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.
[Illustration]
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.
[Illustration]
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.
[Illustration]
_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.
[Illustration]
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.