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REESE LIBRARY
UNIVERSITY OF CALIFORNIA.
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W-/A
HISTORY OF ASTRONOMY,
All rights reserved.
A POPULAR
HISTORY OF ASTRONOMY
DURING
THE NINETEENTH CENTURY
BY
AGNES M. CLERKE
EDINBURGH: ADAM & CHARLES BLACK
MDCCCLXXXV.
3 o
PREFACE.
THE progress of astronomy during the last hundred years has
been rapid and extraordinary. In its distinctive features,
moreover, the nature of that progress has been such as to lend
itself with facility to untechnical treatment. To this circum-
stance the present volume owes its origin. It embodies an
attempt to enable the ordinary reader to follow, with intelli-
gent interest, the course of modern astronomical inquiries,
and to realise (so far as it can at present be realised) the full
effect of the comprehensive change in the whole aspect, pur-
poses, and methods of celestial science introduced by the
momentous discovery of spectrum analysis.
Since Professor Grant's invaluable work on the History of
Physical Astronomy was published, a third of a century has
elapsed. During the interval, a so-called "new astronomy"
has grown up by the side of the old. One effect of its advent
has been to render the science of the heavenly bodies more
popular, both in its needs and in its nature, than formerly.
More popular in its needs, since its progress now primarily
depends upon the interest in, and consequent efforts towards
its advancement of the general public; more popular in its
nature, because the kind of knowledge it now chiefly tends to
accumulate is more easily intelligible less remote from ordi-
nary experience than that evolved by the aid of the calculus
from materials collected by the use of the transit-instrument
and chronograph.
vi PREFACE.
It has thus become practicable to describe in simple language
the most essential parts of recent astronomical discoveries;
and being practicable, it could not be otherwise than desirable
to do so. The service to astronomy itself would be not in-
considerable of enlisting wider sympathies on its behalf; while
to help one single mind towards a fuller understanding of the
manifold works which have, in all ages, irresistibly spoken to
man of the glory of God, might well be an object of no ignoble
ambition.
The present volume does not profess to be a complete or
exhaustive History of Astronomy during the period covered
by it. Its design is to present a view of the progress of
celestial science, on its most characteristic side, since the time
of Herschel. Abstruse mathematical theories, unless in some
of their more striking results, are excluded from consideration.
These, during the eighteenth century, constituted the sum and
substance of astronomy; and their fundamental importance
can never be diminished, and should never be ignored. But,
as the outcome of the enormous development given to the
powers of the telescope in recent times, together with the
swift advances of physical science, and the inclusion, by means
of the spectroscope, of the heavenly bodies within the domain
of its inquiries, much knowledge has been acquired regarding
the nature and condition of those bodies, forming, it might be
said, a science apart, and disembarrassed from immediate de-
pendence upon intricate, and, except to the initiated, unin-
telligible formulas. This kind of knowledge forms the main
subject of the book now offered to the public.
There are many reasons for preferring a history to a formal
treatise on astronomy. In a treatise, what we know is set
forth. A history tells us, in addition, how we came to know
it. It thus places facts before us in the natural order of their
ascertainment, and narrates instead of enumerating. The story
PREFACE. vii
to be told leaves the marvels of imagination far behind, and
requires no embellishment from literary art or high-flown
phrases. Its best ornament is unvarnished truthfulness, and
this at least may confidently be claimed to be bestowed upon
it in the ensuing pages.
In them unity of treatment is sought to be combined with
a due regard to chronological sequence by grouping in sepa-
rate chapters the various events relating to the several depart-
ments of descriptive astronomy. The whole is divided into
two parts, the line between which is roughly drawn at the
middle of the present century. Herschel's inquiries into the
construction of the heavens strike the keynote of the first
part ; the discovery of sun-spot and magnetic periodicity and
of spectrum analysis, determine the character of the second.
Where the nature of the subject required it, however, this
arrangement has been disregarded. Clearness and consist-
ency should obviously take precedence of method. Thus, in
treating of the telescopic scrutiny of the various planets, the
whole of the related facts have been collected into an un-
interrupted narrative. A division, elsewhere natural and help-
ful, would here have been purely artificial, and therefore con-
fusing.
The interests of students have been consulted by a full and
authentic system of references to the sources of information
relied upon. Materials have been derived, as a rule with
very few exceptions, from the original authorities. The system
adopted has been to take as little as possible at second-hand.
Much pains have been taken to trace the origin of ideas,
often obscurely enunciated long before they came to resound
through the scientific world, and to give to each individual
discoverer, strictly and impartially, his due. Prominence has
also been assigned to the biographical element, as underlying
and determining the whole course of human endeavour. The
viii PREFACE.
advance of knowledge may be called a vital process. The
lives of men are absorbed into and assimilated by it. In-
quiries into the kind and mode of the surrender in each
separate case must always possess a strong interest, whether
for study or for example.
The acknowledgments of the writer are due to Professor
Edward S. Holden, director of the Washburn Observatory,
Illinois, and to Dr. Copeland, chief astronomer of Lord
Crawford's .Observatory at Dunecht, for many valuable com-
munications.
CONTENTS.
INTRODUCTION.
PAGE
Three Kinds of Astronomy Progress of the Science during the Eight-
eenth Century Popularity and Rapid Advance during the
Nineteenth Century I
Ipart fc
PROGRESS OF ASTRONOMY DURING THE FIRST
HALF OF THE NINETEENTH CENTURY.
CHAPTER I.
FOUNDATION OF SIDEREAL ASTRONOMY.
State of Knowledge regarding the Stars in the Eighteenth Century
Career of Sir William Herschel Constitution of the Stellar
System Double Stars Herschel's Discovery of their Revolu-
tions His Method of Star-Gauging Discoveries of Nebulae
Theory of their Condensation into Stars Summary of Results . 1 1
CHAPTER II.
PROGRESS OF SIDEREAL ASTRONOMY.
Exact Astronomy in Germany Career of Bessel His Fundamenta
Astronomies Career of Fraunhofer Parallaxes of Fixed Stars
Translation of the Solar System Astronomy of the Invisible
Struve's Researches in Double Stars Sir John Herschel's Ex-
ploration of the Heavens Character of Fifty Years' Progress . 35
x CONTENTS.
CHAPTER III.
PROGRESS OF KNOWLEDGE REGARDING THE SUN.
PAGE
Early Views_as-to-the Nature of Sun-Spots Wilson's Observations
and Reasonings Ilerschel's Theory of the Solar Constitution
Sir John Herschel's Trade- Wind Hypothesis Baily's Beads
Total Solar Eclipse of 1842 Corona and Prominences Eclipse
of 1851 66
CHAPTER IV.
PLANETARY DISCOVERIES.
Bode's Law Search for a Missing Planet Its Discovery by Piazzi
Further Discoveries of Minor Planets Unexplained Disturb-
ance of Uranus Discovery of Neptune Its Satellite An
Eighth Saturnian Moon Saturn's Dusky Ring The Uranian
System 93
CHAPTER V.
COMETS.
Predicted Return of Halley's Comet Career of Olbers Acceleration
of Encke's Comet Biela's Comet Its Duplication Faye's
Comet Comet of 1811 Electrical Theory of Cometary Emana-
tions The Earth in a Comet's Tail Second Return of Halley's
Comet Great Comet of 1843 Results to Knowledge . . 115
CHAPTER VI.
INSTRUMENTAL ADVANCES.
Two Principles of Telescopic Construction Early Reflectors Three
Varieties Herschel's Specula High Magnifying Powers In-
vention of the Achromatic Lens Guinand's Optical Glass The
Great Rosse Reflector Its Disclosures Mounting of Telescopes
Astronomical Circles Personal Equation .... 140
CONTENTS. xi
part HE*
RECENT PROGRESS OF ASTRONOMY.
CHAPTER I.
FOUNDATION OF ASTRONOMICAL PHYSICS.
PAGE
Schwabe's Discovery of a Decennial Sun-Spot Period Coincidence
with Period of Magnetic Disturbance Sun- Spots and Weather
Spectrum Analysis Preliminary Inquiries Fraunhofer Lines
Kirchhoffs Principle Anticipations Elementary Principles of
Spectrum Analysis Unity of Nature 161
CHAPTER II.
SOLAR OBSERVATIONS AND THEORIES.
Black Openings in Spots Carrington's Observations Rotation of
the Sun Kirchhoff s Theory of the Solar Constitution Faye's
Views Solar Photography Kew Observations Whirlpool
Theory of Sun-Spots Volcanic Hypothesis A Solar Outburst
Sun-Spot Periodicity Planetary Influence Nasmyth's Willow
Leaves 185
CHAPTER III.
RECENT SOLAR ECLIPSES.
Expeditions to Spain Great Indian Eclipse New Method of View-
ing Prominences Total Eclipse Visible in North America
Spectrum of the Corona Eclipse of 1870 Young's Reversing
Layer Eclipse of 1871 Corona of 1878 Eclipse Observations
at Sohag and at Caroline Island Recapitulation . . .213
CHAPTER IV.
SPECTROSCOPIC WORK ON THE SUN.
Chemistry of Prominences Study of their Forms Two Classes
Distribution of Prominences Structure of the Chromosphere
xii CONTENTS.
PAGB
Spectroscopic Measurement of Movements in Line of Sight-
Velocities of Transport in the Sun Lockyer's Theory of Dis-
sociation Hydrogen a Solar Constituent Oxygen in the Sun . 238
CHAPTER V.
TEMPERATURE OF THE SUN.
Thermal Power of the Sun Radiation and Temperature Estimates
of Solar Temperature Rosetti's Result Zollner's Method
Langley's Experiment at Pittsburg The Sun's Atmosphere
Selective Absorption by our Air The Sun Blue . . . 257
CHAPTER VI.
' THE SUN'S DISTANCE.
Difficulty of the Problem Oppositions of Mars Transits of Venus
Lunar Disturbance Velocity of Light Transit of 1874 Incon-
clusive Result Opposition of Mars in 1877 Measurements of
Minor Planets Transit of 1882 Conclusions and Limits of
Error . . 269
CHAPTER VII.
PLANETS AND SATELLITES.
Schroter's Life and Work Luminous Appearances during Transits of
Mercury Mountains of Mercury Intra-Mercurial Planets Ro-
tation of Venus Mountains and Atmosphere Ashen Light
Solidity of the Earth Secular Changes of Climate Figure of the
Globe Study of the Moon's Surface Lunar Atmosphere New
Craters Thermal Effects of Moonlight Tidal Friction . . 288
CHAPTER VIII.
PLANETS AND SATELLITES (continued}.
Analogy between Mars and the Earth Martian Snowcaps, Seas, and
Continents Climate and Atmosphere Schiaparelli's Canals
Discovery of Two Martian Satellites Distribution of the Minor
CONTENTS. xiii
PAGE
Planets Their Collective Mass and Estimated Diameters Con-
dition of Jupiter His Spectrum Transits of his Satellites The
Great Red Spot Constitution of Saturn's Rings Period of
Rotation of the Planet Variability of Japetus Equatorial
Markings on Uranus His Spectrum Rotation of Neptune
Trans-Neptunian Planets 319
CHAPTER IX.
THEORIES OF PLANETARY EVOLUTION.
Origin of the World according to Kant Laplace's Nebular Hypo-
thesis Maintenance of the Sun's Heat Meteoric Hypothesis
Radiation the Result of Contraction Regenerative Theory
Origin of the Moon Effects of Tidal Friction . . . .348
CHAPTER X.
RECENT COMETS.
Donati's Comet The Earth again Involved in a Comet's Tail
Comets of the August and November Meteors Star Showers
Comets and Meteors Biela's Comet and the Andromeds
Spectroscopic Analysis of Cometary Light 363
CHAPTER XI.
RECENT COMETS (continued}.
Forms of Comets' Tails Electrical Repulsion Bredichin's Three
Types Great Southern Comet Supposed Previous Appearances
Tebbutt's Comet and the Comet of 1807 Successful Photo-
graphs Schaberle's Comet Comet Wells Sodium Blaze in
Spectrum Great Comet of 1882 Transit Across the Sun Re-
lation to Comets of 1843 and 1880 Cometary Systems Origin
of Comets 384
CHAPTER XII.
STARS AND NEBULAE.
Stellar Chemistry Four Orders of Stars Their Relative Ages-
Variable Stars New Stars Discovery of Gaseous Nebulae
xiv CONTENTS.
PAGE
Variable Nebulas Velocities of Stars in Line of Sight Stellar
and Nebular Photography Construction of the Heavens
Double Stars Status of Nebulae Star Drift . . . .411
CHAPTER XIII.
METHODS OF RESEARCH.
Development of Telescopic Power Silvered Glass Reflectors Giant
Refractors Difficulty of Further Improvement Atmospheric
Disturbance Mountain Observatories The Equatoreal Coude
The Photographic Camera Retrospect and Conclusion . . 440
INDEX 455
ERRATA.
Page 4, line 3 from bottom, for 1748, read 1758.
Page 183, line 10 from bottom, for "the sciences," read "the physical
HISTORY OF ASTRONOMY
DURING THE NINETEENTH CENTURY.
INTRODUCTION. 4
WE can distinguish three kinds of astronomy, each with a
different origin and history, but all mutually dependent, and
composing, in their fundamental unity, one science. First in
order of time came the art of observing the returns and
measuring the places of the heavenly bodies. This was the
sole astronomy of the Chinese and Chaldeans ; r but to it the
vigorous Greek mind added a highly complex geometrical plan
of their movements, for which Copernicus substituted a more
harmonious system, without as yet any idea of a compelling
cause. The planets revolved in circles because it was their
nature to do so, just as laudanum sets to sleep because it
possesses a virtus dormitiva. This first and oldest branch is
known as "observational," or "practical astronomy." Its
business is to note facts as accurately as possible; and it is
essentially unconcerned with schemes for connecting those
facts in a manner satisfactory to the reason.
The second kind of astronomy was founded by Newton.
Its nature is best indicated by the term " gravitational ; " but it
is also called "theoretical astronomy." 1 It is based on the
idea of cause ; and the whole of its elaborate structure is reared
1 The denomination "physical astronomy," first used by Kepler, and
long appropriated to this branch of the science, has of late been otherwise
applied.
A
2 HISTORY OF ASTRONOMY.
according to the dictates of a single law, simple in itself, but
the tangled web of whose consequences can be unravelled only
by the subtle agency of an elaborate calculus.
The third and last division of celestial science may properly
be termed "physical and descriptive astronomy." It seeks
to know what the heavenly bodies are in themselves, leaving
the How ? and the Wherefore ? of their movements to be
otherwise answered. Now such inquiries became possible
only with the invention of the telescope, so that Galileo
was, in point of fact, their originator. But Herschel was the
first to give them a prominence which the whole progress of
science during the nineteenth century has served to confirm
and render more exclusive. Inquisitions begun with the
telescope have been extended and made effective in unhoped-
for directions by the aid of the spectroscope and photographic
camera ; and a large part of our attention in the present volume
will be occupied with the brilliant results thus achieved.
The unexpected development of this new physical-celestial
science is the leading fact in recent astronomical history.
It was out of the regular course of events. In the degree in
which it has actually occurred it could certainly not have been
foreseen. It was a seizing of the prize by a competitor who
had hardly been thought qualified to enter the lists. Orthodox
astronomers of the old school looked with a certain contempt
upon observers who spent their nights in scrutinising the faces
of the moon and planets, rather than in timing their transits,
or devoted daylight energies, not to reductions and computa-
tions, but to counting and measuring spots on the sun. They
were regarded as irregular practitioners, to be tolerated perhaps,
but certainly not encouraged.
The advance of astronomy in the eighteenth century ran in
general an even and logical course. The age succeeding
Newton's had for its special task to demonstrate the universal
validity, and trace the complex results of the law of gravitation.
Its accomplishment occupied just one hundred years. It was
virtually brought to a close when Laplace explained to the
INTRODUCTION. 3
French Academy, November 19, 1787, the cause of the
moon's accelerated motion. As a mere machine, the solar
system, so far as it was then known, was found to be complete
and intelligible in all its parts ; and in the Mecanique Celeste
its mechanical perfections were displayed under a form of
majestic unity which fitly commemorated the successive
triumphs of analytical genius over problems amongst the most
arduous ever dealt with by the mind of man.
Theory, however, demands a practical test. All its data
are derived from observation ; and their insecurity becomes
less tolerable as it advances nearer to perfection. ' Observation,
on the other hand, is the pitiless critic of theory ; it detects
weak points, and provokes reforms which may be the be-
ginnings of discovery. Thus, theory and observation mutually
act and react, each alternately taking the lead in the endless
race of imf>rovement.
Now, while in France Lagrange and Laplace were bringing
the gravitational theory of the solar system to completion,
work of a very different kind, yet not less indispensable to the
future welfare of astronomy, was being done in England. The
Royal Observatory at Greenwich is one of the few useful
institutions which date their origin from the reign of Charles
II. The leading position which it still occupies in the science
of celestial observation was, for near a century and a half after
its foundation, an exclusive one. It was absolutely without a
rival. Systematic observations of sun, moon, stars, and planets
were, during the whole of the eighteenth century, made only
at Greenwich. Here materials were accumulated for the
secure correction of theory, and here refinements were intro-
duced by which the exquisite accuracy of modern practice in
astronomy was eventually attained.
The chief promoter of these improvements was James
Bradley. Few men have possessed in an equal degree with
him the power of seeing accurately, and reasoning on what
they see. He let nothing pass. The slightest inconsistency
between what appeared and what was to be expected, roused
4 HISTORY OF ASTRONOMY.
his keenest attention ; and he never relaxed his mental grip of
a subject until it had yielded to his persistent inquisition. It
was to these qualities that he owed his discoveries of the aber-
ration of light and the nutation of the earth's axis. The first
was announced in 1729. It means that, owing to the circum-
stance of light not being instantaneously transmitted, the
heavenly bodies appear shifted from their true places by an
amount depending upon the ratio which the velocity of light
bears to the speed of the earth in its orbit. Because light
travels with enormous rapidity, the shifting is very slight ; and
each star returns to its original position at the end of a year.
Bradley's second great discovery was finally ascertained in
1748. Nutation is a real "nodding" of the terrestrial axis
produced by the -dragging of the moon at the terrestrial
equatorial protuberance. From it results an apparent dis-
placement of the stars, each of them describing a little ellipse
about its true, or "mean" position, in a period of eighteen
years and about seven months.
Now an acquaintance with the fact and the laws of each
of these minute irregularities is vital to the progress of obser-
vational astronomy for without it the places of the heavenly
bodies could never be accurately known or compared. So
that Bradley, by their detection, at once raised the science to
a higher grade of precision. Nor was this the whole of his
work. Appointed Astronomer- Royal in 1742, he executed
during the years 1750-62 a series of observations which formed
the real beginning of exact astronomy. Part of their superiority
must, indeed, be attributed to the co-operation of John Bird,
who provided Bradley in 1750 with a measuring instrument of
till then unequalled excellence. For not only was the art of
observing in the eighteenth century a peculiarly English art,
but the means of observing were furnished almost exclusively
by British artists. John Dollond, the son of a Spitalfields
weaver, invented the achromatic lens in 1748, removing
thereby the chief obstacle to the development of the powers
of refracting telescopes; James Short, of Edinburgh, was
INTRODUCTION. 5
without a rival in the construction of reflectors ; the sectors,
quadrants, and circles of Graham, Bird, Ramsden, and Gary
were inimitable by Continental workmanship.
Thus practical and theoretical astronomy advanced on
parallel lines in England and France respectively, the im-
provement of their several tools the telescope and the
quadrant on the one side, and the calculus on the other keep-
ing pace. The whole future of the science seemed to be theirs.
The cessation of interest through a too speedy attainment
of the perfection towards which each spurred the other,
appeared to be the only danger it held in store for them.
When, all at once, a rival stood by their side not, indeed,
menacing their progress, but threatening to absorb their
popularity.
The rise of Herschel was the one conspicuous anomaly in
the astronomical history of the eighteenth century. It proved
decisive of the course of events in the nineteenth. It was
unexplained by anything that had gone before; yet all that
came after hinged upon it. It gave a new direction to effort ;
it lent a fresh impulse to thought. It opened a channel for
the widespread public interest which was gathering towards
astronomical subjects to flow in.
Much of this interest was due to the occurrence of events
calculated to arrest the attention and excite the wonder of the
uninitiated. The predicted return of Halley's comet in 1759
verified, after an unprecedented fashion, the computations of
astronomers. It deprived such bodies for ever of their por-
tentous character; it ranked them as denizens of the solar
system. Again, the transits of Venus in 1761 and 1769 were
the first occurrences of the kind since the awakening of science
to their consequence. Imposing preparations, journeys to
remote and hardly accessible regions, official expeditions, inter-
national communications, all for the purpose of observing them
to the best advantage, brought their high significance vividly
to the public consciousness ; a result aided by the facile pen
of Lalande, in rendering intelligible the means by which these
6 HISTORY OF ASTRONOMY.
elaborate arrangements were to issue in an accurate knowledge
of the sun's distance. Lastly, Herschel's discovery of Uranus,
March 13, 1781, had the surprising effect of utter novelty.
Since the human race had become acquainted with the company
of the planets, no addition had been made to their number.
The event thus broke with immemorial traditions, and seemed
to show astronony as still young, and full of unlooked-for
possibilities.
Further popularity accrued to the science from the sequel of
a career so strikingly opened. Herschel's huge telescopes, his
detection by their means of two Saturnian and as many
Uranian moons, his piercing scrutiny of the sun, picturesque
theory of its constitution, and sagacious indication of the route
pursued by it through space ; his discovery of stellar revolving
systems, his bold soundings of the universe, his grandiose ideas,
and the elevated yet simple language in which they were con-
veyed formed a combination powerfully effective to those
least susceptible of new impressions. Nor was the evoked
enthusiasm limited to the British Isles. In Germany, Schroter
followed longo intervallo in Herschel's track. Von Zach
set on foot from Gotha that general communication of ideas
which gives life to a forward movement. Bode wrote much
and well for unlearned readers. Lalande, by his popular lec-
tures and treatises, helped to form an audience which Laplace
himself did not disdain to address in the Exposition du
Systeme du Monde.
This great accession of popularity gave the impulse to the
extraordinarily rapid progress of astronomy in the nineteenth
century. Official patronage combined with individual zeal
sufficed for the elder branches of the science. A few well-
endowed institutions could accumulate the materials needed
by a few isolated thinkers for the construction of theories of
wonderful beauty and elaboration, yet precluded, by their
abstract nature, from winning general applause. But the new
physical astronomy depends for its prosperity upon the favour
of the multitude whom its striking results are well fitted to
INTRODUCTION. 7
attract. It is, in a special manner, the science of amateurs.
It welcomes the most unpretending co-operation. There is no
one " with a true eye and a faithful hand " but can do good
work in watching the heavens. And not unfrequently prizes
of discovery which the most perfect appliances failed to grasp
have fallen to the share of ignorant or ill-provided assiduity.
Observers, accordingly, have multiplied ; observatories have
been founded in all parts of the world ; associations have been
constituted for mutual help and counsel. A formal astro-
nomical congress met in 1798 at Gotha then, under Duke
Ernest II. and Von Zach, the focus of German astronomy
and instituted a combined search for the planet suspected to
revolve undiscovered between the orbits of Mars and Jupiter.
The Astronomical Society of London was established in 1820,
and the similar German institution in 1863. Both have been
highly influential in promoting the interests, local and general,
of the science they are devoted to forward ; while functions
corresponding to theirs have been discharged elsewhere by
older or less specially constituted bodies, and new ones are
springing up on all sides.
Modern facilities of communication have helped to impress
more deeply upon modern astronomy its associative character.
The electric telegraph gives a certain ubiquity which is invalu-
able to an observer of the skies. With the help of a wire, a
battery, and a code of signals, he sees whatever is visible from
any portion of our globe, depending, however, upon other
eyes than his own, and so entering as a unit into a widespread
organisation of intelligence. The press, again, has been a
potent agent of co-operation. It has mainly contributed to
unite astronomers all over the world into a body animated by the
single aim of collecting " particulars " in their special branch for
what Bacon termed a History of Nature, eventually to be inter-
preted according to the sagacious insight of some one among
them gifted above his fellows. The first really effective astro-
nomical periodical was the Monatliche Correspondenz, started
by Von Zach in the year 1800. It was followed in 1822 by
8 HISTORY OF ASTRONOMY.
the Astronomische Nachrichten^ later by the Memoirs and
Monthly Notices of the Astronomical Society, and by the host
of varied publications which now, in every civilised country,
communicate the discoveries made in astronomy to divers
classes of readers, and so incalculably quicken the current of
its onward flow.
Public favour brings in its train material resources. It is
represented by individual enterprise, and finds expression in
an ample liberality. The first regular observatory in the
southern hemisphere was founded at Paramatta by Sir Thomas
Makdougall Brisbane in 1821. The Royal Observatory at the
Cape of Good Hope was completed in 1829. Similar estab-
lishments were set to work by the East India Company at
Madras, Bombay, and St. Helena, during the first third of the
nineteenth century. The organisation of astronomy in the
United States of America was due to a strong wave of popular
enthusiasm. In 1825 John Quincy Adams vainly urged upon
Congress the foundation of a National Observatory; but in
1843 the lectures of Ormsby MacKnight Mitchel on celestial
phenomena stirred an impressionable audience to the pitch of
providing him with the means of erecting at Cincinnati the
first astronomical establishment worthy the name in that great
country. On the ist of January 1882 no less than one hundred
and forty-four were active within its boundaries.
The apparition of the great comet of 1 843 gave an additional
fillip to the movement. To the excitement caused by it the
Cambridge Observatory called the " American Pulkowa "
directly owed its origin ; and the example was not ineffective
elsewhere. Corporations, universities, municipalities, vied with
each other in the creation of similar institutions ; private sub-
scriptions poured in ; emissaries were sent to Europe to
purchase instruments and procure instruction in their use.
In a few years the young Republic was, in point of astro-
nomical efficiency, at least on a level with countries where the
science had been fostered since the dawn of civilisation.
A vast widening of the scope of astronomy has accompanied,
INTRODUCTION. 9
and in part occasioned, the great extension of its area of
cultivation which our age has witnessed. In the last century
its purview was a comparatively narrow one. Problems lying
beyond the range of the solar system were almost unheeded,
because they seemed inscrutable. Herschel first showed the
sidereal universe as accessible to investigation, and thereby
offered to science new worlds majestic, manifold, " infinitely
infinite " to our apprehension in number, variety, and extent
for future conquest. Their gradual appropriation has absorbed,
and will long continue to absorb, the powers which it has
served to develop.
But this is not the only direction in which astronomy has
enlarged, or rather has levelled, its boundaries. The unification
of the physical sciences is perhaps the greatest intellectual feat
of recent times. The process has included astronomy ; so
that, like Bacon, she may now be said to have "taken all
knowledge " (of that kind) " for her province." In return,
she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale for
our benefit. The sun, stars, and nebulae form so many
celestial laboratories, where the nature and mutual relations of
the chemical " elements " may be tried by more stringent tests
than sublunary conditions afford. The laws of terrestrial
magnetism can be completely investigated only with the aid of
a concurrent study of the face of the sun. The positions of
the planets will perhaps one day tell us something of impending
droughts, famines, and cyclones.
Astronomy generalises the results of the other sciences.
She exhibits the laws of Nature working over a wider area,
and under more varied conditions, than ordinary experience
presents. Ordinary experience, on the other hand, has become
indispensable to her progress. She takes in at one view the
indefinitely great and the indefinitely little. The mutual
revolutions of the stellar multitude during tracts of time which
seem to lengthen out to eternity as the mind attempts to
io HISTORY OF ASTRONOMY.
traverse them, she does not admit to be beyond her ken ; nor
is she indifferent to the constitution of the minutest atom of
matter that thrills the ether into light. How she entered
upon this vastly expanded inheritance, and how, so far, she
has dealt with it, is attempted to be set forth in the ensuing
chapters.
( II )
PART I.
PROGRESS OF ASTRONOMY DURING THE FIRST HALF
OF THE NINETEENTH CENTURY.
CHAPTER I.
FOUNDATION OF SIDEREAL ASTRONOMY.
UNTIL nearly a hundred years ago the stars were regarded by
practical astronomers mainly as a number of convenient fixed
points by which the motions of the various members of the
solar system could be determined and compared. Their
recognised function, in fact, was that of milestones on the
great celestial highway traversed by the planets, as well as
on the byeways of space occasionally pursued by comets.
Not that curiosity as to their nature, and even conjecture as
to their origin, were at any period absent. Both were from
time to time powerfully stimulated by the appearance of
startling novelties in a region described by philosophers as
"incorruptible," or exempt from change. The catalogue of
Hipparchus probably, and certainly that of Tycho Brahe,
some seventeen centuries later, owed each its origin to the
temporary blaze of a new star. The general aspect of the
skies was thus (however imperfectly) recorded from age to
age, and with improved appliances the enumeration was
rendered more and more accurate and complete ; but the
secrets of the stellar sphere remained inviolate.
In a qualified, though very real sense, Sir William Herschel
12 HISTORY OF ASTRONOMY.
may be called the Founder of Sidereal Astronomy. Before
his time some curious facts had been noted, and some inge-
nious speculations hazarded, regarding the condition of the
stars, but not even the rudiments of systematic knowledge had
been acquired. The facts ascertained can be summed up in
a very few sentences.
Giordano Bruno was the first to set the suns of space in
motion, but in imagination only. His daring surmise was,
however, confirmed in 1718, when Halley announced 1 that
Sirius, Aldebaran, Betelgeux, and Arcturus had unmistakably
shifted their quarters in the sky since Ptolemy assigned their
places in his catalogue. A similar conclusion was reached by
J. Cassini in 1738, from a comparison of his own observations
with those made at Cayenne by Richer in 1672 ; and Tobias
Mayer drew up in 1756 a list showing the direction and
amount of about fifty-seven proper motions, 2 founded on star-
places determined by Olaus Romer fifty years previously. Thus
the stars were no longer regarded as " fixed," but the question
remained whether the movements perceived were real or only
apparent ; and this it was not yet found possible 1 to answer.
Already, in the previous century, the ingenious Robert Hooke
had suggested an "alteration of the very system of the sun," 3
to account for certain suspected changes in stellar positions ;
Bradley in 1748, and Lambert in 1761, pointed out that such
apparent displacements (by that time well ascertained) were in
all probability a combined effect of motions, both of sun and
stars ; and Mayer actually attempted the analysis, but without
result.
On the 1 3th of August 1596, David Fabricius, an unpro-
fessional astronomer in East Friesland, saw in the neck of the
Whale a star of the third magnitude, which by October had
1 Phil. Trans., vol. xxx. p. 737.
2 Out of eighty stars compared, fifty-seven were found to have changed
their places by more than 10". Lesser discrepancies were at that time
regarded as falling within the limits of observational error. Tobice Mayeri,
Op. Inedita, t. i. pp. 80-8 1, and Herschel in Phil. Trans., vol. Ixxiii. pp.
275-278. 3 Posthumous Works, p. 506.
FOUNDATION OF SIDEREAL ASTRONOMY. 13
disappeared. It was, however, visible in 1603, when Bayer
marked it in his catalogue with the Greek letter o, and was
watched through its phases of brightening and apparent ex-
tinction by a German professor named Holwarda in 1638-39. 1
From Hevelius this first known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud, in 1667,
fixed the length of its cycle of change at 334 days. It was
not a solitary instance. A star in the Swan was perceived by
Janson in 1600 to show fluctuations of light, and Montanari
found in 1669 that Algol in Perseus shared the same peculiarity
to a marked degree. Altogether the class embraced in 1782
half a dozen members. When it is added that a few star-
couples had been noted in singularly, but it was supposed
accidentally, close juxtaposition, and that the failure of repeated
attempts to find an annual parallax pointed to distances at
least 400,000 times that of the earth from the sun, 2 the picture
of sidereal science, when the last quarter of the eighteenth
century began, is practically complete. It included three items
of information that the stars have motions, real or apparent \
that they are immeasurably remote ; and that a few shine with
a periodically variable light. Nor were the facts thus scantily
collected ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They
needed to be both multiplied and marshalled, and it seemed
as if centuries of patient toil must elapse before any reliable
conclusions could be derived from them. The sidereal world
was thus the recognised domain of far-reaching speculations,
which remained wholly uncramped by systematic research
until Herschel entered upon his career as an observer of the
heavens.
The greatest of modern astronomers was born at Hanover,
November 15, 1738. He was the fourth child of Isaac
1 Arago in Annuaire du Bureau des Longitudes, 1842, p. 313.
2 Bradley to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His obser-
vations were directly applicable to only two stars, 7 Draconis and 17 Ursae
Majoris, but some lesser ones were included in the same result.
H HISTORY OF ASTRONOMY.
Herschel, a hautboy-player in the band of the Hanoverian
Guard, and was early trained to follow his father's profession.
On the termination, however, of the disastrous campaign of
1757, his parents removed him from the regiment, and he
went to England to seek his fortune. He was then nearly
nineteen, his military service having lasted four years. Of the
life of struggle and privation which ensued little is known beyond
the circumstances that in 1760 he was engaged in training the
regimental band of the Durham Militia, and that in 1765 he
was appointed organist at Halifax. This post he exchanged
a year later for the more distinguished one of organist at the
Octagon Chapel in Bath. The tide of prosperity now began
to flow for him. The most brilliant and modish society in
England was at that time to be met at Bath, and the young
Hanoverian quickly found himself a favourite and the fashion
in it. Engagements multiplied upon him. He became
director of the public concerts; he conducted oratorios, en-
gaged singers, organised rehearsals, composed anthems, chants,
choral services, besides undertaking private tuitions, at times
amounting to thirty-five or even thirty-eight lessons a week.
He in fact personified the musical activity of a place then
eminently and energetically musical.
But these multifarious avocations did not take up the whole
of his thoughts. His education, notwithstanding the poverty
of his family, had not been neglected, and he had always
greedily assimilated every kind of knowledge that came in his
way. Now that he was a busy and a prosperous man, it might
have been expected that he would run on in the deep pro-
fessional groove laid down for him. On the contrary, his
passion for learning seemed to increase with the diminution of
the time available for its gratification. He studied Italian,
Greek, mathematics ; Maclaurin's Fluxions served to " unbend
his mind;" Smith's Harmonics and Optics and Ferguson's
Astronomy were the nightly companions of his pillow. What
he read stimulated without satisfying his intellect. He desired
not only to know, but to discover. In 1773 he hired a
FOUNDATION OF SIDEREAL ASTRONOMY. 15
small telescope, and through it caught a preliminary glimpse
of the rich and varied fields in which, for so many years, he
was to expatiate. Henceforward the purpose of his life was
fixed. It was to obtain " a knowledge of the construction of
the heavens ; " J and to this sublime ambition he remained
true until the end.
A more powerful instrument was the first desideratum ; and
here his mechanical genius came to his aid. Having purchased
the apparatus of a Quaker optician, he set about the manu-
facture of specula with a zeal which seemed to anticipate the
wonders they were to disclose to him. It was not until fifteen
years later that his grinding and polishing machines were
invented, so the work had at that time to be entirely done by
hand. During this tedious and laborious process (which could
not be interrupted without injury, and lasted on one occasion
sixteen hours), his strength was supported by morsels of food
put into his mouth by his sister, 2 and his mind amused by
her reading aloud to him the Arabian Nights, Don Quixote,
or other light works. At length, after repeated failures, he
found himself provided with a reflecting telescope a five-foot
Newtonian of his own construction. A copy of his first
observation with it, on the great Nebula in Orion an object of
continual amazement and assiduous inquiry to him is pre-
served by the Royal Society. It bears the date March 4,
I774- 3
In the following year he executed his first " review of the
heavens," memorable chiefly as an evidence of the grand and
novel conceptions which already inspired him, and of the
enthusiasm with which he delivered himself up to their guid-
ance. Overwhelmed with professional engagements, he still
contrived to snatch some moments for the stars ; and between
1 Phil. Trans.) vol. ci. p. 269.
2 Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in
the same place, January 9, 1848. She came to England in 1772, and was
her brother's devoted assistant, first in his musical undertakings, and
afterwards, down to the end of his life, in his astronomical labours.
3 Holden, Sir William Herschel, his Life and Works, p. 39.
16 HISTORY OF ASTRONOMY.
the acts at the theatre was often seen running from the harpsi-
chord to his telescope, no doubt with that " uncommon precipi-
tancy which accompanied all his actions." 1 He now rapidly
increased the power and perfection of his telescopes. Mirrors
of seven, ten, even twenty feet focal length were successively
completed, and unprecedented magnifying powers employed.
His energy was unceasing, his perseverance indomitable. In
the course of twenty-one years no less than 430 parabolic
specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the Philosophical Trans-
actions ; yet during the ensuing thirty- nine years his contribu-
tions many of them elaborate treatises numbered sixty-nine,
forming a series of extraordinary importance to the history of
astronomy. As a mere explorer of the heavens his labours
were prodigious. He discovered 2500 nebulae, 806 double stars,
passed the whole firmament in review four several times, counted
the stars in 3400 " gauge-fields," and executed a photometric
classification of the principal stars, founded on an elaborate
(and the first systematically conducted) investigation of their
relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure
accuracy in his observations ; yet in one night he would
examine, singly and carefully, up to 400 separate objects.
The discovery of Uranus was a mere incident of the scheme
he had marked out for himself a fruit gathered, as it were,
by the way. It formed, nevertheless, the turning-point in his
career. From a star-gazing musician he was at once trans-
formed into an eminent astronomer; he was relieved from
the drudgery of a toilsome profession, and installed as royal
astronomer, with a modest salary of ^200 a year; funds were
provided for the construction of the forty-foot reflector, from
the great space-penetrating power of which he expected as yet
unheard-of revelations ; in fine, his future work was not only
rendered possible, but it was stamped as authoritative. 2 On
1 Memoir of Caroline Her schel, p. 37.
2 See Holden's Sir William Herschel, p. 54.
FOUNDATION OF SIDEREAL ASTRONOMY. 17
Whit-Sunday 1782, William and Caroline Herschel played
and sang in public for the last time in St. Margaret's Chapel,
Bath ; in August of the same year the household was moved
to Datchet, near Windsor, and on April 3, 1786, to Slough.
Here happiness and honours crowded on the fortunate dis-
coverer. In 1788 he married Mary, only child of James
Baldwin, a merchant of the city of London, and widow of
John Pitt, Esq., a lady endowed not only with all the
domestic virtues, but with a large share of more substantial,
though less precious goods. The fruit of their union was
one son, of whose work the worthy sequel of his father's
we shall have to speak further on. Herschel was created a
Knight of the Hanoverian Guelphic Order in 1816, and in
1821 he became the first President of the Royal Astronomical
Society, his son being its first Foreign Secretary. But his
health had now for some years been failing, and on August 25,
1822, he died at Slough, in the eighty-fourth year of his age,
and was buried in Upton churchyard.
His epitaph claims for him the lofty praise of having
"burst the barriers of heaven." Let us see in what sense this
is true.
The first to form any definite idea as to the constitution of
the stellar system was Thomas Wright, the son of a carpenter
living at Byer's Green, near Durham. With him originated
what has been called the " Grindstone Theory " of the universe,
which regarded the Milky Way as the projection on the sphere
of a stratum or disc of stars (our sun occupying a position
near the centre), similar in magnitude and distribution to the
lucid orbs of the constellations. 1 He was followed by Kant, 2
who transcended the views of his predecessor by assigning to
nebulas the position they long continued to occupy, rather on
imaginative than on scientific grounds, of " island universes,"
1 An Original Theory or New Hypothesis of the Universe, London,
I75 O - See also De Morgan's summary of his views in Philosophical
Magazine, April 1848.
2 Allgemeine Naturgeschichte und Theorie des Himmels, 1755'
B
i8 HISTORY OF ASTRONOMY.
external to, and co-equal with the Galaxy. Johann Heinrich
Lambert, 1 the tailor's apprentice of Miihlhausen, followed, but
independently. The conceptions of this remarkable man were
grandiose, his intuitions bold, his views on some points a
singular anticipation of subsequent discoveries. The sidereal
world presented itself to him as a hierarchy of systems, starting
from the planetary scheme, rising to throngs of suns within
the circuit of the Milky Way the " ecliptic of the stars," as
he phrased it--expanding to include groups of many Milky
Ways ; these again combining to form the unit of a higher order
of assemblage, and so onwards and upwards until the mind
reels and sinks before the immensity of the contemplated crea-
tions.
" Thus everything revolves the earth round the sun ; the
sun round the centre of his system; this system round a
centre in common to it with other systems ; this group, this
assemblage of systems, round a centre which is common to it
with other groups of the same kind ; and where shall we have
done?" 2
The stupendous problem thus speculatively attempted, Her-
schel undertook to grapple with experimentally. The upshot
of this memorable inquiry was the inclusion for the first time
within the sphere of human knowledge, of a connected body of
facts and inferences from facts regarding the sidereal universe
in other words, the foundation of what may properly be called
a science of the stars.
Tobias Mayer had illustrated the perspective effects which
must ensue in the stellar sphere from a translation of the solar
system, by comparing them to the separating in front and
closing up behind of trees in a forest to the eye of an advanc-
1 Cosmologische Brief e, Augsburg, 1761.
2 The System of the World, p. 125, London, 1800 (a translation of the
above). Lambert regarded nebulae as composed of stars crowded together,
but not as external universes. In the case of the Orion nebula, indeed,
he throws out such a conjecture, but afterwards suggests that it may form
a centre for that one of the subordinate systems composing the Milky Way
to which our sun belongs.
FOUNDATION OF SIDEREAL ASTRONOMY. 19
ing spectator; 1 but the appearances which he thus correctly
described he was unable to detect. By a more searching
analysis of a smaller collection of proper motions, Herschel
succeeded in rendering apparent the very consequences foreseen
by Mayer. He showed, for example, that Arcturus and Vega
did, in fact, appear to recede from, and Sirius and Aldebaran
to approach, each other by very minute amounts ; and, with a
striking effort of divinatory genius, placed the " apex," or point
of direction of the sun's motion, close to the star X in the con-
stellation Hercules, 2 within a few degrees of the spot indicated
by the latest and most refined methods of research. The
validity of this conclusion was long doubted ; but it has been
triumphantly confirmed, and scarcely corrected. The question
as to the "secular parallax" of the fixed stars was in effect
answered.
With their annual parallax, however, the case was very
different. The search for it had already led Bradley to the
important discoveries of the aberration of light and the nuta-
tion of the earth's axis ; it was now about to lead Herschel
to a discovery of a different, but even more elevated char-
acter. Yet in neither case was the object primarily sought
attained.
From the very first promulgation of the Copernican theory
the seeming immobility of the stars had been urged as an
argument against its truth ; for if the earth really travelled
in a vast orbit round the sun, objects in surrounding space
should appear to change their positions, unless their distances
were on a scale which, to the narrow ideas of the universe then
1 Op. In., t. i. p. 79.
2 Phil. Trans., vol. Ixxiii. (1783), p. 273. He resumed the subject in
1805 (Phil. Trans., vols. xcv. and xcvi.), but, though employing a more
rigorous method, was scarcely so happy in his result. It is worthy of
remark that Prevost, almost simultaneously with Herschel, executed an
investigation similar to his with very considerable success. Klugel con-
firmed Herschel's result by an analytical inquiry in 1789.
20 HISTORY OF ASTRONOMY.
prevailing, seemed altogether extravagant. 1 The existence of
such apparent, or " parallactic " displacements was accordingly
regarded as the touchstone of the new views, and their de-
tection became an object of earnest desire to those interested
in maintaining them. Copernicus himself made the attempt ;
but with his "Triquetrum," a jointed wooden rule with the
divisions marked in ink, constructed by himself, he was hardly
able to measure angles of ten minutes, far less fractions
of a second. Galileo, a more impassioned defender of the
system, strained his ears, as it were, from Arcetri, in his
blind and sorrowful old age, for news of a discovery which
two more centuries had still to wait for. Hooke believed
he had found a parallax for the bright star in the Head
of the Dragon, but was deceived. Bradley convinced him-
self that such effects were too minute for his instruments to
measure. Herschel made a fresh attempt by a practically
untried method.
It is a matter of daily experience that two objects situated
at different distances, seem to a beholder in motion to move
relatively to each other. This principle Galileo, in the third
of his Dialogues on the Systems of the World, 2 proposed to
employ for the determination of stellar parallax ; for two stars,
lying apparently close together, but in reality separated by a
great gulf of space, must shift their mutual positions when
observed from opposite points of the earth's orbit ; or rather,
the remoter forms a virtually fixed point, to which the move-
ments of the other can be conveniently referred. By this
means complications were abolished more numerous and
perplexing than Galileo himself was aware of, and the problem
was reduced to one of simple micrometrical measurement.
The " double-star method " was also suggested by James Gre-
gory in 1675, and again by Wallis in i693; 3 Huygens first,
1 " Ingens bolus devorandus est," Kepler admits to Herwart in May
1603.
2 Opere, t. i. p. 415. 3 Phil. Trans., vol. xvii. p. 848.
FOUNDATION OF SIDEREAL ASTRONOMY. 21
and afterwards Dr. Long of Cambridge (about 1750), made
futile experiments with it ; and it eventually led, in the hands
of Bessel, to the successful determination of the parallax of
6 1 Cygni.
Its advantages were not lost upon Herschel. His attempt
to assign definite distances to the nearest stars was no isolated
effort, but part of the settled plan upon which his observations
were conducted. He proposed to sound the heavens, and the
first requisite was a knowledge of the length of his sounding-
line. Thus it came about that his special attention was early
directed to double stars.
" I resolved," he writes, 1 " to examine every star in the
heavens with the utmost attention and a very high power, that
I might collect such materials for this research as would enable
me to fix my observations upon those that would best answer
my end. The subject has already proved so extensive, and
still promises so rich a harvest to those who are inclined to be
diligent in the pursuit, that I cannot help inviting every lover
of astronomy to join with me in observations that must inevit-
ably lead to new discoveries."
The first result of these inquiries was a classed catalogue of
269 double stars, presented to the Royal Society in 1782,
followed, after three years, by an additional list of 434. In
both these collections the distances separating the individuals
of each pair were carefully measured, and (with a few excep-
tions) the directions with reference to an invariable line, of
the lines joining their centres (technically called "angles of
position "), were determined with the aid of a " revolving-wire
micrometer," specially devised for the purpose. Moreover,
an important novelty was introduced by the observation of the
various colours visible in the star-couples, the singular and
vivid contrasts of which were now for the first time described.
Double stars were at that time supposed to be a purely
optical phenomenon. Their components, it was thought,
while in reality indefinitely remote from each other, were
1 Phil. Trans., vol. Ixxii. p. 97.
22 HISTORY OF ASTRONOMY.
brought into fortuitous contiguity by the chance of lying
nearly in the same line of sight from the earth. This view,
however, was not universal. The Rev. John Mitchell, argu-
ing by the doctrine of probabilities, came to a different con-
clusion.
"It is highly probable in particular," he wrote in 1767, *
" and next to a certainty in general, that such double stars as
appear to consist of two or more stars placed very near together,
do really consist of stars placed near together, and under the
influence of some general law." And in 1 784:2 "It is not
improbable that a few years may inform us that some of the
great number of double, triple stars, &c, which have been
observed by Mr. Herschel, are systems of bodies revolving
about each other."
This remarkable speculative anticipation had a practical
counterpart in Germany. Father Christian Mayer, a Jesuit
astronomer at Mannheim, set himself, in January 1776, to
collect examples of stellar pairs, and shortly after published
the supposed discovery of " satellites " to many of the principal
stars. 3 His observations, however, were neither exact nor pro-
longed enough to lead to useful results in such an inquiry.
His disclosures were derided; his planet-stars treated as
results of hallucination. On ria point cru a des chases aussi
extraordinaires, wrote Lalande 4 within one year of a better-
grounded announcement to the same effect.
Herschel at first shared the general opinion as to the merely
optical connection of double stars. Of this the purpose for
which he made his collection is in itself sufficient evidence,
since what may be called the differential method of parallaxes
depends, as we have seen, for its efficacy upon disparity of
distance. It was "much too soon," he declared in T782, 5 "to
form any theories of small stars revolving round large ones;"
1 Phil. Trans., vol. Ivii. p. 249. 2 Ibid., vol. Ixxiv. p. 56.
3 Grundliche Vertheidigung neuer Beobachtungen von Pixsterntrabanten,
1778, and De Novis in Ccelo Sidereo Phcenomenis, 1779.
4 Bibliographic, p. 569. 5 Phil. Trans., vol. Ixxii. p. 162.
FOUNDATION OF SIDEREAL ASTRONOMY. 23
while in the year following, 1 he remarked that the identical
proper motion of the two stars forming, to the naked eye, the
single bright orb of Castor could only be explained as both
equally due to the "systematic parallax" caused by the sun's
movement in space. Plainly showing that the notion of a
physical tie compelling the two bodies to travel together, had
not as yet entered into his speculations. But he was eminently
open to conviction, and had, moreover, by observations un-
paralleled in amount as well as in kind, prepared ample
materials for convincing himself and others. In 1802 he was
able to announce the fact of his discovery, and in the two
ensuing years to lay in detail before the Royal Society, proofs,
gathered from the labours of a quarter of a century, of orbital
revolution in the case of as many as fifty double stars, hence-
forth, he declared, to be held as real binary combinations,
"intimately held together by the bond of mutual attraction." 2
The fortunate preservation in Dr. Maskelyne's notebook of a
remark made by Bradley about 1759, to the effect that the
line joining the two stars of Castor was precisely coincident
with that joining Castor with Pollux, added eighteen years to
the time during which the pair were under scrutiny, and
confirmed the evidence of change afforded by more recent
observations. Approximate periods were fixed for many of
the revolving suns for Castor, 342 years ; for y Leonis, 1200,
d Serpentis, 375, t Bootis, 1681 years; e Lyrae was noted as
a "double-double star," a change of situation having been
detected in each of the two pairs composing the group ;
and the occultation of one star by another in the course
of their mutual revolutions, of which curious phenomenon
two examples (in d Cygni and Herculis) occurred in 1802,
was described.
Thus, by the sagacity and perseverance of a single observer,
a firm basis was at last provided upon which to raise the edifice
of sidereal science. The analogy long presumed to exist
between the mighty star of our system and the bright points of
1 Phil. Trans., vol. Ixxiii. p. 272. 2 Ibid., vol. xciii. p. 340.
24 HISTORY OF ASTRONOMY.
light spangling the firmament, was shown to be no fiction of
the imagination, but a physical reality ; the fundamental quality
of attractive power was proved to be common to matter so far
as the telescope was capable of exploring, and law, subordina-
tion, and regularity to give testimony of supreme and intelligent
design no less in those limitless regions of space, than in our
narrow terrestrial home. The discovery was emphatically (in
Arago's phrase) " one with a future," since it introduced the
element of precise knowledge where more or less probable
conjecture had previously held almost undivided sway; and
precise knowledge tends to propagate itself and advance from
point to point.
We have now to speak of Herschel's pioneering work in the
skies. To explore with line and plummet the shining zone of
the Milky Way, to delineate its form, measure its dimensions,
and search out the intricacies of its construction, was the
primary task of his life, which he never lost sight of, and to
which all his other investigations were subordinate. He was
absolutely alone in this bold endeavour. Unaided he had to
devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on
this sublime subject was prepared, and the greater part of it
anticipated, by him.
The ingenious method of "star-gauging," and its issue in
the delineation of the sidereal system as an irregular stratum
of evenly scattered suns, is the best known part of his work.
But it was, in truth, only a first rude approximation, the prin-
ciple of which maintained its credit in the literature of astro-
nomy a full half-century after its abandonment by its author.
This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars,
it is obvious that the number of stars visible in any particular
direction would be strictly proportional to the range of the
system in that direction, apparent accumulation being pro-
duced by real extent. The process of " gauging the heavens,"
accordingly, consisted in counting the stars in successive
FOUNDATION OF SIDEREAL ASTRONOMY. 25
telescopic fields, and calculating thence the depths of space
necessary to contain them. The result of 3400 such operations
was the plan of the Galaxy familiar to every reader of an
astronomical text-book. Widely varying evidence was, as
might have been expected, derived from an examination of
different portions of the sky. Some fields of view were almost
blank, while others (in or near the Milky Way) blazed with
the radiance of many hundred stars compressed into an area
about one-fourth that of the full moon. In the most crowded
parts 116,000 were stated to have been passed in review within
a quarter of an hour. Here the "length of his sounding-line"
was estimated by Herschel at about 497 times the distance of
Sirius in other words, the bounding orb, or farthest sun of
the system in that direction, so far as was revealed by the
2o-foot reflector, was thus inconceivably remote. But since the
distance of Sirius, no less than of every other fixed star, was
as yet an unknown quantity, the dimensions inferred for the
Galaxy were of course purely relative ; a knowledge of its form
and structure might (admitting the truth of the fundamental
hypothesis) be obtained, but its real or absolute size remained
altogether undetermined.
Even as early as 1785, however, Herschel perceived traces
of a tendency which completely invalidated the supposition
of any approach to an average uniformity of distribution.
This was the action of what he called a " clustering power " in
the Milky Way. " Many gathering clusters " x were already
discernible to him even while he endeavoured to obtain " a true
mean result" on the assumption that each star in space was
separated from its neighbours as widely as the sun from Sirius.
"It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems ; " and
in certain assemblages he was able to trace " a course or tide
of stars setting towards a centre," denoting, not doubtfully,
the presence of attractive forces. 2 Thirteen years later, he
described our sun and his constellated companions as sur-
1 Phil. 7'rans., vol. Ixxv. p. 255. 2 Ibid., vol. Ixxix. pp. 214, 222.
26 HISTORY OF ASTRONOMY.
rounded by "a magnificent collection of innumerable stars,
called the Milky Way, which must occasion a very powerful
balance of opposite attractions to hold the intermediate stars at
rest. For though our sun, and all the stars we see, may truly
be said to be in the plane of the Milky Way, yet I am now
convinced, by a long inspection and continued examination of
it, that the Milky Way itself consists of stars very differently
scattered from those which are immediately about us." "This
immense aggregation," he added, "is by no means uniform.
Its component stars show evident signs of clustering together
into many separate allotments." 1
The following sentences, written in 1811, contain a definite
retractation of the view frequently attributed to him :
" I must freely confess," he says, " that by continuing my
sweeps of the heavens my opinion of the arrangement of the
stars and their magnitudes, and of some other particulars, has
undergone a gradual change ; and indeed, when the novelty of
the subject is considered, we cannot be surprised that many
things formerly taken for granted should on examination prove
to be different from what they were generally but incautiously
supposed to be. For instance, an equal scattering of the
stars may be admitted in certain calculations ; but when we
examine the Milky Way, or the closely compressed clusters
of stars of which my catalogues have recorded so many
instances, this supposed equality of scattering must be given
up." 2
Another assumption, the fallacy of which he had not the
means of detecting since become available, was retained by
him to the end of his life. It was that the brightness of a star
afforded an approximate measure of its distance. Upon this prin-
ciple he founded in 1817 his method of "limiting apertures," 3
by which two stars, brought into view in two precisely similar
telescopes, were "equalised" by covering a certain portion of
the object-glass collecting the more brilliant rays. The distances
1 Phil. Trans., vol. xcii. pp. 479, 495.
2 Ibid., vol. ci. p. 269. 3 Ibid., vol. cvii. p. 311.
FOUNDATION OF SIDEREAL ASTRONOMY. 27
of the orbs compared were then taken to be in the ratio of the
reduced to the original apertures of the instruments with which
they were examined. If indeed the absolute lustre of each
were the same, the result might be accepted with confidence ;
but since we have no warrant for assuming a " standard star "
to facilitate our computations, but much reason to suppose an
indefinite range, not only of size but of intrinsic brilliancy, in
the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.
In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He
was the first to bestow serious study on the enigmatical objects
known as "nebulae." The history of the acquaintance of our
race with them is comparatively short. The only one recog-
nised before the invention of the telescope was that in the
girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sufi ;
and its place was marked with dots on an old Dutch chart
of the constellation, presumably about 1500 A.D. 1 Yet so
little was it noticed, that it might practically be said as far as
Europe is concerned to have been discovered in 1612 by
Simon Marius (Mayer of Genzenhausen), who aptly described
its appearance as that of a " candle shining through horn."
The first mention of the great Orion nebula is by a Swiss
Jesuit named Cysatus, who succeeded Father Scheiner in the
chair of mathematics at Ingolstadt. He used it, apparently
without any suspicion of its novelty, as a term of comparison
for the comet of December i6i8. 2 A novelty, nevertheless,
to astronomers it still remained in 1656, when Huygens
discerned " as it were, an hiatus in the sky, affording a glimpse
of a more luminous region beyond." 3 Halley in 1714 knew
of six nebulae, which he believed to be composed of a " lucid
1 Bullialdus, De Nebulas A StellA in Cingulo Andromeda (1667) ; see also
G. P. Bond, Mem. Am. Ac., vol. iii. p. 75, and Holden's Monograph on
the Orion Nebula, Washington Observations, vol. xxv. 1878 (pub. 1882).
2 Mathemata Astronomica, p. 75. 3 Systema Saturnium i p. 9.
28 HISTORY OF ASTRONOMY.
medium " diffused through the ether of space. 1 He appears,
however, to have been unacquainted with some previously
noticed by Hevelius. Lacaille brought back with him from
the Cape a list of forty-two the first-fruits of observation in
Southern skies arranged in three numerically equal classes ; 2
and Messier (nicknamed by Louis XV. the " ferret of comets " 3 ),
finding such objects a source of extreme perplexity in the
pursuit of his chosen game, attempted to eliminate by metho-
dising them, and drew up a catalogue comprising, in 1781, 103
entries. 4
These preliminary attempts shrank into insignificance when
Herschel began to " sweep the heavens " with his giant tele-
scopes. In 1786 he presented to the Royal Society a de-
scriptive catalogue of 1000 nebulae and clusters, followed, three
years later, by a second of an equal number; to which he
added in 1802 a further gleaning of 500. On the subject of
their nature his views underwent a remarkable change. Find-
ing that his potent instruments resolved into stars many nebulous
patches in which no signs of such a structure had previously
been discernible, he naturally concluded that " resolvability "
was merely a question of distance and telescopic power. He
was (as he said himself) led on by almost imperceptible degrees
from evident clusters, such as the Pleiades, to spots without
a trace of stellar formation, the gradations being so well con-
nected as to leave no doubt that all these phenomena were
equally stellar. The singular variety of their appearance was
thus described by him :
" I have seen," he says, " double and treble nebulae variously
arranged ; large ones with small, seeming attendants ; narrow,
but much extended lucid nebulae or bright dashes ; some of
the shape of a fan, resembling an electric brush, issuing from
a lucid point; others of the cometic shape, with a seeming
1 Phil. Trans., vol. xxix. p. 390. 2 Mem. Ac. des Sciences, 1755.
3 Wolf, Gesch. d. Astr., p. 709.
4 Conn, des Temps, 1784 (pub. 1781), p. 227. A previous list of forty-
five had appeared in Mem. Ac. d. Sc. t 1771.'
FOUNDATION OF SIDEREAL ASTRONOMY. 29
nucleus in the centre, or like cloudy stars surrounded with a
nebulous atmosphere ; a different sort, again, contain a nebu-
losity of the milky kind, like that wonderful, inexplicable
phenomenon about Orionis ; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable
into stars." l
" These curious objects " he considered to be " no less than
whole sidereal systems," 2 some of which might "well outvie
our Milky Way in grandeur." He admitted, however, a wide
diversity in condition as well as compass. The system to which
our sun belongs he described as " a very extensive branching
congeries of many millions of stars, which most probably
owes its origin to many remarkably large, as well as pretty
closely scattered small stars, that may have drawn together the
rest." 3 But the continued action of this same "clustering
power " would, he supposed, eventually lead to the breaking up
of the original majestic Galaxy into two or three hundred
separate groups, already visibly gathering. Such minor neb-
ulae, due to the "decay" of other "branching nebulae" similar
to our own, he recognised by the score, lying, as it were,
stratified in certain quarters of the sky. " One of these nebu-
lous beds," he informs us, "is so rich, that in passing through
a section of it, in the time of only thirty-six minutes, I detected
no less than thirty-one nebulae, all distinctly visible upon a fine
blue sky." The stratum of Coma Berenices he judged to be
the nearest to our system of such layers ; nor did the marked
aggregation of nebulae towards both poles of the circle of the
Milky Way, escape his notice.
By a continuation of the same process of reasoning, he
was enabled (as he thought) to trace the life-history of neb-
ulae from a primitive loose and extended formation, through
clusters of gradually increasing compression, down to the
kind named by him " Planetary " because of the defined
and uniform discs which they present. These he regarded
1 Phil. Trans., vol. Ixxiv. p. 442. 2 Ibid. t vol. Ixxix. p. 213.
3 Ibid.) vol. Ixxv. p. 254.
30 HISTORY OF ASTRONOMY.
as "very aged, and drawing on towards a period of change
or dissolution." 1
"This method of viewing the heavens," he concluded,
" seems to throw them into a new kind of light. They now
are seen to resemble a luxuriant garden, which contains the
greatest variety of productions in different flourishing beds ;
and one advantage we may at least reap from it is, that we
can, as it were, extend the range of our experience to an
immense duration. For, to continue the simile which I have
borrowed from the vegetable kingdom, is it not almost the
same thing whether we live successively to witness the germi-
nation, blooming, foliage, fecundity, fading, withering, and cor-
ruption of a plant, or whether a vast number of specimens,
selected from every stage through which the plant passes
in the course of its existence, be brought at once to our
view?" 2
But already this supposed continuity was broken. After
mature deliberation on the phenomena presented by nebu-
lous stars, Herschel was induced, in 1791, to modify essentially
his original opinion.
" When I pursued these researches," he says, " I was in the
situation of a natural philosopher who follows the various
species of animals and insects from the height of their perfection
down to the lowest ebb of life ; when, arriving at the vegetable
kingdom, he can scarcely point out to us the precise boundary
where the animal ceases and the plant begins ; and may even
go so far as to suspect them not to be essentially different.
But, recollecting himself, he compares, for instance, one of the
human species to a tree, and all doubt upon the subject vanishes
before him. In the same manner we pass through gentle steps
from a coarse cluster of stars, such as the Pleiades . . . till
we find ourselves brought to an object such as the nebula
in Orion, where we are still inclined to remain in the once
adopted idea of stars exceedingly remote, and inconceivably
crowded, as being the occasion of that remarkable appearance.
1 Phil Trans., vol. Ixxix. p. 225. 2 Ibid., vol. Ixxix. p. 226.
r^'
UNI V*
FOUNDATION OF SIDEREAL ASTRONOMY. 31
It seems, therefore, to require a more dissimilar object to set
us right again. A glance like that of the naturalist, who casts
his eye from the perfect animal to the perfect vegetable, is
wanting to remove the veil from the mind of the astronomer.
The object I have mentioned above is the phenomenon that
was wanting for this purpose. View, for instance, the iQth
cluster of my 6th class, and afterwards cast your eye on this
cloudy star, and the result will be no less decisive than that
of the naturalist we have alluded to. Our judgment, I may
venture to say, will be that the nebulosity about the star is not of
a starry nature" 1
The conviction thus arrived at of the existence in space of a
widely diffused " shining fluid " (a conviction long afterwards
fully justified by the spectroscope), led him into a field of
endless speculation. What was its nature? Should it "be
compared to the coruscation of the electric fluid in the aurora
borealis? or to the more magnificent cone of the zodiacal
light?" Above all, what was its function in the cosmos?
And on this point he already gave a hint of the direction
in which his mind was moving by the remark that this
self-luminous matter seemed "more fit to produce a star
by its condensation, than to depend on the star for its exist-
ence." 2
This was not a novel idea. Tycho Brahe had tried to
explain the blaze of the star of 1572 as due to a sudden
concentration of nebulous material in the Milky Way, even
pointing out the space left dark and void by the withdrawal
of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same
track. But under Herschel's treatment the nebular origin of
stars first acquired the consistency of a formal theory. He
meditated upon it long and earnestly, and in two elaborate
treatises, published respectively in 1811 and 1814, he at length
set forth the arguments in its favour. These rested entirely
upon the "principle of continuity." Between the successive
1 Phil. Trans.) vol. Ixxxi. p. 72. z Jbid., vol. Ixxxi. p. 85.
32 HISTORY OF ASTRONOMY.
classes of his progressive assortment of objects, there was, as
he said, " perhaps not so much difference as would be in an
annual description of the human figure, were it given from the
birth of a child till he comes to be a man in his prime." 1
From diffused nebulosity, barely visible in the most powerful
light-gathering instruments, but which he estimated to cover
nearly 152 square degrees of the heavens, 2 to planetary
nebulae, supposed to be already centrally solid, instances
were alleged by him of every stage and phase of condensa-
tion. The validity of his reasoning, however, was evidently
impaired by his confessed inability to distinguish between the
dim rays of remote clusters and the milky light of true
gaseous nebulae.
It may be said that such speculations are futile in them-
selves, and necessarily barren of results. But they gratify an
inherent tendency of the human mind, and, if pursued in a
becoming spirit, should be neither reproved nor disdained.
Herschel's theory still exercises men's thoughts, and not
unworthily, although the testimony of recent discoveries with
regard to it is, at the best, hesitating and inconclusive. It
should be added, that it seems to have been propounded in
complete independence of Laplace's nebular hypothesis as to
the origin of the solar system. Indeed, it dated, as we have
seen, in its first inception, from 1791, while the French geo-
metrician's view was not advanced until 1796.
We may now briefly sum up the chief results of Herschel's
long years of "watching the heavens." The apparent motions
of the stars had been disentangled ; one portion being clearly
shown to be due to a translation towards a point in the con-
stellation Hercules of the sun and his attendant planets;
while a large balance of displacement was left to be accounted
for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to,
if not identical with, gravity, suns of every degree of size and
splendour, and sometimes brilliantly contrasted in colour, were
1 Phil. Trans., vol. ci. p. 271. 2 Ibid., vol. ci. p. 277.
FOUNDATION OF SIDEREAL ASTRONOMY. 33
seen to be held together in systems, consisting of two, three,
four, even six members, whose revolutions exhibited a wide
range of variety both in period and in orbital form. A new
department of physical astronomy was thus created, 1 and rigid
calculation for the first time made possible within the astral
region. : The vast problem of the arrangement and relations
of the millions of stars forming the Milky Way was shown to
be capable of experimental treatment, and of at least partial
solution, notwithstanding the inexhaustible variety and bound-
less complexity seen to prevail, to an extent previously un-
dreamt of, in the adjustments of that majestic system. The
existence of a luminous fluid, diffused through enormous tracts
of space, and intimately associated with stellar bodies, was
virtually demonstrated, and its place and use in creation
attempted to be divined by a bold, but plausible conjecture.
Change on a stupendous scale was observed to be every-
where in progress. One star 55 Herculis vanished, it might
be said, under the very eye of the astronomer, and other dis-
appearances were more than surmised; progressive ebbings
or flowings of light were indicated as probable in many stars
under no formal suspicion of variability ; forces were every-
where perceived to be at work, by which the very structure of
the heavens themselves must be slowly but fundamentally
modified. In all directions groups were seen to be formed or
forming; tides and streams of suns to be setting towards
powerful centres of attraction ; new systems to be in process
of formation, while effete ones hastened to decay or regenera-
tion when the course appointed for them by Infinite Wisdom
was run. And thus, to quote the words of the observer who
" had looked farther into space than ever human being did
before him," 2 "the state into which the incessant action of
the clustering power has brought the Milky Way at present, is
a kind of chronometer that may be used to measure the time
1 Sir J. Herschel, Phil. Trans. , vol. cxiv. part iii. p. I.
2 His own words to the poet Campbell, cited by Holden, Life and
Works, p. 109.
C
34 HISTORY OF ASTRONOMY.
of its past and future existence ; and although we do not know
the rate of going of this mysterious chronometer, it is never-
theless certain that since the breaking up of the parts of the
Milky Way affords a proof that it cannot last for ever, it
equally bears witness that its past duration cannot be admitted
to be infinite." l
1 Phil. Trans., vol. civ. p. 283.
( 35 )
CHAPTER II.
PROGRESS OF SIDEREAL ASTRONOMY.
WE have now to consider labours of a totally different character
from those of Sir William Herschel. Exploration and discovery
do not constitute the whole business of astronomy ; the less
adventurous, though not less arduous, task of gaining a more
and more complete mastery over the problems immemorially
presented to her, may, on the contrary, be said to form her
primary duty. A knowledge of the movements of the heavenly
bodies has, from the earliest times, been demanded by the
urgent needs of mankind ; and science finds prosperity, as in
many cases it has taken its origin, in condescension to practical
claims. Indeed, to bring such knowledge as near as possible
to absolute precision has been denned by no mean authority l
as the true end of astronomy.
Several causes concurred about the beginning of the present
century to give a fresh and powerful impulse to investigations
having this end in view. The rapid progress of theory almost
compelled a corresponding advance in observation; instru-
mental improvements rendered such an advance possible ;
Herschel's discoveries quickened public interest in celestial
inquiries ; royal, imperial, and grand-ducal patronage widened
the scope of individual effort. The heart of the new movement -
was in Germany. Hitherto the observatory of Flamsteed and
Bradley had been the acknowledged centre of practical
astronomy; Greenwich observations were the standard of
1 Bessel, Populdre Vorlesungen, pp. 6, 408.
36 HISTORY OF ASTRONOMY.
reference all over Europe ; and the art of observing prospered
in direct proportion to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of
Astronomer Royal during forty-six years (from 1765 to 1811),
was no unworthy successor to the eminent men who had gone
before him. His foundation of the Nautical Almanac (in
1767) alone constitutes a valid title to fame; he introduced at
the Observatory the important innovation of the systematic
publication of results and the careful and prolonged series of
observations executed by him formed the basis of the improved
theories, and corrected tables of the celestial movements, which
were rapidly being brought to completion abroad. But he had
in him no stirrings of the future. He was fitted rather to
continue a tradition than to found a school. The old ways
were dear to him; and, indefatigable as he was, a definite
purpose was wanting to compel him, by its exigencies, along
the path of progress. Thus, for almost fifty years after
Bradley's death, the acquisition of a small achromatic 1 was
the only notable change made in the instrumental equipment
of the Observatory. The transit, the zenith sector, and the
mural quadrant, with which Bradley had done his incomparable
work, retained their places long after they had become de-
teriorated by time, and obsolete by the progress of invention ;
and it was not until the very close of his career that Maskelyne,
compelled by Pond's detection of serious errors, ordered a
Troughton's circle, which he did not live to employ.
Meanwhile, the heavy national disasters with which Germany
was overwhelmed in the early part of the present century,
seemed to stimulate, rather than impede the intellectual revi-
val already for some years in progress there. Astronomy was
amongst the first of the sciences to feel the new impulse. By
the efforts of Bode, Olbers, Schroter, and Von Zach, just and
elevated ideas on the subject were propagated, intelligence was
diffused, and a firm ground prepared for common action in
mutual sympathy and disinterested zeal. They were powerfully
1 Fitted to the old transit instrument, July n, 1772.
PROGRESS OF SIDEREAL ASTRONOMY. 37
seconded by the foundation, in 1804, by a young artillery officer
'named VonRejchenbach,of an Optical and Mechanical Institute
at Munich. Here the work of English instrumental artists
was for the first time rivalled, and that of English opticians
when Fraunhofer entered the new establishment far surpassed.
The development given to the refracting telescope by this
extraordinary man was indispensable to the progress of that
fundamental part of astronomy which consists in the exact
determination of the places of the heavenly bodies. Reflec-
tors are brilliant engines of discovery, but they lend themselves
with difficulty to the prosaic work of measuring right ascensions
and polar distances. A signal improvement in the art of making
and working flint-glass thus most opportunely coincided with
the rise of a German school of scientific mechanicians, to fur-
nish the instrumental means needed for the reform which was at
hand. Of the leader of that reform it is now time to speak.
Friedrich Wilhelm Bessel was born at Minden in Westphalia,
July 22, 1784. A certain taste for figures, coupled with a still
stronger distaste for the Latin accidence, directed his incli-
nation and his father's choice towards a mercantile career.
In his fifteenth year, accordingly, he entered the house of
Kuhlenkamp & Sons, in Bremen, as an apprenticed clerk.
He was now thrown completely upon his own resources.
From his father, a struggling Government official, heavily
weighted with a large family, he was well aware that he had
nothing to expect ; his dormant faculties were roused by the
necessity for self-dependence, and he set himself to push
manfully forward along the path that lay before him. The
post of supercargo on one of the trading expeditions sent out
from the Hanseatic towns to China and the East Indies, was
the aim of his boyish ambition, for the attainment of which
he sought to qualify himself by the industrious acquisition of
suitable and useful knowledge. He learned English in two or
three months ; picked up Spanish with the casual aid of a
gunsmith's apprentice ; studied the geography of the distant
lands which he hoped to visit; collected information as to
38 HISTORY OF ASTRONOMY.
their climates, inhabitants, products, and the courses of trade.
He desired to add some acquaintance with the art (then much
neglected) of taking observations at sea ; and thus, led on from
navigation to astronomy, and from astronomy to mathematics,
he groped his way into a new world.
It was characteristic of him that the practical problems ot
science should have attracted him before his mind was as yet
sufficiently matured to feel the charm of its abstract beauties.
His first attempt at observation was made with a sextant, rudely
constructed under his own directions, and a common clock.
Its object was the determination of the longitude of Bremen,
and its success, he tells us himself, 1 filled him with a rapture
of delight, which, by confirming his tastes, decided his destiny.
He now eagerly studied B ode's Jahrbuch and Von Zach's
Monatliche Correspondenz, overcoming each difficulty as it
arose with the aid of Lalande's Traite d' Astronomic^ and
supplying, with amazing rapidity, his early deficiency in ma-
thematical training. In two years he was able to attack a
problem which would have tasked the patience, if not the
skill, of the most experienced astronomer. Amongst the Earl
of Egremont's papers, Von Zach had discovered Harriot's
observations on Halley's comet at its appearance in 1607, and
published them as a supplement to Bode's Annual. With an
elaborate care inspired by his youthful ardour, though hardly
merited by their loose nature, Vessel deduced from them an
orbit for that celebrated body, and presented the work to
Olbers, whose reputation in cometary researches gave a special
fitness to the proffered homage. The benevolent physician-
astronomer of Bremen welcomed with surprised delight such a
performance emanating from such a source. Fifteen years
before, the French Academy had crowned a similar perfor-
mance ; now its equal was produced by a youth of twenty,
busily engaged in commercial pursuits, self-taught, and obliged
to snatch from sleep the hours devoted to study. The paper
was immediately sent to Von Zach for publication, with a note
1 Briefwechsd mit Olbers , p. xvi.
PROGRESS OF SIDEREAL ASTRONOMY. 39
from Olbers explaining the circumstances of its author, and
the name of Bessel became the common property of learned
Europe.
He had, however, as yet no intention of adopting astronomy
as his profession. For two years he continued to work in the
counting-house by day, and to pore over the Mecanique Celeste
and the Differential Calculus by night. But the post of assis-
tant in Schroter's observatory at Lilienthal having become
vacant by the removal of Harding to Gottingen in 1805, Olbers
procured for him the offer of it. It was not without a struggle
that he resolved to exchange the desk for the telescope. His
reputation with his employers was of the highest; he had
thoroughly mastered the details of the business, which his
keen practical intelligence followed with lively interest; his
years of apprenticeship were on the point of expiring, and
an immediate, and not unwelcome prospect of comparative
affluence lay before him. The love of science, however, pre-
vailed ; he chose poverty and the stars, and went to Lilien-
thal with a salary of a hundred thalers yearly. Looking back
over his life's work, Olbers long afterwards declared that the
greatest service which he had rendered to astronomy was that of
having discerned, directed, and promoted the genius of Bessel. 1
For four years he continued in Schroter's employment. At
the end of that time the Prussian Government chose him to
superintend the erection of a new observatory at Konigsberg,
which, after many vexatious delays caused by the prostrate
condition of the country, was finished towards the end of 1813.
Konigsberg was the first really efficient German observatory.
It became, moreover, a centre of improvement, not for Germany
alone, but for the whole astronomical world. During two-and-
thirty years it was the scene of Bessel's labours, and BesseFs
labours had for their aim the reconstruction, on an amended
and uniform plan, of the entire science of observation.
A knowledge of the places of the stars is the foundation of
astronomy. 2 Their configuration lends to the skies their
1 R. Wolf, Gesch. der Astron., p. 518. 2 Bessel, Pop. VorL> p. 22.
40 HISTORY OF ASTRONOMY.
distinctive features, and marks out the shifting tracks of more
mobile objects with relatively fixed, and generally unvarying
points of light. A more detailed and accurate acquaintance
with the stellar multitude, regarded from a purely uranogra-
phical point of view, has accordingly formed at all times a
primary object of celestial science, and has, during the present
century, been cultivated with a zeal and success by which all
previous efforts are dwarfed into insignificance. In Lalande's
Histoire Celeste, published in 1801, the places of no less than
47,390 stars were given, but in the rough, as it were, and conse-
quently needing laborious processes of calculation to render
them available for exact purposes. Piazzi set an example of
improved methods of observation, resulting in the publication,
in 1803 and 1814; of two catalogues of about 7000 stars, which
for their time were models of what such works should be.
Stephen Groombridge at Blackheath was similarly and most
beneficially active. But something more was needed than the
diligence of individual observers. A systematic reform was
called for ; and it was this which Bessel undertook and carried
through.
Direct observation furnishes only what has been called the
"raw material" of the positions of the heavenly bodies. 1 A
number of highly complex corrections have to be applied before
their mean can be disengaged from their apparent places on
the sphere. Of these, the most considerable and familiar is
atmospheric refraction, by which objects seem to stand higher
in the sky than they in reality do, the effect being evanescent
at the zenith, and attaining, by gradations varying with condi-
tions of pressure and temperature, a maximum at the horizon.
Moreover, the points from which measurements are taken are
themselves in motion, either continually in one direction, or
periodically to and fro. The precession of the equinoxes is
slowly progressive, or rather retrogressive ; the nutation of the
pole oscillatory in a period of about eighteen years. Added to
which, the successive transmission of light, combined with the
1 Bessel, Pop. Vorl, p. 440.
PROGRESS OF SIDEREAL ASTRONOMY. 41
movement of the earth in its orbit, causes a minute displacement
known as aberration.
Now it is easy to see that any uncertainty in the application
of these corrections saps the very foundations of exact astronomy.
Extremely minute quantities, it is true, are concerned ; but the
life and progress of modern celestial science depends upon the
sure recognition of extremely minute quantities. In the early
years of this century, however, no uniform system of "reduction"
(so the complete correction of observational results is termed)
had been established. Much was left to the individual cap-
rice of observers, who selected for the several " elements " of
reduction such values as seemed best to themselves. Hence
arose much hurtful confusion, tending to hinder united action
and mar the usefulness of laborious researches. For this state
of things, Bessel, by the exercise of consummate diligence, saga-
city, and patience, provided an entirely satisfactory remedy.
His first step was an elaborate investigation of the precious
series of observations made by Bradley at Greenwich from 1750
until his death in 1762. The catalogue of 3222 stars which
he extracted from them, gave the earliest example of the
systematic reduction on a uniform plan of such a body of work.
It is difficult, without entering into details out of place in a
volume like the present, to convey an idea of the arduous nature
of this task. It involved the formation of a theory of the errors
of each of Bradley's instruments, and a difficult and delicate
inquiry into the true value of each correction to be applied
before the entries in the Greenwich journals could be developed
into a finished and authentic catalogue. Although completed
in 1813, it was not until five years later that the results appeared
with the proud, but not inappropriate title of Fundamenta
Astronomic. The eminent value of the work consisted in
this, that by providing a mass of entirely reliable information
as to the state of the heavens at the epoch 1755, it threw back
the beginning of exact astronomy almost half a century. By
comparison with Piazzi's catalogues the amount of precession
was more accurately determined, the proper motions of a
42 HISTORY OF ASTRONOMY.
considerable number of stars became known with certainty, and
definite prediction the certificate of initiation into the secrets
of Nature at last became possible as regards the places of the
stars. Bessel's final improvements in the methods of reduction
were published in 1830 in his Tabula Regiomontancz. They
not only constituted an advance in accuracy, but afforded
a vast increase of facility in application, and were at once
and everywhere adopted. Thus astronomy became a truly
universal science ; uncertainties and disparities were banished,
and observations made at all times and places rendered
mutually comparable. 1
More, however, yet remained to be done. In order to
verify with greater strictness the results drawn from the Bradley
and Piazzi catalogues, a third term of comparison was wanted,
and this Bessel undertook to supply. By a course of 75,011
observations, executed during the years 1821-33, w ^ tn tne
utmost nicety of care, the number of accurately known stars
was brought up to above 50,000, and an ample store of
trustworthy facts laid up for the use of future astronomers. In
this department Argelander, whom he attracted from finance
to astronomy, and trained in his own methods, was his assis-
tant and successor. The great " Bonn Durchmusterung," ' 2 in
which 324,198 stars visible in the northern hemisphere are
enumerated, and the corresponding "Atlas," published in
1857-63, constituting a picture of our sidereal surroundings
of heretofore unapproached completeness, may be justly said
to owe their origin to Bessel's initiative, and to form a sequel
to what he commenced.
But his activity was not solely occupied with the promotion
of a comprehensive reform in astronomy ; it embraced special
problems as well. The long-baffled search for a parallax of
the fixed stars was resumed with fresh zeal as each mechanical
or optical improvement held out fresh hopes of a successful
1 Durege, BesseVs Leben und Wirkert^ p. 28.
2 Banner Beobachtungen> Bd. iii.-v. 1859-62.
PROGRESS OF SIDEREAL ASTRONOMY. 43
issue. Illusory results abounded. Piazzi in 1805 perceived,
as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon ; the truth being that his
instruments were worn out with constant use, and could no
longer be depended upon. 1 His countryman, Calandrelli, was
similarly deluded. The celebrated controversy between the
Astronomer Royal and Dr. Brinkley, director of the Dublin
College Observatory, turned on the same subject. Brinkley,
who was in possession of a first-rate meridian-circle, believed
himself to have discovered relatively large parallaxes for four
of the brightest stars ; Pond, relying on the testimony of the
Greenwich instruments, asserted their nullity. The dispute
was protracted for fourteen years, from iSioto 1824, and was
brought to no definite conclusion ; but the strong presumption
on the negative side was abundantly justified in the event
There was good reason for incredulity in the matter of
parallaxes. Announcements of their detection had become so
frequent as to be discredited before they were disproved ;
and Struve, who made an investigation of the subject at
Dorpat in 1818-21, had clearly shown that the quantities con-
cerned were so small as to lie beyond the reliable measuring
powers of any instrument then in use. Already, however, the
means were being prepared of giving to those powers a large
increase.
On the 2ist July 1801, two old houses in an alley of Munich
tumbled down, burying in their ruins the occupants, of whom
one alone was extricated alive, though seriously injured. This
was an orphan lad of fourteen, named Joseph Fraunhofer.
The Elector Maximilian Joseph was witness of the scene,
became interested in the survivor, and consoled his misfortune
, with a present of eighteen ducats. Seldom was money better
bestowed or better employed. Part of it went to buy books
and a glass-polishing machine, with the help of which young
Fraunhofer studied mathematics and optics, and secretly
exercised himself in the shaping and finishing of lenses ; the
1 Bessel, Pop. Vorl., p. 238.
44 HISTORY OF ASTRONOMY.
remainder purchased his release from the tyranny of one
Weichselberger, a looking-glass maker by trade, to whom he
had been bound apprentice on the death of his parents. A
period of struggle and privation followed, during which, how-
ever, he rapidly extended his acquirements; and was thus
eminently fitted for the task awaiting him, when, in 1806, he
entered the optical department of the establishment founded
two years previously by Von Reichenbach and Utzschneider.
He now zealously devoted himself to the improvement of the
achromatic telescope ; and after prolonged study of the theory
of lenses, and many toilsome experiments in the manufacture
of flint-glass, he succeeded in perfecting, December 12, 1817,
an object-glass of exquisite quality and finish, 9^ inches in
diameter, and of fourteen feet focal length.
This (as it was then considered) gigantic lens was secured
by Struve for the Russian Government, and the " great Dorpat
refractor " the first of the large achromatics which have played
such an important part in modern astronomy was, late in
1824, set up in the place which it still occupies. By inge-
nious improvements in mounting and fitting, it was adapted to
the finest micrometrical work, and thus offered unprecedented
facilities both for the examination of double stars (in which
Struve chiefly employed it), and for such subtle measurements
as might serve to reveal or disprove the existence of a sensible
stellar parallax. Fraunhofer, moreover, constructed for the
observatory of Konigsberg the first really available heliometer.
The principle of this instrument (termed with more propriety
a "divided object-glass micrometer") is the separation, by a
strictly measurable amount, of two distinct images of the same
object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided, are
shifted until the upper star (say) in one image is brought into
coincidence with the lower star in the other, when their distance
apart becomes known by the amount of motion employed. 1
1 The heads of the screws applied to move the halves of the object-
glass^ in the Konigsberg heliometer are of so considerable a size that a
PROGRESS OF SIDEREAL ASTRONOMY. 45
This virtually new engine of research was delivered and
mounted in 1829, three years after the termination of the life
of its deviser. The Dorpat lens had brought to Fraunhofer a
title of nobility and the sole management of the Munich Optical
Institute (completely separated since 1814 from the mechanical
department). What he had achieved, however, was but a small
part of what he meant to achieve. He saw before him the
possibility of nearly quadrupling the light-gathering capacity
of the great achromatic acquired by Struve ; he meditated im-
provements in reflectors as important as those he had already
effected in refractors ; and was besides eagerly occupied with
investigations into the nature of light, the momentous character
of which we shall by-and-bye have an opportunity of estimating.
But his health was impaired, it is said, from the weakening
effects of his early accident combined with excessive and
unwholesome toil, and, still hoping for its restoration from a
projected journey to Italy, he died of consumption, June 7,
1826, aged thirty-nine years. His tomb in Munich bears the
concise eulogy, Approximavit sidera.
Bessel had no sooner made himself acquainted with the
exquisite defining powers of the Konigsberg heliometer, than he
resolved to employ them in an attack upon the now secular
problem of star-distances. But it was not until 1837 that he
found leisure to pursue the inquiry. In choosing his test-star
he adopted a new principle. It had hitherto been assumed
that our nearest neighbours in space must be found amongst
the brightest ornaments of our skies. The knowledge of stellar
proper motions afforded by the critical comparison of recent
with earlier star-places, suggested a different criterion of dis-
tance. It is impossible to escape from the conclusion that the
apparently swiftest-moving stars are, on the whole, also the
nearest to us, however numerous the individual exceptions to
the rule. Now, as early as I792, 1 Piazzi had noted as an
thousandth part of a revolution, equivalent to -^th of a second of arc, can be
measured with the utmost accuracy. Main in R. A. S. Mem., vol. xii. p. 53.
1 Specola Astronomica di Palermo, lib. vi. p. 10, note.
46 HISTORY OF ASTRONOMY.
indication of relative vicinity to the earth, the unusually large
proper motion (5.2" annually) of a double star of the fifth
magnitude in the constellation of the Swan. Still more
emphatically in I8I2 1 Bessel drew the attention of astro-
nomers to the fact, and 61 Cygni became known as the
" flying star." The seeming rate of its flight, indeed, is of so
leisurely a kind, that in a thousand years it will have shifted
its place by less than 3j lunar diameters, and that a quarter of
a million would be required to carry it round the entire circuit
of the visible heavens. Nevertheless, it has few rivals in
rapidity of movement, the apparent displacement of the vast
majority of stars being, by comparison, almost insensible.
This interesting, though inconspicuous object, then, was
chosen by Bessel to be put to the question with his heliometer,
while Struve made a similar, and somewhat earlier trial with
the bright gem of the Lyre, whose Arabic title of the " Falling
Eagle" survives as a time-worn remnant in "Vega." Both
astronomers agreed to use the "differential" method, for
which their instruments and the vicinity to their selected stars
of minute, physically detached companions offered special
facilities. In the last month of 1838 Bessel made known the
result of one year's observations, showing for 61 Cygni a
parallax of about a third of a second (0.3 136"). 2 He then
had his heliometer taken down and repaired, after which he
resumed the inquiry, and finally terminated a series of 402
measures in March i84o. 3 The resulting parallax of 0.3483"
(corresponding to a distance about 600,000 times that of the
earth from the sun), seemed to be ascertained beyond the
possibility of cavil, and is memorable as the first published
instance of the fathom-line, so industriously thrown into celes-
tial space, having really and indubitably touched bottom. It
1 Monatliche Correspondenz, vol. xxvi. p. 162.
2 Astronomische Nachrichten, Nos. 365-366. It should be explained
that what is called the " annual parallax " of a star is only half its apparent
displacement. In other words, it is the angle subtended at the distance of
that particular star by the radius of the earth's orbit.
3 Astr. Nach., Nos. 401-402.
PROGRESS OF SIDEREAL ASTRONOMY. 47
was confirmed in 1842-43 with curious exactness by Peters at
Pulkowa ; but the latest researches show that it requires to be
increased to just half a second. 1
Struve's measurements inspired less confidence. They ex-
tended over three years (1835-38), but were comparatively few,
and were frequently interrupted. Nevertheless the parallax
of about a quarter of a second (0.26 13") which he derived from
them for a Lyrse, and announced in 1840,2 has proved real,
though somewhat excessive. 3
Meanwhile a result of the same kind, but of a more striking
character than either Bessel's or Struve's, had been obtained,
one might almost say casually, by a different method and in a
distant region. Thomas Henderson, originally an attorney's
clerk in his native town of Dundee, had become known for
his astronomical attainments, and was appointed in 1831 to
direct the recently completed observatory at the Cape of Good
Hope. He began observing in April 1832, and the serious
shortcomings of his instrument notwithstanding, executed dur-
ing the thirteen months of his tenure of orifice a surprising
amount of first-rate work. With a view to correcting the
declination of the lustrous double star a Centauri (which ranks
after Sirius and Canopus as the third brightest orb in the
heavens), he effected a number of successive determinations of
its position, and on being informed of its very considerable
proper motion (3.6" annually), he resolved to examine the
observations already made for possible traces of parallactic
displacement. This was done on his return to Scotland, where
he filled the office of Astronomer Royal from 1834 until his
premature death in 1844. The result justified his expectations.
From the declination measurements made at the Cape and
duly reduced, a parallax of about one second of arc clearly
emerged (diminished by Gill's and Elkin's observations, 1882-
1 Dr. Ball's measurements at Dunsink give a parallax of 0.47" for 61
Cygni ; Professor A. Hall's at Washington, 0.48".
2 Addilamentum in Mensuras Micro melricas^ p. 28.
3 Professor Hall in 1 88 1 found the parallax of Vega = 0.18".
4 8 HISTORY OF ASTRONOMY.
1883, to 0.75''), but, by perhaps an excess of caution, was
withheld from publication until fuller certainty was afforded
by the concurrent testimony of Lieutenant Meadows' deter-
minations of the same star's right ascension. 1 When at last,
January 9, 1839, Henderson communicated his discovery to
the Astronomical Society, he could no longer claim the priority
which was his due. Bessel had anticipated him with the par-
allax of 6 1 Cygni by just two months.
Thus from three different quarters, three successful, and
almost simultaneous assaults were delivered upon a long-
beleaguered citadel of celestial secrets. The same work has
since been steadily pursued, with the general result of showing
that, as regards -their overwhelming majority, the stars are far
too remote to show even the slightest trace of optical shift-
ing from the revolution of the earth in its orbit. In about a
score of cases, however, small parallaxes have been determined,
some certainly (that is, within moderate limits of error), others
more or less precariously. The list is an instructive one, in
its omissions no less than in its contents. It includes stars of
many degrees of brightness, from Sirius down to a nameless
telescopic star in the Great Bear; 2 yet the vicinity of this
minute object is so much greater than that of the brilliant
Arcturus, that the latter transported to its place would increase
in lustre fifteen times. Moreover, by far the greater number
of the brightest stars are found to have no sensible parallax,
while most of those ascertained to be nearest to the earth are
of fifth, sixth, even ninth magnitudes. The obvious conclu-
sions follow that the range of variety in the sidereal system is
enormously greater than had been supposed, and that estimates
of distance based upon apparent magnitude must be wholly
futile. To which we may add the probable inference of a real
preponderance of small stars over large that is, of bodies
1 Mem. Roy. Astr. Soc., vol. xi. p. 61.
3 That numbered 21,185 in Lalande's Hist. Cel., found by Argelander
to have a proper motion of 4.734", and by Winnecke a parallax of 0.511".
Month. Not., vol. xviii. p. 289.
PROGRESS OF SIDEREAL ASTRONOMY. 49
inferior to our sun in size and lustre over such giants as Sirius,
Arcturus, Aldebaran, and Capella. At the same time, both the
so-called "optical" and " geometrical" methods of relatively
estimating star-distances are seen to have a foundation of fact,
although so disguised by complicated relations as to be of very
doubtful individual application. On the whole, the chances
are in favour of the superior vicinity of a bright star over a
faint one; and, on the whole, the stars in swiftest apparent
motion are amongst those whose actual remoteness is least.
Indeed, there is no escape from either conclusion, unless on
the supposition of special arrangements in themselves highly
improbable, and, we may confidently say, non-existent.
The distances even of the few stars found to have measure-
able parallaxes are on a scale entirely beyond the powers of
the human mind to conceive. In the attempt both to realise
them distinctly, and to express them conveniently, a new unit
of length, itself of bewildering magnitude, has originated. This
is what we may call the light-journey of one year. The subtle
vibrations of the ether, propagated on all sides from the surface
of luminous bodies, travel at the rate of 186,300 miles a second,
or (in round numbers) six billions of miles a year. Four and
a third such measures are needed to span the abyss that
separates us from the nearest fixed star. In other words, light
takes four years and four months to reach the earth from
Centauri ; yet a Centauri lies some ten billions of miles
nearer to us (so far as is yet known) than any other member
of the sidereal system !
The determination of parallax leads, in the case of binary
systems, to the determination of mass ; for the distance from
the earth of the two bodies forming such a system being ascer-
tained, the seconds of arc apparently separating them from
each other can be at once translated into millions of miles ;
and we only need to add a knowledge of their period to enable
us, by an easy sum in proportion, to find their combined mass
in terms of that of the sun. Thus, since according to the
elements published by Dr. Elkin in 1880 the two stars form-
D
5 o HISTORY OF ASTRONOMY.
ing a Centauri revolve round their common centre of gravity
at a mean distance 23 J times the radius of the earth's orbit, in
a period of 77 J years, the attractive force of the two together
must be fully twice the solar. We may gather some idea of
their relations by placing in imagination, a second luminary
like our sun in circulation between the orbits of Uranus and
Neptune. But systems of still more majestic proportions lie
buried in distance impenetrable by our unaided sight. A
minute double star in the constellation Eridanus, for which
Dr. Gill has detected a small parallax, appears to be more
than thrice as massive as the central orb of our world, while
6 1 Cygni affords an instance of a binary combination in
which only a . fractional part of the solar gravitating power
resides.
Further, the actual rate of proper motions, so far as regards
that part of them which is projected upon the sphere, can be
ascertained for stars at known distances. The annual journey,
for instance, of 61 Cygni across the line of sight amounts to
1000, and that of a Centauri to 436 millions of miles. A
small star, numbered 1830 in Groombridge's Circumpolar Cata-
logue, "devours the way " at the rate of 200 miles a second
a speed, in Newcomb's opinion, beyond the gravitating power
of the entire sidereal system to control ; and Tucanae pos-
sesses, according to Dr. Gill, just half that amazing velocity,
besides whatever movement each may have towards or from
the earth, of which the spectroscope may eventually give an
account.
Herschel's conclusion as to the movement of the sun among
the stars was not admitted as valid by the most eminent of his
successors. Bessel maintained that there was absolutely no
preponderating evidence in favour of its supposed direction
towards a point in the constellation Hercules. 1 Biot, Burck-
hardt, even Herschel's own son, shared his incredulity. But
the appearance of Argelander's prize-essay in 1837 2 changed
the aspect of the question. Herschel's first memorable solution
1 Fund. Astr., p. 309. 2 Mem. Pres. a I' Ac. de St. Peter -sb. t t. iii.
PROGRESS OF SIDEREAL ASTRONOMY. 51
in 1783 was based upon the proper motions of thirteen stars,
imperfectly known ; his second, in 1805, u P n those of no more
than six. Argelander now obtained an entirely concordant result
from the large number of 390, determined with the scrupulous
accuracy characteristic of Bessel's work .and his own. The
reality of the fact thus persistently disclosed could no longer
be doubted ; it was confirmed five years later by the younger
Struve, and still more strikingly in 1847 1 by Galloway's investi-
gation, founded exclusively on the apparent displacements of
southern stars. In 1860, Mr. (now Sir George) Airy and Mr.
Dunkin, 2 employing all the resources of modern science, and
commanding the wealth of material furnished by 1167 pro-
per motions carefully determined by Mr. Main, reached a con-
clusion closely similar to that indicated nearly eighty years
previously by the first great sidereal astronomer ; which Mr.
Plummer's reinvestigation of the subject in 1883 served but
slightly to modify. The general direction of the solar move-
ment may thus be regarded as known ; but as to its rate, the
grounds of inference are much less satisfactory. Otto Struve's
estimate of 154 million miles a year is based upon the assump-
tion of an average annual parallax, for stars of the first mag-
nitude, of about a quarter of a second ; and since only five out
of eighteen stars< of the first magnitude appear to have any
measurable parallax, it is obvious that it merits a very restricted
confidence.
As might have been expected, speculation has not been idle
regarding the purpose and goal of the strange voyage of
discovery through space upon which our system is embarked ;
but altogether fruitlessly. The variety of the conjectures
hazarded in the matter is in itself a measure of their futility.
Long ago, before the construction of the heavens had as yet
been made the subject of methodical inquiry, Kant was
disposed to regard Sirius as the "central sun" of the Milky
Way ; while Lambert surmised that the vast Orion nebula
1 Phil. Trans., vol. cxxxvii .p. 79.
5 Mem. Roy. Astr. Soc., vol. xxviii. 1860, and Month. A T ot., vol.xxiii. p. 168.
52 HISTORY OF ASTRONOMY.
might serve as the regulating power of a subordinate group
including our sun. Herschel threw out the hint that the great
cluster in Hercules (estimated to include 14,000 stars) might
prove to be the supreme seat of attractive force ; 1 Argelander
placed his central body in the constellation Perseus; 2 Fomalhaut,
the brilliant of the Southern Fish, was set in the post of honour
by Boguslawski of Breslau. Madler (who succeeded Struve at
Dorpat in 1839) concluded from a more formal inquiry that
the ruling power in the sidereal system resided, not in any
single preponderating mass, but in the centre of gravity of the
self-controlled revolving multitude. 3 In the former case (as we
know from the example of the planetary scheme), the stellar
motions would be most rapid near the centre ; in the latter,
they would become accelerated with remoteness from it. 4
Madler showed that no part of the heavens could be indicated
as a region of exceptionally swift movements, such as would
result from the vicinity of a gigantic (though possibly obscure)
ruling body; but that a community of extremely sluggish
movements undoubtedly existed in, and in the neighbourhood
of the group of the Pleiades, where, accordingly, he placed the
centre of gravity of the Milky Way. 5 The bright star Alcyone
thus became the " central sun," but in a purely passive sense,
its headship being determined by its situation at the point
of neutralisation of opposing tendencies, and of consequent
rest. The solar period of revolution round this point was, by an
avowedly conjectural method, fixed at 18,200,000 years, imply-
ing, on the extremely hazardous supposition that the distance of
Alcyone is thirty-four million times that of the earth from the
sun, a velocity for our system of about thirty miles a second.
1 Phil. Trans., vol. xcvi. p. 230.
3 Mem. Pres. a fAc. de St. Petersbourg, t. iii. p. 603 (read Feb. 5, 1837).
3 Die Centralsonnt, Astr. Nach., Nos. 566-567, 1846.
4 Sir J. Herschel, note to Treatise on Astronomy, and Phil. Trans., vol.
cxxiii. part ii. p. 502.
5 The position is (as Sir J. Herschel pointed out, Outlines of Astronomy,
p. 631, loth ed.) placed beyond the range of reasonable probability by its
remoteness (fully 26) from the galactic plane.
PROGRESS OF SIDEREAL ASTRONOMY. 53
The scheme of sidereal government framed by the Dorpat
astronomer was, it may be observed, of the most approved con-
stitutional type ; deprivation, rather than increase of influence
accompanying the office of chief dignitary. But while we are
still ignorant, and shall perhaps ever remain so, of the funda-
mental plan upon which the Galaxy is organised, recent investi-
gations tend more and more to exhibit it, not as monarchical
(so to speak), but as federative. The community of proper
motions detected by Madler in the vicinity of the Pleiades
may accordingly possess a significance altogether different from
what he imagined.
Bessel's so-called " foundation of an Astronomy of the
Invisible" now claims attention. 1 His prediction regarding
the planet Neptune does not belong to the present division
of our subject ; a strictly analogous discovery in the sidereal
system was, however, also very clearly foreshadowed by him.
His earliest suspicions of non-uniformity in the proper motion
of Sirius dated from 1834 ; they extended to Procyon in 1840 ;
and after a series of refined measurements with the new
Repsold circle, he announced, in 1844, his conclusion that
these irregularities were due to the presence of obscure bodies
round which the two bright Dog-stars revolved as they pursued
their way across the sphere. 2 He even assigned to each an
approximate period of half a century. "I adhere-to the con-
viction," he wrote later to Humboldt, "that Procyon and
Sirius form real binary systems, consisting of a visible and an
invisible star. There is no reason to suppose luminosity an
essential quality of cosmical bodies. The visibility of count-
less stars is no argument against the invisibility of countless
others." 3
An inference so contradictory to received ideas obtained
little credit, until Peters found, in 1851,* that the apparent
anomalies in the movements of Sirius could be completely
1 Madler in Westermann's Jahrbuch, 1867, p. 615.
2 Letter from Bessel to Sir J. Herschel, Month. Not., vol. vi. p. 139.
3 Wolf, Gesch. d. Astr., p. 743, note. 4 Astr. Nach,, Nos. 745-748.
54 HISTORY OF ASTRONOMY.
explained by an orbital revolution in a period of fifty years.
Bessel's prevision was destined to be still more triumphantly
vindicated. On the 3ist of January 1862, while in the act of
trying a new 1 8-inch refractor, Alvan Clark, jun. (one of the
celebrated firm of American opticians), actually discovered
the hypothetical Sirian companion in the precise position re-
quired by theory. It has now been watched through just half a
revolution (period 49.4 years), and unless there should prove to
be other bodies concerned in disturbing the motion of Sirius
must be very slightly luminous in proportion to its mass. Its
attractive power, in fact, is about half that of its primary, while
it emits only y^^th of its light. Sirius itself, on the other
hand, possesses a far higher radiative intensity than our sun.
It gravitates admitting Dr. Gill's parallax of 0.38 " to be exact
like three suns, but shines like seventy. Possibly it is enor-
mously distended by heat, and undoubtedly its atmosphere
intercepts a very much smaller proportion of its light than in
stars of the solar class. As regards Procyon, visual verifica-
tion is still wanting, but to the mental eye the presence of a
considerable disturbing mass is fully assured by the inquiry
instituted by Auwers in 1862. l A period of forty years is
assigned by him to the system.
But Bessel was not destined to witness the recognition of
" the invisible " as a legitimate and profitable field for astro-
nomical research. He died March 17, 1846, just six months
before the discovery of Neptune, of an obscure disease,
eventually found to be occasioned by an extensive fungus-
growth in the stomach. The place which he left vacant was
not one easy to fill. Rarely indeed shall we find one who
reconciled with the same success the claims of theoretical and
practical astronomy, or surveyed the science which he had
made his own with a glance equally comprehensive, practical,
and profound.
The career of Friedrich Georg Wilhelm Struve illustrates
the maxim that science differentiates as it develops. He
1 Astr. Nach., Nos. 1371-1373.
PROGRESS OF SIDEREAL ASTRONOMY. 55
might be called a specialist in double stars. His earliest re-
corded use of the telescope was to verify HerschePs conclusion
as to the revolving movement of Castor, and he never varied
from the predilection which this first observation at once indi-
cated and determined. He was born at Altona, of a respect-
able yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He
then turned to science, was appointed in 1813 to a professor-
ship of astronomy and mathematics, and began regular work
in the Dorpat Observatory just erected by Parrot for Alexander I.
It was not, however, until 1819 that the acquisition of a
5-foot refractor by Troughton enabled him to take the position-
angles of double stars with regularity and tolerable precision.
The resulting catalogue of 795 stellar systems gave the signal
for a general resumption of the Herschelian labours in this
branch. The extraordinary facilities for observation afforded
by the Fraunhofer achromatic encouraged him to undertake,
February n, 1825, a review of the entire heavens down to 15
south of the celestial equator, which occupied more than two
years, and yielded, from an examination of above 120,000 stars,
a harvest of about 2200 previously unnoticed composite objects.
The ensuing ten years were devoted to delicate and patient
measurements, the results of which were embodied in Mensurce
Micrometrica, published at St. Petersburg in 1837. This monu-
mental work gives the places, positions, distances, colours,
and relative brightness of 3112 double and multiple stars, all
determined with the utmost skill and care. The record is one
which gains in value with the process of time, and will for
ages serve as a standard of reference by which to detect change
or confirm discovery.
It appears from Struve's researches that about one in forty
of all stars down to the ninth magnitude is composite, but
that the proportion is doubled in the brighter orders. 1 This
he attributed to the difficulty of detecting the faint companions
of very remote orbs. It was also noticed, both by him and
1 Ueber die Doppelsttme, Bericht, 1827, p. 22.
56 HISTORY OF ASTRONOMY.
Bessel, that double stars are in general remarkable for large
proper motions. Struve's catalogue included no star of which
the components were more than 32" apart, because beyond
that distance the chances of merely optical juxtaposition
become appreciable; but the immense preponderance of
extremely close over (as it were) loosely yoked bodies is such
as to demonstrate their physical connection, even if no other
proof were forthcoming. Many stars previously believed to
be single divided under the scrutiny of the Dorpat refrac-
tor; while in some cases, one member of a (supposed) binary
system revealed itself as double, thus placing the surprised
observer in the unexpected presence of a triple group of suns.
Five instances were noted of two pairs lying so close together
as to induce a conviction of their mutual dependence ; 1 besides
which, 124 examples occurred of triple, quadruple, and multiple
combinations, the reality of which was open to no reasonable
doubt. 2
It was first pointed out by Bessel that the fact of stars exhi-
biting a common proper motion might serve as an unfailing
test of their real association into systems. This was, accord-
ingly, one of the chief criteria employed by Struve to distin-
guish true binaries from merely optical couples. On this
ground alone, 61 Cygni was admitted to be a genuine double
star ; and it was shown that, although its components appear
to follow almost strictly rectilinear paths, yet the probability
of their forming a connected pair is actually greater than
that of the sun rising to-morrow morning. 3 Moreover,
this tie of an identical movement was discovered to unite
bodies 4 far beyond the range of distance ordinarily separating
the members of binary systems, and to prevail so extensively
1 Uebtr die Doppthterne, p. 25. 2 Mensura Micr. , p. xcix.
3 Stellarum fiixarum imprimis Duplicium et Multiplicium Positiones
Media t pp. cxc., cciii.
4 For instance, the southern stars 36A Ophinchi (itself double) and 30
Scorpii, which are 12' 10" apart. Ibid., p. cciii. Recent investigations
have vastly enlarged the area over which this species of connection extends.
PROGRESS OF SIDEREAL ASTRONOMY. 57
as to lead to the conclusion that single do not outnumber
conjoined stars more than twice or thrice. 1
In 1835 Struve was summoned by the Emperor Nicholas
to superintend the erection of a new observatory at Pulkowa,
near St. Petersburg, destined for the special cultivation of
sidereal astronomy. Boundless resources were placed at his
disposal, and the institution created by him was acknowledged
to surpass all others of its kind in splendour, efficiency, and
completeness. Its chief instrumental glory was a refractor
of fifteen inches aperture by Merz and Mahler (Fraunhofer's
successors), which left the famous Dorpat telescope far behind,
and remained long without a rival. On the completion of this
model establishment, August 19, 1839, Struve was installed
as its director, and continued to fulfil the important duties of
the post with his accustomed vigour until 1858, when illness
compelled his virtual resignation in favour of his son Otto
Struve, born at Dorpat in 1819. He died November 23,
1864.
An inquiry into the laws of stellar distribution, undertaken
during the early years of his residence at Pulkowa, led Struve
to confirm, in the main, the inferences arrived at by Herschel
as to the construction of the heavens. According to his view,
the appearance known as the Milky Way is produced by a
collection of (for the most part) irregularly condensed star-
clusters, within which the sun is somewhat eccentrically placed.
The nebulous ring which thus integrates the light of countless
worlds, he found to differ slightly from the form of a great
circle, and he accounted for this deviation from symmetry by
supposing the stars composing it to be scattered over a bent
or " broken plane," or to lie in two planes slightly inclined
to each other, our system occupying a position near their
intersection. 2 He further attempted to show that the limits
of this vast assemblage must remain for ever shrouded from
human discernment, owing to the gradual extinction of light
1 Stellarum Fixarum, &r., p. ccliii.
2 Etudes d 1 Astronomic Stellaire, 1847, p. 82.
58 HISTORY OF ASTRONOMY.
in its passage through space, 1 and sought to confer upon this
celebrated hypothesis a definiteness and certainty far beyond
the aspirations of its earlier advocates, Cheseaux and Olbers ;
but arbitrary assumptions vitiated his reasonings on this, as
well as on some other points. 2
In his special line as a celestial explorer of the most com-
prehensive type, Sir William Herschel had but one legitimate
successor, and that successor was his son. John Frederick
William Herschel was bom at Slough, March 17, 1792,
graduated with the highest honours from St. John's College,
Cambridge, in 1813, and entered upon legal studies with a
view to being called to the Bar. But his share in an early
compact with Peacock and Babbage, " to do their best to
leave the world wiser than they found it," was not thus to be
fulfilled. The acquaintance of Dr. Wollaston decided his scien-
tific vocation. Already, in 1816, we find him reviewing some
of his father's double stars; and he completed in 1820 the
1 8-inch speculum which was to be the chief instrument of his
investigations. Soon after he undertook, in conjunction with
Mr. (afterwards Sir James) South, a series of observations,
issuing in the presentation to the Royal Society of a paper 3
containing micrometrical measurements of 380 binary stars, by
which the elder Herschel's inferences of orbital motion were,
in many cases, strikingly confirmed. A star in the Northern
Crown, for instance (n Coronas), was found to have completed
more than one entire circuit since its first discovery ; another,
r Serpentarii, had closed up into apparent singleness ; while in
a third, Orionis, the converse change had taken place, and de-
ceptive singleness had been transformed into obvious duplicity. 4
It was from the first confidently believed that the force
retaining double stars in curvilinear paths was identical with
that governing the planetary revolutions. But that identity
1 Etudes cTAstr. Stellaire, p. 86.
2 See Encke's criticism in Astr. Nach., No. 622.
3 Phil. Trans., vol. cxiv. partiii. 1824.
4 Grant, Hist. Phys. Astr., p. 560.
PROGRESS OF SIDEREAL ASTRONOMY. 59
was not ascertained until Savary of Paris showed, in I827, 1
that the movements of a well-known binary in the hind-paw of
the Great Bear (g Ursse) could be represented with all attain-
able accuracy by an ellipse calculated on orthodox gravita-
tional principles with a period of 58^ years. Encke followed
at Berlin with a still more elegant method; and Sir John
Herschel, pointing out the uselessness of analytical refinements
where the data were necessarily so imperfect, described in 1831
a graphical process by which " the aid of the eye and hand "
was brought in " to guide the judgment in a case where
judgment only, and not calculation, could be of any avail." 2
The subject has since been cultivated with diligence, and not
without success ; but our acquaintance with stellar orbits can
hardly yet be said to have emerged from the tentative stage.
In 1825 Herschel undertook, and executed with great assi-
duity during the ensuing eight years, a general survey of the
northern heavens, directed chiefly towards the verification of
his father's nebular discoveries. The outcome was a catalogue
of 2306 nebulae and clusters, of which 525 were observed for
the first time, besides 3347 double stars discovered almost
incidentally. 3 y " Strongly invited," as he tells us himself, " by
the peculiar interest of the subject, and the wonderful nature
of the objects which presented themselves," he resolved to
attempt the completion of the survey in the southern hemi-
sphere. 'With this noble object in view, he embarked his family
and instruments on board the Mount Stewart Elphinstone,
and, after a prosperous voyage, landed at Cape Town on the
1 6th of January 1834. Choosing as the scene of his observa-
tions a rural spot under the shelter of Table Mountain, he
began regular " sweeping " on the 5th of March. The site of
his great reflector is now marked with an obelisk, and the
name of Feldhausen has become memorable in the history of
science ; for the four years' work done there may truly be said to
open the chapter of our knowledge as regards the southern skies.
1 Conn. d. Temps, 1830. z R. A. S. Mem., vol. v. 1833, p. 178.
3 Phil. Trans., vol. cxxiii., and Results, &c., In trod.
60 HISTORY OF ASTRONOMY. .
The full results of Herschel's journey to the Cape were not
made public until 1847, when a splendid volume 1 embodying
them was brought out at the expense of the Duke of Nor-
thumberland. They form a sequel to his father's labours such
as the investigations of one man have rarely received from
those of another. What the elder observer did for the
northern heavens, the younger did for the southern, and
with generally concordant results. Reviving the paternal
method of "star-gauging," he showed, from a count of 2299
fields, that the Milky Way surrounds the solar system as a
complete annulus of minute stars ; not, however, quite sym-
metrically, since it appears that the sun lies somewhat above
its medial plane, as well as somewhat nearer to those por-
tions visible in the southern hemisphere, which accordingly
display a brighter lustre and a more complicated structure
than the northern branches. The singular cosmical agglo-
merations known as the " Magellanic Clouds " were now, for
the first time, submitted to a detailed, though admittedly in-
complete, examination, the almost inconceivable richness and
variety of their contents being such that a lifetime might
with great profit be devoted to their study. In the Greater
Nubecula, within a compass of forty-two square degrees,
Herschel reckoned 278 distinct nebulae and clusters, besides
fifty or sixty outliers, and a large number of stars intermixed
with diffused nebulosity, in all, 919 catalogued objects, and,
for the Lesser Cloud, 244. Yet this was only the most con-
spicuous part of what his twenty-foot revealed. Such an
extraordinary concentration of bodies so various led him to
the inevitable conclusion that "the Nubeculae are to be
regarded as systems sui generis > and which have no analogues
in our hemisphere." 2 He noted also the blankness of sur-
rounding space, especially in the case of Nubecula Minor,
" the access to which on all sides," he remarked, " is
through a desert ; " as if the cosmical material in the neigh-
1 Results of Astronomical Observations made during the years 1834-8 at
the Cape of Good Hope. 2 Results, &c., p. 147.
PROGRESS OF SIDERRAL ASTRONOMY. 61
bourhood had been swept up and garnered in these mighty
groups. 1
Of southern double stars, he discovered and gave careful
measurements of 2102, and described 1708 nebulae, of which
at least 300 were new. The list was illustrated with a number
of drawings, some of them extremely beautiful and elaborate.
Sir John Herschel's views as to the nature of nebulae were
considerably modified by Lord Rosse's success in " resolving "
a crowd of these objects into stars with his great reflectors.
His former somewhat hesitating belief in the existence of phos-
phorescent matter, "disseminated through extensive regions
of space in the manner of a cloud or fog," 2 was changed into
a conviction that no valid distinction could be established
between the faintest wisp of cosmical vapour just discernible
in a powerful telescope, and the most brilliant and obvious
cluster. He admitted, however, an immense range of possible
variety in the size and mode of aggregation of the stellar con-
stituents of various nebulae. Some might appear nebulous
from the closeness of their parts ; some from their smallness.
Others, he suggested, might be formed of " discrete luminous
bodies floating in a non-luminous medium ; " 3 while the an-
nular kind probably consisted of " hollow shells of stars." 4
That a physical, and not merely an optical, connection unites
nebulae with the embroidery (so to speak) of small stars with
which they are in many instances profusely decorated, was
evident to him, as it must be to all who look as closely and
see as clearly as he did. His description of No. 2093 in his
northern catalogue as " a network or tracery of nebula fol-
lowing the lines of a similar network of stars," 5 would alone
suffice to dispel the idea of accidental scattering ; and many
other examples of a like import might be quoted. The
remarkably frequent occurrence of one or more minute stars
in the close vicinity of " planetary " nebulae led him to infer
1 See Proctor's Universe of Stars, p. 92.
5 A Treatise of Astronomy, 1833, p. 406. 3 Results, 6-v., p. 139.
4 Ibid., pp. 24, 142. 5 Phil. Trans., vol. cxxiii. p. 503.
62 HISTORY OF ASTRONOMY.
their dependent condition ; and he advised the maintenance
of a strict watch for evidences of circulatory movements, not
only over these supposed stellar satellites, but also over the
numerous " double nebulas," in which, as he pointed out, " all
the varieties of double stars as to distance, position, and
relative brightness, have their counterparts." He, moreover,
investigated the subject of nebular distribution by the simple
and effectual method of graphic delineation or " charting," and
succeeded in showing that while a much greater uniformity
of scattering prevails in the southern heavens than in the
northern, a condensation is nevertheless perceptible about the
constellations Pisces and Cetus, roughly corresponding to the
" nebular region " in Virgo by its vicinity (within 20 or 30) to
the opposite pole of the Milky Way. He concluded " that the
nebulous system is distinct from the sidereal, though involving,
and perhaps to a certain extent intermixed with, the latter." 1
Towards the close of his residence at Feldhausen, Herschel
was fortunate enough to witness one of those singular changes
in the aspect of the firmament which occasionally challenge
the attention even of the incurious, and excite the deepest
wonder of the philosophical observer. Immersed apparently
in the Argo nebula is a large star denominated v\ Argus. When
Halley visited St. Helena in 1677, it seemed of the fourth
magnitude ; but Lacaille in the middle of the following century,
and others after him, classed it as of the second. In 1827 the
traveller Burchell, being then at St. Paul, near Rio Janeiro,
remarked that it had unexpectedly assumed the first rank, a
circumstance the more surprising to him because he had fre-
quently, when in Africa during the years 1811 to 1815, noted
it as of only fourth magnitude. This observation, however,
did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second,
and had no suspicion of its unusual character until December
1 6, 1837, when he suddenly perceived it with its light almost
tripled. It then far outshone Rigel in Orion, and on the 2d
1 Results, 6c., p. 136.
PROGRESS OF SIDEREAL ASTRONOMY. 63
of January following it very nearly matched a Centauri. From
that date it declined ; but a second and even brighter maxi-
mum occurred in April 1843, when Maclear, then director of
the Cape Observatory, saw it blaze out with a splendour ap-
proaching that of Sirius. Its waxings and wanings were marked
by curious " trepidations " of brightness extremely perplexing
to theory. In 1863 it had sunk below the fifth magnitude,
and in 1869 was barely visible to the naked eye ; but it has
now regained so much of its light as to shine with nearly the
same lustre as when Halley observed it two centuries ago.
There is some reason to believe that its fluctuations are in-
cluded in a cycle of about seventy years, 1 confused probably
by the superposition of more than one secondary period ; but
the extent and character of the vicissitudes to which it is sub-
ject, stamp it as a species of connecting link between regularly
periodic and (so-called) " temporary stars."
Among the numerous topics which engaged Herschel's atten-
tion at the Cape was that of relative stellar brightness. Having
contrived an " astrometer " in which an "artificial star," formed
by the total reflection of moonlight from the base of a prism,
served as a standard of comparison, he was able to estimate
the lustre of the natural stars examined by the distances at
which the artificial object appeared equal respectively to each.
He thus constructed a table of 191 of the principal stars, 2 both
in the northern and southern hemispheres, setting forth the
numerical values of their apparent brightness relatively to that
of a Centauri, which he selected as a unit of measurement.
Further, the light of the full moon being found by him to ex-
ceed that of his standard star 27,408 times, and Dr. Wollaston
having shown that the light of the full moon is to that of the
sun as 1 1801,072 3 (Zollner finds the ratio i : 618,000), it be-
came possible to compare stellar with solar radiance. Hence
was derived, in the case of the few stars at ascertained distances,
a knowledge of real lustre. Alpha Centauri, for example,
1 Loomis in Month, Not., vol. xxix. p. 298.
2 Outlines of Astr. t App. I. 3 Phil. Trans., vol. cxix. p. 27.
64 HISTORY OF ASTRONOMY.
is found to emit four times, Vega nearly forty times as much
light as our sun ; while Arcturus (if its measured parallax of
0.13" can be depended upon) displays the splendour of fully
200 such luminaries.
Herschel returned to England in the spring of 1838, bring-
ing with him a wealth of observation and discovery such as
had perhaps never before been amassed in so short a time.
Deserved honours awaited him. He was created a baronet
on the occasion of the Queen's coronation (he had been
knighted in 1831) ; universities and learned societies vied with
each other in showering distinctions upon him ; and the suc-
cefcs of an enterprise in which scientific zeal was tinctured
with an attractive flavour of adventurous romance, was justly
regarded as a matter of national pride. His career as an
observing astronomer was now virtually closed, and he devoted
his leisure to the collection and arrangement of the abundant
trophies of his father's and his own activity. The result-
ing great catalogue of 5079 nebulae (including all then cer-
tainly known), published in the Philosophical Transactions for
1864, is, and will probably long remain, the leading source of
information on the subject ; x but he unfortunately did not live
to finish the companion work on double stars, for which he
had accumulated a vast store of materials. 2 He died at
Collingwood in Kent, May n, 1871, in the eightieth year of
his age, and was buried in Westminster Abbey, close beside
the grave of Sir Isaac Newton.
The consideration of Sir John Herschel's Cape observations
brings us to the close of the period we are just now engaged
in studying. They were given to the world, as already stated,
three years before the middle of the century, and accurately
1 Dr. Dreyer published in 1878 a supplement to the work, giving the
places of 1880 new nebulae.
2 A list of 10,320 composite stars was drawn out by him in order of
right ascension, and has been published in vol. xl. of Mem. R. A. S. ; but
the data requisite for their formation into a catalogue were not forthcoming.
See Main's and Pritchard's Preface to above, and Dunkin's Obituary
Notices, p. 73.
PROGRESS OF SIDEREAL ASTRONOMY. 65
represent the condition of sidereal science at that date. Look-
ing back over the fifty years traversed, we can see at a glance
how great was the stride made in the interval. Not alone was
acquaintance with individual members of the cosmos vastly
extended, but their mutual relations, the laws governing their
movements, their distances from the earth, masses, and intrinsic
lustre, had begun to be successfully investigated. Begun to be ;
for only regarding a scarcely perceptible minority had even
approximate conclusions been arrived at. Nevertheless the
whole progress of the future lay in that beginning; it was
the thin end of the wedge of exact knowledge. The principle
of measurement had been substituted for that of probability ;
a basis had been found large and strong enough to enable
calculation to ascend from it to the sidereal heavens; and
refinements had been introduced, fruitful in performance, but
still more in promise. Thus, rather the kind than the amount
of information collected was significant for the time to come
rather the methods employed than the results actually secured
rendered the first half of the nineteenth century of epochal
importance in the history of our knowledge of the stars.
( 66 )
CHAPTER III.
PROGRESS OF KNOWLEDGE REGARDING THE SUN.
THE discovery of sun-spots in 1610 by Fabricius and Galileo
first opened a way for inquiry into the solar constitution ; but
it was long before that way was followed with system or profit.
The seeming irregularity of the phenomena discouraged con-
tinuous attention ; casual observations were made the basis
of arbitrary conjectures, and real knowledge received little or
no increase. In 1620 we find Jean Tarde, canon of Sarlat,
arguing that because the sun is "the eye of -the world," and
the eye of the world cannot suffer from ophthalmia, therefore
the appearances in question must be due, not to actual specks
or stains on the bright solar disc, but to the transits of a
number of small planets across it ! To this new group of
heavenly bodies he gave the name of " Borbonia Sidera," and
they were claimed in 1633 for the House of Hapsburg, under
the title of " Austriaca Sidera," by Father Malapertius, a Belgian
Jesuit. 1 A similar view was temporarily maintained against
Galileo by the celebrated Father Scheiner of Ingolstadt, and
later by William Gascoigne, the inventor of the micrometer ;
but most of those who were capable of thinking at all on such
subjects (and they were but few) adhered either to the cloud
theory or to the slag theory of sun-spots. The first was
championed by Galileo, the second by Simon Marius, " astro-
nomer and physician " to the brother Margraves of Branden-
1 Kosmos, Bd. iii. p. 409 ; Lalande, Bibliographic Astronomique, pp.
179, 202.
PROGRESS OF KNOWLEDGE OF THE SUN. 67
burg. The latter opinion received a further notable develop-
ment from the fact that in 1618, a year remarkable for the
appearance of three bright comets, the sun was almost free
from spots; whence it was inferred that the cindery refuse
from the great solar conflagration, which usually appeared as
dark blotches on its surface, was occasionally thrown off in
the form of comets, leaving the sun, like a snuffed taper, to
blaze with renewed brilliancy. 1
In the following century, Derham gathered from observations
carried on during the years 1703-11, "That the spots on the
sun are caused by the eruption of some new volcano therein,
which at first pouring out a prodigious quantity of smoke and
other opacous matter, causeth the spots ; and as that fuliginous
matter decayeth and spendeth itself, and the volcano at last
becomes more torrid and flaming, so the spots decay, and grow
to umbrae, and at last to faculae." 2
The view, confidently upheld by Lalande, 3 that spots were
rocky elevations uncovered by the casual ebbing of a luminous
ocean, the surrounding penumbrae representing shoals or sand-
banks, had even less to recommend it than Derham's volcanic
theory. Both were, however, significant of a growing tendency
to bring solar phenomena within the compass of terrestrial
analogies.
For 164 years, then, after Galileo first levelled his telescope
at the setting sun, next to nothing was learned as to its nature ;
and the facts immediately ascertained of its rotation on an
1 R. Wolf, Die Sonne und ihre Flecken, p. 9. Marius himself, however,
seems to have held the Aristotelian terrestrial-exhalation theory of come-
tary origin. See his curious little tract, Astronomische und Astrologische
Beschreibung des Comelen, Niirnberg, 1619.
2 Phil. Trans., vol. xxvii. p. 274. Umbra (now called penumbrce) are
spaces of half-shadow which usually encircle spots. Facula ("little
torches," so named by Scheiner) are bright streaks or patches closely
associated with spots.
3 Mem. Ac. Sc., 1776 (pub. 1779), p. 507. The merit, however (if merit
it be), of having first put forward (about 1671) the hypothesis alluded to
in the text, belongs to D. Cassini. See Delambre, Hist, de FAstr. Mod.,
t. ii. p. 694, and Kosmos, Bd. iii. p. 410.
68 HISTORY OF ASTRONOMY.
axis nearly erect to the plane of the ecliptic, in a period of
between twenty-five and twenty-six days, and of the virtual
limitation of the spots to a so-called " royal " zone extending
some thirty degrees north and south of the solar equator,
gained little either in precision or development from five gene-
rations of astronomers.
But in November 1769 a spot of extraordinary size engaged
the attention of Alexander Wilson, professor of astronomy in
the University of Glasgow. He watched it day by day, and to
good purpose. As the great globe slowly revolved, carrying the
spot towards its western edge, he was struck with the gradual
contraction and final disappearance of the penumbra on the
side next the centre of the disc ; and when, on the 6th of Decem-
ber, the same spot re-emerged on the eastern limb, he perceived,
as he had anticipated, that the shady zone was now deficient
on the opposite side, and resumed its original completeness as it
returned to a central position. Similar perspective effects were
visible in numerous other spots subsequently examined by him,
and he was thus'in I774 1 able to prove by strict geometrical
reasoning that such appearances were, as a matter of fact, pro-
duced by vast excavations in the sun's substance. It was not,
indeed, the first time that such a view had been suggested.
Father Scheiner's later observations plainly foreshadowed
it ; 2 a conjecture to the same effect was emitted by Leonhard
Rost of Nuremberg early in the eighteenth century; 3 both
by Lahire in 1703 and by J. Cassini in 1719 spots had been
seen to form actual notches on the solar limb ; while Pastor
Schiilen of Essingen convinced himself in 1770, by the care-
ful study of appearances similar to those noted by Wilson, of
the fact detected by him. 4 Nevertheless, Wilson's demonstra-
tion came with all the surprise of novelty, as well as with all
the force of truth.
1 Phil. Trans., vol. Ixiv. part I, pp. 7-11.
2 Rosa Ursina, lib. iv. p. 507.
3 R. Wolf, Die Sonne und ihre Flecken, p. 12.
4 Schellen, Die Spectralanalyse, Bd. ii. p. 56 (3d ed.)
PROGRESS OF KNOWLEDGE OF THE SUN. 69
The general theory by which it was accompanied rested on
a very different footing. It was avowedly tentative, and was
set forth in the modest shape of an interrogatory. " Is it not
reasonable to think," he asks, " that the great and stupendous
body of the sun is made up of two kinds of matter, very
different in their qualities ; that by far the greater part is solid
and dark, and that this immense and dark globe is encom-
passed with a thin covering of that resplendent substance
from which the sun would seem to derive the whole of his
vivifying heat and energy ?" 1 He further suggests that the ex-
cavations or spots may be occasioned " by the working of some
sort of elastic vapour which is generated within the dark
globe," and that the luminous matter being in some degree
fluid, and being acted upon by gravity, tends to flow down and
cover the nucleus. From these hints, supplemented by his
own diligent observations and sagacious reasonings, Herschel
elaborated a scheme of solar constitution which held its ground
until the physics of the sun were revolutionised by the spectro-
scope.
A cool, dark, solid globe, its surface diversified with moun-
tains and valleys, clothed with luxuriant vegetation, and " richly
stored with inhabitants," protected by a heavy cloud-canopy
from the intolerable glare of the upper luminous region, where"
the dazzling coruscations of a solar aurora some thousands of
miles in depth evolved the stores of light and heat which
vivify our world such was the central luminary which Her-
schel constructed with his wonted ingenuity, and described
with his wonted eloquence.
" This way of considering the sun and its atmosphere," he
says, 2 "removes the great dissimilarity we have hitherto been
used to find between its condition and that of the rest of the
great bodies of the solar system. The sun, viewed in this
light, appears to be nothing else than a very eminent, large,
and lucid planet, evidently the first, or, in strictness of speaking,
the only primary one of our system; all others being truly
1 PkiL Trans., vol. Ixiv. p. 20. 2 Ibid., vol. Ixxxv. 1795, p. 63.
70 HISTORY OF ASTRONOMY.
secondary to it. Its similarity to the other globes of the solar
system with regard to its solidity, its atmosphere, and its
diversified surface, the rotation upon its axis, and the fall of
heavy bodies, leads us on to suppose that it is most probably
also inhabited, like the rest of the planets, by beings whose
organs are adapted to the peculiar circumstances of that vast
globe."
We smile at conclusions which our present knowledge con-
demns as extravagant and impossible, but such incidental
flights of fancy in no way derogate from the high value of
Herschel's contributions to solar science. The cloud-like char-
acter which he attributed to the radiant shell of the sun (first
named by Schroter the "photosphere") is borne out by all
recent investigations; he observed its mottled or corrugated
aspect, resembling, as he described it, the roughness on the
rind of an orange ; showed that " faculae " are elevations or
heaped-up ridges of the disturbed photospheric matter ; and
threw out the idea that spots may be caused by an excess of
the ordinary luminous emissions. A certain "" empyreal " gas
was, he supposed (very much as Wilson had done), generated
in the body of the sun, and rising everywhere by reason of its
lightness, made for itself, when in moderate quantities, small
openings or "pores," 1 abundantly visible as dark points on
the solar disc. But should an uncommon quantity be formed,
" it will," he maintained, "burst through the planetary 2 regions
of clouds, and thus will produce great openings ; then, spread-
ing itself above them, it will occasion large shallows (penumbrse),
and, mixing afterwards gradually with other superior gases, it
will promote the increase, and assist in the maintenance of the
general luminous phenomena." 3
This partial anticipation of the modern view that the solar
radiations are maintained by some process of circulation
within the solar mass, was reached by Herschel through pro-
1 Phil. Trans., vol. xci. 1801, p. 303.
2 The supposed opaque or protective stratum was named by him "plane-
tary," from the analogy of terrestrial clouds. 3 Ibid., p. 305.
PROGRESS OF KNOWLEDGE OF THE SUN. 71
longed study of the phenomena in question. The novel and
important idea contained in it, however, it was at that time
premature to attempt to develop. But though many of the
subtler suggestions of Herschel's genius passed unnoticed by
his contemporaries, the main result of his solar researches was
an unmistakable one. It was nothing less than the definitive
introduction into astronomy of the paradoxical conception of
the central fire and hearth of our system as a cold, dark,
terrestrial mass, wrapt in a mantle of innocuous radiance an
earth, so to speak, within a sun without.
Let us pause for a moment to consider the value of this
remarkable innovation. It certainly was not a step in the
direction of truth. On the contrary, the crude notions of
Anaxagoras and Xeno approached more nearly to what we
now know of the sun, than the complicated structure devised
for the happiness of a nobler race of beings than our own, by
the benevolence of eighteenth-century astronomers. And yet
it undoubtedly constituted a very important advance in science.
It was the first earnest attempt to bring solar phenomena
within the compass of a rational system ; to put together into
a consistent whole the facts ascertained ; to fabricate, in short,
a solar machine that would in some fashion work. It is true
that the materials were inadequate and the design faulty.
The resulting construction has not proved strong enough to
stand the wear and tear of time and discovery, but has had to
be taken to pieces and remodelled on a totally different plan.
But the work was not therefore done in vain. None of Bacon's
aphorisms show a clearer insight into the relations between
the human mind and the external world than that which
declares " Truth to emerge sooner from error than from con-
fusion." l A definite theory (even if a false one) gives holding-
ground to thought. Facts acquire a meaning with reference
to it. It affords a motive for accumulating them and a means
of co-ordinating them ; it provides a framework for their
arrangement and a receptacle for their preservation, until they
Organum, lib. ii. aph. 20.
72 HISTORY OF ASTRONOMY.
become too strong and numerous to be any longer included
within arbitrary limits, and shatter the vessel originally framed
to contain them.
Such was the purpose subserved by Herschel's theory of the
sun. It helped to clarify ideas on the subject. The turbid
sense of groping and viewless ignorance gave place to the
lucidity of a plausible scheme. The persuasion of knowledge
is a keen incentive to its increase. Few men care to investigate
what they are obliged to admit themselves entirely ignorant
of; but once started on the road of knowledge (real or sup-
posed), they are eager to pursue it. By. the promulgation of
a confident and consistent view regarding the nature of the
sun, accordingly, research was encouraged, because it was
rendered hopeful, and inquirers were shown a path leading
indefinitely onwards where an impassable thicket had before
seemed to bar the way.
We have called the " terrestrial " theory of the sun's nature
an innovation, and so, as far as its general acceptance is con-
cerned, it may justly be termed; but, like all successful in-
novations, it was a long time brewing. It is extremely curious
to find that Herschel had a predecessor in its advocacy who
never looked through a telescope (nor, indeed, imagined the
possibility of such an instrument), who knew nothing of sun-
spots, was still (mistaken assertions to the contrary notwith-
standing) in the bondage of the geocentric system, and re-
garded Nature from the lofty standpoint of an idealist
philosophy. This was the learned and enlightened Cardinal
Cusa, a fisherman's son from the banks of the Moselle, whose
distinguished career in the Church and in literature extended
over a considerable part of the fifteenth century (1401-64).
In his singular treatise De Docta Ignorantia, one of the most
notable literary monuments of the early Renaissance, the
following passage occurs : " To a spectator on the surface of
the sun, the splendour which appears to us would be invisible,
since it contains, as it were, an earth for its central mass, with
a circumferential envelope of light and heat, and between the
PROGRESS OF KNOWLEDGE OF THE SUN. 73
two an atmosphere of water and clouds and translucent air."
The luminary of Herschel's fancy could scarcely be more
clearly portrayed; some added words, however, betray the
origin of the Cardinal's idea. " The earth also," he says,
" would appear as a shining star to any one outside the fiery
element." It was, in fact, an extension to the sun of the
ancient elemental doctrine ; but an extension remarkable at
that period, as premonitory of the tendency, so powerfully
developed by subsequent discoveries, to assimilate the orbs of
heaven to the model of our insignificant planet, and to extend
the brotherhood of our system and our species to the farthest
limit of the visible or imaginable universe.
In later times we find Flamsteed communicating to Newton,
March 7, 1681, his opinion "that the 'substance of the sun is
terrestrial matter, his light but the liquid menstruum encom-
passing him." 1 Bode in 1776 arrived independently at the
conclusion that " the sun is neither burning nor glowing, but
in its essence a dark planetary body, composed like our earth
of land and water, varied by mountains and valleys, and envel-
oped in a vaporous atmosphere; 2 " and the learned in general
applauded and acquiesced. The view, however, was in 1787
still so far from popular, that the holding of it was alleged as
a proof of insanity in Dr. Elliot when accused of a murder-
ous assault on Miss Boydell. His friend Dr. Simmons stated
on his behalf that he had received from him in the preceding
January a letter giving evidence of a deranged mind, wherein
he asserted " that the sun is not a body of fire, as hath been
hitherto supposed, but that its light proceeds from a dense
and universal aurora, which may afford ample light to the
inhabitants of the surface beneath, and yet be at such a distance
aloft as not to annoy them. No objection, he saith, ariseth to
that great luminary's being inhabited ; vegetation may obtain
there as well as with us. There may be water and dry land,
hills and dales, rain and fair weather ; and as the light, so the
1 Brewster's Life of Newton , vol. ii. p. 103.
2 Beschaftigungen d. Berl. Ges. Naturforschender Freunde , Bd. ii. p. 233.
74 HISTORY OF ASTRONOMY.
season must be eternal, consequently it may easily be con-
ceived to be by far the most blissful habitation of the whole
system ! " The Recorder, however, we are told, objected that
if an extravagant hypothesis were to be adduced as proof of
insanity, the same might hold good with regard to some other
speculators, and desired Dr. Simmons to tell the court what
he thought of the theories of Burnet and Buffon. 1
Eight years later, this same "extravagant hypothesis," backed
by the powerful recommendation of Sir William Herschel,
obtained admittance to the venerable halls of science, there to
abide undisturbed for nearly seven decades. It is true there
were individual objectors, but their arguments made little
impression on the general body of opinion. Ruder blows
were required to shatter an hypothesis flattering to human
pride of invention in its completeness, in the plausible detail
of observations by which it seemed to be supported, and in its
condescension to the natural pleasure in discovering resem-
blance under all but total dissimilarity.
Sir John Herschel included among the results of his multi-
farious labours at the Cape of Good Hope a careful study of
the sun-spots conspicuously visible towards the end of the
year 1836 and in the early part of 1837. They were remark-
able, he tells us, for their forms and arrangement, as well as
for their number and size ; one group, measured on the 29th
of March in the latter year, covering (apart from what may be
called its outlying dependencies) the vast area of five square
minutes or 3780 million square miles. 2 We have at present
to consider, however, not so much these observations in them-
selves, as the chain of theoretical suggestions by which they
were connected. The distribution of spots, it was pointed out,
on two zones parallel to the equator, showed plainly their
intimate connection with the solar rotation, and indicated as
their cause fluid circulations analogous to those producing the
terrestrial trade- and anti-trade winds.
1 Gentleman's Magazine, 1787, p. 636. 2 Remits, &>c., p. 432.
PROGRESS OF KNOWLEDGE OF THE SUN. 75
"The spots, in this view of the subject," he went on to say, 1
"would come to be assimilated to those regions on the earth's
surface where, for the moment, hurricanes and tornadoes pre-
vail ; the upper stratum being temporarily carried downwards,
displacing by its impetus the two strata of luminous matter
beneath, the upper of course to a greater extent than the lower,
and thus wholly or partially denuding the opaque surface of
the sun below. Such processes cannot be unaccompanied by
vorticose motions, which, left to themselves, die away by degrees
and dissipate, with the peculiarity that their lower portions
come to rest more speedily than their upper, by reason of the
greater resistance below, as well as the remoteness from the
point of action, which lies in a higher region, so that their
centres (as seen in our waterspouts, which are nothing but small
tornadoes) appear to retreat upwards. Now this agrees per-
fectly with what is observed during the obliteration of the solar
spots, which appear as if filled in by the collapse of their
sides, the penumbra closing in upon the spot and disappearing
after it."
When, however, it comes to be asked whether a cause can
be found by which a diversity of solar temperature might be
produced corresponding with that which sets the currents of
the terrestrial atmosphere in motion, we are forced to reply
that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equa-
tor, due to the slightly spheroidal or bulging form of its outer
atmospheric envelope, assuredly gives no sufficient account of
such circulatory movements as he supposed to exist. Never-
theless, the view that the sun's rotation is intimately connected
with the formation of spots is so obviously correct, that we can
only wonder it was not thought of sooner, while we are even
now unable to explain with any certainty how it is so con-
nected.
Mere scrutiny of the solar surface, however, is not the only
means of solar observation. We have a satellite, and that
1 Results, &<r., p. 434.
76 HISTORY OF ASTRONOMY.
satellite from time to time acts most opportunely as a screen,
cutting off a part or the whole of those dazzling rays in which
the master-orb of our system veils himself from over-curious
regards. The importance of eclipses to the study of the solar
surroundings is of comparatively recent recognition ; neverthe-
less, much of what we know concerning them has been snatched,
as it were, by surprise under favour of the moon. In former
times, the sole astronomical use of such incidents was the cor-
rection of the received theories of the solar and lunar move-
ments ; the precise time of their occurrence was the main fact
to be noted, and subsidiary phenomena received but casual
attention. Now, their significance as a geometrical test of
tabular accuracy 'is altogether overshadowed by the interest
attaching to the physical observations for which they afford
propitious occasions. This change may be said to date, in
its pronounced form, from the great eclipse of 1842. Although
a necessary consequence of the general direction taken by
scientific progress, it remains associated in a special manner
with the name of. Francis Baily.
The " philosopher of Newbury " was by profession a London
stockbroker, and a highly successful one. Nevertheless, his
services to science were numerous and invaluable, though not
of the brilliant kind which attracts popular notice. Born at
Newbury in Berkshire, April 28, 1774, and placed in the City
at the age of fourteen, he derived from the acquaintance of
Dr. Priestley a love of science which never afterwards left him.
It was, however, no passion such as flames up in the brain of
the destined discoverer, but a regulated inclination, kept well
within the bounds of an actively pursued commercial career.
After travelling for a year or two in what were then the wilds of
North America, he went on the Stock Exchange in 1799, and
earned during twenty-four years of assiduous application to
affairs a high reputation for integrity and ability, to which
corresponded an ample fortune. In the meantime the Astro-
nomical Society (largely through his co-operation) had been
founded ; he had for three years acted as its secretary, and he
PROGRESS OF KNOWLEDGE OF THE SUN. 77
now felt entitled to devote himself exclusively to a subject
which had long occupied his leisure hours. He accordingly
in 1825 retired from business, purchased a house in Tavistock
Place, and fitted up there a small observatory. He was, how-
ever, by preference a computator rather than an observer.
What Sir John Herschel calls the "archaeology of practical
astronomy " found in him an especially zealous student. He
re-edited the star-catalogues of Ptolemy, Ulugh Beigh, Tycho
Brahe, Hevelius, Halley, Flamsteed, Lacaille, and Mayer;
calculated the eclipse of Thales and the eclipse of Agathocles,
and vindicated the memory of the first Astronomer Royal. But
he was no less active in meeting present needs than in revising
past performances. The subject of the reduction of observa-
tions, then, as we have already explained, 1 in a state of deplor-
able confusion, attracted his most earnest attention, and he was
close on the track of Bessel when made acquainted with the
method of simplification devised at Konigsberg. Anticipated
as an inventor, he could still be of eminent use as a promoter
of these valuable improvements ; and, carrying them out on a
large scale in the star-catalogue of the Astronomical Society
(published in 1827), "he put" (in the words of Herschel) "the
astronomical world in possession of a power which may be said,
without exaggeration, to have changed the face of sidereal
astronomy." 2
His reputation was still further enhanced by his renewal,
with vastly improved apparatus, of the method, first used by
Henry Cavendish in 1797-98, for determining the density of
the earth. From a series of no less than 2153 delicate and
difficult experiments, conducted at Tavistock Place during
the years 1838-42, he concluded our planet to weigh 5.66 as
much as a globe of water of the same bulk ; and this result
(slightly corrected) is still accepted as a very close approxima-
tion to the truth.
What we have thus glanced at is but a fragment of the truly
1 See ante, p. 41.
~ Memoir of Francis Baity, Mem. R. A. S., vol. xv. p. 324.
78 HISTORY OF ASTRONOMY.
surprising mass of work accomplished by Baily in the course
of a variously occupied life. A rare combination of qualities
fitted him for his task. Unvarying health, undisturbed equa-
nimity, methodical habits, the power of directed and sustained
thought, joined to form in him an intellectual toiler of the
surest, though not perhaps of the very highest quality. He
was in harness almost to the end. He was destined scarcely
to know the miseries of enforced idleness or of consciously
failing powers. In 1842 he completed the laborious reduction
of Lalande's great catalogue, undertaken at the request of the
British Association, and was still engaged in seeing it through
the press when he was attacked with what proved his last, as it
was probably his first, serious illness. He, however, recovered
sufficiently to attend the Oxford Commemoration of July 2,
1844, where an honorary degree of D.C.L. was conferred
upon him in company with Airy and Struve ; but sank rapidly
after the effort, and died on the 3oth of August following, at
the age of seventy, lamented and esteemed by all who knew
him.
It is now time to consider his share in the promotion of solar
research. Eclipses of the sun, both ancient and modern, were
a speciality with him, and he was fortunate in those which came
under his observation. Such phenomena are of three kinds
partial, annular, and total. In a partial eclipse, the moon,
instead of passing directly between us and the sun, slips. by, as
it were, a little on one side, thus cutting off from our sight
only a portion of his surface. An annular eclipse, on the
other hand, takes place when the moon is indeed centrally
interposed, but falls short of the apparent size required for the
entire concealment of the solar disc, which consequently re-
mains visible as a bright ring or annulus, even when the
obscuration is at its height. In a total eclipse, on the con-
trary, the sun completely disappears behind the dark body of
the moon. The difference of the two latter varieties is due to
the fact that the apparent diameters of the sun and moon are
so nearly equal as to gain alternate preponderance one over
PROGRESS OF KNOWLEDGE OF THE SUN. 79
the other, through the slight periodical changes in their re-
spective distances from the earth.
Now on the i5th of May 1836, an annular eclipse was visible
in the northern parts of Great Britain, and was observed by Baily
at Inch Bonney near Jedburgh. It was here that he saw the phe-
nomenon which obtained the name of " Baily's Beads " from
the notoriety conferred upon it by his vivid description.
" When the cusps of the sun," he writes, " were about 40
asunder, a row of lucid points, like a string of bright beads, irre-
gular in size and distance from each other, suddenly formed
round that part of the circumference of the moon that was about
to enter, or which might be considered as having just entered
on the sun's disc. Its formation indeed was so rapid that it
presented the appearance of having been caused by the ignition
of a fine train of gunpowder." He expected every moment to
see the thread of light completed round the moon, attributing
the serrated aspect of its limb to the projection of lunar moun-
tains. " My surprise however was great," he continues, " on
finding that these luminous points as well as the dark inter-
vening spaces increased in magnitude, some of the contiguous
ones appearing to run into each other like drops of water. . . .
Finally, as the moon pursued her course, these dark intervening
spaces (which, at their origin, had the appearance of lunar
mountains in high relief, and which still continued attached to
the sun's border) were stretched out into long, black, thick,
parallel lines, joining the limbs of the sun and moon ; when
all at once they suddenly gave way, and left the circumference
of the sun and moon in those points, as in the rest, com-
paratively smooth and circular, and the moon perceptibly
advanced on the face of the sun." 1
A lively interest was excited by the communication from
which the above passages are taken. The curious appearances
described in it were not, indeed, an absolute novelty, but they
had previously received only transient or partial notice.
Webber in 1791, and Von Zach in 1820, had seen the
1 Mem. R. A. S., vol. x. pp. 5-6.
8o HISTORY OF ASTRONOMY.
"beads;" Van Swinden had described the "belts" or
"threads." 1 These last were, moreover (as Baily clearly
perceived), completely analogous to the "black ligament"
which formed so troublesome a feature in the transits of
Venus in 1764 and 1769, and which, to the regret and con-
fusion, though no longer to the surprise of observers, was
renewed in that of 1874. No completely satisfactory explana-
tion of the entire phenomenon has yet been offered. Funda-
mentally, no doubt, it is an effect of what is called irradiation^
by which a bright object seems to encroach upon a dark one ;
but other circumstances, both instrumental and atmospheric,
aid in its production; 2 while the inequalities of the moon's
edge complicate the action of other causes.
The immediate result of Baily's observation at Jedburgh
was powerfully to stimulate attention to solar eclipses in their
physical aspect. Never before had an occurrence of the kind
been expected so eagerly or prepared for so actively as that
which was total over Central and Southern Europe on the 8th
of July 1842. Astronomers hastened from all- quarters to the
favoured region. The Astronomer Royal (Airy) repaired to
Turin; Baily to Pavia; Otto Struve threw aside his work
amidst the stars at Pulkowa, and went south as far as Lipeszk ;
Schumacher travelled from Altona to Vienna; Arago from
Paris to Perpignan. Nor did their trouble go unrewarded.
The expectations of the most sanguine were outdone by the
wonders disclosed.
Baily (whose narrative we again have recourse to) had set up
his Dollond's achromatic ($\ feet focal length) in an upper
room of the University of Pavia, and was eagerly engaged in
noting a partial repetition of the singular appearances seen by
him in 1836, when he was "astounded by a tremendous burst
of applause from the streets below, and at the same moment
was electrified at the sight of one of the most brilliant and
splendid phenomena that can well be imagined. For at that
1 Mem. R. A. S., vol. x. pp. 14-17.
2 See Proctor, Transits of Venus, pp. 63-66.
PROGRESS OF KNOWLEDGE OF THE SUN. 81
instant the dark body of the moon was suddenly surrounded
with a corona, or kind of bright glory similar in shape and
relative magnitude to that which painters draw round the
heads of saints, and which by the French is designated an
aureole. Pavia contains many thousand inhabitants, the major
part of whom were, at this early hour, walking about the streets
and squares or looking out of windows, in order to witness
this long-talked-of phenomenon ; and when ..the total obscu-
ration took place, which was instantaneous, there was an uni-
versal shout from every observer, which 'made the welkin
ring,' and, for the moment, withdrew my attention from the
object with which I was immediately occupied. I had indeed
anticipated the appearance of a luminous circle round the
moon during the time of total obscurity ; but I did not expect,
from any of the accounts of preceding eclipses that I had read,
to witness so magnificent an exhibition as that which took
place. . . . The breadth of the corona, measured from the
circumference of the moon, appeared to me to be nearly equal
to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense (indeed I may say
quite dense) close to the border of the moon, and became
gradually and uniformly more attenuate as its distance there-
from increased, assuming the form of diverging rays in a
rectilinear line, which at the extremity were more divided, and
of an unequal length ; so that in no part of the corona could I
discover the regular and well-defined shape of a ring at its
outer margin. It appeared to me to have the sun for its
centre, but I had no means of taking any accurate measures
for determining this point. Its colour was quite white, not
pearl-colour, nor yellow, nor red, and the rays had a vivid and
nickering appearance, somewhat like that which a gaslight
illumination might be supposed to assume if formed into a
similar shape. . . . Splendid and astonishing, however, as
this remarkable phenomenon really was, and although it could
not fail to call forth the admiration and applause of every
beholder, yet I must confess that there was at the same time
82 HISTORY OF ASTRONOMY.
something in its singular and wonderful appearance that was
appalling ; and I can readily imagine that uncivilised nations
may occasionally have become alarmed and terrified at such
an object, more especially at times when the true cause of the
occurrence may have been but faintly understood, and the
phenomenon itself wholly unexpected.
" But the most remarkable circumstance attending the pheno-
menon was the appearance of three large protuberances, ap-
parently emanating from the circumference of the moon, but
evidently forming a portion of the corona. They had the
appearance of mountains of a prodigious elevation ; their
colour was red tinged with lilac or purple ; perhaps the colour
of the peach-blossom would more nearly represent it. They
somewhat resembled the snowy tops of the Alpine mountains
when coloured by the rising or setting sun. They resembled
the Alpine mountains also in another respect, inasmuch as their
light was perfectly steady, and had none of that flickering or
sparkling motion so visible in other parts of the corona. All
the three projections were of the same roseate cast of colour,
and very different from the brilliant vivid white light that
formed the corona ; but they differed from each other in mag-
nitude. . . . The whole of these three protuberances were
visible even to the last moment of total obscuration ; at least,
I never lost sight of them when looking in that direction ; and
when the first ray of light was admitted from the sun, they
vanished, with the corona, altogether, and daylight was instan-
taneously restored." *
Notwithstanding unfavourable weather, the "red flames"
were perceived with little less clearness and no less amazement
from the Superga than at Pavia, and were even discerned by
Mr. Airy with the naked eye. "Their form" (the Astronomer
Royal wrote) "was nearly that of saw-teeth in the position
proper for a circular saw turned round in the same direction
in which the hands of a watch turn ; . . . their colour was
1 Mem. JR. A. S., vol. xv. pp. 4-6.
PROGRESS OF KNOWLEDGE OF THE SUN. 83
a full lake red, and their brilliancy greater than that of any
other part of the ring." l
The height of these extraordinary objects was estimated by
Arago at two minutes of arc, representing, at the sun's distance,
an actual elevation of 56,000 miles. When carefully watched,
the rose-flush of their illumination was perceived to fade
through violet to white as the light returned; the same
changes in a reversed order having accompanied their first
appearance. Their forms, however, during about three minutes
of visibility, showed no change, although ^ of so (apparently)
unstable a character as to suggest to Arago " mountains on the
point of crumbling into ruins " through topheaviness. 2
The corona, both as to figure and extent, presented very
different appearances at different stations. This was no doubt
due to varieties in atmospheric conditions. At the Superga,
for instance, all details of structure seem to have been effaced
by the murky air, only a comparatively feeble ring of light
being seen to encircle the moon. Elsewhere, a brilliant radi-
ated formation was conspicuous, spreading, at four opposite
points, into four vast luminous expansions, compared to feather-
plumes or aigrettes? Arago at Perpignan noticed consider-
able irregularities in the divergent rays ; some appeared curved
and twisted ; a few lay across the others, in a direction almost
tangential to the moon's limb; the general effect being
described as that of a "hank of thread in disorder." 4 At
Lipeszk, where the sun stood much higher above the horizon
than in Italy or France, the corona showed with surprising
splendour. Its apparent extent was judged by Struve to be
no less than twenty-five minutes (more than six times Airy's
estimate), while the great plumes spread their radiance to
three or four degrees from the dark lunar edge. So dazzling
was the light, that many well-instructed persons denied the
totality of the eclipse. Nor was the error without precedent,
although the appearances attending respectively a total and an
. x Mem. R. A. S., vol. xv. p. 16. 2 Annuaire, 1846, p. 409.
3 Ibid., p. 317. 4 Ibid., p. 322.
84 HISTORY OF ASTRONOMY.
annular eclipse are in reality wholly dissimilar. In the latter
case, the surviving ring of sunlight becomes so much enlarged
by irradiation, that the interposed dark lunar body is reduced
to comparative insignificance, or even invisibility. Maclaurin
tells us, 1 that during an eclipse of this character which he
observed at Edinburgh in 1737, "gentlemen by no means
shortsighted declared themselves unable to discern the moon
upon the sun without the aid of a smoked glass ; " and Baily
(who, however, was shortsighted) could distinguish, in 1836,
with the naked eye no trace of " the globe of purple velvet "
which the telescope revealed as projected upon the face of the
sun. 2 Moreover, the diminution of light is described by him
as " little more than might be caused by a temporary cloud
passing over the sun ; " birds continued in full song ; and
" one cock in particular was crowing with all his might while
the annulus was forming."
Very different were the effects of the eclipse of 1842, as to
which some interesting particulars were collected by Arago. 3
Beasts of burthen, he tells us, paused in their labour, and
could by no amount of punishment be induced to move until
the sun reappeared. Birds and beasts abandoned their food ;
linnets were found dead in their cages ; even ants suspended
their toil. Diligence-horses, on the other hand, seemed as
insensible to the phenomenon as locomotives. The convolvulus
and some other plants closed their leaves ; but those of the
mimosa remained open. The little light that remained was of
a livid hue. One observer described the general coloration
as resembling the lees of wine, but human faces showed pale
olive or greenish. We may, then, rest assured that none of
the remarkable obscurations recorded in history were due to
eclipses of the annular kind.
The existence of the corona is no modern discovery.
Indeed, it is too conspicuous an apparition to escape notice
from the least attentive, or least practised observer of a total
1 Phil. Trans., vol. xl. p. 192. 2 Mem. R. A. S., vol. x. p. 17.
3 Ann. du Bureau des Long.> 1846, p. 309,
PROGRESS OF KNOWLEDGE OF THE SUN. 85
eclipse. Nevertheless, explicit references to it are rare in
early times. Both Plutarch, 1 however, and Philostratus in his
Life of Apollonius of Tyana, 2 are unmistakable in their
allusions, the latter describing a " crown," or garland similar to
the iris, by which the sun was encompassed and obscured
during an eclipse. The first to take the phenomenon into
scientific consideration was Kepler. He showed, from the
positions in their orbits at the time of the sun and moon, that
an eclipse observed by Clavius at Rome in 1567 could not
have been annular, 3 as the dazzling coron*al radiance visible
during the obscuration had caused it to be believed. Although
he himself never witnessed a total eclipse of the sun, he care-
fully collected and compared the remarks of those more for-
tunate, and concluded that the ring of " flame-like splendour"
seen on such occasions was caused by the reflection of the
solar rays from matter condensed in the neighbourhood either
of the sun or moon. 4 To the solar explanation he gave his
own decided preference, but, with one of those curious flashes
of half-prophetic insight characteristic of his genius, declared
that "it should be laid by ready for use, not brought into
immediate requisition." 5 So literally was his advice acted
upon, that the theory, which we now know to be (broadly
speaking) the correct one, only emerged from the repository
of anticipated truths after 236 years of almost complete retire-
ment, and even then timorously and with hesitation.
The first eclipse of which the attendant phenomena were
observed with tolerable exactness was that which was central
in the South of France, May 12, 1706. Cassini then put
forward the view that the " crown of pale light " seen round
the lunar disc was caused by the illumination of the zodiacal
light ; 6 but it failed to receive the attention which, as a step
1 Op. Mor. et Phil., vol. ix. p. 682, edit. Lips. 1778.
1 Book viii. chap, xxiii. Both references are due to R. Grant, Astr.
Nach., No. 1838. 3 Astronomia Pars Optica, Op. omnia, t. ii. p. 317.
4 De Stelld Navd, Op., t. ii. pp. 696-697. 6 Astr. Pars. Op., p. 320.
6 Mem. de ?Ac. des Sciences, 1715, p. 119.
86 HISTORY OF ASTRONOMY.
in the right direction, it undoubtedly merited. Nine years
later we meet with Halley's comments on a similar event, the
first which had occurred in London since March 20, 1140. By
nine in the morning of April 22 (O.S.), 1715, the obscuration,
he tells us, " was about ten digits, 1 when the face and colour
of the sky began to change from perfect serene azure blue to a
more dusky livid colour, having an eye of purple intermixt. . . .
A few seconds before the sun was all hid, there discovered
itself round the moon a luminous ring, about a digit or perhaps
a tenth part of the moon's diameter in breadth. It was of a
pale whiteness or rather pearl colour, seeming to me a little
tinged with the colours of the iris, and to be concentric with
the moon, whence I concluded it the moon's atmosphere.
But the great height thereof, far exceeding our earth's atmos-
phere, and the observation of some, who found the breadth of
the ring to increase on the west side of the moon as emersion
approached, together with the contrary sentiments of those
whose judgment I shall always revere" (Newton is most pro-
bably referred to), " makes me less confident, especially in a
matter whereto I confess I gave not all the attention requisite."
He concludes by declining to decide whether the " enlightened
atmosphere," which the appearance " in all respects resembled,"
" belonged to sun or moon." 2
A French Academician, who happened to be in London at the
time, was less guarded in expressing an opinion. The Chevalier
de Louville declared emphatically for the lunar atmospheric
theory of the corona, 3 and his authority carried great weight.
It was, however, much discredited by an observation made by
Maraldi in 1724, to the effect that the luminous ring, instead
of travelling with the moon, was traversed by it. 4 This was
in reality decisive, though, as usual, belief lagged far behind
1 A digit = T \th of the solar diameter.
2 Phil. Trans., vol. xxix. pp. 247-249.
3 Mem de VAc. des Sciences, 1715 ; Histoire, p. 49 ; Memoires, pp. 93-98.
4 Ibid., 1724, p. 178.
PROGRESS OF KNOWLEDGE OF THE SUN. 87
demonstration. Moreover, the advantage accruing from this
fresh testimony was adjudged to the wrong claimant. In 1715
a novel explanation had been offered by Delisle and Lahire, 1
supported by experiments regarded at the time as perfectly
satisfactory. The aureola round the eclipsed sun, they argued,
is simply a result of the diffraction or apparent bending of the
sunbeams that graze the surface of the lunar globe an effect
of the same kind as the coloured fringes of shadows. And
this view prevailed amongst men of science until (and even
after) Brewster showed, with clear and simple decisiveness,
that such an effect could by no possibility be appreciable at
our distance from the moon. 2 Don Jose Joaquim de Ferrer,
who observed a total eclipse of the sun at Kinderhook, in the
State of New York, on June 16, 1806, seems to have been
ignorant that such a refined optical rationale of the phenomenon
was current in the learned world. Two alternative explanations
alone presented themselves to his mind as possible. The
bright ring round the moon must be due to the illumination
either of a lunar or of a solar atmosphere. If the former, he
calculated that it should have a height fifty times that of the
earth's gaseous envelope. " Such an atmosphere," he rightly
concluded, "cannot belong to the moon, but must without
any doubt belong to the sun." 3 He, however, stood alone in
this unhesitating assertion.
The importance of the problem was first brought fully home
to astronomers by the eclipse of 1842. The brilliant and
complex appearance which, on that occasion, challenged the
attention of so many observers, demanded and received, no
longer the casual attention hitherto bestowed upon it, but the
most earnest study of those interested in the progress of
science. Nevertheless, it was only by degrees and through a
process of " exclusions " (to use a Baconian phrase) that the
corona was . put in its right place as a solar appendage. As
1 Mem de fAc. des Sciences, 1715, p. 161, and pp. 166-169.
2 Ed. Ency., art. Astronomy, p. 635.
3 Trans. Am. Phil. See., vol. vi. p. 274.
88 HISTORY OF ASTRONOMY.
every other available explanation proved inadmissible and
dropped out of sight, the broad presentation of Nature's fact
remained, which, though of sufficiently obvious interpretation,
was long and persistently misconstrued. Nor was it until 1869
that absolutely decisive evidence on the subject was forth-
coming, as we shall see farther on.
Sir John Herschel, writing to his venerable aunt, relates that
when the brilliant red flames burst into view behind the dark
moon on the morning of the 8th July 1842, the populace of
Milan, with the usual inconsequence of a crowd, raised the
shout, " Es leben die Astronomen ! " * In reality, none were less
prepared for their apparition than the class to whom the
applause due to the magnificent spectacle was thus adjudged.
And in some measure through their own fault ; for many partial
hints and some distinct statements from earlier observers had
given unheeded notice that some such phenomenon might be
expected to attend a solar eclipse.
What we now call the " chromosphere " is an envelope of
glowing gases, principally hydrogen, by which the sun is
completely covered, and from which the " prominences " are
emanations, eruptive or otherwise. Now, continual indications
of the presence of this fire-ocean had been detected during
eclipses in the eighteenth and nineteenth centuries. Captain
Stannyan, describing in a letter to Flamsteed an occurrence of
the kind witnessed by him at Berne on May i (O.S.), 1706,
says that the sun's " getting out of the eclipse was preceded
by a blood-red streak of light from its left limb." 2 A pre-
cisely similar appearance was noted by both Halley and De
Louville in 1715; during annular eclipses by Lord Aberdour
in I737, 3 and by Short in 1748,* the tint of the ruby border
being, however, subdued to " brown " or " dusky red " by the
surviving sunlight; while observations identical in character
1 Memoir of Caroline Herschel, p. 327.
2 Phil. Trans., vol. xxv. p. 2240. 3 Ibid., vol. xl. p. 182.
4 Ibid., vol. xlv. p. 586.
PROGRESS OF KNOWLEDGE OF THE SUN. 89
were made at Amsterdam in I820, 1 at Edinburgh (by Hender-
son) in 1836, and at New York in i838. 2
" Flames " or " prominences," if more conspicuous, are less
constant in their presence than the glowing stratum from
which they spring. The first to describe them was a Swedish
professor named Vassenius, who observed a total eclipse at
Gottenburg, May 2 (O.S.), I733. 3 His astonishment equalled
his admiration when he perceived, just outside the edge of the
lunar disc, and suspended, as it seemed, in the coronal atmos-
phere, three or four reddish spots or clouds, one of which was
so large as to be detected with the naked eye. As to their
nature, he did not even offer a speculation, further than by
tacitly referring them to the moon, in which position they
appear to have remained so long as the observation was held
in mind. It was repeated in 1778 by a Spanish admiral, but
with no better success in directing efficacious attention to the
phenomenon. Don Antonio Ulloa was on board his ship the
Espagne in passage from the Azores to Cape St. Vincent
on the 24th of June in that year, when a total eclipse of the
sun occurred, of which he has left a valuable description.
His notices of the corona are full of interest ; but what just
now concerns us is the appearance of " a red luminous point "
" near the edge of the moon," which gradually increased in
size as the moon moved away from it, and was visible during
about a minute and a quarter. 4 He was satisfied that it
belonged to the sun because of its fiery colour and growth
in magnitude, and supposed that it was occasioned by some
crevice or inequality in the moon's limb, through which the
solar light penetrated.
1 Mem. R. A. S., vol. i. pp. 145, 148.
2 American Journal of Science, vol. xlii. p. 396.
3 Phil. Trans., vol. xxxviii. p. 134. Father Secchi has, however,
pointed out a tolerably distinct mention of a prominence so far back as
1239 A.D. In a description of a total eclipse of that date it is added, " Et
quoddam foramen erat ignitum in circulo solis ex parte inferiori " (Mura-
tori, Rer. It. Scriptores, t. xiv. col. 1097). The " circulus solis" of course
signifies the corona. 4 Phil. Trans., vol. Ixix. p. 114
9 o HISTORY OF ASTRONOMY.
Allusions less precise, both prior and subsequent, which it
is now easy to refer to similar objects (such as the " slender
columns of smoke" seen by Ferrer), 1 might be detailed; but the
evidence already adduced suffices to show that the prominences
viewed with such amazement in 1842 were no unprecedented,
or even unusual phenomenon.
It was more important, however, to decide what was their
nature than whether their appearance might have been anti-
cipated. They were generally, and not very incorrectly, set
down as solar clouds. Arago believed them to shine by re-
flected light, 2 but the Abbe" Peytal rightly considered them to
be self-luminous. Writing in a Montpelier paper of July
1 6, 1842, he declared that we had now become assured of
the existence of a third or outer solar envelope, composed
of a glowing substance of a bright rose tint, forming moun-
tains of prodigious elevation, analogous in character to the
clouds piled above our horizons. 3 This first extant descrip-
tion of a very important feature of our great luminary was
probably founded on an observation made by Berard at
Toulon during the then recent eclipse, " of a very fine red
band, irregularly dentelated, or, as it were, crevassed here and
there," 4 encircling a large arc of the moon's circumference.
It can hardly, however, be said to have obtained distinct
recognition until the 28th of July 1851. On that day a total
eclipse took place, which was observed with considerable suc-
cess in various parts of Sweden and Norway by a number of
English astronomers. Mr. Hind saw, on the south limb of
the moon, " a long range of rose-coloured flames," 5 described
by Mr. Dawes as "a low ridge of red prominences, resembling
in outline the tops of a very irregular range of hills." 6 Mr.
Airy termed the portion of this " rugged line of projections "
visible to him the sierra, and was struck with its brilliant
1 Trans. Am. Phil. Soc., vol. vi. 1809, p. 267.
2 Annuaire, 1846, p. 460. 3 Ibid,, p. 439, note.
4 Ibid., 1846, p. 416. 6 Mem. R. A. S., vol. xxi. p. 82.
6 Ibid., p. 90.
PROGRESS OF KNOWLEDGE OF THE SUN. 91
light and " nearly scarlet " colour. 1 Its true character of a
continuous solar envelope was inferred from these data by
(amongst others) Grant, Swan, and Littrow ; and was by
Father Secchi formally accepted as established after the
great eclipse of i86o. 2
Several prominences of remarkable forms, especially one
variously compared to a Turkish scimitar, *a sickle, and a
boomerang, were seen in 1851. In connection with them
two highly significant circumstances were pointed out. First,
that of the approximate coincidence between their positions
and those of sun-spots previously observed. 3 Next, that "the
moon passed over them, leaving them behind, and revealing
successive portions as she advanced." 4 This latter fact (as to
which there could be no doubt, since it was separately noted
by at least four first-rate observers) was justly considered by
the Astronomer Royal and others as affording absolute cer-
tainty of the solar dependence of these singular objects.
Nevertheless sceptics were still found. M. Faye of the
French Academy inclined to a lunar origin for them; 5
Professor von Feilitsch of Greifswald published in 1852 a
treatise for the express purpose of proving all the luminous
phenomena attendant on solar eclipses corona, prominences,
and " sierra " (or chromosphere) to be purely optical appear-
ances. 6 Happily, however, the unanswerable arguments of the
photographic camera were soon to be made available against
such hardy incredulity.
Thus, the virtual discovery of the solar appendages, both
coronal and chromospheric, may be said to have been begun
in 1842, and completed in 1851. The current Herschelian
1 Mem. R. A. S., vol. xxi. pp. 7-8. 2 Le Soleil, t. i. p. 386.
3 By Williams and Stanistreet, Mem. R. A. S., vol. xxi. pp. 54, 56.
Santini had made a similar observation at Padua in 1842. Grant, Hist.
A sir., p. 401.
4 Lassell in Month. Not., vol. xii. p. 53.
5 Comptes Rendus, t. xxxiv. p. 155.
6 Optische Untersuchungen> and Zeitschrift fur populart Mittheilungen,
Bod. i. 186, p. 201.
92 HISTORY OF ASTRONOMY.
theory of the solar constitution remained, however, for the
time, intact. Difficulties, indeed, were thickening around it ;
but their discussion was perhaps felt to be premature, and they
were permitted to accumulate without debate, until fortified by
fresh testimony into unexpected and overwhelming preponder-
ance.
( 93 )
CHAPTER IV.
PLANETARY DISCOVERIES.
IN the course of his early gropings towards a law of the
planetary distances, Kepler tried the experiment of setting a
planet, invisible by reason of its smallness, to revolve in the
vast region of (seemingly) desert space separating Mars from
Jupiter. 1 The disproportionate magnitude of the same in-
terval was explained by Kant as due to the overweening size
of Jupiter. The zone in which each planet moved was,
according to the philosopher of Konigsberg, to be regarded as
the empty storehouse from which its materials had been
derived. A definite relation should thus exist between the
planetary masses and the planetary intervals. 2 Lambert, on
the other hand, sportively suggested that the body or bodies
(for it is noticeable that he speaks of them in the plural)
which once bridged this portentous gap in the solar system,
might, in some remote age, have been swept away by a great
comet, and forced to attend its wanderings through space. 3
These speculations were destined before long to assume a
more definite form. Johann Daniel Titius, a professor at
Wittenberg (where he died in 1796), pointed out in 1772, in a
note to a translation of Bonnet's Contemplation de la Nature , 4
1 Op., t. i. p. 107. He interposed, but tentatively only, another similar
body between Mercury and Venus.
2 Allgemeine Naticrgeschichie (ed. 1798), pp. 118-119.
3 Cosmologische Brief e> No. I (quoted by Von Zach, Monat. Corr.,
vol. iii. p. 592).
4 Second ed., p. 7. See Bode, Von dem neucn Hauptplanelen, p. 43, note.
94 HISTORY OF ASTRONOMY.
the existence of a remarkable symmetry in the disposition of
the bodies constituting the solar system. By a certain series of
numbers, increasing in regular progression, 1 he showed that
the distances of the six known planets from the sun might
be represented with a close approach to accuracy. But with
one striking interruption. The term of the series succeeding
that which corresponded to the orbit of Mars was without a
celestial representative. The orderly flow of the sequence was
thus singularly broken. The space where a planet should in
fulfilment of the " Law " have revolved, was, it appeared,
untenanted. Johann Elert Bode, then just about to begin his
long career as leader of astronomical thought and work at
Berlin, marked at once the anomaly, and filled the vacant
interval with an hypothetical planet. The discovery of Uranus
at a distance falling but slightly short of perfect conformity
to the law of Titius, lent weight to a seemingly hazardous
prediction, and Von Zach was actually at the pains, in 1785,
to calculate what he termed "analogical" elements 2 for this
unseen and (by any effect or influence) unfelt body. The
search for it, though confessedly scarcely less chimerical than
that of alchemists for the philosopher's stone, he kept steadily
in view for fifteen years, and at length, September 21, 1800,
succeeded in organising, in combination with five other Ger-
man astronomers assembled at Lilienthal, a force of what he
jocularly termed celestial police, for the express purpose of
tracking and intercepting the fugitive subject of the sun. The
zodiac was accordingly divided for purposes of scrutiny into
twenty-four zones ; their apportionment to separate observers
was in part effected, and the association was rapidly getting into
working order, when news arrived that the missing planet had
1 The representative numbers are obtained by adding 4 to the following
series (irregular, it will be observed, in its first member, which should be
i instead of o) : o, 3, 6, 12, 24, 48, &c. The formula is a purely em-
pirical one, and is, moreover, completely at fault as regards the distance of
Neptune.
2 Monat. Corr., vol. iii. p. 596.
PLANETARY DISCOVERIES. 95
been found, through no systematic plan of search, but by the
diligent, though otherwise directed labours of a distant watcher
of the skies.
Giuseppe Piazzi was born at Ponte in the Valtelline, July
1 6, 1746. He studied at various places and times under
Tiraboschi, Beccaria, Jacquier, and Le Sueur and having en-
tered the Theatine order of monks at the age of eighteen,
he taught philosophy, science, and theology in several of the
Italian cities as well as in Malta until 1780, when the chair of
mathematics in the University of Palermo was offered to and
accepted by him. Prince Caramanico, then viceroy of Sicily,
had scientific leanings, and was easily won over to the project
of building an observatory, a commodious foundation for
which was afforded by one of the towers of the viceregal
palace. This architecturally incongruous addition to an
ancient Saracenic edifice once the abode of Kelbite and
Zirite Emirs was completed in February 1791. Piazzi,
meanwhile, had devoted nearly three years to the assiduous
study of his new profession, acquiring a practical knowledge
of Lalande's methods at the Ecole Militaire, and of Maskelyne's
at the Royal Observatory; and returned to Palermo in 1789,
bringing with him, in the great five-foot circle which he had
prevailed upon Ramsden to construct, the most perfect mea-
suring instrument hitherto employed by an astronomer.
He had been above nine years at work on his star-catalogue,
and was still profoundly unconscious that a place amongst the
Lilienthal band of astronomical detectives was being held in
reserve for him, when, on the first evening of the nineteenth
century, January i, 1801, he noted the position of an eighth-
magnitude star in a part of the constellation Taurus, towards
which an error of Wollaston's had directed his special attention.
On the following night, it seemed to him that the star had slightly
shifted its position to the west; on the 3d, he assured himself of
the fact, and believed that he had chanced upon a new kind of
comet without tail or coma. The wandering body (whatever
its nature) exchanged retrograde for direct motion on January
96 HISTORY OF ASTRONOMY.
13,* and was carefully watched by Piazzi until February n,
when a dangerous illness interrupted his observations. He
had, however, not omitted to give notice of his discovery, but
so precarious were communications in those unpeaceful times,
that his letter to Oriani of January 23 did not reach Milan
until April 5, while a missive of one day later addressed to
Bode came to hand at Berlin, March 20. The delay just
afforded time for the publication, by a young philosopher of
Jena named Hegel, of a " Dissertation " showing, by the
clearest light of reason, that the number of the planets could
not exceed seven, and exposing the folly of certain devotees
of induction who. sought a new celestial body merely to fill a
gap in a numerical series. 2
Unabashed by speculative scorn, Bode had scarcely read
Piazzi's letter when he concluded that it referred to the precise
body in question. The news spread rapidly, and created a
profound sensation not unmixed with alarm lest this latest
addition to the solar family should have been found only to
be again lost. For by that time Piazzi's moving star was too
near the sun to be any longer visible, and in order to redis-
cover it after conjunction a tolerably accurate knowledge of
its path was indispensable. But a planetary orbit had never
before been calculated from such scanty data as Piazzi's ob-
servations afforded ; 3 and the attempts made by nearly every
astronomer of note in Germany to compass the problem were
manifestly inadequate, failing even to account for the positions
in which the body had been actually seen, and a fortiori serv-
ing only to mislead as to the places where, from September
1 80 1, it ought once more to have become discernible. It
was in this extremity that the celebrated mathematician Gauss
came to the rescue. He was then in his twenty-fifth year, and
1 Such reversals of direction in the apparent movements of the planets
are a consequence of the earth's revolution in its orbit.
2 Dissertatio Philosophica de Orbitis Planeiarum, iSoi. See Wolf,
Gcsch. d. Astr., p. 685.
3 Observations on Uranus, as a supposed fixed star, reached back to
1690.
PLANETARY DISCOVERIES. 97
was earning his bread by tuitions at Brunswick, with many
possibilities, but no settled career before him. The news
from Palermo may be said to have converted him from an
arithmetician into an astronomer. He was already in posses- 1
sion of a new and more general method of computing elliptical
orbits, and the system of " least squares," which he had de-
vised though not published, enabled him to extract the utmost
amount of probable truth from a given set of observations.
Armed with these novel powers, he set to work, and the com-
munication in November of his elements and ephemeris for
the lost object revived the drooping hopes of the little band
of eager searchers. Their patience, however, was to be still
further tried. Clouds, mist, and sleet seemed to have con-
spired to cover the retreat of the fugitive; but on the last
night of the year the sky cleared unexpectedly with the setting
in of a hard frost, and there, in the north-western part of Virgo,
nearly in the position assigned by Gauss to the runaway
planet, a strange star was discerned by Von Zach l at Gotha,
and on the subsequent evening the anniversary of the original
discovery by Olbers at Bremen. The name of Ceres (as the
tutelary goddess of Sicily) was, by Piazzi's request, bestowed
upon this first known of the numerous, and probably all but
innumerable, family of the minor planets.
The recognition of the second followed as the immediate
consequence of the detection of the first. Olbers had made
himself so familiar with the positions of the small stars along
the track of the long-missing body, that he was at once struck,
March 28, 1802, with the presence of an intruder near the
spot where he had recently identified Ceres. He at first
believed the newcomer to be a variable star usually incon-
spicuous, but just then at its maximum of brightness ; but
within two hours he had convinced himself that it was no fixed
star, but a rapidly moving object. The aid of Gauss 'was
1 He had caught a glimpse of it on December 7, but was prevented by
bad weather from verifying his suspicion. Monat. Corr., vol. v. p.
171.
G
98 HISTORY OF ASTRONOMY.
again invoked, and his prompt calculations showed that this
fresh celestial acquaintance (named "Pallas" by Olbers) re-
volved round the sun at nearly the same mean distance
as Ceres, and was beyond question of a strictly analogous
character.
This result was perplexing in the extreme. The symmetry
and simplicity of the planetary scheme appeared fatally com-
promised by the admission of many, where room could,
according to old-fashioned rules, only be found for one. A
daring hypothesis of Olbers' invention provided an exit from
the difficulty. He supposed that both Ceres and Pallas were
fragments of a primitive trans-Martian planet, blown to pieces
in the remote past, either by the action of internal forces or by
the impact of a comet ; and predicted that many more such
fragments would be found to circulate in the same region. He,
moreover, pointed out that these numerous orbits, however
much they might differ in other respects, must all have a
common line of intersection, 1 and that the bodies moving in
them must consequently pass, at each revolution, through two
opposite points of the heavens, one situated in the Whale, the
other in the constellation of the Virgin, where already Pallas
had been found, and Ceres recaptured. The intimation that
fresh discoveries might be expected in those particular regions
was 'singularly justified by the detection of the two bodies now
known respectively as Juno and Vesta. The first was found
near the predicted spot in Cetus by Harding, Schroter's
assistant at Lilienthal, September 2, 1804; the second by
Olbers himself in Virgo, after three years of patient scrutiny,
March 29, 1807.
The theory of an exploded planet now seemed to have
everything in its favour. It required that the mean or average
distances of the newly discovered bodies should be nearly the
1 Planetary fragments, hurled in any direction, and with any "velocity
short of that which would for ever release them from the solar sway, would
continue to describe elliptic orbits round the sun, all passing through the
scene of the explosion, and thus possessing a common line of intersection.
PLANETARY DISCOVERIES. 99
same, but admitted a wide range of variety in the shapes and
positions of their orbits, provided always that they preserved
common points of intersection. These conditions were fulfilled
with a striking approach to exactness. Three of the four
" asteroids " (a designation introduced by Sir W. Herschel 1 )
conformed with very approximate precision to " Bode's law "
of distances ; they all traversed, in their circuits round the
sun, nearly the same parts of Cetus and Virgo ; while the
eccentricities and inclinations ,of their paths departed widely
from the planetary type that of Vesta, for example, making
with the ecliptic an angle of no less than 35. The minute-
ness of these bodies appeared further to strengthen the im-
putation of a fragmentary character. Herschel estimated the
diameter of Ceres at 162, that of Pallas at 147 miles. 2 Juno
is smaller than either ; and even Vesta, which surpasses all the
minor planets in size, and may, under favourable circumstances,
be seen with the naked eye, has a diameter scarcely, if at all,
exceeding 500 miles. A suspected variability of brightness in
some of the asteroids, somewhat hazardously explained as due
to the irregularities of figure to be expected in cosmical pot-
sherds (so to speak), was added to the confirmatory evidence. 3
The strong point of the theory, however, lay not in what it
explained, but in what it had predicted. It had been twice
confirmed by actual exploration of the skies, and had produced,
in the recognition of Vesta, the first recorded instance of the
premeditated discovery of a heavenly body.
The view not only commended itself to the facile imagina-
tion of the unlearned, but received the sanction of the highest
scientific authority. The great Lagrange bestowed upon it his
analytical imprimatur ; showing that the explosive forces re-
quired to produce the supposed catastrophe came well within
1 Phil. Trans., vol. xcii. part ii. p. 228.
2 Ibid., p. 218. In a letter to Von Zach of June 24, 1802, he speaks of
Pallas as "almost incredibly small," and makes it only seventy English
miles in diameter. Monat. Corr., vol. vi. pp. 89-90.
3 Olbers, Monat. Corr., vol. vi. p. 88.
ioo HJSTORY OF ASTRONOMY.
the bounds of possibility a velocity of less than twenty times
that of a cannon-ball leaving the gun's mouth- sufficing, ac-
cording to his calculation, to have launched the asteroidal
fragments on their respective paths. Indeed, he was disposed
to regard the hypothesis of disruption as more generally
available than its author had designed it to be, and proposed
to supplement with it, as explanatory of the eccentric orbits of
comets, the nebular theory of Laplace, thereby obtaining, as
he said, " a complete view of the origin of the planetary system
more conformable to Nature and mechanical laws than any yet
proposed." l
Nevertheless .the hypothesis of Olbers has not held its
ground. It seemed as if all the evidence available for its
support had been produced at once and spontaneously, while
the unfavourable items were elicited slowly, and, as it were, by
cross-examination. A more extended acquaintance with the
group of bodies whose peculiarities it was framed to explain, has
shown them, after all, as recalcitrant to any such explanation.
Coincidences at the first view significant and striking have
been swamped by contrary examples; and a hasty general
conclusion has, by a not uncommon destiny, at last perished
under the accumulation of particulars. Moreover, as has
been remarked by Professor Newcomb, 2 mutual perturbations
would rapidly efface all traces of a common disruptive origin,
and the catastrophe, to be perceptible in its effects, should have
been comparatively recent.
A new generation of astronomers had arisen before any addi-
tions were made to the little family of the minor planets. Piazzi
died in 1826, Harding in 1834, Olbers in 1840; all those who
had prepared, or participated in the first discoveries passed
away without witnessing their resumption. In 1830, however,
a certain Hencke, ex-postmaster in the Prussian town of
Driessen, set himself to watch for new planets, and after
fifteen long years his patience was rewarded. The asteroid
found by him December 8, 1845, received the name of Astraea,
1 Conn. d. Terns, for 1814, p. 218. 2 Popular Astronomy, p. 327.
PLANETARY DISCOVERIES. 101
and his further prosecution of the search resulted, July i, 1847,
in the discovery of Hebe. A few weeks later, August 13, Mr.
Hind, after many months' exploration from Mr. Bishop's obser-
vatory in the Regent's Park, picked up Iris, and, October 18,
Flora. 1 The next on the list was Metis, found by Mr. Graham,
April 25, 1848, at Markree in Ireland. 2 At the close of the
period to which our attention is at present limited, the number
of these small bodies known to astronomy was thirteen ; and
the course of discovery has since proceeded still more rapidly
and with less interruption.
Both in itself and in its consequences the recognition of the
minor planets was of the highest importance to science. The
traditional ideas regarding the constitution of the solar system
were enlarged by the admission of a new class of bodies,
strongly contrasted, yet strictly co-ordinate with the old-estab-
lished planetary order; the profusion of resource, so conspi-
cuous in the living kingdoms of Nature, was seen to prevail
no less in the celestial spaces j and some faint preliminary
notion was afforded of the indefinite complexity of relations
underlying the apparent simplicity of the majestic scheme to
which our world belongs. Theoretical and practical astronomy
both derived profit from the admission of these apparently insig-
nificant strangers to the rights of citizenship of the solar system.
The disturbance of their motions by their giant neighbour
afforded a more accurate knowledge of the Jovian mass, which
Laplace had taken about -f^h too small ; the anomalous char-
acter of their orbits presented geometers with highly stimulating
problems in the theory of perturbations ; while the exigencies
of the first discovery had produced the Theoria Motus, and
won Gauss over to the ranks of calculating astronomy. More-
over, the sure prospect of further detections powerfully in-
cited to the exploration of the skies ; observers became more
numerous and more zealous in view of the prizes held out to
them ; star-maps were diligently constructed, and the sidereal
multitude strewn along the great zodiacal belt acquired a fresh
1 Month, Not., vol. vii. p. 299 ; vol. viii. p. I. 2 Ibid., vol. viii. p. 146.
102 HISTORY OF ASTRONOMY.
interest when it was perceived that its least conspicuous
member might be a planetary shred or projectile in the majestic
disguise of a distant sun. Harding's " Celestial Atlas," designed
for the special purpose of facilitating asteroidal research, was
the first systematic attempt to represent to the eye the telescopic
aspect of the heavens. It was while engaged on its construc-
tion that the Lilienthal observer successfully intercepted Juno
on her passage through the Whale in 1804; whereupon pro-
moted to Gottingen, he there completed, in 1822, the arduous
task so opportunely entered upon a score of years previously.
Still more important were the great star-maps of the Berlin
Academy, undertaken at Bessel's suggestion, with the same
object of distinguishing errant from fixed stars, and exe-
cuted, under Encke's supervision, during the years ^830^59.
They have played a noteworthy part in the history of planetary,
discovery, nor of the minor kind alone.
We have now to recount an event unique in scientific history.
The discovery of Neptune has been characterised as the result
of a " movement of the age," * and with some justice. It had
become necessary to the integrity of planetary theory. Until
it was accomplished, the phantom of an unexplained anomaly
in the orderly movements of the solar system must for ever
have haunted the brains of astronomers. Moreover, it was
prepared by many, suggested as possible by not a few, and
actually achieved, simultaneously, independently, and com-
pletely, by two investigators.
The position of the planet Uranus was recorded as that of
a fixed star no less than twenty times between 1690 and
the epoch of its final detection by Herschel. But these early
observations, far from affording the expected facilities for the
calculation of its orbit, proved a source of grievous perplexity.
The utmost ingenuity of geometers failed to combine them
satisfactorily with the later Uranian places, and it became
evident, either that they were widely erroneous, or that the
revolving body was wandering from its ancient track. The
1 Airy, Mem. R. A. S., vol. xvi. p. 386.
PLANETARY DISCOVERIES. 103
simplest course was to reject them altogether, and this was
done in the new Tables published in 1821 by Alexis Bouvard,
the indefatigable computating partner of Laplace. But the
trouble was not thus to be got rid of. After a few years fresh
irregularities began to appear, and continued to increase until
absolutely "intolerable." It may be stated, as illustrative of
the perfection tw which astronomy had been brought, that diver-
gences regarded as menacing the very foundation of its theories
never entered the range of unaided vision. In other words,
if the theoretical and the real Uranus had been placed side by
side in the sky, they would have seemed, to the sharpest eyes,
to form a single body. 1
The idea that these enigmatical disturbances were due to
the attraction of an unknown exterior body was a tolerably
obvious one ; and we accordingly find it suggested in many
different quarters. Bouvard himself was perhaps the first to
conceive it. He kept the possibility continually in view, and
bequeathed to his nephew's diligence the inquiry into its reality
when he felt that his own span was drawing to a close ; but
before any progress had been made with it, he had already
(June 7, 1843) "ceased to breathe and to calculate." The
Rev. T. J. Hussey actually entertained in 1834 the notion, but
found his powers inadequate to the task, of assigning an
approximate place to the disturbing body ; and Bessel, in 1840,
laid his plans for an assault in form upon the Uranian diffi-
culty, the triumphant exit from which fatal illness frustrated
his hopes of effecting or even witnessing.
The problem was practically untouched when, in 1841, an
undergraduate of St. John's College, Cambridge, formed the
resolution of grappling with it. The projected task was an
1 See Newcomb's Pop, Astr., p. 359. The error of Uranus amounted,
in 1844, to 2' j but even the tailor of Breslau, whose extraordinary powers
of vision Humboldt commemorates (Cosmos, Bd. iii. p. 112), could only
see Jupiter's first satellite at its greatest elongation, 2' 15". He might, how-
ever, possibly have distinguished two objects of equal lustre at a lesser
interval. The components of the double star e Lyrae, which Bessel, when
a boy, could see separately with the naked eye, are 3^' apart.
104 HISTORY OF ASTRONOMY.
arduous one. There were no guiding precedents for its
conduct. Analytical obstacles had to be encountered so
formidable as to appear invincible even to such a mathema-
tician as Airy. John Couch Adams, however, had no sooner
taken his degree, which he did as senior wrangler in January
1843, than he set resolutely to work, and by October 1845
was able to communicate to the Astronomer Royal numeri-
cal estimates of the elements and mass of the unknown
planet, together with an indication of its actual place in the
heavens.
Sir George Biddell Airy had begun .in 1835 his long and
energetic administration of Greenwich Observatory, and was
already in possession of data vitally important to the momen-
tous inquiry then on foot. At his suggestion, and under his
superintendence, the reduction of all the planetary observations
made at Greenwich from 1750 downwards had been under-
taken in 1833. The results, published in 1846, constituted a
permanent and universal stock of materials for the correction
of planetary theory. But in the meantime, investigators, both
native and foreign, were freely supplied with the " places and
errors," which, clearly exhibiting the discrepancies between
observation and calculation between what was and what
was expected formed the very groundwork of future improve-
ments.
Mr. Adams had no reason to complain of official discourtesy.
His labours were aided and encouraged ; but they were not
fully believed in. " I have always," Sir George Airy wrote, 1
"considered the correctness of a distant mathematical result to
be a subject rather of moral than of mathematical evidence."
And, in the case actually before him, there was absolutely no
warrant for putting faith in the solution, by a young and
untried man, of a problem before the complexities of which
Laplace himself might have quailed. Moreover, Mr. Adams
unaccountably neglected to answer (until too late) a question
regarded by Sir George Airy in the light of an experimentum
1 Mem. R. A. S., vol. xvi. p. 399.
PLANETARY DISCOVERIES. 105
cruets as to the soundness of the new theory. Nor did he
himself take any steps to obtain a publicity which he was more
anxious to merit than to secure. The investigation conse-
quently remained buried in obscurity. It is now known that
had a search been instituted in the autumn of 1845 for
the remote body -\ahose existence had been so marvellously
foretold, it would have been found within three and a half
lunar diameters (i 49') of the spot assigned to it by Mr.
Adams.
A competitor, however, equally daring and more fortunate
audax fortuna adjutus, as Gauss said of him was even then
entering the field. Urbain Jean Joseph Leverrier, the son of
a small Government employe in Normandy, was born at Saint-
L6, March n, 1811. He studied with brilliant success at the
Ecole Polytechnique, accepted the post of astronomical teacher
there in 1837, and, "docile to circumstance," immediately
concentrated the whole of his vast, though as yet undeveloped
powers upon the formidable problems of celestial mechanics.
He lost no time in proving to the mathematical world that the
race of giants was not extinct. Two papers on the stability of
the solar system, presented to the Academy of Sciences,
September 16 and October 14, 1839, showed him to be the
worthy successor of Lagrange and Laplace, and encouraged
hopes destined to be abundantly realised. His attention
was directed by Arago to the Uranian difficulty in 1845,
when he cheerfully put aside certain intricate cometary re-
searches upon which he happened to be engaged, in order to
obey with dutiful promptitude the summons of the astrono-
mical chief of France. In his first memoir on the subject
(communicated to the Academy, November 10, 1845), ne
proved the inadequacy of all known causes of disturbance
to account for the vagaries of Uranus \ in a second (June i,
1846), he demonstrated that only an exterior body occupying
at a certain date a determinate position in the zodiac could
produce the observed effects; in a third (August 31, 1846),
he assigned the orbit of the disturbing body, and announced
106 HISTORY OF ASTRONOMY.
its visibility as an object with a sensible disc about as bright
as a star of the eighth magnitude.
The question was now visibly approaching an issue. On
September 10 Sir John Herschel declared to the British Asso-
ciation respecting the hypothetical new planet : " We see it
as Columbus saw America from the coast of Spain. Its move-
ments have been felt, trembling along the far-reaching line of
our analysis with a certainty hardly inferior to that of ocular
demonstration." Less than a fortnight later, September 23,
Professor Galle, of the Berlin Observatory, received a letter
from Leverrier requesting his aid in the telescopic part of the
inquiry already analytically completed. He directed his re-
fractor to the heavens that same night, and perceived within
less than a degree of the spot indicated, an object with a
measurable disc nearly three seconds in diameter. Its absence
from Bremiker's recently completed map of that region of the
sky showed it to be no star, and its movement in the predicted
direction confirmed without delay the strong persuasion of its
planetary nature.
In this remarkable manner the existence of the remote
member of our system known as " Neptune " was ascertained.
But the discovery, which faithfully reflected the duplicate char-
acter of the investigation which led to it, had been already
secured at Cambridge before it was announced from Berlin.
Sir George Airy's incredulity vanished in the face of the strik-
ing coincidence between the position assigned by Leverrier
to the unknown planet in June, and that laid down by Mr.
Adams in the previous October ; and on the 9th of July he
wrote to Professor Challis, director of the Cambridge Observa-
tory, recommending a search with the great Northumberland
equatoreal. Had a good star-map been at hand, the process
would have been a simple one ; but of Bremiker's " Hora XXI."
no news had yet reached England, and there was no other
sufficiently comprehensive to be available for an inquiry which,
in the absence of such aid, promised to be both long and
laborious. As the event proved, it might have been neither.
PLANETARY DISCOVERIES. 107
" After four days of observing," Professor Challis wrote, Oc-
tober 12, 1846, to Sir George Airy, "the planet was in my
grasp if' only I had examined or mapped the observations." l
Had he done so, the first honours in the discovery, both
theoretical and optical, would flfave fallen to the University of
Cambridge. But Professor Challis had other astronomical
avocations to attend to, and, moreover, his faith in the precision
of the indications furnished to him was, by his own confes-
sion, a very feeble one. For both reasons he postponed to a
later stage of the proceedings the discussion and comparison
of the data nightly furnished to him by his telescope, and
thus allowed to lie, as it were, latent in his observations the
momentous result which his diligence had ensured, but which
his delay suffered to be anticipated. 2
Nevertheless it should not be forgotten that the Berlin as-
tronomer had two circumstances in his favour apart from which
his swift success could hardly have been achieved. The first
was the possession of a good star-map ; the second was the
clear and confident nature of Leverrier's instructions. " Look
where I tell you," he seemed authoritatively to say, " and you
will see an object such as I describe." 3 And in fact, not only
Galle on the 23d of September, but also Challis on the 29th,
immediately after reading the French geometer's lucid and
impressive treatise, picked out from among the stellar points
strewing the zodiac, a small planetary disc, which eventually
proved to be that of the precise body he had been in search
of during two months.
Personal questions, however, vanish in the magnitude of the
event they relate to. By it the last lingering doubts as to the
universal validity of the Newtonian Law were dissipated.
Recondite analytical methods received a confirmation brilliant
1 Mem. R. A. S., vol. xvi. p. 412.
2 He had recorded the places of 3150 stars (three of which were different
positions of the planet), and was preparing to map them, when, October
I, news of the discovery arrived from Berlin. Prof. Challis's Report ', quoted
in Obituary Notice, Month. Not., Feb. 1803, p. 170.
3 See Airy in Mem. R. A. S. t vol. xvi. p. 411.
io8 HISTORY OF ASTRONOMY.
and intelligible even to the minds of the .vulgar, and emerged
from the patient solitude of the study to enjoy an hour of
clamorous triumph. For ever invisible to the unaided eye of
man, a sister-globe to our earth was shown to circulate, in
perpetual frozen exile, at thirty times its distance from the sun.
Nay, the possibility was made apparent that the limits of our
system were not even thus reached, but that yet profounder
abysses of space might shelter obedient, though little favoured
members of the solar family, by future astronomers to be re-
cognised through the sympathetic thrillings of Neptune, even
as Neptune himself was recognised through the tell-tale devia-
tions of Uranus.
It is curious to find that the fruit of Adams' and Leverrier's
laborious investigations had been accidentally all but snatched
half a century before it was ripe to be gathered, On the 8th,
and again on the loth of May 1795, Lalande noted the posi-
tion of Neptune as that of a fixed star, but perceiving that the
two observations did not agree, he suppressed the first as
erroneous, and pursued the inquiry no further. An immor-
tality which he would have been the last to despise hung in
the balance; the feather-weight of his carelessness, however,
kicked the beam, and the discovery was reserved to be more
hardly won by later comers.
Bode's Law did good service in the quest for a trans-Uranian
planet by affording ground for a probable assumption as to
its distance. A starting-point for approximation was pro-
vided by it; but it was soon found to be considerably at
fault. Even Uranus is about 36 millions of miles nearer to
the sun than the order of progression requires ; and Neptune's
vast distance of 2800 million should be increased by no less
than 800 million miles, and its period of 165 lengthened out to
225 years, 1 in order to bring it into conformity with the curious
and unexplained rule which planetary discoveries have alter-
nately tended to confirm and to invalidate.
Within seventeen days of its identification with the Berlin
1 Ledger, The Sun, its 'Planets, and their Satellites, p. 414.
SJ
CAUf
PLANETARY DISCOVERIES. 109
achromatic, Neptune was found to be attended by a satellite.
This discovery was the first notable performance of the cele-
brated two-foot reflector l erected by Mr.^Lassell at his sugges-
tively named residence of Starfield, near Liverpool. William
Lassell was a brewer by profession, but by inclination an
astronomer. Born at Bolton in Lancashire, June 18, 1799,
he closed a life of eminent usefulness to science, October 5,
1880, thus spanning with his well-spent years almost the
entire of the momentous period which we have undertaken to
traverse. At the age of twenty-one, being without the means
to purchase, he undertook to construct telescopes, and natu-
rally turned his attention to the reflecting sort, as favouring
amateur efforts by the comparative simplicity of its structure.
His native ingenuity was remarkable, and was developed by
the hourly exigencies of his successive enterprises. Their
uniform success encouraged him to enlarge his aims, and in
1844 he visited Birr Castle for the purpose of inspecting the
machine used in polishing the giant speculum of Parsonstown.
In the construction of his new instrument, however, he even-
tually discarded the model there obtained, and worked on a
method of his own, assisted by the supreme mechanical skill
of James Nasmyth. The result was a Newtonian of exquisite
definition, with an aperture of two, and a focal length of twenty
feet, provided by a novel artifice with the equatoreal mount-
ing, previously regarded as available only for refractors.
This beautiful instrument afforded to its maker, October
jo, 1846, a cursory view of a Neptunian attendant. But the
planet was then approaching the sun, and it was not until the
following July that the observation could be verified, which it
was completely, first by Lassell himself, and somewhat later
by Otto Struve and Bond of Cambridge (U.S.) When it
is considered that this remote object shines by reflecting
sunlight reduced by distance to -9 J^th of the intensity with
which it illuminates our moon, the fact of its visibility, even
in the most perfect telescopes, is a somewhat surprising one.
1 Lately presented by the Misses Lassell to the Greenwich Observatory.
i io HISTORY OF ASTRONOMY.
It can only, indeed, be accounted for by attributing to it
dimensions very considerable for a body of the secondary
order. It shares with the moons of Uranus the peculiarity
of retrograde motion ; that is to say, its revolutions, running
counter to the grand current of movement in the solar sys-
tem, are performed from east to west, in a plane inclined
at an angle of 35 to that of the ecliptic. Their swiftness
serves to measure the mass of the globe round which they are
performed. For while our moon takes twenty-seven days
and nearly eight hours to complete its circuit of the earth, the
satellite of Neptune, at a distance not greatly inferior, sweeps
round its primary in five days and twenty-one hours, showing
(according to a very simple principle of computation) that it
is urged by a force seventeen times greater than the terrestrial
pull upon the lunar orb. Combining this result with that of
measurements of the small telescopic disc of this farthest
known planet, it is found that while in mass Neptune equals
seventeen earths, in bulk it is equivalent to eighty-four. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal
heat, as yet unwasted by radiation into space, to about five
times the volume they would occupy in the interior of our
globe. The fact, at any rate, is fairly well ascertained that the
average density of Neptune differs little from that of water.
We must now turn from this late-recognised member of our
system to bestow some brief attention upon the still fruitful
field of discovery offered by one of the immemorial five. The
family of Saturn, unlike that of its brilliant neighbour, has been
gradually introduced to the notice of astronomers. Titan, the
sixth Saturnian moon in order of distance, led the way, being
detected by Huygens, March 25, 1655 ; Cassini made the
acquaintance of four more between 1671 and 1684; while
Mimas and Enceladus, the two innermost, were caught by
Herschel in 1789, as they threaded their lucid way along the
edge of the almost vanished ring. In the distances of these
seven revolving bodies from their primary, an order of pro- '
I
PLANETARY DISCOVERIES. in
gression analogous to that pointed out by Titius in the
planetary intervals was found to prevail 4 but with one con-
spicuous interruption, similar to that which had first suggested
the search for new members of the solar system. Between
Titan and Japetus the sixth and seventh reckoning outwards
there was obviously room for another satellite. It was
discovered, on both sides of the Atlantic simultaneously, on
the i Qth of September 1848. Mr. W. C. Bond, employing
the splendid 1 5-inch refractor of the Harvard Observatory,
noticed, September 16, a minute star situated in the plane
of Saturn's rings. The same object was discerned by Mr.
Lassell on the i8th. On the following evening, both observers
perceived that the problematical speck of light kept up with,
instead of being left behind by, the planet as it moved, and
hence inferred its true character. 1 Hyperion, the seventh by
distance and eighth by recognition of Saturn's attendant train,
is of so insignificant a size when compared with some of its
fellow-moons (Titan is but little inferior to the planet Mars),
as to have suggested to Sir John Herschel 2 the idea that it
might be only one of several bodies revolving very close
together in fact, an asteroidal satellite ; but the conjecture has,
so far, not been verified.
The coincidence of its duplicate discovery was singularly
paralleled two years later. Galileo's amazement when his
" optic glass " revealed to him the " triple " form of Saturn
planeta tergeminus has proved to be, like the laughter of the
gods, "inextinguishable." It must revive in every one who
contemplates anew the unique arrangements of that world apart
known to us as the Saturnian system. The resolution of the
so-called anscz, or " handles," into one encircling ring by
Huygens in 1655; the discovery by Cassini in 1675 f tne
division of that ring into two concentric ones ; the closely con-
cordant determination, theoretically by Laplace and optically
by Herschel, of their period of rotation, 3 constituted, with
1 Grant, Hist, of Astr., p. 271. 2 Month. Not., vol. ix. p. 91.
8 The computed period was loh. 33m. 365.; the observed period, loh.
32m. 155.
112 HISTORY OF ASTRONOMY.
some minor observations, the sum of the knowledge obtained,
up to the middle of the present century, on the subject
of this remarkable formation. The first place in the dis-
covery now about to be related belongs to an American
astronomer.
William Cranch Bond, born in 1789 at Falmouth (now
Portland), in the State of Maine, was a watchmaker whom the
solar eclipse of 1806 attracted to study the wonders of the
heavens. When, in 1815, the erection of an observatory in
connection with Harvard College, Cambridge, was first con-
templated, he undertook a mission to England for the purpose
of studying the working of similar institutions there ; and, on
his return, erected a private observatory at Dorchester, where
he worked diligently for many years. Meanwhile, the time
was approaching for the resumption of the long-postponed
design of the Harvard authorities ; and on the completion of
the new establishment in 1844, Bond, who had for some time
been officially connected with the College, and had carried on
his scientific labours within its precincts, was offered and
accepted the post of its director. Placed thus in possession
of one of the finest instruments in the world a masterpiece
of Merz and Mahler he headed the now long list of dis-
tinguished Transatlantic observers. Like the elder Struve, he
left an heir to his oifice and to his eminence; but George
Bond unfortunately died in 1865, at the early age of thirty-
nine, having survived his father but six years.
On the night of November 15, 1850 the air, remarkably
enough, being so hazy that only the brightest stars could be
perceived with the naked eye William Bond discovered a
third dusky ring, extending about half-way between the inner
bright one and the globe of Saturn. A fortnight later, but
before the observation had been announced in England, the
same appearance was seen by the Rev. W. R. Dawes with the
comparatively small refractor of his observatory at Watering-
bury, and on December 3 was described by Mr. Lassell (then
on a visit with him) as " something like a crape veil covering
PLANETARY DISCOVERIES. 113
a part of the sky within the inner ring." 1 Next morning the
Times containing the report of Bond's discovery reached
Wateringbury. The most surprising circumstance in the
matter was that the novel appendage had remained so long
unrecognised. As the rings opened out to their full extent,
it became obvious with very moderate optical assistance ; yet
some of the most acute observers who have ever lived, using
instruments of vast power, had heretofore failed to detect its
presence. It soon appeared, however, that Galle of Berlin 2
had noticed, June 10, 1838, a veil-like extension of the lucid
ring across half the dark space separating it from the planet ;
but the observation, although communicated at the time
to the Berlin Academy of Sciences, had remained barren.
Traces of the dark ring, moreover, were found in a drawing
executed by Campani in 1664 ; 3 and Picard (June 15, i6y3), 4
Hadley (spring of i72o), 5 and Herschel, 6 had all undoubtedly
seen it under the aspect of a dark bar or belt crossing the
Saturnian globe. It was, then, of no recent origin ; but there
seemed reason to think that it had lately gained considerably
in brightness. The full meaning of this remarkable fact it was
reserved for later investigations to develop.
What we may, in a certain sense, call the closing result of
the race for discovery, in which several observers seemed at
that time to be engaged, was the establishment, on a satis-
factory footing, of our acquaintance with the dependent system
of Uranus. Sir William Herschel, whose researches formed,
in so many distinct lines of astronomical inquiry, the starting-
points of future knowledge, detected, January n, 1787, 7 two
Uranian moons, since called Oberon and Titania, and ascer-
tained the curious circumstance of their motion in a plane
1 Month. Not., vol. xi. p. 21.
2 Astr. Nach., No. 756 (May 2, 1851).
3 F. Secchi, Month. Not., vol. xiii. p. 248.
4 Hind, in ibid., vol. xv. p. 32.
5 Lynn, Observatory, Oct. I, 1883; Hadley, Phil. Trans., vol. xxxii. p. 385.
6 Proctor, Saturn and its System, p. 64.
7 Phil. Trans., vol. Ixxvii. p. 125.
H
ii4 HISTORY OF ASTRONOMY.
almost at right angles to the ecliptic, in a direction contrary to
that of all previously known denizens (other than cometary) of
the solar kingdom. He believed that he caught occasional
glimpses of four more, but never succeeded in assuring himself
of their substantial existence. Even the two first remained
unseen save by himself until 1828, when his son re-observed
them with a 20-foot reflector, similar to that with which they
had been originally discovered. Thenceforward they were kept
fairly within view, but their four questionable companions,
in spite of some false alarms of detection, remained in the
dubious condition in which Herschel had left them. At last,
on October 24, I85I, 1 after some years of fruitless watching,
Mr. Lassell espied " Ariel " and " Umbriel," two Uranian atten-
dants, interior to Oberon and Titania, and of about half their
brightness ; so that their disclosure is still reckoned amongst
the very highest proofs of instrumental power and perfection.
In all probability they were then for the first time seen ; for
although Professor Holden, 2 director of the Washburn Observa-
tory (U.S.), has attempted to identify them with two of Herschel's
doubtful quartette, Mr. Lassell's argument 3 that the glare of
the planet in Herschel's great specula must have rendered
almost impossible the perception of objects so minute and so
close to its disc, appears tolerably decisive to the contrary.
Uranus is thus attended by four moons, and so far as present
knowledge extends by no more. Amongst the most impor-
tant of the "negative results" 4 secured by Mr. Lassell's
observations at Malta during the years 1852-53 and 1861-65,
were the convincing evidence afforded by them that, without
great increase of optical power, no further Neptunian or
Uranian satellites can be perceived, and the consequent re-
legation of Herschel's baffling four notwithstanding the un-
questioned place long assigned to them in astronomical text-
books to the shadowy condition of telescopic "ghosts."
1 Month. Not., vol. xi. p. 248. 2 Ibid., vol. xxxv. pp. 1 6-22.
3 Ibid., p. 26. 4 Ibid., vol. xli. p. 190.
CHAPTER V.
COMETS.
NEWTON showed that the bodies known as "comets," or
hirsute stars, obey the law of gravitation ; but it was by no
means certain that the individual of the species observed by
him in 1680 formed a permanent member of the solar system.
The velocity, in fact, of its rush round the sun was quite
possibly sufficient to carry it off for ever into the depths of
space, there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years
later, the case was different. Edmund Halley, who afterwards
succeeded Flamsteed as Astronomer Royal, calculated its orbit
on Newton's principles, and found it such as to give a period
of revolution of about seventy-six years. He accordingly an-
nounced its probable identity with the comets observed by Peter
Apian in 1531 and by Kepler in 1607, and fixed its return for
1758-59. The prediction was one of the test-questions put
by Science to Nature, on the replies to which largely depend
both the development of knowledge and the conviction of its
reality. In the present instance, the answer afforded may be
said to have laid the foundation of this branch of astronomy.
Halley's comet punctually reappeared on Christmas Day,
1758, and effected its perihelion passage on the i2th of March
following, thus proving beyond dispute that some at least of.
these erratic bodies are domesticated within our system, and
strictly conform, if not to its unwritten customs (so to speak),
at any rate to its fundamental laws. Their movements, in
short, were demonstrated by the most unanswerable of all
i
ii6 HISTORY OF ASTRONOMY.
arguments that of verified calculation to be calculable, and
their investigation was erected into a legitimate department of
astronomical science.
This notable advance was the chief result obtained in the
field of inquiry just now under consideration during the
eighteenth century. But before it closed, its cultivation had
received a powerful stimulus through the invention of an
improved method. The name of Olbers has already been
brought prominently before our readers in connection with
asteroidal discoveries ; these, however, were but chance
excursions from the path of cometary research which he
steadily pursued through life. An early predilection for the
stars was fixed in this particular direction by one of the happy
inspirations of genius. As he was watching, one night in the
year 1779, by the sick-bed of a fellow-student in medicine at
Gottingen, an important simplification in the mode of com-
puting the paths of comets occurred to him. Although not
made public until 1797, " Gibers' method " was then universally
adopted, and is still regarded as the most expeditious and
convenient in cases where absolute rigour is not required. By
its introduction, not only many a toilsome and thankless hour
was spared, but workers were multiplied, and encouraged in
the prosecution of labours more useful than attractive.
The career of Heinrich Olbers is a brilliant example of what
may be done by an amateur in astronomy. He at no time
did regular work in an observatory ; he was never the possessor
of a transit or any other fixed instrument ; moreover, all the
best years of his life were absorbed in the assiduous exercise
of a toilsome profession. In 1781 he settled as a physician in
his native town of Bremen (he was born in 1758 at Arbergen,
a neighbouring village, of which his father was pastor), and
continued in active practice for over forty years. It was thus
only the hours which his robust constitution enabled him to
spare from sleep that were available for his intellectual plea-
sures. Yet his recreation was, as Von Zach remarked, 1 no less
1 Allgemeine Geographische Ephemeriden, vol. iv. p. 287.
COMETS. 117
prolific of useful results than the severest work of other men.
The upper part of his house in the Sandgasse was fitted up with
such instruments and appliances as restrictions of space per-
mitted, and there, night after night during half a century and
upwards, he discovered, calculated, or observed the cometary
visitants of northern skies. Almost as effective in promoting
the interests of science as the valuable work actually done by
him, was the influence of his genial personality. He engaged
confidence by his ready and discerning sympathy ; he inspired
affection by his benevolent disinterestedness ; he quickened
thought and awakened zeal by the suggestions of a lively and
inventive spirit, animated with the warmest enthusiasm for the
advancement of knowledge. Nearly every astronomer in Ger-
many enjoyed the benefits of a (frequently active) correspon-
dence with him, and his communications to the scientific
periodicals of the time were numerous and striking. The
motive power of his mind was thus widely felt and continually
in action. Nor did it wholly cease to be exerted even when
the advance of age and the progress of infirmity rendered him
incapable of active occupation. He was, in fact, alive even to
the last day of his long life of eighty-one years ; and his death,
which occurred March 2, 1840, left vacant a position which a
rare combination of moral and intellectual qualities had con-
spired to render unique.
Amongst the many younger men who were attracted and
stimulated by intercourse with him was Johann Franz Encke.
But while Olbers became a mathematician because he was an
astronomer, Encke became an astronomer because he was a
mathematician. A born geometer, he was naturally sent to
Gottingen and placed under the tuition of Gauss. But
geometers are men ; and the contagion of patriotic fervour
which swept over Germany after the battle of Leipsic did not
spare Gauss's promising pupil. He took up arms in the Han-
seatic Legion, and marched and fought until the oppressor of
his country was safely ensconced behind the ocean-walls of St.
Helena. In the course of his campaigning he met Lindenau,
ii8 HISTORY OF ASTRONOMY.
the militant director of the Seeberg Observatory, and by his
influence was appointed his assistant, and eventually, in 1822,
became his successor. Thence he was promoted in 1825 to
Berlin, where he superintended the building of the new obser-
vatory, so actively promoted by Humboldt, and remained at
its head until within some eighteen months of his death in
August 1865.
On the 26th o'f November 1818, Pons of Marseilles dis-
covered a comet, whose inconspicuous appearance gave little
promise of its becoming one of the most interesting objects in
our system. Encke at once took the calculation of its elements
in hand, and brought out the unexpected result that it revolved
round the sun in a period of about 3^ years. 1 He moreover
detected its identity with comets seen by Mechain in 1786, by
Caroline Herschel in 1795, by Pons, Huth, and Bouvard in
1805, and after six laborious weeks of research into the dis-
turbances experienced by it from the planets during the entire
interval since its first ascertained appearance, he fixed May 24,
1822, as the date of its next return to perihelion. Although
on that occasion, owing to the position of the earth, invisible
in the northern hemisphere, Sir Thomas Brisbane's observatory
at Paramatta was fortunately ready equipped for its recapture,
which Riimker effected quite close to the spot indicated by
Encke's ephemeris.
The importance of this event can be better understood when
it is remembered that it was only the second instance of the
recognised return of a comet (that of Halley's, sixty-three years
previously, having, as already stated, been the first) ; and that
it moreover established the existence of a new class of celestial
objects, somewhat loosely distinguished as " comets of short
period." These bodies (of which a dozen are known to cir-
culate within the orbit of Saturn) are remarkable as showing
certain planetary affinities in the manner of their motions not
at all perceptible in the wider travelling members of their order.
1 Aslr. Jahrbuch, 1823, p. 217. The period (1208 days) of this body
is considerably shorter than that of any other known comet.
COMETS. 119
They revolve, without exception, in the same direction as the
planets from west to east ; they exhibit a marked tendency
to conform to the zodiacal track which limits planetary
excursions north and south ; and their paths round the sun,
although much more eccentric than the approximately circular
planetary orbits, are far less so than the extravagantly long
ellipses in which comets comparatively untrained (as it were)
in the habits of the solar system ordinarily perform their
revolutions.
No great comet is of the "planetary" kind. These are,
indeed, only by exception visible to the naked eye ; they dis-
play extremely feeble tail-producing powers, and give small
signs of central condensation. Thin wisps of cosmical cloud,
they flit across the telescopic field of view without sensibly
obscuring the smallest star. Their appearance, in short,
suggests what some notable facts in their history will presently
be shown to confirm that they are bodies already effete, and
verging towards dissolution. If it be asked what possible
connection can be shown to exist between the shortness of
period by which they are essentially characterised, and what
we may call their superannuated condition, we are not altogether
at a loss for an answer. Kepler's remark, 1 that comets are
consumed by their own emissions, has undoubtedly a measure
of truth in it. The substance ejected into the tail must, in
overwhelmingly large proportion, be for ever lost to the central
mass from which it issues. True, it is of a nature inconceiv-
ably tenuous ; but unrepaired waste, however small in amount,
cannot be persisted in with impunity. The incitement to such
self-spoliation proceeds from the sun ; it accordingly progresses
more rapidly the more numerous are the returns to the solar
vicinity. Comets of short period may thus reasonably be
expected to wear out quickly.
They are, moreover, bodies subject to many adventures and
vicissitudes. Their aphelia or the farthest points of their
1 " Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi
et denique mod." De Cometis, Op. y vol. vii. p. no.
120 HISTORY OF ASTRONOMY.
orbits from the sun are all situated so near to the path either
of Jupiter or of Saturn, as to permit these giant planets to act
as secondary rulers of their destinies. By their influence they
were, in all probability, originally fixed in their present tracks ;
and by their influence, exerted in an opposite sense, they may,
in some cases, be eventually ejected from them. A curious
instance of such capricious dealing on the part of Jupiter, was
afforded by the comet of 1770, found by Lexell of St. Peters-
burg to perform its circuit of the sun in 5^ years, but which
had never previously, and has never since been seen. The
explanation of this anomaly, suggested by. Lexell, and fully con-
firmed by the analytical inquiries both of Laplace and Lever-
rier, was that a very close approach to Jupiter in 1767 had
completely changed the character of its orbit, and brought it
within the range of terrestrial observation; while in 1779,
after having only twice traversed its new path (at its second
return it was so circumstanced as to be invisible from the
earth), it was, by a fresh encounter, diverted into one entirely
different. 1
It can easily be imagined that careers so varied are likely to
prove instructive, and astronomers have not been backward
in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much
curious information, and it may be hoped that its function in
that respect is by no means at an end. The great extent of
the solar system traversed by its eccentric path makes it
peculiarly useful for the determination of the planetary masses.
At perihelion it penetrates within the orbit of Mercury; it
1 Leverrier showed {Comptes Rendus, t. xxv. 1847, p. 564) that the problem
of the disturbances suffered by Lexell's comet was a far less determinate
one than it had been made to appear in the Mecanique Celeste. It is
possible that this body may, in 1779, have been finally thrust out of our
system ; it is also possible (as Laplace concluded) that it may be revolving
too far from the sun to be accessible to our view ; but it is much more
probable that its orbit still retains a family likeness to the one temporarily
assigned to it by Jovian influence in 1767, in which case Leverrier's caldula-
tions afford criteria for its eventual re-identification.
COMETS.
121
considerably transcends at aphelion the farthest excursion of
Pallas. Its vicinity to the first-named planet in August 1835
offered the first convenient opportunity of placing that body
in the astronomical balance. Its weight or mass had pre-
viously been assumed, not ascertained ; and the comparatively
slight deviation from its regular course impressed upon the
comet by its attractive power, showed that it had been assumed
nearly twice too great. 1 That fundamental datum of planetary
astronomy the mass of Jupiter was corrected by similar
means ; and it was reassuring to find the correction in satis-
factory accord with that already introduced from observation
of the asteroidal movements.
The fact that comets contract in approaching the sun had
been noticed by Hevelius; Pingre admitted it with hesitating per-
plexity; 2 the example of Encke's comet rendered it conspicuous
and undeniable. On the 28th of October 1828, the diameter
of the nebulous matter composing this body was estimated at
312,000 miles. It was then about one and a half times as
remote from the sun as the earth is at the time of the equinox.
On the 24th of December following, its distance being re-
duced by nearly two-thirds, it was found to be only 14,000
miles across. 3 That is to say, it had shrunk during those two
months of approach to TT .JoTy tn P art f * ts original volume !
Yet it had still seventeen days' journey to make before reach-
ing perihelion. The same curious circumstance was even more
markedly apparent at its return in 1838. Its bulk, or the
actual space occupied by it, was reduced, as it drew near the
hearth of our system (so far at least as could be inferred from
optical evidence),, in the enormous proportion of 800,000 to i.
A corresponding expansion on each occasion accompanied its
retirement from the sphere of observation. Similar changes
1 From the observed results of a second appulse in 1848, the Mercurian
mass is now estimated at about T .TOr.innr tnat f tne sun > while the inverse
relation assumed by Lagrange to exist between distance from the sun and
density brought it out ^ui.^TTf Laplace, Exposition du Systcme du
Alonde, t. ii. p. 50 (5th ed. 1824).
a Arago, Annuaire, 1832, p. 218. 3 Hind, The Comets, p. 20.
122 HISTORY OF ASTRONOMY.
of volume, though rarely to the same astounding extent, have
been perceived in other comets. They still remain unex-
plained ; but it can scarcely be doubted that they are due to
the action of the same energetic internal forces which reveal
themselves in so many splendid and surprising cometary phe-
nomena.
Another question of singular interest was raised by Encke's
acute inquiries into the movements and disturbances of the
first known "comet of short period." He found from the
first that its revolutions were subject to some influence be-
sides that of gravity. After every possible allowance had
been made for the pulls, now backward, now forward, exerted
upon it by the -several planets, there was still a surplus of
acceleration left unaccounted for. Each return to perihelion
took place about two and" a half hours sooner than received
theories warranted. Here then was a " residual phenomenon "
of the utmost promise for the disclosure of novel truths.
Encke (in accordance with the opinion of Olbers) explained
it as due to the presence in space of some such " subtle
matter " as was long ago invoked by Euler l to be the agent of
eventual destruction for the fair scheme of planetary creation.
The apparent anomaly of accounting for an accelerative effect
by a retarding cause disappears when it is considered that any
check to the motion of bodies revolving round a centre of
attraction causes them to draw closer to it, thus shortening
their periods and quickening their circulation. If space were
filled with a resisting medium capable of impeding, even in
the most infinitesimal degree, the swift course of the planets,
their orbits should necessarily be, not ellipses, but very close
elliptical spirals, along which they would slowly, but inevitably,
descend into the burning lap of the sun. The circumstance
that no such tendency can be traced in their revolutions by no
means sets the question at rest. For it might well be that an
effect totally imperceptible until after the lapse of countless
ages, as regards the solid orbs of our system, might be obvious
1 Phil. Trans., vol. xlvi. p. 204.
COMETS. 123
in the movements of bodies like comets of small mass and
great bulk ; just as a feather or a gauze veil at once yields its
motion to the resistance of the air, while a cannon-ball cuts its
way through with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous
character hang on the time-keeping of comets ; for plainly all
must in some degree suffer the same kind of hindrance as
Encke's, if the cause of that hindrance be the one suggested.
More than half a century, however, elapsed before the slightest
trace of similar symptoms could be detected in any of its con-
geners. At length, in 1880, Professor Oppolzer announced 1
that a comet, first seen by Pons in 1819, and rediscovered by
Winnecke in 1858, having a period of 2052 days (5.6 years),
was accelerated at each revolution precisely in the manner
required by Encke's theory. The " resisting medium " was
thereby generally admitted to have made good its footing. But
Backlund's latest researches 2 (in continuation of those of Von
Asten, cut short by his premature death) into the movements
of Encke's comet have revealed a perplexing circumstance.
They confirm Encke's results for the period covered by them,
but exhibit the acceleration as progressively diminishing from
1865 to 1881. Uniformity of action, however, would seem to
be an indispensable attribute of a true ethereal resistance.
The question is thus reopened, and with a renewal of in-
terest ; for although we have to wait for a definitive answer,
there is much to be learned from even the unsuccessful testing
of various hypotheses. There seems, in the first place, no
reason to suspect any physical change in the comet itself, such
as would render its motion less sensitive to opposition. A
diminution of bulk would have this effect, but the telescope
reports its aspect unaltered. Can the change, we then ask,
be in the condition of inter-planetary space ? The character
of the supposed resistance, it may be remarked, has been
1 Astr. Nach., No. 2314.
2 Mem. de St. Petersbourg, t. xxxii. (yth series), 1884. For a precis of
results, see Bulletin Astronomiqite> t. i. p. 239.
124 HISTORY OF ASTRONOMY.
often misapprehended. What Encke stipulated for was not a
medium equally diffused throughout the visible universe, such
as the ethereal vehicle of the vibrations of light, but a rare
fluid, rapidly increasing in density towards the sun. 1 This
cannot be a solar atmosphere, since it is mathematically certain,
as Laplace has shown, 2 that no envelope partaking of the sun's
axial rotation can extend farther from his surface than nine-
tenths of the mean distance of Mercury. \Vithin such an
envelope Encke's comet can never penetrate. There is, be-
sides, strong evidence of a physical kind that the actual depth
of the solar atmosphere bears a very minute proportion to
the possible depth theoretically assigned to it. That matter,
however, not atmospheric in its nature that is, neither
forming one body with the sun nor altogether aeriform exists
in its neighbourhood, can admit of no reasonable doubt. The
great lens-shaped mass of the zodiacal light, reaching out at
times far beyond the earth's orbit, may be regarded as an
extension of the corona, and, like the corona, is probably com-
posed of matter in very various forms cosmical dust, planetary
refuse, cometary debris, vaporous ejections. Now the changes
in shape and brightness visible in this singular feature of our
system may well be accompanied by changes in the power of
impeding motion of its constituting substances ; and we may
say with confidence that they are intimately connected with
variations in solar activity. The state of the sun and his
appendages at the times of the successive approaches to
perihelion of Encke's comet should thus be taken into account
in studying the problem of its acceleration, evidently a more
intricate one than had been supposed. The comparison may
yet be the means of bringing to light hitherto unsuspected
relations.
The history of the next known " planetary " comet has proved
of even more curious interest than that of the first. It was
discovered by an Austrian officer named Wilhelm von Biela
at Josephstadt in Bohemia, February 27, 1826, and ten days
1 Month. Not., vol. xix. p. 72. 2 Mecanique Celeste, t. ii. p. 197.
COMETS. 125
later by the French astronomer Gambart at Marseilles. Both
observers computed its orbit, showed its remarkable similarity
to that traversed by comets visible in 1772 and 1805, and
connected them together as previous appearances of the body
just detected, by assigning to its revolutions a period of between
six and seven years. The two brief letters conveying these
strikingly similar inferences were printed side by side in the
same number of the Astronomische Nachrichten (No. 94) ; but
Biela's priority in the discovery of the comet was justly re-
cognised by the bestowal upon it of his name.
The object in question was at no time (subsequently to its
appearance in 1805) visible to the naked eye. Its aspect in
Sir John Herschel's great reflector on the 23d of September
1832, was described by him as that of a " conspicuous nebula,"
about 2j to 3 minutes in diameter. No trace of a tail was
discernible. While he was engaged in watching it, a small
knot of minute stars (i6th or i7th magnitude) was directly
traversed by it, "and when on the cluster," he tells us, 1 it
" presented the appearance of a nebula resolvable, and partly
resolved into stars, the stars of the cluster being visible through
the comet." Yet the depth of cometary matter through which
such faint stellar rays penetrated undimmed, was, near the
central parts of the globe, not less than 50,000 miles.
It is curious to find that this seemingly harmless, and we
may perhaps add, effete body, gave occasion to the first (and
not the last) cometary "scare" of this enlightened century.
Its orbit, at the descending node, may be said to have
intersected that of the earth ; since, according as it bulged in
or out under the disturbing influence of the planets, the
passage of the comet was effected inside or outside the terrestrial
track. Now certain calculations published by Olbersin i828 2
showed that, on October 29, 1832, a considerable portion of
its nebulous surroundings would actually sweep over the spot
which, a month later, would be occupied by our planet. It
needed no more to set the popular imagination in a ferment.
1 Month. Not.) vol. ii. p. 117. 2 Astr. Nach., No. 128.
126 HISTORY OF ASTRONOMY.
Astronomers after all could not, by an alarmed public,, be held
to be infallible. Their computations, it was averred, which
a trifling oversight would suffice to vitiate, exhibited clearly
enough the danger, but afforded no guarantee of safety from a
collision, with all the terrific consequences frigidly enumerated
by Laplace. Nor did the panic subside until Arago formally
demonstrated that the earth and comet could by no possibility
approach within less than fifty millions of miles. 1
The return of the same body in 1845-46, was marked by an
extraordinary circumstance. When first seen, November 28,
it wore its usual aspect of a faint round patch of cosmical
fog ; but on December 19, Mr. Hind noticed that it had
become distorted somewhat into the form of a pear ; and ten
days later it had divided into two separate objects. This
singular duplication was first perceived at New Haven in
America, December 29,2 by Messrs. Herrick and Bradley,
and by Lieutenant Maury at Washington, January 13, 1846.
The earliest British observer of the phenomenon was Pro-
fessor Challis. " I see two comets ! " he exclaimed, putting
his eye to the great equatoreal of the Cambridge Observatory
on the night of January 15; then, distrustful of what his
senses had told him, he called in his judgment to correct
their improbable report by resolving one of the dubious objects
into a hazy star. 3 On the 23d, however, both were again
seen by him in unmistakable cometary shape, and until far on
in March (Otto Struve caught a final glimpse of the pair on
the 1 6th of April), 4 continued to be watched with equal
curiosity and amazement by astronomers in every part of the
1 Annuaire, 1832, p. 186.
2 Am. Journal of Science, vol. i. (2d series), p. 293. Prof. Hubbard's
calculations indicated a probability that the definitive separation of the
two nuclei occurred as early as Sept. 30, 1844. Astronomical Journal
(Gould's), vol. iv. p. 5. See also, on the subject of this comet, W. T. Lynn,
Intellectual Observer, vol. xi. p. 208, and the Rev. E. Ledger, Observatory,
August 1883, p. 244. 3 Month. Not., vol. vii. p. 73.
4 Bulletin Ac. Imp. de St. Petersbourg, t. vi. col. 77. The latest observa-
tion of the parent nucleus was that of Argelander, April 27, at Bonn.
COMETS. 127
northern hemisphere. What Seneca reproved Ephorus for
supposing to have taken place in 373 B.C. what Pingre
blamed Kepler for conjecturing in 1618, had then actually
occurred under the attentive eyes of science in the middle of
the nineteenth century !
At a distance from each other of about two-thirds the dis-
tance of the moon from the earth, the twin comets meantime
moved on tranquilly, so far, at least, as their course through
the heavens was concerned. Their extreme lightness, or the
small amount of matter contained in each, could not have
received a more signal illustration than by the fact that their
revolutions round the sun were performed independently ; that
is to say, they travelled side by side without experiencing any
appreciable mutual disturbance, thus plainly showing that at
an interval of only 157,250 miles, their attractive power was
virtually inoperative. Signs of internal agitation, however,
were not wanting. Each fragment threw out a short tail in a
direction perpendicular to the line joining their centres, and
each developed a bright nucleus, although the original comet
had exhibited neither of these signs of cometary vitality. A
singular interchange of brilliancy was, besides, observed to
take place between these small objects, each of which alter-
nately outshone and was outshone by the other, while an arc
of light, apparently proceeding from the more lustrous, at times
bridged the intervening space. Obviously, the gravitational
tie, rendered powerless by exiguity of matter, was here re-
placed by some other form of mutual action, the nature of
which can as yet be dealt with only by conjecture.
Once more, in August 1852, the double comet returned to
the neighbourhood of the sun, but under circumstances not
the most advantageous for observation. Indeed, the companion
was not detected until September 16, by Father Secchi at
Rome, and was then perceived to have increased its distance
from the originating body to a million and a quarter of miles, or
about eight times the average interval at the former appearance.
Both vanished shortly afterwards, and have never since been
128 HISTORY OF ASTRONOMY.
seen, notwithstanding the eager watch kept for objects of such
singular interest, and the accurate knowledge of their track
supplied by Santini's investigations. We can scarcely doubt
that the fate has overtaken them which Newton assigned as
the end of all cometary existence. Diffundi tandem et spargi
per ccelos universes. 1
A telescopic comet with a period of 7^ years, discovered
November 22, 1843, by M. Faye of the Paris Observatory,
formed the subject of a characteristically patient and profound
inquiry on the part of Leverrier, designed to test its suggested
identity with Lexell's lost comet. The result was decisive
against the hypothesis of Valz, the divergences between the
orbits of the two bodies being found to increase instead of to
diminish, as the history of the newcomer was traced backwards
into the last century. 2 Faye's comet pursues the most nearly
circular path of any similar known object ; even at its nearest
approach to the sun it remains farther off than Mars when he
is most distant from it ; and it has been proved by the admir-
able researches of Professor Axel Holier, 3 director of the
Swedish observatory of Lund, to exhibit no trace of the action
of a resisting medium.
No great comet appeared between the " star " which presided
at the birth of Napoleon and the "vintage" comet of 1811.
The latter was first descried by Flaugergues at Viviers, March
26, 1811 ; Wisniewski, at Neu-Tscherkask in Southern Russia,
caught the last glimpse of it August 17, 1812. Two dis-
appearances in the solar rays as the earth moved round in
its orbit, and two reappearances after conjunction, were in-
cluded in this unprecedentedly long period of visibility of 510
days. This relative permanence (so far as the inhabitants of
Europe were concerned) was due to the high northern latitude
attained near perihelion, combined with a certain leisureliness
of movement along a path everywhere external to that of
the earth. The magnificent luminous train of this body, on
1 D' Arrest, Astr. Nach., No. 1624.
2 Comptes Rendus, t. xxv. p. 570. 3 Month. Not., vol. xii. p. 248.
COMETS.
October 15, the day of its nearest terrestrial approach, covered
an arc of the heavens 23^ degrees in length, corresponding to
a real extension of one hundred millions of miles. Its form
was described by Sir William Herschel as that of " an inverted
hollow cone," and its colour as yellowish, strongly contrasting
with the bluish-green tint of the " head," round which it was
flung like a transparent veil. The planetary disc of the head,
127,000 miles across, appeared to be composed of strongly
condensed nebulous matter; but somewhat eccentrically situated
within it was a star-like nucleus of a reddish tinge, which Her-
schel presumed to be solid, and ascertained, with his usual
care, to have a diameter of 428 miles. From the total
absence of phases, as well as from the vivacity of its radiance,
he confidently inferred that its light was not borrowed, but
inherent. 1
This remarkable apparition formed the subject of a memoir 2
by Olbers, the striking, yet steadily reasoned-out suggestions
contained in which there was at that time no means of follow-
ing up with profit. Only of late has the " electrical theory,"
of which Zollner 3 regarded Olbers as the founder, assumed a
definite and measurable form, capable of being tested by the
touchstone of fact, as knowledge makes its slow inroads on the
fundamental mystery of the physical universe.
The paraboloidal shape of the bright envelope separated by
a dark interval from the head of the great comet of 1811, and
constituting, as it were, the root of its tail, seemed to the
astronomer of Bremen to reveal the presence of a double
repulsion ; the expelled vapours accumulating where the two
forces, solar and cometary, balanced each other, and being
then swept backwards in a huge train. He accordingly dis-
tinguished three classes of these bodies : First, comets which
develop no matter subject to solar repulsion. These have no
1 Phil. Trans., vol. cii. pp. 118-124.
2 Ueberden Schweif des grossen Comcten von 181 1, Monat. Corr., vol. xxv.
pp. 3-22. Reprinted by Zollner, Ueber die Natur der Cometen, pp. 3-15.
3 Natur der Cometen, p. 148.
I
130 HISTORY OF ASTRONOMY.
tails, and are probably mere nebulosities, without solid nuclei.
Secondly, comets which are acted upon by solar repulsion only,
and consequently throw out no emanations towards the sun.
Of this kind was a bright comet visible in 1807. 1 Thirdly,
comets, like that of 1811, giving evidence of action of both
kinds. These are distinguished by a dark hoop encompassing
the head and dividing it from the luminous envelope, as well
as by an obscure caudal axis, resulting from the hollow, cone-
like structure of the tail.
Again, the ingenious view recently put forward by M. Bre-
dichin of Moscow as to the connection between the form
of these appendages and the kind of matter composing them,
was very clearly anticipated by Olbers. The amount of tail-
curvature, he pointed out, depends in each case upon the
proportion borne by the velocity of the ascending particles
to that of the comet in its orbit ; the swifter the outrush, the
straighter the resulting tail. But the velocity of the ascending
particles varies with the energy of their repulsion by the sun,
and this again, it may be presumed, with their quality. Thus
multiple tails are developed when the same comet throws off,'
as it approaches perihelion, specifically distinct substances.
The long, straight ray which proceeded from the comet of
1807, for example, was doubtless made up of particles subject
to a much more vigorous solar repulsion than those formed
into the shorter, curved emanation issuing from it nearly in the
same direction. In the comet of 1811, he calculated that the
particles expelled from the head travelled to the remote ex-
tremity of the tail in eleven minutes, indicating by this enormous
rapidity of movement (comparable to that of the transmission
of light) the action of a force greatly more powerful than the
opposing one of gravity. The not uncommon phenomena of
multiple envelopes, on the other hand, he explained as due to
the varying amounts of repulsion exercised by the nucleus
itself on the different kinds of matter developed from it.
The movements and perturbations of the comet of 1811
1 The subject of a classical memoir by Bessel, published in 1810.
COMETS. 131
were no less profoundly studied by Argelander than its
physical constitution by Olbers. The orbit which he assigned
to it is of such vast dimensions as to require no less than
3065 years for the completion of its circuit ; and to carry the
body describing it at each revolution to fourteen times the dis-
tance from the sun of the frigid Neptune. Thus, when it last
visited our neighbourhood, Achilles may have gazed on its
imposing train as he lay on the sands all night bewailing the
loss of Patroclus ; 1 and when it returns, it will perhaps be to
shine upon the ruins of empires and civilisations still deep
buried among the secrets of the coming time.
On the 26th of June 1819, while the head of a comet
passed across the face of the sun, the earth was (in all pro-
bability) involved in its tail. But of this remarkable double
event nothing was known until more than a month later, when
the fact of its past occurrence emerged from the calculations
of Olbers. 2 Nor had the comet itself been generally visible
previous to the first days of July. Several observers, however,
on the publication of these results, brought forward accounts
of singular spots perceived by them upon the sun at the time
of the transit, and the original drawing of one of them, Pas-
torff of Buchholtz, has been preserved. This undoubtedly
authentic delineation 3 represents a round nebulous object with
a bright spot in the centre, of decidedly cometary aspect, and
not in the least like an ordinary solar "macula." Mr. Hind, 4
nevertheless, has shown that its position on the sun is irrecon-
cilable with that which the comet must have occupied ; and
Mr. Ranyard's discovery of a similar smaller drawing by the
same author, dated May 26, i828, 5 reduces to evanescence
the probability of its connection with that body. Indeed,
recent experience renders very doubtful the possibility of such
an observation.
1 If we adopt the chronology of Madler, Reden und Abhandl., p. 118.
2 Astr. Jahrbuch (Bode's), 1823, p. 134.
3 Reproduced in Webb's Celestial Objects, 4th ed.
4 Month. Not.) vol. xxxvi. p. 309. 5 Celestial Objects, p. 40, note.
132 HISTORY OF ASTRONOMY.
The return of Halley's comet in 1835 was looked forward
to as an opportunity for testing the truth of floating cometary
theories, and did not altogether disappoint expectation. As
early as 1817, its movements and disturbances since 1759
were proposed by the Turin Academy of Sciences as the sub-
ject of a prize awarded to Baron Damoiseau. Pontecoulant
was adjudged a similar distinction by the Paris Academy in
1829; while Rosenberger's calculations were rewarded with
the gold medal of the Royal Astronomical Society. 1 The
result entirely disproved the hypothesis (designed to explain
the invariability of the planetary periods) of what may be
described as a vortex of attenuated matter moving with the
planets, and offering, consequently, no resistance to their motion.
For since Halley's comet revolves in the opposite direction
in other words, has a " retrograde " movement it is plain that
if compelled to make head against an ethereal current, it would
rapidly be deprived of the tangential velocity which enables it
to keep at its proper distance from the sun, and would thus
gradually but conspicuously approach, and eventually be pre-
cipitated upon it. No such effect, however, has in this crucial
instance been detected.
On the 6th of August 1835, a "early circular misty object
was seen at Rome not far from the predicted place of the
comet. It was not, however, until the middle of September
that it began to throw out a tail, which by the i5th of
October had attained a length of about 24 degrees (on the
1 9th, at Madras, it extended to fully 3o), 2 the head showing
to the naked eye as a reddish star rather brighter than Alde-
baran or Antares. 3 Some curious phenomena accompanied
the process of tail-formation. An outrush of luminous mat-
ter, resembling in shape a partially opened fan, issued from
the nucleus towards the sun, and at a certain point, like smoke
driven before a high wind, was vehemently swept back-
wards in a prolonged train. The appearance of the comet at
1 See Airy's Address, Mem. R. A. S., vol. x. p. 376.
3 Hind, The Comets, p. 47. 3 Arago, Annuaire, 1836, p. 228.
COMETS. 133
this time was compared by Bessel, 1 who watched it with
minute attention, to that of a blazing rocket. He made the
singular observation that this fan of light, which seemed the
source of supply for the tail, oscillated like a pendulum to and
fro across a line joining the sun and nucleus, in a period of
4| days ; and he was unable to escape from the conclusion 2
that a repulsive force, about twice as powerful as the attractive
force of gravity, was concerned in the production of these
remarkable effects. Nor did he hesitate to recur to the
analogy of magnetic polarity, or to declare, still more em-
phatically than Olbers, " the emission of the tail to be a purely
electrical phenomenon." 3
The transformations undergone by this body were almost as
strange and complete as those which affected the brigands in
Dante's " Inferno" When first seen it wore the aspect of a
nebula ; later it put on the distinctive garb of a comet ; it
next appeared as a star ; finally it dilated, first in a spherical,
then in a paraboloidal form, until May 5, 1836, when it
vanished, as if by melting into adjacent space from the excessive
diffusion of its light. A very uncommon circumstance in its
development was that it lost (it would appear) all trace of
tail previous to its arrival at perihelion on the 1 6th of Novem-
ber. Nor did it begin to recover its elongated shape for
more than two months afterwards. On the 23d of January
Boguslawski perceived it as a star of the sixth magnitude,
without measurable disc.*' Only two nights later, Maclear,
director of the Cape Observatory, found the head to be 131
seconds across. 5 And so rapidly did the augmentation of size
progress, that Sir John Herschel, who was then observing at
Feldhausen, estimated the actual bulk of this singular object to
have increased forty-fold in the ensuing week. " I can hardly
1 A sir. Nach., No. 300.
2 It deserves to be recorded that Robert Hooke drew a very similar
inference from his observations of the comets of 1680 and 1682. Month.
JVot., vol. xiv. pp. 77-83.
3 Briefwechsel zwischen Olbers und Bessel, Bd. ii. p. 390.
4 Herschel. Results, p. 405. 5 Mem. R. A. S., vol. x. p. 92.
134 HISTORY OF ASTRONOMY.
doubt," he remarks, "that the comet was fairly evaporated in
perihelio by the heat, and resolved into transparent vapour,
and is now in process of rapid condensation and re-precipi-
tation on the nucleus." l A plausible, but no longer admissible
interpretation of this still unexplained phenomenon.
By means of an instrument devised by himself for testing
the quality of light, Arago obtained decisive evidence that
some at least of the radiance proceeding from Halley's comet
was derived by reflection from the sun. 2 Indications of the
same kind had been afforded 3 by the comet which sud-
denly appeared above the north-western horizon of Paris,
July 3, 1819, after having enveloped (as already stated) our
terrestrial abode in its filmy appendages; but the " polari-
scope" had not then reached the perfection subsequently
given to it, and its testimony was accordingly far less reliable
than in 1835. Such experiments, however, are in reality more
beautiful and ingenious than instructive, since incandescent
as well as obscure bodies possess the power of throwing back
light incident upon them, and will consequently transmit to us
from the neighbourhood of the sun rays partly direct, partly
reflected, of which a certain proportion will exhibit the peculi-
arity known as polarisation.
The most brilliant comets of the century were suddenly
rivalled if not surpassed by the extraordinary object which
blazed out beside the sun, February 28, 1843. I* was simul-
taneously perceived in Mexico and the United States, in
Southern Europe, and at sea off the Cape of Good Hope,
where the passengers on board the Owen Glendower were
amazed by the sight of a " short, dagger-like object," closely
following the sun towards the western horizon. 4 , At Florence
Amici found its distance from the sun's centre at noon to be
only i23 / ; and spectators at Parma were able, when sheltered
from the direct glare of midday, to trace the tail to a length of
1 Results, p. 401. 2 Annuaire, 1836, p. 233.
3 Cosmos, vol. i. p. 90, note (Otte's trans.)
4 Herschel, Outlines, p. 399 (9th ed.)
COMETS. 135
four or five degrees. The full dimensions of this astonishing
appurtenance began to be disclosed a few days later. On the
3d of March it measured 25, and on the nth, at Calcutta,
Mr. Clerihew observed a second streamer, nearly twice as long
as the first, and making an angle with it of 18, to have been
emitted in a single day. This rapidity of projection, Sir John
Herschel remarks, " conveys an astounding impression of the
intensity of the forces at work." " It is clear," he continues,
" that if we have to deal here with matter, such as we conceive it
viz., possessing inertia at all, it must be under the dominion
of forces incomparably more energetic than gravitation, and
quite of a different nature." *
On the 1 7th of March a silvery ray, some 40 degrees long
and slightly curved at its extremity, shone out above the sun-
set clouds in this country. No previous intimation had been
received of the possibility of such an apparition, and even
astronomers no lightning messages across the seas being as
yet possible were perplexed. The nature of the phenome-
non, indeed, soon became evident, but the wonder of it did
not diminish with the study of its attendant circumstances.
Never before, within astronomical memory, had our system
been traversed by a body pursuing such an adventurous career.
The closest analogy was offered by the great comet of 1680
(Newton's), which rushed past the sun at a distance of only
144,000 miles ; but even this on the cosmical scale scarcely
perceptible interval was reduced nearly one-half in the case
we are now concerned with. The centre of the comet of
1843 approached the formidable luminary within 78,000 miles,
leaving, it is estimated, a clear space of not more than 32,000
between the surfaces of the bodies thus brought into such
perilous proximity. The escape of the wanderer was, how-
ever, secured by the extraordinary rapidity of its flight. It
swept past perihelion at a rate 366 miles a second which,
if continued, would have carried it right round the sun in two
hours ; and in only eleven minutes more than that short period
1 Outlines, p. 398.
136 HISTORY OF ASTRONOMY.
it actually described half the curvature of its orbit an arc of
1 80 although in travelling over the remaining half many
hundreds of sluggish years will doubtless be consumed.
The behaviour of this comet may be regarded as an experi-
mentum crucis as to the nature of tails. For clearly no fixed
appendage many millions of miles in length could be whirled
like a brandished sabre from one side of the sun to the other
in 131 minutes. Cometary trains are then, as Olbers rightly
conceived them to be, emanations, not appendages incon-
ceivably rapid outflows of highly rarefied matter, the greater
part, if not all of which becomes permanently detached from
the nucleus.
That of the comet of 1843 reached, about the time that it
became visible in this country, the extravagant length of 200
millions of miles. 1 It was narrow, and bounded by nearly
parallel and nearly rectilinear lines, resembling to borrow a
comparison of Aristotle's a " road " through the constella-
tions ; and after the 3d of March showed no trace of hollow-
ness, the axis being, in fact, rather brighter than the edges.
Distinctly perceptible in it were those singular aurora-like
coruscations which gave to the "tresses" of Charles V.'s
comet the appearance as Cardan described it of " a torch
agitated by the wind," and have not unfrequently been ob-
served to characterise other similar objects. A consideration
first adverted to by Olbers proves these to originate in our
own atmosphere. For owing to the great differences in the
distances from the earth of the origin and extremity of such
vast effluxes, the light proceeding from their various parts is
transmitted to our eyes in notably different intervals of time.
Consequently a luminous undulation, even though propagated
instantaneously from end to end of a comet's tail, would
appear to us to occupy many minutes in its progress. But the
coruscations in question pass as swiftly as a falling star. They
are, then, of terrestrial production.
1 Boguslawski calculated that it extended on the 2ist of March to
581 millions. Repart Brit. Ass., 1845, P- 8 9-
COMETS. 137
Periods of the utmost variety were by different computators
assigned to the body, which arrived at perihelion, February 27,
1843, at 9.47 p.m. Professor Hubbard of Washington found
that it required 533 years to complete a revolution ; MM.
Laugier and Mauvais of Paris considered the true term to be
35 ; 1 Clausen looked for its return at the end of between six and
seven. All these estimates were indeed admittedly uncertain,
the available data affording no sure means of determining the
value of this element ; yet there seems no doubt that they
fitted in more naturally with a period counted by centuries
than with one reckoned by decades. Nor could any previous
appearance be satisfactorily made out, although the similarity
of the course pursued by a brilliant comet in 1668, known as
the " Spina " of Cassini, made an identification not impossible.
This would imply a period of 175 years, and it was somewhat
hastily assumed that a number of earlier celestial visitants
might thus be connected as returns of the same body.
It may now be asked what were the conclusions regarding
the nature of comets drawn by astronomers from the consider-
able mass of novel experience accumulated during the first
half of this century? The first and best assured was that
the matter composing them is in a state of extreme tenuity.
Numerous and trustworthy observations showed that the
feeblest rays of light might traverse some hundreds of thou-
sands of miles of their substance, even where it was apparently
most condensed, without being perceptibly weakened. Nay,
instances were recorded in which stars were said to have
gained in brightness from the process ! 2 On the 24th of
June 1825, Olbers 3 saw the comet then visible all but
obliterated by the central passage of a star too small to be
distinguished with the naked eye, its own light remaining
wholly unchanged. A similar effect was noted December i,
1 Conrptes Rendus, t. xvi. p. 919.
2 Piazzi noticed a considerable increase of lustre in a very faint star of
the twelfth magnitude viewed through a comet. Madler, Reden, &>c., p.
248, note. 3 Astr. Jahrbuch, 1828, p. 151.
138 HISTORY OF ASTRONOMY.
i8.ii, when the great comet of that year approached so close
to Atair, the lucida of the Eagle, that the star seemed to be
transformed into the nucleus of the comet. 1 Even the central
blaze of Halley's comet in 1835 was powerless to impede the
passage of stellar rays. Struve 2 observed at Dorpat, on
September 1 7, an all but central occultation ; Glaisher 3 one
(so far as he could ascertain) absolutely so eight days later
at Cambridge. In neither case was there any appreciable
diminution of the star's light. Again, on the nth of October
1847, Mr. Dawes, 4 an exceptionally keen observer, distinctly
saw a star of the tenth magnitude through the exact centre
of a comet discovered on the ist of that month by Maria
Mitchell of Nantucket.
Examples, on the other hand, were not wanting of the
diminution of stellar light under similar circumstances; but
probably in general not more than would be accounted for by
the illumination of the background with diffused nebulous
radiance. 5 In one solitary instance, however, on the 28th of
November 1828, a star was alleged to have actually vanished
behind a comet. 6 The observer of this unique phenomenon
was Wartmann of Geneva ; but his instrument was so defective
as to leave its reality open to grave doubt, especially when
it is considered that the eclipsing body was Encke's comet,
which better equipped astronomers have, on various occasions,
found to be perfectly translucent.
From the failure to detect any effects of refraction in the
light of stars occulted by comets, it was inferred (though, as
, we now know, erroneously) that their composition is rather that
1 Madler, Gesch. d. Astr., Bd. ii. p. 412.
2 Recueil de ? Ac. Imp. de St. Petersbourg, 1835, p. 143.
3 Guillemin's World of Comets, trans, by J. Glaisher, p. 294, note.
4 Month. Not., vol. viii. p. 9.
5 A real, though only partial stoppage of light seems indicated by
HerschePs observations on the comet of 1807. Stars seen through the
tail, October 18, lost much of their lustre. One near the head was only
faintly visible by glimpses. Phil. Trans., vol. xcvii. p. 153.
6 Arago, Annuaire, 1832, p. 205.
COMETS. 139
of dust than that of vapour ; that they consist not of any con-
tinuous substance, but of discrete solid particles, very finely
divided and widely scattered. In conformity with this view was
the known smallness of their masses. Laplace had shown that if
the amount of matter forming Lexell's comet had been as much
as -5^7^ of that contained in our globe, the effect of its attrac-
tion, on the occasion of its approach within 1,438,000 miles
of the earth, July i, 1770, must have been apparent in the
lengthening of the year. And that some comets, at any
rate, possess masses immeasurably below this maximum value,
was clearly proved by the undisturbed parallel march of the
two fragments of Biela in 1846.
But the discovery in this branch most distinctive of the
period under review, is that of " short period " comets, of
which four 1 were known in 1850. These, by the character of
their movements, serve as a link between the planetary and
cometary worlds, and by the nature of their construction,
seem to mark a stage in cometary decay. For that comets
are rather transitory agglomerations, than permanent products
of cosmical manufacture, appeared to be demonstrated by the
division and disappearance of one amongst their number, as
well as by the singular and rapid changes in appearance
undergone by many, and the (seemingly) irrevocable diffusion
of their substance visible in nearly all. They might then be
defined, according to the ideas respecting them prevalent
thirty-five or forty years ago, as bodies unconnected by origin
with the solar system, but encountered, and to some extent
appropriated by it in its progress through space, owing their
visibility in great part, if not altogether, to light reflected from
the sun, and their singular and striking forms to the action of
repulsive forces emanating from him, the penalty of their
evanescent splendour being paid in gradual waste and final
dissipation and extinction.
1 Viz., Encke's, Biela's, Faye's, and Brorsen's. A comet with a supposed
period of 5^ years, detected by De Vico at Rome, August 22, 1844, has,
it would appear, made no subsequent return to perihelion.
CHAPTER VI.
INSTRUMENTAL ADVANCES.
IT is impossible to follow with intelligent interest the course
of astronomical discovery without feeling some curiosity as to
the means by which such surprising results have been secured.
Indeed, the bare .acquaintance with what has been achieved,
without any corresponding knowledge of how it has been
achieved, supplies food for barren wonder rather than for
fruitful and profitable thought. Ideas advance most readily
along the solid ground of practical reality, and often find true
sublimity while laying aside empty marvels. Progress is the
result, not so much of sudden flights of genius, as of sustained,
patient, often commonplace endeavour; and the true lesson
of scientific history lies in the close connection which it dis-
closes between the most brilliant developments of knowledge
and the faithful accomplishment of his daily task by each
individual thinker and worker.
It would be easy to fill a volume with the detailed account
of the long succession of optical and mechanical improvements
by means of which the observation of the heavens has been
brought to its present degree of perfection ; but we must here
content ourselves with a summary sketch of the chief amongst
them. The first place in our consideration is naturally claimed
by the telescope.
This marvellous instrument, we need hardly remind our
readers, is of two distinct kinds that in which light is gathered
together into a focus by refraction, and that in which the same
end is attained by reflection. The image formed is in each
case viewed through a magnifying lens, or combination of
INSTRUMENTAL ADVANCES. 141
lenses, called the eye-piece. Not for above a century after the
" optic glasses " invented or stumbled upon by the spectacle-
maker of Middleburg (1608) had become diffused over Europe,
did the reflecting telescope come, even in England, the place
of its birth, into general use. Its principle (a sufficiently
obvious one) had indeed been suggested by Mersenne as early
as 1639 j 1 James Gregory in 1663 2 described in detail a mode
of embodying that principle in a practical shape ; and Newton,
adopting an original system of construction, actually produced
in 1668 a tiny speculum, one inch across, by means of which
the apparent distance of objects was reduced thirty-nine times.
Nevertheless, the exorbitantly long tubeless refractors, intro-
duced by Huygens, maintained their reputation until Hadley
exhibited to the Royal Society in 17233 a reflector sixty-two
inches in focal length which rivalled in performance, and of
course indefinitely surpassed in manageability, one of the
" aerial " kind of 123 feet.
The concave mirror system now gained a decided ascendant,
and was brought to unexampled perfection by James Short
of Edinburgh during the years 1732-68. Its capabilities were,
however, first fully developed by William Herschel. The
energy and inventiveness of this extraordinary man marked an
epoch wherever they were applied. His ardent desire to mea-
sure and gauge the stupendous array of worlds which his specula
revealed to him, made him continually intent upon adding
to their " space-penetrating power " by increasing their light-
gathering surface. These, as he was the first to explain, 4 are
in a constant proportion one to the other. For a telescope with
twice the linear aperture of another will collect four times as
much light, and will consequently disclose an object four times
as faint as could be seen with the first, or, what comes to the
same, an object equally bright at twice the distance. In other
words, it will possess double the space-penetrating power
of the smaller instrument. Herschel's great mirrors the
1 Grant, Hist. Astr., p. 527. - Optica Promota, p. 93.
3 Phil. Trans., vol. xxxii. p. 383. 4 lbid. t vol. xc. p. 65.
I 4 2 HISTORY OF ASTRONOMY.
first examples of the giant telescopes of modern times were
then primarily engines for extending the bounds of the visible uni-
verse ; and from the sublimity of this " final cause " was derived
the vivid enthusiasm which animated his efforts to success.
It seems probable that the seven-foot telescope constructed
by him in 1775 that is, within little more than a year after
his experiments in shaping and polishing metal had begun
already exceeded in effective power any work by an earlier
optician ; and both his skill and his ambition rapidly developed.
His efforts culminated, after mirrors of ten, twenty, and thirty
feet focal length had successively left his hands, in the gigantic
forty-foot, completed August 28, 1789. It was the first re-
flector in which only a single mirror was employed. In the
" Gregorian " form, the focussed rays are, by a second reflection
from a small concave 1 mirror, thrown straight back through a
central aperture in the larger one, behind which the eye-piece
is fixed. The object under examination is thus seen in the
natural direction. The " Newtonian," on the other hand,
shows the object in a line of sight at right angles to the true
one, the light collected by the speculum being diverted to one
side of the tube by the interposition of a small plane mirror
situated at an angle of 45 to the axis of the instrument. Upon
these two systems Herschel worked until 1787, when, becoming
convinced of the supreme importance of economising light
(necessarily wasted by the second reflection), he laid aside the
small mirror of his forty-foot then in course of construction,
and turned it into a " front-view " reflector. This was done
according to the plan proposed by Lemaire in 1732 by
slightly inclining the speculum so as to enable the image formed
by it to be viewed with an eye-glass fixed at the upper margin
of the tube. The observer thus stood with his back turned to
the object he was engaged in scrutinising.
1 Cassegrain, a Frenchman, substituted in 1672 a convex, for a concave
secondary speculum. The tube was thereby enabled to be shortened by
twice the focal length of the mirror in question. The great Melbourne
reflector (four feet aperture, by Grubb) is constructed upon this plan.
INSTRUMENTAL ADVANCES. 143
The advantages of the increased brilliancy afforded by this
modification were strikingly illustrated by the discovery,
August 28 and September 17, 1789, of the two Saturnian satel-
lites nearest the ring. Nevertheless, the monster telescope of
Slough cannot be said to have realised the sanguine expecta-
tions of its constructor. The occasions on .which it could be
usefully employed were found to be extremely rare. It was
injuriously affected by every change of temperature. The great
weight (25 cwt.) of a speculum four feet in diameter rendered it
peculiarly liable to distortion. With all imaginable care, the
delicate lustre of its surface could not be preserved longer than
two years, 1 when the difficult process of repolishing had to be
undertaken. It was never used after 1811, when, having gone
blind from damp, it lapsed by degrees into the condition of a
museum inmate.
The extraordinarily high magnifying powers employed by
Herschel constituted a novelty in optical astronomy scarcely
less striking than the gigantic size of his specula. They had
never previously been approached ; they have never since been
surpassed ; and they seem to mark, for these latitudes at least,
the very outside limit of practicability. The attempt to in-
crease in this manner the efficacy of the telescope is speedily
checked by atmospheric, to say nothing of other difficulties.
Precisely in the same proportion as an object is magnified,
the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights,
the air is never for a moment really still. The rays of light
traversing it are continually broken by minute fluctuations of
refractive power caused by changes of temperature and pressure,
and the currents which these engender. With such luminous
quiverings and waverings the astronomer has always more or
less to reckon ; their absence is simply a question of degree ;
if sufficiently magnified, they are at all times capable of render-
ing observation impossible.
1 Phil. Trans., vol. civ. p. 275, note.
144 HISTORY OF ASTRONOMY.
Thus, such vast powers as 3000, 4000, 5000, even 665 2, 1
which Herschel now and again applied to his great telescopes,
must, save on the rarest occasions, prove an impediment rather
than an aid to vision. They were, however, used by him only
for special purposes, and with the clearest discrimination of
their advantages and drawbacks. It is obvious that per-
fectly different ends are subserved by increasing the aperture
and by increasing the power of a telescope. In the one case,
a larger quantity of light is captured and concentrated ; in the
other, the same amount is distributed over a wider area. A
diminution of brilliancy accordingly attends upon each aug-
mentation of apparent size. For this reason, such faint
objects as nebulae are most successfully observed with moderate
powers applied to instruments of a great capacity for light, the
details of their structure actually disappearing when highly
magnified. With stellar groups the reverse is the case. Stars
cannot be magnified, simply because they are too remote to
have any sensible dimensions ; but the space between them
can. It was thus for the purpose of dividing very close double
stars that Herschel increased to such an unprecedented extent
the magnifying capabilities of his instruments ; and to this
improvement incidentally the discovery of Uranus, March 13,
1 78 1, 2 was due. For by the examination with strong lenses
of an object which, even with a power of 227, presented a
suspicious appearance, he was able at once to pronounce its
disc to be real, and not merely " spurious," and so to dis-
tinguish it unerringly from the crowd of stars amidst which it
was moving.
While the reflecting telescope was astonishing the world by
its rapid development in the hands of Herschel, its unpretend-
ing rival was slowly making its way towards the position which
the future had in store for it. The great obstacle which long
1 Phil. Trans., vol. xc. p. 70. With the 4O-foot, however, only very
moderate powers seem to have been employed, whence Dr. Robinson
argued a deficiency of defining power. Proc. Roy. Irish Ac., vol. ii. p. II.
2 Phil. Trans., vol. Ixxi. p. 492.
INSTRUMENTAL ADVANCES. 145
stood in the way of the improvement of refractors was the
defect known as " chromatic aberration." This is due to no
other cause than that which produces the rainbow and the
spectrum the separation, or "dispersion" in their passage
through a refracting medium, of the variously coloured rays
composing a beam of white light. In an ordinary lens there
is no common point of concentration ; each colour has its own
separate focus ; and the resulting image, formed by the super-
position of as many images as there are hues in the spectrum,
is indistinctly terminated with a tinted border, eminently
baffling to exactness of observation.
The extravagantly long telescopes of the seventeenth century
were designed to avoid this evil (as well as another source of
indistinct vision in the spherical shape of lenses) ; but no
attempt to remedy it was made until an Essex gentleman suc-
ceeded, in 1733, in so combining lenses of flint and crown
glass as to produce refraction without colour. 1 Mr. Chester
More Hall was, however, equally indifferent to fame and
profit, and took no pains to make his invention public. The
effective discovery of the achromatic telescope was, accord-
ingly, reserved for John Dollond, whose method of correcting
at the same time chromatic and spherical aberration was
laid before the Royal Society in 1758. Modern astronomy
may be said to have been thereby rendered possible. Re-
fractors have always been found better suited than reflectors
to the ordinary work of observatories. They are, so to speak,
of a more robust, as well as of a more plastic nature. They
suffer less from vicissitudes of temperature and climate. They
retain their efficiency with fewer precautions and under more
trying circumstances. Above all, they co-operate more readily
with mechanical appliances, and lend themselves with far
greater facility to purposes of exact measurement.
A practical difficulty, however, impeded the realisation of
1 It is remarkable that, as early as 1695, the possibility of an achromatic
combination was inferred by David Gregory from the structure of the
human eye. See his Catoptricce et Dioptrics Spherica Elementa, p. 98.
K
146 HISTORY OF ASTRONOMY.
the brilliant prospects held out by Dollond's invention. It
was found impossible to procure flint-glass, such as was
needed for optical use that is, of perfectly homogeneous
quality except in fragments of insignificant size. Discs of
more than two or three inches in diameter were of extreme
rarity ; and the crushing excise duty imposed upon the article
by the financial unwisdom of the Government, both limited
its production, and, by rendering experiments too costly for
repetition, barred its improvement.
Up to this time, Great Britain had left foreign competitors
far behind in the instrumental department of astronomy.
The quadrants and circles of Bird, Gary, and Ramsden were
unapproached abroad. The reflecting telescope came into
existence and reached maturity on British soil. The re-
fracting telescope was cured of its inherent vices by British
ingenuity. But with the opening of the nineteenth century,
the almost unbroken monopoly of skill and contrivance which
our countrymen had succeeded in establishing was invaded,
and British workmen had to be content to exchange a posi-
tion of supremacy for one of at least partial and temporary
inferiority.
Somewhere about the time that Herschel set about polishing
his first speculum, Pierre Louis Guinand, a Swiss artisan,
living near Chaux-de-Fonds, in the canton of Neuchatel,
began to grind spectacles for his own use, and was thence
led on to the rude construction of telescopes by fixing lenses
in pasteboard tubes. The sight of an English achromatic,
however, stirred a higher ambition, and he took the first
opportunity of procuring some flint-glass from England (then
the only source of supply), with the design of imitating an
instrument the full capabilities of which he was destined to
be the humble means of developing. The English glass
proving of inferior quality, he conceived the possibility, un-
aided and ignorant of the art as he was, of himself making
better, and spent seven years (1784-90) in fruitless experiments
directed to that end. Failure only stimulated him to enlarge
INSTRUMENTAL ADVANCES. 147
their scale. He bought some land near Les Brenets, con-
structed upon it a furnace capable of melting two quintals
of glass, and reducing himself and his family to the barest
necessaries of life, he poured his earnings (he at this time
made bells for repeaters) unstintingly into his crucibles. 1 His
undaunted resolution triumphed. In 1799 he carried to Paris
and there showed to Lalande several discs of flawless crystal
four to six inches in diameter. Lalande advised him to keep
his secret, but in 1805 he was induced to remove to Munich,
where he became the instructor of the immortal Fraunhofer.
His return to Les Brenets in 1814 was signalised by the dis-
covery of an ingenious 'mode of removing striated portions of
glass by breaking and re-soldering the product of each melting,
and he eventually attained to the manufacture of perfect discs
up to 1 8 inches in diameter. An object-glass for which he had
furnished the material to Cauchoix, procured him, in 1823, a
royal invitation to settle in Paris ; but he was no longer equal
to the change, and died at the scene of his labours, February
13 following,
This same lens (12 inches across) was afterwards purchased
by Sir James South, and the first observation made with it,
February 13, 1830, disclosed to Sir John Herschel the sixth
minute star in the central group of the Orion nebula,
known as the "trapezium." 2 A still larger objective (of
nearly 14 inches) made of Guinand's glass was secured
about the same time in Paris, by Mr. Edward Cooper of
Markree Castle, Ireland. The peculiarity of the method dis-
covered at Les Brenets resided in the manipulation, not in
the quality of the ingredients ; the secret, that is to say, was
not chemical, but mechanical. 8 It was communicated by
Henry Guinand (a son of the inventor) to Bontemps, one of
the directors of the glassworks at Choisy-le-Roi, and by him
transmitted to Messrs. Chance of Birmingham, with whom he
entered into partnership when the revolutionary troubles of
1 Wolf, Biographien^ Bd. ii. p. 301. 2 Month. Not., vol. i. p. 153, note.
3 Henrivaux, Encyclopedic Chimique, t. v. fasc. 5, p. 363.
148 HISTORY OF ASTRONOMY.
1848 obliged him to quit his native country. The celebrated
American opticians, Alvan Clark & Sons, have derived from
the Birmingham firm the materials for some of their finest
telescopes, notably the 1 9-inch Chicago and 26-inch Washing-
ton equatoreals.
Two distinguished amateurs, meanwhile, were preparing to
reassert on behalf of reflecting instruments their claim to the
place of honour in the van of astronomical discovery. Of Mr.
Lassell's specula something has already been said. 1 They
were composed of an alloy of copper and tin, with a minute
proportion of arsenic (after the example of Newton 2 ), and
were remarkable for perfection of figure and brilliancy of
surface.
The resources of the Newtonian system were developed still
more fully it might almost be said to the uttermost by the
enterprise of an Irish nobleman. William Parsons, known as
Lord Oxmantown until 1841, when, on his father's death, he
succeeded to the title of Earl of Rosse, was born at York, June
17,1 800. His public duties began before his education was com-
pleted. He was returned to Parliament as member for King's
County while still an undergraduate at Oxford, and continued
to represent the same constituency for thirteen years (1821-34).
From 1845 unt il hi s death, which took place at Birr Castle,
Parsonstown, October 31, 1867, he sat, silent but assiduous,
in the House of Lords as an Irish representative peer; he
held the not unlaborious post of President of the Royal
Society from 1849 to 1854 ; presided over the meeting of the
British Association at Cork in 1843, and was elected Vice-
Chancellor of Dublin University in 1862. In addition to
these extensive demands upon his time and thoughts, were
those derived from his position as (practically) the feudal chief
of a large body of tenantry in times of great and anxious
responsibility, to say nothing of the more genial claims of an
unstinted hospitality. Yet, while neglecting no public or
private duty, this model nobleman found leisure to render to
1 See ante^ p. 109.; 2 Phil. Trans., vol. vii. p. 407. t
INSTRUMENTAL ADVANCES. 149
science services so conspicuous as to entitle his name to a
lasting place in its annals.
He early formed the design of reaching the limits of the
attainable in enlarging the powers of the telescope, and the
qualities of his mind conspired with the circumstances of
his fortune to render the design a feasible one. From re-
fractors, it was obvious that no such vast and rapid advance
could be expected. English glass-manufacture was still in a
backward state. So late as 1839, Simms (successor to the
distinguished instrumentalist Edward Troughton) reported a
specimen of crystal scarcely 7^ inches in diameter, and perfect
only over six, to be unique in the history of English glass-
making. 1 Yet at that time the 15-inch achromatic of Pulkowa
had already left the workshop of Fraunhofer's successors at
Munich. It was not indeed until 1845, when the impost
which had so long hampered their efforts was removed, that
the optical artists of these islands were able to compete on
equal terms with their rivals on the Continent. In the case of
reflectors, however, there seemed no insurmountable obstacle
to an almost unlimited increase of light-gathering capacity ;
and it was here, after some unproductive experiments with
fluid lenses, that Lord Oxmantown concentrated his energies.
He had to rely entirely on his own invention, and to earn
his own experience. James Short had solved the problem of
giving to metallic surfaces a perfect parabolic figure (the only
one by which parallel incident rays can be brought to an exact
focus) ; but so jealous was he of his secret, that he caused all
his tools to be burnt before his death ; 2 nor was anything
known of the processes by which Herschel had achieved his
astonishing results. Moreover, Lord Oxmantown had no
skilled workmen to assist him. His implements, both animate
and inanimate, had to be formed by himself. Peasants taken
from the plough were educated by him into efficient mechanics
and engineers. The delicate and complex machinery needed
in operations of such hairbreadth nicety as his enterprise
1 J. Herschel, The Telescope t p. 39. 2 Month. Not.> vol. xxix. p. 125.
150 HISTORY OF ASTRONOMY.
involved, the steam-engine which was to set it in motion, at
times the very crucibles in which his specula were cast, issued
from his own workshops. 1
In 1827 experiments on the composition of speculum-metal
were set on foot, and the first polishing-machine ever driven
by steam-power was contrived. But twelve arduous years of
struggle with recurring difficulties passed before success began
to dawn. A material less tractable than the alloy selected of
four chemical equivalents of copper to one of tin 2 can scarcely
be conceived. It is harder than steel, yet brittle as glass,
crumbling into fragments with the slightest inadvertence of
handling or treatment ; 3 and the precision of figure requisite
to secure good definition is almost beyond the power of lan-
guage to convey. The quantities involved are so small as
not alone to elude sight, but to confound imagination. Sir
John Herschel tells us that " the total thickness to be abraded
from the edge of a spherical speculum 48 inches in diameter
and 40 feet focus, to convert it into a paraboloid, is only
ar.irsT f an mcn > " 4 Y et upon this minute difference of form
depends the clearness of the image, and, as a consequence, the
entire efficiency of the instrument. " Almost infinite," indeed
(in the phrase of the late Dr. Robinson), must be the exactitude
of the operation adapted to bring about so delicate a result.
At length, in 1840, two specula, each three feet in diameter,
were turned out in such perfection as to prompt a still bolder
experiment. The various processes needed to ensure success
were now ascertained and under control ; all that was neces-
sary was to repeat them on a larger scale. A gigantic mirror,
six feet across and fifty-four in focal length, was accordingly
cast on the t3th of April 1842 ; in two months it was ground
down to figure by abrasion with emery and water, and daintily
1 Month. Not., vol. xxix. p. 129.
2 A slight excess of copper renders the metal easier to work, but liable
to tarnish. Robinson, Proc. Roy. Irish Ac., vol. ii. p. 4.
3 Brit. Ass., 1843, Dr. Robinson's closing Address. Atkenaum, Sept.
23, p. 866. 4 The Tekscope, p. 82.
INSTRUMENTAL ADVANCES. 151
polished with rouge; and by the month of February 1845
the " leviathan of Parsonstown " was available for the exami-
nation of the heavens.
The suitable mounting of this vast machine was a problem
scarcely less difficult than its construction. The shape of a
speculum needs to be maintained with an elaborate care equal
to that used in imparting it. In fact, one of the most for-
midable obstacles to increasing the size of such reflecting
surfaces consists in their liability to bend under their own
weight. That of the great Rosse speculum was no less than
four tons. Yet, although six inches in thickness, and com-
posed of a material only a degree inferior in rigidity to wrought
iron, the strong pressure of a man's hand at its back produced
sufficient flexure to distort perceptibly the image of a star
reflected in it. 1 Thus the delicacy of its form was perishable
equally by the stress of its own gravity, and by the slightest
irregularity in the means taken to counteract that stress. The
problem of affording a perfectly equable support in all possible
positions was solved by resting the speculum upon twenty-
seven platforms of cast iron, felt-covered, and carefully fitted
to the shape of the areas they were to carry, which platforms
were themselves borne by a complex system of triangles and
levers, ingeniously adapted to distribute the weight with com-
plete uniformity. 2
A tube which resembled, when erect, one of the ancient
round towers of Ireland, 3 served as the habitation of the great
mirror. It was constructed of deal staves bound together with
iron hoops, was fifty-eight feet long (including the speculum-
box), and seven in diameter. We are assured that the late
Dean of Ely walked through it with umbrella uplifted. 4 Two
piers of solid masonry, about fifty feet high, seventy long, and
1 Lord Rosse in Phil. Trans., vol. cxl. p. 302.
2 This method is the same in principle with that applied by Grubb in
1834 to a 15-inch speculum for the observatory of Armagh. Phil. Trans.,
vol. clix. p. 145. 3 Robinson, Proc. Roy. Jr. Ac., vol. iii. p. 120.
4 Brewster, North British Review, vol. ii. p. 207.
152 HISTORY OF ASTRONOMY.
twenty-three apart, flanked the huge engine on either side.
Its lower extremity rested on an universal joint of cast iron ;
above, it was slung in chains, and even in a gale of wind
remained perfectly steady. The weight of the entire, although
amounting to fifteen tons, was so skilfully counterpoised, that
the tube could with ease be raised or depressed by two men
working a windlass. Its horizontal range was limited by the
lofty walls erected for its support to about ten degrees on each
side of the meridian ; but it moved vertically from near the
horizon through the zenith as far as the pole. Its construction
was of the Newtonian kind, the observer looking into the side
of the tube near its upper end, which a series of galleries and
sliding stages enabled him to reach in any position. It has
also, though rarely, been used without a second mirror, as a
" Herschelian " reflector.
The splendour of the celestial objects as viewed with this
vast "light-grasper" surpassed all expectation. "Never in
my life," exclaims Sir James South, "did I see such glorious
sidereal pictures ! " l The orb of Jupiter produced an effect
compared to that of the introduction of a coach-lamp into the
telescope ; 2 and certain star-clusters exhibited an appearance
(we again quote Sir James South) " such as man before had
never seen, and which for its magnificence baffles all descrip-
tion." But it was in the examination of the nebulas that the
superiority of the new instrument was most strikingly displayed.
A large number of these misty objects, which the utmost
powers of Herschel's specula had failed to resolve into stars,
yielded at once to the Parsonstown reflector ; while many
others showed under entirely changed forms through the dis-
closure of previously unseen details of structure.
One extremely curious result of the increase of light was the
abolition of the distinction between the two classes of " annular "
and " planetary " nebulae. Up to that time, only four ring-
shaped systems two in the northern and two in the southern
hemisphere were known to astronomers ; they were now
1 Astr. Nach., No. 536. 2 Airy, Month. Not., vol. ix. p. 120.
INSTRUMENTAL ADVANCES. 153
reinforced by five of the planetary kind, the discs of which
were observed to be centrally perforated ; while the sharp
marginal definition visible in weaker instruments was replaced
by ragged edges or filamentous fringes.
Still more striking was the discovery of an entirely new and
highly remarkable species of nebulae. These were termed
" spiral," from the more or less regular convolutions, re-
sembling the whorls of a shell, in which the matter composing
them appeared to be distributed. The first and most con-
spicuous specimen of this class was met with in April 1845 >
it is situated in Canes Venatici, close to the tail of the Great
Bear, and wore, in Sir J. Herschel's instruments, the aspect of
a split ring encompassing a bright nucleus, thus presenting, as
he supposed, a complete analogue to the system of the Milky
Way. In the Rosse mirror it shone out as a vast whirlpool
of light a stupendous witness to the presence of cosmical
activities on the grandest scale, yet regulated by laws as to the
nature of which we are profoundly ignorant. Professor Stephen
Alexander of New Jersey, however, concluded, from an in-
vestigation (necessarily founded on highly precarious data) of
the mechanical condition of these extraordinary agglomerations,
that we see in them " the partially scattered fragments of enor-
mous masses once rotating in a state of dynamical equilibrium."
He further suggested " that the separation of these fragments
may still be in progress," 1 and traced back their origin to the
disruption, through its own continually accelerated rotation,
of a "primitive spheroid" of inconceivably vast dimensions.
Such also, it was added (the curvilinear form of certain out-
liers of the Milky Way, giving evidence of a spiral structure),
is probably the history of our own cluster ; the stars composing
which, no longer held together in a delicately adjusted system
like that of the sun and planets, are advancing through a
period of seeming confusion towards an appointed goal of
higher order and more perfect and harmonious adaptation. 2
1 Astronomical Journal (Gould's), vol. ii. p. 97.
2 Ibid.) vol. ii. p. 160.
154 HISTORY OF ASTRONOMY.
The class of spiral nebulae included, in 1850, fourteen
members, besides several in which the characteristic arrange-
ment seemed partial or dubious. 1 A tendency in the exterior
stars of other clusters to gather into curved branches (as in
our Galaxy) was likewise noted ; and the existence of un-
suspected analogies was proclaimed by the significant com-
bination in the "Owl" nebula (a large planetary in Ursa
Major) 2 of the twisted forms of a spiral, with the perforations
distinctive of an annular nebula.
Once more, by the achievements of the Parsonstown re-
flector, the supposition of a " shining fluid " filling vast regions
of space was brought into (as it has since proved) undeserved
discredit. Although Lord Rosse himself rejected the inference
that because many nebulae had been resolved, all were resolv-
able, very few imitated his truly scientific caution; and the
results of Bond's investigations 3 with the Harvard College
refractor quickened and strengthened the current of prevalent
opinion. It is now certain that the evidence furnished on
both sides of the Atlantic as to the stellar composition of some
conspicuous objects of this class (notably the Orion and
" Dumb-bell " nebulae) was delusive ; but the spectroscope alone
was capable of meeting it with a categorical denial. Mean-
while there seemed good ground for the persuasion, which
now, for the last time, gained the upperhand, that nebulae
are, without exception, true "island-universes," or assemblages
of distant suns.
Lord Rosse's telescope possesses a nominal power of 6000
that is, it shows the moon as if viewed with the naked eye at a
distance of forty miles. But this seeming advantage is neutra-
lised by the weakening of the available light through excessive
diffusion, as well as by the troubles of the surging sea of air
through which the observation must necessarily be made.
1 Lord Rosse in Phil. Trans., vol. cxl. p. 505.
2 No. 2343 of Herschel's (1864) Catalogue. Before 1850 a star was
visible in each of the two larger openings by which it is pierced ; since then
one only. Webb, Celestial Objects (4th ed.), p. 409.
3 Mem. Am. Ac., vol. iii. p. 87 ; and Astr. Nach., No. 6ll.
INSTRUMENTAL ADVANCES. 155
Professor Newcomb, in fact, doubts whether with any telescope
our satellite has ever been seen to such advantage as it would
be if brought within 500 miles of the unarmed eye. 1
The French opticians' rule of doubling the number of milli-
metres contained in the aperture of an instrument to find the
highest magnifying power usefully applicable to it, would give
3600 as the maximum for the leviathan of Birr Castle ; but
in a climate like that of Ireland the occasions must be rare
when even that limit can be reached. Indeed, the experience
acquired by its use plainly shows that atmospheric, rather than
mechanical difficulties impede a still further increase of tele-
scopic power. Its construction may accordingly be said to
mark the ne plus ultra of effort in one direction, and the
beginning of its conversion towards another. It became
thenceforward more and more obvious that the conditions
of observation must be ameliorated before any added efficacy
could be given to it. The full effect of an uncertain climate
in nullifying optical improvements was recognised, and the
attention of astronomers began to be turned towards the
advantages offered by more tranquil and more translucent
skies.
Even more important for the practical uses of astronomy
than the optical qualities of the telescope is the manner of its
mounting. There is a far greater likelihood of getting good
work done with an imperfect instrument skilfully mounted,
than with the most admirable performance of the optician of
which the mechanical accessories are ill-arranged or incon-
venient. Thus the astronomer is ultimately dependent upon
the mechanician ; and so excellently have his needs been
served, that the history of the ingenious contrivances by which
discoveries have been prepared would supply a subject scarcely
inferior in extent and instruction to the history of those dis-
coveries themselves. But the limits of the present work barely
admit of a passing glance at the subject.
There are two chief modes of using the telescope, to which
1 Pop. Astr. t p. 145.
156 HISTORY OF ASTRONOMY.
all others may be considered subordinate. 1 Either it may be
immovably directed towards the south, in other words, fixed
in the plane of the meridian, so as to intercept the heavenly
bodies at the moment of transit across that plane ; or it may
be arranged so as to follow the daily revolution of the sky,
thus keeping the object viewed permanently in sight, instead
of simply noting the instant of its flitting across the telescopic
field. The first plan is that of the " transit instrument," the
second that of. the " equatoreal." Both were, by a remarkable
coincidence, introduced about 1690 2 by Olaus Romer, the
brilliant Danish astronomer who first measured the velocity of
light.
The uses of each are entirely different. With the transit,
the really fundamental task of astronomy the determination
of the movements of the heavenly bodies is mainly accom-
plished ; while the investigation of their nature and peculiarities
is best conducted with the equatoreal. One is the instrument
of mathematical, the other of descriptive astronomy. One
furnishes the materials with which theories are constructed, and
the tests by which they are corrected ; the other registers new
facts, takes note of new appearances, sounds the depths, and
pries into every nook of the heavens.
The great improvement of giving to a telescope equatoreally
mounted an automatic movement by connecting it with clock-
work, was proposed in 1674 by Robert Hooke. Bradley in
1721 actually observed Mars with a telescope "moved by a
machine that made it keep pace with the stars ; " 3 and Von
Zach relates 4 that he had once followed Sirius for twelve hours
1 This statement must be taken in the most general sense. Supple-
mentary observations of great value are now made at Greenwich with the
altitude and azimuth instrument, which likewise served Piazzi to determine
the places of his stars j while a "prime vertical instrument " is prominent
at Pulkowa.
2 As early as 1620, according to R. Wolf (Gesch. der Astr., p. 587),
Father Scheiner made the experiment of connecting a telescope with an
axis directed to the pole. 3 Bradley 1 * Miscellaneous Works, p. 350.
4 Astr. Jahrbuch, 1799 (published 1796), p. 115.
INSTRUMENTAL ADVANCES. 157
with a "heliostat" of Ramsden's construction. But these
eighteenth-century attempts were of no practical effect.
Movement by clockwork was virtually a complete novelty
when it was adapted by Fraunhofer in 1824 to the Dorpat
refractor. By simply giving to an axis unvaryingly directed
towards the celestial pole an equable rotation with a period of
twenty-four hours, a telescope attached to it, and pointed in
any direction, will trace out on the sky a parallel of declination,
thus necessarily accompanying the movement of any star upon
which it may be fixed. It thus forms part of the large sum of
Fraunhofer's merits to have secured this inestimable advantage
to observers.
It was considered by Sir John Herschel that Lassell's appli-
cation of equatoreal mounting to a nine-inch Newtonian in 1840
made an epoch in the history of " that eminently British
instrument, the reflecting telescope." l Nearly a century
earlier, 2 it is true, Short had fitted one of his Gregorians to a
complicated system of circles in such a manner that, by
moving a handle, it could be made to follow the revolution
of the sky ; but the arrangement did not obtain, nor did it
deserve, general adoption. Lassell's plan was a totally dif-
ferent one; he employed the crossed axes of the true
equatoreal, and his success removed, to a great extent, the
fatal objection of inconvenience in use, until then unanswer-
ably urged against reflectors. The very largest of these can
now be mounted equatoreally ; even the Rosse within its
limited range has been for some years provided with a move-
ment by clockwork along declination-parallels.
The art of accurately dividing circular arcs into the minute
equal parts which serve as the units of astronomical measure-
ment, remained, during the whole of the eighteenth century,
almost exclusively in English hands. It was brought to a high
degree of perfection by Graham, Bird, and Ramsden, all of
whom, however, gave the preference to the old-fashioned
mural quadrant and zenith sector over the entire circle, which
1 Month. Not,, vol. xli. p. 189. 2 Phil. Trans. , vol. xlvi. p. 242.
158 HISTORY OF ASTRONOMY.
Romer had already found the advantage of employing. The
five-foot vertical circle, which Piazzi with some difficulty in-
duced Ramsden to complete for him in 1789, was the first
divided instrument constructed in what may be called the
modern style. It was provided with magnifiers for reading off
the divisions (one of the neglected improvements of Romer),
and was set up above a smaller horizontal circle, forming an
"altitude and azimuth" combination (again Romer's inven-
tion), by which both the elevation of a celestial object above
the horizon and its position as referred to the horizon could
be measured. In the same year Borda invented the " repeat-
ing circle" (the principle of which had been suggested by
Tobias Mayer in I756 1 ), a device for exterminating, so far as
possible, errors of graduation by repeating an observation with
different parts of the limb. This was perhaps the earliest
systematic effort to correct the imperfections of instruments by
the manner of their use.
The manufacture of astronomical circles was brought to a
very refined state of excellence early in the present century
by Reichenbach at Munich, and (after 1818) by Repsold at
Hamburg. Bessel states 2 that the "reading-off" on an instru-
ment of the kind by the latter artist was accurate to about
g^th of a human hair. Meanwhile the traditional reputation
of the English school was fully sustained ; and Sir George
Airy did not hesitate to express his opinion that the new
method of graduating circles, published by Troughton in iSop, 3
was the "greatest improvement ever made in the art of in-
strument-making." 4 But a more secure road to improvement
than that of mere mechanical exactness was pointed out by
Bessel. His introduction of a regular theory of instrumental
errors might almost be said to have created a new art of
observation. Every instrument, he declared in memorable
words, 5 must be twice made once by the artist, and again by
1 Grant, Hist, of Astr., p. 487. 2 Pop. VbrL, p. 546.
3 Phil. Trans., vol. xcix. p. 105.
4 Refort Brit. Ass., 1832, p. 132. 5 Pop. Vorl., p. 432.
INSTR UMENTA L ADVA NCES. 1 59
the observer. Knowledge is power. Defects that are ascer-
tained and can be allowed for are as good as non-existent.
Thus the truism that the best instrument is worthless in the
hands of a careless or clumsy observer, became supplemented
by the converse maxim that defective appliances may, by
skilful use, be made to yield valuable results. The Konigsberg
observations of which the first instalment was published in
1815 set the example of regular "reduction " for instrumental
errors. Since then, it has become an elementary part of an
astronomer's duty to study the idiosyncrasy of each one of the
mechanical contrivances at his disposal, in order that its
inevitable, 'but now certified deviations from ideal accuracy
may be included amongst the numerous corrections by which
the pure essence of (even approximate) truth is distilled from
the rude impressions of sense.
Nor is this enough \ for the casual circumstances attending
each observation have to be taken into account with no less
care than the inherent, or constitutional peculiarities of the
instrument with which it is made. There is no "once for all"
in astronomy. Vigilance can never sleep ; patience can never
tire. Variable as well as constant sources of error must be
anxiously heeded ; one infinitesimal inaccuracy must be
weighed against another; all the forces and vicissitudes of
nature frosts, dews, winds, the interchanges of heat, the dis-
turbing effects of gravity, the shiverings of the air, the tremors
6f the earth, the weight and vital warmth of the observer's own
body, nay, the rate at which his brain receives and transmits
its impressions, must all enter into his calculations, and be
sifted out from his results.
It was in 1823 that Bessel drew attention to discrepancies
in the times of transits given by different astronomers. 1 The
quantities involved were far from insignificant. He was
himself nearly a second in advance of all his contemporaries,
Argelander lagging behind him as much as a second and a
1 C. T. Anger, Grundziige der neueren astronomischen Beobachtungs-
Kunst, p. 3.
160 HISTORY OF ASTRONOMY.
quarter. Each individual, in fact, was found to have a certain
definite rate of perception, which, under the name of "personal
equation," now forms an important element in the correction
of observations.
Such are the refinements upon which modern astronomy
depends for its progress. It is a science of hairbreadths and
fractions of a second. It exists only by the rigid enforcement
of arduous accuracy and unwearying diligence. Whatever
secrets the universe still has in store for man will only be
communicated on these terms. They are, it must be acknow-
ledged, difficult to comply with. They involve an unceasing
struggle against the infirmities of his nature and the instabilities
of his position. But the end is not unworthy the sacrifices
demanded. One additional ray of light thrown on the marvels
of creation a single, minutest encroachment upon the strong-
holds of ignorance, is recompense enough for a lifetime of toil.
Or rather, the toil is its own reward, if pursued in the lofty
spirit which alone becomes it. For it leads through the
abysses of space and the unending vistas of time to the very
threshold of that infinity and eternity of which the disclosure
is reserved for a life to come.
161
PART II.
RECENT PROGRESS OF ASTRONOMY.
CHAPTER I.
FOUNDATION OF ASTRONOMICAL PHYSICS.
IN the year 1826, Heinrich Schwabe of Dessau, elated with
the hope of speedily delivering himself from his hereditary
incubus of an apothecary's shop, 1 obtained from Munich a
small telescope and began to observe the sun. His choice
of an object for his researches was instigated by his friend
Harding of Gottingen. It was a peculiarly happy one. The
changes visible in the solar surface were then generally regarded
as no less capricious than the changes in the skies of our tem-
perate regions. Consequently, the reckoning and registering
of sun-spots was a task hardly more inviting to an astronomer
than the reckoning and registering of summer clouds. Cassini,
Keill, Lemonnier, Lalande, were unanimous in declaring that
no trace of regularity could be detected in their appearances or
effacements. 2 Delambre pronounced them " more curious
than really useful." 3 Even Herschel, profoundly as he studied
them, and intimately as he was convinced of their importance
as symptoms of solar activity, saw no reason to suspect that
their abundance and scarcity were subject to orderly alter-
nation. One man alone in the eighteenth century, Christian
1 Wolf, Gesch. der Astr., p. 655.
2 Manuel Johnson, Mem. R. A. Soc., vol. xxvi. p. 197.
3 Astronomic Theorique et Pratique, t. iii. p. 2O.
L
1 62 HISTORY OF ASTRONOMY.
Horrebow of Copenhagen, divined their periodical character,
and foresaw the time when the effects of the sun's vicissitudes
upon the globes revolving round him might be investigated
with success ; but this prophetic utterance was of the nature
of a soliloquy rather than of a communication, and remained
hidden away in an unpublished journal until 1859, when it was
brought to light in a general ransacking of archives. 1
Indeed, Schwabe himself was far from anticipating the dis-
covery which fell to his share. He compared his fortune to
that of Saul, who, seeking his father's asses, found a kingdom. 2
For the hope which inspired his early resolution lay in quite
another direction. His patient ambush was laid for a possible
intra-mercurial planet, which, he thought, must sooner or later
betray its existence in crossing the face of the sun. He took,
however, the most effectual measures to secure whatever new
knowledge might be accessible. During forty-three years his
" imperturbable telescope " 3 never failed (weather and health
permitting), to bring in its daily report as to how many, or if
any, spots were visible on the sun's disc, the information
obtained being day by day recorded on a simple and unvary-
ing system. In 1843 he made his first announcement of a
probable decennial period, 4 but it met with no general atten-
tion ; although Julius Schmidt of Bonn (afterwards director of
the Athens Observatory) and Gautier of Geneva were im-
pressed with his figures, and Littrow had himself, in i836, 5
hinted at the likelihood of some kind of regular recurrence.
Schwabe, however, worked on, gathering each year fresh
evidence of a law such as he had indicated ; and when Hum-
boldt published in 1851, in the third volume of his Kosmos?
a table of the sun-spot statistics collected by him from 1826
downwards, the strength of his case was perceived with, so to
speak, a start of surprise ; the reality and importance of the
1 Wolf, Gesch. der Astr., p. 654. 2 Month. Not., vol. xvii. p. 241.
3 Mem. R. A. Soc., vol. xxvi. p. 200. 4 Astr. Nach., No. 495.
B Gehler's Physikalisches Worlerbuch, art. Sonnenflecken, p. 851.
6 Ziveite Abth., p. 401.
FOUNDATION OF ASTRONOMICAL PHYSICS. 163
discovery were simultaneously recognised, and the persevering
Hofrath of Dessau found himself famous among astronomers.
His merit recognised by the bestowal of the Astronomical
Society's Gold Medal in 1857 consisted in his choice of an
original and appropriate line of work, and in the admirable
tenacity of purpose with which he pursued it. His resources
and acquirements were those of an ordinary amateur ; he was
distinguished solely by the (unfortunately rare) power of turn-
ing both to the best account. He died where he was born
and had lived, April n, 1875, at the "P e a e of eighty- six.
Meanwhile an investigation of a totally different character,
and conducted by totally different means, had been prosecuted
to a very similar conclusion. Two years after Schwabe began
his solitary observations, Humboldt gave the first impulse, at
the Scientific Congress of Berlin in 1828, to a great inter-
national movement for attacking simultaneously, in various
parts of the globe, the complex problem of terrestrial magnetism.
Through the genius and energy of Gauss, Gottingen became its
centre. Thence new apparatus and a new system for its em-
ployment issued; there, in 1833, the first regular magnetic
observatory was founded, while Gottingen mean time was made
the universal standard for magnetic observations. The letter
addressed by Humboldt in April 1836 to the Duke of Sussex
as President of the Royal Society, enlisted the co-operation of
England. A network of magnetic stations was spread all over
the British dominions, from Canada to Van Diemen's Land ;
measures were concerted with foreign authorities, and an expe-
dition was fitted out, under the able command of Captain
(afterwards Sir James) Clark Ross, for the special purpose of
bringing intelligence on the subject from the dismal neighbour-
hood of the South Pole. In 1841, the elaborate organisation
created by the disinterested efforts of scientific " agitators "
was complete ; Gauss's "magnetometers" were vibrating, under
the view of attentive observers, in five continents, and simul-
taneous results began to be recorded.
Ten years later, in September 1851, Dr. John Lamont, the
1 64 HISTORY OF ASTRONOMY.
Scotch director of the Munich Observatory, in reviewing the
magnetic observations made at Gottingen and Munich from
1835 to I ^5j perceived with some surprise that they gave un-
mistakable indications of a period which he estimated at loj
years. 1 The manner in which this periodicity manifested itself
requires a word of explanation. The observations in question
referred to what is called the " declination " of the magnetic
needle that is, to the position assumed by it with reference
to the points of the compass when moving freely in a horizontal
plane. Now this position as was discovered by Graham in
1722 is subject to a small daily fluctuation, attaining its maxi-
mum towards the east about 8 A.M., and its maximum towards
the west shortly before 2 P.M. In other words, the direction
of the needle approaches (in these countries at the present time)
nearest to the true north some four hours before noon, and de-
parts farthest from it between one and two hours after noon.
It was the range of this daily variation that Lamont found to
increase and diminish once in every loj years.
In the following winter, Sir Edward Sabine, ignorant as yet
of Lament's conclusion, undertook to examine a totally diffe-
rent set of observations. The materials in his hands had been
collected at the British colonial stations of Toronto and Hobar-
ton from 1843 to Z 848, and had reference, not to the regular
diurnal swing of the needle, but to those curious spasmodic
vibrations, the inquiry into the laws of which was the primary
object of the vast organisation set on foot by Humboldt and
Gauss. Yet the upshot was practically the same. Once in
about ten years magnetic disturbances (termed by Humboldt
" storms ") were perceived to reach a maximum of violence and
frequency. Sabine was the first to note the coincidence be-
tween this unlooked-for result and Schwabe's sun-spot period.
He showed that, so far as observation had yet gone, the two
cycles of change agreed perfectly both in duration and phase,
maximum corresponding to maximum, minimum to minimum.
What the nature of the connection could be that bound to-
1 Annalen der Physik (Poggendorff's), Bd. Ixxxiv. p. 580.
FOUNDATION OF ASTRONOMICAL PHYSICS. 165
gether by a common law effects so dissimilar as the rents in
the luminous garment of the sun, and the swayings to and fro
of the magnetic needle, was, and still remains beyond the reach
of well-founded conjecture ; but the fact was from the first un-
deniable.
The memoir containing this remarkable disclosure was pre-
sented to the Royal Society, March 18, and read May 6,
1852. l On the 3ist of July following, Rudolf Wolf at Berne, 2
and on the i8th of August Alfred Gautier at Sion, 3 announced,
separately and independently, perfectly similar conclusions.
This triple event is perhaps the most striking instance of the
successful employment of the Baconian method of co-opera-
tion in discovery, by which "particulars" are amassed by one
set of investigators corresponding to the " Depredators " and
" Inoculators " of Solomon's House while inductions are
drawn from them by another and a higher class the " Inter-
preters of Nature." Yet even here the convergence of two
distinct lines of research was wholly fortuitous, and skilful
combination owed the most brilliant part of its success to the
unsought bounty of what we call Fortune.
The exactness of the coincidence thus brought to light was
fully confirmed by further inquiries. A diligent search through
the scattered records of sun-spot observations, from the time of
Galileo and Scheiner onwards, put Wolf 4 in possession of
materials by which he was enabled to correct Schwabe's loosely
indicated decennial period to one of slightly over eleven (i 1. 1 1 )
years ; and he further showed that this fell in with the ebb and
flow of magnetic change even better than Lament's zoj year
cycle. For the first time, also, the analogy was pointed out be-
tween the "light-curve, "or zigzagged line representing on paper
the varying intensity in the lustre of certain stars, and the similar
delineation of spot-frequency ; the ascent from minimum to
1 Phil. Trans., vol. cxlii. p. 103.
2 Mittheilungen der Naturforschenden Gesellschaft, 1852, p. 183.
3 Archives des Sciences, t. xxi. p. 194.
4 Neue Untersuchungen, Mitth. Naturf. Ges., 1852, p. 249.
1 66 HISTORY OF ASTRONOMY.
maximum being, in both cases, usually steeper than the descent
from maximum to minimum ; while an additional point of
resemblance was furnished by the irregularities in height of the
various maxima. In other words, both the number of spots
on the sun and the brightness of variable stars increase, as a
rule, more rapidly than they decrease ; nor does the amount of
that increase, in either instance, show any approach to uni-
formity.
The endeavour, suggested by the very nature of the pheno-
menon, to connect sun-spots with weather was less successful.
The first attempt of the kind was made by Sir William Her-
schel in the first year of the present century, and a very notable
one it was. Meteorological statistics, save of the scantiest and
most casual kind, did not then exist ; but the price of corn
from year to year was on record, and this, with full recognition
of its inadequacy, he adopted as his criterion. Nor was he
much better off for information respecting the solar condition.
What little he could obtain, however, served, as he believed,
to confirm his surmise that a copious emission of light and
heat accompanies an abundant formation of " openings " in the
dazzling substance whence our supply of those indispensable
commodities is derived. 1 He gathered, in short, from his
inquiries very much what he had expected to gather, namely,
that the price of wheat was high when the sun showed an un-
sullied surface, and that food and spots became plentiful to-
gether.
This plausible inference, however, was scarcely borne out
by a more exact collocation of facts. Schwabe failed to detect
any reflection of the sun-spot period in his meteorological
register. Gautier 2 reached a provisional conclusion the reverse
though not markedly the reverse of Herschel's. Wolf,
in 1852, derived from an examination of Vogel's collection of
Zurich Chronicles (1000-1800 A.D.) evidence showing (as he
thought) that minimum years were usually wet and stormy,
1 Phil. Trans., vol. xci. p. 316.
2 Bibliothtque Universdk de Genhse, t. li. p. 336.
FOUNDATION OF ASTRONOMICAL PHYSICS. 167
maximum years dry and genial ; 1 but a subsequent review of
the subject in 1859 convinced him that no relation of any
kind between the two kinds of effects was traceable. 2 With
the singular affection of our atmosphere known as the Aurora
Borealis (more properly Aurora Polaris) the case was different.
Here the Ziirich Chronicles set Wolf on the right track in
leading him to associate such luminous manifestations with
a disturbed condition of the sun ; since subsequent detailed
observation has exhibited the curve of auroral frequency as
following with such fidelity the jagged lines figuring to the eye
the fluctuations of solar and magnetic activity, as to leave no
reasonable doubt that all three rise and sink together under
the influence of a common cause. As long ago as I7I4, 3
Halley had conjectured that the Northern Lights were due to
magnetic " effluvia," but there was nc\ evidence on the subject
forthcoming until Hiorter observed at Upsala in 1741 their
agitating influence upon the magnetic needle. That the effect
was no casual one was made superabundantly clear by Arago's
researches in 1819 and subsequent years. Now both were
perceived to be swayed by the same obscure power of cosmical
disturbance.
The sun is not the only one of the heavenly bodies by which
the magnetism of the earth is affected. Proofs of a similar
kind of lunar action were laid by Kreil in 1841 before the
Bohemian Society of Sciences, and were fully substantiated,
though with minor corrections, by Sabine's more extended
researches. It has thus been ascertained that each lunar day,
or the interval of twenty-four hours and about fifty-four minutes
between two successive meridian passages of our satellite, is
marked by a perceptible, though very small double oscillation
1 Neue Untersuchungen, p. 269.
2 Die Sonne undihre Flecken, p. 30. Arago was the first who attempted
to decide the question by keeping, through a series of years, a parallel
register of sun-spots and weather ; but the data regarding the solar condition
collected at the Paris Observatory from 1822 to 1830 were not sufficiently
precise to found any inference upon.
3 Phil. Trans., vol. xxix. p. 421.
168 HISTORY OF ASTRONOMY.
of the needle two progressive movements from east to west,
and two returns from west to east. 1 Moreover, the lunar, like
the solar influence (as was proved in each case by Sabine's
analysis of the Hobarton and Toronto observations), extends to
all three " magnetic elements," affecting not only the position
of the horizontal or dedinatum-needle, but also the dip and
intensity. It seems not unreasonable to attribute some portion
of the same subtle power to the planets, and even to the stars,
though with effects rendered imperceptible by distance.
We have now to speak of the discovery and application to
the heavenly bodies of a totally new method of investigation.
Spectrum analysis may be shortly described as a mode of dis-
tinguishing the various species of matter by the kind of light
proceeding from each. This definition at once explains how
it is that, unlike every other system of chemical analysis, it has
proved available in astronomy. Light, so far as quality is con-
cerned, ignores distance. No intrinsic change (that we yet
know of) is produced in it by a journey from the farthest
bounds of the visible universe ; so that, provided only that in
quantity it remain sufficient for the purpose, its peculiarities
can be equally well studied whether the source of its vibrations
be one foot or a hundred billion miles distant. Now the most
obvious distinction between one kind of light f and another
resides in colour. But of this distinction the eye takes cognis-
ance in an aesthetic, not in a scientific sense. It finds gladness
in the "thousand tints" of nature, but can neither analyse
nor define them. Here the refracting prism or the combina-
tion of prisms known as the " spectroscope " comes to its aid,
teaching it to measure as well as to perceive. It furnishes, in
a word, an accurate scale of colour. The various rays which,
entering the eye together in a confused crowd, produce a
compound impression made up of undistinguishable elements,
are, by the mere passage through a triangular piece of glass,
separated one from the other, and ranged side by side in
orderly succession, so that it becomes possible to tell at a
1 Phil. Trans., vol. cxliii. p. 558, and vol. cxlvi. p. 505.
FOUNDATION OF ASTRONOMICAL PHYSICS. 169
glance what kinds of light are present, and what absent. Thus,
if we could only be assured that the various chemical sub-
stances, when made to glow by heat, emit characteristic rays
rays, that is, occupying a place in the spectrum reserved for
them, and for them only we should at once be in possession of a
mode of identifying such substances with the utmost readiness
and certainty. This assurance, which forms the solid basis of
spectrum analysis, was obtained slowly and with difficulty.
The first to employ the prism in the examination of various
flames (for it is only in a state of vapour that matter emits dis-
tinctive light) was a young Scotchman named Thomas Melvill,
who died in 1753, at the age of twenty-seven. He studied the
spectrum of burning spirits, into which were introduced suc-
cessively sal ammoniac, potash, alum, nitre, and sea-salt, and
observed the singular predominance, under almost all circum-
stances, of a particular shade of yellow light, perfectly definite
in its degree of refrangibility 1 in other words, taking up a
perfectly definite position in the spectrum. His experiments
were repeated by Morgan, 2 Wollaston, and with far supe-
rior precision and diligence by Fraunhofer. 3 The great
Munich optician, whose work was completely original, re-
discovered MelvilPs deep yellow ray and measured its place
in the colour-scale. It has since become well known as the
" sodium line," and has played a very important part in the
history of spectrum analysis. Nevertheless, its ubiquity and
conspicuousness long impeded progress. It was elicited by
the combustion of a surprising variety of substances sulphur,
alcohol, ivory, wood, paper ; its persistent visibility suggesting
the accomplishment of some universal process of nature rather
than the presence of one individual kind of matter. But if
spectrum analysis were to exist as a science at all, it could only
be by attaining certainty as to the unvarying association of one
special substance with each special quality of light.
1 Observations on Light and Colours, p. 35.
2 Phil. Trans., vol. Ixxv. p. -190.
3 Denkschriften (Munich Ac. of Sc.), 1814-15, Bd. v. p. 197.
170 HISTORY OF ASTRONOMY.
Thus perplexed, Fox Talbot l hesitated in 1826 to enounce
this fundamental principle. He was inclined to believe that the
presence in the spectrum of any individual ray told unerringly
of the volatilisation in the flame under scrutiny of some body
as whose badge or distinctive symbol that ray might be regarded ;
but the continual prominence of the yellow beam staggered
him. It appeared, indeed, without fail where sodium was ;
but it also appeared where it might be thought only reasonable to
conclude that sodium was not. Nor was it until thirty years later
that William Swan, 2 by pointing out the extreme delicacy of
the spectral test, and the singularly wide dispersion of sodium,
made it appear probable (but even then only probable) that
the questionable yellow line was really due invariably to that
substance. Common salt (chloride of sodium) is, in fact, the
most diffusive of solids. It floats in the air ; it flows with
water; every grain of dust has its attendant particle; its abso-
lute exclusion approaches the impossible. And withal the light
that it gives in burning is so intense and concentrated, that if
a single grain be divided into 180 million parts, and one alone
of such inconceivably minute fragments be present in a source
of light, the spectroscope will show unmistakably its charac-
teristic beam.
Amongst the pioneers of knowledge in this direction were
Sir John Herschel 3 who, however, applied himself to the
subject in the interests of optics, not of chemistry W. A.
Miller, 4 and Wheatstone. The last especially made a notable
advance when, in the course of his studies on the " prismatic
decomposition " of the electric light, he reached the significant
conclusion that the rays visible in its spectrum were different
for each kind of metal employed as "electrodes." 5 Thus
1 Edinburgh Journal of Science, vol. v. p. 77. See also Phil. Mag.,
Feb. 1834, vol. iv. p. 112. 2 Ed. Phil. Trans., vol. xxi. p. 411.
3 On the Absorption of Light by Coloured Media, Ed. Phil. Trans., vol.
ix. p. 445 (1823). 4 Phil. Mag., vol. xxvii. (ser. Hi.), p. 81.
5 Report Brit. Ass., 1835, P- IJ (P l ") Electrodes are the terminals
from one to the other of which the electric spark passes, volatilising and
FOUNDATION OF ASTRONOMICAL PHYSICS. 171
indications of a wider principle were to be found in several
quarters, but no positive certainty on any single point was
obtained until, in 1859, Gustav Kirchhoff, professor of physics
in the University of Heidelberg, and his colleague, the eminent
chemist Robert Bunsen, took the matter in hand. By them
the general question as to the necessary and invariable con-
nection of certain rays in the spectrum with certain kinds of
matter, was first resolutely confronted, and first definitively
answered. It was answered affirmatively else there could
have been no science of spectrum analysis as the result of
experiments more numerous, more stringent, and more precise
than had previously been undertaken. 1 And the assurance of
their conclusion was rendered doubly sure by the discovery,
through the peculiarities of their light alone, of two new metals,
named, from the blue and red rays by which they were respec-
tively distinguished, "Caesium," and "Rubidium." 2 Both
were immediately afterwards actually obtained in small quanti-
ties by evaporation of the Diirckheim mineral waters.
The link connecting this important result with astronomy
may now be indicated. In the year 1802 it occurred to
William Hyde Wollaston to substitute for the round hole used
by Newton and his successors for the admittance of light to
be examined with the prism, an elongated " crevice " -^th of an
inch in width. He thereupon perceived that the spectrum,
thus formed of light, as it were, purified by the abolition of
overlapping images, was traversed by seven dark lines. These
he took to be natural boundaries of the various colours, 3 and,
satisfied with this quasi-explanation, allowed the subject to
drop. It was independently taken up after twelve years by
a man of higher genius. In the course of experiments on
light, directed towards the perfecting of his achromatic lenses,
rendering incandescent in its transit some particles of their substance, the
characteristic light of which accordingly flashes out in the spectrum.
1 Phil. Mag., vol. xx. p. 93.
2 Annalen der Physik, Bd. cxiii. p. 357.
8 Phil. Trans., vol. xcii. p. 378.
1/2 HISTORY OF ASTRONOMY.
Fratmhofer, by means of a slit and a telescope, made the
surprising discovery that the solar spectrum is crossed, not by
seven, but by thousands of obscure transverse streaks. 1 Of
these he counted some 600, and carefully mapped 324; while
a few of the most conspicuous he set up (if we may be per-
mitted the expression) as landmarks, measuring their distances
apart with a theodolite, and affixing to them the letters of the
alphabet by which they are still universally known. He went
further. The same system of examination applied to the rest
of the heavenly bodies showed the mild effulgence of the
moon and planets to be deficient in precisely the same rays as
sunlight ; while in the stars it disclosed the differences in like-
ness which are always an earnest of increased knowledge. The
spectra of Sirius and Castor, instead of being delicately ruled
crosswise throughout, like that of the sun, were seen to be
interrupted by three massive bars of darkness two in the
blue and one in the green ; 2 the light of Pollux, on the other
hand, seemed precisely similar to sunlight attenuated by
distance or reflection, and that of Capella, Betelgeux, and
Procyon to share some of its peculiarities. One solar line
especially that marked in his map with the letter D proved
common to all the four last-mentioned stars ; and it was re-
markable that it exactly coincided in position with the con-
spicuous yellow beam (afterwards, as we have said, identified
with the light of glowing sodium) which he had already found to
accompany most kinds of combustion. Moreover, both the
dark solar and the bright terrestrial " D lines " were displayed
by the refined Munich appliances as double.
In this striking correspondence, discovered by Fraunhofer
in 1815, was contained the very essence of solar chemistry;
but its true significance did not become apparent until long
afterwards. Fraunhofer was by profession, not a physicist,
but a practical optician. Time .pressed; he could not and
would not deviate from his appointed track ; all that was
1 Denkschriften, Bd. v. p. 202.
2 Ibid., p. 220; Edin.Jour. of Science, vol. viii. p. 9.
FOUNDATION OF ASTRONOMICAL PHYSICS. 173
possible to him was to indicate the road to discovery, and
exhort others to follow it. 1
Partially and inconclusively at first this was done. The
" fixed lines " (as they were called) of the solar spectrum took
up the position of a standing problem, to the solution of which
no approach seemed possible. Conjectures as to their origin
were indeed rife. An explanation put forward by Zantedeschi 2
and others, and dubiously favoured by Sir David Brewster and
Dr. J. H. Gladstone, 3 was that they resulted from "inter-
ference" that is, a destruction of the motion producing in
our eyes the sensation of light, by the superposition of two
light-waves in such a manner that the crests of one exactly fill
up the hollows of the other. This effect was supposed to be ,
brought about by imperfections in the optical apparatus em-
ployed.
A more plausible view was that the atmosphere of the earth
was the agent by which sunlight was deprived of its missing
beams. For a few of them this is actually the case. Brewster
found in 1832 that certain dark lines, which were invisible when
the sun stood high in the heavens, became increasingly con-
spicuous as he approached the horizon. 4 These are the well-
known " atmospheric lines ; " but the immense majority of
their companions in the spectrum remain quite unaffected by
the thickness of the stratum of air traversed by the sunlight
containing them. They are then obviously due to another
cause.
There remained the true interpretation absorption in the
suris atmosphere ; and this, too, was extensively canvassed.
But a remarkable observation made by Professor Forbes of
Edinburgh 5 on the occasion of the annular eclipse of May 15,
1 Denkschriften, Bd. v. p. 222. 2 Arch, des Sciences, 1849, p. 43.
3 Phil. Trans., vol. cl. p. 159, note.
4 Ed. Phil. Trans., vol. xii. p. 528.
5 Phil. Trans., vol. cxxvi. p. 453. "I conceive," he says, "that this
result proves decisively that the sun's atmosphere has nothing to do with
the production of this singular phenomenon," p. 455. And Brewstei's
174 HISTORY OF ASTRONOMY.
1 836, appeared to throw discredit upon it. If the problematical
dark lines were really occasioned by the stoppage of certain rays
through the action of a vaporous envelope surrounding the sun,
they ought, it seemed, to be strongest in light proceeding from
his edges, which, cutting that envelope obliquely, passed through
a much greater depth of it. But the circle of light left by the in-
terposing moon, and of course derived entirely from the rim of
the solar disc, yielded to Forbes's examination precisely the
same spectrum as light coming from its central parts. This
circumstance helped to baffle inquirers, already sufficiently per-
plexed. It still remains an anomaly, of which no completely
satisfactory explanation has been offered.
Convincing evidence as to the true nature of the solar lines
was however at length, in the autumn of 1859, brought forward
at Heidelberg. KirchhofFs experimentum cruds in the matter
was a very simple one. He threw bright sunshine across a
space occupied by vapour of sodium, and perceived with
astonishment that the dark Fraunhofer line D, instead of being
effaced by flame giving a luminous ray of the same refrangibility,
was deepened and thickened by the superposition. He tried
the same experiment, substituting for sunbeams light from a
Drummond lamp, and with a similar result. A dark furrow, cor-
responding in every respect to the solar D line, was instantly seen
to interrupt the otherwise unbroken radiance of its spectrum.
The inference was irresistible, that the effect thus produced arti-
ficially was brought about naturally in the same way, and that
sodium formed an ingredient in the glowing atmosphere of the
sun. 1 This first discovery was quickly followed up by the identi-
fication of numerous bright rays in the spectra of other metallic
bodies with others of the hitherto mysterious Fraunhofer lines.
Kirchhoff was thus led to the conclusion that (besides sodium)
iron, magnesium, calcium, and chromium are certainly solar
constituents, and that copper, zinc, barium, and nickel are also
well-founded opinion that it had much to do with it was thereby in fact,
overthrown.
1 Monatsberichte, Berlin, 1859, p. 664.
FOUNDATION OF ASTRONOMICAL PHYSICS. 175
present, though in smaller quantities. 1 As to cobalt, he hesitated
to pronounce, but its existence in the sun has since been
established.
These memorable results were founded upon a general
principle first enunciated by Kirchhoff in a communication to
the Berlin Academy, December 15, 1859, and afterwards more
fully developed by him. 2 It may be expressed as follows :
Substances of every kind are opaque to the precise rays which
they emit at the same temperature ; that is to say, they stop
the kinds of light or heat which they are then actually in a
condition to radiate. But it does not follow that cool bodies
absorb the rays which they would give out if sufficiently heated.
Hydrogen at ordinary temperatures, for instance, is almost
perfectly transparent, but if raised to the glowing point as by
the passage of electricity it then becomes capable of arresting,
and at the same time of displaying in its own spectrum light
of four distinct colours.
This principle, well understood, discloses the whole secret
of solar chemistry. It gives the key to the hieroglyphics of
the Fraunhofer lines. The same characters which are written
bright in terrestrial spectra are written dark in the unrolled
sheaf of sun-rays ; the meaning remains unchanged. It must,
however, be remembered that they are only relatively dark.
The substances stopping those particular tints in the neighbour-
hood of the sun are at the same time vividly glowing with the
very same. Remove the dazzling solar background, by contrast
with which they show as obscure, and they will be seen, and
have, under certain circumstances, actually been seen, in all
their native splendour. It is because the atmosphere of the
1 Abhandlungen, Berlin, 1861, pp, 80, 81.
2 Ibid., 1861, p. 77 ; Annalen der Physik, Bd. cxix. p. 275. A similar
conclusion, reached by Balfour Stewart in 1858 for heat-rays (Ed. Phil.
Trans., vol. xxii. p. 13), was, in 1860, without previous knowledge of
Kirchhoff s work, extended to light (Phil. Mag., vol. xx. p. 534) ; but
his experiments wanted the precision of those executed at Heidelberg.
Angstrom, too, had foreshadowed it in 1853 (Phil. Mag., vol. ix. p. 328),
as indeed Euler had done nearly a century earlier.
i?6 HISTORY OF ASTRONOMY.
sun is cooler than the globe it envelopes that the different
kinds of vapour constituting that atmosphere take more than
they give, absorb more light than they are capable of emitting ;
raise them to the same temperature as the sun itself, and their
powers of emission and absorption being brought exactly to
the same level, the thousands of dusky rays in the solar spectrum
will be at once obliterated.
The establishment of the terrestrial science of spectrum
analysis was due, as we have seen, equally to Kirchhoff and
Bunsen, but its celestial application to Kirchhoff alone. He
effected this object of the aspirations, more or less dim, of
many other thinkers and workers, by the union of two separate
though closely related lines of research the study of the
different kinds of light emitted by various bodies, and the study
of the different kinds of light absorbed by them. The latter
branch appears to have been first entered upon by Dr. Thomas
Young in 1803;! it was pursued by the younger Herschel, 2
by William Allen Miller, Brewster, and Gladstone. Brewster
indeed made, in i833, 3 a formal attempt to found what might
be called an inverse system of analysis with the prism, based
upon absorption ; and his efforts were repeated, just a quarter
of a century later, by Gladstone. 4 But no general point of
view was attained ; nor, it may be added, was it by this path
attainable.
Kirchhoff's map of the solar spectrum, drawn to scale with
exquisite accuracy, and printed in three shades of ink to
convey the graduated obscurity of the lines, was published in
the Transactions of the Berlin Academy for 1861 and i862. 5
Representations of the principal lines belonging to various
elementary bodies formed, as it were, a series of marginal notes-
accompanying the great solar scroll, and enabling the veriest
1 Miscellaneous Works, vol. i. p. 189.
2 Ed. Phil. Trans., rol. ix. p. 458. 3 Ibid., vol. xii. p. 519.
4 Quart. Jour. Chem. Soc., vol. x. p. 79.
5 A facsimile accompanied Professor Roscoe's translation of Kirchhoff's
"Researches on the Solar Spectrum" (London, 1862-63).
FOUNDATION OF ASTRONOMICAL PHYSICS. 177
tyro in the new science to decipher its meaning at a glance.
Where the dark solar and bright metallic rays agreed in position,
it might safely be inferred that the metal emitting them was a
solar constituent ; and such coincidences were numerous. In
the case of iron alone, no less than sixty occurred in one-half of
the spectral area, rendering the chances l absolutely overwhelm-
ing against mere casual conjunction. The preparation of this
elaborate picture proved so trying to the eyes that Kirchhoff
was compelled by failing vision to resign the latter half of the
task to his pupil Hofmann. The complete map measured
nearly eight feet in length.
The conclusions reached by KirchhofF were no sooner
announced than they took their place, with scarcely a dis-
senting voice, among the established truths of science. The
broad result, that the dark lines in the spectrum of the sun
afford an index to its chemical composition no less reliable
than any of the tests used in the laboratory, was equally capti-
vating to the imagination of the vulgar, and authentic in the
judgment of the learned ; and, like all genuine advances in
the knowledge of Nature, it stimulated curiosity far more than
it gratified it. Now the history of how discoveries were missed
is often quite as instructive as the history of how they were
made ; it may then be worth while to expend a few words on
the thoughts and trials by which, in the present case, the actual
event was heralded.
Three times it seemed on the verge of being anticipated.
The experiment, which in KirchhofFs hands proved decisive,
of passing sunlight through glowing vapours and examining
the superposed spectra, was performed by Professor W. A.
Miller of King's College in i845. 2 Nay, more, it was per-
formed with express reference to the question, then already
(as has been noted) in debate, of the possible production of
Fraunhofer's lines by absorption in a solar atmosphere. Yet
it led to nothing.
1 Estimated by Kirchhoff at a trillion to one. AbhandL, 1861, p. 79.
2 Phil. Mag., vol. xxvii. (3d series), p. 90.
M
i;8 HISTORY OF ASTRONOMY.
Again, at Paris in 1849, with a view to testing the asserted
coincidence between the solar D line and the bright yellow
beam in the spectrum of the electric arc (really due to the un-
suspected presence of sodium), Leon Foucault threw a ray of
sunshine across the arc and observed its spectrum. 1 He was
surprised to see that the D line was rendered more intensely
dark by the combination of lights. To assure himself still
further, he substituted a reflected image of one of the white-
hot carbon-points for the sunbeam, with an identical result.
The same ray was missing. It needed but another step to
have generalised this result, and thus laid hold of a natural
truth of the highest importance ; but that step was not taken.
Foucault, keen and brilliant though he was, rested satisfied
with the information that the voltaic arc had the power of
stopping the kind of light emitted by it ; he asked no further
question, and was consequently the bearer of no further intelli-
gence on the subject.
The truth conveyed by this remarkable experiment was,
however, divined by one eminent man. Professor Stokes of
Cambridge stated to Sir William Thomson, shortly after it
had been made, his conviction that an absorbing atmosphere
of sodium surrounded the sun. And so forcibly was his
hearer impressed with the weight of the arguments based upon
the absolute agreement of the D line in the solar spectrum
with the yellow ray of burning sodium (then freshly certified
by W. H. Miller), combined with Foucault's " reversal " of that
ray, that he regularly inculcated, in his public lectures on
natural philosophy at Glasgow, five or six years before Kirch-
hoff's discovery, not only the fact of the presence of sodium
in the solar neighbourhood, but also the principle of the study
of solar and stellar chemistry in the spectra of flames. 2 Yet it
does not appear to have occurred to either of these two dis-
tinguished professors themselves amongst the foremost of
their time in the successful search for new truths to verify
1 Ltlnslitut, Feb. 7, 1849, P- 45 '> Phil. Mag., vol. xix. (4th series), p. 193.
2 Ann. d. Phys., vol. cxviii. p. no.
FOUNDATION OF ASTRONOMICAL PHYSICS. 179
practically a sagacious conjecture in which was contained the
possibility of a scientific revolution. It is just to add, that
Kirchhoff was unacquainted, when he undertook his investiga-
tion, either with the experiment of Foucault or the speculation
of Stokes.
It may here be useful since without some clear ideas on
the subject no proper understanding of recent astronomical
progress is possible to take a cursory view of the elementary
principles of spectrum analysis. To many of our readers they
are doubtless already familiar ; but it is better to be trite in
repetition than obscure from lack of explanation.
The spectrum, then, of a body is simply the light proceeding
from it spread out by refraction 1 into a brilliant variegated band,
passing from brownish-red through crimson, orange, yellow,
green, and azure into dusky violet. The reason of this
spreading-out or " dispersion " is that the various colours have
different wave-lengths, and consequently meet with different
degrees of retardation in traversing the denser medium of the
prism. The shortest and quickest vibrations (producing the
sensation we call "violet") are thrown farthest away from
their original path in other words, suffer the widest " devia-
tion ; " the longest and slowest (the red) travel much nearer to
it. Thus the sheaf of rays which would otherwise combine
into a patch of white light are separated through the divergence
of their tracks after refraction by a prism, so as to form a tinted
riband. This visible spectrum is prolonged invisibly at both
ends by a long range of vibrations, either too rapid or too
sluggish to affect the eye as light, but recognisable through
their chemical and heating effects.
Now all incandescent solid or liquid substances, and even
gases ignited under great pressure, give what is called a " con-
tinuous spectrum;" that is to say, the light derived from
them is of every conceivable hue. Sorted out with the prism,
its tints merge imperceptibly one into the other, uninterrupted
1 Spectra may be produced by diffraction as well as by refraction ; but
we are here only concerned with the matter in its simplest aspect.
i So HISTORY OF ASTRONOMY.
by any dark spaces. No colours, in short, are missing. But
gases and vapours rendered luminous by heat emit rays of
only a few tints sometimes of one only which accordingly
form an interrupted spectrum, usually designated as one of
lines or bands. And since these rays are perfectly definite
and characteristic not being the same for any two substances
it is easy to tell what kind of matter is concerned in pro-
ducing them. We may suppose that the inconceivably minute
particles which by their rapid thrillings agitate the ethereal
medium so as to produce light, are free to give out their
peculiar tone of vibration only when floating apart from each
other in gaseous form ; but when crowded together into a con-
densed mass, the .clear ring of the distinctive note is drowned,
so to speak, in an universal molecular clang. Thus prismatic
analysis has no power to identify individual kinds of matter
except when they present themselves as glowing vapours.
A spectrum is said to be " reversed " when lines previously
seen bright on a dark background appear dark on a bright
background. In this form it is equally characteristic of
chemical composition with the " direct " spectrum, being due
to absorption, as the latter is to emission. And absorption
and emission are, by KirchhofFs law, strictly correlative. This
is easily understood by the analogy of sound. For, just as a
tuning-fork responds to sound-waves of its own pitch, but
remains indifferent to those of any other, so those particles of
matter whose nature it is, when set swinging by heat, to vibrate
a certain number of times in a second, thus giving rise to light
of a particular shade of colour, appropriate those same vibrations,
and those only, when transmitted past them, or, phrasing it
otherwise, are opaque to them, and transparent to all others.
It should further be explained that the shape of the bright
or dark spaces in the spectrum has nothing whatever to do
with the nature of the phenomena. The " lines " and " bands "
so frequently spoken of are seen as such for no other reason
than because the light forming them is admitted through a
narrow, straight opening. Change that opening into a fine
FOUNDATION OF ASTRONOMICAL PHYSICS. 181
crescent or a sinuous curve, and the " lines " will at once appear
as crescents or curves.
Resuming in a sentence what has been already explained,
we find that the prismatic analysis of the heavenly bodies was
founded upon three classes of facts : First, the unmistakable
character of the light given by each different kind of glowing
vapour ; secondly, the identity of the light absorbed with the
light emitted by each ; thirdly, the coincidences observed be-
tween rays missing from the solar spectrum and rays absorbed
by various terrestrial substances. Thus, a realm of knowledge,
pronounced by Morinus l in the seventeenth century, and no
less dogmatically by Auguste Comte 2 in the nineteenth, hope-
lessly out of reach of the human intellect, was thrown freely
open, and the chemistry of the sun and stars took its place
amongst the foremost of the experimental sciences.
The immediate increase of knowledge was not the chief
result of KirchhofFs labours ; still more important was the
change in the scope and methods of astronomy, which, set on
foot in 1852 by the detection of a common period affecting at
once the spots on the sun and the magnetism of the earth, was
extended and accelerated by the discovery of spectrum analysis.
The nature of that change is concisely indicated by the heading
of the present chapter; we would now ask our readers to
endeavour to realise somewhat distinctly what is implied by
the " foundation of astronomical physics."
Some two centuries and fourscore years ago, Kepler drew
a forecast of what he called a " physical astronomy " a science
treating of the efficient causes of planetary motion, and holding
the "key to the inner astronomy." 3 What Kepler dreamed
of and groped after, Newton realised. He showed the beauti-
ful and symmetrical revolutions of the solar system to be
governed by a uniformly acting cause, and that cause no other
than the familiar force of gravity which gives stability to all our
1 Astrologia Galllca (1661), p. 189.
2 Pos. Phil., vol. i. pp. 114-115 (Martineau's trans.)
3 Proem. Astronomic Pars Opttca (1604), Op. t t. ii.
1 82 HISTORY OF ASTRONOMY.
terrestrial surroundings. The world under our feet was thus
for the first time brought into physical connection with the
worlds peopling space, and a very tangible relationship was
demonstrated as existing between what used to be called the
" corruptible " matter of the earth and the " incorruptible "
matter of the heavens.
This process of unification of the cosmos this levelling of
the celestial with the sublunary was carried no farther until
the fact unexpectedly emerged from a vast and complicated
mass of observations, that the magnetism of the earth is subject
to subtle influences emanating, certainly from some, and pre-
sumably (were their amount sufficient to be perceptible) from
all of the heavenly bodies ; the inference being thus rendered
at least plausible, that a force not less universal than gravity
itself, but with whose modes of action we are as yet unac-
quainted, pervades the universe, and forms, it might be said,
an intangible bond of sympathy between its parts. Now for
the investigation of this influence two roads are open. It may
be pursued by observation either of the bodies from which it
emanates, or of the effects which it produces that is to say,
either by the astronomer or by the physicist, or, better still, by
both concurrently. Their acquisitions are mutually profitable ;
nor can either be considered as independent of the other. Any
important accession to knowledge respecting the sun, for
example, may be expected to cast a reflected light on the still
obscure subject of terrestrial magnetism ; while discoveries in
magnetism or its alter ego electricity must profoundly aifect
solar inquiries.
The establishment of the new method of spectrum analysis
drew far closer this alliance between celestial and terrestrial
science. Indeed, they have come to merge so intimately one
into the other, that it is no easier to trace their respective
boundaries than it is to draw a clear dividing-line between the
animal and vegetable kingdoms. Yet up to the middle of the
present century, astronomy, while maintaining her strict union
with mathematics, looked with indifference on the rest of the
FOUNDATION OF ASTRONOMICAL PHYSICS. 183
sciences ; it was enough that she possessed the telescope and
the calculus. Now the materials for her inductions are sup-
plied by the chemist, the electrician, the inquirer into the most
recondite mysteries of light and the molecular constitution
of matter. She is concerned with what the geologist, the
meteorologist, even the biologist, has to say ; she can afford to
close her ears to no new truth of the physical order. Her
position of lofty isolation has been exchanged for one of com-
munity and mutual aid. The astronomer has become, in the
highest sense of the term, a physicist ; while the physicist is
bound to be something of an astronomer.
This, then, is what is designed to be conveyed by the
" foundation of astronomical or cosmical physics." It means
the establishment of a science of Nature whose conclusions are
not only presumed by analogy, but are ascertained by observa-
tion, to be valid wherever light can travel and gravity is
obeyed a science by which the nature of the stars can be
studied upon the earth, and the nature of the earth can be
better known by study of the stars a science, in a word, which
is, or aims at being, one and universal, even as Nature the
visible reflection of the invisible highest Unity is one and
universal.
It is not too much to say that a new birth of knowledge has
ensued. The astronomy so signally promoted by Bessel l the
astronomy placed by Comte 2 at the head of the hierarchy of
the sciences was the science of the movements of the heavenly
bodies. And there were those who began to regard it as a
science which, from its very perfection, had ceased to be
interesting whose tale of discoveries was told, and whose
farther advance must be in the line of minute technical im-
provements, not of novel and stirring disclosures. But the
science of the nature of the heavenly bodies is one only in the
beginning of its career. It is full of the audacities, the incon-
sistencies, the imperfections, the possibilities of youth. It
promises everything; it has already performed much; it will
1 Pop. Vorl., pp. 14, 19, 408. 2 Pos. Phil., p. 115.
184 HISTORY OF ASTRONOMY.
doubtless perform much more. The means at its disposal are
vast, and are being daily augmented. What has so far been
secured by them it must now be our task to extricate from
more doubtful surroundings and place in due order before our
readers.
CHAPTER II.
SOLAR OBSERVATIONS AND THEORIES.
THE zeal with which solar studies have been pursued during
the last quarter of a century has already gone far to redeem
the neglect of the two preceding ones. Since Schwabe's
discovery was published in 1851, observers have multiplied,
new facts have been rapidly accumulated, and the previous
comparative quiescence of thought on the great subject of the
constitution of the sun, has been replaced by a bewildering
variety of speculations, conjectures, and more or less justifiable
inferences. It is satisfactory to find this novel impulse not
only shared, but to a large extent guided, by our countrymen.
William Rutter Dawes, one of many clergymen eminent in
astronomy, observed, in 1852, with the help of a solar eye-
piece of his own devising, some curious details of spot-
structure. 1 The umbra heretofore taken for the darkest part
of the spot was seen to be suffused with a mottled, nebulous
illumination, in marked contrast with the striated appearance
of the penumbra ; while through this " cloudy stratum " a
" black opening " permitted the eye to divine further unfathom-
able depths beyond. The hole thus disclosed evidently the
true nucleus was found to be present in all considerable, as
well as in many small maculae.
Again, the whirling motions of some of these objects were
noticed by him. The remarkable form of one sketched at
Wateringbury, in Kent, January 17, 1852, gave him the means
. A. S., vol. xxi. p. 157.
186 HISTORY OF ASTRONOMY.
of detecting and measuring a rotatory movement of the whole
spot round the black nucleus at the rate of 100 degrees in
six days. "It appeared," he said, "as if some prodigious
ascending force of a whirlwind character, in bursting through
the cloudy stratum and the two higher and luminous strata,
had given to the whole a movement resembling its own." l An
interpretation founded, as is easily seen, on the Herschelian
theory, then still in full credit.
An instance of the same kind was observed by the late Mr.
W. R. Birt in i86o, 2 and cyclonic movements are now a well-
recognised feature of sun-spots. They are, however, as Father
Secchi 3 concluded from his long experience, but temporary
and casual. Scarcely three per cent, of all spots visible exhibit
the spiral structure which should invariably result if a conflict
of opposing, or the friction of unequal, currents were essential,
and not merely incidental to their origin. A whirlpool phase
not unfrequently accompanies their formation, and may be
renewed at periods of recrudescence or dissolution ; but it is
both partial and inconstant, sometimes affecting only one side
of a spot, sometimes slackening gradually its movement in one
direction, to resume it, after a brief pause, in the opposite.
Persistent and uniform motions, such as the analogy of terres-
trial storms would absolutely require, are not to be found.
So that the "cyclonic theory" of sun-spots, suggested by
Herschel in 1847,* and urged, from a different point of view,
by Faye in 1872, may be said to have completely broken
down.
The drift of spots over the sun's surface was first systemati-
cally investigated by Carrington. Before narrating what he
did, it is worth while to pause for a moment to consider who
he was. Nor will it take long to tell. Richard Christopher
Carrington was a self-constituted astronomer, with the will and
the courage and the instinct of thoughtful labour in him.
Born at Chelsea in May 1826, he entered Trinity College,
1 Mem. R. A. S., vol. xxi. p. 160. 2 Month. Not., vol. xxi. p. 144.
3 Le Soldi, t. i. pp. 87-90 (2d ed. 1875). 4 See ante, p. 75.
SOLAR OBSERVATIONS AND THEORIES. 187
Cambridge, in 1844. He was intended for the Church, but
Professor Challis's lectures diverted him to astronomy, and he
resolved, as soon as he had taken his degree, to prepare, with
all possible diligence, to follow his new vocation. His father,
who was a brewer on a large scale at Brentford, offered no
opposition ; ample means were at his disposal ; nevertheless,
he chose to serve an apprenticeship of three years as observer in
the University of Durham, as though his sole object had been to
earn a livelihood. He quitted the post only when he found
that its restricted opportunities offered no further prospect of
self-improvement.
He now built an observatory of his own at Redhill in Surrey,
with the design of completing Bessel's and Argelander's survey
of the northern heavens by adding to it the circumpolar stars
omitted from their view. This project, successfully carried out
between 1854 and 1857, had another and still larger one
superposed upon it before it had even begun to be executed.
In 1852, while the Redhill Observatory was in course of erec-
tion, the discovery of the coincidence between the sun-spot and
magnetic periods was announced. Carrington was profoundly
interested, and devoted his enforced leisure to the examination
of records, both written and depicted, of past solar observations.
Struck with their fragmentary and inconsistent character, he
resolved to " appropriate," as he said, by " close and methodical
research," the eleven-year period next ensuing. 1 He calculated
rightly that he should have the field pretty nearly to himself;
for many reasons conspire to make public observatories slow
in taking up new subjects, and amateurs with freedom to
choose, and means to treat them effectually, were even scarcer
then than they are now.
The execution of this laborious task was commenced Novem-
ber 9, 1853. It was intended to be merely a parergon a
" second subject," upon which daylight energies might be spent,
while the hours of night were reserved for cataloguing those
stars that "are bereft of the baths of ocean." Its results, how-
1 Observations at Redhill (&>$}, Introduction.
i88 HISTORY OF ASTRONOMY.
ever, proved of the highest interest, although the vicissitudes
of life barred the completion, in its full integrity, of the original
design. By the death, in 1858, of the elder Carrington, the
charge of the brewery devolved upon his son ; and eventually
absorbed so much of his care that it was found advisable to
bring the solar observations to a premature close, March 24,
1861.
His scientific life may be said to have closed with them.
Attacked four years later with severe, and, in its results, perma-
nent illness, he disposed of the Brentford business, and with-
drew to Churt, near Farnham, in Surrey. There, in a lonely
spot, on the top of a detached conical hill known as the
"Devil's Jump," he built a second observatory, and erected
an instrument which he was no longer able to use with pristine
effectiveness; and there, November 27, 1875, he died of the
rupture of a blood-vessel on the brain, before he had completed
his fiftieth year. 1
His observations of sun-spots were of a geometrical character.
They concerned positions and movements, leaving out of sight
physical peculiarities. Indeed, the prudence . with which he
limited his task to what came strictly within the range of his
powers to accomplish, was one of Carrington's most valuable
qualities. The method of his observations, moreover, was
chosen with the same practical sagacity as their objects. As
early as 1847, Sir John Herschel had recommended the daily
self-registration of sun-spots, 2 and he enforced the suggestion,
with more immediate prospect of success, in i854. 3 The art
of celestial photography, however, was even then in a purely
tentative stage, and Carrington wisely resolved to waste no time
on dubious experiments, but employ the means of registration
and measurement actually at his command. These were very
simple, yet very effective. To the " helioscope " employed by
Father Scheiner 4 two centuries and a quarter earlier a species
of micrometer was added. The image of the sun was pro-
1 Month. Not., vol. xxxvi. p. 142. 2 Cape Observations, p. 435? note.
3 Month. Not., vol. x. p. 158. 4 Rosa Ursina, lib. iii. p. 348.
SOLAR OBSERVATIONS AND THEORIES. 189
jected upon a screen by means of a firmly clamped telescope,
in the focus of which were placed two cross wires forming
angles of 45 with the meridian. The six instants were then
carefully noted at which these were met by the edges of the
disc as it traversed the screen and by the nucleus of the spot
to be measured. 1 A short process of calculation then gave the
exact position of the spot as referred to the sun's centre.
From a series of 5290 observations made in this way, together
with a great number of accurate drawings, Carrington derived
conclusions of great importance on each of the three points
which he had proposed to himself to investigate. These were :
the law of the sun's rotation, the existence and direction of
systematic currents, and the distribution of spots on the solar
surface.
Grave discrepancies were early perceived to exist between
the determinations of the sun's rotation by different observers.
Galileo, with " comfortable generality," estimated the period at
" about a lunar month ; " 2 Scheiner, at twenty-seven days. 3
Cassini, in 1678, made it 25.58 ; Delambre, in 1775, no more
than twenty-five days. Later inquiries brought these diver-
gences within no more tolerable limits. Laugier's result of
25.34 days obtained in 1841 enjoyed the highest credit, yet
it differed widely in one direction from that of Bohm (1852),
giving 25.52 days, and in the other from that of Kysaeus (1846),
giving 25.09 days. Now the cause of these variations was
really obvious from the first, although for a long time strangely
overlooked. Father Scheiner pointed out in 1630 that different
spots gave different periods, adding the significant remark
that one at a distance from the solar equator revolved more
slowly than those nearer to it. 4 But the hint was wasted. For
upwards of two centuries ideas on the subject were either
1 Observations at Redhill, p. 8. 2 Op., t. iii. p. 402.
3 Rosa Ursina, lib. iv. p. 601. Both Galileo and Scheiner spoke of the
apparent or " synodical " period, which is about one and a third days
longer than the true or " sidereal " one. The difference is caused by the
revolution of the earth in its orbit in the same direction with the sun's
rotation on its axis. * Rosa Ursina t lib. iii. p. 260.
I 9 o HISTORY OF ASTRONOMY.
retrograde or stationary. What were called the " proper
motions" of spots were, however, recognised by Schroter, 1
and utterly baffled Laugier, 2 who despaired of obtaining any
concordant result as to the sun's rotation except by taking the
mean of a number of discordant ones. At last, in 1855, a
valuable course of observations made at Capo di Monte, Naples,
in 1845-46, enabled C. H. F. Peters 3 (now of Hamilton Col-
lege, Clinton, N.Y.) to set in the clearest light the insecurity
of determinations based on the assumption of fixity in objects
visibly affected by movements uncertain both in amount and
direction.
Such was the state of affairs when Carrington entered upon
his task. Everything was in confusion ; the most that could be
said was that the confusion had come to be distinctly admitted
and referred to its true source. What he discovered was this :
that the sun, or at least the outer shell of the sun visible to
us, has no single period of rotation, but drifts round, carrying
the spots with it, at a rate continually accelerated from the
poles to the equator. In other words, the time of axial revolu-
tion is shortest at the equator and lengthens with increase of
latitude. Carrington devised a mathematical formula by which
the rate or " law " of this lengthening was conveniently expressed ;
but it was a purely empirical one. It was a concise statement,
but implied no physical interpretation. It summarised, but
did not explain the facts. An assumed " mean period " for the
solar rotation of 25.38 days (twenty-five days nine hours, very
nearly), was thus found to be actually conformed to only in
two parallels of solar latitude (14 north and south), while the
equatorial period was slightly less than twenty-five, and that
of latitudes 50 rose to twenty-seven days and a half. 4 These
curious results gave quite a new direction to ideas on solar
physics.
The other two " elements " of the sun's rotation were also
1 Faye, Comptes Rendus, t. Ix. p. 818. 2 Ibid., t. xii. p. 648.
3 Proc. Am. Ass. Adv. of Science, 1855, p. 85.
4 Observations at Redhill, p. 221,
SOLAR OBSERVATIONS AND THEORIES. 191
ascertained by Carrington with hitherto unattained precision.
He fixed the inclination of its axis to the ecliptic at 82 45' ;
the longitude of the ascending node at 73 40' (both for the
epoch 1850 A.D.) These data which have not yet been
improved upon suffice to determine the position in space of
the sun's equator. Its north pole is directed towards a star
in the coils of the Dragon, midway between Vega and the
Pole-star ; its plane intersects that of the earth's orbit in such
a way that our planet finds itself in the same level on or about
the 3d of June and the 5th of December, when any spots
visible on the disc cross it in apparently straight lines. At
other times, the paths pursued by them seem curved down-
wards between June and December, upwards between De-
cember and June.
A singular peculiarity in the distribution of sun-spots emerged
from Carrington's studies at the time of the minimum of 1856.
Two broad belts of the solar surface, as we have seen, are
frequented by them, of which the limits may be put at 6 and
35 of latitude, one zone lying so far north, the other as much
south of the solar equator. Individual equatorial spots are
not uncommon, but nearer to the poles than 35 they are a
rare exception. Carrington observed as an extreme instance
in July 1858, one in south latitude 44 ; and Peters, in June
1846, watched, during several days, a spot in 50 24' north
latitude. But beyond this no true macula has ever been seen ;
for Lahire's reported observation of one in latitude 70 is now
believed to have had its place on the solar globe erroneously
assigned ; and the " veiled spots " described by Trouvelot in
1875 l as occurring within 10 of the pole can only be regarded
as, at the most, the same kind of disturbance in an undeveloped
form.
But the novelty of Carrington's observations consisted in the
detection of certain changes in distribution concurrent with
the progress of the eleven-year period. As the minimum ap-
proached, the spot-zones contracted towards the equator, and
1 Am. Jour, of Science, vol. xi. p. 169.
192 HISTORY OF ASTRONOMY.
there finally vanished ; then, as if by a fresh impulse, spots
suddenly reappeared in high latitudes, and spread downwards
with the development of the new phase of activity. Scarcely
had this remark been made public, 1 when Wolf 2 found a con-
firmation of its general truth in Btihm's observations during
the years 1833-36 ; and a perfectly similar behaviour was noted
both by Sporer and Secchi at the minimum epoch of 1867.
The ensuing period gave less marked indications ; but it may
be looked upon as established that the activity manifested in
sun-spots widens its range with the growth of its intensity, and
becomes reduced in space and strength simultaneously a
feature of which no theory has yet given any tolerable account.
Gustav Sporer, born at Berlin in 1822, began to observe
sun-spots with the view of assigning the law of solar rotation
in December 1860. His means were at first very limited, but
his assiduity and success attracted attention, and a Government
endowment was procured for the little solar observatory organ-
ised by him at Anclam, in Pomerania. Unaware of Carring-
ton's discovery (first made known in January 1859), he arrived
at and published, in June i86i, 3 a similar conclusion as to the
equatorial quickening of the sun's movement on its axis. His
sun-spot observations were continued until 1867, and he after-
wards became one of the most zealous students of solar pro-
minences, upon which subject he is at present employed at
the " astro-physical " observatory of Potsdam.
The time had now evidently come for a fundamental revision
of current notions respecting the nature of the sun. Herschel's
theory of a cool, dark, habitable globe, surrounded by, and
protected against the radiations of a luminous and heat-giving
envelope, was shattered by the first dicta of spectrum analysis.
Traces of it may be found for a few years subsequent to
1 Month. Not., vol. xix. p. I.
2 Vierteljahrsschrift der Nalurfors. Gesellschaft (Zurich), 1859^.252.
3 Astr. Nach., No. 1315.
4 As late as 1866 an elaborate treatise in its support was written by M.
F. Coyteux, entitled Quest ce que le Soleil ? Peut-il $tre halite 1 and answer-
ing the question in the affirmative.
SOLAR OBSERVATIONS AND THEORIES. 193
but they are obviously survivals from an earlier order of ideas,
doomed to speedy extinction. .It needs only a moment's
consideration of what was implied in the discovery of the
origin of the Fraunhofer lines to see the incompatibility of the
new facts with the old conceptions. It implied, not only the
presence near the sun, as glowing vapours, of bodies highly
refractory to heat, but that these glowing vapours formed the
relatively cool envelope of a still hotter internal mass. KirchhorT,
accordingly, included in his great memoir "On the Solar
Spectrum," read before the Berlin Academy of Sciences, July
u, 1861, an exposition of the views on the subject to which
his memorable investigations had led him. They may be
briefly summarised as follows.
Since the body of the sun gives a continuous spectrum, it
must be either solid or liquid, 1 while the interruptions in its
light prove it to be surrounded by a complex atmosphere
of metallic vapours, somewhat cooler than itself. Spots are
simply clouds due to local depressions of temperature, differing
in no respect from terrestrial clouds except as regards the
kinds of matter composing them. These sun-clouds take their
origin in the zones of encounter between polar and equatorial
currents in the solar atmosphere.
This explanation was liable to all the objections urged
against the " cumulus theory " on the one hand, and the
"trade-wind theory" on the other. Setting aside its pro-
pounder, it was consistently upheld perhaps by no man
eminent in science except Sporer; and his advocacy of it
tended rather to delay the recognition of his own merits than
to promote its general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to
propose a coherent scheme of the solar constitution covering
the whole range of new discovery. The fundamental ideas on
the subject now in vogue here made their first connected
1 The subsequent researches of Pliicker, Frankland, Wiillner, and
others showed that gases strongly compressed give an absolutely unbroken
spectrum.
N
I 9 4 HISTORY OF ASTRONOMY.
appearance. Much, indeed, remained to be modified and
corrected ; but the transition was finally made from the old to
the new order of conceptions. The essence of the change of
view thus effected may be conveyed in a single sentence. The
sun was thenceforth regarded, not as a mere heated body, or
still more remotely from the truth as a cool body unaccount-
ably spun round with a cocoon of fire, but as a vast heat-
radiating machine. The terrestrial analogy was abandoned in
one more particular besides that of temperature. The solar
system of circulation, instead of being adapted, like that of the
earth, to the distribution of heat received from without, was
seen to be directed towards the transportation towards the
surface of the heat contained within. Polar and equatorial
currents, tending to a purely superficial equalisation of tem-
perature, were replaced by vertical currents bringing up suc-
cessive portions of the intensely, heated interior mass, to contri-
bute their share in turns to the radiation into space which might
be called the proper function of a sun.
Faye's views, which were communicated to the Academy of
Sciences, January 16, I865, 1 were avowedly based on the
anomalous mode of solar rotation discovered by Carrington.
This may be regarded either as an acceleration increasing
from the poles to the equator, or as a retardation increasing
from the equator to the poles, according to the rate of revolu-
tion we choose to assume for the unseen nucleus. Faye
preferred to consider it as a retardation produced by as-
cending currents continually left behind as the sphere
widened in which the matter composing them was forced to
travel. He further supposed that the depth from which these
vertical currents started, and consequently the amount of
retardation effected by their ascent to the surface, became
progressively greater as the poles were approached; but this
was plainly an arbitrary expedient of theory, confronted with
inconvenient and uncompromising facts.
The extreme internal mobility betrayed by Carrington's and
1 Comptes Rendus, t. Ix. pp. 89, 138.
SOLAR OBSERVATIONS AND THEORIES. 195
Sporer's observations led to the inference that the matter com-
posing the sun was mainly or wholly gaseous. This had
already been suggested by Father Secchi l a year earlier, and
by Sir John Herschel in April i864; 2 but it first obtained
general currency through Faye's more elaborate presentation.
A physical basis was afforded for the view by Cagniard de la
Tour's experiments in i822, 3 proving that, under conditions of
great heat and pressure, the vaporous state was compatible with
a very considerable density. The position was strengthened
when Andrews showed, an i869, 4 that above a fixed limit of
temperature, varying for different bodies, true liquefaction is
impossible, even though the pressure be so tremendous as to
retain the gas within the same space that enclosed the liquid.
The opinion that the mass of the sun is gaseous now com-
mands a very general assent ; although the gaseity admitted is
of such a nature as to afford the consistence rather of honey
or pitch than of the aeriform fluids with which we are familiar.
On another important point the course of subsequent
thought was powerfully influenced by Faye's conclusions in
1865. Arago somewhat hastily inferred from experiments
with the polariscope the wholly gaseous nature of the visible
disc of the sun. Kirchhoff, on the contrary, believed (errone-
ously, as we now know) that the brilliant continuous spectrum
derived from it proved it to be a wtyte-hot solid or liquid.
Herschel and Secchi 5 indicated a cloud-like structure as that
which would best harmonise the whole of the evidence at
command. The novelty introduced by Faye consisted in re-
garding the photosphere.no longer "as a defined surface, in
the mathematical sense, but as a limit to which, in the general
fluid mass, ascending currents carry the physical or chemical
phenomena of incandescence." 6 Uprushing floods of mixed
1 Bull. Meteor, delt Osservatorio del Coll. Rom., Jan. I, 1864, p. 4.
2 Quart. Jour, of Science, vol. i. p. 222.
3 Ann. de Chim. et de Phys., t. xxii. p. 127.
4 Phil. Trans., vol. clix. p. 575.
. 5 Les Mondes, Dec. 22, 1864, p. 707.
6 Comptes Rendus, t. Ix. p. 147.
196 HISTORY OF ASTRONOMY.
vapours with strong affinities say of calcium or sodium and
oxygen at last attain a region cool enough to permit their
combination ; a fine dust of solid or liquid compound particles
(of lime or soda, for example) there collects into the photo-
spheric clouds, and descending by its own weight in torrents
of incandescent rain, is dissociated by the fierce heat below,
and replaced by ascending and combining currents of similar
constitution.
This first attempt to assign the part played in cosmical
physics by chemical affinities, was marked by the importation
into the theory of the sun of the now familiar phrase dissociation.
It is indeed tolerably certain that no such combinations as
those contemplated by Faye occur at the photospheric level,
since the temperature there must be enormously higher than
would be needed to reduce all metallic earths and oxides ; but
molecular changes of some kind, dependent perhaps in part
upon electrical conditions, in part upon the effects of radiation
into space, most likely replace them. The conjecture (originally
thrown out, it would seem, by Faye himself) was countenanced
by Angstrom, 1 and has recently been advocated by Professor
Hastings of Baltimore, 2 that the photospheric clouds are com-
posed of particles of some member of the carbon-triad 3 pre-
cipitated from its mounting vapour just where the temperature
is lowered by expansion and radiation to the boiling-point of that
substance. But the question is one which must for the present re-
main within the sphere of interesting and admissible speculation.
In Faye's theory, sun-spots were regarded as simply breaks
in the photospheric clouds, where the rising currents had
strength to tear them asunder. It followed that they were
regions of increased heat regions, in fact, where the tempera-
ture was too high to permit the occurrence of the precipitations
to which the photosphere is due. Their obscurity was attributed
to deficiency of emissive power. But here it was irresistibly
1 Recherche* sur le Spectre Solaire, p. 38.
2 Am. Jour, of Science, 1881, vol. xxi. p. 41.
3 Carbon, silicon, and boron.
SOLAR OBSERVATIONS AND THEORIES. 197
objected by Professors Balfour Stewart and Kirchhoff that
emissive and absorptive power being strictly correlative, the
supposed defect of radiation would be exactly compensated
by an increase of transparency. The light from the farther
photosphere would then, shining across the whole body of the sun,
completely fill up, to the eye, the gap in the hither photosphere,
and no macula at all would remain visible. Besides, we now
know that ignited gases under a pressure far less than that
which must exist at even a small distance below the solar
surface, give light equally brilliant and uninterrupted with that
derived from solid bodies.
After every deduction, however, has been made, we still find
that several ideas of permanent value were embodied in this
comprehensive sketch of the solar constitution. The principal
of these were : first, that the sun is a mainly gaseous body ;
secondly, that its stores of heat are rendered available at the
surface by means of vertical convection-currents by the bodily
transport, that is to say, of intensely hot matter upwards, and
of comparatively cool matter downwards; thirdly, that the
photosphere is a surface of condensation, forming the limit set
by the cold of space to this circulating process, and that a
similar formation must attend, at a certain stage, the cooling
of every cosmical body.
To Mr. Warren De la Rue belongs the honour of having
obtained the earliest results of substantial value in celestial
photography. What had been done previously was interesting
in the way of promise, but much could not be claimed for it
as actual performance. Some " pioneering experiments " were
made by Dr. J. W. Draper of New York in 1840, resulting in
the production of a few " moon-pictures " one-inch in diameter ; x
but slight encouragement was derived from them, either to
himself or others. Bond of Cambridge (U.S), however, got
impressions of Vega and Castor in i845, 2 an d in 1850 secured
with the Harvard 1 5-inch refractor that daguerreotype of the
1 H. Draper, Quart. Jour, of St., vol. i. p. 381 ; also Phil. Mag., vol.
xvii. 1840, p. 222. 2 Proc. Roy. Soc., vol. xiii. p. 511.
198 HISTORY OF ASTRONOMY.
moon with which the career of extra-terrestrial photography
may be said to have formally opened. It was shown in Lon-
don at the Great Exhibition of 1851, and determined the
direction of De la Rue's efforts. Yet it did little more than
prove the art to be a possible one.
Warren De la Rue was born in Guernsey in 1815, was
educated at the ficole Sainte-Barbe in Paris, and made a large
fortune as a paper manufacturer in England. The material
supplies for his scientific campaign were thus amply and early
provided. Towards the end of 1853 he took some successful
lunar photographs. They were remarkable as the first examples
of the application to astronomical light-painting of the collodion
process, invented by Archer in 1851 ; and also of the use of
reflectors (Mr. De la Rue's was one of thirteen inches, con-
structed by himself) for that kind of work. The absence of
a driving apparatus was, however, very sensibly felt; the
difficulty of moving the instrument by hand so as accurately
to follow the moon's apparent motion being such as to cause
the discontinuance of the experiments until 1857, when the
want was supplied. Mr. De la Rue's new observatory, built
in that year at Cranford, twelve miles west of Hyde Park, was
specially dedicated to celestial photography; and there he
immediately applied to the heavenly bodies the stereoscopic
method of obtaining relief, and turned his attention to the
delicate business of photographing the sun.
A solar daguerreotype 1 was taken at Paris, April 2, 1845, by
MM. Foucault and Fizeau, acting on a suggestion from Arago.
But the attempt, though far from being unsuccessful, does not,
at that time, seem to have been repeated. Its great difficulty
consisted in the enormous light-power of the object to be
represented, rendering an inconceivably short period of ex-
posure indispensable, under pain of getting completely " burnt-
up" plates. In 1857 Mr. De la Rue was commissioned by
the Royal Society to construct an instrument specially adapted
to the purpose for the Kew observatory. The resulting " photo-
1 Reproduced in Arago's Popular Astronomy, plate xii. vol. i.
SOLAR OBSERVATIONS AND THEORIES. 199
heliograph" may be described as a small telescope (of 3^
inches aperture and 50 focus), with a plate-holder at the eye-
end, guarded in front by a spring-slide, the rapid movement of
which across the field of view secured for the sensitive plate a
virtually instantaneous exposure. By its means the first solar
light-pictures of real value were taken, and the autographic
record of the solar condition recommended by Sir John Her-
schel was commenced and continued at Kew during fourteen
years -1858-72. The work of photographing the sun is now
carried on in every quarter of the globe, from the Mauritius to
Massachusetts, and the days are few indeed on which the
self-betrayal of the camera can be evaded by our chief luminary.
In the year 1883, the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on
340 days ; and the missing twenty-five were doubtless provided
for elsewhere.
The conclusions arrived at by photographic means at Kew
were communicated to the Royal Society in a series of
papers drawn up jointly by Messrs. De la Rue, Balfour
Stewart, and Benjamin Loewy, in 1865 and subsequent years.
They influenced materially the progress of thought on the
subject they were concerned with.
By its rotation the sun itself offers opportunities for bringing
the stereoscope to bear upon it. Two pictures, taken at an
interval of twenty-six minutes, show just the amount of difference
needed to give, by their combination, the maximum effect of
solidity. 1 Mr. De la Rue thus obtained, in 1861, a stereoscopic
view of a sun-spot and surrounding faculae, representing the
various parts in their true mutual relations. " I have ascertained
in this way," he wrote, 2 " that the faculae occupy the highest
portions of the sun's photosphere, the spots appearing like holes
in the penumbrse, which appeared lower than the regions sur-
rounding them ; in one case, parts of the faculae were dis-
covered to be sailing over a spot apparently at some consi-
derable height above it." Thus Wilson's inference as to the
1 Report Brit. Ass., 1859, p. 148. 3 Phil. Trans., vol. clii. p. 407.
200 HISTORY OF ASTRONOMY.
depressed nature of spots received, after the lapse of not far
from a century, proof of the most simple, direct, and convinc-
ing kind. A careful application of Wilson's own geometrical
test gave results only a trifle less decisive. Of 694 spots
observed, 78 per cent, showed, as they traversed the disc, the
expected effects of perspective; 1 and their absence in the
remaining 22 per cent, might be easily explained by internal
commotions producing irregularities of structure. The absolute
depth of spot-cavities at least of their sloping sides was
determined by Father Secchi through measurement of the
" parallax of profundity " 2 that is, of apparent displacements
attendant on the sun's rotation, due to depression below the
sun's surface. He found that it in every case fell short of
4000 miles, and averaged not more than 1321, corresponding,
on the terrestrial scale, to an excavation in the earth's crust
of ii miles. There may be, however, and probably are, depths
below this depth, of which the eye takes not even indirect
cognisance ; so that it would be hasty to pronounce spots to
be a merely superficial phenomenon.
The opinion of the Kew observers as to the nature of such
disturbances was strongly swayed by another curious result of
the "statistical method" of inquiry. They found that of 1137
instances of spots accompanied by faculse, 584 had those
faculse chiefly or entirely on the left, 508 showed a nearly
equal distribution, while 45 only had faculous appendages
mainly on the right side. 3 Now, the rotation of the sun, as we
see it, is performed from left to right ; so that the marked
tendency of the faculse was a lagging one. This was easily
accounted for by supposing the matter composing them to have
been flung upwards from a considerable depth, whence it would
reach the surface with the lesser absolute velocity belonging to
a smaller circle of revolution, and would consequently fall
behind the cavities or " spots " formed by its abstraction.
1 Researches in Solar Physics, part i. p. 20.
2 Both the phrase and the method were suggested by Faye. Comptes
Rendus, t. Ixi. p. 1082. 3 Proc. Roy. Soc., vol. xiv. p. 39.
SOLAR OBSERVATIONS AND THEORIES. 201
The ideas of M. Faye were, on two fundamental points,
contradicted by the Kew investigators. He held spots to be
regions of uprush and of heightened temperature \ they
believed their obscurity to be due to a downrush of compara-
tively cool vapours. On which side does the truth lie ?
Observing at Ville-Urbanne, March 6, 1865, M. Chacornac
saw floods of photospheric matter visibly precipitating them-
selves into the abyss opened by a great spot, and carrying with
them small neighbouring maculae. 1 Similar instances were
repeatedly noted by Father Secchi, who considered the exist-
ence of a kind of suction in spots to be quite beyond question. 2
The tendency in their vicinity, to put it otherwise, is centri-
petal^ not centrifugal ; and this alone seems to negative the
supposition of a central uprush.
A fresh witness was now at hand. The application of the
spectroscope to the direct examination of the sun's surface
dates from March 4, 1866, when Mr. Norman Lockyer began
his inquiry into the cause of the darkening in spots. 3 The
answer was prompt and unmistakable, and was again, in this
case, adverse to the French theorist's view. The obscurations
in question were found to be produced by no deficiency of
emissive power, but by an increase of absorptive action. The
background of variegated light remains unchanged, but more
of it is stopped by the interposition of a dense mass of relatively
cool vapours. The spectrum of a sun-spot is crossed by the
same set of multitudinous dark lines, with some minor differ-
ences, visible in the ordinary solar spectrum. We must then
conclude that the same vapours which are dispersed over the
unbroken solar surface are accumulated in the umbral cavity,
the compression incident to such accumulation being betrayed
by the thickening of certain lines of absorption. But there
is also a general absorption, extending almost continuously
from one end of the spot-spectrum to the other. And this is
explained in Professor Hastings's ingenious speculation by a
1 Lockyer, Contributions to Solar Physics, p. 70.
2 Le Soleil, p. 87. 3 Proc. Roy. Soc., vol. xv. p. 256.
202 HISTORY OF ASTRONOMY.
deposition of soot, or something analogous in other words, by
the presence, as a slowly settling fine dust, of cold, dark par-
ticles of carbon or silicon. 1
An inquiry, however, prosecuted by Professor Young of
Princeton, New Jersey, during the latter half of 1883, has set
the matter in a new light. Using a spectroscope of excep-
tionally high dispersive power, he succeeded to a considerable
extent in "resolving" the supposed continuous obscurity of
spot-spectra into a countless army of fine dark lines set very
close together. 2 The substances producing this darkening or
absorption are then in a gaseous state, and the " soot " theory
collapses. We may add, with some confidence, that their
temperature, although affected by great irregularities, is in
general lower than that of the encircling photosphere. Pro-
fessor Langley in 1875 3 fully confirmed Professor Henry's dis-
covery in 1845, tna t tne nuclei of spots radiate far less heat
than equal areas of the unbroken disc ; but this tells us little
or nothing as to their real thermal condition. The character
of their spectra, however, makes it extremely probable that it
is one of comparative coolness.
As to the movements of the constipated vapours forming spots,
the spectroscope is also competent to supply information. The
principle of the method by which it is procured will be explained
farther on. Suffice it here to say that the transport, at any
considerable velocity, to or from the eye of the gaseous material
giving bright or dark lines, can be measured by the displace-
ment of such lines from their previously known normal positions.
In this way movements have been detected in or above spots
of enormous rapidity, ranging up to 320 miles per second.^
But the result, so far, has been to negative conjectures either
of uprushes or downrushes as part of the regular internal
economy of spots.
A new theory of sun-spots, started by Faye in 1872, and still
advocated by him, is sufficiently plausible to merit some brief
1 Am. Jour .^ vol. xxi. p. 42. 2 Phil. Mag., vol. xvi. p. 460.
3 Comptes Rendus y t.lxxx. p. 746. 4 Young, The Sun, p. 99.
SOLAR OBSERVATIONS AND THEORIES. 203
attention. He had been foremost in pointing out that the
observations of Carrington and Sporer absolutely forbade the
supposition that any phenomenon at all resembling our trade-
winds exists in the sun. The " proper movements " of spots
give no evidence of regular currents either towards or from
the poles. The systematic drift of the photosphere is strictly
a drift in longitude ; its direction is everywhere parallel to the
equator. This fact being once clearly recognised, the " solar
tornado " hypothesis at once fell to pieces ; but M. Faye 1
perceived another source of vorticose motion in the unequal
rotating velocities of contiguous portions of the photosphere.
The " pores " with which the whole surface of the sun is studded
he took to be the smaller eddies resulting from these inequali-
ties ; the spots to be such eddies developed into whirlpools. It
only needs to thrust a stick into a stream to produce the kind
of effect designated. And it happens that the differences of
angular movement adverted to attain a maximum just about
the latitudes where spots are most frequent and conspicuous.
There are, however, two fatal objections. One (already
mentioned) is the total absence of the regular swirling motion
in a direction contrary to that of the hands of a watch north
of the solar equator, in the opposite sense south of it which
should impress itself upon every lineament of a sun-spot if the
cause assigned were a primary producing, and not merely (as
it possibly may be) a secondary determining one. The other,
pointed out by Professor Young, 2 is that the cause is inadequate
to the effect. The difference of movement, or relative drift^
supposed to occasion such prodigious disturbances, amounts,
at the utmost, for two portions of the photosphere 123 miles
apart, to about five yards a minute. Thus the friction of con-
tiguous sections must be quite insignificant.
One other view, remains to be noticed. It is that urged by
Father Secchi in and after the year 1872, and adopted with
some useful modifications by Professor Young. 3 Spots are
manifestly associated with violent eruptive action, giving rise
1 Comptes Rendus, t. Ixxv. p. 1664. 2 The Sun, p. 174. 3 Ibid., p. 175.
204 HISTORY OF ASTRONOMY.
to the faculae and prominences which usually garnish their
borders. It is accordingly contended that upon the withdrawal
of matter from below by the flinging up of a prominence must
ensue a sinking-in of the surface, into which the partially
cooled erupted vapours rush and settle, producing just the
kind of darkening by increased absorption told of by the
spectroscope. Round the edges of the cavity the rupture of
the photospheric shell will form lines of weakness provocative
of further eruptions, which will, in their turn, deepen and
enlarge the cavity. The phenomenon will thus tend to per-
petuate itself, until equilibrium is at last restored by internal
processes. A sun-spot might then be described as an inverted
terrestrial volcano, in which the outbursts of heated matter
take place on the borders instead of at the centre of the
crater, while the cooled products gather in the centre instead
of at the borders. A real analogy is, however, probably masked
by superficial unlikeness. Both in earth and sun but in the
sun to an enormously greater extent the same fundamental
conditions of volcanic action are found. These are heat and
pressure. Matter, in which inconceivable powers of expansion
are lodged by virtue of the suppressed fury of its interstitial
movements, is held down in the rigid grasp of its own weight.
The slightest disturbance of this delicately adjusted balance of
forces suffices to produce an outbreak. The gun is ready
loaded ; it only needs to pull the trigger. It is true that we
cannot, in either case, tell exactly how the trigger is pulled
whether by local increase of heat or local relief of pressure, or
by both in combination ; but it is easy to see that the erup-
tive capacities of our own quiescent little globe must, in the
sun, be intensified to a degree beyond the reach even of
imagination.
The " volcanic hypothesis " of sun-spots makes no attempt to
explain their peculiarities of distribution either in space or
time their preference for two zones of the solar surface, or
their marked periodicity. It is thus far indeed from being
completely satisfactory ; yet it seems the least misleading way
SOLAR OBSERVATIONS AND THEORIES. 205
of conceiving the facts that can be suggested in the present
state of our knowledge.
A singular circumstance has now to be recounted. On the
ist of September 1859, while Carrington was engaged in his
daily work of measuring the positions of sun-spots, he was
startled by the sudden appearance of two patches of peculiarly
intense light within the area of the largest group visible. His
first idea was that a ray of unmitigated sunshine had penetrated
the screen employed to reduce the brilliancy of the image ;
but, having quickly convinced himself to the contrary, he ran
to summon an additional witness of an unmistakably remarkable
occurrence. On his return he was disappointed to find the
strange luminous outburst already on the wane ; shortly after-
wards the last trace vanished. Its entire duration was five
minutes from 11.18 to 11.23 A.M., Greenwich time; and
during those five minutes it had traversed a space estimated at
35,000 miles ! No perceptible change took place in the details
of the group of spots visited by this transitory conflagration,
which, it was accordingly inferred, took place at a considerable
height above it. 1
Carrington's account was precisely confirmed by an obser-
vation made at Highgate. Mr. R. Hodgson described the
appearance seen by him as that "of a very brilliant star of
light, much brighter than the sun's surface, most dazzling to
the protected eye, illuminating the upper edges of the adjacent
spots and streaks, not unlike in effect the edging of the clouds
at sunset." 2
This unique phenomenon seemed as if specially designed to
accentuate the inference of a sympathetic relation between the
earth and the sun. From trie 28th of August to the 4th of
September 1859, a magnetic storm of unparalleled intensity,
extent, and duration, was in progress over the entire globe.
Telegraphic communication was everywhere interrupted
except, indeed, that it was, in some cases, found practicable to
work the lines without batteries^ by the agency of the earth-
1 Month. Not., vol. xx. p. 13. z Ibid., p. 15.
206 HISTORY OF ASTRONOMY.
currents alone ; l sparks issued from the wires ; gorgeous
aurorae draped the skies in solemn crimson over both hemi-
spheres, and even within the tropics ; the magnetic needle lost
all trace of continuity in its movements, and darted to and fro
as if stricken with inexplicable panic. The coincidence was
drawn even closer. At the very instant 2 of the solar outburst
witnessed by Carrington and Hodgson, the photographic appa-
ratus at Kew registered a marked disturbance of all the three
magnetic elements ; while, shortly after the ensuing midnight,
the electric agitation culminated, thrilling the earth with subtle
vibrations, and lighting up the atmosphere from pole to pole
with the coruscating splendours which, perhaps, dimly recall
the times when our ancient planet itself shone as a star.
Here then, at least, the sun was in Professor Balfour
Stewart's phrase " taken in the act " 3 of stirring up terrestrial
commotions. Nor have instances since been wanting of an
indubitable connection between outbreaks of individual spots
and magnetic disturbances four such were recorded in 1882
although the peculiar features of the event of September i,
1859, have not recurred. An attempt was made to explain
them by Professor Piazzi Smyth, 4 who suggested that the flying
luminous objects seen on that occasion were nothing else than
a pair of unusually large meteors ignited by retardation in the
solar atmosphere. But the inadequacy of the conjecture hardly
needs to be pointed out. The sudden development of light
was certainly no accidental occurrence, but marked the climax
of some systematic commotion already for some days in pro-
gress. If we were to look for its terrestrial analogue, we should
rather find it in the "auroral beam" which traversed the
heavens during a vivid display of polar lights, November 17,
1882, and shared, there is every reason to believe, their
electrical origin and character. 5
1 Am. your., vol. xxix. (2d series), pp. 94-95.
2 The magnetic disturbance took place at 11.15 A.M., three minutes
before the solar blaze compelled the attention of Carrington.
8 Phil. 7rans. y vol. cli. p. 428. 4 Month. Not., vol. xx. p. 88.
5 See J. Rand Capron, Phil. Mag., May 1883.
SOLAR OBSERVATIONS AND THEORIES. 207
Meantime M. Rudolf Wolf, transferred to the direction of
the Zurich Observatory, had relaxed none of his zeal in the
investigation of sun-spot periodicity. A laborious revision of the
entire subject with the aid of fresh materials led him, in 1859,!
to the conclusion that while the mean period differed little
from that arrived at in 1852 of n.ii years, very considerable
fluctuations on either side of that mean were rather the rule
than the exception. -Indeed, the phrase "sun-spot period"
must be understood as fitting very loosely the great fact it is
taken to represent ; so loosely, that the interval between two
maxima may rise to sixteen and a half or sink below seven and
a half years. 2 In 1861 3 Wolf showed, and the remark was
fully confirmed by the Kew observations, that the shortest
periods brought the most acute crises, and vice versa ; as if for
each wave of disturbance a strictly equal amount of energy were
available, which might spend itself lavishly and rapidly, or
slowly and parsimoniously, but could in no case be exceeded.
The further inclusion of recurring solar commotions within a
cycle of fifty-five and a half years was simultaneously pointed out;
and Hermann Fritz showed soon after that the aurora borealis is
subject to an identical double periodicity. 4 The same inquirer
has more recently detected both for aurorae and sun-spots a
"secular period" of 222 years, 5 and the Kew observations
indicate for the latter, oscillations accomplished within twenty-
six and twenty-four days. 6 The more closely spot-fluctuations
are looked into, indeed, the more complex they prove. Maxima
1 Mittheilungen Uber die Sonnenflecken, No. ix., Vierteljahrsschrift der
Naturforschenden Gesellschaft in Zurich, Jahrgang 4.
2 Mitth., No. lii. p. 58 (i 88 1).
3 Ibid., No. xii. p. 192. Mr. Joseph Baxendell, of Manchester, reached
independently a similar conclusion. See Month. Not., vol. xxi. p. 141.
4 Wolf, Mitth., No. xv. p. 107, &c. Olmsted, following Hansteen, had
already, in 1856, sought to establish an auroral period of sixty-five years.
Smithsonian Contributions, vol. viii. p. 37.
5 Hahn, Ueber die Beziehtmgen der Sonnenfleckenperiode zu weteorolo-
gischen Erscheinungen, p. 99 (1877).
6 Report Brit. Ass., 1881, p. 518; 1883, p. 418.
208 HISTORY OF ASTRONOMY.
of one order are superposed upon, or in part neutralised by,
maxima of another order; originating causes are masked by
modifying causes ; the larger waves of the commotion are
indented with minor undulations, and these again crisped with
tiny ripples, while the whole rises and falls with the swell of
the great secular wave, scarcely perceptible in its progress
because so vast in scale.
The idea that solar maculation depends in some way upon
the position of the planets occurred to Galileo in I6I2. 1 It
has been industriously sifted by a whole bevy of modern solar
physicists. Wolf in 18592 found reason to believe that the
eleven-year curve is determined by the action of Jupiter, modi-
fied by that of Saturn, and diversified by influences proceeding
from the Earth and Venus. Its tempting approach to agree-
ment with Jupiter's period of revolution round the sun, indeed,
irresistibly suggested a causal connection ; yet it does not seem
that the most skilful " coaxing " of figures can bring about a
fundamental harmony. Carrington pointed out in 1863 that
while, during eight successive periods ; from 1770 downwards,
there were approximate coincidences between Jupiter's aphelion
passages and sun-spot maxima, the relation had been almost
exactly reversed in the two periods preceding that date ; 3 and
the latest conclusion of M. Wolf himself is that the Jovian
origin must be abandoned. 4 Nevertheless it is still held by
M. Duponchel 5 of Paris, who accommodates discrepancies with
the help of perturbations by the large exterior planets ; and it
deserves notice that his prediction of an abnormal lengthening
of the maximum due in 1882, through certain peculiarities in
the positions of Uranus and Neptune about that time, has
been remarkably verified by the event.
That outbreaks of solar activity are modified by influences
depending upon planetary configuration has been tolerably
well ascertained by the Kew observations. This no less signi-
1 Of ere, t. iii. p. 412. 2 Mitth., Nos. viii. and xviii.
3 Observations at Redhiil, p. 248. 4 Comples Rendtts, t. xcv. p. 1249.
5 Ibid., t. xciii. p. 827; t. xcvi. p. 1418.
SOLAR OBSERVATIONS AND THEORIES. 209
ficant than surprising result was imparted by Professor Balfour
Stewart to the Royal Society of Edinburgh, April 18, I864. 1
The method of research by which it was arrived at (said to
have been privately recommended by Galileo 2 ) consisted in
studying the " behaviour " of each spot as it crossed the disc.
This, it was found, was almost always marked, about the same
epochs, with a common character. If one rent in the
photosphere widened as the central meridian of the sun was
approached, those in its train were pretty sure to do likewise ;
if it closed up, its successors followed suit. Moreover, the
controlling power was perceived to travel onwards at a rate
quicker than that of the earth's annual revolution. It followed,
in short, with much fidelity, the orbital movement of Venus.
Its nature is of such a kind as to assuage outbreaks on the
side of the sun turned towards the planet, and to aggravate
them on the opposite hemisphere. 3 The action both of Jupiter
and Mercury is, it would seem, the same in kind though less
in degree. That of the earth is more difficult to determine,
but it can scarcely be doubted that it is similarly exercised.
It has even been attempted to invert the process, and arrive at
the period of an unknown planet through the observation of
sun-spots. Professor Balfour Stewart has shown that inequali-
ties in their development exist corresponding severally to the
revolution of such a body round the sun in twenty-four days,
and to its " synodical periods" or successive meetings with
Jupiter, Venus, and Mercury. 4 But the prediction still awaits
fulfilment.
The question so much discussed, as to the influence of
sun-spots on weather, does not yet admit of a satisfactory
answer. The facts of meteorology are too complex for easy
or certain classification. Effects owning dependence on one
1 Ed. Phil. Trans., vol. xxiii. p. 499.
2 Researches in Solar Physics^ ser. ii. p. 46 (privately printed). The
Rev. Mr. Selwyn is responsible for the statement, for which he gives no
authority. 3 Proc. Roy. Soc. } vols. xiv. p. 59, xx. p. 210.
4 Report Brit. Ass., iSSi, p. 518.
O
210 HISTORY OF ASTRONOMY.
cause often wear the livery of another ; the meaning of observed
particulars may be inverted by situation ; and yet it is only by
the collection and collocation of particulars that we can hope
to reach any general law. There is, however, a good deal of
evidence to support the opinion the grounds for which were
primarily derived from the labours of Mr. Meldrum at the
Mauritius that increased rainfall and atmospheric agitation
attend spot-maxima ; while HerschePs conjecture of a more
copious emission of light and heat about the same epochs is
so far from having been borne out by modern investigations,
that the probabilities seem rather to lean the other way.
The examination of what we may call the texture of the sun's
surface derived new interest from a remarkable announcement
made by Mr. James Nasmyth in I862. 1 He had made (as he
supposed) the discovery that the entire luminous stratum of
the sun is composed of a multitude of elongated shining
objects on a darker background, shaped much like willow-
leaves, of vast size, crossing each other in all possible directions,
and endowed with unceasing relative motions. A lively con-
troversy ensued. In England and abroad, the most powerful
telescopes were directed to a scrutiny encompassed with varied
difficulties. The results, on the whole, were such as to invali-
date the precision of the disclosures made by the Hammerfield
reflector. Mr. Dawes was especially emphatic in declaring
that Nasmyth's "willow-leaves" were nothing more than the
" nodules " of Sir William Herschel seen under a misleading
aspect of uniformity; and there is little doubt that he was
right. It is, however, admitted that something of the kind
may be seen in the penumbras and " bridges " of spots, present-
ing an appearance compared by Dawes himself in 1852 to that
of a piece of coarse straw-thatching left untrimmed at the
edges. 2
The term "granulated," suggested by Dawes in i864, 3 best
describes the mottled aspect of the solar disc as shown by
Report Brit. Ass., 1862, p. 16 (pt. ii.) 2 Mem. R. A. Soc., vol. xxi. p. l6l.
J Month. Not., vol. xxiv. p. 162.
1
SOLAR OBSERVATIONS AND THEORIES. 211
modern telescopes and cameras. The grains, or rather
the " floccules," with which it is thickly strewn, have been
resolved by Langley, under exceptionally favourable condi-
tions, into "granules" not above 100 miles in diameter; and
from these relatively minute elements, composing, jointly,
about one-fifth of the visible photosphere, 1 he estimates that
three-quarters of the entire light of the sun are derived. 2
Janssen goes so far as to say that if the whole surface were as
bright as its brightest parts, its luminous emission would be
ten to twenty times greater than it actually is. 3
The rapid changes in the forms of these solar cloud-summits
are beautifully shown in the marvellous photographs taken
by Janssen at Meudon, with exposures reduced at times to
TFo.V o~o f a second ! By their means, also, the curious phe-
nomenon known as the reseau photo spherique has been made
evident. 4 This consists in the diffusion over the entire disc of
fleeting blurred patches, as if of imperfect definition, due,
doubtless, to agitations in the intervening solar atmosphere.
The same cause may perhaps account for the evanescent
obscurations described by Father Perry of Stonyhurst before
the Royal Astronomical Society, May 9, i884. 5
The " grains," or more brilliant parts of the photosphere, are
now generally held to represent the upper terminations of
ascending and condensing currents, while the darker interstices
(Herschel's " pores ") mark the positions of descending cooler
ones. In the penumbrae of spots, the glowing streams rushing
up from the tremendous sub-solar furnace are bent sideways
by the powerful indraught, so as to change their vertical for a
nearly horizontal motion, and are thus taken, as it were, in
flank by the eye, instead of being seen end-on in mamelon-
form. This gives a plausible explanation of the channelled
1 Am. Jour, of Science, vol. vii. 1874, p. 92.
2 Young, The Sun, p. 103. 3 Ann. Biir. Long., 1879, p. 679.
4 Ibid., 1878, p. 689.
6 Observatory, vol. vii. p. 154. Father Perry sought to identify the
objects observed by him with Trouvelot's " veiled spots ; " Mr. Ranyard
suggested the more probable analogy of the reseau photospherique.
212 HISTORY OF ASTRONOMY.
structure of penumbrae which suggested the comparison to a
rude thatch. Accepting this theory as in the main correct, we
perceive that the very same circulatory process which, in its
spasms of activity, gives rise to spots, produces in its regular
course the singular " marbled " appearance, for the recording
of which we are no longer at the mercy of the fugitive or delu-
sive impressions of the human retina. And precisely this cir-
culatory process it is which gives to our great luminary its
permanence as a sun, or warming and illuminating body.
CHAPTER III.
RECENT SOLAR ECLIPSES.
BY observations made during a series of five remarkable
eclipses, comprised within a period of eleven years, knowledge
of the solar surroundings was advanced nearly to its present
stage. Each of these events brought with it a fresh disclosure
of a definite and unmistakable character. We will now briefly
review this orderly sequence of discovery.
Photography was first systematically applied to solve the
problems presented by the eclipsed sun, July 18; 1860. It is
true that a very creditable daguerreotype, taken by Busch at
Konigsberg during the eclipse of 1851, is still valuable as a
record of the corona of that year ; and some subsequent attempts
were made to register partial phases of solar occultation ; but
the ground remained practically unbroken until 1860.
In that year the track of totality crossed Spain, and thither,
accordingly, Mr. Warren De la Rue transported his photo-helio-
graph, and Father Secchi his six-inch Cauchoix refractor. The
question then primarily at issue was that relating to the nature
of the red protuberances. Although, as already stated, the
evidence collected in 1851 gave a reasonable certainty of their
connection with the sun, objectors were not silenced; and
when the side of incredulity was supported by so considerable
an authority as M. Faye, it was impossible to treat it with
contempt. Two crucial tests were available. If it could be
shown that the fantastic shapes suspended above the edge of
the dark moon were seen under an identical aspect from two
2i 4 HISTORY OF ASTRONOMY.
distant stations, that fact alone would annihilate the theory of
optical illusion or "mirage;" while the certainty that they
were progressively concealed by the advancing moon on one
side, and uncovered on the other, would effectually detach
them from dependence on our satellite, and establish them as
solar appendages.
Now both these tests were eminently capable of being applied
by photography. But the difficulty arose that nothing was
known as to the chemical power of the rosy prominence-light,
while everything depended on its right estimation. A shot had
to be fired, as it were, in the dark. It was a matter of some
surprise, and of no small congratulation, that, in both cases, the
shot took effect.
Mr. De la Rue occupied a station at Rivabellosa, in the
Upper Ebro valley; Father Secchi set up his instrument at
Desierto de las Palmas, about 250 miles to the south-east,
overlooking the Mediterranean. From the totally eclipsed sun,
with its strange garland of flames, each observer derived
several perfectly successful impressions, which were found, on
comparison, to agree in the most minute details. This at once
settled the fundamental question as to the substantial reality of
these objects ; while their solar character was demonstrated by
the passage of the moon in front of them, indisputably attested
by pictures taken at successive stages of the eclipse. That
forms seeming to defy all laws of equilibrium were, neverthe-
less, not wholly evanescent, appeared from their identity at an
interval of seven minutes, during which the lunar shadow was
in transit from one station to the other; and the singular
energy of their "actinic" rays was shown by the record on the
sensitive plates of some prominences invisible in the telescope.
Moreover, photographic evidence strongly confirmed the
inference previously drawn by Grant and others, and now
repeated with fuller assurance by F. Secchi that an unin-
terrupted stratum of prominence-matter encompasses the sun
on all sides, forming a reservoir from which gigantic jets issue,
and into which they subside.
RECENT SOLAR ECLIPSES. 215
Thus a beginning of accurate knowledge regarding the
surroundings of the sun was made, and the value of the brief
moments of eclipse indefinitely increased by supplementing
transient visual impressions with the faithful and lasting records
of the camera.
In the year 1868 the history of eclipse spectroscopy virtually
began, as that of eclipse photography in 1860; that is to say,
the respective methods then first gave definite results. On the
1 8th of August 1868, the Indian and Malayan peninsulas were
traversed by a lunar shadow producing total obscuration during
five minutes and thirty-eight seconds. Two English and two
French expeditions were despatched to the distant regions
favoured by an event so propitious to the advance of know-
ledge, chiefly to obtain the verdict of the prism as to
the composition of prominences. Nor were they despatched
in vain. An identical discovery was made by nearly all the
observers. At Jamkandi, in the Western Ghauts, where
Lieutenant (now Colonel) Herschel was posted, unremitting bad
weather threatened to baffle his eager expectations ; but during
the lapse of the critical five and a half minutes the clouds
broke, and across the driving wrack a " long, finger-like pro-
jection" jutted out over the margin of the dark lunar globe.
In another moment the spectroscope was pointed towards it ;
three bright lines red, orange, and blue flashed out, and the
problem was solved. 1 The problem was solved in this general
sense, that the composition out of glowing vapours of the
objects infelicitously termed "protuberances" or "promi-
nences " was no longer doubtful ; although further inquiry was
needed for the determination of the particular species to which
those vapours belonged.
Similar, but more complete observations were made, with
less atmospheric hindrance, by Tennant and Janssen at Guntoor,
by Pogson at Masulipatam, and by Rayet at Wha-Tonne, on
the coast of the Malay peninsula, the last observer counting as
many as nine bright lines. 2 Amongst them it was not difficult
1 Proc. Roy. Sac., vol. xvii. p. 116. 2 Comptes Rendus^ t. Ixvii. p. 757.
216 HISTORY OF ASTRONOMY.
to recognise the characteristic light of hydrogen ; and it was
generally, though over-hastily, assumed that the orange ray
matched the luminous emissions of sodium. But fuller oppor-
tunities were at hand
The eclipse of 1868 is chiefly memorable for having taught
astronomers to do without eclipses, so far, at least, as one
particular branch of solar inquiry is concerned. Inspired by
the beauty and brilliancy of the variously tinted prominence-
lines revealed to him by his spectroscope, Janssen exclaimed
to those about him, "Je verrai ces lignes-la en dehors des
eclipses ! " On the following morning he carried into execution
the plan which formed itself in his brain while the phenomenon
which suggested it was still before his eyes. It rests upon an
easily intelligible -principle.
The glare of our own atmosphere alone hides the appendages
of the sun from our daily view. To a spectator on an airless
planet, the central globe would appear attended by all its
splendid retinue of crimson prominences, silvery corona, and
far-spreading zodiacal light, projected on the star-spangled,
black background of an absolutely unilluminated sky. Now
the spectroscope offers the means of indefinitely weakening
atmospheric glare by diffusing a constant amount of it over
an indefinitely widened area. But monochromatic or " bright-
line " light is, by its nature, incapable of being so diffused.
It can, of course, be deviated by refraction to any extent
desired ; but it always remains equally concentrated, in what-
ever direction it may be thrown. Hence, when it is mixed up
with continuous light as in the case of the solar flames shining
through our atmosphere it derives a relative, gain in intensity
from every addition to the dispersive power of the spectroscope
with which the heterogeneous mass of beams is analysed.
Employ prisms enough, and eventually the undiminished rays
of persistent colour will stand out from the continually fading
rainbow-tinted band, by which they were at first effectually
veiled.
This Janssen saw by a flash of intuition while the eclipse
RECENT SOLAR ECLIPSES. 217
was in progress ; and this he realised at 10 A.M. next morning,
August 19, 1868 the date of the beginning of spectroscopic
work at the margin of the unobscured sun. During the whole
of that day and many subsequent ones, he enjoyed, as he
said, the advantage of a prolonged eclipse. The intense
interest with which he surveyed the region suddenly laid bare
to his scrutiny, was heightened by evidences of rapid and
violent change. On the i8th of August, during the eclipse, a
vast spiral structure, at least 89,000 miles high, was perceived,
planted in surprising splendour on the rim of the interposed
moon. It was formed, as Major Tennant judged from its
appearance in his photographs, by the encounter of two
mounting torrents of flame, and was distinguished as the
" Great Horn." Next day it was in ruins ; hardly a trace
remained to show where it had been. 1 Janssen's spectro-
scope furnished him besides with the strongest confirmation
of what had already been reported by the telescope and the
camera as to the continuous nature of the scarlet " sierras "
lying at the base of the prominences. Everywhere at the sun's
edge the same bright lines appeared.
It was not until the 4th of September that Janssen thought
fit to send news of his discovery to Europe. He little dreamed
of being anticipated ; nor did he indeed grudge that science
should advance at the expense of his own undivided fame. A
few minutes before his despatch was handed to the Secretary
of the Paris Academy of Sciences, a communication similar
in purport had been received from Mr. Norman Lockyer.
There is no need to discuss the narrow and wearisome question
of priority ; each of the competitors deserves, and has obtained,
full credit for his invention. With noteworthy and confi-
dent prescience, Mr. Lockyer, in 1866, before anything was
yet known regarding the constitution of the " red flames,"
ordered a strongly dispersive spectroscope for the express
purpose of viewing, apart from eclipses, the bright-line spectrum
which he expected them to give. Various delays, however,
1 Comptes Rendus y t. Ixvii. p. 839.
218 HISTORY OF ASTRONOMY.
supervened, and the instrument was not in his hands until
October 16, 1868. On the 20th he picked up the vivid rays,
of which the presence and (approximately) the positions had
in the meantime become known. But there is little doubt
that, even without that previous knowledge, they would have
been found ; and that the eclipse of August 18 only accelerated
a discovery already assured.
Mr. Huggins, too, had been groping for prominence-lines
during two years and a half with the aid of various apparatus
at his observatory of Tulse Hill ; l but not until he knew where
to look did he succeed in seeing them. It should be added
that the principle of the method was suggested to Lieutenant
Herschel by the phenomena of the eclipse, and was briefly
described in his report. 2
Astronomers, thus liberated, by the acquisition of power to
view them at any time, from the necessity of studying pro-
minences during eclipses, were able to concentrate the whole
of their attention on the corona. The first thing to be done
was to ascertain the character of its spectrum. This was seen
in 1868 only as a faintly continuous one ; for Rayet, who
seems to have perceived its characteristic bright line far above
the summits of the flames, connected it, nevertheless, with
those objects. On the other hand, Lieutenant Campbell ascer-
tained on the same occasion the polarisation of the coronal
light in planes passing through the sun's centre, 3 thereby show-
ing that light to be, in whole or in part, reflected sunshine.
But if reflected sunshine, it was objected, the chief at least of
the dark Fraunhofer lines should be visible in it, as they are
visible in moonbeams, sky illumination, and all other sun-
derived light. The objection was well founded, but was pre-
maturely urged, as we shall see.
On the 7th of August 1869, a track of total eclipse crossed
the continent of North America diagonally, entering at Behring's
Straits, and issuing on the coast of North Carolina. It was
1 Month. Not., vol. xxviii. p. 88.
2 Proc. Roy. Soc. t vol. xvii. p. 119. 3 Ibid., p. 123.
RECENT SOLAR ECLIPSES. 219
beset with observers ; but the most effective work was done
in Iowa. At Des Homes, Professor Harkness of the Naval
Observatory, Washington, obtained from the corona an " abso-
lutely continuous spectrum," slightly less bright than that of
the full moon, but traversed by a single green ray. 1 The same
green ray was seen at Burlington and its position measured
by Professor Young of Dartmouth College. 2 It was found to
coincide with that of a dark line of iron in the solar spectrum,
numbered 1474 on KirchhofFs scale. This was perplexing;
since it seemed, at first sight, to compel the inference that the
corona was actually composed of vapour of iron, 3 so attenuated
as to give only one line of secondary importance out of the many
hundreds belonging to it. But in 1876 Young was able, by the
use of greatly increased dispersion, to resolve the Fraunhofer
line " 1474" into a pair, of which one component is due to
iron, the other (the more refrangible) to the coronal gas. 4 This
substance, of which nothing is known to terrestrial chemistry,
is luminous at least half a million of miles above the sun's
surface, and must be considerably lighter even than hydrogen.
A further trophy was carried off by American skill 5 sixteen
months after the determination due to it of the distinctive
spectrum. of the corona. The eclipse of December 22, 1870,
though lasting only two minutes and ten seconds, drew
observers from the New, as well as from the Old World to the
shores of the Mediterranean. Janssen issued from Paris in a
balloon, carrying with him the vital parts of a reflector spe-
cially constructed to collect evidence about the corona. But
he reached Oran only to find himself shut behind a cloud-
curtain more impervious than the Prussian lines. Everywhere
the sky was more or less overcast. Mr. Lockyer's journey
from England to Sicily, and shipwreck in the Psyche^ were
1 Washington Observations, 1867, App. ii., Harkness's Report, p. 60.
2 Am. Jour., vol. xlviii. (2d series), p. 377.
3 This view was never assented to by either Young or Lockyer.
4 Am. Jour., vol. xi. (3d series), p. 429.
5 Everything in such observations depends upon the proper manipulation
of the slit of the spectroscope.
220 HISTORY OF ASTRONOMY.
recompensed with a glimpse of the solar aureola during one
second and a half! Three parties stationed at various heights
on Mount Etna saw absolutely nothing. Nevertheless, impor-
tant information was snatched in despite of the elements.
The prominent event was Young's discovery of the "reversing
layer." As the surviving solar crescent narrowed before the
encroaching moon, " the dark lines of the spectrum," he tells
us, "and the spectrum itself, gradually faded away, until all at
once, as suddenly as a bursting rocket shoots out its stars, the
whole field of view was filled with bright lines more numerous
than one could count. The phenomenon was so sudden, so
unexpected, and so wonderfully beautiful, as to force an
involuntary exclamation." 1 Its duration was about two
seconds, and the impression produced was that of a complete
reversal of the Fraunhofer spectrum that is, the substitution
of a bright for every dark line.
Now something of the kind was theoretically necessary to
account for the dusky rays in sunlight which have taught us so
much, and have yet much more to teach us ; so that, although
surprising from its transitory splendour, the appearance could
not strictly be called " unexpected." Moreover, its premonitory
symptom in the fading out of those rays had been actually
described by Father Secchi in i868, 2 and looked for by Young
as the moon covered the sun in August 1869. But with the
slit of his spectroscope placed normally to the sun's limb, the
bright lines gave a flash too thin to catch the eye. In 1870
the position of the slit was tangential it ran along the shallow
bed of incandescent vapours, instead of cutting across it :
hence his success.
The same observation was made at Xerez de la Frontera
by Mr. Pye, a member of Young's party; and, although an
exceedingly delicate one, has since frequently been repeated.
The whole Fraunhofer series appeared bright (omitting other
instances) to Maclear, Herschel, and Fyers in 1871, at the
beginning or end of totality ; to Pogson during a period (perhaps
1 Mem. R. A. Soc., vol. xli. p. 435. 2 Comptes Rendus, t. Ixvii. p. 1019.
RECENT SOLAR ECLIPSES. 221
erroneously estimated) of from five to seven seconds, at the
break up of an annular eclipse, June 6, 1872; to Stone at
Klipfontein, April 16, 1874, when he saw "the field full of
bright lines." 1 But between the picture presented by the
" veritable pluie de lignes brillantes," 2 which descended into
M. Trepied's spectroscope for three seconds after the disappear-
ance of the sun, May 17, 1882, and the familiar one of the
dark-line solar spectrum, certain differences were perceived,
showing their relation to be not simply that of a positive to a
negative impression.
A " reversing layer," or stratum of mixed vapours, glowing,
but at a lower temperature than that of the actual solar surface,
was an integral part of Kirchhoff's theory of the production of
the Fraunhofer lines. Here it was assumed that the missing
rays were stopped, and here also it was assumed that the
missing rays would be seen bright, could they be isolated from
the overpowering splendour of their background. This isolation
is effected by eclipses, with the result beautifully confirmatory
of theory of reversing, or turning from dark to bright, the
Fraunhofer spectrum. But there is a difficulty. If absorption
be in truth thus localised, it should appear greatly strengthened
near the edges of the solar disc. This, however, is not the
case. Kirchhoff met the objection by giving a great depth to
the reversing stratum, whereby the difference in length of the
paths across that stratum traversed by rays from the sun's
limb and centre, became relatively insignificant. In other
words, he supposed that the chief part of the light absent
from the spectrum was arrested in the region of the corona.
This view is rendered wholly untenable by the character of the
coronal spectrum.
Faye, on the other hand, abolished the reversing layer
altogether (there was at that time no ocular demonstration of
its existence) ; or rather, sunk it out of sight below the visible
level of the photosphere, and got the necessary absorption done
in the interstices of the photospheric clouds by the vapours in
1 Mem. R. A. Sac., vol. xli. p. 43. 2 Comptes Rendus, t. xciv. p. 1640.
222 HISTORY OF ASTRONOMY.
which they float, and from which they condense. It was, how-
ever, at once seen that the lines thus produced would be bright,
not dark, since the brilliant cumuli would be cooled, by their
greater power of radiation, below the temperature of the
surrounding medium. A better explanation was offered by
Professor Hastings of Baltimore in iSSi. 1 He maintains that
Young's stratum, of which the thickness is estimated at about
600 miles, 2 represents only the upper margin of a reversing
ocean, in which the granules of the photosphere float at various
depths. The necessary difference of temperature is derived
from the coolness of the descending vapours, which bathe the
radiating particles and rob them of certain characteristic beams.
We are thus driven to suppose that only a small part of the
absorption betrayed by the Fraunhofer lines takes place in the
complex layer disclosed by eclipses ; 3 so that a strict corre-
spondence between its bright rays and the solar dusky rays
is not to be expected, and would, in fact, prove somewhat
embarrassing. M. Trepied's detection of differences is, for
this reason, especially valuable, and we may hope that, before
long, an instantaneous photograph of the complete " rainbow-
flash " accompanying totality will afford a more stable support
to theory on the subject than it can yet claim.
The last of the five eclipses which we have grouped together
for separate consideration, was visible in Southern India and
Australia, December 12, 1871. Some splendid photographs
were secured by the English parties on the Malabar coast,
showing, for the first time, the remarkable branching forms of
the coronal emanations ; but the most conspicuous result was
1 Am. Jour, of Science, vol. xxi. p. 33.
2 Pulsifer's observations at Fort Worth in 1878 gave a minimum depth
of 524 miles (Am. Jour, of Science, vol. xvii. p. 495).
3 This cannot be due to the shallowness of the layer, since a few feet (or
even, as in the case of sodium, a few millimetres) of glowing vapour can
be experimentally shown capable of producing the amount of absorption
present in the solar spectrum. We must then assume that its temperature
is so nearly on a level with that of the photosphere that it replaces almost
all the light it absorbs.
RECENT SOLAR ECLIPSES. 223
Janssen's detection of some of the dark Fraunhofer lines long
vainly sought in the continuous spectrum of the corona. Chief
amongst these was the D line of sodium, the original index,
it might be said, to solar chemistry. No proof could be
afforded more decisive than this faint echoing back of the
distinctive notes of the Fraunhofer spectrum, that the polari-
scope had spoken the truth in asserting a large part of the
coronal radiance to be reflected sunlight. But it is (especially
at certain epochs) so drenched in original luminous emissions,
that its characteristic features are almost obliterated. Janssen's
success in seizing them was due in part to the extreme purity
of the air at Sholoor, in the Neilgherries, where he was stationed;
in part to the use of an instrument adapted by its large aperture
and short focus to give an image of the utmost possible
luminosity.
His observations further " peremptorily demonstrated " the
presence of hydrogen far outside the region of prominences,
and forming an integral constituent of the corona. This im-
portant fact was simultaneously attested by Lockyer at Baikul,
and by Respighi at Poodacottah, each making separate trial of a
" slitless spectroscope " devised for the occasion. This consists
simply of a prism placed outside the object-glass of a telescope
or the lens of a camera, whereby the radiance encompassing
the eclipsed sun is separated into as many differently tinted
rings as it contains different kinds of light. These tinted rings
were viewed by Respighi through a telescope, and were photo-
graphed by Lockyer, with the same result of showing hydrogen
to ascend uniformly from the sun's surface to a height of fully
200,000 miles. Another notable observation made by Herschel
arid Tennant at Dodabetta showed the green ray " 1474 "
to be just as bright in a " rift " as in the adjacent streamer.
The visible structure of the corona was thus seen to be inde-
pendent of the distribution of the gases which enter into its
composition.
By means, then, of the five great eclipses of 1860-71 it was
ascertained: first, that the prominences, and at least the
224 HISTORY OF ASTRONOMY.
lower part of the corona, are genuine solar appurtenances ;
secondly, that the prominences are composed of hydrogen and
other gases in a state of incandescence, and rise, as irregular
outliers, from a continuous envelope of the same materials,
some thousands of miles in thickness ; thirdly, that the corona
is of a highly complex constitution, being made up in part of
glowing vapours, in part of matter capable of reflecting sun-
light. We may now proceed to consider the results of sub-
sequent eclipses.
These have raised and have helped to solve some very
curious questions. Indeed, every carefully watched total
eclipse of the sun stimulates, as well as appeases curiosity,
and leaves a legacy of outstanding doubt, continually, as time
and inquiry go on, removed, but continually replaced. It
cannot be denied that the corona is a perplexing phenomenon,
and that it does not become less perplexing as we know more
about it. It presented itself under quite a new and strange
aspect on the occasion of the eclipse which visited the Western
States of North America July 29,1878. The conditions of obser-
vation were peculiarly favourable. The weather was superb ;
above the Rocky Mountains the sky was of such purity as to
permit the detection, with the naked eye, of Jupiter's satellites
on several successive nights. The opportunity of advancing
knowledge was made the most of. Nearly a hundred astrono-
mers (including several Englishmen) occupied twelve separate
posts, and prepared for an attack in force.
The question had often suggested itself, and was a natural
one to ask, whether the corona sympathises with the general
condition of the sun ? whether, either in shape or brilliancy,
it varies with the progress of the sun-spot period ? A more
propitious moment for getting this question answered could
hardly have been chosen than that at which the eclipse occurred.
Solar disturbance was just then at its lowest ebb. The devel-
opment of spots for the month of July 1878 was represented
on Wolf s system of "relative numbers'" by the fraction o.i,
as against 135.4 for December 1870, an epoch of maximum
RECENT SOLAR ECLIPSES. 225
activity. The " chromosphere " l was, for the most part,
shallow and quiescent; its depth, above the spot-zones, had
sunk from about 6000 to 2000 miles ; prominences were few
and faint. Obviously, if a type of corona corresponding to a
minimum of sun-spots existed, it should be seen then or never.
It was seen ; but while, in some respects, it agreed with anti-
cipation, in others it completely set it at naught.
The corona of 1878, as compared with those of 1869, 1870,
and 1871, was generally admitted to be shrunken in its main
outlines, and much reduced in brilliancy. Mr. Lockyer pro-
nounced it ten times fainter than in 1871 ; Professor Harkness
estimated its light at less than one-seventh that derived from
the mist-blotted aureola of 1870.2 In shape, too, it was
markedly different. When sun-spots are numerous, the corona
appears to be most fully developed above the spot-zones, thus
offering to our eyes a rudely quadrilateral contour. The four
great luminous sheaves forming the corners of the square are
made up of rays curving together from each side into " syn-
clinal " or ogival groups, each of which may be compared to
the petal of a flower. To Janssen, in 187 1, the eclipsing moon
seemed like the dark heart of a gigantic dahlia, painted in
light on the sky ; and the similitude to the ornament on a
compass-card used by Sir George Airy in 1851, well conveys
the decorative effect of the beamy, radiated kind of aureola
never, it would appear, absent when solar activity is at a
tolerably high pitch. In his splendid volume on eclipses, 3
Mr. Ranyard first generalised the peculiarity of the synclinal
structures by a comparison of records ; but the symmetry of
their arrangement, though frequently striking, is liable to be
confused by secondary formations. Nothing of all this, how-
ever, was visible in 1878. Instead, there was seen, as the
1 The rosy envelope of prominence-matter was so named by Lockyer in
1868 (Phil. Trans., vol. clix. p. 430) ; and the appellation, its defiance
of Greek grammar notwithstanding, has had vitality to survive and prevail.
2 Bull Phil. Soc. Washington, vol. iii, p. 1 1 8.
3 Mem. R. A. Soc., vol. xli. 1879.
P
226 HISTORY OF ASTRONOMY.
groundwork of the corona, a ring of pearly light, nebulous to
the eye, but shown by telescopes and in photographs to have
a fibrous texture, as if made up of bundles of fine hairs. North
and south a series of short, vivid, electrical-looking flame-
brushes diverged with conspicuous regularity from each of
the solar poles. Their direction was not towards the centre
of the sun, but towards each summit of his axis, so that the
farther rays on either side started almost tangentially to the
surface. It is difficult not to connect this unusual display of
polar activity 1 with the great relative depth of the chromo-
sphere in those regions, noticed by Trouvelot previous to the
eclipse. 2
But the leading, and a truly amazing, characteristic of the
phenomenon was formed by two vast, faintly-luminous wings
of light, expanded on either side of the sun in the direction of
the ecliptic. These were missed by very few careful onlookers ;
but the extent assigned to them varied with skill in, and
facilities for, seeing. By far the most striking observations
were made by Newcomb at Separation (Wyoming), by Cleveland
Abbe from the shoulder of Pike's Peak, and by Langley at its
summit, an elevation of 14,100 feet above the sea. Never
before had an eclipse been viewed from anything approaching
that altitude, or under so translucent a sky. A proof of the
great reduction in atmospheric glare was afforded by the
perceptibility of the corona for above four minutes after
totality was over. For the 165 seconds of its duration, the
remarkable streamers above alluded to continued "persistently
visible," stretching away right and left of the sun to a distance
of at least ten million miles ! One branch was traced over
an apparent extent of fully twelve lunar diameters, without
sign of a definite termination having been reached ; and there
1 Professor W. A. Norton observed a similar phenomenon in 1869,
accompanied by some symptoms of equatorial emission. This is the more
remarkable as 1869 was a year of many sun-spots. His evidence, though
unsupported, and adverse to the theory of varying types, should not be
overlooked. See Am. Jour, of Sc., vol. i. (3d ser.), p. I.
2 Wash. Obs., 1876, App. iii. p. 80.
RECENT SOLAR ECLIPSES. 227
were no grounds for supposing the other more restricted.
The axis of the longest ray was found to coincide exactly, so
far as could be judged, with the ecliptic. 1 Pale cross-beams
were seen by Young and Abbe.
The resemblance to the zodiacal light was striking ; and a
community of origin between that enigmatical member of our
system and the corona was irresistibly suggested. We should,
indeed, expect to see, under such exceptionally favourable
atmospheric conditions as Professor Langley enjoyed on Pike's
Peak, the roots of the zodiacal light presenting near the sun
just such an appearance as he witnessed; but we can ima-
gine no reason why their visibility should be associated with
a low state of solar activity. Nevertheless this seems to be the
case with the streamers which astonished astronomers in 1878.
Once before, in August 1867, similar emanations had been de-
scribed and depicted by Grosch 2 of the Santiago Observatory ;
and then, too, sun-spots were at a minimum. Moreover, they
were seen combined with the same symptoms of polar excitement
visible eleven years later. The reality of the presumed con-
nection will be solidly established should the peculiar corona
of 1867 and 1878 reappear in 1889.
An alternative explanation was offered by the meteoric
hypothesis. Professor Cleveland Abbe was fully persuaded
that the long rays carefully observed by him were nothing else
than streams of meteorites rushing towards or from perihelion ;
and it is quite certain that the solar neighbourhood must be
crowded with such bodies. But there are no grounds for
supposing that they affect the ecliptic more than any other of
the infinite number of planes passing through the sun's centre.
On the contrary, everything we know leads us to believe that
meteorites, like their cometary allies, yield no obedience to the
rules of the road which bind the planets, but travel in either
direction indifferently, and in paths inclined at any angle to
the fundamental plane of our system. Besides, the peculiar
1 Wash. Obs., 1876, App. iii. p. 209. 2 Astr. Nach., No. 1737.
228 HISTORY OF ASTRONOMY.
structure at the base of the streamers displayed in the photo-
graphs, the curved rays meeting in pointed arches like Gothic
windows, the visible upspringing tendency, the filamentous
texture, speak unmistakably of the action of forces proceed-
ing from the sun, not of extraneous matter circling round
him.
Again, it may be asked what possible relation can exist
between the zodiacal plane and the sun's internal activity ? For
it is a remakable fact that to this approximately, and not to
the level of the solar equator, the streamers conformed. We
are acquainted with no such relation ; but it may be remarked
that the coronal axis of symmetry has frequently been observed
during eclipses to be inclined at an appreciable angle to the
solar axis of rotation, and the corresponding "magnetic equa-
tor " might quite conceivably be the scene of emanations in-
duced by some form of electrical repulsion.
The surest, though not the most striking, proof of sympathetic
change in the corona is afforded by the analysis of its light. In
1878 the bright lines so conspicuous in the coronal spectrum
in 1870 and 1871 were discovered to have faded to the very
limits of visibility. Several skilled observers failed to see them
at all ; but Young and Eastman succeeded in tracing both the
hydrogen and the green " 1474 " rays all round the sun, to a
height estimated at 340,000 miles. The substances emitting
them were thus present, though in a low state of incandescence.
The continuous spectrum was relatively strong ; a faint reflec-
tion of the Fraunhofer lines was traced in it ; and polarisation
was undoubted, increasing towards the limb, whereas in 1870
it reached a maximum at a considerable distance from it.
Experiments with Edison's tasimeter showed that the corona
radiates a sensible amount of heat.
The next promising eclipse occurred May 17, 1882. The
concourse of astronomers which has become usual on such
occasions assembled this time at Sohag, in Upper Egypt.
Rarely have seventy- four seconds been turned to such account.
To each observer a special task was assigned, and the advan-
RECENT SOLAR ECLIPSES. 229
tages of a strict division of labour were visible in the variety
and amount of the information gained.
The year 1882 was one of numerous sun-spots. On the
eve of the eclipse twenty-three separate maculae were counted.
If there were any truth in the theory which connected coronal
forms with fluctuations in solar activity, it might be anticipated
that the vast ecliptical expansions and polar "brushes" of 1878
would be found replaced by the star-like structure of 1871.
This expectation was literally fulfilled. No zodiacal streamers
were to be seen. The universal failure to perceive them, after
express search in a sky of the most transparent purity, justifies
the emphatic assertion that they were not there. Instead, the
type of corona observed in India eleven years earlier was repro-
duced, with its shining aigrettes, complex texture, and brilliant
decorative effect.
Concordant testimony was given by the spectroscope. The
reflected light derived from the corona was weaker than in
1878, while its original emissions were proportionately intensi-
fied. A number of new bright lines were discovered. Tacchini
determined four in the red end of the spectrum j Thollon per-
ceived several in the violet; and Dr. Schuster measured and
photographed about thirty. 1 The Fraunhofer lines autographi-
cally recorded in the continuous spectrum were not less nume-
rous. This was the first successful attempt to photograph the
spectrum of the corona as seen with an ordinary slit- spectroscope.
The slitless spectroscope, or " prismatic camera," although its
statements are necessarily of a far looser character, was, however,
also profitably employed. The uncommon strength in the chro-
mospheric regions of the violet light concentrated in the two
lines H and K, attributed to calcium, was strikingly brought
out by it ; and Dr. Schuster, using plates sensitised in the infra-
red by Captain Abney's newly invented process, obtained an
annular impression of the solar nimbus, probably corresponding
to an invisible red hydrocarbon band made known by Captain
Abney's researches. 2
1 Proc. Roy. Soc. t vol. xxxv. p. 154. 2 Observatory, vol. v. p. 209.
2 3 o HISTORY OF ASTRONOMY.
Dr. Schuster's photographs of the corona itself were the
most extensive, as well as the most detailed, of any .yet
secured. One rift imprinted itself on the plates to a distance
of nearly a diameter and a half from the limb ; and the trans-
parency of the streamers was shown by the delineation through
them of the delicate tracery beyond. The singular and
picturesque feature was added of a bright comet, self-depicted
in all the exquisite grace of swift movement betrayed by the
fine curve of its tail, hurrying away from, possibly, its only
visit to our sun, and rendered momentarily visible by the
withdrawal of the splendour in which it had been, and was
again quickly veiled.
From a careful study of these valuable records Dr. Huggins
derived the idea of a possible mode of photographing the
corona without an eclipse^ As already stated, its ordinary
invisibility is entirely due to the "glare" or reflected light
diffused through our atmosphere. But Dr. Huggins found, on
examining Schuster's negatives, that a large proportion of the
light in the coronal spectrum, both continuous and interrupted,
is collected in the violet region between the Fraunhofer lines
G and H. There, then, he hoped that, all other rays being
excluded, it might prove strong enough to vanquish inimical
glare, and stamp on prepared plates, through local superiority
in illuminative power, the forms of the appendage by which it
is emitted.
His experiments were begun towards the end of May 1882,
and by September 28 he had obtained a fair earnest of success.
The exclusion of all other qualities of light save that with
which he desired to operate, was at first effected by the inter-
position of screens of purple glass, or other similarly absorbing
media ; later, however, his purpose was more simply and
efficaciously realised by using chloride of silver as his sensitive
material, that substance being chemically inert to all other but
1 Proc. Roy. Soc., vol. xxxiv. p. 409. Experiments directed to the same
end had been made by Dr. O. Lohse at Potsdam, 1878-80 ; not without
some faint promise of ultimate success. Astr. Nach., No. 2486.
RECENT SOLAR ECLIPSES. 231
those precise rays in which the corona has the advantage. 1
Of the genuineness of the impressions left upon his plates there
can be no question. Their satisfactory agreement with the
Egyptian photographs fully attest the truth of their pretensions
as coronal autographs. "Not only the general features,"
Captain Abney bore them witness, 2 " are the same, but details,
such as rifts and streamers, have the same position and form."
It was found, moreover, that the corona photographed during
the total eclipse of May 6, 1883, was intermediate in shape be-
tween the coronas photographed by Dr. Huggins before and
after that event, each picture taking its proper place in a series
of progressive modifications highly interesting in themselves,
and emphatic in their testimony to the value of the method
employed to record them. In this climate, however, and near
the sea-level, it can never be brought to the perfection of
which it gives promise.
The prosperous result of the Sohag observations stimulated
the desire to repeat them on the first favourable opportunity.
This offered itself one year later, May 6, 1883, yet not with-
out the drawbacks incident to terrestrial conditions. The
eclipse promised was of rare length, giving no less than five
minutes and twenty-three seconds of total obscurity, but its path
was almost exclusively a " water-track." It touched land only
on the outskirts of the Marquesas group in the Southern Pacific,
and presented, as the one available foothold for observers, a coral
reef named Caroline Island, seven and a half miles long by one
and a half wide, unknown previous to 1874, and visited only for
"the sake of its stores of guano. Seldom has a more striking
proof been given of the vividness of human curiosity as to
the condition of the worlds outside our own, than in the
assemblage of a group of distinguished men from the chief
centres of civilisation, on a barren ridge, isolated in a vast and
1 The sensitiveness of chloride of silver extends from h to H ; that is,
over the upper or more refrangible half of the space in which the main
part of the coronal light is concentrated.
2 Proc. Roy. Soc., vol. xxxiv. p. 414.
232 HISTORY OF ASTRONOMY.
tempestuous ocean, at a distance, in many cases, of 11,000
miles and upwards from the ordinary scene of their labours.
And all these sacrifices the cost and care of preparation,
the transport and readjustment of delicate instruments, the
contrivance of new and more subtle means of investigating
phenomena on the precarious chance of a clear sky during
one particular five minutes ! The event, though fortunate,
emphasised the hazard of the venture. The observation of
the eclipse was made possible only by the happy accident of a
serene interval between two storms.
The American expedition was led by Professor Edward S.
Holden, and to it were courteously permitted to be attached
Messrs. Lawrance and Woods, photographers, sent out by the
Royal Society of London. M. Janssen was chief of the French
Academy mission; he was accompanied from Meudon by
Trouvelot, and joined from Vienna by Palisa, and from Rome
by Tacchini. A large share of the work done was directed to
assuring or negativing previous results. The circumstances
of an eclipse favour illusion. A single observation by a single
observer, made under unfamiliar conditions, and at a moment
of peculiar excitement, can scarcely be regarded as offering
more than a suggestion for future inquiry. But incredulity
may be carried too far. Janssen, for instance, felt compelled
by the survival of unwise doubts, to devote some of the pre-
cious minutes of obscurity at Caroline Island to confirming
what, in his own persuasion, needed no confirmation that is,
the presence of reflected Fraunhofer lines in the spectrum of
the corona. Trouvelot and Palisa, on the other hand, instituted
an exhaustive, but fruitless search for the spurious " intra-
mercurial" planet announced by Swift and Watson in 1878.
New information, however, was not deficient. The corona
proved identical in type with that of 1882, agreeably to what
was expected at an epoch of protracted solar activity. The
characteristic aigrettes (of which five appeared in Mr. Dixon's
sketch) were of even greater brilliancy than in the preceding
year, and the chemical intensity of the coronal light then
RECENT SOLAR ECLIPSES. 233
first measured with some precision was found to exceed that
of full moonlight. Janssen's photographs, owing to the con-
siderable apertures (six and eight inches) of his object-glasses,
and the long exposures permitted by the duration of totality,
were singularly perfect ; they gave a greater extension to the
corona than could be traced with the telescope, 1 and showed
its forms as absolutely fixed and of remarkable complexity.
The English pictures, taken with exposures up to sixty
seconds, were likewise of great value. They exhibited details
of structure from the limb to the tips of the streamers, which
terminated definitely, and as it seemed actually, where the
impressions on the plates ceased. The coronal spectrum
was also successfully photographed, with a number of bright
and dark lines ; and a print was caught of some of the more
prominent rays of the reversing layer just before and after
totality. The use of the prismatic camera was baffled by the
anomalous scarcity of prominences.
A highly suggestive observation was made during this eclipse
by Professor Tacchini. One of the aigrettes of the corona
displayed in his spectroscope, on a feebly continuous back-
ground, two of the bright bands familiar in the hydrocarbon
spectrum of comets. 2 This requires confirmation; neverthe-
less, the analogy which it hints at is a tempting one. The
resemblance of the silvery sheaves of the corona to the tails of
comets had already given rise to much fruitless speculation ;
and the exertion of a repulsive force, such as is obviously at
work in comets, by the sun on his surroundings, has been
considered, by some solar physicists, absolutely necessary to
explain the lowness of atmospheric pressure at his surface.
The presence of carbon in the sun's atmosphere was inferred
by Mr. Lockyer in 1878 from a comparison of photographs
of the solar and electric-arc spectra; 3 Dr. Schuster, as has
been mentioned, obtained indications of the same kind in
1882 ; and Captain Abney finds hydrocarbon bands in the
1 Comftes Rendus, t. xcvii. p. 592. 2 Ibid., p. 594.
3 Proc. Roy. Sec., vol. xxvii. p. 308.
234 HISTORY OF ASTRONOMY.
invisible or infra-red part of the Fratmhofer spectrum. 1 But
the subject needs to be further investigated.
Another of the observations made at Caroline Island,
although probably through some unexplained cause delusive,
merits some brief notice. Using an ingenious apparatus for
viewing simultaneously the spectrum from both sides of the
sun, Professor Hastings saw (as he supposed), certain alter-
nations, with the advance of the moon, in the respective
heights above the right and left solar limbs of the coronal line
" 1474," which were thought to imply an unexpected strength
of diffusive action in our atmosphere. If this were true, then
spectroscopic evidence as to the extent of the sun's gaseous
surroundings should at once be discarded as misleading; but
the simple consideration that if diffusion caused the observed
effect, it should extend bright lines across the disc of the moon
no less than on either side of it, suffices to show the fallacy of
the inference.
The controversy is an old one as to the part played by our
air in producing the radiance visible round the eclipsed sun.
In its original form, it is true, it came to an end when Pro-
fessor Harkness, in 1869^ pointed out that the shadow of the
moon falls equally over the air and on the earth, and that if
the sun had no luminous appendages, a circular space of
almost absolute darkness would consequently surround the
apparent places of the superposed sun and moon. Mr. Proc-
tor, 3 with his usual ability, impressed this mathematically cer-
tain truth (the precise opposite of the popular notion) upon
public attention ; and Sir John Herschel calculated that the
diameter of the " negative halo " thus produced would be, in
general, no less than 23.
But about the same time a noteworthy circumstance relat-
ing to the state of things in the solar vicinity was brought
into view. On February n, 1869, Messrs. Frankland and
Lockyer communicated to the Royal Society a series of experi-
1 Report Brit. Ass., 1881, p. 524. 2 Wash. Obs., 1867, App. ii. p. 64.
3 The Sun, p. 357.
RECENT SOLAR ECLIPSES. 235
ments on gaseous spectra under varying conditions of heat and
density, leading them to the conclusion that the higher solar
prominences exist in a medium of excessive tenuity, and that
even at the base of the chromosphere the pressure is far below
that at the earth's surface. 1 This inference was fully borne
out by the researches of Wiillner ; and Janssen expressed the
opinion that the chromospheric gases are rarefied almost to
the degree of an air-pump vacuum. 2 Hence was derived a
general and fully justified conviction that there could be out-
side, and incumbent upon the chromosphere no such vast
atmosphere as the corona appeared to represent. Upon the
strength of which conviction the " glare " theory entered,
chiefly under the auspices of Mr. Lockyer, upon the second
stage of its existence.
The genuineness of the "inner corona " to a height of 5' or
6' from the limb was admitted ; but it was supposed that by
the detailed reflection of its light in our air the far more ex-
tensive "outer corona" was optically created, the irregularities
of the moon's edge being called in to account for the rays and
rifts by which its structure was varied. This view received
some countenance from Maclear's observation, during the
eclipse of 1870, of bright lines "everywhere" even at the
centre of the lunar disc. Here, indeed, was an undoubted
case of atmospheric diffusion ; but here, also, was a safe index
to the extent of its occurrence. Light scatters equally in all
directions ; so that when the moon's face at the time of an
eclipse shows (as is the common case) a blank in the spectro-
scope, it is quite certain that the corona is not noticeably
enlarged by atmospheric causes. A sky drifted over with thin
cirrous clouds and air charged with aqueous vapour amply
accounted for the abnormal amount of scattering in 1870.
But even in 1870 positive evidence was obtained of the
substantial reality of the radiated outer corona, in the appear-
ance on the photographic plates exposed by Willard in Spain
and by Brothers in Sicily, of identical dark rifts. The truth is,
1 Proc. Roy. Soc., vol. xvii. p. 289. 2 Comptes Rendus t t. Ixxiii. p. 434.
236 HISTORY OF ASTRONOMY.
that far from being developed by misty air, it is peculiarly liable
to be effaced by it. The purer the sky, the more extensive,
brilliant, and intricate in the details of its structure the corona
appears. Take as an example General Myer's description of
the eclipse of 1869, as seen from the summit of White Top
Mountain, Virginia, at an elevation above the sea of 5530 feet,
in an atmosphere of peculiar clearness.
"To the unaided eye," he wrote, 1 "the eclipse presented,
during the total obscuration, a vision magnificent beyond
description. As a centre stood the full and intensely black
disc of the moon, surrounded by the aureola of a soft bright
light, through which shot out, as if from the circumference of
the moon, straight, massive, silvery rays, seeming distinct and
separate from each, other, to a distance of two or three dia-
meters of the solar disc ; the whole spectacle showing as on a
background of diffused rose-coloured light."
On the same day, at Des Moines, Newcomb could perceive,
through somewhat hazy air, no long rays, and the four-pointed
outline of the corona reached at its farthest only a single
semi-diameter of the moon from the limb. The plain fact,
that our atmosphere acts rather as a veil to hide the coronal
radiance than as the medium through which it is visually
formed, emerges from the records of innumerable other ob-
servations.
Summing up what we have learned about the corona during
some forty minutes of scrutiny in as many years, we may state,
to begin with, that it is not a solar atmosphere. It does not
gravitate upon the sun's surface and share his rotation, as our
air gravitates upon and shares the rotation of the earth ; and
this for the simple reason that there is no visible growth of
pressure downwards (such as the spectroscope would infallibly
give notice of) in its gaseous constituents ; whereas under the
sole influence of the sun's attractive power, their density should
be multiplied many million times in the descent through a mere
fraction of their actual depth.
1 Wash. Obs., 1867, App. ii. p. 195.
((
N^C4tIf
RECENT SOLAR ECLIPSES. 237
The corona is properly described as a solar appendage ; and
may be conjecturally denned as matter in a perpetual state of
efflux from, and influx to our great luminary, under the stress
of electrical repulsion in one direction and of gravity in the
other. 1 Its constitution is of a composite character. It is
partly made up of self-luminous gases, chiefly hydrogen, and
the unknown substance giving the green ray "1474;" partly
of solid or liquid particles, seen by reflected sunlight. There
is a strong probability that it is affected by the periodic ebb
and flow of solar activity, the rays emitted by the gases con-
tained in it fading, and the continuous spectrum brightening,
at times of minimum sun-spots, as if by a fall of temperature
producing, on the one hand, a decline in luminosity of the
incandescent materials existing near the sun, and, on the other,
a condensation of vapours previously invisible into compact
particles of some reflective capacity.
The most important lesson, however, derived from eclipses
is that of independence of them. Some of its fruits in the
daily study of prominences the next chapter will collect ; while
the attainment, through Dr. Huggins's photographic method, of
a corresponding power as regards the corona, may be expected
to mark an epoch in the investigation of that still problematical
phenomenon.
1 Professor W. A. Norton, of Yale College, appears to have been the
earliest formal advocate of the Expulsion Theory of the solar surroundings,
in the second (1845) and later editions of his Treatise on Astronomy. J
CHAPTER IV.
SPECTROSCOPIC WORK ON THE SUN.
THE new way struck out by Janssen and Lockyer was at once
and eagerly followed. In every part of Europe, as well as in
North America, observers devoted themselves to the daily
study of the chromosphere and prominences. Foremost
among these were Lockyer in England, Zollner at Leipzig,
Sporer at Anclam, Young at Hanover, New Hampshire,
Secchi and Respighi at Rome. There were many others, but
these names are conspicuous from the outset,
The first point to be cleared up was that of chemical
composition. Leisurely measurements verified the presence
above the sun's surface of hydrogen in prodigious masses, but
showed that sodium had nothing to do with the orange-yellow
ray identified with it in the haste of the eclipse. From its
vicinity to the D pair (than which it is slightly more refrangible),
the prominence-line was, however, designated D3, and the
unknown substance emitting it was named by Frankland
" helium." Young is inclined to associate with it two other
faint but persistent lines in the spectrum of the chromo-
sphere; 1 and Messrs. Liveing and Dewar pointed out, in
1879^ tnat tne wave-lengths of all three are bound together
with that of the coronal ray " 1474 " by numerical ratios virtually
the same with those underlying the vibrations of hydrogen,
and also conformed to by certain lines of lithium and magne-
sium. This obscure but interesting subject deserves further
1 Phil. Mag., vol. xlii. 1871, p. 380.
2 Proc. Roy. Soc., vol. xxviii. p. 475.
SPECTROSCOPIC WORK ON THE SUN. 239
inquiry. It should be added that Mr. Lockyer attributes
both the 03 and " 1474" lines to a modification of hydrogen;
but the actual relation would seem to be one of analogy
rather than of identity.
Hydrogen and helium form the chief and unvarying materials
of the solar sierra and its peaks; but a number of metallic
elements make their appearance spasmodically under the
influence of disturbances in the layers beneath. In September
1871, Young 1 drew up at Dartmouth College a list of 103
lines significant of injections into the chromosphere of iron,
titanium, calcium, magnesium, and many other substances.
During two months' observation in the pure air of Mount
Sherman (8335 feet high) in the summer of 1872, these tell-
tale lines mounted up to 273 ; and he believes their number
might still be doubled by steady watching. Indeed, both
Young and Lockyer have more than once seen the whole field
of the spectroscope momentarily inundated with bright rays,
as if the " reversing layer " seen at the beginning and end of
eclipses had been suddenly thrust upwards into the chromo-
sphere, and as quickly allowed to drop back again. It would
thus appear that the two form one continuous region, of which
the lower parts are habitually occupied by the heaviest vapours,
but where orderly arrangement is continually overturned by
violent eruptive disturbances.
The study of the forms of prominences practically began
with Dr. Huggins's observation of one through an " open slit,"
February 13, i869. 2 At first it had been thought possible to
study them only in sections that is, by admitting mere narrow
strips or " lines " of their various kinds of light ; while the
actual shape of the objects emitting those lines had been
arrived at by such imperfect devices as that of giving to the
slit of the spectroscope a vibratory movement rapid enough
to enable the eye to retain the impression of one part while
others were successively presented to it. It was an immense
gain to find their rays strong enough to bear so much of dilu-
1 Phil. Mag. } vol. xlii. p. 377. 2 Proc. Roy. Sac., vol. xvii. p. 302.,
240 HISTORY OF ASTRONOMY.
tion with ordinary light as was involved in opening the spectro-
scopic shutter wide enough to exhibit the tree-like, or horn-
like, or flame-shaped bodies rising over the sun's rim in their
undivided proportions. Three images of each prominence
are formed in the spectroscope a crimson, a deep yellow, and
a bluish green. The crimson, however (built up out of the C
line of hydrogen), is the most intense, and is commonly used
for purposes of observation and illustration.
Friedrich Zollner was, by a few days, beforehand with
Huggins in describing the open-slit method, but was some-
what less prompt in applying it. His first survey of a complete
prominence, pictured in, and not simply intersected by, the
slit of his spectroscope, was obtained July i, I869. 1 Shortly
afterwards the plan was successfully adopted by the whole
band of investigators.
A difference in kind was very soon perceived to separate
these objects into two well-marked classes. Its natural and
obvious character was shown by its having struck several
observers independently. The distinction of "cloud-promin-
ences " from " flame-prominences " was announced by Lockyer,
April 27, by Zollner, June 2, and by Respighi, December 4,
1870.
The first description is tranquil and relatively permanent,
sometimes enduring without striking change for many days.
They mimic terrestrial cloud-scenery now appearing like
fleecy cirrus transpenetrated with the red glow of sunset
now like prodigious masses of cumulo-stratus hanging heavily
above the horizon. These solar clouds, however, have the
peculiarity of possessing stems. Slender columns can ordinarily
be seen to connect the surface of the chromosphere with its
outlying portions. Hence the fantastic likeness to forest
scenery presented by the long ranges of fiery trunks and
foliage at times seeming to fringe the sun's limb. But while
this formation suggests an actual outpouring of incandescent
material, certain facts require a different interpretation. At
1 Astr. Nach., No. 1769.
SPECTROSCOPIC WORK ON THE SUN. 241
a distance, and quite apart from the chromosphere, prominences
have been perceived, both by Secchi and Young, to form, just
as clouds form in a clear sky, condensation being replaced by
ignition. Filaments were then thrown out downwards towards
the chromosphere, and finally the usual appearance of a
" stemmed prominence " was assumed. Still more remarkable
was an observation made by Trouvelot at Harvard College
Observatory, June 26, iSy^.. 1 A gigantic comma-shaped pro-
minence, 82,000 miles high, vanished from before his eyes by
a withdrawal of light as sudden as the passage of a flash of
lightning. The same observer has frequently witnessed a
gradual illumination or gradual extinction of such objects,
testifying to changes in the thermal or electrical condition of
matter already in situ.
The chemistry of "cloud-prominences" is very simple.
Hydrogen and helium are their only constituents. " Flame-
prominences," on the other hand, show, in addition, the
characteristic rays of a number of metals, amongst which iron,
titanium, barium, sodium, and magnesium are conspicuous.
They are intensely brilliant ; sharply denned in their varying
forms of jets, spikes, fountains, waterspouts ; of rapid formation
and speedy dissolution, seldom attaining to the vast dimensions
of the more tranquil kind. They are visibly of eruptive origin,
and are closely connected with spots ; the materials ejected as
"flames " cooling and settling down, according to Father Secchi, 2
as dark, depressed patches of increased absorption. The two
classes of phenomena, at any rate, stand in a most intimate
relation ; they obey the same law of periodicity, and are con-
fined to the same portions of the sun's surface, while quiescent
prominences may be found right up to the poles and close to
the equator.
The general distribution of prominences, including both
species, follows that of faculae much more closely than that of
spots. From Father Secchi and Professor Respighi's observa-
tions, 1869-71, were derived the first clear ideas on the subject,
1 Am. Jour, of Sc., vol. xv. p. 85. 2 Le Soleil t t. ii. p. 294.
Q
242 HISTORY OF ASTRONOMY.
which have been supplemented and modified by the later
researches of Professors Tacchini and Ricco at Rome and
Palermo. The results are somewhat complicated, but may be
stated broadly as follows. The district of greatest prominence-
frequency covers and overlaps by several degrees that of greatest
spot-frequency. That is to say, it extends to about 40 north and
south of the equator. 1 There is a visible tendency to a second pair
of maxima nearer the poles. The poles themselves, as well as
the equator, are regions of minimum occurrence. Distribution
in time is governed by the spot-cycle, but the maximum lasts
longer for prominences than for spots.
The structure of the chromosphere was investigated in 1869
and subsequent years by Professor Respighi, director of the
Capitoline Observatory, as well as by Sporer, and by Bredichin
of the Moscow Observatory. They found this supposed solar
envelope to be of the same eruptive nature as the vast pro-
trusions from it, and to be made up of a congeries of minute
flames 2 set close together like blades of grass. " The appear-
ance," Professor Young writes, 3 "which probably indicates a
fact, is as if countless jets of heated gas were issuing through
vents and spiracles over the whole surface, thus clothing it
with flame which heaves and tosses like the blaze of a con-
flagration."
The summits of these filaments of fire are commonly inclined,
as if by a wind sweeping over them, when the sun's activity
is near its height, but erect during his phase of tranquillity.
Sporer, in 1871, inferred the influence of permanent polar
currents, 4 but Tacchini showed in 1876 that the deflections
upon which this inference was based, ceased to be visible as
the spot-minimum drew near. 5
Another peculiarity of the chromosphere, denoting the re-
1 L'Astronomie, August 1884, p. 292 (Ricco).
2 Averaging about 100 miles across and 300 high. Le Soleil, t. ii. p. 35.
3 The Sun, p. 180. 4 Astr. Nach., No. 1854.
5 Mem. degli Spettroscopisti Italian^ t. v. p. 4. Restated by Secchi,
Ibid., t. vi. p. 56.
SPECTROSCOPIC WORK ON THE SUN. 243
moteness of its character from that of a true atmosphere, 1 is
the irregularity of its distribution over the sun's surface. There
are no signs of its bulging out at the equator, as the laws of
fluid equilibrium in a rotating mass would require ; but there
are some that the fluctuations in its depth are connected with
the phases of solar agitation. At times of minimum it seems
to accumulate and concentrate its activity at the poles ; while
maxima probably bring a more equable general distribution,
with local depressions at the base of great prominences and
above spots.
The reality of the appearance of violent disturbance pre-
sented by the " flaming " kind of prominence can be tested in
a very remarkable manner. Christian Doppler, 2 professor of
mathematics at Prague, enounced in 1842 the theorem that
the colour of a luminous body, like the pitch of a sonorous
body, must be changed by movements of approach or recession.
The reason is this. Both colour and pitch are physiological
effects, depending, not upon absolute wave-length, but upon
the number of waves entering the eye or ear in a given interval
of time. And this number, it is easy to see, must be increased
if the source of light or sound is diminishing its distance, and
diminished if it is increasing it. In the one case, the vibrating
body pursues and crowds together the waves emanating from
it ; in the other, it retreats from them, and so lengthens out
the spa.ce covered by an identical number. The principle
may be thus illustrated. Suppose shots to be fired at a target
at fixed intervals of time. If the marksman advances, say
twenty paces between each discharge of his rifle, it is evident
that the shots will fall faster on the target than if he stood still ;
if, on the contrary, he retires by the same amount, they will
strike at correspondingly longer intervals. The result will of
course be the same whether the target or the marksman be in
movement.
1 Its non-atmospheric character was early defined by Proctor, Month.
Not., vol. xxxi. p. 196.
2 Abh. d. Kon. Bohrn. Ges. d. Wiss., Bd. ii. 1841-42, p. 467.
244 HISTORY OF ASTRONOMY.
So far Doppler was altogether right. As regards sound, any
one can convince himself that the effect he predicted is a real
one, by listening to the alternate shrilling and sinking of the
steam-whistle when an express train rushes through a station.
But in applying this principle to the colours of stars he went
widely astray ; for he omitted from consideration the double
range of invisible vibrations which partake of, and to the eye
exactly compensate, changes of refrangibility in the visible rays.
There is, then, no possibility of finding a criterion of velocity
in the hue of bodies shining, like the sun and stars, with con-
tinuous light. There is a slight shift of the entire spectrum
up or down in the scale of refrangibility ; certain rays normally
visible become exalted or degraded (as the case may be) into
invisibility, and certain other rays at the opposite end undergo
the converse process ; but the sum-total of impressions on the
retina continues the same.
We are not, however, without the means of measuring this
sub-sensible transportation of the light-gamut. Once more
the wonderful Fraunhofer lines came to the rescue. They
were called by the earlier physicists " fixed lines ; " but it is
just because they are not fixed that, in this instance, we find
them useful. They share, and in sharing betray, the general
shift of the spectrum. This aspect of Doppler's principle was
adverted to by Fizeau in I848, 1 and the first tangible results
in the estimation of movements of approach and recession
between the earth and the stars, were communicated by Dr.
Muggins to the Royal Society, April 23, 1868. Eighteen
months later, Zollner devised his " reversion-spectroscope " 2 for
doubling the measurable effects of line-displacements ; aided by
which ingenious instrument, and following a suggestion of its
inventor, Professor H. C. Vogel succeeded at Bothkamp, June 9
1 87 1, 3 in detecting effects of that nature due to the solar rota
1 In a paper read before the Societe Philomathique de Paris, December
23, 1848, and first published in exienso in Ann. de Chim. etde Phys. t t. xiy.
p. 211 (1870).
2 Astr. Nack. t No. 1772. 3 Ibid., No. 1864.
SPECTROSCOPIC WORK ON THE SUN. 245
tion. This application constitutes at once the test and the
triumph of the method.
The eastern edge of the sun is continually moving towards
us with an equatorial speed of about a mile and a quarter per
second, the western edge retreating at the same rate. The
displacements towards the violet on the east, towards the red
on the west corresponding to this velocity are very small ; so
small that it seems hardly credible that they should have been
laid bare to perception. They amount to but T ^th part of the
interval between the two constituents of the D line of sodium ;
and the D line of sodium itself can be separated into a pair
only by a powerful spectroscope. Nevertheless, Professor
Young 1 was able to show quite satisfactorily, in 1876, not only
deviations in the solar lines from their proper places indicating
a velocity of rotation (1.42 miles per second) slightly in excess
of that given by observations of spots, but the exemption of
terrestrial lines (those produced by absorption in the earth's
atmosphere) from the general push upwards or downwards.
Shortly afterwards, Professor Langley, director of the Allegheny
Observatory, having devised a means of comparing with great
accuracy light from different portions of the sun's disc, found that
while the obscure rays in two juxtaposed spectra derived from
the solar poles were absolutely continuous, no sooner was the
instrument rotated through ninety degrees, so as to bring its
luminous supplies from opposite extremities of the equator,
than the same rays became perceptibly " notched." The
telluric lines, meanwhile, remained unaffected, so as to be
" virtually mapped " by the process. 2 This rapid and unfailing
mode of distinction was used by Cornu with perfect ease during
his investigation of atmospheric absorption near Loiret in
August and September i883. 3
A beautiful experiment of the same kind was performed by
M. Thollon, of M. Bischoffsheim's observatory at Nice, in the
summer of i88o. 4 He confined his attention to one delicately
1 Am. Jour, of Sc. t vol. xii. p. 321. 2 Ibid., vol. xiv. p. 140.
Bull. Astronom., Feb. 1884, p. 77. 4 Comptes Rendus^ t. xci. p. 368.
246 HISTORY OF ASTRONOMY.
defined group of four lines in the orange, of which the inner
pair are solar (iron) and the outer terrestrial. At the centre
of the sun the intervals separating them were sensibly equal ;
but when the light was taken alternately from the right and
left limbs, a relative shift in alternate directions of the solar,
towards and from the stationary telluric rays became apparent.
This amounts to a demonstration that results of this kind are
worthy of confidence ; and since they are, in certain cases, such
as to startle it, it is important to make sure of their founda-
tions.
Mr. Lockyer 1 was the first to perceive the applicability of
this subtle and surprising discovery to the study of prominences,
the discontinuous light of which affords precisely the same
means of detecting movement without seeming change of
place, as do lines of absorption in a continuous spectrum.
Indeed, his observations at the sun's edge almost compelled
him to have recourse to an explanation made available just
when the need of it began to be felt. He saw bright lines, not
merely pushed aside from their normal places by a barely
perceptible amount, but bent, torn, broken, as if by the stress
of some tremendous violence. These remarkable appearances
were quite simply interpreted as the effects of movements
varying in amount and direction in the different parts of the
extensive mass of incandescent vapours falling within a single
field of view. Very commonly they are of a cyclonic character.
The opposite distortions of the same coloured rays betray the
fury of "counter-gales" rushing along at the rate of 120 miles
a second; while their undisturbed sections prove the persistence
of a " heart of peace " in the midst of that unimaginable fiery
whirlwind. Velocities up to 250 miles a second, or 15,000
times that of an express train at the top of its speed, were
thus observed by Young during his trip to Mount Sherman,
August 3, 1872.
Motions ascertainable in this way near the limb are, of
course, horizontal as regards the sun's surface } the analogies
1 Proc. Roy. Soc., vols. xvii. p. 415 ; xviii. p. 120.
SPECTROSCOPIC WORK ON THE SUN. 247
they present might, accordingly, be styled meteorological rather
than volcanic. But vertical displacements on a scale no less
stupendous can also be shown to exist. Observations of the
spectra of spots centrally situated (where motions in the line
of sight are vertical) disclose the progress of violent uprushes
and downrushes of ignited gases, for the most part in the
penumbral or outlying districts. They appear to be occasioned
by fitful and irregular disturbances, and have none of the
systematic quality which would be required for the elucidation
of sun-spot theories. Indeed, they almost certainly take place
at a great height above the actual opening in the photosphere.
As to vertical motions above the limb, on the other hand,
we have direct visual evidence of a truly amazing kind. The
projected glowing matter has, by the aid of the spectroscope,
been watched in transit. On September 7, 1871, Young exa-
mined at noon a vast hydrogen-cloud, 100,000 miles long, as
it showed to the eye, and 54,000 high. It floated tranquilly
above the chromosphere at an elevation of some 15,000 miles,
and was connected with it by three or four upright columns,
presenting the not uncommon aspect compared by Lockyer to
that of a grove of banyans. Called away for a few minutes at
12.30, on returning at 12.55 the observer found
" That in the meantime the whole thing had been literally
blown to shreds by some inconceivable uprush from beneath.
In place of the quiet cloud I had left, the air, if I may use the
expression, was filled with flying debris a mass of detached,
vertical, fusiform filaments, each from 10" to 30" long by 2"
or 3" wide, 1 brighter and closer together where the pillars had
formerly stood, and rapidly ascending. They rose, with a
velocity estimated at 166 miles a second, to fully 200,000
miles above the sun's surface, then gradually faded away like
a dissolving cloud, and at 1.15 only a few filmy wisps, with
some brighter streamers low down near the photosphere, re-
mained to mark the place." 2
1 At the sun's distance, one second of arc represents about 450 miles.
2 Am. Jour, of Sc., vol. ii. 1871, p. 468.
248 HISTORY OF ASTRONOMY.
A velocity of projection of at least 500 miles per second has
been calculated by Proctor 1 to be necessary in order to account
for this extraordinary display. It was marked by the simul-
taneous record at Greenwich of a magnetic disturbance, and
was succeeded, the same evening, by a fine aurora. It has
proved by no means an isolated occurrence. Young saw its
main features repeated, October 7, i88i, 2 on a still vaster
scale; for the exploded prominence attained, this time, an
altitude of 350,000 miles the highest yet chronicled. Mr.
Lockyer, moreover, has seen a prominence 40,000 miles high
blown to pieces in ten minutes; while uprushes have been
witnessed by Respighi, of which the initial velocities were
judged by him to be 400 or 500 miles a second. When it is
remembered that a body starting from the sun's surface at the
rate of 379 miles a second would, if it encountered no resist-
ance, escape for ever from his control, it is obvious that we
have, in the enormous forces of eruption or repulsion mani-
fested in the outbursts just described, the means of accounting
for the vast diffusion of matter in the solar neighbourhood.
Nor is it possible to explain them away, as Cornu, 3 Faye, 4 and
others have sought to do, by substituting for the rush of matter
in motion, progressive illumination through electric discharges,
or even through the mere reheating of gases cooled by ex-
pansion. 5 All the appearances are against such evasions of the
difficulty presented by velocities stigmatised as " fabulous "
and' "improbable," but which, there is the strongest reason
to believe, really exist.
On the 1 2th of December 1878, Mr. Lockyer formally
expounded before the Royal Society his now famous hypothesis
of the compound nature of the "chemical elements." 6 He
was led to it by several converging lines of research. In a
1 Month. Not., vol. xxxii. p. 51. 2 Nature, vol. xxiii. p. 281.
3 Comptes Rendus, t. Ixxxvi. p. 532. 4 Ibid., t. xcvi. p. 359.
5 Such prominences as have been seen to grow by the spread of incan-
descence are of the quiescent kind, and present no deceptive appearance
of violent motion.
6 Proc. Roy. Soc., vol. xxviii. p. 157.
SPECTROSCOPIC WORK ON THE SUN. 249
letter to M. Dumas, dated December 3, 1873, he had sketched
out the successive stages of " celestial dissociation " which he
conceived to be represented in the sun and stars. The absence
from the solar spectrum of metalloidal absorption he explained
by the separation, in the fierce solar furnace, of such substan-
ces as oxygen, nitrogen, sulphur, carbon, &c., into simpler
constituents possessing unknown spectra; while metals were
at that time still admitted to be capable of existing there in a
state of integrity. Three years later he made a further step.
He announced, as the result of a comparative study of the
Fraunhofer and electric-arc spectra of calcium, that the
" molecular grouping " of that metal, which at low tempera-
tures gives a spectrum with its chief line in the blue, is nearly
broken up in the sun into another or others with lines in the
violet. 1 The further progress of his work showed him this
discrepancy between solar and terrestrial spectra as no ex-
ception, but " a truly typical case." 2
From 1875 onwards this unwearied student of nature was
engaged in the construction of a map of the solar spectrum on
a scale of magnitude such that, when completed down to the
infra-red, it will be 315 feet, or about half a furlong in length.
The attendant laborious investigation, by the aid of photo-
graphy, of metallic spectra, afforded him the supposed dis-
covery of "basic lines." These are lines occurring in the
spectra of two or more metals after all possible " impurities "
have been eliminated, and were held to attest the presence of
a common substratum of matter in a simpler state of aggrega-
tion than any with which we are ordinarily acquainted. Now
it is a singular fact that these "basic lines" are precisely those
which appear, with a persistence altogether out of proportion
to their actual numbers, in the spectrum of the chromosphere
1 Proc. Roy. Soc., vol. xxiv. p. 353. The remarkable pair of lines in
the violet (H and K) attributed to calcium stand urgently in need of being
cleared up. Vogel discovered in 1879 a hydrogen line coincident with H
(Monatsb. Preuss. Ak. t Feb. 1879, p. 115). Young attributes both H and
K to that substance, on the ground of their anomalous behaviour in pro-
minences (Natttre, vol. xxiii. p. 281). 2 Proc. Roy. Soc. } vol. xxviii. p. 444.
250 HISTORY OF ASTRONOMY.
when agitated by eruptive injections. The presence of iron,
for example, instead of being signified by the flashing out of
some of the strong representative lines which are the first to
appear and the last to disappear in its laboratory-spectrum,
makes itself known by the brightening of some inconspicuous
ray, claimed, moreover, with an equal title, by (say) calcium
or titanium. What more natural than to conclude, with Mr.
Lockyer, that the erupted substance is not really iron at all,
but some more elementary form of matter entering into the
composition of iron as well as of calcium and titanium, the
reduction having been brought about by the inconceivable
heat of the sub-photospheric regions ?
There is, nevertheless, a difficulty in accepting this plausible
view. The foundation of fact upon which it rests is insecure.
The lines called basic are probably not really identical, but
only very closely coincident. They are formed of doublets or
triplets merged together by insufficient dispersion. Out of
Thalen's original list of seventy rays common to several
spectra, 1 only seven (besides about five which by their situa-
tion elude scrutiny) have so far resisted Tholloris and Young's
powerful spectroscopes ; and the process of resolution will
almost certainly be carried farther. Thus the argument from
community of lines to community of substance may be regarded
as already half extinct. The circumstance, however, still
requires explanation, that these twin-lines these spots of
rendezvous, it might be said, for different sets of vibrations
are specially selected for display in solar disturbances are
predominantly brightened in flames and thickened in spots.
But the really strong point of the " dissociation theory " has
yet to be mentioned. It is that the contortions or displace-
ments due to motion are frequently seen to affect a single line
belonging to a particular -substance, while the other lines of
that same substance remain imperturbable. Now, how is this
most singular fact, which seems at first sight to imply that a
1 Many of these were shown by Mr. Lockyer, who was the first to sift the
matter, to be due to very slight admixtures of the several metals concerned.
SPECTROSCOPIC WORK ON THE SUN. 251
body may be at rest and in motion at one and the same instant,
to be accounted for ? It is accounted for, on Mr. Lockyer's
hypothesis, easily enough, by supposing that the rays thus
discrepant in their testimony, do not belong to one kind of
matter, but to several, combined, at ordinary temperatures, to
form a body in appearance " elementary." Of these different
vapours, one or more may of course be rushing rapidly towards
or from the observer, while the others remain still ; and since
the line of sight across the average prominence region pene-
trates, at the sun's edge, a depth of about 300,000 miles, 1 all the
incandescent materials separately occurring along which line are
projected into a single " flame " or " cloud," it will be perceived
that there is ample room for diversities of behaviour.
The alternative mode of escape from the perplexity consists
in assuming that the vapour in motion is rendered luminous
under conditions which reduce its spectrum to one or two rays,
the unaffected lines being derived from a totally distinct
mass of the same substance shining with its ordinary emis-
sions. 2 The supposition is by no means a violent one, since
both hydrogen and nitrogen can readily be brought, in the
laboratory, into the state of monochromatic radiation; and
even sodium has, by careful manipulation, been induced to
give a spectrum from which the all but ubiquitous D line
is missing. 3 The results to the eye would, on either supposi-
tion, be the same.
Mr. Lockyer's view has the argument from continuity in its
favour. It only asks us to believe that processes which we
know to take place on the earth under certain conditions, are
carried further in the sun, where the same conditions are,
it may be presumed, vastly exalted. We find that the bodies we
1 Thollon's estimate (Comptes Rendus, t. xcvii. p. 902) of 300.000 kilo-
metres seems considerably too low. Limiting the " average prominence
region " to a shell 54,000 miles deep (2' of arc as seen from the earth), the
visual line will, at mid-height (27,000 miles from the sun's surface), travel
through (in round numbers) 320,000 miles of that region.
2 Liveing and Dewar, Phil, Mag., vol. xvi. (5th ser.), p. 407.
3 Lockyer, Proc. Roy. Soc., vol. xxix. p. 140.
252 HISTORY OF ASTRONOMY.
call " compound " split asunder at fixed degrees of heat within
the range of our resources. Why should we hesitate to admit
that the bodies we call " simple " do likewise at degrees of heat
without the range of our resources? There is no intrinsic
difference separating them. The term " element " simply
expresses terrestrial incapability of reduction. That, in celes-
tial laboratories, the means and their effect here absent should
be present, would be an inference challenging, in itself, no
expression of incredulity.
Yet there are grave objections to assent when the actual
circumstances of the case are attentively considered. Of these
objections we need at present advert to only one ; but it is
fundamental. Far from being a simplification, the hypothesis
in question introduces an enormous complication into the
workings of nature. We now recognise sixty-four " elements "
provisionally so called ; for no chemist supposes them to be
essentially and ab origine distinct kinds of matter. But, if
Mr. Lockyer's reasoning be admitted as valid, these sixty-
four should be multiplied many times ; for it asserts that each
body known to us upon the earth is broken up in the sun into
several constituents, and the evidence in favour of the " basic "
nature of any of these constituents has, as we have seen, virtually
collapsed. Thus hydrogen is " dissociated " into at least three
separate substances entirely independent of any others, and the
components of iron should be counted by the score. Nay, if
the principle be admitted, which is the implied postulate under-
lying the arguments used, that a truly elementary body can
radiate but one kind of light gives, in other words, a spectrum
of one bright line it is difficult to stop short of the conclusion
that each of the multitudinous coloured rays in the spectra of
our sixty-four " elements " is the individual representative of a
distinct species of matter.
There can be no doubt that the spectra of bodies are an
index to changes in their molecular constitution of every kind
and degree, from a complete disruption of the molecule into
atoms, homogeneous or heterogeneous, to some unspeakably
SPECTROSCOPIC WORK ON THE SUN. 253
minute, yet orderly and harmonious rearrangement of parts in
the complex little system of which the movements are the source
of light. Mr. Lockyer's " working hypothesis " thus raises ques-
tions which science is not yet prepared to answer. It brings us
face to face with the mysteries of the ultimate constitution of
matter, and of its relations to the vibrating medium filling space.
It makes our ignorance on the subject seem at once more dense
and more definite. Nevertheless, this in itself (though the saying
appear paradoxical) constitutes an advance and gives hope of
progress. The mustering, drafting, and drilling of facts due to
Mr. Lockyer's diligence, must in the end tell for truth, although
their interpretation be for a time doubtful.
Professor A. J. Angstrom of Upsala takes rank after
Kirchhoff as a subordinate founder, so to speak, of solar
spectroscopy. His anticipation of its fundamental principle
(equivalence of emission and absorption) had, perhaps, scarcely
the absolute character claimed for it ; but his work in the
development of that principle was of extraordinary value.
His great map of the " normal " solar spectrum l was published
in 1868, two years before he died. Robert Thalen was his
coadjutor in its execution, and the immense labour which it
cost was amply repaid by its eminent and lasting usefulness.
It is still the universal standard of reference in all spectroscopic
inquiries within the range of the visible emanations.
The discovery that hydrogen exists in the atmosphere of the
sun was made by Angstrom in 1862. His list of solar elements
published in that year, 2 the result of an investigation separate
from, though conducted on the same principle as Kirchhoff 's,
included the substance which we now know to be predominant
amongst them. Dr. Pliicker of Bonn had identified in 1859
1 The normal spectrum is that depending exclusively upon wave-length
the fundamental constant given by nature as regards light. It is obtained
by the interference of rays, in the manner first exemplified by Fraunhofer,
and affords the only unvarying standard for measurement. In the refrac-
tion-spectrum (upon which Kirchhoffs map was founded), the relative
positions of the lines vary with the material of the prisms.
2 Ann. d. Pkys., Bd. cxvii. p. 296.
254 HISTORY OF ASTRONOMY.
the Fraunhofer line F with the green ray of hydrogen, but
drew no inference from his observation. The agreement was
verified by Angstrom ; two further coincidences were estab-
lished ; and in 1866 a fourth hydrogen line in the extreme
violet (named h) was detected in the solar spectrum. With
Thalen, he besides added manganese, aluminium, and tita-
nium to the constituents of the sun enumerated by Kirchhoff,
and raised the number of identical rays in the solar and
terrestrial spectra of iron to no less than 460. l
Thus, when Mr. Lockyer entered on that branch of inquiry
in 1872, fourteen substances were recognised as common to
the earth and sun. Early in 1878 he was able, by applying
the test of length in lieu of that of strength in the comparison
of lines (looking, that is, rather to their persistence through a
wide range of temperature, than to their brilliancy at any one
temperature), to increase the list provisionally to thirty-three. 2
All these are metals ; for there is strong reason to believe that
hydrogen presents a solitary instance of an ordinarily gaseous
metal, just as mercury does of an ordinarily liquid one. Up
to 1877 the fourteen metalloids (non-metallic elements) were
conspicuous by absence.
But in that year the late Dr. Henry Draper of New York
announced a discovery of very wide significance. As the
upshot of an investigation lasting several years, he found oxygen
to be revealed in the sun, not, like the metals, by the reversal
of its spectral rays, but by their direct presence. Each one of
eighteen bright lines in its photographed spectrum was seen
to be represented by a strictly corresponding brilliant band in
the analysed light of the sun. 3 The reality of these coincidences
having been doubted, Dr. Draper set to work afresh, and on
the i3th of June 1879 4 laid before the Royal Astronomical
Society photographs on a scale four times that of the original
ones, in which the solar counterpart of the laboratory-spectrum
1 Comptes Rendus, t. Ixiii. p. 647. 2 Ibid., t. Ixxxvi. p. 317.
3 Am. Jour, of St., vol. xiv. p. 89; Nature, vol. xvi. p. 364.
4 Month. Not., vol. xxxix. p. 440.
SPECTROSCOPIC WORK ON THE SUN. 255
of oxygen was no less apparent than before. Mr. Ranyard
remarked that, by this fourfold dispersion, the evidential value
of the eighteen observed coincidences was increased 4 18 , or (in
round numbers) 68,719 million times ; but the rigid numerical
test of probability does not in this case carry its full weight
of conviction. The discrimination of bright lines from a
very slightly less lucid background must, it is plain, be always
a matter of much delicacy and some uncertainty, especially
when the lines to be discriminated are not sharp, but more
or less blurred and widened. Nevertheless the correspond-
ences in Dr. Draper's photographs are far too striking to be
overlooked, and afford strong ground for accepting his con-
clusion (recommended, besides, by our innate tendency to
complete an analogy) that the most widely prevalent superficial
constituent of the earth is not missing from the sun.
The peculiarity of its showing bright, instead of dark lines
may be said to have given a new turn to the spectrum analysis
of the heavenly bodies. It illustrates the endless variety in
nature's modes of proceeding, and accentuates the danger of
negative inferences. That a substance displays none of its
distinctive beams in the spectrum of the sun or of a star, no
longer affords even a presumption against its presence there.
For it may be situated below the level where absorption occurs,
or under a pressure such as to efface lines by continuous lustre ;
it may be at a temperature so high that it gives out more light
than it takes up, and yet its incandescence may be masked by
the absorption of other bodies ; finally, it may just balance
absorption by emission, with the result of complete spectral
neutrality. An instructive example is that of helium, the
enigmatical chromospheric element. Father Secchi remarked
in 1868 1 that there is no dark line in the solar spectrum
matching its light ; and the faint traces of 03 absorption since
detected would probably never have been observed, had not
the substance producing them been otherwise known to exist.
Indications are not altogether wanting as to the cause of the
1 Comptes Rendus, t. Ixvii. p. 1123.
256 HISTORY OF ASTRONOMY.
sun's oxygen attesting its presence as it does. The inner
organisation of the oxygen molecule is a considerably plastic
one. It is readily modified by heat, and these modifications
are reflected in its varying modes of radiating light. Dr.
Schuster enumerated in 1879 x four distinct oxygen spectra,
corresponding to various stages of temperature, or phases of
electrical excitement; and a fifth has been added by M.
Egoroff's discovery in 1 883 2 that certain well-known groups of
dark lines in the red end of the solar spectrum (Fraunhofer's
A and B) are due to absorption by the cool oxygen of our air.
Now, of these five different systems of luminous emission,
three are, in all probability, represented one, as just stated,
through terrestrial, the others through solar action in analysed
sunlight. The brilliant range of lines detected by Dr. Draper
belong to the maximum heat developed by high-tension elec-
tricity. The oxygen producing it certainly lies at a low level
in the sun, since its lines never appear in the spectrum of the
chromosphere ; and we may conclude that it forms part of the
hottest layers of which we receive the radiations. The next, or
" compound-line spectrum," produced at a considerably lower
stage of thermal excitement, Dr. Schuster has found, with
evidence " little short of absolute certainty," to be dark in the
sun. 3 And here (as he pointed out) some prospect seems to
open of meeting with a definite criterion of the solar tem-
perature. For evidently the degree of heat (whatever that may
be) at which spectrum No. i changes to spectrum No. 2
occurs somewhere between the stratum giving Draper's bright
lines and the stratum giving Schuster's dusky lines. This
brings us to the subject of the next chapter.
1 Phil. Trans., vol. clxx. p. 46. 2 Comptes Rendus, t. xcvii. p. 555.
3 Nature, vol. xvii. p. 148. A
CHAPTER V.
TEMPERATURE OF THE SUN.
NEWTON was the first who attempted to measure the quantity
of heat received by the earth from the sun. His object in
making the experiment was to ascertain the temperature en-
countered by the comet of 1680 at its passage through peri-
helion. He found it, by multiplying the observed heating
effects of direct sunshine according to the familiar rule of the
"inverse squares of the distances," to be about 2000 times
that of red-hot iron. 1
Determinations of the sun's thermal power made with some
scientific exactness, date, however, from 1837. A few days
previous to the beginning of that year, Herschel began ob-
serving at the Cape of Good Hope with an " actinometer,"
and obtained results agreeing quite satisfactorily with those
derived by Pouillet from experiments made in France some
months later with a " pyrheliometer." 2 Pouillet found that
the vertical rays of the sun falling on each square centimetre
of the earth's surface are competent (apart from atmospheric
absorption) to raise the temperature of 1.7633 grammes of
water one degree centigrade per minute. This number (1.7633)
he called the " solar constant ; " and the unit of heat chosen is
known as the "calorie." Hence it was computed that the
total amount of solar heat received during a year would suffice
to melt a layer of ice covering the entire earth to a depth of
30.89 metres, or 100 feet ; while the heat emitted would melt,
1 Principle p. 498 (ist ed.) 2 Comptes Rendus, t. vii. p. 24.
258 HISTORY OF ASTRONOMY.
at the sun's surface, a stratum 11.80 metres thick each minute.
A careful series of observations showed that nearly half the
heat incident upon our atmosphere is stopped in its passage
through it.
Herschel got somewhat larger figures, though he assigned
only a third as the spoil of the air. Taking a mean between
his own and Pouillet's, he calculated that the ordinary expen-
diture of the sun per minute would have power to melt a
cylinder of ice 184 feet in diameter, reaching from his surface
to that of a Centauri ; or, putting it otherwise, that an ice-rod
45.3 miles across, continually darted into the sun with the
velocity of light, would scarcely consume, in dissolving, the
thermal supplies now poured abroad into space. 1 It is nearly
certain that this estimate should be increased by about two-
thirds in order to bring it up to the truth.
Nothing would, at first sight, appear simpler than to pass
from a knowledge of solar emission a strictly measurable
quantity to a knowledge of the solar temperature ; this being
defined as the temperature to which a surface thickly coated
with lamp-black (that is, of standard radiating, power) should
be raised to enable it to send us, from the sun's distance, the
amount of heat actually received from the sun. Sir John
Herschel showed that heat-rays at the sun's surface must be
192,000 times as dense as when they reach the earth; but it
by no means follows that either the surface emitting, or a body
absorbing those heat -rays must be 192,000 times hotter than a
body exposed here to the full power of the sun. The reason
is, that the rate of emission consequently the rate of absorp-
tion, which is its correlative increases very much faster than
the temperature. In other words, a body radiates or cools at
a continually accelerated pace as it becomes more and more
intensely heated above its surroundings.
Newton, however, took it for granted that radiation and
temperature advance paripassu that you have only to ascertain
the quantity of heat received from, and the distance of a remote
1 Results of Astr. Observations ; p. 446.
TEMPERATURE OF THE SUN. 259
body in order to know how hot it is. 1 And this principle,
which is known as "Newton's Law" of cooling, has still a
limited number of adherents. Its validity was never questioned
until De la Roche pointed out, in i8i2, 2 that it was approxi-
mately true only over a low range of temperature ; and five
years later, Dulong and Petit generalised experimental results
into the rule, that while temperature grows by arithmetical,
radiation increases by geometrical progression. 3 Adopting
this formula, Pouillet derived from his observations on solar
heat a solar temperature of somewhere between 1461 and
1761 Cent. Now, the higher of these points which is nearly
that of melting platinum is undoubtedly surpassed at the
focus of certain burning-glasses which have been constructed
of such power as virtually to bring objects placed there within
a quarter of a million of miles of the photosphere. In the
rays thus concentrated, platinum and diamond become rapidly
vaporised, notwithstanding the great loss of heat by absorption,
first in passing through the air, and again in traversing the
lens. Pouillet's maximum is then manifestly too low, since it
involves the absurdity of supposing a radiating mass capable
of heating a distant body more than it is itself heated.
Less demonstrably, but scarcely less surely, Mr. J. J.
Waterston, who attacked the problem in 1860, erred in the
opposite direction. Working up, on Newton's principle,
data collected by himself in India and at Edinburgh, he got
for the "potential temperature" of the sun 12,880,000
Fahr., 4 equivalent to 7,156,093 Cent. The phrase potential
temperature (for which Violle substituted, in 1876, effective
temperature] was designed to express the accumulation in a
single surface, postulated for the sake of simplicity, of the
radiations not improbably received from a multitude of separate
1 "Est enim calor soils ut radiorum densitas, hoc est, reciproce ut
quadratum distantise locorum a sole." Principle p. 508 (3d ed. 1726).
2 Jour, de Physique, t. Ixxv. p. 215.
3 Ann. de CJiimie, t. vii. 1817, p. 365.
4 Phil. Mag., vol. xxiii. (4th ser. ), p. 505.
260 HISTORY OF ASTRONOMY.
solar layers reinforcing each other; and might thus (it was
explained) be considerably higher than the actual temperature
of any one stratum.
At Rome, in 1861, Father Secchi repeated Waterston's ex-
periments, and reaffirmed his conclusion ; x while Soret's
observations, made on the summit of Mont Blanc in i867, 2
furnished him with materials for a fresh and even higher
estimate of ten million degrees centigrade. 3 Yet from the
very same data, substituting Dulong and Petit's for Newton's
law, Vicaire deduced in 1872 a provisional solar temperature
of i398. 4 This is below that at which iron melts, and we
know that iron-vapour exists high up in the sun's atmosphere.
The matter was taken into consideration on the other side of
the Atlantic by Ericsson in 1871. He attempted to re-esta-
blish the shaken credit of Newton's principle, and arrived, by
its means, at a temperature of four million degrees of Fahren-
heit. 5 More recently, what he considers an " underrated com-
putation," based upon observation of the quantity of heat
received by his "sun motor," has given him three million
degrees. This, he rightly thinks, must be accepted, if it be
granted that the temperature produced by radiant heat is pro-
portional to its density, or inversely as its diffusion. 6 Could
this be granted, the question would be much simplified ; but
there is little doubt that the case is far otherwise when heat
becomes intensified.
In 1876 the sun's temperature was proposed as the subject
of a prize by the Paris Academy of Sciences ; but although
the essay of M. Jules Violle was crowned, the problem was
declared to remain unsolved. Violle (who adhered to Dulong
and Petit's formula) arrived at an effective temperature of 1500
C., but considered that it might actually reach 2500 C., owing
1 Nuovo amenta, t. xvi. p. 294. z Comptes Rendus, t. Ixv. p. 526.
3 The direct result of 5 million degrees was doubled in allowance for
absorption in the sun's own atmosphere. Comptes Rendus, t. Ixxiv. p. 26.
4 Ibid., p. 31. 5 Nature^ vols. iv. p. 204; v. p. 505.
6 Natiire^ vol. xxx. p. 467.
TEMPERATURE OF THE SUN. 261
to a probable inferiority in emissive power of the photospheric
clouds to the lamp-black standard. 1 Experiments made in
April and May 1881 giving a somewhat higher result, he raised
this figure to 3000 C. 2
Appraisements so outrageously discordant as those of Water-
ston, Secchi, and Ericsson on the one hand, and those of the
French savans on the other, served only to show that all were
based upon a vicious principle. Professor F. Rosetti, 3 accord-
ingly, of the Paduan University, at last perceived the necessity
for getting out of the groove of "laws " plainly in contradic-
tion with facts. The temperature, for instance, of the oxy-
hydrogen flame was fixed by Bunsen at 2800 C. an estimate
certainly not very far from the truth. But if the two systems
of measurement applied to the sun be used to determine the
heat of a solid body rendered incandescent in this flame, it
comes out, by Newton's mode of calculation, 45,000 C. ; by
Dulong and Petit's, 870 C. 4 Both, then, are justly discarded,
the first as convicted of exaggeration, the second of under-
valuation. The formula substituted by Rosetti was tested
successfully up to 2000 C. ; but since it is, like its pre-
decessors, a purely empirical rule, is guaranted by no principle,
and can, in consequence, not be trusted out of sight, it may,
like them, break down at still higher elevations. All that can
be said is that it gives the most plausible results. Radiation,
so far as it obeys this new prescription, increases as the square
of the absolute temperature that is, of the number of degrees
counted from the "absolute zero" of 273 C. Its employ-
ment gives for the sun's radiating surface an effective tempera-
ture of 20,380 C. (including a supposed loss of one-half in
the solar atmosphere) ; and when a probable deficiency in
emission (as compared with lamp-black) is set against a pro-
bable mutual reinforcement of superposed strata, Professor
1 Ann. de Chim., t. x. (5th sen), p. 361.
2 Comptes JRendus, t. xcvi. p. 254.
3 Phil. Mag., vol. viii. 1879, p. 324.
4 Ibid., p. 325.
262 HISTORY OF ASTRONOMY.
Rosetti thinks that " effective " may be taken as nearly equiva-
lent to " actual " temperature.
A new line of inquiry was struck out by Zollner in 1870.
Instead of tracking the solar radiations backwards with the
dubious guide of empirical formulae, he investigated their
intensity at their source. He showed l that, considering pro-
minences as simple effects of the escape of powerfully com-
pressed gases, it was possible, from the known mechanical laws
of heat and gaseous constitution, to deduce minimum values
for the temperatures prevailing in the area of their develop-
ment. These came out 27,700 C. for the strata lying imme-
diately above, and 68,400 C. for the strata lying immediately
below the photosphere, the former being regarded as the
region into which, and the latter as the region from which the
eruptions took place. In this calculation, no prominences
exceeding 50,000 miles (1.5') in height were included. But
in 1884, G. A. Hirn of Colmar, taking into account the enor-
mous velocities of projection observed in the interim, fixed two
million degrees centigrade as the lowest internal temperature
by which they could be accounted for; although of opinion
that the condensations, presumed to give origin to the photo-
spheric clouds, were incompatible with a higher external tem-
perature than 50,000 to 100,000 C. 2
This method of going straight to the sun itself, observing
what goes on there, and inferring conditions, has much to
recommend it ; but its profitable use demands knowledge we
are still very far from possessing. We are quite ignorant, for
instance, of the actual circumstances attending the birth of
the solar flames. The assumption that they are nothing but
phenomena of elasticity is a purely gratuitous one. Spectro-
scopic indications, again, give hope of eventually affording a
fixed point of comparison with terrestrial heat-sources ; but
their interpretation is still beset with uncertainties ; nor can,
indeed, the expression of transcendental temperatures in de-
grees of impossible thermometers be, at the best, other than
1 Astr. Nock., Nos. 1815-16. 2 L* Astronomic, Sept. 1884, p. 334.
TEMPERATURE OF THE SUN. 263
a futile attempt to convey notions respecting a state of things
altogether outside the range of our experience.
A more tangible, as well as a less disputable proof of solar
radiative intensity than any mere estimates of temperature,
was provided in some experiments made by Professor Langley
in iSyS. 1 Using means of unquestioned validity, he found
the sun's disc to radiate 87 times as much heat, and 5300 times
as much light as an equal area of metal in a Bessemer con-
verter after the air-blast had continued about twenty minutes.
The brilliancy of the incandescent steel, nevertheless, was so
blinding, that melted iron, flowing in a dazzling white-hot stream
into the crucible, showed " deep brown by comparison, pre-
senting a contrast like that of dark coffee poured into a white
cup." Its temperature was estimated (not quite securely, as
Young has pointed out) 2 at 1800 to 2000 C. ; and no allow-
ances were made, in computing relative intensities, for atmos-
pheric ravages on sunlight, for the extra impediments to its
passage presented by the smoke-laden air of Pittsburg, or for
the obliquity of its incidence. Thus a very large balance of
advantage lay on the side of the metal.
A further element of uncertainty in estimating the intrinsic
strength of the sun's rays has still to be considered. From
the time that his disc first began to be studied with the tele-
scope, it was perceived to be less brilliant near the edges.
Lucas Valerius of the Lyncean Academy seems to have been
the first to note this fact, which, strangely enough, was denied
by Galileo in a letter to Prince Cesi of January 25, i6i3. 3
Father Scheiner, however, fully admitted it, and devoted some
columns of his bulky tome to the attempt to find an appropriate
explanation. 4 In 1729, Bouguer measured, with much accur
racy, the amount of this darkening ; and from his data, Laplace,
adopting a principle of emission now known to be erroneous,
concluded that the sun loses eleven-twelfths of his light through
1 Jour, of Science, vol. i. (3d ser.), p. 653.
- The Sun, p. 269. 3 Op., t. vi. p. 198.
4 Rosa Ursina, lib. iv. p. 618.
264 HISTORY OF ASTRONOMY.
absorption in his own atmosphere. 1 The real existence of this
atmosphere, which is totally distinct from the beds of ignited
vapours producing the Fraunhofer lines, is not open to doubt,
although its nature is still a matter of conjecture. The sepa-
rate effects of its action on luminous, thermal, and chemical
rays were carefully studied by Father Secchi, who in iSyo, 2
inferred the total absorption to be -f^ of all radiations taken
together, and added the important observation that the light
from the limb is no longer white, but reddish-brown. Selective
absorption was thus seen to be at work ; and this could
evidently be studied to advantage only by taking the various
rays of the spectrum separately, and finding out how much
each had suffered in transmission.
This was done by H. C. Vogel in 1877.3 Using a polarising
photometer, he found that only 13 per cent, of the violet rays
escape at the edge of the solar disc, 16 of the blue and green,
25 of the yellow, and 30 per cent, of the red. Midway between
centre and limb, 88.7 of violet light and 96.7 of red penetrate
the absorbing envelope, the removal of which would leave the
sun's visible spectrum of just three times its present intensity
in the most, and once and a half times in the least refrangible
parts. The nucleus of a small spot was ascertained to be of
the same luminous intensity as a portion of the unbroken
surface about two and a half minutes from the limb. These
experiments having been made during a spot-minimum, when
there is reason to think that absorption is below its average
strength, Vogel suggested their repetition at a time of greater
activity.
Professor Langley went farther in the same direction.
Reliable determinations of the "energy" of the individual
spectral rays were, for the first time, rendered possible by
his invention of the "bolometer" in 1880.* This exquisitely
sensitive instrument affords the means of measuring heat,
1 Mec. CeL, liv. x. p. 323. 2 Le Soleil (ist ed.), p. 136.
3 Monatsber., Berlin, 1877, P IO 4-
4 Am. Jour, of Sc., vol. xxi. p. 187.
TEMPERATURE OF THE SUN. 265
not directly, like the thermopile, but in its effects upon the
conduction of electricity. It represents, in the phrase of
the inventor, the finger laid upon the throttle-valve of a
steam-engine. A minute force becomes the modulator of
a much greater force, and thus from imperceptible becomes
conspicuous. By locally raising the temperature of an incon-
ceivably fine strip of platinum serving as the conducting-
wire in a circuit, the flow of electricity is impeded at that
point, and the included galvanometer records a disturbance
of the electrical flow. Amounts of heat have, in this way, been
detected in less than ten seconds, which, expended during a
thousand years on the melting of a kilogramme of ice, would
leave a part of the work still undone.
The heat contained in the diffraction spectrum is, with equal
dispersions, barely one-tenth of that in the prismatic spectrum.
It had, accordingly, never previously been found possible to
measure it in detail that is, ray by ray. But it is only from
the diffraction, or normal spectrum that any true idea can be
gained as to the real distribution of energy amongst the various
constituents, visible and invisible, of a sunbeam. The effect
of passage through a prism is to crowd together the red rays
very much more than the blue. To this prismatic distortion
was owing the establishment of a pseudo-maximum of heat in
the infra-red, which disappeared when the natural arrangement
by wave-length was allowed free play. Professor Langley's
bolometer has shown that the hottest part of the normal
spectrum virtually coincides with its most luminous part, both
lying in the orange, close to the D line. 1 Thus the last shred
of evidence in favour of the threefold division of solar radia-
tions vanished, and it became obvious that the varying effects
thermal, luminous, or chemical produced by them are due,
not to any distinction of quality in themselves, but to the
different properties of the substances they impinge upon. They
are simply bearers of vis viva, conveyed in shorter or longer
1 For J. W. Draper's partial anticipation of this result, SZQ Am. Jour, of
Sc., vol. iv. 1872, p. 174.
266 HISTORY OF ASTRONOMY.
vibrations ; upon the capacity of the material particles meeting
them for taking up those shorter or longer vibrations, and
turning them variously to account in their inner economy,
depends the result in each separate case.
A long series of experiments at Allegheny was completed in
the summer of 1881 on the crest of Mount Whitney in the
Sierra Nevada. Here, at an elevation of 14,887 feet, in the
driest and purest air, perhaps, in the world, atmospheric absorp-
tive inroads become less sensible, and the indications of the
bolometer, consequently, surer and stronger. An enormous ex-
pansion was at once given to the invisible region in the solar
spectrum below the red. Captain Abney had got chemical
effects from undulations twelve ten-thousandths of a millimetre
in length. These- were the longest recognised as, or indeed
believed, on theoretical grounds, to be capable of existing.
Professor Langley now got heating effects from rays of above
twice that wave-length, his delicate thread of platinum groping
its way down to thirty ten-thousandths of a millimetre, or three
" microms." The known extent of the solar spectrum was thus
at once more than doubled. Its visible portion, covers a range
of about one octave ; bolometric indications comprise between
three and four. The great importance of the newly explored
region appears from the fact that three-fifths of the entire energy
of sunlight reside in the infra-red, while scarcely more than
one-hundredth part of that amount is found in the better known
ultra-violet space. 1
Atmospheric absorption had never before been studied with
such precision as it was by Professor Langley on Mount
Whitney. Aided by simultaneous observations from Lone Pine,
at the foot of the Sierra, he was able to calculate the intensity
belonging to each ray before entering the earth's gaseous
envelope in other words, to construct an extra-atmospheric
curve of energy in the spectrum. The result showed that the
blue end suffered far more than the red, absorption varying
1 Phil. Mag., vol. xiv. p. 179 (March 1883).
TEMPERATURE OF THE SUN. 267
inversely as wave-length. This property of stopping predomi-
nantly the quicker vibrations is shared, as both Vogel and
Langley l have conclusively shown, by the solar atmosphere.
The effect of this double absorption is as if two plates of
reddish glass were interposed betwen us and the sun, the with-
drawal of which would leave his orb, not only three or four
times more brilliant, but in colour of a distinct greenish-
blue, not very different from the tint of the second (F) line
of hydrogen. 2
The fact of the unveiled sun being blue has an important
bearing upon the question of his temperature, to afford a
somewhat more secure answer to which was the ultimate
object of Professor Langley's persevering researches ; for it
is well known that, as bodies grow hotter, the proportionate
representation in their spectra of the more refrangible rays
becomes greater. The lowest stage of incandescence is the
familiar one of red heat. As it gains intensity, the quicker
vibrations come in, and an optical balance of sensation is
established at white heat. The final term of blue heat, as
we now know, is attained by the photosphere. On this
ground alone, then, of the large original preponderance
of blue light, we must raise our estimate of solar heat :
and actual measurements show the same upward tendency.
Until quite lately, Pouillet's figure of 1.7 calories per minute
per square centimetre of terrestrial surface, was the received
value for the " solar constant." v Forbes had, it is true, got
2.85 from observations on the Faulhorn in i842; 3 but they
failed to obtain the confidence they merited. Pouillet's re-
sult was not definitively superseded until Violle, from actino-
metrical measures at the summit and base of Mont Blanc in
1875, computed the intensity of solar radiation at 2.54, 4 and
Crova, about the same time, at Montpellier, showed it to be
above two calories. 5 Langley went higher still. His pre-
1 Comptes Rendus, t. xcii. p. 701. 2 Nature, vol. xxvi. p. 589.
3 Phil. Trans. , vol. cxxxii. p.'_273. 4 Ann. de Chirn., t. x. p. 321.
5 Ibid., t. xi. p. 505.
268 HISTORY OF ASTRONOMY.
liminary estimate, December 30, 1882, agreed with Forbes's ;
and his definitive one, when the results of the Mount Whitney
expedition are fully worked out, is likely to fall scarcely short
of three calories, as the amount of heat reaching the outskirts
of our atmosphere. 1 Thus, modern inquiries, though they
give no signs of agreement, within any tolerable limits of error,
as to the probable temperature of the sun, tend, with growing
certainty, to render more and more evident the vastness of the
thermal stores contained in the great central reservoir of our
system.
1 rhil. Mag., vol. xiv. p. 181.
CHAPTER VI.
THE SUN'S DISTANCE.
THE question of the sun's distance arises naturally from the
consideration of his temperature, since the intensity of the
radiations emitted as compared with those received and
measured, depends upon it. But the knowledge of that
distance has a value quite apart from its connection with
solar physics. The semi-diameter of the earth's orbit is our
standard measure for the universe. It is the great fundamental
datum of astronomy the unit of space, any error in the
estimation of which is multiplied and repeated in a thousand
different ways, both in the planetary and sidereal systems.
Hence its determination has been called by Airy " the noblest
problem in astronomy." It is also one of the most difficult.
The quantities dealt with are so minute that their sure grasp
tasks all the resources of modern science. An observational
inaccuracy which would set the moon nearer to, or farther
from us than she really is by one hundred miles, would vitiate
an estimat..- of the sun's distance to the extent of sixteen
million ! 1 What is needed in order to attain knowledge of
the desired exactness is no less than this : to measure an angle
about equal to that subtended by a halfpenny 2000 feet from
the eye, within a little more than a thousandth part of its
v> lue.
The angle thus represented is what is called the " horizontal
parallax" of the sun. By this amount the breadth of a half
penny at 2000 feet he is, to a spectator on the rotating earth.
removed at rising and setting from his meridian place in the
1 Airy, Month. Not., vol. xvii. p. 210.
270 HISTORY OF ASTRONOMY.
heavens. Such, in other terms, would be the magnitude of
the terrestrial radius as viewed from the sun. If we knew this
magnitude with certainty and precision, we should also know
with certainty and precision the dimensions of the earth
being, as they are, well ascertained the distance of the sun.
In fact, the one quantity commonly stands for the other in
works treating professedly of astronomy. But this angle of
parallax or apparent displacement cannot be directly measured
cannot even be perceived with the finest instruments. Not
from its smallness. The parallactic shift of the nearest of the
stars as seen from opposite sides of the earth's orbit, is many
times smaller. But at the sun's limb, and close to the horizon,
where the visual angle in question opens out to its full extent,
atmospheric troubles become overwhelming, and altogether
swamp the far more minute effects of parallax.
There remain indirect methods. Astronomers are well
acquainted with the proportions which the various planetary
orbits bear to each other. They are so connected, in the
manner expressed by Kepler's Third Law, that the periods
being known, it only needs to find the interval between any
two of them in order to infer at once the distances separating
them all from one another and from the sun. The plan is
given ; what we want to discover is the scale upon which it is
drawn ; so that, if we can get a reliable measure of the distance
of a single planet from the earth, our problem is solved.
Now some of our fellow-travellers in our unending journey
round the sun, come at times well within the scope of celestial
trigonometry. The orbit of Mars lies at one point not more
than thirty-five million miles outside that of the earth, and
when the two bodies happen to arrive together in or near the
favourable spot a conjuncture which recurs every fifteen years
the desired opportunity is granted. Mars is then "in op-
position," or on the opposite side of us from the sun, crossing
the meridian consequently at midnight. 1 It was from an
opposition of Mars, observed in 1672 by Richer at Cayenne in
1 Mars comes into opposition once in about 780 days ; but owing to the
THE SUN'S DISTANCE. 271
concert with Cassini in Paris, that the first scientific estimate
of the sun's distance was derived. It appeared to be nearly
eighty-seven millions of miles (parallax 9.5"); while Flamsteed
deduced 81,700,000 (parallax 10") from his independent ob-
servations of the same occurrence a difference quite insignifi-
cant at that stage of the inquiry. But Picard's result was just
half Flamsteed' s (parallax 20" ; distance forty-one million
miles) ; and Lahire considered that we must be separated from
the hearth of our system by an interval of at least 136 million
miles. 1 So that uncertainty continued to be on a gigantic
scale.
Venus, on the other hand, comes closest to the earth when
she passes between it and the sun. At such times of " inferior
conjunction " she is, however, still twenty-six million miles, or
(in round numbers) 109 times as distant as the moon. More-
over, she is so immersed in the sun's rays that it is only when
her path lies across his disc that the requisite facilities for
measurement are afforded. (These " partial eclipses of the sun
by Venus" (as Encke terms them) are coupled together in
pairs, 2 of which the components are separated by eight years,
recurring at intervals alternately of 105^ and 121^ years,
^hus, the first calculated transit took place in December 1631,
and its companion (observed by Horrocks) in the same month
(N.S.) 1639. Then, after the lapse of 121 J years, came the
eccentricity of both orbits, his distance from the earth at those epochs
varies from thirty-five to sixty-two million miles.
1 J. D. Cassini, Hist. Abrtgee de la Parallaxe du Soleil, p. 122, 1772.
2 The present period of coupled eccentric transits will, in the course of
ages, be succeeded by a period of single, nearly central transits. The
alignments by which transits are produced, of the earth, Venus, and the sun,
close to the place of intersection of the two planetary orbits, now occur, the
first a little in front of, the second, after eight years less two and a half days,
a little behind the node. But when the first of these two meetings takes place
very near the node, giving a nearly central transit, the second falls too far
from it, and the planet escapes projection on the sun. The reason of the
liability to an eight -yearly recurrence is that eight revolutions of the earth
are accomplished in only a very little more time than thirteen revolutions
of Venus.
V
272 HISTORY OF ASTRONOMY.
June couple of 1761 and 1769 ; and again, after 105 J, the two
recently observed December 8, 1874, and December 6, 1882.
Throughout the twentieth century there will be no transit of
Venus ; but the astronomers of the twenty-first will only have
to wait four years for the first of a June pair. The rarity of
these events is due to the fact that the orbits of the earth and
Venus do not lie in the same plane. If they did, there would
be a transit each time that our twin-planet overtakes us in her
more rapid circling that is, on an average, every 584 days.
As things are actually arranged, she passes above or below the
sun, except when she happens to be very near the line of inter-
section of the two tracks.
Such an occurrence as a transit of Venus seems, at first
sight, full of promise for solving the problem of the sun's
distance. For nothing would appear easier than to determine
exactly either the duration of the passage of a small, dark orb
across a large brilliant disc, or the instant of its entry upon or
exit from it.; And the differences in these times (which, owing
to the comparative nearness of Venus, are quite considerable),
as observed from remote parts of the earth, can be translated
into differences of space that is, into apparent or parallactic dis-
placements, whereby the distance of Venus becomes known,
and thence, by a simple sum in proportion, the distance of the
sun. But in that word " exactly " what snares and pitfalls lie
hid ! It is so easy to think and to say ; so indefinitely hard to
realise. The astronomers of the eighteenth century were full
of hope and zeal. They confidently expected to attain, through
the double opportunity offered them, to something like a
permanent settlement of the statistics of our system. They
were grievously disappointed. The uncertainty as to the sun's
distance, which they had counted upon reducing to a few
hundred thousand miles, remained at many millions.
In 1822, however, Encke, then director of the Seeberg
Observatory near Gotha, undertook to bring order out of
the confusion of discordant, and discordantly interpreted
observations. His combined result for both transits (1761
THE SUN'S DISTANCE. 273
and 1769) was published in I824, 1 and met universal acquies-'
cence. The parallax of the sun thereby established was
8.5776", corresponding to a mean distance 2 of 95^ million
miles. Yet this abolition of doubt was far from being so
satisfactory as it seemed. Serenity on the point lasted exactly
thirty years. It was disturbed in 1854 by Hansen's announce-
ment 3 that the observed motions of the moon could be drawn
into accord with theory only on the terms of bringing the sun
considerably nearer to us than he was supposed to be.
Dr. Matthew Stewart, professor of mathematics in the Uni-
versity of Edinburgh, had made a futile attempt in 1763 to
deduce the sun's distance from his disturbing power over our
satellite. 4 ) Tobias Mayer of Gottingen, however, whose short
career was so fruitful of suggestions, struck out the right way
to the same end ; and Laplace, in the seventh book of the
Mecanique Celeste? gave a solar parallax derived from the lunar
" parallactic inequality " substantially identical with that issuing
from Encke's subsequent discussion of the eighteenth-century
transits. Thus two wholly independent methods the trigo-
nometrical, or method by survey, and the gravitational, or
method by perturbation seemed to corroborate each the
upshot of the use of the other until the nineteenth century was
well past its meridian. It is singular how often errors con-
spire to lead conviction astray.
Hansen's note of alarm in 1854 was echoed by Leverrier in
He found that an apparent monthly oscillation of the
1 Die Enffernung der Sonne : Fortsetzung, p. 108. Encke slightly cor-
rected his result of 1824 in Berlin Abh., 1835, p. 295.
2 Owing to the ellipticity of its orbit, the earth is nearer to the sun in
January than in June by 3,100,000 miles. The quantity to be determined,
or " mean distance," is that lying midway between these extremes is, in
other words, half the major axis of the ellipse in which the earth travels.
3 Month. Not., vol. xv. p. 9.
4 The Distance of the Sun from the Earth determined by the Theory of
Gravity, Edinburgh, 1763. 5 Opera, t. iii. p. 326.
6 Comptes Rendus, t. xlvi. p. 882. The parallax 8.95" derived by
Leverrier from the " parallactic inequality " in the earth's motion, was
corrected by Stone to 8.91". Month. Not., vol. xxviii. p. 25.
S
274 HISTORY OF ASTRONOMY.
sun which reflects a real monthly movement of the earth round
its common centre of gravity with the moon, and which
depends for its amount solely on the mass of the moon and
the distance of the sun, required a diminution in the admitted
value of that distance by fully four million miles. Three years
later he pointed out that certain perplexing discrepancies
between the observed and computed places both of Venus and
Mars, would vanish on the adoption of a similar measure. 1
Moreover, a favourable opposition of Mars gave the oppor-
tunity in 1862 for fresh observations, which, separately worked
out by Stone and Winnecke, agreed with all the newer investi-
gations in fixing the great unit at slightly over 91 million miles.
In Newcomb's hands they gave 92 J million. O The accumulat-
ing evidence in favour of a large reduction in the sun's dis-
tance was just then reinforced by an auxiliary result of a totally
different and unexpected kind.
The discovery that light does not travel instantaneously from
point to point, but takes some short time in transmission, was
made by Olaus Romer in 1675, through observing that the
eclipses of Jupiter's satellites invariably occurred later, by a
considerable interval, when the earth was on the far side, than
when it was on the near side of its orbit. [ Half this interval,
or the time spent by a luminous vibration in crossing the " mean
radius " of the earth's orbit, is called the " light-equation ; " and
the determination of its precise value has claimed the minute
care distinctive of modern astronomy. Delambre in 1792
made it 493.2 seconds. Glasenapp, a Russian astronomer,
raised the estimate in 1874 to 500.84 seconds and this, from
the extreme care employed, can hardly, at the outside, be
more than a couple of seconds astray. Hence, if we had any
independent means of ascertaining how fast light travels, we
could tell at once how far off the sun is.
There is yet another way by which knowledge of the swiftness
of light would lead us straight to the goal. The heavenly
1 Month. A 7 ot., vol. xxxv. p. 156.
2 Wash. Obs., 1865, App. ii. p. 28.
THE SUN'S DISTANCE. 275
bodies are perceived, when carefully watched and measured, to
be pushed forwards out of their true places, in the direction of
the earth's motion, by a very minute quantity. This effect
(already adverted to) has been known since Bradley's time as
"aberration." It arises from a combination of the two move-
ments of the earth round the sun and of the light-waves through
the ether. If the earth stood still, or if light spent no time on the
road from the stars, such an effect would not exist. Its amount
represents the proportion between the velocities with which
the earth and the light rays pursue their respective journeys.
This proportion is, roughly, one to ten thousand. So that
here again, if we knew the rate per second of luminous trans-
mission, we should also know the rate per second of the earth's
movement, consequently the size of its orbit and the distance
of the sun.
But, until lately, instead of finding the distance of the sun
from the velocity of light, there has been no means of ascer-
taining the velocity of light except through the imperfect know-
ledge possessed as to the distance of the sun. The first suc-
cessful terrestrial experiments on the point date from 1849;
and it is certainly no slight triumph of human ingenuity to have
taken rigorous account of the delay of a sunbeam in flashing
from one mirror to another. Fizeau led the way, 1 and he was
succeeded, after a few months, by Leon Foucault, 2 who, in
1862, had so far perfected Wheatstone's method of revolving
mirrors, as to be able to announce with authority that light
travelled slower, and that the sun was in consequence nearer,
than had been supposed. 3 Thus a third line of separate
research was found to converge to the same point with the two
others.
1 Comptes Kendus, t. xxix. p. 90.
2 Ibid., t. xxx. p. 551.
3 Ibid., t. Iv. p. 501. The previously admitted velocity was 308 million
metres per second ; Foucault reduced it to 298 million. Combined with
Struve's "constant of aberration " this gave 8.86" for the solar parallax,
which exactly agreed with Cornu's result from a repetition of Fizeau's
experiments in 1872. Comptes Rendus, t. Ixxvi. p. 338.
276 HISTORY OF ASTRONOMY.
Such a conspiracy of proof was not to be resisted, and at
the anniversary meeting of the Royal Astronomical Society
in February 1864, the correction of the solar distance took the
foremost place in the annals of the year. Lest, however, a
sudden bound of four million miles nearer to the centre of
our system should shake public faith in astronomical accuracy,
it was explained that the change in the solar parallax corre-
sponding to that huge leap, amounted to no more than the
breadth of a human hair 125 feet from the eye I 1 From 1866
the improved value of 8.90" was adopted in the Nautical Al-
manac, while Newcomb's result of 8.85" has appeared since
1869 in the Berlin Ephemeris. In astronomical literature the
change was initiated by Sir Edmund Beckett in the first edi-
tion (1865) of his Astronomy without Mathematics.
If any doubt remained as to the misleading character of
Encke's deduction, so long implicitly trusted in, it was removed
by Powalky's and Stone's rediscussions, in 1864 and 1868
respectively, of the transit observations of 1769. Using im-
proved determinations of the longitude of the various stations,
and a selective judgment in dealing with their materials, which,
however indispensable, did not escape adverse criticism, they
brought out results confirmatory of the no longer disputed
necessity for largely increasing the solar parallax, and propor-
tionately diminishing the solar distance.
Conclusions on the subject, however, were still regarded as
purely provisional. A transit of Venus was fast approaching,
and to its arbitrament, as to that of a court of final appeal, the
pending question was to be referred. It is true that the verdict
in. the same case by the same tribunal a century earlier had
proved of so indistinct a character as to form only a starting-
point for fresh litigation ; but that century had not passed in
vain, and it was confidently anticipated that observational
difficulties, then equally unexpected and insuperable, would
yield to the elaborate care and skill of forewarned modern
preparation.
1 Month. Aot., vol. xxiv. p. 103.
THE SUN'S DISTANCE. 277
The conditions of the transit of December 8, 1874, were
sketched out by the then Astronomer-Royal (Sir George Airy)
in 185 7, 1 and formed the subject of eager discussions in this
and other countries down to the very eve of the occurrence.
In these Mr. Proctor took a leading part, supplying official
omissions, and working out, with geometrical accuracy, the
details of the relations between the different parts of the earth
and Venus's shadow-cone ; and it was due to his urgent repre-
sentations that provision was made for the employment of the
method identified with the name of Halley, 2 which had been too
hastily assumed inapplicable to the first of each transit-pair. It
depends upon the difference in the length of time taken by the
planet to cross the sun's disc, as seen from various points of the
terrestrial surface, and requires, accordingly, the visibility of
both entrance and exit at the same station. Since these were,
in 1874, about three and a half hours, and are often much
farther apart, the choice of posts for the successful use of the
" method of durations " is a matter of some difficulty.
The system described by Delisle in 1760, on the other hand,
involves merely noting the instant of ingress or egress (accord-
ing to situation) from opposite extremities of a terrestrial dia-
meter ; the disparity in time giving a measure of the planet's
apparent displacement, hence of its actual rate of travel in
miles per minute, from which its distances severally from earth
and sun are immediately deducible. Its chief attendant
difficulty is the necessity for accurately fixing the longitudes of
the points of observation. This, however, was much more
sensibly felt a century ago than it is now, and the improved
facility and certainty of modern determinations have tended to
give the Delislean plan a decided superiority over its rival.
These two traditional methods were supplemented in 1874
by the camera and the heliometer. From photography, above
all, much was expected. Observations made by its means
1 Month. Not., vol. xvii. p. 208.
2 Because closely similar to that proposed by him in Phil. Trans, for
1716.
278 HISTORY OF ASTRONOMY.
would have the advantages of impartiality, multitude, and
permanence. Peculiarities of vision and bias of judgment
would be eliminated; the slow progress of the phenomenon
would permit an indefinite number of pictures to be taken,
their epochs fixed to a fraction of a second ; while subsequent
leisurely comparison and measurement could hardly fail, it was
thought, to educe approximate truth from the mass of accumu-
lated evidence. The use of the heliometer (much relied on by
German observers) was so far similar to that of the camera
that the object aimed at by both was the determination of the
relative positions of the centres of the sun and Venus viewed,
at the same absolute instant, from opposite sides of the globe.
So that the two older methods seek to ascertain the exact times
of meeting between the solar and planetary limbs ; while the two
modern methods" work by measurement of the position of the
dark body already thrown into complete relief by its shining
background. The former are " methods by contact," the latter
"methods by projection."
Every country which had a reputation to keep or to gain for
scientific zeal was forward to co-operate in the great cosmo-
politan enterprise of the transit. France and' Germany each
sent out six expeditions ; twenty-six stations were in Russian,
twelve in English, eight in American, three in Italian, one
(equipped with especial care) in Dutch occupation. In all, at
a cost of nearly a quarter of a million, some fourscore dis-
tinct posts of observation were provided ; amongst them such
inhospitable, and all but inaccessible rocks in the bleak
Southern Ocean, as St. Paul's and Campbell Islands, swept by
hurricanes, and fitted only for the habitation of seabirds, where
the daring votaries of science, in the wise prevision of a long
leaguer by the elements, were supplied with stores for many
months, or even a whole year. Siberia and the Sandwich
Islands were thickly beset with observers ; parties of three
nationalities encamped within the mists of Kerguelen Island,
expressively termed the " Land of Desolation," in the sanguine,
though not wholly frustrated hope of a glimpse of the sun at
THE SUN'S DISTANCE. 279
the right moment. M. Janssen narrowly escaped destruction
from a typhoon in the China seas on his way to Nagasaki ;
Lord Lindsay (now Earl of Crawford and Balcarres) equipped,
at his private expense, an expedition to the Mauritius, which
was in itself an epitome of modern resource and ingenuity.
During several years the practical methods best suited to
ensure success for the impending enterprise, formed a subject
of " European debate." Official commissions were appointed
to receive and decide upon evidence ; and experiments were
in progress for the purpose of defining the actual circumstances
of the contacts, the accurate determination of which constituted
the only tried, though by no means an assuredly safe road to
the end in view. In England, America, France, and Germany,
artificial transits were mounted, and the members of the various
expeditions were carefully trained to unanimity in estimating
the phases of junction and separation between a moving dark
circular body and a broad illuminated disc. In the last cen-
tury, a formidable and prevalent phenomenon had swamped
all pretensions to rigid accuracy. This was an effect analogous
to " Baily's Beads," which acquired notoriety as the " Black
Drop " or " Black Ligament." It may be described as substi-
tuting adhesion for contact, the limbs of the sun and planet,
instead of meeting and parting with the desirable clean definite-
ness, dinging together as if made of some glutinous material,
and prolonging their connection by means of a dark band or
dark threads stretched between them. Some astronomers
ascribe this baffling appearance entirely to instrumental im-
perfections ; others to atmospheric agitation ; others again to
the optical encroachment of light upon darkness known as
" irradiation." It is probable that all these causes conspire, in
various measure, to produce it ; and it is certain that by suit-
able precautions, combined with skill in the observer and a
reasonably tranquil air, its conspicuous appearance may, in most
cases, be obviated.
The organisation of the British forces reflected the utmost
credit on the energy and ability of Lieutenant-Colonel Tupman,
280 HISTORY OF ASTRONOMY.
of the Royal Marine Artillery, who was responsible for the
whole. No useful measure was neglected. Each observer
went out ticketed with his "personal equation," his senses
drilled into a species of martial discipline, his powers absorbed,
so far as possible, in the action of a cosmopolitan observing
machine. Instrumental uniformity and uniformity of method
were attainable, and were attained ; but diversity of judgment
unhappily survived the best-directed efforts for its extirpation.
The eventful day had no sooner passed than telegrams
began to pour in, announcing an outcome of considerable,
though not unqualified success. The weather had proved
generally favourable ; all the manifold arrangements had (save
for some casual mishaps) worked well.; contacts had been
plentifully observed ; photographs in lavish abundance had
been secured ; a store of materials, in short, had been laid up,
of which it would take years to work out the full results by
calculation. Gradually, however, it came to be known that
the hope of a definitive issue must be abandoned. Unanimity
was found to be as remote as ever. The dreaded "black
ligament " gave, indeed, less trouble than was expected ; but
another appearance supervened which took most observers by
surprise. This was the illumination due to the atmosphere of
Venus. Astronomers, it is true, were not ignorant that the
planet had, on previous occasions, been seen girdled with a
lucid ring ; but its power to mar observations by the distorting
effect of refraction had scarcely been reckoned with. It
proved, however, to be very great. Such was the difficulty of
determining the critical instant of internal contact, that (in
Colonel Tupman's words) " observers side by side, with ade-
quate optical means, differed as much as twenty or thirty
seconds in the times they recorded for phenomena which they
have described in almost identical language." l
Such uncertainties in the data admitted of a corresponding
variety in the results. From the British observations of ingress
and egress Sir George Airy 2 derived, in 1877, a solar parallax
1 Month. Not., vol. xxxviii. p. 447. 2 Ibid., p. 1 1.
THE SUN'S DISTANCE. 281
of 8.76" (corrected to 8.754"), indicating a mean distance of
93,375,000 miles. Mr. Stone obtained a value of ninety-two
millions (parallax 8.88"), and held any parallax less than 8. 84"
or more than 8.93" to be " absolutely negatived " by the docu-
ments available. 1 Yet, from the same, Colonel Tupman de-
duced 8.8i", 2 implying a distance 700,000 miles greater than
Stone had obtained. The French observations of contacts
gave (the best being selected) a parallax of about 8.88" ; French
micrometric measures the obviously exaggerated one of 9-05". 3
Photography, as practised by most of the European parties,
was a total failure. Utterly discrepant values of the micro-
scopic displacements designed to serve as sounding-lines for
the solar system, issued from attempts to measure even the
most promising pictures. " You might as well try to measure
the zodiacal light," it was remarked to Sir George Airy.
Those taken on the American plan (adopted by Lord Lindsay),
of using telescopes of so great focal length as to afford, without
further enlargement, an image of the requisite size, gave notably
better results. From an elaborate comparison of these (some
dating from Vladivostock, Nagasaki, and Pekin, others from
Kerguelen and Chatham Islands), Mr. D. P. Todd, of the
American Nautical Almanac, deduced a solar distance of about
ninety-two million miles (parallax 8.883" + 0.034"), 4 a value, as
Mr. Stone has pointed out, favoured by a considerable accumu-
lation of independent evidence.
On the whole, estimates of the great spatial unit cannot be
said to have gained any security from the combined effort of
1874. A few months before the transit, Mr. Proctor considered
that the uncertainty then amounted to 1,448,000 miles ; 5 five
years after the transit, Professor Harkness judged it to be still
T >575>95 miles; 6 yet it had been hoped that it would have
been brought down to 100,000. As regards the end for which
1 Month. Not., vol. xxxviii. p. 294. 2 Ibid., p. 334.
3 Comptes Rendus, t. xcii. p. 812. 4 Observatory, No. 51, p. 205.
5 Transits of Venus, p. 89 ( 1st ed.)
6 Am. four, of Sc., vol. xx. p. 393.
282 HISTORY OF ASTRONOMY.
it had been undertaken, the grand campaign had come to
nothing. Nevertheless, no sign of discouragement was apparent.
There was a change of view, but no relaxation of purpose.
The problem, it was seen, could be solved by no single heroic
effort, but by the patient approximation of gradual improve-
ments. Astronomers, accordingly, looked round for fresh
means or more refined expedients for applying those already
known. A new phase of exertion was entered upon.
On September 5, 1877, Mars came into opposition hear the
part of his orbit which lies nearest to that of the earth, and Dr.
Gill (now Her Majesty's Astronomer at the Cape of Good
Hope) took advantage of the circumstance to appeal once more
to him for a decision on the qu&stio vexata of the sun's distance.
He chose, as the scene of his labours, the Island of Ascension,
and for their plan a method recommended by Airy in 1857^
but never before fairly tried. This is known as the "diurnal
method of parallaxes." Its principle consists in substituting
successive morning and evening observations from the same
spot, for simultaneous observations from remote spots, the
rotation of the earth supplying the necessary difference in the
points of view. Its great advantage is that of unity in per-
formance. A single mind, looking through the same pair of
eyes, reinforced with the same optical appliances, is employed
throughout, and the errors inseparable from the combination
of data collected under different conditions are avoided.
There are many cases in which one man can do the work of
two better than two men can do the work of one. The result
of Dr. Gill's skilful determinations (made with Lord Lindsay's
heliometer) was a solar parallax of 8.78", corresponding to a
distance of 93,080,000 miles. 2 The bestowal of the Royal
Astronomical Society's gold medal stamped the merit of this
distinguished service.
But there are other subjects for this kind of inquiry besides
Mars and Venus. ' Professor Galle of Breslau suggested in
1 Month. Not, vol. xvii. p. 219.
2 Mem. Roy. Astr. Soc., vol. xlvi. p. 163.
THE SUN'S DISTANCE. 283
1872 1 that some of the minor planets might be got to repay
astronomers for much disinterested toil spent in unravelling
their motions, by lending aid to their efforts towards a correct
celestial survey. Ten or twelve come near enough, and are
bright enough for the purpose; and, in fact, the absence of
sensible magnitude is one of their chief recommendations,
since a point of light offers far greater facilities for exact
measurement than a disc. The first attempt to work this new
vein was made at the opposition of Phocaea in 1872; and
from observations of Flora in the following year at twelve
observatories in the northern and southern hemispheres, Galle
deduced a solar parallax of S.S;". 2 At the Mauritius in 1874,
Lord Lindsay and Dr. Gill applied the " diurnal method " to
Juno, then conveniently situated for the purpose; and the
continued use of similar occasions affords, in the opinion of
the latter, the best available means for improving knowledge
of the sun's distance. They frequently recur ; they need no
elaborate preparation ; a single astronomer armed with a
heliometer can do all the requisite work. The recommenda-
tion of Dr. Gill was accordingly acted upon in 1882, when
favourable oppositions of both Victoria and Sappho took
place ; and it is probable that each future event of the kind
will be made to serve as a step towards the desired level of
accuracy.
The second of the nineteenth-century pair of Venus-transits
was looked forward to with much abated enthusiasm. Russia
refused her active co-operation in observing it, on the ground
that oppositions of the minor planets were trigonometrically
more useful, and financially far less costly ; and her example
was followed by Austria, while Italian astronomers limited
their sphere of action to their own peninsula. Nevertheless, it
was generally held that a phenomenon which the world could
not again witness until it was four generations older should, at
the price of any effort, not be allowed to pass in neglect.
1 Astr. Nach., No. 1897.
2 Hilfiker, Bern Mittheilungen, 1878, p. 109.
284 HISTORY OF ASTRONOMY.
An International Conference, accordingly, met at Paris in
1 88 1 with a view to concerting a plan of operations. America,
however, preferring independent action, sent no representative ;
and the European break-down of photography in recording
transit-phases was admitted by its official abandonment. It
was decided to give Delisle's method another trial; and the
ambiguities attending and marring its use were sought to be
obviated by careful regulations for ensuring agreement in the
estimation of the critical moments of ingress and egress. 1 But,
in fact (as M. Puiseux had shown 2 ), contacts between the
limbs of the sun and planet, so far from possessing the geo-
metrical simplicity long attributed to them, are really made up
of a prolonged succession of various and varying phases, im-
possible either to predict or identify with anything like rigid
exactitude. Dr. Ball compares the task of determining the pre-
cise instant of their meeting or parting, to that of telling the
hour with accuracy on a watch without a minute-hand ; and
the comparison is admittedly inadequate. For not only is
the apparent movement of Venus across the sun extremely
slow, being but the excess of her real motion over that of the
earth ; but three distinct atmospheres the solar, terrestrial,
and cytherean combine to deform outlines and mask the
geometrical relations which it is desired to connect with a strict
count of time.
The result was very much what had been expected. The
arrangements were excellent, and were only in a few cases
disconcerted by bad weather. The British parties, under the
experienced guidance of Mr. Stone, the RadclirTe observer,
took up positions scattered (not at random) over the globe
from Queensland to Bermuda, and accumulated an ample
supply of skilful observations ; the Americans gathered in a
whole library of photographs, amongst them a fine series taken
at the new Lick Observatory on Mount Hamilton : the Germans
and Belgians trusted to the heliometer ; the French used the
1 Comptes Rendus, t. xciii. p. 569. 2 lbid. t t. xcii. p. 481.
THE SUN'S DISTANCE. 285
camera as an adjunct to the method by contacts. Yet little or
no approach was made to solving the problem. The range of
doubt as to the sun's distance remained as wide as before.
The value published by M. Houzeau, late director of the
Brussels Observatory, in I884, 1 forcibly illustrates this un-
welcome conclusion. From 606 measures of Venus on the
sun, taken with a new kind of heliometer at St. Jago in
Chili, he derives a solar parallax of 8.9 n", and a distance of
91,727,000 miles. But the "probable error" of this determi-
nation amounts to 0^084" either way ; that is, it is subject to
a "more or less" of 900,000 miles, or to a total uncertainty
of 1,800,000.
The state and progress of knowledge on this important
subject have thus not materially altered since they were
summed up by Faye and Harkness in i88i. 2 The methods
employed in its investigation fall (as we have seen) into three
separate classes the trigonometrical, the gravitational, and the
" phototachymetrical " an ungainly adjective used to describe
the method by the velocity of light. Each has its special
difficulties and sources of error ; each has counter-balancing
advantages. The last of the three is that in which M. Faye
places most confidence. As its mean result he finds a parallax
of 8.813", implying a distance of 92,750,000 miles exact, in
his opinion, to 104,000. And this agrees admirably with
Todd's mean result, from the same method, of 92,800,000
miles (parallax 8.8o3 // ). 3 On the other hand, considerably
divergent values have high authorities in their favour. Cornu
(as already stated) obtained 8. 86"; and Harkness, from a
combination of Glasenapp's "light-equation" with Michelson's
light- velocity, deduces 8.758".
By a beautiful series of experiments on Foucault's principle,
Master A. A. Michelson, of the United States Navy, fixed in
1879 the rate of luminous transmission at 299,930 kilometres a
1 In Annales de PObs., t. v. 1884. See Observatory, vol. vii. p. 212.
2 Comptes Rendus, t. xcii. p. 375 ; Am. Jour, of Sc., vol. xxii. p. 375.
3 Am. Jour, of Sc., vol. xix. 1880, p. 64.
286 HISTORY OF ASTRONOMY.
second. 1 This determination claims, and doubtless possesses,
a high degree of accuracy. Todd believes it to be entitled to
four times as much confidence as any previous one ; and its
credit extends to the values of parallax arrived at by its means.
Nevertheless there are still difficulties. Experiments on the
velocity of light are necessarily made in air at the ordinary
pressure ; the results are then " corrected for a vacuum ; " but
we cannot be sure that even thus they give the precise rate at
which it flies from planet to planet. Further, an uncertainty
of several hundredths of a second still prevails as to the precise
amount of the aberration of light. 2 Nor are the eclipses of
Jupiter's satellites, upon which the value of the light-equation
depends, by any means instantaneous phenomena ; so that the
apparent times of their occurrence may easily be erroneous by
one or two seconds. All these sources of uncertainty are on
an extremely minute scale ; they become, however, enormously
magnified in the resulting distance of the sun.
On the whole, the most promising plan of investigation at
present is the "diurnal method" applied to minor planets in
opposition, as exemplified by Lord Lindsay and Dr. Gill in
1874. But the method by lunar and planetary disturbances
is unlike all the others in having time on its side. It is this
which Leverrier declared with emphasis must inevitably pre-
vail, because its accuracy is continually growing. 3 The scarcely
perceptible errors which still impede its application are of such
a nature as to accumulate year by year ; eventually, then, they
will challenge, and must receive, a more and more perfect cor-
rection.
The best authorities now concur in placing the sun some-
where between ninety-two and ninety-three millions of miles
from us. Mr. Stone abides by 92,000,000 ; Professor Harkness
prefers 92,365,000; M. Faye, 92,750,000; Professor Young,
1 Am. Jour, of Sc., vol. xviii. p. 393.
2 Struve's "constant of aberration," 20.445", ^ as lately been increased
to 20.517" by M. Magnus Nyren of St. Petersburg.
3 Month. Not. } vol. xxxv. p. 401.
THE SUN'S DISTANCE. 287
92,885,ooo; 1 Dr. Ball, 93,ooo,ooo. 2 If the accord is not all
that could be desired, it is encouraging to remember that
throughout the first half of the last century doubt claimed a
margin of fully twenty million miles ; now possible error
amounts to little more than one and a half millions, and/w-
bable error is of even less extent.
1 The Sun, p. 278. See, however, his lecture on " Pending Problems in
Astronomy," Nature, September 18, 1884, in which he admits his previous
confidence to be somewhat shaken by the results of the last transit of
Venus.
2 In his discourse " On the Sun's Distance " at the Southport Meeting
of the British Association, September 21, 1883.
( 288 )
CHAPTER VII.
PLANETS AND SATELLITES.
JOHANN HIERONYMUS ScHROTER was the Herschel of Germany.
He did not, it is true, possess the more brilliant gifts of his
rival. Herschel's piercing discernment, comprehensive in-
telligence, and inventive splendour were wanting to him. He
was, nevertheless, the founder of descriptive astronomy in
Germany, as Herschel was in England.
Born at Erfurt in 1745, he prosecuted legal studies at Got-
tingen, and there imbibed from Kastner a life-long devotion
to science. From the law, however, he got the means of living,
and, what was to the full as precious to him, the means of
observing. Entering the sphere of Hanoverian officialism in
1788, he settled a few years later at Lilienthal near Bremen, as
" Oberamtmann," or chief magistrate. Here he built a small
observatory, enriched in 1785 with a seven-foot reflector by
Herschel, then one of the most powerful instruments to be
found anywhere out of England. It was soon surpassed,
through his exertions, by the first-fruits of native industry in
that branch. Schrader of Kiel transferred his workshops to
Lilienthal in 1792, and constructed there, under the super-
intendence and at the cost of the astronomical Oberamtmann,
a thirteen-foot reflector, declared by Lalande to be the finest
telescope in existence, and one twenty-seven feet in focal
length, probably as inferior to its predecessor in real efficiency
as it was superior in size.
Thus, with instruments of gradually increasing power,
Schroter studied during thirty-four years the topography of the
PLANETS AND SATELLITES. 289
moon and planets. The field was then almost untrodden ; he
had but few and casual predecessors, and has since had no
equal in the sustained and concentrated patience of his hourly
watchings. Both their prolixity and their enthusiasm are faith-
fully reflected in his various treatises. Yet the one may be
pardoned for the sake of the other, especially when it is re-
membered that he struck out a substantially new line, and that
one of the main lines of future advance. Moreover, his
infectious zeal communicated itself; he set the example of
observing when there was scarcely an observer in Germany ;
and under his roof Harding and Bessel received their training
as practical astronomers.
But he was reserved to see evil days. Early in 1813 the
French under Vandamme occupied Bremen. On the night of
April 20, the Vale of Lilies was, by their wanton destructiveness,
laid waste with fire ; the Government offices were destroyed,
and with them the chief part of Schroter's property, includ-
ing the whole stock of his books and writings. There was
worse behind. A few days later, his observatory, which
had escaped the conflagration, was broken into, pillaged, and
ruined. His life was wrecked with it. He survived the
catastrophe three years without the means to repair, or the
power to forget it, and gradually sank from disapppointment
into decay, terminated by death, August 29, 1816. He had,
indeed, done all the work he was capable of ; and though not
of the first quality, it was far from contemptible. He laid the
foundation of the comparative study of the moon's surface, and
the descriptive particulars of the planets laboriously collected
by him constituted a store of more or less reliable information
hardly added to during the ensuing half century. They rested,
it is true, under some shadow of doubt ; but the most recent
observations have tended on several points to rehabilitate the
discredited authority of the Lilienthal astronomer. We may
now briefly resume, and pursue in its further progress the
course of his studies, taking the planets in the order of their
distances from the sun,
T
290 HISTORY OF ASTRONOMY.
In April 1792 Schroter first saw reason to conclude, from
the gradual degradation of light on its partially illuminated
disc, that Mercury possesses a tolerably dense atmosphere. 1
During the transit of May 7, 1799, he was, moreover, struck
with the appearance of a ring of softened luminosity encircling
the planet to an apparent height of three seconds, or about a
quarter of its own diameter. 2 Although a " mere thought "
in texture, yet it remained persistently visible both with the
seven-foot and the thirteen-foot reflectors, armed with powers
up to 288. It had a well-marked greyish boundary, and
reminded him, though indefinitely fainter, of the penumbra of
a sun-spot. A similar appendage, but more distinctly bright,
had been noticed by De Plantade at Montpellier, November
n, 1736, and again in 1786 and 1789 by Prosperin and
Flaugergues. Mercury projected on the sun, November 9,
1802, appeared to Ljunberg at Copenhagen surrounded with a
dark zone ; but Herschel, on the same day, saw its " preceding
limb cut the luminous solar clouds with the most perfect
sharpness." 3 The presence, however, of a " halo," appearing
to some observers a little darker, to others a little brighter than
the solar surface, was unmistakable in 1832. Professor Moll
of Utrecht described it as " a nebulous ring of a darker tinge,
approaching to the violet colour." 4 'To Huggins and Stone,
November 5, 1868, it showed as lucid and most distinct. No
change in the colour of the glasses used, or the powers applied,
could get rid of it, and it lasted throughout the transit. 5 ) It was
again well seen by Christie and Dunkin at Greenwich, May 6,
i878, 6 and with much precision of detail by Trouvelot at
Cambridge (U.S.) 7 No observations of much interest were
made during the transit of November 8, 1881. Dr. Little, at
Shanghai, perceived an unvarying " darkish halo," of which,
1 Neueste Beytrdge zur Erweiterung der Sternkunde, Bd. iii. p. 14 (1800).
2 Ibid., p. 24. 3 Phil. Trans., vol. xciii. p. 215.
4 Mem. Roy. Astr. Soc., vol. vi. p. 116.
6 Month. Not., vol. xxix. pp. II, 25. 6 Ibid., vol. xxxviii. p. 398.
7 Am. Jour, of Sc., vol. xvi. p. 124.
PLANETS AND SATELLITES. 291
however, neither Mr. Ellery at Melbourne, nor Mr. Tebbutt at
Windsor, New South Wales, saw any trace. 1 They, on the other
hand, took note of a certain whitish spot on the planet's disc,
which, ever since 1697, when it was detected by Wurzelbauer at
Erfurt, has been one of the most frequent attendant phenomena
of a transit of Mercury. It is not always centrally situated, and
is sometimes seen in duplicate, so that Powell's explanation by
diffraction is obviously insufficient. Nevertheless there can
scarcely be a doubt that it is an optical effect of some kind.
As to the " halo," it is less easy to decide. That Mercury
possesses a considerably refractive atmosphere is certified by
the observation of De Plantade in 1736,2 and the still more
definite observation of Simms in i832, 3 of a luminous edge to
the part of the disc outside the sun at ingress or egress. The
natural complement to this appearance would be a dusky
annulus round the planet on the sun precisely such as was
seen by Moll and Little due to the imperfect transparency of
its gaseous envelope. But the brilliant ring vouched for by'
others is not so readily explicable. Airy has shown that it
cannot possibly be caused by refraction, and must accordingly
be set down as " strictly an ocular nervous phenomenon." 4
It is the less easy to escape from this conclusion that we find
the virtually airless moon capable of exhibiting a like appendage.
Professor Stephen Alexander of the United States Survey,
with two other observers, perceived, during the eclipse of the
sun of July 18, 1860, the advancing lunar limb to be bordered
with a bright band ; 5 and photographic effects of the same
kind appear in pictures of transits of Venus and partial solar
eclipses.
In the case of Mercury, a real effect is perhaps complicated
with an illusory one. Different eyes are very differently sensitive
to degrees of light and shade. Absorption by a Mercurian
atmosphere is doubtless in some degree present, and it may be
1 Month. Not., vol. xlii. pp. 101-104.
2 Mini, de fAc., 1736, p. 440. 3 Month. No!., vol. ii. p. 103.
4 Ibid., vol. xxiv. p. 1 8. 5 Ibid., vol. xxiii. p. 234 (Challis).
292 HISTORY OF ASTRONOMY.
that the faintly shadowed ring produced by it impresses some
observers, by contrast with the ink-black disc of the planet,
as bright. The further investigation of this curious subject
must wait for the next transit of Mercury, May 9, 1891.
As to the constitution of this planet, the spectroscope has
little to tell. Its light is of course that of the sun reflected,
and its spectrum is consequently a faint echo of the Fraunhofer
spectrum. Dr. H. C. Vogel, who first examined it in April
1871, suspected traces of the action of an atmosphere like ours, 1
but, it would seem, on slight grounds. It is, however, certainly
very poor in blue rays.
On March 26, 1800, Schroter, observing with his 1 3-foot
reflector in a peculiarly clear sky, perceived the southern horn
of Mercury's crescent to be quite distinctly blunted. 2 Inter-
ception of sunlight by a Mercurian mountain rather more than
eleven English miles high, explained the effect to his satis-
faction. By carefully timing its recurrence, he concluded
rotation on an axis in a period of 24 hours 4 minutes. This
was the first determination of the kind, and was the reward of
twenty years' unceasing vigilance. It was confirmed by watch-
ing the successive appearances of a dusky streak and blotch in
May and June 1 80 1 . 3 These, however, were inferred to be no
permanent markings on the body of the planet, but atmos-
pheric formations, the streak at times drifting forwards (it was
thought) under the fluctuating influence of Mercurian breezes.
From a rediscussion of these observations Bessel inferred that
Mercury rotates on an axis inclined 70 to the plane of its orbit
in 24 hours 53 seconds. A close analogy would thus exist
between the alternations of its seasons and those of the earth,
save that their effects must (except within the polar circles) be
well-nigh swallowed up in the larger vicissitudes produced by
the considerable eccentricity of its path, causing its distance
from the sun to vary from 29 to 43 million miles, and the
1 Untersuchungen iiber die Spectra der Ptaneten, p. 9.
2 Neueste Beytrage, Bd. iii. p. 50.
3 Astr. Jahrbuch, 1804, pp. 97-102.
PLANETS AND SATELLITES. 293
light and heat received, from four to ten times the amount
reaching our planet.
The rounded appearance of the southern horn seen by
Schroter was more or less doubtfully caught by Noble (1864),
Burton, and Franks (iSyy); 1 but was obvious to Mr. W. F.
Denning at Bristol on the morning of November 5, i882. 2 He
also discerned brilliant and dusky spaces, the displacements of
which, during four days, indicated rotation in about twenty-
five hours. The general aspect of the planet reminded him
of that of Mars ; 3 but the difficulties in the way of its observa-
tion are enormously enhanced by its constant close attendance
on the sun.
The theory of Mercury's movements has always given trouble.
In Lalande's, 4 as in Mastlin's time, the planet seemed to exist
for no other purpose than to throw discredit on astronomers ;
and even to Leverrier's powerful analysis it long proved recal-
citrant. On the 1 2th of September 1859, however, he was
able to announce before the Academy of Sciences 5 the terms
of a compromise between observation and calculation. They
involved the addition of a new member to the solar system.
The hitherto unrecognised presence of a body about the size
of Mercury itself, revolving at somewhat less than half its mean
distance from the sun (or, if farther, then of less mass, and vice
versa), would, it was pointed out, produce exactly the effect
required, of displacing the perihelion of the former planet 38
seconds a century more than could otherwise be accounted
for. The planes of the two orbits, however, should not lie far
apart, as otherwise a nodal disturbance would arise not per-
ceived to exist. It was added that a ring of asteroids similarly
placed would answer the purpose equally well, and was more
likely to have escaped notice.
Upon the heels of this forecast followed promptly a seeming
verification. Dr. Lescarbault, a physician residing at Orgeres,
1 Webb, Celestial Objects, p. 46 (4th ed.)
2 L ' Astronomic, t. ii. p. 141. 3 Observatory, No. 82, p. 40.
4 Hist, de tAstr., p. 682. 5 Comptes Rendus, t. xlix. p. 379.
294 HISTORY OF ASTRONOMY.
whose slender opportunities had not blunted his hopes of
achievement, had, ever since 1845, when he witnessed a transit
of Mercury, cherished the idea that an unknown planet might
be caught thus projected on the solar background. Unable to
observe continuously until 1858, he, on March 26, 1859, saw
what he had expected a small, perfectly round object slowly
traversing the sun's disc. The fruitless expectation of re-
observing the phenomenon, however, kept him silent, and it
was not until December 22, after the news of Leverrier's pre-
diction had reached him, that he wrote to acquaint him with
his supposed discovery. 1 The Imperial Astronomer thereupon
hurried down to Orgeres, and by personal inspection of the
simple apparatus used, by searching cross-examination and
local inquiry, convinced himself of the genuine character and
substantial accuracy of the reported observation. (He named
the new planet "Vulcan," and computed elements giving it a
period of revolution slightly under twenty days. 2 But it has
never since been seen. M. Liais, director of the Brazilian
Coast Survey, thought himself justified in asserting that it
never had been seen. Observing the sun for twelve minutes
after the supposed ingress recorded at Orgeres, -he noted those
particular regions of its surface as "tres uniformes d'in-
tensite." 3 He subsequently, however, admitted Lescarbault's
good faith, at first rashly questioned. The planet-seeking doctor
was, in truth, only one among many victims of similar illusions.
Waning interest in the subject was revived by a fresh an-
nouncement of a transit witnessed, it was asserted, by Weber
at Peckaloh, April 4, i8y6. 4 The pseudo-planet, indeed, was
detected shortly afterwards on the Greenwich photographs,
and was found to have been seen by M. Ventosa at Madrid
in its true character of a sun-spot without penumbra ; but
Leverrier had meantime undertaken the investigation of a list
or twenty similar dubious appearances, collected by Haase, and
1 Comptes Rendus, t. 1. p. 40. z Ibid., p. 46.
3 Astr. Nach., Nos. 1248 and 1281.
4 Comptes Rendus, t. Ixxxiii, pp. 510, 561.
PLANETS AND SATELLITES. 295
republished by Wolf in iSyz. 1 From these five were picked
out as referring in all likelihood to the same body, the reality
of whose existence was now confidently asserted, and of which
more or less probable transits were fixed for March 22, 1877,
and October 15, i882.' 2 But, widespread watchfulness not-
withstanding, no suspicious object came into view at either
epoch.
The next announcement of the discovery of " Vulcan " was
on the occasion of the total solar eclipse of July 29, 1878.*
This time it was stated to have been seen at some distance
south-west of the obscured sun, as a ruddy star with a minute
planetary disc ; and its simultaneous detection by two observers
the late Professor James C. Watson, stationed at Rawlins
(Wyoming Territory), and Professor Lewis Swift at Denver
(Colorado) was at first readily admitted. But their separate
observations could, on a closer examination, by no possibility
be brought into harmony, and, if valid, certainly referred to
two distinct objects, if not to four ; each astronomer eventually
claiming a pair of planets. Nor could any one of the four be '
identified with Lescarbault's and Leverrier's Vulcan, which,
if a substantial body revolving round the sun, must then (as
Oppolzer showed) 4 have been found on the east side of that
luminary. The most feasible explanation of the puzzle seems
to be that Watson and Swift merely saw each the same two
stars in Cancer : haste and excitement doing the rest. 5 Never- *
theless they strenuously maintained their opposite conviction. 6
Intra-Mercurial planets have since been diligently searched
for when the opportunity of a total eclipse offered, especially
during the long obscuration at Caroline Island. Not only did
Professor Holden "sweep" in the solar vicinity, but Palisa
1 Handbuch der Mathematik, Bd. ii. p. 327.
2 Comptes Rendus, t. Ixxxiii. p. 721.
3 Nature, vol. xviii. pp. 461, 495, 539. 4 Astr. Nach., No. 2239.
5 Astr. Nock., Nos. 2253-2254 (C. H. F. Peters).
6 Ibid., Nos. 2263 and 2277. t See also Tisserand \^Ann. Bur. des Long.,
1882, p. 729.
296 HISTORY OF ASTRONOMY.
and Trouvelot agreed to divide the field of exploration, and
thus make sure of whatever planetary prey there might be
within reach ; yet with only negative results. Belief in the
presence of any considerable body or bodies within the orbit
of Mercury is, accordingly, now at a low ebb. Yet the exist-
ence of the anomaly in the Mercurian movements indicated by
Leverrier has been made only surer by further research. 1 Its
elucidation constitutes one of the " pending problems " of
astronomy. It need only be remarked that, owing to the
absence of extra-disturbance of the nodes, neither a condensa-
tion inwards of the matter showing to us as the zodiacal light,
nor any accumulation of meteors revolving far from the plane
of Mercury's orbit, will meet the requirements of the situation.
From the observation at Bologna in 1666-67 of some very
faint spots, Domenico Cassini concluded a rotation or libra-
tion of Venus he was not sure which in about twenty-three
hours. 2 By Bianchini in 1726 the period was augmented to
twenty-four days eight hours. J. J. Cassini, however, in 1740,
showed that the data collected by both observers were con-
sistent with rotation in twenty-three hours twenty minutes. 3
So the matter rested until Schroter's time. After watching
nine years in vain, he at last, February 28, 1788, perceived the
ordinarily uniform brightness of the planet's disc to be marbled
with a filmy streak, which returned periodically to the same
position in about twenty-three hours twenty-eight minutes.
This approximate estimate was corrected by the application of
a more definite criterion. On December 28, 17^9, the southern
horn of the crescent Venus was seen truncated, an outlying
lucid point interrupting the darkness beyond. Precisely the
same appearance recurred two years later, giving for the planet's
1 See J. Bauschinger's Unterstichungen (1884), summarised in Bull.
Astr,, t. i. p. 506. Newcomb finds the anomalous motion of the peri-
helion to be even larger (43" instead of 38") than Leverrier made it. Month.
Not., Feb. 1884, p. 187.
2 Jour, des Sfavans, Dec. 1667, p. 122.
3 Elemens (TAstr., p. 525.
PL A NE TS A ND SA TELLITES. 297
rotation a period of twenty three hours twenty-one minutes. 1
To this only twenty-two seconds were added by De Vico, as
the result of over 10,000 observations made with the Cauchoix
refractor of the Collegio Romano, 1839-41. The axis of
rotation was found to be much more bowed towards the orbital
plane than that of the earth, the equator making with it an
angle of 53 n' 26". Of fundamental importance as regards
our views of the planet's constitution, is the fact that De Vico
plainly identified the individual markings drawn by Bianchini
113 years earlier. 2 They cannot, then (if this conclusion be ac-
curate), possess the evanescent atmospheric character attributed
to them by Schroter, but must be inherent peculiarities of surface.
Of the frequently mountainous nature of that surface there
appears to be no reasonable doubt. Francesco Fontana at
Naples in 1643 noticed irregularities along the inner edge of
the crescent. 3 De la Hire in 1700 considered them regard
being had to difference of distance to be much more strongly
marked than those visible in the moon. 4 Schroter's assertions
to the same effect, though scouted with some unnecessary
vehemence by Herschel, 5 have since been repeatedly confirmed ;
amongst others by Madler, De Vico, Langdon, who in 1873
saw the broken line of the " terminator " (the boundary between
light and darkness) with peculiar distinctness through a veil of
auroral cloud ; 6 by Denning, 7 March 30, 1881, despite prelimi-
nary impressions to the contrary, as well as by C. V. Zenger at
Prague, January 8, 1883. The great mountain mass, presumed
to occasion the periodical blunting of the southern horn, was
precariously estimated by the Lilienthal observer to rise to the
prodigious height of nearly twenty-seven miles, or just five
times the elevation of Mount Everest ! Yet the phenomenon
persists, whatever may be thought of the explanation. More-
1 Beobachtungen iiber die sehr belrdchtlichen Gebirge und Rotation der
Venus, 1793, p. 45. Schroter's final result in 1811 was 23!!. 2im. 7.9773.
Monat. Corr., Bd. xxv. p. 367.
2 Astr. Nach., No. 404. 3 Nova Observationes, p. 92.
4 Mem. deFAc., 1700, p. 296. 5 Phil. Trans., vol. Ixxxiii. p. 201.
6 Webb, Cel. Objects, p. 58. / Month. Not., vol. xlii. p. in.
298 HISTORY OF ASTRONOMY.
over, the speck of light beyond, interpreted as the visible sign
of a detached peak rising high enough above the encircling
shadow to catch the first and last rays of the sun, was frequently
discerned by Baron van Ertborn in 1876 ; x while an object
near the northern horn of the crescent, strongly resembling a
lunar ring-mountain, was delineated both by De Vico in 1841
and by Denning. forty years later. Another curious circum-
stance, first observed by Schroter in August 1793, and since
abundantly verified, is that the phases of the crescent Venus
are continually retarded, and of the waning Venus accelerated
by several days. In both cases the disc is illuminated over a
much more, restricted area than it ought to be from its position.
The same applies to Mercury. Schroter's explanation by the
arrest of nearly level sunlight through the intervention of lofty
ranges is far from satisfactory ; but no other has been offered.
We are almost equally sure that Venus, as that the earth
is encompassed with an atmosphere. Yet, notwithstanding
luminous appearances plainly due to refraction during the
transits both of 1 7 6 1 and 1769, Schroter, in 1 7 9 2, took the initia-
tive in coming to a definite conclusion on the subject. 2 It was
founded, first, on the rapid diminution of brilliancy towards
the terminator, attributed to atmospheric absorption ; next, on
the extension beyond a semicircle of the horns of the crescent ;
lastly, on the presence of a bluish gleam illuminating the early
hours of the Cytherean night with what was taken to be
genuine twilight. Even Herschel admitted that sunlight, by
the same effect through which the heavenly bodies show visibly
above our horizons while still geometrically below them, appeared
to be bent round the shoulder of the globe of Venus. Ample
confirmation of the fact has since been afforded. (At Dorpat
in May 1849, the planet being within 3 26' of inferior con-
junction, Madler found the arms of waning light upon the disc
to embrace no less than 240 of its extent ; 3 and in December
1 Btdl. Ac. de Brnxelles, t. xliii. p. 22.
2 Phil. Trans., vol. Ixxxii. p. 309 ; Aphroditographische Fragments, p.
85 (1796). a Astr. Nach., No. 679,
PLANETS AND SATELLITES.
UNI VJ
JLJF
1842, Mr. Guthrie, of Bervie, N.B., actually observed, under
similar conditions, the whole circumference to be lit up with a
faint nebulous glow. 1 Here the solar rays evidently pierced
the planet's atmosphere from behind, pursuing a curved path,
as if through a lens. The same curious phenomenon was
intermittently seen by Mr. Leeson Prince at Uckfield in
September i863; 2 but with more satisfactory distinctness by
Mr. C. S. Lyman of Yale College, 3 before and after the con-
junction of December n, 1866, and during nearly five hours
previous to the transit of 1874, when the yellowish ring of
refracted light showed at one point an approach to interruption,
it might be presumed through the intervention of a bank of
clouds. These effects can be accounted for, as Mr. Neison
pointed out, 4 only by supposing the atmosphere of Venus to
be nearly twice as dense at the surface of its globe, and to
possess nearly twice as much refractive power as that of the
earth. )
Similar appearances are conspicuous during transits. But
while the Mercurian halo is characteristically seen on the sun,
the " silver thread " round the limb of Venus commonly shows
on the part ^"the sun. There are, however, instances of each
description in both cases. Mr. Grant, in collecting the records
of physical phenomena accompanying the transits of 1761 and
1769, remarks that no one person saw both kinds of annulus,
and argues thence a dissimilarity in their respective modes of
production. 5 Such a dissimilarity probably exists, in the sense
that the inner section of the ring is due to absorption, the outer
to refraction by the same planetary atmosphere ; but the dis-
tinction of separate visibility has not been borne out by recent
experience. Several of the Australian observers during the
transit of 1874 witnessed the complete phenomenon. Mr. J.
Macdonnell, at Eden, saw a "shadowy nebulous ring" sur-
round the whole disc when ingress was two-thirds accomplished ;
1 Month. Not., vol. xiv. p. 169. 2 Ibid., vol. xxiv. p. 25.
3 Am. four, of Sc. t vol. xliii. p. 129 (2d ser.) ; vol. ix. p. 47 (3d ser.)
4 Month. Not., vol. xxxvi. p. 347. 5 Hist. Phys. Astr., p. 431.
300 HISTORY OF ASTRONOMY.
Mr. Tornaghi, at Goulburn, perceived a halo, entire and un-
mistakable, at half egress. 1 Similar observations were made
at Sydney, 2 and were renewed in 1882 by Lescarbault at
Orgeres, by Metzger in Java, and by Barnard at Vanderbilt
University. 3
Spectroscopic indications of aqueous vapour as present in
ihe atmosphere of Venus, were obtained in 1874 and 1882, by
Tacchini and Riccb in Italy, and by Young in New Jersey. 4
Janssen, however, who made a special study of the point
subsequently to the transit of 1882, found them much less
certain than his earlier expectations led him to expect ; 5 and
Vogel, by repeated examinations, 1871-73, could detect only
the very slightest variations from the pattern of the solar
spectrum. ) Some additions there indeed seem to be in the
thickening of certain water-lines, and also of a group (B) since
shown by EgororT to be developed through the absorptive
action of cool oxygen ; but so nearly evanescent as to induce
the persuasion that the light we receive from Venus is reflected
from a heavy cloud-stratum, and has traversed, consequently,
only the rarer upper portion of its atmosphere. 6 This would
also account for the extreme brilliancy of the planet. On the
26th and 27th of September 1878, a close conjunction gave
Mr. James Nasmyth the rare opportunity of watching Venus
and Mercury for several hours side by side in the field of his
reflector ; when the former appeared to him like clean silver,
the latter as dull as lead or zinc. 7 Yet the light incident upon
Mercury is, on an average, three and a half times as strong as
the light reaching Venus. Thus, the reflective power of Venus
must be singularly strong. And we find accordingly, from
1 Mem. Roy. Astr. Soc., vol. xlvii. pp. 77, 84.
- Astr. Reg., vol. xiii. p. 132.
3 L'Astronomie, t. ii. p. 27; Astr. Nach., No. 2021 ; Am. Jour, of Sc.,
vol. xxv. p, 430.
4 Mem. Spettr. ItaL, Dicembre 1882; Am. Jour. ofSc., vol. xxv. p. 328.
5 Comptes Rendus, t. xcvi. p. 288.
6 Vogel, Unters. iiber die Spectra der Planeten, p. 15.
7 Nature, vol. xix. p. 23.
PLANETS AND SATELLITES. 301
a combination of Zollner's with Pickering's results, that its
" albedo " is but little inferior to that of new-fallen snow ; in
other words, it gives back 72 J per cent, of the luminous rays
impinging upon it.
This view, that we see only the cloud-canopy of Venus, is
manifestly inconsistent with the supposed permanency of its
spots, or with the perception of shadow effects on a rugged
crust. It is, however, with some reservation, shared by Mr. E.
L. Trouvelot, who since 1875 has pursued a diligent telescopic
study of the planet at Cambridge (U.S.) Not the least sur-
prising fact about this sister-globe is that the axis on which it
rotates is hooded at each end with some shining substance.
These polar appendages were discovered in 1813 by Gruit-
huisen, 1 who set them down as polar snow-caps like those
of Mars. Nor is it altogether certain that he was wrong.
Trouvelot, indeed, in January 1878, perceived (or thought that
he perceived) the southern one to be composed of isolated
peaks thrown into relief against the sky, and hence concluded
both to represent lofty groups of mountains penetrating the
vapour-stratum supposed to form the greater part of the visible
disc. He pointed out, moreover, that the place of the southern
spot might be called identical with that of a projection above
the limb detected by MM. Bouquet de la Grye and Arago in
measuring photographs of Venus in transit taken at Puebla
and Port-au-Prince in i882. 2 This projection corresponded
to a real elevation of about sixty-five miles. But it was more
probably due to "photographic irradiation" from a local
excess of brilliancy, the result according to the French in-
vestigators' conjecture of accumulations of ice and snow, or
the continuous formation of vast cloud-masses.
The same photographs show that in figure Venus very
closely resembles our earth, the equatorial bulging produced
by rotation being ^ J^- of its mean radius.
The "secondary," or "ashen light" of Venus was first
1 Nova Acta Acad. Natura Curwsorum, Bd. x. p. 239.
2 Observatory, vols. iii. p. 416, vii. p. 239.
302 HISTORY OF ASTRONOMY.
noticed by Riccioli in 1643; it was seen by Derham about
I 7 I 5> by Kirch in 1721, by Schrb'ter and Harding in I806; 1
and the reality of the appearance has since been authenticated
by numerous and trustworthy observations. It is precisely
similar to that of the " old moon in the new moon's arms ; "
and Zenger, who witnessed it with unusual distinctness,
January 8, 1883,2 supposes it due to the same cause namely,
to the faint gleam of reflected earth-light from the night-side
of the planet. ] When we remember, however, that "full earth-
light " on Venus, at its nearest, has little more than T o-,^^ its
intensity on the moon, we see at once that the explanation is
inadequate. Nor can Professor Schafarik's, 3 by phosphor-
escence of the warm and teeming oceans with which Zollner 4
regarded the globe of Venus as mainly covered, be seriously
entertained. Vo'gePs suggestion is more plausible. He and
Lohse, at Bothkamp, November 3-11, 1871, saw the dark
hemisphere partially illuminated by secondary light, extending
30 from the terminator, and thought the effect might be
produced by a very extensive twilight. 5 An atmospheric
diffusion of sunlight seems, in fact, the best answer to the
riddle. It involves difficulties, but probably" none that are
insuperable.
The third planet encountered in travelling outwards from the
sun is the abode of man. He has in consequence opportunities
of studying its physical habitudes altogether different from the
baffling glimpses afforded to him of the other members of the
solar family. Regarding the earth, then, a mass of knowledge
so varied and comprehensive has been accumulated as to
form a science or rather several sciences apart. But under-
neath all lie astronomical relations, the recognition and investi-
1 Astr. Jahrbuch, 1809, p. 164. 2 Month. Not., vol. xliii. p. 331.
3 Report Biit. Ass., 1873, p. 407. The paper contains a valuable record
of observations of the phenomenon.
4 Photom. Untersuchungen, p. 301.
6 Beobachtnngen zu Bothkamp, Heft ii. p. 126,
PLANETS AND SATELLITES. 303
gation of which constitute one of the most significant intellectual
events of the present century.
It is indeed far from easy to draw a line of logical distinction
between items of knowledge which have their proper place
here, and those which should be left to the historian of geology.
There are some, however, of which the cosmical connections
are so close that it is impossible to overlook them. (Amongst
these'is the ascertainment of the solidity of the globe. At first
sight it seems difficult to conceive what the apparent positions
of the stars can have to do with subterranean conditions ; yet
it was from star measurements alone that Hopkins, in 1839,
concluded the earth to be solid to a depth of at least 800 or
1000 miles. 1 His argument was, that if it were a mere shell
filled with liquid, precession and nutation would be much
larger than they are observed to be. For the shell alone would
follow the pull of the sun and moon on its equatorial girdle,
leaving the liquid behind ; and being thus so much the lighter,
would move the more readily. There is, it is true, grave reason
to doubt whether this reasoning corresponds with the actual
facts of the case ; 2 but the conclusion to which it led has been
otherwise affirmed and extended.
Indications to an identical effect have been derived from
another kind of external disturbance, affecting our globe through
the same agencies. Sir William Thomson pointed out in
1862 3 that tidal influences are brought to bear on land as well
as on water, although obedience to them is perceptible only in
the mobile element. Some bodily distortion of the earth's
figure must however take place, unless we suppose it of absolute
or " preternatural " rigidity, and the amount of such distortion
1 Phil. Trans., 1839, 1841, 1842.
2 Delaunay objected (Comptes Rendus, t. Ixvii. p. 65) that the viscosity
of the contained liquid (of which Hopkins took no account) would, where
the movements were so excessively slow as those of the earth's axis, almost
certainly cause it to behave like a solid. Sir W. Thomson, however (Report
Brit. Ass., 1876, ii. p. l), considers Hopkins's argument valid as regards the
comparatively quick solar semi-annual and lunar fortnightly nutations,
3 Phil. Trans., vol. cliii. p. 573.
304 HISTORY OF ASTRONOMY.
Yt
can be determined from its effect in diminishing oceanic tides
below their calculated value. For if the earth were perfectly
plastic to the stresses of solar and lunar gravity, tides in the
ordinary sense would not exist. Continents and oceans would
swell and subside together. It is to the difference in the
behaviour of solid and liquid terrestrial constituents that the
ebb and flow of the waters are due.
Six years later, the distinguished Glasgow professor suggested
that this criterion might, by the aid of a prolonged series of
exact tidal observations, be practically applied to test the interior
condition of our planet. 1 In 1882, accordingly, suitable data
extending over thirty-three years having at length become
available, Mr. G. H. Darwin performed the laborious task
of their analysis, with the general result that the "effective
rigidity " of the earth's mass must be at least as great as that of
steel. 2
In a paper read before the Geological Society, December 15,
i83o, 3 Sir John Herschel threw out the idea that the perplexing
changes of climate revealed by the geological record might be
explained through certain slow fluctuations in the eccentricity
of the earth's orbit, produced by the disturbing action of the
other planets. Shortly afterwards, however, he abandoned the
position as untenable ; 4 , and it was left to Mr. James Croll, in
1864 and subsequent years, to reoccupy and convert it into a
strong, if not an impregnable one. Within restricted limits
(as Lagrange, and, more certainly and definitely, Leverrier
proved), the path pursued by our planet round the sun alter-
nately contracts, in the course of ages, into a moderate ellipse,
and expands almost to a circle, the major axis, and conse-
quently the mean distance, remaining invariable. Even at pre-
sent, when the eccentricity approaches a minimum, the sun is
nearer to us in January than in July by above three million miles,
1 Report Brit. Ass., 1868, p. 494. 2 Ibid., 1882, p. 474.
3 Trans. Geol. Soc., vol. iii. (2d ser.), p. 293.
4 See his Treatise on Astronomy, p. 199 (1833).
5 Phil. Mag., vol. xxviii. (4th sen), p. 121.
PLANETS AND SATELLITES. 305
and some 850,000 years ago this difference was more than four
times as great. Mr. Croll has brought together 1 a mass of
evidence to support the view that, at epochs of considerable
eccentricity, the hemisphere of which the winter, occurring at
aphelion, was both intensified and prolonged, must have under-
gone extensive glaciation ; while the opposite hemisphere, with
a short, mild winter, and long, cool summer, enjoyed an
approach to perennial spring. These conditions were exactly
reversed at the end of 10,500 years, through the shifting of
the perihelion combined with the precession of the equinoxes,
the frozen hemisphere blooming into a luxuriant garden as
its seasons came round to occur at the opposite sides of the
terrestrial orbit, and the vernal hemisphere subsiding simul-
taneously into ice-bound rigour. Thus a plausible explana-
tion was offered of the anomalous alternations of glacial and
semi-tropical periods, attested, on incontrovertible geological
evidence, as having succeeded each other in times past over
what are now temperate regions. The most recent glacial
epoch is placed by Mr. Croll about 200,000 years ago, when
the eccentricity of the earth's orbit was 3.4 times as great
as it now is. At present, a faint representation of such a state
of things is afforded by the southern hemisphere. One con-
dition of glaciation in /the coincidence of winter with the
maximum of remoteness from the sun, is present ; the other
a high eccentricity is deficient. Yet the ring of ice-bound
territory hemming in the southern pole is well known to be
far more extensive than the corresponding region in the
north.
This ingenious hypothesis has certainly made good its footing
among the better-warranted speculations of science. The pre-
cise nature of the connection between geological and astro-
nomical events indicated by it may be questioned, but there
can no longer be any doubt that, in some form, such a rela-
tion exists. Its ascertainment marks one further step in that
process of unification between things celestial and things ter-
1 Climate and Time, 1875.
U
306 HISTORY OF ASTRONOMY.
restrial which forms, it might be said, the vast presiding idea of
astronomical history during the last three centuries.
The first attempt at an experimental estimate of the " mean
density" of the earth was Maskelyne's observation in 1774
of the deflection of a plumb-line through the attraction of
Schehallien. The conclusion thence derived, that our globe
weighs 4! times as much as an equal bulk of water, 1 was not
very exact. It was considerably improved upon by Cavendish,
who, in 1798, brought into use the "torsion-balance" con-
structed for the same purpose by John Michell. The resulting
estimate of 5.48 was raised to 5.66 by Francis Baily's elaborate
repetition of the process in 1838-42. From the latest ex-
periments on the subject those made in 1872-73 by Cornu
and Bailie the slightly inferior value of 5.56 was derived ; and
it was further shown that the data collected by Baily, when
corrected for a systematic error, gave a practically identical
result (5-S5). 2 Newton's guess at the average weight of the
earth as five or six times that of water has thus been curiously
verified.
Operations for determining the figure of the earth have been
carried out during the present century on an. unprecedented
scale. The Russo-Scandinavian arc, of which the measure-
ment was completed under the direction of the elder Struve
in 1855, reached from Hammerfest to Ismailia on the
Danube, a length of 25 20'. But little inferior to it was the
Indian arc, begun by Lambton in the first'years of the century,
continued by Everest, revised and extended by Walker. The
general upshot is to show that the polar compression of the
earth is somewhat greater than had been supposed. The
admitted fraction until lately was -g^; that is to say, the
thickness of the protuberant equatorial ring was taken to be
3 J^ of the mean radius. But Sabine's pendulum experiments,
discussed by Airy in 1826, gave ^|-g-; 3 and arc measurements
tend more and more towards agreement with this figure. A
1 Phil Trans., vol. Ixviii. p. 783. 2 Comptes Rendus, t. Ixxvi. p. 954.
3 Phil. 7rans., vol. cxvi. p. 548.
PLANETS AND SATELLITES. 307
fresh investigation led the late J. B. Listing in 1878 l to state
the dimensions of the terrestrial spheroid as follows : equatorial
radius = 6,377,377 metres; polar radius = 6,355,270 metres;
ellipticity = ^.^s-
It is, however, far from certain that the figure of the earth is
one of strict geometrical regularity. Nay, it is by no means
clear that even its main outlines are best represented by what
is called an " ellipsoid of revolution " in other words, by a
globe flattened at top and bottom, but symmetrical on every
side. From a survey of geodetical results all over the world,
Colonel Clarke concludes that different meridians possess
different amounts of curvature ; 2 so that the equator, instead
of being a circle, as it should be apart from perturbing causes
in a rotating body, must, on this view, be itself an ellipse,
and our planet be correctly described as in shape " an ellip-
soid of three unequal axes." But the point is still sub judice.
Operations towards its decision are in active progress both in
Europe and India.
The moon possesses for us an unique interest. She in all pro-
bability shared the origin of the earth ; she perhaps prefigures
its decay. She is at present its minister and companion. Her
existence, so far as we can see, serves no other purpose than to
illuminate the darkness of terrestrial nights, and to measure,
by swiftly-recurring and conspicuous changes of aspect, the
long span of terrestrial time. Inquiries stimulated by visible
dependence, and aided by relatively close vicinity, have re-
sulted in a wonderfully minute acquaintance with the features
of the single lunar hemisphere open to our inspection.
Selenography, in the modern sense, is not yet a hundred
years old. It originated with the publication in 1791 of
Schroter's Selenotopographische Fragmented Not but that the
lunar surface had already been diligently studied, chiefly by
1 Astr. Nach., No. 2228.
2 Phil. Mag., vol. vi. (5th ser.), p. 92.
3 The second volume was published at Gottingen in 1802.
3 o8 HISTORY OF ASTRONOMY.
Hevelius, Cassini, and Tobias Mayer ; the idea, however, of
investigating the moon's physical condition, and detecting
symptoms of the activity there of natural forces through
minute topographical inquiry, first obtained effect at Lilienthal.
Schroter's delineations, accordingly, imperfect though they
were, afforded a starting-point for a comparative study of the
superficial features of our satellite.
The first of the curious objects which he named "rills"
was noted by him in 1787. Before 1801 he had found eleven ;
Lohrmann added 75 ; Madler 55 ; Schmidt published in 1866
a catalogue of 425, of which 278 had been detected by himself; 1
and he eventually brought the number up to nearly 1000.
They are, then, a very persistent lunar feature, though wholly
without terrestrial analogue. There is no difference of opinion
as to their nature. They are quite obviously clefts in a rocky
surface, TOO to 500 yards deep (the depression of the great
rill near Aristarchus was estimated by Schmidt at 554 yards),
usually a couple of miles across, and pursuing straight, curved,
or branching tracks up to 150 miles in length. As regards
their origin, the most probable view is that they are fissures
produced in cooling ; but Neison inclines to " consider them
r:.'. 1 -?^ as dried watercourses. 2
On February 24, 1792, Schroter perceived what he took to
be distinct .traces of a lunar twilight, and continued to observe
them during nine ensuing years. 3 They indicated, he thought,
the presence of a shallow atmosphere (not reaching a height
of more than 8400 feet), about -^th as dense as our own.
Bessel, on the other hand, considered that the only way of
" saving " a lunar atmosphere was to deny it any refractive
power, the sharpness and suddenness of star-occultations nega-
tiving the possibility of gaseous surroundings exceeding in
density (as he computed on an extreme supposition) ^ J^th that
of terrestrial air. 4 Newcomb places the maximum at ^J^. Sir
1 Ueber Rillen auf dem Monde, p. 13. 2 The Moon, p. 73.
3 Selen. Fragm., Th. ii. p. 399.
4 Astr. Nach., No. 263 (1834) ; Pop. Vorl., pp. 615-620 (1838).
i
PLANETS AND SATELLITES. 309
John Herschel concluded " the non-existence of any atmosphere
at the moon's edge having one-ipSoth part of the density of
the earth's atmosphere." l
This decision was fully borne out by Dr. Huggins's spectro-
scopic observation of the disappearance behind the moon's
limb of the small star & Piscium, January 4, 1 865.2 Not the
slightest sign of selective absorption or unequal refraction was
discernible. The entire spectrum went out at once, as if a
slide had suddenly dropped over it from above. The spectro-
scope has uniformly told the same tale ; for M. Thollon's
observation during the total solar eclipse at Sohag of a sup-
posed thickening at the moon's rim, of certain dark lines in
the solar spectrum, is now all but admitted to have been
illusory. Moonlight, analysed with the prism, is found to
be pure reflected sunlight, diminished in quantity, owing to
the low reflective capability of the lunar surface, to about
one-sixth its incident intensity, but wholly unmodified in
quality.
Yet there is little or no doubt that the diameter of the moon,
as determined from occultations, is 4" smaller than it appears
by direct measurement. This fact, which emerged from Sir
George Airy's discussion, in i865, 3 of an extensive series of
Greenwich and Cambridge observations, would naturally result
from lunar atmospheric refraction. He showed, however, that
even if the entire eifect were thus produced (a certain share is
claimed by irradiation) the atmosphere involved would be
2000 times thinner than our own air at the sea-level. A
gaseous stratum of such extreme tenuity could scarcely pro-
duce any spectroscopic effect. It is certain (as Mr. Neison
has pointed out 4 ) that a lunar atmosphere of very great extent
and of no inconsiderable mass would possess, owing to the low
power of lunar gravity, a very small surface density, and might
thus escape direct observation while playing a very important
part in the economy of our satellite. Some renewed evidence
1 Outlines of Astr., par. 431. 2 Month. Not., vol. xxv. p. 61.
3 Month. Not., vol. xxv. p. 264. 4 The Moon, p. 25.
310 HISTORY OF ASTRONOMY.
of actual crepuscular gleams on the moon has, besides, been
lately furnished to MM. Paul and Prosper Henry of the Paris
Observatory by their skilful use of a powerful telescope. 1
The first to emulate Schroter's selenographical zeal was
Wilhelm Gotthelf Lohrmann, a land-survevor of Dresden, who,
in 1824, published four out of twenty-five sections of the first
scientifically executed lunar chart, on a scale of 37 J inches
to a lunar diameter. His sight, however, began to fail three
years later, and he died in 1840, leaving materials from which
the work was completed and published in 1878 by Dr. Julius
Schmidt, late director of the Athens Observatory. Much had
been done in the interim. Beer and Madler began at Berlin
in 1830 their great trigonometrical survey of the lunar surface,
as yet neither revised nor superseded. ( A map, issued in four
parts, 1834-36, on nearly the same scale as Lohrmann's, but
more detailed and authoritative, embodied the results. It was
succeeded, in 1837, by a descriptive volume bearing the im-
.posing title, Der Mond ; oder allgemeine vergleichende Seleno-
graphie. This summation of knowledge in that branch, though
in truth leaving many questions open, had an air of finality
which tended to discourage further inquiry. 2 It gave form to
a reaction against the sanguine views entertained by Hevelius,
Schroter, Herschel, and Gruithuisen as to the possibilities of
agreeable residence on the moon, and relegated the " Selenites,"
one of whose cities Schroter thought he had discovered, and
of whose festal processions Gruithuisen had not despaired of
becoming a spectator, to the shadowy land of the Ivory Gate.
All examples of change in lunar formations were, moreover,
dismissed as illusory. The light contained in the work was, in
short, a "dry light," not stimulating to the imagination. "A
mixture of a lie," Bacon shrewdly remarks, "doth ever add
pleasure." For many years, accordingly, Schmidt had the field
of selenography almost to himself.
Reviving interest in the subject was at once excited and dis-
played by the appointment, in 1864, of a Lunar Committee of
1 Webb, Cel. Objects, p. 79. 2 Neison, The Moon, p. 104.
PLANETS AND SATELLITES. 311
the British Association. The indirect were of greater value
than the direct fruits of its labours. An English school of seleno-
graphy rose into importance. Popularity was gained for the
subject by the diffusion of works conspicuous for ingenuity and
research. Messrs. Nasmyth's and Carpenter's beautifully illus-
trated volume (1874) was succeeded, after two years, by a still
more weighty contribution to lunar science. Mr. Neison's book
was accompanied by a map, based on the survey of Beer and
Madler, but adding some 500 measures of position, besides
the representation of several thousand new objects. With
Schmidt's Charte der Gebirge des Mondes, Germany once more
took the lead. This splendid delineation the result of thirty-
two years' labour was built upon Lohrmann's foundation, but
embraces the detail contained in upwards of 3000 original
drawings. No less than 32,856 craters are represented in it.
The scale is seventy-five inches to a diameter. An additional
help to lunar inquiries was provided at the same time in this
country by the establishment, through the initiative of the late
Mr. W. R. Birt, of the Selenographical Society.
But the strongest incentive to diligence in studying the rugged
features of our celestial helpmate has been the idea of probable
or actual variation in them. A change always seems to the
inquisitive intellect of man like a breach in the defences of
Nature's secrets, through which it may hope to make its way
to the citadel. What is desirable easily becomes credible ; and
thus statements and rumours of lunar convulsions have suc-
cessively, during the last hundred years, obtained credence,
and successively, on closer investigation, been rejected. The
subject is one as to which illusion is peculiarly easy. Our
view of the moon's surface is a bird's-eye view. Its conforma-
tion reveals itself indirectly through irregularities in the dis-
tribution of light and darkness. The forms of its elevations
and depressions can be inferred only from the shapes of the
black, unmitigated shadows cast by them. But these shapes
are in a state of perpetual and bewildering fluctuation, partly
through changes in the angle of illumination, partly through
3 I2 HISTORY OF ASTRONOMY.
changes in our point of view, caused by what are called
the moon's "librations." l The result is/ that no single obser-
vation can be exactly repeated by the same observer, since
identical conditions recur only after the lapse of a great num-
ber of years.
Local peculiarities of surface, besides, are liable to produce
perplexing effects. The reflection of earth-light at a particular
angle from certain bright summits completely, though tem-
porarily deceived Herschel into the belief that he had wit-
nessed, in 1783 and 1787, volcanic outbursts on the dark side
of the moon/ The persistent recurrence, indeed, of similar
appearances under circumstances less amenable to explanation,
inclined Webb to the view that effusions of native light actually
occur. 2 More cogent proofs, must, however, be adduced
before a fact so intrinsically improbable can be admitted as
true.
But from the publication of Beer and Madler's work until
1866, the received opinion was that no genuine sign of activity
had ever been seen, or was likely to be seen, on our satellite ;
that her face was a stereotyped page, a fixed and irrevisable
record of the past. A profound sensation, accordingly, was
produced by Schmidt's announcement, in October 1866, that
the well-known crater " Linne " had disappeared, 3 effaced, as
it was supposed, by an igneous outflow. The case seemed
undeniable, and is still dubious. Linne had been known to
Lohrmann and Madler, 1822-32, as a deep crater, five or six
'miles in diameter, the third largest in the dusky plain known
as the "Mare Serenitatis ; " and Schmidt had observed and
1 The combination of a uniform rotational, with an unequal orbital move-
ment causes a slight swaying of the moon's globe, now east, now west, by
which we are enabled to see round the edges of the averted hemisphere.
There is also a " parallactic " libration, depending on the earth's rotation ;
and a species of nodding movement the "libration in latitude" is pro-
duced by the inclination of the moon's axis to her orbit, and by her changes
of position with regard to the terrestrial equator. Altogether, about j^-
of the invisible side come into view. 2 Cel, Objects, p. 58 (4th ed.)
3 Astr. Nach., No. 1631.
PLANETS AND SATELLITES. 313
drawn it, 1840-43, under a practically identical aspect. Now
it appears under high light as a whitish spot, in the centre of
which, as the rays begin to fall obliquely, a pit, probably under
two miles across, emerges into view. The crateral character
of this comparatively minute depression was detected by
Father Secchi, February u, 1867.
This, however, is not all. Schroter's description of Linnd,
as seen by him November 5, 1788, tallies quite closely with
modern observation j 1 while its inconspicuousness in 1797 is
shown by its omission from Russell's lunar globe and maps. 2
We are thus driven to adopt one of two suppositions : either
Lohrmann, Madler, and Schmidt were entirely mistaken in the
size and importance of Linne, or a real change in its outward
semblance supervened during the first half of this century, and
has since passed away, perhaps again to recur. The latter
hypothesis seems the more probable ; and its probability is
strengthened by much evidence of actual obscuration or
variation of tint in other parts of the lunar surface, more
especially on the floor of the great "walled plain" named
" Plato." 3
An instance of an opposite kind of change was alleged
by Dr. Hermann J. Klein of Cologne in March 1878.* In
Linne, the obliteration of an old crater had been assumed ; in
" Hyginus N.," the formation of a new crater was asserted.
Yet, quite possibly, the same cause may have produced the
effects thought to be apparent in both. It is, however, far
from certain that any real change has affected the neighbour-
hood of Hyginus. The novelty of Klein's observation of May
19, 1877, ni ay have consisted simply in the detection of a
hitherto unrecognised feature. The region is one of complex
formation, consequently of more than ordinary liability to
deceptive variations in aspect under rapid and entangled
1 Respighi, Les Mondes, t. xiv. p. 294; Huggins, Month. Not., vol.
xxvii. p. 298. 2 Birt, ibid n p. 95.
3 Report Brit. Ass., 1872, p. 245.
4 Astr. Reg., vol. xvi. p. 265 ; Astr. Nach., No. 2275.
314 HISTORY OF ASTRONOMY.
fluctuations of light and shade. 1 Moreover, it seems to be
certain, from Messrs. Pratt and Capron's attentive study, that
"Hyginus N." is no true crater, but a shallow, saucer-like
depression, difficult of clear discernment 2 Under suitable
illumination, nevertheless, it contains, and is marked by, an
ample shadow. 3
In both these controverted instances of change, lunar photo-
graphy was invoked as a witness ; but, notwithstanding the
great advances made in the art by Mr. De la Rue in this
country, by Dr. Henry Draper, and above all by Mr. Lewis
M. Rutherfurd, in America, without decisive results. Auto-
graphic records, it may be expected, will gain increasing
authority on such points in the future.
Melloni was the first to get undeniable heating effects from
moonlight. His experiments were made at Naples early in
1846,* and were repeated with like result by Zantedeschi at
Venice four years later. A rough measure of the intensity of
those effects was arrived at by Piazzi Smyth at Guajara, on the
Peak of Teneriffe, in 1856. At a distance of fifteen feet from
the thermomultiplier, a Price's candle was found to radiate
just twice as much heat as the full moon. 5 But by far the
most exact and extensive series of observations on the subject
were those made by the present Earl of Rosse, 1869-72. The
lunar radiations, from the first to the last quarter, displayed,
when concentrated with the Parsonstown three-foot mirror, ap-
preciable thermal energy, increasing with the phase, and largely
due to " dark heat," distinguished from the quicker-vibrating
sort by inability to traverse a plate of glass. This was sup-
posed to indicate an actual heating of the surface, during the
long lunar day of 300 hours,, to about 500 F., 6 the moon thus
1 See Lord Lindsay and Dr. Copeland in Month. Not., vol. xxxix. p.
195.
2 Observatory, vols. ii. p. 296 ; iv. p. 373. Mr. N. G. Green (Astr. Reg.,
vol. xvii. p. 144) concludes the object a mere " spot of colour," dark under
oblique light. 3 Webb, Cd. Objects, p. 101.
4 Comptes Rendus, t. xxii. p. 541. 5 Phil. Trans., vol. cxlviii. p. 502.
6 Proc. Roy. Soc., vol. xvii. p. 443.
PLANETS AND SATELLITES. 315
acting as a direct radiator no less than as a reflector of heat.
These results, though not fully borne out by further and more
careful trials executed at Parsonstown by Dr. Copeland, 1 have
lately received some countenance from Professor Langley's
experiments with the bolometer, showing that moonlight un-
deniably contains a proportion of obscure thermal rays.
This implies some kind of atmospheric clothing. For, en-
tirely denuded of such, Professor Langley has shown 2 that
even under the fiercest sunshine the lunar surface must abide
frostbound at somewhere below 50 Fahr. ; that is to say,
mercury, and a fortiori water, could never liquefy on an airless
moon. That it is capable of sending us any perceptible heat
on its own account that is, apart from its office as a reflector of
solar radiations proves conclusively that it is preserved from
immediate contact with the cold of space by the survival of
some thin remnant of aerial covering.
Although that fundamental part of astronomy known as
" celestial mechanics " lies outside the scope of this work, and
we must therefore pass over in silence the immense labours of
Plana, Damoiseau, Hansen, Delaunay, and Airy in reconciling
the observed and calculated motions of the moon, there is one
slight, but significant discrepancy which is of such importance
to the physical history of the solar system, that some brief
mention must be made of it.
Halley discovered in 1693, by examining the records of
ancient eclipses, that the moon was going faster then than
2000 years previously so much faster, as to have got ahead
of the place in the sky she would otherwise have occupied, by
about two of her own diameters. It was one of Laplace's
highest triumphs to have found an explanation of this puzzling
fact. He showed, in 1787, that it was due to a very slow
change in the ovalness of the earth's orbit, tending, during the
present age of the world, to render it more nearly circular.
The pull of the sun upon the moon is thereby lessened ; the
1 Phil. Trans., vol. clxiii. p. 625.
2 Nature, vol. xxvi. p. 316.
316 HISTORY OF ASTRONOMY.
counter-pull of the earth gets the upper hand ; and our satellite,
drawn nearer to us by something less than an inch each year, 1
proportionally quickens her pace. Many thousands of years
hence the process will be reversed ; the terrestrial orbit will
close in at the sides, the lunar orbit will open out under the
growing stress of solar gravity, and our celestial chronometer
will lose instead of gaining time.
This is all quite true as Laplace put it ; but it is not enough.
Adams, the virtual discoverer of Neptune, found with surprise
in 1853 that the received account of the matter was "essen-
tially incomplete," and explained, when the requisite correction
was introduced, only half the observed acceleration. 2 What
was to be done with the remaining half? Here Delaunay, the
eminent French mathematical astronomer, unhappily drowned
at Cherbourg in 1872 by the capsizing of a pleasure-boat,
came to the rescue. 3
It is obvious to any one who considers the subject a little
attentively, that the tides must act to some extent as a friction-
brake upon the rotating earth. In other words, they must
bring about an almost infinitely slow lengthening of the day.
For the two masses of water piled -up by lunar influence on
the hither and farther sides of our globe, strive, as it were, to
detach themselves from the unity of the terrestrial spheroid,
and to follow the movements of the moon. The moon, accord-
ingly, holds them against the whirling earth, which revolves
like a shaft in a fixed collar, wasting its momentum as heat
dissipated through space. This must go on (so far as we can
see) until the periods of the earth's rotation and of the moon's
revolution coincide. Nay, the process will be continued
should our oceans survive so long by the feebler tide-raising
power of the sun, ceasing only when day and night cease to
alternate, when one side of our planet is plunged in perpetual
darkness and the other seared by unchanging light.
1 Airy, Observatory, No. 37, p. 420.
2 Phil. Trans., vol. cxliii. p. 397 ; Proc. Roy. Soc., vol. vi. p. 321.
3 Comptes Rendus, t. Ixi. p. 1023.
PLANETS AND SATELLITES. 317
Here, then, we have the secret of the moon's turning always
the same face towards the earth. It is that in primeval times,
when the moon was liquid or plastic, an earth-raised tidal
wave rapidly and forcibly reduced her rotation to its present
exact agreement with her period of revolution. This was
divined by Kant l nearly a century before the necessity for
such a mode of action presented itself to any other thinker.
In a weekly paper published at Konigsbergin 1754, the modern
doctrine of " tidal friction " was clearly outlined by him, both
as regards its effects actually in progress on the rotation of the
earth, and as regards its effects already consummated on the
rotation of the moon the whole forming a preliminary attempt
at what he called a " natural history " of the heavens. His
sagacious suggestion, however, remained entirely unnoticed
until revived it would seem independently by Julius Robert
Mayer in 1848^ while similar, and probably original conclu-
sions were reached by William Ferrel of Allensville, Kentucky,
in 1853.3
Delaunay was not then the inventor or discoverer of tidal
friction ; he merely displayed it as an effective cause of change.
He showed reason for believing that its action in checking the
earth's rotation, far from being, as Ferrel had supposed, com-
pletely neutralised by its contraction through cooling, was a
fact to be reckoned with in computing the movements, as well
as in speculating on the history of the heavenly bodies. The
outstanding acceleration of the moon was thus at once ex-
plained. It was explained as apparent only the reflection of
a real lengthening, by one second in 100,000 years, of the day.
But on this point the last word has not yet been spoken.
Professor Newcomb undertook in 1870 the formidable task
of a complete rediscussion of the lunar theory. The results,
published in 1878,* have proved somewhat perplexing. They
1 Sammtl. Werke (ed. 1839), Th. vi. pp. 5-12. See also Mr. C. J.
Monro's useful indications in Nature, vol. vii. p. 241.
2 Dynamik des Himmels, p. 40.
3 Goiilcfs Astr.Jour., vol. iii. p. 138.
4 Wash. Obs. for 1875, vol. xxii. App. ii.
318 HISTORY OF ASTRONOMY.
tend, in general, to reduce the amount of acceleration left
unaccounted for by Laplace's gravitational theory, and pro-
portionately to diminish the importance of the part played by
tidal friction. But, in order to bring about this diminution,
and at the same time conciliate Alexandrian and Arabian
observations, it is necessary to reject as total the ancient solar
eclipses known as those of Thales and Larissa. This may be
a necessary, but it must be admitted to be a hazardous ex-
pedient.
It was further shown that small residual irregularities are
still found in the movements of our satellite, inexplicable either
by any known gravitational influence, or by any uniform value
that could be assigned to secular acceleration. 1 ) If set down
to the account of imperfections in the "time-keeping" of the
earth, it could only be on the arbitrary supposition of fluctua-
tions in its rate of going themselves needing explanation.
This, it is true, might be found, as Sir W. Thomson pointed
out in i876, 2 in very slight changes of figure, not altogether
unlikely to occur. But into this cloudy and speculative region
astronomers for the present decline to penetrate. They prefer,
if possible, to deal only with calculable causes, and thus to
preserve for their " most perfect of sciences " its special prero-
gative of assured prediction.
1 Newcomb, Pop. Astr. (4th ed.), p. IOI.
2 Report Brit. Ass., 1876, p. 12.
319 )
CHAPTER VIII.
PLANETS AND SATELLITES (contimted\
" THE analogy between Mars and the earth is perhaps by far
the greatest in the whole solar system." So Herschel wrote
in I783, 1 and so it may safely be repeated to-day, after an ad-
ditional hundred years of scrutiny. This circumstance lends
a particular interest to inquiries into the physical habitudes of
our exterior planetary neighbour.
Fontana was the first to catch glimpses, at Naples in 1636
and 1638,2 of dusky stains on the ruddy disc of Mars. They
were next seen by Hooke and Cassini in 1666, and this time
with sufficient distinctness to serve as indexes to the planet's
rotation, determined by the latter as taking place in a period of
twenty-four hours forty minutes. 3 } Increased confidence was
given to this result through Maraldi's precise verification of it in
1 7 19.* Amongst the spots observed by him, he distinguished
two as stable in position, though variable in size. They were
of a peculiar character, showing as bright patches round the
poles, and had already been noticed during sixty years back.
A current conjecture of their snowy nature obtained validity
when Herschel connected their fluctuations in extent with the
progress of the Martian seasons. It was hard to resist the
inference of frozen precipitations when once it was clearly per-
ceived that the shining polar zones did actually diminish alter-
nately and grow with the alternations of summer and winter in
the corresponding hemisphere.
1 Phil. Trans., vol. Ixxiv. p. 260. 2 Nova Observations , p. 105.
3 Phil. Trans., vol. i. p. 243. 4 Mem. de I'Ac., 1720, p. 146.
320 HISTORY OF ASTRONOMY.
This, it may be said, was the opening of our acquaintance
with the state of things prevailing on the surface of Mars. It
was accompanied by a steady assertion, on Herschel's part,
of permanence in the dark markings, notwithstanding partial
obscurations by clouds and vapours floating in a " considerable
but moderate atmosphere." Hence the presumed inhabitants
of the planet "probably enjoy a situation in many respects
similar to ours." l
Schroter, on the other hand, went altogether wide of the
truth as regards Mars. He held that the surface visible to us
is a mere shell of drifting cloud, deriving a certain amount of
apparent stability from the influence on evaporation and con-
densation of subjacent but unseen areographical features ; 2
and his opinion prevailed with his contemporaries. It was,
however, rejected by Kunowsky in 1822, and finally overthrown
by Beer and Madler's careful studies during five consecutive
oppositions, 1830-39. They identified at each the same dark
spots, frequently blurred with mists, especially when the local
winter prevailed, but fundamentally unchanged. 3 In 1862
Mr. Lockyer established a "marvellous agreement " with Beer
and Madler's results of 1830, leaving no doubt -as to the com-
plete fixity of the main features, amid " daily, nay, hourly,"
variations of detail through transits of clouds. 4 On seventeen
nights of the same opposition, F. Kaiser of Leyden obtained
drawings in which nemy all the markings noted in 1830 at
Berlin reappeared, besides spots frequently seen respectively
by Arago in 1813, by Herschel in 1783, and one sketched by
Huygens in 1672 with a writing-pen in his diary. 5 From these
data the Leyderi observer arrived at a period of rotation of
24?]. 37m. 22.625., being just one second shorter than that
deduced, exclusively from their own observations, by Beer
1 Phil. Trans., vol. Ixxiv. p. 273.
2 A large work, entitled Areographische Fragmente, in which Schroter
embodied ihe results of his labours on Mars, 1785-1803, narrowly escaped
the conflagration of 1813, and was published at Leyden in 1881.
3 Beitrage, p. 124. 4 Mem. R. A. Soc., vol. xxxii. p. 183.
5 Astr. Nach., No. 1468.
PLANETS AND SATELLITES. 321
and Madler. But the exactness of even this result has been
surpassed. Taking a drawing by Hooke of March 12, 1666
(N.S.), as a starting-point, and delineations by Browning in 1867
and 1869 as termini, Mr. Proctor was enabled to measure the
rotation of Mars by means of an interval of about 203 years. 1
Provided that the right count be kept in the number of entire
rotations performed (which is easily secured by comparison with
intermediate observations), extraordinary accuracy can in this
way be obtained; for an almost infinitesimal error becomes
multiplied by frequent repetition into something so considerable
as to compel correction. Mr. Proctor, for instance, showed
that an estimate astray by so much as the tenth of a second
would, when carried back to Hooke's time, throw the planet
out of its true position by 2 hours 20 seconds. The period then
adopted of 24!}. 37m. 22.7355. is possibly one or two hundredths
of a second too long, but is undoubtedly of a precision un-
approached in the case of any other heavenly body save the
earth itself.
Two facts bearing on the state of things at the surface of
Mars were, then, fully acquired to science in or before the year
1862. The first was that of the seasonal fluctuations of the
polar spots; the second, that of the permanence of certain
dark grey or greenish patches, perceived with the telescope as
standing out from the deep yellow ground of the disc. The
opinion has steadily gained consistency during the last halt-
century that these varieties of tint correspond to the real
diversities of a terraqueous globe, the " ripe cornfield" 2 sections
representing land, the dusky spots and streaks, oceans and
straits. Sir J. Herschel in 1830 led the way in ascribing the
redness of the planet's light to an inherent peculiarity of soil. 3
Previously it had been assimilated to our sunset glows rather
than to our red sandstone formations set down, that is, to an
atmospheric stoppage of blue rays. But the extensive Martian
1 Month, Not., vols. xxviii. p. 37 ; xxix. p. 232 ; xxxiii. p. 552.
2 Flammarion, L? Astro no mie, t. i. p. 266,
3 Smyth, Cel. Cycle, vol. i. p. 148 (ist ed.)
X
322 HISTORY OF ASTRONOMY.
atmosphere, implicitly believed in on the strength of some
erroneous observations by Cassini and Romer in the seven-
teenth century, vanished before the sharp occultation of a
small star in Leo, witnessed by Sir James South in I822; 1
and Dawes's observation in i865, 2 that the ruddy tinge is
deepest near the central parts of the disc, certified its non-
atmospheric origin. The absolute whiteness of the polar
snow-caps was alleged in support of the same inference by
Dr. Huggins in i867. 3
All recent .observations tend to show that the atmosphere of
Mars is much thinner than our own. This was to have been
expected a priori^ since the same proportionate mass of air
would, owing to the small size and inferior specific gravity of
Mars, as compared with the earth, form a very much sparser
covering over each square mile of his surface. 4 Besides,
gravity there possesses much less than half its force here, so
that this sparser covering would weigh less, and be less con-
densed than if it enveloped the earth. Atmospheric pressure
would accordingly be of about two and a quarter, instead of
fifteen terrestrial pounds per square inch. This corresponds
with what the telescope shows us. It is extremely doubtful
whether any features of the earth's actual surface could be
distinguished by a planetary spectator, however well pro-
vided with optical assistance. Professor Langley's inquiries 5
have led him to conclude that fully twice as much light
is absorbed by our air as had previously been supposed say
forty per cent, of vertical rays in a clear sky. Of the sixty
reaching the earth, less than a quarter would be reflected even
from white sandstone; and this quarter would again pay its
toll of forty per cent, in escaping back to space. Thus not
more than eight or nine out of the original hundred sent by
the sun would, under the most favourable circumstances, and
1 Phil. Trans., vol. cxxi. p. 417. * Month. Not., vol. xxv. p. 227.
3 Phil. Mag., vol. xxxiv. p. 75-
4 Proctor, Quart. Jour, of Science, vol. x. p. 185 ; Maunder, Sunday
Mag., Jan., Feb., March, 1882. 5 Am. Jotir. of St., vol. xxviii. p. 163.
PLANETS AND SATELLITES. 323
from the very centre of the earth's disc, reach the eye of a
Martian or lunar observer. The light by which he views our
world is, there is little doubt, light reflected from the various
strata of our atmosphere, cloud- or mist-laden or serene, as the
case may be, with an occasional snow-mountain figuring as a
permanent white spot.
This consideration at once shows us how much more tenu-
ous the Martian air must be, since it admits of topographical
delineations of the Martian globe. The clouds, too, that form
in it seem to be rather of the nature of ground-mists than of
heavy cumulus. 1 There is, indeed, plenty of aqueous vapour
present. A characteristic group of dark rays, due to its
absorptive action, was detected by Dr. Huggins in the analysed
light of the planet in 1867,2 and serves to raise the conjecture
of " snowy poles " to a verisimilitude scarcely to be distin-
guished from certainty.
The climate of Mars seems to be unexpectedly mild. The
polar snows are both less extensive and less permanent than
those on the earth. The southern white hood, always eccen-
trically situated, was noticed by Schiaparelli in 1877 to have
survived the summer only as a small lateral patch, the pole
itself being quite free from snow. But we might expect
to see the whole wintry hemisphere, at any rate, frostbound,
since the sun radiates less than half as much heat on Mars as
on the earth. Water seems, nevertheless, to remain, as a rule,
uncongealed everywhere outside the polar regions. We are
at a loss to imagine by what beneficent arrangement the
rigorous conditions naturally to be looked for, can be modified
into a climate which might be found tolerable by creatures
constituted like ourselves.
Martian topography may be said to form now-a-days a
separate sub -department of descriptive astronomy. The
amount of detail become legible by close scrutiny on a little
disc which, once in fifteen years, attains a maximum of about
1 Burton, Trans. Roy. Dublin Soc., vol. i. 1880, p. 169.
2 Month. Not., vol. xxvii. p. 179.
324 HISTORY OF ASTRONOMY.
WSTT tne area f *he full moon, must excite surprise, and
might provoke incredulity. Spurious discoveries, however,
have little chance of holding their own where there are so
many competitors quite as ready to dispute as to confirm.
The first really good map of Mars was constructed in 1869 by
Mr. Proctor from drawings by Dawes. Kaiser of Leyden fol-
lowed in 1872 with a representation founded upon data of his
own providing in 1862-64 ; and M. Terby, in his valuable
Areographie, presented to the Brussels Academy in I874 1 a
careful discussion of all important observations from the time
of Fontana downwards, thus virtually adding to knowledge by
summarising and digesting it. The memorable opposition of
September 5, 1877, marked a fresh epoch in the study of Mars.
While executing a trigonometrical survey (the first attempted)
of the disc, then of the unusual size of 25" across, Signor G.
V. Schiaparelli, director of the Milan Observatory, detected a
novel and curious feature. What had been taken for Martian
continents were found to be, in point of fact, agglomerations
of islands, separated from each other by a network of so-called
" canals." These are obviously extensions of the " seas,"
originating and terminating in them, and sharing their grey-
green hue, but running sometimes to a length of three or four
thousand miles in a straight line, and preserving throughout a
nearly uniform breadth of about sixty miles. Further inquiries
have fully substantiated the discovery made at the Brera
Observatory. " The " canals " of Mars are an actually existent
and permanent phenomenon. An examination of the draw-
ings in his possession showed M. Terby that they had been
seen, though not distinctively recognised, by Dawes, Secchi, and
Holden ; several were independently traced out by Burton at
the opposition of 1879 > an< ^ au * were recovered by Schiaparelli
himself in 1879 and 1881-82.
When the planet culminated at midnight, and was therefore
in opposition, December 26, 1881, its distance was greater,
and its apparent diameter less than in 1877, in the proportion
1 Memoires Couronnes, t. xxxix.
PLANETS AND SATELLITES. 325
of sixteen to twenty-five. Its atmosphere was, however, more
transparent, and ours of less impediment to northern observers,
the object of scrutiny standing considerably higher in northern
skies. Never before, at any rate, had the true aspect of Mars
come out so clearly as at Milan with the 8|-inch Merz refractor
of the observatory, between December 1881 and February
1882. The canals were all again there, but this time they were
in as many as twenty cases seen in duplicate. That is to
say, a twin-canal ran parallel to the original one at an interval
of 200 to 400 miles. 1
We are here brought face to face with an apparently insoluble
enigma. Schiaparelli regards the " gemination " of his canals
as a periodical phenomenon depending on the Martian seasons ;
but it is as yet premature to form an opinion. Fresh evidence
will, it is to be hoped, become available during the next
favourable opposition in 1892.
Meanwhile, the closeness of the terrestrial analogy remains
somewhat impaired. The distribution of land and water on
Mars, at any rate, appears to be of a completely original type.
The interlacing everywhere of continents with arms of the sea
(if that be the correct interpretation of the visual effects)
implies that their levels scarcely differ ; 2 and it is held by
Schiaparelli and others that their outlines are not absolutely
constant, encroachments of dusky upon bright tints suggesting
the possibility of extensive inundations. Mr. N. E. Green's
noteworthy observations at Madeira in 1877 seem to indicate,
on the other hand, a rugged south polar region. The contour
of the snow-cap not only appeared indented, as if by valleys
and promontories, but brilliant points were discerned outside
the white area, attributed to isolated snow-peaks. 3 Still more
elevated, if similarly explained, must be the "ice island" first
seen in a comparatively low latitude by Dawes in January 1865.
Mars was gratuitously supplied with a pair of satellites long
1 Mem. Spettr. Italian^ t. xi. p. 28.
2 Flammarion, L 'Astronomic^ t. i. p. 206.
3 Month. A'ot. } vol. xxxviii. p. 41.
326 HISTORY OF ASTRONOMY.
before he was found actually to possess them. Kepler inter-
preted Galileo's anagram of the " triple " Saturn in this sense ;
they were perceived by Microme'gas on his long voyage through
space; and the Laputan astronomers had even arrived at a
knowledge, curiously accurate under the circumstances, of
their distances and periods. But terrestrial observers could
see nothing of them until the night of August n, 1877. The
planet was then within one month of its second nearest ap-
proach to the earth during this century; and in 1845 the
Washington 26-inch refractor was not in existence. 1 Professor
Asaph Hall, accordingly, determined to turn the conjuncture
to account for an exhaustive inquiry into the surroundings of
Mars. Keeping his glaring disc just outside the field of view,
a minute attendant speck of light was "glimpsed" August IT.
Bad weather however intervened, and it was not until the i6th
that it was ascertained to be what it appeared a satellite. On
the following evening a second, still nearer to the primary,
was discovered, which, by the bewildering rapidity of its pas-
sages hither and thither, produced at first the effect of quite a
crowd of little moons. 2
Both these delicate objects have since been repeatedly
observed, both in Europe and America, even with compara-
tively small instruments. But at each opposition since that of
1877 the distance of the planet has been increasing, and in
1884 was too great to permit of their detection elsewhere than
at Washington. It is unlikely that they will be again seen
before 1888 or 1890.
The names chosen for them were taken from the Iliad, where
" Deimos " and " Phobos " (Fear and Panic) are represented
as the companions in battle of Ares. In several respects, they
are interesting and remarkable bodies. As to size, they may
be said to stand midway between meteorites and satellites.
From careful photometric measures executed at Harvard in
1877 and 1879, Professor Pickering concluded their diame-
1 See Mr. Wentworth Erck's remarks in Trans. Roy. Dublin Sec., vol.
i. p. 29. 2 Month. Not., vol. xxxviii. p. 206.
PLANETS AND SATELLITES. 327
ters to be respectively six and seven miles. 1 This is on
the assumption that they reflect the same proportion of the
light incident upon them that their primary does. But it
may very well be that they are less reflective, in which case
they would be more extensive. The albedo of Mars, accord-
ing to Zollner, is 0.2762 ; his surface, in other words, returns
27.62 per cent, of the rays striking it. If we put the albedo of
his satellites equal to that of our moon, 0.1736, their diameters
will be increased from six and seven to 9! and u| miles,
Phobos, the inner one, being the larger. Their actual dimen-
sions do not, in all probability, exceed this estimate. It is in-
teresting to note that Deimos, according to Professor Pickering's
very distinct perception, does not share the reddish tint of Mars.
Both satellites move quickly in small orbits. Deimos com-
pletes a revolution in thirty hours eighteen minutes, at a dis-
tance from the surface of its ruling body of 12,500 miles;
Phobos in seven hours thirty-nine minutes twenty-two seconds,
at a distance of only 3760 miles. This is the only known
instance of a satellite circulating faster than its primary rotates,
and is a circumstance of some importance as regards theories
of planetary development. To a Martian spectator the curious
effect would ensue of a celestial object, seemingly exempt from
the general motion of the sphere, rising in the west, setting
in the east, and culminating three, or even four times a day.
The detection of new members of the solar system has come
to be one of the most ordinary of astronomical events. Since
1846 no single year has passed without bringing its tribute of
asteroidal discovery. In the last of the seventies alone, a full
score of miniature planets were distinguished from the throng-
ing stars amid which they seem to move ; 1875 brought seven-
teen such recognitions ; their number touched a minimum of
one in 1881 ; it rose in 1882 to eleven, dropped to fourin 1883,
and remounted as far as nine in 1884. At the present date
(September 1885), 250 asteroids are known to revolve between
the orbits of Mars and Jupiter. Of these, no less than forty-
1 Annals Harvard Coll. Obs.^ vol. xi. pt. ii. p. 317.
328 HISTORY OF ASTRONOMY.
eight are claimed by a single observer Professor J. Palisa of
Vienna ; Dr. C. H. F. Peters of Clinton, N.Y., comes in a good
second with forty-three ; Watson, Borrelly, Luther, Hind, Gold-
schmidt, Tempel, and many others, have each contributed
numerously to swell the sum-total. The construction by
Chacornac and his successors at Paris, and more recently by
Peters at Clinton, of ecliptical charts showing all stars down
to the thirteenth and fourteenth magnitudes respectively, ren-
ders the picking out of moving objects above that brightness
a mere question of time and diligence. Far more onerous is
the task of keeping them in view once discovered of tracking
out their paths, fixing their places, and calculating the disturb-
ing effects upon them of the mighty Jovian mass. These
complex operations have come to be centralised at Berlin under
the superintendence of Professor Tietjen, and their results are
given to the public through the medium of the Berliner As-
tronomisches Jahrbuch.
The crowd of orbits thus disclosed invites attentive study.
D'Arrest remarked in 185 1, 1 when only thirteen minor planets
were known, that supposing their paths to be represented by
solid hoops, not one of the thirteen could be "lifted from its
place without bringing the others with it. The complexity of
interwoven tracks thus illustrated has grown almost in the
numerical proportion of discovery. Yet no two actually in-
tersect, because no two lie exactly in the same plane, so that
the chances of collision are at present nil. There is only one
case, indeed, in which it seems to be eventually possible.
M. Lespiault has pointed out that the curves traversed by
" Fides " and " Mai'a " approach so closely that a time may
arrive when the bodies in question will either coalesce or
unite to form a binary system. 2
The maze threaded by the 250 asteroids contrasts singularly
with the harmoniously ordered and rhythmically separated
orbits of the larger planets. Yet the seeming confusion is not
1 Astr. Nack. y No. 752.
2 L. Niesten, Anmtaire, Bruxelles, 1881, p. 269.
PLANETS AND SATELLITES. 329
without a plan. The established rules of our system are far
from being totally disregarded by its minor members. The
orbit of Vesta, with its inclination of 34 42', touches the limit
of departure from the ecliptic-level ; the average plane of the
asteroidal paths differs by only about one degree from that of
the sun's equator ; their mean eccentricity is below that of the
curve traced out by Mercury, and all without exception are
pursued in the planetary direction from west to east.
The zone in which these small bodies travel is about three
times as wide as the interval separating the earth from the
sun. It extends perilously near to Jupiter, and actually en-
croaches upon the sphere of Mars. In one of his lectures at
Gresham College in iSyg, 1 Mr. Ledger remarked that the
minor planet Aethra, when in perihelion, gets inside Mars in
aphelion by as much as five millions of miles, though at so
different a level in space that there is no close approach.
The distribution of the asteroids over the zone frequented
by them is very unequal. They are most densely congregated
about the place where a single planet ought, by Bode's Law,
to revolve ; it may indeed be said that only stragglers from the
main body are found more than fifty million miles within
or without a mean distance from the sun 2.8 times that
of the earth. Significant gaps, too, occur where some force
prohibitive of their presence would seem to be at work.
What the nature of that force may be, Professor Daniel
Kirkwood of the Indiana University indicated, first in 1866
when the number of known asteroids was only eighty-eight,
and again with more confidence in 1876 from the study
of a list then run up to 172.2 It appears that these bare
spaces are found just where a revolving body would have a
period connected by a simple relation with that of Jupiter.
It would perform two or three circuits to his one, five to his
two, nine to his five, and so on. Kirkwood's inference is that
the gaps in question were cleared of asteroids by the attractive
influence of Jupiter. For disturbances recurring time after
1 Sun and Planets, p. 267. 3 Smiths. Report, 1876, p. 358.
330 HISTORY OF ASTRONOMY.
time owing to commensurability of periods nearly at the
same part of the orbit, would have accumulated until the
shape of that orbit was notably changed. The body thus dis-
placed would have come in contact with other cosmical particles
of the same family with itself then, it may be assumed, more
evenly distributed than now would have coalesced with them,
and permanently left its original track. In this way the re-
gions of maximum perturbation would gradually have become
denuded of their occupants.
We can scarcely doubt that this law of commensurability
has largely influenced the present distribution of the asteroids.
The correspondence of the facts with the hypothesis is in
general striking. At the same time it is not perfect. The
minor planet Menippe, for example, revolves almost exactly
five times while Jupiter revolves once ; and (as Professor
Newcomb has pointed out 1 ) several of its companions have
periods nearly three-eighths that of the disturbing planet.
The clue offered by Professor Kirkwood is not therefore to be
rejected ; but further inquiry, here as elsewhere, is needed.
Leverrier fixed, in 1853,2 one-fourth of the earth's mass as
the outside limit for the combined masses of all the bodies
circulating between Mars and Jupiter; but it is far from
probable that this maximum is at all nearly approached. M.
Niesten estimated that the whole of the 216 asteroids dis-
covered up to August 1880 amounted in volume to only ^oVs
of our globe, 3 and we may safely add since they are tolerably
certain to be lighter, bulk for bulk, than the earth that their
proportionate mass is smaller still. Professor Pickering, from
determinations of light-intensity, assigns to Vesta a diameter of
319 miles, to Pallas 167, to Juno 94, down to twelve and
fourteen for the smaller members of the group. 4 An albedo
equal to that of Mars is assumed as the basis of the calculation.
Professor M. W. Harrington, director of the Ann Arbor
1 Pop. Astr.) p. 338 (2d ed.) 2 Comptes Rendus, t. xxxvii. p. 797.
3 Annuaire, Bruxelles, 1881, p. 243.
4 Harvard Annals, vol. xi. part ii. p. 294.
PLANETS AND SATELLITES. 331
Observatory, on the other hand, concludes Vesta, from the size
of her visible disc, to be as much as 520 miles across. 1 But
if this be so, her surface is singularly absorptive of light,
returning only ten per cent, of the rays striking it. The same
observer holds Vesta and Flora to be together nearly equal in
bulk to the whole of their remaining companions. 2 He has
also ascertained, with much probability, the variability of
Vesta to the extent of one stellar magnitude, and attributes the
changes to a rapid axial rotation combined with an unequally
reflective surface.
There is no good reason to suppose that any of the minor
planets possess atmospheres. The aureolas seen by Schroter
to surround Ceres and Pallas have been dissipated by optical
improvements. Vogel in 1872 thought he had detected an
air-line in the spectrum of Vesta ; 3 but admitted that its pre-
sence required confirmation, which has not been forthcoming.
Crossing the zone of asteroids on our journey outward from
the sun, we meet with a group of bodies widely different from
the "inferior" or ten^strir.! planets. Their gigantic size, low
specific gravity, and rapid rotation, obviously from the first
threw the " superior " planets into a class apart ; and modern
research has added qualities still more significant of a dis-
similar physical constitution. Jupiter, a huge globe 86,000
miles in diameter, stands pre-eminent amongst them. He is,
however, only primus inter pares ; all the wider inferences
regarding his condition may be extended, with little risk of
error, to his fellows ; and inferences in his case rest on surer
grounds than in the case of the others, from the advantages
offered for telescopic scrutiny by his comparative nearness.
Now the characteristic modern discovery concerning Jupiter
is that he is a body midway between the solar and terrestrial
stages of cosmical existence a decaying sun or a developing
earth, as we choose to put it whose vast unexpended stores
1 Am. Jour, of Sc., vol. xxvi. (3d sen), p. 464.
2 Observatory, vol. vii. p. 339. 3 Spectra der Planeten, p. 24.
332 HISTORY OF ASTRONOMY.
of internal heat are mainly, if not solely, efficient in producing
the interior agitations betrayed by the changing features of his
visible disc. This view was anticipated in the last century.
Buffon wrote in his Epoques de la Mature (zyyS): 1 "La
surface de Jupiter est, comme Ton sait, sujette a des change-
mens sensibles, qui semblent indiquer que cette grosse planete
est encore dans un etat d'inconstance et de bouillonnement."
Primitive incandescence, attendant, in his fantastic view, on
planetary origin by cometary impacts with the sun, combined,
he concluded, with vast bulk to bring about this result. Jupiter
had not yet had time to cool. Kant thought similarly in 1 785 ; 2
but the idea did not commend itself to the astronomers of the
time, and dropt out of sight until Mr. Nasmyth arrived at it
afresh in i853. 3 Even still, however, terrestrial analogies held
their ground. The dark belts running parallel to the equator,
first seen at Naples in 1630, continued to be associated as
Herschel had associated them in 1781 with Jovian trade-
winds, in raising which the deficient power of the sun was
supposed to be compensated by added swiftness of rotation.
But opinion was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some
remarkable indications from experiments on the light of
Jupiter. 4 They showed that fourteen times more of the photo-
graphic rays striking it are reflected by the planet than by our
moon, and that, unlike the moon, which sends its densest rays
from the margin, Jupiter is brightest near the centre. But the
most perplexing part of his results was that Jupiter actually
seemed to give out more light than he received. The question
of original luminosity thus definitely raised can assuredly not
be answered with an unqualified negative. Bond, however,
considered his data too uncertain for the support of so bold an
assumption, and, even if the presence of native light were
proved, thought that it might emanate from auroral clouds of
1 Tom. i. p. 93. 2 Berlinische Monatsschrift, 1785, p. 211.
3 Month. A 7 ot., vol. xiii. p. 40. 4 Mem. Am. Ac., vol. viii. p. 221.
PLANETS AND SATELLITES. 333
the terrestrial kind. The conception of a sun-like planet was
still a remote, and seemed an extravagant one.
Only since it was adopted and enforced by Zollner in I865, 1
can it be regarded as permanently acquired to science. The
rapid changes in the cloud-belts both of Jupiter and Saturn,
he remarked, attest a high internal temperature. For we
know that all atmospheric movements on the earth are sun-
heat transformed into motion. But sun-heat at the distance
of Jupiter possesses but ^Y* at tnat of Saturn -j-J^ of its force
here. The large amount of energy, then, obviously exerted
in those remote firmaments must have some other source, to
be found nowhere else than in their own active and all-pervad-
ing fires, not yet banked in with a thick solid crust.
The same acute investigator dwelt, in 187 1, 2 on the simi-
larity between the modes of rotation of the great planets and
of the sun, applying the same principles of explanation to each
case. The fact of this similarity is undoubted. Cassini 8 and
Schroter both noticed that markings on Jupiter travelled
quicker the nearer they were to his equator; and Cassini even
hinted at their possible assimilation to sun-spots. 4 It is now
well ascertained that, as a rule (not altogether without excep-
tions), equatorial spots give a period some 5j minutes shorter
than those in latitudes of about 30. But, as Mr. Denning
has pointed out, 5 no single period will satisfy the observations
either of different markings at the same epoch, or of the same
markings at different epochs. Accelerations and retardations,
depending upon internal conditions, take place in very much
the same kind of way as in solar maculae, inevitably suggesting
a similar eruptive origin.
Amongst popular writers, Mr. Proctor has been foremost in
realising the highly primitive condition of these giant orbs, and
in impressing the facts and their logical consequences upon
the public mind. The inertia of ideas on the subject has been
1 Photom. Unters., p. 303. 2 Astr. Nach., No. 1851.
3 Mtm. de FAc., t. x. p. 514. 4 Ibid., 1692. p. 7.
6 Month. Not., vol. xliv. p. 63.
334 HISTORY OF ASTRONOMY.
overcome largely through the arguments reiterated in the
various and well-known works published by him since 1870.
It should be added that Mr. Mattieu Williams in his Fuel of
the Sun adopted, equally early, similar views.
The interesting query as to Jupiter's surface-incandescence
has been studied since Bond's time with the aid of all the
appliances furnished to physical inquirers by modern inven-
tiveness, yet without bringing to it a categorical reply.
Zollner in 1865 estimated his albedo at 0.62, that of fresh-
fallen snow being 0.78, and of white paper 0.70. 1 But the
disc of Jupiter is by no means purely white. The general
ground is tinged with ochre, the polar zones are leaden or
fawn-coloured, large spaces are at times stained or diffused
with chocolate-browns and rosy hues. It is occasionally seen
ruled from pole to pole with dusky bars, and is never wholly
free from obscure markings. The reflection then by it, as a
whole, of 62 per cent, of the rays impinging upon it, might
well suggest some original reinforcement.
Nevertheless, the spectroscope gives little countenance to
the supposition of any considerable permanent light-emission.
The spectrum of Jupiter, as examined by Huggrns, 1862-64,
and by Vogel, 1871-73, shows the familiar Fraunhofer rays
belonging to reflected sunlight. But it also shows lines of
native absorption. Some of these are identical with those
produced by the action of our own atmosphere, especially one
or more groups due to aqueous vapour ; others are of unknown
origin, and it is remarkable that one amongst the latter a
strong band in the red agrees in position with a dark line in
the spectra of some ruddy stars. 2 There is, besides, a general
absorption of blue rays, intensified as Le Sueur observed at
Melbourne in 1869 3 in the dusky markings, evidently through
an increase of depth in the atmospheric strata traversed by
the light proceeding from them.
All these observations, hcvrever (setting aside the stellar
1 Photom. Unters., pp. 165, 273. 2 Vogel, Sp. d. Planeten, p. 33, note.
3 Proc. Roy. Soc., vol. xviii. p. 250.
PLANETS AND SATELLITES. 335
line as of doubtful significance), point to a cool planetary
atmosphere. ' There is, we believe, only one on record evincing
unmistakably the presence of intrinsic light. On September
27, 1879, Dr. Henry Draper obtained a photograph of Jupiter's
spectrum, in which a strengthening of the impression was
visible in the parts corresponding to the planet's equatorial
regions. 1 This is just the right sort of evidence, but it is
altogether exceptional. We are driven then to conclude that
native emissions from Jupiter's visible surface are local and
fitful, not permanent and general. Indeed, the total disap-
pearance of his satellites on entering his shadow-cone, suf-
ficiently proves that they receive from him no sensible illumina-
tion. This conclusion, however, by no means invalidates that
of his excessively high internal temperature.
The curious phenomena attending Jovian satellite-transits
may be explained, partly as effects of contrast, partly as due
to temporary obscurations of the small discs projected on the
large disc of Jupiter. At their first entry upon its marginal
parts, which are two or three times less luminous than those
near the centre, they usually show as bright spots, then vanish,
and re-emerge dusky against the more lustrous background
met in their gradual advance. But they sometimes appear
bright throughout ; while, on the other hand, instances are not
rare, more especially of the third and fourth satellites stand-
ing out in such inky darkness as to be mistaken for their
own shadows. The earliest witness of a " black transit " was
Cassini, September 2, 1665; Romer in 1677, and Maraldi in
1707 and 1713, made similar observations, which have been
multiplied during the present century. In some cases, the
process of darkening has been visibly attended by the forma-
tion, or emergence into view, of spots on the transiting body, as
noted by the two Bonds at Harvard, March 18, 1848.2 The
third satellite was seen by Dawes, half dark, half bright, when
1 Month. Not., vol. xl. p. 433.
2 Engelmann, Ueber die Helligkeitsverhaltnisst der Jupiterstrabanten,
P- 59-
33^ HISTORY OF ASTRONOMY.
crossing Jupiter's disc, August, 21, 1867 ; x one-third dark by
Davidson of California, January 15, 1884, under the same
circumstances ; 2 and unmistakably spotted, both on and off
the planet, by Schroter, Secchi, Dawes, and Lassell.
The different effects produced by Jupiter's satellites in
transit result then intelligibly from the marked variability of
their light ; 3 and their variability seems, in some degree, to
depend upon their orbital positions. This amounts to saying
that, as Herschel concluded in 1797, they always, like our
moon, turn the same face towards their primary, thus always
presenting to us when in the same relative situations, the same
obscure or brilliant sections of their globes. As regards the
outer satellite, Engelmann's researches in 1871, and the late
C. E. Burton's in 1873, make this almost certain ; and there
is a strong probability that it also applies to the other three.
The phenomena, however, are quite too irregular to be com-
pletely rationalised on so simple and obvious a principle. We
are also driven to assume changes in the power of reflecting
light of the satellites themselves, which Vogel's detection of
lines in their spectra or traces of such indicative of gaseous
envelopes similar to that of Jupiter, entitle us tc regard as
possibly of atmospheric production.
In the course of his observations on Jupiter at Brussels in
1878, M. Niesten was struck with a rosy cloud attached to a
whitish zone beneath the dark southern equatorial band. 4 Its
size was enormous. At the distance of Jupiter, its measured
dimensions of 13" by 3" implied a real extension in longitude
of 30,000, in latitude of something short of 7000 miles. The
earliest record of its appearance seems to be by Professor
Pritchett, director of the Morrison Observatory (U.S.), who
figured and described it July 9, i878. 5 It was again delineated
1 Month. Not., vol. xxviii. p. II. 2 Observatory, vol. vii. p. 175.
3 There is a consensus among observers as to the marked variability of
all Jupiter's satellites, though Pickering strangely finds no trace of it in his
exact measures of their light. See Harvard Annals, vol. xi. pt. ii. p. 245.
4 Bull. Ac. R. Biuxelles, t. xlviii. p. 607. 5 Astr. Nach., No. 2294.
PLANETS AND SATELLITES. 337
August 9, by Tempel at Florence. 1 In the following year it
attracted the wonder and attention of almost every possessor
of a telescope. Its colour had by that time deepened into
a full brick-red, and was set off by contrast with a white
equatorial spot of unusual brilliancy. During three ensuing
years these remarkable objects continued to offer a visible
and striking illustration of the compound nature of the planet's
rotation. The red spot completed a circuit in nine hours
fifty-five minutes thirty-six seconds ; the white spot in about
five and a half minutes less." Their relative motion was thus
no less than 260 miles an hour, bringing them together in the
same meridian at intervals of forty-four days ten hours forty-
two minutes. Neither, however, preserved continuously the
same uniform rate of travel. The period of each had lengthened
by some seconds in 1883, while sudden displacements, asso-
ciated with the recovery of lustre after recurrent semi-efface-
ments, were observed in the position of the white spot, 2 recall-
ing the leap forwards of a reviving sun-spot. The analogy was
extended to the red spot by a shining aureola of " faculae,"
described by Bredichin at Moscow, and by Lohse at Potsdam,
as encircling it in September 1 879.3
The conspicuous visibility of this astonishing object lasted
three years, and may, it is thought, shortly recur. When the
planet returned to opposition in 1882-83, it had faded so con-
siderably that Riccb's uncertain glimpse of it at Palermo, May
31, 1883, was expected to be the last. It had, nevertheless,
begun to recover in December, and was seen, "reduced to a
mere skeleton " by internal wasting of substance and colour, by
Mr. Denning, February 18, 1885.* In this emaciated condition
it presented a striking likeness to an " elliptical ring " observed
in the same latitude by Mr. Gledhill at Halifax in 1869-70.
This, indeed, might be called the preliminary sketch for the
1 Astr. Nach., No. 2284.
2 Denning, Month. Not., vol. xliv. pp. 64, 66; Nature, vol. xxv. p. 226.
3 Astr. Nach., Nos. 2280, 2282.
4 Observatory, vol. viii. p. 95 ; Nature, July 4, 1885.
Y
338 HISTORY OF ASTRONOMY.
famous object brought to perfection ten years later, but which
Mr. H. C. Russell of Sydney saw and drew in June 1876,1 in
what might be called an unfinished condition, before it had
separated from its matrix, the dusky south-tropical belt. In
earlier times, too, a marking " at once fixed and transient " had
been repeatedly perceived attached to the southernmost of the
central belts. It gave Cassini in 1665 a rotation-period of nine
hours fifty-six minutes, 2 reappeared and vanished eight times
during the next forty-three years, and was last seen by Maraldi
in 1713. It was, however, very much smaller than the recent
object, and showed no unusual colour.
The assiduous observations made by Mr. Denning at Bristol
and by Professor Hough at Chicago on the " Great Red Spot "
of 1879-82 afforded grounds only for negative conclusions as
to its nature. It certainly did not represent the outpourings
of a Jovian volcano ; it was in no sense attached to the Jovian
soil if the phrase have any application to that planet ; it was
not a mere disclosure of a glowing mass elsewhere seethed
over by rolling vapours. To say that its origin was in some
way eruptive is to say almost nothing ; yet this is about all
that can safely be affirmed on the subject. It" might be de-
scribed, again, with some probability as an accidental excres-
cence on the general circulatory system of a strongly heated
and cooling body. There is some reason to suppose that its
surface was depressed below the average cloud level, and that
the cavity was filled with vapours. But it was almost certainly
not self-luminous, a satellite projected upon it in transit having
been seen to show as bright as upon the dusky equatorial bands.
In 1870, Mr. Ranyard, 3 acting upon an earlier suggestion
of Dr. Huggins, collected records of unusual appearances on
the disc of Jupiter, with a view to investigate the question of
their recurrence at regular intervals. He concluded that the
development of the deeper tinges of colour, and of the equa-
torial " port-hole " markings girdling the globe in regular
1 Proc. Roy. Soc. N. S. Wales, vol. xiv. p. 68.
2 Phil. Trans., vol. i. p. 143. 3 Month. Not., vol. xxxi. p. 34.
PLANETS AND SATELLITES. 339
alternations of bright and dusky, agreed, so far as could be
ascertained, with epochs of sun-spot maximum. The further
inquiries of Dr. Lohse at Bothkamp in 1873 1 went to strengthen
the coincidence, which had been anticipated h priori by
Zollner in iSyi. 2 Yet subsequent experience has rather added
to than removed doubts as to the validity of that first con-
clusion. It may, indeed, be taken for granted that what
Hahn terms the universal pulse of the solar system 3 affects
the vicissitudes of Jupiter ; but the law of those vicissitudes
is far from being so obviously subordinate to the rhythmical
flow of central disturbance as are certain terrestrial pheno-
mena. The fundamental agreement which probably exists is
confused in its display by secondary causes.
It is likely that Saturn is in a still earlier stage of planetary
development than Jupiter. He is the lightest for his size of
all the planets. In fact, he would float in water. And since
his density is shown, by the amount of his equatorial bulging,
to increase centrally, 4 it follows that his superficial materials
must be of a specific gravity so low as to be inconsistent, on
any probable supposition, with the solid or liquid states.
Moreover, the chief arguments in favour of the high tempera-
ture of Jupiter apply, with increased force, to Saturn ; so
that it may be concluded, without much risk of error, that a
large proportion of his bulky globe, 70,000 miles in diameter,
is composed of heated vapours, kept in active and agitated
circulation by the process of cooling.
His unique set of appendages has, since the middle of the
century, formed the subject of searching and fruitful inquiries,
both theoretical and telescopic. The mechanical problem of
the stability of Saturn's rings was left by Laplace in a very
unsatisfactory condition. Considering them as rotating solid
bodies, he pointed out that they could not maintain their
1 Beobachtungen, Heft ii. p. 99.
2 Ber. Sachs. Ges. der Wtss., 1871, p. 553.
3 Beziehungen der Sonnenfleckentxriode, p. 175*
4 A. Hall, Astr. Nach., No. 2269.
340 HISTORY OF ASTRONOMY.
position unless their weight were in some way unsymmetrically
distributed ; but made no attempt to determine the kind or
amount of irregularity needed to secure this end. Some
observations by Herschel gave astronomers an excuse for
taking for granted the fulfilment of the condition thus vaguely
postulated ; and the question remained in abeyance until
once more brought prominently forward by the discovery of
the inner dusky ring in 1850.
The younger Bond led the way, among modern observers,
in denying the solidity of the structure. The fluctuations in
its aspect were, he asserted in 185 1, 1 inconsistent with such an
hypothesis. The fine dark lines of division, frequently de-
tected in both bright rings, and as frequently relapsing into
imperceptibility, were due, in his opinion, to the real mobility
of their particles, and indicated a fluid formation. Professor
Benjamin Peirce of Harvard University immediately followed
with a demonstration, on abstract grounds, of their non-soli-
dity. 2 Streams of some fluid denser than water were, he
maintained, the physical reality giving rise to the anomalous
appearance first disclosed by Galileo's telescope. .
The mechanism of Saturn's rings, proposed as the subject
of the Adams Prize, was dealt with by the late James Clerk
Maxwell in 1857. His investigation forms the groundwork of
all that is at present known in the matter. Its upshot was to
show that neither solid nor fluid rings could continue to exist,
and that the only possible composition of the system was by
an aggregated multitude of unconnected particles, each revolv-
ing independently in a period corresponding to its distance
from the planet. 3 This idea of a satellite-formation had been,
remarkably enough, several times entertained and lost sight of.
It was first put forward by Roberval in the seventeenth century,
again by Jacques Cassini in 1715, and with perfect definiteness
by Wright of Durham in 1750.* Little heed, however, was taken
1 Astr. Jour. (Gould's), vol. ii. p. 17. 2 Ibid., p. 5.
3 On the Stability of the Motion of Saturris Rings, p. 67.
4 Mem. deFAc., 1715, p. 47; Montucla, Hist, des Math., t. iv. p. 19;
An Original Theory of the Universe, p. 115.
PLANETS AND SATELLITES. 341
of these casual anticipations of a truth which reappeared, a
virtual novelty, as the legitimate outcome of the most refined
modern methods.
The details of telescopic observation accord, on the whole,
admirably with this hypothesis. The displacements or dis-
appearance of secondary dividing-lines the singular striated
appearance, first remarked by Short in the eighteenth century,
last by Perrotin and Lockyer at Nice, March 18, I884, 1 show
the effects of waves of disturbance traversing a moving mass
of gravitating particles ; 2 the broken and changing line of the
planet's shadow on the ring gives evidence of variety in the
planes of the orbits described by those particles. There is
but one serious discrepancy.
On the satellite-theory, the obscure inner ring is formed of
similar small bodies to those aggregated in the lucid members
ot the system, only much more thinly strewn, and reflecting,
consequently, much less light. It is not, however, easy to see
why these sparser flights should show as a dense dark shading
on the body of Saturn. Yet this is invariably the case. The
objection, long felt, has recently been urged by Professor
Hastings of Baltimore. The brightest parts of these appendages,
he remarks, 3 are more lustrous than the globe they encircle ;
but if the inner ring consist of identical materials, possess-
ing presumably an equal reflective capacity, the mere fact of
their scanty distribution would not cause them to show as
dark against the same globe. The conclusion seems inevitable,
that the bright and dark rings are not composed of identical
materials.
A question of singular interest, and one which we cannot
refrain from putting to ourselves, is whether we see in the
rings of Saturn a finished structure, destined to play a per-
manent part in the economy of the system ; or whether they
represent merely a stage in the process of development out of
1 Comptes Rendus, t. xcviii. p. 718.
2 Proctor, Saturn and his System (1865), p. 125.
3 Smiths. Report, 1880 (Holden).
342 HISTORY OF ASTRONOMY.
the chaotic state in which it is impossible to doubt that the
materials of all planets were originally merged. M. Otto
Struve has attempted to give a definite answer to this im-
portant query.
A study of early and later records of observations disclosed
to him, in 1851, an apparent progressive approach of the inner
edge of the bright ring to the planet. The rate of approach he
estimated at about fifty-seven English miles a year, or 11,000
miles during the 194 years elapsed since the time of Huygens. 1
Were it to continue, a collapse of the system must be far
advanced within three centuries. But was the change real
or illusory a plausible, but deceptive inference from in-
secure data? M. Struve resolved to put it to the test. A
set of minutely careful micrometrical measures of the dimen-
sions of Saturn's rings, executed by himself at Pulkowa in the
autumn of 1851, was provided as a standard of future com-
parison ; and he was enabled to renew them, under closely
similar circumstances, in i882. 2 But the expected diminution
of the space between Saturn's globe and his rings had not
taken place. There was, indeed, a slight extension in the
width of the system, both outwards and inwards ;" but so slight
that it could hardly be considered to lie outside the limits
of probable error. Still it is worth notice that just such a
separation of the rings was indicated by Clerk Maxwell's theory,
so that there is an a priori likelihood of its being in progress.
Moreover, since 1657, when Huygens described the interval
between the ring and the planet as rather exceeding the width
of the ring, it is all but certain that a growth inwards has
actually occurred. For the two bright rings together, instead
of being narrower than the interval, are now more than one
and a half times as broad. Hence the expressions used by
Huygens, no less than most of the old drawings, are glaringly
inconsistent with the planet's present appearance.
1 Mem. de FAc. Imp. (St. Petersb.), t. vii. 1853, p. 464.
2 Astr. Nach., No. 2498.
PLANETS AND SATELLITES. 343
There seems reason to admit that Kirkwood's law of com-
mensurability has had some effect in bringing about the pre-
sent distribution of the matter composing these appendages.
Here the disturbing bodies are Saturn's moons, while the
divisions and boundaries of the rings represent the spaces
where their disturbing action conspires to eliminate revolving
particles. Kirkwood, in fact, showed, in I867, 1 that a body
circulating in the chasm between the bright rings known as
" Cassini's division," would have a period nearly commensurable
with those of four out of the eight moons; and Dr. Meyer of
Geneva has recently calculated all such combinations with the
result of bringing out coincidences between regions of maximum
perturbation and the limiting and dividing lines of the system. 2
This is in itself a strong confirmation of the view that the rings
are made up of independently revolving small bodies.
On December 7, 1876, Professor Asaph Hall discovered at
Washington a bright equatorial spot on Saturn, which he
followed and measured through above sixty rotations, each
performed in ten hours fourteen minutes twenty-four seconds. 3
He is careful to add that this represents the period, not
necessarily of the planet, but only of the individual spot.
The only previous determination of Saturn's axial movement
(setting aside some insecure estimates by Schroter) was
Herschel's in 1794, giving 4 period of ten hours sixteen
minutes.
Saturn's outermost satellite, Japetus, is markedly variable
so variable that it sends us, when brightest, just 4^ times as
much light as when faintest. Moreover, its fluctuations depend
upon its orbital position in such a way as to make it a con-
spicuous telescopic object when west, a scarcely discernible
one when east of the planet. Herschel's inference 4 of a
partially obscured globe turning always the same face towards
1 Meteoric Astronomy, chap. xii. He carried the subject somewhat
farther in 1871. See Observatory, vol. vi. p. 335.
2 Astr. Nach., No. 2527. 3 Am. Jour of Sc., vol. xiv. p. 325.
4 Phil. Irans.) vol. Ixxxii. p. 14.
344 HISTORY OF ASTRONOMY.
its primary, seems the only admissible one, and is confirmed
by Pickering's measurements of the varying intensity of its
light. He remarks further that the dusky and brilliant* hemi-
spheres must be so posited as to divide the disc, viewed from
Saturn, into nearly equal parts ; so that this Saturnian moon,
even when "full," appears very imperfectly illuminated over
one-half of its surface. 1
The spectrum of Saturn is closely similar to that of Jupiter.
It shows the distinctive dark line in the red, which we may
call the " red-star line ; " and J.anssen, examining it from the
summit of Etna in I86;, 2 found unmistakable traces of aqueous
absorption. The light from the ring is much less modified by
original atmospheric action.
Uranus can now -easily be seen with the naked eye as a star
somewhat below the fifth magnitude. He thus appears con-
siderably brighter than when discovered 105 years ago. Not,
however, through any intrinsic change. He is at present con-
spicuous simply because he has but lately passed perihelion. 3
This circumstance has enabled astronomers, provided with the
powerful telescopes of modern times, to make -some highly
interesting observations on this remote planet
It will be remembered that Uranus presents the unusual
spectacle of a system of satellites travelling nearly at right
angles to the plane of the ecliptic. The existence of this
anomaly gives a special interest to investigations of his axial
movement, which the analogy of the other planets might lead
us to presume to be executed in the same tilted plane. Yet
this, strange to say, does not seem to be the case.
Mr. BufTham in 1870-72 caught traces of bright markings
on the Uranian disc, suggesting, with much uncertainty, a
rotation in about twelve hours in a plane not coincident with
that in which his satellites circulate. 4 Dusky bands resembling
1 Smiths. Report, 1880. * Comptes Rendus, t. Ixiv. p. 1304.
3 Tebbutt, Jrans. Roy. Soc. N. S. Wales, vol. xiv. p. 23.
4 Month, Not., vol. xxxiii. p. 164.
II U IN J V K
v r*
PL A NETS A ND SA TELLITES. 345
those of Jupiter, but very faint, were barely perceptible to
Professor Young at Princeton in 1883. Yet, though inevitably
inferred to be equatorial, they made a considerable angle with
the trend of the satellites' orbits. 1 More distinctly by the
brothers Henry, with the aid of their fine refractor, two grey
parallel rulings, separated by a brilliant zone, were discerned
every clear night at Paris from January to June iSS/j.. 2 What
were taken to be the polar regions appeared comparatively
dusky. The direction of the equatorial rulings (for so we may
safely call them) made an angle of 40 with the satellites' line
of travel. Similar observations were made at Nice by MM.
Perrotin and Thollon, March to June 1884, a lucid spot near
the equator, in addition, indicating rotation in a period of
about ten hours. 3
Measurements of the little sea-green disc which represents
to us the massive bulk of Uranus, give, however, a different
result. Young, Schiaparelli, 4 and Schafarik have each found it
to be quite distinctly bulged ; and all agree that the bulging
lies just in the plane of the satellites' orbits. If this be so,
there can be no question but that the same plane is that of
the planet's rotation, the spheroidal shape of a rotating globe
being the necessary consequence of the greater equatorial
velocity of its particles. But the "equatorial" markings
visibly assert a rapid whirling in a direction removed by nearly
half a right angle from that plane. Which are we to believe ?
Where such minute quantities are concerned as in the differences
between the various diameters of a disc about four seconds
across, conclusions are of necessity highly precarious. They
cannot weigh against the positive assurance conveyed by the
parallel bands seen at Nice and Paris that Uranus now rotates
in a plane widely removed from that in which the bodies
dependent upon him circulate. This discrepancy may possibly
be the result of a violent change in the axis of rotation ; and
we might conjecture that the planet still retains the shape im-
1 Astr. Nach., No. 2545. 2 Comptes Rendus, t. xcviii. p. 1419.
3 Ibid., pp. 718, 967. 4 Astr. Nach., No. 2526.
346 HISTORY OF ASTRONOMY.
pressed by former conditions of movement, were it not that a
globe almost certainly plastic, if not largely vaporous, would at
once accommodate its form to their change, j
The spectrum of Uranus was first examined by Father
Secchi in 1869, and later, though with more advantages for
accuracy, by Huggins and Vogel. It is a very remarkable
one. In lieu of the reflected Fraunhofer lines, imperceptible
perhaps through feebleness of light, six broad bands of origi-
nal absorption appear, 1 one corresponding to the blue-green
ray of hydrogen (F), another to the " red-star line " of Jupiter
and Saturn, the rest as yet unidentified. The hydrogen band
seems much too strong and diffused to be the mere echo of a
solar line, and implies accordingly the presence of free hydro-
gen in the Uranian atmosphere, where a temperature must
thus prevail sufficiently high to reduce water to its constituent
elements.
Judging from the indications of an almost evanescent spec-
trum, Neptune, as regards physical condition, is the twin of
Uranus, as Saturn of Jupiter. Of the circumstances of his
rotation we are as good as completely ignorant. Mr. Maxwell
Hall, indeed, noticed at Jamaica, in November and December
1883, certain rhythmical fluctuations of brightness, suggesting
revolution on an axis in slightly less than eight hours ; 2 but
Professor Pickering reduces the supposed variability to an
amount altogether too small for certain perception, and Dr.
G. Miiller denies its existence in toto. It is true their observa-
tions were not precisely contemporaneous with those of Mr.
Hall, 3 who believes the partial obscurations recorded by him-
self to have been of a passing kind, and to have suddenly
ceased after a fortnight of prevalence. Their less conspicuous
renewal was visible to him in November 1884, confirming a
rotation period of 7.92 hours.
1 Vogel, Annalen der Phys., vol. clviii. p. 470.
3 Month. Not., vol. xliv. p. 257.
3 Observatory, vol. vii. pp. 134, 221, 264.
PLANETS AND SATELLITES. 347
The possibility that Neptune may not be the most remote
body circling round the sun has been contemplated ever since
he has been known to exist. Within the last few years the
position at a given epoch of a planet far beyond him has been
approximately fixed by two separate investigators. Its actual
discovery is perhaps one of the prizes reserved for the astrono-
mers of the future.
Professor George Forbes of Edinburgh hit upon in 1880 a
novel plan of search for unknown members of the solar system.
It depends upon the movements of comets. It is well known
that those of moderately short periods are, for some reason,
connected with the larger planets in such a way that the
cometary aphelia fall near some planetary orbit. Jupiter
claims above a dozen of such partial dependants, Neptune
owns six, and there are two considerable groups, the farthest
distances of which from the sun lie respectively near 100 and
300 times that of the earth. At each of these vast intervals,
one involving a period of 1000, the other of 5000 years, Pro-
fessor Forbes maintains that an unseen planet circulates. He
has even computed elements for the nearer of the two, and
fixed its place on the celestial sphere. 1
In the meantime, Mr. D. P. Todd of Washington had been
groping for the same object by the help of a totally different
set of indications. The old approved method of perturbations
was that adopted by him ; but those of Neptune have scarcely
yet had time to develop, so that he was thrown back upon
the " residual errors " of Uranus. They gave him a virtually
identical situation for the new planet with that arrived at by
Professor Forbes. 2 If this be a coincidence, it is a very re-
markable one, the more so as each inquirer worked in complete
ignorance of the results of the other.
1 Proc. Roy. Sec. Edinb., vol. x. p. 429 ; Observatory > vol. iii. p. 439.
2 Am. Jour, of Sc., vol. xx. p. 225.
( 348 )
CHAPTER IX.
THEORIES OF PLANETARY EVOLUTION.
WE cannot doubt that the solar system, as we see it, is the
result of some process of growth that, during innumerable
ages, the forces of Nature were at work upon its materials,
blindly modelling them into the shape appointed for them
from the beginning by Omnipotent Wisdom. To set ourselves
to inquire what that process was, may be an audacity, but it
is a legitimate, nay, an inevitable one. For man's implanted
instinct to " look before and after " does not apply to his own
little life alone, but regards the whole history of creation, from
the highest to the lowest from the microscopic germ of an alga
or a fungus to the visible frame and furniture of the heavens.
Kant considered that the inquiry into the mode of origin of
the world was one of the easiest problems set by Nature ; but
it cannot be said that his own solution of it was a satisfactory
one. He, however, struck out in 1755 a track which thought
still pursues. In his Allgemeine Naturgeschichte the growth of
sun and planets was traced from the cradle of a vast and form-
less mass of evenly diffused particles, and the uniformity of
their movements was sought to be accounted for by the
uniform action of attractive and repulsive forces, under the
dominion of which their development was carried forwards.
In its modern form, the " Nebular Hypothesis " made its
appearance in 1796. x It was presented by Laplace with
diffidence, as a speculation unfortified by numerical buttresses
of any kind, yet with visible exultation in having, as he thought,
1 Exposition du Systlme du Monde, t. ii. p. 295.
THEORIES OF PLANETARY EVOLUTION. 349
penetrated the birth-secret of our system. He demanded,
indeed, more in the way of postulates than Kant had done.
He started with a sun ready-made, 1 and surrounded with a
vast glowing atmosphere, extending into space out beyond the
orbit of the farthest planet, and endowed with a slow rotatory
motion. As this atmosphere or nebula cooled, it contracted ;
and as it contracted, its rotation, by a well-known mechani-
cal law, became accelerated. At last, a point arrived when
centrifugal force at the equator increased beyond the power
of gravity to control, and equilibrium was restored by the
separation of a nebulous ring revolving in the same period as
the generating mass. After a time, the ring broke up into
fragments, all eventually reunited in a single revolving and
rotating body. This was the first and farthest planet.
Meanwhile the parent nebula continued to shrink and whirl
quicker and quicker, passing, as it did so, through successive
crises of instability, each resulting in, and terminated by, the
formation of a planet, at a smaller distance from the centre,
and with a shorter period of revolution than its predecessor.
In these secondary bodies the same process was repeated on
a reduced scale, the birth of satellites ensuing upon their con-
traction, or not, according to circumstances. Saturn's ring,
it was added, afforded a striking confirmation of the theory of
annular separation, 2 and appeared to have survived in its
original form in order to throw light on the genesis of the
whole solar system ; while the four first discovered asteroids
offered an example in which the debris of a shattered ring had
failed to coalesce into a single globe.
This scheme of cosmical evolution was a characteristic
bequest from the eighteenth century to the nineteenth. It
possessed the self-sufficing symmetry and entireness appro-
priate to the ideas of a time of renovation, when the com-
plexity of nature was little accounted of in comparison with the
imperious orderliness of the thoughts of man. Since it was
1 In later editions a retrospective clause was added admitting a prior
condition of all but evanescent nebulosity. 2 Mec. Cel., lib. xiv. ch. iii.
350 HISTORY OF ASTRONOMY.
propounded, however, knowledge has transgressed many
boundaries, and set at naught much ingenious theorising.
How has it fared with Laplace's sketch of the origin of the
world? It has at least not been discarded as effete. The
groundwork of speculation on the subject is still furnished by
it. It is, nevertheless, admittedly inadequate. Of much that
exists it gives no account, or an erroneous one. It is certain
that the march of events did not everywhere it is doubtful
whether it anywhere followed the exact path prescribed for it.
Yet modern science attempts to supplement, but scarcely ven-
tures to supersede it.
Thought has, in many directions, been profoundly modified
by Mayer's and Joule's discovery, in 1842, of the equivalence
between heat and motion. Its corollary was the grand idea of
the " conservation of energy," now one of the cardinal principles
of science. This means that, under the ordinary circum-
stances of observation, the old maxim ex nihilo nihil fit applies
to force as well as to matter. The supplies of heat, light,
electricity, must be kept up, or the stream will cease to flow.
The question of the maintenance of the sun's heat was thus
inevitably raised ; and with the question of maintenance that
of origin is indissolubly connected.
Dr. Julius Robert Mayer, a physician residing at Heilbronn,
was the first to apply the new light to the investigation of what
Sir John Herschel had termed the " great secret." He showed
that if the sun were a body either simply cooling or in a state
of combustion, it must long since have "gone out." Had an
equal mass of coal been set alight, four or five centuries
after the building of the Pyramid of Cheops, and kept burn-
ing at such a rate as to supply solar light and heat during
the interim, only a few cinders would now remain in lieu of
our undiminished glorious orb. Mayer looked round for an
alternative. He found it in the " meteoric hypothesis " of
solar conservation. 1 The importance in the economy of our
system of the bodies known as falling stars was then (in 1848)
1 Beitrdge zur Dynamik des Himmels, p. 12.
THEORIES OF PLANETARY EVOLUTION. 351
beginning to be recognised. It was known that they revolved
in countless swarms round the sun ; that the earth daily en-
countered millions of them ; and it was surmised that the cone
of the zodiacal light represented their visible condensation
towards the attractive centre. From the zodiacal light, then,
Mayer derived the store needed for maintaining the sun's
radiations. He proved that, by the stoppage of their motion
through falling into the sun, bodies would evolve from 4600
to 9200 times as much heat (according to their ultimate
velocity) as would result from the burning of equal masses of
coal, their precipitation upon the sun's surface being brought
about by the resisting medium observed to affect the revolu-
tions of Encke's comet. There was, however, a difficulty.
The quantity of matter needed to keep, by the sacrifice of its
movement, the hearth of our system warm and bright, would
be very considerable. Mayer's lowest estimate put it at
94,000 billion kilogrammes per second, or a mass equal to
that of our moon bi-annually. But so large an addition to the
gravitating power of the sun would quickly become sensible in
the movement of the bodies dependent upon him. Their
revolutions would be notably accelerated. Mayer admitted
that each year would be shorter than the previous one by a
not insignificant fraction of a second, and postulated an un-
ceasing waste of substance, such as Newton had supposed
must accompany emission of the material corpuscles of light,
to neutralise continual reinforcement.
Mayer's views obtained a very small share of publicity, and
owned Mr. Waterston as their independent author in this
country. The meteoric, or "dynamical" theory of solar sus-
tentation was expounded by him before the British Associa-
tion in 1853. It was developed with his usual ability by Sir
William Thomson in the following year. The inflow of
meteorites, he remarked, " is the only one of all conceivable
causes of solar heat which we know to exist from independent
evidence." l We know it to exist, but we now also know it to
1 Trans. Roy. 0c. of Edinburgh, vol. xxi. p. 66.
352 HISTORY OF ASTRONOMY.
be entirely insufficient. The supplies presumed to be con-
tained in the zodiacal light would be quickly exhausted; a
constant inflow from space would be needed to meet the
demand. But if moving bodies were drawn into the sun at
anything like the required rate, the air, even out here at
ninety-three millions of miles distance, would be thick with
them ; the earth would be red-hot from their impacts ; l geo-
logical deposits would be largely meteoric ; 2 to say nothing
of the effects on the mechanism of the heavens. Sir William
Thomson himself urged the inadmissibility of the "extra-
planetary" theory of meteoric supply on the very tangible
ground that, if it were true, the year would be shorter now,
actually by six weeks, than at the opening of the Christian era.
The " intra-planetary " supply, however, is too scanty to be
anything more than a temporary makeshift.
The meteoric hypothesis was naturally extended from the
maintenance of the sun's heat to the formation of the bodies
circling round him. The earth no less doubtless than the
other planets is still growing. Cosmical matter in the shape
of falling stars and aerolites, to the amount, adopting Professor
Newton's estimate, of 100 tons daily, is swept u'p by it as it
pursues its orbital round. Inevitably the idea suggested itself
that this process of appropriation gives the key to the life-
history of our globe, and that the momentary streak of fire in
the summer sky represents a feeble survival of the glowing
hail-storm by which, in old times, it was fashioned and warmed.
Mr. E. W. Brayley supported this view of planetary produc-
tion in i864, 3 and it has recommended itself to Haidinger,
Helmholtz, Proctor, and Faye. But the negative evidence of
geological deposits appears fatal to it.
The theory of solar energy now generally regarded as the
true one, was enounced by Helmholtz in a popular lecture
in 1854. It depends upon the same principle of the equi-
1 Newcomb, Pop. Astr., p. 521 (2d ed.)
2 M. Williams, Nature, vol. iii. p. 26.
3 Conip. Brit. Almanac, p. 94.
THEORIES OF PLANETARY EVOLUTION. 353
valence of heat and motion which had suggested the meteoric
hypothesis. But here the movement surrendered and trans-
formed belongs to the particles, not of any foreign bodies, but
of the sun itself. Drawn together by the force of their own
gravity from a wide ambit, their fall towards the sun's centre
must have engendered a vast thermal store, of which f f are
computed to be already spent. Presumably, however, this
stream of reinforcement is still flowing. In the very act of
parting with heat, the sun develops a fresh stock. His
radiations, in short, are the direct result of shrinkage through
cooling. A diminution of the solar diameter by 300 feet
yearly (Langley) wou\d just suffice to cover the present rate of
emission. But the process, though not terminated, is strictly
a terminable one. In five million years, the sun will have
contracted to half its present bulk. In seven million more, it
will be as dense as the earth. It is difficult to believe that it
will then be a luminous body. 1 Nor can an unlimited past
duration be admitted. Helmholtz considered that radiation
might have gone on with its actual intensity for twenty-two,
Langley allows only eighteen million years. The period can
scarcely be stretched, by the most generous allowances, to
double the latter figure. But this is far from meeting the
demands of geologists and biologists.
An ingenious attempt has lately been made to supply the
sun with machinery analogous to that of a regenerative furnace,
enabling it to consume the same fuel over and over again, and
so to prolong indefinitely its beneficent existence. The inor-
dinate "waste" of energy, which shocks our thrifty ideas, was
simultaneously abolished. The earth stops and turns variously
to account one 225o-millionth part of the solar radiations;
each of the other planets and satellites takes a proportionate
share ; the rest, being all but an infinitesimal fraction of the
whole, is dissipated through endless space, to serve what pur-
pose we know not. Now, on the late Sir William Siemens's
plan, this reckless expenditure would cease ; the solar incomings
1 Newcomb, Pop. Astr., pp. 521-525.
354 HISTORY OF ASTRONOMY.
and outgoings would be regulated on approved economic
principles, and the inevitable final bankruptcy would be staved
off to remote ages. Let us see how it is to be done.
We must first imagine space to be filled with combustible
substances hydrogen, hydro-carbons, and oxygen in an ex-
cessively rarefied state. Next, that the sun keeps up, by its
rotation, a fan-like action on this floating matter, drawing it
inwards at the polar surfaces, and projecting it outwards at the
equator " in a continuous disc-like stream." x But it will not
travel from the sun unchanged. Combustion will have inter-
vened. In other words, the particles sucked in will have
surrendered their stored-up energy in the shape of heat and
light, and they will depart, no longer combustible, but the
mere inert products of combustion. By the very power of
the radiations they had contributed to supply, however, they
may be restored to activity. Sir W. Siemens obtained some
experimental evidence that carbonic acid and water may
possibly be dissociated in space, as they undoubtedly are in
the leaves of plants, by the power of direct sunshine. Their
particles, thus compulsorily separated, and by the act restocked
with energy, are ready to rush together again with fresh evo-
lution of heat and light. A mechanical circulation is, in this
way, combined with a pendulum-swing of chemical change,
and the round might go on for ever, if only one condition
were granted. That one condition is an unlimited supply of
motive power. It is, however, an inexorable law of nature
that there is no work without waste. Ex nihilo nihil fit
In this case, the heart-throb of the circulating system resides
in the rotation of the sun. Therein is contained a certain
definite amount of mechanical power enough, according to
Sir W. Thomson, if directly converted into heat, to keep up
the sun's emission during 116 years and six days a mere
moment in cosmical time. More economically applied, it
would no doubt go farther. Its exhaustion would neverthe-
less, under the most favourable circumstances, ensue in a
1 Proc. Roy. Soc., vol. xxxiii. p. 393.
THEORIES OF PLANETARY EVOLUTION. 355
comparatively short period. 1 Many other objections equally
unanswerable have been urged to the " regenerative " hypo-
thesis, but this one suffices.
There remains, then, as the only intelligible rationale of solar
sustentation, Helmholtz's shrinkage theory. And this has a
very important bearing upon the nebular view of planetary
formation : it may, in fact, be termed its complement. For
it involves the idea that the sun's materials, once enormously
diffused, gradually condensed to their present volume with
development of heat and light, and, it may plausibly be added,
the separation of dependent globes. The data furnished by
spectrum analysis, too, favour the supposition of a common
origin for sun and planets by showing their community of
substance ; while gaseous nebulae present examples of vast
masses of tenuous vapour, such as our system may plausibly
be conjectured to have primitively sprung from.
But recent science raises many objections to the details, if
it supplies some degree of confirmation to the fundamental
idea of Laplace's cosmogony. The detection of the retrograde
movement of Neptune's satellite made it plain that the anoma-
lous conditions of the Uranian world were due to no extra-
ordinary disturbance, but to a systematic variety of arrangement
at the outskirts of the solar domain. So that, were a trans-
Neptunian planet discovered, we should be fully prepared to
find it rotating, and surrounded by satellites circulating from
east to west. The uniformity of movement, upon the proba-
bilities connected with which the French geometer mainly
based his scheme, thus at once vanishes.
The excessively rapid revolution of the inner Martian moon
is a further stumbling-block. On the nebular view, no satellite
can revolve in a shorter time than its primary rotates ; for in
its period of circulation survives the period of rotation. of the
1 To this hostile argument, as urged by Mr. E. Douglas Archibald, Sir
W. Siemens opposed the increase of rotative velocity through contrac-
tion (Nature, vol. xxv. p. 505). But contraction cannot restore lost mo-
mentum.
356 HISTORY OF ASTRONOMY.
parent mass which filled the sphere of its orbit at the time of
giving it birth. And rotation quickens as contraction goes on ;
therefore, the older time of axial rotation should invariably
be the longer. There is, however, a way out of this difficulty,
presently to be adverted to.
More serious is one connected with the planetary periods,
pointed out by Babinet in iS6i. 1 In order to make them fit
in with the hypothesis of successive separation from a rotating
and contracting body, certain arbitrary assumptions have to
be made of fluctuations in the distribution of the matter
forming that body at the various epochs of separation. Such
expedients usually merit the distrust which they inspire.
Again, it was objected by Professor Kirkwood in 1869 2 that
there could be no sufficient cohesion in such an enormously
diffused mass as the planets are supposed to have sprung from,
to account for the wide intervals between them. The matter
separated, through the growing excess of centrifugal speed,
would have been cast off, not by rarely recurring efforts, but
continually, fragmentarily, pari passu with condensation and
acceleration. Each wisp of nebula, as it found itself unduly
hurried, would have declared its independence," and set about
revolving and condensing on its own account. The result
would have been a meteoric, not a planetary system.
Moreover, it is a question whether the relative ages of the
planets do not follow an order just the reverse of that con-
cluded by Laplace. Professor Newcomb holds the opinion
that the rings which eventually constituted the planets, divided
from the main body of the nebula almost simultaneously,
priority, if there were any, being on the side of the inner and
smaller ones ; 3 while, in M. Faye's ingenious supplement to
the nebular cosmogony, 4 the retrograde motion of the systems
formed by the two outer planets is ascribed on grounds, it is
true, of dubious validity to their comparatively late origin.
1 Comptes Rendus, t. lii. p. 481. See also Kirkwood, Observatory, vol.
iii. p. 409. 2 Month. Not., vol. xxix. p. 96.
3 Pop. Astr., p. 527. 4 Nature, vol. xxxi. p. 194.
THEORIES OF PLANETARY EVOLUTION. 357
We now come to a most remarkable investigation one,
indeed, unique in its profession to lead us back with mathe-
matical certainty towards the origin of a heavenly body. We
refer to Mr. G. H. Darwin's recent inquiries into the former
relations of the earth and moon. 1
They deal exclusively with the effects of tidal friction, and
primarily with those resulting, not from oceanic, but from
" bodily " tides, such as the sun and moon must have raised in
past ages on a liquid or viscous earth. The immediate effect
of either is, as already explained, to destroy the rotation of
the body on which the tide is raised, as regards the tide-raising
body, bringing it to turn always the same face towards its
disturber. This, we can see, has been completely brought
about in the case of the moon. There is, however, a secon-
dary or reactive effect. Action is always mutual. Precisely as
much as the moon pulls the terrestrial tidal wave backward,
the tidal wave pulls the moon forward. But pulling a body
forward in its orbit implies the enlargement of that orbit ; that
is to say, the moon is, as a consequence of tidal friction, very
slowly receding from the earth. This will go on (other cir-
cumstances remaining unchanged) until the lengthening day
overtakes the more tardily lengthening month, when each will
be of about 1400 hours. A position of what we may call tidal
equilibrium between earth and moon will (apart from disturb-
ance by other bodies) then be attained.
If, however, it be true that, in the time to come, the moon
will be much farther from us, it follows that in the time past
she was much nearer to us than she now is. Tracing back
her history by the aid of Mr. Darwin's clue, we at length find
her revolving in a period of somewhere between two and four
hours, almost in contact with an earth rotating just at the same
rate. This was before tidal friction had begun its work of
grinding down axial velocity and expanding orbital range.
But the position was not one of stable equilibrium. The
slightest inequality must have set on foot a series of uncom-
1 Phil. Trans., vol. clxxi. p. 713.
358 HISTORY OF ASTRONOMY.
pensated changes. If the moon had whirled the least iota
faster than the earth spun, she must have been precipitated
upon it. Her actual existence shows that the trembling
balance inclined the other way. By a second or two to begin
with, the month exceeded the day ; the tidal wave crept ahead
of the moon ; tidal friction came into play, and our satellite
started on its long spiral journey outward from the parent
globe. This must have occurred, it is computed, at least fifty-
four million years ago.
Assuming the exactness of the physical data involved a
proviso which may cover a good deal of doubt these con-
clusions are, in the opinion of those most competent to judge,
mathematically certain. An irresistible conjecture carries us
one step beyond them. The moon's time of revolution, when
so near the earth as barely to escape contact with it, must
have been, by Kepler's Law, more than two, and less than two
and a half hours. Now it happens that the most rapid rate of
rotation of a fluid mass of the earth's average density, con-
sistent with spheroidal equilibrium, is two hours and twenty
minutes. Quicken the movement but by one second, and the
globe must fly asunder. Hence the inference that the earth
actually did fly asunder through over-fast spinning, the ensuing
disruption representing the birth-throes of the moon. It is
likely that the event was hastened or helped by solar tidal
disturbance.
To recapitulate. Analysis tracks backward the two bodies
until it leaves them in very close contiguity, one rotating and
the other revolving in approximately the same time, and that
time certainly not far different from, and quite possibly
identical with, the critical period of instability for the terrestrial
spheroid. " Is this," Mr. Darwin asks, " a mere coincidence,
or does it not rather point to the break-up of the primeval
planet into two masses in consequence of a too rapid rota-
tion ? " J Few will hesitate as to the answer.
i
1 Phil. Trans., vol. clxxi. p. 835.
THEORIES OF PLANETARY EVOLUTION. 359
This investigation was communicated to the Royal Society,
December 18, 1879. It was followed, January 20, iSSi/by an
inquiry on the same principles into the earlier condition of
the entire solar system. The results were a warning against
hasty generalisation. They showed that the lunar terrestrial
system, far from being a pattern for their development, was a
singular exception amongst the bodies swayed by the sun.
Its peculiarity resides in the fact that the moon is proportion-
ately by far the most massive attendant upon any known
planet. Its disturbing power over its primary is thus abnor-
mally great, and tidal friction has, in consequence, played a
predominant part in bringing their mutual relations into their
present state.
The comparatively late birth of the moon tends to ratify
this inference. The dimensions of the earth did not differ
(according to Mr. Darwin) very greatly from what they now
are when her solitary offspring came, somehow, into existence.
This is found not to have been the case with any other of the
planets. It is unlikely that the satellites of Jupiter, Saturn, or
Mars (we may safely add of Uranus or Neptune) ever revolved
in much narrower orbits than those they now traverse ; it is
practically certain that they did not originate close to the
present surfaces of their primaries, like our moon. 2 What
follows ? The tide-raising power of a body grows with nearness
in a rapidly accelerated ratio. Lunar tides must then have
been on an enormous scale when the moon swung round just
outside the earth's atmosphere. But no other satellite with
which we are acquainted occupied at any time a corresponding
position. Hence no -other satellite ever possessed tide-raising
capabilities in the least comparable to those of the moon. We
conclude once more that tidal friction had an influence here
quite different from its influence elsewhere.
There is, however, another branch of the same subject. We
know that the sun as well as the moon causes tides in our
oceans. There must then be solar, no less than lunar tidal
1 FhiL Trans., vol. clxxii. p. 491. 2 Jbid. t p. 530.
360 HISTORY OF ASTRONOMY.
friction. The question at once arises : What part has it played
in the development of the solar system ? Has it ever been
one of leading importance, or has its influence always been, as
it now is, subordinate, almost negligeable ? To this, too, Mr.
Darwin supplies an answer.
It can be stated without hesitation that the sun did not give
birth to the planets, as the earth may have given birth to the
moon, by the disruption of its already condensed, though
plastic and glowing mass, pushing them then gradually back-
ward from its surface into their present places. For the utmost
possible increase in the length of the year through tidal friction
is one hour ; and five minutes is a more probable estimate. 1
So far as the pull of tide- waves raised on the sun by the
planets is concerned, then, the distances of the latter have
never been notably different from what they now are ; though
that cause may have converted the paths traversed by them
from circles into ellipses.
Over their physical history, however, it was probably in a
large measure influential. The first vital issue for each of
them was satellites or no satellites ? Were they to be gover-
nors as well as governed, or should they revolve in sterile
isolation throughout the aeons of their future existence ? Here
there is strong reason to believe that solar tidal friction
was the overruling power. It is remarkable that planetary
fecundity increases at least so far outward as Saturn with
distance from the sun. Can these two facts be in any way
related? In other words, is there any conceivable way by
which tidal influence could prevent or impede the throwing-
off of secondary bodies ? We have only to think for a moment
in order to see that this is precisely one of its direct results.
Tidal friction, whether solar or lunar, tends to reduce the
axial movement of the body it acts upon. But the separation
of satellites depends according to the received view upon
the attainment of a disruptive rate of rotation. Hence, if
solar tidal friction were strong enough to keep down the pace
1 Phil. Trans., vol. clxxii. p. 533.
THEORIES OF PLANETARY EVOLUTION. 361
below this critical point, the contracting mass would remain
intact there would be no satellite^production. This, in all
probability, actually occurred in the case both of Mercury
and Venus. They cooled without dividing, because the solar
friction-brake applied to them was too strong to permit
acceleration to pass the limit of equilibrium. The earth
barely escaped the same fate of loneliness. Her first and
only epoch of instability was retarded until she had nearly
reached maturity. The late appearance of the moon accounts
for its large relative size through the increased cohesion of
an already strongly condensed parent mass and for the dis-
tinctive peculiarities of its history and influence on the produc-
ing globe.
Solar tidal friction is still considerably effective at the dis-
tance of Mars. It did not, indeed, hinder the formation of
two minute dependants, but it explains the anomalously rapid
revolution of one of them. Phobos, we have seen, completes
more than three revolutions while Mars rotates once. But
this was probably not always so. The two periods were origin-
ally nearly equal. The difference was brought about by tidal
waves raised on the viscous spheroid of Mars by the sun.
Rotatory velocity was thereby destroyed, the Martian day
slowly lengthened, and, as a secondary consequence, the period
of the inner satellite, become shorter than the augmented day,
began progressively to diminish. So that Phobos, unlike our
moon, was in the beginning farther from its primary than now.
The attraction of the tidal wave raised by the sun on the globe
of Mars is gradually drawing it inward, and threatens to effect
its eventual precipitation upon his surface. The same destiny,
it may be added, awaits our own satellite, should the present
order of things endure long enough to enable solar tidal fric-
tion to bring about that indefinitely remote end.
Outside the orbit of Mars, this agency can scarcely be said
to possess any sensible power. In the systems of Jupiter,
Saturn, Uranus, and Neptune, tides are probably effective
362 HISTORY OF ASTRONOMY.
chiefly on the rotation of satellites, compelling them to turn
always the same faces towards their primaries.
The general outcome of Mr. Darwin's researches has been
to leave Laplace's cosmogony untouched. He concludes
nothing against it, and, what perhaps tells with more weight in
the long run, has nothing to substitute for it. In one fonn or
the other, if we speculate at all on the development of the
planetary system, our speculations are driven into conformity
with the broad lines of the Nebular Hypothesis so far, at
least, as admitting an original material unity and motive uni-
formity. But we can see now, better than formerly, that these
supply a bare and imperfect sketch of the truth. We should
err gravely were we to suppose it possible to reconstruct, with
the help of any knowledge our race is ever likely to possess,
the real and complete history of our admirable system. "The
subtlety of nature," Bacon says, "transcends in many ways
the subtlety of the intellect and senses of man." By no mere
barren formula of evolution, indiscriminately applied all round,
the results we marvel at, and by a fragment of which our life
is conditioned, were brought forth ; but by the manifold play
of interacting forces, variously modified and variously prevail-
ing, according to the local requirements of the design they
were appointed to execute.
( 363 )
CHAPTER X.
RECENT COMETS.
ON the 2d of June 1858, Giambattista Donati discovered at
Florence a feeble round nebulosity in the constellation Leo,
about one-tenth the diameter of the full moon. It proved to
be a comet approaching the sun. But it changed little in
apparent place or brightness for some weeks. The gradual
development of a central condensation of light was the first
symptom of coming splendour. At Harvard, in the middle of
July, a strong stellar nucleus was seen; on August 14 a tail
began to be thrown out. As the comet wanted still over six
weeks of the time of its perihelion-passage, it was obvious that
great things might be expected of it. They did not fail of
realisation.
Not before the early days of September was it generally
recognised with the naked eye, though it had been detected
without a glass at Pulkowa, August 19. But its growth was
thenceforward a surprisingly rapid one, as it swept with ac-
celerated motion under the hindmost foot of the Great Bear,
and past the starry locks of Berenice. A sudden leap upward
in lustre was noticed on September 12, when the nucleus
shone with about the brightness of the pole-star, and the tail,
notwithstanding large fore-shortening, could be traced with
the lowest telescopic power over six degrees of the sphere.
The appendage, however, attained its full development only
after perihelion,' September 30, by which time, too, it lay nearly
square to the line of sight from the earth. On October 10
it stretched in a magnificent scimitar-like curve over a third
364 HISTORY OF ASTRONOMY.
and upwards of the visible hemisphere, representing a real
extension in space of fifty-four million miles. But the most
striking view was presented on October 5, when the brilliant
star Arcturus became involved in the brightest part of the tail,
and during many hours contributed, its lustre undiminished
by the interposed nebulous screen, to heighten the grandeur
of the most majestic celestial object of which living memo-
ries retain the impress. Donati's comet was, according to
Admiral Smyth's testimony, 1 outdone " as a mere J7gv$/-object "
by the great comet of 1811 ; but what it lacked in splendour,
it surely made up in grace, and variety of what we may call
"scenic" effects.
Some of these were no less interesting to the student than
they were impressive to the spectator. At Pulkowa, on the
1 6th September, Winnecke 2 observed a faint outer envelope
resembling a veil of almost evanescent texture flung some-
what widely over the head. Next evening, the first of the
" secondary " tails appeared, possibly as part of the same
phenomenon. This was a narrow, straight ray, forming a
tangent to the strong curve of the primary tail, and reaching
to a still greater distance from the nucleus. ' It continued
faintly visible for about three weeks, during part of which time
it was seen in duplicate. For from the chief train itself, at a
point where its curvature abruptly changed, issued, as if through
the rejection of part of its materials, a second beam nearly
parallel to the first, the rigid line of which contrasted singularly
with the softly diffused, and waving aspect of the plume of
light from which it sprang. Olbers's theory of unequal repul-
sive forces was never more beautifully illustrated. The triple
tail was a visible solar analysis of cometary matter.
The processes of luminous emanation going on in this body
forcibly recalled the observations made on the comets of 1 744
and 1835. From the middle of September, the nucleus, esti-
mated by Bond to be under five hundred miles in diameter,
1 Month. Not, vol. xix. p. 27.
2 Mem. de F Ac. Imp., t. ii. 1859, p. 46.
RECENT COMETS. 365
was the centre of action of the most energetic kind. Seven
distinct "envelopes" were detached in succession from the
nebulosity surrounding the head, and after rising towards the
sun during periods of from four to seven days, finally cast their
material backward to form the right and left branches of the
great train. The separation of these by an obscure axis ap-
parently as black, quite close up to the nucleus, as the sky
indicated for the tail a hollow, cone-like structure ; l while
the repetition of certain spots and rays in the same correspond-
ing situation on one envelope after another, served to show
that the nucleus to some local peculiarity of which they were
doubtless due had no proper rotation, but merely shifted suffi-
ciently on an axis to preserve the same aspect towards the sun
as it moved round it. 2 This observation of Bond's was strongly
confirmatory of Bessel's hypothesis of opposite polarities in
such bodies' opposite sides.
The protrusion towards the sun, on September 25, of a
brilliantly luminous, fan-shaped sector completed the resem-
blance to Halley's comet. The appearance of the head was
now somewhat that of a " bat's-wing " gaslight. There were,
however, no oscillations to and fro, such as Bessel had seen
and speculated upon in 1835. As the size of the nucleus
contracted with approach to perihelion, its intensity augmented.
On October 2, it outshone Arcturus, and for a week or ten
days was a conspicuous object half an hour after sunset. Its
lustre setting aside the light emitted from the tail was, at
that date, 6300 times what it had been in June 15, though
theoretically taking into account, that is, only the differences
of distance from sun and earth it should have been only -^
of that amount. Here, it might be thought, was convincing
evidence of the comet itself becoming ignited under the
growing intensity of the solar radiations. Experiments with
the polariscope were, however, interpreted in an adverse sense,
and Bond's conclusion that the comet sent us virtually un-
mixed reflected sunshine was generally acquiesced in. It did
1 Harvard Annals, vol. iii. p. 368. 2 Ibid., p. 371.
366 HISTORY OF ASTRONOMY.
not, nevertheless, survive the first application of the spectro-
scope to these bodies.
Very few comets have been so well or so long observed as
Donati's. It was visible to the naked eye during 112 days ;
it was telescopically discernible for 275, the last observation
having been made by Dr. Mann at the Cape of Good Hope,
March 4, 1859. Its course through the heavens combined
singularly with the orbital place of the earth to favour curious
inspection. The tail, when near its greatest development,
lost next to nothing by the effects of perspective, and at the
same time lay in a plane sufficiently inclined to the line of
sight to enable it to display its exquisite curves to the greatest
advantage. Even the weather was, on both sides of the
Atlantic, propitious during the period of greatest interest, and
the moon as little troublesome as possible. The splendid
volume compiled by the younger Bond is a monument to the
care and skill with which these advantages were turned to
account. Yet this stately apparition marked no turning-point
in the history of cometary science. By its study knowledge
was indeed materially advanced, but along the old lines.
No quick and vivid illumination broke upon its' path. Quite
insignificant objects as we have already partly seen have
often proved more vitally instructive.
Donati's comet has been identified with no other. Its path
is an immensely elongated ellipse, lying in a plane far apart
from that of the planetary movements, carrying it at perihelion
considerably within the orbit of Venus, and at aphelion out into
space to 5 1 times the distance from the sun of Neptune. The
entire circuit occupies over 2000 years, and is performed
in a retrograde direction, or against the order of the Signs.
Before its next return, about the year 4000 A.D., the enigma
of its presence and its purpose may have been to some extent
though we may be sure not completely penetrated.
On June 30, 1861, the earth passed, for the second time in
this century, through the tail of a great comet. Many of our
readers must remember the unexpected disclosure, on the
RECENT COMETS. 367
withdrawal of the sun below the horizon on that evening, of an
object so remarkable as to challenge universal attention. A
golden-yellow planetary disc, wrapt in dense nebulosity, shone
out while the June twilight of these latitudes was still in its first
strength. The number and complexity of the envelopes sur-
rounding the head produced, according to the late Mr. Webb, 1
a magnificent effect. Portions of six distinct emanations were
traceable. " It was as though a number of light, hazy clouds
were floating round a miniature full moon." As the light faded,
the tail came out. 2 Although in brightness and sharpness of
definition it could not compete with the display of 1858, its
dimensions proved to be extraordinary. It reached upwards
beyond the zenith when the head had already set. By some
authorities its extreme length was stated at 118, and it showed
no trace of curvature. Most remarkable, however, was the
appearance of two widely divergent rays, each pointing towards
the head, though cut off from it by sky- illumination, of which
one was seen by Mr. Webb, and both by Mr. Williams at
Liverpool, a quarter of an hour before midnight. There seems
no doubt that Mr. Webb's interpretation was the true one,
and that these beams were, in fact, " the perspective repre-
sentation of a conical or cylindrical tail, hanging closely above
our heads, and probably just being lifted up out of our atmos-
phere." 3 The cometary train was then rapidly receding from
the earth, so that the sides of the "outspread fan" of light
which it showed when we were right in the line of its axis,
must have appeared (as they did) to close up in departure.
The swiftness with which the visually opened fan shut, proved
its vicinity ; and indeed Mr. Hind's calculations showed that
we were not so much near, as actually within its folds at that
very time.
Already M. Liais, from his observations at Rio de Janeiro,
June ii to 14, had anticipated, as a probability, such an
encounter, and had subsequently proved that it must have
1 Month. Not.) vol. xxii. p. 306. 2 Stothard in ibid., vol. xxi. p. 243.
3 Intell. Observer, vol. i. p. 65.
368 HISTORY OF ASTRONOMY.
occurred in such a way as to cause an immersion of the earth in
cometary matter to a depth of 300,000 miles. 1 The comet
then lay between the earth and the sun at a distance of about
fourteen million miles from the former ; its tail stretched out-
ward just along the line of intersection of its own with the terres-
trial orbit to an extent of fifteen million miles ; so that our globe,
happening to pass at the time, found itself during some hours
involved in the flimsy appendage.
No perceptible effects were produced by the meeting ; it was
"known to have occurred by theory alone. A peculiar glare in
the sky, thought by some to have distinguished the evening
of June 30, was, at best, inconspicuous. Nor were there any
symptoms of unusual electric excitement. The Greenwich
instruments were, indeed, disturbed on the following night ;
but it would be rash to infer that the comet had art or part in
their agitation.
The perihelion-passage of this body occurred June IT,
1861 ] and its orbit has been shown by M. Kreutz of Bonn,
from a very complete investigation founded on observations
extending over nearly a year, to be an ellipse traversed in a
period of 409 $ years. 2
Towards the end of August 1862, a comet became visible
to the naked eye high up in the northern hemisphere, with a
nucleus equalling in brightness the lesser stars of the Plough
and a feeble tail 20 in length. It thus occupied quite a
secondary position among the members of its class. It was,
nevertheless, a splendid object in comparison with a telescopic
nebulosity discovered by Tempel at Marseilles, December 19,
1865. This, the sole comet of 1866, slipped past perihelion
January n, without pomp of train or other appendages, and
might have seemed hardly worth the trouble of pursuing.
Fortunately, however, this was not the view entertained by
observers and computers ; since upon the knowledge acquired
of the movements of these two bodies has been founded one
of the most significant discoveries of modern times. The first
1 Comptes Rendus, t. Ixi. p. 953. ? Smiths. Report^ 1881 (Holden).
RECENT COMETS. 369
of them is now styled the comet (1862 iii.) of the August
meteors, the second (1866 i.) that of the November meteors.
The steps by which this curious connection came to be ascer-
tained were many, and were taken in succession by a number
of individuals. But the final result was reached by Schiaparelli
of Milan, and remains deservedly associated with his name.
The idea prevalent in the last century as to the nature of
shooting stars was that they were mere aerial ignes fatui in-
flammable vapours accidentally kindled in our atmosphere.
But Halley had already entertained the opinion of their
cosmical origin; and Chladni in 1794 formally broached the
theory that space is filled with minute circulating atoms, which,
drawn by the earth's attraction, and ignited by friction in its
gaseous envelope, produce the luminous effects so frequently
witnessed. 1 Acting on his suggestion, Brandes and Benzenberg,
two students at the University of Gottingen, began in 1798 to
determine the heights of falling stars by simultaneous obser-
vations at a distance. They soon found that they move with
planetary velocities in the most elevated regions of our atmos-
phere, and by the ascertainment of this fact laid a foundation
of distinct knowledge regarding them. Some of the data
collected, however, served only to perplex opinion, and even
caused Chladni temporarily to renounce his. Many high
authorities, headed by Laplace in 1802, declared for the lunar-
volcanic origin of meteorites ; but thought on the subject was
turbid, and inquiry seemed only to stir up the mud of ignor-
ance. It needed one of those amazing spectacles, at which
man assists, no longer in abject terror for his own frail for-
tunes, but with keen curiosity and the vivid expectation of
new knowledge, to bring about a clarification.
On the night of November 12-13, I ^33> a tempest of
falling stars broke over the earth. North America bore the
brunt of its pelting. From the Gulf of Mexico to Halifax,
until daylight with some difficulty put an end to the display,
the sky was scored in every direction with shining tracks and
1 Ueber den Ursprung der von Pallas gefundenen Eisenmassen, p. 24.
2 A
370 HISTORY OF ASTRONOMY.
illuminated with majestic fireballs. At Boston, the frequency
of meteors was estimated to be about half that of flakes of
snow in an average snowstorm. Their numbers, while the
first fury of their coming lasted, were quite beyond counting ;
but as it waned, a reckoning was attempted, from which it was
computed, on the basis of that much-diminished rate, that
240,000 must have been visible during the nine hours they
continued to fall. 1
Now there was one very remarkable feature common to the
innumerable small bodies which traversed, or were consumed
in our atmosphere that night. They all seemed to come from
the same part of the sky. Traced backwards, their paths were
invariably found to converge to a point in the constellation
Leo. Moreover, that point travelled with the stars in their
nightly round. In other words, it was entirely independent of
the earth and its rotation. It was a point in inter-planetary
space.
The effective perception of this fact 2 amounted to a discovery,
as Olmsted and Twining, who had "simultaneous ideas" on
the subject, were the first to realise. Denison Olmsted was
then professor of mathematics in Yale College. He showed
early in i834 3 that the emanation of the showering meteors
from a fixed "radiant" proved their approach to the earth
along nearly parallel lines, appearing to diverge by an- effect of
perspective ; and that those parallel lines must be sections of
orbits described by them round the sun and intersecting that
of the earth. For the November phenomenon was now seen
to be a periodical one. On the same night of the year 1832,
although with less dazzling and universal splendour than in
America in 1833, it had been witnessed over great part of
Europe and in Arabia. Olmsted accordingly assigned to the
cloud of cosmical particles (or " comet," as he chose to call
1 Arago, Annuaire, 1836, p. 294.
2 Humboldt had noticed the emanation of the shooting stars of 1799
from a single point, or "radiant," as Greg long afterwards termed it ; but
no reasoning was founded on the observation.
3 Am. Jour, of Sc., vol. xxvi. p. 132.
RECENT COMETS. 371
it), by terrestrial encounters with which he supposed the ap-
pearances in question to be produced, a period of about 182
days ; its path a narrow ellipse, meeting, near its farthest end
from the sun, the place occupied by the earth on November 12.
Once for all, then, as the result of the star-drift of 1833, the
study of luminous meteors became an integral part of astronomy.
Their membership of the solar system was no longer a theory
or a conjecture it was an established fact. The discovery
might be compared to, if it did not transcend in importance,
that of the asteroidal group. " C'est un nouveau monde plane-
taire," Arago wrote, 1 " qui commence a se reveler a nous."
Evidences of periodicity continued to accumulate. It was.
remembered that Humboldt and Bonpland had been the spec-
tators, at Cumana, after midnight of November 12, 1799, f
a fiery shower little inferior to that of 1833, and reported
to have been visible from the equator to Greenland. More-
over, in 1834 and some subsequent years, there were waning
repetitions of the display, as if through the gradual thinning-out
of the meteoric supply. The extreme irregularity of its dis-
tribution was noted by Olbers in 1837, who conjectured that
we might have to wait until 1867 to see the phenomenon re-
newed on its former scale of magnificence. 2 This was the
first hint of a thirty-three or thirty-four year period.
The falling stars of November did not alone attract the
attention of the learned. Similar appearances were tradition-
ally associated with August 10 by the popular phrase in which
they figured as " the tears of St. Lawrence." But the associa-
tion could not be taken on trust from mediaeval authority. It
had to be proved scientifically, and this Quetelet of Brussels
succeeded in doing in December i836. 3
A second meteoric revolving system was thus shown to
exist. But its establishment was at once perceived to be fatal
to the " cosmical cloud " hypothesis of Olmsted. For if it
be a violation of probability to attribute to one such agglomera-
1 Annuairc, 1836, p. 297.
8 Ann. de fObserv., Bruxelles, 1839, p. 248 3 Ibid., 1837, p. 272.
372 HISTORY OF ASTRONOMY.
tion a period of an exact year, or sub-multiple of a year, it
would be plainly absurd to suppose the movements of two
or more regulated by such highly artificial conditions. An
alternative was proposed by Adolf Erman of Berlin in I839. 1
No longer in clouds^ but in closed rings ^ he supposed meteoric
matter to revolve round the sun. Thus the mere circumstance
of intersection by a meteoric, of the terrestrial orbit, without
any coincidence of period, would account for the earth meet-
ing some members of the system at each annual passage
through the "node" or point of intersection. This was an
important step in advance, yet it decided nothing as to the
forms of the orbits of such annular assemblages ; nor was it
followed up in any direction for a quarter of a century.
Professor Hubert A. Newton of Yale College took up,
however, the dropped thread of inquiry in 1 864.2 A search
through old records carried the November phenomenon back
to the year 902 A.D., long distinguished as " the year of the
stars." For in the same night in which Taormina was captured
by the Saracens, and the cruel Aghlabite tyrant Ibrahim ibn
Ahmed died " by the judgment of God " before Cosenza, stars
fell from heaven in such abundance as to amaze and terrify be-
holders far and near. This was on October 13, and recurrences
were traced down through the subsequent centuries, always
with a day's delay in about seventy years. It was easy, too,
to derive from the dates a cycle of 33^ years, so that Professor
Newton did not hesitate to predict the exhibition of an un-
usually striking meteoric spectacle for November 13-14, i866. 3
For the astronomical explanation of the phenomena, recourse
was had to a method introduced by Erman of computing
meteoric orbits. It was found, however, that conspicuous
recurrences every thirty-three or thirty-four years could be
explained on the supposition of five widely different periods,
combined with varying degrees of extension in the revolving
1 Astr. Nach., Nos. 385, 390.
2 Am. Jour, of Sc., vol. xxxvii. (2d ser.), p. 377-
3 2 bid., vol. xxxviii. p. 61.
RECENT COMETS. 373
group. Professor Newton himself gave the preference to the
shortest of the five of 354^ days but indicated the means
of deciding with certainty upon the true one. It was furnished
by the advancing motion of the node, or that day's delay of the
November shower every seventy years, which the old chroni-
cles had supplied data for detecting. For this is a strictly
measurable effect of gravitational disturbance by the various
planets, the amount of which naturally depends upon the
course pursued by the disturbed bodies. Here the great
mathematical resources of Professor Adams were brought to
bear. By laborious processes of calculation, he ascertained
that four out of Newton's five possible periods were entirely
incompatible with the observed nodal displacement, while ior
the fifth that of 33 J years a perfectly harmonious result was
obtained. 1 This was the last link in the chain of evidence
proving that the November meteors or "Leonids," as they
had by that time come to be called revolve round the sun in
a period of 33.27 years, in an ellipse spanning the vast gulf
between the orbits of the earth and Uranus, the group
being so extended as to occupy six or eight years in defiling
past the scene of terrestrial encounters. But before it was
completed in March 1867, the subject had assumed a new
aspect and importance.
Professor Newton's prediction of a remarkable star-shower
in November 1866 was punctually fulfilled. This time,
Europe served as the main target of the celestial projectiles,
and observers were numerous and forewarned. The display,
although, according to Mr. BaxendelPs memory, 2 inferior to
that of 1833, was of extraordinary impressiveness. Dense
crowds of meteors, equal in lustre to the brightest stars, and
some rivalling Venus at her best, 3 darted from east to west
across the sky with enormous apparent velocities, and with a
certain determinateness of aim, as if let fly with a purpose, and
1 Month. Not., vol. xxvii. p. 247.
2 Am. Jour. o/Sc., vol. xliii. (2d ser.), p. 87.
3 Grant, Month. Not., vol. xxvii. p. 29.
374 HISTORY OF ASTRONOMY.
at some definite object. 1 Nearly all left behind them trains
of emerald-green or clear blue light, which occasionally lasted
many minutes, before they shrivelled, and curled up out of sight.
The maximum rush occurred a little after one o'clock on the
morning of November 14, when attempts to count were over-
powered by frequency. But during a previous interval of seven
minutes five seconds, four observers at Mr. Bishop's observatory
at Twickenham reckoned 514, and during an hour ii2o. 2
Before daylight the earth had fairly cut her way through the
star-bearing stratum ; the " ethereal rockets " had ceased to fly.
This event brought the subject of shooting stars once more
vividly to the notice of astronomers. Schiaparelli had, in-
deed, been already attracted by it. The results of his studies
were made known in four remarkable letters, addressed, before
the close of the year 1866, to Father Secchi, and published in
the Bullettino of the Roman Observatory. 3 Their upshot was
to show, in the first place, that meteors possess a real velocity
considerably greater than that of the earth, and travel, accord-
ingly, to enormously greater distances from the sun, along
tracks resembling those of comets in being very eccentric, in
lying at all levels indifferently, and in being pursued in either
direction. It was next inferred that comets and meteors
equally have an origin foreign to the solar system, but are
drawn into it temporarily by the sun's attraction, and occa-
sionally fixed in it by the backward pull of some planet. But
the crowning fact was reserved for the last. It was the as-
tonishing one that the August meteors move in the same orbit
with the bright comet of 1862 that the comet, in fact, is but a
larger member of the family of Perseids (so named because
their radiant point is situated in the constellation Perseus).
This discovery was quickly capped by others of the same
kind. Leverrier published, January 21, i867, 4 elements for
the November swarm, founded on the most recent and authen-
1 P. Smyth, Month. Not., vol. xxvii. p. 256. 2 Hind, ibid., p. 49.
3 Reproduced in Les Mondes, t. xiii.
4 Comptes Rendus, t. Ixiv. p. 96.
RECENT COMETS. 375
tic observations ; at once identified by Dr. Peters of Altona,
the late distinguished editor of the Astronomische Nach
richten, with Oppolzer's elements for Tempel's comet of I866. 1
A few days later, Schiaparelli, having re-calculated the orbit of
the meteors from improved data, arrived at the same conclu-
sion ; while Professor Weiss of Vienna pointed to the agreement
between the orbits of a comet which had appeared in 1861
and of a star-shower found to recur on April 20 (Lyraids), as
well as between those of Biela's comet and certain conspicuous
meteors of November 28. 2
These instances do not seem to be exceptional. The num-
ber of known or suspected accordances of cometary tracks
with meteor streams contained in a list drawn up in 1878 3 by
Professor Alexander S. Herschel (who has made the subject
peculiarly his own); amounts to seventy-six ; although the four
first detected still remain the most conspicuous examples of a
relation as significant as it was, to most astronomers, unexpected.
There had, indeed, been anticipatory ideas. Not that
Kepler's comparison of shooting stars to "minute comets,"
or Maskelyne's "forse risultera che essi sono comete,"
in a letter to the Abate Cesaris, December 12, 1783,* need
count for much. But Chladni, in iSip, 5 considered both to be
fragments or particles of the same primitive matter, irregularly
dispersed through space as nebulae ; and Morstadt of Prague
suggested about 1837 6 that the November meteors might be
dispersed atoms from the tail of Biela's comet, the path of
which is cut across by the earth near that epoch. Professor
Kirkwood, however, by a luminous intuition, penetrated the
whole secret, so far as it has yet been made known. In an
article published, or rather buried, in the Danville Quarterly
Review for December 1861, he argued from the observed
division of Biela, and other less noted instances of the same
1 Astr. Nach., No. 1626. z Ibid., No. 1632.
3 Month. Not.) vol. xxxviii. p. 369.
4 Schiaparelli, Le Stelle Cadenti, p. 54.
5 Ueber Feuer-Meteore, p. 406. 6 Astr. Nach., No. 347 (Madler).
376 HISTORY OF ASTRONOMY.
kind, that the sun exercises a "divellent influence" on the
nuclei of comets, which may be presumed to continue its action
until their corporate existence (so to speak) ends in complete
pulverisation. " May not," he continued, " our periodic me-
teors be the debris of ancient, but now disintegrated comets,
whose matter has become distributed round their orbits ? " 1
The gist of Schiaparelli's discovery could not be more
clearly conveyed. For it must be borne in mind that
with the ultimate destiny of comets' tails this had nothing to
do. The tenuous matter composing them is, no doubt,
permanently lost to the body from which it emanated; but
science does not pretend to track its further wanderings
through space. It can, however, state categorically that these
will no longer be conducted along the path forsaken under
solar compulsion.. From the central, and probably solid parts
of comets, on the other hand, are derived the granules by the
swift passage of which our skies are seamed with periodic
fires. It is certain that a loosely agglomerated mass (such as
there is every reason to believe cometary nuclei to be) must
gradually separate through the unequal action of gravity on its
various parts through, in short, solar tidal influence. Thence-
forward its fragments will revolve independently in parallel
orbits, at first as a swarm, finally when time has been given
for the full effects of the lagging of the slower moving particles
to develop as a closed ring. The first condition is still,
more or less, that of the November meteors ; those of August
have already arrived at the second. For this reason, Leverrier
pronounced, in 1867, the Perseid to be of older formation
than the Leonid system. He even assigned a date at which
the introduction of the last-named bodies into their present
orbit was probably effected through the influence of Uranus. 2
In 126 A.D. a close approach must have taken place between
. 1 Nature, vol. vi. p. 148.
2 Mr. Proctor's recent inquiries have shown that the effect of no single
planetary encounter can suffice (as the "capture theory" of comets re-
quires that it should) to compel a body approaching the sun from an
RECENT COMETS. 377
the planet and the parent comet of the November stars, after
which its regular returns to perihelion, and the consequent
process of its disintegration, set in. Though not complete,
it is already far advanced.
The view that meteorites are the dust of decaying comets,
was now to be put to a definite test of prediction. Biela's
comet had not been seen since its duplicate return in 1852.
Yet it had been carefully watched for with the best telescopes ;
its path was accurately known ; every perturbation it could
suffer was scrupulously taken into account. Under these cir-
cumstances, its repeated failure to come up to time might
fairly be thought to imply a cessation from visible existence.
Might it not, however, be possible that it would appear under
another form that a star-shower might have sprung from, and
would commemorate its dissolution ?
An unusually large number of falling stars was seen by
Brandes, December 7, 1798. Similar displays were noticed
in the years 1830, 1838, and 1847 (a day earlier on the two
latter occasions), and the point from which they emanated
was shown by Heis at Aix-la-Chapelle to be situated near the
bright star y Andromedae. 1 Now this is precisely the direction
in which the orbit of Biela's comet would seem to lie, as it runs
down to cut the terrestrial track very near the place of the
earth at the above dates. The inference was then an easy one
that the meteors were pursuing the same path with the comet ;
and it was separately arrived at, early in 1867, by Weiss,
D' Arrest, and Galle. 2 But Biela travels in the opposite
direction to Tempel's comet and its attendant " Leonids ; "
its motion is direct, or from west to east, while theirs is
retrograde. Consequently, the motion of its node is in the
opposite direction too. In other words,.the meeting-place of
its orbit with that of the earth retreats (and very rapidly) along
indefinite distance to revolve thenceforth in an orbit having its aphelion
near the meeting-place. Several successive encounters, however, may
have done the work.
1 A. S. Herschel, Month. Not., vol. xxxii. p. 355.
2 Astr. Nach., Nos. 1632, 1633, l6 35-
3?S HISTORY OF ASTRONOMY.
the ecliptic, instead of advancing. So that if the " Andromeds "
possessed the intimate relation supposed to Biela's comet,
they might be expected to anticipate the times of their recur-
rence by as much as a week (or thereabouts) in half a century.
All doubt as to the fact may be said to have been removed by
Signor Zezioli's observation of the annual shower in more than
usual abundance, at Bergamo, November 30, 1867.
The missing comet was next due at perihelion in the year
1872, and the probability was contemplated by both Weiss and
Galle of its being replaced by a somewhat dense drift of falling
stars. The precise date of the occurrence was not easily
determinable, but Galle thought the chances in favour of
November 28. The event anticipated the prediction by
twenty-four hours. Scarcely had the sun set in Western
Europe on November 27, when it became evident that Biela's
comet was shedding over us the pulverised products of its
disintegration. The meteors came in volleys from the foot of
the Chained Lady, their numbers at times baffling the attempt
to keep a reckoning. At Moncalieri, about 8 P.M., they con-
stituted (as Father Denza said 1 ) a "real rain of fire." Four
observers counted, on an average, four hundred each minute
and a half; and not a few fireballs equalling the moon in diameter
traversed the sky. On the whole, however, the stars of 1872,
though about equally numerous, were less brilliant than those
of 1866 ; the phosphorescent tracks marking their passage were
comparatively evanescent, and their movements sluggish. This
is easily understood when we remember that the Andromeds
overtake the earth, while the Leonids rush to meet it ; the
velocity of encounter for the first class of bodies being under
twelve, for the second above forty-four miles a second. The
spectacle was, nevertheless, magnificent. It presented itself
successively to various parts of the earth, from Bombay and
the Mauritius to New Brunswick and Venezuela, and was most
diligently and extensively observed. Here it had well-nigh
terminated by midnight 2
1 Nature, vol. vii. p. 122.
2 A. S. Herschel, Keport Brit. Ass., 1873, P- 39-
RECENT COMETS. 379
It was attended by a slight aurora, and although Tacchini
had telegraphed that the state of the sun rendered some show
of polar lights probable, it has too often figured as an ac-
companiment of star-showers to permit the coincidence to
rank as fortuitous. Admiral Wrangel was accustomed to
describe how, during the prevalence of an aurora on the
Siberian coast, the passage of a meteor never failed to extend
the luminosity to parts of the sky previously dark ; l and the
power of exciting electrical disturbance seems to belong to
all such flitting cosmical atoms.
A singular incident connected with the meteors of 1872 has
now to be recounted. The late Professor Klinkerfues, who
had observed them very completely at Gottingen, was led to
believe that not merely the debris strewn along its path, but
the comet itself, must have been in the closest proximity to
the earth during their appearance. 2 If so, it might be possible,
he thought, to descry it as it retreated in the diametrically
opposite direction from that in which it had approached. On
November 30, accordingly, he telegraphed to Mr. Pogson, the
Madras astronomer, " Biela touched earth November 27; search
near Theta Centauri " the " anti-radiant," as it is called, being
situated close to that star. Bad weather prohibited observation
during thirty-six hours, but when the rain-clouds broke on the
morning of December 2, there a comet was, just in the indi-
cated position. In appearance it might have passed well
enough for one of the Biela twins. It had no tail, but a
decided nucleus, and was about 45 seconds across, being thus
altogether below the range of naked-eye discernment. It was
again observed December 3, when a short tail was perceptible ;
but overcast skies supervened, and it has never since been seen.
Its identity accordingly remained in doubt. It seems tolerably
certain, however, that it was not the lost comet, which ought to
have passed that spot twelve weeks earlier, and was subject to
no conceivable disturbance capable of delaying to that extent
1 Humboldt, Cosmos, vol. i. p. 114 (Otte's trans.)
2 Month. Not., vol. xxxiii. p. 128.
380 HISTORY OF ASTRONOMY.
its revolution. On the other hand, there is the strongest likeli-
hood that it belonged to the same system l that it was a third
fragment, torn from the parent-body of the Andromeds at a
period anterior to our first observations of it. Nor did the
meteors of November 27 directly replace the vanished comet.
They too must have separated from it at a much earlier stage
of its history.
Biela does not offer the only example of cometary disruption.
Setting aside the un authentic reports of early chroniclers, we
meet the " double comet " discovered by Liais at Olinda
(Brazil), February 27, 1860, of which the division appeared
recent, and about to be carried farther. 2 But a division once
established, separation must continually progress. The periodic
times of the fragments will never be identical ; one must drop
a little behind the other at each revolution, until at length
they come to travel in remote parts of nearly the same orbit.
Thus the comet predicted by Klinkerfues and discovered by
Pogson had already lagged to the extent of twelve weeks, and
we shall meet instances farther on where the retardation is
counted, not by weeks, but by years. Here, original identity
emerges only from calculation and comparison of orbits.
Comets then die, as Kepler wrote long ago, sicut bombyces
filo fundendo. This certainty, anticipated by Kirkwood in
1 86 1, we have at least acquired from the discovery of their
generative connection with meteors. Nay, their actual ma-
terials become, in smaller or larger proportions, incorporated
with our globe. Whether, indeed, the ponderous masses of
which, according to Daubree's estimate, 3 600 or 700 fall an-
nually from space upon the earth, ever formed part of the
bodies known to us as comets, is a question. Some follow
Tschermak in attributing to aerolites a totally different origin
from that of periodical shooting-stars. That no clear line of
1 Even this was denied by Bruhns, Astr. Nach., No. 2054.
2 Month. Not., vol. xx. p. 336.
3 Newton, Ency. Brit., vol. xvi. p. 109.
RECENT COMETS. 381
demarcation can be drawn is no valid reason for asserting that
no real distinction exists ; and it is certainly remarkable that a
meteoric fusillade may be kept up for hours without a single
solid projectile reaching its destination. It would seem as if
the celestial army had been supplied with blank cartridges.
There is, indeed, much probability that few of the components
of the recent brilliant showers attained the dimensions of a
canary-seed.
It would gratify curiosity to think that we might, by actual
inspection and analysis, ascertain the composition of those
mysterious visitors, the "brandishing" of whose "crystal
tresses" in our skies was wont, in times past, to " import
change of times and states." But if this be denied us, another
way has been laid open towards the same end.
The first successful application of the spectroscope to such
bodies was by Donati in I864. 1 A comet discovered by
Tempel, July 4, brightened until it appeared like a star some-
what below the second magnitude, with a feeble tail 30 in
length. It was remarkable as having, on August 7, almost
totally eclipsed a small star a very rare occurrence. 2 On
August 5, Donati admitted its light through his train of prisms,
and found it, thus analysed, to consist of three bright bands
yellow, green and blue separated by wider dark intervals.
This implied a good deal. Comets had previously been con-
sidered, as we have seen, to shine mainly, if not wholly, by re-
flected sunlight. They were now perceived to be self-luminous,
and to be formed, to a large extent, of glowing gas. The next
step was to determine what kind of gas it was that was thus
glowing in them; and this was taken by Dr. Huggins in i868. 3
A comet of subordinate brilliancy, known as comet 1868 ii.,
or sometimes as Winnecke's, was the subject of his experi-
ment. On comparing its spectrum with that of an olefiant-
gas "vacuum tube" rendered luminous by electricity, he
found the agreement exact. It has since been abundantly
1 Asfr. Nach., No. 1488. 2 Annuaire, Paris, 1883, p. 185.
3 Phil, 7^rans., vol. clviii. p. 556.
382 HISTORY OF ASTRONOMY.
confirmed. All the eighteen comets of which the light had
been analysed down to 1880, showed the typical hydro-carbon
spectrum l common to the whole group of those compounds,
but probably due immediately to the presence of acetylene.
Some slight apparent anomalies have been almost certainly
caused, not by any real differences of constitution, but by
deficient light-power, rendering observations difficult and in-
secure. The brighter the comet, the more perfect proved
its conformity to the type.
The earliest comet of first-class lustre to present itself for
spectroscopic examination, was that discovered by Coggia at
Marseilles, April 17, 1874. Invisible to the naked eye until
June, it blazed out in July a splendid ornament of our northern
skies, with a just perceptibly curved tail, reaching more than
half way from the horizon to the zenith, and a nucleus sur-
passing in brilliancy the brightest stars in the Swan. Bredi-
chin, Vogel, and Huggins 2 were unanimous in pronouncing
its spectrum to be that of marsh, or olefiant gas. Father Secchi,
in the clear sky of Rome, was able to push the identification
even closer than had heretofore been done. The complete
hydro-carbon spectrum consists of five zones of variously
coloured light. Three of these only the three central ones
had till then been obtained from comets ; it was supposed,
because their temperature was not high enough to develop
the others. The light of Coggia's comet, however, was found
to contain all five, traces of the violet band emerging June 4,
of the red, July 2. 3 Presumably, all five would show universally
in cometary spectra, were the dispersed rays strong enough to
enable them to be seen.
The gaseous surroundings of comets are then made up of
a compound of hydrogen with carbon. Other materials are
also present, as will be seen by and by ; but the hydro-carbon
element is probably unfailing and predominant. Its luminosity
1 Hasselberg, Mem. de F Ac. Imp. de St. Petersbourg, t. xxviii. (7th ser.),
No. 2, p. 66. 2 Proc. Roy. Soc., vol. xxiii. p. 154.
3 Hasselberg, loc. cit., p. 58.
RECENT COMETS. 383
is, there is little doubt, an effect of electrical excitement.
Zollner showed in 1872 1 that, owing to evaporation, and other
changes produced by rapid approach to the sun, electrical pro-
cesses of considerable intensity must take place in comets ;
and that 'their original light is immediately connected with
these, and is only an indirect result of solar radiation, may be
considered a truth permanently acquired to science. 2 They
are not, it thus seems, bodies incandescent through heat, but
glowing by electricity ; and this is compatible with a relatively
low temperature.
The gaseous spectrum of comets is accompanied, in varying
degrees, by a continuous spectrum. This is usually derived
most strongly from the nucleus, but extends, more or less, to
the nebulous appendages. In part, it is certainly due to
reflected sunlight ; in part, it is likely, to the ignition of minute
solid particles.
1 Ueber die Natur. tier Cometen, p. 1 1 2.
2 Hasselberg, loc. cit., p. 38.
( 384 )
CHAPTER XL
RECENT COMETS (continued].
THE mystery of comets' tails has been to some extent pene-
trated ; so far, at least, that, by making certain assumptions
strongly recommended by the facts of the case, their forms can
be, with very approximate precision, calculated beforehand.
We have, then, the assurance that these extraordinary appen-
dages are composed of no ethereal or super-sensual stuff, but
of matter such as we know it, and subject to the ordinary laws
of motion, though in a state of extreme tenuity. This is
unquestionably one of the most remarkable discoveries of our
time.
Olbers, as already stated, originated in 1812 the view that
the tails of comets are made up of particles subject to a force
of electrical repulsion proceeding from the sun. It was
developed and enforced by Bessel's discussion of the appear-
ances presented by Halley's comet in 1835. He, moreover,
provided a formula for computing the movement of a particle
under the influence of a repulsive force of any given intensity,
and thus laid firmly the foundation of a mathematical theory
of cometary emanations. Professor W. A. Norton of Yale Col-
lege considerably improved this by inquiries begun in 1844,
and resumed on the apparition of Donati's comet ; and Dr.
C. F. Pape at Altona l gave numerical values for the impulses
outward from the sun, which must have actuated the materials
1 Astr. Nach., Nos. 1172-74.
RECENT COMETS. 385
respectively of the curved and straight tails adorning the same
beautiful and surprising object.
The physical theory of repulsion, however, was, it might be
said, still in the air. Nor did it assume an aspect of even
moderate plausibility until Zollner took it in hands in 187 1. 1
It is perfectly well ascertained that the energy of the push
or pull produced by electricity depends (other things being
the same) upon the surface of the body acted on ; that of
gravity, upon its mass. The efficacy of solar electrical repul-
sion relatively to solar gravitational attraction grows, conse-
quently, as the size of the particle diminishes. Make this
small enough, and it will virtually cease to gravitate, and will
unconditionally obey the impulse to recession.
This principle Zollner was the first to realise in its applica-
tion to comets. It gives the key to their constitution. Admit-
ting (as we seem bound to do) that the sun and they are simi-
larly electrified, their more substantially aggregated parts will still
follow the solicitations of his gravity, while the finely-divided
particles escaping from them will, simply by reason of their
minuteness, fall under the sway of his repellent electric power-
They will, in other words, form " tails." Nor is any extrava-
gant assumption called for as to the intensity of the electrical
charge concerned in producing these effects. Zollner, in fact,
showed 2 that it need not be higher than that attributed by the
best authorities to the terrestrial surface.
It is now nearly a quarter of a century since M. Bredichin,
late director of the Moscow Observatory, directed his attention
to these curious phenomena. His persistent inquiries on the
subject, however, date from the appearance of Coggia's comet
in 1874. On computing the value of the repulsive force
exerted in the formation of its tail, and comparing it with
values of the same force arrived at by him in 1862 for some
other conspicuous comets, it struck him that the numbers
representing them fell into three well-defined classes. " I
1 Berichte Sachs. Ges., 1871, p. 174.
2 Natur der Cometen, p. 124; Astr. Nach., No. 2086.
2 B
386 HISTORY OF ASTRONOMY.
suspect," he wrote in 1877, "that comets are divisible into
groups, for each of which the repulsive force is perhaps the
same." 1 This idea was confirmed on fuller investigation. In
1882 the appendages of thirty-six well-observed comets had
been re-constructed in the study, without a single exception
being met with to the rule of the three types.
In the first of these, the repellent energy of the sun is twelve
times as strong as his attractive energy ; the particles forming
the enormously long, straight rays projected outwards from
this kind of comet, leave the nucleus with a velocity of 4^
kilometres per second, which becoming constantly accelerated,
carries them in a few days to the limit of visibility. The great
comets of 1811, 1843, and 1861, that of 1744 (so far as its
principal tail was concerned), and Halley's comet at its various
apparitions, belonged to this class. For the second type, the
value of the repulsive force employed is less narrowly limited.
It may range as high as 2^ (2.6) times, or descend as low as
y 8 ^ the power of solar gravity ; 2 but, on an average, it is just
equal to it. The corresponding initial velocity is 900 metres
a second, and the resulting appendage a scimitar-like or plumy
tail, such as Donati's and Coggia's comets furnished splendid
examples of. Tails of the third type are constructed with a force
of repulsion from the sun one-fifth (or, at the most, three-tenths)
that of his gravity, producing an accelerated movement of
attenuated matter from the nucleus, beginning at the leisurely
rate of 300 metres a second. They are short, strongly bent,
brush-like emanations, and in bright comets seem to be only
found in combination with tails of the higher classes. Multiple
tails, indeed that is, tails of different types emitted simul-
taneously by one comet are perceived, as experience advances
and observation becomes closer, to be rather the rule than
the exception. 3
Now what is the meaning of these three types? Is any
translation of them into physical fact possible? To this
1 Annaks de PObs. de Moscou, t. iii. pt. i. p. 37.
2 Ibid., t. vii. pt. ii. p. 56. 3 Faye, Comptes Rendus, t. xciii. p. 13.
RECENT COMETS. 387
question Bredichin supplied in 1879 a plausible answer. 1 It
was already a current conjecture that multiple tails are com-
posed of different kinds of matter, differently acted on by the
sun. Both Olbers and Bessel had suggested this explanation
of the straight and curved emanations from the comet of 1807 ;
Norton had applied it to the faint light-tracks proceeding from
that of Donati ; 2 Winnecke, to the varying deviations of its
more brilliant plumage. Bredichin went further. He under-
took to determine (provisionally as yet) the several kinds of
matter appropriated severally to the three classes of tails.
These he found to be, hydrogen for the first, hydro-carbons for
the second, and iron for the third. The ground of this ap-
portionment is that the atomic weights of these substances
bear to each other the same inverse proportion as the repulsive
forces employed in producing the appendages they are sup-
posed to form; and Zollner had pointed out in 1875 that the
" heliofugal " power by which comets' tails are developed,
would in fact be effective just in that ratio. 3 Hydrogen, as the
lightest known element that is, the least under the influence
of gravity was naturally selected as that which yielded most
readily to the counter-persuasions of electricity. Hydro-carbons
had been shown by the spectroscope to be present in comets,
and were fitted by their specific weight, as compared with that
of hydrogen, to form tails of the second type ; while the atoms
of iron were just heavy enough to compose those of the third,
and, from the plentifulness of their presence in meteorites,
might be presumed to enter, in no inconsiderable proportion,
into the mass of comets. These three substances, however,
were by no means supposed to be the sole constituents of the
appendages in question. On the contrary, the great breadth
of what, for the present, were taken to be characteristically
" iron " tails, was attributed to the presence of many kinds of
matter of high, and slightly different specific weights ; 4 while
1 Annales, t. v. pt. ii. p. 137.
2 Am. Jour, of 'St., vol. xxxii. (2d ser.), p. 57.
3 Astr. Nach.) No. 2082. 4 Annales, t. vi. pt. i. p. 60.
388 HISTORY OF ASTRONOMY.
the expanded plume of Donati was shown to be, in reality, a
whole system of tails, made up of many substances, each
spreading into a separate hollow cone, more or less deviating
from, and partially superposed upon the others.
Never was a theory more promptly or profusely illustrated
than this of Bredichin. Within three years of its promulgation
five bright comets made their appearance, each presenting
some distinctive peculiarity by which knowledge of these
curious objects was materially helped forward. The first of
these is remembered as the " Great Southern Comet." It was
never visible in these latitudes, but made a short, though
stately progress through southern skies. Its earliest detection
was at Cordoba on the last evening of January, 1880 ; and it
was seen on February i as a luminous streak, extending just after
sunset from the south-west horizon towards the pole, in New
South Wales, at Monte Video, and the Cape of Good Hope.
The head was lost in the solar rays until February 4, when Dr.
Gould, director of the National Observatory of the Argentine
Republic at Cordoba, caught a glimpse of it very low in the
west ; and on the following evening, Mr. Eddie, at Graham's
Town, discovered a faint nucleus, of a straw : coloured tinge,
about the size of the annular nebula in Lyra. Its condensation,
however, was very imperfect, and the whole apparition was of
an exceedingly filmy texture. The tail was enormously long.
On February 5 it extended large perspective retrenchment
notwithstanding over an arc of 50 ; but its brightness nowhere
exceeded that of the Milky Way in Taurus. There was little
curvature perceptible ; the edges of the appendage ran parallel,
forming a nebulous causeway from star to star ; and the compari-
son to an auroral beam was appropriately used. The aspect of
the famous comet of 1843 was forcibly recalled to the memory
of Mr. Janisch, governor of St. Helena ; and the resemblance
proved not merely superficial. But the comet of 1880 was
less brilliant, and even more evanescent. After only eight
days of visibility, it had faded so much as no longer to strike,
though still discoverable by, the unaided eye ; and on February
RECENT COMETS. 389
20 it was invisible with the great Cordoba equatoreal pointed
to its known place.
But the most astonishing circumstance connected with this
body is the identity of its path with that of its predecessor in
1843. This is undeniable. Dr. Gould, 1 Mr. Hind, and Dr.
Copeland, 2 each computed a separate set of elements from the
first rough observations, and each was struck with an agree-
ment between the two orbits so close as to render them
virtually indistinguishable. " Can it be possible," Mr. Hind
wrote to Sir George Airy, " that there is such a comet in the
system, almost grazing the sun's surface in perihelion, and
revolving in less than thirty-seven years ? I confess I feel a
difficulty in admitting it, notwithstanding the above extra-
ordinary resemblance of orbits." 3
Mr. Hind's difficulty was shared by other astronomers. It
would, indeed, be a violation of common sense to suppose
that a celestial visitant so striking in appearance had been for
centuries back an unnoticed frequenter of our skies. Various
expedients accordingly were resorted to for getting rid of the
anomaly. The most promising at first sight was that of the
"resisting medium." It was hard to believe that a body,
largely vaporous, shooting past the sun at a distance of less
than a hundred thousand miles from his surface, should have
escaped powerful retardation. It must have passed through the
very midst of the corona. It might easily have had an actual
encounter with a prominence. Escape from such proximity
might, indeed, very well have been judged beforehand to be
impossible. Even admitting no other kind of opposition than
that met with by Encke's and Winnecke's comets, the effect
in shortening the period ought to be of the most marked kind.
It was proved by Oppolzer 4 that if the comet of 1843 had entered
our system from stellar space with parabolic velocity, it would,
by the action of a medium such as Encke postulated (varying
in density inversely as the square of the distance from the sun),
1 Astr. Nach., No. 2307. 2 Ibid., No. 2304.
3 Observatory, vol. iii. p. 390. 4 Astr. Nach., No. 2319.
390 HISTORY OF ASTRONOMY.
have been brought down, by its first perihelion passage, to
elliptic movement in a period of twenty-four years, with such
rapid diminution that its next return would be in about ten.
But such restricted observations as were on either occasion of
its visibility available, gave no sign of such a rapid progress
towards engulfment.
Another form of the theory was advocated by Klinkerfues.
He supposed that four returns of the same body had been
witnessed within historical memory the first in 371 B.C., the
next in 1668, besides those of 1843 and 1880; an original
period of 2039 years being successively reduced by the with-
drawal at each perihelion passage of y^Vg- of the velocity
acquired by falling from the far extremity of its orbit towards
the sun, to 175 and 37 years. A continuance of the process
would bring the comet of 1880 back in 1897.
Unfortunately, the earliest of these apparitions cannot be
identified with the recent ones unless by doing violence to the
plain meaning of Aristotle's words in describing it. He states
that the comet was first seen " during the frosts and in the
clear skies of winter," setting due west nearly at the same time
as the sun. 1 This implies some considerable- north latitude.
But the objects lately observed had practically no north lati-
tude. They accomplished their entire course above the ecliptic
in two hours and a quarter, during which space they were
barely separated a hand's-breadth (one might say) from the
sun's surface. For the purposes of the desired assimilation,
Aristotle's comet should have appeared in March. It is not
credible, however, that even a native of Thrace should have
termed March "winter."
With the comet of 1668 the case is more dubious. The
circumstances of its appearance are barely reconcilable with
the identity attributed to it, although too vaguely known to
render certainty one way or the other attainable. It might,
however, be expected that recent observations would at least
decide the questions whether the comet of 1843 could have
1 Meteor^ lib. i. cap. 6.
RECENT COMETS. 391
returned in less than thirty-seven years, and whether the comet
of 1880 was to be looked for at the end of 17^. But the
truth is that both these objects were observed over so small
an arc 8 and 3 respectively that their periods remained
virtually undetermined. For while the shape and position of
their orbits could be and were fixed with a very close approach
to accuracy, the length of those orbits might vary enormously
without any very sensible difference being produced in the
small part of the curves traced out near the sun. It is, how-
ever, remarkable that Dr. Wilhelm Meyer arrived, by an ela-
borate discussion, at a period of thirty-seven years for the
comet of iSSo, 1 while the observations of 1843 are admittedly
best fitted by Hubbard's ellipse of 533 years; but these Dr.
Meyer supposes to be affected by some constant source of
error, such as would be produced by a mistaken estimate of
the position of the comet's centre of gravity. He infers finally
that, in spite of previous non-appearances, we really have to do
with a regular denizen of our system, returning once in thirty-
seven years along an orbit of such extreme eccentricity that its
movement might be described as one of precipitation towards,
and rapid escape from the sun, rather than of sedate circula-
tion round it.
The geometrical test of identity has hitherto been the only
one which it was possible to apply to comets, and in the case
before us it may fairly be said to have broken down. We
may, then, tentatively, and with much hesitation, try a physical
test, though scarcely yet, properly speaking, available. We
have seen that the comets of 1843 an( ^ 1880 were strikingly
alike in general appearance, though the absence of a formed
nucleus in the latter, and its inferior brilliancy, detracted from
the convincing effect of the resemblance. Nor was it main-
tained when tried by exact methods of inquiry. M. Bredichin
found that the gigantic ray emitted in 1843 belonged to his type
No. i ; that of 1880 to type No. 2. 2 The particles forming the
1 Mem. Soc. Phys. de Geneve, t. xxviii. p. 23.
2 AnnaJes, t. vii. pt. i. p. 60.
392 HISTORY OF ASTRONOMY.
one were actuated by a repulsive force ten times as powerful
as those forming the other. It is true that a second noticeably
curved tail was seen in Chili, March i, and at Madras, March
n, 1843 > an d M. Bredichin, accordingly, thinks the conjecture
justified that the materials composing on that occasion the
principal appendage having become exhausted, those of the
secondary one remained predominant, and reappeared alone
in the "hydro-carbon" train of 1880. But the one known
instance in point is against such a supposition. Halley's
comet, the only great comet of which the returns have been
securely authenticated and carefully observed, has preserved
its " type " unchanged through many successive revolutions.
The dilemma presented to astronomers by the Great Southern
Comet of 1880 was unexpectedly renewed in the following
year.
On the 22d of May 1881, Mr. John Tebbutt of Windsor,
New South Wales, scanning the western sky, discerned a hazy-
looking object which he felt sure was a strange one. A marine
telescope at once resolved it into two small stars and a comet,
the latter of which quickly attracted the keen attention of
astronomers; for Dr. Gould, computing its orbit from his
first observations at Cordoba, found it to agree so closely with
that arrived at by Bessel for the comet of 1807, that he
telegraphed to Europe, June i, announcing the unexpected
return of that body. So unexpected, that theoretically it
was not possible before the year 3346 ; and Bessel's in-
vestigation was one which inspired, and eminently deserved
confidence. Here then once more the perplexing choice had
to be made between a premature and unaccountable re-
appearance, and the admission of a plurality of comets moving
nearly in the same path. But in this case facts proved
decisive.
Tebbutt's comet passed the sun June 16, at a distance of
sixty-eight millions of miles, and became visible in Europe
six days later. It was, in the opinion of some, the finest
object of the kind since 1861. In traversing the constella-
RECENT COMETS. 393
tion Auriga, on its dtbut in these latitudes, it outshone
Capella. On June 24 and some subsequent nights, it was
unmatched in brilliancy by any star in the heavens. In the
telescope, the "two interlacing arcs of light" which had
adorned the head of Coggia's comet were reproduced ; while
a curious dorsal spine of strong illumination formed the axis
of the tail, which extended in clear skies over an arc of 20.
It belonged to the same "type" as Donati's great plume;
the particles composing it being driven from the sun by a
force twice as powerful as that urging them towards it. 1 But
the appendage was, for a few nights, and by two observers,
perceived to be double. Tempel on June 27, and Lewis
Boss, at Albany (N.Y.), June 26 and 28, saw a long straight
ray corresponding to a far higher rate of emission than the
curved train, and shown by Bredichin to be a member of the
(so-called) hydrogen class. It had vanished by July i, but
made a temporary reappearance July 22. 2
The appendages of this comet were of remarkable trans-
parency. Small stars shone wholly undimmed across the
tail, and a very nearly central transit of the head over one of
the seventh magnitude on the night of June 29, produced if
any change an increase of brilliancy in the object of this
spontaneous experiment. 3 Yet Dr. Meyer, at the Geneva
Observatory, found distinct evidence of refraction suffered by
stellar rays under these circumstances. Three times he pursued
with micrometric measurements the course of a star across
the cometary surroundings ; and on each occasion the unifor-
mity of its progress was disturbed in a manner corresponding
to the optical action of a gaseous mass increasing in density
and refractive power as the square of the distance from the
nucleus diminished. Supposing olefiant gas to be in question,
1 Bredichin, Annales, t. viii. p. 68.
2 Am. Jour, of Sc., vol. xxii. p. 305.
3 Messrs. Burton and Green observed a dilatation of the stellar image
into a nebulous patch by the transmission of its rays through a nuclear
jet of the comet. Am. Jour. o/Sc., vol. xxii. p. 163.
394 HISTORY OF ASTRONOMY.
its density, 102,000 kilometres from the nucleus, was estimated
to be YTJ^ that of our atmosphere at the sea-level. 1 This was
the first successful attempt to measure the effects of cometary
refraction, and will doubtless be renewed on a favourable
opportunity.
The track pursued by this comet gave peculiar advantages
for its observation. Ascending from Auriga through Camelo-
pardus, it stood, July 19, on a line between the Pointers and
the Pole, within 8 degrees of the latter, thus remaining for a
considerable period constantly above the horizon of northern
observers. Its brightness, too, was no transient blaze, but had
a lasting quality which enabled it to be kept steadily in view
during nearly nine months. Visible to the naked eye until
the end of August, the last telescopic observation of it was
made February 14, 1882, when its distance from the earth
considerably exceeded 300 million miles. Under these cir-
cumstances, the knowledge acquired of its orbit was of more
than usual accuracy, and showed conclusively that the comet
was not a simple return of Bessel's ; for this would involve a
period of seventy-four years, whereas Tebbutt's comet cannot
revisit the sun until after the lapse of close upon three millen-
niums. Nevertheless, the two bodies move so nearly in the
same path that an original connection of some kind is obvious ;
and the recent example of Biela readily suggested a conjecture
as to what the nature of that connection might have been.
The comets of 1807 and 1881 are then regarded with much pro-
bability as fragments of a primitive disrupted body, one follow-
ing in the wake of the other at an interval of seventy-four
years.
Tebbutt's comet was the first of which a satisfactory photo-
graph was obtained. The difficulties to be overcome were
very great. The chemical intensity of cometary light is,
to begin with, extraordinarily small. Janssen estimates it at
f moonlight. 2 So that, if the ordinary process by
1 Archives des Sciences, t. viii. p. 535. Meyer founded his conclusions
on the theory of M. Gustave Cellerier. 2 Annuaire, Paris, 1882, p. 781.
RECENT COMETS. 395
which lunar photographs are taken had been applied to the
comet of 1 88 1, an exposure of at least three days would have
been required in order to get an impression of the head with
about a tenth part of the tail. But by that time a new
method of vastly increased sensitiveness had been rendered
available, by which dry gelatine-plates were substituted for the
wet collodion-plates hitherto in use ; and this improvement
alone reduced the necessary time of exposure to two hours.
It was brought down to half an hour by Janssen's employment
of a reflector specially adapted to give an image illuminated
eight or ten times as strongly as that produced in the focus of
an ordinary telescope. 1
The photographic feebleness of cometary rays was not the
only obstacle in the way of success. The proper motion of
these bodies is so rapid as to render the usual devices for
keeping a heavenly body steadily in view quite inapplicable.
The machinery by which the diurnal movement of the sphere
is followed, must be specially modified to suit each eccentric
career. This too was done, and on June 30, 1881, Janssen
secured a perfect photograph of the brilliant object then visible,
showing the structure of the tail with beautiful distinctness to
a distance of 2^ from the head. An impression to nearly
10 was obtained about the same time by Dr, Henry Draper
at New York, with an exposure of 162 minutes. 2
Tebbutt's (or comet 1881 iii.) was also the first comet of
which the spectrum was so much as attempted to be chemi-
cally recorded. Both Dr. Huggins and Dr. Draper were
successful in this respect, but Dr. Huggins the more com-
pletely so. 3 The importance of the feat consisted in its
throwing open to investigation a part of the spectrum invisible
to the eye, and so affording an additional test of cometary
constitution. The result was fully to confirm the origin from
carbon-compounds assigned to the visible rays, by disclosing
additional bands belonging to the same series in the ultra-
1 Annuaire, p. 776. 2 Am. Jour, of Sc., vol. xxii. p. 134.
3 Report Brit. Ass., 1881, p. 520.
396 HISTORY OF ASTRONOMY.
violet ; as well as to establish unmistakably the presence of a
not inconsiderable proportion of reflected solar light by the
clear impression of some of the principal Fraunhofer lines.
Thus the polariscope was found to have told the truth, though
not the whole truth.
The photograph so satisfactorily communicative was taken
by Dr. Huggins on the night of June 24; and on the 29th,
at Greenwich, the tell-tale Fraunhofer lines were perceived to
interrupt the visible range of the spectrum. This was at first
so vividly continuous, that the characteristic cometary bands
could scarcely be detached from their bright background.
But as the" nucleus faded towards the end of June, they came
out strongly, and were more and more clearly seen, both at
Greenwich and at Princeton, to agree, not with the spectrum
of hydro-carbons lit up in a vacuum tube by an electric dis-
charge, but with that of the same substances burning in a
Bunsen flame. 1 Here we have an additional clue to the mole-
cular condition of cometary materials. It need not, however,
be inferred that they are really in a state of combustion. This,
from all that we know, may be called an impossibility. The
truth pointed to seems rather to be that the- electricity by
which comets are rendered luminous is of very low intensity. 2
The spectrum of the tail was, in this comet, found to be not
essentially different from that of the head. Professor Wright
of Yale College ascertained a large, but probably variable per-
centage of its light to be polarised in a plane passing through
the sun, and hence to be reflected sunlight. 3 A faint con-
tinuous spectrum corresponded to this portion of its radiance \
but gaseous emissions were also present. At Potsdam, on
June 30, the hydro-carbon bands were traced by Vogel to the
very end of the tail ; 4 and they were kept in sight by Young at
a greater distance from the nucleus than the more equably dis-
persed light. There seems little doubt that, as in the solar
1 Month. Not., vol. xlii. p. 14 ; Am. Jour, of St., vol. xxii. p. 136.
2 Piazzi Smyth, Nature, vol. xxiv. p. 430.
3 Astr. Nach., No. 2395. 4 Ibid.
RECENT COMETS. 397
corona, the relative strength of the two orders of spectrum is
subject to fluctuations.
The comet 1881 iii. was thus of signal service to science.
It afforded, when compared with the comet of 1807, the first
undeniable example of two such bodies travelling so nearly in
the same orbit as to leave absolutely no doubt of the existence
of a genetic tie between them. Cometary photography came
to its earliest fruition with it ; and cometary spectroscopy made
a notable advance by means of it. Before it was yet out of
sight, it was provided with a successor.
At Ann Arbor Observatory, Michigan, on July 14, a comet
was discovered by Dr. Schaberle, which, as his claim to priority
is undisputed, is often allowed to bear his name. In strict
scientific parlance, however, it is designated comet 1881 iv.
It was observed in Europe after three days, became just dis-
cernible by the naked eye at the end of July, and brightened
consistently up to its perihelion passage, August 22, when it was
still about fifty million miles from the sun. During many days
of that month, the uncommon spectacle was presented of two
bright comets circling together, though at widely different
distances, round the north pole of the heavens. The new-
comer, however, never approached the pristine lustre of its
predecessor. Its nucleus, when brightest, was comparable to
the star Cor Caroli, a narrow, perfectly straight ray proceed-
ing from it to a distance of 10. This was easily shown by
Bredichin to belong to the hydrogen type of tails ; l while a
" strange, faint second tail, or bifurcation of the first one,"
observed by Captain Noble, August 24,2 fell into the hydro-
carbon class of emanations. It was seen, August 22 and 24, by
Dr. F. Terby of Louvain, 3 as a short nebulous brush, like the
abortive beginnings of a congeries of curving trains ; but
appeared no more. Its well-attested presence was, however,
significant of the complex constitution of such bodies, and the
manifold kinds of action progressing in them.
1 Astr. Nach. t No. 2411. 2 Month. Not., vol. xlii. p. 49.
3 Astr. Nach., No. 2414,
398 HISTORY OF ASTRONOMY.
The only peculiarity in the spectrum of Schaberle's comet
consisted in the almost total absence of continuous light. The
carbon-bands were nearly isolated and very bright. Barely
from the nucleus proceeded a rainbow-tinted streak, indicative
of solid or liquid matter, which, in this comet, must have been
of very scanty amount. Its visit to the sun in 1881 was, so
far as is known, the first. The elements of its orbit showed no
resemblance to those of any previous comet, nor any marked
signs of periodicity. So that, although it may be considered
probable, we do not know that it is moving in a closed curve, or
will ever again penetrate the precincts of the solar system. It
was last seen from the southern hemisphere, October 19, 1881.
The third of a quartette of lucid comets visible within six-
teen months, was discovered by Mr. C. S. Wells at the Dudley
Observatory, Albany, March 17, 1882. Two days later it was
described by Mr. Lewis Boss as " a great comet in miniature,"
so well defined and regularly developed were its various parts
and appendages. It was discernible without optical aid early
in May ; and on June 5 it was observed on the meridian at
Albany just before noon an astronomical event of extreme
rarity. Comet Wells, however, never became an- object so con-
spicuous as to attract general attention, owing to its immersion
in the evening twilight of our northern June.
But the study of its spectrum revealed new facts of the utmost
interest. All the comets till then examined had been found
to conform to one invariable type of luminous emission. In-
dividual distinctions there had been, but no specific differences.
Now all these bodies had kept at a respectful distance from
the sun; for of the great comet of 1880 no spectroscopic in-
quiries had been made. Comet Wells, on the other hand,
approached his surface within little more than five million
miles on June 10, 1882 ; and it is not doubtful that to this cir-
cumstance the novel feature in its incandescence was due.
During the first half of April its spectrum was of the normal
type, though the carbon bands were unusually weak ; but with
increasing vicinity to the sun they died out, and the entire
RECENT COMETS. 399
light seemed to become concentrated into a narrow, unbroken,
brilliant streak, hardly to be distinguished from the spectrum
of a star. This unusual behaviour excited attention, and a
strict watch was kept. It was rewarded at the Dunecht Ob-
servatory (Lord Crawford's), May 27, by the discernment of
what had never before been seen in a comet the yellow ray
of sodium. 1 By June i, this had kindled into a blaze over-
powering all other emissions. The light of the comet was
practically monochromatic ; and the image of the entire head,
with the root of the tail, could be observed, like a solar pro-
minence, depicted, in its new saffron vesture of vivid illumina-
tion, within the jaws of an open slit.
At Potsdam, the bright yellow line was perceived with
astonishment by Vogel on May 31, and was next evening
identified with Fraunhofer's " D." Its character led him to
infer a very considerable density in the glowing vapour
emitting it, 2 Hasselberg founded an additional argument in
favour of the electrical origin of cometary light on the changes
in the spectrum of comet Wells. 3 , For they were closely
paralleled by some earlier experiments of Wiedemann, in
which the gaseous spectra of vacuum tubes were at once
effaced on the introduction of metallic vapours. It seemed
as if the metal had no sooner been rendered volatile by heat,
than it usurped the entire office of carrying the discharge, the
resulting light being thus exclusively of its production. Had
simple incandescence by heat been in question, the effect
would have been different ; the two spectra would have been
superposed without prejudice to either. Similarly, the replace-
ment of the hydro-carbon bands in the spectrum of the comet
by the sodium line, proved electricity to be the exciting agent.
For the increasing thermal power of the sun might, indeed,
have ignited the sodium, but it could not have extinguished
the hydro-carbons.
Dr. Huggins succeeded in photographing the spectrum of
1 Copernicus, vol. ii. p. 229.
9 A sir. Nach., Nos. 2434, 2437. 3 Ibid., No. 2441.
400 HISTORY OF ASTRONOMY.
comet Wells by an exposure of one hour and a quarter. 1 The
result was to confirm the novelty of its character. None of
the ultra-violet carbon-groups were apparent ; but certain
bright rays, as yet unidentified, had imprinted themselves.
Otherwise the spectrum was strongly continuous, uninterrupted
even by the Fraunhofer lines detected in the spectrum of
Tebbutt's comet. Hence it was concluded that a smaller
proportion of reflected light was mingled with the native
emissions of the later arrival.
All that is certainly known about the extent of the orbit
traversed by the first comet of 1882 is that it came from, and
is now retreating towards, vastly remote depths of space. An
American computer 2 found a period indicated for it of no
less than 400,000 years ; A. Thraen of Dingelstadt arrived at
one of 361 y. 3 Both are perhaps equally insecure.
We have now to give some brief account of one of the
most remarkable cometary apparitions on record, and with
the single exception of that identified with the name of Halley
the most instructive to astronomers. The lessons learned
from it were as varied and significant as its aspect was splendid ;
although from the circumstance of its being visible in general
only before sunrise, the spectators of its splendour were com-
paratively few.
The discovery of a great comet at Rio Janeiro, September
n, 1882, became known in Europe through a telegram from
M. Cruls, director of the observatory at that place. It had,
however (as appeared subsequently), been already seen on the
8th by Mr. Finlay, assistant at the Cape Observatory, and at
Auckland as early as September 3. A later, but very singu-
larly conditioned detection, quite unconnected with any of the
preceding, was effected by Mr. Common at Ealing. Since the
eclipse of May 17, when a comet named "Tewfik" in honour
of the Khedive of Egypt was caught on Dr. Schuster's photo-
1 Report Brit. Ass., 1882, p. 442.
2 J. J. Parsons, Am. Jour, of Science, vol. xxvii. p. 34.
3 Astr. Nach., No. 2441.
RECENT COMETS. 401
graphs, entangled, one might almost say, in the outer rays of
the corona, he had scrutinised the neighbourhood of the sun
on the infinitesimal chance of intercepting another such body
on its rapid journey thence or thither. We record with wonder
that, after an interval of exactly four months, that infinitesimal
chance turned up in his favour.
On the forenoon of Sunday, September 17, he saw a great
comet close to, and rapidly approaching the sun. It was, in
fact, then within a few hours of perihelion. Some measures of
position were promptly taken ; but a cloud-veil covered the
interesting spectacle before midday was long past. Mr. Finlay
at the Cape was more completely fortunate. Divided from
his fellow-observer by half the world, he unconsciously finished,
under a clearer sky, his interrupted observation. The comet,
of which the silvery radiance contrasted strikingly with the
reddish-yellow glare of the sun's margin it drew near to, was
followed " continuously right into the boiling at the limb"
a circumstance without precedent in cometary history. 1 Dr.
Elkin, who watched the progress of the event with another
instrument, thought the intrinsic brilliancy of the nucleus
scarcely surpassed by that of the sun's surface. Nevertheless
it had no sooner touched it than it vanished as if annihilated.
So sudden was the disappearance (at 4h. 5om. 585. Cape mean
time), that it was at first thought that the comet must have
passed behind the sun. But this proved not to have been the
case. The observers at the Cape had witnessed a genuine
transit. Nor could non-visibility be explained by equality of
lustre. For the gradations of light on the sun's disc are amply
sufficient to bring out against the dusky background of the
limb any object matching the brilliancy of the centre ; while
an object just equally luminous with the limb must inevitably
show dark at the centre. The only practicable view, then, is
that the bulk of the comet was of too filmy a texture, and its
presumably solid nucleus too small, to intercept any notice-
1 Observatory, vol. v. p. 355.
2 C
4C2 HISTORY OF ASTRONOMY.
able part of the solar rays a piece of information worth re-
membering.
On the following morning, the object of this unique ob-
servation showed (in Dr. Gill's words) " an astonishing brilli-
ancy as it rose behind the mountains on the east of Table
Bay, and seemed in no way diminished in brightness when the
sun rose a few minutes afterward. It was only necessary to
shade the eye from direct sunlight with the hand at arm's
length, to see the comet with its brilliant white nucleus and
dense, white, sharply-bordered tail of quite half a degree in
length." 1 All over the world, wherever the sky was clear
during that day, September 18, it was obvious to ordinary
vision. Since 1 843 nothing had been seen like it. From Spain,
Italy, Algeria, Southern France, despatches came in announc-
ing the extraordinary appearance. At Cordoba, in South
America, the " blazing star near the sun " was the one topic
of discourse. 2 Moreover, and this is altogether extraordinary,
the records of its daylight visibility to the naked eye extend
over three days. At Reus, near Tarragona, it showed bright
enough to be seen through a passing cloud when only three
of the sun's diameters from his limb, just before its final rush
past perihelion on September 17 ; while at Carthagena in
Spain, on September 19, it was kept in view during two hours
before, and two hours after noon, and was similarly visible in
Algeria on the same day. 3
But still more surprising than the appearance of the body
itself, were the nature and relations of the path it moved in.
The first rough elements computed for it by Mr. S. C. Chandler
of Harvard, and by Mr. White, assistant at the Melbourne
Observatory, showed at once a striking resemblance to those
of the twin comets of 1843 and 1880. This suggestive fact
became known in this country, September 27, through the
medium of a Dunecht Circular. It was fully confirmed by
subsequent inquiries, for which ample opportunities were
i Observatory, vol. v. p. 354. 2 Gould, Astr.*Nach., No. 2481.
3 Flammarion, Comptes Rcndiis^ t. xcv. p. 55^-
RECENT COMETS. 403
luckily provided. The likeness was not, indeed, so absolutely
perfect as in the previous case ; it included some slight, though
real differences ; but it bore a strong and unmistakable stamp,
broadly challenging explanation.
Two hypotheses only were really available. Either the
comet of 1882 was an accelerated return of those of 1843 and
1880, or it was a fragment of an original mass to which they
also had belonged. For the purposes of the first view the
" resisting medium " was brought into full play ; the opinion
invoking it was, for some time, both prevalent and popular,
and formed the basis, moreover, of something of a sensational
panic. For a comet which, at a single passage through the
sun's atmosphere, encountered sufficient resistance to shorten
its period from thirty-seven, to two years and eight months,
must, in the immediate future, be brought to rest on his surface ;
and the solar conflagration thence ensuing was represented in
some quarters, with more license of imagination than coun-
tenance from science, as likely to be of catastrophic import to
the inhabitants of our little planet.
But there was a test available in 1882 which it had not been
possible to apply either in 1843 or m *88o. The two bodies
visible in those years had been observed only after they had
already passed perihelion ; l the third member of the group,
on the other hand, was accurately followed for a week before
that event, as well as during many months after it. Mr.
Finlay's and Dr. Elkin's observation of its disappearance at
the sun's edge formed, besides, a peculiarly delicate test of its
motion. The opportunity was thus afforded, by directly com-
paring the comet's velocity before and after its critical plunge
through the solar surroundings, of ascertaining with some
approach to certainty whether any considerable retardation
had been experienced in the course of that plunge. The
answer distinctly given was that there had not. The computed
and observed places on both sides of the sun, fitted harmo
1 Captain Ray's sextant-observation of the comet of 1843 a few hours
before perihelion, was too rough to be of use.
404 HISTORY OF ASTRONOMY.
niously together. The effect, if any were produced, was too
small to be perceptible.
This result is, in itself, a memorable one. It seems to give
the coup de grace to Encke's theory somewhat discredited, in
addition, by Backlund's investigation of a resisting medium
growing rapidly denser inwards. For the perihelion distance
of the comet of 1882, though somewhat greater than that of
its predecessors, was nevertheless extremely small. It passed
at less than 300,000 miles of the sun's surface. But the
ethereal substance long supposed to obstruct the movement of
Encke's comet, would there be nearly 2000 times denser than
at the perihelion of the smaller body, and must have exerted
a conspicuous retarding influence. That none such could be
detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the
question of identity. The " Great September Comet " of 1882
was in no hurry to withdraw itself from curious terrestrial
scrutiny. It was discerned with the naked eye at Cordoba
as late as March 7, 1884, and still showed in the field of the
great equatoreal on June i as an " excessively faint whiteness." x
It was then about 470 millions of miles from the earth a
distance to which no other comet save the exceptional one
of 1729 has been pursued. 2 Moreover, an arc of 340 out of
the entire 360 degrees of its circuit had been described under
the eyes of astronomers ; so that its course came to be very
well known. It may then be taken as ascertained that its
movement is in a very eccentric ellipse, traversed in several
hundred years. The lowest estimate of period, founded on suffi-
ciently extensive data, is of 65 2 \ years (Morrison) ; the highest
deserving any confidence that by Kreutz of 843. 3 There is
reason to believe that this last is not very far from the truth.
Now this conclusion of a period to be counted by centuries,
must be taken to apply to all the three bodies so curiously
related by the nature of their movements. For to assert (as
1 Astr. Nach., No. 2538. z Nature, vol. xxix. p. 135.
^ 3 Astr. Nach., No. 2482.
RECENT COMETS. 405
many astronomers of repute still do) that the comets of 1843
and 1880 are one and the same body revolving regularly in
nearly thirty-seven years, is virtually to cut off all connection
between them and the comet of 1882. If the length of the
ellipses they respectively trace out be thus totally and widely
different, then the likeness between their other elements must
be purely superficial a mere freak of circumstance and
means nothing. But this no one has ever ventured to assert.
We have no alternative, then, but to regard all three as moving
in nearly the same orbit, with nearly the same period that is,
as individually distinct, though members of a single system.
So that the visibility of none of them can again be looked for
until the twenty-sixth or twenty- seventh century, when they
will probably return successively to perihelion in the same
order, and presenting much the same appearances, as in the
nineteenth.
The idea of cometary systems was first suggested by Thomas
Clausen in 1831. 1 It was developed by the acute inquiries of
the late M. Hoek, director of the Utrecht Observatory, in 1865
and some following years. 2 He found that in quite a consider-
able number of cases, the paths of two or three comets had a
common point of intersection far out in space, indicating with
much likelihood a community of origin. This consisted,
according to his surmise, in the disruption of a parent mass
during its sweep round the star latest visited. Be this as it
may, the fact is undoubted that numerous comets fall into
groups, in which affinity of geometrical relations betrays a pre-
existent physical connection. Never before, however, had
geometrical affinity been so notorious as between the three
comets now under consideration ; and never before, in a comet
still, it might be said, in the prime of life, had physical peculi-
arities tending to account for that affinity been so obvious as
in the last-comer of the group.
Observation of a granular structure in cometary nuclei dates
1 Gruithuisen's Analekten^ Heft vii. p. 48.
2 Month. Not., vols. xxv., xxvi., xxviii.
406 HISTORY OF ASTRONOMY.
far back into the seventeenth century, when Cysatus and
Hevelius described the central parts of the comets of 1618 and
1652 respectively, as made up of a congeries of minute stars.
Analogous symptoms of a loose state of aggregation have of
late been not unfrequently detected in telescopic comets, be-
sides the instances of actual division offered by those connected
with the names of Biela and Liais. The forces concerned in
producing these effects seem to have been peculiarly energetic
in the great comet of 1882.
The segmentation of the nucleus was first noticed in the
United States and at the Cape of Good Hope, September 30.
It proceeded rapidly. At Kiel, on October 5 and 7, Professor
Kriiger perceived two centres of condensation. A definite and
progressive separation into three masses was observed by Pro-
fessor Holden, October 13 and ly. 1 A few days later, M.
Tempel found the head to consist of four lucid aggregations,
ranged nearly along the prolongation of the caudal axis ; 2 and
Mr. Common, January 27, 1883, saw five nuclei in a line " like
pearls on a string." 3 This remarkable character was preserved
to the last moment of the comet's distinct visibility.
There were, however, other curious proofs, of a marked
tendency in this body to disaggregation. On October 8,
Schmidt discovered at Athens a nebulous object 4 south-west
of the great comet, and travelling in the same direction. It
remained visible for a few days, and, from Oppenheim's and
Hind's calculations, there can be little doubt that it was really
the offspring by fission of the body it accompanied. 4 This is
rendered more probable by the unexampled spectacle offered,
October 14, to Mr. E. E. Barnard of Nashville, Tennessee, of
six or eight distinct cometary masses within 6 south by west
of the comet's head, none of which reappeared on the next
opportunity for a search. 5 A week later, however, one similar
object was discerned by Mr. Brooks, of Phelps, N.Y., in the
1 Nature, vol. xxvii. p. 246. 2 Astr. Nach., No. 2468.
3 Athenaum, Feb. 3, 1883. 4 Astr. Nach., Nos. 2462, 2466.
5 Ibid., No. 2489.
RECENT COMETS. 407
opposite direction from the comet. Thus, space appeared to
be strewn with the filmy debris of this extraordinary body all
along the track of its retreat from the sun.
Its tail was only equalled (if it were equalled) in length by
that of the comet of 1843. It extended in space to the vast
distance of two hundred millions of miles from the head ; but,
so imperfectly were its proportions displayed to terrestrial
observers, that it at no time covered an arc of the sky of more
than 30. This apparent extent was attained, during a few
days previous to September 25, by a faint, thin, rigid streak,
noticed only by a few observers by Elkin at the Cape Ob-
servatory, Eddie at Grahamstown, and Cruls at Rio Janeiro.
It diverged at a low angle from the denser curved train, and
was produced, according to Bredichin, 1 by the action of a
repulsive force twelve times as strong as the counter-pull of
gravity. It belonged, that is, to type i ; while the great forked
appendage, obvious to all eyes, corresponded to the lower rate
of emission characteristic of type 2. This was remarkable for
the perfect defmiteness of its termination, for its strongly forked
shape, and for its unusual permanence. Down to the end of
January 1883, its length, according to Schmidt's observations,
was still 93 million miles \ and a week later it remained visible
to the naked eye, without notable abridgment.
Most singular of all was an anomalous extension of the
appendage towards the sun. During the greater part of Octo-
ber and November, a luminous " tube " or " sheath " of
prodigious dimensions seemed to surround the head, and
project in a direction nearly opposite to that of the usual out-
pourings of attenuated matter. Its diameter was computed
by Schmidt to be, October 15, no less \hd3\four million miles,
and it was described by Cruls as a " truncated cone of nebu-
losity," stretching 3 or 4 sunwards. 2 There can be little
doubt that this abnormal kind of efflux was a consequence of
the tremendous physical disturbance suffered at perihelion ;
1 Annales, Moscow, t. ix. pt. ii. p. 52.
2 Comptes Rcndus, t. xcvii. p. 797.
408 HISTORY OF ASTRONOMY.
and it is worth remembering that something analogous was
observed in the comet of 1680 (Newton's), also noted for its
excessively close approach to the sun. The only plausible
hypothesis as to the mode of its production is that of an
opposite state of electrification in the particles composing the
ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a
repetition of that of its immediate predecessor, thus confirming
the inference that the previously unexampled sodium-blaze
was in both a direct result of the intense solar action to which
they were exposed. But the D-line was, this time, not seen
alone. At Dunecht, on September 17, Drs. Copeland and
Lohse succeeded in identifying six brilliant rays in the green
and yellow with as many prominent iron-lines ; l a very sig-
nificant addition to our knowledge of cometary constitution,
and one which goes far to justify Bredichin's assumption of
various kinds of matter issuing from the nucleus with velocities
inversely as their atomic weights. All the lines equally showed
a slight displacement, indicating recession from the earth at
the rate of 3 7 to 46 miles a second. A similar observation
made by M. Thollon at Nice on the following day, supplied
a highly satisfactory test of the accuracy of the spectroscopic
method of estimating movement in the line of sight. Before
anything was as yet known of the comet's path or velocity,
he announced, from the position of the double sodium line
alone, that at three P.M. on September 18 it was increasing
its distance from our planet by from 61 to 76 kilometres per
second. 2 M. Bigourdan's subsequent calculations showed that
its actual swiftness of recession was at that moment 73 kilo-
metres.
Changes in the inverse order to those seen in the spectrum
of Comet Wells, soon became apparent. In the earlier body,
carbon-bands had died out with approach to perihelion, and
had been replaced by sodium-emissions; in its successor,
sodium-emissions became weakened and disappeared with
1 Copernicus, vol. ii. p. 235. 2 Comptes Rendus> t. xcvi. p. 371.
RECENT COMETS. 409
retreat from perihelion, and found their substitute in carbon-
bands. Professor Ricco was, in fact, able to infer, from the
sequence of prismatic phenomena, that the comet had already
passed the sun ; thus establishing a novel criterion for deter-
mining the position of a comet in its orbit by the varying
quality of its radiations.
Recapitulating what has been learnt from the five con-
spicuous comets of 1880-82, we find that the leading facts
acquired to science were these three. First, that groups of
comets may be met with pursuing each other, after intervals
of many years, in the same, or nearly the same track ; so that
identity of orbit can no longer be regarded as a sure test of
individual identity. Secondly, that no appreciable resistance
to motion is experienced by such bodies in traversing the
sun's corona. Finally, that their chemical constitution is a
highly complex one, and that they possess, in some cases at
least, a metallic core resembling the meteoric masses which
occasionally reach the earth from planetary space.
As to the origin of comets, there has been of late years much
speculation, ingenious or inane, which, however, it were quite
superfluous to review. Yet we are not wholly without the
guidance of ascertained fact on the subject. Laplace assumed
that the fundamental shape of comets' orbits, when unmodified
by planetary perturbations, is that of a hyperbola a circum-
stance which, if true, would imply their total disconnection
from our system, save by fortuitous encounter. But Gauss
and Schiaparelli separately proved, on the contrary, that these
bodies move naturally in prodigiously long ellipses, 1 the hyper-
bolic form, in the extremely rare cases where it may exist,
being a result of disturbance. This being so, it follows that
their condition previous to being attracted by the sun was
one of relative repose. 2 In other words, they shared the
movement of translation through space of the solar system.
This significant conclusion had been indicated, on other
1 Thury and Meyer, Arch, des Sciences, t. vi. (3d ser.), p. 187.
8 W. Forster, Pop. Mitth., 1879, p. 7.
410 HISTORY OF ASTRONOMY.
grounds, as the upshot of researches undertaken independently
by Carrington 1 and Mohn 2 in 1860, with a view to ascertain-
ing the anticipated existence of a relationship between the
general lie of the paths of comets, and the direction of the
sun's journey. It is tolerably obvious that, if they wander at
haphazard through the interstellar regions, a preponderance
of their apparitions should seem to arrive from the vicinity of
the constellation Hercules; that is to say, we should meet
considerably more comets than would overtake us. Just for
the very same reason that falling stars are more numerous
after than before midnight. Moreover, the comets met by
us should be apparently swifter-moving objects than those
coming up with us from behind ; because, in the one case, our
own real movement would be added to, in the other, subtracted
from theirs. But nothing of all this can be detected. Comets
approach the sun indifferently from all quarters, and with
velocities quite independent of direction.
We conclude then, with Schiaparelli and Forster, that the
" cosmical current " which bears the solar system towards its
unknown goal, carries also with it nebulous masses of undefined
extent, and at an undefined remoteness, fragments detached
from which, continually entering the sphere of the sun's attrac-
tion, flit across our skies under the form of comets. These
are, however, almost certainly so far strangers to our system
that they had no part in the long processes of development by
which its present condition was attained. They are, perhaps,
survivals of an earlier, and by us scarcely and dimly con-
ceivable state of things, when the chaos from which sun and
planets were, by a supreme edict, to emerge, had not as yet
separately begun to be.
1 Mem. R. A. Soc., vol. xxix. p. 355.
2 Month. Not., vol. xxiii. p. 203.
CHAPTER XII.
STARS AND NEBULA.
THAT a science of stellar chemistry should not only have
become possible, but should already have made material
advances, is assuredly one of the most amazing features in the
swift progress of knowledge our age has witnessed. Custom
can never blunt the wonder with which we must regard the
achievement of compelling rays emanating from a source
devoid of sensible magnitude through immeasurable dis-
tance, to reveal, by its peculiarities, the composition of that
source. The discovery of revolving double stars assured us
that the great governing force of the planetary movements, and
of our own material existence, sways equally the courses of the
farthest suns in space; the application of prismatic analysis
certified to the presence in the stars of the familiar materials,
no less of the earth we tread, than of the bodies built up out
of its dust and circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained
the generic participation of stellar light in the peculiarity by
which sunlight, spread out by transmission through a prism,
shows numerous transverse rulings of interrupting darkness.
No sooner had Kirchhoff supplied the key to the hidden mean-
ing of those ciphered characters, than it was eagerly turned to
the interpretation of the dim scrolls unfolded in the spectra of
the stars. Donati made at Florence, in 1860, the first efforts
in this direction ; but with little result, owing to the imper-
fections of the instrumental means at his command. His
comparative failure, however, was a prelude to others' success.
412 HISTORY OF ASTRONOMY.
Almost simultaneously, in 1862, the novel line of investigation
was entered upon by Huggins and Miller near London, by
Father Secchi at Rome, and by Lewis M. Rutherfurd in New
York. Fraunhofer's device of using a cylindrical lens for the
purpose of giving a second dimension to stellar spectra, was
adopted by all, and was indeed indispensable. For a lumi-
nous point, such as a star appears, becomes, when viewed
through a prism, a variegated line, which, until broadened
into a band by the intervention of a cylindrical lens, is all but
useless for purposes of research. This process of rolling out
involves, it is true, much loss of light a scanty and precious
commodity, as coming from the stars ; but the loss is an inevi-
table one. And so fully is it compensated by the great light-
grasping power of modern telescopes, that important information
can now be gained from the spectroscopic examination of stars
far below the range of the unarmed eye.
The effective founders of stellar spectroscopy, then (since
Rutherfurd shortly turned his efforts elsewhither), were Father
Secchi, the eminent Jesuit astronomer of the Collegio Romano,
where he died, February 26, 1878, and Dr. Huggins, with
whom the late Professor W. A. Miller was associated. The
work of each was happily directed so as to supplement that of
the other. With less perfect appliances, the Roman astronomer
sought to render his extensive rather than precise ; at Upper
Tulse Hill, searching accuracy over a narrower range was aimed
at and attained. To Father Secchi is due the merit of having
executed the first spectroscopic survey of the heavens. Above
4000 stars were in all passed in review by him, and classified
according to the varying qualities of their light. His provi-
sional establishment (1863-67) of four types of stellar spectra 1
has proved a genuine aid to knowledge through the facilities
afforded by it for the arrangement and comparison of rapidly
accumulating facts. Moreover, it is scarcely doubtful that
1 Report Brit. Ass., 1868, p. 166. Rutherfurd gave a rudimentary
sketch of a classification of the kind in December 1862, but based on
imperfect observations. See Am. Jour, of Sc.^ vol. xxxv. p. 77.
STARS AND NEBULAE. 413
these spectral distinctions correspond to differences in physical
condition of a marked kind.
The first order comprises more than half the visible stars,
and a still larger proportion of those eminently lustrous.
Sirius, Vega, Regulus, Altair, are amongst its leading members.
Their spectra are distinguished by the breadth and intensity of
the four dark bars due to the absorption of hydrogen, and by
the extreme faintness of the metallic lines, of which, never-
theless, hundreds are disclosed by careful examination. The
light of these "Sirian " orbs is white or bluish ; and it is found
to be rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of
stars, which is about one-sixth less numerously represented
than the first. Their spectra are quite closely similar to that
of sunlight, in being ruled throughout by innumerable fine dark
lines ; and they share its yellowish tinge.
The third class includes most red and variable stars (com-
monly synonymous), of which Betelgeux in the shoulder of
Orion, and " Mira " in the Whale are noted examples. Their
characteristic spectrum is of the "fluted" description. It
shows like a strongly illuminated colonnade seen in perspec-
tive, the light falling from the red end towards the violet.
This kind of absorption is produced by the vapours of metal-
loids or of compound substances.
To the fourth order of stars belongs also a colonnaded
spectrum, but reversed ; the light is thrown the other way.
The individuals composing it are few, and apparently insigni-
ficant, the brightest of them not exceeding the fifth magnitude.
They are commonly distinguished by a deep red tint, and
gleam like rubies in the field of the telescope. Father Secchi,
who detected the peculiarity of their analysed light, ascribed
it to the presence of carbon in some form in their atmos-
pheres ; and this has been confirmed by the latest researches
of H. C. Vogel, 1 now director of the Astro-physical Observatory
at Potsdam. The hydro-carbon bands, in fact, seen bright in
1 Publicationen, Potsdam, No. 14, 1884, p. 31.
4H HISTORY OF ASTRONOMY.
comets, are dark in these singular objects the only ones in
the heavens (save, perhaps, a coronal streamer or a rare meteor) l
which display a cometary analogy of the fundamental sort
revealed by the spectroscope.
The members of all four orders are, however, emphatically
suns. They possess, it would appear, photospheres radiating
all kinds of light, and differ from each other (so far as we are
able to judge) solely in the varying qualities of their absorp-
tive atmospheres. The principle that the colours of stars
depend, not on the intrinsic nature of their light, but on the
kinds of vapours surrounding them, and stopping out certain
portions of that light, was laid down by Huggins in i864. 2
The phenomena of double stars seem to indicate a connection
between the state of the investing atmospheres by the action
of which their often brilliantly contrasted tints are produced,
and their mutual physical relations. A remarkable tabular
statement put forward by Professor Holden in June i88o 3
made it, at any rate, clear that inequality of magnitude be-
tween the components of binary systems accompanies unlike-
ness in colour, and that stars more equally matched in one
respect are pretty sure to be so in the other. Besides, blue
and green stars of a decided tinge are never (so far as is cer-
tainly known) solitary ; they invariably form part of systems.
So that association has undoubtedly a predominant influence
upon colour.
Nevertheless, the crude notion thrown out by Zollner in
1 865,* that yellow and red stars are simply white stars in various
stages of cooling, has obtained undeserved currency. D'Arrest,
it is true, protested against it, but Vogel adopted it in 1874 as
the "rational" basis of his classification. 5 This differs from
Father Secchi's only in presenting his third and fourth types as
1 Von Konkoly once derived from a slow-moving meteor a hydro-carbon
spectrum. A. S. Herschel, Nature, vol. xxiv. p. 507.
2 Phil. Trans., vol. cliv. p. 429.
3 Am. Jour, of St., vol. xix. p. 467. 4 Photom. Unters., p. 243.
5 Astr. Nach. t No. 2000.
STARS AND NEBULA. 415
subdivisions of the same order ; but the seductive, though possi-
bly misleading idea of progressive development is added. Thus,
the white Sirian stars are represented as the youngest because
the hottest of the sidereal family j those of the solar pattern as
having already wasted much of their store by radiation, and
being well advanced in middle life ; while the red stars with
banded spectra figure as effete suns, hastening rapidly down
the road to final extinction. 1
Now the truth is, that we are just as ignorant of the relative,
as of the absolute ages of the stars, the arguments employed
on the point being, as it were, reversible. For instance, if
there be any truth in the theory of nebular condensation, we
should expect to find a forming sun surrounded by a dense
and extensive atmosphere, not unlike that of a " hydro-carbon "
star ; while the decay of luminous power would probably be
attended by a falling-off in absorptive action, resulting in a
feebly continuous spectrum. Stars of the latter description
may exist ; but the absence of characterisation, no less than
of intensity in their light renders them both a difficult, and an
unattractive subject of study.
A spectroscopic star-catalogue (the first attempted) is now
in course of preparation at Potsdam and Lund by Drs. Vogel
and Duner. It will include all stars down to magnitude 7^
situated between the north pole of the heavens and one degree
south of the equator. The first part, giving the .results of obser-
vations upon the spectra of 4051 stars (12,000 were incidentally
examined), was published in 1883,2 an d a further instalment
will shortly follow. The provision of such a vast and accurate
store of data for future reference is a duty, in Vogel's estima-
tion, which the present generation owes to posterity, and may
prove of inestimable importance to the progress of discovery.
1 Mr. J. Birmingham, in the Introduction to his valuable Catalogue of
Red Stars, comments upon this "singular conceit," and alleges various
instances of change of colour in a direction the opposite of that which it
supposes to be the inevitable result of time. Trans. R. Irish Ac., vol.
xxvi. pp. 251-253. 2 Publicationen, No. II, Potsdam, 1883.
416 HISTORY OF ASTRONOMY.
A fairly complete answer to the question, What are the
stars made of? was given by Dr. Huggins in I864. 1 By
laborious processes of comparison between stellar dark lines
and the bright rays emitted by terrestrial substances, he made
quite sure of his conclusions, though at much cost of time
and pains. He assures us, indeed, that taking into account
restrictions by weather and position the thorough investiga-
tion of a single star-spectrum would be the work of some years.
Of two, however those of Betelgeux and Aldebaran he was
able to furnish detailed and accurate drawings. The dusky
flutings in the prismatic light of the first of these stars have
not been identified with the absorption of any particular sub-
stance ; but associated with them are dark lines telling of the
presence of sodium, iron, calcium, magnesium, and bismuth.
Hydrogen rays are also inconspicuously present. That an
exalted temperature reigns, at least in the lower strata of the
atmosphere, is certified by the vaporisation there of matter so
refractory to heat as iron. 2
Nine elements those identified in Betelgeux, with the addi-
tion of tellurium, antimony, and mercury were recognised as
having stamped their signature on the spectrum of Aldebaran ;
while the existence in Sirius, and nearly all the other stars
inspected, of hydrogen, sodium, iron, and magnesium was
rendered certain or highly probable. This was admitted to be
a bare gleaning of results ; nor is there reason to suppose any
of his congeners inferior to our sun in complexity of consti-
tution.
The evidence given by the spectroscope of fluctuations in
quality as well as in quantity, in the light of variable stars,
suggests a rationale of the surprising appearances presented
by them, which may eventually be expected to supersede all
others. Speaking generally, stellar variability is an accom-
paniment of a ruddy tint and a banded spectrum. In other
1 Phil. Trans., vol. cliv. p. 413. Some preliminary results were em-
bodied in a "- note " communicated to the Royal Society, February 19,
1863 (Proc. Roy. Soc., vol. xii. p. 444). 2 Ibid., p. 429, note.
STARS AND NEBULA. 417
words, it prevails in stars surrounded by powerfully absorptive
atmospheres. Moreover, the strength of their absorption in-
creases as the light diminishes ; perhaps we might say, the
light diminishes because it increases.
Heretofore the explanations of variability chiefly entertained
had been these two : rotation on an axis, showing alternately
a darker and a brighter side, and the interposition of a non-
luminous body, revolving round a star periodically eclipsed by
it. But in truth, the facts, for the most part, fitted very ill
with either, and were, not unfrequently, in glaring disaccord
with both. There are, however, a few exceptional cases to
which the " eclipse " theory has been thought to be peculiarly
applicable; and assuredly, if it fail in them, it will succeed
nowhere else. The leading member of this small group is the
star called Algol in the head of Medusa.
It stands apart in several respects from most other variables.
In the first place, it is a white star, and shows a spectrum of
the Sirian pattern two circumstances highly favourable to
stability in lustre. Further, the diminution of its light is by a
strictly impartial process ; no individual rays are attacked more
than others ; it remains unchanged in kind even when reduced
to a sixth of its original amount. Finally, the accomplishment
of its decline and revival, instead of being distributed, with
more or less of irregularity, over its entire period, is restricted
to a perfectly definite fractional part of it. During somewhat
more than two days and a half it shines quite steadily as a star
of the second magnitude; its fall to, and recovery from the
fourth are hurried over in about seven hours.
This manner of procedure suggested to Goodricke, who dis-
covered in 1782 the periodical variability of Algol, the inter-
position of a large satellite; and the explanation has been
generally accepted. The conditions under which it must be
available were, however, first seriously investigated by Professor
Pickering in iSSo. 1 He found that the appearances in ques-
1 Proc. Ant. Ac. Sc., vol. xvi. p. 17 ; Observatory, vol. iv. p. 116. For a
preliminary essay by T. S. Aldis in 1870, see Phil. Mag. vol. xxxix. p. 363.
2 D
4x8 HISTORY OF ASTRONOMY.
tion could be quite satisfactorily accounted for by admitting
the revolution round the star of an obscure body 0.764
of its own diameter, in a period of two days twenty hours
forty-nine minutes. It needs, indeed, a mind trained to the
docile adoption of views authoritatively recommended, to con-
template without some measure of incredulity a system in
which a satellite of the same relative magnitude that 446
Jupiters would bear to our sun, circulates in a relative con-
tiguity to its primary only a little less close than that of his
inner satellite to Mars. But, as Professor Pickering remarks
in a similar connection, "what could be more improbable than
the phenomenon itself, were it not verified by observation ? " 1
The Algol class of variables includes only seven or eight
members. If the hypothesis of an eclipsing body (for which
a cloud of meteorites may be substituted) represent the truth
in one case, it must be capable of adaptation to all. But the
attempt to fit it to a remarkable star in the constellation
Cepheus, discovered byM. Ceraski at Moscow, June 23, 1880,
may be said to have broken down. Its phases are of the
same rapid and well-defined description as those of Algol, and
recur in the still shorter period of two and a half days. Its
bluish white rays, however, turn ruddy at minimum, which
implies, not mere stoppage, but selective absorption. Besides,
the interposing satellite should (according to Pickering's cal-
culations) be almost as large as its primary, so that the eclipse-
rationale seems here burdened with intolerable difficulties.
The number of recognised variables of all classes already
reaches some hundreds, and is continually increasing. Indeed,
Dr. Gould is of opinion that most stars fluctuate slightly
in brightness through surface-alternations similar to, but on a
larger scale than those of the sun. The solar analogy might,
perhaps, be push'ed somewhat further. It may be found to
contain a clue to much that is perplexing in stellar behaviour.
Wolf pointed out in 1852 the striking resemblance in character
between curves representing sun-spot frequency, and curves
1 Proc. Am. Ac., vol. xvi. p. 259.
STARS AND NEBULA. 419
representing the changing luminous intensity of many variable
stars. There were the same steep ascent to maximum and
more gradual decline to minimum, the same irregularities in
heights and hollows, and, it may be added, the same tendency
to a double maximum, and complexity of superposed periods.
It is impossible to compare the two sets of phenomena thus
graphically portrayed, without reaching the conclusion that
they are of closely related origin that our sun, in fact, is of
the kindred of variable stars, though the family peculiarities
have, for some reason, remained comparatively undeveloped.
Every kind and degree of variability is exemplified in the
heavens. At the bottom of the scale are stars like the sun,
of which the lustre is tried by our instrumental means
sensibly steady. At the other extreme are ranged the astound-
ing apparitions of " new," or " temporary " stars. Within the
last score of years three of these stellar guests (as the Chinese
call them) have presented themselves, and we meet with a
fourth no farther back than April 27, 1848. But of the "new
star " in Ophiuchus found by Mr. Hind on that night, little
more could be learnt than of the brilliant objects of the same
kind observed by Tycho and Kepler. The spectroscope had
not then been invented. Let us hear what it had to tell of
later arrivals.
Between thirty and fifteen minutes before midnight of May
12, 1866, Mr. John Birmingham of Millbrook, near Tuam, in
Ireland, saw with astonishment a bright star of the second
magnitude unfamiliarly situated in the constellation of the
Northern Crown. Four hours earlier, Schmidt of Athens had
been surveying the same part of the heavens, and was able to
testify that it was not visibly there. That is to say, a few
hours, or possibly a few minutes, sufficed to bring about a
conflagration, the news of which may have occupied hundreds
of years in travelling to us across space. The rays which were
its messengers, admitted within the slit of Dr. Huggins's
spectroscope, May 16, proved to be of a composition highly
significant as to the nature of the catastrophe. The star which
420 HISTORY OF ASTRONOMY.
had already declined below the third magnitude showed what
was described as a double spectrum. To the dusky flutings
of Secchi's third type four brilliant rays were added. 1 The
chief of these agreed in position with lines of hydrogen ; so that
the immediate cause of the outburst was plainly perceived to
have been the eruption, or ignition, of vast masses of that
subtle kind of matter, the universal importance of which
throughout the cosmos is one of the most curious facts re-
vealed by the spectroscope.
T Coronse(as the new star was called) quickly lost its adven-
titious splendour. Nine days after its discovery it was again
invisible to the naked eye. It is now a pale yellow, slightly
variable star near the tenth magnitude, and finds a place as
such in Argelander's charts. It was thus obscurely known
before it made its sudden leap into notoriety.
The mantle of gaseous incandescence in which it was tem-
porarily wrapt, is a recurrent, or even a permanent feature in
some other stars. Two of these & Lyrse, a white star variable
(by a rare exception) in a period of twelve days and nearly
twenty-two hours, and 7 Cassiopeise were noticed by Father
Secchi at the outset of his spectroscopic inquiries. Both show
bright lines of hydrogen and ' helium,' so that the peculiarity
of their condition probably consists in the unusual extent, and
intense ignition of their chromospheric surroundings. But
this condition is subject to fluctuations. The brilliant rays
indicative of it died out during nine years, 1874-83, and the
first symptom of their reappearance was caught by M. Eugen
von Gothard in a twinkling of the crimson C line in the
spectrum of 7 Cassiopeiae, August 13, 1883.2 Before the end
of the month, the whole range was vividly apparent, and the
Lyre variable followed suit in the course of the autumn. An
ebb and flow of brightness in a period of seven days, has since
been found by M. von Gothard to affect the hydrogen and
1 Proc. Roy. Sec., vol. xv. p. 146.
2 A sir. Nach. t Nos. 2539, 2548, 2581.
STARS AND NEBULA. 421
helium spectrum of the latter star, and he suspects an analo-
gous inconstancy in the emissions of its fellow. 1
These two luminaries formed the nucleus of what is now
generally regarded as a distinct stellar class. To it belong the
extraordinary variable y Argus, with 7 in the same constellation,
examined by Respighi in 1871 ; and it includes three small
stars in the Swan, the peculiar character of which was
discovered in 1867 by MM. Wolf and Rayet of the Paris
Observatory. 2 Their light betrayed a mainly gaseous origin,
separating into three bands, identical in each star, but corre-
sponding to no known substance, and scarcely connected by
an almost evanescent continuous spectrum. No sign of change
has been detected in them. Three analogous objects have
since been discovered by Professor Pickering, and five more
were found by Dr. Copeland in 1883 in the course of an ex-
cursion exploratory of visual possibilities in the Andes. 3
Now the question arises, have we here to do with stars in
the ordinary sense at all? that is, with suns like our own,
reduced by the immensity of their distance to sparkling points
of light? We should reply in the negative were the above
definition to be adopted ; but our readers will have already
gathered that it requires much extension and qualification.
How far we may yet be led to extend it, and how profoundly
our ideas of what constitutes a "star" may eventually have to
be modified, a recent noteworthy event has gone a great way
to indicate.
On the 24th of November 1876, at Athens, Dr. Schmidt
discovered a new star in the constellation Cygnus. It was
then nearly of the third magnitude, and in its previous state
must have been below the ninth, since Argelander had made
no record of its existence. Its spectrum was examined
December 2, by Cornu at Paris, 4 and a few days later by
Vogel and Lohse at Potsdam. 5 It proved of a closely similar
1 Bull Astr., t. ii. p. 149. 2 Comptes Rendus, t. Ixv. p. 292.
3 Copernicus, vol. iii. p. 207. 4 Comptes Rendus, t. Ixxxiii. p. 1172.
5 Monatsb., Berlin, 1877, pp. 241, 826.
422 HISTORY OF ASTRONOMY.
character to that of T Coronae. A range of bright lines, in-
cluding those of hydrogen, helium, and perhaps of the coronal
gas (1474), stood out from a continuous background strongly
" fluted " by absorption. It may be presumed that in reality
the gaseous substances, which, by their sudden incandescence
had produced the apparent conflagration, lay comparatively
near the surface of the star, while the screen of cooler materials
rhythmically intercepting large portions of its light, was situated
at a considerable elevation in its atmosphere.
The object, meanwhile, steadily faded. By the end of the
year it was of no more than seventh magnitude. After the
second week of March 1877, strengthening twilight combined
with the decline of its radiance to arrest further observation.
It was resumed, September 2, at Dunecht, with a strange
result. Practically the whole of its scanty light (it had then
sunk below the tenth magnitude) was perceived to be gathered
into a single bright line in the green, and that the most
characteristic line of gaseous nebulae. 1 The star had, in fact,
so far as outward appearance was concerned, become trans-
formed into a planetary nebula, many of which are so minute
as to be distinguishable from small stars only by the quality of
their radiations. The nebular phase, however, seems to have
been transient. In the course of 1880, Professor Pickering
found that Nova Cygni gave an ordinary stellar spectrum of
barely perceptible continuous light ; 2 and his observation was
negatively confirmed at Dunecht, February i, 1881.
This enigmatical object has now dropt to (if not below) the
fourteenth magnitude, being thus out of reach of spectroscopic
scrutiny, save (possibly) with a few of the most powerful tele-
scopes in the world. The lesson learnt from its changes ap-
pears to be no less than this : That no clear dividing-line can
be drawn between stars and nebulae ; but that in what are
called "planetary nebulae" on the one side, and in "gaseous
stars" (those giving a spectrum of bright lines) on the other,
we meet with transitional forms, serving to bridge the gap
1 Copernicus, vol. ii. p. 101. 2 Annual Report, 1880, p. 7.
STARS AND NEBULA. 423
between such vast and highly finished orbs if we may be per-
mitted the expression as Sirius, and the inchoate, faintly-
lucent stuff which curdles round the trapezium of Orion.
We have been compelled somewhat to anticipate our narra-
tive as regards inquiries into the nature of this latter kind of
object. The fluctuations of opinion on the point came to an
abrupt end with the application to them of the spectroscope.
On August 29, 1864, Dr. Huggins sifted through his prisms
the rays of a bright planetary nebula in Draco. 1 To his
infinite surprise, they proved to be mainly of one colour. In
other words, they avowed their origin from a mass of glowing
vapour. As to what kind of vapour it might be by which
Herschel's conjecture of a " shining fluid " variously diffused
throughout the cosmos, was thus unexpectedly verified, an
answer was also at hand. The conspicuous bright line of the
Draco nebula was found to belong very probably to nitrogen ;
of its two fainter companions, one was unmistakably the F
line of hydrogen, while the other, in position intermediate
between the two, still remains unidentified. The extreme
faintness of nebular light was experimentally shown to be
reason sufficient for the solitariness in its spectrum of the lines
emanating respectively from nitrogen and hydrogen ; the sur-
viving nebular rays being precisely those which resist extinc-
tion longest.
By 1868, Dr. Huggins had satisfactorily examined the
spectra of about seventy nebulae, of which one-third dis-
played a gaseous character. 2 In all of these (and the rule has
hitherto proved without exception) the nitrogen line appeared ;
though in some cases as "the Dumb-bell" nebula in Vul-
pecula it appeared alone. On the other hand, a fourth line,
the dark blue of hydrogen in addition to the normal three,
was subsequently detected in the light of the great Orion
nebula. But, fundamentally, the composition of all bodies of
this class may be assumed the same. The differences in their
radiations seem to be of intensity, not of kind. All planetary
1 Phil. Trans., vol. cliv. p. 437. 2 Z6uf. t vol. clviii. p. 540.
424 HISTORY OF ASTRONOMY.
and annular nebulae belong to it, as well as those termed
"irregular" which frequent the region of the Milky Way.
Thus the signs of resolvability noted at Parsonstown and
Cambridge (U.S.) in Orion and the " Dumb-bell," were proved
fallacious, so far, at least, as they had been taken to indicate
a stellar constitution ; though they may have quite faithfully
corresponded to the existence in discrete masses of the glow-
ing vapours elsewhere more equably diffused.
The well-known nebula in Andromeda, and the great spiral
in Canes Venatici are amongst the more remarkable of those
giving a continuous spectrum ; and, as a general rule, the
emissions of all such nebulae as present the appearance of star-
clusters grown misty through excessive distance, are of the
same kind. It would, however, be eminently rash to conclude
thence that they are really aggregations of sun-like bodies.
The improbability of such an inference has been vastly en-
hanced by the recent outbreak of a new star apparently in the
very heart of the Andromeda nebula. First seen by Mr. Isaac
W. Ward, August 19, 1885, as an ordinary yj magnitude star,
it already shows a diminished lustre. It gives a continu-
ous spectrum precisely similar to that of the nebula that is,
truncated in the red as if by absorptive action." Hence, the
cause of its sudden development of light must have been a
totally different one from that occasioning the flaming appari-
tions in Corona and Cygnus.
Among the ascertained analogies between the stellar and
nebular systems is that of variability of light. On October
n, 1852, Mr. Hind discovered a small nebula in Taurus.
Chacornac observed it at Marseilles in 1854, but was con-
founded four years later to find it vanished. D'Arrest missed
it October 3, and re-detected it December 29, 1861. It was
easily seen in 1865-66, but invisible in the most powerful
instruments iSyy-So. 1 This was the first undisputed instance
of nebular variability. Brought to the notice of astronomers
1 Chambers, Descriptive Astronomy (3d ed.), p. 543 ; Flammarion,
UUnivers Sidtral, p. 818.
STARS AND NEBULAE. 425
by D'Arrest in I862, 1 it has since been confirmed by others of
the same nature. Two such have recently been adduced by
Winnecke ; 2 and Professor Holden, having co-ordinated in his
admirable " Monograph of the Nebula of Orion " 3 the results
of all the more prominent inquiries into the structure of that
marvellous object since 1758, reaches the conclusion, that
while the figure of its various parts has (with only one possible
exception) remained the same, their brightness has been and
is in a state of continual fluctuation. This accords precisely
with the conviction expressed by O. Struve in i857, 4 and may
now be safely accepted an an ascertained fact.
More dubious is the case of the "trifid" nebula in Sagit-
tarius, investigated by Professor Holden in 1877. 5 That
change of some kind has occurred, is indeed established by a
comparison of his own and others' observations with those of
the two Herschels ; but he inclines to the view that motion,
with or without an accompanying variation in light, is here the
agent of change. What is certain is, that a remarkable triple
star which, during the years 1784-1833, was centrally situated
in a dark space between the three great lobes of the nebula,
has since become involved in one of them ; and since the
star gives no sign of sensible displacement, the movement if
movement there should prove to be must be thrown upon
the nebula.
A similar example was last year alleged by Mr. H. Sadler, 6
but the evidence upon which it rests is disputed. The as-
certainment of true proper motion in a nebula would be the
more interesting from its absolute novelty. Hitherto this
class of bodies have shown no sign of sharing the busy journey-
ings of the stars. 7 They have remained as seemingly fixed in
their places as if exempt from all relation with the multitudi-
1 Astr. Nach., No. 1366.
2 Month. Not., vol. xxxviii. p. 105 ; Astr. Nach., No. 2293.
3 Wash. Obs., vol. xxv. App. I. 4 Month. Not., vol. xvii. p. 230.
5 Am. Jour, of Sc., vol. xiv. p. 433. 6 Observatory, vol. viii. p. 127.
7 For some instances of supposed orbital movement in " double "
nebulae, see Flammarion, Comptts Rendus, t. Ixxxviii. p. 27.
426 HISTORY OF ASTRONOMY.
nous hosts of the galactic world. This singular immobility
might, on a casual view, be set down to the account of enor-
mous distance, no single nebula having, so far, exhibited the
faintest trace of parallactic displacement. But there is a
method of estimating motion independent of distance, and to
this also nebulae have hitherto proved unresponsive.
The principle upon which " motion in the line of sight " can
be detected and measured with the spectroscope, has already
been explained. 1 It depends, as our readers will remember,
upon the removal of certain lines, dark or bright (it matters
not which), from their normal places by almost infinitesimal
amounts. The whole spectrum of the moving object, in fact,
is very slightly shoved hither or thither, according as it is
travelling towards or from the eye; but, for convenience of
measurement, one line is usually picked out from the rest,
and attention concentrated upon it. The application of this
method to the stars, however, is encompassed with difficulties.
It needs a powerfully dispersive spectroscope to show line-
displacements of the minute order in question ; and powerful
dispersion involves a strictly proportionate enfeeblement of
light. This, where the supply is already to a deplorable
extent niggardly, can ill be afforded; and it ensues that the
operation of determining a star's approach or recession is,
even apart from atmospheric obstacles, an excessively deli-
cate one.
It was first successfully executed by Dr. Huggins early in
i868. 2 The brightest star in the heavens was selected as the
most promising subject of experiment, and proved amenable.
In the spectrum of Sirius, the F line was perceived to- be just
so much displaced towards the red as to indicate (the orbital
motion of the earth being deducted) recession at the rate of
twenty-nine miles a second. Of this an undetermined propor-
tion was no doubt attributable to the advance through space
of the solar system, for which Struve's estimate of four miles a
1 See ante, p. 243. 2 Phil. Trans., vol. clviii. p. 529.
STARS AND NEBULAE. 427
second was almost certainly too small. Still there remained a
large surplus of Sirian proper motion. Its reality and direction
were placed beyond doubt by Vogel and Lohse's observation,
March 22, 1871, of a similar, but even more considerable dis-
placement. 1 The inquiry was resumed by Dr. Huggins with
improved apparatus in the following year, when the movements
of thirty stars were approximately determined. 2 The retreat
of Sirius was now diminished in estimated velocity to about
twenty miles per second, and it was discovered to be shared, at
rates varying from twelve to twenty-nine miles, by Betelgeux,
Rigel, Castor, Regulus, and five of the principal stars in the
Plough. Arcturus, on the contrary, gave signs of rapid ap-
proach (fifty-five miles a second), as well as Pollux, Vega,
Deneb in the Swan, and the brightest of the Pointers.
The realisation of this method of investigating stellar motions
has an importance far beyond that of which the idea is con-
veyed by the bare enumeration of its preliminary results. It
may confidently be expected to play a leading part in the
unravelment of the vast and complex relations which we can
dimly detect as prevailing amongst the innumerable orbs of
the sidereal world ; for it supplements the means which we
possess of measuring by direct observation movements trans-
verse to the line of sight, and thus completes our knowledge
of the courses and velocities of stars at ascertained distances,
while supplying for all a valuable index to the amount of
perspective foreshortening of apparent movement. Thus some,
even if an imperfect, knowledge may at length be gained of
the revolutions of the stars of the systems they unite to form,
of the paths they respectively pursue, and of the forces under
the compulsion of which they travel.
Already, though the method can scarcely be said to have
passed the tentative stage, a most curious fact has been
brought to light. Since 1874, spectroscopic measures of the
visual component of stellar motions have been made part of
1 Schellen, Die Spectralanalyse, Bd. ii. p. 326 (ed. 1883).
2 Proc. Roy. Soc., vol. xx. p. 386.
428 HISTORY OF ASTRONOMY.
the regular work at the Royal Observatory, Greenwich. The
results have proved, on the whole, strongly confirmatory of
Dr. Huggins's. But in the movement of Sirius a perplexing
change has taken place. In March 1876 it was estimated to
be adding to its distance from the earth by twenty-seven miles
each second. In 1877 a slackening was perceived; and
this progressively advanced, until, in 1882, the rate of reces-
sion was diminished to, or below, seven miles a second. A
reversal of direction was even anticipated, and shortly occurred.
The spectrum was markedly shifted towards the blue end,
November 16, I883; 1 and a series of forty-five measures exe-
cuted by Mr. Maunder on thirteen nights in 1884, gave to
the star a mean motion of approach of twenty-two miles a
second. 2 It does not appear that the known elliptic revolution
of Sirius round its companion will account for these vicissitudes,
although it is remarkable that they are suspected also to affect,
in some degree, the course of Procyon, a star similarly cir-
cumstanced to Sirius in its vicinity to a comparatively obscure
source of disturbance. The further development of these
significant changes will be of the highest interest.
None of the nebulae hitherto examined show the slightest
trace of displacement in the line of sight. 3 And this conclusion,
unlike estimates of apparent movement across the sky, has
absolutely no connection with their greater or less remoteness.
So that we seem compelled to draw an inference which must
largely affect our ideas of the whole structure of the heavens j
namely, that nebulae, as a class, are very much slower-moving
bodies than stars.
The uses of photography in celestial investigations become
every year more manifold and more apparent. The earliest
chemical star-pictures were those of Castor and Vega, obtained
1 Month. Not., vol. xliv. p. 91.
2 Observatory, vol. viii. p. 109. Dr. Huggins in 1872 anticipated as a
possible consequence of its circulation in an orbit the occurrence of such
changes in the movement of Sirius as have actually been observed. Proc.
Roy. Sec,, vol. xx. p. 387.
3 Huggins, Proc. Roy. Soc., vol. xxii. p. 251.
STARS AND NEBULA. 429
with the Cambridge refractor in 1845 by Whipple of Boston
under the direction of W. C. Bond. Double-star photography
was inaugurated under the same auspices in 1857, with an
impression of Mizar, the middle star in the handle of the
Plough, and its small companion Alcor, the old Arab test of
keen eyesight, but now a comparatively easy naked-eye object.
The matter next fell into the able hands of Rutherfurd, who
completed in 1864 a fine object-glass, corrected for the ultra-
violet rays, consequently useless for visual purposes. The
sacrifice was recompensed by conspicuous success. A set of
measurements from his photographs of nearly fifty stars in the
Pleiades, enabled Dr. Gould in 1866 to ascertain, by com-
parison with Bessel's places for the same stars, that during the
intervening quarter of a century no changes of importance
had occurred in their relative positions. 1 The construction of
photographic star-maps of real and permanent value was thus
demonstrated to be a possibility, and is rapidly being con-
verted into a reality of the utmost moment to the future of
science. In carrying on the work of ecliptical charting, left
half completed by Chacornac, the MM. Henry encountered
sections of the Milky Way which defied the enumerating
efforts of eye and hand, and resolved in consequence to have
recourse to the camera. The perfect success of some pre-
liminary trials made with an instrument constructed expressly
for the purpose, was announced to the Academy of Sciences
at Paris, May n, 1885. By its means, stars down to the
sixteenth magnitude clearly record their presence and their
places ; and we are hence doubtless on the eve of seeing the co-
operative photographic survey of the heavens, recommended by
Dr. Gill, carried into execution. It will include the uncounted
host of separate stars, showing the significant character-
istics of their distribution ; will individualise the hundreds, or
1 Gould on Celestial Photography, Observatory, vol. ii. p. 16. Professor
Pritchard communicated to the Roy. Astr. Soc., May 9, 1884, his detection
of some small movements inter se of members of the Pleiades group.
Observatory, vol. vii. p. 163.
430 HISTORY OF ASTRONOMY.
even thousands, of components forming each of those strange
systems apart, known to us as " star-clusters ; " will determine
the configurations and apparent distances of the members of
binary and multiple groups, with an enormous saving of labour,
and with the elimination of vexatious personal peculiarities in
error ; besides faithfully recording the forms and positions of
those baffling crowds of nebulae, the yearly discoveries of which
are counted by the score ; thus providing in all branches of
sidereal astronomy a sure criterion of future change.
In the use of photography as an engine of research into the
physical condition of the stars, Dr. Huggins led the way. In
March 1863 he obtained with his coadjutor, Dr. Miller, micro-
scopic prints of the spectra of Sirius and Capella. 1 But they
told nothing. No lines were visible in them. They were mere
characterless streaks of light. He tried again in 1876, when
the 1 8-inch speculum of the Royal Society had come into
his possession, using prisms of Iceland spar, and lenses of
quartz ; and this time with better success. A photograph of the
spectrum of Vega showed seven strong lines. 2 Still he was
not satisfied. He waited and worked for three years longer.
At length, on December 18, 1879, he was able to communicate
to the Royal Society 3 results answering to his expectations.
The delicacy of eye and hand needed to attain them may be
estimated from the single fact, that the image of a star had to
be kept, by continual minute adjustments, exactly projected
upon a slit -3--$ of an inch in width during nearly an hour, in
order to give it time to imprint the characters, of its analysed
light upon a gelatine plate raised to the highest pitch of sensi-
tiveness.
The ultra-violet spectrum of the white stars of which Vega
was taken as the type was by this means shown to be a very
remarkable one. Twelve strong lines, arranged at intervals
diminishing regularly upwards, intersected it. They belonged
presumably to one substance ; and since the two least refrangible
1 Month. Not.) vol. xxiii. p. 180. 2 Proc. Roy. Soc., vol. xxv. p. 446.
3 2 hil. Trans., vol. clxxi. p. 669.
STARS AND NEBULA. 431
were known hydrogen rays, that substance could scarcely be
any other than hydrogen This was rendered certain by direct
photographs of the hydrogen-spectrum taken by H. W. Vogel
at Berlin a few months earlier. 1 In them seven of the white-
star series were visible; and the remaining five were absent
only because the higher rays failed to get through the glass
prism employed.
In yellow stars, such as Capella and Arcturus, the same rhyth-
mical series was partially represented, but associated with a
great number of other lines ; their state, as regards ultra-violet
absorption, thus approximating to that of the sun ; while the
redder stars betrayed so marked a deficiency in actinic rays,
that from Betelgeux, with an exposure forty times that required
for Sirius, only a faint spectral impression could be obtained,
and from Aldebaran, in the strictly invisible region, almost
none at all.
The same process was successfully applied to the Orion
nebula, March 7, i882. 2 Five lines in all stamped themselves
upon the plate during forty-five minutes of exposure. Of
these, four were the known visible rays, and the fifth seemed
to agree with one of the hydrogen set displayed by Vega.
Almost simultaneously, this notable feat in celestial photography
was achieved by Dr. Draper at New York, 3 and with the
additional result of obtaining from the nebulous " knots "
preceding the trapezium, a continuous spectrum. This was
thought to indicate an advance of central condensation
possibly even the beginning of the long birth-process of an
orderly revolving system, reserved for the future habitation of
rational beings. It may be so ; the ways of creative power
are dark. Yet we cannot help remarking that the presence
of so many stars fully formed, yet seemingly wrapt up and
involved in the prodigious masses of nebulosity filling that
portion of the sky, appears in some degree to discount the
expectation of stellar development from them.
1 Astr. Nock., No. 2301. 2 Proc. Roy. Soc., vol. xxxiii. p. 425.
3 Comftes Rendus, t. xciv. p. 1243.
432 HISTORY OF ASTRONOMY.
The first promising photograph of the Orion nebula itself
was obtained by Draper, September 30, iSSo. 1 The marked
approach towards a still more perfectly satisfactory result
shown by his plates of March 1881 and 1882, was unhappily
cut short by his premature death. Meanwhile, M. Janssen
was at work in the same field from 1881, with his accustomed
success. 2 But Mr. Ainslie Common left all competitors far
behind with a splendid picture, taken January 30, 1883, by
means of an exposure of thirty-seven minutes in the focus of
his three-foot silvered glass-mirror. 3 Photography may thereby
be said to have definitively assumed the office of historiographer
to the nebulae ; since this one impression embodies a mass of
facts hardly to be compassed by months of labour with the
pencil, and affords a record of shape and relative brightness in
the various parts of the stupendous object it delineates, which
must prove invaluable to the students of its future condition.
The sublime problem of the construction of the heavens
has not been neglected amid the multiplicity of tasks imposed
upon the cultivators of astronomy by its rapid development.
But data of a far higher order of precision, and indefinitely
greater in amount, than those at the disposal of Herschel or
Struve, must be accumulated before any definite conclusions
on the subject are possible. The first organised effort towards
realising this desideratum, was made by the German Astro-
nomical Society in 1865, two years after its foundation at
Heidelberg. The scheme, as originally proposed, consisted
in the ^^/determination of the places of about 100,000 stars,
from the re-observation of which, say, in the year 1950,
astronomers of two or three generations hence may gather a
vast store of knowledge directly of the apparent motions,
indirectly of the mutual relations binding together the suns
and systems of space. Fourteen observatories in Europe and
America joined in the work, which is now far advanced.
1 Wash. Obs. t vol. xxv. App. i. p. 226.
2 Comptes Rendus, t. xcii. p. 261.
3 Month. Not., vol. xliii. p. 255.
STARS AND NEBULA. 433
Its scope, however, has, since its inception, been widened so as
to include southern zones as far as the Tropic of Capricorn ;
and a preliminary survey of the new region on Argelander's
plan has just been made by Schonfeld at Bonn.
Through Dr. Gould's unceasing labours, during his fifteen
years' residence at Cordoba, a detailed acquaintance with
southern stars has at length been brought about. His Urano-
metria Argentina (1879) enumerates the magnitudes of 8198
out of 10,649 stars visible to the naked eye under those trans-
parent skies; 73,160 down to 9! magnitude are embraced in
his "zones;" besides which, he has brought back with him to
Boston materials for a catalogue including 35,000 entries.
Valuable work of the same kind is being done at Virginia by
Professor O. Stone ; while the present Radcliffe observer's
"Cape Catalogue for 1880" affords an aid to the practical
astronomer south of the line, of which it would be difficult to
over-estimate the importance. Moreover, the gigantic task
undertaken in 1860 by Dr. C. H. F. Peters, director of the
Litchfield Observatory, Clinton (N.Y.), and of which a large
instalment was finished in 1882, deserves honourable mention.
It is nothing less than to map all stars down to, and even below,
the fourteenth magnitude, situated within 30 degrees on either
side of the ecliptic, and so to afford " a sure basis for drawing
conclusions with respect to the changes going on in the starry
heavens." 1
In the arduous matter of determining star-distances, too,
progress has been made. Together, yet independently, Drs.
Gill, and Elkin carried out, at the Cape Observatory in 1882-83,
an investigation of remarkable accuracy into the parallaxes of
nine southern stars. One of these was the famous a Centauri,
the distance of which from the earth was ascertained to be just
one-third greater than Henderson had made it. The parallax
of Sirius, on the other hand, was doubled, or its distance
halved ; while Canopus was discovered to be quite immeasur-
1 Gilbert, Sidereal Messenger^ vol. i. p. 288.
2 E
434 HISTORY OF ASTRONOMY.
ably remote a circumstance which, considering that, amongst
all the stellar multitude, it is outshone only by the radiant
Dog-star, gives a stupendous idea of its real splendour and
dimensions.
Dr. Ball, the Astronomer Royal for Ireland, has recently
devoted much attention to inquiries of this kind. Besides
approximately confirming Struve's parallax of half a second
of arc for 61 Cygni, he discovered in 1881 that another
very similar double star in the same constellation is situated
at a sensibly equal distance from us; 1 and by a sweeping
search for (so-called) "large" parallaxes disposed of certain
baseless conjectures of comparative nearness to the earth, in
the case of red and temporary stars. 2 Amongst other note-
worthy results may be mentioned Otto Struve's detection of
a parallax of half a second for Aldebaran, and Professor A.
Hall's measures of 61 Cygni and Vega with the great Washing-
ton refractor, 1 880-81.
Foremost among living observers of double stars ranks
Mr. S. W. Burnham of Chicago. His discoveries in this line
numbered one thousand (including some of the most difficult
objects known) in May 1882, when he brought his regular
astronomical work to a close. 3 The curious phenomenon of
one star revolving round another in a period shorter than that
in which Jupiter circulates round the sun, came to his notice
in i883- 4 The very close pair in question, discovered by Otto
Struve in 1852, is known as d Equulei, and the period pro-
bably assigned to it is of 10.8 years by far the shortest
attributable to any member of a stellar system.
Another fact of interest in this connection is that 61
Cygni at length gives signs of yielding up its secret. The
seemingly parallel tracks followed by its components during
a century and a quarter of observation, were found by Struve
in 1875 to exhibit deviations countenancing the inference
1 Nature, vol. xxvii. p. 210. 2 Ibid., vol. xxiv. p. 91.
3 Mem. R. A. Soc., vol. xlvii. p. 178.
4 Observatory, vol. vii. p. 13.
STARS AND NEBULA. 435
of mutual revolution; for which, in 1880, a period of about
eleven hundred years was arrived at as a first approximation. 1
From a fresh discussion three years later, Mr. N. M. Mann
of Rochester (N.Y.) concluded it 1159 years, giving (with a
parallax taken at 0.55") a value for the combined mass of the
connected bodies only one-seventh the solar mass. 2
Stellar photometry, initiated by the elder Herschel, has of
late years assumed the importance of a separate department
of astronomical research. More systematically than elsewhere
it has been cultivated at Harvard, under the direction of Pro-
fessor Pickering. His photometric catalogue of 4260 stars,
constructed from ninety thousand observations of light-intensity
during the years 1879-82, constitutes one more of the precious
seeds of discovery laid in the ground by the present generation
of astronomers, for their successors to reap the fruits of.
Meanwhile, thought cannot be held aloof from the great
subject upon the future illustration of which so much patient
industry is being expended. Nor are partial glimpses denied
to us of relations fully discoverable perhaps only by the slow
efflux of time. Some important points in cosmical economy
have, indeed, become quite clear within the last thirty years,
and scarcely any longer admit of a difference of opinion.
One of these is that of the true status of nebulae.
This was virtually settled by Sir J. Herschel's description in
1847 f tne structure of the Magellanic clouds; but it was
not until Whewell in 1853, and Herbert Spencer in i858, 3
enforced the conclusions necessarily to be derived therefrom,
that the conception of the nebulae as remote galaxies, which
Lord Rosse's resolution of many into stellar points had ap-
peared to support, began to withdraw into the region of dis-
carded and half-forgotten speculations. In the Nubeculae,
as Whewell insisted, 4 " there co-exists, in a limited compass,
1 Mtm. de V Ac., St. Petersbourg, t. xxvii. p. 16.
2 Sidereal Messenger, vol. ii. p. 22.
3 Essays (2d ser.), The Nebular Hypothesis.
4 On t,(e Plurality of Worlds, p. 214 (2d ed.)
436 HISTORY OF ASTRONOMY.
and in indiscriminate position, stars, clusters of stars, nebulae,
regular and irregular, and nebulous streaks and patches.
These, then, are different kinds of things in themselves, not
merely different to us. There are such things as nebulae side
by side with stars and with clusters of stars. Nebulous matter
resolvable occurs close to nebulous matter irresolvable."
This argument from co-existence in nearly the same region
of space, was reiterated and reinforced, with others, by Mr.
Spencer, and has more lately been urged with his accustomed
force and freshness by Mr. Proctor. It is unanswerable.
There is no maintaining nebulae to be simply remote worlds
of stars in the face of an agglomeration like the Nubecula
Major, containing in its (certainly capacious) bosom both stars
and nebulae. Add the evidence of the spectroscope to the
effect that a large proportion of these perplexing objects are
gaseous, with the facts of their distribution telling of an inti-
mate relation between the mode of their scattering and the
lie of the Milky Way, and it becomes impossible to resist the
conclusion that both nebular and stellar systems are parts of
a single scheme. 1
As to the stars themselves, the presumption of their approxi-
mate uniformity in size and brightness has been effectually
dissipated. Differences of distance can no longer be invoked
to account for dissimilarity in lustre. Minute orbs, altogether
invisible without optical aid, are found to be indefinitely nearer
to us than such radiant objects as Capella, Regulus, or Procyon.
Moreover, intensity of light is perceived to be a very imperfect
index to real magnitude. Brilliant suns are swayed from their
courses by the attractive power of massive, yet imperfectly
luminous companions, and are suspected of suffering eclipse
from obscure interpositions. Besides, effective lustre is now
known to depend no less upon the qualities of the investing
atmosphere, than upon the extent and radiative power of the
stellar surface. Red stars must be far larger in proportion to
1 Proctor, Month. Not., vol. xxix. p. 342.
STARS AND NEBULAE. 437
the light diffused by them than white stars. 1 It is highly
probable that our sun would at least double its brightness
were the absorption suffered by its rays to be reduced to the
Sirian standard ; and, on the other hand, that it would lose half
its present efficiency as a light-source, if the atmosphere par-
tially veiling its splendours were rendered as dense as that
of Aldebaran.
Thus, variety of all kinds is seen to abound in the heavens ;
and it must be admitted that the inevitable abolition of all
hypotheses as to the relative distances of the stars singularly
complicates the question of their allocation in space. Never-
theless, something has been learnt even on that point ; and the
tendency of modern research is, on the whole, strongly con-
firmatory of the views expressed by Herschel in 1802. He
then no longer regarded the Milky Way as the mere visual
effect of an enormously extended stratum of stars, but as an
actual aggregation, highly irregular in structure, made up of
stellar clouds and groups and nodosities. All the facts since
ascertained fit in with this conception ; and to them Mr.
Proctor has added, what we may almost call the discovery
that the stars forming the galactic stream are not only situated
more closely together, but are also really, as well as apparently,
of smaller dimensions than the lucid orbs studding our skies.
By the laborious process of isographically charting the whole
of Argelander's 324,000 stars, he made it clear, in 1871, 2 that
the brighter stars show, in their distribution, a detailed relation-
ship to the complex branchings of the Milky Way, avoiding, to
a marked extent, its vacuities, and thronging its denser con-
volutions. It follows that they must be actually intermingled
with them. So that, for every triton sun there are doubtless
swarms of minnows bodies not perhaps larger than our own
little planet, yet self-luminous and diffusive of beneficent in-
fluences according to the inscrutable design of the Creator.
The first step towards the unravelment of the tangled web of
1 This remark is due to the late Mr. J. Birmingham.
2 Month. Xot., vols. xxxi. p. 175 ; xxxii. p. I.
438 HISTORY OF ASTRONOMY.
stellar movements was taken when Herschel established the
reality, and indicated the direction of the sun's journey.
But the gradual shifting backwards of the whole of the
celestial scenery amid which we advance, accounts for only a
part of the observed displacements. The stars have motions
of their own besides those reflected upon them from ours.
All attempts, however, to grasp the general scheme of these
motions, have hitherto failed. Yet they have not remained
wholly fruitless. The community of slow movement in Taurus,
upon which Madler based his famous theory, has proved to be
a fact, and one of very extended significance.
In 1870 Mr. Proctor undertook to chart down the directions
and proportionate amounts of about 1600 proper motions, as
determined by Messrs Stone and Main, with the result of bring-
ing to light the remarkable phenomenon termed by him " star-
drift." 1 Quite unmistakably, large groups of stars, otherwise
apparently disconnected, were seen to be in progress together,
in the same direction, and at the same rate, across the sky. A
striking instance of this kind of unanimity is afforded by the
five intermediate stars of the Plough. So clearly were they
marked out from their companions in the same asterism, that
Mr. Proctor ventured to invite the application of the spectro-
scope as a sure means of ratifying the distinction. And so
indeed it proved. The five associated stars were discerned by
Dr. Huggins in 1872 2 to be in rapid retreat from the earth,
while the brightest of the Pointers, and the last star in the tail
of the Great Bear, verified their surmised independence by dis-
playing, the one a diametrically opposite, the other a widely
different rate of motion.
Here then we have a system on a scale so vast that the
imagination shrinks from the effort to conceive it. None of
the stars forming it have any sensible parallax, so that they
certainly surpass our sun many, perhaps thousands of times in
dimensions and splendour. Moreover, the distances separating
them one from the other must be enormous to be reckoned
1 Proc. Roy. Soc., vol. xviii. p. 169. 2 Ibid., vol. xx. p. 392. ,
STARS AND NEBULA. 439
by billions of miles, or years of light-travel. Yet a special tie
unites them; they are subject to the stress of an identical
force, swaying their movements into harmonious accord ; and
they doubtless shed one upon the other mutual influences
apart from which their function in the cosmos would be imper-
fectly fulfilled.
And this is by no means a solitary example. Particular
association, indeed as was surmised by Michell six-score years
ago appears to be the rule, rather than an exception in the
sidereal scheme. Stars are bound together by twos, by threes,
by dozens, by hundreds. Our own sun is perhaps not exempt
from this gregarious tendency. Dr. Gould conjectures that it
belongs to a group of about four hundred of the brightest
visible stars, forming a subordinate system within the confines
of the Milky Way. 1 Such another would be the Pleiades. The
laws and revolutions of such majestic communities lie, for the
present, far beyond the range of possible knowledge ; centuries
may elapse before even a rudimentary acquaintance with them
begins to develop; while the economy of the higher order of
association, which we must reasonably believe that they unite
to compose, will possibly continue to stimulate and baffle
human curiosity to the end of time.
1 Month. Not., vol. xl. p. 249.
( 44 )
CHAPTER XIII.
METHODS OF RESEARCH.
COMPARING the methods now available for astronomical in-
quiries with those in use thirty years ago, we are at once struck
with the fact that they have multiplied. .The telescope has
been supplemented by the spectroscope and the photographic
camera. Now this -really involves a whole world of change.
It means that astronomy has left the place where she dwelt
apart in rapt union with mathematics, indifferent to all things
on earth save only to those mechanical improvements which
should aid -her to penetrate further into the heavens, and has
descended into the forum of human knowledge, at once a
suppliant and a patron, alternately invoking help from, and
promising it to each of the sciences, and patiently waiting upon
the advance of all. The science of the heavenly bodies has,
in a word, become a branch of terrestrial physics, or rather a
higher kind of integration of all their results. It has, however,
this leading peculiarity, that the materials for the whole of its
inquiries are telescopically furnished. They are such as the
unarmed eye takes no, or a very imperfect cognisance of.
Spectroscopic and photographic apparatus are simply ad-
ditions to the telescope. They do not supersede, or render it
of less importance. On the contrary, the efficacy of their action
depends primarily upon the optical qualities of the instrument
they are attached to. Hence the development, to their fullest
extent, of the powers of the telescope is of vital moment to
the progress of modern physical astronomy, while the older
METHODS OF RESEARCH. 441
mathematical astronomy could afford to remain comparatively
indifferent to it.
The colossal Rosse reflector still marks, as to size, the ne plus
ultra of performance in that line. No existing mirror comes
nearer to it than that, four feet in diameter, sent out to
Melbourne by the late Thomas Grubb of Dublin in 1870.
This is mounted in the Cassegrainian manner; so that the
observer looks straight through it towards the object viewed,
of which he really sees a twice-reflected image. It is of
excellent definition and rare convenience in management ;
but the dust-laden atmosphere of Melbourne is said to impede
very seriously its usefulness.
It may be doubted whether so large a speculum will ever
again be constructed. A new material for the mirrors of re-
flecting telescopes was introduced by Leon Foucault in I857, 1
which has already in a great measure superseded the use of
a metallic alloy. This is glass upon which a thin film of silver
has been deposited by a process known as Drayton's. It gives
a peculiarly brilliant reflective surface, throwing back more
light than a metallic mirror of the same area, in the proportion
of about sixteen to nine. Liability to tarnish in part counter-
acts this great advantage. The largest instrument successfully
turned out on this plan is Mr. Common's 36-inch reflector,
finished in 1879. To its excellent qualities his triumphs in
celestial photography are largely due.
It is, however, in the construction of refracting telescopes
that the most conspicuous advances have recently been made.
The Harvard College 15 -inch achromatic was mounted and
ready for work in June 1847. A similar instrument had already
for some years been in its place at Pulkowa. It was long
before the possibility of surpassing these masterpieces of
German skill presented itself to any optician. For fifteen
years it seemed as if a line had been drawn just there. It was
first transgressed in America. A portrait-painter of Cambridge-
port, Massachusetts, named Alvan Clark, had for some time
1 Comptes Rendus, t. xliv. p. 339.
442 HISTORY OF ASTRONOMY.
amused his leisure with grinding lenses, the singular excellence
of which was discovered in England by Mr. Dawes in I853. 1
Seven years passed, and then an order came from the University
of Mississippi for an object-glass of the unexampled size of
eighteen inches. An experimental glance through it to test
its definition resulted, as we have seen, in the detection of the
companion of Sirius, January 31, 1862. It never reached its
destination in the South. War troubles supervened; and it
was eventually sent to Chicago, where it has served Professor
Hough in his investigations of Jupiter, and Mr. Burnham in
his scrutiny of double stars.
The next step was an even longer one, and it was again
taken by a self-taught optician, Thomas Cooke, the son of a
shoemaker at Allerthorpe, in the East Riding of Yorkshire.
Mr. Newall of Gateshead ordered from him in 1863 a 25-inch
object-glass. It was finished early in 1868, but at the cost of
shortening the life of its maker, who died October 19, 1869,
before the giant refractor he had toiled at for five years, was
completely mounted. Although believed to be still the finest
telescope in England, its high qualities have been largely
neutralised by an unfavourable situation.
Close upon its construction followed that of the Washington
26-inch, for which twenty thousand dollars were paid to Alvan
Clark. Set to work in 1873, the most illustrious point in its
career, so far, has been the discovery of the satellites of Mars.
Once known to be there, these were, indeed, found to be
perceptible with very moderate optical means (Mr. Wentworth
Erck saw Deimos with a nine-inch Clark) ; but the first detection
of such minute objects is a feat of a very different order from
their subsequent observation.
For a little over eight years the Washington refractor held
the primacy. It had to yield the place of honour in December
1880 to a giant achromatic, twenty-seven inches in aperture,
built by Howard Grubb (son and successor of Thomas Grubb)
for the Vienna Observatory. This, in its turn, has been sur-
1 Newcomb, Pop. Astr., p. 137.
METHODS OF RESEARCH. 443
passed by one of thirty inches sent by Alvan Clark to Pulkowa ;
and an object-glass, fully three feet in diameter, is now in course
of construction by the same firm for the Lick Observatory in
California. The difficulties, however, encountered in procuring
discs of glass of the size and purity required for this last
venture, seem to indicate that a term to progress in this direction
is near at hand. The flint was indeed cast with comparative
ease in the workshops of M. Feil at Paris. The flawless mass
weighed 170 kilogrammes, was over 38 inches across, and cost
2000 pounds. But with the crown part of the designed achro-
matic combination, things have gone less smoothly. The pro-
duction of a successful disc was preceded by nineteen failures,
involving a delay of more than two years, and postponing the
probable completion of the great telescope until the year 1887
or 1888. 1
Nor is the difficulty in obtaining suitable material almost
overwhelming though it be the only obstacle to increasing
the size of refractors. Colour-fringes also step in and bar
the way, their complete, or approximately complete, correc-
tion demanding, in the case of such vast apertures as have
recently been attempted, a focal length so exorbitant as to
be practically, under the ordinary conditions of mounting, out
of the question. Besides, a refracting telescope loses one of
its chief advantages over a reflector when its size is increased
beyond a certain limit. That advantage is the greater lumi-
nosity of the images given by it. Considerably more light
is transmitted through a glass lens than is reflected from an
equal metallic surface. But only so long as both are of
moderate dimensions. For the glass necessarily grows in
thickness as its area augments, and consequently stops a larger
percentage of the rays it refracts. So that a point at length
arrives fixed by the late Dr. Robinson at a diameter a little
short of three feet 2 where the glass and the metal are, in this
respect, on an equality ; while above it, the metal has the
1 Holden, Observatory, vol. viii. p. 84.
2 H. Grubb, Trans. Roy. Dub. Soc., vol. i. (new ser.), p. 2.
444 HISTORY OF ASTRONOMY.
advantage. And since silvered glass gives back considerably
more light than speculum-metal, the stage of equalisation with
lenses is reached proportionately sooner where this material is
employed.
It will thus probably be long before the light-grasp of Mr.
Common's three-foot mirror is surpassed by a refractor. But in
the inquiries for which the great telescopes of modern times
are more especially designed, light-grasp is everything. For
the spectroscopic examination of stars, for the measurement of
their motions in the line of sight, for the study of nebulas, for
stellar and nebular photography, the cry continually is, " More
light." Apart from the exigencies of these, and a few other
enticing branches of research, there would be little to be gained
in adding to the power of optical apparatus. And there is
much lost. The penalties of bigness are heavy. Perfect
definition becomes, with increasing size, more and more diffi-
cult to attain ; once attained, it becomes more and more
difficult to keep. For the huge masses of material employed
to form great object-glasses or specula, tend, with every move-
ment, to become deformed by their own weight. Gravity
exacts the further toll of unwieldiness. Each glance through
a large instrument is highly paid for in time and trouble. Nor
is the glance thus paid for often a satisfactory one. Atmos-
pheric troubles intervene.
These are the worst plagues of all those that afflict the astro-
nomer. No mechanical skill avails to neutralise or alleviate
them. They augment, in a rapidly increasing ratio, with each
addition to the aperture of the telescope, or of the magnifying
powers applied to it. To them chiefly is due the growing dis-
content with the performance of the colossal instruments of
modern times. It is admitted on all hands that, for the ordinary
work of an observatory, an aperture of ten or twelve inches is
the outside limit of usefulness. But it is also found, with
disappointment, that even in the field of descriptive research,
where it might be expected that luminosity and magnification
would be all-important, results fall far short of anticipation.
METHODS OF RESEARCH.
445
Schiaparelli, with an eight-inch achromatic, obtained views of
Mars such as were never vouchsafed to Harkness or Hall,
though using the Washington 26-inch; and, according to Mr.
Denning, 1 details of the Jovian surface are shown by an
insignificant 4|-inch, which remain invisible with the majestic
refractor of the Dearborn Observatory, Chicago.
Now this is due to no imperfections inherent in the instru-
ments themselves ; it is due to the conditions of our habitation
on an air-wrapt globe. It is not only that much less than half
the light incident upon the surface of the atmospheric ocean
penetrates to the bottom of it. That loss might, in some
measure, be repaired; but what no optical contrivance can
get rid of, is the disturbance suffered by the rays that reach us.
The twinkling of stars to the naked eye is but a faint symptom
of their behaviour in the telescope ; while the images of sun,
moon, and planets " boil " at the edges, or are suffused and dis-
torted by waves of agitation caused by the magnified surgings
of the turbulent vapours we see through. The mischief, Dr.
Robinson estimated in the case of reflectors, grows with the
cubes of their diameters ; and it is commonly found, in practice,
that the " seeing " will be perfectly good with a small telescope,
but altogether intolerable with a large one standing beside it.
Under such skies as ours, in fact, there are not more than
three or four nights in the year when an aperture of as much
as eighteen inches can be used to real advantage; and Mr.
Newall remarked in 1885 that during fifteen years he had
known but one fine night 2 fine, that is, in the sense of avail-
ability for observation with his great refractor.
Thus it seems clear that we have reached a turning-point
in the history of telescopic improvement. Not alone have the
material obstacles to any further increase of size become all
but insuperable, but the conviction is forced upon us that, were
instruments of greater power than any now possessed by astro-
nomers actually in their hands, they must remain wholly useless
save on one condition that of an improved climate.
Observatory, vol. viii. p. 79.
Ibid., p. So.
446 HISTORY OF ASTRONOMY.
Ever since the Parsonstown telescope was built, it has been
obvious that the limit of profitable augmentation of aperture
had been reached, if not overpassed ; and Lord Rosse himself
was foremost to discern the need of pausing to look round the
world for a clearer and stiller air than was to be found within
the bounds of the United Kingdom. With this express object
Mr. Lassell transported his two-foot Newtonian to Malta in
1852, and mounted there, in 1860, a similar instrument of
four-fold capacity, with which in the course of about two years
600 new nebulae were discovered. Professor Piazzi Smyth's
experiences during a trip to the Peak of TenerifTe in 1856 in
search of astronomical opportunities, 1 gave countenance to the
most sanguine hopes of deliverance, at suitably elevated stations,
from some of the oppressive conditions of low-level star-gaz-
ing; yet for a number of years nothing effectual was done
for their realisation. Now at last, however, mountain observa-
tories are not only an admitted necessity, but an accomplished
fact ; and Newton's long forecast of a time when astronomers
would be compelled, by the developed powers of their tele-
scopes, to mount high above the " grosser clouds " in order to
use them,' 2 has been justified by the event.
Mr. James Lick, a millionaire of San Francisco, had already
chosen when he died, October i, 1876, a site for the new
observatory, to the building and endowment of which he had
devoted a part of* his large fortune. The establishment now
only awaits the completion of the 36-inch refractor and its
great sheltering dome, to be in a state of perfect efficiency.
Indeed, its present instrumental outfit including a twelve-
inch Clark's achromatic is one of high excellence. The
situation of the " Lick " Observatory is exceptional and splen-
did. Planted on one of the three peaks of Mount Hamil-
ton, a crowning summit of the Californian Coast Range, at an
elevation of 4200 feet above the sea, in a climate scarce
rivalled throughout the world, it commands views both celestial
and terrestrial which the lover of nature and astronomy may
1 Phil. Trans. , vol. cxlviii. p. 465. 2 Optice, p. 107 \2d ed., 1719.)
METHODS OF RESEARCH. 447
alike rejoice in. Impediments to observation are there found
to be most materially reduced. Professor Holden tells us that
during six or seven months of the year an unbroken serenity
prevails, and that half the remaining nights are clear. 1 The
power of continuous work thus afforded is of itself an inestim-
able advantage; and when combined with the high visual
excellences testified to by Mr. Burnham's discovery, during a
two months' trip to Mount Hamilton in the autumn of 1879,
of forty-two new double stars with a six-inch achromatic, it
gives hopes of a brilliant future for the Lick establishment.
A higher altitude than the comparatively modest one at
which it is placed, would hardly prove suitable to a great per-
manent observatory ; but considerably more elevated posts for
temporary astronomical occupation are being provided, and
will shortly be looked upon as indispensable. One such was
fitted up near the summit of Mount Etna in 1882. The build-
ing is the highest in Europe, standing 9655 feet above the
sea, and includes within its walls the " Casa Inglese," in which
travellers were used to seek repose before attempting the final
ascent of the cone. Splendid telescopic opportunities are
indicated by Professor Langley's experimental observations,
carried through under every disadvantage in the winter of
1879-80; and the Merz equatoreal of nearly fourteen inches
aperture, provided for the Etnean establishment, may be ex-
pected, freed from the impeding mists and restless currents of
the lower atmosphere, to prove of singular efficiency.
The Pic du Midi, too, is destined for astronomical occupa-
tion. A meteorological observatory was in 1881, thanks to
the enterprise of General de Nansouty and M. Vaussenat,
opened on its summit, at an altitude of 9600 feet ; and the
glowing account given by MM. Thollon and Trepied in 1883 2
of the advantages offered by the dark translucency of its sky,
determined Admiral Mouchez upon founding there a species
of succursale to the Paris Observatory, whither despondent
astronomers might repair within a few hours, in the sure hope
1 Observatory, vol. viii. p. 85. 2 Comptes Rendus, t. xcvii. p. 834.
448 HISTORY OF ASTRONOMY.
of leaving their too-familiar weather-troubles behind, and of
finding the heavens laid bare of all but the clearest and
thinnest remnant of their atmospheric vesture. An eight-inch
equatoreal has been appropriated to use on the Pic, but
funds are not as yet forthcoming for the erection of a
dome.
The diminution of " glare " at such elevated posts is all-
important for solar inquiries ; and if Dr. Huggins's ingenious
devices for photographing the corona are not to remain a mere
curiosity of science, but are to be turned to practical account
for the increase of knowledge, it can only be by experiments
liberated from the obliterating effects of confused reflections in
dense air. For stellar and nebular photography, on the other
hand, luminous and untroubled images are the chief requisite,
and these can generally be secured by a judicious ascent.
Indeed a store of materials may be collected during a few
weeks' sojourn at a high altitude, for the due discussion and
elucidation of which the whole year besides will hardly afford
leisure. In the spectroscopy of the stars, Dr. Copeland's
flying observations amongst the Andes show what can be done
by climbing towards them. Peculiarities previously invisible
become obvious : measurement is rendered easy ; discoveries
of curious interest crowd upon the enterprising observer. It
may indeed be safely predicted that knowledge of the spectra
of faint stars will never be made extensive and precise until
ample means are available for studying them in the finer air of
the mountains.
Vapours and air-currents, however, do not alone embarrass
the use cf giant telescopes. Mechanical difficulties also
threaten to oppose an insuperable barrier to any further growth
in size. But what seems to be an insuperable barrier often
proves to be only a fresh starting-point; and signs are not
wanting that it may be found so in this case. It is possible
that the monumental domes and huge movable tubes of our
present observatories will, in a few decades, be as much things
of the past as Huygens's " aerial " telescopes. It is certain
METHODS OF RESEARCH. 449
that the thin edge of the wedge of innovation has been driven
into the old plan of equatoreal mounting.
M. Loewy, the present sub-director of the Paris Observatory,
proposed to Delaunay in 1871 the erection of a telescope on a
novel system. The design seemed feasible, and was adopted ;
but the death of Delaunay and the other untoward circumstances
of the time interrupted its execution. Its resumption, after
some years, was rendered possible by M. Bischoffsheim's gift
of 25,000 francs for expenses, and the Coude or "bent" equa-
toreal has been, since 1882, one of the leading instruments at
the Paris establishment.
Its principle is briefly this. The telescope is, as it were, its
own polar axis. The anterior part of the tube is supported at
both ends, and is thus fixed in a direction pointing towards the
pole, with only the power of twisting axially. The posterior
section is joined on to it at right angles, and presents the
object-glass accordingly to the celestial equator, in the plane
of which it revolves. Stars in any other part of the heavens
have their beams reflected upon the object-glass by means of
a plane rotating mirror placed in front of it. The observer,
meanwhile, is looking steadfastly down the bent tube towards
the invisible southern pole. He would naturally see nothing
whatever, were it not that a second plane mirror is fixed at the
" elbow " of the instrument, so as to send the rays which have
traversed the object-glass to his eye. He never needs to move
from his place. He watches the stars seated in an arm-chair
in a warm room, with as perfect convenience as if he were
examining the seeds of a fungus with a microscope. Nor is
this a mere gain of personal ease. The abolition of hardship
includes a vast accession of power. 1
Amongst other advantages of this method of construction
are, first, that of added stability, the motion given to the or-
dinary equatoreal being transferred, in part, to an auxiliary
mirror. Next, that of increased focal length. The fixed part
of the tube can be made almost indefinitely long without in-
1 Loewy, Bull. Astr.^ t. i. p. 286 ; Nature, vol. xxix. p. 36.
2 F
450 HISTORY OF ASTRONOMY.
convenience, and with enormous advantage to the optical
qualities of a large instrument. Finally, the costly and un-
manageable cupola is got rid of, a mere shed serving all
purposes of protection required for the " CoudeV'
The desirability of some such change as that which M. Loewy
has realised, has been felt by others. Professor Pickering
sketched in 1881 a plan for fixing large refractors in a per-
manently horizontal position, and reflecting into them, by
means of a shifting mirror, the objects desired to be observed. 1
An instrument with " siderostatic " mounting by Mr. Howard
Grubb has actually been in use at the Queen's College Obser-
vatory, Cork, since 1882 ; and in a paper read before the
Royal Society, January 21, 1884, he proposed to carry out the
principle on a more extended scale. 2 The chief honours,
however, remain to the Paris inventor. None of the prog-
nosticated causes of failure have proved effective. The loss of
light from the double reflection is insignificant. The menaced
deformation of images is, through the exquisite skill of the
MM. Henry in producing plane mirrors of all but absolute
perfection, quite imperceptible. The definition of the novel
loj-inch equatoreal is admitted to be singularly good. Dr.
Gill states that he had never measured a double star so easily
as he did 7 Leonis by its means. 3 Mr. Lockyer believes it to
be " one of the instruments of the future ; " and the principle
of its construction has already been adopted by the directors of
the Besangon and Algiers Observatories. At elevated stations
especially, the abolition of the hitherto indispensable massive
dome, obnoxious to all the winds of heaven, which there blow
at times with exceeding violence, ought to be decisive in its
favour ; while its adaptation to reflectors 4 may be expected to
turn the scale in favour of silvered glass mirrors as the great
coming engines of physical research in astronomy.
Celestial photography is but forty years old ; yet its earliest
beginnings already seem centuries behind its present perfor-
1 Nature, vol. xxiv. p. 389. 2 Ibid., vol. xxix. p. 470.
Observatory, vol. vii. p. 167. 4 Loewy, Bttll. Astr., L i. p. 265.
METHODS OF RESEARCH.
451
mances. The details of its gradual, yet rapid improvement
are of too technical a nature to find a place in these pages.
Suffice it to say that the " dry-plate " process, with which such
wonderful results have been obtained, appears to have been
first made available by Dr. Huggins in photographing the
spectrum of Vega in 1876, and was then successively adopted
by Common, Draper, and Janssen. Nor should Captain
Abney's remarkable extension of the powers of the camera
be left unnoticed. He began his experiments on the chemical
action of red and infra-red rays in 1874, and at length succeeded
in obtaining a substance the " blue " bromide of silver
highly sensitive to these slower vibrations of light. With its
aid he explored a vast, unknown, and for ever invisible region
of the solar spectrum, presenting to the Royal Society, Decem-
ber 5, 1879^ a detailed map of its infra-red portion (wave-
lengths 7600 to 10,750), from which valuable inferences may
yet be derived as to the condition of the various kinds of matter
ignited in the solar atmosphere.
The chemical plate has two advantages over the human
retina. 2 First, it is sensitive to rays which are utterly powerless
to produce any visual effect ; next, it can accumulate impres-
sions almost indefinitely, while from the retina they fade after
one-tenth part of a second, leaving it a continually renewed
tabula rasa.
It is accordingly quite possible to photog aph objects so faint
as to be altogether beyond the power of any telescope to reveal ;
and we may thus eventually learn whether a blank space in the
sky truly represents the end of the stellar universe in that
direction, or whether farther and farther worlds roll and shine
beyond, veiled in the obscurity of immeasurable distance.
The means at the disposal of astronomers have not multi-
plied faster than the tasks imposed upon them. Looking back
to the year 1800, we cannot fail to be astonished at the change.
The comparatively simple and serene science of the heavenly
1 Phil. Trans., vol. clxxi. p. 653.
2 Janssen, L? Astronomic, t. ii. p. 121.
452 HISTORY OF ASTRONOMY.
bodies known to our predecessors, almost perfect so far as it
went, incurious of what lay beyond its grasp, has developed
into a body of manifold powers and parts, each with its separate
mode and means of growth, full of strong vitality, but animated
by a restless and unsatisfied spirit, haunted by the sense of
problems unsolved, and tormented by conscious impotence to
sound the immensities it perpetually confronts.
Knowledge might then be said to be bounded by the solar
system ; but even the solar system presented itself under an
aspect strangely different from that it now wears. It consisted
of the sun, seven planets, and twice as many satellites, all
circling harmoniously in obedience to an universal law, by the
compensating action of which the indefinite stability of their
mutual relations was secured. The occasional incursion of a
comet, or the periodical presence of a single such wanderer
chained down from escape to outer space by planetary attrac-
tion, availed nothing to impair the symmetry of the majestic
spectacle.
Now, not alone the ascertained limits of the system have
been widened by a thousand millions of miles, with the addition
of one more giant planet and six satellites to the ancient classes
of its members, but a complexity has been given to its con-
stitution baffling description or thought. Two hundred and
fifty circulating planetary bodies bridge the gap between
Jupiter and Mars, the complete investigation of the movements
of any one of which would overtask the energies of a lifetime.
Meteorites, strangers apparently to the fundamental ordering
of the solar household, swarm, nevertheless, by millions in
every cranny of its space, returning at regular intervals like the
comets so singularly associated with them, or sweeping across
it with hyperbolic velocities, brought perhaps from some dis-
tant star. And each of these cosmical grains of dust has a
theory far more complex than that of Jupiter ; it bears within
it the secret of its origin, and fulfils a function in the universe.
The sun itself is no longer a semi- fabulous, fire-girt globe, but
the vast scene of the play of forces as yet imperfectly known
METHODS OF RESEARCH. 453
to us, offering a boundless field for the most arduous and
inspiring researches. Amongst the planets, the widest variety
in physical habitudes is seen to prevail, and each is recognised
as a world apart, inviting inquiries which, to be effective,
must necessarily be special and detailed. Even our own moon
threatens to break loose from the trammels of calculation, and
commits " errors " which sap the very foundations of the
lunar theory, and suggest the formidable necessity for its
revision. Nay, the steadfast earth has forfeited the implicit
confidence placed in it as a time-keeper, and questions
relating to the stability of the earth's axis, and the constancy
of the earth's rate of rotation, are amongst those which it
behoves the future to answer. Everywhere there is multi-
formity and change, stimulating a curiosity which the rapid
development of methods of research offers the possibility of at
least partially gratifying.
Outside the solar system, the problems which demand a
practical solution are all but infinite in number and extent.
And these have all arisen and crowded upon our thoughts
within less than a hundred years. For sidereal science became
a recognised branch of astronomy only through Herschel's
discovery of the revolutions of double stars in 1802. Yet
already it may be, and has been called, " the astronomy of the
future." So rapidly has the development of a keen and uni-
versal interest attended and stimulated the growth of power
to investigate this sublime subject. What has been done is
little is scarcely a beginning ; yet it is much in comparison
with the total blank of a century past. And our knowledge
will, we are easily persuaded, appear in turn the merest
ignorance to those who come after us. Yet it is not to be
despised, since by it we reach up groping fingers to touch the
hem of the garment of the Most High.
INDEX.
ABBE, Cleveland, corona of 1878,
226, 227
Aberration, discovered by Bradley,
4, 19 ; an uranographical correc-
tion, 41 ; distance of sun determined
by, 275, 286
Abney, infra-red photography, 229,
266, 451 ; Muggins's coronal im-
pressions, 231 ; hydro-carbon bands
in solar spectrum, 233
Absorption, spectra, 176; terrestrial
atmospheric, 173, 256, 258, 266,
322 ; solar, 255, 263-4, 267 ;
correlative with emission, 175,
253
Adams, elements of Neptune, 104-5
lunar acceleration, 316; orbit of
November meteors, 373
Airy, solar translation, 51 ; pro-
minences, 82 ; sierra, 90 ; Astro-
nomer Royal, 104 ; search for
Neptune, 106 ; corona of 1851,
225 ; transit of Venus, 277 ; solar
parallax, 280 ; lunar atmosphere,
309
Albedo, of Venus, 301 ; of Mars,
327 ; of minor planets, 330 ; of
Jupiter, 332, 334
Algol, a variable star, 13, 417 ; inter-
posing satellite. 418
Altitude and azimuth instrument, 156
note, 158
Amici, observation of comet of 1843,
134
Andrews, conditions of liquefaction,
J 9S
Andromeda nebula, 27, 424
Angstrom, equivalence of emission
and absorption, 175 note ; solar
spectroscopy, 253-4
Arago, eclipse of 1842, 83-4 ; pro-
minences, 90 ; polarisation of corne-
tary light, 134; gaseous nature of
photosphere, 195 ; meteor-systems,
371
Argelander, Bonn Durchmusterung,
42, 437 ; solar motion, 50-1, 5 2 >
comet of 1811, 131
Aristotle, description of a comet,
390
Asteroids, minor planets so desig-
nated by Herschel, 99
Astronomical circles, 1 58
Astronomical physics, 181-3, 440
Astronomical Society, founded, 7 ;
Herschel first president, 17
Astronomy, classification, I ; rapid
progress, 6 ; observational, 35 ; in
Germany, 36 ; reform, 37 ; of the
Invisible, 53 ; physical, 181
Atmosphere, solar, 124, 236 ; ot
Venus, 280, 298-300 ; of Mercury,
290, 292; lunar, 308, 309, 315;
of Mars, 322-3 ; of minor planets,
331
Aurorse, periodicity, 167, 207 ; ex-
cited by meteors, 379
Auwers, system of Procyon, 54
BABINET, objection to nebular hypo-
thesis, 356
Backlund, researches into movements
of Encke's comet, 123, 404
Baily, early life and career, 76-8 ;
solar eclipses, 79-82 ; density of
the earth, 77, 306
Baily's Beads, 79-80, 279
Ball, contacts between limbs of Venus
and sun, 284 ; solar distance, 287 ;
parallaxes of stars, 434
456
INDEX.
Barnard and Brooks, debris of a
comet, 406
Basic lines, 249, 250
Beckett, Sir E., improved value of
solar parallax, 276
Beer and Madler, survey of lunar
surface, 310, 311, 312 ; studies of
Mars, 320 ,
Bessel, biographical sketch, 37-9 ;
reduction of Bradley's observa-
tions, 41 ; sidereal survey, 42 ;
parallax of 61 Cygni, 45-6 ; dis-
turbed movement of Sirius and
Procyon, 53 ; death, 54 ; trans-
Uranian planet, 103 ; Halley's
comet, 133 ; personal equation,
159; lunar atmosphere, 308; op-
posite polarities in comets, 365 ;
mathematical theory of cometary
emanations, 384 ; multiple tail?,
387
Biela, discovery of comet, 124-5
Birmingham, relative ages of stars,
415 note; discovery of T Coronse,
419
Birt, rotation of a sun spot, 186;
establishment of Selenographical
Society, 311
Black Ligament, 279, 280
Bode, solar constitution, 73 ; a planet
missed, 94 ; found, 96
Bode's Law, 94, 99, 108, 329
Boguslawski, centre of sidereal revo-
lutions, 52 ; observation of Halley's
comet, 133
Bohm, solar observations, 189,
192
Bolometer described, 264-5
Bond, G. P., his father's successor,
112; light of Jupiter, 332; flu-
idity of Saturn's rings, 340 ; Do-
nati's comet, 364-6
Bond, W. C., discovery of Hyperion,
ill; of Saturn's dusky ring, 112;
sketch of life, 112; resolution of
nebulse, 154, 424 ; celestial photo-
graphs, 197, 428 ; satellite-transit
on Jupiter, 335
Bonn Durchmusterung, 42
Borda, repeating circle, 158
Boss, observations on comets, 393,
398
Bouguer, solar atmospheric absorp-
tion, 263
Bouquet de la Grye and Arago, pho-
tographs of Venus on the sun,
301
Bouvard, Tables of Uranus, 103
Bradley, powers of observation, 3 ;
discoveries, 4 ; Astronomer Royal,
4 ; star distances, 13, 20 ; observa-
tion on Castor, 23 ; instruments,
36, 156; observations reduced by
Bessel, 41
Brahe, Tycho, star of 1572, 31
Brandes, observation of Andromeds,
377
Brandes and Benzenberg, heights of
meteors, 369
Brayley, meteoric origin of planets,
352
Bredichin, structure of chromosphere,
242 ; red spot on Jupiter, 337 ;
spectrum of Coggia's comet, 382 ;
repulsive forces in comets' tail?,
385-6, 407 ; three types, 387, 391,
392, 393
Brewster, atmospheric lines in solar
spectrum, 173; absorption spectra,
176
Brinkley, supposed stellar parallaxes,
43
Brisbane, observatory at Paramatta, 8
Bruno, Giordano, stars in motion, 12
Buffham, rotation of Uranus, 344
Buffon, internal heat of Jupiter, 332
Bunsen, discovery of spectrum ana-
lysis, 171, 176
Burnham, discoveries of double stars,
434, 447
Burton, canals of Mars, 324 ; rotation
of Jupiter's satellites, 336
Busch, daguerreotype of eclipsed sun,
213
CAMPBELL, polarisation of corona,
218
Carrington, astronomical career, 186-
8 ; sun-spot observations, 188-9;
solar rotation, 190 ; spot-distribu-
tion, 191 ; luminous outburst on
the sun, 205 ; Jovian and sun-spot
periods, 208 ; origin of comets,
410
Cassini, Domenico, discoveries of Sa-
turnian satellites, 1 10 ; of division
in ring, in ; period of solar rota-
tion, 189; solar parallax, 271 ; ro-
INDEX.
457
tation of Venus, 296 ; of Mars, 319;
spots on Jupiter, 333,338; satellite-
transit, 335
Cassini, J. J., stellar proper motions,
12 ; sun's limb notched by a spot,
68 ; theory of corona, 85 ; rotation
of Venus, 296 ; structure of Saturn's
rings, 340
Cavendish experiment, 77, 306
Ceraski, new variable, 418
Chacornac, observation on a sun spot,
201 ; ecliptical star-maps, 328, 429;
variable nebula, 424
Challis, search for Neptune, 106-7 >
duplication of Biela's comet, 126
Chladni, origin of meteors, 369, 375
Chromosphere, early indications, 88 ;
distinct recognition, 90-1, 214 ;
depth, 225 ; eruptive character,
242 ; metallic injections, 239, 249
Clark, Alvan, large refractors, 148,
441-2
Clark, Alvan, jun., discovery of Sirian
companion, 54, 442
Clarke, Colonel, figure of the earth, 307
Clausen, cometary systems, 405
Clerihew, secondary tail of 1843 co-
met, 135
Coggia, discovery of comet, 382
Comet, Halley's, return in 1759, 5,
1 15 ; orbit computed by Bessel, 38 ;
return in 1835, I 3 2 ~4> 384 ; star-
occultations by, 138 ; type of tail,
386, 392; Newton's, 115, 408;
Encke's, 118; expansion in ap-
proaching sun, 121 ; acceleration,
122-4; Lexell's, 120, 139; Win-
necke's, 123, 381 ; Biela's, 124-
7> 379 5 star-shower in connec-
tion with, 377-8 ; Faye's, 128 ; of
1811, 128-131, 386; of 1807, 130,
387, 392, 394, 397J of 1819, I3i>
134; of 1843, 134-7; type of tail,
386, 391 ; shortening of period,
389; Tewfik, 230, 400; Donati's,
363-6 ; type of tail, 386, 387 ;
comet of 1861, 366-8 ; type of tail,
386 ; of the August meteors, 368,
374 ; of the November meteors,
368, 375, 377; Klinkerfues's, 379,
380; of 1864, 381 ; Coggia's, 382,
385, 386 ; southern, of 1880, 388-
392; Aristotle's, 390; Tebbutt's,
392-7; Schaberle's, 397-8; Wells's,
398-400 ; of September 1882, 400-
409 ; of 1652, 406 ; Schmidt's, 406
Comets of 1618, 67, 406; obey law
of gravitation, 115 ; contract in
approaching sun, 121, 134; trans-
lucency, 125, 137-8, 393; polari-
sation of light, 134, 396; refrac-
tion by, 138, 393 ; smallness of
masses, 139 ; travel in same orbits
with meteor-systems, 374-5 ; dis-
integration and disruption, 376,
380; spectra, 381-2, 395-6, 398-
400 ; luminous by electricity, 383,
396, 3995 systems, 405; origin,
409-10
Comets' tails, repulsive forces pro-
ducing, 129, 130, 132-3, 384-7;
velocity of projection, 130, 135-
6, 386 ; coruscations, 136 ; three
types, 386-7, 391, 397; multiple,
130, 364, 386, 387, 392, 393. 397
Common, A., daylight discovery of
great comet, 400 ; five nuclei, 406 ;
photograph of Orion nebula, 432 ;
36-inch reflector, 441, 444
Comte, celestial chemistry, 181; as-
tronomy, 183
Cooke, 25-inch refractor, 442
Copeland, lunar radiation, 315; co-
mets of 1843 and 1880, 389 ; spec-
trum of great comet, 408 ; gaseous
stars, 421 ; observations in the
Andes, 448
Copernicus, stellar parallax, 20
Cornu, telluric lines in solar spec-
trum, 245 ; reillumination of pro-
minences, 248; solar parallax by
light velocity, 275, 285 ; spectrum
of new star, 421
Cornu and Bailie, density of the
earth, 306
Corona, of 1842, 81, 83 ; early records
and theories, 85-7 ; photographs,
213, 222, 230, 231, 233 ; spec-
trum, 219, 223, 228, 229 ; consti-
tution, 223, 224, 227, 236-7 ; of
1878, 224, 226, 228 ; of 1867, 227;
of 1882, 229 ; cometary analogy,
233 J glare theory, 234-6 ; expul-
sion theory, 237
Croll, secular changes of climate,
34-S
Crova, solar constant, 267
Cruls, great comet of 1882, 400, 407
458
INDEX.
Cusa, Cardinal, solar constitution,
72-3
Cysatus, nebula in Orion, 27 ; comet
of 1652, 406
D' ARREST, orbits of minor planets,
328; Biela meteors, 377; variable
nebula, 424
Darwin, G. H., rigidity of the earth,
304 ; origin of the moon, 357-8 ;
development of solar system, 359 ;
solar tidal friction, 360-2
Davidson, satellite-transit on Jupiter,
336
Dawes, prominences, 90 ; Saturn s
third ring, 1 12; a star behind a
comet, 138 ; solar observations,
185, 210; ice-island on Mars, 325;
satellite-transit on Jupiter, 335
Delambre, light-equation, 274
De la Roche, Newton's law of cool-
ing, 259
De la Rue, celestial photography,
197-9, 314; solar investigations,
199, 200 ; expedition to Spain,
213-14
De la Tour, experiments on liquefac-
tion, 195
Delaunay, tidal friction, 316-17
Delisle, diffraction-theory of corona,
87 ; method of observing transits
of Venus, 277, 284
Denning, rotation of Mercury, 293 ;
mountains of Venus, 297-8 ; ro-
tation of Jupiter, 333 ; red spot,
337-8 ; giant telescopes, 445
Denza, Father, meteors of 1872, 378
Derham, theory of sun spots, 67
Diffraction, corona explained by, 87,
91 ; spectrum, 179 note, 253, 265
Dissociation in the sun, 196, 249-53 ;
in space, 354
Dollond, discovery of achromatic tele-
scope, 4, 145
Donati, discovery of comet, 363 ;
analysis of cometary light, 381 ;
of stellar light, 411
Doppler,refrangibility of light changed
by motion, 243
Dorpat refractor, 44, 56-7
Draper, J. W., lunar photographs,
197 ; distribution ot energy in
spectrum, 265 note
Draper, H., oxygen in sun, 254-5 ;
? holographs of the moon, 314 ; of
upiter's spectrum, 335 ; of Teb-
butt's comet, 395 ; of Orion nebula,
43 i -2
Dulong and Petit, law of radiation,
259-261
Duponchel, sun-spot period, 208
EARTH, the, body of science regard-
ing, 302 ; rigidity, 303-4 ; secular
changes of climate, 304-5 ; mean
density, 306 ; figure, 306-7 ; rota-
tion retarded by tidal friction,
316-17; possible irregularities, 318,
453 J bodily tides, 357 ; primitive
disruption, 358
Eclipse, solar, of 1836,. 79 ; of 1842,
80-4, 88; of 1851, 90-1, 213 ; of
1860, 213-14; of 1868, 215-17;
of 1869, 218; of 1870, 219-20;
of 1871, 222; 011878,224-8; of
1882, 228 ; of 1883, 231-4
Eclipses, solar, importance, 76, 213 ;
varieties, 78 ; results, 223-4 ; an-
cient, 318
Edison, tasimeter, 228
Egoroff, telluric lines in spectrum,
256, 300
Elements, chemical, supposed disso-
ciation, 248-53
Elkin, transit of great comet of 1882,
401 ; secondary tail, 407 ; paral-
laxes of southern stars, 433
Elliott, Dr., opinions regarding the
sun, 73
Encke, a pupil of Gauss, 117? dis-
covery of comet, 118; hypothesis
of a resisting medium, 122 ; dis-
tance of the sun, 272
Engelmann, rotation of Jupiter's sa-
tellites, 336
Ericsson, solar temperature, 260-1
Erman, revolving meteoric rings, 372
Ertborn, mountain in Venus, 298
Evolution of solar system, 348, 349,
360, 362
FABRICIUS, David, discovery of vari-
able star Mira, 12
Fabricius, John, discovery of sun
spots, 66
Faye, nature of prominences, 91 ; dis-
co very of a comet, 128; theory of
solar constitution, 193-7; solar
INDEX.
459
absorption, 221 ; progressive illu
mination of prominences, 248 ; dis
tance of the sun, 285, 286 ; origin
of the planets, 356
Feilitsch, solar appendages, 91
Ferrel, tidal friction, 317
Ferrer, origin of the corona, 87
Finlay, observations of great comet
of 1882, 400, 401
Fizeau, daguerreotype of the sun, 198 ;
Doppler's principle, 244 ; velocity
of light, 275
Flamsteed, nature of the sun, 73 ;
distance, 271
Flaugergues, detection of 181 1 comet,
128
Fontana, mountains of Venus, 297 ;
spots on Mars, 319
Forbes, Prof. George, trans - Nep-
tunian planets, 347
Forbes, James D., solar spectrum
during annular eclipse, 173 ; solar
constant, 267
Foucault, spectrum of voltaic arc,
178 ; first photographic impression
of the sun, 198 ; velocity of light,
275 j silvered glass reflectors,
441
Fraunhofer, early accident, 43 ; im-
provement of refractors, 44 ; death,
45 ; spectra of flames, 169 ; of sun
and stars, 172
Fraunhofer lines, mapped, 172; na-
ture, 174-5 > solar absorption pro-
ducing, 221-2; reflected in coro-
nal spectrum, 223, 229, 232 ; a
criterion of motion, 244 ; reflected
in cometary spectra, 396, 400
Fritz, auroral periodicity, 207
GALILEO, originator of descriptive
astronomy, 2 ; double-star method
of parallaxes, 20 ; discovery of sun-
spots, 66 ; solar rotation, 189 ;
planets and sun - spots, 208-9 >
darkening at sun's edge, 263
Galle, discovery of Neptune, 1 06 ;
Saturn's dusky ring, 113; distance
of the sun, 282 ; Biela's comet
and Andromeds, 377, 378
Galloway, solar translation, 51
Gambart, discovery of Biela's comet,
I2 5
Gauss, orbits of minor planets, 96-8 ;
7'heoria Motus^ 101 ; magnetic ob-
servations, 163; cometary orbits,
409
Gautier, sun - spot and magnetic
periods, 165 ; sun - spots and
weather, 166
German Astronomical Society, 7, 432
Gill, expedition to Ascension, 282 ;
diurnal method of parallaxes, 283,
286 ; great comet, 402 ; photo-
graphic survey of the heavens, 429 ;
parallaxes of southern stars, 433 ;
Coude telescope, 450
Gladstone, Dr. J. H., spectrum ana-
lysis, 173, 176
Glaisher, star-occultation by Halley's
comet, 138
Glasenapp, light-equation, 274, 285
Glass, optical, excise duty on, in Eng-
land, 146, 149 ; Guinand's, 147
Gledhill, spot on Jupiter, 337
Goodricke, periodicity of Algol, 417
Gotha, astronomical congress at, 7
Gothard, bright-line stellar spectra,
420
Gould, southern comet of 1880, 388,
389; comets of 1881 and 1807,
392 ; fluctuations in stellar light,
418 ; Uranometria Argentina, 433 ;
Pleiades, 429
Graham, discovery of Metis, 101
Grant, solar envelope, 91, 214 ; lumi-
nous phenomena attending tran-
sits of Venus, 299
Green, N. E., observations of Mars,
^325 .
Greenwich observations, 35, 41, 104
Gregory, David, achromatic lens, 145
note.
Gregory, James, reflecting telescope,
141-2
roombridge, star-catalogue, 40
Czrosch, coronal streamers, 227
rubb, Howard, Vienna refractor,
442
rubb, Thomas, Melbourne reflector,
441
ruithuisen, snow-caps of Venus, 301 ;
lunar inhabitants, 310
Cruinand, improvement of optical
glass, 146-7
Griithrie, nebulous glow round Venus,
299
4 6o
INDEX.
HADLEY, reflecting telescope, 141
Hall, Professor A., satellites of Mars,
326 ; rotation of Saturn, 343 ;
parallaxes of 61 Cygni and Vega,
434
Hall, Maxwell, rotation of Neptune,
346
Halley, stellar proper motions, 12 ;
nebulae, 27; eclipse of 1715, 86;
orbit of comet, 115 ; solar parallax
from transits of Venus, 277 ; lunar
acceleration, 315; origin of meteors,
369
Hansen, solar parallax from lunar
theory, 273
Harding, discovery of Juno, 98 ;
Celestial Atlas, 102
Harkness, spectrum of corona, 219 ;
corona of 1878, 225; shadow of
the moon in solar eclipses, 234 ;
distance of sun, 285-6
Harrington, diameter of Vesta, 331
Harriot, observations .on Halley's
comet, 38
Hasselberg, cometary spectra, 382, 399
Hastings, composition of photosphere,
196 ; absorption in sun-spots, 201 ;
Fraunhofer lines, 222 ; observation
at Caroline Island, 234 ; Saturn's
dusky ring, 341
Hegel, number of the planets, 96
Heis, radiant of Andromeds, 377
Heliometer, 44, 278, 285
Helium, a constituent of prominences,
238, 241 ; a supposed modification
of hydrogen, 239 ; slight absorp-
tion in solar spectrum, 255
Helmholtz, gravitational theory of
solar heat, 352-3, 355
Hencke, discoveries of minor planets,
101
Henderson, parallax of a Centauri,
47-8
Henry, Paul and Prosper, lunar twi-
light, 310; markings on Uranus,
345 ; stellar photography, 429
Henry, Professor, radiation from sun
spots, 202
Herschel, Professor A. S., accordances
of cometary and meteoric orbits,
375 ; Andromeds, 377-8
Herschel, Caroline, her brother's as-
sistant, 15 ; observation of Encke's
comet, 118
Herschel, Sir John, life and work,
58-64 ; sun-spots, 73-4 ; solar
flames, 88 ; discovery of Neptune,
106 ; Biela's comet, 125 ; Halley's
comet, 133 ; comet of 1843, 135 ;
spectrum analysis, 170; solar con-
stitution, 195 ; negative halo round
eclipsed sun, 234 ; actinometrical
experiments, 257 ; solar heat, 258 ;
climate and eccentricity, 304 ; sur-
face of Mars, 322 ; Magellanic
Clouds, 60, 435
Herschel, Lieut. -Colonel, spectrum
of prominences, 2i5> 218 ; of co-
rona, 233
Herschel, Sir William, services to as-
tronomy, 5-6 ; discovery of Uranus,
6 ; founder of sidereal astronomy,
12, 453 ; biographical sketch, 13-
17 ; discovery of the sun's motion in
space, 19, 438 ; revolutions of double
stars, 23 ; structure of Milky Way,
24-6, 437 ; study of nebulae, 27-
32 ; results of astronomical labour?,
32 ; centre of sidereal system, 52 ;
theory of the sun, 69-72, 92 ; dis-
coveries of Saturnian and Uranian
satellites, no, 143, 113-14; reflect-
ing telescopes, 141-4 ; sun-spots
and weather, 166 ; transit of Mer-
cury, 290 ; refraction in Venus,
298 ; lunar volcanoes, 312 ; simi-
larity of Mars to the earth, 319-20;
Jovian trade-winds, 332 ; rotation
of Jupiter's satellites, 336; rotation
of Saturn, 343
Hevelius, granular structure of a
comet, 406
Hind, solar flames, 90 ; Iris and Flora
discovered by, 101 ; distortion of
Biela's comet, 126; transit of a
comet, 131 ; the earth in a comet's
tail, 367 ; comets of 1843 and 1880,
389; new star, 419; variable ne-
bula, 424
Hodgson, luminous outburst on the
sun, 205
Hoek, cometary systems, 405
Holden, Uranian satellites, 114;
eclipse-expedition, 232 ; intra-Mer-
curial planets, 295 ; disintegration
of comet, 406 ; Orion and trifid
nebulae, 425
Hooke, solar translation, 12 ; stellar
INDEX.
461
parallax, 20 ; repulsive force in
comets, 133 note; automatic move-
ment of telescopes, 156 ; spots on
Mars, 319, 321
Hopkins, solidity of the earth,
33
Horrebow, sun - spot periodicity,
162
Hough, red spot on Jupiter, 338
Houzeau, solar parallax, 285
Huggins, spectroscopic observation
of prominences, 218; open-slit
method, 239 ; extra-eclipse photo-
graphs of corona, 230-1, 237 ;
motions of stars in line of sight,
244, 426, 438 ; occultation of e
Piscium, 309 ; snow-caps on Mars,
322 ; spectrum of Mars, 323, of
Jupiter, 334, of Uranus, 346 ;
cometary spectra, 381-2, photo-
graphed, 395, 399 ; stellar chemis-
try, 412, 416 ; colours of stars, 414 ;
spectrum of new star, 419, of ne-
bulae, 423 ; photographs of stellar
and nebula spectra, 430-1, 451
Humboldt, sun-spot period, 162 ;
magnetic observations, 163 ; star
shower, 371
Hussey, search for Neptune, 103
Huygens, stellar parallax, 20; ne-
bula in Orion, 27 ; discovery of
Titan, no; Saturn's ring, in,
342 ; spot on Mars, 320
Hydrogen, a constituent of promi-
nences, 216, 238, 241 ; spectrum,
238, 249 note, 254, 431 ; dissocia-
tion, 252 ; absorption in sun, 253 ;
a gaseous metal, 254 ; in comets'
tails, 387 ; in stars, 413, 416, 431 ;
in nebulae, 423 ; ignited in new
stars, 420, 422
JANSSEN, solar photographs, 211 ;
extra-eclipse observations of pro-
minences, 216-17; escape from
Paris in a balloon, 219 ; spectrum
of corona, 223, 232 ; corona of
1871, 225 ; photographs of corona
of 1883, 233 ; rarefaction of chro-
mospheric gases, 235 ; spectrum of
Venus, 300, of Saturn, 344 ; photo-
graphs of Tebbutt's comet, 394-5
Jupiter, mass corrected, 101, 121 ;
physical condition, 331-3 ; spec-
trum, 334; satellite-transits, 335;
red spot, 336-8 ; periodicity of
markings, 338-9
KAISER, rotation of Mars, 320 ; map
of Mars, 324
Kant, position of nebulae, 17 ; Sirius
the central sun, 51 ; planetary in-
tervals, 93 ; tidal friction, 317 ;
condition of Jupiter, 332 j cosmo-
gony, 348
Kepler, star of 1604, 31 ; nature of
corona, 85 ; missing planets, 93 ;
comets, 119; physical astronomy,
181
Kirchhoff, foundation of spectrum
analysis, 171, 174-5 ; map of solar
spectrum, 176 ; theory of the sun,
193, 195, 221
Kirkwood, law of commensurability
in distribution of minor planets,
329, in divisions of Saturn's rings,
343 ; origin of planets, 356 ; comets
and meteors, 375
Klein, Hyginus N., 313
Klinkerfues, prediction of comet,
379> 3^0 ; apparitions of southern
comet, 390
Kreil, lunar-magnetic action, 167
Kriiger, segmentation of great comet
of 1882, 406
LACAILLE, southern nebulae, 28
Lagrange, gravitational theory of
solar system, 3 ; planetary disrup-
tion, 99
Lahire, distance of the sun, 271 ;
mountains of Venus, 297
Lalande, Histoire Celeste, 40 ; nature
of sun spots, 67 ; observations, of
Neptune, 108
Lambert, solar motion, 12 ; construc-
tion of the universe, 18, 51
Lament, magnetic period, 164
Langdon, mountains of Venus,
297
Langley, solar granules, 21 1 ; corona
of 1878, 226 ; spectroscopic effects
of solar rotation, 245 ; solar radia-
tive intensity, 263 ; bolometer,
264 ; distribution of energy in
solar spectrum, 265-6 ; colour of
sun, 267 ; solar constant, 268 ;
lunar radiation, 315; atmospheric
462
INDEX.
absorption, 322 ; age of the sun,
353 ; observations from Etna,
447
Laplace, lunar acceleration, 2, 315 ;
Exposition du Systeme du Monde,
6 ; nebular hypothesis, 32, 348-9,
355-6, 362 ; solar atmospheric ab-
sorption, 263 ; solar distance by
lunar theory, 273 ; stability of
Saturn's rings, 339 ; origin of
meteors, 369, of comets, 409
Lassell, discovery of Neptune's satel-
lite, 109, of Hyperion, in ; dusky
ring of Saturn, 112 ; Uranian
moons, 114; specula, 148; equa-
toreal mounting of reflectors, 157 ;
observations in Malta, 446
Laugier, solar rotation, 189, 190
Lescarbault, pseudo - discovery of
Vulcan, 293 ; halo round Venus
in transit, 300
Le Sueur, spectrum of Jupiter, 334
Leverrier, discovery of Neptune,
105-7 ; Lexell's comet, 120 ; dis-
tance of sun, 273, 286 ; prediction
of Vulcan, 293-4 ; mass of aste-
roids, 330 ; orbit of November
meteors, 374 ; Perseids and
Leonids, 376
Lexell, comet of 1770, 120
Liais, supposed transit of Vulcan,
294; comet of 1861, 367 ; division
of a comet, 380
Lick, foundation of observatory,
446
Light, velocity, 49, 275, 285 ; extinc-
tion, 57~8 ; refrangibility changed
by movement, 243, 426
Light-equation, 274, 285, 286
Lindsay, Lord, expedition to the
Mauritius, 279, 283
Line of sight, movements in, 243 ;
spectroscopically determinable, 244,
426 ; of solar limbs, 245 ; within
prominences, 246, 250-1 ; of stars,
426-8
Listing, dimensions of the globe,
307
Liveing and Dewar, numerical ratios
of wave - lengths, 238 ; line - dis-
placements in sun, 251
Lockyer, solar spectroscopy, 201,
254 ; extra-eclipse study of pro-
minences, 217, 238 ; slitless spec-
troscope, 223 ; corona of 1878,
225 ; chromosphere, 225 note ; car-
bon in solar atmosphere, 233; glare
theory of corona, 235 ; reversing
layer, 239 ; classification of pro-
minences, 240 ; solar cyclones,
246 ; solar dissociation, 248-53 ;
spots on Mars, 320
Loewy, equatoreal Coude, 449
Lohrmann, lunar chart, 310, 311 ;
Linne, 312
Louville, nature of corona, 86
Lyman, atmosphere of Venus, 299
MACLEAR, atmospheric diffusion dur-
ing eclipse, 235
Madler, Alcyone the central sun, 52 ;
atmosphere of Venus, 298 ; aspect
of Linne, 312
Magellanic Clouds, 60, 435
Mann, period of 61 Cygni, 435
Maraldi, observation on the corona,
86; rotation of Mars, 319; satel-
lite-transits on Jupiter, 335 ; spot
on Jupiter, 338
Marius, Simon, nebula in Andro-
meda, 27 ; sun-spots, 66 ; origin of
comets, 67 note
Mars, oppositions, 270 ; solar paral-
lax from, 271, 274,. 282 ; spots on
disc, 319-20; rotation, 319, 321;
atmosphere, 322-3 ; canals, 324-5 ;
satellites, 326-7, 361
Maskelyne, Astronomer Royal, 36 ;
experiment at Schehallien, 306 ;
comets and meteors, 375
Maxwell, J. Clerk, structure of Saturn's
rings, 340, 342
Mayer, Father Christian, star-satel-
lites, 22
Mayer, Julius Robert, meteoric sus-
tentation of sun's heat, 350-1
Mayer, Tobias, stellar proper mo-
tions, 12; solar translation, 18 ;
repeating circle, 158 ; solar distance
by lunar theory, 273
Meldrum, sun-spots and cyclones,
210
Melloni, lunar heat, 314
Melvill, spectra of flames, 169
Mercury, mass, 121 ; luminous ap-
pearances during transits, 290-2 ;
INDEX.
463
rotation, 292-3 ; theory of move-
ments, 293, 296
Messier, catalogue of nebulae, 28
Meteors, heat evolved by falling into
the sun, 351 ; agglomeration into
planets, 352 ; origin, 369 ; Leonids,
369-74 ; relation to comets, 374-
377, 380; Perseids, 371, 374, 376;
Andromeds, 377-8
Meyer, Dr. W., divisions of Saturn's
rings, 343 ; period of comet of
1880, 391 ; refraction by a comet,
393
Michel), double stars, 22 ; torsion-
balance, 306 ; star-systems, 439
Michelson, velocity of light, 285
Milky Way, the, grindstone theory,
17 ; ecliptic of the stars, 18; clus-
tering power, 25, 33 ; structure,
26, 437; centre of gravity, 51-2;
Struve's theory, 57; Sir J. Her-
schel's, 60
Miller, W. A., spectrum analysis,
170, 176, 177; stellar chemistry,
412
Mitchel, lectures at Cincinnati, 8
M oiler, Faye's comet, 128
Mohn, origin of comets, 410
Moon, the, magnetic influence, 167-
168 ; solar parallax from disturb-
ance of, 273 ; study of surface,
307 ; rills, 308 ; atmosphere, 308-
309, 315; charts, 310-11, 313^;
librations, 312 ; the crater Linne,
312; Hyginus N., 313-14; heating
effects, 314; acceleration, 3, 315,
317-18; rotation, 317; theory,
315, 4535. origin, 357-8, 359
Morstadt, Biela meteors, 375
Munich, Optical Institute, 37
Myer, description of solar eclipse,
236
NASMYTH, solar willow-leaves, 210 ;
comparative lustre of Mercury and
Venus, 300 ; condition of Jupiter,
332
Nasmyth and Carpenter, The Moon,
3 11
Nebula, Andromeda, early observa-
tions, 27 ; new star in, 424
Nebula, Orion, observed by Her-
schel, 15 ; first mentioned, 27 ; re-
solvability, 154, 424 ; spectrum,
423,431 ; monograph, 425; photo-
graphs, 431
Nebulae, first discoveries, 27-8 ; cata-
logues, 28, 59, 64 ; composition,
29, 6l, 423 ; distribution, 29, 62,
435-6; resolution, 61, 154, 424;
double, 62, 425 note; spiral, 153-4,
424 ; spectra, 423 ; variable, 424 ;
movements, 425, 428
Nebular hypothesis, Herschel's, 31-2;
Laplace's, 348-9, 355-6, 362
Nelson, atmosphere of Venus, 299 ;
of the moon, 309 ; work on The
Moon, 311
Neptune, discovery, 102-8 ; satel-
lite, 109; density, no; rotation,
346
Newcomb, corona of 1878, 226 ; dis-
tance of the sun, 274, 276 ; lunar
theory, 317; formation of planets,
356
Newton, H. A., periodicity of No-
vember meteors, 372-3
Newton, Sir Isaac, founder of theo-
retical astronomy, I ; law of gravi-
tation obeyed by comets, 115; first
speculum, 141 ; solar radiations,
257 ; law of cooling, 259-60 ;
mountain observatories, 446
Niesten, volume of asteroids, 330 ;
red spot on Jupiter, 336
Norton, expulsion theory of solar
surroundings, 237 note; comets'
tails, 384, 387
Nutation, discovered by Bradley, 4,
19 ; an uranographical correction,
40
OBSERVATORIES, founded in various
places, 8
Observatory, Greenwich, 3, 36 ; Pa-
ramatta, 8, 118; Konigsberg, 39;
Palermo, 45 ; Dorpat, 55 ; . Pul-
kowa, 57; Harvard College, 112;
Anclam, 192 ; Kew, 198 ; Dun-
echt, 399, 422 ; Lick, 446 ; Etna,
447 ; Pic du Midi, 447
Occultations, by comets, 125, 137-8,
393 ; by the moon, 309 ; by Mars,
322
Olbers, Bessel's first patron, 38-9 ;
discoveries of minor planets, 97-8 ;
origin by explosion, 98-100 ; career,
116-17 ; electrical theory of comets,
464
INDEX.
129, 384 ; multiple tails, 130, 364,
387; comet of 1819, 131 ; comet-
ary coruscations, 136 ; observation
of a star through a comet, 137 ;
November meteors, 371
Olmsted, radiant of November me-
teors, 370; orbit, 371
Oppolzer, acceleration of Winnecke's
comet, 123 ; position of Vulcan,
295 ; acceleration of 1843 comet,
3^9
Oxygen in the sun, 254-6
PALISA, discoveries of minor planets,
328
Pape, tails of Donati's comet, 384
Parallax, annual, of stars, 19-21 ; in-
vestigation resumed by Bessel, 42 ;
of 61 Cygni, 46, 47, 434; of Vega,
47, 434 J of a Centauri, 47, 433 ;
of Sirius, 54,433; horizontal of sun,
269; early estimates, 271 ; Encke's
result, 273 ; improved values, 276,
281-3, latest, 285
Parallaxes, diurnal method of, 282,
283, 286
Pastorff, drawings of the sun, 131
Peirce, structure of Saturn's rings,
340
Perrotin, striation of Saturn's rings,
341 ; markings on Uranus, 345
Perry, Father, solar obscurations, 211
Personal equation, 159
Peters, C. A. F., movements of Sirius,
53
Peters, C. F. W., orbit of November
meteors and Tempel's comet, 375
Peters, C. H. F., sun-spot observa-
tions, 190-1 ; discoveries of minor
planets 328 ; star maps, 328, 433
Peytal, first description of chromo-
sphere, 90
Photography, celestial, 1 88, 197,
428-9, 440, 451 ; solar, 198-9, 21 1 ;
during eclipses, 213-14, 222 ; co-
ronal impressions, 230, 233, 237;
use in transits of Venus, 277, 280,
281, 284, 301 ; lunar, 314; comet-
ai 7 394-5, 399; of stellar and
nebular spectra, 430-1 ; of nebulae,
432
Photosphere, named by Schroter, 70;
structure, 195, 211
Piazzi, star-catalogues. 40 ; star-paral-
laxes, 43 ; motion of 61 Cygni, 45;
birth and training, 95 ; discovery
of Ceres, 95-6 ; five-foot circle, 158
Pickering, photometric measures of
satellites of Mars, 326, of minor
planets, 330 ; variability of Japetus,
344, of Neptune, 346, of Algol,
417-18 ; gaseous stars, 421 ; spec-
trum of Nova Cygni, 422 ; photo-
metric catalogue, 435 ; plan for
mounting a telescope, 450
Planets, influence on sun-spots, 208-9 >
periods and distances, 270 ; inferior
and superior, 331 ; origin, 349, 352,
356 ; intra-Mercurial, predictions,
209, 293, 295 ; supposed disco-
veries, 294, 295 ; trans-Neptunian,
347
Planets, minor, existence anticipated,
93-4; discoveries, 95, 101, 327;
solar parallax from, 283, 286 ; dis-
tribution of orbits, 328 ; law of
commensurability, 329-30 ; collec-
tive volume, 330 j atmospheres,
331
Pleiades, community of movement,
S 2 , 53, 439 ; photographs, 429
Plummer, solar translation, 51
Pogson, discovery, of a comet, 379-
380
Pond, controversy with Brihkley, 43
Pouillet, solar constant, 257, 267
Pritchett, red spot on Jupiter, 336
Proctor, glare theory of corona, 234 ;
initial velocity of matter ejected
from sun, 248 ; transit of Venus,
277 ; distance of sun, 281 ; rotation
of Mars, 321 ; map of Mars, 324;
condition of giant - planets, 333 ;
capture-theory of comets, 376 note ;
status of nebulse, 435 ; structure of
Milky Way, 437 ; star-drift, 438
Prominences, observed in 1842, 82-
83 ; described by Vassenius, 89 ;
observed in 1857, 90-1 ; photo-
graphed, 214 ; composition, 215,
238, 241 ; extra-eclipse observa-
tions, 217-18, 238-40 ; forms, 239 ;
two classes, 240; distribution, 241-
242 ; cyclonic movements in, 246 ;
velocities of projection, 247-8
QUETELET, periodicity of August
meteors, 371
INDEX.
465
RANYARD, volume on eclipses, 225 ;
periodicity of Jupiter's markings,
338
Rayet, spectrum of prominences, 215,
218
Reduction of observations, 40 ; Bes-
sel's improvements, 41-2 ; Baily's,
77
Refraction, atmospheric, 40 ; in
Venus, 298
Reichenbach, foundation of Optical
Institute, 37
Repsold, astronomical circles, 158
Resisting medium, 122-4, 389, 403-4
Respighi, slitless spectroscope, 223 ;
prominences and chromosphere,
240, 241-2 ; solar uprushes, 248
Reversing layer, detected, 220 ; theo-
retically necessary, 221 ; depth, 222
Riccioli, secondary light of Venus,
302
Ricco, spectroscopic changes in great
comet of 1882, 409
Roberval, structure of Saturn's rings,
34p
Robinson, reflectors and refractors,
443 ; atmospheric impediments,
445
Romer, star-places, 12 ; invention of
equatoreal and transit-instrument,
156, of altazimuth, 158 ; velocity
of light, 274
Rosetti, temperature of the sun, 261
Rosse, third Earl of, biographical
sketch, 148 ; improvement of tele-
scopes, 149, 446 ; great specula,
150-2; spiral nebulas, 153 ; resolu-
tion of nebulae, 154, 435
Rosse, fourth Earl of, lunar radiation,
3H
Rost, nature of sun-spots, 68
Russell, red spot on Jupiter, 337
Rutherfurd, lunar photography, 314 ;
star-spectra, 412; photographs o
the Pleiades, 429
SABINE, magnetic and sun - spo
periods, 164 ; pendulum - experi
ments, 306
Sadler, motion of a nebula, 425
Satellites, discoveries, 109-11, 113-
114, 143, 326 ; transits, 335 ; varia
bility, 336, 343 ; origin, 349, 360-1
Saturn, low specific gravity, 339 ; ro-
tation, 343 ; spectrum, 344
Saturn's rings, early knowledge of,
in ; detection of dusky ring, 112-
113; stability, 339, 342; satellite-
theory, 340-1
Savary, orbits of double stars, 59
Schaberle, discovery of comet, 397
Schafarik, secondary light of Venus,
302 ; compression of Uranus,
345
Scheiner, nature of sun-spots, 66, 68 ;
solar rotation, 189 ; darkening of
sun's edge, 263
Schiaparelli, snow-cap of Mars, 323 ;
canals, 324-5 ; compression of
Uranus, 345 ; comets and meteors,
374-6 ; origin of comets, 409-10
Schmidt, lunar rills, 308 ; lunar
maps, 310-11 ; disappearance of
Linne, 312-13 ; discovery of co-
met, 406 ; appendages of great
comet of 1882, 407 ; new stars,
419, 421
Schioter, a follower of Herschel,
6, 288 ; biographical sketch, 288-
289 ; observations on Mercury,
290, 292 ; on Venus, 296-8, 302 ;
on the moon, 307-8 ; a lunar city,
310; Linne, 313; spots on Mars,
320, on Jupiter, 333
Schiilen, nature of sun-spots, 68
Schuster, coronal spectrum, 229 ;
photographs of corona, 230 ; car-
bon in sun, 234; oxygen spectra,
256
Schwabe, discovery of sun-spot perio-
dicity, 161-3
Secchi, cyclonic movements in sun-
spots, 1 86 ; structure of photos-
phere, 159 ; nature of spots, 203,
241 ; photograph of eclipsed sun,
214; reversing layer, 220; obser-
vations of prominences, 238, 241 ;
temperature of the sun, 260-1 ;
solar atmospheric absorption, 264 ;
spectrum of Uranus, 346, of
Coggia's comet, 382 ; stellar chem-
istry, 412 ; four spectral types, 413
Short, reflectors, 4, 141, 149, 157 ;
striation of Saturn's rings, 341
Sidereal science, foundation, n, 453 ;
condition at beginning of nineteenth
century, 13 ; progress, 65
2 G
466
INDEX.
Siemens, Sir W., regenerative theory
of the sun, 353-4
Sirius, a double star, 53 ; mass and
luminosity, 54 ; parallax, 54, 433 ;
spectrum, 172, 413 ; movement in
line of sight, 426-8
Smyth, Piazzi, solar outburst, 206 ;
lunar radiations, 314; expedition
to Teneriffe, 446
Solar Spectrum, purified by use of a
slit, 171 ; fixed lines, 173 ; maps
by Fraunhofer, 172, Kirchhoff,
176, Lockyer, 249, Angstrom,
253 ; distribution of energy, 265-6
Solar System, translation, 18-19, 5~
51 ; development, 348-9, 352, 359-
362 ; complexity, 452
Soret, solar temperature, 260
South, Sir James, occultation by
Mars, 322
Spectrum Analysis, defined, 168;
first experiments, 169-70 ; defini-
tive results, 171-7; Kirchhoff s
theorem, 175 ; elementary prin-
ciples, 179-81 ; effects on science,
182 ; application to stars, 172, 411-
412; to comets, 381 ; to nebulae,
423 ; an implement of astronomical
research, 440
Spencer, position of nebulae, 435
Sporer, solar rotation, 192
Stannyan, early observation of chro-
mosphere, 88
Star-catalogues, 40, 41, 78, 433;
spectroscopic, 415 ; photometric,
435
Star-drift, 438
Star-gauging, 24-5, 60
Star-maps, 102, 328, 437; photo-
graphic, 429
Stars, movements, 12, 46, 50, 426-8 ;
photometric estimates, 16, 63, 435 ;
disappearances, 33; distances, 46-9,
433-4 J chemistry, 41 1,416; spectro-
scopic orders, 413 ; colours, 414 ;
relative ages, 415 ; distribution,
437 ; systems, 438-9
Stars, double, catalogues, 21, 55, 64 ;
discovery of revolutions, 21, 453 ;
masses, 49-50 ; Struve's researches,
55-6 ; discoveries, 58^9, 61, 434,
447 ; orbits, 59, 434 ; photographs,
429
Stars, gaseous, 421
Stars, temporary, 31, 63, 419, 421-2
Stars, variable or periodical, early in-
stances, 12-13 J "Q Argus, 62-3, 421 ;
sun-spot analogy, 165, 418 ; hypo-
theses in explanation, 417 ; Algol
class, 418
Stewart, Balfour, Kirchhoff's prin-
ciple, 175 note; Faye's theory of
sun - spots, 197 ; solar investiga-
tions, 199-200, 209
Stewart, Matthew, solar distance by
lunar theory, 273
Stokes, anticipation of spectrum ana-
lysis, 178
Stone, reversal of Fraunhofer spec-
trum, 221 ; distance of the sun,
274, 276, 281, 286 ; transit of
Venus, 284 ; Cape Catalogue, 433
Struve, F. G. W., stellar parallax, 43,
47 ; career and investigations, 54-7 ;
occultation by Halley's comet, 138 ;
Russo-Scandinavian arc, 306
Struve, Otto, solar translation, 51 ;
his father's successor at Pulkowa,
57 ; eclipse of 1842, 80, 83 ;
changes in Saturn's rings, 342 ; in
Orion nebula, 425; parallax of
Aldebaran, 434
Sun, the, Herschel's theory, 69-74,
192; trade- wind analogy, 74-5;
circulatory movements, 74, 194,
212; chemical composition, 174,
253-6; mode of rotation, 190-1 ;
Kirchhoff's theory, 193 ; Faye's,
193-7 ; luminous outburst, 205 ;
repulsive action, 233, 237, 385-6 ;
explosions, 247-8 ; dissociation,
248-52 ; heat-emissions, 257-8,
263, 267-8 ; temperature, 259-60,
262 ; problem of distance, 269 ;
results from transits of last century,
271 ; from transit of 1874, 281 ;
from opposition of Mars, 1877, 282;
from transit of 1882, 285 ; range of
uncertainty, 287 ; maintenance of
heat, 350-5 ; age, 353
Sun - spots, early conjectures, 66-7 ;
Wilson's demonstration, 68 ; distri-
bution, 74, 191-2 ; cyclone theory,
75, 186, 202-3 ; periodicity, 162,
165, 207 j relation to magnetic
changes, 164; to weather, 166-7,
209 ; to auroras, 167 ; spectrum,
201-2 ; volcanic hypothesis, 203-4 ;
INDEX.
467
planetary influence, 208-9 5 con '
nection with markings on Jupiter,
339
Swan, sodium-line, 170
Swift, supposed discovery of Vulcan,
295
TACCHINI, spectrum of corona, 229,
233 ; distribution of prominences,
242
Talbot, Fox, spectrum analysis, 170
Tarde, sun-spots, 66
Tebbutt, discovery of comet, 392
Telescope, the reflecting and refract-
ing, 140, 443 ; achromatic, 145 ;
Rosse, 150-5, 441 ; equatoreal,
156-7 ; Melbourne, 441 ; Com-
mon's silvered glass, 441, 444;
NewalPs 25-inch, 442; Lick re-
fractor, 443, 446 ; difficulty of fur-
ther improvement, 444-5 ; Coude,
449-50
Telescopes, reflecting, Short's, 4, 141,
149, 157; HerschePs, 16, 141-4;
Lassell's, 109, 148, 157, 446 ; New-
ton's, 141 ; three varieties, 142 ;
silvered glass, 441 ; refracting, need
for improvement, 37 ; Fraunhofer's,
44 ; Clark's, 148, 442
Tempel, discoveries of minor planets,
328; of comets, 368,381 ; second-
ary tail of comet, 393 ; multiple
nuclei, 406
Tennant, eclipse-observations, 215,
217, 223
Terby, surface of Mars, 324 ; second-
ary tail of comet, 397
Thalen, basic lines, 250 ; map of solar
spectrum, 253-4
Thollon, line-displacements by mo-
tion, 245, 408 ; lunar atmospheric
absorption, 309
Thomson, Sir W., solar chemistry,
178; tidal strains, 303-4; rotation
of the earth, 318 ; dynamical theory
of solar heat, 351-2, 354
Tidal friction, 316-17, 357-9 ; solar,
359-61
Titius, law of planetary intervals, 93-
94
Todd, solar distance, 281, 285 ; trans-
Neptunian planet, 347
Transit-instrument, 156
Trepied, reversal of Fraunhofer spec-
trum, 221-2
Trouvelot, veiled spots, 191, 211, note;
chromosphere in 1878, 226; intra-
Mercurial planets, 232, 296 ; obser-
vation of a prominence, 241 ; moun-
tains of Venus, 301
Tupman, transit-expedition, 279 ; re-
sults, 280, 281
ULLOA, eclipse of 1778, 89
United States, observatories founded,
8
Uranus, discovery, 6, 144 ; unex-
plained disturbances, 102-3, 347 ;
rotation, 344 ; equatoreal markings,
345 ; spectrum, 346
VASSENIUS, first description of pro-
minences, 89
Venus, transits, 5, 271-2, 276 ; transit
of 1874, 277-82, of 1882, 283-5 J
rotation, 296 ; mountainous surface,
297, 301 ; atmosphere, 298 ; lumi-
nous phenomena, 299 ; spectrum,
300; secondary light, 301-2
Vesta, largest minor planet, 99, 330 ;
inclination of orbit, 329 ; spectrum,
331
Vicaire, solar temperature, 260
Vico, rotation of Venus, 297 ; ring-
mountain, 298
Violle, solar temperature, 259, 260 ;
solar constant, 267
Vogel, H. C., solar rotation spectro-
scopically displayed, 244 ; solar
atmospheric absorption, 264, 267 ;
spectra of Mercury, 292, Venus,
300, Vesta, 331, Jupiter, 334,
Jupiter's satellites, 336, Uranus,
346, comets, 382, 396, 399;
secondary light of Venus, 302 ;
carbon in stars, 413 ; star spectra,
414 ; spectroscopic star- catalogue,
415 ; movements of Sirius, 427
Vogel, H. W., spectrum of hydrogen,
431
Vulcan, supposed discoveries, 294-5
WARD, new star in Andromeda ne-
bula, 424
Wartmann, occultation by a comet,
138
Waterston, solar temperature, 259,
468
INDEX.
261 ; sustentation by meteoric
infalls, 351
Watson, supposed discovery of Vul-
can, 295
Webb, comet of 1861, 367
Weiss, comets and meteors, 375, 377,
378
Wells, discovery of comet, 398
Wheatstone, spectrum of electric arc,
170
Whewell, stars and nebulae, 435
Wilson, observations of sun-spots, 68 ;
theory, 69
Winnecke, Donati's comet, 364, 387 ;
variable nebula, 425
Wolf, R., sun - spot and magnetic
periodicity, 165, 207-8 ; sun-spots
and variable stars, 165, 418 ; sun-
spots and auroras, 167 ; suspicious
transits, 295
Wolf and Rayet, gaseous stars, 421
Wolleston, flame-spectra, 169 ; dark
lines in solar spectrum, 171
Wrangel, aurorse and meteors, 379
Wright, Professor, polarisation of
light from comet's tail, 396
Wright, Thomas, Grindstone theory
of Milky Way, 17 ; structure of
Saturn's rings, 340
YOUNG, C. A., spectra of sun-spots,
202 ; corona, 219 ; prominences,
238-9 ; Venus in transit, 300 ;
origin of sun-spots, 203 ; reversing
layer, 22O ; corona of 1878, 227-8 ;
prominences and chromosphere,
241, 242 ; spectroscopic measure-
ment of sun's rotation, 245 ; solar
cyclones and explosions, 246-8 ;
distance of sun, 287 ; markings on
Uranus, 345
ZACH, BARON VON, promotion of as-
tronomy, 7, 36 ; search for missing
planet, 94 ; re-discovery of Ceres,
97; use of a heliostat, 156
Zantedeschi, fixed lines in solar spec-
trum, 173 ; lunar radiation, 314
Zenger, observations on Venus, 297,
302
Zezioli, observation of Andromeds,
378
Zodiacal light and resisting medium,
124; resemblance to coronal
streamers, 227 ; meteoric constitu-
tion, 351-2
Zollner, observations of promi-
nences, 238 ; classification, 240 ;
reversion-spectroscope, 244 ; solar
temperature, 262 ; condition of
Venus, 302, of great planets, 333 ;
albedo of Mars, 327, of Jupiter,
334 ; sun-spots and Jovian mark-
U1 g s > 339 ; electric origin of comet-
ary light, 383 ; repulsive action in
comets, 385, 387; ages of stars, 414
THE END.
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