THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

HISTORY OF ASTRONOMY

A POPULAR

HISTORY OF ASTRONOMY

DURING

THE NINETEENTH CENTUEY

BY

AGNES M.

JUPITER 1879

SATURN 1885

THIRD EDITION

LONDON: ADAM & CHARLES BLACK
1893

CU

PREFACE TO THE THIRD EDITION.

THE interval of six years since the publication of the Second
Edition of the present work has been marvellously fruitful of
astronomical discoveries. Hence a searching revision was called
for, and has been executed without stint of care or pains.
Additions and substitutions have been freely made ; and it has
been the aim of the writer, not only to furnish the large amount
of requisite new information, but to incorporate it so completely
with the pre-existing text as to leave no gaps in the narrative
suggesting " interpolations " to the refined critical sense. The
book has thus grown, and been brought down to date, by a
process of assimilation, rather than of mere accretion. Its
adjuncts, too, have been renovated. The foot-note references
have been multiplied ; the Index has been rendered more copious ;
the Chronological Table has been considerably extended; and
several new Tables of data, likely to be useful, while not every-
where accessible to students, have been appended. Besides the
Frontispiece and Vignette, retained from the Second Edition, for
which the writer renews her thanks to Dr. Common and the
MM. Henry, five Plates have been prepared, with the most
kind assistance of Mr. Kanyard, especially for the Third. Their
availability is due to the courtesy of the Council of the Eoyal
Astronomical Society, of Professor George E. Hale, of Professor

M368106

vi PREFACE TO THE THIRD EDITION.

Barnard, and of Dr. and Mrs. Huggins, to all of whom the
writer offers grateful acknowledgments. She ventures to hope
that the remodelled work will enjoy no less favour with the
public than was shown to : lts earlier issues.

LONDON, May 1893.

PREFACE TO THE FIRST EDITION.

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 intelligent
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, purposes, and
methods of celestial science introduced by the momentous disco-
very 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-

viii PREFACE TO THE FIRST EDITION.

nary experience than that evolved by the aid of the calculus
from materials collected by the use of the transit-instrument
and chronograph.

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 advance 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 formulse. 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

PREFACE TO THE FIRST EDITION. ix

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
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 helpful,
would here have been purely artificial, and therefore confusing.
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

x PREFACE TO THE FIRST EDITION.

also been assigned to the biographical element, as underlying
and determining the whole course of human endeavour. The
advance of knowledge may be called a vital process. The lives
of men are absorbed into ancl assimilated by it. Inquiries 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,
Wisconsin, and to Dr. Copeland, chief astronomer of Lord
Crawford's Observatory at Dunecht, for many valuable com-
munications.

LONDON, September 1885.

CONTENTS.

INTRODUCTION.

Three Kinds of Astronomy Progress of the Science during the
Eighteenth Century Popularity and Rapid Advance during the
Nineteenth Century .

PART I.

PROGRESS OF ASTRONOMY DURING THE FIRST HALF
OF THE NINETEENTH CENTURY.

CHAPTER I.

FOUNDATION OF SIDEEEAL ASTBONOMY.

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 Revolutions His
Method of Star-Gauging Discoveries of Nebulae Theory of their
Condensation into Stars Summary of Results . . . Page 10

CHAPTER II.

PKOGEESS OF SIDEREAL ASTRONOMY.

Exact Astronomy in Germany Career of Bessel His Fundamenta
Astronomice 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 Exploration
of the Heavens Character of Fifty Years' Progress . . Page 32

xii CONTENTS.

CHAPTER III.

PEOGEESS OF KNOWLEDGE EEGAEDING THE SUN.

Early Views as to the Nature of Sun-Spots Wilson's Observations and
Reasonings Herschel's Theory,f the Solar Constitution Sir John
Herschel's Trade-Wind Hypothesis Baily's Beads Total Solar
Eclipse of 1842 Corona and Prominences Eclipse of 1851 Page 62

CHAPTER IV.
PLANETAEY DISCOVEEIES.

Bode's Law Search for a Missing Planet Its Discovery by Piazzi
Further Discoveries of Minor Planets Unexplained Disturbance of
Uranus Discovery of Neptune Its Satellite An Eighth Saturnian
Moon Saturn's Dusky Ring The Uranian System . . Page 87

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 Emanations The
Earth in a Comet's Tail Second Return of Halley's Comet Great
Comet of 1843 Results to Knowledge .... Page 109

CHAPTER VI.
INSTEUMENTAL ADVANCES.

Two Principles of Telescopic Construction Early Reflectors Three
Varieties Herschel's Specula High Magnifying Powers Invention
of the Achromatic Lens Guinand's Optical Glass The Great Rosse
Reflector Its Disclosures Mounting of Telescopes Astronomical
Circles Personal Equation Page 134

PART II.

RECENT PROGRESS OF ASTRONOMY.
CHAPTER I.

FOUNDATION OF ASTEONOMICAL PHYSICS.

Schwabe's Discovery of a Decennial Sun-Spot Period Coincidence with
Period of Magnetic Disturbance Sun-Spots and Weather Spectrum
Analysis Preliminary Inquiries Fraunhof er Lines Kirchhoff 's Prin-
ciple Anticipations Elementary Principles of Spectrum Analysis
Unity of Nature p a ge 155

CONTENTS. xiii

CHAPTER II.

SOLAR OBSERVATIONS AND THEORIES.

Black Openings in Spots Carrington's Observations Rotation of the
Sun KirchhofE 's Theory of the Solar Constitution Faye's Views
Solar Photography Kew Observations Spectroscopic Method
Cyclonic Theory of Sun-Spots Volcanic Hypothesis A Solar Out-
burst Sun-Spot Periodicity Planetary Influence Nasmyth's Willow
Leaves Page 178

CHAPTER III.

RECENT SOLAR ECLIPSES.

Expeditions to Spain Great Indian Eclipse New Method of Viewing
Prominences Total Eclipse Visible in North America Spectrum of
the Corona Eclipse of 1870 Young's Reversing Layer Eclipse of
1871 Corona of 1878 Varying Types Egyptian Eclipse Extra-
Eclipse Coronal Photography Observations at Caroline Island
Diffraction Theory of Corona Photographs of Corona in 1886
and 1889 Mechanical Theory Mathematical Theory Nature of
Corona Page 207

CHAPTER IV.

SPECTROSCOPIC WORK ON THE SUN.

Chemistry of Prominences Study of their Forms Two Classes Distri-
bution of Prominences Structure of the Chromosphere Spectro-
scopic Measurement of Movements in Line of Sight Spectroscopic
Determination of Solar Rotation Velocities of Transport in the Sun
Lockyer's Theory of Dissociation Solar Constituents Absence of
Oxygen Page 241

CHAPTER V.

TEMPERATURE OF THE SUN.

Thermal Power of the Sun Radiation and Temperature Estimates of
Solar Temperature Rosetti's Result Zollner's Method Larigley's
Experiment at Pittsburg The Sun's Atmosphere Selective Absorp-
tion by our Air The Sun Blue The Solar Constant . . Page 268

xiv CONTENTS.

CHAPTEE VI.

THE SUN'S DISTANCE.

Difficulty of the Problem Oppositions of Mars Transits of Venus
Lunar Disturbance Velocity of Light Transit of 1874 Inconclusive
Result Opposition of Mars in 1877 Measurements of Minor Planets
Transit of 1882 Newcomb's Determination of the Velocity of Light
Combined Eesult . . . . . J^ * . Page 280

CHAPTER VII.

PLANETS AND SATELLITES.

Schroter's Life and Work Luminous Appearances during Transits of
Mercury Mountains of Mercury Rotation Intra-Mercurian Planets
Schiaparelli's Conclusion as to the Rotation of Venus Mountains
and Atmosphere of Venus Ashen Light Illusory Satellite
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 . Page 299

CHAPTER VIII.

PLANETS AND SATELLITES

Analogy between Mars and the Earth Martian Snowcaps, Seas, and
Continents Climate and Atmosphere Schiaparelli's Canals Dis-
covery of Two Martian Satellites Distribution of the Minor Planets
Their Collective Mass and Estimated Diameters Condition of
Jupiter His Spectrum Transits of his Satellites Their Mode of
Rotation Discovery of an Inner Satellite 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 Page 336

CHAPTER IX.
THEOEIES OP PLANETAEY EVOLUTION.

Origin of the World according to Kant Laplace's Nebular Hypothesis-
Maintenance of the Sun's Heat Meteoric Hypothesis Radiation the
Result of Contraction Regenerative Theory Faye's Scheme of
Planetary Development Origin of the Moon Effects of Tidal
Friction . ' ' . . p a( j e 374

CONTENTS. xv

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 Andromedes Orbits of Meteorites
Meteors with Stationary Radiants Spectroscopic Analysis of
Cometary Light Page 392

CHAPTER XI.

EECENT 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 Photographs Schaeberle's
Comet Comet Wells Sodium Blaze in Spectrum Great Comet of
1882 Transit Across the Sun Relation to Comets of 1843 and *88o
Cometary Systems Origin of Comets Rediscovery of Lexell's
Comet Swift's Comet Holmes's Comet .... Page 417

CHAPTER XII.
STAES AND NEBULA.

Stellar Chemistry Four Orders of Stars Their Relative Ages Gaseous
Stars Spectroscopic Star-Catalogues Stellar Chemistry The Draper
Catalogue Velocities of Stars in Line of Sight Spectroscopic
Binaries Eclipses of Algol New Stars Outbursts in Nebula3
Nova Aurigas Gaseous Nebulae Variable Nebulas Movements of
Nebulas Stellar and Nebular Photography Nebulas in the Pleiades
Photographic Star-Charting Stellar Parallax Double Stars
Stellar Photometry Status of Nebulas Photographs and Drawings
of the Milky Way Star Drift Page 450

CHAPTER XIII.
METHODS OF BESEAECH.

Development of Telescopic Power Silvered Glass Reflectors Giant
Refractors Comparison with Reflectors The Lick Telescope Atmo-
spheric Disturbance Mechanical Difficulties The Equatoreal Coude
The Photographic Camera Retrospect and Conclusion . Page 512

APPENDIX 531

INDEX 555

LIST OF ILLUSTRATIONS.

Photograph of the Great Nebula in Orion (1883) . . Frontispiece

Photographs of Jupiter (1879) and of Saturn (1885) . Vignette
Plate I. Photographs of the Solar Chromosphere and

Prominences To face p. 246

,, IT. The Great Comet of September 1882 photo-
graphed at the Cape .... 434

III. Photographs of Swift's Comet (1892) . . ., 446

IV. Photographic and Visual Spectrum of Nova

Aurigae 478

,, V. Photograph of the Milky Way in Sagittarius 508

HISTORY OF ASTRONOMY

DURDsG THE NINETEENTH CENTURY.

INTRODUCTION.

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 ; 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
dormittta. This first and oldest branch is known as " observa-
tional.^ 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." l It is based on the idea of
cause ; and the whole of its elaborate structure is reared accord-
ing to the dictates of a single law, simple in itself, but the

1 The denomination "physical astronomy," first used by Kepler, and long
appropriated to this branch of the science, has of late been otherwise applied.

2 HISTORY OF ASTRONOMY.

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 computations, 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. The accom-
plishment of that task occupied just one hundred years. It was
virtually brought to a close when Laplace explained to the
French Academy, November 19, 1787, the cause of the moon's
accelerated motion. As a mere machine, the solar system, so far

INTRODUCTION. 3

as it was then known, was found to be complete and intelligible
in all its parts ; and in the Mtfcanique Celeste its mechanical per-
fections were displayed under a form of majestic unity which
fitly commemorated the successive triumphs of analytical genius
over problems among 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 beginnings of
discovery. Thus, theory and observation mutually act and react,
each alternately taking the lead in the endless race of improve-
ment.

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 lead-
ing 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. Delambre remarked that, had all other materials
of the kind been destroyed, the Greenwich records alone would
suffice for the restoration of astronomy ; 1 and the establishment
was indeed 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 introduced 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 his keenest attention ;

1 Histoire de V Astronomic au xviii 6 Stide, p. 267.

4 HISTORY OF ASTRONOMY.

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 aberration of light and the
nutation of the earth's axis. ..The first was announced in 1729.
What is meant by it is that, owing to the circumstance 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 pro-
duced by the dragging of the moon at the terrestrial equatorial
protuberance. From it results an apparent displacement 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 observational
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-Koyal 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 Brad-
ley 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 1758, removing thereby the chief obstacle to
the development of the powers of refracting telescopes ; James
Short, of Edinburgh, was without a rival in the construction of
reflectors; the sectors, quadrants, and circles of Graham, Bird,

INTRODUCTION. 5

Kamsden, and Gary were inimitable by Continental workman-
ship.

Thus practical and theoretical astronomy advanced on parallel
lines in England and France respectively, the improvement of
their several tools the telescope and the quadrant on the one
side, and the calculus on the other keeping 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 porten-
tous 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, international
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 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

6 HISTORY OF ASTRONOMY.

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
astronomy as still young and fujl 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 "[Iranian
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 conveyed
formed a combination powerfully effective to those least suscep-
tible 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 un-
learned readers. Lalande, by his popular lectures and treatises,
helped to form an audience which Laplace himself did not
disdain to address in the Exposition du Syst&me 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
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.

INTRODUCTION. 7

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 astronomical
congress met in 1789 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 undis-
covered between the orbits of Mars and Jupiter. The Astrono-
mical 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 were organised to serve ; while functions corre-
sponding to theirs have been discharged elsewhere by older or
less specially constituted bodies, and new ones of a more popular
character 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 com-
bination 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
interpreted according to the sagacious insight of some one
among them gifted above his fellows. The first really effective
astronomical periodical was the Monatliclie Correspondent, started
by Von Zach in the year 1800. It was followed in 1822 by the
Astronomische Nachricliten, later by the Memoirs and Monthh
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

8 HISTORY OF ASTRONOMY.

ample liberality. The first regular observatory in the Southern
Hemisphere was founded at Paramatta by Sir Thomas Mak-
dougall Brisbane in 1821. The Royal Observatory at the Cape
of Good Hope was completed in 1829. Similar establishments
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 1 843 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 establish-
ment worthy the name in that great country. On the 1st of
January 1882 no less than one hundred and forty-four were
active within its boundaries.

The apparition of the great comet of 1843 g ave an additional
fillip to the movement. To the excitement caused by it the
Harvard College 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 such institutions ; private subscrip-
tions poured in ; emissaries were sent to Europe to purchase
instruments and to procure instruction in their use. In a few
years the young Eepublic was, in point of astronomical 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,
and in part occasioned, the great extension of its area of cultiva-
tion 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,

INTRODUCTION. 9

and will long continue to absorb, the powers wliicli 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 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 traverse them, she does not
admit to be beyond her ken ; nor is she indifferent to the con-
stitution of the minutest atom of matter that thrills the ether
into light. How she entered upon this vastly expanded inherit-
ance, and how, so far, she has dealt with it, is attempted to be
set forth in the ensuing chapters.

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 byways 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
may be called the Founder of Sidereal Astronomy. Before his
time some curious facts had been noted, and some ingenious

CHAP. i. SIDEREAL ASTRONOMY. u

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 Homer fifty years previously. Thus the
stars were 110 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 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
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 extinc-
tion by a Dutch professor named Holwarda in I638-39. 4 From

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 obser-
vational error. Tobice Mayeri Up. Inedita, t. i., pp. 80-81, and Herschel in
Phil. Trans., vol. Ixxiii., pp. 275-278. 3 Posthumous Works, p. 701. 4 Arago
in Annuaire du Bureau des Longitudes, 1842, p. 313.

12 HISTORY OF ASTRONOMY. PARTI.

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 singu-
larly, but it was supposed accidentally, close juxtaposition, and
that the failure of repeated attempts to find an annual parallax
pointed to distances for the stars at least 400,000 times that of the
earth from the sun, 1 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,
NoverrJber 15, 1738. He was the fourth child of Isaac Herschel,
a hautboy-player in the band of the Hanoverian Guard, and was
early trained to follow his father's profession. On the termina-
tion, however, of the disastrous campaign of 1757, his parents
removed him from the regiment, there is reason to believe, in a
somewhat unceremonious manner. Technically, indeed, he in-
curred the penalties of desertion, remitted according to the
Duke of Sussex's statement to Sir George Airy by a formal
pardon handed to him personally by George III. on his presenta-

1 Bradley to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His observa-
tions were directly applicable to only two stars, y Draconis and t] Ursas
Majoris, but i5ome lesser ones were included in the same result.

CHAP. i. SIDEREAL ASTRONOMY. 13

tion in I782. 1 At the age of nineteen, then, his military service
having lasted four years, he came to England to seek his fortune.
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. Engage-
ments multiplied upon him. He became director of the public
concerts ; he conducted oratorios, engaged 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 professional 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 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;" 2 and to this sublime am-
bition he remained true until the end.

1 Holden, Sir William Herschel, his Life and Works, p. 17.

2 Phil. Trans., vol. ci., p. 269.

I 4 HISTORY OF ASTRONOMY. PARTI.

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, 1 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 him-
self provided with a reflecting telescope a 5 J-foot Gregorian
of his own construction. A copy of his first observation with
it, on the great Nebula in Orion an object of continual amaze-
ment and assiduous inquiry to him is preserved by the Royal
Society. It bears the date March 4, I774- 2

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 enthu-
siasm with which he delivered himself up to their guidance.
Overwhelmed with professional engagements, he still contrived
to snatch some moments for the stars ; and between the acts at
the theatre was often seen running from the harpsichord to his
telescope, no doubt with that " uncommon precipitancy which
accompanied all his actions." 3 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

1 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. 2 Holden, op. cit.
p. 39. 3 Memoir of Caroline fferschel, p. 37.

CHAP. i. SIDEREAL ASTRONOMY. 15

hands. He had entered upon his forty-second year when he
sent his first paper to the Philosophical Transactions; yet
during the ensuing thirty-nine years his contributions 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 bright-
ness. 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
attentively, 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 Eoyal
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. 1 On
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 discoverer. In
1788 he married Mary, only child of James Baldwin, a merchant
of the city of London, and widow of Mr. John Pitt a lady
whose domestic virtues were enhanced by the possession of a
large jointure. The fruit of their union was one son, of whose
work the worthy sequel of his father's we shall have to

1 See Holden's Sir William Herschel, p. 54.

16 HISTORY OF ASTRONOMY. PARTI.

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 Eoyal Astronomical Society, his son being
its first Foreign Secretary. Bjut 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 nebulae
the position they long continued to occupy, rather on imagina-
tive than scientific grounds, of "island universes," external to,
and co-equal with the Galaxy. Johann Heinrich Lambert, 3 a
tailor's apprentice from 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 creations.

1 An Original Theory or New Hypothesis of the Universe, London, 1750.
See also De Morgan's summary of his views in Philosophical Magazine, April
1848. 2 Allgemeine Naturgeschichte und Theorie des Himmels, 1755. U Cosmo-
logische Brief e, Augsburg, 1761.

CHAP. i. SIDEREAL ASTRONOMY. 17

" Thus everything revolves the earth around the sun ; the
sun around the centre of his system ; this system round a centre
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 ? ' ?1

The stupendous problem thus speculatively attempted, Herschel
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 advancing
spectator; 2 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, 3 within a few degrees of the spot indicated
by later and indefinitely more refined methods of research. He
resumed the subject in i8c>5, 4 but though employing a more
vigorous method was scarcely so happy in his result. In i8o6, 5

1 The System of the World, p. 125, London, 1800 (a translation of the last-
mentioned work). Lambert regarded nebulae as composed of stars crowded
together, but not as external universes. In the case of the Orion nebula, in-
deed, 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. - Opera Inedita, t. i., p. 79. 3 Phil Trans.,
vol. Ixxiii. (i783),p. 273. It is worthy of remark that Prevost, almost
simultaneously with Herschel, executed an investigation similar to his with
very considerable success. Klugel confirmed Herschel's result by an analytical
inquiry in 1789. 4 Phil. Trans., vol. xcv., p. 233. 5 Ibid., vol. xcvi., p. 205.

i8 HISTORY OF ASTRONOMY. PARTI.

lie made a preliminary attempt to ascertain the speed of the
sun's journey, which he no doubt greatly under-estimated in
fixing it at about three miles a second. Yet the validity of his
general conclusion as to the line of solar travel, though long
doubted, has been triumphantly confirmed. 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 nutation
of the earth's axis ; it was now about to lead Herschel to a
discovery of a different, but even more elevated character. 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 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 detection became an
object of earnest desire to those interested in maintaining them.
Copernicus himself made the attempt; but with his "Trique-
trum," 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 himself
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

1 "Ingens bolus devorandus est," Kepler admitted to Herwart in May
1603.

CHAP. i. SIDEREAL ASTRONOMY. 19

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, 1 proposed to apply
to 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 movements 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 Gregory in 1675, and again by
Wallis in i693; 2 Huygens first, 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 deter-
mination 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, 3 " 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 inevitably 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,

1 Opere, t. i., p. 415. 2 Phil Trans., vol. xvii., p. 848.
3 Phil. Trans., vol. Ixxii., p. 97.

20 HISTORY OF ASTRONOMY. PARTI.

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 exceptions) the direc-
tions with reference to a celestial meridian, 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 brought into fortuitous
contiguity by the chance of lying nearly in the same line of sight
from the earth. Yet Bradley had noticed a change of 30, be-
tween 1718 and 1759, in the position-angle of the two stars
forming Castor, and was thus within a hair's breadth of the dis-
covery of their physical connection. 1 While the Rev. John
Michell, arguing by the doctrine of probabilities, wrote as follows
in 1767: "It is highly probable in particular, 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." 2 And in 1784 : 3 " 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 Christia.n Mayer, a Jesuit
astronomer at Mannheim, set himself, in January 1776, to col-
lect examples of stellar pairs, and shortly afterwards published
the supposed discovery of " satellites " to many of the princi-
pal stars. 4 His observations, however, were neither exact nor
prolonged enough to lead to useful results in such an inquiry.

1 Doberck, Observatory, vol. ii., p. no. - Phil. Trans., vol. Ivii., p. 249.
3 Ibid., vol. Ixxiv., p. 56. 4 Beobachtungen von Fixsterntrdbanten, 1778;
and De Novis in Ccelo tiidereo Phcenomenis, 1779.

CHAP. i. SIDEREAL ASTRONOMY. 21

His disclosures were derided ; his planet-stars treated as results
of hallucination. On ria point cru a des choses aussi extraordi-
naires, wrote Lalande 1 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 colllection 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 1782, 2 "to
form any theories of small stars revolving round large ones ; "
while in the year following, 3 he remarked that the identical,
proper motions 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 Eoyal 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." 4
The fortunate preservation in Dr. Maskelyne's note-book of a
remark made by Bradley about 1759, to the effect that the line
joining the components of Castor was an exact prolongation of
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, 8 Serpentis, 375,

1 Bibliographic, p. 569. - Phil. Trans., vol. Ixxii., p. 162. 3 Ibid., vol.
Ixxiii., p. 272. 4 Phil. Trans., vol. xciii., p 340.

22 HISTORY OF ASTRONOMY. PART i.

g Bootis, 1 68 1 years ; c Lyrae was noted as a " double-double-star,"
a change of relative 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,
was described as having been exemplified in 1795 by the rapidly
circulating system of Herculis.

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 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, subordination,
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 con-
jecture 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 principle of

CHAP. i. SIDEREAL ASTRONOMY. 23

which maintained its credit in the literature of astronomy 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 produced by real extent. The pro-
cess of " gauging the heavens," accordingly, consisted in counting
the stars in successive 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

1 Phil. Trans., vol. Ixxv., p. 255.

24 HISTORY OF ASTRONOMY. PARTI.

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. 1 Thirteen years later, he described our sun
and his constellated companions as surrounded by "a magni-
ficent collection of innumerable stars, called the Milky Way,
which must occasion a very powerful balance of opposite attrac-
tions 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." ' 2

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." 3

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

1 Phil. Trans., vol. Ixxix., pp. 214, 222. - Hid., vol. xcii., pp. 479, 495.
a Ibid., vol. ci., p. 269.

CHAP. i. SIDEREAL ASTRONOMY. 25

principle he founded in 1817 his method of "limiting aper-
tures," 1 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 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 con-
fidence ; 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 recognised
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 marked
with dots on an old Dutch chart of the constellation, presumably
about 1500 A.D. 2 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. 3 A novelty,
nevertheless, to astronomers it still remained in 1656, when
Huygens discerned, "as it were, an hiatus in the sky, affording

1 Phil. Trans., vol. cvii., p. 311. 2 Bullialdus, De Nebulosd Stella in Cingulo
Andromedce (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). s Mathemata Astronomica, p. 75.

26 HISTORY OF ASTRONOMY. PARTI.

a glimpse of a more luminous region beyond." * Halley in 1716
knew of six nebulae, which he believed to be composed of a " lucid
medium " diffused through the ether of space. 2 He appears,
however, to have been unacquainted with some previously noticed
by Hevelius. Lacaille brought Iback with him from the Cape a list
of forty-two the first-fruits of observation in Southern skies
arranged in three numerically equal classes ; 3 and Messier
(nicknamed by Louis XV. the "ferret of comets"), finding-
such objects a source of extreme perplexity in the pursuit of
his chosen game, attempted to eliminate by methodising 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 lloyal Society a descriptive
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. Finding that his
potent instruments resolved into stars many nebulous patches in
which no signs of such a structure had previously been dis-
cernible, 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 connected 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 nucleus in
the centre, or like cloudy stars surrounded with a nebulous

1 8y 'sterna Saturnium, p. 9. " Phil. Trans., vol. xxix., p. 390. 3 Mem.
Ac. des Sciences, 1755. 4 Conn, des Temps, 1784 (pub. 1781), p. 227. A
previous list of forty-five had appeared in Mem. Ac. d. Sc. 1771.

CHAP. i. SIDEREAL ASTRONOMY. 27

atmosphere ; a different sort, again, contain a nebulosity of the
milky kind, like that wonderful, inexplicable phenomenon about
6 Orionis ; while others shine with a fainter, mottled kind of
light, which denotes their being resolvable into stars." *

"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 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 nebulas, due to the
"decay "of other "branching nebulas" similar to our own, he
recognised by the score, lying, as it were, stratified in certain
quarters of the sky. "One of these nebulous 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 nebulas, 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
nebulas 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 nebulas 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 as " very aged, and drawing on
towards a period of change or dissolution." 4

" This method of vie wing the heavens," he concluded, " seems
to throw them into a new kind of light. They now are seen to

1 Phil. Trans., vol. Ixxiv., p. 442. 2 Ibid., vol. Ixxix., p. 213. 3 Ibid.,
vol. Ixxv., p. 254. * Ibid., vol. Ixxix., p. 225.

28 HISTORY OF ASTRONOMY. PART r.

resemble a luxuriant garden which contains the greatest variety
of productions in different nourishing 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 germination, blooming, foliage,
fecundity, fading, withering, and corruption 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 ? " x

But already this supposed continuity was broken. After
mature deliberation on the phenomena presented by nebulous
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 king-
dom, 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, recollect-
ing 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 exceed-
ingly remote and inconceivably crowded, as being the occasion of
that remarkable appearance. 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 ipth cluster of my 6th class, and afterwards cast

1 Pldl. Trans., vol. Ixxix., p. 226.

CHAP. i. SIDEREAL ASTRONOMY. 29

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." x

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 mov-
ing 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 existence." 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 consist-
ency of a formal theory. He meditated upon it long and
earnestly, and in two elaborate treatises, published respect-
ively in 1811 and 1814, he at length set forth the arguments
in its favour. These rested entirely upon the " principle of con-
tinuity." Between the successive 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." 3 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, 4 to planetary nebulae, supposed to be already
centrally solid, instances were alleged by him of every stage and

1 Phil. Trans., vol. Ixxxi., p. 72. - Ibid., p. 85. 3 Ibid., vol. ci., p. 271.
i Ibid., p. 277.

30 HISTORY OF ASTRONOMY. PARTI.

phase of condensation. 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 themselves,
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 holds the field, the testimony of recent discoveries
with regard to it having proved strongly confirmatory of its
principle, although not of its details. Strangely enough, 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 incep-
tion, from 1791, while the French geometrician'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 constel-
lation 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 splen-
dour, and sometimes brilliantly contrasted in colour, were 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 variety and complexity seen to prevail, to
an extent previously undreamt of, in the arrangement of that

1 Sir J. Herschel, Phil. Trans., vol. cxvi., part iii., p. i.

CHAP. i. SIDEREAL ASTRONOMY. 31

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
inferred or observed to be everywhere in progress. Periodical
stars shone out and again decayed ; progressive ebbings or
flowings of light were indicated as probable in many stars under
no formal suspicion of variability ; forces were everywhere
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 regeneration 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," 1 "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 of its past and future existence ; and
although we do not know the rate of going of this mysterious
chronometer, it is nevertheless 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." 2

1 His own words to the poet Campbell, cited by Holden, Life and Works,
p. 109. 2 ' Phil. Tram., vol. civ., p. 283.

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 its advantage, 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 1 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 ; instrumental
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 Elamsteed and Bradley
had been the acknowledged centre of practical astronomy;
Greenwich observations were the standard of reference all over
Europe ; and the art of observing prospered in direct proportion

1 Bessel, Populdre Vorlesungen, pp. 6, 408.

CHAP. ii. SIDEREAL ASTRONOMY. 33

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
of fame ; he introduced at the Observatory the important inno-
vation 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. His catalogue of thirty-six " fundamental"
stars was besides excellent in its way, and most serviceable. Yet
he was devoid of Bradley's instinct for divining the needs of the
future. He was fitted rather to continue a tradition than to
found a school. The old ways were dear to him; and, inde-
fatigable 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 instru-
mental 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 deteriorated 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 revival 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 seconded by the

1 Fitted to the old transit instrument, July 11, 1772.
3

34 HISTORY OF ASTRONOMY. PARTI.

foundation, in 1804, by a young artillery officer named Von
Keichenbach, 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. Reflectors 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 furnish 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 inclination
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-depend-
ence, 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 indus-
trious 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 their climates, inhabitants, products, and the courses of
trade. He desired to add some acquaintance with the art (then

CHAP. ii. SIDEREAL ASTRONOMY. 35

much neglected) of taking observations at sea ; and thus, led on
from navigation to astronomy, and from astronomy to mathe-
matics, he groped his way into a new world.

It was characteristic of him that the practical problems of
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 Bode's Jahrbuch and Von
Zach's Monatliclie Correspondents, overcoming each difficulty as it
arose with the aid of Lalande's Traitt d' Astronomic, and supply-
ing, with amazing rapidity, his early deficiency in mathematical
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, an ^ 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, Bessel deduced from them an orbit for that
celebrated body, and presented the work to Olbers, whose repu-
tation 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 performance ; 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 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
1 Brieficechsel mit Olbers, p. xvi.

36 HISTORY OF ASTRONOMY. PART i.

as his profession. For two years lie continued to work in the
counting-house by day, and to pore over the Mteanique Cdestc
and the Differential Calculus by night. But the post of
assistant 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 imme-
diate, and not unwelcome prospect of comparative affluence lay
before him. The love of science, however, prevailed ; he chose
poverty and the stars, and went to Lilienthal 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 Bessel's
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 dis-
tinctive 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

1 R. Wolf, Gescli. der Astron., p. 518. - Bessel, Pop. For/., p. 22.

CHAP. ii. SIDEREAL ASTRONOMY. 37

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
Histoirc Cdlestc, published in 1801, the places of no less than
47.390 stars were given, but. in the rough, as it were, and con-
sequently 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 7600 stars the
second being a revision and enlargement of the first 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 atmo-
spheric 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 conditions of
pressure and temperature, a maximum at the horizon. Moreover,
the points to which measurements are referred 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 non-
instantaneous transmission of light, combined with the move-
ment 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

] Bessel, Pop. Vorl., p. 440.

38 HISTORY OF ASTRONOMY. PART i.

sure recognition of extremely minute quantities. In the early
years of this century, however, no uniform system of " reduc-
tion " (so the complete correction of observational results is
termed) had been established. .. Much was left to the individual
caprice 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,
sagacity, 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 deter-
mined, the proper motions of a 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 Tabulce Regiomontance. 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

CHAP. ii. SIDEREAL ASTRONOMY. 39

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 182133, with the
utmost nicety of care, the number of accurately known stars
was brought up to above $0,000, and an ample store of trust-
worthy 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 assistant
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, con-
stituting 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 issue.
Illusory results had for some time abounded. Piazzi in. 1 805 per-
ceived, as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon ; the truth being that his instru-
ments were worn out with constant use, and could no longer be de-
pended upon. 3 His countryman, Calandrelli, was similarly de-
luded. 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 rela-
tively large parallaxes for four of the brightest stars ; Pond, rely-

1 Durege, HesseVs Leben und Wirken, p. 28. '- Banner JBeobachtungen,
Bd. iii.-v., 1859-62. s Bessel, Pop. Vorl., p. 238.

40 HISTORY OF ASTRONOMY. PART i.

ing 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 par-
allaxes. Announcements of their detection had become so
frequent as to be discredited before they were disproved ; and
Struve, who investigated the subject at Dorpat in 1818-21. had
clearly shown that the quantities concerned 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 2 1st 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.
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 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, however, he rapidly extended his acquirement ; 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 Yon Reichenbach and Utzschneider. He
now zealously devoted himself to the improvement of the
achromatic telescope ; and, after a 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, g\ inches in diameter
and of fourteen feet focal length.

CHAP. ii. SIDEREAL ASTRONOMY. 41

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
ingenious improvements in mounting and fitting, it was adapted
to the finest micrometrical work, and thus offered unprecedented
facilities both for tlie 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 at 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 examina-
tion, 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 em-
ployed. 1

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 improve-
ments in reflectors as important as those he had already effected
in refractors ; and was besides eagerly occupied with investiga-
tions into the nature of light, the momentous character of

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 thousandth
part of a revolution, equivalent to ^ ff of a second of arc, can be measured with
the utmost accuracy. Main, in li. A. #. Mem., vol. xii., p. 53.

42 HISTORY OF ASTRONOMY. PARTI.

which we shall by-and-by 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 un-
wholesome toil, and, still hoping for its restoration from a pro-
jected journey to Italy, he died of consumption, June 7, 1826,
aged thirty-nine years. His tomb in Munich bears the concise
eulogy, Approximamt 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 neighbour 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 distance.
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 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 constella-
tion of the Swan. Still more emphatically in i8i2 2 Bessel drew
the attention of astronomers 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 3^ 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

1 Specola Astronomica di Palermo, lib. vi., p. 10, note. ' 2 Monatlictie Corre-
spondent, vol. xxvi., p. 162.

CHAP. ii. SIDEREAL ASTRONOMY. 43

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 instru-
ments and the vicinity to their selected stars of minute, physi-
cally 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 (O.3I36"). 1 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. 2 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 celestial space, having really and indubitably touched
bottom. It was confirmed in 1842-43 with curious exactness by
C. A. F. Peters at Pulkowa ; but later researches showed that it
required increase to nearly half a second. 3

Struve's measurements inspired less confidence. They ex-
tended over three years (1835-38), but were comparatively few,
and were frequently interrupted. The parallax, accordingly, of
about a quarter of a second (0.26 13") which he derived from
them for a Lyrge, and announced in 1840,* has proved consider-
ably too large. 5

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 dis-
tant 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

1 Astronomische NachricJiten, Nos. 365-366. It should be explained that
which is called the "annual parallax " of a star is only half its apparent dis-
placement. In other words, it is the angle subtended at the distance of that
particular star by the radius of the earth's orbit. 2 Astr. Nach., Nos. 401-
402. 3 Sir E. Ball's measurements at Dun sink give to 61 Cygni a parallax
of 0.47" ; Professor Pritchard has obtained, by photographic determinations,
one of 0.43". * Additamentum in Mensuras Micrometricas, p. 28. 5 Elkin's
corrected result (in 1892) for the parallax of Vega is 0.092".

44 HISTORY OF ASTRONOMY. PARTI.

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 during the thirteen
months of his tenure- of office 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 in-
formed of its very considerable proper motion (3.6" annually), he
resolved to examine the observations already made for possible
traces of paral lactic displacement. This was done on his return
to Scotland, where he filled the office of Astronomer Koyal 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,
1882-1883, 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' determinations
of the same star's right ascension. 1 When at last, January 9, 1 839,
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 parallax of 61 Cygni by just
two months.

Thus from three different quarters, three successful and
almost simultaneous assaults were delivered upon a long-
beleagured 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 shifting from
the revolution of the earth in its orbit. In more than fifty 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

] Mem. Roy. Astr. Soc., vol. xi., p. 61.

CHAP. ii. SIDEREAL ASTRONOMY. 45

Great Bear ; l yet the vicinity of this minute object is so much
greater than that of the brilliant Vega, that the latter trans-
ported to its place would increase in lustre thirty times.
Moreover, many of the brightest stars are found to have no
sensible parallax, while the majority of those ascertained to be
nearest to the earth are of fifth, sixth, even ninth magnitudes.
The obvious conclusions 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. Thus, the splendid Canopus, Arcturus,
and Rigel, can be inferred, from their indefinite remoteness, to
exceed our sun thousands of times in size and lustre ; while
many inconspicuous objects, which prove to be in our relative
vicinity, must be notably his inferiors. The limits of real stellar
magnitude are then set very widely apart. 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 measurable
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

1 That numbered 21,185 in Lalande's Hist. Gel., found by Argelander to
have a proper motion of 4.734", and by Winnecke a parallax of 0.51 1". Month.
Not., vol. xviii., p. 289.

46 HISTORY OF ASTRONOMY. PARTI.

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 a
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
ascertained, the seconds of arc apparently separating them from
each other can be 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 Dr.
Doberck's elements the two stars forming a Centauri revolve
round their common centre of gravity at a mean distance nearly
25 times the radius of the earth's orbit, in a period of 88 years,
the attractive force of the two together must be just 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 Neptune and Uranus. But systems of
still more majestic proportions are reduced by extreme remote-
ness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is
ascribed on the authority of 0. Struve, appears to be more than
nine times as massive as the central orb of our world ; while a
much less conspicuous pair 85 Pegasi exerts, if the available
data can be depended upon, fully thirteen times the solar
gravitating power.

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
IOOO, and that of a Centauri to 446 millions of miles. A small
star, numbered 1830 in Groombridge's Circumpolar Catalogue,
" devours the way" at the rate of 230 miles a second a speed,
in Newcomb's opinion, beyond the gravitating power of the

CHAP. ii. SIDEREAL ASTRONOMY. 47

entire sidereal system to control; and Toucange possesses,
according to Dr. Gill, nearly 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 i837 2 changed the
aspect of the question. Herschel's first memorable solution in
1783 was based upon the motions of thirteen stars, imperfectly
known; his second, in 1805, upon 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 i847 3 by Galloway's investigation, founded
exclusively on the apparent displacements of southern stars.
In 1859 and 1863, the late Sir George Airy and Mr. Dunkin, 4
employing all the resources of modern science, and commanding
the wealth of material furnished by 1167 proper motions care-
fully determined by Mr. Main, reached conclusions closely
similar to that indicated nearly eighty years previously by the
first great sidereal astronomer ; which Mr. Plummer's reinvesti-
gation of the subject in i883 5 served but slightly to modify.
Yet astronomers were not satisfied. Dr. Auwers of Berlin
finished in 1886 a splendid piece of work, 6 for which he received
in 1888 the Gold Medal of the Koyal Astronomical Society.
It consisted in reducing afresh, with the aid of the most
refined modern data, Bradley's original stars, and comparing

1 Fund. Astr., p. 309. - Mem. Pres. a ?Ac. de St. Peter si., t. iii. 3 Phil.
Trans., vol. cxxxvii., p. 79. 4 Mem. Roy. Astr. Soc., vols. xxviii. and xxxii.
"' Ibid., vol. xlvii., p. 327. 6 Not yet completely published (1893).

48 HISTORY OF ASTRONOMY. PARTI.

their places thus obtained for the year 1755 with those assigned
to them from observations made at Greenwich after the lapse of
ninety years,. In the interval, as was to be anticipated, most of
them were found to have travelled over some small span of the
heavens, and there resulted a stock of nearly three thousand
highly authentic proper motions. These ample materials were
turned to account by M. Ludwig Struve 1 for a discussion of the
sun's motion, of which the upshot was to shift its point of aim
to the bordering region of the constellations Hercules and Lyra.
Still later, Professor Lewis Boss of Albany, N.Y., 2 and M. Oscar
Stumpe of Bonn, 3 made similar experiments with variously
assorted lists of stars, by which the more easterly position of
the solar apex was fully confirmed. The brilliant Vega may
indeed be said to mark a centre round which newly determined
apexes tend loosely to group themselves. 4 The general direc-
tion of the solar movement may nevertheless be regarded as
fairly well known ; but as to its rate, the grounds of inference
long remained unsatisfactory. Otto Struve's estimate of 154
million miles a year is based upon the assumption of an average
annual parallax, for stars of the first magnitude, of about a
quarter of a second ; an assumption negatived by actual experi-
ence. Fortunately, however, as will be seen further on, a method
of determining the sun's velocity independently of any knowledge
of star-distances is now rapidly becoming available.

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 enquiry, Kant was disposed
to regard Sirius as the '-'central sun" of the Milky Way ; while
Lambert surmised that the vast Orion nebula might serve as the
regulating power of a subordinate group including our sun.

1 Memoires de St. Petersburg, t. xxxv., No. 3, 1887. ~ Astronomical
Journal, No. 213. 3 Astr. Nach., Nos. 2999, 3000. 4 Cf. Month. Notices +
vol. li., p. 243 ; Nature, vol. xliv., p. 572.

CHAP. ii. SIDEREAL ASTRONOMY. 49

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 Bogus-
lawski 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 prepon-
derating 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 presence of a
gigantic (though possibly obscure) ruling body ; but that a
community of extremely sluggish movements undoubtedly
existed in and near the group of the Pleiades, where, accord-
ingly, 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 situa-
tion 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, implying, 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.

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

1 Phil. Trans., vol. xcvi., p. 230. 2 Mem. Pris. a I' Ac. de St. Petersboura,
t. iii., p. 603 (read Feb. 5, 1837). 3 Die Centralsonne, 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.

4

50 HISTORY OF ASTRONOMY PARTI.

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 irregu-
larities 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 conviction," 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 countless 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 explained by an
orbital revolution in a period of fifty years. Bessel's prevision
was destined to be still more triumphantly vindicated. On the
3 1st of January 1862, while in the act of trying a new 1 8-inch
refractor, Mr. Alvan G. Clark (one of the celebrated firm, of
American opticians) actually discovered the hypothetical Sirian

1 Madler in Westmnann's Jahrbuch, 1867, p. 615. ~ Letter from Bessel to
Sir J. Herschel, Month. Xot., vol. vi., p. 139. 3 Wolf, Gesch. d, Astr.
p. 743, note. 4 Astr. Nach., Nos. 745-748.

-CHAP. ii. SIDEREAL ASTRONOMY. 51

companion in the precise position required by theory. It has
now been watched through three-fifths of a revolution (period
49.4 years), and proves to be very slightly luminous in proportion
to its mass. Its attractive power, in fact, is nearly half that of
its primary, while it emits only 1 1 p-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 two suns, but shines like sixty-three. Possibly it
is enormously 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 verification
is still wanting, but to the mental eye the presence of a con-
siderable disturbing mass was fully assured through the inquiry
instituted by Auwers in I862. 1 The period of forty years then
.assigned to the system appears confirmed by recent observations.

But Bessel was not destined to witness the recognition of
"the in visible "as