "force" is meant pri-marily muscular action not accompanied bymotion. Our sense of this teaches us thatspace, though roomy, is not empty: it gives usour second scientific inference what we call"matter."

Again we do not stop at this bare inference.By another sense, that of pain, or mere sensa-tion, we discriminate between masses of matterin apparently intimate relation with ourselves,18

A CONNECTING MEDIUM

and other or foreign lumps of matter; and weuse the first portion as a measure of the extentof the second. The human body is our standardof size. We proceed also to subdivide our ideaof matter according to the varieties of resist-ance with which it appeals to our muscularsense into four different states, or "elements,"as the ancients called them viz., the solid, theliquid, the gaseous, and the ethereal. Theresistance experienced when we encounter one

or other of these forms of material existencevaries from something very impressive thesolid ; through something nearly impalpablethe gaseous ; up to something entirely imagina-tive, fanciful, or inferential viz., the ether.

The ether does not in any way affect our senseof touch (i.e., of force) ; it does not resist motionin the slightest degree. Not only can our bodiesmove through it, but much larger bodies, planetsand comets, can rush through it at what we arepleased to call a prodigious speed (being fargreater than that of an athlete) without showing

the least sign of friction. I myself, indeed, havedesigned and carried out a series of delicateexperiments to see whether a whirling mass ofiron could to the smallest extent grip the etherand carry it round, with so much as a thousandthpart of its own velocity. These shall be de-scribed further on, but meanwhile the resultarrived at is distinct. The answer is, no; I

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THE ETHER OF SPACE

cannot find a trace of mechanical connectionbetween matter and ether, of the kind knownas viscosity or friction.

Why, then, if it is so impalpable, should weassert its existence? May it not be a merefanciful speculation, to be extruded from physicsas soon as possible? If we were limited for ourknowledge of matter to our sense of touch, thequestion would never even have presented itself;we should have been simply ignorant of theether, as ignorant as we are of any life or mindin the universe not associated with some kindof material body. But our senses have attaineda higher stage of development than that. Weare conscious of matter by means other than itsresisting force. Matter acts on one small por-tion of our body in a totally different way, andwe are said to taste it. Even from a distance it

is able to fling off small particles of itself sufficientto affect another delicate sense. Or again, if itis vibrating with an appropriate frequency, an-other part of our body responds; and the uni-verse is discovered to be not silent but eloquentto those who have ears to hear. Are there anymore discoveries to be made ? Yes ; and alreadysome have been made. All the senses hithertomentioned speak to us of the presence of or-dinary matter gross matter, as it is sometimescalled though when appealing to our sense ofsmell, and more especially to a dog's sense of20

A CONNECTING MEDIUM

smell, it is not very gross; still, with the senseshitherto enumerated we should never have be-come aware of the ether. A stroke of lightningmight have smitten our bodies back into theirinorganic constituents, or a torpedo-fish mighthave inflicted on us a strange kind of torment;but from these violent tutors we should havelearnt little more than a school-boy learns from

the once ever-ready cane.

But it so happens that the whole surface of ourskin is sensitive in yet another way, and a smallportion of it is asftoundingly and beautifullysensitive, to an impression of an altogether dif-ferent character one not necessarily associatedwith any form of ordinary matter one that willoccur equally well through space from which allsolid, liquid, or gaseous matter has been removed.


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Hold your hand near a fire, put your face in thesunshine, and what is it you feel? You are nowconscious of something not arriving by ordinarymatter at all. You are now as directly consciousas you can be of the ethereal medium. True theprocess is not very direct. You cannot apprehendthe ether as you can matter, by touching ortasting or even smelling it; but the process isanalogous to the kind of perception we mightget of ordinary matter if we had the sense ofhearing alone. It is something akin to vibra-tions in the ether that our skin and our eyes feel.

It may be rightly asserted that it is not the21

THE ETHER OF SPACE

ethereal disturbances themselves, but other dis-turbances excited by them in our tissues, that ourheat nerves feel; and the same assertion canbe made for our more highly developed and

specialised sight nerves. All nerves must feelwhat is occurring next door to them, and candirectly feel nothing else; but the "radiation,"the cause which excited these disturbances,travelled througi the ether not through anyotherwise known material substance.

It should be a commonplace to rehearse howwe know this. Briefly, thus: Radiation con-spicuously comes to us from the sun. If anyfree or ordinary matter exists in the interveningspace, it must be an exceedingly rare gas. Inother words, it must consist of scattered par-

ticles of matter, some big enough to be calledlumps, some so small as to be merely atoms, buteach with a considerable gap between it and itsneighbor. Such isolated particles are absolute-ly incompetent to transmit light. And, paren-thentically, I may say that no form of ordinarymatter, solid, liquid, or gaseous, is competentto transmit a thing travelling with the speedand subject to the known laws of light. Forthe conveyance of radiation or light all ordinarymatter is not only incompetent, but hopelesslyand absurdly incompetent. If this radiationis a thing transmitted by anything at all, it must

be by something sui generis.22

A CONNECTING MEDIUM

But it is transmitted; for it takes time on thejourney, travelling at a well-known and definitespeed; and it is a quivering or periodic disturb-


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ance, falling under the general category ofwave-motion. Nothing is more certain thanthat. No physicist disputes it. Newton him-self, who is commonly and truly asserted tohave promulgated a rival theory, felt the ne-cessity of an ethereal medium, and knew thatlight consisted essentially of waves.

Sight.

A small digression here, to avoid any possibleconfusion due to the fact that I have purposelyassociated together temperature nerves and sightnerves. They are admittedly not the same,but they are alike in this, that they both affordevidence of radiation; and, were we blind, wemight still know a good deal about the sun, andif our temperature nerves were immensely in-creased in delicacy (not all over, for that wouldbe merely painful, but in some protected region) ,we might even learn about the moon, planets,and stars. In fact, an eye, consisting of a pupil(preferably a lens) and a sunken cavity linedwith a surface sensitive to heat, could readily be

imagined, and might be somewhat singularlyeffective. It would be more than a light recorder;it could detect all the ethereal quiverings caused

2 3

THE ETHER OF SPACE

by surrounding objects, and hence would seeperfectly well in what we call "the dark." Butit would probably see far too much for con-

venience, since it would necessarily be affectedby every kind of radiation in simple proportionto its energy; unless, indeed, it were providedwith a supply of screens with suitably selectedabsorbing powers. But whatever might be theadvantage or disadvantage of such a sense-organ, we as yet do not possess one. Our eyedoes not act by detecting heat; in other words,it is not affected by the whole range of etherealquiverings, but only by a very minute andapparently insignificant portion. It whollyignores the ether waves whose frequency iscomparable with that of sound; and, for thirty

or forty octaves above this, nothing about usresponds; but high up, in a range of vibrationof the inconceivably high pitch of four to sevenhundred million million per second a rangewhich extremely few accessible bodies are ableto emit, and which it requires some knowledgeand skill artificially to produce to those wavesthe eye is acutely, surpassingly, and most in-telligently sensitive.


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This little fragment of total radiation is initself trivial and negligible. Were it not formen, and glow-worms, and a few other forms oflife, hardly any of it would ever occur, on such amoderate- sized lump of matter as the earth.24

A CONNECTING MEDIUM

Except for an occasional volcano, or a flash oflightning, only gigantic bodies like the sun andstars have energy enough to produce these high-er flute-like notes ; and they do it by sheer mainforce and violence the violence of their gravi-tative energy producing not only these, butevery other kind of radiation also. Glow-worms, so far as I know, alone have learned thesecret of emitting the physiologically usefulwaves, and none others.

Why these waves are physiologically usefulwhy they are what is called "light," while other

kinds of radiation are "dark," is a question tobe asked, but, at present, only tentatively an-swered. The answer must ultimately be givenby the Physiologist; for the distinction betweenlight and non-light can only be stated in termsof the eye, and its peculiar specialised sensitive-ness; but a hint may be given him by thePhysicist. The ethereal waves which affect theeye and the photographic plate are of a size notwholly incomparable with that of the atoms ofmatter. When a physical phenomenon is con-cerned with the ultimate atoms of matter, it isoften relegated at present to the field of knowl-

edge summarized under the head of Chemistry.Sight is probably a chemical sense. The retinamay contain complex aggregations of atoms,shaken asunder by the incident light vibrations,and rapidly built up again by the living tissues

3 25

THE ETHER OF SPACE

in which they live ; the nerve endings meanwhile

appreciating them in their temporarily dissoci-ated condition. A vague speculation! Not tobe further countenanced except as a workinghypothesis leading to examination of fact; but,nevertheless, the direction in which the thoughtsof some physicists are tending a directiontoward which many recently discovered ex-perimental facts point. 1

Gravitation and Cohesion.


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It would take too long to do more than suggestsome other functions for which a continuousmedium of communication is necessary. Weshall argue in Chapter VIII that technical actionat a distance is impossible. A body can onlyact immediately on what it is in contact with;it must be by the action of contiguous particlesthat is, practically, through a continuousmedium, that force can be transmitted acrossspace. Radiation is not the only thing theearth feels from the sun ; there is in addition itsgigantic gravitative pull, a force or tension morethan what a million million steel rods, each seven-teen feet in diameter, could stand (see Chap. IX).What mechanism transmits this gigantic force?Again, take a steel bar itself: when violently

1 Cf. sections i$7A, 143, 187, and chap, xvi., of myModern Views of Electricity.

26

A CONNECTING MEDIUM

stretched, with how great tenacity its parts clingtogether! Yet its particles are not in absolutecontact, they are only virtually attached to eachother by means of the universal connectingmedium the ether a medium that must becompetent to transmit the greatest stresses whichour knowledge of gravitation and of cohesionshows us to exist.

O

Electricity and Magnetism.

Hitherto I have mainly confined myself to theperception of the ether by our ancient sense ofradiation, whereby we detect its subtle anddelicate quiverings. But we are growing a newsense; not perhaps an actual sense-organ, thoughnot so very unlike a new sense-organ, though theportions of matter which go to make the organare not associated with our bodies by the usuallinks of pain and disease; they are more analo-gous to artificial teeth or mechanical limbs, and

can be bought at an instrument-maker's.

Electroscopes, galvanometers, telephonesdelicate instruments these ; not yet eclipsing oursense-organs of flesh, but in a few cases comingwithin measurable distance of their surprisingsensitiveness. And with these what do we do?Can we smell the ether, or touch it, or what isthe closest analogy ? Perhaps there is no usefulanalogy; but nevertheless we deal with it, and


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27

THE ETHER OF SPACE

that closely. Not yet do we fully realise whatwe are doing. Not yet have we any dynamicaltheory of electric currents, of static charges, andof magnetism. Not yet, indeed, have we anydynamical theory of light. In fact, the etherhas not yet been brought under the domain ofsimple mechanics it has not yet been reducedto motion and force : and that probably becausethe force aspect of it has been so singularlyelusive that it is a question whether we oughtto think of it as material at all. No, it is apartfrom mechanics at present. Conceivably itmay remain apart ; and our first additional cate-gory, wherewith the foundations of physicsmust some day be enlarged, may turn out tobe an ethereal one. And some such inclusionmay have to be made before we can attempt toannex vital or mental processes. Perhaps they

will all come in together.

Howsoever these things be, this is the kind ofmeaning lurking in the phrase that we do not yetknow what electricity or what the ether is. Wehave as yet no dynamical explanation of eitherof them; but the past century has taught uswhat seems to their student an overwhelmingquantity of facts about them. And when thepresent century, or the century after, lets usdeeper into their secrets, and into the secrets ofsome other phenomena now in course of beingrationally investigated, I feel as if it would be

28

A CONNECTING MEDIUM

no merely material prospect that will be openingon our view, but some glimpse into a region ofthe universe which Science has never enteredyet, but which has been sought from far, and

perhaps blindly apprehended, by painter andpoet, by philosopher and saint.

Note on ike Spelling of Ethereal.

The usual word "ethereal" suggests something un-substantial, and is so used in poetry; but for theprosaic treatment of Physics it is unsuitable, andetheric has occasionally been used instead. No justderivation can be given for such an adjective, how-


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ever; and I have been accustomed simply to spelletherial with an * when no poetic meaning was intend-ed. This alternative spelling is not incorrect; butMilton uses the variant "ethereous," in a sense sug-gestive of something strong and substantial (Par.Lost, vi, 473). Either word, therefore, can be em-ployed to replace "ethereal " in physics : and in succeed-ing chapters one or other of these is for the most partemployed.

Ill

INFLUENCE OF MOTION ON VARIOUSPHENOMENA

NOTWITHSTANDING its genuine physicalnature and properties, the ether is singularlyintangible and inaccessible to our senses, and ac-cordingly is a subject on which it is extremelydifficult to try experiments. Many have been

the attempts to detect some phenomena de-pending on its motion relative to the earth.The earth is travelling round the sun at the rateof 19 miles a second, and although this is slowcompared with light being, in fact, just aboutio^ooth of the speed of light yet it would seemfeasible to observe some modification of opticalphenomena due to this motion through the ether.And one such phenomenon is indeed knownnamely, the stellar aberration discovered byBradley in 1729. The position of objects noton the earth, and not connected with the solarsystem, is apparently altered by an amount

comparable to one part in ten thousand, by theearth's motion; that is to say, the apparent place

30

INFLUENCE OF MOTION

of a star is shifted from its true place by an angle

io^oth of a "radian," 1 or about 20 seconds of arc.

This is called Astronomical Aberration, and isextremely well known. But a number of otherproblems open out in connection with it, and onthese it is desirable to enter into detail. For ifthe ether is stationary while the earth is flyingthrough it at a speed vastly faster than anycannon-ball, as much faster than a cannon-ballas an express train is faster than a saunter on


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foot it is for all practical purposes the same asif the earth were stationary and the ether stream-ing past it with this immense velocity in theopposite direction. And some consequence ofsuch a drift might at first sight certainly beexpected. It might, for instance, seem doubtfulwhether terrestrial surveying operations can beconducted, with the extreme accuracy expectedof them, without some allowance for the violentrush of the light-conveying medium past andthrough the theodolite of the observer.

Let us therefore consider the whole subjectfurther.

ABERRATION.

Everybody knows that to shoot a bird on thewing you must aim in front of it. Every one will

1 Radian is the name given by Prof. James Thomsonto a unit angle of circular measure, an angle whose arcequals its radius, or about 57.

3 1

THE ETHER OF SPACE

readily admit that to hit a squatting rabbit froma moving train you must aim behind it.

These are examples of what may be called"aberration" from the sender's point of view,from the point of view of the source. And theaberration, or needful divergence, between the

point aimed at and the thing hit has oppositesign in the two cases the case when receiver ismoving, and the case when source is moving.Hence, if both be moving, it is possible for thetwo aberrations to neutralize each other. So tohit a rabbit running alongside the train you mustaim straight at it.

If there were no air, that is all simple enough.But every rifleman knows to his cost thatthough he fixes both himself and his targettightly to the ground, so as to destroy allaberration proper, yet a current of air is very

competent to introduce a kind of spuriousaberration of its own, which may be called wind-age; and that he must not aim at the target ifhe wants to hit it, but must aim a little in theeye of the wind.

So much from the shooter's point of view.Now attend to the point of view of the target.

Consider it made of soft enough material to be


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completely penetrated by the bullet, leaving alongish hole wherever struck. A person behindthe target, whom we may call a marker, byapplying his eye to the hole immediately after

3 2

INFLUENCE OF MOTION

the hit, may be able to look through it at theshooter, and thereby to spot the successful man.I know that this is not precisely the function ofan ordinary marker, but it is more completethan his ordinary function. All he does usuallyis to signal an impersonal hit ; some one else has torecord the identity of the shooter. I am ratherassuming a volley of shots, and that the markerhas to allocate the hits to their respective sourcesby means of the holes made in the target.

Well, will he do it correctly? Assuming, ofcourse, that he can do so if everything is station-

ary, and ignoring all curvature of path, whethervertical or horizontal curvature. If you thinkit over you will perceive that a wind will notprevent his doing it correctly; the line of holewill point to the shooter along the path of hisbullet, though it will not point along his lineof aim. Also, if the shots are fired from a mov-ing ship, the line of hole in a stationary targetwill point to the position the gun occupied at theinstant the shot was fired, though it may havemoved since then. In neither of these cases(moving medium and moving source) will therebe any error.

But if the target is in motion, on an armouredtrain for instance, then the marker will be atfault. The hole will not point to the man whofired the shot, but to an individual ahead of him.The source will appear to be displaced in the

33

THE ETHER OF SPACE

direction of the observer's notion. This is com-mon aberration. It is the simplest thing inthe world. The easiest illustration of it is thatwhen you run through a vertical shower, you tiltyour umbrella forward; or, if you have not gotone, the drops hit you in the face; more ac-curately, your face as you run forward hits thedrops. So the shower appears to come from acloud ahead of you, instead of from one over-head.


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We have thus three motions to consider, thatof the source, of the receiver, and of the medium;and, of these, only motion of receiver is able tocause an aberrational error in fixing the positionof the source.

So far we have attended to the case of pro-jectiles, with the object of leading up to light.But light does not consist of projectiles, it con-sists of waves; and with waves matters are alittle different. Waves crawl through a mediumat their own definite pace ; they cannot be flungforward or sideways by a moving source; theydo not move by reason of an initial momentumwhich they are gradually expending, as shotsdo; their motion is more analogous to that of abird or other self-propelling animal, than it is tothat of a shot. The motion of a wave in amoving medium may be likened to that ofa rowing-boat on a river. It crawls forwardwith the water, and it drifts with the water;

34

INFLUENCE OF MOTION

its resultant motion is compounded of the two,but it has nothing to do with the motionof its source. A shot from a passing steamerretains the motion of the steamer as well as thatgiven it by the powder. It is projected, there-fore, in a slant direction. But a boat loweredfrom the side of a passing steamer, and rowingoff, retains none of the motion of its source; it is

not projected, it is self-propelled. That is likethe case of a wave.

The diagram illustrates the difference. Fig. ishows a moving cannon or machine-gun, movingwith the arrow, and firing a succession of shotswhich share the motion of the cannon as well astheir own, and so travel slant. The shot firedfrom position 1 has reached A, that fired fromposition 2 has reached B, and that fired fromposition 3 has reached C, by the time the fourthshot is fired at D. The line A B C D is a pro-longation of the axis of the gun ; it is the line of

aim, but it is not the line of fire; all the shotsare travelling aslant this line, as shown by thearrows. There are thus two directions to bedistinguished. There is the row of successiveshots, and there is the path of any one shot.These two directions enclose an angle. It maybe called an aberration angle, because it is dueto the motion of the source, but it need not giverise to any aberration. True direction may stillbe perceived from the point of view of the receiver.


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35

THE ETHER OF SPACE

To prove this let us attend to what is happeningat the target. The first shot is supposed to beentering at A, and if the target is stationary willleave it at Y. A marker looking along Y A willsee the position whence the shot was fired. Thismay be likened to a stationary observer lookingat a moving star. He sees it where and as it waswhen the light started on its long journey. He

FIG. i. Shots or Disturbances with Momentumfrom a Moving Gun.

does not see its present position, but there is noreason why he should. He does not see its

physical state or anything as it is now. He seesit as it was when it sent the information whichhe has just received. There is no aberrationcaused by motion of source.

But now let the receiver be moving at samepace as the gun, as when two grappled ships arefiring into each other. The motion of the targetcarries the point Y forward, and the shot Aleaves it at Z, because Z is carried to where Ywas. So in that case the marker looking along

36

INFLUENCE OF MOTION

Z A will see the gun, not as it was when firing,but as it is at the present moment; and he willsee likewise the row of shots making straightfor him. This is like an observer looking at aterrestrial object. Motion of the earth does notdisturb ordinary vision.

Fig. 2 shows as nearly the same sort of thing as

possible for the case of emitted waves. Thetube is a source emitting a succession of disturb-ances without momentum. A B C D may bethought of as horizontally flying birds, or as crestsof waves, or as self-swimming torpedoes; or theymay even be thought of as bullets, if the gunstands still every time it fires, and only movesbetween whiles.


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fr-

FIG. 2. Waves or Disturbances without Momentumfrom a Moving Source.

The line A B C D is now neither the line of firenor the line of aim: it is simply the locus ofdisturbances emitted from the successive posi-tions 1234.

37

THE ETHER OF SPACE

A stationary target will be penetrated in thedirection A Y, and this line will point out thecorrect position of the source when the receiveddisturbance started. If the target moves, a dis-turbance entering at A may leave it at Z, or

at any other point according to its rate ofmotion; the line Z A does not point to theoriginal position of the source, and so there willbe aberration when the target moves. Other-wise there would be none.

Now, Fig. 2 also represents a parallel beam oflight travelling from a moving source, andentering a telescope or the eye of an observer.

FIG. 3. Beam from a Revolving Lighthouse.

The beam lies along A B C D, but this is not thedirection of vision. The direction of vision, to astationary observer, is determined not by thelocus of successive waves, but by the path ofeach wave. A ray may be denned as the pathof a labelled disturbance. The line of visionis Y A 1, and coincides with the line of aim;

38

INFLUENCE OF MOTION

which in the projectile case (Fig. i) it didnot.

The case of a revolving lighthouse, emittinglong parallel beams of light and brandishingthem rapidly round, is rather interesting. Fig. 3may assist the thinking out of this case. Suc-


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cessive disturbances A, B, C, D, lie along a spiralcurve, the spiral of Archimedes; and this is theshape of the beams, as seen illuminating the dustparticles, though the pitch of the spiral is toogigantic to be distinguished from a straight line.At first sight it might seem as if an eye lookingalong those curved beams would see the light-house slightly out of its true position; but it isnot so. The true rays or actual paths of eachdisturbance are truly radial ; they do not coincidewith the apparent beam. An eye looking atthe source will not look tangentially along thebeam, but will look along A S, and will see thesource in its true position. It would be other-wise for the case of projectiles from a revolvingturret.

Thus, neither translation of star nor rotationof sun can affect direction. There is no aberra-tion so long as the receiver is stationary.

But what about a wind, or streaming of themedium past source and receiver, both station-ary? Look at Fig. i again. Suppose a row

of stationary cannon firing shots, which getblown by a cross wind along the slant 1 A Y

39

THE ETHER OF SPACE

(neglecting the curvature of path which wouldreally exist) : still the hole in the target fixes thegun's true position, the marker looking alongY A sees the gun which fired the shot. There is

no true deviation from the point of view of thereceiver, provided the drift is uniform every-where, although the shots are blown aside andthe target is not hit by the particular gun aimedat it.

With a moving cannon combined with an op-posing wind, Fig. i would become very like Fig. 2.

(N.B. The actual case, even without com-plication of spinning, etc., but merely with thecurved path caused by steady wind-pressure, isnot so simple, and there would really be an

aberration or apparent displacement of thesource toward the wind's eye: an apparentexaggeration of the effect of wind shown in thediagram.)

In Fig. 2 the result of a wind is much the same,though the details are rather different. Themedium is supposed to be drifting downward,across the field. The source may be taken asstationary at S. The horizontal arrows show the


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direction of waves in the medium; the dottedslant line shows their resultant direction. Awave centre drifts from D to 1 in the same timeas the disturbance reaches A, travelling downthe slant line D A. The angle between dottedand full lines is the angle between ray and wave40

INFLUENCE OF MOTION

normal. Now, if the motion of the medium in-side the receiver is the same as it is outside, thewave will pass straight on along the slant toZ, and the true direction of the source is fixed.But if the medium inside the target or telescopeis stationary, the wave will cease to drift as soonas it gets inside under cover, as it were ; it willproceed along the path it has been really pur-suing in the medium all the time, and make itsexit at Y. In this latter case of differentmotion of the medium inside and outside thetelescope the apparent direction, such as Y A,

is not the true direction of the source. The rayis in fact bent where it enters the differently movingmedium (as shown in Fig. 4).

FIG. 4. Ray through a Moving Stratum.

A slower moving stratum bends an obliqueray, slanting with the motion, in the samedirection as if it were a denser medium. Aquicker stratum bends it oppositely. If a

4 4 I

THE ETHER OF SPACE

medium is both denser and quicker moving, itis possible for the two bendings to be equal andopposite, and thus for a ray to go on straight.Parenthetically, I may say that this is preciselywhat happens, on Fresnel's theory, down theaxis of a water-filled telescope exposed to thegeneral terrestrial ether drift.

In a moving medium waves do not advance intheir normal direction, they advance slantways.The direction of their advance is properly calleda ray. The ray does not coincide with thewave-normal in a moving medium.

All this is well shown in Fig. 5.

S is a stationary source emitting successive


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waves, which drift as spheres to the right. Thewave which has reached M has its centre at C,and C M is its normal; but the disturbance, M,has really travelled along S M, which is thereforethe ray. It has advanced as a wave from S to P,and has drifted from P to M. Disturbancessubsequently emitted are found along the ray,precisely as in Fig. 2. A stationary telescopereceiving the light will point straight at S. Amirror, M, intended to reflect the light straightback must be set normal to the ray, not tangen-tial to the wave front.

The diagram also equally represents the case

of a moving source in a stationary medium. The

source, starting at C, has moved to S, emitting

waves as it went ; which waves, as emitted, spread

42

INFLUENCE OF MOTION

out as simple spheres from the then position ofsource as centre. Wave-normal and ray nowcoincide : S M is not a ray, but only the locus ofsuccessive disturbances. A stationary telescope

FIG. 5. Successive Wave Fronts in aMoving Medium.

would look not at S, but along M C to a pointwhere the source was when it emitted the waveM ; a moving telescope, if moving at same rate assource, will look at S. Hence S M is sometimescalled the apparent ray. The angle S M C is the

43

THE ETHER OF SPACE

aberration angle, which in Chap. X we denoteby .

Fig. 6 shows normal reflection for the case ofa moving medium. The mirror M reflects lightreceived from S t to a point S 2 just in time tocatch the source there if that is moving with the


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it must be stationary altogether: there must beno boundary between stationary and moving46

EXPERIMENTS ON THE ETHER

ether, no plane of slip, no quicker motion evenin some regions than in others. For (referringback to the remarks preceding Fig. 4) if the etherin receiver is stagnant while outside it is moving,a wave which has advanced and drifted as far asthe telescope will cease to drift as soon as it getsinside, but will advance simply along the wavenormal. And in general, at the boundary ofany such change of motion a ray will be bent,and an observer looking along the ray willsee the source not in its true position, noteven in the apparent position appropriate tohis own motion, but lagging behind that po-sition.

Such an aberration as this, a lag or negative

aberration, has never yet been observed; but ifthere is any slip between layers of ether, if theearth carries any ether with it, or if the ether,being in motion at all, is not equally in motioneverywhere throughout every transparent sub-stance, then such a lag or negative aberrationmust occur: in precise proportion to the amountof the carriage of ether by moving bodies

(cf- P- 63).

On the other hand, if the ether behaves as aperfectly frictionless in viscid fluid, or if for any

other reason there is no rub between it andmoving matter, so that the earth carries noether with it at all, then all rays will be straight,aberration will have its simple and well-known

47

THE ETHER OF SPACE

value, and we shall be living in a virtual etherstream of 19 miles a second, by reason of the

orbital motion of the earth.

It may be difficult to imagine that a great masslike the earth can rush at this tremendous pacethrough a medium without disturbing it. It isnot possible for an ordinary sphere in an ordinaryfluid. At the surface of such a sphere there is aviscous drag, and a spinning motion diffuses outthence through the fluid, so that the energy ofthe moving body is gradually dissipated. The


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persistence of terrestrial and planetary motionsshows that etherial viscosity, if existent, is small;or at least that the amount of energy thus gotrid of is a very small fraction of the whole. Butthere is nothing to show that an appreciable layerof ether may not adhere to the earth and travelwith it, even though the force acting on it be butsmall.

This, then, is the question before us:

Does the earth drag some ether with it? ordoes it slip through the ether with perfect free-dom? (Never mind the earth's atmosphere;the part it plays is known and not impor-tant.)

In other words, is the ether wholly or partiallystagnant near the earth, or is it streaming pastus with the opposite of the full terrestrial velocityof nineteen miles a second? Surely if we areliving in an ether stream of this rapidity we48


ought to be able to detect some evidence of itsexistence. 1

It is not so easy a thing to detect as you wouldimagine. We have seen that it produces nodeviation or error in direction. Neither does itcause any change of colour or Doppler effect;that is, no shift of lines in spectrum. No steadywind can affect pitch, simply because it cannotblow waves to your ear more quickly than theyare emitted. It hurries them along, but itlengthens them in the same proportion, and theresult is that they arrive at the proper fre-quency. The precise effects of motion on pitchare summarised 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 andvelocity in exactly compensatory manner.

What other phenomena may possibly resultfrom motion? Here is a list:


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Phenomena resulting from Motion

(i) Change or apparent change in direction;observed by telescope, and called aberration.

1 The word "stationary" is ambiguous. I proposeto use "stagnant," as meaning stationary with respectto the earth i.e., as opposed to stationary in space.

49

THE ETHER OF SPACE

(2) Change or apparent change in frequency;observed by spectroscope, and called Dopplereffect.

(3) Change or apparent change in time ofjourney; observed by lag of phase or shift ofinterference fringes.

(4) Change or apparent change in intensity;

observed by energy received by thermopile.

What we have arrived at so far is the fol-lowing :

Motion of either source or receiver can alterfrequency ; motion of receiver can alter apparentdirection ; motion of the medium can do neither.

But the question must be asked, can it nothurry a wave so as to make it arrive out of phasewith another wave arriving by a different path,and thus produce or modify interference effects ?

Or again, may it not carry the waves downstream more plentifully than up stream, andthus act on a pair of thermopiles, arranged foreand aft at equal distances from a source, withunequal intensity?

And once more, perhaps the laws of reflectionand refraction in a moving medium are not thesame as they are if it be at rest. Then, more-over, there is double refraction, colours of thinplates and thick plates, polarisation angle, ro-tation of the plane of polarisation; all sorts of

optical phenomena that need consideration.

50


It may have to be admitted, perhaps, that inempty space the effect of an ether drift is dif-


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ficult to detect, but will not the presence ofdense matter especially the passage throughdense transparent matter make the detectioneasier? So a great number of questions arise,all of which have been, from time to time, seri-ously discussed.

Interference.

As an instance of such discussion, considerNo. 3 of the phenomena tabulated above. Iexpect that every reader understands inter-ference, but I may just briefly say that twosimilar sets of waves "interfere" whenever andwherever the crests of one set coincide with andobliterate the troughs of the other set. Lightadvances in any given direction when crests inthat direction are able to remain crests, andtroughs to remain troughs. But if we contriveto split a beam of light into two halves, to sendthem round by different paths, and make themmeet again, there is no guarantee that crest willmeet crest and trough trough; it may be justthe other way in some places, and wherever that

opposition of phase occurs there there will belocal obliteration or "interference." Two re-united half -beams of light may thus producelocal stripes of darkness, and these stripes arecalled interference bands.

THE ETHER OF SPACE

It is not to be supposed that there is anydestruction of light, or any dissipation of energy :it is merely a case of redistribution.

The bright parts are brighter just in propor-tion as the dark parts are darker. The screen isilluminated in stripes and no longer uniformly,but its total illumination is the same as if therewere no interference.

PROJECTION OP INTERFERENCE BANDS.

It is not easy to project these interferencebands on a screen so as to make them visible

to an audience, partly because the bands orstripes of darkness are exceedingly narrow;indeed, I had not previously seen the experimentattempted. But by means of what I call aninterference kaleidoscope, consisting of twomirrors set at an angle with a third semi-trans-parent mirror between them, it is possible toget the bands remarkably clear and bright, sothat they can readily be projected: and I showedthese at a lecture to the Royal Institution of


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Great Britain in 1892.

Each mirror is mounted on a tripod withadjustable screw feet, which stand on a thickiron slab, which again rests on hollow india-rubber balls. Looking down on the mirrors theplan is as in the diagram Fig. 7, which indicatessufficiently the geometry of the arrangement,

52


and shows that the two half beams, into whichthe semi-transparent plate divides the light, willeach travel round the same contour A B C inopposite directions, and will then reunite andtravel together toward the point of the arrow.

FIG. 7. Plan of Interference Kaleidoscope with threemirrors.

The arrow-feather ray is bifurcated at A by a semi-transparentmirror of thinly silvered glass; and the two halves reunite alongthe arrow-head after traversing a triangular contour A B C inopposite directions. The simple geometrical relations which permitthis are sufficiently indicated in the figure. The arrangementwould suit Fizeau's experiment.

53*

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A parallel beam from an electric lantern, whenthus treated, depicts bright and broad inter-ference bands on a screen. And the arrangementis very little sensitive to disturbance, becausethe paths of the two halves of the beam areidentical, and because of the mounting. Apiece of good glass can be interposed withoutdisturbance, and the table can be struck a heavyblow without confusing the bands.

The only regular and orderly way of causing ashift of the bands is to accelerate one half of thebeam and to retard the other half by moving atransparent substance along the contour. Forinstance, let the sides of the triangle A B C, orone of them, consist of a tube of water in whicha rapid stream is maintained; then the streamhas a chance of accelerating one half the beamand retarding the other half, thereby shiftingthe fringes from their normal position by a


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measurable amount. This is the experimentmade in 1859 by Fizeau. (Appendix 3.)

Now that most interesting and important, andI think now well-known, experiment of Fizeauproves quite simply and definitely that if lightbe sent along a stream of water, travelling insidethe water as a transparent medium, it will goquicker with the current than against it.

You may say that is only natural; a windassists sound one way and retards it the oppositeway. Yes, but then sound travels in air; and

54


wind is a bodily transfer of air; hence, of course,gives the sound a ride. Whereas light does notreally travel in water, but always in ether; andit is by no means obvious whether a stream of

water can help or hinder it. Experiment decides,however, and answers in the affirmative. Ithelps it along with just about half the speed ofthe water; not with the whole speed, which iscurious and important, and really means thatthe moving water has no effect whatever onthe ether of space, though we must defer ex-plaining how this comes about. Suffice forpresent purposes the fact that the velocityof light inside moving water, and thereforepresumably inside all transparent matter, isaltered to some extent by motion of thatmatter.

Does not this fact afford an easy way of de-tecting a motion of the earth through the ether ?Every vessel of stagnant water is really travel-ling along through the ether at the rate of nine-teen miles a second. Send a beam of lightthrough it one way, and it will be hurried; itsvelocity, instead of being 140,000 miles a second,will be 140,009 miles. Send a beam of light theother way, and its velocity will be 139,991 ; justas much less. Bring these two beams together;surely some of their wave-lengths will interfere.M. Hoek, Astronomer at Utrecht, tried the ex-

periment in this very form; here is a diagram of

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his apparatus (Fig. 8). Babinet had tried an-other form of the experiment previously. Hoek


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expected to see interference bands from the twohalf-beams which had traversed the water, one

FIG. 8. Hoek's arrangement.

The light from source S is reflected so as to travel half throughstagnant water and half through air on its direct journey, the pathbeing inverted on the return journey, after whch it enters the eye.

in the direction of the earth's motion and theother against it. But no interference bandswere seen. The experiment gave a negativeresult.

An experiment, however, in which nothing isseen is never a very satisfactory form of a nega-tive experiment ; it is, as Mascart calls it, " doublynegative," and we require some guarantee thatthe conditions were right for seeing what mighl

really have been in some sort there. Henc(Mascart and Jamin's modification of the experi-ment is preferable (Fig. 9). The thinglooked for is a shift of already existing inter-ference bands, when the above apparatus isturned so as to have different aspects with re-

56


spect to the earth's motion; but no shift wasseen.

Interference methods all fail to display anytrace of relative motion between earth and ether.

Try other phenomena, then. Try refraction.The index of refraction of glass is known to de-pend on the ratio of the speed of light outside tothe speed inside the glass. If, then, the ether bestreaming through glass, the velocity of lightwill be different inside according as it travelswith the stream or against it, and so the index of

refraction may be different. Arago was the firstto try this experiment by placing an achromatic

FIG. 9. Arrangement of Mascart and Jamin.

A modification of Fig. 8, with the beam split definitely into twohalves by reflection from a thick glass plate and reunited before


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observation. The two half beams go through stagnant water inopposite directions.

prism in front of a telescope on a mural circle

and observing the deviation it produced on stars.

Observe that it was an achromatic prism,

^ 57

THE ETHER OF SPACE

treating all wave-lengths alike; he looked at thedeviated image of a star, not at its dispersedimage or spectrum else he might have detectedthe change-of- frequency-effect due to motion ofsource or receiver first actually seen by Sir W.Huggins. I do not think Arago would have seenit, because I do not suppose his arrangementswere delicate enough for that very small effect;but there is no error in the conception of his

experiment, as Professor Mascart has inadver-tently suggested there was.

Then Maxwell repeated the attempt in a muchmore powerful manner, a method which couldhave detected a very minute effect indeed, andMascart has also repeated it in a simple form.All are absolutely negative.

Well, then, what about aberration? If onelooks through a moving stratum, say a spinningglass disk, there ought to be a shift caused bythe motion (see Fig. 4). That particular ex-

periment has not been tried, but I entertain nodoubt about its result, though a high speed andconsiderable thickness of glass or other mediumwould be necessary to produce even a microscopicapparent displacement of objects seen through it.

But the speed of the earth is available, and thewhole length of a telescope tube may be filledwith water; surely that is enough to displacerays of light appreciably.

Sir George Airy tried it at Greenwich on a star,

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with an appropriate zenith-sector full of water.Stars were seen through the water-telescopeprecisely as through an air telescope. A nega-tive result again! (The theory is fully dealt


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with in Chapter X and Appendix 3.)

Stellar observations, however, are un-necessarily difficult. Fresnel had pointed outthat a terrestrial source of light would do justas well. He had also (being a man of exceedinggenius) predicted that nothing would happen.Hoek has now tried it in a perfect manner andnothing did happen.

But these facts are not at all disconcerting;they are just what ought to be anticipated, in thelight of true theory. The absence of all effectcaused by stagnant dense matter inserted in thepath of a beam of light, that is of dense transpar-ent matter not artificially moved with referenceto the earth or rather with reference to sourceand receiver is explicable on Fresnel' s theoryconcerning the behaviour of ether inside matter.

If the index of refraction of the matter is calledp, that means that the speed of light inside it isJ: th of the speed outside or in vacuo. And thatis only another way of saying that the virtual

etherial density inside it is represented by /**,since the velocity of waves is inversely as thesquare root of the density of the medium whichconveys them; the elasticity being reckoned asconstant, and the same inside as out.

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But then if the ether is incompressible its

density must really be constant, so how can itbe denser inside matter than it is outside ? Theanswer is that presumably the ether is not reallyextra dense, but is, as it were, loaded by thematter. The atoms of matter, or the constituentelectrons, must be presumed to be shaken by thepassage of the waves of light, as they obviouslyare in fluorescent substances; and accordinglythe speed of propagation will be lessened by theextra loading which the waves encounter. Itis not a real increase of density, but a virtualincrease, which is really due to the addition of acertain fraction of material inertia to the inertia

of the ether itself. The density of ether out-side being 1, and that of the loaded ether insidebeing p. 2 , the effect of the load is expressible as/i 2 1, while the free ether is the same inside as out.

Suppose now that the matter is moved along.The extra loading, being part of the matter, ofcourse travels with it, and thereby affects thespeed of light to the extent of the load that isto say, by an amount proportional to p 2 1 as


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contrasted with p 2 .

This is Fresnel's predicted ratio (^ I):/* 2 ,or 1 ^; and in Fizeau's experiment with run-ning water especially as repeated later, withmodern accuracy, by Michelson this representsexactly the amount of observed effect upon thelight.

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But if, instead of running water, stagnantwater is used that is stationary with respectto the earth, though still moving violentlythrough the ether then the (/z 2 1) effect ofthe load will be fixed to the matter, and can pro-duce no extra or motile effect. The only part

that could produce an effect of that kind wouldbe the free ether, of density i. But then thison the above view is absolutely stationary, notbeing carried along by the earth at all ; hence thiscan give no effect either. Consequently thewhole effect of an ether-drift past the earth iszero, on optical experiments, according to thetheory of Fresnel; and that is exactly what allthe experiments just described have confirmed.

Since then Professor Mascart, with great per-tinacity, has attacked the phenomena of thickplates, Newton's rings, double refraction, and

the rotatory phenomenon of quartz; but he hasfound absolutely nothing attributable to astream of ether past the earth.

The only positive result ever supposed to beattained was in a very difficult polarisationobservation by Fizeau in 1859. Unless this hasbeen repeated, it is safest to ignore it; but Ibelieve that Lord Rayleigh has repeated it, andobtained a negative result.

Fizeau also suggested, but did not attempt,what seems an easier experiment, with fore and

aft thermopiles and a source between them, to61

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observe the drift of a medium by its convection


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of energy; but arguments based on the law ofexchanges 1 tend to show, and do show as I think,that a probable alteration of radiating powerdue to motion through a medium would justcompensate the effect otherwise to be expected.We may summarise most of these statementsas follows:

Summary.

Source alonemoving pro-duces

Medium alonemoving, orsource andreceiver mov-ing together,produces . .

A real and apparent change ofwave-length.

A real but not apparent errorin direction.

No lag of phase or change ofintensity, except that ap-propriate to altered wave-length.

No change of frequency.

No error in direction.

A real lag of phase, but un-detectable without controlover the medium.

A change of intensity corre-sponding to different dis-tance, but compensated bychange of radiating power.

1 Lord Rayleigh, "Nature," March 25, 1892.62



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Receiver alonemoving pro-duces

An apparent change of wave-length.

An apparent error in direction.

No change of phase or of in-tensity, except that appro-priate to different virtualvelocity of light.

I may say, then, that not a single optical phe-nomenon is able to show the existence of an etherstream near the earth. All optics go on precise-ly as if the ether were stagnant with respect to

the earth.

Well, then, perhaps it is stagnant. The ex-periments I have quoted do not prove that it isso. They are equally consistent with its perfectfreedom and with its absolute stagnation,though they are not consistent with any in-termediate position. Certainly, if. the etherwere stagnant nothing could be simpler thantheir explanation.

The only phenomena then difficult to explainwould be those depending on light coming from

distant regions through all the layers of more orless dragged ether. The theory of astronomicalaberration would be seriously complicated ; in itspresent form it would be upset (p. 47) . But it isnever wise to control facts by a theory ; it is bet-ter to invent some experiment that will give a

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different result in stagnant and in free ether.None of those experiments so far described arereally discriminative. They are, as I say, con-sistent with either hypothesis, though not veryobviously so.

B


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m

FIG. 10. The course of the light and of the two halfbeams in Michelson's most famous experiment.

The light is split at A, one half sent toward B and back, the otherhalf to C and back. (Compare with Pig. 7.)

Michelson Experiment.

Mr. Michelson, however, of the United States,invented a plan that looked as if it really woulddiscriminate; and, after overcoming many diffi-culties, he carried it out. It is described in thePhilosophical Magazine for 1887.

Michelson's famous experiment consists inlooking for interference between two half beams

of light, of which one has been sent to and froacross the line of ether drift, and the other hasbeen sent to and fro along the line of ether drift.64


A semi-transparent mirror set at 45 is em-ployed to split the beam, and a pair of normaland ordinary mirrors, set perpendicular to thetwo half beams, are employed to return them

back whence they came, so that they can enterthe eye through an observing telescope.

It differs essentially from the interferencekaleidoscope, Fig. 7, inasmuch as there is nowno luminous path B C, and no contour enclosedby the light. Each half beam goes to and froon its own path, and these paths, instead ofbeing coincident, are widely separate one northand south, for instance, and the other eastand west.

Under these conditions the bands are much

more tremulous than they were in the arrange-ment of Fig. 7, and are subject to every kind ofdisturbance. The apparatus has to be ex-cessively steady, and no fluctuation even oftemperature must be permitted in the path ofeither beam. To secure this, the source, themirrors, and the observing telescope were allmounted upon a massive stone slab; and thiswas floated in a bath of mercury.


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The slab could then be slowly turned round,so that sometimes the path A B and sometimesthe path A C lay approximately along orathwart the direction of the earth's motion inspace.

And inasmuch as the motion along would take

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a little longer than the motion across, thougheverything else was accurately the same, someshift of the interference bands might be expectedas the slab rotated.

But whereas in all the experiments previouslydescribed the effect looked for was a first-ordereffect, of magnitude one in ten or twenty thou-sand depending, that is to say, on the firstpower of the ratio of speed of earth to speed of

light the effect now to be expected dependson the square of that same ratio, and thereforecannot be greater, even in the most favourablecircumstances, than i part in a hundred million.

It is easy to realise, therefore, that it is anexceptionally difficult experiment, and that itrequired bot

key ether of space

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