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JOHN TYNDALL

LIGHT

PROFhSSOS JOHN TYNBALL'S WOEKS.

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Six Lectures on Light. Delivered in America iu
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i^-ii^i^

SIX LECTURES ON LIGHT

DELIVERED IN AMERICA IN 1872-1873

JOHN TYNDALL, D.C.L. LL.D. F.RS.

PHOFKSSOR Of NATURAL PHILOSOPHY IN THB ROYAL INBTITOTIOK

SECOND EDITION

NEW YORK:

D. APPLETON AND COMPANY,

I, 3, AND 5 BOND STREET.

I 886.

,/t^"R%

^ Ff e 2 8 is/a

ILLUSTRATIONS.

Thomas Young . Frontujpicoe

Plumes produced bf the Ceystai-lization of

VVatbs . To/aof p. 24H

PEEFACE

TO THE

SECOND ENGLISH EDITION.

The reasons for givingf these Lectures are briefly set
forth in the Introduction to the first of them ; while
the grounds of their publication are stated in the
' Preface to the American Edition.'

During their delivery, the experimental facts were
before the audience, forming visible links in the
logic of each discourse. Here, by the use of plain
language, I have endeavoured to reproduce distinct
images of these facts, and to show them in their proper
relations.

With a view to this end I have sought to raise the
Wave-theory of Light to adequate clearness in the
reader's mind, and to show its power as an organizer of
optical phenomena.

From what has been recently written on such ques-
tions, it is to l)e inferred that the origin, scope, and

vi PEEFACE TO THE

warrant, of physical theories generally, constitute a
theme of considerable interest to thoughtful minds.
On these points I have ventured, particularly in the
second and third Lectures, to state the views which my
own reflections have suggested to me.

To produce a systematic treatise on Light wat«, ot'
course, quite wide of my aim. My desire rather was t<>
throw into a small compass, an exposition for which I
should have been grateful at a certain period of my
own studies. I wished in the first place, as the prime
condition of all satisfactory progress, to clear the reader's
mind of all indistinctness regarding elementary facts
and conceptions ; and to whet incidentally the desire
for further knowledge.

I wished, moreover, for the sake of that numerous
portion of the community who are interested in the
material results of science, to trace effects to their causes,
by showing how such results receive their primary
vitalization from the thoughts of men with no material
end in view. The ' Summary and Conclusion,' whicli
might be read as an introduction, is for the most part
devoted to this object. I have added in an Appendix
three brief Addresses by distinguished Americans, whicli
possess more than a passing interest.

To the first English edition of these Lectures, Dr.
Young's ' Reply to the Edinburgh Reviewers ' was ap-
pended. Numbers of scientific men were, to my
knowledge, but imperfectly acquainted with this grent

SECOND ENGLISH EDITION. vii

discussion ; while the general public knew nothing
whatever about it. The end contemplated having been
gained, the ' Reply ' is here omitted ; and in lieu of
it a portrait of Dr. Young, engraved with great success
by Mr. Adlard, forms the frontispiece of the volume.

John Tyndall.

Royal Institution : May 1875.

PEEFACE

THE AMERICAN EDITION.

Mt eminent friend Professor Joseph Henry, of the
Smitlisonian Institution, Washington, did me the
honour of taking these Lectures under his personal
direction, and of arranging the times and places at
which they were to be delivered.

Believing that my home duties could hardly be
suspended for a longer period, I did not, at the outset,
expect to be able to prolong my visit to the United
States beyond the end of 1872.

Thus limited as to time, Professor Henry began
in the North, and, proceeding southwards, arranged
for the successive delivery of the lectures in Boston,
New York, Philadelphia, Baltimore, and Washington.

By this arrangement, which circumstances at the
time rendered unavoidable, the lectures in New York
would have been rendered coincident with the period of
the presidential election. This was deemed unsatisfac-

X PREFACE TO

tory ; and tbe fact being represented to me, I at once
ofifered to extend tlie time of my visit so as to make the
lectures in New York succeed those in "Washington.
This proposition was cordially accepted by my friends.

To me personally this modified arrangement has
proved both pleasant and beneficial. It gave me a
much-needed and delightful holiday at Niagara Falls ;
it, moreover, rendered the successive stages of my
work a kind of growth, which reached its most im-
pressive development in New York and Brooklyn.

My reception throughout has been that of a friend
by friends ; and now that my visit has become virtually
a thing of the past, I look back upon it as a memory
without a single stain of unpleasantness. Excepting
one inexorable event, nothing has occurred that I could
wish not to have occurred ; while from beginning to
end I have been met by expressions of good-will on the
part of the American people never, on my part, to be
forgotten. Indeed, ' good-will ' is not the word to ex-
press the kindness manifested towards me in the United
.States.

Would that it had been in my powerto meet the wishes
of my fi-iends more completely, by responding to the invi-
tations sent to me from the great cities of the Interior and
the West, and from Canada. But the character of the
lectures, and their weight of instrumental appliances,
involved such heavy labour that the need of rest alone
would be a sufficient reason for my pausing here.

THE AMERICAN EDITION. xi

Besides this, each successive mail from London brings
me intelligence of work suspended and duties post-
poned through my absence.

The Royal Institution possesses an honorary secretary
who has devoted the best years of an active professional
life and the best energies of a strong mind to its
interests. And if anything of the kind should ever be
founded here, the heartiest wish that I could offer for
its success would be, that it may be served with tlie
singleness of purpose, and self-sacrificing love, bestowed
by its managers and its members on the Royal Institu-
tion ; and by none more unceasingly tlian by Dr. Bence
Jones. But he, on whom I might rely, is now struck
down by a distressing illness ; * and, though others are
willing to aid me in all possible ways, there can be no
doubt as to my line of duty. I ought to be at home.
I ask my friends in the Interior and the West, and in
Canada, to take these things into consideration ; and to
think of me not as one insensible to their kindness,
but as one who, with a warmth commensurate with their
own, would comply with all their wishes if he could.

One other related point deserves mention. On quit-
ting England I had no intention of publishing these
Lectures, and, except a fragment or two, they were wholly
unwritten when I arrived in this city. Since that time,
besides lecturing in New York, Brooklyn, and New

' Hedied, working for the Institution to the last, on Sunday morning,
April 20, 1873.

SU PREFACE TO

Haven, the Lectures have been written out and carritd
through the Press. Many evidences of the rapidity of
their production will appear ; but I thought it due to
those who listened to them with such unwavering atten-
tion, as also to those who wished to hear them, but were
unable to do so, to leave them behind me in an approxi-
mately authentic form.

The constant application which this work rendered
necessary has cut me off from many social pleasures ; it
has prevented me from making myself acquainted with
the working of institutions in which I feel a deep
interest, and from availing myself of the generous
hospitality offered to me by the clubs of New York.
In short, it has made me an unsociable man. But, find-
ing social pleasure and hard work incompatible, I took
the line of devoting such energy as I could command,
not to the society of my intimate friends alone, but to
the people of the United States.

In the opening lecture are mentioned the names of
gentlemen to whom I am under lasting obligations for
their friendly, and often laborious aid. The list might
readily be extended, for in every city visited willing
helpers were at hand. I must not, however, omit the
name of Mr. Rhees, Professor Henkt's private secretary,
who not only in Washington, but in Boston, gave me
most important assistance. To the Trustees of the
Cooper Institute my acknowledgments are due ; and to
the Directors of the Mercantile Library at Brooklyn.

THE AMERICAN EDITION. xiii

I would add to these a brief but grateful reference to
my high-minded friend and kinsman General Hector
Tyndale, for his long-continued care of me, and for the
thoughtful tenderness by which he and his family
softened, both to me and to the parents of the youth,
the grief occasioned by the death of myjimior assistant
in Philadelphia.

Finally, I have to mention with warm commendation
the integrity, ability, and devotion with which, from
first to last, I have been aided by my principal assistant,
Mr. John Cottrell.

John Tyndaix.

New York : Eelruary 1878

CONTENTS.

LECTURE I.

PAoa

Introduetxjry— Uses of Experiment — Early Scientific Notions —

Sciences of Observation — Knowledge of the Ancients regapdiag;
Light — Defects of the Eye — Our Instruments — Eectilineal Pro-
pagation of Light — Law of Incidence and Reflection — Sterility
of the Middle Ages — Refraction — Discovery of Snell — Partial and-
Tot:il Reflection — Velocity of Light — Roemer, Bradley, Foucault,
and Fizeau — Principle of Least Action — Descartes and the Rain-
bow— Newton's Experiments on the Composition of Solar Light —
His Mistake regarding Achromatism — Synthesis of White Light
—Yellow and Blue Lights produce White by their Mixture —
Colours of Natural "Rndipg — /\l)f;ni'ptinn. — MJTtiirw of Pigments
contrasted with Mixture of Lights ...... 1

LHUTURi: II.

Oiigin of Physical Theories —Scope of the Imagination — Newton
and the Emission Theory — Verification of Physical Theories —
The Luminiferous Ether— Wave-theory of Light — Thomas
Young — Fresnel and Arago — Conception of Wave-motion —
Interference of Waves — Constitution of Sound-waves — Analogies
of Sound an(i_Light — Illustrations of Wave-motion — Interference
of Sound Waves — Optical Illustrations — Pitch and Colour —
Lengths of the Waves of Li^ht and Rates of Vibration of the
Ether-particles — Interference of Light — Phenomena which first
suggested the Undulatory Theory — Boyle and Ilooke — The
Colours of thin Plates — The Soap-bubble — Newton's Rings —
Theory of 'Fits ' — Its Explanation of the Rings — Overthrow of
the Theory — Diffraction of Light — Colours produced by DilTrac-
I ion- -Colours of Motlicr-of-po;irl ...... 42

xvi CONTENTS.

LECTUEE III.

PAGH

K elation of Theories to Experience — Origin of the Notion of the
Attraction of Grayitation — Notion of Polarity, how generated —
Atomic Polarity — Structural Arrangements due to Polarity —
Architecture of Crystals considered as an Introduction to their
Action upon Light — Notion of Atomic Polarity applied to Crys-
talline Structure — Experimental Illustrations — Crystallization
of Water — Expansion by Hpat and by Cold — Deportment of
Water considered and explained — Bearings of Crystallization on
Optical Phenomena — Refraction — Double Eefraction — Polariza-
tion— Action of Tourmaline — Character of the Beams emergent
fiom Iceland Spar — Polarization by ordinary Refraction and
Reflection — Depolarization T .95

LECTURE IV.

Chromatic Phenomena produced by Crj'stals in Polarized Liglit —
The Nicol Prism — Polarizer and Analyzer — Action of Thick aud
Thin Plates of Selenite — Colours dependent on Thickness — Reso-
lution of Polarized Beam into two others by the Selenite — One
of them more retarded than the other — Recompounding of the
two Systems of Waves by the Analyzer — Interference thns
rendered possible — Consequent Production of Colours — Action
of Bodies mechanically strained or pressed — Action of Sonorous
Vibrations — Action of Glass strained or pressed by Heat — Cir-
cular Polarization — Chromatic Phenomena produced by Quartz
— The Magnetization of Light— Rings surrounding the Axes of
Cr}-stals — Biaxal and Uniaxal Crystals — Grasp of the Undu-
liitorv Theory — Th" Colour and Polarization of Sky -light —
Ceneritiiin of Artificial Skies ....... J24

LECTURE V.

liauge of Vijioii not commensurate with Range of Radiation — The
Ultra-violet Rays — Fluorescence — Rendering invisible Rays
visible — Vision not the only Sense appealed to by the Solar and
Electric Beam— Heat of Beam — Combustion by Total Beam at
the Foci of Mirrors and Lenses — Combustion through Ice-lens
— Ignition of Diamond — Search for the effective Rays — Sir
William Herschel's Discovery of dark Solar Rays — Invisible Rays
the Basis of the Visible — Detachment by a Ray-filter of the
Invisible Rays from the Visible — Combustion at Dark Foci —
Conversion of He.it-rays into Light-rays — Calorescenee — Part

CONTENTS.

xvn

played in Nuture by Dark Rays^Identity of Light and Radiant
Heat — Invisible Images — Reflection, Refraction, Plane Polariza-
tion, Depolarization, Circular Polarization, Double Refraction,
and MaL;Detization of Radiant Heat . . . , . . 1C2

LECTURE VI.

Principles of Spactrum Analysis — Prismatic Analysis of the Light
of Incandescent Vapours — Discontinuous Spectra — Spectrum
Bands proved by Bunsen and Kirchhoff to be characteristic of the
Vapour — Discovery of Rubidium, Caesium, and Thallium —
Relation of Emission to Absorption — The Lines of Fraunhofer
— Their Explanation by Kirchhoff — Solar Chemistry involved in
this Explanation — Foucault's Experiment— Principles of Ab-
sorption— Analogy of Sound and Light — Experimental Demon-
stration of this Analogy — Recent Applications of the Spectro-
scope— Summary and Conclusion ......

192

APPENDIX.

President Barnard's Address
Professor Draper's Ac'dress .
President White's Remarks .
Professor Tyndall's Remarks
Measurement of the Waves of Light
Water Crystallization .
On the Spectra of Polarized Light

229
235
238
242
247
249
250

Lndkx

265

^u^

ON LIGHT.

■ao\iii,00—

/:yCtJn^

E c T u E E

DfTEODUCTORr — USES OF EXPKIilMEXT — EARLY SCIENTIFIC NOTIONS —
SCIENCES OF OBSEKTATION — KNOWLEDGE OF THE ANCIENTS HEGAED-

INO LIGHT DEFECTS OF THE EYE — OUB INSTEUMENTS — EECTILINEAL

PEOPAGATION OF LIGHT — LAW OF INCTDENCB AND EEFLKCTION — STERIL-
ITY OF THE MIDDLE AGES — REFRACTION DISCOVERY OP SNBLL

PABTIAl AND TOTAL REFLECTION — VELOCITY OF LIGHT — ECEMEE,

BRADLEY, FOXJCAULT, AND FIZEAU PRINCIPLE OF LEAST ACTION —

DESCARTES AND THE RAINBOW NEWTON'S EXPERIMENTS ON THE

COMPOSITION OF SOLAR LIGHT HIS MISTAKE AS REGARDS ACHROMAT-
ISM SYNTHESIS OF 'VATIITB LIGHT — YELLOW AND BLUE LIGHTS PRODUCE

WHITB BY THEIR MIXTURE — COLOURS OF NATURAL BODIES — ABSORP-
TION— MIXTURE OF PIGMEITTS CONTRASTED WITH MIXTURE OF
LIGHTS.

§ 1. Introduction.

SOME twelve years ago I published, in England, a
little book entitled the ' Glaciers of the Alps,' and,
a couple of years subsequently, a second book, en-
titled ' Heat as a Mode of Motion.' These volumes were
followed by others, written with equal plainness, and with
a similar aim, that aim being to develope and deepen
sympathy between science and the world outside of
science. I agreed witli thoughtful men ' who deemed
it good for neither world to be isolated from the other,

• Among whom may ho mentioned, especially, the late Sir Edmund
Head, Bart., with whom I had many conversations on this sulijcet.

2 ON LIGHT. LECT.

or unsympathetic towards the other, and, to lessen
this isolation, at least in one department of science, I
swerved aside from those original researches which had
previously been the pursuit and pleasure of my life.

The works here referred to were, for the most part,
republished by the Messrs. Appleton of New York, '
under the auspices of a man who is untiring in his
efforts to diffuse sound scientific knowledge among the
people of the United States ; whose energy, ability,
and single-mindedness, in the prosecution of an arduous
task, have won for him the sympathy and support
of many of us in ' the old country.' I allude to
Professor Youmans. Quite as rapidly as in England,
the aim of these works was understood and appreciated
in the United States, and they brought me from this
side of the Atlantic innumerable evidences of good-
will. Year after year invitations reached me^ to
visit America, and last year I was honoured with a
request so cordial, signed by five-and-twenty names so
distinguished in science, in literature, and in adminis-
trative position, that I at once resolved to respond to
it by braving not only the disquieting oscillations
of the Atlantic, but the far more disquieting ordeal
of appearing in person before the people of the
United States.

This invitation, conveyed to me by my accom-
plished friend Professor Lesley, of Philadelphia, and
preceded by a letter of the same purport from your
scientific Nestor, the celebrated Joseph Henry, of

' At "whose hands it gives me pleasure to stiite I have always ex-
perienced honourable and liberal treatment.

* One of the earliest of these came from Mr. John Amory Lowell of
Boston.

I. INTRODUCTORY, USES OF EXPERIMENT. 3

Washington, desired that I would lecture in some of
the principal cities of the Union. This I agreed to
do, though much in the dark as to a suitable subject.
In answer to my inquiries, however, I was given to
understand that a course of lectures showing the uses of
experiment in the cultivation of Natural Knowledge
would materially promote scientific education in this
country. And though such lectures involved the selec-
tion of weighty and delicate instruments, and their
transfer from place to place, I at once resolved to meet
the wishes of ray friends as far as the time and means
at my disposal would allow.

§ 2. Subject of the Course. Source of LigJd
employed.

Experiments have two great uses — a use in dis-
covery and verification, and a use in tuition. They
were long ago defined as the investigator's language
addressed to Nature, to which she sends intellio-ible
replies. These replies, however, usually reach the ques-
tioner in whispers too feeble for the public ear. But
after the discoverer comes the teacher, whose function
it is so to exalt and modify the experiments of his pre-
decessor as to render them fit for public presentation.
This secondary function I shall endeavour, in the present
instance, to fulfil.

I propose to take a single department of natural
philosophy, and illustrate, by means of it, the growth
of scientific knowledge under the guidance of experi-
ment. I wish, in this first lecture, to make you
acquainted with certain elementary phenomena ; then
to point out to you how those theoretic principles by
which phenomena are explained, take root, and flourish

4 ON LIGHT. tErr.

in the human mind, and afterwards to apply these prin-
ciples to the whole body of knowledge covered by the
lectures. The science of optics lends itself to this
mode of treatment, and on it, therefore, I propose to
draw for the materials of the present course. It will
be best to begin with the few simple facts regarding
light which were known to the ancients, and to pass
from them in historic gradation to the more abstruse
discoveries of modern times.

All our notions of Nature, however exalted or how-
ever grotesque, have some foundation in experience.
The notion of personal volition in Nature had this basis.
In the fury and the serenity of natural phenomena the
savage saw the transcript of his own varying moods,
and he accordiugly ascribed these phenomena to beings
of like passions with himself, but vastly transcending
him in power. Thus the notion of causality — the as-
sumption that natural things did not come of them-
selves, but had unseen antecedents — lay at the root of
even the savage's interpretation of Nature. Out of
this bias of the human mind to seek for the ante-
cedents of phenomena all science has sprung.

We will not now go back to man's first intellectual
gropings ; much less shall we enter upon the thorny dis-
cussion as to how the groping man arose. We will take
him at a certain stage of his development, when, by evo-
lution or sudden endowment, he became possessed of the
apparatus of thought and the power of using it. For
a time — and that historically a long one — he was limited
to mere observation, accepting what Nature offered,
and confining intellectual action to it alone. The ap-
parent motions of sun and stars first drew towards them
the questionings of the intellect, and accordingly astro-

I. PROGRESS OF THE ANCIENTS. ft

nomy was the first science developed. Slowly, and with
difficulty, the notion of natural forces took root in the
human mind. Slowly, and with difficulty, the science
of mechanics had to grow out of this notion ; and slowly
at last came the full application of mechanical princi-
ples to the motions of the heavenly bodies. We trace
the progress of astronomy through Hipparchus and
Ptolemy ; and, after a long halt, through Copernicus,
Galileo, Tycho Brahe, and Kepler ; while from the high
table-land of thought raised by these men Newton shoots
upward like a peak, overlooking all others from his
dominant elevation.

But other objects than the motions of the stars at-
tracted the attention of the ancient world. Light was
a familiar phenomenon, and from the earliest times we
find men's minds busy with the attempt to render some
account of it. But without experiment, which belongs
to a later stage of scientific development, little progress
could be made in this subject. The ancients, accord-
ingly, were far less successful in dealing with light
than in dealing with solar and stellar motions. Still
they did make some progress. They satisfied them-
selves that light moved in straight lines ; they knew
also that light was reflected from polished surfaces, and
tliat the angle of incidence of the rays of light was
equal to the angle of reflection. These two results of
ancient scientific curiosity constitute the starting-point
of our present course of lectures.

But in the first place it will be useful to say a few

words regarding the source of light to be employed in

our experiments. The rusting of iron is, to all intents

and purposes, the slow burning of iron. It dcvelopes

6 ON LIGHT. uiCT.

heat, and, if the heat be preserved, a high temperature
may be thus attained. The destruction of the first
Atlantic cable was probably due to heat developed in
this way. Other metals are still more combustible
tlian iron. You may light strips of zinc in a candle
flame, and cause them to burn almost like strips of
paper. But we must now expand our definition of
combustion, including under this term not only com-
bustion in air, but also combustion in liquids. Water,
for example, contains a store of oxygen, which may
unite with and consume a metal immersed in it ; it is
from this kind of combustion that we are to derive the
heat and light employed in our present course.

The generation of this light and of this heat merits
a moment's attention. Before you is an instrument —
a small voltaic battery — in which zinc is immersed in
a suitable liquid. An attractive force is at this
moment exerted between the metal and the oxygen
of the liquid ; actual union, however, being in the
first instance avoided. Uniting the two ends of the
battery by a thick wire, the attraction is satisfied,
the oxygen unites with the metal, zinc is consumed,
and heat, as usual, is the result of the combustion. A
power which, for want of a better name, we call an
electric current, passes at the same time through the
wire.

Cutting the thick wire in two, let the severed ends
be united by a thin one. It glows with a white heat.
Whence comes that heat ? The question is well worthy
of an answer. Suppose in the first instance, when
the thick wire is employed, that we permit the action
to continue until 100 grains of zinc are consumed, the
amount of heat generated in the bakery would be

I. SOURCE OF LIGHT. 7

capable of accurate numerical expression. Let the
action then continue, with the thin wire glowing, until
100 grains of zinc are consumed. Will the amount of
heat generated in the battery be the same as before ?
No, it will be less by the precise amount generated in
the thin wire outside the battery. In fact, by adding
the internal heat to the external, we obtain for the
combustion of 100 grains of zinc a total which never
varies. We have here a beautiful example of that law of
constancy as regards natural energies, the establisliment

Fi«. 1.

of which is the greatest achievement of modern scientific
philosophy. By this arrangement, then, we are able to
burn our zinc at one place, and to exhibit the effects
of its combustion at a distance. In New York, for
example, we may have our grate and fuel; but the heat
and liglit of our fire may be made to appear at San
Francisco.

Eemoving the thin wire and attaching to the severed
ends of the thick one two rods of coke, we obtain, on
bringing tlie rods together (as in fig. 1), a small star uf

8 ON LIGHT. tBCT.

light. Now, the light to be employed in our lectures
is a simple exaggeration of this star. Instead of being
produced by ten cells, it is produced by fifty. Placed
in a suitable camera, provided with a suitable lens,
this powerful source will give us all the light necessary
for our experiments.

And here, in passing, I am reminded of the common
delusion that the works of Nature, the human eye in-
cluded, are theoretically perfect. The eye has grown
for ages towards perfection ; but ages of perfect-
ing may be still before it. Lo»king at the dazzling
light from our large battery, I see a luminous globe,
but entirely fail to see the shape of the coke-points
whence the light issues. The cause may be thus made
clear ; On the screen before you is projected an image
of the carbon points, the whole of the lens in front
of the camera being employed to form the image. It
is not sharp, but surrounded by a halo which nearly
obliterates the carbons. This arises from an imperfec-
tion of the lens, called its spherical aberration, due to
the fact that the circumferential and central rays have
not the same focus. The human eye labours under a
similar defect, and from this and other causes it arises
that when the naked light from fifty cells is looked at,
the blur of light upon the retina is sufficient to destroy
the definition of the retinal image of the carbons. A
long list of indictments might indeed be brought against
the eye — its opacity, its want of symmetry, its lack of
achromatism, its absolute blindness, in part. All these
taken together caused Helmholtz to say that, if any
optician sent him an instrument so full of defects, he
would be justified in sending it back with the severest
censure. But the eye is not to be judged from the

1. KECTILINEAL PROPAGATION. 9

stand-point of theory. It is not perfect, as I have said,
but on its -way to perfection. As a practical instrument,
and taking the adjustments by which its defects are
neutralized into account, it must ever remain a marvel
to the reflecting mind.

§ 3. Rectilineal Projpagation of Light. Elementary
ExpeH'inents. Law of Reflection.

The ancients were aware of the rectilineal pro-
pagation of light. They knew that an opaque body,
placed between the eye and a point of light, intercepted
the light of the point. Possibly the terms * ray ' and
' beam ' may have been suggested by those straight
spokes of light which, in certain states of the atmo-
sphere, dart from the sun at his rising and his setting.
The rectilineal propagation of light may be illustrated
by permitting the solar light to enter by a small
aperture in a window-shutter a dark room in which
a little smoke has been dififused. In pure air you can-
not see the beam, but in smoke you can, because
the light, which passes unseen through the air, is scat-
tered and revealed by the smoke particles, among which
the beam pursues a straight course.

The following instructive experiment depends on the
rectilineal propagation of light. Make a small hole in
a closed window-shutter, before which stands a house or
a tree, and place within the darkened room a white
screen at some distance from the orifice. Every straight
ray proceeding from the house or tree stamps its colour
upon the screen, and the sum of all the rays will, there-
fore, be an image of the object. But, as the rays cross
each other at the orifice, the image is inverted. At
present we may illustrate and expand the subject thus :

ON LIGHT.

LECT.

In front of our camera is a large opening (L, fig. 2),
from which the lens has been removed, and which is
closed at present by a sheet of tin-foil. Pricking by
means of a common sewing-needle a small aperture in
the tin-foil, an inverted image of the carbon-points
starts forth upon the screen. A dozen apertures will
give a dozen images, a hundred a hundred, a thousand
a thousand. But, as the apertures come closer to each
other, that is to say, as the tin-foil between the aper-
tures vanishes, the images overlap more and more.

Fig. 2.

Eemoving the tin-foil altogether, the screen becomes
uniformly illuminated. Hence the light upon the
screen may be regarded as the overlapping of innumer-
able images of the carbon-points. In like manner the
light upon every white wall on a cloudless day may be
regarded as produced by the superposition of innumer-
able images of the sun.

The law that the angle of incidence is equal to the
angle of reflection has a bearing upon a theory, to be
subsequently mentioned, which renders its simple illus-
tration here desirable. A straight lath (pointing to

EEFLECTION OF LIGHT.

the figure 5 in fig. 3) is fixed as an index perpendicular
to a small looking-glass (M) capable of rotation.
A beam of light is first received upon the glass and re-
flected back along the line of its incidence. The index
being turned, the mirror tiu-ns along with it, and at
each side of the index the incident and the reflected
beams (L o, o R) track themselves through the dust of
the room. The mere inspection of the two angles

Fig. 3.

enclosed between the index and the two beams suffices
to show their equality, while if the graduated quadrant
be consulted, the arc from 5 to m is found accurately
equal to the arc from 5 to n. A card placed edgeways
upon a table without inclination to tlie riglit or to the
left is said to be perpendicular to the plane of the table.
The complete expression of the law of reflection is that
the angles of incidence and reflection are equal ; and
that the incident and reflected rays always lie in a
plane perpendicular to the reflecting surface.

Tliis simple apparatus enables us to illustrate another

12 ON LIGHT. i.Bcr,

law of great practical importance, namely, that, -when a
mirror rotates, the angular velocity of a beam reflected
from it is twice that of the reflecting mirror. A simple
experiment will make this plain. The arc (m -n, fig. 3)
before you is divided into ten equal parts, and when
the incident beam and the index cross the zero of the
graduation, both the incident and reflected beams are
horizontal. Moving the index of the mirror to 1,
the reflected beam cuts the arc at 2 ; moving the index
to 2, the arc is cut at 4 ; moving the index to 3, the arc
is cut at 6 ; moving the index to 4, the arc is cut at 8 ;
finally, moving the index to 5, the arc is cut at 10 (as
in the figure). In every case the reflected beam
moves through twice the angle passed over by the
mirror.

One of the problems of science, on which scientific
progress mainly depends, is to help the senses of man,
by carrying them into regions which could never be
attained without such help. Thus we arm the eye with
the telescope when we want to sound the depths of
space, and with the microscope when we want to ex-
. plore motion and structure in their infinitesimal dimen-
, sions. Now, this law of angular reflection, coupled
, with the fact that a beam of light possesses no weight,
gives us the means of magnifying small motions to an
extraordinary degree. Thus, by attaching mirrors to
his suspended magnets, and by watching the images of
divided scales reflected from the mirrors, the celebrated
Gauss was able to detect the slightest thrill of variation
on the part of the earth's magnetic force. By a similar
arrangement the feeble attractions and repulsions of
the diamagnetic force have been made manifest. The
minute elongation of a bar of metal by the mere warmth

I. SIIRIT OF THE MIDDLE AGES. 13

of the hand may be so magnified by this method as to
cause the index-beam to move through 20 or 30 feet.
The lengthening of a bar of iron when it is magnetized
may be also thus demonstrated. Helmholtz long ago
employed this method to render evident to his students
the classical experiments of Du Bois Eaymond on animal
electricity ; while in Sir William Thomson's reflecting
galvanometer the principle receives one of its latest, and
most important applications.

§ 4. The Refraction of Light. Total Reflection.

For more than a thousand years no step was taken
in optics beyond this law of reflection. The men of
the Middle Ages, in fact, endeavoured on the one hand
to develope the laws of the universe a "priori out of
their own consciousness, while many of them were so
occupied with the concerns of a future world that they
looked with a lofty scorn on all things pertaining to
this one. Speaking of the natural philosophers of his
time, Eusebius says, ' It is not through ignorance of the
tilings admired by them, but through contempt of their
useless labour, that we think little of these matters,
turning our souls to the exercise of better things.' So
also Lactantius — ' To search for the causes of things ;
to inquire whether the sun be as large as he seems ;
whether the moon is convex or concave ; whether the
stars are fixed in the sky, or float freely in the air ;
of what size and of what material are the heavens;
whether they be at rest or in motion ; what is the mag-
nitude of the earth ; on what foundations is it suspended
or balanced ; — to dispute and conjecture upon such
matters is just as if we chose to discuss what we think

14 ON LIGHT. tHCT.

of a city in a remote country, of which we never heard
but the name.' '

As regards the refraction of light, the course of
real inquiry was resumed in 1100 by an Arabian
philosopher named Alhazen. Then it was taken up in
succession by Roger Bacon, Vitellio, and Kepler. One
of the most important occupations of science is the
determination, by precise measurements, of the quan-
titative relations of phenomena ; the value of such
measurements depending greatly upon the skill and
conscientiousness of the man who makes them. Vitellio
appears to have been both skilful and conscientious,
while Kepler's habit was to rummage through the
observations of his predecessors, to look at them in all
lights, and thus distil from them the principles which
united them. He had done this with the astronomical
measurements of Tycho Brahe, and had extracted from
Ihem the celebrated ' laws of Kepler.' He did it also
with Vitellio's measurements of refraction. But in
this case he was not successful. The principle, though

' The spirit of tbose ancient heroes of the faith is still to be fonnd
in unexpected places. In the April number of the Contemporary Review,
after describing liow modern science came to be what it is, my friend
Dr. Acland puts the following language into the mouth of Bishop
Wilson : — ' What is surprising to me is the labour that you hare taken
to attain so very little. You deserve for this the utmost credit a reason-
able being can desire; for you, being so accurate and so painstaking,
seem well aware of the uncerbiinty of some of your data, and of tlie pos-
sible futility, therefore, of some of your conclusions. For I am told that,
with all your pains, your sciences contain within them so many examples
of proved errors, that, being candid men, you must often feel the material
ground under your feet to be very slippery.' Schelling thus expresses
his contempt for experimental knowledge: 'Newton's Optics is the
greatest illustration of a whole structure of fallacies, which in all its
parts is founded on observation and experiment.' There are some small
imitators of Schelling still in Germany.

I. REFRACTION OF LIGHT, SNELL. 15

a simple one, escaped liim, and it was first discovered
by Willebrord Snell, about the year 1621.

Less with the view of dwelling upon the phenome-
non itself than of introducing it in a form which will
render intelligible to you, subsequently, the play of
theoretic thought in Newton's mind, the fact of refrac-
tion may be here demonstrated. I will not do this by
drawing the course of the beam with chalk on a black

Fig. 4.

board, but by causing it to mark its own white track
before you. A shallow circular vessel (R I Gr, fig. 4),
with a glass face, half filled with water rendered barely
turbid by the admixture of a little milk or the precijii-
tation of a little mastic, is placed with its glass face
vertical. By means of a small plane reflector (M), and
through a slit (I) in the hoop surrounding the vessel, a
beam of light is admitted in any required direction.
It impinges upon the water (at 0), enters it, and tracks
itself through the liquid in a sharp, bright band (0 G).
Meanwhile the beam passes unseen through the air
above the water, for the air is not competent to scatter
the light. A puff of tobacco smoke into this space at

ON LIGHT.

LECT.

once reveals the track of the incident-beam. If the
incidence be vertical, the beam is unrefracted. If
oblique, its refraction at the common sm'face of air and
water (at 0) is rendered clearly visible. It is also
seen that reflection (along 0 R) accompanies refraction,
the beam dividing itself at the point of incidence into
a refracted and a reflected portion.^

The law by which Snell connected together all the

Fig. 5.

\»».

/. \wi

■x/

•'■"' \

/ ,.''

'7^

7i'

/ /

^\2_

«^

^—-^

measurements executed up to his time, is this : Let
AB C D (Fig. 5) represent the outline of our circular
vessel, A C being the water-line. "When the beam is
incident along B E, which is perpendicular to A C,
there is no refraction. When it is incident along m E,
there is refraction : it is bent at E and strikes the
circle at n. When it is incident along mf E, there is
also refraction at E, tlie beam striking the point n'.
From the ends of the incident beams, let the perpen-
diculars m o, mf o' be drawn upon B D, and from the

' It will be subsequently shown how this simple apparatiis may l>e
employed to determine the ' polarising angle ' of a liquid.

I. PAETIAL EEFLECTION. 17

ends of the refracted beams let the perpendiculars
p n, -p' n' be also drawn. Measure the lengths of o m
and of p n, and divide the one by the other. You
obtain a certain quotient. In like manner divide w' o'
by the corresponding perpendicular _p' n' ; you obtain
in each case the same quotient. Snell, in fact, found
this quotient to be a constant quantity for each par-
ticular substance, though it varied in amount from
substance to substance. He called the quotient the
index of refraction.

In all cases where the light is incident from air
upon the surface of a solid or a liquid, or, more gene-
rally still, when the incidence is from a less highly
refracting to a more highly refracting medium, the
reflection is partial. In this case the most powerfully
reflecting substances either transmit or absorb a portion
of the incident light. At a perpendicular incidence
water reflects only 18 rays out of every 1,000; glass
reflects only 25 rays, while mercury reflects 666.
When the rays strike the surface obliquely the reflec-
tion is augmented. At an incidence of 40°, for ex-
ample, water reflects 22 rays, at 60° it reflects 65 rays,
at 80° 333 rays ; while at an incidence of 891°, where
the light almost grazes the surface, it reflects 721 rays
out of every 1,000. Thus, as the obliquity increases,
the reflection from water approaches, and finally quite
overtakes, the reflection from mercury ; but at no inci-
dence, however great, when the incidence is from air,
is the reflection from water, mercury, or any other sub-
stance, total.

Still, total reflection may occur, and with a view to
understanding its subsequent application in the Nicol's
prism, it is necessary to state when it occurs. This

ON LIGHT.

LECT.

leads me to the enunciation of a principle which under-
lies all optical phenomena — the principle of reversi-
bility.^ In the case of refraction, for instance, when
the ray passes obliquely from air into water, it is bent
towards the perpendicular ; when it passes from water
to air, it is bent /rom the perpendicular, and accurately
reverses its course. Thus in fig. 5, if 971 e ti be the track
taken by a ray in passing from air into water, n B m
will be its track in passing from water into air. Let
us push this principle to its consequences. Supposing
the light, instead of being incident along wi e or m' e,
were incident as close as possible along c e (fig. 6) ;
suppose, in other words, that it just grazes the surface

before entering the water. After refraction it will pur-
sue the course e v/\ Conversely, if the light start from
v/', and be incident at e, it will, on escaping into the
air, just graze the surface of the water. The question
now arises, what will occur supposing the ray from the
water follows the course n'" E, which lies beyond n" e ?
The answer is, it will not quit the water at all, but will
be totally reflected (along e x). At the under surface

' From this principle Sir John Herschel deduces in a simple and
elegant manner the fundamental law of reflection. — See Familiar
Lectures, p. 236.

T. TOTAL KEFLECTION. 19

of the water, moreover, the law is just the same as at
its upper surface, the angle of incidence (deh'^') being
here also equal to the angle of reflection (d e x).

Total reflection may be thus simply illustrated : —
Place a shilling in a drinking-glass, and tilt the glass
so that the light from the shilling shall fall with the
necessary obliquity upon the water surface above it.
Look upwards towards that surface, and you see the
image of the shilling shining there as brightly as the
shilling itself. Thrust the closed end of a glass test-
tube into water, and incline the tube. When the in-
clination is sufficient, horizontal light falling upon the
tube cannot enter the air within it, but is totally re-
flected upward : when looked down upon, such a tube
looks quite as bright as burnished silver. Pour a little
water into the tube ; as the liquid rises, total reflection
is abolished, and with it the lustre, leaving a gradually
diminishing shining zone, which disappears wholly
when the level of the water within the tube reaches
that without it. Any glass tube, with its end stopped
water-tight, will produce this effect, which is both
beautiful and instructive.

Total reflection never occurs except in the attempted
passage of a ray from a more refracting to a less re-
fracting medium ; but in this case, when the obliquity
is sufiicient, it always occurs. The mirage of the desert,
and other phantasmal appearances in the atmosphere,
are in part due to it. When, for example, the sun
lieats an expanse of sand, the layer of air in contact
with the sand becomes lighter and less refracting than
the air above it ; consequently, the rays from a distant
object, striking very obliquely on the surface of the
heated stratum, are sometimes totally reflected upwards,

20 ON LIGHT.

Ut'CT.

thus producing images similar to those produced by
water. I have seen the image of a rock called Mont
Tombeline distinctly reflected from the heated air of the
strand of Normandy near Avranches ; and by such de-
lusive appearances the thirsty soldiers of the French
army in Egypt were greatly tantalized.

The angle which marks the limit beyond which total
reflection takes place is called the limiting angle (it is
marked in fig. 6 by the strong line E n"y It must evi-
dently diminish as the refractive index increases. For
water it is 48^°, for flint glass 38° 41' , and for diamond
23° 42'. Thus all the light incident from two complete
quadrants, or 180**, in the case of diamond, is con-
densed into an angular space of 47° 22' (twice 23° 42')
by refraction. Coupled with its great refraction, are
the great dispersive and great reflective powers of dia-
mond; hence the extraordinary radiance of the gem,
both as regards white light and prismatic light.

§ 5. Velocity of LigJd. Abei^ration. Principle
of least Action.

In 1676 an impulse was given to optics by astronomy.
In that year Olav Eoemer, a learned Dane, was engaged
at the Observatory of Paris in observing the eclipses of
Jupiter's moons. The planet, whose distance from the
sun is 475,693,000 miles, has four satellites. We are
now only concerned with the one nearest to the planet.
Eoemer watched this moon, saw it move round in front
of the planet, pass to the other side of it, and then
plunge into Jupiter's shadow, behaving like a lamp sud-
denly extinguished : at the second edge of the shadow
he saw it reappear, like a lamp suddenly lighted. The
moon thus acted the part of a signal light to the

I. VELOCITY OF LIGHT, RCEMER. 21

astronomer, and enabled him to tell exactly its time of
revolution. The period between two successive light-
ings up of the lunar lamp he found to be 42 hours, 28
minutes, and So seconds.

This measurement of time was so accurate, that
having determined the moment when the moon emerged
from the shadow, the moment of its hundredth appear-
ance could also be determined. In fact, it would be
100 times 42 hours, 28 minutes, 35 seconds, after the
first observation.

Ecemer's first observation was made when the earth
was in the part of its orbit nearest Jupiter. About
six months afterwards, the earth being then at the
opposite side of its orbit, when the little moon ought
to have made its hundredth appearance, it was found
unpunctual, being fully 15 minutes behind its calcu-
lated time. Its appearance^" moreover, had been grow-
ing gradually later, as the earth retreated towards the
part of its orbit, most distant from Jupiter. Ecemer
reasoned thus : — ' Had I been able to remain at the
other side of the earth's orbit, the moon might have
appeared always at the proper instant ; an observer
placed there would probably have seen the moon 15
minutes ago, the retardation in my case being due to
the fact that the light requires 15 minutes to travel
from the place where my first observation was made to
my present position.'

This flash of genius was immediately succeeded by
another. ' If this surmise be correct,' Ecemer reasoned,
' then as I approach Jupiter along the other side of the
earth's orbit, the retardation ought to become gradu-
ally less, and when I reach the place of my first obser-
vation, there ought to be no retardation at all.' lie

22 ON LIGHT.

found this to be the case, and thus not only proved that
light required time to pass through space, but also
determined its rate of propagation.

The velocity of light, as determined by Eoemer, is
192,500 miles in a second.

For a time, however, the observations and reasonings
of Eoemer failed to produce conviction. They were
doubted by Cassini, Fontenelle, and Hooke. Subse-
quently came the unexpected corroboration of Eoemer by
the English astronomer, Bradley, who noticed that the
fixed stars did not really appear to be fixed, but that they
describe little orbits in the heavens every year. The
result perplexed him, but Bradley had a mind open to
suggestion, and capable of seeing, in the smallest fact,
a picture ?f the largest. He was one day upon the
Thames in a boat, and noticed that as long as his
course remained unchanged, the vane upon his mast-
Lead showed the wind to be blowing constantly in the
same direction, but that the wind appeared to vary with
every change in the direction of his boat. ' Here,' as
Whewell says, ' was the image of his case. The boat
was the earth, moving in its orbit, and the wind was
the light of a star.'

We may ask in passing, what without the faculty
which formed the ' image,' would Bradley's wind and
vane have been to him ? A wind and vane, and nothing
more. You will immediately understand the meaning
of Bradley's discovery. Imagine yourself in a motion-
less railway-train, with a shower of rain descending
vertically downwards. The moment the train begins
to move the rain-drops begin to slant, and the quicker
the motion of the train the greater is the obliquity. In
a precisely similar manner the rays from a star verti-

I. ABEERATION OF LIGHT, BRADLEY. 23

cally overhead are caused to slant by the motion of the
earth through space. Knowing the speed of the train,
and the obliquity of the falling rain, the velocity of
the drops may be calculated ; and knowing the speed
of the earth in her orbit, and the obliquity of the rays
due to this cause, we can calculate just as easily the
velocity of light. Bradley did this, and the ' aberra-
tion of light,' as his discovery is called, enabled him
to assign to it a velocity almost identical with that
deduced by Koemer from a totally different method
of observation. Subsequently Fizeau, and quite recently
Cornu, employing not planetary or stellar distances, but
simply the breadth of the city of Paris, determined the
velocity of light : while Foucault — a man of the rarest
mechanical genius — solved the problem without quitting
his private room. Owing to an error in the determi-
nation of the earth's distance from the sun, the velocity
assigned to light by both Eoemer and Bradley is too
great. With a close approximation to accuracy it may
be regarded as 186,000 miles a second.

By Roemer's discovery, the notion entertained by
Descartes, and espoused by Hooke, that light is pro-
pagated instantly through space, was overth rown. But
the establishment of its motion through stellar space
led to speculations regarding its velocity in transparent
terrestrial substances. The index of refraction of a ray
passing from air into water is ^. Newton assumed these
numbers to mean that the velocity of light in water
being 4, its velocity in air is 3 ; and he deduced the
phenomena of refraction from this assumption. The
reverse has since been proved to be the case — that is to
Bay, the velocity of light in water being 3, its velocity
in air is 4 : but both in Newton's time and ours the

24 ON LIGHT. I.KCT.

same great principle determined, and determines, the
course of light in all cases. In passing from point to
point, whatever be the media in its path, or however it
may be reflected, light takes the course which occupies
least time. Thus in fig. 4, taking its velocity in air
and in water into account, the light reaches Gr from I
more rapidly by travelling first to 0, and there changing
its course, than if it proceeded straight from I to G.
This is readily comprehended, because in the latter case
it would pursue a greater distance through the water,
which is the more retarding medium.

§ 6. Descartes' Explanation of the Rainbow.

Snell's law of refraction is one of the comer-stones of
optical science, and its applications to-day are million-
fold. Immediately after its discovery Descartes applied it
to the explanation of the rainbow. A beam of solar light
falling obliquely upon a xain-drop is refracted on enter-
ing the drop. It is in part reflected at the back of the
drop, and on emerging it is again refracted. By these
two refractions, at its entrance and at its emergence, the
beam of light is decomposed, quitting the drop resolved
into coloured constituents. The light thus reaches
the eye of an observer facing the drop, and with his
back to the sun.

Conceive a line drawn from the sun to the observer's
eye, and prolonged beyond the observer. Conceive
another line drawn through the eye, enclosing an angle
of 42|° with the line drawn from the sun, and prolonged
to the falling shower. Along this second line a rain-
drop, at its remote end, when struck by a sunbeam, will
send a ray of red light. Every other drop similarly
situated, that is, every drop at an angular distance of

I. THE RAINBOW, DESCARTES. 25

42^° from the line aforesaid, will do tlie same. A cir-
cular band of red light is thus formed, which may be re-
garded as the boundary of the base of a cone, having
the rays which form the band for its surface, and its
apex at the observer's eye. Because of the magnitude
of the sun, the angular width of this red band will be
half a degree.

From the eye of the observer conceive another line
to be drawn, enclosing an angle, not of 42^°, but of
40^% with the prolongation of the line drawn to the
sun. Along this line a solar beam striking a rain-drop
will send violet light to the eye. All drops at the same
angular distance will do the same, and we shall there-
fore obtain a band of violet light of the same width as
the red band. These two bands constitute the limiting
colours of the rainbow, and between them the bands
corresponding to the other colours lie.

Thus the line drawn from the eye to the oniddle
of the bow, and the line drawn through the eye to the
sun, always enclose an angle of about 41° ; to account
for this was the great difficulty, which remained un-
solved up to the time of Descartes.

Taking a pen in hand, and calculating by means
of Snell's law the track of every ray through a rain-
drop, Descartes found that, at one particular angle,
the rays, reflected at its back, emerged from the drop
almost parallel to each other. They were thus enabled
to preserve their intensity through long atmospheric
distances. At all other angles the rays quitted the drop
divergent, and through this divergence became so
enfeebled as to be practically lost to the eye. The
angle of parallelism here referred to was that of forty-
one degrees, which observation had pro\ed to be in-
variably associated with the rainbow.

26 ON LIGHT. LECT.

From wliat has been said, it is clear that two ob-
servers standing beside each other, or one above the
other, nay, that even the two eyes of the same observer,
do not see exactly the same bow. The position of the
base of the cone changes with that of its apex. And
here we have no difficulty in answering a question often
asked — namely, whether a rainbow is ever seen reflected
in water. Seeing two bows, the one in the heavens,
the other in the water, you might be disposed to infer
that the one bears the same relation to the other that a
tree upon the water's edge bears to its reflected image.
The rays, however, which reach an observer's eye after
reflection, and which form a bow, would, were their
course uninterrupted, converge to a point vertically
under the observer, and as far below the level of the
water as his eye is above it. But under no cir-
cumstances could an eye above the water-level, and one
below it, see the same bow — in other words, the self-
same drops of rain cannot form the reflected bow
and the bow seen directly in the heavens. The re-
flected bow, therefore, is not, in the usual optical sense
of the term, the image of the bow seen in the sky.

$ 7. Analysis and Synthesis of Light. DodHne
of Colours.

In the rainbow a new phenomenon was introduced
— the phenomenon of colour. And here we arrive
at one of those points in the history of science, when
great men's labours so intermingle that it is difficult
to assign to each worker his precise meed of honour.
Descartes was at the threshold of the discovery of the
composition of solar light ; but for Newton was
reserved the enunciation of the true law. He went

ANALYSIS OF LIGHT, NEWTOX.

to work in this way : Through the closed window-
shutter of a room he pierced an orifice, and allowed
a thin sunbeam to pass through it. The beam stamped
a round white image of the sun on the opposite wall
of the room. In the path of this beam Newton placed
a prism, expecting to see the beam refracted, but also
expecting to see the image of the sun, after refraction,
still round. To his astonishment, it was drawn out to
an image with a length five times its breadth. It was,

Fio. 7.

moreover, no longer white, but divided into bands of
different colours. Newton saw immediately that solar
light was coriiposite, not simple. His elongated image
revealed to him the fact that some constituents of the
light were more deflected by the prism than others,
and he concluded, therefore, that white solar light was
a mixture of lights of different colours, of different
degrees of refrangibility.

Let us reproduce this celebrated experiment. On
the screen is now stamped a luminous disk, which may

28 ON LIGHT.

LECT,

stand for Newton's image of the sun. Causing the heam
(from L, fig. 7) wliich produces the disk to pass through
a lens (E) which forms an image of the aperture, and
then through a prism (P), we obtain Newton's coloured
image, with its red and violet ends, which he called a
spectrum. Newton divided the spectrum into seven
parts — red, orange, yellow, green, blue, indigo, violet ;
which are commonly called the seven primary or pris-
matic colours. The drawing out of the white light
into its constituent colours is called dispersion.

This was the first analysis of solar light by Newton ;
but the scientific mind is fond of verification, and never
neglects it where it is possibje. Newton completed his
proof by synthesis in this way : The spectrum now
before you is produced by a glass prism. Causing the
decomposed beam to pass through a second similar
prism, but so placed that the colours are refracted back
and reblended, the perfectly white luminous disk is
restored.

In this case, refraction and dispersion are simulta-
neously abolished. Are they always so ? Can we have
the one without the other ? It was Newton's conclu-
sion that we could not. Here he erred, and his error,
which he maintained to the end of his life, retarded
the progress of optical discovery. Dollond «jabse-
quently proved that, by combining two difi'erent kinds
of glass, the colours can be extinguished, still leaving
a residue of refraction, and he employed this residue
in the construction of achromatic lenses — lenses
yielding no colour — which Newton thought an impossi-
bility. By setting a water-prism — water contained in
a wedge-shaped vessel with glass sides (B, fig. 8) — in

ACHEOMATISM, DOLLOND.

opposition to a wedge of glass (to the right of B), this
point can be illustrated before you. We have first
of all the position (dotted) of the unrefracted beam
marked upon the screen ; then we produce the narrow
water-spectrum (W) ; finally, by introducing a flint-
glass prism, we refract the beam back, until the colour
disappears (at A). The image of the slit is now ichite ;
but though the dispersion is abolished, there remains
a very sensible amount of refraction.

Fio. 8.

This is the place to illustrate another point bearing
upon the instrumental means employed in these lec-
tures. Bodies differ widely from each other as to their
powers of refraction and dispersion. Note the position
of the water-spectrum upon the screen. Alteriug in
no particular the wedge-shaped vessel, but simply
substituting for the water the transparent bisulphide
of carbon, you notice how much higher the beam is
thrown, and how much richer is the display of colour.
3

ON LIGHT.

LECT

To augment the size of our spectrum we here employ
(at L) a slit, instead of a circular aperture.*

The synthesis of white light may be effected in
three ways, all of which are worthy of attention :
Here, in the first instance, we have a rich spectrum
produced by the decomposition of the beam (from L,
fig. 9). One face of the prism (P) is protected by a

Fio. 9.

diaphragm (not shown in the figure), with a longitu-
dinal slit, through which the beam passes into the prism.

* The low dispersive power of water masks, as Helmholtz has re-
marked, the imperfect achromatism of the eye. With the naked eye I
can see a distant blue disk sharply defined, but not a red one. I can also
Bee the lines which mark the upper and lower boundaries of a horizon-
tally refracted spectrum sharp at the blue end, but ill-defined at the red
end. Projecting a luminous disk upon a screen, and covering one semi-
circle of the aperture with a red and the other with a blue or green glass,
the difference between the apparent sizes of the two semicircles is in
my case, and in numerous other cases, extraordinary. Many per-
sons, however, see the apparent sizes of the two semicircles reversed.
If with a spectacle glass I correct the dispersion of the red light over
the retina, then the blue ceases to give a sharply-defined image. Thu!^
examined the departure of the eye from achromatism appears very gross
indeed.

u COMPLEMENTARY COLOUES, HELMHOLTZ. 31

It emerges decomposed at tte other side. I permit the
colours to pass through a cylindrical lens (C), which
so squeezes them together as to produce upon the
screen a sharply-defined rectangular image of the
longitudinal slit. In that image the colours are re-
blended, and it is perfectly white. Between the prism
and the cylindrical lens may be seen the colours,
tracking themselves through the dust of the room.
Cutting off the more refrangible fringe by a card, the
rectangle fs seen red ; cutting off the less refrangible
fringe, the rectangle is seen blue. By means of a thin
glass prism (W), I deflect one portion of the colours, and
leave the residual portion. On the screen are now two
coloured rectangles produced in this way. These are
compleTnentary colours — colours which, by their union,
produce white. Note that, by judicious management,
one of these colours is rendered yellow, and the other
blue. I withdraw the thin prism ; yellow and blue
immediately commingle, and we have ivhite as the result
of their \mion. On our way, then, we remove the
fallacy, first exposed by Helmholtz, that the mixture
of blue and yellow lights produces green.

Restoring the circular aperture, we obtain once
more a spectrum like that of Newton. By means
of a lens, we gather up these colours, and build them
together, not to an image of the aperture, but to an
image of the carbon-points themselves.

Finally, in virtue of the persistence of impressions
upon the retina, by means of a rotating disk, on which
are spread in sectors the colours of the spectrum, we
blend together the prismatic colours in the eye itself,
and thus produce the impression of whiteness.

Ilavinji: unravelled the interwoven constituents of

32 ■ ON LIGHT.

UICT.

white light, we have next to inquire, What part the
constitution so revealed enables this agent to play in
Nature ? To it we owe all the phenomena of colour,
and yet not to it alone ; for there must be a certain rela-
tionship between the ultimate particles of natural bodies
and white light, to enable them to extract from it the
luxury of colour. But the function of natural bodies is
here selective, not creative. There is no colour generated
by any natural body whatever. Natural bodies have
showered upon them, in the white light of the sun,
the sum total of all possible colours, and their action
is limited to the sifting of that total, the appropri-
ating from it of the colours which really belong to them,
and the rejecting of those which do not. It will fix
this subject in your minds if I say, that it is the portion
of light which they reject, and not that which belongs
to them, that gives bodies their colours.

Let us begin our experimental inquiries here by
asking, What is the meaning of blackness ? Pass a
black ribbon through the colours of the spectrum ; it
quenches all of them. The meaning of blackness is
thus revealed — it is the result of the absorption of all
the constituents of solar light. Pass a red ribbon
through the spectrum. In the red light the ribbon is a
vivid red. Why ? Because the light that enters the
ribbon is not quenched or absorbed, but in great part
sent back to the eye. Place the same ribbon in the green
of the spectrum ; it is black as jet. It absorbs the green
light, and leaves the space on which it falls a space of
intense darkness. Place a green ribbon in the green
of the spectrum. It shines vividly with its proper
colour ; transfer it to the red, it is black as jet. Here

1. COLOURS PRODUCED EY ABSORPTION. 33

it absorbs all the light that falls upon it, and offers
mere darkness to the eye.

Thus, when white light is employed, the red sifts
it by quenching the green, and the green sifts it
by quenching the red, both exhibiting the residual
colour. The process through which natural bodies
acquire their colours is therefore a negative one.
The colours are produced by subtraction, not by addi-
tion. This red glass is red because it destroys all the
more refrangible rays of the spectrum. This blue
liquid is blue because it destroys all the less refrangible
rays. Both together are opaque because the light
transmitted by the one is quenched by the other. In
this way, by the union of two transparent substances
we obtain a combination as dark as pitch to solar light.
This other liquid, finally, is purple because it destroys
the green and the yellow, and allows the terminal
colours of the spectrum to pass unimpeded. From the
blending of the blue and the red this gorgeous purple
is produced.

One step further for the sake of exactness. The light
which falls upon a body is divided into two portions,
one of which is reflected from the surface of the
body ; and this is of the same coloiir as the incident
light. If the incident light be white the superficially
reflected light will also be white. Solar light, for
example, reflected from the surface of even a black body,
is white. The blackest camphine smoke in a dark
room through which a sunbeam passes from an aperture
in the window-shutter, renders the track of the beam
white, by the light scattered from the surfaces of the
soot particles. The moon appears to us as if

' Clolhcfl in white samite, mystic, wonderful ;'

34 ON LIGHT.

LECT.

but were she covered with the blackest velvet she would
still hang in the heavens as a white orb, shining upon
our world substantially as she does now.

§ 8. Colours of Pigments as distinguished frora
Colours of Light.

The second portion of the light enters the body,
and upon its treatment there the colour of the body
depends. And here a moment may properly be given
to the analysis of the action of pigments upon light.
They are composed of particles mixed with a vehicle ;
but how intimately soever the particles may be blended,
they, still remain particles, separated it may be by
exceedingly minute distances, but still separated. To
use the scientific phrase, they are not optically continu-
ous. Now, wherever optical continuity is ruptured we
have reflection of the incident light. It is the multi-
tude of reflections at the limiting surfaces of the
particles that prevents light from passing through glass,
or rock-salt, when these transparent substances are
pounded into powder. The light here is exhausted in
a waste of echoes, not extinguished by true absorption.
It is the same kind of reflection that renders the
thunder-cloud so impervious to light. Such a cloud is
composed of particles of water mixed with particles of
air, both separately transparent, but practically opaque
when thus mixed together.

In the case of pigments, then, the light is reflected
at the limiting surfaces of the particles, but it is in
part absorbed within the particles. The reflection is
necessary to send the light back to the eye ; the absorp-
tion is necessary to give the body its colour. The same

I. COLOUKS OF PIGMENTS. 85

remarks apply to flowers. The rose is red in virtue,
not of the light reflected from its surface, but of light
which has entered its substance, which has been re-
flected from surfaces within, and which in returning
through the substance has had its green extinguislied.
A similar process in the case of hard green leaves ex-
tinguishes the red, and sends green light from the body
of the leaves to the eye.

All bodies, even the most transparent, are more
or less absorbent of light. Take the case of water :
in small quantities it does not sensibly affect light.
A glass cell of clear water interposed in the track of
our beam does not perceptibly change any one of the
colours of the spectrum derived from the beam. Still
absorption, though insensible, has here occurred, and
to render it sensible we have only to increase the depth
of the water through which the light passes. Instead of
a cell an inch thick, let us take a layer, ten or fifteen
feet thick : the colour of the water is then very evide^iL
By augmenting the thickness we absorb moi-e of the
light, and by making the thickness very great we absorb
the light altogether. Lampblack or pitch can do
no more, and the only difference between them and
water is that a very small depth in their case suffices
to extinguish all the light. The difference between
the highest known transparency, and the highest known
opacity, is one of degree merely.

If, then, we render water sufficiently deep to quench
all the light ; and if from the interior of the water no
light reaches the eye, we have the condition necessary
to produce l^lackncss. Looked properly down iipon
there are portions of the Atlantic Ocean to which one
would hardly ascribe a trace of colour : at the most a

36 ON LIGHT.

LECI.

tint of dark indigo reaches the eye. The water, in
fact, is practically black, and this is an indication both
of its depth and purity. But the case is entirely
changed when the ocean contains solid particles in a
state of mechanical suspension, capable of sending
light back to the eye.

Throw, for example, a white pebble into the blackest
Atlantic water ; as it sinks it becomes greener and
greener, and, before it disappears, it reaches a vivid
blue green. Break such a pebble into fragments, these
will behave like the unbroken mass : grind the pebble
to powder, every particle will yield its modicum of
green ; and if the particles be so fine as to remain
suspended in the water, the scattered light will be a
uniform green. Hence the greenness of shoal water.
You go to bed with the black water of the Atlantic
around you. You rise in the morning, find it a vivid
green, and correctly infer that you are crossing the
bank of Newfoundland. Such water is found charged
with fine matter in a state of mechanical suspension.
The light from the bottom may sometimes come into
play, but it is not necessary. The subaqueous foam
generated by the screw or paddle-wheels of a steamer
also sends forth a vivid green. The foam here fur-
nishes a reflecting surface, the water between the eye
and it the absorbing medium.

Nothing can be more superb than the green of the
Atlantic waves when the circumstances are favourable
to the exhibition of the colour. As long as a wave
remains unbroken no colour appears, but when the foam
just doubles over the crest like an Alpine snow-cornice,
under the cornice we often see a display of the most
exquisite green. It is metallic in its brilliancy. But

I. COLOUR OF WATER. 37

foam is necessary to its production. The foam is first
illuminated, and it scatters the light in all direc-
tions ; the light which passes through the higher
portion of the wave alone reaches the eye, and gives
to that portion its matchless colour. The folding of the
wave, producing, as it does, a series of longitudinal
protuberances and fmrows which act like cylindrical
lenses, introduces variations in the intensity of the
light, and materially enhances its beauty.

We are now prepared for the further consideration
of a point already adverted to, and regarding which
error long found currency. You will find it stated
in many books that blue and yellow lights mixed
together produce green. But blue and yellow have
been just proved to be complementary colours, pro-
ducing white by their mixture. The mixture of blue
and yellow pigments undoubtedly produces green, but
the mixture of pigments is totally different from the
mixtiue of lights.

Helmholtz has revealed the cause of the green in
the case of a .mixture of blue and yellow pigments. No
natural colour is pure. A blue liquid or a blue powder
permits not only the blue to pass through it, but a por-
tion of the adjacent green. A yellow powder is trans-
parent not only to the yellow light, but also in part to
the adjacent green. Now, when blue and yellow are
mixed together, the blue cuts off the yellow, the orange,
and the red ; the yellow, on the other hand, cuts off the
violet, the indigo, and the blue. Green is the only
colour to which both are transparent, and the conse-
quence is that, when white light falls upon a mixture
of yellow and blue powders, the green alone is sent
back to the eye. You have already seen that the fine

38 ON LIGHT.

LECT.

blue ammonia-sulphate of copper transmits a large
portion of green, while cutting off all the less re-
frangible light. A yellow solution of picric acid also
allows the green to pass, but quenches all the more
refrangible light. What must occur when we send a
beam through both liquids ? The experimental answer
to this question is now before you : the green band of
the spectrum alone remains upon the screen.

The impurity of natural colours is strikingly illus-
trated by an observation recently communicated to
me by Mr. Woodbury. On looking through a blue
glass at green leaves in sunshine, he saw the super-
ficially reflected light blue. The light, on the con-
trary, which came from the body of the leaves was
crimson. On examination, I found that the glass em-
ployed in this observation transmitted both ends of the
spectrum, the red as well as the blue, and that it
quenched the middle. This furnished an easy explana-
tion of the effect. In the delicate spring foliage the blue
of the solar light is for the most part absorbed, and a
light, mainly yellowish green, but containing a con-
siderable quantity of red, escapes from the leaf to the
eye. On looking at such foliage through the violet
glass, the green and the yellow are stopped, and the red
alone reaches the eye. Thus regarded, therefore, the
leaves appear like faintly-blushing roses, and present a
very beautiful appearance. With the blue ammonia-
sulphate of copper, which transmits no red, this effect
is not obtained.

As the year advances the crimson gradually hardens
to a coppery red ; and in the dark green leaves of old
ivy it is almost absent. Permitting a concentrated
beam of white light to fall upon fresh leaves in a dark

T. COLOUES OF FOLIAGE. 39

room, the sudden change from green to red, and from
red back to green, when the violet glass is alternately
introduced across the beam and withdrawn, is very
surprising. Looked at through the same glass, the
meadows in May appear of a warm purple. With a
solution of permanganate of potash, which, while it
quenches the centre of the spectrum, permits its ends
to pass more freely than the violet glass, striking effects
are also obtained.^

This question of absorption, considered with refer-
ence to its molecular mechanism, is one of the most
subtle and difficult in physics. We are not yet in a
condition to grapple with it, but we shall be by-and-
by. jNIeanwhi^e we may profitably glance back on
the web of relations which these experiments reveal
to us. We have in the first place in solar light an agent
of exceeding complexity, composed of innumerable
constituents, refrangible in different degrees. We find,
secondly, the atoms and molecules of bodies gifted
with the power of sifting solar light in the most vari-
ous ways, and producing by this sifting the colours
observed in nature and art. To do this they must pos-
sess a molecular structure commensurate in complexity
with that of light itself. Thirdly, we have the human
eye and brain, so organized as to be able to take in and

' Eoth in foliage and in flowers vre have striking difierences of ab-
sorption. The copper beech and the green beech, for example, take in
diflferent rays. But the very growth of the tree is due to some of the
rays thus taken in. Are the chemical rays, then, the same in the
copper and the green beech ? In two such flowers as the primrose and
the violet, where the absorptions, to judge by the colours, are almost com-
plementary, are the chemically active rays the same ? The general re-
lation of colour to chemical action is worthy of tlio application of the
method by which Dr. Draper proved so conclusively the chemical potency
of the yellow rays.

40 ON LIGHT. LECT.

distinguish tlie multiiide of impressions thus generated.
The light, therefore, at starting is complex ; to sift and
select it as they do, natural bodies must be complex; while
to take in the impressions thus generated, the human
eye and brain, however we may simplify our conceptions
of their action,^ must be highly complex. "Whence this
triple complexity ? If what are called material pur-

' Young, Helmholtz, and Maxwell reduce all diiFerences of hue to
combinations in different proportions of three primary colours. It is
demonstrable by experiment that from tne red, green, and violet all the
other colours of the spectrum may be obtained.

Sir Charles Wheatstone has recently drawn my attention to a work
by Christian Ernst Wiinsch, Leipzig, 1792, in which the author an-
nounces the proposition that there are neither fire nor seven, but only
three simple colours in white light. "Wiinsch produces fire spectra,
with fire prisms and five small apertures, and he mixes the colours first
in pairs, and afterwards in other ways and proportions. His result is that
'red is a simjple colour incapable of being decomposed; that orange is
compounded of intense red and weak green ; that yellow is a mixture of
intense red and intense green ; that green is a simple colour ; that bine
is compounded of saturated green and saturated violet ; that indigo is a
mixture of saturated violet and weak green ; while violet is a pure simple
colour. He also finds that yellow and indigo blue produce white by
their mixture. Yellow with bright blue (hochblau) also produces white,
which seems, however, to have a tinge of green, while the pigments of
these two colours when mixed always give a more or less beautiful
green. Wiinsch very emphatically distinguishes the mixture of pigments
from that of lights. Speaking of the generation of yellow, he says,
• I say expressly red and green light, because I am speaking about light-
colours (Lichtfarben), and not about pigments.' However faulty his
theories may be, Wiinsch's experiments appear in the main to be
precise and conclusive. Nearly ten years subsequently Young adopted
red, green, and violet as the three primary colours, each of them
capable of producing three sensations, one of which, however, pre-
dominates over the two others. Helmholtz adopts, elucidates, and
enriches this notion. (Popular Lectures, p. 249. The beautiful paper
of Helmholtz on the mixture of colours, translated by myself, is pub-
lished in the 'Philosophical Magazine' for 1852. Maxwell's excellent
memoir on the Theory of Compound Colours is published in the • Philo-
Bophical Transactions,' vol. 150, p. 57.)

I. PRIMAEY COLOUES. 41

poses were the only end to be served, a much simpler
mechanism woidd be suffiaient. But, instead of sim-
plicity, we have prodigality of relation and adaptation —
and this apparently for the sole purpose of enabling us
to see things robed in the splendours of colour. Would
it not seem that Nature harboured the intention of edu-
cating us for other enjoyments than those derivable
from meat and drink ? At all events, whatever Nature
meant — and it would be mere presumption to dogmatize
as to what she meant — we find ourselves here, as
the upshot of her operations, endowed with capacities
to enjoy not only the materially useful, but endowed
with others of indefinite scope and application, which
deal alone with the beautiful and the true.

42 ON LIGHT.

I£OT.

LECTUEE II.

OEIGm OF PHYSICAL THEORIES — SCOPE OF THE IMAGINATION — NEVTTOS

AND THE EMISSION THEOEY ^VERIFICATION OF PHYSICAL THEORIES

THE LUMrNIFEKOrS ETHER "WATB-THEORY OF LIGHT THOMAS YOCNG

FRESNBL ANB ARAGO CONCEPTION OF TVAVB-MOTION— rINTERFERENCB

OF WAVES CONSTITUTION OF SOIIND-'WAVES — ANALOGIES OF SOUND

AND LIGHT ILLUSTEATI0W3 OP "SVAVE-MOTION INTERFERENCE OF

SOUNDWAVES — OPTICAL ILLUSTRATIONS PITCH AND COLOUR — LENGTHS

OF THE WAVES OF LIGHT AND KATES OF VIBRATION OF THE ETHER-
PARTICLES INTERFERENCE OF LIGHT — PHENOMENA WHICH FIRST

SUGGESTED THE UNDULATORY THEORY — BOYLE AND HOOKE THE

COLOURS OF THIN PLATES THE SOAP-BUBBLE — NEWTON's RINGS

THEORY OF ' FITS ' ITS EXPLANATION OF THE RINGS OVERTHROW

OF THE THEORY — DIFFRACTION OF LIGHT — COLOURS PRODUCED BY
DIFFRACTION COLOURS OF MOTHER-OF-PEARL.

§ 1 . Origin and Scope of Physical Theories.

WE might vary and extend our experiments on
Light indefinitely, and they certainly would prove
us to possess a wonderful mastery over the phenomena.
But the vesture of the agent only would thus be re-
vealed, not the agent itself. The human mind, how-
ever, is so constituted and so educated, as regards
natural things, that it can never rest satisfied with this
outward view of them. Brightness and freshness take
possession of the mind when it is crossed by the light
of principles, shewing the facts of Nature to be organ-
ically connected.

Let us, then, inquire what this tiling is that we

II. CONCEPTION OF PHYSICAL THEORY. 43

have been generating, reflecting, refracting and analyz-
ing.

In doing this, we shall learn that the life of the
experimental philosopher is twofold. He lives, in his
vocation, a life of the senses, using his hands, eyes, and
ears in his experiments : but such a question as that
now before us carries him beyond the margin of the
senses. He cannot consider, much less answer, the
question, 'What is light?' without transporting liim-
self to a world which underlies the sensible one, and
out of which spring all optical phenomena. To realize
this subsensible world, if I may use the term, the mind
must possess a certain pictorial power. It must be
able to form definite images of the things which that
world contains ; and to say that, if such or such a state
of things exist in that world, then the phenomena
which appear in ours must, of necessity, grow out of
this state of things. If the picture be correct, the phe-
nomena are accounted for ; a physical theory has been
enunciated which unites and explains them all.

This conception of physical theory implies, as you
perceive, the exercise of the imagination. Do not be
afraid of this word, which seems to render so many
respectable people, both in the ranks of science and
out of them, uncomfortable. That men in the ranks of
science should feel thus is, I think, a proof that they
have suffered themselves to be misled by the popular
definition of a great faculty instead of observing its
operation in their own minds. Without imagination
we cannot take a step beyond the bourne of the mere
animal world, perhaps not even to the edge of this one.
But, in speaking thus of imagination, I do not mean a
riotous power whicli deals capriciously with facts, but

44 ON LIGHT.

ISCT.

a well-ordered and disciplined power, whose sole func-
tion is to form conceptions which the intellect im-
peratively demands. Imagination, thus exercised, never
really severs itself from the world of fact. This is the
storehouse from which the materials for all its pictures
are derived ; and the magic of its art consists, not in
creating things anew, but in so changing the magnitude,
position, and other relations of sensible things, as to
render them fit for the requirements of the intellect in
the subsensible world.'

Descartes imagined space to be filled with some-
thing that transmitted light instantaneously. Firstly,
because, in his experience, no measurable interval was
known to exist between the appearance of a flash of
light, however distant, and its efifect upon consciousness ;
and secondly, because, as far as his experience went, no
physical power is conveyed from place to place without

' The following charming extract, bearing upon this point, -was dis-
covered and -Hritten out for me by my deeply lamented friend Dr.
Bence Jones, late Hon. Secretary to the Royal Institution :

' In every kind of magnitude there is a degree or sort to which our
sense is proportioned, the perception and knowledge of which is of the
greatest use to mankind. The same is the groundwork of philosophy ;
for, though all sorts and degrees are equally the object of philosophical
speculation, yet it is from those which are proportioned to sense that a
philosopher must set out in his inquiries, ascending or descending after-
wards as his pursuits may require. He does well indeed to take his
views from many points of sight, and supply the defects of sense by a
well-regulated imagination ; nor is he to be confined by any limit in
space or time ; but, as his knowledge of Nature is founded on the ob-
servation of sensible things, he must begin with these, and must often
retxim to them to examine his progress by them. Here is his secure
hold ; and as he sets out from thence, so if he likewise trace not often
his steps backwards with caution, he will be in hazard of losing his way
in the labyrinths of Nature.' — {Maclaurin: An Account of Sir I. New-
ton'» Philosophical Discoveries. Written 1728; second edition, 1750;
pp. 18, 19.)

n. BASIS OF THE EMISSION THEORY. 45

a vehicle. But his imagination helped itself farther
by illustrations drawn from the world of fact. ' When,'
he says, ' one walks in darkness with staff in hand, the
moment the distant end of the staff strikes an obstacle
the hand feels it. This explains what might otherwise
be thought strange, that the light readies us instan-
taneously from the sun. I wish thee to believe that
light in the bodies that we call luminoas is nothing
more than a very brisk and violent motion, which, by
means of the air and other transparent media, is con-
veyed to the eye exactly as the shock through the
walking-stick reaches the hand of a blind man. This
is instantaneous, and would be so even if the intervening
distance were greater than that between earth and
heaven. It is therefore no more necessary that any-
thing material should reach the eye from the luminous
object, than that something should be sent from the
ground to the hand of the blind man when he is con-
scious of the shock of his staff.' The celebrated Robert
Hooke first threw doubt upon this notion of Descartes,
but afterwards substantially espoused it. The belief in
instantaneous transmission was destroyed by the dis-
covery of Roemer referred to in our last lecture.

§ 2. Tlie Emission Theoiy of Light.

The case of Newton still more forcibly illustrates
the position, that in forming physical theories we draw
for our materials upon the world of fact. Before he
began to deal with light, he was intimately acquainted
with the laws of elastic collision, which all of you have
Been more or less perfectly illustrated on a billiard-table.
As regards the collision of sensible masses, Newton knew

40 ON LIGHT.

LKCT.

the angle of incidence to be equal to the angle of re-
flection, and he also knew that experiment, as shewn in
our last lecture (fig. 3), had established the same law
with regard to light. He thus found in his previous
knowledge the material for theoretic images. He had
only to change the magnitude of conceptions already
in his mind to arrive at the Emission Theory of Light.
He supposed light to consist of elastic particles of in-
conceivable minuteness shot out with inconceivable
rapidity by luminous bodies, and that such particles
impinging upon smooth surfaces were reflected in ac^
cordance with the ordinary law of elastic collision.
The fact of optical reflection certainly occurred as if
light consisted of such particles, and this was Newton's
sole justification for introducing them.

But this is not all. In another important particu-
lar, also, Newton's conceptions regarding the nature of
light were influenced by his previous knowledge. He
had been pondering over the phenomena of gravitation,
and had made himself at home amid the operations of
this universal power. Perhaps his mind at this time was
too freshly and too deeply imbued with these notions
to permit of his forming an unfettered judgement re-
garding the nature of light. Be that as it may, Newton
saw in Eefraction the action of an attractive force ex-
erted on the light-particles. He carried his conception
out with the most severe consistency. Dropping ver-
tically downwards towards the earth's surface, the mo-
tion of a body is accelerated as it approaches the earth.
Dropping in the same manner downwards on a horizontal
surface, say throvigh air on glass or water, the velocity
of the light-particles, when they came close to the sur-
face, was, according to Newton, also accelerated. Ap-

n. TEST OF THEOEY. 47

proaching such a surface obliquely, he supposed the
particles, when close to it, to be drawn down upon it,
as a projectile is drawn by gravity to the surface of the
earth. This deflection was, according to Newton, the
refraction seen in our last lecture (fig. 4). Finally, it
was supposed that differences of colour might be due to
differences in the size of the particles. This was the
physical theory of light enunciated and defended by
Newton ; and you will observe that it simply consists
in the transference of conceptions born in the world of
the senses to a subsensible world.

But, though the region of physical theory lies thus
behind the world of senses, the verifications of theory
occur in that world. Laying the theoretic conception
at the root of matters, we determine by deduction what
are the phenomena which must of necessity grow out of
this root. If the phenomena thus deduced agree with
those of the actual world, it is a presumption in favour
of the theory. If, as new classes of phenomena arise,
they also are found to harmonize with theoretic de-
duction, the presumption becomes still stronger. If,
finally, the theory confers prophetic vision upon the
investigator, enabling him to predict the occurrence of
phenomena which have never yet been seen, and if those
predictions be found on trial to be rigidly correct, the
persuasion of the truth of the theory becomes over-
powering.

Thus working backwards from a limited number of
phenomena, genius, by its own expansive force, reaches
a conception which covers them all. There is no
more wonderful performance of the intellect than
this ; but we can render no account of it. Like the
scriptural gift of the Spirit, no man can tell whence

48 ON LIGHT. LECT.

it Cometh. The passage from fact to principle is
sometimes slow, sometimes rapid, and at all times a
source of intellectual joy. When rapid, the pleasure
is concentrated and becomes a kind of ecstasy or in-
toxication. To any one who has experienced this
pleasure, even in a moderate degree, the action of
Archimedes when he quitted the bath, and ran naked,
crying 'Eureka!' through the streets of Syracuse,
becomes intelligible.

How, then, did it fare with the Emission Theory when
the deductions from it were brought face to face with
natural phenomena? Tested by experiment, it was
found competent to explain many facts, and with tran-
scendent ingenuity its author sought to make it account
for them all. He so far succeeded, that men so cele-
brated as Laplace and Malus, who lived till 1812, and
Biot and Brewster, who lived till our own time, were
found among his disciples.

§ 3. The Undulatory Theory of Light.

Still, even at an early period of the existence of the
Emission Theory, one or two great names were found
recording a protest against it ; and they furnish another
illustration of the law that, in forming theories, the
scientific imagination must draw its materials from the
world of fact and experience. It was known long ago
that sound is conveyed in waves or pulses through the
air ; and no sooner was this truth well housed in the
mind than it was transformed into a theoretic concep-
tion. It was supposed that light, like sound, might
also be the product of wave-motion. But what, in this
case, could be the material forming the waves ? For the

11. BASIS OF THE UNDULATOEY THEORY. 49

waves of sound we have the air of our atmosphere ; but
the stretch of imagination which filled all space with a
luminiferous ether trembling with the waves of light
was so bold as to shock cautious minds. In one of my
latest conversations with Sir David Brewster, he said to
me that his chief objection to the undulatory theory
of light was that he could not think the Creator guilty
of so clumsy a contrivance as the filling of space with
ether in order to produce light. This, I may say, is
very dangerous ground, and the quarrel of science with
Sir David, on this point, as with many estimable persons
on other points, is, that they profess to know too much
about the mind of the Creator.

This conception of an ether was advocated, and in-
deed applied to various phenomena of optics, by the
celebrated astronomer, Huyghens. It was espoused
and defended by the celebrated mathematician, Euler
They were, however, opposed by Newton, whose au-
thority at the time bore them down. Or shall we say it
was authority merely ? Not quite so. Newton's pre-
ponderance was in some degree due to the fact that,
though Huyghens and Euler were right in the main,
they did not possess sufficient data to prove themselves
right. No human authority, however high, can main-
tain itself against the voice of Nature speaking through
experiment. But the voice of Nature may be an un-
certain voice, through the scantiness of data. This was
the case at the period now referred to, and at such a
period by the authority of Newton all antagonists were
naturally overborne.

Still, this great Emission Theory, which held its
ground so long, resembled one of those circles which,
according to your countryman Emerson, the force of

50 ON LIGHT.

racT.

genius periodically draws round the operations of the
intellect, but which are eventually broken through by
pressure from behind. In the year 1773 was born, at
Milverton, in Somersetshire, one of the most remarkable
men that England ever produced. He was educated for
the profession of a physician, but was too strong to be
tied down to professional routine. He devoted him-
self to the study of natural philosophy, and became in
all its departments a master. He was also a master of
letters. Languages, ancient and modern, were housed
within his brain, and, to use the words of his epitaph,
' he first penetrated the obscurity which had veiled for
ages the hieroglyphics of Egypt.' It fell to the lot of
this man to discover facts in optics which Newton's
theory was incompetent to explain, and his mind
roamed in search of a sufficient theory. He had made
himself acquainted with all the phenomena of wave-
motion ; with all the phenomena of sound ; working
successfully in this domain as an original discoverer. ,
Thus informed and disciplined, he was prepared to i
detect any resemblance which might reveal itself be- / i
tween the phenomena of light and those of wave-motion.
Such resemblances he did detect ; and, spurred on by 7
the discovery, he pursued his speculations and his .
experiments, until he finally succeeded in placing on an ;
immovable basis the Undulatory Theory of Light. '

The founder of this great theory was Thomas Young,
a name, perhaps, unfamiliar to many of you, but which
ought to be familiar to you all. Permit me, there-
fore, by a kind of geometrical construction which I once
ventured to employ in London, to give you a notion
of the magnitude of this man. Let Newton stand
erect in his age, and Young in liis. Draw a straight

H. YOUNG, FRESNEL, ARAGO, BROUGHAM. 51

line from Newton to Young, tangent to the heads of
both. This line would slope downwards from Newton to
Young, because Newton was certainly the taller man
of the two. But the slope would not be steep, for the
difference of stature was not excessive. The line
would form what engineers call a gentle gradient from
Newton to Young. Place underneath this line the
biggest man born in the interval between both. It may
be doubted whether he would reach the line ; for if he did
he would be taller intellectually than Young, and there
was probably none taller. But I do not want you to
rest on English estimates of Young; the German,
Helmholtz, a kindred genius, thus speaks of him : ' His
was one of the most profound minds that the world
has ever seen ; but he had the misfortune to be too
much in adv^ance of his age. He excited the wonder
of his contemporaries, who, however, were unable to
follow him to the heights at which his daring intellect
was accustomed to soar. His most important ideas
lay, therefore, buried and forgotten in the folios of the
Koyal Society, until a new generation gradually and
painfully made the same discoveries, and proved the
exactness of his assertions and the truth of his de-
monstrations.'

It is quite true, as Helmholtz says,, that Young was
in advance of his age ; but something is to be added
which illustrates the responsibility of our public writers.
For twenty years this man of genius was quenched —
hidden from the appreciative intellect of his country-
men— deemed in fact a dreamer, through the vigorous
sarcasm of a writer who had then possession of the
public ear, and who in the Edinburgh Review poured
ridicule upon Young and liis speculations. To the cele-

52 ON LIGHT.

LECT.

bleated Frenchmen Fresnel and Arago he was first
indebted for the restitution of his rights ; for they,
3specially Fresnel, remade independently, as Helm-
holtz says, and vastly extended his discoveries. To
the students of his works Young has long since ap-
peared in his true light, but these twenty blank years
pushed him from the public mind, which became in turn
filled with the fame of Young's colleague at the Eoyal
Institution, Davy, and afterwards with the fame of
Faraday. Carlyle refers to a remark of Novalis, that
a man's self-trust is enormously increased the moment
he finds that others believe in him. If the opposite
remark be true — if it be a fact that public disbelief
weakens a man's force — there is no calculating the
amount of damage these twenty years of neglect may
have done to Young's productiveness as an investi-
gator. It remains to be stated that his assailant was
Mr. Henry Brougham, aftei-wards Lord Chancellor of
England.

§ 4. Wave-motion, Interference of Waves, ' Whirlpool
Hapids ' of Niagara.

Our hardest work is noAV before us. But the
capacity for hard work depends in a great measure
on the antecedent winding up of the will ; I would
call upon you, therefore, to gird up your loins for our
coming labours. If we succeed in climbing the hill
which faces us to-night, our future difficulties will not
be insurmountable.

In the earliest writings of the ancients we find the
notion that sound is conveyed by the air. Aristotle
gives expression to this notion, and the great architect

II.

WAVE-MOTION. 53

Vitruvius compares the waves of sound to waves of
water. But the real mechanism of wave-motion was
hidden from the ancients, and indeed was not made
clear imtil the time of Newton. The central difficulty
of the subject was, to distinguish between the motion
of the wave itself, and the motion of the particles which
at any moment constitute the wave.

Stand upon the sea-shore and observe the advancing
rollers before they are distorted by the friction of the
bottom. Every wave has a back and a front, and, if
you clearly seize the image of the moving wave, you
will see that every particle of water along the front of
the wave is in the act of rising, while every particle
along its back is in the act of sinking. The particles
in front reach in succession the crest of the wave, and
as soon as the crest is passed they begin to fall. They
then reach the furrow or sinus of the wave, and can
sink no farther. Immediately afterwards they become
the front of the succeeding wave, rise again until they
reach the crest, and then sink as before. Thus, while
the waves pass onward horizontally, the individual
particles are simply lifted up and down vertically.
Observe a sea-fowl, or, if you are a swimmer, abandon
yourself to tlie action of the waves ; you are not carried
forward, but simply rocked up and down. The propa-
gation of a wave is the propagation of a form, and not
the transference of the substance wliich constitutes the
wave.

The length of the wave is the distance from crest to
crest, while the distance through which the individual
particles oscillate is called the amplitude of the oscil-
lation. You will notice that in this description the
4

54 ON LIGHT. i£CT.

particles of water are made to vibrate across the line of
propagation.'

And now we have to take a step forwards, and it is
the most important step of all. You can picture two
series of waves proceeding from different origins
tlirough the same water. When, for example, you
throw two stones into still water, the ring-waves pro-
ceeding from the two centres of disturbance intersect
each other. Now, no matter how numerous these waves
may be, the law holds good that the motion of every
particle of the water is the algebraic sum of all the
motions imparted to it. If crest coincide with crest
and furrow with furrow, the wave is lifted to a double
height above its sinus ; if furrow coincide with crest,
the motions are in opposition, and their sum is zero.
We have then stili water. This action of wave upon
wave is technically called interference, a term to be
remembered.

To the eye of a person conversant with these princi-
ples, nothing can be more interesting than the crossing
of water ripples. Through their interference the water-
surface is sometimes shivered into the most beautiful
mosaic, trembling rhythmically as if with a kind of
visible music. When waves are skilfully generated in
a dish of mercury, a strong light thrown upon the
shining surface, and reflected on to a screen, reveals
the motions of the liquid metal. The shape of the
vessel determines the forms of the figures produced. In

' I do uot wish to encumber the conception here with the det:iils of
the motion, but I may draw attention to Ihe beautiful model of Prof.
Lyman, wherein waves are shown to be produced by the circular motion
of the particles. This, as proved by the brothers Weber, is the real
motion in the case of water-waves.

II. INTERFERENCE OF WATER-WAVES. 56

a circular dish, for example, a disturbance at the centre
propagates itself as a series of circular waves, which,
after reflection, again meet at the centre. If the point
of disturbance be a little way removed from the centre,
the interference of the direct and reflected waves
produces the magnificent chasing shown in the annexed

Fig. 10.

figure.^ The light reflected from such a surface yields
a pattern of extraordinary beauty. When the mercury
is slightly struck by a needle-point in a direction
concentric with the surface of the vessel, the lines of
light run round in mazy coils, interlacing and unravel-
ling themselves in a wonderful manner. When the vessel
is square, a splendid chequer-work is produced by the

' Copied from Wuber's Wdlenlehre.

56 ON LIGHT.

I-BCT.

cro-^sing of the direct and reflected waves. Thus, in
the case of wave-motion, the most ordinary causes give
rise to most exquisite effects. The words of your coun-
tryman, Emerson, are perfectly applicable here: —

' Thou can'st not ■wave thy staff in the air.
Or dip thy paddle in the lake,
But it carves the brow of beauty there,

And the ripples in rhymes the oars forsake.'

The most impressive illustration of the action of
waves on waves that I have ever seen occurs near
Niagara. For a distance of two miles, or thereabouts,
below the Falls, the river Niagara flows unruffled
through its excavated gorge. The bed subsequently
narrows, and the water quickens its motion. At the
place called the ' Whirlpool Eapids,' I estimated the
width of the river at 300 feet, an estimate confirmed
by the dwellers on the spot. When it is remembered
that the drainage of nearly half a continent is com-
pressed into this space, the impetuosity of the river's
escape tiirough this gorge may be imagined.

Two kinds of motion are here obviously active, a
motion of translation and a motion of undulation — the
race of the river through its gorge, and the great waves
generated by its collision with the obstacles in its way.
In the middle of the stream, the rush and tossing are
most violent ; at all events, the impetuous force of the
individual waves is here most strikingly displayed.
Vast pyramidal heaps leap incessantly from the river,
some of them with such energy as to jerk their summits
into the air, where they hang suspended as bundles of
liquid pearls, which, when shone upon by the sun, are of
indescribable beauty.

11. 'WHIRLPOOL EAPIDS" OF NIAGARA. 57

The first impression, and, indeed, the current ex-
planation of these Rapids is, that the central bed of the
river is cumbered with large boulders, and that the
jostling, tossing, and wild leaping of the Avater there
are due to its impact against these obstacles. A very
different explanation occurred to me upon the spot.
Boulders derived from the adjacent cliffs visibly cumber
the sides of the river. Against these the water rises
and sinks rhythmically but violently, large waves being
thus produced. On the generation of each wave there
is an immediate compounding of the wave-motion with
the river-motion. The ridges, w^hich in still water
would proceed in circular curves round the centre of
disturbance, cross the river obliquely, and the result is,
that at the centre waves commingle which have really
been generated at the sides. This crossing of waves
may be seen on a small scale in any gutter after rain ;
it may also be seen on simply pouring water from a
wude-lipped jug. Where crest and furrow cross each
other, the wave is annulled ; where furrow and
furrow cross, the river is ploughed to a greater depth ;
and where crest and crest aid each other, we have that
astonishing leap of the water which breaks the co-
hesion of the crests, and tosses them shattered into
the air. The phenomena observed at the Whirlpool
Rapids constitute, in fact, one of the grandest illustra-
tions of the principle of interference,

§ 5. Analogies of Sound and Light.

Thomas Young's fundamental discovery in optics
was that the principle of Interference was applicable to
light. Long prior to his time an Italian philosopher,

58 ON LIGHT. LBCT.

Grimaldi, had stated that under certain circumstances
two thin beams of light, each of which, acting singly,
produced a luminous spot upon a white wall, when caused
to act together, partially quenched each other and
darkened the spot. This was a statement of fundamental
significance, hut it required the discoveries and the
genius of Young to give it meaning. How he did so will
gradually become clear to you. You know that air is
compressible ; that by pressure it can be rendered more
dense, and that by dilatation it can be rendered more
rare. Properly agitated, a tuning-fork now sounds in
a manner audible to you all, and most of you know that
the air through which the sound is passing is parcelled
out into spaces in which the air is condensed, followed
by other spaces in which the air is rarefied. These
condensations and rarefactions constitute what we call
waves of sound. You can imagine the air of a room
traversed by a series of such waves, and you can imagine
a second series sent through the same air, and so related
to the first that condensation coincides with condensa-
tion and rarefaction with rarefaction. The consequence
of this coincidence would be a louder sound than that
produced by either system of waves taken singly. But
you can also imagine a state of things where the con-
densations of the one system fall upon the rarefactions
of the other system. In this case the two systems
would completely neutralize each other. Each of them
taken singly produces sound ; both of them taken
together produce no sound. Thus, by adding sound
to sound we produce silence, as Grimaldi in his experi-
ment produced darkness by adding light to light.

The analogy between sound and light here flashes
upon the mind. Yoimg generalized this observation. He

LONGITUDINAL WAVES.

discovered a multitude of similar cases, and determined
their precise conditions. On the assumption that
light was wave-motion, all his experiments on inter-
ference were explained ; on the assumption that light
was flying particles, nothing was explained. In the
time of Huyghens and Euler a medium had been
assumed for the transmission of the waves of light ;
but Newton raised the objection that, if light consisted
of the waves of such a medium, shadows could not

Fio. 11.

exist. The waves, he contended, would bend round
opaque bodies and produce the motion of light behind
them, as sound turns a comer, or as waves of water
wash round a rock. It was proved that the bending
round referred to by Newton actually occurs, but that
the inflected waves abolish each other by their mutual
interference. Young also discerned a fundamental
difference between the waves of light and those of
sound. Could you see the air tlirough which sound-

GO ON LIGHT. LECT.

waves are passing, you would observe every individual
particle of air oscillating to and fro in the direction of
propagation. Could you see the luminiferous ether,
you would also find every individual particle making a
small excursion to and fro ; but here the motion, like
that assigned to the water-particles above referred to,
would be across the line of propagation. The vibra-
tions of the air are longitudinal, those of the ether
transversal.

It is my desire that you should realize with clearness
the character of wave-motion, both in ether and in air.
And, with this view, I bring before you an experiment
wherein the air-particles are represented by small spots
of light (K 0, fig. 11). They are derived from a clean
spiral, drawn upon a circle of blackened glass (D), so
that when the circle rotates, the spots move in successive
pulses over the screen.' In this experiment you have
clearly set before you how the pulses travel incessantly
forward, while their component particles perform oscilla-
tions to and fro. This is the picture of a sound-wave, in
which the vibrations are longitudinal. By another glass
wheel (D, fig. 12) we produce an image of a transverse
wave (0 K), and here we observe the waves travelling in
succession over the screen, while each individual spot of
light performs an excursion to and fro across the line of
propagation.

Notice what foUows when the glass wheel is turned
very quickly. Objectively considered, the transverse
waves propagate themselves as before, but subjectively
the effect is totally changed. Because of the reten-
tion of impressions upon the retina, the spots of light

' The apparatus is constructed by that excellent acoustic mechanician,
M. Rudolf Konig, of Paris.

It.

TRANSVERSE WAVES.

simply describe a series of parallel luminous lines upon
the screen, the length of these lines marking the ampli-
tude of the vibration. Here the impression of wave-
motion has totally disappeared.

The most familiar illustration of the interference of
sound-waves is furnished by the beats produced by
two musical sounds slightly out of imison. When two
tuning-forks in perfect imison are agitated together

Fio. 12.

the two sounds flow without roughness, as if they
were but one. But, by attaching with wax to one
of the forks a little weight, we cause it to vibrate
more slowly than its neighbour. Suppose that one
of them performs 101 vibrations in the time re-
quired by the other to perform 100, and suppose that
at starting the condensations and rarefactions of both
forks coincide. At the 101st vibration of the quickest
fork they will again coincide, that fork at this point
having gained one whole vibration, or one whole wave-

62 ON LIGHT.

tBCT.

length upon the other. But a little reflection will
make it clear that, at the 50th vibration, the two forks
are in opposition ; here the one tends to produce a
condensation where the other tends to produce a rare-
faction ; by the united action of the two forks, therefore,
the sound is quenched, and we have a pause of silence.
This occurs where one fork has gained half a wave-
length upon the other. At the 101st vibration, as
already stated, we have coincidence, and, therefore,
augmented sound ; at the 1 50th vibration we have
again a quenching of the sound. Here the one fork is
three half-waves in advance of the other. In general
terms, the waves conspire when the one series is an
even number of half-wave lengths, and they destroy each
other when the one series is an odd number of half- wave
lengths in advance of the other. With two forks so cir-
cumstanced, we obtain those intermittent shocks of
sound separated by pauses of silence, to which we give
the name of beats. By a suitable arrangement, more-
over, it is possible to make one sound wholly extinguish
another. Along four distinct lines, for example, the
vibrations of the two prongs of a tuning-fork completely
blot each other out.'

The pitch of sound is wholly determined by tlie
rapidity of the vibration, as the intensity is by the am-
plitude. What pitch is to the ear in acoustics, colour
is to the eye in the undulatory theory of light. Though
never seen, the lengths of the waves of light have been
determined. Their existence is proved by their effects,
and from their effects also their lengths may be accu-
rately deduced. This may, moreover, be done in many

' Sound, 1st and 2nd eJ., Lecture VIL; and 3rd ed., Chap. VIII.
Longmans.

II. INTERFERENCE OF SOUND. 63

ways, and, when the dififerent determinations are com-
pared, the strictest harmony is found to exist between
them. This consensus of evidence is one of the strong-
est points of the undulatory theory. The shortest
waves of the visible spectrum are those of the extreme
violet ; the longest, those of the extreme red ; while the
other colours are of intermediate pitch or wave-length.
The length of a wave of the extreme red is such that it
would require 36,918 of them, placed end to end, to
cover one inch, while 64,631 of the extreme violet
waves would be required to span the same distance.

Now, the velocity of light, in round numbers, is
190,000 miles per second. Eeducing this to inches,
and multiplying the number thus found by 36,918, we
find the number of waves of the extreme red, in
190,000 miles, to be four hundred and fifty -^one millions
of millions. All these waves enter the eye, and strike
the retina at the back of the eye in one second. In a
similar manner, it may be found that the number of
shocks corresponding to the impression of violet is seven
hundred and eighty-nine millions of millions.

All space is filled with matter oscillating at such
rates. From every star waves of these dimensions
move, with the velocity of light, like spherical shells
in all directions. And in ether, just as in water, the
motion of every particle is the algebraic sum of ail the
separate motions imparted to it. One motion does not
blot out the other; or, if extinction occur at one point,
it is strictly atoned for, by augmented motion, at some
other point. Every star declares by its light its un-
damaged individuality, as if it alone had sent its thrill
through space.

64 ON LIGHT. i-bct.

§ 6. Interference of Light.

The principle of interference, as proved by Young,
applies to the waves of light as it does to the waves
of water and the waves of sound. And the conditions
of interference are the same in all three. If two
series of light-waves of the same length start at the
same moment from a common origin (say A, fig. 13),
crest coincides with crest, sinus with sinus, and the two

Fig, 13.

systems blend together to a single system (A m n) of
double amplitude. If both series start at the same
moment, one of them being, at starting, a whole wave-
length in advance of the other, they also add them-
selves together, and we have an augmented luminous
effect. The same occurs when the one system of waves
is any even number of semi-undulations in advance of
the other. But if the one system be half a wave-length
(as at A' a', fig. 14), or any odd number of half wave-
lengths in advance, then the crests of the one fall upon
the sinuses of the other ; the one system, in fact, tends
to lift the particles of ether at the precise places where
the other tends to depress them ; hence, through the
joint action of these opposing forces (indicated by the
arrows) the light-ether remains perfectly still. This

n. INTERFERENCE OF LIGHT. 65

stillness of the ether is what we call darkness, which
corresponds with a dead level in the case of water.

Fig. 14.

It was said in our first lecture, with reference to
the colours produced by absorption, that the function
of natural bodies is selective, not creative ; that they ex-
tinguish certain constituents of the white solar light,
and appear in the colours of the unextinguished light.
It must at once flash upon your minds that, inasmucli as
we have in interference an agency by which light may
be self-extinguished, we may have in it the conditions
for the production of colour. But this would imply that
certain constituents are quenched by interference, while
others are permitted to remain. This is the fact ; and
it is entirely due to the difTerence in the lengths of the
waves of light.

§ 7. Colours of thin Films. Observations of Boyle

and Hooke.

This subject may be illustrated by the class of
phenomena which first suggested the undulatory theory
to the mind of Hooke. These are the colours of thin
transparent films of all kinds, known as the colours
of thin plates. In this relation no object in the world
possesses a deeper scientific interest than a common
soap-bubble. And here let me say emerges one of the
difliculties which the student of pure science encounters
in the presence of practical ' communities like those of

66 ON LIGHT. tECT.

America and England ; it is not to be expected that
such communities can entertain any profound sympathy
with labours which seem so far removed from the domain
of practice as many of the labours of the man of science
are. Imagine Dr. Draper spending his days in blowing
soap-bubbles and in studying their colom'S ! Would
you show him the necessary patience, or grant him the
necessary svipport ? And yet be it remembered it was
thus that minds like those of Boyle, Newton and Hooke
were occupied ; and that on such experiments has been
founded a theory, the issues of which are incalculable.
I see no other way for you, laymen, than to trust the
scientific man with the choice of his inquiries ; he stands
before the tribunal of his peers, and by their verdict on
his labours you ought to abide.

Whence, then, are derived the colours of the soap-
bubble ? Imagine a beam of white light impinging
on the bubble. When it reaches the first surface of the
film, a known fraction of the light is reflected back.
But a large portion of the beam enters the film, reaches
its second surface, and is again in part reflected. The
waves from the second surface thus turn back and hotly
pursue the waves from the first surface. And, if the
thickness of the film be such as to cause the necessary
retardation, the two systems of waves interfere with each
other, producing augmented or diminished light, as the
case may be.

But, inasmuch as the waves of light are of different
lengths, it is plain that, to produce self- extinction in
the case of the longer waves, a greater thickness of
film is necessary than in the case of the shorter ones.
Different colours, therefore, must appear at different
thicknesses of the film.

COLOURS OF THIN PLATES. 67

Take with you a little bottle of spirit of turpentine,
and pour it into one of your country ponds. You
will then see the flashing of those coloiurs over tlie
surface of the water. On a small scale we produce them
thus : A common tea-tray is filled with water, beneath
the surface of which dips the end of a pipette. A beam
of light falls upon the water, and is reflected by it to
the screen. Spirit of turpentine is poured into the
pipette ; it descends, issues from the end in minute
drops, which rise in succession to the surface. On
reaching it, each drop spreads suddenly out as a film,
and glowing colours immediately flash forth upon the
screen. The colours change as the thickness of the
film changes by evaporation. They are also arranged
in zones, in consequence of the gradual diminution of
thickness from the centre outwards.

Any film whatever will produce these colours. The
film of air between two plates of glass squeezed together,
exhibits, as shown by Hooke, rich fringes of colour. A
particularly fine example of these fringes is now before
you. Nor is even air necessary ; the rupture of optical
continuity suffices. Smite with an axe the black, trans-
parent ice — black, because it is pure and of great depth
— under the moraine of a glacier ; you readily produce in
the interior flaws which no air can reach, and from these
flaws the colours of thin plates sometimes break like fire.
But the source of most historic interest is, as already
stated, the soap-bubble. With one of these mixtures
employed by the eminent blind philosopher Plateau in
his researches on the cohesion figures of thin films, we
obtain in still air a bubble ten or twelve inches in
diameter. You may look at the bubble itself, or you
may look at its projection upon the screen ; rich colours

68 ON LIGHT. i-ECT.

arranged in zones are, in both cases, exhibited. Ren-
dering the beam parallel, and permitting it to impinge
upon the sides, bottom, and top of the babble, gorgeous
fans of colour overspread the screen, rotating as the
beam is carried round the circumference of the bubble.
By this experiment the internal motions of the film are
also strikingly displayed.

Not in a moment are great theories elaborated : the
facts which demand them are first called into pro-
minence by observant minds ; then, to the period of
observation, succeeds a period of pondering and of
tentative explanation. By such efforts the human
mind is gradually prepared for the final theoretic
illumination. The colours of thin plates, for ex-
ample, occupied the attention of the celebrated Eobert
Boyle. In his ' Experimental History of Colours ' he
contends against the schools which affirmed that colour
was ' a penetrative quality that reaches to the inner-
most parts of the object,' adducing opposing facts.
' To give you a first instance,' he says, ' I shall need
but to remind you of what I told you a little after
the beginning of this essay, touching the blue and red
and yellow that may be produced upon a piece of
tempered steel ; for these colours, though they be very
vivid, yet if you break the steel they adorn they will
appear to be but superficial.' He then describes, in
phraseology which shows the delight he took in his
work, the following beautiful experiment : —

* We took a quantity of clean lead, and melted it
with a strong fire, and then immediately pouring it out
into a clean vessel of convenient shape and matter
(we used one of iron, that the great and sudden heat
might not injure it), and then carefully and nimbly

11. BOYLE'S OBSERVATIONS. 69

taking off the scum that floated on the top, we per-
ceived, as we expected, the smooth and glossy surface
of the melted matter to be adorned with a very glorious
colour, which being as transitory as deliglitful, did
almost immediately give place to another vivid colour,
and that was as quickly succeeded by a third, and this,
as it were, chased away by a fourth ; and so these wonder-
fully vivid colours successively appeared and vanished
till the metal ceasing to be hot enough to hold any
longer this pleasing spectacle, the colours that chanced
to adorn the surface when the lead thus began to cool
remained upon it, but were so superficial that how
little soever we scraped off the surface of the lead, we
did, in such places, scrape off all the colour.' ' Tliese
things,' he adds, 'suggested to me some thoughts or
ravings which I have not now time to acquaint you
with.' 1

He extends his observations to chemical essential
oils and spirit of wine, ' which being shaken till they
have good store of bubbles, those bubbles will (if atten-
tively considered) appear adorned with various and
lovely colours, which all immediately vanish upon the
retrogressing of the liquid Avhich affords these bubbles
their skins into the rest of the oil.' He also refers to
the colours of glass films. ' I have seen one that was
skilled in fashioning glasses by the help of a lamp blow-
ing some of them so strongly as to burst them ; where-
upon it was found that the tenacity of the metal was
such that before it broke it suffered itself to be reduced
into films so extremely thin that they constantly showed
upon their surfaces the varying colours of the rainbow.''

' Boyle's Works, Birch's edition, p. 675. '•' Page 743.

70 ON LIGHT. I.ECT.

Subsequent to Boyle the colours of thin plates
occupied the attention of the celebrated Robert Hooke,
in whose writings we find a dawning of the undulatory
theory. He describes with great distinctness the colours
obtained with thin flakes of ' Muscovy glass ' (talc), also
those surrounding flaws in crystals where optical con-
tinuity is destroyed. He shows very clearly the de-
pendence of the colour upon the thickness of the film,
and proves by microscopic observation that plates of a
uniform thickness yield uniform colours. ' If,' he says,
'you take any small piece of the Muscovy glass, and
with a needle, or some other convenient instrument,
cleave it oftentimes into thinner and thinner laminae,
you shall find that until you come to a determinate
thinness of them they shall appear transparent and
colourless ; but if you continue to split and divide them
further, you shall find at last that each plate shall
appear most lovely tinged or imbued with a determinate
colour. If, further, by any means you so flaw a pretty
thick piece that one part begins to cleave a little from
the other, and between these two there be gotten some
pellucid medium, those laminated or pellucid bodies
that fill that space shall exhibit several rainbows or
coloured lines, the colours of which will be disposed
and ranged accordino: to the various thicknesses of the
several parts of the plate.' He then describes fully
and clearly the experiment with pressed glasses already
referred to : —

' Take two small pieces of ground and polished look-
ing-glass plate, each about the bigness of a shilling :
take these two dry, and with your forefingers and
thumbs press them very hard and close together, and
you shall find that when they approach each other

II. hooke's observations. 71

very near there will appear several irises or coloured
lines, in the same manner almost as in the Muscovy
glass ; and you may very easily change any of the
colours of any part of the interposed body by pressing
the plates closer and harder together, or leaving them
more lax — that is, a part which appeared coloured with
a red may be presently tinged with a yellow, blue,
green, purple, or the like. Any substance,' he says,
' provided it be thin and transparent, will sho-\;r these
colours.' Like Boyle, he obtained them with glass
films ; he also ' produced them with bubbles of pitch,
rosin, colophony, turpentine, solutions of several gums,
as gum arabic in water, any glutinous liquor, as wort,
wine, spirit of wine, oyl of turpentine, glare of snails,
&c.'

Hooke's writings show that even in his day the idea
that both light and heat are modes of motion had taken
possession of many minds. ' First,' he says, ' that all kind
offieiy burning bodies have their parts in motion I think
will be very easily granted me. That the spark struck
from a flint and steel is in rapid agitation I have else-
where made probable ; . . . . that heat argues a motion
of the internal parts is (as I said before) generally

granted ; and that in all extremely hot shining

bodies there is a very quick motion that causes light,
as well as a more robust that causes heat, may be
argued from the celerity wherewith the bodies are dis-
solved. Next, it must be a vibrative motion^ His
reference to the quick motion of light and the more
robust motion of heat is a remarkable stroke of sagacity ;
but Hooke's direct insight is better than his reasoning ;
for the proofs he adduces that light is 'a vibrating
motion' have no particular bearing upon the question.

72 ON LIGHT. lECT.

Still the Undulatory Theory was undoubtedly dawn-
ing upon the mind of this remarkable man. In endea-
vouring to account for the colours of thin plates, he again
refers to the relation of colour to thickness : he dwells
upon the fact that the film which shows these colours
must be transparent, proving this by showing that
however thin an opaque body was rendered no colours
were produced. ' This,' he says, ' I have often tried by
pressing a small globule of mercury between two smooth
plates of glass, whereby I have reduced that body to a
much greater thinness than was requisite to exhibit
the colours with a transparent body.' Then follows
the sagacious remark that to produce the colours
' there must be a considerable reflecting body adjacent
to the under or further side of the lamina or plate :
for this I always found, that the greater that reflection
was, the more vivid were the appearing colours. From
which observations,' he continues, ' it is most evident,
that the reflection from the under or further side of
the body is the principal cause of the production of
these colours.^

He draws a diagram, correctly representing the
reflection at the two surfaces of the film ; but here
his clearness ends. He ascribes the colours to a
coalescence or confusion of the two reflected pulses;
the principle of interference being unknown to him, he
could not go further in the way of explanation.

§ 8. Newton's Rings. Relation of Colour to
Thickness of Film.

In this way, then, by the active operation of difierent
minds, facts are observed, examined, and the precise

n. NEWTON'S OBSERVATIONS. 73

conditions of their appearance determined. All such
work in science is the prelude to other work ; and the
efforts of Boyle and Hooke cleared the way for the
optical career of Newton. He conquered the difficulty
which Hooke had found insuperable, and determined by
accurate measurements the relation of the thickness of
the film to the colour of displays. In doing this his
first care was to obtain a film of variable and calculable
depth. On a plano-convex glass lens (D B E, fig. 15)

Fig. 15.

of very feeble curvature he laid a plate of glass (AC)
with a plane surface, thus obtaining a film of air of
gradually increasing depth from the point of contact (B)
outwards. On looking at the film in monochromatic
light he saw, with the delight attendant on fulfilled
prevision, surrounding the place of contact a series
of bright rings separated from each other by dark
ones, and becoming more closely packed together as
the distance from the point of contact augmented
(as in fig. 16). When he employed red light, his rings
had certain diameters ; when he employed blue light,
the diameters were less. In general terms, the more
refrangible the light the smaller were the rings.
Causing his glasses to pass through the spectrum
from red to blue, the rings gradually contracted ;
when tlie passage was from blue to red, the
rings expanded. This is a beautiful experiment, and
appears to have given Newton tho most lively satis-
faction. "NVlien white light fell upon tlie glasses,

ON LIGHT.

ij:ct.

inasmuch as the colours were not superposed, a series
of iris-coloured circles was obtained. A magnified
image of Newton^s rings is now before you, and, by
employing in succession red, blue, and white light, we
obtain all the effects observed by Newton. You notice
that in monochromatic light the rings run closer and

Fig. 15.

closer together as they recede frona the centre, lliis is
due to the fact that at a distance the film of air thickens
more rapidly than near the centre. When white light
is employed, this closing up of the rings causes the
various eoloura to be superposed, so that after a certain
thickness they are blended together to white light, the
rings then ceasing altogether. It needs but a moment's
reflection to understand that the colours of thin plates
are never unmixed or monochromatic.

Newton compared the tints obtained in this way
with the tints of his soap-bubble, and he calculated the
corresponding thickness. How he did this may be thus
made plain to you : Suppose the water of the ocean to

• I. NEWTON'S KINGS. 75

be absolutely smooth ; it would then accurately repre-
sent the earth's curved surface. Let a perfectly hori-
zontal plane touch the surface at any point. Knowing
the earth's diameter, any engineer or mathematician
in this room could tell you how far the sea's surface
will lie below this plane, at the distance of a yard, ten
yards, a hundred yards, or a thousand yards from the
point of contact of the plane and the sea. It is common,
indeed, in levelling operations, to allow for the curva-
ture of the earth. Newton's calculation was precisely
similar. His plane glass was a tangent to his curved
one. From its refractive index and focal distance he
determined the diameter of the sphere of which his
curved glass formed a segment, he measured the dis-
tances of his rings from the place of contact, and he
calculated the depth between the tangent plane and
the curved surface, exactly as the engineer would
calculate the distance between his tangent plane and
the surface of the sea. The wonder is, that, where
such infinitesimal distances are involved, Newton, with
the means at his disposal, could have worked with such
marvellous exactitude.

To account for these rings was the greatest difficulty
that Newton ever encountered. He quite appreciated
the difficulty. Over his eagle-eye there was no film — no
vagueness in his conceptions. At the very outset his
theory was confronted by the question, Why, when a
beam of light is incident on a transparent body, are
some of the light-particles reflected and some trans-
mitted ? Is it that there are two kinds of particles,
the one specially fitted for transmission and the other
for reflection ? This cannot be the reason ; for, if
we allow a beam of light wliich has been reflected

76 ON LIGHT. LECT.

from one piece of glass to fall upon another, it, as a
general rule, is also divided into a reflected and a trans-
mitted portion. The particles once reflected are not
always reflected, nor are the particles once transmitted
always transmitted. Newton saw all this ; he knew he
had to explain why it is that the self-same particle is at
one moment reflected and at the next moment trans-
mitted. It could only be through some change in the
condition of the particle itself. The self-same par-
ticle, he aflarmed, was affected by ' fits ' of easy trans-
mission and reflection.

§ 9. Theory of ' Fits ' applied to NewtorCs Rings.

If you are willing to follow me in an attempt to
reveal the speculative groundwork of this theory of
fits, the intellectual discipline will, I think, repay you
for the necessary effort of attention. Newton was chary
of statinof what he considered to be the cause of the
fits, but there can hardly be a doubt that his mind
rested on a physical cause. Nor can there be a doubt
that here, as in all attempts at theorising, he was
compelled to fall back upon experience for the materials
of his theory. Let us attempt to restore his course of
thought and observation. A magnet would furnish
him with the notion of attracted and repelled poles ;
and he who habitually saw in the visible an image of
the invisible would naturally endow his light-particles
with such poles. Turning their attracted poles towards
a transparent substance, the particles would be sucked
in and transmitted ; turning their repelled poles, they
would be driven away or reflected. Thus, by the
ascription of poles, the transmission and reflection of

II. THEORY OF 'FITS.' 77

tlie self-same particle at different times might be ac-
counted for.

Regard these rings of Newton as seen in pure red
light: they are alternately bright and dark. The film
of air corresponding to the outermost of them is not
thicker than an ordinary soap-bubble, and it becomes
thinner on approaching the centre ; still Newton, as I
have said, measured the thickness corresponding to
every ring, and showed the difference of thickness be-
tween ring and ring. Now, mark the result. For the
sake of convenience, let us cull the thickness of the film
of air corresponding to the first dark ring d ; then
Newton found the distance corresponding to the second
dark ring 2 d ; the thickness corresponding to the third
dark ring 3 d ; the thickness corresponding to the tenth
dark ring 10 d, and so on. Surely there must be some
hidden meaning in this little distance d, which turns
up so constantly ? One can imagine the intense interest
with which Newton pondered its meaning. Observe
the probable outcome of his thought. He had endowed
his light-particles with poles, but now he is forced to
introduce the notion of periodic recunxnce. Here his
power of transfer from the sensible to the subsensible
would render it easy for him to suppose the light-par-
ticles animated, not only with a motion of translation,
but also with a motion of rotation. Newton's astrono-
mical knowledge rendered all such conceptions familiar
to him. The earth has such a double motion. In the
time occupied in passing over a million and a half of
miles of its orbit — that is, in twenty-four hours — our
planet performs a complete rotation, and, in the time
required to pass over the distance d, Newton's light-
particle must be supposed to perform a complete r-ita-
6

78 ON LIGHT. LBCT.

tion. True, the light-particle is smaller than the planet,
and the distance d, instead of being a million and a
half of miles, is a little over the ninety thousandth of
an inch. But the two conceptions are, in point of in-
tellectual quality, identical.

Imagine, then, a particle entering the film of air
where it possesses this precise thickness. To enter tlie
film, its attracted end must be presented. Within the
film it is able to turn once completely round ; at the
other side of the film its attracted pole will be again
presented ; it will, therefore, enter the glass at the op-
posite side of the film and be lost to the eye. All round
the place of contact, wherever the film possesses this
precise thickness, the light will equally disappear — we
shall therefore have a ring of darkness.

And now observe how well this conception falls in
with the law of proportionality discovered by Newton.
When the thickness of the film is 2 d, the particle has
time to perform two complete rotations within the
film ; when the thickness is 3 (i, three complete rota-
tions ; when 10 c?, ten complete rotations are per
formed. It is manifest that in each of these cases, on
arriving at the second surface of the film, the attracted
pole of the particle will be presented. It will, there-
fore, be transmitted ; and, because no light is sent to
the eye, we shall have a ring of daikness at each of
these places.

The bright rings follow immediately from the same
conception. They occur between the dark rings, tlie
thicknesses to which they correspond being also inter-
mediate between those of the dark ones. Take tlie case
of the first bright ring. The thickness of the film is
\d\ in this interval the rot;iting particle can perfoim

11. APPLICATION OF THEORY. 79

only half a rotation. When, therefore, it reaches the
second surface of the film, its repelled pole is pre-
sented ; it is, therefore, driven back and reaches the
eye. At all distances round the centre correspond-
ing to this thickness the same effect is produced, and
the consequence is a ring of brightness. The other
bright rings are similarly accounted for. At the second
one, where the thickness is 1^ d, a. rotation and a half
is performed ; at the third, two rotations and a half;
and at each of these places the particles present their
repelled poles to the lower surface of the film. They
are therefore sent back to the eye, and produce there
the impression of brightness. This analysis, though
involving difficulties when closely scrutinised, enables
us to see how the theoiy of fits may have grown into
consistency in the mind of Newton.

It has been already stated that the Emission Theory
assigiied a gi'eater velocity to light in glass and water
ihan in air or stellar space; and that on this point
it was at direct issue with the theory of undulation,
which makes the velocity in air or stellar space greater
than in glass or water. By an experiment proposed by
Arago, and executed with consummate skill by Fou-
ca\dt and Fizeau, this question was brought to a crucial
test, and d(>cided in favour of the theory of undula-.

tion.

In the present instance also the two theories are at

variance. Newton assumed that the action which pro-
duces the alternate bright and dark rings took place at a
single surface ; that is, the second surface of the film.
The undulatory theory affirms that the rings are caused
by the interference of waves reflected from both sur-
faces. This als(» has l»een demonstrated by experiment.

80 ON LIGHT. LBCT.

By a proper arrangement, as we shall afterwards learn,
we may abolish reflection from one of the surfaces of
the film, and when this is done the rings vanish alto-
gether.

Rings of feeble intensity are also formed by trans-
7)iitted light. These are referred by the undulatory
theory to the interference of waves which have passed
directly through the film, with others which have suf-
fered two reflections within the film. They are thus
completely accounted for.

§ 10. The Diffraction of Light.

Newton's espousal of the emission theory is said to
have retarded scientific discovery. It might, however,
be questioned whether, in the long run, the errors
of great men have not really their effect in ren-
dering intellectual progress rhythmical, instead of
permitting it to remain uniform, the ' retardation ' in
each case being the prelude to a more impetuous
advance. It is confusion and stagnation, rather than
error, that we ought to avoid. Thus, though the undu -
latory theory was held back for a time, it gathered
strength in the interval, and its development within
the last half century has been so rapid and trium-
pliant as to leave no rival in the field. We have now
to turn to the investigation of new classes of pheno-
mena, of which it alone can render a satisfactory
account.

Newton, who was familiar with the idea of an ether,
and who introduced it in some of his speculations,
objected, as already stated, that if light consisted of
waves shadows could not exist ; for that the waves
would bend round the edges of opaque bodies and

II. DIFFRACTION. 8 1

agitate the ether behind them. He was right in
affirming that this bending ought to occur, but wrong
in supposing that it does not occur. The bending is
real, though in all ordinary cases it is masked by the
action of interference. This inflection of the light
receives the name of Diffraction.

To study the phenomena of diffraction it is necessary
that our source of light should be a physical point,
or a fine line ; for when luminous surfaces are employed,
the waves issuing from different points of the surface
obscure and neutralize each other. A point of light of
high intensity is obtained by admitting the parallel rays
of the sun through an aperture in a window-shutter, and
concentrating the beam by a lens of sliort focus. The
small solar image at the focus constitutes a suitable
point of light. The image of the sun formed on the
convex surface of a glass bead, or of a watch-glass
blackened within, though less intense, will also answer.
An intense line of light is obtained by admitting the
sunlight through a slit, and sending it through a
strong cylindrical lens. The slice of light is contracted to
a physical line at the focus of the lens. A glass tube
blackened within and placed in the light, reflects from
its surface a luminous line wliich, thougli less intense,
also answers the purpose.

In the experiment now to be described a vertical
slit of variable width is placed in front of the electric
lamp, and this slit is looked at from a distance through
another vertical slit, also of variable aperture, and held
in the hand.

The light of the lamp being, in the first place,
rendered monochromatic by placing a pure red glass in
front of the slit, when the eye is placed in the straiglit

ON LIGHT.

LKCT.

line drawn through both slits an extraordinary appear-
ance (shown in fig. 17) is observed. Firstly, the slit
in front of the lamp is seen as a vivid rectangle of light,
but right and left of it is a long series of rectangles,
decreasing in vividness, and separated from each other
by intervals of absolute darkness.

The breadth of these bands is seen to vary with the
width of the slit held before the eye. When the slit
is widened the bands become narrower, and they crowd
more closely together ; when the slit is narrowed, the

Fig. 17.

individual bands widen and also retreat from each otlier,
leaving between them wider spaces of darkness than
before.

Leaving everything else unchanged, let a blue glass
or a solution of ammonia-sulphate of copper, which
gives a very pure blue, be placed in the path of the
light. A series of blue bands is thus obtained, exactly
like the former in all respects save one ; the blue
rectangles are narrower, and they are closer together
than the red ones.

If we employ colours of intermediate refrangibilities,
which we may do by causing the different colours of a
spectrum to shine through the slit, we obtain bands of
colour intermediate in width and occupying interme-

II.

DIFFRACTION BANDS.

diate positions between those of the red and blue. The
aspect of the bands in red, green, and violet light is
represented in fig. 18. Wlien white light, therefore,
passes through the slit the various colours are not
superposed, and instead of a series of monochromatic
bands, separated from each other by intervals of dark-
ness, we have a series of coloured spectra placed side
by side. When the distant slit is illuminated by a
candle flame, instead of the more intense electric light,
or when a distant platinum wire raised to a white heat

Fig. 18.

by an electric current is employed, substantially the
same effects are observed.

§ 11. Appiication of the Wave-theoinj to tfte Phe-
nomena of Diffraction.

Of these and of a multitude of similar effects the
Emission Theory is incompetent to offer any satisfactory
explanation. Let us see how they are accounted forl)y
the Theory of Undulation.

And here, with tlie view of reaching absolute clear-
ness, I must make an appeal to tliat facidty tin'
importance of which I have dwelt upon so earnestly

84 ON LIGHT lect.

here and elsewhere — the faculty of imagiuatioD. Figure
yourself upon the sea-shore, with a well-formed wave
advancing. Take a line of particles along the front of
the wave, all at the same distance below the crest ; they
are all rising in the same manner and at the same rate.
Take a similar line of particles on the back of the wave,
they are all falling in the samie manner and at the
same rate. Take a line of particles along the crest,
they are all in the same condition as regards the motion
of the wave. The same is true for a line of particles
along the furrow of the wave.

The particles referred to in each of these cases re-
spectively being in the same condition as regards the
motion of the wave, are said to be in the same phase
of vibration. But if you compare a particle on the
front of the wave with one at the back ; or more
generally, if you compare together any two particles
not occupying the same position in the wave, tlieir
conditions of motion not being the same, they are said
to be in different phases of vibration. If one of the
particles lie upon the crest, and the other on the furrow
of the wave, then, as one is about to rise and the other
about to fall, they are said to be in opposite phases of
vibration.

There is still another point — and it is one of the
utmost importance as regards our present subject — to
be cleared up. Let 0 (fig. 19) be a point in still
water which, when disturbed, produces a series of
circular waves : the disturbance necessary to produce
these waves is simply an oscillation up and down of the
point 0. Let 7n nhe the position of the ridge of one
of the waves at any moment, and on' n' its position a
second or two afterwards. Now every particle of water.

n. PRINCIPLE OF HUYGHENS. 85

as the wave passes it, oscillates, as we have learned, up
and down. If, then, this oscillation be a sufficient
origin of wave-motion, then each distinct particle of

Fig. 19.

the wave m n ought to give birth to a series of circular
waves. This is the important point up to which I
wished to lead you. Every particle of the wave m n does
act in this way. Taking each particle as a centre, and
surrounding it by a circular wave with a radius equal
to the distance between m n and m/ n', the coalescence
of all these little waves would build up the larger
ridge m' n' exactly as we find it built up in nature.
Here, in fact, -vve resolve the wave-motion into its
elements, and having succeeded in doing this we shall
have no great difficulty in applying our knowledge to
optical phenomena.

Now let us return to our slit, and, for the sake of
simplicity, we will first consider the case of monochro-
matic light. Conceive a series of waves of ether
advancing from the first slit towards the second, and
finally filling the second slit. When each wave passes
through the latter it not only pursues its direct course

ON LIGHT.

LBCT.

to the retina, but diverges right and left, tending to
throw into motion the entire mass of the ether
behind the slit. In fact, as already explained, evei^
'point of the ivave which fills the slit is itself a centre
of a new wave-system, which is transmitted in all
directions through the ether behind the slit. This is
the celebrated principle of Huyghens : we have now
to examine how these secondary waves act upon each
other.

Let us first regard the central band of the series. Let
A P (fig. 20) be the width of the aperture held before the

eye, grossly exaggerated of course, and let the dots across
the aperture represent ether particles, all in the same
phase of vibration. Let E T represent a portion of the
retina. From 0, in the centre of the slit, let a per-
pendicular 0 E be imagined drawn upon the retina. The
motion communicated to the point K will then be the
sum of all the motions emanating in this direction
from the ether particles in the slit. Considering the
extreme narrowness of the aperture, we may, without
sensible error, regard all points of the wave A P as
equally distant from K. No one of the parti::!

n. EXPLANATION OF BANDS. 87

waves lags sensibly behind the others : hence, at R, and
in its immediate neighbourhood, we have no sensible
reduction of the light by interference. This undi-
minished light produces the brilliant central band of
the series.

Let us now consider those waves which diverge
laterally behind the slit. In this case, the waves from
the two sides of the slit have, in order to converge
upon the retina, to pass over unequal distances. Let
A P (fig. 21) represent, as before, the width of tbe

second slit. We have now to consider the action of
the various parts of the wave A P upon a point R' of
the retina, not situated in the line joining the slits.
Let us take the particular case in which tlie difference
in path from the two marginal points A, P, to the retina
is a whole wave-length of the red light; liow musttliis
difference affect the final illumination of the retina ?

Let us fix our attention upon the particular oblique
line that passes through the centre 0 of the tlit to the
retina at K'. The difference of path between the waves
which pass along this line and those from the two

88 ON LIGHT. LECT.

margins is, in the case here supposed, lialf a wave-
length. Make e R' equal to P E', join P and e, and
draw 0 d parallel to P e. A e is then the length of a
wave of light, while A c? is half a wave-length. Now
the least reflection will make it clear that not only
is there discordance between the central and marginal
waves, but that every line of waves such as x R', on
the one side of 0 R', finds a line x' R' upon the other
side of 0 R, from which its path differs by half an
undulation, with which, therefore, it is in complete
discordance. The consequence is that the light on the
one side of the central line will completely abolish the
light on the other side of that line, absolute darkness
being the result of their coalescence. The first dark
interval of our series of bands is thus accounted for.
It is produced by an obliquity of direction which causes
the paths of the marginal waves to be a whole wave-
length different from each other.

When the diflference between the paths of the mar-
ginal waves is half a wave-length, a partial destruction
of the light is effected. The luminous intensity corre-
sponding to this obliquity is a little less than one-half
— accurately 0*4 — that of the undiffracted light.

If the paths of the marginal waves be three semi-
undulations different from each other, and if the whole
beam be divided into three equal parts, two of these
parts will, for the reasons just given, completely neu-
tralize each other, the third only being effective.
Corresponding, therefore, to an obliquity which pro-
duces a difference of three semi-undulations in the
marginal waves, we have a luminous band, but one of
considerably less intensity than the undiffracted cen-
tral band.

Ti. DIFFRACTION THROUGH SEVERAL APERTURES. 89

With a marginal difference of path of foiir semi-
undulations we have a second extinction of the entire
beam, because here the beam can be divided into four
equal parts, every two of which quench each other.
A second space of absolute darkness will therefore
correspond to the obliquity producing this difference.
In this way we might proceed further, the general
result being that, whenever the direction of wave-
motion is such as to produce a marginal difference of
path of an even number of semi-undulations, we have
complete extinction ; while, when the marginal dif-
ference is an odd number of semi-undulations, we have
only partial extinction, a portion of the beam remaining
as a luminous band.

A moment's reflection will make it plain that the
wider the slit the less will be the obliquity of direction
needed to produce the necessary difference of path. With
a wide slit, therefore, the bands, as observed, will be closer
together than with a narrow one. It is also plain that
the shorter the wave, the less will be the obliquity re-
quired to produce the necessary retardation. The maxima
and minima of violet light must therefore fall nearer to
the centre than the maxima and minima of red light.
The maxima and minima of the other colours fall
between these extremes. In this simple way tlie
undulatory theory completely accounts for the extra-
ordinary appearance above referred to.

When a slit and telescope are used, instead of the
slit and naked eye, the effects are magnified and ren-
dered more brilliant. Looking, moreover, through a
properly adjusted telescope with a small circular aper-
ture in front of it, at a distant point of light, the point
is seen encircled by a series of coloured bands. If

ON LIGHT.

i.F.cr.

taonochromatic light be used, these bands are simply
bright and dark, but with white light the circles display
iris-colours. If a slit be shortened so as to form a
square aperture, we have two series of spectra at righ'
angles to each other. The effects, indeed, are capabL
of endless variation by varying the size, shape, and
number of the apertures througli which the point of

FiQ. 22.

light is observed. Througli two square apertures, with
their corners touching each other as at A, Schwerd
observed the appearance shown in fig. 22. Adding two
others to them, as at B, he observed the appearance
represented in fig. 23. The position of every band
of light and shade in such figures has been calculated
from theory by Fresnel, Fraunhofer, Herschel, Schwerd,
and others, and completely verified by experiment.

SCKWERD'S OBSERVATIONS.

Yoiir eyes could not tell you with j^reiiter certainty of
the existence of these bands than the theoretic calcu-
lation.

The street-lamps at night, looked at through the
meshes of a handkerchief, show diffraction phenomena.
The diffraction effects obtained in looking through a
bird's feathers are, as shown by Schwerd, very brilliant.

Fio. 23.

€J^tepinafeiDB3

^JDrwA^MLm

The iridescence of certain Alpine clouds is also an effect
of diffraction which may be imitated by the spores
of Lycopodium. When shaken over a glass plate
these spores cause a point of light, looked at througli
the dusted plate, to be surroimded by coloured circles,
which rise to actual splendour when the light becoes
intense. Shaken in the air the spores produce the same

92 ON LIGHT.

LEcr.

effect. The diffraction phenomena obtained during
the artificial precipitation of clouds from the vapours
of various liquids in an intensely illuminated tube are
exceedingly fine.

One of the most interesting cases of diffraction by
small particles that ever came before me was that of
an artist whose vision was disturbed by vividly-coloured
circles. He was in great dread of losing his sight ;
assigning as a cause of his increased fear that the
circles were becoming larger and the colours more
vivid. I ascribed the colours to minute particles in
the humours of the eye, and ventured to encourage
him by the assurance that the increase of size and
vividness on the part of the circles indicated that the
diffracting particles were becoming smaller^ and that
they might finally be altogether absorbed. The predic-
tion was verified. It is needless to say one word on the
necessity of optical knowledge in the case of the prac-
tical oculist.

Without breaking ground on the chromatic pheno-
mena presented by crystals, two other sources of colour
may be mentioned here. By interference in the earth's
atmosphere the light of a star, as shovm by Arago, is
self-extinguished, the twinkling of the star and the
changes of colour which it undergoes being due to this
cause. Looking at such a star through an opera-
glass, and shaking the glass so as to cause the
image of the star to pass rapidly over the retina,
you produce a row of coloured beads, the spaces
between which correspond to the periods of extinction.
Fine scratches drawn upon glass or polished metal
reflect the waves of light from their sides; and
some, being reflected from opposite sides of the

II. COLOUES OF STEIATED SUEFACES. 93

snme scratch, interfere with and quench each other.
But the obliquity of reflection which extinguishes
the shorter waves docs not extinguish tlie longer
ones, hence the phenomena of colour. These are
called the colours of striated surfaces. They are
beautifully illustrated by mother-of-pearl. This shell
is composed of exceedingly thin layers, which, when cut
across by the polishing of the shell, expose their edges
and furnish the necessary small and regular grooves.
The most conclusive proof that the colours are due to
the mechanical state of the siurface is to be found in
the fact, established by Brewster, that by stamping the
shell carefully upon black sealing-wax, we transfer the
grooves, and produce upon the wax the colours of
mother-of-pearl.

ON LIGHT.

LE'.T

(II.

LECTURE IIL

BKLATION OF THEOKI>;S TO EXPEKIKNCB — OUIfilN OF TIIK NOTIHN OF TlIK
ATTKACriON OF GKAVITATION — KOTION OF POLARITY, IU)W (iHNKllATED
— ATOMIC POLARITY — STRUCTURAL ARRANiJEMENTS DUE TO POLARITT
• — AHCHITECTURE OF CRYSTALS CONSIDERED AS AN INTRODUCTION TO

TIIEIE ACTION UPON LIGHT NOTION OF ATOMIC POLARITY APPLIED TO

CRYSTALLINE STRUCTURE EXPERIMENTAL ILLUSTKATIONS CRYSTAL-
LIZATION OF TVATER EXPANSION IIY HEAT AND BY COLD — DEPORTMENT

OF WATER CONSIDERED AND EXPLAINED BEARINGS OF CRYSTALLIZA-
TION ON OPTICAL PHENOMENA REFRACTION — DOUBLE REFRACTION —

POLARIZATION — ACTION OF TOURJLILINE — CHARACTER OF THE BEAMS
EMERGENT FROM ICELAND SPAR — POLARIZATION BT ORDINARY i:K-
FRACTION AND EEFLKCTION DEPOLARIZATION.

§ 1. Derivation of Theoretic Conceptions /rem
Experience.

One of tlie objects of our last lecture, and that not the
least important, was to illustrate the manner in which
scientific theories are formed. They, in the first place,
take their rise in the desire of the mind to penetrate
to the sources of phenomena. From its infinitesi-
mal beginnings, in ages long past, this desire has
grown and strengthened into an imperious demand of
man's intellectual nature. It long ago prompted
CsDsar to say that he would exchange his victories for
a glimpse of the sources of the Nile ; it wrouglit itself
into the atomic tlieories of Lucretius ; it impels Darwin
to tliose daring speculations which of late years have
eo agitated the public mind. But in no case in framing

96 ON LIGHT. LECT.

theories does the imagination create its materials. It
expands, diminishes, moulds and refines, as the case
may be, materials derived from the world of fact and
observation.

This is more evidently the case in a theory like that
of light, where the motions of a subsensible medium,
the ether, are presented to the mind. But no theory
escapes the condition. Newton took care not to en-
cumber the idea of gravitation with unnecessary physi-
cal conceptions; but we know that he indulged in
them, though he did not connect them with his
theory. But even the theory as it stands did not
enter the mind as a revelation dissevered from the
world of experience. The germ of the conception
that the sim and planets are held together by a force
of attraction is to be found in the fact that a
magnet had been previously seen to attract iron. The
notion of matter attracting matter came thus from
without, not from within. In our present lectui e the
magnetic force must serve us as the portal into a new
subsensible domain; but in the first place we must
master its elementary phenomena.

The general facts of magnetism are most simply
illustrated by a magnetized bar of steel, commonly
called a bar magnet. Placing such a magnet upright
upon a table, and bringing a magnetic needle near its
bottom, one end of the needle is observed to retreat
from the magnet, while the other as promptly ap-
proaches. The needle is held quivering there by some
invisible influence exerted upon it. Eaising the needle
along the magnet, but still avoiding contact, the ra-
pidity of its oscillations decreases, because the force
acting upon it becomes weaker. At the centre the oscil-

III.

EXTENSION OF MAGNETISM TO MOLECULES. 97

lations cease. Above the centre, the end of the needle
which had heen previously drawn towards the magnet
retreats, and the opposite end approaches. As we as-
cend higher, the oscillations become more violent,
because the force becomes stronger. At the upper end
of the magnet, as at the lower, the force reaches a
maximum ; but all the lower half of the magnet, from
E to S (fig. 25), attracts one end of the needle, while
all the upper half, from E to N, attracts the opposite
end. This douhleness of the magnetic force is called

Fig. 25.

r ^

_S_

"polarity^ and the points near the ends of the magnet in
which the forces seem concentrated are called its poles.
What, then, will occur if we break this magnet in
two at the centre E ? Shall we obtain two magnets,
each with a single pole ? No ; each half is in itself a
perfect magnet, possessing two poles. Tins may be
proved by breaking something of less value than the
magnet — tlie steel of a lady's stays, for example,
hardened and magnetized. It acts like tlie magnet.
"When broken, eacli lialf acts like tlie whole ; and when

r»8 ON LIGHT.

lECT,

these parts are again broken, we have still the perfect
magnet, possessing, as in the first instance, two poles.
Push your breaking to its utmost sensible limit, you
cannot stop there. The bias derived from observa-
tion will infallibly carry you beyond the bourne of
the senses, and compel you to regard this thing
that we call magnetic polarity as resident in the
ultimate particles of the steel. You come to the
conclusion that each atom of the magnet is endowed
with this polar force.

Like all other forces, this force of magnetism is
amenable to mechanical laws ; and, knowing the direc-
tion and magnitude of the force, we can predict its
action. Placing a small magnetic needle near a bar
magnet, it takes up a determinate position. That
position might be deduced theoretically from the
mutual action of the poles. Moving the needle round
the magnet, for each point of the surrounding space
there is a definite direction of the needle, and no
other. A needle of iron will answer as well as the
magnetic needle ; for the needle of iron is magnetized
by the magnet, and acts exactly like a steel needle
independently magnetized.

If we place two or more needles of iron near the mag-
net, the action becomes more complex, for then the
needles are not only acted on by the magnet, but
they act upon each other. And if we pass to smaller
masses of iron — to iron filings, for example — we find
that they act substantially as the needles, arranging
themselves in definite forms, in obedience to the mag-
netic action.

Placing a sheet of paper or glass over a bar
magnet and showering iron filings upon the paper, I

ill.

POLARITY AND STRUCTURE.

notice a tendency of the filings to arrange themselves
in determinate lines. They cannot freely follow this
tendency, for they are hampered by the friction against
the paper. They are helped by tapping the paper ;

Fig. 26.

N is the nozzle of >ho lamp ; 51 a plane mirror, reflecting the Deam upwards. At
P the magnets ■Mid iron filings are placed; L is a lens wliieh f.)rms an image of
the magnets and lilings ; and R is a totally-reflecting prism, which casta the image
G upon the screen.

each tap releasing them for a moment, and enabling
them to follow their tendencies. But this is an experi-
ment which can only be seen by myself. To enable you
all to see it, I take a pair of small magnets and by a
simple optical arrangement throw the magnified images
of the magnets iijjon tlie screen. Scattering iron lilings
over the glass plate to which the small magnets are
attached, and tapping the jjlate, you see the arrange-

100 ON LIGHT. iBCT.

ment of the iron filings in those magnetic curves which
have been so long familiar to scientific men.^

The aspect of these curves so fascinated Faraday
that the greater portion of his intellectual life was de-
voted to pondering over them. He invested the space
through which they run with a kind of materiality ;
and the probability is that the progress of science, by \
connecting the phenomena of magnetism with the lumi- \
niferous ether, will prove these 'lines of force,' as \
Faraday loved to call them, to represent a condition of 1
this mysterious substratum of all radiant action.

But it is not the magnetic curves, as such, but
their relationship to theoretic conceptions that we have
now to consider. By the action of the bar magnet upon
the needle we obtain a notion of a polar force ; by the
breaking of the strip of magnetized steel, we attain the
notion that polarity can attach itself to the ultimate
particles of matter. The experiment with the iron
filings introduces a new idea into the mind ; the idea,
namely, of structural arrangement. Every pair of
filings possesses four poles, two of which are attractive
and two repulsive. The attractive poles approach, the
repulsive poles retreat ; the consequence being a certain
definite arrangement of the particles with reference to
each other.

§ 2. Theory of Cin/stallization.

Now, this idea of structure, as produced by polar
force, opens a way for the intellect into an entirely new

' Very beautiful specimens of these curves have Leen recently
obtained and Jia:ed by my distinguished friend, Prof. Mayer, of Hoboken,
to whom I am indebted for the original of the woodcut placed in front
of this Lecture,

ni. CRYSTALS BUILT BY POLAE FORCE. 101

region, and the reason you are asked to accompany me
into this region is, that our next enquiry relates to the
action of crystals upon light. Prior to speaking of this
action, I wish you to realise intellectually the process
of crystalline architecture. Look then into a granite
quarry, and spend a few minutes in examining the
rock. It is not of perfectly uniform texture. It is
rather an agglomei-ation of pieces, which, on examina-
tion, present curiously-defined forms. You have there
what mineralogists call quartz, you have felspar, you
have mica. In a mineralogical cabinet, where these
substances are preserved separately, you will obtain
some notion of their forms. You will see there, also,
specimens of beryl, topaz, emerald, tourmaline, heavy
spar, fluor-spar, Iceland spar — possibly a full-formed
diamond, as it quitted the hand of Nature, not yet
having got into the hands of the lapidary.

These crystals, you will observe, are put together ac-
cording to law ; they are not chance productions ; and, if
you care to examine them more minutely, you will find
their architecture capable of being to some extent
revealed. They often split in certain directions before
a knife-edge, exposing smooth and shining surfaces,
which are called planes of cleavage ; and by following
these planes you sometimes reach an internal form,
disguised beneath the external form of the crystal.
Ponder these beautiful edifices of a hidden builder.
You cannot help asking yourself how they were built ;
and familiar as yon now are with the notion of a polar
force, and the ability of that force to produce structural
arrangement, your inevitable answer will be, that those
crystals are built by the play of polar forces with which
their molecules are endowed. In virtue of these forces,
6

102 ON LIGHT.

LECT.

atom lays itself to atom in a perfectly definite way,
the final visible form of the crystal depending upon
this play of its molecules.

Everywhere in Nature we observe this tendency to
run into definite forms, and nothing is easier than to
give scope to this tendency by artificial arrangements.
Dissolve nitre in water, and allow the water slowly to
evaporate ; the nitre remains, and the solution soon
becomes so concentrated that the liquid condition can
no longer be preserved. The nitre-molecules approach
each other, and come at length within the range of
their polar forces. They arrange themselves in obedi-
ence to these forces, a minute crystal of nitre being at
first produced. On this crystal the molecules continue
to deposit themselves from the surrounding liquid.
The crystal grows, and finally we have large prisms of
nitre, each of a perfectly definite shape. Alum crys-
tallizes with the utmost ease in this fashion. The
resultant crystal is, however, different in shape from
that of nitre, because the poles of the molecides are
differently disposed. If they be only nursed with
proper care, crystals of these substances may be caused
to grow to a great size.

The condition of perfect crystallization is, that the
crystallizing force shall act with deliberation. There
should be no hurry in its operations ; but every mole-
cule ought to be permitted, without disturbance from
its neighbours, to exercise its own rights. If the crys-
tallization be too sudden, the regularity disappears.
Water may be saturated with sulphate of soda, dissolved
when the water is hot, and afterwards permitted to cool.
When cold the solution is supersaturated ; that is to say,
more solid matter is contained in it than corresponds to

m. ILLUSTRATIONS OF CRYSTALLIZATION. 103

its temperature. Still the molecules show no sign of
building themselves together.

This is a very remarkable, though a very common
fact. The molecules in the centre of the liquid are so
hampered by the action of their neighbours that freedom
to follow their own tendencies is denied to them. Fix your
mind's eye upon a molecule within the mass. It wishes
to unite with its neighbour to the right, but it wishes
equally to unite with its neighbour to the left ; the
one tendency neutralizes the other, and it unites with
neither. But, if a crystal of sulphate of soda be dropped
into the solution, the molecular indecision ceases. On
the crystal the adjacent molecules will immediately
precipitate themselves ; on these again others will be
precipitated, and this act of precipitation will continue
from the top of the flask to the bottom, until the
solution has, as far as possible, assumed the solid form.
Ths crystals here produced are small, and confusedly
arranged. The process has been too hasty to admit of
the pure and orderly action of the crystallizing force.
It typifies the state of a nation in which natural and
healthy change is resisted, until society becomes, as it
were, supersaturated with the desire for change, the
change being then effected through confusion and revo-
lution.

Let me illustrate the action of crystallizing force by
two examples of it : Nitre might be employed, but
another well-known substance enables me to make the
experiment in a better form. The substance is com-
mon sal-ammoniac, or chloride of ammonium, dissolved
in water. Cleansing perfectly a glass plate, the solu-
tion of the chloride is poured over the glass, to which,
when the plate is set on edge, a thin film of the liquid

104 ON LIGHT. i-ECT.

adheres. Warming the glass slightly, evaporation is
promoted, but by evaporation the water only is removed.
The plate is then placed in a solar microscope, and an
image of the film is thrown upon a white screen. The
warmth of the illuminating beam adds itself to that
already imparted to the glass plate, so that after a
moment or two the dissolved salt can no longer exist in
the liquid condition. Molecule then closes with mole-
cule, and you have a most impressive display of crystal-
lizing energy overspreading the whole screen. You
may produce something similar if you breathe upon the
frost-ferns which overspread your window-panes in
winter, and then observe through a pocket lens the sub-
sequent recongelation of the film.

In this case the crystallizing force is hampered by
the adhesion of the film to the glass ; nevertheless, the
play of power is strikingly beautiful. Sometimes the
crystals start from the edge of the film and run through
it from that edge, for, the crystallization being once
started, the molecules throw themselves by preference
on the crystals already formed. Sometimes the crys-
tals start from definite nuclei in the centre of the film-;
every small crystalline particle which rests in the film
furnishing a starting-point. Throughout the process
you notice one feature which is perfectly unalterable,
and that is, angular magnitude. The spiculse branch
from the trunk, and from these branches others shoot ;
but the angles enclosed by the spiculse are unalterable.
In like manner you may find alum-crystals, quartz-
crystals, and all other crystals, distorted in shape. They
are thus far at the mercy of the accidents of crystalliza-
tion ; but in one particular they assert their superiority

m. CRYSTALLIZATION OF WATER. 105

over all such accidents — angular magnitude is always
rigidly preserved.

My second example of the action of crystallizing force
is this : By sending a voltaic current through a liquid,
you know that we decompose the liquid, and if it con-
tains a metal, we liberate this metal by the electrolysis.
This small cell contains a solution of acetate of lead,
which is chosen for our present purpose, because lead lends
itself freely to this crystallizing power. Into the cell
are dipped two very thin platinum wires, and these are
connected by other wires with a small voltaic battery.
On sending the voltaic current through the solution,
the lead will be slowly severed from the atoms with
which it is now combined ; it will be liberated upon
one of the wires, and at the moment of its liberation it
will obey the polar forces of its atoms, and produce
crystalline forms of exquisite beauty. They are now
before you, sprouting like ferns from the wire, appear-
ing indeed like vegetable growths rendered so rapid as
to be plainly visible to the naked eye. On reversing
the current, these wonderful lead-fronds will dissolve,
while from the other wire filaments of lead dart through
the liquid. In a moment or two the growth of the lead-
trees recommences, but they now cover the other wire.

In the process of crystallization. Nature first reveals
herself as a builder. Where do her operations stop ?
Does she continue by the play of the same forces to
form the vegetable, and afterwards the animal 1 What^
ever the answer to these questions may be, trust me that
the notions of the coming generations regarding this mys-
terious thing, which some have called 'brute matter,'
will be very diflferent from those of the generations past.

106 ON LIGHT.

LECr.

There is hardly a more beautiful and instructive
example of this play of molecular force than that
furnished by the case of water. You have seen the
exquisite fern-like forms produced by the crystallization
of a film of water on a cold window-pane.' You have
also probably noticed the beautiful rosettes tied together
by tlie crystallizing force during the descent of a snow-
shower on a very calm day. The slopes and summits
of the Alps are loaded in winter with these blossoms
of the frost. They vary infinitely in detail of beauty,
but the same angular magnitude is preserved through-
out : an inflexible power binding spears and spiculse to
the angle of 60 degrees.

The common ice of our lakes is also ruled in its
deposition by the same angle. You may sometimes
see in freezing water small crystals of stellar shapes,
each star consisting of six rays, with this angle of
60° between every two of them. This structure may
be revealed in ordinary ice. In a sunbeam, or,
failing that, in our electric beam, we have an in-
strument delicate enough to unlock the frozen mole-
cules without disturbing the order of their architecture.
Cutting from clear, sound, regularly-frozen ice a slab
parallel to the planes of freezing, and sending a sun-
beam through such a slab, it liquefies internally at
special points, round each point a six-petalled liquid
flower of exquisite beauty being formed. Crowds of
such flowers are thus produced. From an ice-house we
sometimes take blocks of ice presenting misty spaces in
the otherwise continuous mass ; and when we enquire

' A specimen of the plumes produced by water crystallization k
figured, and an account of it given, in the Appendix.

III. EXPANSION BY COLD; PROPOSED EXPLANATION. 107

into the cause of this mistiness, we find it to be due to
myriads of small six-petalled flowers, into which the ice
has been resolved by the mere heat of conduction.

A moment's furtlier devotion to the crystallization
of water will be well repaid ; for the sum of qualities
which renders this substance fitted to play its part in
Nature may well excite wonder and stimulate thought.
Like almost all other substances, water is expanded by
heat and contracted by cold. Let this expansion and
contraction be first illustrated :

A small flask is filled with coloured water, and
stopped with a cork. Through the cork passes a glass
tube water-tight, the liquid standing at a certain
height in the tube. The flask and its tube resemble
the bulb and stem of a thermometer. Applying the
heat of a spirit-lamp, the water rises in the tube, and
finally trickles over the top. Expansion by heat is thus
illustrated.

Removing the lamp and piling a freezing mixture
round the flask, the liquid coliunn falls, thus showing
the contraction of the water by the cold. But let
the freezing mixture continue to act : the falling of
the column continues to a certain point ; it then
ceases. The top of the column remains stationary for
some seconds, and afterwards begins to rise. The con-
traction has ceased, and expaasionhy cold sets in. Let
the expansion continue till the liquid trickles a second
time over the top of the tube. The freezing mixture has
here produced to all appearance the same effect as the
flame. In the case of water, contraction by cold ceases,
and expansion by cold sets in at the definite tempera-
ture of 39" Fahr. Crystallization has virtually here
commenced, the molecules preparing themselves for the

108 ON LIGHT.

tEcr.

subsequent act of solidification which occurs at 32°, and
in which the expansion suddenly culminates. In virtue
of this expansion, ice, as you know, is lighter than
water in the proportion of 8 to 9.'

A molecular problem of great interest is here in-
volved, and I wish now to place before you, for the
satisfaction of your minds, a possible solution of the
problem : —

Consider, then, the ideal case of a number of magnets
deprived of weight, but retaining their polar forces. If
we had a mobile liquid of the specific gravity of steel,
we might, by making the magnets float in it, realize this
state of things, for in such a liquid the magnets would
neither sink nor swim. Now, the principle of gravi-
tation enunciated by Newton is that every particle of
matter, of every kind, attracts every other particle with
a force varying as the inverse square of the distance. In
virtue of the attraction of gravity, then, the magnets, if
perfectly free to move, would slowly approach each other.

But besides the impolar force of gravity, which be-
longs to matter in general, the magnets are endowed
with the polar force of magnetism. For a time, however,
the polar forces do not come sensibly into play. In this

' In a little volume entitled ' Forms of Water,' I have mentioned
that cold iron floats upon molten iron. In company with my friend Sir
William Armstrong, I had repeated opportunities of -witnessing this
fact in his -works at Elswick, 1863. Faraday, I remember, spoke to me
subsequently of the completeness of iron castings as probably due to the
swelling of the metal on solidification. Beyond this, I hare given the
subject no special attention ; and I know that many intelligent iron-
founders doubt the fact of expansion. It is quite possible that the
solid floats because it is not wetted by the molten iron, its volume being
virtually augmented by capillary repulsion. Certain flies walk freely
upon -water in virtue of an action of this kind. With bismuth, however,
it is easy to burst iron bottles by the force of solidification.

III. UNDULATORY THEORY OF REFRACTION. 109

condition the magTiets resemble our water-molecules
at the temperature say of 50°. But the magnets come
at length sufficiently near each other to enable their
poles to interact. From this point the action ceases
to be solely a general attraction of the masses. An
attraction of special points of the masses and a repul-
sion of other points now come into play ; and it is
easy to see that the rearrangement of the magnets con-
sequent upon the introduction of these new forces may
be such as to require a greater amount of room. This, I
take it, is the case with our water-molecules. Like the
magnets, they approach each other for a time as wholes.
Previous to reaching the temperature 39° Fahr., the
polar forces had doubtless begun to act, but it is at
this temperature that their action exactly balances the
contraction due to cold. At lower temperatures, as
regards change of volume, the polar forces predoniinate.
But they carry on a struggle with the force of contrac-
tion until the freezing temperature is attained. The
molecules then close up to form solid crystals, a con-
siderable augmentation of volume being the immediate
consequence.

§ 3. Ordinary Refraction of Light explained by
the Undulatd'y Theory.

We have now to exhibit the bearings of this act of
crystallization upon optical phenomena. According to
the undulatory theory, the velocity of light in water and
glass is less than in air. Consider, then, a small por-
tion of a wave issuing from a point of light so distant
that the portion may be regarded as practically plane.
Moving vertically downwards, and impinging on an
horizontal surface of glass or water, the wave would go

110

ON LIGHT.

l/ECT,

through the medium without change of direction. But,
as the velocity in glass and water is less than the
velocity in air, the wave would be retarded on passing
into the denser medium.

But suppose the wave, before reaching the glass, to
be oblique to the surface ; that end of the wave which
first reaches the medium will be the first retarded by
it, the other portions as they enter the glass being re-
tarded in succession. It is easy to see that this
retardation of the one end of the wave must cause it
to swing round and change its front, so that when the
wave has fully entered the glass its course is oblique to

its original direction. According to the undulatory
theory, light is thus refracted.

With these considerations to guide us, let us follow
the course of a beam of monochromatic light through
our glass prism. The velocity in air is to its velocity in
glass as 3 : 2. Let a B c (fig. 27) be the section of our
prism, and a h the section of a plane wave approach-
ing it in the direction of the arrow. When it reaches
c d^ one end of the wave is on the point of entering the

in. APPLICATION OF THEORY TO CRYSTALS. Ill

glass, and while the portion of the wave still in the air
passes over the distance c e, the wave in the glass will
have passed over only two-thirds of this distance, or
df. The line ef now marks the front of the wave.
Immersed wholly in the glass it pursues its way to g h,
where the end g of the wave is on the point of escaping
into the air. During the time required by the end h
of the wave to pass over the distance h Jc to the surface
of the prism, the other end g, moving more rapidly,
will have reached the point i. The wave, therefore,
has again changed its front, so that after its emergence
from the prism it will pass on to I m, and subsequently
in the direction of the arrow. The refraction of the
beam is thus completely accounted for ; and it is, more-
over, based upon actual experiment, which proves that
the ratio of the velocity of light in glass to its velocity
in air is that here mentioned. It is plain that if the
change of velocity on entering the glass was greater,
the refraction also would be greater.

§ 4. Double Refraction of Light explained by the
JJndulatory Theory,

The two elements of rapidity of propagation, both
of sound and light, in any substance whatever, are
elasticity and density, the speed increasing with the
former and diminishing with the latter. The enormous
velocity of light in stellar space is attainable because
the ether is at the same time of infinitesimal density
and of enormous elasticity. Now the ether surrounds
the atoms of all bodies, but it is not independent of them.
In ponderable matter it acts as if its density were in-
creased without a proportionate increase of elasticity ;

112 ON LIGHT.

ia:cT.

and this accounts for the diminished velocity of light in
refracting bodies. We here reach a point of cardinal im-
portance. In virtue of the crystalline architecture that
we have been considering, the ether in many crystals
possesses different densities, and hence different elastici-
ties, in two different directions ; and the consequence is,
that some of these media transmit light with two diffe-
rent velocities. But as refraction depends wholly upon
the change of velocity on enteringthe refracting medium,
and is greatest where the change of velocity is greatest,
we have in many crystals two different refractions.
By such crystals a beam of light is divided into two.
This effect is called double refraction.

In ordinary water, for example, there is nothing
in the grouping of the molecules to interfere with the
perfect homogeneity of the ether ; but, when water crys-
tallizes to ice, the case is different. In a plate of ice
the elasticity of the ether in a direction perpendicular to
the surface of freezing is different from what it is
parallel to the surface of freezing ; ice is, therefore, a
double refracting substance. Double refraction is dis-
played in a particularly impressive manner by Iceland
spar, which is crystallized carbonate of lime. The
difference of ethereal density in two directions in this
crystal is very great, the separation of the beam into
the two halves being, therefore, particularly striking.

I am unwilling to quit this subject before raising it
to unmistakable clearness in your minds. The vibra-
tions of light being transversal, the elasticity concerned
in the propagation of any ray is the elasticity at right
angles to the direction of propagation. In Iceland
spar there is one direction round which the crystalline
molecules are symmetrically built. This direction is

ni.

DOUBLE KEFRACTION.

113

called the axis of the crystal. In consequence of this
symmetry the elasticity is the same in all directions
perpendicular to the axis, and hence a ray transmitted
along the axis suffers no double refraction. But the
elasticity along the axis is greater than the elasticity
at right angles to it. Consider, then, a system of
waves crossing the crystal in a direction perpendicular
to the axis. Two directions of vibration are open to
such waves : the ether particles can vibrate parallel
to the axis or perpendicular to it. They do both, and
hence immediately divide themselves into two systems
propagated with different velocities. Double refraction
is the necessary consequence.

By means of Iceland spar cut in the proper direction,

Fig. 28.

double refraction is capable of easy illustration. Causing
the beam which builds the image of our carbon-points
to pass through the spar, the single image is instantly
divided into two. Projecting (by the lens E, fig. 28)
an image of the aperture (L) through which the light
issues from the electric lamp, and introducing the spar

114 ON LIGHT. i^CT.

(P), two luminous disks (E 0) appear immediately
upon the screen instead of one.

The two beams into which the spar divides the
single incident-beam have been subjected to the closest
examination. They do not behave alike. One of them
obeys the ordinary law of refraction discovered by Snell,
and is, therefore, called the ordinary ray : its index of
refraction is 1-654. The other does not obey this law.
Its index of refraction, for example, is not constant,
but varies from a maximum of 1*654 to a minimum
of 1*483 ; nor in this case do the incident and refracted
rays alwTays lie in the same plane. It is, therefore,
called the extraordinary ray. In calc-spar, as just
stated, the ordinary ray is the most refracted. One
consequence of this merits a passing notice. Pour
water and bisulphide of carbon into two cups of the
same depth ; the cup that contains the more strongly-
refracting liquid will appear shallower than the other.
Place a piece of Iceland spar over a dot of ink ; two
dots are seen, the one appearing nearer than the other
to the eye. The nearest dot belongs to the most
strongly-refracted ray, exactly as the nearest cup-
bottom belongs to the most highly refracting liquid.
"WTien you turn the spar round, the extraordinary image
of the dot rotates round the ordinary one, which
remains fixed. This is also the deportment of our two
disks upon the screen.

§ 5. Polarization of Light explained by the
Undulatory Theory.

The double refraction of Iceland spar was first
treated in a work published by Erasmus Bartholinus, in
1669. The celebrated Huyghens sought to account for

II!.

POLARIZATION OF LIGHT. 115

this phenomenon on the principles of the wave theory,
and he succeeded in doing so. He, moreover, made
highly important observations on the distinctive cha-
racter of the two beams transmitted by the spar,
admitting, with resigned candom-, that he had not
solved them, and leaving that solution to future times.
Newton, reflecting on the observations of Huyghens,
came to the conclusion that each of the beams trans-
mitted by Iceland spar had two sides ; and from the
analogy of this two-sidedness with the two-endedness
of a magnet, wherein consists its polarity, the two
beams came subsequently to be described as polarized.

We may begin the study of the polarization of
light, with ease and profit, by means of a crystal of
tourmaline. But we must start with a clear conception
of an ordinary beam of light. It has been already
explained that the vibrations of the individual ether-
particles are executed across the line of propagation.
In the case of ordinary light we are to figure the ether-
particles as vibrating in all directions, or azimuths, as
it is sometimes expressed, across this line.

Now, in the case of a plate of tourmaline cut
parallel to the axis of the crystal, a beam of light
incident upon the plate is divided into two, the one
vibrating parallel to the axis of the crystal, the other
at right angles to the axis. The grouping of the
molecules, and of the ether associated with the mole-
cules, reduces all the vibrations incident upon the
crystal to these two directions. One of these beams,
namely, that whose vibrations are perpendicular to
the axis, is quenched with exceeding rapidity by the
tourmaline. To such vibrations many specimens of
the crystal are highly opaque ; so that, after having

116

ON LIGHT.

tKCT.

passed through a very small thickness of the tourmaline,
the light emerges with all its vibrations reduced to a
single plane. In this condition it is what we call
plane polarized light.

A moment's reflection will show that, if what is
here stated be correct, on placing a second plate of
tourmaline with its axis parallel to the first, the light
will pass through both ; but that, if the axes be crossed,
the light that passes through the one plate will be
quenched by the other, a total interception of the light
being the consequence. Let us test this conclusion by
experiment. The image of a plate of tourmaline {t t,
fig. 29) is now before you. I place parallel to it another

Fin. 29.

plate {if t'): the green of the crystal is a little
deepened, nothing more ; this agrees with our conclu-
sion. By means of an endless screw, I now turn one of
the crystals gradually round, and you observe that as
long as the two plates are oblique to each other, a
certain portion of light gets through ; but that when
they are at right angles each other, the space

ni. DISCOVERY OF MALUS. 117

common lo both is a space of darkness (fig. 30), Our con-
clusion, arrived at prior to experiment, is thus verified.

Let us now return to a single plate ; and here let
me say that it is on the green light transmitted by the
tourmaline that you are to fix your attention. We have
to illustrate the two-sidedness of that green light, in
contrast to the all-sidedness of ordinary light. The
light surrounding the green image, being ordinary light,
is reflected by a plane glass mirror in all directions ;
the green light, on the contrary, is not so reflected.
The image of the tourmaline is now horizontal ; re-
flected upwards, it is still green; reflected sideways,
the image is reduced to blackness, because of the in-
competency of the green light to be reflected in this
direction. Making the plate of tourmaline vertical,
and reflecting it as before, it is in the upper image that
the light is quenched ; in the side image you have now
the green. This is a result of the greatest significance.
If the vibrations of light were longitudinal, like those
of sound, you could have no action of this kind ; and
this very action compels us to assume that the vibra-
tions are transversal. Picture the thing clearly. In the
one case the mirror receives, as it were, the impact of the
edges of the waves, the green light being then quenched.
In the other case the sides of the waves strike the mir-
ror, and the green light is reflected. To render the
extinction complete, the light must be received upon
the mirror at a special angle. What this angle is we
shall learn presently.

The quality of two-sidedness conferred upon light
V)y bi-refracting crystals may also be conferred upon it
by ordinary reflection. Mains made this discovery in
1808, while looking through Iceland spar at the light of

118 ON LIGHT.

I-ECT.

the sun reflected from the windows of the Luxembourg
palace in Paris. I receive upon a plate of window-glass
the beam from our lamp ; a great portion of the light
reflected from the glass is polarized. The vibrations of
this reflected beam are executed, for the most part,
parallel to the surface of the glass, and when the glass
is held so that the beam shall make an angle of 58°
with the perpendicular to the glass, the ivhole of the
reflected beam is polarized. It was at this angle that
the image of the tourmaline was completely quenched in
our former experiment. It is called thepoiarizing angle.

Sir David Brewster proved the angle of polarization
of a medium to be that particular angle at which tlie
refracted and reflected rays inclose a right angle.* The
polarizing angle augments with the index of refraction.
For water it is 52^° ; for glass, as already stated, 58° ;
while for diamond it is 68°.

And now let us try to make substantially the
experiment of Malus. The beam from the lamp is
received at the proper angle upon a plate of glass
and reflected through the spar. Instead of two images,
you see but one. So that the light, when polarized, as
it now is by reflection, can only get through the spar in
one direction, and consequently produce but one image.
Why is this ? In the Iceland spar, as in the tourmaline,
all the vibrations of the ordinary light are reduced to

' This beautiful law is usually thus expressed : The index of refrac-
tion of any substance is the tangent of its polarizing angle. With the
aid of this law and an apparatus similar to that figured at page 15, we
can readily determine the index of refracting any liquid. The refracted
and reflected beams being visible, they can readily be caused to enclose
a right angle. The polarizing angle of the liquid may be thus found
with the sharpest precision. It is then only necessary to seek out its
natural tangent to obtain the index of refraction.

TTi. BEAMS FROM SPAR TESTED BY TOURM.\LINE. 119

two planes at right angles to each other; but, unlike
the tourmaline, both beams are transmitted with equal
facility by the spar. The two beams, in short, emergent
from the spar, are polarized, their directions of vibration
being at right angles to each other. It is important to
remember this. When, therefore, the light was polar-
ized by reflection, the direction of vibration in the spar
which coincided with the direction of vibration of the
polarized beam transmitted it, and that direction only.
Only one image, therefore, was possible under the con-
ditions.

You will now observe that such logic as connects
our experiments is simply a transcript of the logic of
Nature. On the screen before you are two disks of
light produced by the double refraction of Iceland spar.
They are, as you know, two images of the aperture
through which the liji:ht issues from the camera. Plac-
ing the tourmaline in front of the aperture, two images
of the crystal will also be obtained ; but now let us
reason out beforehand what is to be expected from this
experiment. The light emergent from the tourmaline
is polarized. Placing the crystal with its axis hori-
zontal, the vibrations of its transmitted light will be
horizontal. Now the spar, as already stated, has two
directions of vibration, one of which at tlie present
moment is vertical, the other horizontal. ^Miat are
we to conclude ? That the green light will be trans-
mitted along the latter, which is parallel to tlie axis of
the tourmaline, and not along the former, which is
perpendicular to that axis. Hence we may infer that
one image of the tourmaline will show the ordinary
green light of the crystal, while the other image will

ON LIGHT.

tsCT.

be black. Tested by experiment, our reasoning is veri-
fied to the letter (fig. 31).

¥iG. 31.

Let us push our test still further. By means of an
endless screw, the crystal can be turned ninety degrees
round. The black image, as I turn, becomes gradually

Fia. 32.

brighter, and the bright one gradually darker ; at an
angle of forty-five degrees both images are equally
bright (fig. 32) ; while, when ninety degrees have been

Fig. 33.

obtained, the axis of the crystal being then vertical,
the bright and black images have changed places

III. TESTED BY REFLECTION AND REFRACTION. 121

exactly as reasoning would have led us to suppose
(fig. 33).

Given the two beams transmitted through Iceland
spar, it is perfectly manifest that we have it in our
power to determine instantly, by means of a plate of
tourmaline, the directions in which the ether-particles
vibrate in the two beams. The double refracting spar
might be placed in any position whatever. A minute's
trial with the tourmaline would enable you to deter-
mine the position which yields a black and a bright
image, and from this you would at once infer the direc-
tions of vibration.

Let us reason still further together. The two
beams from the spar being thus polarized, it is plain

Fig. 34.

(B is the hi-refractlng spar, dividing the incident light into the two beams o and e,
a is the mirror.) The beam is here reflected laterally. When the reflection la up-
uardt, the other beam is reflected as shown in flg. 35.

that if they be suitably received upon a plate of glass
at the polarizing angle, one of them will l)e reflected,
the other not. This is a simple inference from our pre-
vious knowledge ; but you observe that the inference ia
justified by experiment. (Figs. 34 and 35.)

122

ON LIGHT.

LKCT.

I have said that the whole of the beam reflected
from glass at the polarizing angle is polarized ; a word
must now be added regarding the far larger portion of
the light which is transmitted by the glass. The

Fig. 35.

transmitted beam contains a quantity of polarized
light equal to the reflected beam : but this is only
a fraction of the whole transmitted light. By tak-
ing two plates of glass instead of one, we augment 4
the quantity of the transmitted polarized light ; and by
taking a bundle of plates, we so increase the quantity
as to render the transmitted beana, for all practical pur-
poses, 'perfectly polarized. Indeed, bundles of glass
plates are often employed as a means of furnisliing
polarized light. Interposing such a bundle at the
proper angle into the paths of the two beams emergent
from Iceland spar, that which, in the last experiment,
failed to be reflected, is here transmitted. The plane
of vibration of this transmitted light is at right angles
to that of the reflected light.

One word more. When the tourmalines are crossed,
the space where they cross each other is black. But

in.

DEPOLARIZATION. 1 2 3

we have seen that the least obliquity on the part of
the crystals permits light to get through both. Now
suppose, when the two plates are crossed, that we in-
terpose a third plate of tourmaline between them, with
its axis oblique to both. A portion of the light trans-
mitted by the first plate will get through this inter-
mediate one. But, after it has got through, its plane
of vibration is changed : it is no longer perpendicular
to the axis of the crystal in front. Hence it will get
through that crystal. Thus, by pure reasoning, we
infer that the interposition of a third plate of tourma-
line will in part abolish the darkness produced by the
perpendicular crossing of the other two plates. I have
not a third plate of tourmaline ; but the talc or mica
which you employ in your stoves is a more convenient
substance, which acts in the same way. Between the
crossed tourmalines, I introduce a film of this crystal
with its axis oblique to theirs. You see the edge of the
film slowly descending, and as it descends, light takes
the place of darkness. The darkness, in fact, seems
scraped away, as if it were something material. This
effect has been called, naturally but improperly,
depolarization. Its proper meaning will be disclosed
in our next lecture.

These experiments and reasonings, if only thorouglily
studied and understood, will form a solid groundwork
for the analysis of the splendid optical phenomena next
to be considered.

124 ON LIGHT.

LECT.

LECTURE IV.

CHEOMATIC PHENOMENA PHODTJCED BY CEYSTALS IN POLAHIZED LIGHT

THE NICOL PBI8M POLABIZEE AND ANALYZES — ACTION OF THICK

AND THIN PLATES OF SELENITB — COLOUES DEPEIHJENT ON THICK-
NESS— EBSOLXTTION OF POLAHIZED BEAM INTO TWO OTHEES BY THE
SELENITE — ONE OF THEM MOBB EETAEDED THAN THE OTHEE — EB-
COMPOUNDINQ OF THE TWO SYSTEMS OF WAVES BY THE ANALYZES

INTEEFEEENCB THUS EENDEEED POSSIBLE — CONSEQUENT PEODUC-

TIOK OF COLOUES — ACTION OF BODIES MECHANICALLY STEAINED OS

PHESSED — ACTION OF SON'iEOUS TIBEATIONS ACTION OF GLASS

STEAINED OR PEESSED BY HEAT — CIECULAE POLAEIZATION CHEOMA-

TIC PHENOMENA PEODUCED BY QUAETZ THE MAGNETIZATION OP

LIGHT — EINGS SUEEOUNDINQ THE AXES OF CEYSTALS — BIAXAL AND
UNIAXAL CEYSTALS — GEASP OF THE UNDULATOEY THEOEY — THE COLOUE
AND POLAEIZATION OF SKY-LIGHT GENEEATION OF AETIFICIAL SKIES.

§ 1. Action of Crystals on Polarized Light: the
Nicol Prism.

\Vb have this evening to examine and illustrate the
chromatic phenomena produced by the action of crystals,
and double-refracting bodies generally, upon polarized
light, and to apply the Undulatory Theory to tlieir eluci-
dation. For a long time investigators were compelled
to employ plates of tourmaline for this purpose, and
the progress they made with so defective a means of
inquiry is astonishing. But these men had their hearts
in their work, and were on this account enabled to
extract great results from small instrumental appliances.
But for our present purpose we need far larger appa-
ratus ; and, happily, in these later times this need has

THE NICOL'S PRISM. 125

been to a great extent satisfied. We have seen and
examined the two beams emergent from Iceland spar,
and have proved them to be polarized. If, at the
sacrifice of half the light, we could abolish one of these,
the other would place at our disposal a beam of polarized
light, incomparably stronger than any attainable from
tourmaline.

The beams, as you know, are refracted differently, and
from this, as made plain in § 4. Lecture I., we are able
to infer that the one may be totally reflected, when
the other is not. An able optician, named Nicol, cut a
crystal of Iceland spar in two halves in a certain direc-
tion. He polished the severed surfaces, and reunited
them by Canada balsam, the surface of union being
so inclined to the beam traversing the spar that the
ordinary ray, which is the most highly refracted, was
totally reflected by the balsam, while the extraordinary
ray was permitted to pass on.

Let bx,cy (fig. 36) represent the section of an elon-
gated rhomb of Iceland spar cloven from the crystal. Let
this rhomb be cut along the plane b c ; and the two
severed surfaces, after having been polished, reunited
by Canada balsam. We learned, in our first lecture,
that total reflection only takes place when a ray seeks
to escape from a more refracting to a less refracting
medium, and that it always, under these circumstances,
takes place when the obliquity is sufficient. Now the
refractive index of Iceland spar is, for the extraordinary
ray less, and for the ordinary greater, than for Canada
balsam. Hence, in passing from the spar to the balsam,
the extraordinary ray passes from a less refracting to
a more refracting medium, where total reflection cannot
occui* ; while the ordinary ray passes from a more
7

126

ON LIGHT.

I-ECT.

refracting to a less refracting medium, where total
reflection can occur. The requisite obliquity is secured
by making the rhomb of such a length that the plane

Fia. 36.

of which 6 c IS the section shall be perpendicular, or
nearly so, to the two end surfaces of the rhomb
b X, c y.

The invention of the Nicol prism was a great step in
practical optics, and quite recently such prisms have
been constructed of a size and purity which enable
audiences like the present to witness the chromatic phe-
nomena of polarized light to a degree altogether unat-
tainable a short time ago. The two prisms here before
you belong to my excellent friend Mr. William Spottis-
woode, and they were manufactured by Mr. Ladd. I
have with me another pair of very noble prisms, still
larger than these, manufactured for me by Mr. Browning,

IV. THICK AND THIN PLATES OY SELENITE. 127

who has gained so high and well-merited a reputation
in the construction of spectroscopes.'

§ 2. Colours of Films of Seleniie in Polarized Lir/ht.

These two Nicol prisms play the same part as the
two plates of tourmaline. Placed with their directions
of vibration parallel, the light passes through both ;
while when these directions are crossed the light is
quenched. Introducing a film of mica between the
prisms, the light, as in the case of the tourmaline, is
restored. But notice, when the film of mica is thin
you have sometimes not only light, but coloured light.
Oiu: work for some time to come will consist of the ex-
amination of such colours. With this view, I will take
a representative crystal, one easily dealt with, because
it cleaves with great facility — the crystal gypsum, or
selenite, which is crystallized svdphate of lime. Between
the crossed Nicols I place a thick plate of this crystal ;
like the mica, it restores the light, but it produces no
colour. With my penknife I take a thin splinter from
the crystal and place it between the prisms ; the image
of the splinter glows with the richest colours. Turning
the prism in front, these colours gradually fade and
disappear, but, by continuing the rotation until the
vibrating sections of the prisms are parallel to each
other, vivid colours again arise, but these colours are
complementary to the former ones.

Some patches of the splinter appear of one colour,
some of anotlier. These differences are due to the
different thicknesses of the film. As in the case of

' The largest and purest prism hitherto made has been recently con-
Btruited for Mr. Spottiswoode by Messrs. Tisley & Spiller.

128 ON LIGHT,

LECT,

Hooke's tliin plates, if the thickness be uniform, the
colour is uniform. Here, for instance, is a stellar shape,
every lozenge of the star being a film of gypsum of
uniform thickness : each lozenge, you observe, shows a
brilliant and uniform colour. It is easy, by shaping
our films so as to represent flowers or other objects, to
exhibit such objects in hues unattainable by art. Here,
for example, is a specimen of heart's-ease, the colours of
which you might safely defy the artist to reproduce.
By turning the front Nicol 90 degrees round, we pass
through a colourless phase to a series of colours com-
plementary to the former ones. This change is still
more strikingly represented by a rose-tree, which is
now presented in its natural hues — a red flower and
green leaves ; turning the prism 90 degrees round, we
obtain a green flower and red leaves. All these wonder-
ful chromatic effects have definite mechanical causes in
the motions of the ether. The principle of interference
duly applied and interpreted explains them all.

§ 3. Colours of Ciystals in Polarized Light explained
by the Undulatory Theoi^y.

By this time you have learned that the word ' light '
may be used in two different senses; it may mean
the impression made upon consciousness, or it may
mean the physical agent which makes the impression.
It is with the agent that we have to occupy ourselves
at present. That agent is a substance which fills all
space, and sm-rounds the atoms and molecules of bodies.
To this interstellar and interatomic medium definite
mechanical properties are ascribed, and we deal with it
in our reasonings and calculations as a body possessed of

IT. THEOKETIC ANALYSIS OF VIBRATIONS. 129

these properties. In mechanics we have the composition
and resolution of forces and of motions, extending to the
composition and resolution of vibrations. We treat the
luminiferous ether on mechanical principles, and, from
the composition, resolution, and interference of its vi-
brations we deduce all the phenomena displayed by
crystals in polarized light.

Let us take, as an example, the crystal of tourmaline,
with which we are now so familiar. Let a vibration
cross this crystal oblique to its axis. Experiment has
assured us that a portion of the light will pass through.
The quantity which passes we determine in this way. Let
A B (fig. 37) be the axis of the tourmaline, and let a b

Fig. 37.

represent the amplitude of the ethereal vibration before
it reaches A B. From a and b let the two perpendicu-
lars a c and b dhe drawn upon the axis : then c d will
be the amplitude of the transmitted vibration.

I shall immediately ask you to follow me while
I endeavour to explain the effects observed when
a film of gypsum is placed between the two Nicol's
prisms. But, prior to this, it will be desirable to esta-
blish still further the analogy between the action of the
prisms and that of the two plates of tourmaline. The
magnified images of these plates, with their axes at right-
angles to each other, are now before you. Introducing
between them a film of selenite, you observe that by
turning the film round it may be placed in a position

130

ON LIGHT.

LECT.

where it has no power to abolish the darkness of the
superposed portions of the tourmalines. Why is this ?
The answer is, that in the gypsum there are two direc-
tions, at right angles to each other, in which alone vibra-
tions can take place, and that in our present experiment
one of these directions is parallel to one of the axes
of the tourmaline, and the other parallel to the other
axis. When this is the case, the film exercises no
sensible action upon the light. But now I turn the
film so as to render its directions of vibration oblique
to the two tourmaline axes ; then, you see it exercises
the power, demonstrated in the last lecture, of restoring
the light.

Let us now mount our Nicol's prisms, and cross
them as we crossed the tourmalines.

Introducing

Fia. 38.

our film of gypsum between them, you notice that in
one particular position the film has no power what-
ever over the field of view. But, when the film is
turned a little way round, the light passes. We have
now to understand the mechanism by which this is
effected.

IV. ArPLICATION OF THEORY TO SELENITE, 131

Firstly, then, we have a prism which receives
the light from the electric lamp, and which is called
the polarizer. Then we have the plate of gypsum
^supposed to be placed at S, fig. 38), and then the
prism in front, which is called the analyzer. On its
emergence from the first prism, the light is polarized ;
and, in the particular case now before us, its vibrations
are executed in a horizontal plane. We have to ex-
amine what occurs when the two directions of vibration
in the gypsum are oblique to the horizon. Draw a
rectangular cross (A B, C D, fig. 39) to represent these

Fig. 39.

two dii'ections. Draw a line (a h) to represent the
amplitude of the vibration on the emergence of the
light from the first Nicol. Let fall from the two ends
of this line two perpendiculars on each of the arms of
the cross ; then the distances (c d^ e f) between the
feet of these perpendiculars represent the amplitudes of
two rectangular vibrations, which are the components
of the first single vibration. Thus the polarized ray,
when it enters the gypsum, is resolved into its two
equivalents, which vibrate at right angles to each
other.

In one of the.se two rectangular directions the ether

132 ON LIGHT. LECT.

within the gypsum is more sluggish than in the
other ; and, as a consequence, the waves that follow
this direction are more retarded than the others. In
fact, in both cases the undulations are shortened when
they enter the gypsum, hut in the one case they
are more shortened than in the other. You can
readily imagine that in this way the one system of
waves may get half a wave-length, or indeed any num-
ber of half-wave lengths, in advance of the other. The
possibility of interference here at once flashes upon the
mind. A little consideration, however, will render it
evident that, as long as the vibrations are executed at
right angles to each other, they cannot quench each
other, no matter what the retardation may be. This
brings us at once to the part played by the analyzer.
Its sole function is to recompound the two vibrations
emergent from the gypsum. It reduces them to a
single plane, where, if one of them be retarded by
the proper amount, extinction will occur.

But here, as in the case of thin films, the different
lengths of the waves of light come into play. Eed wiU
require a greater thickness to produce the retardation
necessary for extinction than blue ; consequently, when
the longer waves have been withdrawn by interference,
the shorter ones remain, the film of gypsum shining
with the colours which they confer. Conversely, when
the shorter waves have been withdrawn, the thickness
is such that the longer waves remain. An elementary
consideration suffices to show that, when the directions
of vibration of the prisms and the gypsum enclose an
angle of forty-five degrees, the colours are at their maxi-
mum brilliancy. When the film is turned from this
direction, the colours gradually fade, until, at the point

IT. RELATION OF THICKNESS TO COLOUR. 133

where the directions of vibration are parallel, they dis-
appear altogether.

The best way of obtaining a knowledge of these phe-
nomena is to construct a model of thin wood or paste-
board, representing the plate of gypsum, its planes of
vibration, and also those of the polarizer and analyzer.
Two parallel pieces of the board are to be separated by
an interval which shall represent the thickness of the
film of gypsum. Between them, two other pieces,
intersecting each other at a right angle, are to repre-
sent the planes of vibration within the film ; while at-
tached to the two parallel surfaces outside are two other
pieces of board to represent the planes of vibration of
the polarizer and analyzer. On the two intersecting
planes the waves are to be drawn, showing the resolu-
tion of the first polarized beam into two others, and
then the subsequent reduction of the two systems of vi-
brations to a common plane by the analyzer. Follow-
ing out rigidly the interaction of the two systems of
waves, we are taught by such a model that all the phe-
nomena of colour obtained by the combination of the
waves when the planes of vibration of the two Nicols
are parallel are displaced by the complementary phe-
nomena when the planes of vibration are perpendicular
to each other.

In considering the next point, we will operate, for
the sake of simplicity, with monochromatic light — with
red light, for example, which is easily obtained pure by
red glass. Supposing a certain thickness of the gypsum
produces a retardation of half a wave-length, twice this
thickness will produce a retardation of two half wave-
lengths, three times this thickness a retardation of three
half-wave lengths, and so on. Now, when the Nicols

134 ON LIGHT. i-ECT,

are parallel, the retardation of half a wave-length, or
of any odd number of half wave-lengths, produces ex-
tinction ; at all thicknesses, on the other hand, which
correspond to a retardation of an even number of half
wave-lengths, the two beams support each other, when
they are brought to a common plane by the analyzer.
Supposing, then, that we take a plate of a wedge-form,
which grows gradually thicker from edge to back,
we ought to expect in red light a series of recurrent
bands of light and darkness ; the dark bands occurring
at thicknesses which produce retardations of one, three,
five, etc., half wave-lengths, while the bright bands
occur between the dark ones. Experiment proves the
wedge-shaped film to show these bands. They are also
beautifully shown by a circular film, so worked as to
be thinnest at the centre, and gradually increasing in
thickness from the centre outwards. A splendid series
of rings of light and darkness is thus produced.

When, instead of employing red light, we employ
blue, the rings are also seen : but, as they occur at
thinner portions of the film, they are smaller than the
rings obtained with the red light. The consequence
of employing white light may be now inferred ; inas-
much as the red and the blue fall in different places,
we have iris-coloured rings produced by the white
light.

Some of the chromatic effects of irregular crystal-
lization are beautiful in the extreme. Could I intro-
duce between our Nicols a pane of glass covered by
those frost-ferns which the cold weather renders now so
frequent, rich colours would be the result. The beau-
tiful effects of the irregular crystallization of tartaric
acid and other substances on glass plates, now presented

IV. COMPLEMENTARY PHENOMENA. 135

to you, illustrate what you might expect from the
frosted window-pane. And not only do crystalline
bodies act thus upon light, but almost all bodies that
possess a definite structure do the same. As a general
rule, organic bodies act thus upon light ; for their
architecture implies an arrangement of the molecules,
and of the ether, wliich involves double refraction. A
film of horn, or the section of a shell, for example,
yields very beautiful colours in polarized light. In a
tree, the ether certainly possesses different degrees of
elasticity along and across the fibre ; and, were wood
transparent, this peculiarity of molecular structure
would infallibly reveal itself by chromatic phenomena
like those that you have seen.

§ 4. Colours 'produced by Strain and Pressure.

But not only do natural bodies behave in this way,
but it is possible, as shown by Brewster, to confer, by
artificial strain or pressure, a temporary double-refract-
ing structure upon non-crystalline bodies, such as
common glass. This is a point worthy of illustration.
When I place a bar of wood across my knee and seek to
break it, what is the mechanical condition of the bar ?
It bends, and its convex surface is strained longitudi-
nally ; its concave surface, that next my knee, is longitu-
dinally pressed. Both in the strained portion and in the
pressed portion the ether is thrown into a condition
which would render the wood, were it transparent, double-
refracting. For, in cases like the present, the drawing
of the molecules asunder longitudinally is always ac-
companied by their approach to each other laterally ;
wliile the longitudinal squeezing is accompanied by

136 ON LIGHT. UBCT.

lateral retreat. Each half of the bar exhibits this anti-
thesis, and is therefore double-refracting.

Let us now repeat this experiment "with a bar of
glass. Between the crossed Nicols I introduce such a
bar. By the dim residue of light lingering upon the
screen, you see the image of the glass, but it has no
eifect upon the light. I simply bend the glass bar
with my finger and thumb, keeping its length oblique
to the directions of vibration in the Nicols. Instantly
light flashes out upon the screen. The two sides of
the bar are illuminated, the edges most, for here the
strain and pressure are greatest. In passing from
longitudinal strain to longitudinal pressure, we cross a
portion of the glass where neither is exerted. This is
the so-called neutral axis of the bar of glass, and along
it you see a dark band, indicating that the glass along
this axis exercises no action upon the light. By em-
ploying the force of a press, instead of the force of my
finger and thumb, the brilliancy of the light is greatly
augmented.

Again, I have here a square of glass which can be
inserted into a press of another kind. Introducing
the uncompressed square between the prisms, its neu-
trality is declared ; but it can hardly be held suffi-
ciently loosely in the press to prevent its action from
manifesting itself. Already, though the pressure is
infinitesimal, you see spots of light at the points where
the press is in contact with the glass. On tirrning a
screw the image of the square of glass flashes out upon
the screen. Luminous spaces are seen separated from
each other by dark bands.

Every two adjacent luminous spaces are in oppo-
site mechanical conditions. On one side of the dark

IT. EFFECT OF MECHANICAL STRAINS AND PRESSURES. 137

band we have strain, on the other side pressure ;
while the dark band marks the neutral axis between
both. I now tighten the vice, and you see colour ;
tighten still more, and the colours appear as rich as
those presented by crystals. Eeleasing the vice, the
colours suddenly vanish ; tightening suddenly, they
reappear. From the colours of a soap-bubble Newton
was able to infer the thickness of the bubble, thus
uniting by the bond of thought apparently incongruous
things. From the colours here presented to you, the
magnitude of the pressure employed might be in-
ferred. Indeed, the late M. Wertheim, of Paris,
invented an instrument for the determination of strains
and pressures, by the colours of polarized liglit, which
exceeded in accuracy all previous instruments of the
kind.

And now we have to push these considerations to a
final illustration. Polarized light may be turned to
account in various ways as an analyzer of molecular
condition. It may, for instance, be applied to reveal
the condition of a solid body when it becomes sonorous.
A strip of glass six feet long, two inches wide, and a
quarter of an inch thick, is held at the centre between
the finger and thumb. On sweeping a wet woollen rag
over one of its halves, you hear an acute sound due to the
vibrations of the glass. What is the condition of the
glass while the sound is heard ? This : its two halves
lengthen and shorten in quick succession. Its two ends,
therefore, are in a state of quick vibration ; but at the
centre the pulses from the two ends alternately meet
and retreat from each other. Between their opposing
actions, the glass at the centre is kept motionless ;
but, on the other hand, it is alternately strained and

138 ON LIGHT. LECT.

compressed. The state of the glass may be illustrated by

Fig. 40.

.r-^-." "t:

a row of spots of light, as the propagation of a sonorous

I'lo. 41.

rr. ACTION OF SONOROUS VIBRATIONS. 139

pulse was illustrated in our second lecture. By a simple
mechanical contrivance the spots are made to vibrate to
and fro : the terminal dots have the largest amplitude
of vibration, while those at the centre are alternately
crowded together and drawn asunder, the centre one
not moving at all. (In fig. 40, A B may be taken to
represent the glass rectangle with its centre condensed ;
while A' B' represents the same rectangle with its
centre rarefied. The ends of the strip suffer neither
condensation nor rarefaction.)

If we introduce the strip of glass (s s', fig. 41 ) between
the crossed Nicols, taking care to keep it oblique to the
directions of vibration of the Nicols, and sweep our wet
rubber over the glass, this may be expected to occur :
At every moment of compression the light will flash
through ; at every moment of strain the light will also
flash through ; and these states of strain and pressure
will follow each other so rapidly that we may expect a
permanent luminous impression to be made upon the
eye. By pure reasoning, therefore, we reach the con-
clusion that the light will be revived whenever the glass
. is sounded. That it is so, experiment testifies : at every
sweep of the rubber, a fine luminous disk (o) flashes
out upon the screen. The experiment may be varied
in this way : Placing in front of the polarizer a plate of
unannealed glass, you have a series of beautifully
coloured rings, intersected by a black cross. Every
sweep of the rubber not only abolishes the rings, but
introduces complementary ones, the black cross being,
for the moment, supplanted by a white one. This is a
modification of a beautiful experiment which we owe
to Biot. His apparatus, however, confined the obser-
vation of it to a single person at a time.

140 ON LIGHT. u'CT.

§ 5. Colours of Unannealed Glass.

Bodies are usually expanded by heat and con-
tracted by cold. If the heat be applied with perfect
uniformity, no local strains or pressures come into play ;
but, if one portion of a solid be heated and other
portions not, the expansion of the heated portion intro-
duces strains and pressures which reveal themselves
under the scrutiny of polarized light. When a square
of common window-glass is placed between the
Nicols, you see its dim outline, but it exerts no
action on the polarized light. Held for a moment
over the flame of a spirit-lamp, on reintroducing
it between the Nicols, light flashes out upon the
screen. Here, as in the case of mechanical action,
you have luminous spaces of strain divided by dark
neutral axes from spaces of pressure.

Let us apply the heat more symmetrically. A
small square of glass is perforated at the centre, and
into the orifice a bit of copper wire is introduced.
Placing the square between the prisms, and heating
the wire, the heat passes by conduction to the
glass, through which it spreads from the centre out-
wards. You immediately see, bounding four lumi-
nous quadrants, a dim cross, which becomes gradually
blacker by comparison with the adjacent brightness.
And as, in the case of pressure, we produced colours, so
here also, by the proper application of heat, gorgeous
chromatic effects may be produced. The condition
necessary to the production of these colours may be
rendered permanent by first heating the glass sufii-
ciently, and then cooling it, so that the chilled mass

lY.

ACTION OF UNANNEALED GLASS.

141

shall remain in a state of permanent strain and pressure.
Two or three examples will illustrate this point. Figs.

Fig. 42.

Fig. 43.

42 and 43 represent the figiures obtained with two
pieces of glass thus prepared. Two rectangular pieces

142 ON LIGHT. lECT.

of unannealed glass, crossed and placed between the
polarizer and analyzer, exhibit the beautiful iris fringes
represented in fig. 44.

§ 6. Circular Polarization.

But we have to follow the ether still further into its
hiding-places. Suspended before you is a pendulum,
which, when drawn aside and liberated, oscillates to and
fro. If, when the pendulum is passing the middle point
of its excursion, I impart a shock to it tending to drive
it at right angles to its present course, what occurs ?
The two impulses compound themselves to a vibration
oblique in direction to the former one, but the pen-
dulum still oscillates in a plane. But, if the rect-
angular shock be imparted to the pendulum when it is
at the limit of its swing, then the compounding of the
two impulses causes the suspended ball to describe not
a straight line, but an ellipse ; and, if the shock be
competent of itself to produce a vibration of the same
amplitude as the first one, the ellipse becomes a circle.

Why do I dwell upon these things ? Simply to make
known to you the resemblance of these gross mechanical
vibrations to the vibrations of light. I hold in my hand
a plate of quartz cut from the crystal perpendicular to its
axis. The crystal thus cut possesses the extraordinary
power of twisting the plane of vibration of a polarized
ray to an extent dependent on the thickness of the
crystal. And the more refrangible the light the greater
is the amount of twisting ; so that, when white light
is employed, its constituent colours are thus drawn
asunder. Placing the quartz between the polarizer
and analyzer, you see this vivid red, and, turning the
analyzer in front, from right to left, the other colours

IT. ACTION OF QUAETZ CRYSTALS. 143

of the spectrum appear in succession. Specimens of
quartz have been found which require the analyzer to
be turned from left to right to obtain the same succes-
sion of colours. Crystals of the first class are therefore
called right-handed, and of the second class, left-handed
crystals.

With profound sagacity, Fresnel, to whose genius
we mainly owe the expansion and final triumph of the
undulatory theory of light, reproduced mentally the
mechanism of these crystals, and showed their action to
be due to the circumstance that, in them, the waves of
ether so act upon each other as to produce the condition
represented by our rotating pendulum. Instead of
being plane polarized, the light in rock crystal is cir-
cularly polarized. Two such rays, transmitted along
the axis of the crystal, and rotating in opposite direc-
tions, when brought to interference by the analyzer,
are demonstrably competent to produce all the observed
phenomena.

§ 7. Complementary Colours of Bi-refr acting Spar in
Circularly Polarized Light. Proof that Yellow and
Blue are Complementai^.

I now remove the analyzer, and put in its place the
piece of Iceland spar with which we have already illus-
trated double refraction. The two images of the car-
bon-points are now before you, produced, as you know,
by two beams vibrating at right angles to each other.
Introducing a plate of quartz between the polarizer
and the spar, the two images glow with complementary
colours. Employing the image of an aperture instead
of that of the carbon-points, we have two coloured

144

ON LIGHT.

LECT.

circles. As the analyzer is caused to rotate, the colours
pass through various changes ; but they are always
complementary. When the one is red, the other is
green ; when the one is yellow, the other is blue.
Here we have it in our power to demonstrate afresh a
statement made in our first lecture, that, although the
mixture of blue and yellow pigments produces green,
the mixtm-e of blue and yellow lights produces white.
By enlarging our aperture, the two images produced
by the spar are caused to approach each other, and
finally to overlap. The one is now a vivid yellow.

Fig. 45.

9 h

1#^

the other a vivid blue, and you notice that where the
colours are superposed we have a pure white. (See fig.
45, where N is the end of the polarizer, Q the quartz
plate, L a lens, and B the bi-refracting spar. The two
images overlap at 0, and produce white by their mix-
ture.)

§ 8. The Magnetization of Light.

This brings us to a point of our inquiries which,
though rarely illustrated in lectures, is nevertheless
60 likely to affect profoundly the future course of

IT. MAGNETIZATION OF LIGHT. 145

scientific thought that I am unwilling to pass it over
without reference. I refer to tlie experiment whicli
Faraday, its discoverer, called the ' magnetization of
light.' The arrangement for this celebrated experiment
is now before you. We have first om: electric lamp,
then a Nicol prism, to polarize the beam emergent
from the lamp ; then an electro-magnet, then a second
Nicol, and finally our screen. At the present moment
the prisms are crossed, and the screen is dark. I
place from pole to pole of the electro-magnet a cylin-
der of a peculiar kind of glass, first made by Faraday,
and called Faraday*s heavy glass. Through this glass
the beam from the polarizer now passes, being inter-
cepted by the Nicol in front. On exciting the magnet
light instantly appears upon the screen. By the action
of the magnet upon the ether contained within the
heavy glass, the plane of vibration is caused to rotate,
the light being thus enabled to get through the
analyzer.

The two classes into which quartz-crystals are di-
vided have been already mentioned. In my hand I hold
a compound plate, one half of it taken from a right-
handed, and the other from a left-handed crystal.
Placing the plate in front of the polarizer, I turn one
of the Nicols until the two halves of the plate show a
conimon puce colour. This yields an exceedingly sensi-
tive means of rendering visible the action of a magnet
upon light. By turning either the polarizer or the
analyzer through the smallest angle, the uniformity of
the colour disappears, and the two halves of the quartz
show different colours. The magnet produces an effect
equivalent to this rotation. The puce-coloured circle
is now before you on the screen. (See fig. 46, where

146

ON LIGHT.

LBCT.

N is the nozzle of the lamp, H the first Nicol, Q the
biquartz plate, L a lens, M the electro-magnet, with
the heavy glass across its poles, and P the second Nicol.)
Exciting the magnet, one half of the image becomes
suddenly red, the other half green. Interrupting the
current, the two colours fade away, and the primitive
puce is restored.

Fig. 46.

The action, moreover, depends upon the polarity
of the magnet, or, in other words, on the direction of
the current which surrounds the magnet. Eeversing
the current, the red and green reappear, but they
have changed places. The red was formerly to the
right, and the green to the left ; the green is now to
the right, and the red to the left. With the most ex-
quisite ingenuity, Faraday analyzed all those actions
and stated their laws. This experiment, however, long
remained rather a scientific curiosity than a fruitful
germ. That it would bear fruit of the highest impor-
tance, Faraday felt profoundly convinced, and recent re-
searches are on the way to verify his conviction.

IT. RINGS ROUND AXES OF CRYSTALS. 147

§ 9. Iris-rings surrounding the Axes of Cry.stals.

A few words more are necessary to complete our
knowledge of the wonderful interaction between pon-
derable molecules and the ether interfused among them.
Symmetry of molecular arrangement implies symmetry
on the part of the ether ; atomic dissymmetry, on the
other hand, involves the dissymmetry of the ether, and,
as a consequence, double refraction. In a certain class
of crystals the structure is homogeneous, and such
crystals produce no double refraction. In certain other
crystals the molecules are ranged symmetrically round
a certain line, and not aroimd others. Along the
former, therefore, the ray is undivided, while along all
the others we have double refraction. Ice is a familiar
example : its molecules are built with perfect symmetry
around the perpendiculars to the planes of freezing,
and a ray sent through ice in this direction is not
doubly refracted; whereas, in all other directions, it is.
Iceland spar is another example of the same kind : its
molecules are built symmetrically round the line unit-
ing the two blunt angles of the rhomb. In this direc-
tion a ray suffers no double refraction, in all others it
does. This direction of no double refraction is called
the optic axis of the crystal.

Hence, if a plate be cut from a crystal of Iceland spar
perpendicular to the axis, all rays sent across this plate
in the direction of the axis will produce but one image.
But, tlie moment we deviate from the parallelism with
the axis, double refraction sets in. If, therefore, a
beam that has been rendered conical by a converging
lens be sent through the spar so that the central ray of

148 ON LIGHT.

LEC7.

the cone passes along the axis, this ray only will escape
double refraction. Each of the others will he divided
into an ordinary and an extraordinary ray, the one
moving more slowly through the crystal than the
other; the one, therefore, retarded with reference to
the other. Here, then, we have the conditions for
interference, when the waves are reduced by the ana-
lyzer to a common plane.

Placing the plate of Iceland spar between the crossed
Nicol's prisms, and employing the conical beam, we have

Fig. 47.

upon the screen a beautiful system of iris-rings sur-
rounding the end of the optic axis, the circular bands
of colour being intersected by a black cross (fig. 47 ).
The arms of this cross are parallel to the two directions
of vibration in the polarizer and analyzer. It is
easy to see that those rays whose planes of vibration
within the spar coincide with the plane of vibration
of either prism, cannot get through both. This com-
plete interception produces the arms of the cross.
With monochromatic light the rings would be simply
bright and black — the bright rings occurring at those
thicknesses of the spar which cause the rays to con-

IV.

UNIAXAL AND BIAXAL CRYSTALS.

14'J

spire ; the black rings at those thicknesses which cause
them to quench each other. Turning the analyzer 90°
round, we obtain the complementary phenomena. The
black cross gives place to a bright one, and every dark
ring is supplanted also by a bright one (fig. 48). Here,

Fig. 48.

as elsewhere, the different lengths of the light-waves
give rise to iris-colom-s when white light is employed.
Besides the regular crystals which produce double

Fia. 49.

refraction in no direction, and tlie uniaxal crystals
which produce it in all directions but one, Brewster
discovered that in a large class of crystals there are
huo directions in which double refraction does not take
place. These are called hiaxal crystals. When plates
of these crystals, suitably cut, are placed between the
8

loU ON LIGHT. MCT.

polarizer and analyzer, the axes (A A', fig 49) are seen
surrounded, not by circles, but by curves of another order
and of a perfectly definite mathematical character. Each
band, as proved experimentally by Herschel, forms a
leniniscata ; but the experimental proof was here, as
in numberless other cases, preceded by the deduction
which showed that, according to the undulatory theory,
the bands must possess this special character.

§ 10. Power of the Undulatory Theory.

I have taken this somewhat wide range over polar-
ization itself, and over the phenomena exhibited by
crystals in polarized light, in order to give you some
notion of the firmness and completeness of the theory
which grasps them all. Starting from the single
assumption of transverse undvdations, we first of all
determine the wave-lengths, and find all the pheno-
mena of colour dependent on this element. The wave-
lengths may be determined in many independent ways.
Newton virtually determined them when he measured
the periods of his Fits : the length of a fit, in fact, is
that of a quarter of an undulation. The wave-lengths
may be determined by diffraction at the edges of a slit
(as in the Appendix to these Lectures) ; they may be
deduced from the interference fringes produced by
reflection ; from the fringes produced by refraction ; also
by lines drawn with a diamond upon glass at measured
distances asunder. And when the lengths determined
by these independent methods are compared together,
the strictest agreement is found to exist between them.

With the wave-lengths at our disposal, we follow the
ether into the most complicated cases of interaction

tv. GRASP OF THE UNDULATORY THEORY. 151

between it and ordinary matter, ' the theory is equal
to them all. It makes not a single new physical
hypothesis ; but out of its original stock of principles
it educes the counterparts of all that observation shows.
It accounts for, explains, simplifies the most entangled
cases ; corrects known laws and facts ; predicts and dis-
closes unknown ones ; becomes the guide of its former
teacher Observation ; and, enlightened by mechanical
conceptions, acquires an insight which pierces through
shape and colour to force and cause.' '

But, while I have thus endeavoured to illustrate be-
fore you the power of the undulatory theory as a solver
of all the difficulties of optics, do I therefore wish you
to close your eyes to any evidence that may arise
against it? By no means. You may urge, and justly
J* nrge, that a hundred years ago another theory was held
r by the most eminent men, and that, as the theory then
1^ held had to yield, the undulatory theory may have to
|>|. • yield also. This seems reasonable ; but let us under-
\ » stand the precise value of the argument. In similar
^ language a person in the time of Newton, or even in
^A our time, might reason tlius: Hipparchus and Ptolemy,

tand numbers of great men after them, believed that
the earth was the centre of the solar system. But this
deep-set theoretic notion had to give way, and the
theory of gravitation may, in its turn, have to give
way also. This is just as reasonable as the first argti-
ment. Wherein consists the strength of the theory of
gravitation ? Solely in its competence to account for
all the phenomena of the solar system. Wherein con-
sists the strength of the theory of undulation ? Solely
in its competence to disentangle and explain phenomena

' Whcwell.

ia2 ON LIGHT. LEOT.

a hundred-fold more complex than tliose of the solar
system. Accept if you will the scepticism of Mr.
Mill' regarding the undulatory theory ; but if your
scepticism be philosophical, it will wrap the theory of
gravitation in the same or greater doubt.'

§ 11. The Blue of the Shy.

I am unwilling to quit these chromatic phenomena
without referring to a source of colour which has often
come before me of late in the blue of your skies at
noon, and the deep crimson of your horizon after the
set of sun. I will here summarise and extend what I have
already said upon this subject in another place. Proofs
of tlie most cogent description could be adduced
to show that the blue light of the firmament is
reflected light. That light comes to us across the direc-
tion 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. The solar light, moreover, is not reflected by
the sky in the proportions which produce white. The
sky is blue, which indicates an excess of the smaller waves.
The blueness of the air has been given as a reason for
the blueness of the sky ; but then the question arises.
How, if the air be blue, can the light of sunrise and sun-
set, which travels through vast distances of air, be yellow,
orange, or even red ? The passage of the white solar

' Removed from us since these words were -written.

* The only essay known to me on the Undulatory Theory, from the
pen of an American writer, is an excellent one by President Barnard,
published in the Smithsonian Report for 1862.

17. ^ SCATTERING BY SMALL PARTICLES. 153

light through a blue medium could by no possibility
redden the light ; the hypothesis of a blue air is there-
fore untenable. In fact the agent, whatever it be,
which sends us the liglit of the sky, exercises in so
doing a dichroitic action. The liglit reflected is blue,
the light transmitted is orange or red. A marked dis-
tinction is thus exhibited between reflection from the
sky and that from an ordinary cloud, which exercises
no such dichroitic action.

The cloud, in fact, takes no note of size on the part
of the waves of ether, but reflects them all alike. 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 scatter them all indifferentl}^ A broad cliff re-
flects an Atlantic roller as easily as a ripple produced
by a sea-bird's wing ; and in the presence of large re-
flecting surfaces, the existing differences of magnitude
among the waves of ether may also disappear. But
supposing the reflecting particles, instead of being very
large, to be very small, in comparison with the size of
the waves. Then, instead of the whole wave being
fronted and in great part thrown back, a small portion
only is shivered off by the obstacle. Suppose, then,
such minute foreign particles to be diffused in our at-
mosphere. Waves of all sizes impinge upon them,
and at every collision a portion of the impinging wave
is struck off. All the waves of the spectrum, from the
extreme red to the extreme violet, are thus acted upon ;
but in what proportions will they be scattered ? Large-
ness is a thing of relation ; and the smaller tlie wave,
the greater is the relative size of any particle on which
the wave impinges, and the greater also the relative re-
flection.

154 ON LIGHT.

IBCT.

A small pebble placed in the way of the ring-ripples
produced by heavy rain-drops on a tranquil pond will
throw back a large fraction of each ripple incident upon
it, while the fractional part of a larger wave thrown
back by the same pebble might be infinitesimal. Now
to preserve the solar light white, its constituent pro-
portions must not be altered ; but in the scattering of
the light by these very small particles we see that the
proportions are altered. The smaller waves are in
excess, and, as a consequence, in the scattered light
blue will be the predominant colour. The other
colom's of the spectrum must, to some extent, be asso-
ciated 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 thus 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 scat-
tered by those particles would be exactly such as we
observe in our azure skies. And, indeed, when this
light is analysed, all the colours of the spectrum
are found in the proportions indicated by our con-
clusion.

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 in-
volved, will appear yellowish. But as the sun sinks
towards the horizon the atmospheric distance increases,
and consequently the number of the scattering particles.
They weaken in succession the violet, the indigo, the
blue, and even disturb the proportions of green. The

IV. AETIFICIAI- SKY. 155

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 re-
flected 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.

But can small particles be really proved to act in the
manner indicated ? No doubt of it. Each one of you
can submit the question to an experimental test.
Water will not dissolve resin, but spirit will ; and when
spirit which holds resin in solution is dropped into
water, the resin immediately separates in solid particles,
which render the water milky. The coarseness of this
precipitate depends on the quantity of the dissolved
resin. Professor Briicke has given us the proportions
which produce particles particularly suited to our pre-
sent purpose. One gramme of clean mastic is dissolved
in eighty-seven grammes of absolute alcohol, and the
transparent solution is allowed to drop into a beaker
containing clear water briskly stirred. An exceedingly
fine precipitate is thus formed, which declares its
presence by its action upon light. Placing a dark sur-
face behind the beaker, and permitting the light to fall
into it from the top or front, the medium is seen to be
of a very fair sky-blue. A trace of soap in water gives
a tint of blue. London milk makes an approximation
to the same colour through the operation of the same
cause : and Helmholtz has irreverently disclosed the
fact that a blue eye is simply a turbid medium.

§ 12. Artificial Sky.

But wc have it in our power to imitate far more
closely the natural conditions of this problem. We can

Iu6 ' ON LIGHT.

LBCT.

generate in air artificial skies, and prove their perfect
identity with the natural one, as regards the exhibition
of a number of wholly unexpected phenomena. It has
been recently shown in a great number of instances that
waves of ether issuing from a strong source, such as the
sun or the electric light, are competent to shake asun-
der the atoms of gaseous molecules. The apparatus
used to illustrate this consists of a glass tube about a
yard in length, and from 2^ to 3 inches internal diame-
ter. The gas or vapour to be examined is introduced
into this tube, and upon it the condensed beara of the
electric lamp is permitted to act. The vapour is so
chosen that one, at least, of itsproductsof decomposition,
as soon as it is formed, shall be precipitated to a kind
of cloud. By graduating the quantity of the vapom',this
precipitation may be rendered of any degree of fineness,
forming particles distinguishable by the naked eye, or
particles which are probably far beyond the reach of
our highest microscopic powers. 1 have no reason to
doubt that particles may be thus obtained whose
diameters constitute but a very small fraction of the
length of a wave of violet light.

Now, in all such cases when suitable vapours are
employed in a sufficiently attenuated state, no matter
what the vapour may be, the visible action commences
with the formation of a blue cloud. Let me guard my-
self at the outset against all misconception as to the use
of this term. The blue cloud here referred to is totally
invisible in ordinary daylight. To be seen, it requires
to be surrounded by darkness, it only being illuminated
by a powerful beam of light. This cloud differs in
many important particulars from the finest ordinary
clouds, and might justly have assigned to it an inter-

TV. rOLARIZATION OF SKY-LIGHT. 157

mediate position between these clouds and true cloud-
less vapour.

It is possible to make the particles of this actinic
cloud grow from an infinitesimal and altogether ultra-
microscopic size to particles of sensible magnitude ; and
by mieans of these, in a certain stage of their growth,
we produce a blue which rivals, if it does not transcend,
that of the deepest and purest Italian sky. Introduc-
ing into our tube a quantity of mixed air and nitrite
of butyl vapour sufficient to depress the mercurial
column of an air-pump one-twentieth of an inch,
adding a quantity of air and hydrochloric acid sufficient
to depress the mercury half an inch further, and send-
ing through this compound and highly attenuated atmo-
sphere, the beam of the electric light ; gradually within
the tube arises a splendid azure, which strengthens for
a time, reaches a maximum of depth and purity, and
then, as the particles grow larger, passes into whitish
blue. This experiment is representative, and it illus-
trates a general principle. Various other colourless
substances of tlie most diverse properties, optical and
chemical, might be employed for this experiment. The
incipient cloud, in every case, would exhibit this superb
blue; thus proving to demonstration that particles of
infinitesimal size, without any colour of their own, and
irrespective of those optical properties exhibited by the
substance in a massive state, are competent to produce
the blue colour of the sky.

§ 13. Polarization of Shj-l'ight.

But there is another subject connected with our
firmament, of a more subtle and recondite character

158 ON LIGHT.

UJCT.

than even its colour. I mean that ' mysterious and
beautiful phenomenon,' the polarization of the light
of the sky. Looking at various points of the blue
firmament through a Nicol's prism, and turning the
prism round its axis, we soon notice variations of
brightness. In certain positions of the prism, and
from certain points of the firmament, the light appears
to be wholly transmitted, while it is only necessary to
turn the prism round its axis through an angle of
ninety degrees to materially diminish the intensity of
the light. Experiments of this kind prove that the
blue light sent to us by the firmament is polarized,
and on close scrutiny it is also found that the direction
of most perfect polarization is perpendicular to the
solar rays. Were the heavenly azure like the ordinary
light of the sun, the turning of the prism would have
no effect upon it ; it would be transmitted equally
during the entire rotation of the prism. The light of
the sky is in great part quenched, because it is in great
part polarized.

The same phenomenon is exhibited in perfection by
our actinic clouds, the only condition necessary to its
production being the smallness of the particles. In all
cases, and with all substances, the cloud formed at the
commencement, when the precipitated particles are
sufficiently fine, is blue. In all cases, moreover, this
fine blue cloud polarizes perfectly the beam which
illuminates it, the direction of polarization enclosing
an angle of 90° with the axis of the illuminating
beam.

It is exceedingly interesting to observe both the
growth and the decay of this polarization. For ten or
fifteen minutes after its first appearance the light from

IT. POLARIZATION BY ARTIFICIAL SKY. 151)

a vividly illuminated incipient cloud, looked at hori-
zontally, is absolutely quenched by a Nicol's prism
with its longer diagonal vertical. But as the sky-blue
is gradually rendered impure by the introduction of
particles of too large a size, in other words, as real
clouds begin to be formed, the polarization begins to
deteriorate, a portion of the light passing through the
prism in all its positions, as it does in the case of sky-
light. It is worthy of note that for some time after
the cessation of perfect polarization the residual light
which passes, when the Nicol is in its position of
minimum transmission, is of a gorgeous blue, the
whiter light of the cloud being extinguished. "When
the cloud texture has become suflBciently coarse to ap-
proximate to that of ordinary clouds, the rotation of
the Nicol ceases to have any sensible efifect on the
quantity of the light discharged at right angles to the
beam.

The perfection of the polarization in a direction
perpendicular to the illuminating beam was also illus-
trated by the following experiment executed with many-
vapours. A Nicol's prism large enough to embrace
the entire beam of the electric lamp was placed
between the lamp and the experimental tube. Send-
ing the beam polarized by the Nicol through the
tube, I placed myself in front of it, the eyes being on a
level with its axis, my assistant occupying a similar
position behind the tube. The short diagonal of the
large Nicol was in the first instance vertical, the plane
of vibration of the emergent beam being therefore also
vertical. As the light continued to act, a superb blue
cloud visible to both my assistant and myself was slowly
formed. But this cloud, so deep and rich when looked at

160 ON LIGHT. lECT.

from the positions mentioned, utterly disappeared when
looked at vertically downwards, or vertically upwards.
Keflection from the cloud was not possible in these
directions. When the large Nicol was slowly turned
round its axis, the eye of the observer being on the
level of the beam, and the line of vision perpendicular
to it, entire extinction of the light emitted hori-
zontally occurred when the longer diagonal of the
large Nicol was vertical. But a vivid blue cloud
was seen when looked at downwards or upwards. This
truly fine experiment, which I should certainly have
made without suggestion, was, as a matter of fact, first
definitely suggested by a remark addressed to me in
a letter by Professor Stokes.

All the phenomena of colour and of polarization
observable in the case of skylight are manifested by
those actinic clouds ; and they exhibit additional phe-
nomena which it would be neither convenient to
pursue, nor perhaps possible to detect, in the actual
firmament. They enable us, for example, to follow
the polarization from its first appearance on the barely
visible blue to its final extinction in the coarser cloud.
These changes, as far as it is now necessary to refer to
them, may be thus summed up : —

1. The actinic cloud, as long as it continues blue,
discharges polarized light in all directions, but the
direction of maximum polarization, like that of sky-
light, is at right angles to the direction of the illumin-
ating beam.

2. As long as the cloud remains distinctly blue the
light discharged from it at right angles to the illumi-
nating beam is ^perfectly polarized. It may be utterly
quenched by a Nicol's prism, the cloud from which it

IV. SUMMARY OF PHENOMENA. 161

issues being caused to disappear. Any deviation frona
the perpendicular enables a portion of the light to get
through the prism.

3. The direction of vibration of the polarized light
is at right angles to the illuminating beam. Hence a
plate of tourmaline, with its axis parallel to the beam,
stops the light, and with the axis perpendicular to the
beam transmits the light.

4. A plate of selenite placed between the Nicol and
the actinic cloud shows the colours of polarized light ;
in fact, the cloud itself plaj/s the part of a polarizing
Nicol.

5. The particles of the blue cloud are immeasurably
small, but they increase gradually in size, and at a certain
period of their growth cease to discharge perfectly
polarized light. For some time afterwards the light
that reaches the eye through the Nicol is of a mag-
nificent blue, far exceeding in depth and purity that
of the purest sky ; thus the waves that first feel the
influence of size, at both limits of the polarization,
are the shortest waves of the spectrum. These are
the first to accept polarization, and they are the first
to escape from it.

162 ON LIGHT.

UECT.

LECTUEE V.

KANGE OF VISION NOT COJOIENSURATE "WITH EANGE OF EADIATION — TIIV

ULTRA-VIOLET EAYS FLUORESCENCE RENDERING INVISIBLE RAYS

VISIBLE VISION NOT THE ONLY SENSE APPEALED TO BY THE SOLAB

AND ELECTRIC BEAM — HEAT OF BEAM CONBUSTION BY TOTAL BEAM

AT THE FOCI OF MIRRORS AND LENSES — COMBUSTION THROUGH ICE-
LENS — IGNITION OF DIAMOND — SEARCH FOR THE EAY3 HERE EFFEC-
TIVE— SIR VTILLIAM HBRSCHEl's DISCOVERY OF DARK SOLAR RAYS

INVISIBLE RAYS THE BASIS OF THE VISIBLE — DETACHMENT BY A RAY-
FILTBE OF THE INVISIBLE RAYS FROM THE VISIBLE — COMBUSTION AT
DARK FOCI — CONVERSION OF HEAT-RAYS INTO LIGHT-RAYS — CALOR-
ESCENCE — PART PLAYED IN NATURE BY DARK HAYS — IDENTITY OF LIGHT
AND RADIANT HEAT — INVISIBLE IMAGES — REFLECTION, REFRACTION,
PLANE POLARIZATION, DEPOLARIZATION, CIRCULAR POLARIZATION,
DOUBLE REFRACTION, AND MAGNETIZATION OF RADIANT HEAT.

§ 1. Range of Vision and of Radiation.

The first question that we have to consider to-night
is this : Is the eye, as an organ of vision, commensurate
with the whole range of solar radiation — is it capable
of receiving visual impressions from all the rays emitted
by the sun ? The answer is negative. If we allowed
ourselves to accept for a moment that notion of gradual
growth, amelioration, and ascension, implied by the
term evolution^ we might fairly conclude that there
are stores of visual impressions awaiting man, far
greater than those now in his possession. Eitter dis-
covered in 1801 that beyond the extreme violet of the
spectrum there is a vast efflux of rays which are totally

r. ULTRA-VIOLET RAYS. 163

useless as regards our present powers of visiou. These
ultra-violet waves, however, thougli incompetent to
awaken the optic nerve, can shake asunder the mole-
cules of certain compound substances on which they
impinge, thus producing chemical decomposition.

But though the blue, violet, and ultra-violet rays
can act thus upon certain substances, the fact is hardly
sufficient to entitle them to the name of ' chemical rays,'
usually applied to distinguish them from the other
constituents of the spectrum. As regards their action
upon the salts of silver, and many other substances
they may perhaps merit this title ; but in the case o
the grmdest example of the chemical action of light —
the decomposition of carbonic acid in the leaves of
plants, with which my eminent friend Dr. Draper has
so indissolubly associated his name — the yellow rays are
found to be most active.

There are substances, however, on which the violet
and idtra-violet waves exert a special decomposing
power ; and, by permitting the invisible spectrum to
fall upon surfaces prepared with such substances, we
veveal both the existence and the extent of the ultra-
;iolet spectrum.

§ 2. Ultra-violet Bays : Fluorescence.

The method of exhibiting the action of the ultra-
violet rays by their chemical action has been long
known ; indeed, Thomas Young photograplied the ultra-
violet rings of Newton. We have now to demonstrate
their presence in another way. As a general rule,
bodies either transmit light or absorb it ; but there is a
third case in which the light falling upon the body is

1G4 ON LIGHT. LECT

neither transmitted nor absorbed, but converted into
light of another kind. Professor Stokes, the occupant
of the chair of Newton in the University of Cambridge,
has demonstrated this change of one kind of light into
another, and has pushed his experiments so far as to
render the invisible rays visible.

A large number of substances examined by Stokes,
when excited by the invisible ultra-violet waves, have
been proved to emit light. You know tlie rate of vibra-
tion corresponding to the extreme violet of the spectrum ;
you are aware that to produce the impression of this
colour, the retina is struck 789 millions of millions of
times in a second. At this point, the retina ceases to
be useful as an organ of vision, for though struck by
waves of more rapid recurrence, they are incompetent
to awaken the sensation of light. But when such non-
visual waves are caused to impinge upon the molecules
of certain substances — on those of sulphate of quinine,
for example — they compel those molecules, or their
constituent atoms, to vibrate ; and the peculiarity is,
that the vibrations thus set up are of slower jperiod
than those of the exciting waves. By this lowering of
the rate of vibration through the intermediation of the
sulphate of quinine, the invisible rays are brought
within the range of vision. We shall subsequently
have abundant opportunity for learning that trans-
parency to the visible by no means involves transparency
to the invisible rays. Our bisulphide of carbon, for
example, which, employed in prisms, is so eminently
suitable for experiments on the visual rays, is by no
means so suitable for these ultra-violet rays. Flint
glass is better, and rock crystal is better than flint

V. FLUOKESCENCE, 1 65

glass. A glass prism, however, will suit our present
purpose.

Casting by means of such a prism a spectrum, not
upon the white surface of our screen, but upon a sheet
of paper which has been wetted with a saturated
solution of the sulphate of quinine, and afterwards dried,
an obvious extension of the spectrum is revealed. We
have, in the first instance, a portion of the violet
rendered whiter and more brilliant; but, besides this,
we have the gleaming of the colour where, in the case
of unprepared paper, nothing is seen. Other substances
produce a similar effect, A substance, for example, re-
cently discovered by President Morton, and named by
him Thallene, produces a very striking elongation of
the spectrum, the new light generated being of peculiar
brilliancy.

Fluor spar and some other substances, when raised to
a temperature still under redness, emit light. During the
ages which have elapsed since their formation, this capa-
city of shaking the ether into visual tremors appears to
have been enjoyed by these substances. Light has been
potential within them all this time ; and, as well ex-
plained by Draper, the heat, though not itself of visual
intensity, can unlock the molecules so as to enable them
to exert their long-latent power of vibration. This de-
portment of fluor spar determined Stokes in his choice
of a name for his great discovery : he called this ren-
dering visible of the ultra-violet rays Fluorescence.

By means of a deeply-coloured violet glass, we cut
off almost the whole of the light of our electric beam ;
but this glass is pecidiarly transparent to the violet and
ultra-violet rays. The violet beam now crosses a large
jar filled witli water, into which I pour a solution of

16© ON LIGHT.

LECT.

sulphate of quinine. Clouds, to all appearance opaque,
instantly tumble downwards. Fragments of horse-
chestnut bark thrown upon the water also send down
beautiful cloud-like striae. But these are not clouds ;
there is nothing precipitated here : the observed action
is an action of Tnolecules, not of particles. The me-
dium before you is not a turbid medium, for when you
look through it at a luminous surface it is perfectly
clear.

If we paint upon a piece of paper a flower or a
bouquet with the sulphate of quinine, and expose it to
the full beam, scarcely anything is seen. But on inter-
posing the violet glass, the design instantly flashes forth
in strong contrast with the deep surrounding violet.
A most beautiful example of such a design has been pre-
pared for me with his thallene by President Morton :
placed in the violet light it exhibits a peculiarly
brilliant fluorescence. From the experiments of Dr.
Bence Jones, it would seem that there is some sub-
i tance in the hiunan body resembling the sulphate of
quinine, which causes all the tissues of the body to be
more or less fluorescent. The crystalline lens of the
eye exhibits the effect in a very striking manner.
When, for example, I plunge my eye into this violet
beam, I am conscious of a whitish-blue shimmer filling
the space before me. This is caused by fluorescent
light generated in the eye itself. Looked at from with-
out, the crystalline lens at the same time is seen
to gleam vividly.

Long before its physical origin was understood this
fluorescent light attracted attention. Boyle, as Sir
Charles Wheatstone has been good enough to point
out to me, describes it with great fullness and exact-

V BOYLE AND GOETHE. 167

iiess. ' We have sometimes,' he says, ' found in the
shops of our druggists a certain wood which is there
called Lignum Nephrlt'icuTn, because the inhabitants
of the country where it grows are wont to use the
infusion of it, made in fair water, against the stone in
the kidneys. This wood may afford us an experiment
which, besides the singularity of it, may give no small
assistance to an attentive considerer towards the detec-
tion of the nature of colours. Take Lignum Nephri-
ticum, and with a knife cut it into thin slices : put
about a handful of these slices into two or three or
four pounds of the purest spring water. Decant this
impregnated water into a glass phial ; and if you hold
it directly between the light and your eye, you shall
see it wholly tinted with an almost golden colour.
But if you hold this phial from the light, so that your
eye be placed betwixt the window and the phial, the
liquid will appear of a deep and lovely ceruleous
colour.'

' These,' he continues, ' and other phenomena which
I have observed in this delightful experiment, divers
of my friends have looked upon, not without some
wonder ; and I remember an excellent oculist, finding
by accident in a friend's chamber a phial full of
this liquor, which I had given that friend, and having
never heard anything of the experiment, nor having
anybody near him who could tell him what this strange
liquor might be, was a great while apprehensive, as he
presently afterwards told me, that some strange new
distemper was invading his eyes. And I confess that
the unusualness of the phenomenon made me very
solicitous to find out the cause of this experiment ; and
though I am far from pretending to have found it, yet

168 ON LIGHT.

LECT.

my enquiries have, I suppose, enabled me to give such
hints as may lead your greater sagacity to the discovery
of the cause of this wonder.''

Goethe in his ' Farbenlehre ' thus describes the
fluorescence of horse-chestnut bark : — ' Let a strip of
fresh horse-chestnut bark be taken and clipped into a
glass of water ; the most perfect sky-blue will be imme-
diately produced.'^ Sir John Herschel first noticed
and described the fluorescence of the sulphate of quin-
ine, and showed that the light proceeded from a thin
stratum of the solution adjacent to the surface where
the lig"ht enters it. He showed, moreover, that the
incident beam, although not sensibly weakened in lumi-
nous power, lost, in its transmission through the solution
of sulphate of quinine, the power of producing the
blue fluorescent light. Sir David Brewster also
worked at the subject ; but to Professor Stokes we are
indebted not only for its expansion, but for its full
and final explanation.

§ 3. The Heat of the Electric Beam. Ignition through
a Lens of Ice. Possible Coraetary Temperature.

But the waves from our incandescent carbon-points
appeal to another sense than that of vision. They not
only produce light, but heat, as a sensation. The
magnified image of the carbon-points is now upon the
screen; and with a suitable instrument the heating
power of the rays which form that image might be
readily demonstrated. In this case, however, the heat
is spread over too large an area to be very intense.

' Boyle's Works, Birch's edition, vol. i. pp. 729 and 730.
' Werke, b. xxix. p. 24.

HEAT OF ELECTRIC BEAil.

1G9

Pushing out the lens, and causing a movable screen to
approach our lamp, the image is seen to become
smaller and smaller ; the rays at the same time be-
coming more and more concentrated, until finally
they are able to pierce black paper with a burning ring.
Pushing back the lens so as to render the rays parallel
and receiving them upon a concave mirror, they are
brought to a focus ; paper placed at that focus is caused
to smoke and burn. Heat of this intensity may be
obtained with our ordinary camera and lens, and a
concave mirror of very moderate power.

We will now adopt stronger measures with thi
radiation. In this larger camera of blackened tin is

Fig. 60,

placed a lamp, in all particulars similar to those already
employed. But instead of gathering up the rays from
the carbon-points by a condensing lens, we gatlier them
up by a concave mirror {m m/, fig 50), silvered in front
and placed behind the carbons (P). By this mirror we
can cause the rays to issue tlirough the orifice in front
of the camera, either parallel or convergent. They
are now parallel, and therefore, to a certain extent,

170 ON LIGHT.

lUCT.

diffused. We place a convex lens (L) in the path of
the beam ; the light is converged to a focus (C), and at
that focus paper is not only pierced and a burning ring
formed, but it is instantly set ablaze.

Many metals may be burned np in the same way.
In our first lecture the combustibility of zinc was men-
tioned. Placing a strip of sheet-zinc at this focus, it
is instantly ignited, burning with its characteristic
purple flame. And now I will substitute for our glass
lens (L) one of a more novel character. In a smooth
iron mould a lens of pellucid ice has been formed.
Placing it in the position occupied a moment ago by
the glass lens, I can see the beam brought to a sharp
focus. At the focus I place a bit of black paper, with
a little gun-cotton folded up within it. The paper
immediately ignites and the cotton explodes. Strange,
is it not, that the beam should possess such heating*
power after having passed through so cold a substance ?
In his arctic expeditions Dr. Scoresby succeeded in
exploding gunpowder by the sun's rays converged by
large lenses of ice ; here we have succeeded in pro-
ducing the effect with a small lens, and with a terres-
trial source of heat.

In this experiment, you observe that, before the beam
reaches the ice-lens, it has passed through a glass cell
containing water. The beam is thus sifted of con-
stituents, which, if permitted to fall upon the lens,
would injure its surface, and blur the focus. And this
leads me to say an anticipatory word regarding trans-
parency. In our first lecture we entered fully into the
production of colom-s by absorption, and we spoke re-
peatedly of the quenching of the rays of light. Did
this mean that the light was altogether annihilated ?

V. IIERSCHEL'S DISCOVERY. 171

By no means. It was simply so lowered in refrangi-
bility as to escape the visual range. It was converted
into heat. Om* red ribbon in the green of the spectrum
quenched the gTcen, but if suitably examined its tem-
perature would have been found raised. Our green
ribbon in the red of the spectrum quenched the red,
but its temperature at the same time was augmented
to a degree exactly equivalent to the light extinguished.
Our black ribbon, when passed through the spectrum,
was found competent to quench all its colours ; but at
every stage of its progress an amount of heat was
generated in the ribbon exactly equivalent to the light
lost. It is only when absorption takes place that
heat is thus produced ; and heat is always a result of
absorption.

Examine the water, then, in front of the lamp after
the beam has passed through it : it is sensibly warm,
and, if permitted to remain there long enough, it might
be made to boil. This is due to the absorption, by the
water, of a certain portion of the electric beam. But a
portion passes through unabsorbed, and does not at all
contribute to the heating of the water. Now, ice is
also in great part transparent to these latter rays, and
therefore is but little melted by them. Hence, by
employing this particular portion of the beam, we
are able to keep our lens intact, and to produce b^
means of it a sharply-defined focus. Placed at that
focus, white paper is not ignited, because it fails to
absorb the rays emergent from the ice-lens. At tlie
same place, however, black paper instantly burns, be-
cause it absorbs the transmitted light.

And here it may be useful to refer to an estimate by
Newton, based upon doubtful data, but repeated by

172 ON LIGHT. LKCT.

various astronomers of eminence since his time. The
comet of 1680, when nearest to the sun, was only a sixth
of the sun's diameter from his surface. Newton esti-
mated its temperature, in this position, to be more than
two thousand times that of molten iron. Now it is clear
from the foregoing experiments that the temperature of
the comet could not be inferred from its nearness to the
sun. If its power of absorption were sufficiently low,
the comet might carry into the sun's neighbourhood the
temperature of stellar space.

§ 4. Combustion of Diamond by Radiant Heat.

Faraday thus describes the burning of a diamond in
oxygen by the concentrated rays of the sun. It was
effected at Florence, in presence of Sir Humphry Davy,
on Tuesday the 27th of March, 1814: — 'To-day we made
the grand experiment of burning the diamond, and
certainly the phenomena presented were extremely
beautiful and interesting. A glass globe containing
about 22 cubical inches was exhausted of air, and filled
with pure oxygen. The diamond was supported in the
centre of this globe. The Duke's burning-glass was the
instrument used to apply heat to the diamond. It con-
sists of two double convex lenses, distant from each
other about 3^ feet; the large lens is about 14 or 15
inches in diameter, the smaller one about 3 inches in
diameter. By means of the second lens the focus is
very much reduced, and the heat, when the sun shines
brightly, rendered very intense. The diamond was
placed in the focus and anxiously watched. On a
sudden Sir H. Davy observed the diamond to burn

V. ULTEA-EED EAYS. 17 li

visibly, and when removed from the focus it was found
to be in a state of active and rapid combustion.'

The combustion of the diamond had never been
effected by radiant heat from a terrestrial source. I
tried to accomplish tliis before crossing the Atlantic,
and succeeded in doing so. The small diamond now in
my hand is held by a loop of platinum wire. To pro-
tect it as far as possible from air currents, and also to
concentrate the heat upon it, it is surrounded by a hood
of sheet platinum. Bringing ajar of oxygen underneath,
I cause the focus of the electric beam to fall upon the
diamond. A small fraction of the time expended in
the experiment described by Faraday, suffices to raise
the diamond to a brilliant red. Plunging it then into
the oxygen, it glows like a little white star, and it would
continue to burn and glow until wholly consumed.
The focus can also be made to fall upon the diamond
in oxygen, as in the Florentine experiment : the result
is the same. It is simply to secure more complete
mastery over the position of the focus, so as to cause it
to fall accurately upon the diamond, that tlie mode of
experiment here described was resorted to.

§ 5. Ultra-red Rays : Calorescence.

In the path of the beam issuing from our lamp I
now place a cell witli glass sides containing a solution of
alum. All the lir/ht of the beam passes through this
solution. This light is received on a powerfully con-
verging mirror silvered in front, and brought to a focus
by the mirror. You can see the conical beam of re-
flected light tracking itself througli the dust of the
room. A scrap of wliite paper placed at the focus
9

174 ON LIGHT. lbct.

shines there with dazzling brightness, but it is not even
charred. On removing the alum cell, however, the
paper instantly inflames. There must, therefore, be
something in this beam besides its light. The light is
not absorbed by the white paper, and therefore does
not burn the paper ; but there is something over and
above the light which is absorbed, and which provokes
combustion. What is this something ?

In the year 1800 Sir William Herschel passed a
thermometer through the various colours of the solar
spectrum, and marked the rise of temperature corre-
sponding to each colour. He found the heating effect
to augment from the violet to the red ; he did not, how-
ever, stop at the red, but pushed his thermometer into
the dark space beyond it. Here he found the tempera-
ture actually higher than in any part of the visible
spectrum. By this important observation, he proved
that the sun emitted heat-rays which are entirely unfit
for the purposes of vision. The subject was subse-
quently taken up by Seebeck, Melloni, Miiller, and
others, and within the last few years it has been found
capable of unexpected expansions and applications. I
have devised a method whereby the solar or electric
beam can be so filtered as to detach from it, and pre-
serve intact, this invisible ultra-red emission, while the
visible and ultra-violet emissions are wholly intercepted.
We are thus enabled to operate at will upon the purely
ultra-red waves.

In the heating of solid bodies to incandescence this
non-visual emission is the necessary basis of the visual.
A platinum wire is stretched in front of the table, and
through it an electric current flows. It is warmed by
the current, and may be felt to be warm by the hand.

T. HEAT FILTEEED FROM LIGHT. 175

It emits waves of heat, but no light. Augmenting the
strength of the current, the wire becomes hotter; it
finally glows with a sober red light. At this point Dr.
Draper many years ago began an interesting investiga-
tion. He employed a voltaic current to heat his
platinum, and he studied, by means of a prism, the suc-
cessive introduction of the colours of the spectrum.
His first colour, as here, was red ; then came orange,
then yellow, then green, and lastly all the shades of
blue. Thus as the temperature of the platinum was
gradually augmented, the atoms were caused to vibrate
more rapidly ; shorter waves were thus introduced, until
finally waves were obtained corresponding to the entire
spectrum. As each successive colour was introduced,
the colours preceding it became more vivid. Now the
vividness or intensity of light, like that of sound, de-
pends not upon the length of the wave, but on the am-
plitude of the vibration. Hence, as the less refrangible
colours grew more intense as the more refrangible ones
were introduced, we are forced to conclude that side by
side with the introduction of the shorter waves we had
an augmentation of the amplitude of the longer ones.

These remarks apply not only to the visible emission
examined by Dr. Draper, but to the invisible emission
which precedes the appearance of any light. In the
emission from the white-hot platinum wire now before
you the very waves exist with which we started, only
their intensity has been increased a thousand-fold by
the augmentation of temperature necessary to the pro-
duction of tliis white light. Both effects are bound
together : in an incandescent solid, or in a molten
solid, you cannot have the shorter waves without tliis
intensification of the longer ones. A sun is possilJe

176 ON LIGHT.

tECT.

only on these conditions ; hence Sir William Herschel's
discovery of the invisible ultra-red solar emission.

The invisible heat, emitted both by dark bodies and
by luminous ones, flies through space with the velocity
of light, and is called radiant heat. Now, radiant heat
may be made a subtle and powerful explorer of mole-
cular condition, and, of late years, it has given a new
significance to the act of chemical combination. Take,
for example, the air we breathe. It is a mixture of
oxygen and nitrogen ; and it behaves towards radiant
heat like a vacuum, being incompetent to absorb it in
any sensible degree. But permit the same two gases
to unite chemically ; then, without any augmentation
of the quantity of matter, without altering the gaseous
condition, without interfering in any way with the
transparency of the gas, the act of chemical union is
accompanied by an enormous diminution of its diather-
mancy, or perviousness to radiant heat.

The researches which established this result also
proved the elementary gases, generally, to be highly
transparent to radiant heat. This, again, led to the
proof of the diathermancy of elementary liquids, like
bromine, and of solutions of the solid elements sulphur,
phosphorus, and iodine. A spectrum is now before
you, and you notice that the transparent bisulphide of
carbon has no effect upon the colours. Dropping into
the liquid a few flakes of iodine, you see the middle of
the spectrum cut away. By augmenting the quantity of
iodine, we invade the entire spectrum, and finally cut
it off altogether. Now, the iodine, which proves itself
thus hostile to the light, is perfectly transparent to the
ultra-red emission with which we have now to deal,
It, therefore, is to be our ray-filter.

▼. CALORESCENCE. 177

Placing the alum-cell again in front of the electric
lamp, we assiure ourselves, as before, of the utter in-
ability of the concentrated light to fire white paper.
Introducing a cell containing the solution of iodine, the
light is entirely cut off; and then, on removing the
alum-cell, the white paper at the dark focus is instantly
set on fire. Black paper is more absorbent than white
for these rays ; and the consequence is, that with it the
suddenness and vigour of tlie combustion are augmented.
Zinc is burnt up at the same place, magnesium bursts
into vivid combustion, while a sheet of platinized
platinum placed at the focus is heated to whiteness.

Looked at through a prism, the white-hot platinum
jaelds all the colours of the spectrum. Before im-
pinging upon the platinum, the waves were of too slow
recurrence to awaken vision ; by the atoms of the
platinum, these long and sluggish waves are broken up
into shorter ones, being thus brought within tlie vi&ual
range. At the other end of the spectrum, by the
interposition of suitable substances, Professor Stokes
lowered the refrangibility, so as to render the non-
visual rays visual, and to this change he gave the name
of Fluorescence. Here, by the intervention of the
platinum, the refrangibility is raised, so as to render
the non-visual visual, and to this change I have given
the name of Calorescence.

At the perfectly invisible focus where these effects
are produced, the air may be as cold as ice. Air, as
already stated, does not absorb the radiant heat, and is
therefore not warmed by it. Nothing could more
forcibly illustrate the isolation, if I may use the term,
of the luminiferous ether from the air. The wave-
motion of the one is heaped up, without sensible effect

178 ON LIGHT. LBCT.

upon Liie other. I may add that, with suitable pre-
cautions, the eye may be placed in a focus competent
to heat platinum to vivid redness, without experiencing
any damage, or the slightest sensation either of light
or heat.

The important part played by these ultra-red rays in
Nature may be thus illustrated : I remove the iodine
filter, and concentrate the total beam upon a test-tube
containing water. It immediately begins to sputter,
and in a minute or two it hoils. What boils it ?
Placing the alum solution in front of the lamp, the
boiling instantly ceases. Now, the alum is pervious to
all the luminous rays ; hence it cannot be these rays
that caused the boiling. I now introduce the iodine,
and remove the alum ; vigorous ebullition immediately
recommences at the invisible focus. So that we here
fix upon the invisible ultra-red rays the heating of the
water.

We are thus enabled to understand the momentous
part played by these rays in Nature. It is to them
that we owe the warming and the consequent evapora-
tion of the tropical ocean ; it is to them, therefore, that
we owe our rains and snows. They are absorbed close
to the surface of the ocean, and warm the superficial
water, while the luminous rays plunge to great depths
without producing any sensible effect. But we can
proceed further than this. Here is a large flask con-
taining a freezing mixture, which has so cliilled the
flask, that the aqueous vapour of the air has been con-
densed and frozen upon it to a white fur. Introducing
the alum-cell, and placing the coating of hoar-frost at
the intensely luminous focus, not a spicula of the daz-
zling frost is melted. Introducing the iodine-cell, and

T. THE THEEMO-PILE. 17 'J

removing the alum, a broad space of the frozen coating
is instantly melted away. Hence we infer that the snow
and ice, which feed the Rhone, the Rhine, and other
rivers with glaciers for their sources, are released from
their imprisonment upon the mountains by the in-
visible ultra-red rays of the sun.

§ 6. Identity of Light and Radiant II eat. Reflection
from Plane and Curved Surfaces. Total Reflec-
tion of Heat.

The growth of science is organic. That which to-
day is an end becomes to-morrow a means to a remoter
end. Every new discovery in science is immediately
made the basis of other discoveries, or of new methods
of investigation. Thus about fifty years ago, CErsted,
of Copenhagen, discovered the deflection of a magnetic
needle by an electric current ; and about the same
time Thomas Seebeck, of Berlin, discovered thermo-
electricity. These great discoveries were soon after-
wards turned to account, by Nobili and Melloni, in the
construction of an instrument which has vastly aug-
mented our knowledge of radiant heat. This instru-
ment, which is called a tltermo-electric pile, con-
sists of thin bars of bismuth and antimony, soldered
alternately together at their ends, but separated from
each other elsewhere. From the ends of this * thermo-
pile' wires pass to a galvanometer, which consists of a
coil of covered wire, within and above which are sus-
pended two magnetic needles, joined to a rigid system,
and carefully defended from currents of air.

The action of the arrangement is this : the heat, falling
on the pile, produces an electric current ; the current.

180 ON LIGHT.

UBCT.

passing through the coil, deflects the needles, and the
magnitude of the deflection may be made a measure of
the heat. The upper needle moves over a graduated dial
far too small to be directly seen. It is now, however,
strongly illuminated ; and above it is a lens which, if
permitted, would form an image of the needle and dial
upon the ceiling. There, however, it could not be con-
veniently viewed. The beam is therefore received upon
a looking-glass, placed at the proper angle, which throws
the image upon a screen. In this way the motions of
this small needle may be made visible to you all.

The delicacy of this apparatus is such that in a room
filled, as this room now is, with an audience physically
warm, it is exceedingly difficult to work with it. My
assistant stands several feet off". I turn the pile towards
him : the heat from his face, even at tliis distance, pro-
duces a deflection of 90°. I turn the instrument
towards a distant wall, judged to be a little below the
average temperature of the room. The needle descends
nd passes to the other side of zero, declaring by this
negative deflection that the pile feels the chill of the
wall. Possessed of this instrument, of our ray-filter,
and of our large Nicol prisms, we are in a condition to
investigate a subject of great philosophical interest ;
one which long engaged the attention of some of our
foremost scientific workers — the substantial identity
DJ light and radiant heat.

That they are identical in all respects cannot of
course be the case, for if they were they would act
in the same manner upon all instruments, the eye
included. The identity meant is such as subsists
between one colour and another, causing them to
behave alike as regards reflection, refraction, double

IDENTITY OF LIGHT AND HEAT.

181

refraction, and polarization. Let us here run rapidly
over tlie resemblances of light and heal. As regards
reflection from plane surfaces, we may employ a looking-
glass to reflect the light. Marking any point in tlie
track of the reflected beam, cutting off the light by
the dissolved iodine, and placing the pile at the marked
point, the needle immediately starts aside, showing that
the heat is reflected in the same direction as the light.
This is true for every position of the mirror. Eesuming,
for example, the experiments made with the apparatus

Fio. 51.

employed in our first lecture (fig. 3, p. 11); moving
the index attached to the mirror along the divisions
of our graduated arc (M 0), and determining by the
pile the positions of the invisible reflected beam, we
prove that the angular velocity of the heat-beam, like
that of the light-beam, is twice that of the mirror.

As roofards reflection from curved surfaces, the
identity also holds good. Receiving the beam from
our electric lamp on a concave mirror (m m, fig. 51),
it is gathered up into a cone of reflected light ; mark-

182

ON LIGHT.

LECT,

ing the apex of the cone by a pointer, and cutting off
the light by the iodine solution (T),a momsnt's exposure
of the pile (P) at the marked point produces a violent
deflection of the needle.

The common and total reflection of a beam of radiant
heat may be simultaneously demonstrated. From the
nozzle of the lamp (L, fig. 52) a beam impinges upon
a plane mirror (M N), is reflected upwards, and enters
a right-angled prism, of which a 6 c is the section.

Fig, 62.

It meets the hypothenuse at an obliquity greater
than the limiting angle,' and is therefore totally re-
flected. Quenching the light by the ray-filter at F,
and placing the pile at P, the totally-reflected heat-
beam is immediately felt by the pile, and declared by
the galvanometric deflection.

• D-:fined in Lecture I.

V. INVISI13LE IMACiES. POLARIZATION OF HEAT. 183

§ 7. Tniisible Images formed hy Radiant Heat.

Perhaps no experiment more conclusively proves the
substantial identity of light and radiant heat, than the
formation of invisible heat-images. Employing the
mirror already used to raise the beam to its highest
state of concentration, we obtain, as is well known, an
inverted image of the carbon points, formed by the
light rays at the focus. Cutting off the light by the

Fig. 53.

ray-fiiter, and placing at the focus a thin sheet of
platinized platinum, the invisible rays declare their
presence and distribution by stamping upon the plati-
num a white-hot image of the carbons. (See fig. 53.)

§ 8. Polarization of Heat.

Whether radiant heat be capable of polarization or
not was for a long time a subject of discussion. Bcrard
had announced affirmative results, but Powell and
Lloyd failed to verify them. The doubts thus thrown
upon the question were removed by the experiments

184

ON LIGHT.

LECT.

of Forbes, who first established tlie polarization and
'depolarization of heat. The subject was subsequently
followed up by Melloni, an investigator of consummate
ability, who sagaciously turned to account his own dis-
covery, that the obscure rays of luminous sources are
in part transmitted by black glass. Intercepting by a
plate of this glass the light from an oil flame, and
operating upon the transmitted invisible heat, he
obtained effects of polarization, far exceeding in mag-
nitude those which could be obtained with non-lumi-
nous sources. At present the possession of our more
perfect ray-hlter, and more powerful source of heat,

Pig. 51.

enables us to pursue this identity question to its utmost
practical limits.

Mounting our two Nicols (B and C, fig. 54) in front
of the electric lamp, with their principal sections
crossed, no light reaches the screen. Placing pur
thermo-electric pile (D) behind the prisms, with its
face turned towards the source, no deflection of the
giilvanometer is observed. Interposing between the

V. DOUBLE EEfRACTION OF HEAT. 185

Ump (A) and the first prism (B) our ray-filter, the
light previously transmitted through the first Nicol
is quenched ; and now the slightest turning of either
Nicol opens a way for the transmission of the heat,
a very small rotation sufficing to send the needle up
to 90°. When the Nicol is turned back to its first
position, the needle again sinks to zero, thus demon-
strating in the plainest manner the polarization of
the hea*".

When the Nicols are crossed and the field is dark,
you have seen, in the case of light, the effect of introduc-
ing a plate of mica between the polarizer and analyzer.
In two positions the mica exerts no sensible influence ;
in all others it does. A precisely analogous deportment
is observed as regards radiant heat. Introducing our
ray-filter, the thermo-pile, playing the part of an eye as
regards the invisible radiation, receives no heat when
the eye receives no light ; but when the mica is so
turned as to make its planes of vibration oblique to
those of the polarizer and analyzer, the heat immedi-
ately passes through. So strong does the action be-
come, that the momentary plunging of the film of mica
into the dark space between the Nicols suffices to send
the needle up to 90°. This is the effect to which the
term ' depolarization ' has been applied ; the experi-
ment really proving that with light and heat we have
the same resolution by the plate of mica, and recom-
pounding by the analyzer, of the ethereal vibrations.

liemoving the mica and restoring the needle once
more to 0°, I introduce between the Nicols a plate of
quartz cut perpendicular to the axis ; the immediate
deflection of the needlp declares the transmission of the
heat, and wlien the transmitted beam is p operly

186

ON LIGHT.

I.EC'

examined, it is found to be circularly polarized, exactly
as a beam of light is polarized under the same con-
ditions.

§ 9. Double Refraction of Heat.

I will now abandon the Nicols, and send through
the piece of Iceland spar (B, fig. 55), already employed
to illustrate the double refraction of light, our sifted

Fra. 65.

beam of invisible heat. To determine the positions of
the two images, let us first operate upon the total
beam. Marking the places of the light-images, we
introduce between N and L our ray-filter (not in the
figure) and quench the light. Causing the pile to
approach one of the marked points, the needle remains
unmoved until the point has been attained ; here the
pile at once detects the heat. Pushing the pile acrofs

V. MiVGNETIZATION OF HEAT. 187

the interval separating the two marks, the needle first
fulls to 0^, and then rises again to 90° in the second
position. This proves the double refraction of the
heat.

I now turn the Iceland spar : the needle remains
fixed : there is no alteration of the deflection. Pass-
ing the pile rapidly across to the other mark, the
deflection is maintained. Once more I turn the spar,
but now the needle falls to 0°, rising, however, again
to 90° after a rotation of 360°. We know that in the
case of light the extraordinary beam rotates round the
ordinary one ; and we have here been operating on the
extraordinary heat-beam, which, as regards double re-
fraction, behaves exactly like a beam of light.

§ 10. Magnetization of Heat.

To render our series of comparisons complete, we must
demonstrate the magnetization of heat. But here a
slight modification of our arrangement will be necessary.
In repeating Faraday's experiment on the magnetiza-
tion of light, we had, in the first instance, our Nicols
crossed and the field rendered dark, a flash of light ap-
pearing upon the screen when the magnet was excited.
Now the quantity of liglit transmitted in this case is
really very small, its effect being rendered striking
through contrast with the preceding darkness. When
we so place the Nicols that their principal sections en-
close an angle of 45°, the excitement of the magnet
causes a far greater positive augmeutatiun of the light,
though the augmentation is not so well seeti througli
lack of contrast, because here, at starting, the field is
illuminated.

18S ON LIGHT.

I-ECT,

In trying to magnetize our beam of heat, we will
adopt this arrangement. Here, however, at the outset,
a considerable amount of heat falls upon one face of the
pile. This it is necessary to neutralize, by permitting
rays from another source to fall upon the opposite face
of the pile. The needle is thus brought to zero. Cut-
ting off the light by our ray-filter, and exciting the mag-
net, the needle is instantly deflected, proving that the
magnet has opened a door for the heat, exactly as in
Faraday's experiment it opened a door for the light.
Thus, in every case brought under our notice, the sub-
stantial identity of light and radiant heat has been
demonstrated.

By the refined experimentsof Knoblauch, who worked
long and successfully at this question, the double refrac-
tion of heat, by Iceland spar, was first demonstrated ;
but though he employed the luminous heat of the sun,
the observed deflections were exceedingly small. So,
likewise, those eminent investigators De la Povostaye
and Desains succeeded in magnetizing a beam of heat ;
but though, in their case also, the luminous solar heat
was employed, the deflection obtained did not amount
to more than two or three degrees. With obscure
radiant heat the effect, prior to these experiments, had
not been obtained ; but, with the arrangement here de-
scribed, we obtain deflections from purely invisible heat,
equal to 150 of the lower degrees of the galvanometer.

§ 11. Distribution of Heat in the Electric Spectrum.

We have finally to determine the position and mag-
nitude of the invisible radiation which produces these
resvdts. For this purpose we employ a particular form

V. DISTRIBUTION OF HEAT IN SPECTRUM. 18i)

of the thermo-pile. Its face is a rectangle, which by
movable side-pieces can be rendered as narrow as de-
sirable. Throwing a small and concentrated spectrum
upon a screen, by means of an endless screw we move
the rectangular pile through the entire spectrum, and
determine in succession the thermal power of all its
colours.

When this instrument is brought to the violet end
of the spectrum, the heat is found to be almost insen-
sible. As the pile gradually moves from the violet
towards the red, it encounters a gradually augmenting
heat. The red itself possesses the highest heating
power of all the colours of the spectrum. Pushing the
pile into the dark space beyond the red, the heat rises
suddenly in intensity, and at some distance beyond
the red it attains a maximum. From this point the
heat falls somewhat more rapidly than it rose, and after-
wards gradually fades away.

Drawing a horizontal line to represent the length of
the spectrum, and erecting along it, at various points,
perpendiculars proportional in lengtli to the heat exist-
ing at those points, we obtain a curve which exhibits
the distribution of heat in our spectrum. It is repre-
sented in the adjacent figure. Beginning at the blue,
tlie curve rises, at first very gradually ; towards the red
it rises more rapidly, the line C D (fig 56, next page)
representing the strength of the extreme red radiation.
Beyond the red it shoots upwards in a steep and massive
peak to B ; whence it falls, rapidly for a time, and after-
wards gradually fades from the perception of tlie pile.
This figure is the result of more tlian twelve careful
series of measurements, for each of which the curve
was constructed. On superposing all these curves, a

190

ON LIGHT.

UICT.

!i5

N
ic

T. VERIFICATION OF RESULT. 191

satisfactory agreement was found to exist between them.
So that it may safely be concluded that the areas of the
dark and white spaces, respectively, represent the rela-
tive energies of the visible and invisible radiation.
Tlie one is 7-7 times the other.

But in verification, as already stated, consists the
strength of science. Determining in the first place the
total emission from the electric lamp ; then by means
of the iodine filter determining the ultra-red emission ;
the difference between both gives the luminous emis-
sion. In this way, it is found that the energy of
the invisible emission is eight times that of the visible.
No two methods could be more opposed to each other,
and hardly any two results could better harmonize. I
think, therefore, you may rely upon the accuracy of the
distribution of heat here assigned to the prismatic
spectrum of the electric light. There is nothing vague
in the mode of investigation, or doubtful in its con-
clusions.

192 ON LIGHT.

LBCr.

LECTUEE VI.

PUINCITLES OF SPECTBUM ANALYSIS PRISMATIC AWALYSIS OF THE LIGHT

OF INCANDESCENT VAPOURS — DISCONTINUOUS SPECTRA SPECTRUM

BANDS PROVED BY BUNSEN AND KIRCHHOFF TO BE CHARACTERISTIC OF

THE VAPOUR DISCOVERY OF RUBIDIUM, CJESIUM, AND THALLIUM

RELATION OF EMISSION TO ABSORPTION — THE LINES OF FKAUNHOFEB

THEIR EXPLANATION BY KIRCHHOFF — SOLAR CHEMISTRY INVOLVED

IN THIS EXPLANATION — FOUCAULT's EXPERLMENT — PRINCIPLES OF
ABSORPTION ANALOGY OF SOUND AND LIGHT — EXPERIMENTAL DE-
MONSTRATION OF THIS ANALOGY — RECENT APPLICATIONS OF THB
SPECTROSCOPE— SUMMARY AND CONCLUSION.

We have employed as our source of light in these
lectures the ends of two rods of coke, rendered incan-
descent by electricity. Coke is particularly suitable for
this purpose, because it can bear intense heat without
fusion or vaporization. It is also black, which helps tlie
light ; for, other circumstances being equal, as shown
experimentally by Professor Balfour Stewart, the blacker
the body the brighter will be its light when incandes-
cent. Still, refractory as carbon is, if we closely ex-
amined our voltaic arc, or stream of light between the
carbon-points, we should find there incandescent carbon-
vapour. And if we could detach the light of this vapour
from the more dazzling light of the solid points, we
should find its spectrum not only less brilliant, but of a
totally different character from the spectra that we have
already seen. Instead of being an unbroken succession
of colours from red to violet, the carbon-vapour would

Ti. SPECTRA OF INCANDESCENT VAPOURS. 193

yield a few bands of colour with spaces of darkness be-
tween them.

What is true of the carbon is true in a still more
striking degree of the metals, the most refractory
of which can be fused, boiled, and reduced to vapour by
the electric current. From the incandescent vapour the
light, as a general rule, flashes in groups of rays of
definite degrees of refrangibility, spaces existing be-
tween group and group, which are unfilled by rays
of any kind. But the contemplation of the facts will
render this subject more intelligible than words can
make it. Within the camera is now placed a cylindei
of carbon hollowed out at the top to receive a bit of
metal ; in the hollow is placed a fragment of the metal
thallium. Down upon this we bring the upper carbon
point, and then separate the one from the other. A
stream of incandescent thallium vapour passes between
them, the magnified image of which is now seen upon
the screen. It is of a beautiful green colour. What is
the meaning of that green ? We answer the question
by subjecting the light to prismatic analysis. Sent
through the prism, its spectrum is seen to consist of a
single refracted band. Light of one degree of refrangi-
bility, and that corresponding to this particular green, is
emitted by the thallium vapour.

We will now remove the thallium and put a bit of
silver in its place. The arc of silver is not to be dis-
tinguished from that of thallium ; it is not only green,
but the same shade of green. Are they then alike ?
Prismatic analysis enables us to answer the question.
However impossible it is to distinguish the one colour
from the other, it is equally impossible to confound tlie
nvectrum of incandescent silver vapour with that of

194 ON LIGHT. i^cT.

thallium. In the case of silver, we have two green bands
instead of one.

If we add to the silver in our camera a bit of thal-
lium, we shall obtain the light of both metals. After
waiting a little, we see that the green of the thallium lies
midway between the two greens of the silver. Hence
this similarity of colour.

But why have we to wait a little before we see
this effect ? The thallium band at first almost masks
the silver bands by its superior brightness. Indeed,
the silver bands have wonderfully degenerated since
the bit of thallium was put in, and for a reason worth
knowing. It is the resistance offered to the passage
of the electric current from carbon to carbon, that calls
forth the power of the current to produce heat. If the
resistance were materially lessened, the heat would be
materially lessened ; and if all resistance were abolished,
there would be no heat at all. Now, thallium is a much
more fusible and vaporizable metal than silver ; and its
vapour facilitates the passage of the current, to such a
degree, as to render it almost incompetent to vaporize the
more refractory silver. But the thallium is gradually con-
sumed ; its vapour diminishes, the resistance rises, until
finally you see the two silver bands as brilliant as they
were at first.*

We have in these bands a perfectly unalterable
characteristic of the two metals. You never get other
bands than these two green ones from the silver, never
other than the single green band from the thallium,
never other than the three green bands from the
mixture of both metals. Every known metal has its

' This circumstance ought not to be lost sight of in the examination
of compound spectra. Other similar instances might be cited.

Ti, SPECTRUM ANALYSIS. 195

own particular bands, and in no known case are the bands
oftwo different metals alike in refrangibility. It follows,
therefore, that these spectra may be made a sure test
for the presence or absence of any particular metal.
If we pass from the metals to their alloys, we find no
confusion. Copper gives green bands ; zinc gives blue
and red bands ; brass, an alloy of copper and zinc, gives
the bands of both metals, perfectly unaltered in position
or character.

But we are not confined to the metals themselves ;
the salts of these metals yield the bands of the metals.
Chemical union is ruptured by a sufficiently high heat ;
the vapour of the metal is set free, and it yields its cha-
racteristic bands. The chlorides of the metals are parti-
cularly suitable for experiments of this character. Com-
mon salt, for example, is a compound of chlorine and
sodium ; in the electric lamp it yields the spectrum of
the metal sodium. The chlorides of copper, lithium,
and strontium yield, in like manner, the bands of these
metals.

When, therefore, Bunsen and Kirchhoff, the cele-
brated founders oi specb^ni analysis^dLiiex having estab-
lished by an exhaustive examination the spectra of all
known substances, discovered a spectrum containing bands
different from any known bands, they immediately in-
ferred the existence of a new metal. They were operat-
ing at the time upon a residue, obtained by evaporating
one of the mineral waters of Germany. In that water they
knew the unknown metal was concealed, but vast quanti-
ties of it had to be evaporated before a residue could be
obtained, sufficiently large to enable ordinary chemistry to
grapple with the metal. Tliey, however, hunted it down,
and it now stands among chemical substances as the metal

196 ON LIGHT.

LECT

Rubidium. They subsequently discovered a second
metal, which they called Ccesium. Thus, having first
placed spectrum analysis on a sure foundation, they de-
monstrated its capacity as an agent of discovery. Soon
afterwards Mr. Crookes, pursuing the same method,
discovered the bright green band of thallium, and ob-
tained the salts of the metal which yielded it. The
metal itself was first isolated in ingots by M. Lamy, a
French chemist.

All this relates to chemical discovery upon earth,
where the materials are in our own hands. But it was
soon shown how spectrum analysis might be applied
to the investigation of the sun and stars ; and this
result was reached through the solution of a problem
which had been long an enigma to natural philosophers.
The scope and conquest of this problem we must now
endeavour to comprehend. A spectrum is pure in which
the colours do not overlap each other. We purify the
spectrum by making our slit narrow, and by augmenting
the number of our prisms. When a pure spectrum of
the sun has been obtained in this way, it is found to be
furrowed by innumerable dark lines. Four of them were
first seen by Dr. Wollaston, but they were afterwards
multiplied and measured by Fraunhofer with such
masterly skill, that they are now universally known as
Fraunhofer's lines. . To give an explanation of these
lines was, as I have said, a problem which long chal-
lenged the attention of philosophers, and to Kirchhoff,
Professor of Physics in the University of Heidelberg,
belongs the honour of having first conquered this
problem.

(The positions of the principal lines, lettered accord-
ing to Fraunhofer, are shown in the annexed sicetcl)

VI.

FRAUNHOFER'S LINES.

197

(fig. 57) of the solar spectrum. A is supposed to stand
near the extreme red, and J near the extreme violet.)

The brief memoir of two pages, in which this im •
mortal discovery is recorded, was communicated to
the Berlin Academy on October 27, 1859. Fia. 67.
Fraunhofer had remarked in the spectrum
of a candle flame two bright lines, which
coincide accurately, as to position, with the
double dark line D of the solar spectrum.
These briglit lines are produced with par- ^
ticular intensity by the yellow flame derived
from a mixture of salt and alcohol. They
are in fact the lines of sodium vapour.
KirchhofF produced a spectrum by permit-
ting the sunlight to enter his telescope by a
slit and prism, and in front of the slit he
placed the yellow sodium flame. As long
as the spectrum remained leeble, there always
appeared two bright lines, derived from the
flame, in the place of the two dark lines D
of the spectrum. In this case, such absorp-
tion as the flame exerted upon the sunlight
was more than atoned for by the radiation
from the flame. When, however, the solar
spectrum was rendered sufficiently intense,
the bright bands vanished, and the two dark
Fraunhofer lines appeared with much greater
sharpness and distinctness than when the
flame was not employed.

This result, be it noted, was not due to o
any real quenching of the bright lines of the "^
flame, but to tlie augmentation of the in-
tensity of the adjacent spectrum. The experi-
10

198 ON LIGHT.

LECT

ment proved to demonstration, that when the white light
sent through the flame was sufficiently intense, the
quantity which the flame absorbed was far in excess of
that which it radiated.

Here then is a result of the utmost significance.
Kirchhoff immediately inferred from it that the salt
flame, which could intensify so remarkably the dark
lines of Fraunhofer, ought also to be able to produce
them. The spectrum of the Drummond light is known
to exhibit the two bright lines of sodium, which, however,
gradually disappear as the modicum of sodium, contained
as an impurity in the incandescent lime, is exhausted.
Kirchhoff formed a spectrum of the lime-light, and after
the two bright lines had vanished, he placed his salt
flame in front of the slit. The two dark lines D imme-
diately started forth. Thus, in the continuous spectrum
of the lime-light, he evoked, artificially, the lines D of
Fraunhofer.

Kirchhoff knew that this was an action not peculiar
to the sodium flame, and he immediately extended his
generalization to all coloured flames which yield sharply
defined bright bands in their spectra. White light, with
all its constituents complete, sent through such flames,
would, he inferred, have those precise constituents
absorbed, whose refrangibilities are the same as those
of the bright bands ; so that after passing through such
flames, the white light, if sufficiently intense, would
have its spectrum furrowed by bands of darkness. On
the occasion here referred to, Kirchhoff also succeeded
in reversing a bright band of lithium.

The long-standing difficulty of Fraunhofer's lines fell
to pieces in the presence of facts and reflections like
these, which also carried with them an immeasurable

ri.

SOLAR CHEMISTEY.

199

extension of the chemist's power. Kirchhoff saw that
from the agreement of the lines i)i the spectra of terres-
trial substances with Fraimhofer's lines, the presence
of these substances in the sun and fixed stars might
be immediately inferred. Thus the dark lines D in
the solar spectrum proved the existence of sodium
in the solar atmosphere; while the bright lines dis-
covered by Brewster in a nitre flame, which had
been proved to coincide exactly with certain dark lines
between A and B in the solar spectrum, proved the
existence of potassium in the sun.

All subsequent research verified the accuracy of these
first daring conclusions. In his second paper, commu-
nicated to the Berlin Academy before the close of 1859,
Kirchhoff proved the existence of iron in the sun.
The bright lines of the spectrum of iron vapour are
exceedingly numerous, and 65 of them were subse-
quently proved by Kirchhoff to be absolutely identical

in position with 65 dark Fraunhofer's lines. Angstrom
and Thalen pushed the coincidences to 450 for iron,
while, according to the same excellent investigators, the
following numbers express the coincidences, in the case
of the respective metals to which they are attached : —

Calcium .

. 7n

Nickel .

Barium

. 11

Cobalt

Magnesium

. 4

Hydrogen

Manganese

. 57

Aluminium

Titanium .

. 118

Zinc

Chromium

. 18

Copper

The probability is overwhelming that all these sul)-.
Btanoes exist in the atmosphere of tlie sun.

Kirchhoffs discovery profoundly modified the cun-
ceptions previously entertained regarding the consti
tution of the sun, leading him to views of that con

200 ON LIGHT. LKCT. I

stitution wliich, though they may be modified in detail,
will, I believe, remain substantially valid to the end of
time. The sun consists of a nucleus which is surrounded
by a flaming atmosphere of lower temperature. That
nucleus may, in part, be clouds, mixed with, or under-
lying true vapour. The light of the nucleus would
give us a continuous spectrum, like that of the Drum-
mond light ; but having to pass through the photo-
sphere, as KirchhofiTs beam passed through the sodium
flame, those rays of the nucleus which the photosphere
can emit are absorbed, and shaded lines, corresponding
to the rays absorbed, occur in the spectrum. Abolish
the solar nucleus, and we should have a spectrum
showing a bright line in the place of every dark line of
Fraunhofer, just as, in the case of Kirchhoffs second
experiment, we should have the bright sodium lines of
the flame if the lime light were withdrawn. These ^
lines of Fraunhofer are therefore not absolutely dark, ~/
but dark by an amount corresponding to the difierence « /^
between the light intercepted and the light emitted by ^-^
the photosphere. 'Jf

Almost every great scientific discovery is approached y
contemporaneously by many minds, the fact that one , "^
mind usually confers upon it the distinctness of demon- - ^
stration being an illustration, not of genius isolated, but
of genius in advance. Thus Foucault, in 1849, came
to the verge of Kirchhofl's discovery. By converging
an image of the sun upon a voltaic arc, and thus ob-
taining the spectra of both sun and arc superposed,
he found that the two bright lines which, owing to the
presence of a little sodium in the carbons, or in the
air, are seen in the spectrum of the arc, coincide with
the dark lines D of the solar spectrum. The lines D he

VI. PHYSICAL CAUSE OF ABSOEPTION. 201

found to be considerably strengthened by the passage
of the solar light through the voltaic arc.

Instead of the image of the sun, Foucault then pro-
jected upon the arc the image of one of the solid in-
candescent carbon points, which of itself would give a
continuous spectrum ; and he found that the lines
D were thus generated in that spectrum. Foucault's
conclusion from this admirable experiment was ' that
the arc is a medium which emits the rays D on its
own account, and at the same time absorbs them
when they come from another quarter.' Here he
stopped. He did not extend his observations beyond
the voltaic arc ; he did not offer any explanation of the
lines of Fraunhofer ; he did not arrive at any conception
of solar chemistry, or of the constitution of the sun.
His beautiful experiment remained a germ without
fruit, until the discernment, ten years subsequently, of
the whole class of phenomena to which it belongs,
enabled Kirchhoff to solve these great problems.

Soon after the publication of Kirchhoff's discovery.
Professor Stokes, who, ten years prior to the discovery,
had nearly anticipated it, borrowed an illustration from
sound, to show the reciprocity of radiation and absorp
tion. A stretched string responds to aerial vibra-
tions which synchronize with its own. A great number
of such strings stretched in space would roughly repre-
sent a medium ; and if the note common to them all
were sounded at a distance they would absorb the vibra-
tions corresponding to that note. That is to say, they
would absorb the vibrations which they can emit.

When a violin-bow is drawn across this tuning-fork,
tlie room is immediately filled with a musical sound,
which may be regarded as the radiation or emission

202 ■ ON LIGHT. LECT.

of sound from the fork. A few days ago, on sound-
ing this fork, I noticed that when its vibrations were
quenched, the sound seemed to be continued, though
more feebly. It appeared, more(»ver, to come from
under a distant table, where stood a number of tuning-
forks of different sizes and rates of vibration. One of
these, and one only, had been started by the sounding
fork, and it was the one whose rate of vibration was the
same as that of the fork which started it. This is an
instance of the absorption of the sound of one fork by
another. Placing two unisonant forks near each other,
sweeping the bow over one of them, and then quench-
ing the agitated fork, the other continues to sound;
this other can re-excite the former, and several transfers
of sound between the two forks can be thus effected.
Placing a cent-piece on each prong of one of the forks,
we destroy its perfect synchronism with the other, and
no such communication of sound from the one to the
other is then possible.

I have now to bring before you, on a suitable scale,
the demonstration that we can do with light what has
been here done with sound. For several days in 1861
I endeavoured to accomplish this, with only partial
success. In iron dishes a mixture of dilute alcohol
and salt was placed, and warmed so as to promote
vaporization. The vapour was ignited, and through
the yellow flame thus produced the beam from the elec-
tric lamp was sent ; but a faint darkening only of the
yellow band of a projected spectrum could be obtained.
A trough was then made which, when fed with the salt
and alcohol, yielded a flame ten feet thick ; but the re-
sult of sending the light through this depth of flame
was still unsatisfactory. Remembering that the direct

VI. EXPEKIMENTAL ILLUSTRATION. 203

combustion of sodium in a Bunsen's flame produces a
yellow far more intense than that of the salt flame, and
inferring that the intensity of the colour indicated the
copiousness of the incandescent vapour, I sent through
the flame from metallic sodium the beam of the
electric lamp. The success was complete; and this
experiment I wish now to repeat in your presence.^

Firstly then you notice, when a fragment of sodium
is placed in a tin spoon and introduced into a Bun-
sen's flame, an intensely yellow light is produced. It
corresponds in refrangibility with the yellow band of
the spectrum. Like our tuning-fork, it emits waves
of a special period. When the white light from the
electric lamp is sent through that flame, you will have
ocular proof that the yellow flame intercepts the yellow
of the spectrum ; in other words, that it absorbs waves
of the same period as its own, thus producing, to all in-
tents and purposes, a dark Fraunhofer's band in the place
of the yellow.

In front of the slit (at L, fig. 58) through which the
beam issues is placed a Bunsen's burner (6) protected by
a chimney (C). This beam, after passing through a lens,
traverses the prism (P) (in the real experiment there
was a pair of them), is there decomposed, and forms a
vivid continuous spectrum (S S) upon the screen. In-
troducing a tin spoon with its pellet of sodium into the
Bunsen's flame, the pellet first fuses, colours the flame
intensely yellow, and at length bursts into violent
combustion. At the same moment the spectrum is

' The dark Land produced when the sodium is phicod within the
l.imp was ohsfrrcd on the same occasion. Tlicn was also oliservcd for
the first time the magnificent blue band of lithium which the Uunsen's
iiivae fails to bring out.

204

ON LIGHT.

LECT.

furrowed by an intensely dark band (D), two inches wide
and two feet long. Introducing and withdrawing the
sodium flame in rapid succession, the sudden appearance
and disappearance of the band of darkness is shown in
a most striking manner. In contrast with the adjacent
brightness this band appears absolutely black, so vigor-
ous is the absorption. The blackness, however, is but
relative, for upon the dark space falls a portion of the
liofht of the sodium flame.

Fig. 58.

I have already referred to the experiment of Fou-
cault ; but other workers also had been engaged on the
borders of this subject before it was taken up by Bunsen
and Kirchhoff. With a few modifications here intro-
duced, I have already used the following language re-
garding the precursors of the discovery of spectrum
analysis, and solar chemistry: — 'Mr. Talbot had ob-
served the bright lines in the spectra of coloured flames,
and both he and Sir John Herschel pointed out the pos-
sibility of making prismatic analysis a chemical test of
exceeding delicacy, though not of entire certainty.
More than a (quarter of a century ago Dr. Miller gave

VI. KIRCHHOFF Ai?D HIS PRECURSORS. 205

drawings and descriptions of the spectra of various
coloured flames. Wheatstone, with his accustomed
acuteness, analyzed the light of the electric spark, and
proved that the metals between which the spark passed
determined the bright bands in its spectrum. In an
investigation described by Kirchhoflf as ' classical,' Swan
had shown that ^^^i^^ of a grain of sodium in a Bunsen's
flame could be detected by its spectrum. He also
proved the constancy of the bright lines in the spectra
of hydro-carbon flames. Masson published a prize essay
on the bands of the induction spark ; while Van der
Willigen, and more recently Pliicker, have also given
us beautiful drawings of spectra obtained from the
same source.

' But none of these distinguished men betrayed the
least knowledge of the connection between the bright
bands of the metals, and the dark lines of the solar
spectrum ; nor could spectrum analysis be said to be
placed upon anything like a safe foundation prior to
the researches of Bunsen and KirchhofF. The man
who, in a published paper, came nearest to the philo-

sophy of the subject was Angstrom. In that paper
translated by myself, and published in the- " Philoso-
phical jNIagazine" for 1855, he indicates that the rays
which a body absorbs are precisely those which it can
emit, when rendered luminous. In another place, he
speaks of one of his spectra giving the general im-
pression of the reversal of the solar spectrum. But his
memoir, philosophical as it is, is distinctly marked by
the uncertainty of his time. Foucault, Thomson, and
Balfour Stewart have all been near the discovery, while,
as already stated, it was almost hit by the acute but
unpublished conjecture of Stokes.'

206 ON LIGHT. I.ECT.

Mentally, as well as physically, every year of the
world's age is the outgrowth and offspring of all preceding
years. Science proves itself to be a genuine product
of Nature by growing according to this law. We have
no solution of continuity here. All great discoveries
are duly prepared for in two ways : first, by other dis-
coveries which form their prelude ; and, secondly, by the
sharpening of the enquiring intellect. Thus Ptolemy
grew out of Hipparchus, Copernicus out of both, Kepler
out of all three, and Newton out of all the four. New-
ton did not rise suddenly from the sea-level of the
intellect to his amazing elevation. At the time that
he appeared, the table-land of knowledge was already
high. He juts, it is true, above the table-land, as a
massive peak ; still he is supported by it, and a great
part of his absolute height is the height of humanity
in his time. It is thus with the discoveries of Kirch-
hoff. Much had been previously accomplished; this
he mastered, and then by the force of individual genius
went beyond it. He replaced uncertainty by certainty,
vagueness by definiteness, confusion by order; and I
do not think that Newton has a surer claim to the
discoveries that have made his name immortal, than
Kirchhoflf has to the credit of gathering up the frag-
mentary knowledge of his time, of vastly extending it,
and of infusing into it the life of great principles.

With one additional point we will wind up our illus-
trations of the principles of solar chemistry. Owing
to the scattering of light by matter floating mechani-
cally in the earth's atmosphere, the sun is seen not
sharply defined, but surrounded by a luminous glare.
Now, a loud noise will drown a whisper, an intense
light will quench a feeble one, and so this circumsolar

VT. ROSE-COLOURED SOLAR PROMINENCES. 207

glare prevents us from seeing many striking appearances
round the border of the sun. The glare is abolished in
total eclipses, when the moon comes between the earth
and the sun, and there are then seen a series of rose-
coloured protuberances, stretching sometimes tens of
thousands of miles beyond the dark edge of the moon.
They are described by Vassenius in the ' Philosophical
Transactions' for 1733; and were probably observed
even earlier than this. In 1842 they attracted
great attention, and were then compared to Alpine
snow-peaks reddened by the evening sun. That
these prominences are flaming gas, and principally
hydrogen gas, was first proved by M. Janssen during
an eclipse observed in India, on the 18th of August,
1868.

But the prominences may be rendered visible in full
sunshine ; and for a reason easily understood. You
have seen in these lectures a single prism employed
to produce a spectrum, and you have seen a pair of
prisms employed. In the latter case, the dispersed
white light, being diffused over about twice the area,
had all its colours proportionately diluted. You have
also seen one prism and a pair of prisms employed to
produce the bands of incandescent vapours; but here
the liglit of each band, being absolutely monochro-
matic, was incapable of further dispersion by the second
prism, and could not therefore be weakened by such
dispersion.

Apply these considerations to the circumsolar region.
The glare of white light round the sun can be dispersed
and weakened to any extent, by augmenting the number
of prisms ; while a monochromatic light, mixed with
this glare, and masked by it, would retain its intensity

208 ON LIGHT, I.ECT. VI.

unenfeebled by dispersion. Upon this consideration
ha8 been founded a method of observation, applied in-
dependently by M. Janssen in India and by Mr. Lockyer
in England, by which the monochromatic bands of the
prominences are caused to obtain the mastery, and to
appear in broad daylight. By searching carefully and
skilfully round the sun's rim, Mr. Lockyer has proved
these prominences to be mere local juttings from a
fiery envelope which entirely clasps the sun, and which
he has called the ChroTnosphere.

It would lead us far beyond the object of these lec-
tures to dwell upon the numerous interesting and impor-
tant results obtained by Secchi, Eespighi, Young, and
other distinguished men who have worked at the
chemistry of the sun and its appendages. Nor can
I do more at present than make a passing reference
to the excellent labours of Dr. Huggins in connexion
with the fixed stars, nebulae, and comets. They, more
than any others, illustrate the literal truth of the
statement, that the establishment of spectrum analysis,
and the explanation of Fraunhofer's lines, carried with
them an immeasurable extension of the chemist's
range. But my object here is to make principles plain,
rather than to follow out the details of their illustration.
This latter would be a task requiring only time for its
execution, but requiring more of it than I have now at
my command.

209

SUMMAKY AND COXCLUSIOK

My desire in these lectures has been to show you, with as
little breach of continuity as possible, something of the
past growth and present aspect of a department of science,
in which have laboured some of the greatest intellects
the world has ever seen. My friend Professor Henry,
in introducing me at Washington, spoke of me as an
apostle ; but the only apostolate that I intended to
fulfil was to place, in plain words, my subject before
you, and to permit its own intrinsic attractions to act
upon your minds. I have sought to confer upon
each experiment a distinct intellectual value, for
experiments ought to be the representatives and
expositors of thought — a language addressed to the
eye as spoken words are to the ear. In association
with its context, nothing is more impressive or instruc-
tive than a fit experiment ; but, apart from its context,
it rather suits the conjuror's purpose of surprise, than
that purpose of education which ought to be the ruling
motive of tlie scientific man.

And now a brief summary of our work will not be
out of place. Our present mastery over the laws and
phenomena of light has its origin in the desire of man
to know. We have seen the ancients busy with this
problem, but, like a child who uses his arms aimlessly,
for want of the necessary muscular exercise, so these
early men speculated vaguely and confusedly regarding

210 ON LIGHT.

natural phenomena, not liavinghad the discipline needed
to give clearness to their insight, and firmness to their
grasp of principles. They assured themselves of the rec-
tilineal propagation of light, and that the angle of inci-
dence was equal to the angle of reflection. For more
than a thousand years — I might say, indeed, for more
than fifteen hundred years subsequently — the scientific
intellect appears as if smitten with paralysis, the fact
being that, during this time, the mental force, which
might have run in the direction of science, was diverted
into other directions.

The course of investigation, as regards light, wa3
resumed in 1100 by an Arabian philosopher named
Alhazan. Then it was taken up in succession by Koger
Bacon, Vitellio, and Kepler. These men, though fail-
ing to detect the principle which ruled the facts, kept
the fire of investigation constantly burning. Then
came the fundamental discovery of Snell, that corner-
stone of optics, as I have already called it, and imme-
diately afterwards we have the application by Descartes
of Snell's discovery to the explanation of the rainbow.
Following this we have the overthrow, by Esemer, of
the notion of Descartes, that light was transmitted
instantaneously through space. Then came Newton's
crowning experiments on the analysis and synthesis of
white light, by which it was proved to be compounded
of various kinds of light of different degi'ees of re-
frangibility.

Up to his demonstration of the composition of white
light, Newton had been everywhere triumphant — tri-
umphant in the heavens, triumphant on the earth, and
his subsequent experimental work is, for the most
part, of immortal value. But infallibility is not the

CONCLUSION. 211

property of man, and, soon after his discovery of the
nature of white light, Newton proved himself human.
He supposed that refraction and dispersion went hand in
hand, and that you could not abolish the one without
at the same time abolishing the other. Here Dollond
corrected him.

But Newton committed a graver error than this.
Science, as I sought to make clear to you in our
second lecture, is only in part a thing of the senses.
The roots of phenomena are embedded in a region be-
yond the reach of the senses, and less than the root
of the matter will never satisfy the scientific mind.
We find, accordingly, in this career of optics the great-
est minds constantly yearning to break the bounds of
the senses, and to trace phenomena to their subsensible
foundations. Thus impelled, they entered the region of
theory, and here Newton, though drawn from time to
time towards the truth, was drawn still more strongly
towards the eiTor, and made it his substantial choice.
His experiments are imperishable, but his theory has
passed away. For a century it stood like a dam across
the course of discovery ; but, like all barriers that rest
upon authority, and not upon truth, the pressure from
beliind increased, and eventually swept the barrier away.
This, as you know, was done mainly through the labours
of Thomas Young, and his illustrious French fellow-
worker Fresnel.

In 1808 Malus, looking through Iceland spar at
the sun reflected from the window of the Luxembourg
Palace in Paris, discovered the polarization of light
by reflection. In 1811 Arago discovered the splendid
chromatic phenomena which we have had illustrated
by the deportment of plates of gypsum in polarized

212 ON LIGHT.

light ; he also discovered the rotation of the plane of
polarization by quartz-crystals. In 1813 Seebeck dis-
covered the polarization of light by tourmaline. That
same year Brewster discovered those magnificent bands
of colour that surround the axes of biaxal crystals.
In 1814 Wollaston discovered the rings of Iceland spar.
All these effects, which, without a theoretic clue, would
leave the human mind in a jungle of phenomena with-
out harmony or relation, were organically connected by
the theory of undulation.

The theory was applied and verified in all direc-
tions, Airy being especially conspicuous for the severity
and conclusiveness of his proofs. The most remark
able verification fell to the lot of the late Sir William
Hamilton, of Dublin, who, taking up the theory where
Fresnel had left it, arrived at the conclusion that at
four special points of the ' wave-surface ' in double-
refracting crystals, the ray was divided, not into two
parts, but into an infinite number of parts ; forming at
these points a continuous conical envelope instead of
two images. No human eye had ever seen this
envelope when Sir William Hamilton inferred its ex-
istence. He asked Dr. Lloyd to test experimentally
the truth of his theoretic conclusion. Lloyd, taking
a crystal of arragonite, and following with the most
scrupulous exactness the indications of theory, cutting
the crystal where theory said it ought to be cut, observ-
ing it where theory said it ought to be observed, dis-
covered the luminous envelope which had previously
been a mere idea in the mind of the mathematician.

Nevertheless this great theory of undulation, like
many another truth, which in the long run has proved
a blessing to humanity, had to establish, by hot con-

CONCLUSION. 213

flict, its right to existence. Great names were arrayed
against it. It had been enunciated by Hooke, it had
been applied by Huyghens, it had been defended by
Euler. But they made no impression. And, indeed,
the theory in their hands was more an analogy than a
demonstration. It first took the form of a demon
strated verity in the hands of Thomas Young. He
brought the waves of light to bear upon each other,
causing them to support each other, and to extinguish
each other at will. From their mutual actions he de-
termined their lengths, and applied his knowledge in
all directions. He finally showed that the difficulty of
polarization yielded to the grasp of theory.

After him came Fresnel, whose transcendent mathe-
matical abilities enabled him to give the theory a
generality unattained by Young. He seized it in
its entirety ; followed the ether into the hearts of
crystals of the most complicated structure, and into
bodies subjected to strain and pressure. He showed
that the facts discovered by Mains, Arago, Brewster,
and Biot were so many ganglia, so to speak, of his
theoretic organism, deriving from it sustenance and
explanation. With a mind too strong for the body
with which it was associated, that body became a wreck
long before it had become old, and Fresnel died, leav-
ing, however, behind him a name immortal in the
annals of science.

One word more I should like to say regarding
Fresnel. There are things better even than science.
Character is higher than Intellect, but it is especially
pleasant to those who wish to think well of human
nature when high intellect and upright cliaractcr are
found combined. They were, I believe, combined in

214 ON LIGHT.

this young Frenchman. In those hot conflicts of the
imdulatory theory, he stood forth as a man of integ--
rity, claiming no more than his right, and ready to
concede their rights to others. He at once recog-
nized and acknowledged the merits of Thomas Young.
Indeed, it was he, and his fellow-countryman Arago,
who first startled England into the consciousness of the
injustice done to Young in the Edinburgh Review.

I should like to read to you a hrief extract from a
letter written by Fresnel to Young in 1824, as it
throws a pleasant light upon the character of the
French philosopher. ' For a long time,' says Fresnel,
' that sensibility, or that vanity, which people call love
of glory has been much blunted in me. I labour
much less to catch the suffrages of the public, than to
obtain that inward approval which has always been the
sweetest reward of my efforts. Without doubt, in
moments of disgust and discouragement, I have often
needed the spur of vanity to excite me to pursue my
researches. But all the compliments I have received
from Arago, De la Place, and Biot never gave me so
much pleasure as the discovery of a theoretic truth, or
the confirmation of a calculation by experiment.'

This, then, is the core of the whole matter as regards
science. It must be cultivated for its own sake, for the
pure love of truth, rather than for the applause or profit
that it brings. And now my occupation in America is
well-nigh gone. Still I will bespeak your tolerance for
a few concluding remarks, in reference to the men who
have bequeathed to us the vast body of knowledge of
which I have sought to give you some faint idea in these
lectures What was the motive tliat spurred them on ?

CONCLUSION. 215

What urged them to those battles and those victories
over reticent Nature, which have become the heritage
of the human race ? It is never to be forgotten that
not one of those great investigators, from Aristotle down
to Stokes and Kirchhoff, had any practical end in view,
according to the ordinary definition of the word ' prac-
tical.' They did not propose to themselves money as
an end, and knowledge as a means of obtaining it. P'or
the most part, they nobly reversed this process, made
knowledge their end, and such money as they possessed
the means of obtaining it.

We see to-day the issues of their work in a thousand,
practical forms, and this may be thought suflBcient
to justify, if not ennoble their efforts. But they did
not work for such issues ; their reward was of a totally
different kind. In what way different ? AYe love
clothes, we love luxuries, we love fine equipages, we love
money, and any man who can point to these as the re-
sult of his efforts in life, justifies these results before all
the world. In America and England, more especially, he
is a ' practical' man. But I would appeal confidently to
this assembly whether such things exhaust the demands
of human nature ? The very presence here for six
inclement nights of this great audience, embodying so
much of the mental force and refinement of this vast
city,* is an answer to pay question. I need not tell such
an assembly that there are joys of the intellect as well as
joys of the body, or that these pleasures of the spirit
constituted the reward of our great investigators. Led
on by the whisperings of natuial truth, through pain

' Now York: for more than a dee&ie no such weallicr bad been ex-
jierioncod. The suow was so deep that tlio ordinary moans of locomo-
cioj were for a time suspended.

216 ON LIGHT.

and self-denial, they often pursued their work. Witli
the ruling passion strong in death, some of them, when
no longer able to hold a pen, dictated to their friends
the results of their labours, and then rested from them
for ever.

Could we have seen these men at work, without any
knowledge of the consequences of their work, what
should we have thought of them ? To the uninitiated,
in their day, they might often appear as big children
playing with soap-bubbles and other trifles. It is
so to this hour. Could you watch the true investi-
gator— your Henry or your Draper, for example — in
his laboratory, unless animated by his spirit, you could
hardly understand what keeps him there. Many of
the objects which rivet his attention might appear to
you utterly trivial; and, if you were to ask him
what is the use of his work, the chances are that
you would confound him. He might not be able
to express the use of it in intelligible terms. He
might not be able to assure you that it will put a
dollar into the pocket of any human being, living or to
come. That scientific discovery 7nay put not only
dollars into the pockets of individuals, but millions into
the exchequers of nations, the history of science amply
proves ; but the hope of its doing so never was, and
it never can be, the motive power of the investigator.

I know that some risk is run in speaking thus before
practical men. I know what De Tocqueville says of
you. ' The man of the North,' he says, ' has not only
experience, but knowledge. He, however, does not
care for science as a pleasure, and only embraces it
with avidity when it leads to useful applications.' But
what, I would ask, are the hopes of useful applications

CONCLUSION. 217

which have caused you so many times to fill this place,
in spite of snow-drifts and biting cold ? What, I may
ask, is the origin of that kindness which drew me from
my work in London to address you here, and which, if
I permitted it, would send me home a millionaire ? Not
because I had taught you to make a single cent by
science am I here to-night, but because I tried to the
best of my ability to present science to the world as an
intellectual good. Surely no two terms were ever so
distorted and misapplied with reference to man, in his
higher relations, as these terms useful and practical.
Let us expand our definitions until they embrace all
the needs of man, his highest intellectual needs inclu-
sive. It is specially on this ground of its adminis-
tering to the higher needs of the intellect ; it is mainly
because I believe it to be wholesome, not only as a
source of knowledge but as a means of discipline, that
I urge the claims of science upon your attention.

But with reference to material needs and joys, surely
pure science has also a word to say. People sometimes
speak as if steam had not been studied before James
Watt, or electricity before Wheatstone and Morse ;
whereas, in point of fact, \N'att and Wlieatstone and
JNIorse, with all their practicality, were the mere out-
come of antecedent forces, which acted without refer-
ence to practical ends. This also, I think, merits a
moment's attention. You are delighted, and with good
reason, with your electric telegraphs, proud of your
steam-engines and your factories, and charmed with
the productions of photograpliy. You see daily, with
just elation, the creation of new forms of industry —
new powers of adding to the wealth and comfort of
society. Industrial England is heaving with forces

218 ON LIGHT.

tending to this end ; and the pulse of industry beats
still stronger in the United States. And yet, when
analyzed, what are industrial America and industrial
England ?

If you can tolerate freedom of speech on my part,
I will answer this question by an illustration. Strip
a strong arm, and regard the knotted muscles when the
hand is clenched and the arm bent. Is this exhibition
of energy the work of the muscle alone ? By no means.
The muscle is the channel of an influence, without which
it would be as powerless as a lump of plastic dough.
It is the delicate unseen nerve that unlocks the power
of the muscle. And without those filaments of genius,
which have been shot like nerves through the body of
society by the original discoverer, industrial America,
and industrial England, would be very much in the con-
dition of that plastic dough.

At the present time there is a cry in England for
technical education, and it is a cry in which the most
commonplace intellect can join, its necessity is so
obvious. But there is no cry for original investi-
gation. Still without this, as surely as the stream
dwindles when the spring dies, so surely will ' technical
education' lose all force of growth, all power of repro-
duction. Our great investigators have given us
suSicient work for a time ; but if their spirit die out,
we shall find ourselves eventually in the condition of
those Chinese mentioned by De Tocqueville, who,
having forgotten the scientific origin of what they
did, were at length compelled to copy without varia-
tion the inventions of an ancestry wiser than them-
selves, who had drawn their inspiration direct from
Natui'e.

CONCLUSION. 219

Both England and America have reason to bear those
things in mind, for the largeness and nearness of ma-
terial results are only too likely to cause both countries
to forget the small spiritual beginnings of such results,
in the mind of the scientific discoverer. You mul-
tiply, but he creates. And if you starve him, or other-
wise kill him — nay, if you fail to secure for him free
scope and encouragement — you not only lose the mo-
tive power of intellectual progress, but infallibly sever
yourselves from the springs of industrial life.

What has been said of technical operations holds
equally good for education, for here also the original
investigator constitutes the fountain-head of know-
ledge. It belongs to the teacher to give this knowledge
the requisite form ; an honourable and often a difficult
task. But it is a task which receives its final sanctifica-
tion, when the teacher himself honestly tries to add a
rill to the great stream of scientific discovery. Indeed,
it may be doubted whether the real life of science can
be fully felt and communicated by the man who has not
himself been taught by direct communion with Nature.
We may, it is true, have good and instructive lectures
from men of ability, the whole of whose knowledge is
second-hand, just as we may have good and instructive
sermons from intellectually able and unregenerate men.
But for that power of science, which corresponds to
what the Puritan fathers would call experimental re-
ligion in the heart, you must ascend to the original
investigator.

To keep society as regards science in healthy play,
three classes of workers are necessary : Firstly, the in-
vestigator of natural truth, whose vocation it is to pur-
sue that truth, and extend the field of discovery for the

220 ON LIGHT.

truth's owu sake, and without reference to practical
ends. Secondly, the teacher of natural truth, whose
vocation it is to give public diffusion to the knowledge
already won by the discoverer. Thirdly, the applier of
natural truth, whose vocation it is to make scientific
knowledge available for the needs, comforts, and luxu-
I'ies of civilized life. These three classes ought to co-
exist and interact. Now, the popular notion of science,
both in this country and in England, often relates not
to science strictly so called, but to the applications of
science. Such applications, especially on this continent,
are so astounding— they spread themselves so largely
and umbrageously before the public eye — that they often
shut out from view those workers who are engaged in
the quieter and profounder business of original investi-
gation.

Take the electric telegraph as an example, which
has been repeatedly forced upon my attention of late.
I am not here to attenuate in the slightest degree the
services of those who, in England and America, have
given the telegraph a form so wonderfully fitted for
public use. They earned a great reward, and they
have received it. But I should be untrue to you and
to myself if I failed to tell you that, however high in
particular respects their claims and qualities may be,
your practical men did not discover the electric tele-
graph. The discovery of the electric telegraph implies
the discovery of electricity itself, and the develop-
ment of its laws and phenomena. Such discoveries
are not made by practical men, and they never will
be made by them, because their minds are beset by
ideas which, though of the highest value from one

CONCLUSION. 221

point of view, are not those which stimulate the original
discoverer.

The ancients discovered the electricity of amber ; and
Gilbert, in the year 1600, extended the discovery to
other bodies. Then followed Boyle, Von Griiericke,
Gray, Canton, Du Fay, Kleist, CunEeus, and your
own Franklin. But their form of electricity, though
tried, did not come into use for telegraphic pur-
poses. Then appeared the great Italian Volta, who
discovered the source of electricity which bears his
name, and applied the most profound insight, and
the most delicate experimental skill, to its develop-
ment. Then arose the man who added to the powers
of his intellect all the graces of the human heart,
Michael Faraday, the discoverer of the great domain of
magneto-electricity. OErsted discovered the deflection
of the magnetic needle, and Arago and Sturgeon the
magnetization of iron by the electric current. The
voltaic circuit finally found its theoretic Newton in
Ohm; while Henry, of Princeton, who had the sagacity
to recognise the merits of Olim while they were still
decried in his own country, was at this time in the van
of experimental inquiry.

In the works of these men you have all the materials
employed at this hour, in all the forms of the electric
telegraph. Nay, more ; Gauss, the celebrated astrono-
mer, and Weber, the celebrated natural philosopher,
both professors in the University of Gottingen, wishing
to establish a rapid mode of communication between
the observatory and the physical cabinet of the uni-
versity, did this by means of an electric telegraph.
Thus, before your practical men appeared upon the
scene, tlie force had been discovered, its laws investi-
11

'IT2. ON LIGHT.

g9.ted and made sure, the most complete mastery of its
phenomena had been attained — nay, its applicability to
telegraphic purposes demonstrated — by men whose sole
reward for their labours was the noble excitement of
research, and the joy attendant on the discovery of
natural truth.

Are we to ignore all this ? We do so at our peril.
For I say again that, behind all our practical appli-
cations, there is a region of intellectual action to which
practical men have rarely contributed, but from which
they draw all their supplies. Cut them off from this
region, and they become eventually helpless. In no case
is the adage truer, * Other men laboured, but ye are
entered into their labours,' than in the case of the dis-
coverer and applier of natural truth. But now a word
on the other side. While practical men are not the men
to make the necessary antecedent discoveries, the cases
are rare, though, in our day, not absent, in which the dis-
coverer knows how to turn his labours to practical ac-
count. Different qualities of mind and habits of thought
aie usually needed in the two cases ; and while I wish
to give emphatic utterance to the claims of those whose
position, owing to the simple fact of their intellectual
elevation, is often misunderstood, I am not here to
exalt the one class of workers at the expense of the
other. They are the necessary complements of each
other. But remember that one class is sure to be taken
care of. All the material rewards of society are already
within their reach, while that same society habitually
ascribes to them intellectual achievements which were
never theirs. This cannot but act to the detriment of
those studies out of which, not only oiu: knowledge
of nature, but our present industrial arts themselves

CONCLUSIOX. 223

have sprung, and from which the rising genius of tlie
country is incessantly tempted away.

Pasteur, one of the most eminent members of the
Institute of France, in accounting for the disastrous
overthrow of his country and the predominance of
Germany in the late war, expresses himself thus : ' Few
persons comprehend the real origin of the marvels of
industry and the wealth of nations. I need no further
proof of this than the employment more and more fre-
quent in official language, and in writing of all sorts,
of the erroneous expression applied science. The
abandonment of scientific careers by men capable of
pursuing them with distinction, was recently deplored
in the presence of a minister of the greatest talent.
The statesman endeavoured to show that we ought not
to be surprised at this result, because in our day the
reign of theoretic science yielded place to that of ap-
plied science. Nothing could be more erroneous than
this opinion, nothing, I venture to say, more dangerous,
even to practical life, than the consequences which miglit
flow from these words. They have rested in my mind
as a proof of the imperious necessity of reform in our
superior education. There exists no category of the
sciences, to which the name of applied science could
be rightly given. We have science, and the appli/-
cations of science, which are united together as the
tree and its fruit,'

And Cuvier, the great comparative anatomist, writes
thus upon the same theme : ' These grand practical
innovations are the mere applications of truths of a
higher order, not sought with a practical intent, but
pursued for their own sake, and solely through an
ardour for knowledge. Those wlio applied them could

224 ON LIGHT.

not have discovered them ; those who discovered them
had no inclination to pursue them to a practical end.
Engaged in the high regions whither their thoughts
had carried them, they hardly perceived these practical
issues, though born of their own deeds. These rising
workshops, these peopled colonies, those ships which
furrow the seas — this abundance, this luxury, this
tumult — all this comes from discoverers in science, and
it all remains strange to them. At the point where
science merges into practice they abandon it ; it con-
cerns them no more.'

When the Pilgrim Fathers landed at Plymouth
Eock, and when Penn made his treaty with the Indians,
the new-comers had to build their houses, to chasten
the earth into cultivation, and to take care of their
souls. In such a community science, in its more ab-
stract forms, was not to be thought of. And at the
present hour, when your hardy Western pioneers stand
face to face with stubborn Nature, piercing the moun-
tains and subduing tlie forest and the prairie, the pur-
suit of science, for its own sake, is not to be expected.
The first need of man is food and shelter ; but a vast
portion of this continent is already raised far beyond
this need. The gentlemen of New York, Brooklvn,
Boston, Philadelphia, Baltimore, and Washington, have
already built their houses, and very beautiful they are :
they have also secured their dinners, to the excellence
of which I can also bear testimony. They have, in
fact, reached that precise condition of well-being and
independence when a culture, as high as humanity has
yet reached, may be justly demanded at their hands.
They have reached that maturity, as possessors of
wealth and leisure, when the investigator of natural

CONCLUSION. 225

trutli, for the truth's own sake, ought to find among
them promoters and protectors.

Among the many problems before them the}^ have
this to solve, whether a republic is able to foster the
highest forms of genius. You are familiar with the
writings of De Tocqueville, and must be aware of
the intense sympathy which he felt for your institu-
tions ; and this sympathy is all the more valuable from
the philosophic candoiur with which he points out not only
yovu- merits, but your defects and dangers. Now if I
come here to speak of science in America in a critical
and captious spirit, an invisible radiation from my
words and manner will enable you to find me out, and
will guide your treatment of me to-night. But if I in
no unfriendly spirit — in a spirit, indeed, the reverse of
unfriendly — venture to repeat before you what this
great historian and analyst of democratic institutions
said of America, I am persuaded that you will hear me
out. He wrote some three and twenty years ago, and,
perhaps, would not write the same to-day ; but it will
do nobody any harm to have his words repeated, and, if
necessary, laid to heart.

In a work published in 1850, De Tocqueville says: 'It
must be confessed that, among the civilized peoples of our
age, there are few in which the highest sciences have made
so little progress as in the United States.' * He declares
his conviction that, had you been alone in the universe,
you would soon have discovered that you cannot long
make progress in practical science, without cultivating

' ' II faut rcconnaitre que parmi les peuples ci\n'lise8 do nos jours il
en est peu chez qui los hautes sciences aicnt fait nioins do progresqu'aux
Ktats-Unis, ou qui aiont fouriii moins do grands arlistos, de poetos illns-
Ires et do c^liibros ^crivains.' (' i>e la L>6mocrutie en Am^rique,' etc.,
tome ii. p. 36)

226 ON LIGHT.

theoretic science at the same time. But, according to
De Tocqueville, you are not thug alone. He refuses to
separate America from its ancestral home; and it is
there, he contends, that you collect the treasures of the
intellect, without taking the trouble to create them.

De Tocqueville evidently doubts the capacity of a
democracy to foster genius as it was fostered in the
ancient aristocracies. ' The future,' he says, ' will
prove whether the passion for profound knowledge, so
rare and so fruitful, can be born and developed so
readily in democratic societies as in aristocracies. As
for me,' he continues, 'I can hardly believe it.' He
speaks of the unquiet feverishness of democratic com-
munities, not in times of great excitement, for such
times may give an extraordinary impetus to ideas,
but in times of peace. There is then, he says, ' a small
and uncomfortable agitation, a sort of incessant attri-
tion of man against man, which troubles and distracts
the mind without imparting to it either loftiness or
animation.' It rests with you to prove whether these
things are necessarily so — whether scientific genius
cannot find, in the midst of you, a tranquil home.

I should be loth to gainsay so keen an observer and
so profound a political writer, but, since my arrival in
this country, I have been unable to see anything in
the constitution of society, to prevent a student, with
the root of the matter in him, from bestowing the most
steadfast devotion on pure science. If great scientific
results are not achieved in America, it is not to the small
agitations of society that I should be disposed to ascribe
the defect, but to the fact that the men among you who
possess the endowments necessary for profound scientific
inquiry, are laden with duties of administration, or

CONCLUSION. 227

tuition, so heavy as to be utterly incompatible with the
continuous and tranquil meditation which original inves-
tigation demands. It may well be asked whether Henry
would have been transformed into an administrator, or
whether Draper would have forsaken science to write
history, if the original investigator had been honoured
as he ought to be in this land. I hardly think they woidd.
Still I do not imagine this state of things likely to last.
In America there is a willingness on the part of indi-
viduals to devote their fortunes, in the matter of educa-
tion, to the service of the commonwealth, which is pro-
bably without a parallel elsewhere : and this willingness
requires but wise direction to enable you effectually to
wipe away the reproach of De Tocqueville.

Your most difficult problem will be not to build
institutions, but to discover men. You may erect
laboratories and endow them ; you may furnish them
with all the appliances needed for enquiry ; in so do-
ing you are but creating opportunity for the exercise
of powers which come from sources entirely beyond your
reach. You cannot create genius by bidding for it. In
biblical language, it is the gift of God ; and the most
you could do, were your wealth, and your willingness to
apply it, a million-fold what they are, would be to
make sure that this glorious plant shall have the free-
dom, light, and warmth necessary for its development.
We see from time to time a noble tree dragged down
by parasitic runners. These the gardener can remove,
though the vital force of the tree itself may lie beyond
him : and so, in many a case, you men of wealth can
liberate genius from the hampering toils which tlie
struggle for existence often casts around it.

Drawn by your kindness, I have come here to give

228 ON LIGHT.

these lectures, and now that my visit to America has
become almost a thing of the past, I look back upon
it as a memory without a single stain. No lecturer was
ever rewarded as I have been. From this vantage-
ground, however, let me remind you that the work of
the lecturer is not the highest work ; that, in science,
the lecturer is usually the distributor of intellectual
wealth amassed by better men. And though lecturing
and teaching, in moderation, will in general promote
their moral health, it is not solely, or even chiefly, as
lecturers, but as investigators, that your highest men
ought to be employed. You have scientific genius
amongst you - not sown broadcast, believe me, it is sown
thus nowhere — but still scattered here and there. Take
all unnecessary impediments out of its way. Keep your
sympathetic eye upon the originator of knowledge.
Give him the freedom necessary for his researches, not
overloading him, either with the duties of tuition or
of administration, not demanding frora him so-called
practical results — above all things, avoiding that
question which ignorance so often addresses to genius,
' What is the use of your work ? ' Let him make
truth his object, however unpractical for the time
being it may appear. If you cast your bread thus
upon the waters, then be assured it will return to you,
though it may be after many days.

APPENDIX.

My work in the United States was wound up by a social
meeting in New York, under the presidency of the Hon. W.
M. Evarts, a name as familiar to English as to American ears.
Of the able addresses delivered on that occasion, I here present
three,* which have a special bearing upon scientific and edu-
cational questions. The first by Dr. Barnard, the learned
President of Columbia College, New York ; the second by Dr.
Draper, so well and favourably known in England, both as a
historian and man of science; and the third by Dr. White,
President of Cornell University. To these I have ventured to
add a few remarks of my own, made upon the same occasion.

PRESIDENT BARNAPvD'S ADDRESS.

I am expected to deal, this evening, with a theme which,
under the actual circumstances, it is somewhat difficult to
handle. The degree to which our systems of education tend
to foster or discourage original investigation into the truths of
Nature is a topic which might better befit an assembly more
gravely disposed than the present. Dulce est desipere in loco —
it is pleasant to put on the cap and bells when circumstances
favour, says Horace, and he says quite truly ; but he does not
say, dijjlcile est sapere inter pocula — it is hard to imitate the
solemnity of ^linerva's bird when champagne is on the board,
as I think he ought to have said, and as he would, perhaps,

' With certain personal references orcifted.

230 APPENDIX.

have said if prosody had allowed, and which would have been
equally true. I shall not aim at such an imitation. I do not
mean to be didactic if I can help it. If I am so, I trust you
will be indulgent.

I say, then, that our long- established and time-honoured
system of liberal education — and when I speak of the system
I mean the whole system, embracing not only the colleges,
but the tributary schools of lower grade as well — does not
tend to form original investigators of Nature's truths ; and
the reason that it does not is, that it inverts the natural
order of proceeding in the business of mental culture, and
fails to stimulate in season the powers of observation. And
when I say this, I must not be charged with treason to my
craft — at least not with treason spoken for the first time here,
for I have uttered the same sentiment more than once before
in the solemn assemblies of the craft itself.

I suppose, Mr. President, that at a very early period of
your life you may have devoted, like so many other juvenile
citizens, a portion of your otherwise unemployed time to
experiments in horticulture. In planting leguminous seeds
you could not have failed to observe that the yoxmg plants
come up with their cotyledons on their heads. If, in ponder-
ing this phenomenon, you arrived at the same conclusion that
I did, you must have believed that Nature had made a
mistake, and so have pulled up your plants and replanted
them upside-down. Men and women are but children of a
larger growth. They see the tender intellect shooting up in
like manner, with the perceptive faculties all alive at top ;
and they, too, seem to think that Nature has made a mistake,
and so they treat the mind as the child treats his bean-plant,
and turn it upside-down to make it grow better. They bury
the promising young buds deep in a musty mould formed of
the decay of centuries, under the delusion that out of such
debris they may gather some wholesome nourishment ; when
we know all that they want is the light and warmth of the
Bun to stimulate them and the free air of heaven in which

APPENDIX. 231

to unfold themselves. What heartless cruelty pursues the
little child-martyr every day and all the day long, at home or
at school alike ; in this place bidden to mind his book and not
to look out of the window — in that, told to hold his tongue
and to remember that children must not ask questions ! A
lash from a whalebone switch upon the tender little fingers
too eagerly outstretched could not sting more keenly, or be
felt with a sharper sense of wrong, than such a rebuke coming
across the no less eagerly extended tentacles of the dawning
and inquiring intellect.

Now, a system of education foimded on a principle like
this is not going to fit men to engage successfully in that haz-
ardous game of life, in which, in Prof. Huxley's beautiful
simile, we are all of us represented as playing with an unseen
antagonist, who enforces against us relentlessly every minutest
rule of the game, whether known to us or not. Still less can
it fit them to bring to light new rules of this difficult game,
never yet detected by any human intelligence. Yet it is pre-
cisely of this kind of men that the world has present need.
For grand as are the triumphs of scientific investigation
already achieved, it is impossible to doubt that there are still
grander yet behind to reward the zealous labourers of the time
to come. I know that it sometimes seems to us otherwise. I
know that the very grandeur of the achievements of the
past makes us sometimes doubtful of the future; for it is
generally true that the portals of Nature's secret chambers,
yet unexplored, are only dimly discernible before they are
tmlocked.

I remember a lime — it is now long gone by — when this
sceptical feeling as to the possibilities of large scientific
progress iu the time to come was extremely prevalent, so
prevalent that a learned professor of a neighbouring college
thought it wortli his while to combat, in an energetic public
address, the discouraging notion that Nature has no longer
any important secrets to yield. Subsequently history has mag-
nificently corroborated his argument. For that was a ticoe

232 APPENDIX.

when, as yet, no Faraday had drawn a living spark from the
lifeless magnet ; no Daniel, or Grove, or Bunsen, had given
us an enduring source of electro-dynamic power ; no Ohm
had taught us how to measure such a power when obtained ;
no Bessell had detected the parallaxes of the fixed stars ; no
Adams or Leverrier had thrown his grapple into space, and
felt the influence of an unseen planet trembling, to use the
beautiful language of Herschel, along the delicate line of his
analysis ; no Draper, or Daguerre, or Talbot, had revealed
the wonders of actinism ; no Mayer or Joule had laid a sure
foundation for the grand doctrine of the conservation of
force ; no Carpenter had unravelled the intricacies of nervous
physiology, or analyzed the relations of mind and brain ; no
Agassiz had ridden down the Alps on the backs of the gla-
ciers and proved their steady flow ; no Darwin had lifted the
veil from the mysteries of organic development ; no Schiapa-
relli or Newton had put the harness of universal gravitation
upon the Avayward movements' of the shooting-stars; no
Mallet had presented an intelligible theory of volcanic flames
and of the earth's convulsive tremors ; no KirchhofF had fur-
nished a key to the intimate constitution of celestial bodies
or a gauge of stellar drift ; no Huggins, or Secchi, or Young,
had applied the key thus presented to enter the secret cham-
bers of the sun, the comets, the fixed stars, and the nebulae;
no Stokes had made the darkness visible which lies beyond
tlie violet. In short, that period of presumed scientific omni-
science seems now, as we look back to it, but the faint dawn-
ing of a day of glorious discovery, which we dare not, even
yet, pronounce to be approaching its meridian.

liow much of all this has been due to our systems of edu-
cation ? Among the great promoters of scientific progress
before or since, how large is the number who may, in strict
propriety, be said to have educated themselves ? Take, for
illustration, such fomiliar names as those of William Herschel,
and Franklin, and Rumford, and Rittenhouse, and Davy, and
Faraday, and Henry. Is it not evident that Nature herself,

APPENDIX. 233

to those who will follow her teachings, is a better guide to
the study of her own phenomena than all the training of our
schools? And is not this because Nature invariably begins
with the training of the observing faculties ? Is it not be-
cause the ample page which she spreads out before the
learner is written all over, not with words, but with substan-
tial realities ? Is it not because her lessons reach beyond the
simple understanding and impress the immediate intuition ?
That what she furnishes is something better than barren in-
formation passively received ; it is positive knowledge actively
gathered ?

If, then, in the future we would fit man properly to cul-
tivate Nature, and not leave scientific research, as, to a great
extent, we have done heretofore, to the hazard of chance, we
must cultivate her own processes. Our earliest teachings
must be tilings, and not Avords. The objects first presented
to the tender mind must be such as address the senses, and
such as it can grasp. Store it first abundantly with the ma-
terial of thought, and the process of thinking will be spon-
taneous and easy.

This is not to depreciate the value of other subjects or of
other modes of culture. It is only to refer them to their
proper place. Grammar, philology, logic, human history,
belles-lettres, philosophy — all these things will be seized with
avidity and pursued with pleasure by a mind judiciously pre-
pared to receive them. On this point we shall do well to
learn, and I believe we are beginning to learn something,
from contemporary' peoples upon the Continent of Europe.
Object-teaching is beginning to be inft-oduced, if only spar-
ingly, into our primary schools. It should be so introduced
universally. And in all our schools, but especially in those
in which the foundation is laid of what is called a liberal
education, the knowledge of visible things should be made to
precede the study of the artificial structure of language, and
the intricacies of grammatical rules and forms.

The knowledge of visible things — I repeat these words

234 APPENDIX.

tliat I may emphasize them, and, when I repeat tliem, ob-
serve that I mean knowledge of visible things, and not infor-
mation abont them — knowledge acquired by the learner's
own conscious efforts, not crammed into his mind in set
forms of words out of books. Our methods of education
manifest a strong tendency in these modern times to degener-
ate in such a sort of cramming. Forty years ago, the printed
helps to learning now supphed to the young men of our col-
leges in so lavish profusion were almost unknown ; and teach-
ers lent about as little aid, at least during the earlier years, as
books. What the student learned then he learned for him-
self by positive hard labour. Now we have made the task so
easy, we have built so many royal roads to learning in all its
departments, that it may be well doubted if the young men
of our day, with all their helps, acquire as much as those of
that earlier period acquii-ed without them.

The moral of this experience is, that mental culture is not
secured by pouring information into passive recipients; it
comes from stimulating the mind to gather knowledge for
itself. When our systems of education shall have been re-
modelled from top to bottom, with due attention to this
principle, then, if we have minds among us which are capable
of pursuing Nature into her yet uncaptured strongholds, we
shall find them out and set them at their work. Then
neither * mute, inglorious Miltons ' on the one hand, nor on
the other silent, unsuspected Keplers, nor Newtons ' guilt-
less ' of universal gravitation, shall live unconscious of their
powers, or die and make no sign. Then the progress of
science will no longer be dependent, as in the past it has been
to so great a degree, on the chance struggles of genius rebel-
ling against circumstances, such as have given us a Herschel,
a Franklin, a Hugh Miller, or a Henry ; nor will the world
be any more astonished to see the most brilliant of the
triumphs of the intellect achieved by men who have cloven
their own way to the forefront, in defiance of all its educa-
tional traditions.

APPENDIX. 235

PROFESSOR DRAPER'S ADDRESS.

When I was in London a year or two ago I passed some
pleasant liours with my friend Prof. Tyndall. Among these,
I think that, perhaps, the most pleasant were those of one
afternoon that we spent together in the laboratory of the
Royal Institution, where Davy discovered potassium and
sodium, and decomposed the earths; where Young first
announced the grand and fertile principle of interference, and
placed on firm foundations the wave-theory of light ; where
Faraday made his great discoveries in electricity and magne-
tism. On that occasion Dr. Tyndall was showing me the
action of ether-waves of short period upon gaseous matter,
clouds formed by actinic decomposition. I saw the superb
sky-blue light and verified its polarized condition. It wiis
like the light of heaven.

Well, as I laid down the Nicol prism we had been using,
I could not help thinking that there was an unseen Presence
in the place — a genius loci — that inspired men to make such
discoveries. Who was it that brought that genius there ?

At the time of the American Revolution there resided in
the town of Rumford, N. H., one Benjamin Thompson, who
occupied himself in teaching a school. He embraced, as we
Americans would say, the wrong side of the question on that
occasion — he sided with the king's Government. He went
to England, became a man of mark, and was knighted. Then
he went on the Continent, again distinguished himself by liis
scientific attainments, again was titled, and this time, in
memory of his American home, was called Count Rumford.

On his return to London, Count Rumford founded the
Royel Institution, and thus to a native Ameiican the world
owes that establishment which has been glorified by Davy,
and Young, and Faraday. Had it not been fur Rumford,
Davy might have spent his life in fiUiiig gas-bags for Dr.
Beddoes's patients, and Faraday might liave been a book-
binder

236 ArPENDIX.

But if Benjamin Thompson, an American, founded tha
Royal Institution, James Smithson, an Englishman, shortly
afterwards, founded that noble Institution in Washington
■which bears his name, and which, under the enlightened care
of Prof Henry, has so greatly ministered to the advancement
and diffusion of science. You, sir, have called on me to
respond to your toast, ' English and American Science,' and
I think these facts show you how closely they have been
associated.

Now Prof. Tyndall is on the point of leaving us. When
he gets back to Albemarle Street, he will remember Broad-
way. I am sure that you will all join me in wishing him a
pleasant voyage over the Atlantic. But I wish him some-
thing better than that, I will add — a safe return to America.
There is a great deal for him to do here yet. He may tell his
friends that he has been to America, but he must not teU
them that he has seen the Americans. We who are living
on the Atlantic verge of the continent are only modified
Europeans — very slightly modified, indeed. One must go
beyond the Alleghanies — yes, and over to the Pacific coast,
before he can say he has seen what the American really is.
I suppose that Dr. Tyndall has finished his glacier expe-
ditions to Switzerland. Is there nothing here that can
tempt him ? He and other members of the Alpine Club need
not go about the streets of London weeping like so many
broken-hearted Alexanders that there are no more worlds
to conquer. Let them take a look at the Eocky Mountains
and tell us what they think of them. Dr. Tyndall is a lover
of Nature. Well ! we can show him all kinds of scenery,
from where the half-frozen Mackenzie is lazily flowing through
a waste of snows on its way toward the Arctic Ocean, to where
oranges are growing on the Gulf. Or, if he is tired of inani-
mate Nature, and is in the mood of Dr. Johnson — you know
the story. Boswell said to Johnson one day : ' See ! What a
beautiful afternoon ; let us take a walk in the green fields.'
* No, I won't,' replied the grim and gruff lexicog^pher

APPENDIX. 237

' Yvo s( en green fields ; one green field is like another green
field. They are all alike. No, sir ! I'll walk down Cheap-
side. I like to look at men ; ' — if Dr. Tyndall is in that mood,
can we not satisfy his curiosity ? Another friend of mine,
Mr. Froude, has set us all talking about Ireland. We can
show Dr. Tyndall how we take the Irish immigrant, in his
corduroy knee-breeches, his smashed-down hat, and his shil-
lalah in his fist, and in a generation or so turn him into an
ornament of professional life, make him a successful merchant,
or familiarize him with all the amenities of elegant society.
If that's not enough, we Avill show him how we take the
German, and, wonderful to be said, make him half forget
his fatherland and half his mother-tongue, and become an
English-speaking American citizen. If that's not enough,
Ave will show him how we have purged the African, the
woolly- headed black man, of the paganism of his forefathers,
and are now trying our hand at Darwinizing him into a re-
spectable voter. If that's not enough, we will show him
how, in the trans-Mississippi plains, we are improving the
red Indian — alas, I fear my friend will say, improving him
off the face of the earth ! If that's not enough, we will show
him where we have got tens of thousands of Chinese, with
picks and shovels, digging Pacific railways. We are mixing
European and Asiatic, red Indians and black Africans to-
gether, and I suppose certain English naturalists will tell us
that the upshot of the thing will be a survival of the fittest.
In San Francisco we can show Dr. Tyndall the church, the
chapel, the joss-hoiase, all in a row ; and perhaps, considering
his forlorn, celibate condition, he may be conscience-stricken
when we display before his astonished eyes the much-mar-
ried men of Mormondom.

Nowhere in the world are to be found more imposing
political problems than those to be settled here ; nowhere a
greater need o£ scientific knowledge. I am not speaking of
ours.'Ives alone, but also of our Canadian friends on the other
aide of tlie St. Lawrence. We must join together in generous

238 APPENDIX.

emulation of the best that is done in Europe. In her Ma-
jesty's representative, Lord Dufferin, they will find an eager
appreciation of all that they may do. Together we must try
to refute what De Tocqueville has said about us, that com-
munities such as ours can never have a love of pure science.
But, whatever may be the glory of our future intellectual
life, let us both never ibrget what we owe to England. Hers
is the language that we speak, hers are all our ideas of liberty
and law. To her literature as to a fountain of light we re-
pair. The torch of science that is shining here was kindled
at her midnight lamp.

PRESIDENT WHITE'S REMARKS.

There is a legend well known to most of us — and which
has an advantnge over most legends, in that it is substantially
true — that a very distinguished man of science in this country
was once approached by an eminent practical man, and urged
to turn his great powers in scientific investigation and ex-
position to effect in making a fortune.

And to the great surprise of that man of business, the man
of science responded, * but, my dear sir, / have no time to
waste in maJcing mone]]?

^f all the recent great results of science, I think, sir, that
those words have struck deepest and sped farthest in the
average carnal mind on our side the Atlantic.

* No time to waste in making money ! ' I have stood, sir,
in the presence of a very eminent man of affairs — one whose
word is a power in the great marts of the world, and watched
him as he heard for the first time this astonishing dictum.
He stood silent — apparently in awe. The words seemed to
reverberate among the convolutions of his brain, and to be
re-echoed far away, back, from depth to depth among the
deepest recesses of his consciousness — ' No time to waste in
making money ! '

The toast, sir, to which yoii ask me to speak is, * The
Relation of Science to Political Progress.'

APPENDIX. 239

Now, sir, I maintain that the true spirit of scientific re-
search, embracing as it does zeal in search for truth, devotion
to duty which such a search imposes, faith in good as the
normal and necessary result of such a search — that such a
spirit is, at this moment, one of the most needed elements in
the political progress of our country.

I might go on to show how usefully certain scientific
methods might be brought to bear on the formation of politi-
cal judgments, and in determining courses of political action.
I might show how even a very moderate application of scien-
tific principles would save us from what is constantly going
on in municipal, State, or national legislation — the basing of
important statutes, to-day, on the supposition that two and
two make four, and to-morrow on the theory that two and
two make forty ; but the hour is late, and I spare you ; I
will confine myself simply to the value, in our political pro-
gress, of the spirit and example of some of the scientific
workers of our day and generation.

What is the example which reveals that spirit ? It is an
example of zeal — zeal in search for the truth, sought for truth's
sake — and not for the sake of material advantage ; it is an
example of thoroughness — of the truth sought in its whole-
ness, not in dilutions or adaptations, or siippressions, supposed
to be healthy for this man's mind, or that man's soul ; it is
an example of bravery — the fearlessness that leads a truth-
seeker to brave all outcry and menace ; it is an example of
devotion to duty ; without which, for a steady force, as Prof.
Tyndall just now observed, no worthy scientific work can bo
accomplished ; and, finally, an example of faith — of a high
and holy faith that the results of earnest truth-seeking cannot
b» other than good — faith that truth and goodness are insep-
arable— faith that there is a Power in the universe which
forbids any honest tnith-seeking to lead to lasting evil. A
faith, thi.s is, which has had its ' noble army of martyrs '
from long before lloger Bacon down to this present — mar-
tyrs not less real than that devoted aaint, from whom, us I

240 APPENDIX.

understand, our guest takes his name, who perished in the
flames as a martvr to religious duty.

What I maintain, then, is, that this zeal for truth as truth,
this faith in the good as for .ever allied to the true, this de-
votion to duty, as the result of such faith and zeal, consti-
tute probably the most needed element at this moment in the
political regeneration of this country, and that, therefore, the
example of our little army of true devotees of science has an
exceeding preciousness.

Said a justly distinguished senator to me yesterday, in
Washington : ' The true American idea of education is, to
give all children a good and even start ; then to hold up the
prizes of life before them ; then to say to them : " Go in and
win ; let the smartest have the prizes." '

Who of the common herd shall dispute the conclusions
of a senator beneath the great cast-iron dome at Washing-
ton ? — But here, in this presence, I may venture to say that
such a theory of education is one of the main causes of our
greatest national danger and disgrace. No theory can be
more false, or^ in the long run, more fatal. Look at it for
a moment : —

We are greatly stirred, at times, as this fraud or that
scoundrel is dragged to light, and there rise cries and moans
over the corruption of the times; but, my friends, these
frauds and these scoundrels are not the ' corruption of the
times.' They are the mere pustules which the body politic
throws to the surface. Thank God, that there is vitality
enough left to throw them to the surface ! The disease is
below all this ; infinitely more wide-spread.

What is that disease ? I believe that it is, first of all,
indifference — indifference to truth 'as truth ; next, scepticism,
by which I do not mean inability to believe this or that
dogma, but the scepticism which refuses to believe that there
is any power in the universe strong enough, large enough, good
enough, to make the thorough search for truth safe in every
line of investigation ; next, infidelity, fey which I do not mean

APPEisDIX. 241

want of fidelity to this or that dominant croeil, but want of
fidehty to that which underlies all creeds, the idea that the
true and the good are one ; and, finally, materialism, by which
I do not mean this or that scientific theory of the universe,
but that devotion to the mere husks and rinds of good, that
struggle for place and pelf, that faith in mere material com-
fort and wealth which eats out of human hearts all patriotism,
and which is the very opposite of the spirit that gives energy
to scientific achievement.

The education which our senatorial friend approved
leads naturally to just this array of curses.

On the other hand, I believe that the little army of scien-
tific men furnish a very precious germ from which better
ideas may spring.

And we should strengthen them. We have already mul-
titudes of foundations and appliances for the dilution of
truth — for the stunting of truth — for the promotion of half-
truths — for the development of this or that side of truth.

We have no end of intellectual hot-house arrangements
for the cultivation of the plausible rather than the true; and
therefore it is that Ave ought to attach vast value to the men
who with calmness and determination seek the truth, in
its wholeness, on whatever line of investigation, not dilut-
ing it or masking it.

Their zeal, their devotion, their faith, furnish one of those
very protests which are most needed against that low tone
of political ideas which in its lower strata is political cor-
ruption. Their life gives' that very example of a high spirit,
aim, and work, which the time so greatly needs.

The reverence for scientific achievement, the revelation
of the high honours which are in store for those who seek for
truth in science — the inevitable comparison between a life
devoted to that great pure search, on the one hand, and a lifi:
devoted to place-hunting or pelf-grasping on the other — all
ti.ese shall come to the minds of thoughtful men in Ioik Ij'

242 APPENDIX.

garrets of our cities, in remote cabins on our prairies, and
thereby shall come strength and hope for higher endeavour.

And, Mr. Chairman, as this influence for good spreads
and strengthens, I have faith that gratitude Avill bring in
results for political good of yet another kind.

Many predecessors of our friend have, as literary men,
strengthened tin ties that bind together the old land and
the new ; and I trust that love, admiration, and gratitude,
between men of science on both sides the Atlantic, may add
new cords and give strength to old cords which unite the
hearts of the two great English-speaking nations.

PROFESSOR TYNDALL'S REMARKS.

There is one point in your speech, Mr. President, which
requires simple honesty and little wit on my part to respond to.
That point is symbolized by those united flags of America and
England which I now see before me. You spoke of the
sympathy existing between the intellect of England and t' at
of the United States, and of the smallness of our differences
compared with the area of our coincidences. Coming from you,
sir, these "words had a peculiar weight and worth to me.
I am persuaded that they are not the words of mere con-
ventional compliment, but that they embody your con-
victions. And I am equally persuaded that they are the
expression of a fact which will become more and more prom-
inent as time rolls on, and as international knowledge is
increased.

During my four months' residence in the United States I
have not heard a single whisper hostile to England ; and this
accoTUitsfor a certain change of feeling on my part, accompanied
by a cori-esponding change of expression in my lectures. At a
time when the political relations of America and England
were critical in the extreme, I received from the United States-
letters expressing the emphatic opioion that if men of science,

APPENDIX. 243

on both sides of the Atlantic, could be persuaded to inter-
change friendly visits, it Avould, as far as the United States
were concerned, do more than diplomacy to soften the asperities
arising out of political differences. I said something bearing
upon this point in Boston ; but uf late nothing. And this,
because I saw that any reference to it would have been out of
place ; resembling, as Mr. Emerson would say, the sound of a
scythe in December, when there is nothing to mow. We are
not angels on either side of the Atlantic, nor am I aware that
we desire to be angels ; but as men I believe there exists
between us a strength of brotherhood competent to weld to-
gether the two kindred nations almost as closely as the various
parts of your own vast community are welded to each other.

And now let us turn for a moment to science. The
interest shown in the lectures to which you have so kindly
referred is not, in my opinion, the creation of the hour.
Every such display of public sympathy must have its prelude,
during Avhich men's minds are prepared, a desire for knowledge
created, an intelligent curiosity aroused. Then in the nick of
time comes a person, who, though but an accident, touches a
spring Avhich permits tendency to flow into fact, and public
feeling to pass from the potential to the actual. In no other
way can I account for my four months' experience in the
United States. The soil had been prepared, and the good seed
sown long before I came among you. And it is on the belief
that the subject has a root deeper than the curiositv of tlie
hour, that I found my hopes of its not passing rapidly from
tlie public mind.

It would be a great thing for this land of incalculable
destinies to supplement its achievements in the industrial
arts by those higher investigations from which our mastery
over Nature and over industrial art itself has been derived, and
whicli, when applied in a true catholic spirit to man himself,
will assuredly render him permanently healthier and nobler
than he now is. To no other country is the cultivation of
Bcience, in its highest forms, of more importance thaa to yours.

244 APPENDIX.

In no other country would it exert a more benign and ele-
vating influence. What, then, is to be done toward so desir-
able a consummation ? Here, I think, you must take counsel
of your leading scientific men. As regards physical science,
I think, they are likely to assure you that it is not the
statical element of buildings that you require, so much as
the dynamical element of brains. Making use as fir as
possible of existing institutions, let chairs be founded, suffi-
ciently, but not luxuriously endowed, which shall have original
research for their main object and ambition. With such
vital centres among you, all your establishments of education
would feel their influence ; without such centres, even your
primary instruction will never flourish as it ought. I would
by no means sever tuition from investigation ; but, as in the
institution to which I have the honour to belong, the one
ought, in the cases now in view, to be made subservient to
the other. The Eoyal Institution gives lectures — indeed, it
lives in part by lectures, though mainly by the contributions
of its members, and the bequests of its friends. But the chief
feature of its existence — a feature never lost sight of by its
wise and honourable Board of Managers — is that it should
be a school of research and discovery. Though a by-law gives
them the power to interfere, for the twenty years during which I
have been there, no manager or member of the Institution has
ever interfered wit i my researches. It is this wise freedom,
accompanied by a never-failing sympathy, extended to the
great men who preceded me, that has given to the Eoyal Insti-
tution its imperishable renown.

As to the source of the funds necessary for founding
such chairs as those referred to, it is not for me to off'er an
ojiinion. Without raising the disputed question of State aid,
it is possible in this country to do a great deal without it.
The willingness of American citizens to throw their fortunes
into the cause of public education is, as I have already stated,
without a parallel in my experience. Hitherto their eflbrta

APPENDIX. 245

have been directed to the practical side of science ; and this is
why I sought in my lectures to show the dependence of practice
upon principles. On the ground, then, of mere practical,
material utility, pure science ought to be cultivated. But
assuredly among your men of wealth there are those willing
to listen to an appeal on higher grounds. Into this plea I
would pour all my strength. Not as a servant of Mammon do
I ask you to take science to your hearts, but as the strengtheuer
and enlightener of the mind of man.

Might I now address a word or two to those who in the
ardour of youth feel themselves draivn towards science as a
vocation. They must, I think, be prepared to suffer a little at
times for the sake of scientific righteousness, not refusing,
should occasion demand it, to live low and lie hard to achieve
the object of their lives. I do not here urge upon my younger
friends any thing that I should have been unwilling to do
myself when young. A simple statement of my student-lile
on the Continent would prove this to demonstration. And it
is with the view of giving others the chance that I then
enjoyed, among my noble and disinterested German teachers,
that I propose, aft*ir deducting, with strict accuracy, the sums
which have been actually expended on my lectures, to devote
every cent of the money which you have so generously poured
in upon me, to the education of young American philosophers
in Germany. I ought not, for their sake, to omit one additional
motive which upheld me during my student life — a sense
of duty. Every young man of high aims must, I think,
have a spice of this principle within him. There are sure
to be hours in bis life when his outlook will be dark, his
work difficult, and his intellectual future uncertain. Over
Buch periods, when the stimulus of success is absent, he
must be carried by his sense of duty. It may not be so
quick an incentive as glory, but it is a nobler one, and gives
a tone to character which the hope of glory cannot impart.
That unflinching devotion to work, without which no real
emitience in science- is now attainable, implies the writing at
12

^4b APPENDIX.

certain times of the stern resolve upon the student's character:
* I work, not because I love to work, but because I ought to
work.' In science, however, love and duty are sure to be re-
conciled in the end.

And now, gentlemen, all is nearly over, and in a day or
two I quit these shores. I read a day or two ago an article
in the Galaxy, in which the writer, who had been in Eng-
land, and who had had what you call * a good time ' in
England, spoke nevertheless of the deep pleasure of reaching
his own home. The words struck a sympathetic chord with-
in me. And it is a curious psychical fact, that this home-
yearning, in my case, is not only unopposed, but is actually
aided by the feeling that since I came to this country Amer-
ica has been a home to me. It is not a case of two opposing
attractions, but a case in which, one of the attractions being
satisfied, I am left not only free to be acted on, but more
ready to be acted on by the other. "Were there any linger-
ing doubt, as to my visit, at the bottom of my mind ; did I
feel that I had blundered — and with the best and purest in-
tentions I might, through an error of judgment, have blun-
dered— so as to cause you discontent, I should now be wish-
ing to abolish the doubt or to repair the blunder. This
would be so much withdrawn from the pleasurable thought
of home. But there is no drawback of this kind ; and,
therefore, as I have said, the fulness of my content here, en-
hances the prospective pleasure of meeting my older friends.
By some means or other the people of this country have be-
gotten and fostered a strange confidence in me towards them.
I feel as if I, a simple scientific student, who never taught
the world to be a cent richer, who merely sought to present
science to the world as an intellectual good, am leaving, not a
group of friends merely, not merely a friendly city, but a
friendly continent behind me. The very disappointment of the
West I take as a measure of the West's goodwill. Tested
and true friends are awaiting me at the other side, and, think-
ing of them and you, the pure cold intellect is for the

APPENDIX. 247

moment deposed, and the * human heart ' is master of the
situation. But lest it, in the waywardness of strong emotion,
should utter any thing which the re-enthroned intellect of
to-morrow might condemn, I will pause here — hoping, not
for the entire consummation, for that would be a hope too
daring, but hoping, as the generations pass, that the attachment
which binds me to America, on the one side, and ' the Old
Country,' on the other, may be more and more approached and
realized by the nations themselves.

MEASUREMENT OF THE WA VES OF LIGHT.

The diffraction fringes described in Lecture II., instead c]
being formed on the retina, may be formed on a screen, or
upon ground glass, when they can be looked at through a
magnifying lens from behind, or they can be observed in the
air when the ground glass is removed. Instead of permitting
them to form on the retina, we ■will suppose them formed on
a screen. This places us in a condition to understand, even
without trigonometry, the solution of the important problem
of measuring the length of a wave of light.

We will suppose the screen so distant that the rays falling
upon it from the two margins of the slit are sensibly parallel.
We have learned in Lecture IL that the first of the dark
bands corresponds to a difference of marginal path of one un-
dulation ; the second dark band to a difference of path of two
undulations ; the third dark band to a difference of three
undulations, and so on. Now the angular distance of the
bands from the centre is capable of exact measurement ; this
distance depending, as already stated, on the width of the slit.
With a slit 1'35 ' millimeters wide, Schwerd found the angular
distance of the first dark band from the centre of the field to

' The millimeter is about j'^tb of aa inclu

248

APPENDIX.

be 1' 38" ; the angular distances of the second, third, fourth
dark band being twice, three times, four times this quantity.
Let A B, fig. 59, be the plate in which the slit is cut, and
C D the grossly exaggerated width of the slit, with the beam
of red light proceeding from it at the obliquity corresponding
to the first dark band. Let fall a perpendicular from one

Fig. 59.

edge, D, of the slit on the marginal ray of the other edge at
d. The distance, C c7, between the foot of this perpendicular
and the other edge is the length of a wave of the light. The
angle C D d, moreover, being equal to R C R', is, in the case
now under consideration, 1' 38". From the centre D, with
the width D C as radius, describe a semicircle ; its radius
D C being 1'35 millimeters, the length of this semicircle is found
by an easy calculation to be 4'248 millimeters. The length
C (i is so small that it sensibly coincides with the arc of the
circle. Hence the length of the semicircle is to the length
C d of the wave as 180° to 1' 38", or, reducing all to seconds,
as 648, 000" to 98", Thus, we have the proportion —

648, 000 : 98 : : 4-248 to the wave-length C d.

Making the calculation, we find the wave-length for this
particular kind of light to be 0000643 of a millimeter, or
0-000026 of an inch.

PLUlSfES PRODUCED BY THE CPvYSTALLIZATION OF WATER.

Photographed by Professor Lockett

APPENDIX. 249

JFA TER CR YSTA LLIZA TION.

Tilt following letter from my excellent friend Professor
Joseph Henry refers to a surprising case of crystallization
here figured, and for -which I" am indebted to the kindness of
Professor Lockett • —

' Smithsonian Institution, Washington;

'March 2i, 1873.

' Mr DEAR Peofessor Tyndall, — Accompanying this I
send you a photograph, at the request of Professor S. H.
Lockett, of the Louisiana State University, of which the
following is his explanation : —

* " In my drawing room I kept a wash-basin in which to
rinse out the colour from my water-colour brushes. This
colour gradually formed a uniform sediment of an indefinite
tint over the bottom of the basin. On the night of the 26th
of December last, which was an unusually cold one for this
climate, the water in the basin froze. On the melting of the
ice the next day, the beautiful figure you see on the pho-
tographs was left in the sediment. I carefully poured the
water from the basin, let the sediment dry, and thus perfectly
preserved the figure. It has been accurately photographed
by an artist in this city. The negative is preserved, and if
you would like to have any more copies they can readily be
obtained.

' " We are not much accustomed in this warm country of
ours to the beautiful 'forms of water,' and this has struck me
as a little remarkable, and worthy of being kept."

' The fact that the results have been produced by coloured
sediment indicates a method of exliibiting the eflfects of crys-
tallization in an interesting manner.

' Joseph Henry,
* Secretary, Smithsonian Instituti('n.'

250 APPENDIX.

ON THE SPECTKA OF POLAKIZED LIGHT.

Mr. William Spottiswoode has recently introduced
to the members of the Royal Institution, in a very
striking form, a series of experiments on the spectra of
polarized light. With his large Nicol's prisms he first
repeated and explained the experiments of Foucault
and Fizeau, and subsequently enriched the subject by
very beautiful additions of his own. I here append a
portion of the abstract of his discourse : —

* It is well known that if a plate of selenite sufficiently thin
be placed between two such Nicol's prisms, or, more technically
speaking, between a polarizer and analyzer, colour will be
produced. And the question proposed is, What is the nature
of that colour ? is it simply a pure colour of the spectrum, or
is it a compound, and if so, what are its component parts ?
The answer given by the wave theory is in brief this : In its
passage through the selenite plate the rays have been so
separated in the direction of their vibrations and in the
velocity of their transmission, that, when re-compounded by
means of the analyzer, they have in some instances neutralized
one another. If this be the case, the fact ought to be visible
when the beam emerging from the analyzer is dispersed by
the prism ; for then we have the rays of all the different
colours ranged side by side, and if any be wanting, their
absence will be shown by the appearance of a dark band in
their place in the spectrum. But not only so ; the spectrum
ought also to give an account of the other phenomena ex-
hibited by the selenite when the analyzer is turned l-ound,
viz. that when the angle of turning amounts to 45°, all trace
of colour disappears ; and also that when the angle amounts
to 90°, colour reappears, not, however, the original colour, but
one complementary to it.

APPENDIX. 251

* You see in the spectrum of the reddish h,'^ht produced by
the selenite a broad but dark band in the blue ; when the
analyzer is turned round tlie band becomes less and less dark,
until when the angle of turning amounts to 45° it has entirely
disappeared. At this stage each part of the spectrum has its
own proportional intensity, and the whole produces the
colourless image seen without the spectroscope. Lastly, as
the turning of the analyzer is continued, a dark band appears
in the red, the part of the spectrum complementary to that
occupied by the first band ; and the darkness is most complete
when the turning amounts to 90°. Thus we have from the
spectroscope a complete account of what has taken phice to
produce the original colour and its changes.

' It is further well known that tlie colour produced by a
selenite, or other crystal plate, is dependent upon the thickness
of the plate. And, in iiict, if a series of plates be taken,
giving different colours, t eir spectra are found to show bands
arranged in different positions. The thinner plates show bands
in the parts of the spectrum nearest to the violet, wliere the
waves are shorter, and consequently give rise to redder
colours ; Avhile the thicker show bands nearer to the red,
Avhere the waves are longer and consequently supply bluer
tints.

' When the thickness of the plate is continually increased,
so that the colour })roduced has gone through the complete
cycle of the spectrum, a further increase of thickness causes a
reproduction of the colours in the same order ; but it will be
noticed that at each recurrence of the cycle the tints become
paler, until when a number of cycles have been performed,
and the thickness of the plate is considerable, all trace ot
colour is lost. Let us now take a series of plates, the first two
of which, as you see, give colours ; with the others which are
successively of greater thickness the tints are so feeble that
they can scarcely be distinguished. The spectrum of the first
shows a single band ; that of the second, two; showing that
the second series of tints is not identical with the first, but

252 APPENDIX.

that it is produced by the extinction of two colotirs from the
components of white light. The spectra of the others show
series of bands more and more numerous in proportion to the
thickness of the plate, an array which may be increased inde-
finitely. The total light, then, of which the spectrum is
deprived by the thicker plates is taken from a greater number
of its parts ; or, in other words, the light which still remains
is distributed more and more evenly over the spectrum ; and
in the same proportion the sum total of it approaches more
and more nearly to white light.

* These experiments were made more than thirtj' years ago
by the French philosophers, MM. Foucault and Fizeau.

* If instead of selenite, Iceland spar, or other ordinary
crystals, we use plates of quartz cut perpendicularly to the
axis, and turn the analyzer round as before, the light, instead
of exhibiting only one colour and its complementary with an
intermediate stage in which colour is absent, changes con-
tinuously in tint ; and the order of the colour depends partly
upon the direction in which the analyzer is turned, and partly
upon the character of the crystal, i.e. whether it is right-
handed or left-handed. If we examine the spectrum in this
case we find that the dark band never disappears, but marches
from one end of the spectrum to another, or vice versa, pre-
cisely in such a direction as to give rise to the tints seen by
direct projection.

* The kind of polarization effected by the quartz plates is
called circular, while that effected by the other class of crystals
is called plane, on account of the form of the vibrations exe-
cuted by the molecules of asther ; and this leads us to ex-
amine a little more closely the nature of the polarization of
different parts of these spectra of polarized light,

' Now, two things are clear : first, that if the light be plane-
polarized, that is, if all the vibrations throughout the entire
ray are rectilinear and in one plane, they must in all their bear-
ings have reference to a particular direction in space, so that
they will be differently affected by different positions of the

APPENDIX. 253

analyzer. Secondly, that if the vibrations be circular, they
will be affected in precisely the same way (whatever that may
be) in all positions o£ the analyzer. This statement merely
recapitulates a fundamental point in polarization. In fact,
plane-polarized light is alternately transmitted and extinguished
by the analyzer as it is turned through 90° ; while circularly-
polarized light [if we could get a single ray] remains to all ap-
pearance unchanged. And if we examine carefully the spectrum
of light which has passed through a selenite,or other ordinary
crystal, we shall find that, commencing with two consecutive
bands in position, the parts occupied by the bands and tho.se
midway between them are plane polarized, for they become
alternately dark and bright ; while the intermediate parts, i.e.
the parts at one-fourth of the distance from one band to the
next, remain permanently bright. These are, in fact, circu-
larly polarized. But it would be incorrect to conclude from
this experiment alone that such is really the case, because the
siime appearance would be seen if those parts were unpolarized,
i.e. in the condition of ordinary lights. And on such a sup-
position we should conclude with equal justice that the parts
on either side of the parts last mentioned {e.g. the parts sepa-
rated by eighth parts of the interval between two bands) were
partially polarized. But there is an instrument of very simple
construction, called a " quarter-undulation plate," a plate
usually of mica, who.se thickness is an odd multiple of a
quarter of a wave length, which enables us to discriminate
between light unpolarized and circularly polarized. The exact
mechanical effect produced upon the ray could hardly be ex-
plained in detail within our present limits of time ; but suffice
it for the present to say that when placed in a proper position,
the plate transforms plane into circular and circular into plane
j)olarization. That being so, the parts which Avore originally
landed ought to remain bright, and those which originally
remained bright ought to become banded during the rotation
of the analyzer. The general effect to the eye will conse-

254 APPENDIX.

quently be a general shifting of the bands through one-fourth
of the space which separates each pair.

* Circular polarization, like circular motion generally, may
of course be of two kinds, which differ only in the direction
of the motion. And, in fact, to convert the circular polariza-
tion produced by this plate from one of these kinds to the other
(say from right-handed to left-handed, or vice versa), we
have only to turn the plate round through 90°. Conversely
right-handed circular polarization will be changed by the
plate into plane polarization in one direction, while left-
handed will be changed into plane at right angles to the first.
Hence, if the plate be turned round through 90° we shall see
that the bands are shifted in a direction opposite to that in
which they were moved at first. In this therefore we have
evidence not only that the polarization immediately on either
side of a band is circular ; but also that that immediately on
the one side is right-handed, while that immediately on the
other is left-handed. [']

' If time permitted, I might enter still further into datail,
and show that the polarization between the plane and the
circular is elliptical, and even the positions of the longer and
shorter axes and the direction of motion in each case. But
suflicient has, perhaps, been said for our present purpose.

' Before proceeding to the more varied forms of spectral
bands, which I hope presently to bring under your notice, I
should like to ask your attention for a few minutes to the
peculiar phenomena exhibited when two plates of selenite
giving complementary colours are used. The appearance of
the spectrum varies with the relative position of the plates.
If they are similarly placed — that is, as if they were one plate
of crystal — they will behave as a single plate, whose thickness
is the sum of the thicknesses of each, and will produce double

[' At these points the two rectangular vibrations into which the
original polarized ray is resolved by the plates of gypsum act upon eacli
other like the two rectangular impulses imparted to our pendulum in
Lecture IV., one being given when the pendulum is at the limit of its
swing. Vibration is thus converted into rotation.]

APPENDIX. 25o

the number of bands which one alone would give ; and when
the analyzer is turned, the bands will disappear and re-appear
in their complementary positions, as usual in the case of plane-
polarization. If one of them be turned round through 45°, a
single band will be seen at a particular position in the spec-
trum. This breaks into two, which recede from one another
towards the red and violet ends respectively, or advance
towards one another according to the direction in which the,
analyzer is turned. If the plate be turned through 45° iu
the opposite direction, the effects will be reversed. The
darkness of the bands is, however, not equally complete
during their whole passage. Lastly, if one of the plates be
turned through 90°, no bands will be seen, and the spectrum
will be alternately bright and dark, as if no plates were used,
except only that the polarization is itself turned through 90°.

' If a wedge-shaped crystal be used, the bands, instead of
being straight, will cross the spectrum diagonally, the direc-
tion of the diagonal (dexter or sinister) being determined by
the position of the thicker end of the wedge. If two similar
wedges be used with their thickest ends together, they will
act as a wedge whose angle and whose thickness is double ot
the first. If they be placed in the reverse position they will act
as a flat plate, and the bands will again cross the spectrum in
straight lines at right angles to its length.

' If a concave plate be used the bands will dispose them-
selves in a fanlike arrangement, their divergence depending
upon the distance of the slit from the centre of concavity.

* If two quartz wedges, one of which has the optic axis
parallel to the edge of the refractory angle, and the other
perpendicular to it, but in one of the planes containing the
angle (Babinet's Compensator), the appearances of the bands
are very various.

* The diagonal bands, beside sometimes doubling themselves
as with ordinary wedges, sometimes combine so as to form
longitudinal (instead of transverse) bands ; and sometimes
sross one another so as to form a diaper pattern with bright

256 APPENDIX.

compartments in a dark framework, and vice versa, according
to the position of the plates.

* The effects of different dispositions of the interposed crys-
tals might be varied indefinitely ; but enough has perhaps
been said to show the delicacy of the method of spectrum
analysis as applied to the examination of polarized light.'

The singular and beautiful effect obtained with a
circular plate of selenite, thin at the centre, and
gradually thickening towards the circumference, is
easily connected with a similar effect obtained with
Newton's rings. Let a thin slice of light fall upon the
glasses which show the rings, so as to cover a narrow
central vertical zone passing through them all. The
image of this zone upon the screen is crossed by por-
tions of the iris rings. Subjecting the reflected beam
to prismatic analysis, the resultant spectrum may be
regarded as an indefinite number of images of the zone
placed side by side. In the image before dispersion
we have iris-rings, the extinction of the light being
nowhere complete ; but when the different colours are
separated by dispersion, each colour is crossed trans-
versely by its own system of dark interference bands,
which become gradually closer with the increasing
refrangibility of the light. Tlje complete spectrum,
therefore, appears furrowed by a system of continuous
dark bands, crossing the colours transversely, and ap-
proaching each other as they pass from red to blue.

In the case of the plate of selenite, a slit is placed
in front of the polarizer, and the film of selenite is
held close to the slit, so that the light passes through
the central zone of the film. As in the case of New-
ton's rings, the image of the zone is crossed by iris-

APPENDIX. 257

coloured bands ; but when subjected to prismatic dis-
persion, the light of the zone yields a spectrum furrowed
by bands of complete darkness exactly as in the case of
Newton's rings, and for a similar reason. This is the
beautiful eflfect described by Mr. Spottiswoode as the
fanlike arrangement of the bands — the fan opening
out at the red end of the spectrum.

PREFACE DU TEADUCTEUR FEANgAIS.

La LuMiERE de M. Tyndall ne ressemble a aucun des Traites
qu'il m'a ete donne d'etudier et d'analyser, en si grand nombie,
dans mon ' Kepertoire d'Optique moderne ' et plus tard.

Ce n'est pas un Traite elementaire, car il comprend les
phenomenes les plus delicats, ceux meme que nos programmes
du baccalaureat ecartent de I'enseignement classique ; et ce
n'est pas non plus un Traite d'Optique superieure, puisqu'il
est la redaction de Legons faites devant un auditoire d'hommes
et de femmes du monde.

Ce n'est pas ua Traite d'Optique physique, car il donne la
raison mecanique de chaque plienoniene, et ce n'est pas un
Traite d'Optique matliomatique, car le calcul n'y joue aucun
role.

Ce n'est pas un Traite d'Optique experimentale, car le
raisonnement domine tout, car I'analyse et la synthase y
jirennent une part considerable; et ce n'est pas un Traite
d'Optique rationnelle, car il se resume tout entier dans la
production des phenomenes sur la plus large echelle qu'on
puisse imaginer, en les rendant visibles a un immense audi-
toire.

Qu'est-il done ct comment le caracteriserons-nous ? En
disant qu'il est tout k la fois, dans son unit4 merveilleuse,
Elementaire et sup^rieur, physique et matliomatique, experi-
mental ct rationnel ; qu'il constitue un mode d'enscigncment

2oS APPENDIX.

silt generis, analytique et sjnthetique a la fois, vraiment
nouveau et admirable en soi.

Singulifere dans le fond, la Lumiere est non moins singu-
liere dans la forme. Au premier aspect, et parce qu'elle est
en realite la redaction de Le9ons improvisees, cette forme
semble kisser beaucoup a desirer : on la dirait imparfaite ;
rnais, k mesure qu'on se I'assimile la Lumiere etonne et ravit
par sa verite, sa precision, sa nettete, et Ton est force de
reconnattre qu'elle est en son genre un modele incomparable
de perfection.

Ce qui la specifie et lui donne une superiorite incontes-
table, c'est son mode d'exposltion, d'une transparence en
quelque sorte infinie. Elle donne la vision intuitive des
faits et plus encore de la raison des faits ou du mecanisme
des phenomfenes. Je croyais savoir I'Optique, que j'ai etudiee
et redigee sous toutes ses formes depuis trente ans ; mais je
nuis force de reconnaitre que je la sais incomparablement
mieux depuis que j'ai traduit la Lumiere. Les petites
formules de M. Tyndall penetrent beaucoup mieux au coeur
des phenom^nea que les plus savantes equations des Traites
d'Optique math^matique.

Comme chefs-d'cEUvre du genre je citerai la troisieme
Le9on, sur la polarite et la polarisation rectiligne, et la qua-
trieme, sur la polarisation chromatique et les interferences.
Jamais enseignement n'avait fait sur mon esprit un effet aussi
extraordinaire. Non-seulement je voyais les phenomenrs, mais
je m'identifiaia avec eux; ils n'etaient plus que des modifi-
cations de mon intelligence, ma propre pensee. La sensation
que j'eprouvais alors etait vraiment delicieuse. La vue
intuitive des phenomenes, de leurs causes, de leur mecanisme
est en efFet le triomphe supreme de I'enseignement. Et voila
surtout a quel point de vue je recommande la Lumiere de
M. Tyndall comme le seul livre qui, a ma connaissance, initie
pleinement aux mysteres de I'Optique. Je voudrais le voir
dans les mains de tous les amis de la Science. Elle n'est pas
aussi profonde, aussi savante, aussi encyclopedique que la
Chaleur] mais elle va bien plus directement au but etl'attemt

APPENDIX. 259

plus completement. J'ai fait d'ailleura tout ce que j'ai pu
pour que, sans avoir une couleur trop anglaise, ma traduction
rendit parfaitement le texte original.

Me sera-t-il permis de faire remarquer que, dans son
expose de I'analyse spectrale, M. Tyndall fait la part vraiment
trop belle a M. KirchhofF? S'il est vrai que le celebre pliy-
sicien de Heidelberg a fait, le premier, la chimie so)aire, il
n'est pas vrai au meme degre qu'il ait fait le premier, par
I'analyse spectrale, la chimie des substances terrestres. M.
Tyndall parle bien de quelques precurseurs de M. KirchhofF,
mais il oublie lea pi-incipaux, M. Plucker par exemple, qui
apprit h, MM. Kirchhoff et Bunsen a analyser par le prisme la
lumiere des flammes qu'ils s'effor9aient peniblement de resoudre
dans ses principes constituanta a I'aide de verres colores ou
absorbants. Le Memoire de M. KirchhofF a ete lu a 1' Academic
de Berlin, le 27 octobre 1859 ; et neuf ans auparavant, comme
sir William Thomson, le collogue et glorieux emule de M.
Tyndall, a bien voulu le rappeler dans son discours inaugural
de r Association Britannique (^Edimbourg, septembre 1871),
je disais (' Repertoire d'Optique moderne,' publie en Janvier
1860, t. III. p. 243) : ' M. Foucault a publie une Note curieuse
sur les spectres produits par les corps qui brulent entre les
deux pointes de charbon fixees aux poles d'une forte pile.
Nous avons repete ces belles experiences avec M. Soleil ; nous
jx)uvons meme dire que nous leur avons donne un plus grand
eclat en projetant ces spectres sur un 4cran, sans leur rien
faire perdre de leur splendeur. Dans la combustion de
I'argent et du cuivre, il y a une raie verte qui surpasse lu
intensite les rayons les plus illumines du spectre solaire. L<3
f pectre du cuivre etait facilement reconnu par ses raies vertts,
celui du zinc par ses merveilleuses raies violettes. Dans la
flamme du laiton, compost de cuivre et de zinc, on admirait
les raies vertes du cuivre et les raies violettes du zinc ; le
maillechort a presente des apparences beaucoup plus splendidcs :
I'oeil ne ee lassait pas de contempler toutes les raies lumineuses
des m^taux qui entrent dans la composition de cet alliage
multiple. Avec un peu d'expeuience on arrive A f.*iue,

260 APPENDIX.

PAR l'oBSERVATION DES RAIES, l'aNALYSE SINON QUAKTITATIVE,

DU MOiNS QUALITATIVE des combinaisons les plus complexes de
nietaux tres-dissemblables.'

Dans mon coriipte rendu d'une matinee scientlfique donnee
par M. Soleil et moi, dans les salons de M. Emile de Girardin,
en 1849, j'avais decrit ces memes experiences et aiBrme plua
nettement que les spectres de radiation ou d'absorption de tous
les corps de la nature avaient leurs raies sombres ou brillante.s,
caracteristiques et specifiques de leur nature intime. Ma con-
viction a cet egard etait si profonde, que je priai mon illustre
maitre A. Cauchy de formuler la tbeorie generale de ces raies.
II le fit dans une Note tres-courte, mais tres-explicite, pub-
liee dans les * Comptes rendus des stances de I'Academie des
Sciences' et ins^ree en 1847 dans le second volume de mon
* Repertoire d'Optique,' p. 238 et 239. Qu'il me soit permis de
reproduire ici les quelques lignes qui la terminaient et qui
n'ont pas ete assez remarquees : * Observons encore que I'etat
initial d'un system de molecules ou plutot d'une portion de ce
systeme etant arbitraire, le systems d'ondes planes qui repre-
sente cet ^tat initial, et qui s'en deduit par une formula con-
nue, pent varier a I'infini comme cet etat lui-meme. II en
resulte que, parmi les ondes planes correspondant aux di-
verses longueiirs d'ondulations, les unes doivent etre tres-
sensibles, tandis que les autres peuvent I'Stre beaucoup moins
et disparaitre presque entierement. On ne devra done pas
etre surpris de voir, dans la tbeorie de la lumiere, les rayons
doues de refrangibilites diverses, lorsqu'on les dispersera par
le moyen du prisme, ofFrir des intensites variables non-
seulement avec les longueurs d'ondulations correspondantes,
mais encore avec la nature des corps dont ils emanent ou
qu'ils traversent, et Ton devrait s'etonner, au contraire, qu'il
en fut autrement. Ainsi doivent etre evidemment expliquees
les raies brillantes et obscures decouvertes dans le spectre
solaire et dans ceux que fournissent les autres corps lumineiix.
Les raies du spectre ne doivent pas nous surprendre : leur
absence seule, dans le plus grand nombre des cas, serait
inexplicable.' Voila ce que nous ecrivions en 1847. La

APPENDIX. 261

chimie spectrale etait done a cette epoque une verite essen-
tiflle ct incontestable: M. Kirchhoff n'avait done pas a la
creer. Cette affirmation no ressortira pas moins d'une autre
citation que je tiens a fuire ici : M. Adolphe Erman, dans un
Memoire adresse a TAcadeniie de.s Sciences de Paris, en
octobre 1844, sur les raies d'absorption du chlore, de I'iode et
du brome, disait en termes formels: 'II est etonnant que la
theorie des phenomenes d'absorption (les phenomenes solaires
expliques par Kirchhoff sont des phenomenes d'absorption) ait
encore fait si peu de progres, puisque la marehe des reclierches
qui doivent y conduire etait nettement indiquee d'avance. 11
me serable, en effet, que ees recherches doivent se borner:
1° a decomposer, a I'aide du prisma, la lumiere sur laquelle
I'absorption a agi ; 2° a caracteriser les rayons qvii ont ete
eteints par le seul moyen que I'Optique nous foiu-nit pour cet
effet, je veux dire par la mesure des longueurs d"ondes; et 3^
a voir si les longueurs d'ondes des rayons observes sont liees
par quelques lois qui expliquent leur disparition.' C'est la
evidemment toute I'analyse spectrale, plus complete qu'on ne
I'a fiiite jusqu'iei. En dehors de ees vues theoriques, il y
avait a decouvrir le fait qu'une flamme arrete les ondes qu'elle
peut elle-meme engendrer, et la gloire de cette deeouverte est
essentiellement fran9aise: elle appartient a M. Leon Foucault;
M. Kirchhoff n'a fait que la formuler mieux et la generaliser.'

.11 est dans la Lumiere quelques passages hasarde.s que je
ne puis laisser passer sans explications, preeisement parce que,
en les traduisant, j'en assume a un certain degre la rcsponsa-
bilite.

Page 9, iigne 11, M. Tyndall se fait I'echo d'une boutade
humoristique d'une de ses plus grandes admirations, ISI. Helm-
holtz: * On pourrait en realite dresser contre roeil une longue
litite d'accusations : son opacite, son delaut de symetrie, son
man(jue d'achromatisme, sa cecite absolue ou partielle. Toutes
cea raisons prises ensemble amenerent M. Ilelmlioltz a dire,

' What M. Foucault and M. Kirchhoff rospectively accomplished is
stated in this volume. I have awarded to M. Kirchhoff nothing that,
can bo justly denied to him. — J.T.

262 APPENDIX.

que, si un opticien lui livrait un instrument si plein de defauts
il se croirait autorise a le renvoyer avec les reproches les plus
sev^res.' Sous cette forme, I'appreciation de I'oeil est vraiment
impardonnable. L'oeil n'est pas essentiellement ou absolument
achromatique : cela est vrai, cela meme est necessairement
vrai, puisque aucune ceuvre finie ne peut etre infiniment
parfaite et que ]a perfection absolue est le propre de I'etre
infini. Mais, par cela meme qu'aucun horame n'a conscience
de ce defaut d'achromatisine, qu'il faut, pour le mettre en
evidence, des experiences tres-de!icatesfaites avec de puissants
instruments ; qu'il ne modifie en rien pratiquement les couleurs
des objeta, I'cBil est exactement ce qu'il doit etre. On ne
pourrait probablement faire disparaitie ces imperfections
inseparables de tout etre cr6e et fini sans en faire naitre
d'autres beaucoup plus grandes. M. Helmboltz attribue le
defaut d'achromatisme de l'oeil au fait que la densite des
milieux de Tocil ne surpasse guere la densite de I'eaxi. Or
connait-il assez la constitution de l'oeil et les innombi-ables
conditions qu'il doit remplir pour affirmer qu'une densite plus
grande de ses milieux n'aurait pas des inconvenients tre.s-
graves, n'amenerait pas des epancliements ou des infiltrations ?
Je remercie M. Tyndall d'avoir oppose a ce jugement incon-
sidere cette conclusion trfes-sage (page 9): * Comme instrument
pratique et en faisant entrer en ligne de compte les accommo-
demeuts par lesquels ses defauts sont neutralises, l'oeil n^en
reste pas moins vne merveille pour tout esprit capable de re-
flexion!'

Page 132, M. Tjndall semble regarder conmie fondee et
insoluble I'objection faite a I'^Eglise catholique, de prouver sa
divinite ou son infaillibilite par I'authenticite de Tficriture
sainte, et d'affirmer I'authenticite et I'inspiration divine des
saintes Ecritures par I'autorite de I'Eglise infiillible, ce qui
constituerait un cercle vicieux. On a prouve, depuis bien
longtemps, que ce cercle vicieux n'existe pas pour nous
catholiques. En eiFet nous avons, independamment de toute
citation des Ecritures, par la tradition et la succession du
ministfere apottolique, la certitude de I'apostolicite de I'^glise

APPENDIX. 263

romaine, et aussi du fait que les apotres ont donn^ au.\
^glises qu'ils ont fondees tels ou tels livres, et non d'autrcs,
comme Ecriture sainte et parole de Dieu ; en un mot nous
prouvons, par la tradition non interrompue, I'authenticite et,
par consequent, rinspiration des saintes Ecritures, puis, par
I'inspiration divine de I'JEcriture, I'infaillibilite de TEglise.
Ce n'est pas \k un cercle vicieux, c'est, au contraire, k I'egard
des protestants, qui admettent la divinite de TEcriture sainte,
a priori, au point de recuser toute autre preuve, lui argument
personnel. Ce sont les protestants qui tcmbent, eux, dans un
cercle vicieux, en prouvant la divinity de I'Ecriture 2">ar une
pretendue persuasion interieure du Saint-Esprit, et se pre-
tendent assures de cette assistance par le temoignage des
Ecritures qui la leur promettent. Si M. Tyndall s'etait
interdit cette comparaison, je me serais de mon cot^ interdit
cette observation que je regrette d'etre force de consigner ici.
Non erat hie locus!

Pages 112 et 113, M. Tyndall se hasarde a dire: 'C'est
par cet acte de cristallisation que la Nature se revele d'abord
a nous comme architecte. Ou s'arreteront scs operations?
cnntinuera-t-elle, par le jeu des menies forces, a former des
vegetaux et des animaux 7 Quelle que puisse etre la reponse
a ces questions, croyez-moi, les notions des generations a
venir, sur cette chose mysterieuse que quelques-uns appellent
la matiere brute, seront tres-difFerentes de celles des gene-
rations passees.' C'est le germe de cette confession etrange
qui lui est ^chappee dans son discours d'inauguration de
Belfast : ' Quand je prolonge ma vision en arriere, k travers
les limites de r^vidence experimentale, je discerne en cette
matiere, que, dans notre ignorance et sans le respect dd k son
createur, nous avons jusqu'ici couverte d'opprobres, la piomesse
et la puissance d'engendrer toutes les formes et. toutes les qualites
de la vie.' Nous avons expliqu^ cet ^cart d'un esprit tr^s-
eieve, par cette remarque bien simple, qu'apres avoir perdu
la notion du Dieu createur, et fait la matiere etcrnelle, il
dcvait f.talement lui attribuer les proprietds et les facultes
divines; et que, sans s'cn doutcr, il rcndait hommagc au Dieu

264 APPENDIX.

des Chretiens, devenu pour lui le Dieu inconnu, Jgnoto
Deo.

Dans I'Appendice sur les rapports de la cristallisation avec
la vie, M. Tyndall s'abandonne de nouveau a ses conceptions
materialistes, qui n'aboutissent, helas ! qu'a faire mieux com-
prendre que la vie est rest^e, pour cet esprit eminent, un
niystere impenetrable. II se complait a la representer comme
le resultat des attractions et des repulsions polaires, dont il
dote les atomes et les molecules des corps a la fa9on des
aimants. Mais, m^me dans la vie de son arbre, il y a autre
chose que des directions moleculaires, efFets de forces plutot
Btatiques que dynamiques : il y a mouvement et tiansport,
qui supposent une force vive. J'aurais voulu qu'il eut place
le phenomena de la vie a cote des phenomenes de la sensation
et de la pensee, dont il dit : ' Le cerveau de I'homme lui-
meme est assurement un assemblage de molecules arrangees
suivant des lois physiques; mais, si vous me demandez de
deduire de cette assemblage le plus petit des phenomenes de
la sensation ou de la pensee, je me prosterne dans la poussiere,
et je reconnais I'impuissance humaine. Cette fois la specu-
lation etendrait ses ailes bien au dela de la region oil il n'est
plus de milieu capable de soutenir son vol.'

Sur le terrain purement scientifique, nul plus que moi
n'admire en M. Tyndall le penseur original et profond, le
mattre eminemment habile ; mais, avec son esprit si eclaire
et si eleve, il comprendra qu'en le combattant sur le terrain
de la religion et de la metaphysique, j'use d'un droit legitime,
je remplis un devoir sacre.

F. MOIGNO.

INDEX.

ABS

ABSORPTION, principles of, 202
Addresses at Social Meeting in
New York, 229

Airy, Sir George, severity and con-
clusiveness of his proofs, 212

Alhazen,his inquiry respecting light,
14,210

Analyzer, polarizer and, 131, recom-
pounding of the two systems of
waves by the analj'zer, 133

Angstrom, his paper on spectrum
analysis, 205

Arago, Francois, and Dr. Young, 52

— his discoveries respecting light,
211, 212

Atomic polarity, 98-101

BACON, Eogor, his inquiry re-
specting light, 14, 210

Barnard, President, his address at
Social Meeting in New York, 229

Bartholinus, Erasmus, on Iceland
spar, 114

B^rard on polarization of h°at,
183

Blackness, meaning of, 32

Boyle, Robert, his obser\'ations on
colours, 68, 69

— his remarks on fluorescence,
166-168

Bradley, James, discovers the aber-
ration of light, 22, 23

CKT

Brewster, Sir David, his chief objec-
tion to the undulatory theory of
light, 49

— his discovery in biazal crystals,
212

Brougham, Mr. (afterwards Lord),
ridicules Dr. T. Young's specula-
tions, 51, 52

Browning, Mr., his prisms, 126

CESIUM, discovery of, 196
Calorescence, 177
Clouds, actinic, 155-157

— polarization of, 158
Colours of thin plates, 67 -

— Boyle's observations on, 68, 69

— Hooke on the colours of thin
plates, 70

— of striated surfaces, 92, 93
Comet of 1080, Newton's estimate

of the temperature of, 171 note
Crookes, Mr., his discovery of tlial-

lium, 196
Crystals, action of, upon light,

100

— built by polar force, 101

— illustrationsof crystallization, 102

— architecture of, considered as an
introduction to their action upon
light, 101

— bearings of crystallization u{X)n
optical plienomena, 109

266

INDEX.

GET

Crystals, rings surrounding the axes
of, uniaxal and biaxal, 149

Cuvier on ardour for knowledge,
223

DE TOCQUEVILLE, writings of,
218, 225, 226
Descartes, his explanation of the
rainbow, 24, 25

— his ideas respecting the transmis-
sion of light, 44

— his notion of light, 210
Diamond, ignition of a, in oxygen,

172
Diathermancy, 176
Diffraction of light, phenomena of, 81

— bands, 81, 82

— explanation of, 83

— colours produced by, 92
DoUond, his experiments on achro-
matism, 28

Draper, Dr., his investigation on
heat, 175; his address at Social
Meeting in New York, 235

Drummond light, spectrum of, 198

"UAETH, daily orbit of, 77

-*-^ Electric beam, heat of the,

169
Electricity, discoveries in, 220, 221
Emission theory of light, bases of

the, 46

— Newton espouses the theory, and
the results of this espousal,
79

Ether, Huyghens and Euler advo-
cate and defend the conception of
an, 49, 59

— objected to by Newton, 69
Euler espouses and defends the

conception of an ether, 49, 59

6BA

Eusebius on the natural philosophers

of his time, 13
Expansion by cold, 107
Experiment, uses of, 3
Eye, the, its imperfections, grown for

ages towards perfection, 8
— imperfect achromatism of the,

30 note

FARADAY, Michael, his dis-
eoTery of magneto-electricity,
221
' Fits,' theory of, 76

— its explanation of Newton's rings,
77

— overthrow of the theory, 80
Fizeau determines the velocity of

light, 23
Fluorescence, Stokes's discovery of
164

— the name, 176

Forbes, Professor, polarizes and

depolarizes heat, 184
Foucault, determines the velocity of

light, 23

— his experiments on absorption,
200, 201, 204

Fraunhofer, his theoretical calcula-
tions respecting diffraction, 91

— his lines, 196

their explanation by Kirch-

hoff", 197
Fresnel, and Dr. Young, 52

— his theoretical calculations re-
specting diffraction, 90

— his mathematical abilities and
immortal name, 213

GOETHE, on fluorescence, 168
Gravitation, origin of the no-
tion of the attraction of, 96

INDEX.

267

OBA

Gra^dtation, strength of the theory

of, 150
Grimaldi, his discovery with respect

to light, 58
— Young's generalizations of, 58

TTAMILTON, Sir William, of
-'-L Dublin, his discovery of coni-
cal refraction, 212
Ueat, generation of, 6

— Dr. Draper's investigation re-
specting, 175

nelmholtz, his estimate of the genius
of Young, 51

— on the imperfect achromatism of
the eye, 30 note, 31

— reveals the cause of green in the
case of pigments, 37

Ilenry, Professor Joseph, his invita-
tion, 2

llersehel. Sir John, his theoretical
ailculations respecting diffraction,
90

— first notices and describes the
fluorescence of sulphate of quinine,
168

— his experiments on spectra,
201

Herschol, Sir William, his experi-
ments on the heat of the vari-
ous colours of the solar spectrum,
174

Hooke, Robert, on the colours of
thin plates, 70

■ — his remarks on the idea that
light and heat are modes of
motion, 71

Horse-chostnut bark, fluorescence
of, 168

Huggins Dr., his labours, 208

Huyghens advocates the lonception
of ether, 43, 59

LAC

Huyghens, his celebrated principle,
86

— on the double refraction of Ice-
land spar, 114

TCELAND spar, 112
-*- — double refraction caused by,
113

— this double refraction first
treated by Erasmus Bartholinus,
114

— character of the beams emergent
from, 117

— tested by tourmaline, 119

— Knoblauch's demonstration of
the double refraction of, 188

Ice-lens, combustion through, 170
Imagination, scope of the, 43

— note by Maclaurin on this point,
44 note

TANSSEN, M., on the rose-coloured

solar prominences, 207

Jupiter, Eoemer's observations of

the moons of, 20
Jupiter's distance from the sun, 20

KEPLER, his investigations on
the refraction of light, 14, 210
Kirchhoff, Professor, his explanation
of Fraunhofer's lines, 197

— his precursors, 204

— his claims, 206

Knoblauch, his demonstration of
the double refraction of heat of
Iceland spar, 188

LACTANTIUS, on tho natunij
philosophers of his time, 13

268

INDEX.

LAM

Lamy, M., isolates thallium in

ingots, 196
Lesley, Professor, his invitation, 2
Light familiar to the ancients, 5

— generation of, 6, 7

— spherical aberration of, 8

— the rectilineal propagation of,
and mode of producing it, 9

— illustration showing that the
angle of incidence is equal to the
angle of reflection, 10, 11

— sterility of the Middle Ages, 13

— history of refraction, 14

— demonstration of the fact of
refraction, 15

— partial and total reflection of,
17-20

— velocity of, 21

— - Bradley's discovery of the aber-

ration of light, 22, 23

— principle of least action, 23

— Descartes and the rainbow, 24

— Newton's analysis of, 27

— synthesis of white light, 30

— complementary colours, 31

— - yellow and blue lights produce

white by their mixture, 31

— what is the meaning of black-
ness ? 32

— analysis of the action of pigments
upon, 34

— absorption, 35

— mixture of pigments contrasted
with mixture of lights, 37

— Wiinsch on three simple colours
in white light, 41

— Newton arrives at the emission
theory, 46

— Young's discovery of the undula-
tory theory, 50

— illustrations of wave-motion, 52,
i3

— interference of sound-waves, 61

LIO

Light, velocity of, 63

— principle of interference of waves
of, 64

— phenomena which first suggested
the undulatory theory, 65-72

— soap-bubbles and their colours,
65-73

— Newton's rings, 73-79

— his espousal of the emission
theory, and the results of thia
espousal, 79, 80

— transmitted light, 80

— diffraction, 81, 92

— origin of the notion of the attrac-
tion of gravitation, 96

— polarity, how generated, 97

— action of crystals upon, 101

— refraction of", 110

— elasticity and density. 111

— double refraction, 112

— chromatic phenomena produced
by crystals in polarised, 124

— - the Nicol prism, 125

— mechanism of, 129

— vibrations, 129

— composition and resolution of
vibrations, 131

— polarizer and analyzer, 131

— recompounding the two systeme
of waves by the analyzer, 133

— interference thus rendered possi-
ble, 134

— chromatic plienomena produced
by quartz, 142

— magnetization of, 145

— rings surrounding the axis of
crystals, 146

— colour and polarization of sky
150-60

— range of vision incommensurate
with range of radiation, 162

— effect of thallene on the spec-
trum 165

INDEX.

2G9

LIG

Liglit, fluorescence, 165

— transparency, 171

— the ultra-red rays, 17*

— part played by Nature by these
rays, 178

— conversion of heat-rays into light-
rays, 179

— identity of radiant heat and, 180

— polarization of heat, 183

— principles of spectrum analysis,
192

— spectra of incandescent vapours,
193

— Fraunhofer'slines,andKirclihoffe
explanation of them, 197

— solar chemistry, 198-200

— demonstration of analogy be-
tween sound and, 201, 202

— Kirchhoff and his precursors, 204

— rose-coloured solar prominences,
207

— results obtained by Tarious
workers, 208

— summary and conclusion, 209

— measurement of the wavea of,

247

— polarised, the spectra of, 2.50
Lignum Nephriticum, fluorpscence

of, 167
Lloyd, Dr., on polarization of heat,

183, 212
Lockj'er, Mr., on the rose-coloured

solar prominences, 208
Lycopodium,diffraction effectscaused

by the spores of, 91

MAGNETIZATION of light, 145
Malus, his discovery respect-
ing reflected light through Iceland
spar, 117
— discovers the polariaition of light
by reflpcti'in, 21 1

NIC

Masson, his essivy on the bands of

the induction spark, 205
Melloni, on the polarization of heat,

184
Metals, combustion of, 5, 6

— spectrum analysis of, 193

— spectrum bands proved by Bun-
sen and Kirchhoff to be charac-
teristic of the vapour of, 195

Mill, John Stuart, his scepticism
regarding the undulatory theory,
152

Miller, Dr., his drawings and de-
scriptions of the spectra of various
coloured flames, 204

Morton, Professor, his discovery of
tballene, 165

Mother-of-pearl, colours of, 93

NATURE, a savage's interpreta-
tion of, 4
New York, addresses at Social

Meeting in, 229
Newton, Sir Isaac, his experiments
on the composition of solar light,
28

— his spectrum, 28

— dispersion, 28

— arrives at the emission theory of
light, 46

— his objection to the conception of
an ether espoused and defended
by Huyghens and Eulor, 59

— his optical career, 73

— his rings, 73-79

— espouses the emission theory, 79

— ■ effects of this espousal, 80 «

— his rings explained by the tlu'ory
of ' fits,' 76

— his idea of gravitation, 9fl

— his errors, 211
Nicol prism, the, 125

270

INDEX.

OCE

OCEAN, colour of the, 35
(Ersted, discovers the deflec-
tion of a magnetic needle by an
electric current, 178
Optics, science of, 3

pASTEUR referred to, 223

*- Physical theories, origin of,

42-45
Pigments, analysis of the action of,

upon light, 34

— ■ mixture of, contrasted with mix-
ture of lights, 37

— Helmholtz reveals the cause of the
green in the case of mixed blue
and yellow pigments, 37

— impurity of natural colours, 38
Pitch of sound, 61

Pliieker, his drawings of spectra,

205
Polariscope, stained glass in the,

134, 135

— unannealed glass in the, 1 39
Polarity, notion of, how generated,

— atomic, 98

— structural arrangements due to,
100

— polarization of light, 115

— tested by tourmaline, 119

~ and by reflection and refraction,
121

— depolarization, 123
Polarization of light, 115

— circular, 143

— sky-light, 152, 158, 160

— of artificial sky, 159

— of radiant heat, 1 83
Polarizer and analyzer, 1 3 1
Poles of a magnet, 97

Powell, Professor, on polarization

of heat, 181
Prism, the Nicol, 125

SEB

QUARTZ, chromatic phenomena
produced by, 142

"DADIANTheat, 176
-*-*' — diathermancy, or pervious-
ness to radiant heat, 1 76

— conversion of heat-rays into
light-rays, 177

— formation of invisible heat-
images, 183

— polarization of, 183

— double refraction, 1 85

— magnetization of, 187
Rainbow, Descartes' explanation of

the, 24, 25
Refraction, demonstration of, 15
Refraction of light, 1 1 0

— double, 112

Reflection, partial and total, 17-20
Respighi, results obtained by, 208
Ritter, his discovery of the ultra-
violet rays of the sun, 162
Roemer, Olav, his observations of
Jupiter's moons, 20

— his determination of the velocity
of light, 21

Rubidium, discovery of, 195
Rusting of iron, what it is, 5

SCHWERD, his observations re-
specting diffraction, 91
Science, growth of, 179, 206
Schelling, his contempt for experi-
mental knowledge, 14 note
Scoresby, Dr., succeeds in exploding
gunpowder by the sun's rays con-
veyed by large lenses of ice, 1 70
Secchi, results obtained by, 208
Seebeck, Thomas, discovers thermo-
electricity, 179

— discovers the polarization of
light by tourmaline, 212

INDEX.

271

SEL
Solet'ite, experiments with thick

and thin plates of, 127
Silver spectrum, analysis of, 193, 194
Sky-light, colour and polarization of,

152, 157

— generation of artificial skies, 155
Snell, Willebrord, his discovery, 15

— his law, 16, 24
Soap-bubbles and their colours, 65,

66
Sound, early notions of the ancients
respecting, 52

- interference of waves of, 61

— pitch of, 61

— analogies of light and, 63

— demonstration of analogy be-
tween, and light, 201, 202

Sonorous vibrations, action of, 137
Spectrum analysis, principles of, 192
Spectra of incandescent vapours, 193

— discontinuous, 194, 105

— of polarized light, 250
Spectrum bands proved by Bunsen

and Kirchhoff to be characteristic
of the vapour, 195

— its capacity as an agent of dis-
covery, 196

— analysis of the sun and stars, 196
Bpottiswoode, Mr. William, 126
Stewart, Professor Balfour, 205
Stokes, Professor, results of his ex-
amination of substances excited
by the ultra-violet waves, 164

— his discovery of fluorescence, 165

— on fluorescence, 1 68

— nearly anticipates KirchhoflTs dis-
covery, 201, 205

Striated surfaces, colours of, 93
Sulphate of quinine first noticed and

described by Sir John Herschel, 168
Sun, chemistry of the, 199
Sun, rose-coloured solar promi nonces,

207

UND
'^PALBOT, Mr., his experiment- ,
-L 204

Tartaricacid,irregularcry8billization

of, and its effects, 134
Tliallene, its effect on the spectrum,

165
Thallium, npectnim analysis of, 193,

194

— discovery of, 196

— isolated in ingots by AT. Lamy,
196

Theory, rchition of, to experience,

95
Thermo-electric pile, 179
Thermo-electricity, discovery of, 179
Tombeline, Mont, inverted image of,

20
Tourmaline, polarization of light by

means of, 115
Transmitted light, reason for, 80
Transparency, remarks on, 170
Tyndall, Professor, his remarks at

Social Meeting in New York, 242

TTLTRA-VIOLET sun-ray«, dis-
^ covered by Eitter, 162

effects of, 163

Ultra-red rays of the solar spectrum,

174

part played by the, 176

Undulatory theory of light, bases of

the, 48
Sir David Brewster's chief

objection to the, 49

Young's foundation of the, 50

phenomena which first sug-
gested the, 65, 72
Undulatory theory of light, Mr.

Mill's scepticism regarding the,

152
a demonstrated verity in

the hands of Young, 213

272

INDEX.

VAS

VASSENIUS describes the rose-
coloured Bolar prominences in
1733, 207
Vitellio, his skill and conscientious-
ness, 14
• — his investigations respectinglight,

210
Voltaic battery, use of, and its
production of heat, 6, 7

WATER, cr>-stallization of, 104,
249
— deportment of, considered and
explained, 108, 109
Waves of water, 52

— length of a wave, 53

— interference of waTes, 54-56
Wertheim, M., his instrument for

the determination of strains and
pressures by the colours of polar-
ized light, 137
Wheatstone, Sir Charles, 166

— his analysis of the light of the
' electric spark, 205

Whirlpool Eapids, illustration of
the principle of the interference of
wares at the, 57

TOU

White, President, his remarks at
Social Meeting in New York, 238

Willigen, Van der, his drawings of
spectra, 205

WoUaston, Dr., 196

— discovers the rings of Iceland
spar, 212

Woodbury, Mr., on the impurity of

natural colours, 38
Wiinsch, Christian Ernst, on the

three simple colours in white

lights, 41

— his experiments, 41

YOUNG, Dr. Thomas, his dis-
covery of I^yptian hierogly-
phics, 50; and the undulatory
theory of light, 50
Young, Dr. Thomas, Helmholtz's
estimate of him, 51

— ridiculed by Brougham in the
' Edinburgh Review,' 51

— generalizes Grimaldi's observa-
tion on light, 68

— photographs the ultra-violet rings
of Newton, 163

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