Chapter I: Part 1

ESSAYS

ON THE

USE AND LIMIT

OF THE

IMAGINATION IN SCIENCE.

BY

JOHN TYNDALL, LL.D., F.R.S.

LONDON:
LONGMANS, GREEN, AND CO.
1870.

EXPLANATORY NOTE.

To a Second Edition of a Discourse on the Scientific Use of the Imagination, delivered before the British Association at Liverpool on September 16, 1870, are here added an Address on the Limit of the Imagination in Science, delivered before the Mathematical and Physical Section of the Association at Norwich on August 19, 1868, and a short Essay, entitled ‘Earlier Thoughts.’

The Address and the Essay were meant to be brief, but definite statements of the relation of Life and Consciousness to Matter and Force.

As in the case of the recent Discourse, opinion was divided with regard to the objects and merits of the Norwich Address. On the one hand, two eminent clergymen, one of the Church of England, the other a Dissenter, proposed and seconded respectively a vote of thanks, which was liberally carried by the section; on the other hand, I was publicly warned that, as a consequence of my impiety, the bolts of heaven were in a state of potential suspension above my head, ready to descend if further drawn upon.

My main object, both at Norwich and at Liverpool, was, firstly, to dissipate the repugnance, and indeed terror, which in many minds are associated with the thought that science has abolished the mystery of man’s relation to the universe; and, secondly, to remove the hindrance which popular notions regarding the origin of life oppose to legitimate scientific speculation.

ATHENÆUM CLUB: _November_ 1870.

CONTENTS.

PAGE

PROS AND CONS TOUCHING THE FIRST EDITION 1

SCIENTIFIC USE OF THE IMAGINATION 13

SCIENTIFIC LIMIT OF THE IMAGINATION 52

EARLIER THOUGHTS 66

PROS AND CONS

TOUCHING THE FIRST EDITION.

_From the_ TIMES, _Sept._ 19, 1870.

THE GLORY of a Natural Philosopher appears to depend less on the power of his imagination to explore minute recesses or immeasurable space than on the skill and patience with which, by observation and experiment, he assures us of the certainty of these invisible operations. Newton’s glory is founded, not on the sudden imagination by which he leapt ‘from a falling apple to a falling moon,’ but on that astonishing tenacity of investigation by which he reduced his guesses to moral certainties, and enabled us to witness a practical verification of his laws in every almanack we use. When the movements of the heavenly bodies have been discovered by this laborious process, the imagination is excellently employed in picturing to the mind’s eye what transcends the physical vision; and, perhaps, the labour of the investigation itself would be unendurable unless the attention could be relieved by the constant pictorial aid afforded by the imagination....

In a word, it seems to us worth consideration whether that use of the imagination in Natural Philosophy of which the Professor speaks needed any encouragement at the present day. The discoveries of science have been so astonishing, the new worlds they have opened to us are so vast, that there is, perhaps, more danger of our imagination being exercised too freely than of its not being exercised sufficiently upon them. No one will dispute the claim of the Professor to be one of the privileged spirits of whom he speaks, if there are any such, and he proceeds accordingly to exercise his imagination, not upon the little germ-cells, but upon that vast primordial nebulous envelope out of which, according to the opinion to which some philosophers incline, all the infinite complexity of the existing world has been developed. We are not privileged spirits, and we own ourselves not altogether able to follow him when he leads us into the imaginary realms of the original chaos. He confesses that Mr. Darwin ‘has drawn heavily upon time and adventurously upon matter.’ We ask ourselves again whether we are listening to one experimental philosopher describing the achievements of another experimental philosopher. We had been under the impression that Natural Philosophers drew no bills. We do not presume to say one word about the Evolution Hypothesis. We neither affirm nor deny that Professor Tyndall existed in a nebulous state an infinite number of centuries ago. We only venture to suggest that when the British Association amuse the public with these speculations they are illustrating, not the scientific use of the imagination, but the imaginative use of science.

_From the_ SATURDAY REVIEW, _Sept._ 24, 1870.

Here, too, we question whether Sir William Thomson will be content with this definition of the process by which he has been guided to his most recent advance in molecular physics. In the splendid series of inductions verified step after step by rigorous experiment and observation, and kept in exactest continuity by the chain of mathematical evolution, is imagination the faculty to which are to be given the chief honours of this conquest of a new realm of physics? This would surely be to force upon us a new and arbitrary classification or analysis of the powers of the intellect. If we follow Professor Tyndall himself through the masterly train of reasoning whereby he leads us to the laws of reflection and transmission of light as the cause of the azure of the firmament, is what we admire the leap of imagination, or the firmly balanced and duly graduated tread of a mind trained in the discipline of logic, and careful to plant every step on the assured ground of fact or experience? It is simply a misnomer to apply the name of imagination to the process or the faculty to which this onward march into the realm of unexplored nature is really due. As well describe as a triumph of the imagination the connected and organized plan of the great strategist which has drawn round Paris a living cordon of 300,000 men.

_From a Lecture addressed to Teachers at the South Kensington Museum,
April 30, 1861, by_ J. TYNDALL.

Here, then, is an exhibition of power which we can call forth or cause to disappear at pleasure. We magnetize our strip of steel by drawing it along the pole of a magnet; we can demagnetize it, or reverse its magnetism, by properly drawing it along the same pole in the opposite direction. What, then, is the real nature of this wondrous change? What is it that takes place among the atoms of the steel when the substance is magnetized? The question leads us beyond the region of sense, and into that of imagination. This faculty, indeed, is the divining rod of the man of science. Not, however, an imagination which catches its creations from the air, but one informed and inspired by facts, capable of seizing firmly on a physical image as a principle, of discerning its consequences, and of devising means whereby these forecasts of thought may be brought to an experimental test. If such a principle be adequate to account for all the phenomena, if from an assumed cause the observed facts necessarily follow, we call the assumption a theory, and once possessing it, we can not only revive at pleasure facts already known, but we can predict others which we have never seen. Thus, then, in the prosecution of physical science, our powers of observation, memory, imagination, and inference, are all drawn upon. We observe facts and store them up; imagination broods upon these memories, and by the aid of reason tries to discern their interdependence. The theoretic principle flashes, or slowly dawns upon the mind, and then the deductive faculty interposes to carry out the principle to its logical consequences. A perfect theory gives dominion over natural facts; and even an assumption which can only partially stand the test of a comparison with facts, may be of eminent use in enabling us to connect and classify groups of phenomena.

_From the_ GUARDIAN, _Sept._ 21, 1870.

He held some pieces of paper in his hand, but he rarely referred to them. Thoroughly possessed by his subject, his thoughts seemed to flow forth with perfect ease, fresh minted as they were in the most appropriate and perspicuous words. He led his hearers gently and almost unconsciously through the most perplexed mazes and subtlest passages of thought, keeping their ears enchained and their fancy charmed by the endless succession of apt metaphor; and yet, whenever he felt or fancied that their overstrained attention began to flag, he was able to turn aside into light and pleasant banter, and after this interlude of welcome refreshment, to resume again with renewed power the unbroken thread of his serious discourse. It was the manifest work of a master in his art, handling with ease and grace the weighty tools which long use had made familiar to his hand.

_From the_ ENGLISH CHURCHMAN, _Sept._ 29, 1870.

What astonishes us beyond measure is, that a man of Professor Tyndall’s real ability and earnestness should sneer at the second verse of the Bible, and speak of it as a legend! Why, surely, he has here the very thing which he is searching--the true origin of life. When chaos ruled over the world, and the earth was void of life; it was the Divine Spirit that breathed over the lifeless mass, and light and life sprang into existence. Without stopping to point out the evidence of the highest Christian doctrine in this passage, we have at least the solution of the enigma of the origin of life, in the revealed truth that it was caused by the Creative Spirit.

_From the_ RECORD, _Sept._ 23, 1870.

_Discovery of Motives, No. I._--But why did Professor Tyndall make such an appeal to the imagination of his hearers? His imaginary picture of the occult operations of light was introduced as a plea for the wildness of such weaker brethren, as he calls Mr. Darwin, in speaking of his theory of natural selection, succeeded by his supplementary theory of pangenesis. It is objected to Mr. Darwin’s theories by Christian philosophers that these theories are essentially atheistic. That they are framed for the express purpose of blotting out of the page of nature some of the most marvellous evidences of design--of the most patent revelations of the book of nature, that there is an all-wise, all-mighty Creator, God. That these theories deprive man of all those prerogatives that raise him above the brute. That the facts of nature contradict them. The first theory of Mr. Darwin, that of natural selection, is an attempt to account for the formation of all animal and vegetable beings from an hypothetical germ.... Even this does not pall the imagination of Professor Tyndall. He accepts the theory of the avowed atheist Louis Buchner, in his ‘Force and Matter,’ that the theory of evolution requires us to imagine not only that all the structures, animal and vegetable, were once potentially present in the fire-mist of the nebulous theory, but also that all mental powers--Plato, Shakespeare, Newton, and Raphael--are potential in the fires of the sun.

Professor Tyndall appeals to believers in the Bible as God’s Word not to style such a theory wicked or impious. He says men can hold it, and manifest in their lives what he terms so-called Christian virtues. He says, ‘They who keep such questions open and will not tolerate any unlawful limitation of the horizon of their souls, have as little fellowship with the atheist who says there is no God, as with the theist who professes to know the mind of God.’ These men then are theists, but what kind of a God does their free speculation require? The theists who are sneered at by Professor Tyndall, who believe in a God who has revealed his will and his mind to man, have far higher and more convincing proofs that He has so revealed Himself than Professor Tyndall can ever accumulate for his belief in the undulatory theory of light.

Professor Tyndall’s philosophy regards the universe as a huge mechanical, self-supporting, self-sustaining, self-evolving, material machine--untended by a loving Father’s sustaining and providential care. His God is the God of the Epicureans, who created and started the machine into motion and then left it for ever to itself. Such a philosophy, the child of unbridled pride of intellect, may appeal to the wildest imagination of corrupted human nature, but it has no sympathy with all the higher yearnings of the soul.

_From the_ LANCET, _Sept._ 24, 1870.

_Discovery of Motives, No II._--Now, Professor Tyndall’s object was to preach about germs, and he proceeded to accomplish it in somewhat the following manner. He first set forth that it was a wholesome use of the imagination to apply our knowledge of aërial sound-waves to the solution of the question--what is the cause of the phenomena of light? And he then proceeded to draw one of those charming word-pictures for which he is so famous, showing the rippling of the ethereal light-waves against molecules in the atmosphere, the greater proportionate reflection of the shorter wave of blue, and the consequent preponderance of blue rays in the light reflected to us from the sky, and of red and yellow rays in the light coming unreflected from the sun.... Now, the aim of all this was to seek and show that the air is filled by an infinite multitude of suspended particles, so minute that they do not produce darkness, and that these particles may be germs. Professor Tyndall does not say that they are germs, but, by the aid of a special disclaimer, he prevented his audience from forgetting that they might be. We should be very loth to accuse him of disingenuousness, but we are unable entirely to acquit him of special pleading. We feel that his lecture was a very skilful attempt to familiarise the public mind with the existence of atmospheric particles, and to lead up to and encourage, without absolutely expressing the idea that germs are particles, and that particles may be germs.

_To the Editor of the_ RECORD.

SIR,--It is a grave error on your part to represent me as calling Mr. Darwin ‘one of the weaker brethren.’ Were I asked to name the highest representative of the stronger ones, I should probably name him. But in your article you link my name with that of a writer whom I do rank among the weaker brethren; weak through a defect common to him and his antagonists--the incompetence, namely, to look round a great question and see its bearings on all sides.

I am, Sir, your obedient servant,
JOHN TYNDALL.

ATHENÆUM CLUB, October 4.

_From the_ PALL MALL GAZETTE, _September_ 20, 1870.

Why does Professor Tyndall attribute to Goethe the ‘notion,’ as he calls it, that matter is ‘the living garment of God’? We are not aware that it is to be found in his works. In ‘Faust’ Goethe introduces the spirit of the earth, who describes his own operations as consisting in weaving into one vast fabric the ‘tumults of human life, the storm of actions,’ births and deaths, and the affairs of us mortals, and working thereout ‘a living garment for the Divinity.’ Whether the phrase be a piece of cant, or a piece of sublimity, it has no semblance of the meaning which the Professor attributes to it.

_From the_ SPECTATOR, _September_ 24, 1870.

Professor Tyndall concluded his lecture by a passage on the development theory, in which he contended that if our traditional view of matter had been Goethe’s view, that matter is ‘the living garment of God,’ instead of Young’s, who looked upon it as foreign to mind, and taking all its laws from outside itself, the development theory would not seem to us what we now mean by materialistic. The ‘Pall Mall’ falls severely on Professor Tyndall for misquoting Goethe, and shows that the passage in ‘Faust’ probably referred to, where the Erdgeist speaks of weaving a ‘living garment for the Divinity,’ did not refer to external nature at all. No doubt the special quotation was a little wide of the mark, but does the critic in the ‘Pall Mall’ doubt that Professor Tyndall was interpreting quite accurately Goethe’s conception, as elsewhere expressed with sufficient elaboration? If he does not, his criticism is a cavil. If he does, let him study Goethe more thoroughly--‘Gott und Welt,’ for example, of the proem to which a friend has sent us a faithful version to-day, which we print in another column. What can be stronger than this?--

Was wär’ ein Gott der nur von aussen stiesse
Im Kreis das All am Finger laufen liesse!
Ihm ziemt’s, die Welt im Innern zu bewegen,
Natur in Sich, Sich in Natur zu hegen.

_A Translation of Goethe’s Proemium to ‘Gott und Welt.’_

To Him who from eternity, self-stirred,
Himself hath made by His creative Word!
To him, supreme, who causeth Faith to be,
Trust, Hope, Love, Power, and endless Energy!
To Him, who, seek to name Him as we will,
Unknown within Himself abideth still!

Strain ear and eye, till sight and sense be dim;
Thou’lt find but faint similitudes of Him:
Yea, and thy spirit in her flight of flame
Still strives to gauge the symbol and the name:
Charmed and compelled thou climb’st from height to height,
And round thy path the world shines wondrous bright;
Time, Space, and Size, and Distance cease to be,
And every step is fresh infinity.

What were the God who sat outside to scan
The spheres that ’neath his finger circling ran?
God dwells within, and moves the world and moulds,
Himself and Nature in one form enfolds:
Thus all that lives in Him and breathes and is,
Shall ne’er His puissance, ne’er His spirit miss.

The soul of man, too, is an universe:
Whence follows it that race with race concurs
In naming all it knows of good and true
God,--yea, its own God; and with homage due
Surrenders to His sway both earth and heaven;
Fears Him, and loves, where place for love is given.

J. A. S.

From the SPECTATOR, September 24.

_From the_ TIMES, _October_ 3, 1870.

But the most serious obstacle of a public nature which can possibly impede the progress of science--an obstacle before which all others sink into absolute insignificance--is _the reign of prejudice_, or the unwillingness to adopt the teachings of science, and to accept her legitimate conclusions through certain preconceived opinions, the result of a faulty education or a vicious temperament. To this, indeed, we think the British Association cannot pay too much attention, and we were not a little gratified, in consequence, at the eloquent lecture on the use of the imagination in science delivered by Professor Tyndall. The importance of such a discourse at such a time to clear the atmosphere of the clouds of prejudice which a mistaken zeal has raised in the minds of a large class cannot be over-estimated. Since it is certain that religious intolerance and religious bigotry are the largest sources of prejudice, the removal of these ought to be a primary object of the association, and when the undertaking is made with the same spirit of reverence, the same earnestness of purpose, and philosophical acumen which distinguished Professor Tyndall’s discourse, it seems impossible to doubt that much benefit must ultimately result thereby to the cause of truth. The impression produced on our minds by that philosophical masterpiece will not be easily effaced. As we listened in that crowded hall with admiration to the thoughtful investigator who was unfolding to us the workings of a mind much more than ordinarily acute, we pictured to ourselves the effect which it was so well calculated to produce in the mind of the sceptic in science. We saw in imagination the victory of conscience and reason, the emancipation of a soul, the new birth of an intelligence. As the speaker welded one link to another of the long chain of ratiocination, his ardour rising with the progress of his argument, we thought that it had never been our good fortune to listen to so splendid a discourse. But the end was not yet. The grand appeal had still to be made. In a magnificent peroration Professor Tyndall concluded an argument of no common order--an argument not fitted, indeed, to assuage the terrors of a vicious imagination; an argument which may perchance have grated harshly on the sacerdotal ear; but an argument which elicited thunders of applause from an audience more than usually critical--with an appeal of unrivalled eloquence to abandon dogmatism for ever, and fairly bring every hypothesis before the bar of a disciplined reason. The place and the people were worthy of the man. In a vast hall whose name recalls the finest relics of the old English ballad were gathered together all that the most populous and intelligent shire of Britain could produce of talent and influence, while grouped around the presidential chair were many of the most brilliant ornaments of British science and the representatives of foreign philosophy. It may, indeed, be surmised that of the three thousand souls who listened to Professor Tyndall’s lay sermon there were few who entered into the discussion prepared to embrace his views. Yet we think that there were few also who left that hall at all events without a doubt that the search after truth which is the sole object of the investigation of nature is neither so prosaic nor so dangerous a quest as the false prophets and the Philistines would assert--that philosophy is not ‘harsh and crabbed as dull fools suppose’--

But musical as is Apollo’s lute,
And a perpetual feast of nectar’d sweets,
Where no crude surfeit reigns.

SCIENTIFIC USE OF THE IMAGINATION.

I CARRIED with me to the Alps this year the heavy burden of this evening’s work. In the way of new investigation I had nothing complete enough to be brought before you; so all that remained to me was to fall back upon such residues as I could find in the depths of consciousness, and out of them to spin the fibre and weave the web of this discourse. Save from memory I had no direct aid upon the mountains; but to spur up the emotions, on which so much depends, as well as to nourish indirectly the intellect and will, I took with me two volumes of poetry, Goethe’s ‘Farbenlehre,’ and the work on ‘Logic’ recently published by Mr. Alexander Bain.[1] The spur, I am sorry to say, was no match for the integument of dulness it had to pierce. In Goethe, so glorious otherwise, I chiefly noticed the self-inflicted hurts of genius, as it broke itself in vain against the philosophy of Newton. For a time, Mr. Bain became my principal companion. I found him learned and practical, shining generally with a dry light, but exhibiting at times a flush of emotional strength, which proved that even logicians share the common fire of humanity. He interested me most when he became the mirror of my own condition. Neither intellectually nor socially is it good for man to be alone, and the griefs of thought are more patiently borne when we find that they have been experienced by another. From certain passages in his book I could infer that Mr. Bain was no stranger to such sorrows. Take this passage as an illustration. Speaking of the ebb of intellectual force, which we all from time to time experience, Mr. Bain says, ‘The uncertainty where to look for the next opening of discovery brings the pain of conflict and the debility of indecision.’ These words have in them the true ring of personal experience. The action of the investigator is periodic. He grapples with a subject of enquiry, wrestles with it, overcomes it, exhausts, it may be, both himself and it for the time being. He breathes a space, and then renews the struggle in another field. Now this period of halting between two investigations is not always one of pure repose. It is often a period of doubt and discomfort, of gloom and ennui. ‘The uncertainty where to look for the next opening of discovery brings the pain of conflict and the debility of indecision.’ Such was my precise condition in the Alps this year; in a score of words Mr. Bain has here sketched my mental diagnosis; and it was under these evil circumstances that I had to equip myself for the hour and the ordeal that are now come.

Gladly, however, as I should have seen this duty in other hands, I could by no means shrink from it. Disloyalty would have been worse than failure. In some fashion or other--feebly or strongly, meanly or manfully, on the higher levels of thought, or on the flats of common-place,--the task had to be accomplished. I looked in various directions for help and furtherance; but without me for a time I saw only ‘antres vast,’ and within me ‘deserts idle.’ My case resembled that of a sick doctor who had forgotten his art and sorely needed the prescription of a friend. Mr. Bain wrote one for me. He said, ‘Your present knowledge must forge the links of connection between what has been already achieved and what is now required.’[2] In these words he admonished me to review the past and recover from it the broken ends of former investigations. I tried to do so. Previous to going to Switzerland I had been thinking much of light and heat, of magnetism and electricity, of organic germs, atoms, molecules, spontaneous generation, comets, and skies. With one or another of these I now sought to re-form an alliance, and finally succeeded in establishing a kind of cohesion between thought and Light. The wish grew within me to trace, and to enable you to trace, some of the more occult operations of this agent. I wished, if possible, to take you behind the drop-scene of the senses, and to show you the hidden mechanism of optical action. For I take it to be well worth the while of the scientific teacher to take some pains, and even great pains, to make those whom he addresses copartners of his thoughts. To clear his own mind in the first place from all haze and vagueness, and then to project into language which shall leave no mistake as to his meaning--which shall leave even his errors naked--the definite ideas he has shaped. A great deal is, I think, possible to scientific exposition conducted in this way. It is possible, I believe, even before an audience like the present, to uncover to some extent the unseen things of nature; and thus to give, not only to professed students, but to others with the necessary bias, industry, and capacity, an intelligent interest in the operations of science. Time and labour are necessary to this result, but science is the gainer from the public sympathy thus created.

How then are those hidden things to be revealed? How, for example, are we to lay hold of the physical basis of light, since, like that of life itself, it lies entirely without the domain of the senses? Now philosophers may be right in affirming that we cannot transcend experience. But we can, at all events, carry it a long way from its origin. We can also magnify, diminish, qualify, and combine experiences, so as to render them fit for purposes entirely new. We are gifted with the power of Imagination,--combining what the Germans call Anschauungsgabe and Einbildungskraft--and by this power we can lighten the darkness which surrounds the world of the senses. There are tories even in science who regard imagination as a faculty to be feared and avoided rather than employed. They had observed its action in weak vessels and were unduly impressed by its disasters. But they might with equal justice point to exploded boilers as an argument against the use of steam. Bounded and conditioned by cooperant Reason, imagination becomes the mightiest instrument of the physical discoverer. Newton’s passage from a falling apple to a falling moon was, at the outset, a leap of the imagination. When William Thomson tries to place the ultimate particles of matter between his compass points, and to apply to them a scale of millimetres, he is powerfully aided by this faculty. And in much that has been recently said about protoplasm and life, we have the outgoings of the imagination guided and controlled by the known analogies of science. In fact, without this power, our knowledge of nature would be a mere tabulation of coexistences and sequences. We should still believe in the succession of day and night, of summer and winter; but the soul of Force would be dislodged from our universe; causal relations would disappear, and with them that science which is now binding the parts of nature to an organic whole.

I should like to illustrate by a few simple instances the use that scientific men have already made of this power of imagination, and to indicate afterwards some of the further uses that they are likely to make of it. Let us begin with the rudimentary experiences. Observe the falling of heavy rain-drops into a tranquil pond. Each drop as it strikes the water becomes a centre of disturbance, from which a series of ring-ripples expand outwards. Gravity and inertia are the agents by which this wave-motion is produced, and a rough experiment will suffice to show that the rate of propagation does not amount to a foot a second. A series of slight mechanical shocks is experienced by a body plunged in the water as the wavelets reach it in succession. But a finer motion is at the same time set up and propagated. If the head and ears be immersed in the water, as in an experiment of Franklin’s, the shock of the drop is communicated to the auditory nerve--the _tick_ of the drop is heard. Now this sonorous impulse is propagated, not at the rate of a foot a second, but at the rate of 4,700 feet a second. In this case it is not the gravity, but the _elasticity_ of the water that is the urging force. Every liquid particle pushed against its neighbour delivers up its motion with extreme rapidity, and the pulse is propagated as a thrill. The incompressibility of water, as illustrated by the famous Florentine experiment, is a measure of its elasticity, and to the possession of this property in so high a degree the rapid transmission of a sound-pulse through water is to be ascribed.

But water, as you know, is not necessary to the conduction of sound; air is its most common vehicle. And you know that when the air possesses the particular density and elasticity corresponding to the temperature of freezing water the velocity of sound in it is 1,090 feet a second. It is almost exactly one-fourth of the velocity in water; the reason being that though the greater weight of the water tends to diminish the velocity, the enormous molecular elasticity of the liquid far more than atones for the disadvantage due to weight. By various contrivances we can compel the vibrations of the air to declare themselves; we know the length and frequency of sonorous waves, and we have also obtained great mastery over the various methods by which the air is thrown into vibration. We know the phenomena and laws of vibrating rods, of organ-pipes, strings, membranes, plates, and bells. We can abolish one sound by another. We know the physical meaning of music and noise, of harmony and discord. In short, as regards sound we have a very clear notion of the external physical processes which correspond to our sensations.

In these phenomena of sound we travel a very little way from downright sensible experience. Still the imagination is to some extent exercised. The bodily eye, for example, cannot see the condensations and rarefactions of the waves of sound. We construct them in thought, and we believe as firmly in their existence as in that of the air itself. But now our experience has to be carried into a new region, where a new use is to be made of it. Having mastered the cause and mechanism of sound, we desire to know the cause and mechanism of light. We wish to extend our enquiries from the auditory nerve to the optic nerve. Now there is in the human intellect a power of expansion--I might almost call it a power of creation--which is brought into play by the simple brooding upon facts. The legend of the Spirit brooding over chaos may have originated in a knowledge of this power. In the case now before us it has manifested itself by transplanting into space, for the purposes of light, an adequately modified form of the mechanism of sound. We know intimately whereon the velocity of sound depends. When we lessen the density of a medium and preserve its elasticity constant we augment the velocity. When we heighten the elasticity and keep the density constant we also augment the velocity. A small density, therefore, and a great elasticity, are the two things necessary to rapid propagation. Now light is known to move with the astounding velocity of 185,000 miles a second. How is such a velocity to be obtained? By boldly diffusing in space a medium of the requisite tenuity and elasticity. Let us make such a medium our starting point, endowing it with one or two other necessary qualities; let us handle it in accordance with strict mechanical laws; give to every step of our deduction the surety of the syllogism; carry it thus forth from the world of imagination to the world of sense, and see whether the final outcrop of the deduction be not the very phenomena of light which ordinary knowledge and skilled experiment reveal. If in all the multiplied varieties of these phenomena, including those of the most remote and entangled description, this fundamental conception always brings us face to face with the truth; if no contradiction to our deductions from it be found in external nature, but on all sides agreement and verification; if, moreover, as in the case of Conical Refraction and in other cases, it has actually forced upon our attention phenomena which no eye had previously seen, and which no mind had previously imagined, such a conception, which never disappoints us, but always lands us on the solid shores of fact, must, we think, be something more than a mere figment of the scientific fancy. In forming it that composite and creative unity in which reason and imagination are together blent, has, we believe, led us into a world not less real than that of the senses, and of which the world of sense itself is the suggestion and justification.

Far be it from me, however, to wish to fix you immovably in this or in any other theoretic conception. With all our belief of it, it will be well to keep the theory plastic and capable of change. You may, moreover, urge that although the phenomena occur _as if_ the medium existed, the absolute demonstration of its existence is still wanting. Far be it from me to deny to this reasoning such validity as it may fairly claim. Let us endeavour by means of analogy to form a fair estimate of its force. You believe that in society you are surrounded by reasonable beings like yourself. You are perhaps as firmly convinced of this as of anything. What is your warrant for this conviction? Simply and solely this, your fellow-creatures behave as if they were reasonable; the hypothesis, for it is nothing more, accounts for the facts. To take an eminent example: you believe that our President is a reasonable being. Why? There is no known method of superposition by which any one of us can apply himself intellectually to another so as to demonstrate coincidence as regards the possession of reason. If, therefore, you hold our President to be reasonable, it is because he behaves _as if_ he were reasonable. As in the case of the ether, beyond the ‘_as if_’ you cannot go. Nay I should not wonder if a close comparison of the data on which both inferences rest, caused many respectable persons to conclude that the ether had the best of it.

This universal medium, this light-ether as it is called, is a vehicle, not an origin of wave-motion. It receives and transmits, but it does not create. Whence does it derive the motions it conveys? For the most part from luminous bodies. By this motion of a luminous body I do not mean its sensible motion, such as the flicker of a candle, or the shooting out of red prominences from the limb of the sun. I mean an intestine motion of the atoms or molecules of the luminous body. But here a certain reserve is necessary. Many chemists of the present day refuse to speak of atoms and molecules as real things. Their caution leads them to stop short of the clear, sharp, mechanically intelligible atomic theory enunciated by Dalton, or any form of that theory, and to make the doctrine of multiple proportions their intellectual bourne. I respect the caution, though I think it is here misplaced. The chemists who recoil from these notions of atoms and molecules accept without hesitation the Undulatory Theory of Light. Like you and me they one and all believe in an ether and its light producing waves. Let us consider what this belief involves. Bring your imaginations once more into play and figure a series of sound-waves passing through air. Follow them up to their origin, and what do you there find? A definite, tangible, vibrating body. It may be the vocal chords of a human being, it may be an organ-pipe, or it may be a stretched string. Follow in the same manner a train of ether waves to their source; remembering at the same time that your ether is matter, dense, elastic, and capable of motions subject to and determined by mechanical laws. What then do you expect to find as the source of a series of ether waves? Ask your imagination if it will accept a vibrating multiple proportion--a numerical ratio in a state of oscillation? I do not think it will. You cannot crown the edifice by this abstraction. The scientific imagination, which is here authoritative, demands as the origin and cause of a series of ether waves a particle of vibrating matter quite as definite, though it may be excessively minute, as that which gives origin to a musical sound. Such a particle we name an atom or a molecule. I think the seeking intellect when focussed so as to give definition without penumbral haze, is sure to realise this image at the last.

To preserve thought continuous throughout this discourse, to prevent either lack of knowledge or failure of memory from producing any rent in our picture, I here propose to run rapidly over a bit of ground which is probably familiar to most of you, but which I am anxious to make familiar to you all. The waves generated in the ether by the swinging atoms of luminous bodies are of different lengths and amplitudes. The amplitude is the width of swing of the individual particles of the wave. In water-waves it is the height of the crest above the trough, while the length of the wave is the distance between two consecutive crests. The aggregate of waves emitted by the sun may be broadly divided into two classes: the one class competent, the other incompetent, to excite vision. But the light-producing waves differ markedly among themselves in size, form, and force. The length of the largest of these waves is about twice that of the smallest, but the amplitude of the largest is probably a hundred times that of the smallest. Now the force or energy of the wave, which, expressed with reference to sensation, means the intensity of the light, is proportional to the square of the amplitude. Hence the amplitude being one-hundredfold, the energy of the largest light-giving waves would be ten-thousandfold that of the smallest. This is not improbable. I use these figures not with a view to numerical accuracy, but to give you definite ideas of the differences that probably exist among the light-giving waves. And if we take the whole range of solar radiation into account--its non-visual as well as its visual waves--I think it probable that the force or energy of the largest wave is a million times that of the smallest.

Turned into their equivalents of sensation, the different light-waves produce different colours. Red, for example, is produced by the largest waves, violet by the smallest, while green is produced by a wave of intermediate length and amplitude. On entering from air into more highly refracting substances, such as glass or water, or the sulphide of carbon, all the waves are retarded, but the smallest ones most. This furnishes a means of separating the different classes of waves from each other; in other words, of analysing the light. Sent through a refracting prism, the waves of the sun are turned aside in different degrees from their direct course, the red least, the violet most. They are virtually pulled asunder, and they paint upon a white screen placed to receive them ‘the solar spectrum.’ Strictly speaking, the spectrum embraces an infinity of colours, but the limits of language and of our powers of distinction cause it to be divided into seven segments: red, orange, yellow, green, blue, indigo, violet. These are the seven primary or prismatic colours. Separately, or mixed in various proportions, the solar waves yield all the colours observed in nature and employed in art. Collectively, they give us the impression of whiteness. Pure unsifted solar light is white; and if all the wave-constituents of such light be reduced in the same proportion, the light, though diminished in intensity, will still be white. The whiteness of Alpine snow with the sun shining upon it, is barely tolerable to the eye. The same snow under an overcast firmament is still white. Such a firmament enfeebles the light by reflection, and when we lift ourselves above a cloud-field--to an Alpine summit, for instance, or to the top of Snowdon--and see, in the proper direction, the sun shining on the clouds, they appear dazzlingly white. Ordinary clouds, in fact, divide the solar light impinging on them into two parts--a reflected part and a transmitted part, in each of which the proportions of wave-motion which produce the impression of whiteness are sensibly preserved.

It will be understood that the conditions of whiteness would fail if all the waves were diminished _equally_, or by the same absolute quantity. They must be reduced _proportionately_, instead of equally. If by the act of reflexion the waves of red light are split into exact halves, then, to preserve the light white, the waves of yellow, orange, green, and blue must also be split into exact halves. In short, the reduction must take place, not by absolutely equal quantities, but by equal fractional parts. In white light the preponderance as regards energy of the larger over the smaller waves must always be immense. Were the case otherwise, the physiological correlative, _blue_, of the smaller waves would have the upper hand in our sensations.

My wish to render our mental images complete, causes me to dwell briefly upon these known points, and the same wish will cause me to linger a little longer among others. But here I am disturbed by my reflections. When I consider the effect of dinner upon the nervous system, and the relation of that system to the intellectual powers I am now invoking--when I remember that the universal experience of mankind has fixed upon certain definite elements of perfection in an after-dinner speech, and when I think how conspicuous by their absence these elements are on the present occasion, the thought is not comforting to a man who wishes to stand well with his fellow-creatures in general, and with the members of the British Association in particular. My condition might well resemble that of the ether, which is scientifically defined as an assemblage of vibrations. And the worst of it is that unless you reverse the general verdict regarding the effect of dinner, and prove in your own persons that a uniform experience need not continue uniform--which will be a great point gained for some people--these tremors of mine are likely to become more and more painful. But I call to mind the comforting words of an inspired though uncanonical writer, who admonishes us in the Apocrypha that fear is a bad counsellor. Let me then cast him out, and let me trustfully assume that you will one and all postpone that balmy sleep, of which dinner might under the circumstances be regarded as the indissoluble antecedent, and that you will manfully and womanfully prolong your investigations of the ether and its waves into regions which have been hitherto crossed by the pioneers of science alone.

Not only are the waves of ether reflected by clouds, by solids, and by liquids, but when they pass from light air to dense, or from dense air to light, a portion of the wave-motion is always reflected. Now our atmosphere changes continually in density from top to bottom. It will help our conceptions if we regard it as made up of a series of thin concentric layers, or shells of air, each shell being of the same density throughout, and a small and sudden change of density occurring in passing from shell to shell. Light would be reflected at the limiting surfaces of all these shells, and their action would be practically the same as that of the real atmosphere. And now I would ask your imagination to picture this act of reflection. What must become of the reflected light? The atmospheric layers turn their convex surfaces towards the sun, they are so many convex mirrors of feeble power, and you will immediately perceive that the light regularly reflected from these surfaces cannot reach the earth at all, but is dispersed in space.

But though the sun’s light is not reflected in this fashion from the aërial layers to the earth, there is indubitable evidence to show that the light of our firmament is reflected light. Proofs of the most cogent description could be here adduced; but we need only consider that we receive light at the same time from all parts of the hemisphere of heaven. The light of the firmament comes to us across the direction of the solar rays, and even against the direction of the solar rays; and this lateral and opposing rush of wave-motion can only be due to the rebound of the waves from the air itself, or from something suspended in the air. It is also evident that, unlike the action of clouds, the solar light is not reflected by the sky in the proportions which produce white. The sky is blue, which indicates a deficiency on the part of the larger waves. In accounting for the colour of the sky, the first question suggested by analogy would undoubtedly be, is not the air blue? The blueness of the air has in fact been given as a solution of the blueness of the sky. But reason basing itself on observation, asks in reply, How, if the air be blue, can the light of sunrise and sunset, which travels through vast distances of air, be yellow, orange, or even red? The passage of the white solar light through a blue medium could by no possibility redden the light. The hypothesis of a blue air is therefore untenable. In fact the agent, whatever it is, which sends us the light of the sky, exercises in so doing a dichroitic action. The light reflected is blue, the light transmitted is orange or red. A marked distinction is thus exhibited between the matter of the sky and that of an ordinary cloud, which latter exercises no such dichroitic action.

By the force of imagination and reason combined we may penetrate this mystery also. The cloud takes no note of size on the part of the waves of ether, but reflects them all alike. It exercises no selective action. Now the cause of this may be that the cloud particles are so large in comparison with the size of the waves of ether as to reflect them all indifferently. A broad cliff reflects an Atlantic roller as easily as a ripple produced by a sea-bird’s wing; and in the presence of large reflecting surfaces, the existing differences of magnitude among the waves of ether may disappear. But supposing the reflecting particles, instead of being very large, to be very small, in comparison with the size of the waves. In this case, instead of the whole wave being fronted and in great part thrown back, a small portion only is shivered off. The great mass of the wave passes over such a particle without reflection. Scatter then a handful of such minute foreign particles in our atmosphere, and set imagination to watch their action upon the solar waves. Waves of all sizes impinge upon the particles, and you see at every collision a portion of the impinging wave struck off by reflection. All the waves of the spectrum, from the extreme red to the extreme violet, are thus acted upon. But in what proportions will the waves be scattered? A clear picture will enable us to anticipate the experimental answer. Remembering that the red waves are to the blue much in the relation of billows to ripples, let us consider whether those extremely small particles are competent to scatter all the waves in the same proportion. If they be not--and a little reflection will make it clear to you that they are not--the production of colour must be an incident of the scattering. Largeness is a thing of relation; and the smaller the wave, the greater is the relative size of any particle on which the wave impinges, and the greater also the ratio of the reflected portion to the total wave. A pebble placed in the way of the ring-ripples produced by our heavy rain-drops on a tranquil pond will throw back a large fraction of the ripple incident upon it, while the fractional part of a larger wave thrown back by the same pebble might be infinitesimal. Now we have already made it clear to our minds that to preserve the solar light white, its constituent proportions must not be altered; but in the act of division performed by these very small particles we see that the proportions are altered; an undue fraction of the smaller waves is scattered by the particles, and, as a consequence, in the scattered light, blue will be the predominant colour. The other colours of the spectrum must, to some extent, be associated with the blue. They are not absent but deficient. We ought, in fact, to have them all, but in diminishing proportions, from the violet to the red.

We have here presented a case to the imagination, and, assuming the undulatory theory to be a reality, we have, I think, fairly reasoned our way to the conclusion, that were particles, small in comparison to the size of the ether waves, sown in our atmosphere, the light scattered by those particles would be exactly such as we observe in our azure skies. When this light is analysed, all the colours of the spectrum are found; but they are found in the proportions indicated by our conclusion.

Let us now turn our attention to the light which passes unscattered among the particles. How must it be finally affected? By its successive collisions with the particles the white light is more and more robbed of its shorter waves; it therefore loses more and more of its due proportion of blue. The result may be anticipated. The transmitted light, where short distances are involved, will appear yellowish. But as the sun sinks towards the horizon the atmospheric distances increase, and consequently the number of the scattering particles. They abstract in succession the violet, the indigo, the blue, and even disturb the proportions of green. The transmitted light under such circumstances must pass from yellow through orange to red. This also is exactly what we find in nature. Thus, while the reflected light gives us at noon the deep azure of the Alpine skies, the transmitted light gives us at sunset the warm crimson of the Alpine snows. The phenomena certainly occur _as if_ our atmosphere were a medium rendered slightly turbid by the mechanical suspension of exceedingly small foreign particles.