INTRODUCTION

resistance would be above 600000000 times less than that of Water. And so small a resistance would scarce make any sensible alteration in the Motions of the Planets in ten thousand Years."

That the ether, if there be such a thing in space, can pass readily into or through matter is often 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 us beyond them into a region of greater certainty and knowledge; so that now I am able to advo- cate a view of the Ether which makes it not only uniformly present and all-pervading, but also massive and substantial beyond conception. It is turning out to be by far the most substantial thing perhaps the only substantial thing in the material universe. Compared to ether the densest matter, such as lead or gold, is a filmy gossamer structure; like a comet's tail or a milky way, 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 yet largely unrecognized importance, of the Ether of Space, the following chapters have been written. Some of them represent the expanded notes of lectures which have been given in various places chiefly the Royal Institution; while the first chapter represents a lecture before the Ashmo- lean Society of the University of Oxford in June, 1889. One chapter (viz., Chap. II) has already been printed as part of an appendix to the third edition of Modern Views of Electricity, as well as in the Fortnightly and North American Reviews; but no other chapters have yet been published, though parts appear in more elaborate form in Proceedings 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 units of ordinary matter, has not yet been solved. > Much work has been done in this direction by various mathematicians, but much more re- mains to be done. And until it is done, some scepticism is reasonable perhaps laudable. Meanwhile there are few physicists who will xix

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 in forming a consistent idea of the constitution of the aether, there can be no doubt that the interplanetary and interstellar spaces are not empty, but are occupied by a material substance or body, which is certainly the largest, and probably the most uniform body of which we have any knowledge."

THE ETHER OF SPACE

THE ETHER OF SPACE

THE LUMINIFEROUS ETHER AND THE MODERN THEORY OF LIGHT

THE oldest and best known function for an ether is the conveyance of light, and hence the name " luminif erous " was applied to it; though at the present day many more functions are known, and more will almost certainly be discovered.

To begin with, it is best to learn what we can concerning the properties of the Interstellar Ether from the phenomena of Light.

For now well-nigh a century we have had a wave theory of light; and a wave theory of light is quite certainly true. It is directly demonstrable that light consists of waves of some kind or other, and that these waves travel at a certain well-known velocity, achieving a distance equal to seven times the circumference of the earth every second; from New York to London

THE ETHER OF SPACE

and back in the thirtieth part of a second; and taking only eight minutes on the journey from the sun to the earth. This propagation in time of an undulatory disturbance necessarily in- volves a medium. If waves setting out from the sun exist in space eight minutes before striking our eyes, there must necessarily be in space some medium in which they exist and which conveys them. Waves we cannot have, unless they be waves in something.

No ordinary matter is competent to transmit waves at anything like the speed of light: the rate 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 of substance; and it is called the ether. The luminiferous ether it used to be called, because the conveyance of light was all it was then known to be capable of; but now that it is known to do a variety of other things also, the qualifying adjective may be dropped. But, inasmuch as the term "ether" is also applied to a familiar organic compound, we may distinguish the ultra- material luminiferous medium by calling it the Ether of Space.

Wave motion in ether, light certainly is; but what does one mean by the term wave? The popular notion is, I suppose, of something heav- ing up and down, or perhaps of something break-

2

THEORY OF LIGHT

ing on a shore. But if you ask a mathematician what he means by a wave, he will probably reply that the most general wave i such a function of x and y and t as to satisfy the differential equation

• y_v**.

dt 2 dx 2 ' while the simplest wave is

y = a sin (x - vt) .

And he might possibly refuse to give any other answer.

And in refusing to give any other answer than this, or its equivalent in ordinary words, he is entirely justified; that is what is meant by the term wave, and nothing less general would be all-inclusive.

Translated into ordinary English, the phrase signifies, with accuracy and comprehensive com- pleteness, the full details of "a disturbance periodic both in space and time." Anything thus doubly periodic is a wave; and all waves whether in air as sound waves, or in ether as light waves, or on the surface of water as ocean waves can be comprehended in the definition.

What properties are essential to a medium capable of transmitting wave motion ? Roughly, we may say two : elasticity and inertia. Elasticity in 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. Any medium possessing these two properties can transmit waves, and unless a medium possesses these properties in some form or other, or some equivalent for them, it may be said with moderate security to be incompetent to transmit waves. But if we make this latter statement, one must be prepared to extend to the terms elasticity and inertia their very largest and broadest signification, so as to include any possible kind of restoring force, and any possible kind of persistence of motion, respectively.

These matters may be illustrated in many ways, but perhaps a simple loaded lath, or spring, in a vise will serve well enough. Pull it to one side, and its elasticity tends to make it recoil ; let it go, and its inertia causes it to over- shoot its normal position. That is what inertia is : power of overshooting a mark, or, more accurately, power of moving for a time even against driving force power to rush up hill. Both causes together make it swing to and fro till its energy is exhausted. This is a disturb- ance simply periodic in time. A regular series of such springs, set at equ'al intervals and started vibrating at regular intervals of time one after the 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 as well, and if set swinging in orderly fashion will furnish at once an example and an appearance of wave motion which the most casual observer must recognise as such. The row of springs obviously possesses elasticity and inertia; and any wave-transmitting medium must similarly possess some form of elasticity and some form of inertia.

But now proceed to ask what is this Ether which in the case of light is thus vibrating? What corresponds to the elastic displacement and recoil of the spring or pendulum? What corresponds to the inertia whereby it overshoots its mark? Do we know these properties in the ether in any other way?

The answer, given first by Clerk-Maxwell, and now reiterated and insisted on by experiments performed in every important laboratory in the world, is:

The elastic displacement corresponds to electrostatic charge roughly speaking, to electricity.

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

Let me attempt to illustrate the meaning of this statement, by reviewing some fundamental electrical facts in the light of these analogies:

5

THE ETHER OF SPACE

The old and familiar operation of charging a Leyden jar the storing up of energy in a strained dielectric any electrostatic charging whatever is quite analogous to the drawing aside of our flexible spring. It is making use of the elasticity of the ether to produce a tendency to recoil. Letting go the spring is analogous to permitting a discharge of the jar permitting the strained dielectric to recover itself the electrostatic disturbance to subside.

In nearly all the experiments of electrostatics etherial elasticity is manifest.

Next consider inertia. How would one illus- trate the fact that water, for instance, possesses inertia the power of persisting in motion against obstacles the power of possessing kinetic energy? The most direct way would be to take a stream of water and try suddenly to stop it. Open a water-tap freely and then suddenly shut it. The impetus or momentum of the stopped water makes itself manifest by a violent shock to the pipe, with which everybody must be familiar. This momentum of water is utilised by engineers in the "water-ram."

A precisely analogous experiment in Electricity is what Faraday ca led "the extra current." Send a current through a coil of wire Around a piece of iron, or take any other arrangement for developing 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 sudden enough a flash which means the bursting of the insulating air partition by the accumulated electromagnetic momentum. The scientific name for this electrical inertia is "self-induction."

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

Now return to consider what happens when a charged conductor (say a Leyden jar) is dis- charged. The recoil of the strained dielectric causes a current, the inertia of this current causes it to overshoot the mark, and for an instant the charge of the jar is reversed; the current now flows backward and charges the jar up as at first; back again flows the current; and so on, charging and reversing the charge, with rapid oscillations, until the energy is all dissipated into heat. The operation is precisely analogous to the release of a strained spring, or to the pluck- ing of a stretched string.

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

7

THE ETHER OF SPACE

Etherial waves can, therefore, be actually pro- duced by direct electrical means. I discharge here a jar, and the room is for an instant filled with light. With light, I say, though you can see nothing. You can see and hear the spark, indeed ; but that is a mere secondary disturbance we can for the present ignore I do not mean any secondary disturbance. I mean the true etherial waves emitted by the electric oscillation going on in the neighbourhood of the recoiling dielectric. You pull aside the prong of a tuning- fork and let it go : vibration follows and sound is produced. You charge a Leyden jar and let it discharge: vibration follows and light is ex- cited.

It is light, just as good as any other light. It travels at the same pace, it is reflected and re- fracted according to the same laws; every ex- periment known to optics can be performed with this etherial radiation electrically produced and yet you cannot see it. Why not? For no fault of the light; the fault (if there be a fault) is in the eye. The retina is incompetent to respond to these vibrations they are too slow. The vibrations set up when this large jar is dis- charged are from a hundred thousand to a million per second, but that is too slow for the retina. It responds only to vibrations between 400 billion and 700 billion per second. The vibrations are too quick for the ear, which re- 8

THEORY OF LIGHT

spends only to vibrations between 40 and 40,000 per second. Between the highest audible and the lowest visible vibrations there has been hitherto a great gap, which these electric oscilla- tions go far to fill up. There has been a great gap simply because we have no intermediate sense organ to detect rates of vibration between 40,000 and 400,000,000,000,000 per second. It was therefore an unexplored territory. Waves have 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 in getting electric oscillations so slow as to be audible the lowest I had got in 1889 were 125 per second, and for some way above this the sparks emit a musical note; but no one has yet succeeded in directly making electric oscillations which are visible though indirectly everyone does it when they light a candle.

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

With the large condensers and self-inductances employed in modern cable telegraphy, it is easy to 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 ordinary signalling instrument or siphon recorder.

These electromagnetic waves in space have been known on the side of theory ever since 1865, but interest in them was immensely quick- ened by the discovery of a receiver or detector for them. The great though simple discovery by Hertz, in 1888, of an "electric eye," as Lord Kelvin called it, made experiments on these waves for the first time easy or even possible. From that time onward we possessed a sort of artificial sense organ for their appreci- ation an electric arrangement which can vir- tually "see" these intermediate rates of vibra- tion.

Since then Branly discovered that metallic powder could be used as an extraordinarily sensi- tive detector; and on the basis of this discovery, the "coherer" was employed by me for distant signalling 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 of optical experiments can be performed. They can be reflected by plain sheets of metal, concen- trated by parabolic reflectors, refracted by prisms, and concentrated by lenses. I have made, for instance, a large lens of pitch, weigh- ing over three hundredweight, for concentrating 10

THEORY OF LIGHT

them to a focus. 1 They can be made to show the phenomenon of interference, and thus have their wave-length accurately measured. They are stopped by all conductors, and transmitted by all insulators. Metals are opaque; but even imperfect insulators, such as wood or stone, are strikingly 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 of transparency has long been clear in Maxwell's theory of light, and these electrically produced waves only illustrate and bring home the well- known facts. The experiments of Hertz are, in fact, the apotheosis of Maxwell's theory.

Thus, then, in every way, Clerk-Maxwell's bril- liant perception or mathematical deduction, in 1865, of the real nature of light is abundantly justified; and for the first time we have a true theory of light no longer based upon analogy with sound, nor upon the supposed properties of some hypothetical jelly or elastic solid, but capable of being treated upon a substantial basis of its own, in alliance with the sciences of Electricity and of Magnetism.

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

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

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 of what we are doing, and of the precise mode of its vibration.

It remains to find out how to shorten down the waves to hurry up the vibration until the light becomes visible. Nothing is wanted but quicker modes of vibration. Smaller oscillators must be used very much smaller oscillators not much bigger than molecules. In all probability one may almost say certainly ordinary light is the result of electric oscillation in the molecules or atoms of hot bodies, or sometimes of bodies not hot as in the phenomenon of phosphorescence.

The direct generation of visible light by electric means, so soon as we have learnt how to 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, in Chapter XIV of Modern Views of Electricity. But here we abandon further consideration of this aspect of our great subject.

II

THE INTERSTELLAR ETHER AS A CONNECTING MEDIUM

SO far I have given a general idea of the present condition of the wave theory of light, both from its theoretical and from its experimental 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 and time, and travelling with a known and tremen- dous speed through the ether of space. Their very existence depends upon the ether, and their speed of propagation is its best known and most certain quantitative property.

A statement of this kind does not even initially express a tithe of our knowledge on the subject; nor does our knowledge exhaust any large part of the region of discoverable fact; but the state- ment above made may be regarded as certain, although the absence of mechanics or ordinary dynamics about it removes it, or seems to remove it, from the category of the historically soundest

13

THE ETHER OF SPACE

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

There is, I believe, a general tendency to under- rate the certainty of some of the convictions to which natural philosophers have gradually, in the course of their study of nature, been im- pelled; more especially when those convictions have reference to something intangible and occult. The existence of a continuous space- filling medium, for instance, is probably regarded by most educated people as a more or less fanciful hypothesis, a figment of the scientific imagina- tion a mode of collating and welding together a certain number of observed facts, but not in any physical sense a reality, as water and air are realities.

I am speaking purely physically. There may be another point of view from which all material reality can be denied, but with those questions physics proper has nothing to do; it accepts the evidence of the senses, regarding them as the tools or instruments wherewith man may hope to understand one definite aspect of the uni- verse; and it leaves to philosophers, equipped from a different armory, the other aspects which the material universe may nay, must possess.

By a physical "explanation" is meant a clear

14

A CONNECTING MEDIUM

statement of a fact or law in terms of something with which daily life has made us familiar. We are all chiefly familiar, from our youth up, with two apparently simple things, motion and force. We have a direct sense for both these things. We do not understand them in any deep way, prob- ably we do not understand them at all, but we are accustomed to them. Motion and force are our primary objects of experience and con- sciousness; and in terms of them all other less familiar occurrences may conceivably be stated and grasped. Whenever a thing can be so clearly and definitely stated, it is said to be ex- plained, or understood; we are said to have "a dynamical theory" of it. Anything short of this may be a provisional or partial theory, an explanation of the less known in terms of the more known, but Motion and Force are postulated in physics as the completely known: and no at- tempt is made to press the terms of an explana- tion further than that. A dynamical theory is recognized as being at once necessary and sufficient.

Now, it must be admitted at once that of very few things have we at present such a dynamical explanation. We have no such explanation of matter, for instance, or of gravitation, or of electricity, 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 force may perhaps be essentially involved. Still, physics is bound to push the search for an ex- planation to its furthest limits; and so long as it does not hoodwink itself by vagueness and mere phrases a feebleness against which its leaders are mightily and sometimes cruelly on their guard, preferring to risk the rejection of worthy ideas rather than permit a semi-acceptance of anything fanciful and obscure so long as it vigorously probes all phenomena within its reach, seeking to reduce the physical aspect of them to terms of motion and force so long it must be upon a safe track. And, by its failure to deal with certain phenomena, it will learn it already begins to suspect, its leaders must long have surmised the existence of some third, as yet unknown, category, by incorporating which the physics of the future may rise to higher flights and an enlarged scope.

I have said that the things of which we are permanently conscious are motion and force, but there is a third thing which we have likewise been all our lives in contact with, and which we know even more primarily, though perhaps we are so immersed in it that our knowledge realises itself later viz., life and mind. I do not now pretend to define these terms, or to speculate as to whether the things they denote are essentially one and not two. They exist, in the sense in 16

A CONNECTING MEDIUM

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

Still, it is open to a physicist to state how the universe appears to him, in its broad character and physical aspect. If I were to make the attempt, I should find it necessary, for the sake of clearness, to begin with the simplest and most fundamental ideas; in order to illustrate, by facts and notions in universal knowledge, the kind of process which essentially occurs in con- nection with the formation of higher and less familiar 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, I should sketch the matter thus:

We have muscles and can move. I cannot analyze motion I doubt if the attempt is wise it is a simple immediate act of perception, a direct sense of free unresisted muscular action. We may indeed move without feeling it, and that teaches us nothing, but we may move so as to feel it, and this teaches us much, and leads to our first scientific inference viz., space; that is, simply, room to move about. We might

THE ETHER OF SPACE

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

Of course we do not stop at this baldness of inference : our educated faculty leads us to realise the existence of space far beyond the possibility of direct sensation; and, further, by means of the direct appreciation of speed in connection with motion of uniform and variable speed we be- come able to formulate the idea of "time," or uniformity of sequence; and we attain other more complex notions acceleration, and the like upon a consideration of which we need not now enter.

But our muscular sense is not limited to the perception of free motion: we constantly find it restricted or forcibly resisted. This "muscular action impeded" is another direct sense, that of "force"; and attempts to analyze it into any- thing simpler than itself have hitherto resulted only in confusion. By "force" is meant pri- marily muscular action not accompanied by motion. Our sense of this teaches us that space, though roomy, is not empty: it gives us our 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 matter in apparently intimate relation with ourselves, 18

A CONNECTING MEDIUM

and other or foreign lumps of matter; and we use the first portion as a measure of the extent of the second. The human body is our standard of size. We proceed also to subdivide our idea of matter according to the varieties of resist- ance with which it appeals to our muscular sense into four different states, or "elements," as the ancients called them viz., the solid, the liquid, the gaseous, and the ethereal. The resistance experienced when we encounter one or other of these forms of material existence varies from something very impressive the solid ; through something nearly impalpable the gaseous ; up to something entirely imagina- tive, fanciful, or inferential viz., the ether.

The ether does not in any way affect our sense of touch (i.e., of force) ; it does not resist motion in the slightest degree. Not only can our bodies move through it, but much larger bodies, planets and comets, can rush through it at what we are pleased to call a prodigious speed (being far greater than that of an athlete) without showing the least sign of friction. I myself, indeed, have designed and carried out a series of delicate experiments to see whether a whirling mass of iron could to the smallest extent grip the ether and carry it round, with so much as a thousandth part of its own velocity. These shall be de- scribed further on, but meanwhile the result arrived at is distinct. The answer is, no; I

19

THE ETHER OF SPACE

cannot find a trace of mechanical connection between matter and ether, of the kind known as viscosity or friction.

Why, then, if it is so impalpable, should we assert its existence? May it not be a mere fanciful speculation, to be extruded from physics as soon as possible? If we were limited for our knowledge of matter to our sense of touch, the question would never even have presented itself; we should have been simply ignorant of the ether, as ignorant as we are of any life or mind in the universe not associated with some kind of material body. But our senses have attained a higher stage of development than that. We are conscious of matter by means other than its resisting force. Matter acts on one small por- tion of our body in a totally different way, and we are said to taste it. Even from a distance it is able to fling off small particles of itself sufficient to affect another delicate sense. Or again, if it is vibrating with an appropriate frequency, an- other part of our body responds; and the uni- verse is discovered to be not silent but eloquent to those who have ears to hear. Are there any more discoveries to be made ? Yes ; and already some have been made. All the senses hitherto mentioned speak to us of the presence of or- dinary matter gross matter, as it is sometimes called though when appealing to our sense of smell, and more especially to a dog's sense of 20

A CONNECTING MEDIUM

smell, it is not very gross; still, with the senses hitherto enumerated we should never have be- come aware of the ether. A stroke of lightning might have smitten our bodies back into their inorganic constituents, or a torpedo-fish might have inflicted on us a strange kind of torment; but from these violent tutors we should have learnt little more than a school-boy learns from the once ever-ready cane.

But it so happens that the whole surface of our skin is sensitive in yet another way, and a small portion of it is asftoundingly and beautifully sensitive, to an impression of an altogether dif- ferent character one not necessarily associated with any form of ordinary matter one that will occur equally well through space from which all solid, liquid, or gaseous matter has been removed. Hold your hand near a fire, put your face in the sunshine, and what is it you feel? You are now conscious of something not arriving by ordinary matter at all. You are now as directly conscious as you can be of the ethereal medium. True the process is not very direct. You cannot apprehend the ether as you can matter, by touching or tasting or even smelling it; but the process is analogous to the kind of perception we might get of ordinary matter if we had the sense of hearing 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 the 21

THE ETHER OF SPACE

ethereal disturbances themselves, but other dis- turbances excited by them in our tissues, that our heat nerves feel; and the same assertion can be made for our more highly developed and specialised sight nerves. All nerves must feel what is occurring next door to them, and can directly feel nothing else; but the "radiation," the cause which excited these disturbances, travelled througi the ether not through any otherwise known material substance.

It should be a commonplace to rehearse how we know this. Briefly, thus: Radiation con- spicuously comes to us from the sun. If any free or ordinary matter exists in the intervening space, it must be an exceedingly rare gas. In other words, it must consist of scattered par- ticles of matter, some big enough to be called lumps, some so small as to be merely atoms, but each with a considerable gap between it and its neighbor. Such isolated particles are absolute- ly incompetent to transmit light. And, paren- thentically, I may say that no form of ordinary matter, solid, liquid, or gaseous, is competent to transmit a thing travelling with the speed and subject to the known laws of light. For the conveyance of radiation or light all ordinary matter is not only incompetent, but hopelessly and absurdly incompetent. If this radiation is 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 the journey, travelling at a well-known and definite speed; and it is a quivering or periodic disturb- ance, falling under the general category of wave-motion. Nothing is more certain than that. No physicist disputes it. Newton him- self, who is commonly and truly asserted to have promulgated a rival theory, felt the ne- cessity of an ethereal medium, and knew that light consisted essentially of waves.

Sight.

A small digression here, to avoid any possible confusion due to the fact that I have purposely associated together temperature nerves and sight nerves. They are admittedly not the same, but they are alike in this, that they both afford evidence of radiation; and, were we blind, we might still know a good deal about the sun, and if our temperature nerves were immensely in- creased in delicacy (not all over, for that would be 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 lined with a surface sensitive to heat, could readily be imagined, and might be somewhat singularly effective. 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 see perfectly well in what we call "the dark." But it would probably see far too much for con- venience, since it would necessarily be affected by every kind of radiation in simple proportion to its energy; unless, indeed, it were provided with a supply of screens with suitably selected absorbing powers. But whatever might be the advantage or disadvantage of such a sense- organ, we as yet do not possess one. Our eye does not act by detecting heat; in other words, it is not affected by the whole range of ethereal quiverings, but only by a very minute and apparently insignificant portion. It wholly ignores the ether waves whose frequency is comparable with that of sound; and, for thirty or forty octaves above this, nothing about us responds; but high up, in a range of vibration of the inconceivably high pitch of four to seven hundred million million per second a range which extremely few accessible bodies are able to emit, and which it requires some knowledge and skill artificially to produce to those waves the eye is acutely, surpassingly, and most in- telligently sensitive.

This little fragment of total radiation is in itself trivial and negligible. Were it not for men, and glow-worms, and a few other forms of life, hardly any of it would ever occur, on such a moderate- sized lump of matter as the earth. 24

A CONNECTING MEDIUM

Except for an occasional volcano, or a flash of lightning, only gigantic bodies like the sun and stars have energy enough to produce these high- er flute-like notes ; and they do it by sheer main force and violence the violence of their gravi- tative energy producing not only these, but every other kind of radiation also. Glow- worms, so far as I know, alone have learned the secret of emitting the physiologically useful waves, and none others.

Why these waves are physiologically useful why they are what is called "light," while other kinds of radiation are "dark," is a question to be asked, but, at present, only tentatively an- swered. The answer must ultimately be given by the Physiologist; for the distinction between light and non-light can only be stated in terms of the eye, and its peculiar specialised sensitive- ness; but a hint may be given him by the Physicist. The ethereal waves which affect the eye and the photographic plate are of a size not wholly incomparable with that of the atoms of matter. When a physical phenomenon is con- cerned with the ultimate atoms of matter, it is often relegated at present to the field of knowl- edge summarized under the head of Chemistry. Sight is probably a chemical sense. The retina may 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 to be further countenanced except as a working hypothesis leading to examination of fact; but, nevertheless, the direction in which the thoughts of some physicists are tending a direction toward which many recently discovered ex- perimental facts point. 1

Gravitation and Cohesion.

It would take too long to do more than suggest some other functions for which a continuous medium of communication is necessary. We shall argue in Chapter VIII that technical action at a distance is impossible. A body can only act immediately on what it is in contact with; it must be by the action of contiguous particles that is, practically, through a continuous medium, that force can be transmitted across space. Radiation is not the only thing the earth feels from the sun ; there is in addition its gigantic gravitative pull, a force or tension more than 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 my Modern Views of Electricity.

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A CONNECTING MEDIUM

stretched, with how great tenacity its parts cling together! Yet its particles are not in absolute contact, they are only virtually attached to each other by means of the universal connecting medium the ether a medium that must be competent to transmit the greatest stresses which our knowledge of gravitation and of cohesion shows us to exist.

O

Electricity and Magnetism.

Hitherto I have mainly confined myself to the perception of the ether by our ancient sense of radiation, whereby we detect its subtle and delicate quiverings. But we are growing a new sense; not perhaps an actual sense-organ, though not so very unlike a new sense-organ, though the portions of matter which go to make the organ are not associated with our bodies by the usual links 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, telephones delicate instruments these ; not yet eclipsing our sense-organs of flesh, but in a few cases coming within measurable distance of their surprising sensitiveness. And with these what do we do? Can we smell the ether, or touch it, or what is the closest analogy ? Perhaps there is no useful analogy; but nevertheless we deal with it, and 27

THE ETHER OF SPACE

that closely. Not yet do we fully realise what we are doing. Not yet have we any dynamical theory of electric currents, of static charges, and of magnetism. Not yet, indeed, have we any dynamical theory of light. In fact, the ether has not yet been brought under the domain of simple mechanics it has not yet been reduced to motion and force : and that probably because the force aspect of it has been so singularly elusive that it is a question whether we ought to think of it as material at all. No, it is apart from mechanics at present. Conceivably it may remain apart ; and our first additional cate- gory, wherewith the foundations of physics must some day be enlarged, may turn out to be an ethereal one. And some such inclusion may have to be made before we can attempt to annex vital or mental processes. Perhaps they will all come in together.

Howsoever these things be, this is the kind of meaning lurking in the phrase that we do not yet know what electricity or what the ether is. We have as yet no dynamical explanation of either of them; but the past century has taught us what seems to their student an overwhelming quantity of facts about them. And when the present century, or the century after, lets us deeper into their secrets, and into the secrets of some other phenomena now in course of being rationally investigated, I feel as if it would be 28

A CONNECTING MEDIUM

no merely material prospect that will be opening on our view, but some glimpse into a region of the universe which Science has never entered yet, but which has been sought from far, and perhaps blindly apprehended, by painter and poet, 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 the prosaic treatment of Physics it is unsuitable, and etheric has occasionally been used instead. No just derivation can be given for such an adjective, how- ever; and I have been accustomed simply to spell etherial with an * when no poetic meaning was intend- ed. This alternative spelling is not incorrect; but Milton 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 part employed.

Ill

INFLUENCE OF MOTION ON VARIOUS PHENOMENA

This is called Astronomical Aberration, and is extremely well known. But a number of other problems open out in connection with it, and on these it is desirable to enter into detail. For if the ether is stationary while the earth is flying through it at a speed vastly faster than any cannon-ball, as much faster than a cannon-ball as an express train is faster than a saunter on foot it is for all practical purposes the same as if the earth were stationary and the ether stream- ing past it with this immense velocity in the opposite direction. And some consequence of such a drift might at first sight certainly be expected. It might, for instance, seem doubtful whether terrestrial surveying operations can be conducted, with the extreme accuracy expected of them, without some allowance for the violent rush of the light-conveying medium past and through the theodolite of the observer.

Let us therefore consider the whole subject further.

ABERRATION.

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

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

3 1

THE ETHER OF SPACE

readily admit that to hit a squatting rabbit from a 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 the aberration, or needful divergence, between the point aimed at and the thing hit has opposite sign in the two cases the case when receiver is moving, and the case when source is moving. Hence, if both be moving, it is possible for the two aberrations to neutralize each other. So to hit a rabbit running alongside the train you must aim straight at it.

If there were no air, that is all simple enough. But every rifleman knows to his cost that though he fixes both himself and his target tightly to the ground, so as to destroy all aberration proper, yet a current of air is very competent to introduce a kind of spurious aberration of its own, which may be called wind- age; and that he must not aim at the target if he wants to hit it, but must aim a little in the eye 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 completely penetrated by the bullet, leaving a longish hole wherever struck. A person behind the target, whom we may call a marker, by applying his eye to the hole immediately after

3 2

INFLUENCE OF MOTION

the hit, may be able to look through it at the shooter, and thereby to spot the successful man. I know that this is not precisely the function of an ordinary marker, but it is more complete than his ordinary function. All he does usually is to signal an impersonal hit ; some one else has to record the identity of the shooter. I am rather assuming a volley of shots, and that the marker has to allocate the hits to their respective sources by means of the holes made in the target.

Well, will he do it correctly? Assuming, of course, that he can do so if everything is station- ary, and ignoring all curvature of path, whether vertical or horizontal curvature. If you think it over you will perceive that a wind will not prevent his doing it correctly; the line of hole will point to the shooter along the path of his bullet, though it will not point along his line of aim. Also, if the shots are fired from a mov- ing ship, the line of hole in a stationary target will point to the position the gun occupied at the instant the shot was fired, though it may have moved since then. In neither of these cases (moving medium and moving source) will there be any error.

But if the target is in motion, on an armoured train for instance, then the marker will be at fault. The hole will not point to the man who fired 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 in the world. The easiest illustration of it is that when you run through a vertical shower, you tilt your umbrella forward; or, if you have not got one, the drops hit you in the face; more ac- curately, your face as you run forward hits the drops. So the shower appears to come from a cloud ahead of you, instead of from one over- head.

We have thus three motions to consider, that of the source, of the receiver, and of the medium; and, of these, only motion of receiver is able to cause an aberrational error in fixing the position of 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 a little different. Waves crawl through a medium at their own definite pace ; they cannot be flung forward or sideways by a moving source; they do not move by reason of an initial momentum which they are gradually expending, as shots do; their motion is more analogous to that of a bird or other self-propelling animal, than it is to that of a shot. The motion of a wave in a moving medium may be likened to that of a rowing-boat on a river. It crawls forward with 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 motion of its source. A shot from a passing steamer retains the motion of the steamer as well as that given it by the powder. It is projected, there- fore, in a slant direction. But a boat lowered from the side of a passing steamer, and rowing off, retains none of the motion of its source; it is not projected, it is self-propelled. That is like the case of a wave.

The diagram illustrates the difference. Fig. i shows a moving cannon or machine-gun, moving with the arrow, and firing a succession of shots which share the motion of the cannon as well as their own, and so travel slant. The shot fired from position 1 has reached A, that fired from position 2 has reached B, and that fired from position 3 has reached C, by the time the fourth shot 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 shots are travelling aslant this line, as shown by the arrows. There are thus two directions to be distinguished. There is the row of successive shots, and there is the path of any one shot. These two directions enclose an angle. It may be called an aberration angle, because it is due to the motion of the source, but it need not give rise to any aberration. True direction may still be perceived from the point of view of the receiver.

35

THE ETHER OF SPACE

To prove this let us attend to what is happening at the target. The first shot is supposed to be entering at A, and if the target is stationary will leave it at Y. A marker looking along Y A will see the position whence the shot was fired. This may be likened to a stationary observer looking at a moving star. He sees it where and as it was when the light started on its long journey. He

FIG. i. Shots or Disturbances with Momentum from a Moving Gun.

does not see its present position, but there is no reason why he should. He does not see its physical state or anything as it is now. He sees it as it was when it sent the information which he has just received. There is no aberration caused by motion of source.

But now let the receiver be moving at same pace as the gun, as when two grappled ships are firing into each other. The motion of the target carries the point Y forward, and the shot A leaves it at Z, because Z is carried to where Y was. 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 will see likewise the row of shots making straight for him. This is like an observer looking at a terrestrial object. Motion of the earth does not disturb ordinary vision.

Fig. 2 shows as nearly the same sort of thing as possible for the case of emitted waves. The tube is a source emitting a succession of disturb- ances without momentum. A B C D may be thought of as horizontally flying birds, or as crests of waves, or as self-swimming torpedoes; or they may even be thought of as bullets, if the gun stands still every time it fires, and only moves between whiles.

fr-

FIG. 2. Waves or Disturbances without Momentum from a Moving Source.

The line A B C D is now neither the line of fire nor the line of aim: it is simply the locus of disturbances emitted from the successive posi- tions 1234.

37

THE ETHER OF SPACE

A stationary target will be penetrated in the direction A Y, and this line will point out the correct position of the source when the received disturbance 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 of motion; the line Z A does not point to the original position of the source, and so there will be aberration when the target moves. Other- wise there would be none.

Now, Fig. 2 also represents a parallel beam of light travelling from a moving source, and entering 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 the direction of vision. The direction of vision, to a stationary observer, is determined not by the locus of successive waves, but by the path of each wave. A ray may be denned as the path of a labelled disturbance. The line of vision is Y A 1, and coincides with the line of aim;

38
INFLUENCE OF MOTION

which in the projectile case (Fig. i) it did not.

The case of a revolving lighthouse, emitting long parallel beams of light and brandishing them rapidly round, is rather interesting. Fig. 3 may assist the thinking out of this case. Successive disturbances A, B, C, D, lie along a spiral curve, the spiral of Archimedes; and this is the shape of the beams, as seen illuminating the dust particles, though the pitch of the spiral is too gigantic to be distinguished from a straight line. At first sight it might seem as if an eye looking along those curved beams would see the lighthouse slightly out of its true position; but it is not so. The true rays or actual paths of each disturbance are truly radial ; they do not coincide with the apparent beam. An eye looking at the source will not look tangentially along the beam, but will look along A S, and will see the source in its true position. It would be other- wise for the case of projectiles from a revolving turret.

Thus, neither translation of star nor rotation of sun can affect direction. There is no aberration so long as the receiver is stationary.

But what about a wind, or streaming of the medium past source and receiver, both stationary? Look at Fig. i again. Suppose a row of stationary cannon firing shots, which get blown by a cross wind along the slant 1 A Y

39

THE ETHER OF SPACE

(neglecting the curvature of path which would really exist) : still the hole in the target fixes the gun's true position, the marker looking along Y A sees the gun which fired the shot. There is no true deviation from the point of view of the receiver, provided the drift is uniform every- where, although the shots are blown aside and the target is not hit by the particular gun aimed at 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 the curved path caused by steady wind-pressure, is not so simple, and there would really be an aberration or apparent displacement of the source toward the wind's eye: an apparent exaggeration of the effect of wind shown in the diagram.)

In Fig. 2 the result of a wind is much the same, though the details are rather different. The medium is supposed to be drifting downward, across the field. The source may be taken as stationary at S. The horizontal arrows show the direction of waves in the medium; the dotted slant line shows their resultant direction. A wave centre drifts from D to 1 in the same time as the disturbance reaches A, travelling down the slant line D A. The angle between dotted and full lines is the angle between ray and wave 40

INFLUENCE OF MOTION

normal. Now, if the motion of the medium in- side the receiver is the same as it is outside, the wave will pass straight on along the slant to Z, and the true direction of the source is fixed. But if the medium inside the target or telescope is stationary, the wave will cease to drift as soon as it gets inside under cover, as it were ; it will proceed along the path it has been really pur- suing in the medium all the time, and make its exit at Y. In this latter case of different motion of the medium inside and outside the telescope the apparent direction, such as Y A, is not the true direction of the source. The ray is in fact bent where it enters the differently moving medium (as shown in Fig. 4).

FIG. 4. Ray through a Moving Stratum.

A slower moving stratum bends an oblique ray, slanting with the motion, in the same direction as if it were a denser medium. A quicker stratum bends it oppositely. If a

4 4 I

THE ETHER OF SPACE

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

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

All this is well shown in Fig. 5.

S is a stationary source emitting successive waves, which drift as spheres to the right. The wave 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 therefore the ray. It has advanced as a wave from S to P, and has drifted from P to M. Disturbances subsequently emitted are found along the ray, precisely as in Fig. 2. A stationary telescope receiving the light will point straight at S. A mirror, M, intended to reflect the light straight back 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 of source as centre. Wave-normal and ray now coincide : S M is not a ray, but only the locus of successive disturbances. A stationary telescope

FIG. 5. Successive Wave Fronts in a Moving Medium.

would look not at S, but along M C to a point where the source was when it emitted the wave M ; a moving telescope, if moving at same rate as source, will look at S. Hence S M is sometimes called the apparent ray. The angle S M C is the

43

THE ETHER OF SPACE

aberration angle, which in Chap. X we denote by .

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

Parenthetically, I may say that the time taken on the double journey, Sj M S 2 , when the medium is moving, is not quite the same as the double journey SMS, when all is stationary; and that this is the principle of Michelson's great experi- ment; which must be referred to later.

FIG. 6. Normal Reflection in Moving Medium.

The angle M S X is the angle in the theory of Michelson's experiment described in Chapter IV.

The ether stream we speak of is always to be considered merely as one relative to matter. Ab- solute velocity of matter means velocity through

44

INFLUENCE OF MOTION

the ether which is stationary. If there were no such physical standard of rest as the ether if all motion were relative to matter alone then the contention of Copernicus and Galileo would have had no real meaning.

IV EXPERIMENTS ON THE ETHER

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 observer, 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 tele- scope, there is no aberration at all.

Surveying operations are not rendered the least inaccurate by the existence of a universal ethe- rial drift; and they therefore afford no means of detecting it.

But observe that everything depends on the ether's motion being uniform everywhere, inside as well as outside the telescope, and along the whole path of the ray. If stationary anywhere it must be stationary altogether: there must be no boundary between stationary and moving 46

EXPERIMENTS ON THE ETHER

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 even in the apparent position appropriate to his own motion, but lagging behind that po- sition.

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 sub- stance, then such a lag or negative aberration must occur: in precise proportion to the amount of the carriage of ether by moving bodies

(cf- P- 63).

On the other hand, if the ether behaves as a perfectly frictionless in viscid 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

47

THE ETHER OF SPACE

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 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 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 48

EXPERIMENTS ON THE ETHER

ought to be able to detect some evidence of its existence. 1

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; 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 fre- quency. The precise effects of motion on pitch are 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 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.

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

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THE ETHER OF SPACE

(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.

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

Motion of either source or receiver can alter frequency ; motion of receiver can alter apparent direction ; motion of the medium can do neither.

But the question must be asked, can it not 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, may it not 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 once more, perhaps the laws of reflection and refraction in a moving medium are not the same as they are if it be at rest. Then, more- over, there is double refraction, colours of thin plates and thick plates, polarisation angle, ro- tation of the plane of polarisation; all sorts of optical phenomena that need consideration.

50

EXPERIMENTS ON THE ETHER

It may have to be admitted, perhaps, that in empty space the effect of an ether drift is dif- ficult to detect, but will not the presence of dense matter especially the passage through dense transparent matter make the detection easier? 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, consider No. 3 of the phenomena tabulated above. I expect that every reader understands inter- ference, 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 re- united half -beams of light may thus produce local stripes of darkness, and these stripes are called interference bands.

THE ETHER OF SPACE

It is not to be supposed that there is any destruction 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 is illuminated in stripes and no longer uniformly, but its total illumination is the same as if there were no interference.

PROJECTION OP INTERFERENCE BANDS.

It is not easy to project these interference bands on a screen so as to make them visible to an audience, partly because the bands or stripes of darkness are exceedingly narrow; indeed, I had not previously seen the experiment attempted. But by means of what I call an interference kaleidoscope, consisting of two mirrors set at an angle with a third semi-trans- parent mirror between them, it is possible to get the bands remarkably clear and bright, so that they can readily be projected: and I showed these at a lecture to the Royal Institution of Great Britain in 1892.

Each mirror is mounted on a tripod with adjustable screw feet, which stand on a thick iron slab, which again rests on hollow india- rubber balls. Looking down on the mirrors the plan is as in the diagram Fig. 7, which indicates sufficiently the geometry of the arrangement,

52

EXPERIMENTS ON THE ETHER

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

FIG. 7. Plan of Interference Kaleidoscope with three mirrors.

The arrow-feather ray is bifurcated at A by a semi-transparent mirror of thinly silvered glass; and the two halves reunite along the arrow-head after traversing a triangular contour A B C in opposite directions. The simple geometrical relations which permit this are sufficiently indicated in the figure. The arrangement would suit Fizeau's experiment.

53*

THE ETHER OF SPACE

A parallel beam from an electric lantern, when thus treated, depicts bright and broad inter- ference bands on a screen. And the arrangement is very little sensitive to disturbance, because the paths of the two halves of the beam are identical, and because of the mounting. A piece of good glass can be interposed without disturbance, and the table can be struck a heavy blow without confusing the bands.

The only regular and orderly way of causing a shift of the bands is to accelerate one half of the beam and to retard the other half by moving a transparent substance along the contour. For instance, let the sides of the triangle A B C, or one of them, consist of a tube of water in which a rapid stream is maintained; then the stream has a chance of accelerating one half the beam and retarding the other half, thereby shifting the fringes from their normal position by a measurable amount. This is the experiment made in 1859 by Fizeau. (Appendix 3.)

Now that 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 assists sound one way and retards it the opposite way. Yes, but then sound travels in air; and

54

EXPERIMENTS ON THE ETHER

wind is a bodily transfer of air; hence, of course, gives the sound a ride. Whereas light does not really travel in water, but always in ether; and it is by no means obvious whether a stream of water can help or hinder it. Experiment decides, however, and answers in the affirmative. 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 we must defer ex- plaining how this comes about. Suffice for present purposes the fact that the velocity of light inside moving water, and therefore presumably inside all transparent matter, is altered to some extent by motion of that matter.

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 light through it one way, and it will be hurried; its velocity, instead of being 140,000 miles a second, will be 140,009 miles. Send a beam of light the other way, and its velocity will be 139,991 ; just as 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

55

THE ETHER OF SPACE

his apparatus (Fig. 8). Babinet had tried an- other form of the experiment previously. Hoek expected to see interference bands from the two half-beams which had traversed the water, one

FIG. 8. Hoek's arrangement.

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

in the direction of the earth's motion and the other against it. But no interference bands were seen. The experiment gave a negative result.

An experiment, however, in which nothing is seen is never a very satisfactory form of a nega- tive experiment ; it is, as Mascart calls it, " doubly negative," and we require some guarantee that the 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 thing looked for is a shift of already existing inter- ference bands, when the above apparatus is turned so as to have different aspects with re-

56

EXPERIMENTS ON THE ETHER

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

Interference methods all fail to display any trace 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 to the speed inside the glass. If, then, the ether be streaming through glass, the velocity of light will be different inside according as it travels with the stream or against it, and so the index of refraction may be different. Arago was the first to 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 two halves by reflection from a thick glass plate and reunited before observation. The two half beams go through stagnant water in opposite 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,

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EXPERIMENTS ON THE ETHER

Theory of Michelson Experiment.

The theory of the Michelson experiment can be expressed thus: its optical diagram being the same as is expressed geometrically in Fig. 6.

If a relatively fixed source and receiver move through the ether with velocity u, such that u/v=a the aberration constant; then the time of any to-and-fro journey S M, inclined at angle 6 to the direction of the drift, is increased, above what it would be if there were no drift, in the ratio

V (1 - a 2 sin 2 0) 1-a 2

This follows from merely geometrical consider- ations.

Hence if a ray is split, and half sent so that 0=o while the other half is sent so that = 90 (as in Fig. 10), the one will lag behind the other by a distance \cP times the distance travelled; which, though very small, may be a perceptible fraction of a wave-length, and therefore may cause a perceptible shift of the bands.

But when the experiment is properly per- formed, no such shift is observed.

The experiment thus seems to prove that

there is no motion through the ether at all, that

there is no etherial drift past the earth, that the

ether immediately in contact with the earth is

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THE ETHER OF SPACE

stagnant or that the earth to that extent carries all neighbouring ether with it.

If we wish to evade this conclusion, there is no easy way of doing so. For it depends on no doubtful properties of transparent substances, but on the straightforward fundamental prin- ciple underlying all such simple facts as that It takes longer to row a certain distance and back, up and down stream, than it does to row the same distance in still water; or that it takes longer to run up and down a hill than to run the same distance laid out flat; or that it costs more to buy a certain number of oranges at three a penny and an equal number at two a penny than it does to buy the whole lot at five for twopence.

Hence, although there may be some way of getting round Mr. Michelson's experiment, there is no obvious way; and if the true conclusion be not that the ether near the earth is stagnant, it must lead to some other important and unknown fact.

That fact has now come clearly to light. It was first suggested by the late Prof. G. F. Fitz- Gerald, of Trinity College, Dublin, while sitting in my study at Liverpool and discussing the matter with me. The suggestion bore the im- press of truth from the first. It independently occurred also to Prof. H. A. Lorentz, of Leiden, into whose theory it completely fits, and who has 68

EXPERIMENTS ON THE ETHER

brilliantly worked it into his system. It may be explained briefly thus:

Electric charges in motion constitute an electric current. Similar charges repel each other, but cur- rents in the same direction attract. Consequently two similar charges moving in parallel lines will repel each other less than if stationary less also than if moving one after the other in the same line. Likewise two opposite charges, a fixed distance apart, attract each other less when moving side by side than when chasing each other. The modi- fication of the static force, thus caused, depends on the squared ratio of their joint speed to the velocity of light.

Atoms of matter are charged; and cohesion is a residual electric attraction (see end of Appendix i). So when a block of matter is moving through the ether of space its cohesive forces across the line of motion are diminished, and consequently in that direction it expands, by an amount proportioned to the square of aberration magnitude.

A light journey, to and fro, across the path of a relatively moving medium is slightly quicker than the same journey, to and fro, along (see p. 67). But if the journeys are planned or set out on a block of matter, they do not remain quite the same when it is conveyed through space : the journey across the direction of motion becomes longer than the other journey, as we have just seen. And the extra dis- tance compensates or neutralises the extra speed; so that light takes the same time for both.

SPECIAL EXPERIMENT ON ETHERIAL VISCOSITY

THE balance of evidence at this stage seems to incline in the sense that there is no ether drift, that the ether near the earth is stagnant, that the earth carries all or the greater part of the neighbouring ether with it a view which, if true, must singularly complicate the theory of ordinary astronomical aberration: as is explained at the beginning of the last chapter.

But now put the question another way. Can matter carry neighbouring ether with it when it moves? Abandon the earth altogether; its motion is very quick but too uncontrollable, and it always gives negative results. Take a lump of matter that you can deal with, and see if it pulls any ether along.

That is the experiment which I set myself to perform, and which in the course of the years 1891-97 I performed. It may be thus described in essence:

Take a steel disk, or rather a couple of large steel disks a yard in diameter clamped together 70

SPECIAL EXPERIMENT

with a space between. Mount the system on a vertical axis, and spin it like a teetotum as fast as it will stand without flying to pieces. Then take a parallel beam of light, split it into two by a semi-transparent mirror, M, a piece of glass silvered so thinly that it lets half the light through and reflects the other half, somewhat as in Fig. 7; and send the two halves of this split beam round and round in opposite directions in the space between the disks. They may thus travel a distance of 20 or 30 or 40 feet. Ultimately they are allowed to meet and enter a telescope. If they have gone quite identical distances they need not interfere, but usually the dis- tances will differ by a hundred-thousandth of an inch or so, which is quite enough to bring about interference.

The mirrors which reflect the light round and round between the disks are shown in Fig. n. If they form an accurate square the last two images will coincide, but if the mirrors are the least inclined to one another at any unaliquot part of 360 the last image splits into two, as in the kaleidoscope is well known, and the in- terference bands may be regarded as resulting from those two sources. The central white band bisects normally the distance between them, and their amount of separation determines the width of the bands. There are many interesting op- tical details here, but I shall not go into them.

THE ETHER OF SPACE

The thing to observe is whether the motion of the disks is able to replace a bright band by a dark one, or vice versa. If it does, it means

FIG. ii. Diagrammatic Plan of Optical Frame for Ether Machine; with Steel Disks, one yard in diameter, inside the frame. (The actual apparatus is shown in Figs. 13 and 14 and Fig. 12.)

M is a semi-transparent mirror, reflecting half an incident, beam and transmitting the other half. The two half beams each go three times round the square contour, in opposite directions, and then reunite. It is an extension of the idea of Fig. 7.

that one of the half beams viz., that which is travelling in the same direction as the disks is helped on a trifle, equivalent to a shortening of journey by some quarter millionth of an inch or so in the whole length of 30 feet; while the other half beam viz., that travelling against the

72

SPECIAL EXPERIMENT

motion of the disks is retarded, or its path vir- tually lengthened, by the same amount.

If this acceleration and retardation actually occur, waves which did not interfere on meeting before the disks moved, will interfere now; for one will arrive at the common goal half a length behind the other.

Now a gradual change of bright space to dark, and vice versa, shows itself, to an observer looking at the bands, as a gradual change of position of the bright stripes, or a shift of the bands. A shift of the bands, and especially of the middle white band, which is much more stable than the others, is what we look for. The middle band is, or should be, free from the "concertina "-like motion which is liable to in- fect the others.

At first I saw plenty of shift. In the first experiment the bands sailed across the field as the disks got up speed until the crosswire had traversed a band and a half. The conditions were such that had the ether whirled at the full speed of the disks I should have seen a shift of three bands. It looked very much as if the light was helped along at half the speed of the moving matter, just as it is inside water.

On stopping the disks the bands returned to their old position. On starting them again in the opposite direction, the bands ought to have shifted the other way too, if the effect was

6 73

THE ETHER OF SPACE

genuine; but they did not; they went the same way as before.

The shift was therefore wholly spurious; it was caused by the centrifugal force of the blast of air thrown off from the moving disks. The mirrors and frame had to be protected from this. Many other small changes had to be made, and gradually the spurious shifts have been reduced and reduced, largely by the skill and patience of my assistant, Mr. Benjamin Davies, until pres- ently there was barely a trace of them.

But the experiment is not an easy 'one. Not only does the blast exert pressure, but at high speeds the churning of the air makes it quite hot. Moreover, the tremor of the whirling machine, in which from four to nine horse- power is sometimes being expended, is but too liable to communicate itself to the optical part of the apparatus. Of course elaborate pre- cautions are taken against this. Although the two parts, the mechanical and the optical, are so close together, their supports are entirely in- dependent. But they have to rest on the same earth, and hence communicated tremors are not absent. They are the cause of most of the slight residual trouble.

The whole experiment is described in fairly full detail in the Philosophical Transactions of the Royal Society for 1893 and 1897. And there also are described some further modifications where-

74

SPECIAL EXPERIMENT

by the whirling disks are electrified likewise without optical effect, and are also magnetised; or rather a great iron mass, strongly magnetised by a current, is used to replace the steel disks.

The effect was always zero, however, when spurious results were eliminated; and it is clear that at no practicable speed does either electri- fication or magnetisation confer upon matter any appreciable viscous grip upon the ether. Atoms must be able to throw it into vibration, if they are oscillating or revolving at sufficient speed; otherwise they would not emit light or any kind of radiation; but in no case do they appear to drag it along, or to meet with resistance in any uniform motion through it. Only their accelera- tion is effectual.

In the light of Larmor's electron theory, we know now that acceleration of atoms, or rather of a charge upon an atom, necessarily generates radiation, proportional in amount to the square of the acceleration whether that be tangential or normal. There is no theoretical reason for assuming any influence on uniform velocity. And even the influence on acceleration is exceedingly small under ordinary circumstances. Only dur- ing the violence of collision are ether waves free- ly excited. The present experiment, however, has nothing to do with acceleration: it is a test of viscosity. An acceleration term exists in motion through even a perfect fluid.

75

FIG. 12. General view of whirling part of Ether machine, with pair of steel disks, and motor.

SPECIAL EXPERIMENT

The conclusion at which I arrived in 1892 and 1893 is tnus expressed (p. 777 of vol. 184 Philo- sophical Transactions of the Royal Society} : "I feel confident either that the ether between the disks is quite unaffected by their motion, or, if affected at all, by something less than the thousandth part. At the same time, so far as rigorous proof is concerned, I should prefer to assert that the velocity of light between two steel plates moving together in their own plane an inch apart is not increased or diminished by so much as the jfrth part of their velocity.

The following illustrations give an idea of the apparatus employed :

Fig. 12 shows a photograph of the whirling machine before being bolted down to its stone pier; with the pair of disks at top ready to be whirled by an armature on the shaft, which is supplied with a current sometimes of nine horse- power. The armature winding was of low re- sistance, and was specially braced, so as to give high speed without flying out, and without

77

THE ETHER OF SPACE

generating too much back-E M F. The ampere- meter and volt-meter and the carbon rheostat (in armature circuit), for regulating the speed, are plainly seen. The smooth pulley on the shaft is for applying a brake. The small disk above it is perforated to act as a syren for es- timation of speed; but other arrangements for this purpose were subsequently added. The two large disks at top were of the best circular- saw steel ; they are somewhat thicker at middle than at edge, and are strongly bolted up be- tween iron cheeks, which are attached to the shaft. The lower end of the shaft is a step- bearing of hardened steel in a vessel of oil. The upper collar is elastic, so as to allow for a steady- ing teetotum action at high speeds.

Fig. 13 is a photograph of the optical square, which was ultimately to be placed in position surrounding the disks. The slit and collimator are shown; the micrometer end of the observing telescope is out of the picture.

The mirrors on the sides of the square are accurately plane; they are adjustable on geo- metric principles, and are pressed against their bearings by strong spiral springs. They were made by Hilger.

A drawing of the arrangement is given in Fig. 14, and here the double micrometer eyepiece is visible.

In Fig. 15 the whole apparatus is shown

78

1

THE ETHER OF SPACE

mounted. The whirling machine strongly bolt- ed down to a stone pier independent of the floor ; the optical frame independently supported by a gallows frame from other piers. The centrifugal mercury speed-indicator is visible in front, and Mr. Davies is regulating the speed. At the back is seen a boiler-plate screen for the observer with his eye at the telescope. (See Frontispiece.)

The expense of the apparatus was borne by my friend, the late George Holt, shipowner, of Liverpool.

Fig. 1 6 exhibits something like the appearance seen in the eye-piece, with the interference bands on each side of the middle band, and with the

FIG. 1 6. Approximate appearance of the inter- ference bands and micrometer Tires as seen in the eye-piece of the telescope of the Ether machine.

micrometer wires set in position each moved by

an independent micrometer head. The straight

vertical wire was usually set in the centre of the

80

FiG.J+ Plan of optuxLl Pram* *n*Jt, neoL ctiate in. po ffOfrM O T*pr*ffnu ooe or tfvt panea of o shcx*-rt~, and. por6 of <A* fijnny of th* four m from*, aaafi. twrw hfltL Jy a, bntes plain U. us pressed, by the spring bolts sfi+

SPECIAL EXPERIMENT

middle white band, and the X wire on the yellow of the first coloured band on one side or the other.

The method of observation now consists in setting a wire of the micrometer accurately in the centre of the middle band, while another wire is usually set on the first band to the left. Then the micrometer heads are read, and the setting repeated once or twice to see how closely and dependably they can be set in the same position. Then we begin to spin the disks, and when they are going at some high speed, meas- ured by a siren note and in other ways, the micrometer wires are reset and read reset several times and read each time. Then the disks are stopped and more readings are taken. Then their motion is reversed, the wires set and read again; and finally the motion is once more stopped and another set of readings taken. By this means the absolute shift of middle band, and its relative interpretation in terms of wave- length, are simultaneously obtained; for the distance from the one wire to the other, which is often two revolutions of a micrometer head, rep- resents a whole wave-length shift.

In the best experiments I do still often see something like a fiftieth of a band shift ; but it is caused by residual spurious causes, for it repeats itself with sufficient accuracy in the same direc- tion when the disks are spun the other way round.

Of real reversible shift, due to motion of the 81

THE ETHER OF SPACE

ether, I see nothing. I do not believe the ether moves. It does not move at a five-hundredth part of the speed of the steel disks. Further experience confirms and strengthens this es- timate, and my conclusion is that such things as circular-saws, flywheels, railway trains, and all ordinary masses of matter do not appreciably carry the ether with them. Their motion does not seem to disturb it in the least.

The presumption is that the same is true for the earth ; but the earth is a big body it is con- ceivable that so great a mass may be able to act when a small mass would fail. I would not like to be too sure about the earth at least, not on a strictly experimental basis. What I do feel sure of is that if moving matter disturbs ether in its neighbourhood at all, it does so by some minute action, comparable in amount perhaps to gravitation, and possibly by means of the same property as that to which gravitation is due not by anything that can fairly be likened to etherial viscosity. So far as experiment has gone, our conclusion is that the viscosity or fluid friction of the ether is zero. And that is an en- tirely reasonable conclusion.

MAGNETISATION.

For testing the effect of magnetism, an oblate spheroid was made of specialty selected soft iron, 82

SPECIAL EXPERIMENT

SPECIAL EXPERIMENT

cordingly in the eyepiece the iron surfaces could be seen, above and below, as well as the inter- ference bands in the luminous gap. The whole appearance is depicted in Fig. 18.

ELECTRIFICATION.

For the electrification experiment, a third and insulated disk was clamped between the two steel disks and kept electrified to sparking tension. The arrangement is shown diagrammatically on a smaller scale in Fig. 19.

FIG. 19. Arrangement for electrifyirig a third or mid- dle steel disk to sparking potential while spinning.

The electrification test was exceptionally easy to apply by connecting the insulated charging pin to a Voss machine in action: because when the disks were spinning and the bands in good condition, the electrification could be instan- taneously applied, taken off, reversed, or what- ever was desired; and the effect of the sudden lowering of potential by sparks passing between the revolving plates could be exactly looked for.

The conclusion of my second Philosophical Transactions paper that of 1897 is that neither

85

THE ETHER OF SPACE

an electric nor a magnetic transverse field confers viscosity upon ike ether, nor enables moving matter to grip and move it rotationally.

QUESTION OF A POSSIBLE LONGITUDINAL MAGNETIC DRIFT.

Later I tried a longitudinal magnetic field also ; arranging a series of four large electric bobbins or long coils along the sides of a square inscribed at 45 Q in the optical square (Figs. 1 1 and 13), so that the light went along their axes.

The details of this experiment have been only partially recorded, but the salient points are to be found stated in the Philosophical Magazine for April, 1907, pages 495-500.

There may be reasons for supposing that some much slower drift or conveyance than this 86

SPECIAL EXPERIMENT

is really caused in the ether by a magnetic field ; but if so, the ether must be regarded as so ex- cessively dense that the amount of such a drift for any practicable magnetic field seems almost hopelessly beyond experimental means of de- tection .

VI ETHERIAL DENSITY

THIS leads us to enter upon the question of whether it is possible to determine with any approach to accuracy the actual density or mas- siveness of the ether of space, compared with those forms of matter to which our senses have made us accustomed.

The arguments on which an estimate may be made of the density or massiveness of the ether as compared with that of matter depend on the following considerations, the validity of which again is dependent upon an electrical theory of matter. In this theory, or working hypothesis, an assumption has to be made: but it is one for which there is a large amount of justification, and the reasons for it are given in many books among others in my book on Electrons, and like- wise at the end of the new edition of Modern Views of Electricity, also in my Romanes Lecture, published by the Clarendon Press in 1903. Put briefly, the assumption is that matter is com- posed, in some way or other, of electrons; which

ETHER DENSITY

again must be considered to be essentially pe- culiarities, or singularities, or definite structures, in the ether itself. Indeed, a consideration of electrons alone is sufficient for the argument, provided it be admitted that they have the mass which experiment shows them to possess, and the size which electrical theory deduces for them: the basis of the idea which, indeed, is now ex- perimentally proved being that their inertia is due to their self-induction i.e., to the magnetic field with which they must be surrounded as long as they are in motion.

The mass, or inertia, of an electron is com- parable to the thousandth part of that of the atom of hydrogen. Its linear dimension, let us say its diameter, is comparable to the one hun- dred thousandth part of what is commonly known as molecular or atomic dimensions ; which itself is the ten-millionth part of a millimeter.

Hence, the mass and the bulk of an electron being known, its density is determined, provided we can assume that its mass is all dependent on what is contained within its periphery. But that last assumption is one that quite definitely cannot be made: its mass is for the most part outside itself, and has to be calculated by mag- netic considerations. (See Appendix 2.)

These details are gone into in my paper in the Philosophical Magazine for April, 1907, and in Chapter XVII of Modern Views of Electricity. 7 89

THE ETHER OF SPACE

But without repeating arguments here, it will suffice to say that although the estimates may be made in various ways, differing entirely from each other, yet the resulting differences are only slight; the calculated densities come out all of the same order of magnitude namely, some- thing comparable to io 12 c.g.s. units; that is to say, a million million grammes per cubic centi- meter, or, in other words, a thousand tons to the cubic millimeter.

But, throughout, we have seen reason to assert that the ether is incompressible; arguments for this are given in Modern Views of Electricity, Chapter I. And, indeed, the fundamental medium filling all space, if there be such, must, in my judgment, be ultimately incompressible; otherwise it would be composed of parts, and we should have to seek for something still more fundamental to fill the interstices.

The ether being incompressible, and an elec- tron being supposed composed simply and solely of ether, it follows that it cannot be either a con- densation or a rarefaction of that material, but must be some singularity of structure, or some portion otherwise differentiated. It might, for instance, be something analogous to a vortex ring, differentiated kinetically i.e., by reason of its rotational motion, from the remainderof the ether; or it might be differentiated statically, and be something which would have to be called 90

ETHER DENSITY

a strain-centre or a region of twist, or something which cannot be very clearly at present imagined with any security; though various suggestions have been made in that direction.

The simplest plan for us is to think of it some- what as we think of a knot on a piece of string. The knot differs in no respect from the rest of the string, except in its tied-up structure ; it is of the same density with the rest, and yet it is differ- entiated from the rest ; and, in order to cease to be a knot, would have to be untied a process which as yet we have not yet learned how to apply to an electron. If ever such a procedure becomes possible, then electrons will thereby be resolved into the general body of the undif- ferentiated ether of space that part which is independent of what we call "matter."

The important notion for present purposes is merely this: that the density of the undiffer- entiated or simple ether, and the density of the tied-up or beknotted or otherwise modified ether constituting an electron, are one and the same. Hence the argument above given, at least, when properly worked out, tends to es- tablish the etherial density as of the order io 12 times that of water.

There ought to be nothing surprising (though I admit that there is something very surprising) in such an estimate ; inasmuch as many converg- ing lines of argument tend to show that ordinary

THE ETHER OF SPACE

matter is a very porous or gossamer-like sub- stance, with interspaces great as compared with the spaces actually occupied by the nuclei which constitute it. Our conception of matter, if it is to be composed of electrons, is necessarily rather like the conception of a solar system, or rather of a milky way; where there are innumerable dots here and there, with great interspaces be- tween. So that the average density of the whole of the dots or material particles taken together that is to say, their aggregate mass compared with the space they occupy is excessively small.

In the vast extent of the Cosmos, as a whole, the small bulk of actual matter, compared with the volume of empty space, is striking as we shall show directly; and now on the small scale, among the atoms of matter, we find the con- ditions to be similar. Even what we call the densest material is of extraordinarily insignificant massiveness as compared with the unmodified ether which occupies by far the greater propor- tion of its bulk.

When we speak of the density of matter, we are really though not consciously expressing the group-density of the modified ether which con- stitutes matter not estimated per unit, but per aggregate; just as we might estimate the group or average density of a cloud or mist. Reckoned per unit, a cloud has the density of water; reckoned per aggregate, it is an impal- 92

ETHER DENSITY

pable filmy structure of hardly any density at all. So it is with a cobweb, so perhaps it is with a comet's tail, so also with the Milky Way, with the cosmos and, as it now turns out, with ordinary matter itself.

For consider the average density of the material cosmos. It comes out almost incred- ibly small. In other words, the amount of matter in space, compared with the volume of space it occupies, is almost infinitesimal. Lord Kelvin argues that ultimately it must be really infinitesimal (Philosophical Magazine, Aug., 1901, and Jan., 1902); that is to say that the volume of space is infinitely greater than the total bulk of matter which it contains. Other- wise the combined force of gravity or at least the aggregate gravitational potential on which the velocity generated in material bodies ulti- mately depends, would be far greater than ob- servation shows" it to be.

The whole visible universe, within a paral- lax of isoo second of arc, is estimated by Lord Kelvin as the equivalent of a thousand million of our suns ; and this amount of matter, distrib- uted as it is, would have an average density of 1.6 X 10-23 grammes per c.c. It is noteworthy how exceedingly small is this average or aggre- gate density of matter in the visible region of space. The estimated density of icr 23 c.g.s. means that the visible cosmos is as much rarer

93

THE ETHER OF SPACE

than a "vacuum" of a hundred million ths of an atmosphere, as that vacuum is itself rarer than lead.

It is because we have reason to assert that any ordinary mass of matter consists, like the cosmos, of separated particles, with great intervening distances in proportion to their size, that we are able to maintain that the aggregate density of ordinary stuff, such as water or lead, is very small compared with the continuous medium in which they exist, and of which all particles are supposed to be really composed. So that lead is to the ether, as regards density, very much as the " vacuum" above spoken of is to lead. The fundamental medium itself must be of uni- form density everywhere, whether materialised or free.

VII

FURTHER EXPLANATIONS CONCERNING THE DENSITY AND ENERGY OF THE ETHER

A READER may suppose that in speaking of f\ the immense density or massiveness of ether, and the absurdly small density or specific gravity of gross matter by comparison, I intend to signify that matter is a rarefaction of the ether. That, however, is not my intention. The view I advocate is that the ether is a perfect con- tinuum, an absolute plenum, and that, therefore, no rarefaction is possible. The ether inside matter is just as dense as the ether outside, and no denser. A material unit say, an electron is only a peculiarity or singularity of some kind in the ether itself, which is of perfectly uniform density everywhere. What we "sense" as mat- ter is an aggregate or grouping of an enormous number of such units.

How, then, can we say that matter is millions of times rarer or less substantial than the ether of which it is essentially composed ? Those who feel any difficulty here, should bethink them-

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THE ETHER OF SPACE

selves of what they mean by the average or ag- gregate density of any discontinuous system, such as a powder, or a gas, or a precipitate, or a snow-storm, or a cloud, or a milky way.

If it be urged that it is unfair to compare an obviously discrete assemblage like the stars, with an apparently continuous substance like air or lead, the answer is that it is entirely and accu- rately fair; since air, and every other known form of matter, is essentially an aggregate of particles, and since it is always their average density that we mean. We do not even know for certain their individual atomic density.

The phrase "specific gravity or density of a powder" is ambiguous. It may mean the specific gravity of the dry powder as it lies, like snow ; or it may mean the specific gravity of the particles of which it is composed, like ice.

So also with regard to the density of matter ; we might mean the density of the fundamental material of which its units are made which would be ether; or we might, and in practice do, mean the density of the aggregate lump which we can see and handle ; that is to say, of water or iron or lead, as the case may be.

In saying that the density of matter is small I mean, of course, in the last, the usual, sense. In saying that the density of ether is great I mean that the actual stuff of which these highly porous aggregates are composed is of immense, of

DENSITY AND ENERGY

well-nigh incredible, density. It is only another way of saying that the ultimate units of matter are few and far between i.e., that they are ex- cessively small as compared with the distances between them; just as the planets of the solar system, or worlds in the sky, are few and far between the intervening distances being enor- mous as compared with the portions of space actually occupied by lumps of matter.

It may be noted that it is not unreasonable to argue that the density of a continuum is necessa- rily greater than the density of any disconnected aggregate: certainly of any assemblage whose particles are actually composed of the material of the continuum. Because the former is " all there," everywhere, without break or inter- mittence of any kind; while the latter has gaps in it it is here and there, but not everywhere.

Indeed, this very argument was used long ago by that notable genius Robert Hooke, and I quote a passage which Professor Poynting has discovered in his collected posthumous works and kindly copied out for me:

" As for matter, that I conceive in its essence to be immutable, and its essence being expatiation determinate, it cannot be altered in its quantity, either by condensation or rarefaction; that is, there cannot be more or less of that power or reality, whatever it be, within the same expatia- tion or content; but every equal expatiation

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THE ETHER OF SPACE

contains, is filled, or is an equal quantity of materia; and the densest or heaviest, or most powerful body in the world contains no more materia than that which we conceive to be the rarest, thinnest, lightest, or least powerful body of all; as gold for instance, and cziker, or the substance that fills the cavity of an exhausted vessel, or cavity of the glass of a barometer above the quicksilver. Nay, as I shall afterwards prove, this cavity is more full, or a more dense body of aether, in the common sense or acceptation of the word, than gold is of gold, bulk for bulk; and that because the one viz., the mass of aether, is all aether: but the mass of gold, which we con- ceive, is not all gold; but there is an intermixture, and that vastly more than is commonly sup- posed, of aether with it ; so that vacuity, as it is commonly thought, or erroneously supposed, is a more dense body than the gold as gold. But if we consider the whole content of the one with that of the other, within the same or equal quantity of expatiation, then are they both equally containing the materia or body." From the Posthumous Works of Robert Hooke, M.D., F.R.S., 1705, pp. 171-2 (as copied in Memoir of Dalton, by Angus Smith}.

Newton's contemporaries did not excel in power of clear expression, as he himself did; but Professor Poynting interprets this singular at- tempt at utterance thus: "All space is filled

DENSITY AND ENERGY

with equally dense materia. Gold fills only a small fraction of the space assigned to it, and yet has a big mass. How much greater must be the total mass filling that space."

The tacit assumption here made is that the particles of the aggregate are all composed of one and the same continuous substance practically that matter is made of ether; and that assump- tion, in Hooke's day, must have been only a spec- ulation. But it is the kind of speculation which time is justifying, it is the kind of truth which we all feel to be in process of establishment now. 1

We do not depend on that sort of argument, however; what we depend on is experimental measure of the mass, and mathematical estimate of the volume, of the electron. For calculation shows that however the mass be accounted for whether electrostatically, or magnetically, or hy- drodynamically the estimate of ratio of mass to effective volume can differ only in a numerical coefficient, and cannot differ as regards order of magnitude. The only way out of this conclu- sion would be the discovery that the negative electron is not the real or the main matter-

1 It does not seem to have been noticed that in Query 22, quoted in the Introduction to the present book, Newton seems to throw out a curious hint in this same direction, though he immediately abandons it again. He does not appear to have carefully edited his queries; probably they were published posthumously.

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THE ETHER OF SPACE

unit, but is only a subsidiary ingredient ; where- as the main mass is the more bulky positive charge. That last hypothesis, however, is at present too vague to be useful. Moreover, the mass of such a charge would in that case be un- explained, and would need a further step ; which would probably land us in much the same sort of etherial density as is involved in the estimate which I have based on the more familiar and tractable negative electron. (See Appendix 2.) It may be said, Why assume any definite density for the ether at all ? Why not assume that, as it is infinitely continuous, so it is infinitely dense whatever that may mean and that all its properties are infinite ? This might be possible were it not for the velocity of light. By trans- mitting waves at a finite and measurable speed, the ether has given itself away, and has let in all the possibilities of calculation and numerical statement. Its properties are thereby exhibited as essentially finite however infinite the whole extent of it may turn out to be. Parenthetically, we may remark that "gravitation" has not yet exhibited any similar kind of finite property; and that is why we know so little about it.

ETHERIAL ENERGY.

Instead, then, of saying that the density of the ether is great, the clearest mode of expression 100

DENSITY AND ENERGY

is to say that the density of matter is small. Just as we can say that the density of the visible cosmos is small, although in individual lumps its density is comparable to that of iron or rock.

At the risk of repetition, I have explained this over again, because it is a matter on which con- fusion may easily arise. The really important thing about ether is not so much its density, considered in itself, as the energy which that density necessarily involves, on any kinetic theory of its elasticity. For it is not impossible however hopeless it may seem now that a modicum of that energy may some day be par- tially utilised.

Lord Kelvin's incipient kinetic theory of elas- ticity is a complicated matter, and I will only briefly enter upon it. But before doing so, I want to remove an objection which is some- times felt, as to the fluid and easily permeable character of a medium of this great density that is to say, as to the absence of friction or viscosity the absence of resistance to bodies moving through it. As a matter of fact there is no necessary connection whatever between density and viscosity.

"Density" and "Viscosity" are entirely dif- ferent things; and, if there is no fluid friction, a fluid may have any density you please without interposing any obstacle to constant velocity.

IOZ

THE ETHER OF SPACE

To acceleration it does indeed oppose an obstacle, but that appears as essentially a part of the in- ertia or massiveness of the moving body. It contributes to its momentum; and, if the fluid is everywhere present, it is impossible to dis- criminate between, or to treat separately, that part of the inertia which belongs to the fluid displaced, and that part which belongs to the body moving through it except by theory.

Consequently the pull of the earth on the moon is

6 x io 21

80 x 3600

tons weight.

A pillar of steel which could transmit this force, provided it could sustain a tension of 40 tons to the square inch, would have a diameter of about 400 miles; as stated in the text, page 112. 126

STRENGTH OF THE ETHER

If this force were to be transmitted by a forest of weightless pillars each a square foot in cross- section, with a tension of 30 tons to the square inch throughout, there would have to be 5 million million of them.

Arithmetical Calculation of the Pull of the Sun on the Earth.

The mass of the earth is 6x io 21 tons.- The intensity of solar gravity at the sun's surface is 25 times ordinary terrestrial gravity.

At the earth's distance, which is nearly 200 solar radii, solar gravity will be reduced in the ratio of i : 200 squared.

Hence the force exerted by the sun on the earth is

2< X 6 X IO 21 .

(200 ) 2 tons weight "

That is to say, it is approximately equal to the weight of 37 X io 17 ordinary tons upon the earth's surface.

Now steel may readily be found which can stand a load of 37 tons to every square inch of cross- section. The cross-section of a bar of such steel, competent to transmit the sun's pull to the earth, would therefore have to be

io 17 square inches;

or, say, 700 x io 12 square feet,

127

THE ETHER OF SPACE

And this is equivalent to a million million round rods or pillars each 30 feet in diameter.

Hence the statement in the text (page 26) is well within the mark.

The Pull of the Earth on the Sun.

The pull of the earth on the sun is, of course, equal and opposite to the pull of the sun on the earth, which has just been calculated; but it furnishes another mode of arriving at the result, and may be regarded as involving simpler data i.e., data more generally known. All we need say is the following:

The mass of the Sun is 316,000 times that of the Earth.

The mean distance of the sun is, say, 23,000 earth radii.

Hence the weight or pull of the sun by the earth is

7 i 6000

r, x 6 x io 21 tons weight. (2 3 ooo) 2

In other words, it is approximately equal to the ordinary commercial weight of 36x10" tons, as already calculated.

The Centripetal Force acting on the Earth.

Yet another method of calculating the sun's pull is to express it in terms of the centrifugal force of the earth; namely, its mass, multiplied 128

STRENGTH OF THE ETHER

by the square of its angular velocity, multiplied by the radius of its orbit ; that is to say,

F=:M (1

where T is the length of a year.

The process of evaluating this is instructive, owing to the manipulation of units which it involves :

47r 2 x 02 x io 8 miles

F = 6 x io 21 tons x "

(365* days) 2

which of course is a mass multiplied by an acceleration. The acceleration is

40 X 02 X IO 8

; r-s miles per hour per hour

133300 X (2 4 ) 2

3680 x io 8 x 5280

Consequently the centripetal acceleration is

4 7T 8 X8X IO 6 .. , . 1

--g miles per day per day; which comes

• oo.0=

ds

and ignoring possible variations in the minute correction factor i a 2 between the points A and B, it becomes

Time of _f B cos_ _. ds _ ?B - ?A = minimum . journey I x _ a V V 2 (i-a a )

J A

Now the second term depends only on end points, and therefore has no effect on path. The

139

THE ETHER OF SPACE

first term contains only the second power of aberration magnitude; and hence it has much the same value as if everything were stationary. A ray that was straight will remain straight in spite of motion. Whatever shape it had, that it will retain.

Only cos e, and variations in a 2 , can produce any effect on path ; and effects so produced must be very small, since the value of cos e is

V (i-a 2 sin 2 0).

A second-order effect on direction may there- fore be produced by irrotational motion, but not a first-order effect. A similar statement applies to the time of journey round any closed periphery.

Michelson's Experiment.

We conclude, therefore, that general etherial drift does not affect either the path of a ray, or the time of its journey to and fro, or round a complete contour, to any important extent. But that taking second-order quantities into account, the time of going to and fro in any direction inclined at angle to a constant drift is, from the above expression,

T -L T 2 T cos _/(!- ct 2 sin 2 0)

lj T 1 2 5 5 A 2 1 ,

i - a 2 i-a 2

where 2T is the ordinary time of the double journey without any drift. 140

ABERRATION THEORY

Hence some slight modification of interference effects by reason of drift would seem to be pos- sible; since the time of a to and fro light- journey depends subordinately on the inclination of ray to drift.

The above expression applies to Michelson's remarkable experiment 1 of sending a split beam to and fro, half along and half across the line of the earth's motion; and is, in fact, a theory of it. There ought to be an effect due to the difference between 6=0 and 6=90. But none can be detected. Hence, either something else happens, or the ether near the earth is dragged with it so as not to stream through our instruments.

Alternative Explanation.

But if the ether is dragged along near moving matter, it behaves like a viscous fluid, and all idea of a velocity-potential must be abandoned. This would complicate the theory of aberration (pp. 47 and 64), and moreover is dead against the experimental evidence described in Chapter V.

The negative result of Mr. Michelson's is, how- ever, explicable in another way namely, by the FitzGerald-Lorentz theory that the linear dimen- sions of bodies are a function of their motion through the ether. And such an effect it is reasonable to expect; since, if cohesion forces are

1 Philosophical Magazine, December, 1887. 141

THE ETHER OF SPACE

electrical, they must be affected by motion, to a known and calculable amount, depending on the square of the ratio of the speed to the velocity of light. (See end of Chap. IV.)

The theory of Prof. H. A. Lorentz, accord- ingly, shows that the shape of Michelson's stone supporting block will be distorted by the mo- tion; its dimensions across and along the line of ether drift being affected differently. And the amount of the change will be such as pre- cisely to compensate and neutralise the optical effect of motion which might otherwise be per- ceived. This theory is now generally accepted.

It is this neutralising or compensatory effect which acts equally on to and fro motion of light, to and fro motion of electric currents, and on the shape of material bodies that renders any positive result in experiments on ether-drift so difficult or impossible to obtain ; so that, in spite of the speed with which we are rushing through space, no perceptible influence is felt on either electrical or optical phenomena, except those which are due to relative motion of source and observer.

Some Details in the Theory of the Doppler Effect,

or Effect of Motion on Dispersion

by Prism or Grating.

When light is analysed by a prism or grating

into a spectrum, the course of each ray is de-

142

ABERRATION THEORY

fleeted refracted or diffracted by an amount corresponding to its frequency of vibration or wave-length.

Motion of the medium, so long as it is steady, affects neither frequency nor wave-length, and accordingly is without influence on the result. It produces no Doppler effect except when waxing or waning.

Motion of the source alone crowds the waves together on the advancing side and spreads them out on the receding side. An observer, therefore, whom the source is approaching receives shorter waves, and one from whom the source is reced- ing receives longer waves, than normal. At any fixed point waves will arrive, therefore, with modified frequency.

So long as a source is stationary the wave- lengths emitted are quite normal, but motion of an observer may change the frequency with which they are received, in an obvious way ; they are swept up faster if the receiver is approaching, they have a stern chase if it is receding.

All this is familiar, and was geometrically illus- trated in Chapter III, but there are some minor and rather curious details which are worthy of brief consideration.

Grating Theory.

For suppose a "grating" is used to analyse the light. Its effect can depend on nothing kinetic ;

THE ETHER OF SPACE

it must be regulated by the merely geometric width of the ruled spaces on it. Consequently it can only directly apprehend wave-lengths, not frequencies.

In the case of a moving source, therefore, when the wave-length is really changed, a grating will appreciate the fact, and will show a true Doppler effect. But in the case of a moving observer, when all the waves received are of normal length, though swept up with abnormal frequency, the grating must still indicate wave-length alone, and accordingly will show no true Doppler effect.

But inasmuch as the telescope or line of vision is inclined at the angle of disperson to the direc- tion of the incident ray, ordinary aberration must come in, as it always does when an ob- server moves athwart his line of vision; and so there will be a spurious or apparent Doppler effect due to common aberration. That is to say a spectrum line will not be seen in its true place, but will appear to be shifted by an amount almost exactly imitative of a real Doppler effect the imitation being correct up to the second order of aberration magnitude. The slight out- standing difference between them is calculated in my Philosophical Transactions paper, 1893, page, 787. It is too small to observe.

It is not an important matter, but as it is rather troublesome to work out the diffraction observed by a grating advancing toward the

144

ABERRATION THEORY

source of light, it may be as well to record the result here.

The following are the diffracted rays which require attention with the inclination of each to the grating-normal specified:

The diffracted ray if all were stationary, ;

The real diffracted ray when grating is advancing, <f> ;

The ray as perceived, allowing for aberration, ;

The equivalent diffracted ray if all were stationary and the wave-length really shortened, 0,.

As an auxiliary we use the aberration angle e, such that sin c = a sin 0, where a v/V.

Among these four angles the following relations hold; so that, given one of them, all are known.

f\ j

sin 0j = ( i a) sin 9

cos e + cos

\

148

, instead of

ABERRATION THEORY

So the equivalent air thickness, instead of being (p i)z, is

2L5 z =

/I* COS - a COS 9 \

( t, -v O z '

a

cos H cos 9

or, to the first order of minutiae, (jjii)zaz cos 9;

6 being the angle between ray and ether drift inside the medium.

So the extra equivalent air layer due to the motion is approximately a z cos 6, a quantity independent of /*.

Hence, no plan for detecting this first order effect of motion is in any way assisted by the use of dense stationary substances ; their extra ether, being stationary, does not affect the lag caused by motion, except indeed in the second order of small quantities, as shown above.

Direct experiments made by Hoek,' 1 and by Mascart, on the effect of introducing tubes of water into the path of half beams of light, are in entire accord with this negative conclusion.

Thus, then, we find that no general motion of the entire medium can be detected by changes in direction, or in frequency, or in phase; for on

1 Archives Neerlandaises (1869), Vol. IV, p. 443. or Nature, Vol. XXVI, p. 500. Also Chapter IV, above.

none of them has it any appreciable (i.e., first order) effect, even when assisted by dense matter.

Another mode of stating the matter is to say that the behaviour of ether inside matter is such as to enable a potential-function,

/ n z v cos Ods,

to exist throughout all transparent space, so far as motion of ether alone is concerned (see Ap- pendix 3).

The existence of this potential function readily accounts for the absence of all effect on direction due to the general drift of the medium, whether in the presence of dense matter (such as water- filled telescopes) or otherwise. Whatever may be the path of a ray by reason of reflection or refraction in a stationary ether, it is precisely the same in a moving one if this condition is satisfied, although the wave-normals and wave- fronts are definitely shifted.

However matter affects or loads the ether in- side it, it cannot on this theory be said either to hold it still, or to carry it with it. The general ether stream must remain unaffected, not only near, but inside matter, if rays are to retain precisely the same course as if it were relatively stationary.

ABERRATION THEORY

But it must be understood that the etherial motion here contemplated is the general drift of the entire medium; or its correlative, the uniform motion of all the matter concerned. There is nothing to be said against aberration effects being producible or modifiable by motion of parts of the medium, or by the artificial motion of transparent bodies and other partitioned-off regions. Artificial motion of matter may readily alter both the time of journey and the path of a ray, for it has no potential conditions to satisfy; it may easily describe a closed contour, and may take part in conveying light.

But I must repeat that this conveyance of light by moving matter is an effect due to the material load only; it represents no disturbance of the ether of space. Fresnel's law, in fact, definitely means that moving transparent matter does not appreciably disturb the ether of space. Direct experiment, as recorded in Chapter V, shows that close to rapidly moving opaque matter there is no disturbance either.

I regard the non-disturbance of the ether of space by moving matter as established.

SUMMARY.

The estimates of this book, and of Modern Views of Electricity, are that the ether of space is a continuous, incompressible, stationary, fundamental substance or perfect fluid, with what is equivalent to an inertia-coefficient of io 12 grammes per c.c.; that matter is composed of modified and electrified specks, or minute structures of ether, which are amenable to mechanical as well as to electrical force and add to the optical or electric density of the medium ; and that elastic-rigidity and all potential energy are due to excessively fine-grained etherial circulation, with an intrinsic kinetic energy of the order io 33 ergs per cubic centimeter.

APPENDIX 1. ON GRAVITY AND ETHERIAL TENSION

IN the arithmetical examples of Chapter IX we reckon merely the force between two bodies; but the Newtonian tension mentioned in Chapter VIII does not signify that force, but rather a cer- tain condition or state of the medium, to variations in which, from place to place, the force is due. This Newtonian tension is a much greater quantity than the force to which it gives rise ; and, moreover, it exists at every point of space, instead of being integrated all through an attracted body.

It rises to a maximum value near the surface of any spherical mass ; and if the radius be R and the gravitational intensity is g, the tension at the sur- face is T = gR. At any distance r, further away, the tension is T = gR 2 /r.

This follows at once thus:

Expanding all this into words:

The etherial tension near the earth's surface, required to explain gravity by its rate of variation, is of the order 6 x io u c.g.s. units. The tension near the sun is 2500 times as great (p. 112). With dif- ferent spheres in general, it is proportional to the density and to the superficial area. Hence, near a bullet one inch in diameter, it is of the order io' 6 ; and near an atom or an electron about io~ 21 c.g.s.

If ever the tension rose to equal the constitution- al elasticity or intrinsic kinetic energy of the ether which we have seen is io 33 dynes per square centi- meter (or ergs per c.c.) or io 22 tons weight per square millimeter it seems likely that something would give way. But no known mass of matter is able to cause anything like such a tension.

and the energy per unit volume everywhere is

But a magnetic field has been thought of by many mathematicians as a circulation of fluid along the lines of magnetic induction which are always closed curves at some unknown velocity w.

So consider the energy per unit volume any- where : it can be represented by the equivalent ex- pressions

wherefore

e .sin 9 .

• v(w)

The velocity of the hypothetical circulation must be a maximum at the equator of the sphere, where r = a and 9 = tpo ; so, calling this iv ot

and

w a 2 sin0

wherefore the major part of the circulation is limited to a region not far removed from the sur- face of the electron.

The energy of this motion is

• r ji:* "'** **'*'

whence, substituting the above value of w, the energy comes out equal to $irpa 3 w *.

Comparing this with a niass moving with speed u,

• =!

In any solenoid, the ampere-turns per linear inch furnish a measure of the speed of the supposed magnetic circulation along the axis no matter what the material of the core may be in millimi- crons per sec.

i micron=io%22 6 meter; i millimicron is io%22 9 meter = io- 7 centimeter, or a millionth of a millimeter.

To get up an ethereal speed of i centimeter per second such as might be detected experimentally by refined optical appliances, through its effect in accelerating or retarding the speed of light sent along the lines of magnetic force would need a solenoid of great length, round every centimeter of which 1000 amperes circulated 3000 times. That is to say, a long field of four million c.g.s. units of intensity.

In other words, any streaming along magnetic lines of force, such as could account for the energy of a magnetic field, must be comparable, in centi- meters per second, to one four-millionth of the number of c.g.s. units of intensity in the magnetic field.

APPENDIX 3.

FRESNEL'S LAW A SPECIAL CASE OF A UNIVERSAL POTENTIAL FUNCTION

THE modern view of Fresnel's Law may be worded thus:

Inside a region occupied by matter, in addition to the universal ether of space, are certain modified or electrified specks, which build up the material atoms. These charged particles, when they move, have specific inertia, due to the magnetic field sur- rounding each of them. And by reason of this property, and as a consequence of their discon- tinuity, they virtually increase the optical density of the ether of space, acting in analogy with weights distributed along a flexible cord. Thus they re- duce the velocity of light in the ratio of the re- fractive index ft : i, and therefore may be taken as increasing the virtual density of the ether in the ratio i : /* 2 .

That is to say, their loading makes the ether be- have to optical waves as if being a homogeneous medium without these discontinuous loads it had a density /* 2 times that which it has in space out- side matter. Catling the density outside i, the extra density inside must be /* 2 - 1 , so as to make up the total to /* 2 .

The/* 2 - 1 portion is that which we call "matter," and this portion is readily susceptible to locomo- tion, being subject to that is, accelerated by me- chanical force. The free portion of normal density i is absolutely stationary as regards locomotion, whether it be inside or outside a region occupied by ordinary matter, for it is not amenable to either mechanical or electric forces. They are transmitted by it, but never terminate upon it; except, indeed, at the peculiar structure called a wave-front, which simulates some of the properties of matter.

(If free or unmodified ether can ever be moved at all, it must be by means of a magnetic field; along the lines of which it has, in several theories, been supposed to circulate. Even this, however, is not real locomotion.)

Fizeau tested that straightforward consequence of this theory which is known as Fresnel's law, and ascertained by experiment that a beam of light was accelerated or retarded by a stream of water, ac- cording as it travelled with or against the stream. And he found the magnitude of the effect precisely in accordance with the ratio of the locomotive portion of the ether to the whole the fraction (/i 2 - i)/V 2 of the speed of the water being added to or subtracted from the velocity of light, when a beam was sent down or up the stream.

But even if another mode of expression be adopt- ed, the result to be anticipated from this experi- ment would be the same.

For instead of saying that a modified portion of the ether is moving with the full velocity of the body while the rest is stationary, it is permissible for some purposes to treat the whole internal ether as moving with a fraction of the velocity of the body.

On this method of statement the ether outside a moving body is still absolutely stationary, but, as the body advances, ether may be thought of as continually condensing in front, and, as it were, evaporating behind; while, inside, it is streaming through the body in its condensed condition at a pace such that what is equivalent to the normal quantity of ether in space may remain absolutely stationary. To this end its speed backward rela- tively to the body must be u/n 2 and accordingly its speed forward in space must be u ( i - i /ft 2 ) .

For consider a slab of matter moving flatways with velocity u; let its internal etherial density be p*, and let the external ether of density i be sta- tionary. Let the forward speed of the internal ether through space be xu, so that a beam of light therein would be hurried forward with this velocity. Then consider two imaginary parallel planes mov- ing with the slab, one in advance of it and the other inside it, and express the fact that the amount of ether between those two planes must continue constant. The amount streaming relatively back- ward through the first plane as it moves will be measured by u times the external density, while the amount similarly streaming backward through the second plane will be (u xu) times the internal density. But this latter amount must equal the former amount. In other words,

u x i must equal (u xu) x /t j .

Consequently % comes out x = (/* 2 - i)/V; which is Fresnel's incontrovertible law for the convec- tive effect of moving transparent matter on light inside it.

The whole subject, however, may be treated more generally, and for every direction of the ray, on the lines of Chapter X, thus:

Inside a transparent body light travels at a speed V/V; and the ether, which outside drifts at velocity v, making an angle with the ray, inside may be drifting with velocity v' and angle 0'.

Hence the equation to a ray inside such matter is

T , _ r ds_ min

~~ J (V//i) cos e' + v' cos 9' ~

sin ' v'