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A Popular History of Astronomy During the Nineteenth Century

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Title: A Popular History of Astronomy During the Nineteenth Century

Author: Agnes M. Clerke


        
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A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY

       *       *       *       *       *

BY THE SAME AUTHOR

  PROBLEMS IN ASTROPHYSICS.
     Demy 8vo., cloth. Containing over 100
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  THE SYSTEM OF THE STARS.
     Second Edition. Thoroughly revised and
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   A. AND C. BLACK, SOHO SQUARE, LONDON, W.

       *       *       *       *       *

[Illustration: THE GREAT NEBULA IN ORION, 1883

_See p. 408_]


A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY

by

AGNES M. CLERKE







[Illustration: JUPITER 1879

               SATURN 1885]



London
Adam and Charles Black
1908

First Edition, Post 8vo., published 1885
Second Edition, Post 8vo., published 1887
Third Edition, Demy 8vo., published 1893
Fourth Edition, Demy 8vo., published 1902
Fourth Edition, Post 8vo., reprinted February, 1908




PREFACE TO THE FOURTH EDITION

Since the third edition of the present work issued from the press, the
nineteenth century has run its course and finished its record. A new era
has dawned, not by chronological prescription alone, but to the vital
sense of humanity. Novel thoughts are rife; fresh impulses stir the
nations; the soughing of the wind of progress strikes every ear. "The
old order changeth" more and more swiftly as mental activity becomes
intensified. Already many of the scientific doctrines implicitly
accepted fifteen years ago begin to wear a superannuated aspect.
Dalton's atoms are in process of disintegration; Kirchhoff's theorem
visibly needs to be modified; Clerk Maxwell's medium no longer figures
as an indispensable factotum; "absolute zero" is known to be situated on
an asymptote to the curve of cold. Ideas, in short, have all at once
become plastic, and none more completely so than those relating to
astronomy. The physics of the heavenly bodies, indeed, finds its best
opportunities in unlooked-for disclosures; for it deals with
transcendental conditions, and what is strange to terrestrial experience
may serve admirably to expound what is normal in the skies. In celestial
science especially, facts that appear subversive are often the most
illuminative, and the prospect of its advance widens and brightens with
each divagation enforced or permitted from the strait paths of rigid
theory.

This readiness for innovation has undoubtedly its dangers and drawbacks.
To the historian, above all, it presents frequent occasions of
embarrassment. The writing of history is a strongly selective operation,
the outcome being valuable just in so far as the choice what to reject
and what to include has been judicious; and the task is no light one of
discriminating between barren speculations and ideas pregnant with
coming truth. To the possession of such prescience of the future as
would be needed to do this effectually I can lay no claim; but diligence
and sobriety of thought are ordinarily within reach, and these I shall
have exercised to good purpose if I have succeeded in rendering the
fourth edition of _A Popular History of Astronomy during the Nineteenth
Century_ not wholly unworthy of a place in the scientific literature of
the twentieth century.

My thanks are due to Sir David Gill for the use of his photograph of the
great comet of 1901, which I have added to my list of illustrations, and
to the Council of the Royal Astronomical Society for the loan of glass
positives needed for the reproduction of those included in the third
edition.

London, _July_, 1902.



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 circumstance 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 realize (so far as it can at present be realized) the full effect
of the comprehensive change in the whole aspect, purposes, and methods
of celestial science introduced by the momentous discovery of spectrum
analysis.

Since Professor Grant's invaluable work on the _History of Physical
Astronomy_ was published, a third of a century has elapsed. During the
interval a so-called "new astronomy" has grown up by the side of the
old. One effect of its advent has been to render the science of the
heavenly bodies more popular, both in its needs and in its nature, than
formerly. More popular in its needs, since its progress now primarily
depends upon the interest in, and consequent efforts towards its
advancement of the general public; more popular in its nature, because
the kind of knowledge it now chiefly tends to accumulate is more easily
intelligible--less remote from ordinary 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 inconsiderable 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 dependence upon
intricate, and, except to the initiated, unintelligible formulæ. This
kind of knowledge forms the main subject of the book now offered to the
public.

There are many reasons for preferring a history to a formal treatise on
astronomy. In a treatise, _what_ we know is set forth. A history tells
us, in addition, _how_ we came to know it. It thus places facts before
us in the natural order of their ascertainment, and narrates instead of
enumerating. The story 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 separate chapters the various
events relating to the several departments 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
discoveries 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 consistency 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 uninterrupted
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 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 and 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 communications.

London, _September_, 1885.




CONTENTS

                             _INTRODUCTION_

                                                                      page
  Three Kinds of Astronomy--Progress of the Science during the
  Eighteenth Century--Popularity and Rapid Advance during the Nineteenth
  Century                                                                1


                                 PART I

  _PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY_


                                CHAPTER I

                    FOUNDATION OF SIDEREAL ASTRONOMY

  State of Knowledge regarding the Stars in the Eighteenth Century--
  Career of Sir William Herschel--Constitution of the Stellar System--
  Double Stars--Herschel's Discovery of their Revolutions--
  His Method of Star-gauging--Discoveries of Nebulæ--Theory of their
  Condensation into Stars--Summary of Results                            9


                                CHAPTER II

                     PROGRESS OF SIDEREAL ASTRONOMY

  Exact Astronomy in Germany--Career of Bessel--His _Fundamenta
  Astronomiæ_--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--Fifty Years' Progress                     27


                                CHAPTER III

                  PROGRESS OF KNOWLEDGE REGARDING THE SUN

  Early Views as to the Nature of Sun-spots--Wilson's Observations and
  Reasonings--Sir William Herschel's Theory of the Solar
  Constitution--Sir John Herschel's Trade-Wind Hypothesis--Baily's
  Beads--Total Solar Eclipse of 1842--Corona and Prominences--Eclipse of
  1851                                                                  52


                                CHAPTER IV

                          PLANETARY DISCOVERIES

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                           71


                                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                                                             88


                                CHAPTER VI

                           INSTRUMENTAL 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                                           108



                                  PART II

                       _RECENT PROGRESS OF ASTRONOMY_


                                 CHAPTER I

                     FOUNDATION OF ASTRONOMICAL PHYSICS

  Schwabe's Discovery of a Decennial Sun-spot Period--Coincidence with
  Period of Magnetic Disturbance--Sun-spots and Weather--Spectrum
  Analysis--Preliminary Inquiries--Fraunhofer Lines--Kirchhoff's
  Principle--Anticipations--Elementary Principles of Spectrum
  Analysis--Unity of Nature                                            125


                                CHAPTER II

                     SOLAR OBSERVATIONS AND THEORIES

  Black Openings in Spots--Carrington's Observations--Rotation of the
  Sun--Kirchhoff's Theory of the Solar Constitution--Faye's Views--Solar
  Photography--Kew Observations--Spectroscopic Method--Cyclonic Theory
  of Sun-spots--Volcanic Hypothesis--A Solar Outburst--Sun-spot
  Periodicity--Planetary Influence--Structure of the Photosphere       143


                                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 Coronal Types--Egyptian
  Eclipse--Daylight Coronal Photography--Observations at Caroline
  Island--Photographs of Corona in 1886 and 1889--Eclipses of 1896,
  1898, 1900, and 1901--Mechanical Theory of Corona--Electro-Magnetic
  Theories--Nature of Corona                                           166


                                CHAPTER IV

                            SOLAR SPECTROSCOPY

  Chemistry of Prominences--Study of their Forms--Two
  Classes--Photographs and Spectrographs of Prominences--Their
  Distribution--Structure of the Chromosphere--Spectroscopic Measurement
  of Radial Movements--Spectroscopic Determination of Solar
  Rotation--Velocities of Transport in the Sun--Lockyer's Theory of
  Dissociation--Solar Constituents--Oxygen Absorption in Solar
  Spectrum                                                             194


                                CHAPTER V

                          TEMPERATURE OF THE SUN

  Thermal Power of the Sun--Radiation and Temperature--Estimates of
  Solar Temperature--Rosetti's and Wilson's Results--Zöllner's Method
  --Langley's Experiment at Pittsburg--The Sun's Atmosphere--Langley's
  Bolometric Researches--Selective Absorption by our Air--The Solar
  Constant                                                             216


                                CHAPTER 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 Result                                   227


                                CHAPTER VII

                           PLANETS AND SATELLITES

  Schröter's Life and Work--Luminous Appearances during Transits of
  Mercury--Mountains of Mercury--Intra-Mercurian Planets--Schiaparelli's
  Results for the Rotation of Mercury and Venus--Illusory
  Satellite--Mountains and Atmosphere of Venus--Ashen Light--Solidity of
  the Earth--Variation of Latitude--Secular Changes of Climate--Figure
  of the Globe--Study of the Moon's Surface--Lunar Atmosphere--New
  Craters--Thermal Energy of Moonlight--Tidal Friction                 243


                                CHAPTER VIII

                     PLANETS AND SATELLITES--(_continued_)

  Analogy between Mars and the Earth--Martian Snowcaps, Seas, and
  Continents--Climate and Atmosphere--Schiaparelli's Canals--Discovery
  of Two Martian Satellites--Photographic Detection of Minor
  Planets--Orbit of Eros--Distribution of the Minor Planets--Their
  Collective Mass and Estimated Diameters--Condition of Jupiter--His
  Spectrum--Transits of his Satellites--Discovery of a Fifth
  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                                                              274


                                CHAPTER IX

                      THEORIES OF PLANETARY EVOLUTION

  Origin of the World according to Kant--Laplace's Nebular
  Hypothesis--Maintenance of the Sun's Heat--Meteoric
  Hypothesis--Radiation as an Effect of Contraction--Regenerative
  Theory--Faye's Scheme of Planetary Development--Origin of the
  Moon--Effects of Tidal Friction                                      308


                                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--Holmes's Comet--Deflection
  of the Leonids--Orbits of Meteorites--Meteors with Stationary
  Radiants--Spectroscopic Analysis of Cometary Light--Comet of
  1901--Coggia's Comet                                                 323


                                CHAPTER XI

                        RECENT COMETS--(_continued_)

  Forms of Comets' Tails--Electrical Repulsion--Brédikhine'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
  1880--Cometary Systems--Spectral Changes in Comet of 1882--Brooks's
  Comet of 1889--Swift's Comet of 1892--Origin of Comets               345


                                CHAPTER XII

                             STARS AND NEBULÆ

  Stellar Chemistry--Four Orders of Stars--Their Relative Ages--Gaseous
  Stars--Spectroscopic Star-Catalogues--Stellar Chemistry--Hydrogen
  Spectrum in Stars--The Draper Catalogue--Velocities of Stars in Line
  of Sight--Spectroscopic Binaries--Eclipses of Algol--Catalogues of
  Variables--New Stars--Outbursts in Nebulæ--Nova Aurigæ--Nova
  Persei--Gaseous Nebulæ--Variable Nebulæ--Movements of Nebulæ--Stellar
  and Nebular Photography--Nebulæ in the Pleiades--Photographic
  Star-charting--Stellar Parallax--Double Stars--Stellar
  Photometry--Status of Nebulæ--Photographs and Drawings of the Milky
  Way--Star Drift                                                      372


                                CHAPTER XIII

                             METHODS OF RESEARCH

  Development of Telescopic Power--Silvered Glass Reflectors--Giant
  Refractors--Comparison with Reflectors--The Yerkes
  Telescope--Atmospheric Disturbance--The Lick Observatory--Mechanical
  Difficulties--The Equatoreal _Coudé_--The Photographic
  Camera--Retrospect and Conclusion                                    428


                                APPENDIX

  Chronology, 1774-1893--Chemical Elements in the Sun (Rowland,
  1891)--Epochs of Sun-spot Maximum and Minimum from 1610 to
  1901--Movements of Sun and Stars--List of Great Telescopes--List of
  Observatories employed in the Construction of the Photographic Chart
  and Catalogue of the Heavens                                         443


  INDEX                                                                471


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. 198_

  Plate II. Photograph of the Great Comet of May,
    1901 (Taken at the Royal Observatory, Cape
    of Good Hope)                                          "      343

  Plate III. The Great Comet of September, 1882
    (Photographed at the Cape of Good Hope)                "      359

  Plate IV. Photographs of Swift's Comet, 1892             "      368

  Plate V. Photographic and Visual Spectrum of
    Nova Aurigæ                                            "      396

  Plate VI. Photograph of the Milky Way in Sagittarius     "      424




HISTORY OF ASTRONOMY DURING 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 dormitiva_. This first and oldest
branch is known as "observational," or "practical astronomy." Its
business is to note facts as accurately as possible; and it is
essentially unconcerned with schemes for connecting those facts in a
manner satisfactory to the reason.

The second kind of astronomy was founded by Newton. Its nature is best
indicated by the term "gravitational"; but it is also called
"theoretical astronomy."[1] It is based on the idea of cause; and the
whole of its elaborate structure is reared according to the dictates of
a single law, simple in itself, but the tangled web of whose
consequences can be unravelled only by the subtle agency of an elaborate
calculus.

The third and last division of celestial science may properly be termed
"physical and descriptive astronomy." It seeks to know what the heavenly
bodies are in themselves, leaving the How? and the Wherefore? of their
movements to be otherwise answered. Now, such inquiries became possible
only through the invention of the telescope, so that Galileo was, in
point of fact, their originator. But Herschel first gave them a
prominence which the whole progress of science during the nineteenth
century 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 accomplishment 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 as it was then known, was found to be complete and
intelligible in all its parts; and in the _Mécanique Céleste_ its
mechanical perfections were displayed under a form of majestic unity
which fitly commemorated the successive triumphs of analytical genius
over problems amongst the most arduous ever dealt with by the mind of
man.

Theory, however, demands a practical test. All its data are derived from
observation; and their insecurity becomes less tolerable as it advances
nearer to perfection. Observation, on the other hand, is the pitiless
critic of theory; it detects weak points, and provokes reforms which may
be the beginnings of discovery. Thus, theory and observation mutually
act and react, each alternately taking the lead in the endless race of
improvement.

Now, while in France Lagrange and Laplace were bringing the
gravitational theory of the solar system to completion, work of a very
different kind, yet not less indispensable to the future welfare of
astronomy, was being done in England. The Royal Observatory at Greenwich
is one of the few useful institutions which date their origin from the
reign of Charles II. The leading position which it still occupies in the
science of celestial observation was, for near a century and a half
after its foundation, an exclusive one. Delambre remarked that, had all
other materials of the kind been destroyed, the Greenwich records alone
would suffice for the restoration of astronomy. The establishment was
indeed absolutely without a rival.[2] 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; 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 produced 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
nearly nineteen years.

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-Royal in 1742, he executed during the
years 1750-62 a series of observations which formed the real beginning
of exact astronomy. Part of their superiority must, indeed, be
attributed to the co-operation of John Bird, who provided Bradley in
1750 with a measuring instrument of till then unequalled excellence. For
not only was the art of observing in the eighteenth century a peculiarly
English art, but the means of observing were furnished almost
exclusively by British artists. John Dollond, the son of a Spitalfields
weaver, invented the achromatic lens in 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,
Ramsden, and Cary were inimitable by Continental workmanship.

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 portentous 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 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 full of unlooked-for possibilities.

Further popularity accrued to the science from the sequel of a career so
strikingly opened. Herschel's huge telescopes, his detection by their
means of two Saturnian and as many Uranian moons, his piercing scrutiny
of the sun, picturesque theory of its constitution, and sagacious
indication of the route pursued by it through space; his discovery of
stellar revolving systems, his bold soundings of the universe, his
grandiose ideas, and the elevated yet simple language in which they were
conveyed--formed a combination powerfully effective to those least
susceptible of new impressions. Nor was the evoked enthusiasm limited to
the British Isles. In Germany, Schröter followed--_longo intervallo_--in
Herschel's track. Von Zach set on foot from Gotha that general
communication of ideas which gives life to a forward movement. Bode
wrote much and well for unlearned readers. Lalande, by his popular
lectures and treatises, helped to form an audience which Laplace himself
did not disdain to address in the _Exposition du Système du Monde_.

This great accession of public interest 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.

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 undiscovered between the orbits of Mars and Jupiter. The
Astronomical Society of London was established in 1820, and the similar
German institution in 1863. Both have been highly influential in
promoting the interests, local and general, of the science they are
devoted to forward; while functions corresponding to theirs have been
discharged elsewhere by older or less specially constituted bodies, and
new ones 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 invaluable to an observer of the
skies. With the help of a wire, a battery, and a code of signals, he
sees whatever is visible from any portion of our globe, depending,
however, upon other eyes than his own, and so entering as a unit into a
widespread organisation of intelligence. The press, again, has been a
potent agent of co-operation. It has mainly contributed to unite
astronomers all over the world into a body animated by the single aim of
collecting "particulars" in their special branch for what Bacon termed a
History of Nature, eventually to be interpreted according to the
sagacious insight of some one among them gifted above his fellows. The
first really effective astronomical periodical was the _Monatliche
Correspondenz_, started by Von Zach in the year 1800. It was followed in
1822 by the _Astronomische Nachrichten_, later by the _Memoirs_ and
_Monthly Notices_ of the Astronomical Society, and by the host of varied
publications which now, in every civilised country, communicate the
discoveries made in astronomy to divers classes of readers, and so
incalculably quicken the current of its onward flow.

Public favour brings in its train material resources. It is represented
by individual enterprise, and finds expression in an ample liberality.
The first regular observatory in the Southern Hemisphere was founded at
Paramatta by Sir Thomas Makdougall Brisbane in 1821. The Royal
Observatory at the Cape of Good Hope was completed in 1829. Similar
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 1843 the lectures on celestial phenomena of Ormsby
MacKnight Mitchel stirred an impressionable audience to the pitch of
providing him with the means of erecting at Cincinnati the first
astronomical establishment worthy the name in that great country. On the
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 gave 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. The United States Naval
Observatory was built in 1844, Lieutenant Maury being its first
Director. Corporations, universities, municipalities, vied with each
other in the creation of such institutions; private subscriptions poured
in; emissaries were sent to Europe to purchase instruments and to
procure instruction in their use. In a few years the young Republic 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 cultivation which our age
has witnessed. In the last century its purview was a comparatively
narrow one. Problems lying beyond the range of the solar system were
almost unheeded, because they seemed inscrutable. Herschel first showed
the sidereal universe as accessible to investigation, and thereby
offered to science new worlds--majestic, manifold, "infinitely infinite"
to our apprehension in number, variety, and extent--for future conquest.
Their gradual appropriation has absorbed, and will long continue to
absorb, the powers which it has served to develop.

But this is not the only direction in which astronomy has enlarged, or
rather has levelled, its boundaries. The unification of the physical
sciences is perhaps the greatest intellectual feat of recent times. The
process has included astronomy; so that, like Bacon, she may now be said
to have "taken all knowledge" (of that kind) "for her province." In
return, she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale for our
benefit. The sun, stars, and nebulæ 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 solar spectrum
will perhaps one day, by its recurrent modifications, tell us something
of impending droughts, famines, and cyclones.

Astronomy generalises the results of the other sciences. She exhibits
the laws of Nature working over a wider area, and under more varied
conditions, than ordinary experience presents. Ordinary experience, on
the other hand, has become indispensable to her progress. She takes in
at one view the indefinitely great and the indefinitely little. The
mutual revolutions of the stellar multitude during tracts of time which
seem to lengthen out to eternity as the mind attempts to traverse them,
she does not admit to be beyond her ken; nor is she indifferent to the
constitution of the minutest atom of matter that thrills the ether into
light. How she entered upon this vastly expanded inheritance, and how,
so far, she has dealt with it, is attempted to be set forth in the
ensuing chapters.


FOOTNOTES:

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

[Footnote 2: _Histoire de l'Astronomie au xviii^e Siècle_, p. 267.]




                                 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 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[3] 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,[4] founded on star-places determined by Olaus Römer fifty years
previously. Thus the stars were no longer regarded as "fixed," but the
question remained whether the movements perceived were real or only
apparent; and this it was not yet found possible to answer. Already, in
the previous century, the ingenious Robert Hooke had suggested an
"alteration of the very system of the sun,"[5] 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 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star of the
third magnitude, which by October had disappeared. It was, nevertheless,
visible in 1603, when Bayer marked it in his catalogue with the Greek
letter Omicron, and was watched, in 1638-39, through its phases of
brightening and apparent extinction by a Dutch professor named
Holwarda.[6] From Hevelius this first-known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud in 1667 fixed the length
of its cycle of change at 334 days. It was not a solitary instance. A
star in the Swan was perceived by Janson in 1600 to show fluctuations of
light, and Montanari found in 1669 that Algol in Perseus shared the same
peculiarity to a marked degree. Altogether the class embraced in 1782
half-a-dozen members. When it is added that a few star-couples had been
noted in singularly, but it was supposed accidentally, close
juxtaposition, and that the failure of repeated attempts to measure
stellar parallaxes pointed to distances _at least_ 400,000 times that of
the earth from the sun,[7] 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 these scantily
collected facts ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They needed to be
both multiplied and marshalled, and it seemed as if centuries of patient
toil must elapse before any reliable conclusions could be derived from
them. The sidereal world was thus the recognised domain of far-reaching
speculations, which remained wholly uncramped by systematic research
until Herschel entered upon his career as an observer of the heavens.

The greatest of modern astronomers was born at Hanover, November 15,
1738. He was the fourth child of Isaac Herschel, a hautboy-player in the
band of the Hanoverian Guard, and was early trained to follow his
father's profession. On the termination, however, of the disastrous
campaign of 1757, his parents removed him from the regiment, there is
reason to believe, in a somewhat unceremonious manner. Technically,
indeed, he incurred 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 presentation in 1782.[8]
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. In the
following year he removed to Bath as oboist in Linley's orchestra, and
in October 1767 was promoted to the post of organist in the Octagon
Chapel. The tide of prosperity now began to flow for him. The most
brilliant and modish society in England was at that time to be met at
Bath, and the young Hanoverian quickly found himself a favourite and the
fashion in it. Engagements multiplied upon him. He became director of
the public concerts; he conducted oratorios, 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 1772 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";[9] and this sublime
ambition he cherished to the end.

A more powerful instrument was the first desideratum; and here his
mechanical genius came to his aid. Having purchased the apparatus of a
Quaker optician, he set about the manufacture 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,[10] and his mind amused by her reading aloud to him the Arabian
Nights, Don Quixote, or other light works. At length, after repeated
failures, he found himself provided with a reflecting telescope--a
5-1/2-foot Gregorian--of his own construction. A copy of his first
observation with it, on the great Nebula in Orion--an object of
continual amazement and assiduous inquiry to him--is preserved by the
Royal Society. It bears the date March 4, 1774.[11]

In the following year he executed his first "review of the heavens,"
memorable chiefly as an evidence of the grand and novel conceptions
which already inspired him, and of the enthusiasm with which he
delivered himself up to their 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."[12] He now rapidly increased the
power and perfection of his telescopes. Mirrors of seven, ten, even
twenty feet focal length, were successively completed, and unprecedented
magnifying powers employed. His energy was unceasing, his perseverance
indomitable. In the course of twenty-one years no less than 430
parabolic specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the _Philosophical 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 2,500 nebulæ,
806 double stars, passed the whole firmament in review four several
times, counted the stars in 3,400 "gauge-fields," and executed a
photometric classification of the principal stars, founded on an
elaborate (and the first systematically conducted) investigation of
their relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure accuracy in
his observations; yet in one night he would examine, singly and
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 transformed into an eminent astronomer; he was
relieved from the drudgery of a toilsome profession, and installed as
Royal Astronomer, with a modest salary of £200 a year; funds were
provided for the construction of the forty-foot reflector, from the
great space-penetrating power of which he expected unheard-of
revelations; in fine, his future work was not only rendered possible,
but it was stamped as authoritative.[13] 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 speak further on. Herschel was created a Knight of the
Hanoverian Guelphic Order in 1816, and in 1821 he became the first
President of the Royal Astronomical Society, his son being its first
Foreign Secretary. But his health had now for some years been failing,
and on August 25, 1822, he died at Slough, in the eighty-fourth year of
his age, and was buried in Upton churchyard.

His epitaph claims for him the lofty praise of having "burst the
barriers of heaven." Let us see in what sense this is true.

The first to form any definite idea as to the constitution of the
stellar system was Thomas Wright, the son of a carpenter living at
Byer's Green, near Durham. With him originated what has been called the
"Grindstone Theory" of the universe, which regarded the Milky Way as the
projection on the sphere of a stratum or disc of stars (our sun
occupying a position near the centre), similar in magnitude and
distribution to the lucid orbs of the constellations.[14] He was
followed by Kant,[15] who transcended the views of his predecessor by
assigning to nebulæ the position they long continued to occupy, rather
on imaginative than scientific grounds, of "island universes," external
to, and co-equal with, the Galaxy. Johann Heinrich Lambert,[16] a
tailor's apprentice from Mühlhausen, 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.

"Thus everything revolves--the earth round the sun; the sun round 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?"[17]

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;[18] 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
Lambda in the constellation Hercules,[19] within a few degrees of
the spot indicated by later and indefinitely more refined methods of
research. He resumed the subject in 1805,[20] but though employing a
more rigorous method, was scarcely so happy in his result. In 1806,[21]
he made a preliminary attempt to ascertain the speed of the sun's
journey, fixing it, by doubtless much too low an estimate, 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 Copernician 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.[22] 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
"Triquetrum," a jointed wooden rule with the divisions marked in ink,
constructed by himself,[23] 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
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,[24] proposed to employ for the determination of
stellar parallax; for two stars, lying apparently close together, but in
reality separated by a great gulf of space, must shift their mutual
positions when observed from opposite points of the earth's orbit; or
rather, the remoter forms a virtually fixed point, to which the
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 1693;[25] 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 determination of the parallax of 61 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,[26] "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, 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 angles made with the
hour-circle by 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°, between 1718 and
1759, in the position-angle of the two stars forming Castor, and was
thus within a hair's breadth of the discovery of their physical
connection.[27] 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."[28] And in 1784:[29] "It is not improbable that a few
years may inform us that some of the great number of double, triple
stars, etc., which have been observed by Mr. Herschel, are systems of
bodies revolving about each other."

This remarkable speculative anticipation had a practical counterpart in
Germany. Father Christian Mayer, a Jesuit astronomer at Mannheim, set
himself, in January 1776, to collect examples of stellar pairs, and
shortly after published the supposed discovery of "satellites" to many
of the principal stars.[30] But his observations were neither exact nor
prolonged enough to lead to useful results in such an inquiry. His
disclosures were derided; his planet-stars treated as results of
hallucination. _On n'a point cru à des choses aussi extraordinaires_,
wrote Lalande[31] within one year of a better-grounded announcement to
the same effect.

Herschel at first shared the general opinion as to the merely optical
connection of double stars. Of this the purpose for which he made his
collection is in itself sufficient evidence, since what may be called
the _differential_ method of parallaxes depends, as we have seen, for
its efficacy upon disparity of distance. It was "much too soon," he
declared in 1782,[32] "to form any theories of small stars revolving
round large ones;" while in the year following,[33] 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 unparalleled in amount as well as in kind, prepared ample
materials for convincing himself and others. In 1802 he was able to
announce the fact of his discovery, and in the two ensuing years, to lay
in detail before the Royal Society proofs, gathered from the labours of
a quarter of a century, of orbital revolution in the case of as many as
fifty double stars, henceforth, he declared, to be held as real binary
combinations, "intimately held together by the bond of mutual
attraction."[34] 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 Gamma Leonis,
1200, Delta Serpentis, 375, Eta Bootis, 1681 years; Eta Lyræ
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 was described of one star by another in the
course of their mutual revolutions, as exemplified in 1795 by the
rapidly circulating system of Zeta 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 conjecture had previously held
almost undivided sway; and precise knowledge tends to propagate itself
and advance from point to point.

We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the
intricacies of its construction, was the primary task of his life, which
he never lost sight of, and to which all his other investigations were
subordinate. He was absolutely alone in this bold endeavour. Unaided, he
had to devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on this
sublime subject was prepared, and the greater part of it anticipated, by
him.

The ingenious method of "star-gauging," and its issue in the delineation
of the sidereal system as an irregular stratum of evenly-scattered suns,
is the best-known part of his work. But it was, in truth, only a first
rude approximation, the principle of 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
process 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 3,400 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 could be
seen with the 20-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"[35] were already discernible to him even while he endeavoured
to obtain a "true _mean_ result" on the assumption that each star in
space was separated from its neighbours as widely as the sun from
Sirius. "It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems"; and in certain
assemblages he was able to trace "a course or tide of stars setting
towards a centre," denoting, not doubtfully, the presence of attractive
forces.[36] Thirteen years later, he described our sun and his
constellated companions as surrounded by "a magnificent collection of
innumerable stars, called the Milky Way, which must occasion a very
powerful balance of opposite attractions to hold the intermediate stars
at rest. For though our sun, and all the stars we see, may truly be said
to be in the plane of the Milky Way, yet I am now convinced, by a long
inspection and continued examination of it, that the Milky Way itself
consists of stars very differently scattered from those which are
immediately about us." "This immense aggregation," he added, "is by no
means uniform. Its component stars show evident signs of clustering
together into many separate allotments."[37]

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."[38]

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 principle he founded in 1817 his
method of "limiting apertures,"[39] 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 confidence; but since we have no warrant
for assuming a "standard star" to facilitate our computations, but much
reason to suppose an indefinite range, not only of size but of intrinsic
brilliancy, in the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.

In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was the first
to bestow serious study on the enigmatical objects known as "nebulæ."
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-Sûfi; and marked
with dots on Spanish and Dutch constellation-charts of the fourteenth
and fifteenth centuries.[40] 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 1618.[41] A novelty,
nevertheless, to astronomers it still remained in 1656, when Huygens
discerned, "as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond."[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a "lucid medium" diffused through
the ether of space.[43] He appears, however, to have been unacquainted
with some previously noticed by Hevelius. Lacaille brought back with him
from the Cape a list of forty-two--the first-fruits of observation in
Southern skies--arranged in three numerically equal classes;[44] 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.[45]

These preliminary attempts shrank into insignificance when Herschel
began to "sweep the heavens" with his giant telescopes. In 1786 he
presented to the Royal Society a descriptive catalogue of 1,000 nebulæ
and clusters, followed, three years later, by a second of as many more;
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 discernible, he naturally
concluded that "resolvability" was merely a question of distance and
telescopic power. He was (as he said himself) led on by almost
imperceptible degrees from evident clusters, such as the Pleiades, to
spots without a trace of stellar formation, the gradations being so well
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 nebulæ variously arranged;
large ones with small, seeming attendants; narrow, but much extended
lucid nebulæ 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 atmosphere; a different sort, again, contain a
nebulosity of the milky kind, like that wonderful, inexplicable
phenomenon about Theta Orionis; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable into
stars."[46]

"These curious objects" he considered to be "no less than whole sidereal
systems,"[47] 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."[48] 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 nebulæ, due to the "decay" of other "branching nebulæ" 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 nebulæ, 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 nebulæ 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 nebulæ 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."[49]

"This method of viewing the heavens," he concluded, "seems to throw them
into a new kind of light. They now are seen to resemble a luxuriant
garden which contains the greatest variety of productions in different
flourishing beds; and one advantage we may at least reap from it is,
that we can, as it were, extend the range of our experience to an
immense duration. For, to continue the simile which I have borrowed from
the vegetable kingdom, is it not almost the same thing whether we live
successively to witness the 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?"[50]

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 kingdom, he can scarcely point out
to us the precise boundary where the animal ceases and the plant begins;
and may even go so far as to suspect them not to be essentially
different. But, recollecting himself, he compares, for instance, one of
the human species to a tree, and all doubt upon the subject vanishes
before him. In the same manner we pass through gentle steps from a
coarse cluster of stars, such as the Pleiades ... till we find ourselves
brought to an object such as the nebula in Orion, where we are still
inclined to remain in the once adopted idea of stars exceedingly remote
and inconceivably crowded, as being the occasion of that remarkable
appearance. 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 19th cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less
decisive than that of the naturalist we have alluded to. Our judgment, I
may venture to say, will be, that _the nebulosity about the star is not
of a starry nature_."[51]

The conviction thus arrived at of the existence in space of a widely
diffused "shining fluid" (a conviction long afterwards fully justified
by the spectroscope) led him into a field of endless speculation. What
was its nature? Should it "be compared to the coruscation of the
electric fluid in the aurora borealis? or to the more magnificent cone
of the zodiacal light?" Above all, what was its function in the cosmos?
And on this point he already gave a hint of the direction in which his
mind was moving by the remark that this self-luminous matter seemed
"more fit to produce a star by its condensation, than to depend on the
star for its existence."[52]

This was not a novel idea. Tycho Brahe had tried to explain the blaze of
the star of 1572 as due to a sudden concentration of nebulous material
in the Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same track. But under
Herschel's treatment the nebular origin of stars first acquired the
consistency of a formal theory. He meditated upon it long and earnestly,
and in two elaborate treatises, published respectively in 1811 and 1814,
he at length set forth the arguments in its favour. These rested
entirely upon the "principle of continuity." Between the successive
classes of his assortment of developing 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."[53] 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,[54] to planetary
nebulæ, supposed to be already centrally solid, instances were alleged
of every stage and 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 nebulæ.

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 inception, 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 constellation Hercules of the sun and his
attendant planets; while a large balance of displacement was left to be
accounted for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to, if not
identical with, gravity, suns of every degree of size and splendour, and
sometimes brilliantly contrasted in colour, were 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,[55] 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 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,"[56] 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.[57]


FOOTNOTES:

[Footnote 3: _Phil. Trans._, vol. xxx., p. 737.]

[Footnote 4: Out of eighty stars compared, fifty-seven were found to
have changed their places by more than 10". Lesser discrepancies were at
that time regarded as falling within the limits of observational error.
_Tobiæ Mayeri Op. Inedita_, t. i., pp. 80, 81, and Herschel in _Phil.
Trans._, vol. lxxiii., pp. 275-278.]

[Footnote 5: _Posthumous Works_, p. 701.]

[Footnote 6: Arago in _Annuaire du Bureau des Longitudes_, 1842, p.
313.]

[Footnote 7: Bradley to Halley, _Phil. Trans._, vol. xxxv. (1728), p.
660. His observations were directly applicable to only two stars,
Gamma Draconis and Eta Ursæ Majoris, but some lesser ones
were included in the same result.]

[Footnote 8: Holden, _Sir William Herschel, his Life and Works_, p. 17.]

[Footnote 9: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 10: 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.]

[Footnote 11: Holden, _op. cit._, p. 39.]

[Footnote 12: _Memoir of Caroline Herschel_, p. 37.]

[Footnote 13: See Holden's _Sir William Herschel_, p. 54.]

[Footnote 14: _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.]

[Footnote 15: _Allgemeine Naturgeschichte und Theorie des Himmels_,
1755.]

[Footnote 16: _Cosmologische Briefe_, Augsburg, 1761.]

[Footnote 17: _The System of the World_, p. 125, London, 1800 (a
translation of _Cosmologische Briefe_). Lambert regarded nebulæ as
composed of stars crowded together, but _not_ as external universes. In
the case of the Orion nebula, indeed, he throws out such a conjecture,
but afterwards suggests that it may form a centre for that one of the
subordinate systems composing the Milky Way to which our sun belongs.]

[Footnote 18: _Opera Inedita_, t. i., p. 79.]

[Footnote 19: _Phil. Trans._, vol. lxxiii. (1783), p. 273. Pierre
Prévost's similar investigation, communicated to the Berlin Academy of
Sciences four months later, July 3, 1783, was inserted in the _Memoirs_
of that body for 1781, and thus _seems_ to claim a priority not its due.
Georg Simon Klügel at Halle gave about the same time an analytical
demonstration of Herschel's result. Wolf, _Gesch. der Astronomie_, p.
733.]

[Footnote 20: _Phil. Trans._, vol. xcv., p. 233.]

[Footnote 21: _Ibid._, vol. xcvi., p. 205.]

[Footnote 22: "Ingens bolus devorandus est," Kepler admitted to Herwart
in May, 1603.]

[Footnote 23: Described in "Præfatio Editoris" to _De Revolutionibus_,
p. xix. (ed. 1854).]

[Footnote 24: _Opere_, t. i., p. 415.]

[Footnote 25: _Phil. Trans._, vol. xvii., p. 848.]

[Footnote 26: _Ibid._, vol. lxxii., p. 97.]

[Footnote 27: Doberck, _Observatory_, vol. ii., p. 110.]

[Footnote 28: _Phil. Trans._, vol. lvii., p. 249.]

[Footnote 29: _Ibid._, vol. lxxiv., p. 56.]

[Footnote 30: _Beobachtungen von Fixsterntrabanten_, 1778; and _De Novis
in Coelo Sidereo Phænomenis_, 1779.]

[Footnote 31: _Bibliographie_, p. 569.]

[Footnote 32: _Phil. Trans._, vol. lxxii., p. 162.]

[Footnote 33: _Ibid._, vol. lxxiii., p. 272.]

[Footnote 34: _Ibid._, vol. xciii., p. 340.]

[Footnote 35: _Phil. Trans._, vol. lxxv., p. 255.]

[Footnote 36: _Ibid._, vol. lxxix., pp. 214, 222.]

[Footnote 37: _Ibid._, vol. xcii., pp. 479, 495.]

[Footnote 38: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 39: _Ibid._, vol. cvii., p. 311.]

[Footnote 40: Bullialdus, _De Nebulosâ Stellâ in Cingulo Andromedæ_
(1667); see also G. P. Bond, _Mém. Am. Ac._, vol. iii., p. 75, Holden's
Monograph on the Orion Nebula, _Washington Observations_, vol. xxv.,
1878 (pub. 1882), and Lady Huggins's drawing, _Atlas of Spectra_, p.
119.]

[Footnote 41: _Mathemata Astronomica_, p. 75.]

[Footnote 42: _Systema Saturnium_, p. 9.]

[Footnote 43: _Phil. Trans._, vol. xxix., p. 390.]

[Footnote 44: _Mém. Ac. des Sciences_, 1755.]

[Footnote 45: _Conn. des Temps_, 1784 (pub. 1781), p. 227. A previous
list of forty-five had appeared in _Mém. Ac. des Sciences_, 1771.]

[Footnote 46: _Phil. Trans._, vol. lxxiv., p. 442.]

[Footnote 47: _Ibid._, vol. lxxix., p. 213.]

[Footnote 48: _Ibid._, vol. lxxv., p. 254.]

[Footnote 49: _Ibid._, vol. lxxix., p. 225.]

[Footnote 50: _Phil. Trans._, vol. lxxix., p. 226.]

[Footnote 51: _Ibid._, vol. lxxxi., p. 72.]

[Footnote 52: _Ibid._, p. 85.]

[Footnote 53: _Phil. Trans._, vol. ci., p. 271.]

[Footnote 54: _Ibid._, p. 277.]

[Footnote 55: J. Herschel, _Phil. Trans._, vol. cxvi., part iii., p. 1.]

[Footnote 56: His own words to the poet Campbell cited by Holden, _Life
and Works_, p. 109.]

[Footnote 57: _Phil. Trans._, 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 defined by no mean authority[58] as the true end of
astronomy.

Several causes concurred about the beginning of the last 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 Flamsteed 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 to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of Astronomer
Royal during forty-six years (from 1765 to 1811), was no unworthy
successor to the eminent men who had gone before him. His foundation of
the _Nautical Almanac_ (in 1767) alone constitutes a valid title to
fame; he introduced at the Observatory the important innovation of the
systematic publication of results; and the careful and prolonged series
of observations executed by him formed the basis of the improved
theories, and corrected tables of the celestial movements, which were
rapidly being brought to completion abroad. 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, indefatigable as
he was, a definite purpose was wanting to compel him, by its exigencies,
along the path of progress. Thus, for almost fifty years after Bradley's
death, the acquisition of a small achromatic[59] was the only notable
change made in the instrumental equipment of the Observatory. The
transit, the zenith sector, and the mural quadrant, with which Bradley
had done his incomparable work, retained their places long after they
had become 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 nineteenth 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, Schröter, 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 received powerful aid
through the foundation, in 1804, by a young artillery officer named Von
Reichenbach, 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 and Sons, in Bremen, as
an apprenticed clerk. He was now thrown completely upon his own
resources. From his father, a struggling Government official, heavily
weighted with a large family, he was well aware that he had nothing to
expect; his dormant faculties were roused by the necessity for
self-dependence, and he set himself to push manfully forward along the
path that lay before him. The post of supercargo on one of the trading
expeditions sent out from the Hanseatic towns to China and the East
Indies was the aim of his boyish ambition, for the attainment of which
he sought to qualify himself by the industrious acquisition of suitable
and useful knowledge. He learned English in two or three months; picked
up Spanish with the casual aid of a gunsmith's apprentice; studied the
geography of the distant lands which he hoped to visit; collected
information as to their climates, inhabitants, products, and the courses
of trade. He desired to add some acquaintance with the art (then much
neglected) of taking observations at sea; and thus, led on from
navigation to astronomy, and from astronomy to mathematics, he groped
his way into a new world.

It was characteristic of him that the practical problems 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,[60] 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
_Monatliche Correspondenz_, overcoming each difficulty as it arose with
the aid of Lalande's _Traité d'Astronomie_, and supplying, 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, and published them as a
supplement to Bode's Annual. With an elaborate care inspired by his
youthful ardour, though hardly merited by their loose nature, Bessel
deduced from them an orbit for that celebrated body, and presented the
work to Olbers, whose reputation in cometary researches gave a special
fitness to the proffered homage. The benevolent physician-astronomer of
Bremen welcomed with surprised delight such a performance emanating from
such a source. Fifteen years previously, the French Academy had crowned
a similar work; 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 as his
profession. For two years he continued to work in the counting-house by
day, and to pore over the _Mécanique Céleste_ and the Differential
Calculus by night. But the post of assistant in Schröter's observatory
at Lilienthal having become vacant by the removal of Harding to
Göttingen in 1805, Olbers procured for him the offer of it. It was not
without a struggle that he resolved to exchange the desk for the
telescope. His reputation with his employers was of the highest; he had
thoroughly mastered the details of the business, which his keen
practical intelligence followed with lively interest; his years of
apprenticeship were on the point of expiring, and an immediate, and not
unwelcome prospect of comparative affluence lay before him. The love of
science, however, 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.[61]

For four years he continued in Schröter's employment. At the end of that
time the Prussian Government chose him to superintend the erection of a
new observatory at Königsberg, which after many vexatious delays, caused
by the prostrate condition of the country, was finished towards the end
of 1813. Königsberg 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.[62] Their configuration lends to the skies their distinctive
features, and marks out the shifting tracks of more mobile objects with
relatively fixed, and generally unvarying points of light. A more
detailed and accurate acquaintance with the stellar multitude, regarded
from a purely uranographical point of view, has accordingly formed at
all times a primary object of celestial science, and was, during the
last century, cultivated with a zeal and success by which all previous
efforts were dwarfed into insignificance. In Lalande's _Histoire
Céleste_, published in 1801, the places of no less than 47,390 stars
were given, but in the rough, as it were, and consequently 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 7,600 stars--the second being a revision and enlargement of the
first--which for their time were models of what such works should
be.[63] 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.[64] A number of
highly complex corrections have to be applied before their _mean_ can be
disengaged from their _apparent_ places on the sphere. Of these, the
most considerable and familiar is atmospheric refraction, by which
objects seem to stand higher in the sky than they in reality do, the
effect being evanescent at the zenith, and attaining, by gradations
varying with 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 movement of
the earth in its orbit, causes a small annual displacement known as
_aberration_.

Now it is easy to see that any uncertainty in the application of these
corrections saps the very foundations of exact astronomy. Extremely
minute quantities, it is true, are concerned; but the life and progress
of modern celestial science depends upon the sure recognition of
extremely minute quantities. In the early years of the nineteenth
century, however, no uniform system of "reduction" (so the complete
correction of observational results is termed) had been established.
Much was left to the individual 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 3,222 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 Astronomiæ_. The eminent value of the
work consisted in this, that by providing a mass of entirely reliable
information as to the state of the heavens at the epoch 1755, it threw
back the beginning of _exact_ astronomy almost half a century. By
comparison with Piazzi's catalogues the amount of precession was more
accurately determined, the proper motions of a 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 _Tabulæ
Regiomontanæ_. They not only constituted an advance in accuracy, but
afforded a vast increase of facility in application, and were at once
and everywhere adopted. Thus astronomy became a truly universal science;
uncertainties and disparities were banished, and observations made at
all times and places rendered mutually comparable.[65]

More, however, yet remained to be done. In order to verify with greater
strictness the results drawn from the Bradley and Piazzi catalogues, a
third term of comparison was wanted, and this Bessel undertook to
supply. By a course of 75,011 observations, executed during the years
1821-33, with the utmost nicety of care, the number of accurately known
stars was brought up to above 50,000, and an ample store of trustworthy
facts laid up for the use of future astronomers. In this department
Argelander, whom he attracted from finance to astronomy, and trained in
his own methods, was his assistant and successor. The great "Bonn
Durchmusterung,"[66] in which 324,198 stars visible in the northern
hemisphere are enumerated, and the corresponding "Atlas" published in
1857-63, constituting a picture of our sidereal surroundings of
heretofore unapproached completeness, may be justly said to owe their
origin to Bessel's initiative, and to form a sequel to what he
commenced.

But his activity was not solely occupied with the promotion of a
comprehensive reform in astronomy; it embraced special problems as well.
The long-baffled search for a parallax of the fixed stars was resumed
with fresh zeal as each mechanical or optical improvement held out fresh
hopes of a successful issue. Illusory results abounded. Piazza in 1805
perceived, as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon; the truth being that his instruments
were worn out with constant use, and could no longer be depended
upon.[67] His countryman, Calandrelli, was similarly deluded. The
celebrated controversy between the Astronomer Royal and Dr. Brinkley,
Director of the Dublin College Observatory, turned on the same subject.
Brinkley, who was in possession of a first-rate meridian-circle,
believed himself to have discovered relatively large parallaxes for four
of the brightest stars; Pond, relying on the testimony of the Greenwich
instruments, asserted their nullity. The dispute, protracted for
fourteen years, from 1810 until 1824, was brought to no definite
conclusion; but the strong presumption on the negative side was
abundantly justified in the event.

There was good reason for incredulity in the matter of parallaxes.
Announcements of their detection had become so frequent as to be
discredited before they were disproved; and Struve, who investigated the
subject at Dorpat in 1818-21, had clearly shown that the quantities
concerned were too small to come within 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 21st 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
acquirements; and was thus eminently fitted for the task awaiting him,
when, in 1806, he entered the optical department of the establishment
founded two years previously by Von Reichenbach and Utzschneider. He now
zealously devoted himself to the improvement of the achromatic
telescope; and, after 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, 9-1/2 inches in diameter, and of 14 feet focal
length.

This (as it was then considered) gigantic lens was secured by Struve for
the Russian Government, and the "great Dorpat refractor"--the first of
the large achromatics which have played such an important part in modern
astronomy--was, late in 1824, set up in the place which it still
occupies. By ingenious improvements in mounting and fitting, it was
adapted to the finest micrometrical work, and thus offered unprecedented
facilities both for the examination of double stars (in which Struve
chiefly employed it), and for such subtle measurements as might serve to
reveal or disprove the existence of a sensible stellar parallax.
Fraunhofer, moreover, constructed for the observatory at Königsberg the
first really available heliometer. The principle of this instrument
(termed with more propriety a "divided object-glass micrometer") is the
separation, by a strictly measurable amount, of two distinct images of
the same object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided are shifted
until the upper star (say) in one image is brought into coincidence with
the lower star in the other, when their distance apart becomes known by
the amount of motion employed.[68]

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 improvements in
reflectors as important as those he had already effected in refractors;
and was besides eagerly occupied with investigations into the nature of
light, the momentous character of which we shall by-and-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
unwholesome toil, and, still hoping for its restoration from a projected
journey to Italy, he died of consumption, June 7, 1826, aged thirty-nine
years. His tomb in Munich bears the concise eulogy, _Approximavit
sidera_.

Bessel had no sooner made himself acquainted with the exquisite defining
powers of the Königsberg heliometer, than he resolved to employ them in
an attack upon the now secular problem of star-distances. But it was not
until 1837 that he found leisure to pursue the inquiry. In choosing his
test-star he adopted a new principle. It had hitherto been assumed that
our nearest neighbours in space must be found among 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 1792,[69] 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 constellation of the Swan. Still more emphatically in 1812[70]
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 3-1/2 lunar diameters, and that a quarter
of a million would be required to carry it round the entire circuit of
the visible heavens. Nevertheless, it has few rivals in rapidity of
movement, the apparent displacement of the vast majority of stars being,
by comparison, almost insensible.

This interesting, though inconspicuous object, then, was chosen by
Bessel to be put to the question with his heliometer, while Struve made
a similar and somewhat earlier trial with the bright gem of the Lyre,
whose Arabic title of the "Falling Eagle" survives as a time-worn
remnant in "Vega." Both astronomers agreed to use the "differential"
method, for which their instruments and the vicinity to their selected
stars of minute, physically detached companions offered special
facilities. In the last month of 1838 Bessel made known the result of
one year's observations, showing for 61 Cygni a parallax of about a
third of a second (0·3136").[71] 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 1840.[72] 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.[73]

Struve's measurements inspired less confidence. They extended over three
years (1835-38), but were comparatively few, and were frequently
interrupted. The parallax, accordingly, of about a quarter of a second
(0·2613") which he derived from them for Alpha Lyræ, and announced
in 1840,[74] has proved considerably too large.[75]

Meanwhile a result of the same kind, but of a more striking character
than either Bessel's or Struve's, had been obtained, one might almost
say casually, by a different method and in a distant region. Thomas
Henderson, originally an attorney's clerk in his native town of Dundee,
had become known for his astronomical attainments, and was appointed in
1831 to direct the recently completed observatory at the Cape of Good
Hope. He began observing in April, 1832, and, the serious shortcomings
of his instrument notwithstanding, executed 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 Alpha
Centauri (which ranks after Sirius and Canopus as the third brightest
orb in the heavens), he effected a number of successive determinations
of its position, and on being informed of its very considerable proper
motion (3·6" annually), he resolved to examine the observations already
made for possible traces of parallactic displacement. This was done on
his return to Scotland, where he filled the office of Astronomer Royal
from 1834 until his premature death in 1844. The result justified his
expectations. From the declination measurements made at the Cape and
duly reduced, a parallax of about one second of arc clearly emerged
(diminished by Gill's and Elkin's observations, 1882-1883, to O·75");
but, by perhaps an excess of caution, was withheld from publication
until fuller certainty was afforded by the concurrent testimony of
Lieutenant Meadows's determinations of the same star's right
ascension.[76] When at last, January 9, 1839, Henderson communicated his
discovery to the Astronomical Society, he could no longer claim the
priority which was his due. Bessel had anticipated him with the parallax
of 61 Cygni by just two months.

Thus from three different quarters, three successful and almost
simultaneous assaults were delivered upon a long-beleaguered citadel of
celestial secrets. The same work has since been steadily pursued, with
the general result of showing that, as regards their overwhelming
majority, the stars are far too remote to show even the slightest trace
of optical shifting from the revolution of the earth in its orbit. In
nearly a hundred cases, however, small parallaxes have been determined,
some certainly (that is, within moderate limits of error), others more
or less precariously. The list is an instructive one, in its omissions
no less than in its contents. It includes stars of many degrees of
brightness, from Sirius down to a nameless telescopic star in the Great
Bear;[77] yet the vicinity to the earth of this minute object is so much
greater than that of the brilliant Vega, that the latter transported to
its place would increase in lustre thirty-eight 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, Betelgeux,
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, the so-called "optical" and "geometrical"
methods of relatively estimating star-distances are both 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 realize them distinctly, and to express them
conveniently, a new unit of length, itself of bewildering magnitude, has
originated. This is what we may call the _light-journey_ of one year.
The subtle vibrations of the ether, propagated on all sides from the
surface of luminous bodies, travel at the rate of 186,300 miles a
second, or (in round numbers) six billions of miles a year. Four and a
third such measures are needed to span the abyss that separates us from
the nearest fixed star. In other words, light takes four years and four
months to reach the earth from Alpha Centauri; yet Alpha 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 stars revolving in
known orbits, to the determination of mass; for the distance from the
earth of the two bodies forming a binary 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 components of Alpha 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
remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed on
the authority of O. Struve, appears to be above eight 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, no less than
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 distance. The annual journey, for instance, of 61 Cygni _across
the line of sight_ amounts to 1,000, and that of Alpha Centauri to
446 millions of miles. A small star, numbered 1,830 in Groombridge's
Circumpolar Catalogue, "devours the way" at the rate of at least 150
miles a second--a speed, in Newcomb's opinion, beyond the gravitating
power of the entire sidereal system to control; and Mu Cassiopeiæ
possesses above two-thirds of that surprising velocity; while for both
objects, radial movements of just sixty miles a second were disclosed
by Professor Campbell's spectroscopic measurements.

Herschel's conclusion as to the advance 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.[78] Biot, Burckhardt, even Herschel's own son, shared his
incredulity. But the appearance of Argelander's prize-essay in 1837[79]
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 1847[80] by Galloway's
investigations, founded exclusively on the apparent displacements of
southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin
(1821-1898),[81] employing all the resources of modern science, and
commanding the wealth of material furnished by 1,167 proper motions
carefully 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 reinvestigation of the subject
in 1883[82] served but slightly to modify. Yet astronomers were not
satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of
work, for which he received in 1888 the Gold Medal of the Royal
Astronomical Society. It consisted in reducing afresh, with the aid of
the most refined modern data, Bradley's original stars, and comparing
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[83] 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. And the more easterly position of the solar apex was fully
confirmed by the experiments, with variously assorted lists of stars, of
Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85] Fresh
precautions of refinement were introduced into the treatment of the
subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by
Newcomb[88] and Porter[89] in America, who ably availed themselves of
the copious materials accumulated before the close of the century. Their
results, although not more closely accordant than those of their
predecessors, combined to show that the journey of our system is
directed towards a point within a circle about ten degrees in radius,
having the brilliant Vega for its centre. To determine its rate was a
still more arduous problem. It involved the assumption, very much at
discretion, of an average parallax for the stars investigated; and Otto
Struve's estimate of 154 million miles as the span yearly traversed was
hence wholly unreliable. Fortunately, however, as will be seen further
on, a method of determining the sun's velocity independently of any
knowledge of star-distances, has now become 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 inquiry, 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. Herschel threw out the hint that
the great cluster in Hercules might prove to be the supreme seat of
attractive force;[90] Argelander placed his central body in the
constellation Perseus;[91] Fomalhaut, the brilliant of the Southern
Fish, was set in the post of honour by Boguslawski of Breslau. Mädler
(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 prepondering mass, but in the centre of gravity of the
self-controlled revolving multitude.[92] 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.[93] Mädler 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,
accordingly, he placed the centre of gravity of the Milky Way.[94] The
bright star Alcyone thus became the "central sun," but in a purely
passive sense, its headship being determined by its situation at the
point of neutralisation of opposing tendencies, and of consequent rest.
By an avowedly conjectural method, the solar period of revolution round
this point was fixed at 18,200,000 years.

The scheme of sidereal government framed by the Dorpat astronomer was,
it may be observed, of the most approved constitutional type;
deprivation, rather than increase of influence accompanying the office
of chief dignitary. But while we are still ignorant, and shall perhaps
ever remain so, of the fundamental plan upon which the Galaxy is
organised, recent investigations tend more and more to exhibit it, not
as monarchical (so to speak), but as federative. The community of proper
motions detected by Mädler 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.[95] His prediction regarding the planet Neptune does
not belong to the present division of our subject; a strictly analogous
discovery in the sidereal system was, however, also very clearly
foreshadowed by him. His earliest suspicions of non-uniformity in the
proper motion of Sirius dated from 1834; they extended to Procyon in
1840; and after a series of refined measurements with the new Repsold
circle, he announced in 1844 his conclusion that these irregularities
were due to the presence of obscure bodies round which the two bright
Dog-stars revolved as they pursued their way across the sphere.[96] 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."[97]

An inference so contradictory to received ideas obtained little credit,
until Peters found, in 1851,[98] 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 31st of January, 1862,
while in the act of trying a new 18-inch refractor, Mr. Alvan G. Clark
(one of the celebrated firm of American opticians) actually discovered
the hypothetical Sirian companion in the precise position required by
theory. It has now been watched through nearly an entire 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 1/10000th of its light. Sirius
itself, on the other hand, possesses a far higher radiative intensity
than our sun. It gravitates--admitting Sir David Gill's parallax of
0·38" to be exact--like two suns, but shines like twenty. Possibly it is
much 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 was awaited until November 13,
1896, when Professor Schaeberle, with the great Lick refractor, detected
the long-sought object in the guise of a thirteenth-magnitude star. Dr.
See's calculations[99] showed it to possess one-fifth the mass of its
primary, or rather more than half that of our sun.[100] Yet it gives
barely 1/20000th of the sun's light, so that it is still nearer to total
obscurity than the dusky satellite of Sirius. The period of forty years
assigned to the system by Auwers in 1862[101] appears to be singularly
exact.

But Bessel was not destined to witness the recognition of "the
invisible" as a legitimate and profitable field for astronomical
research. He died March 17, 1846, just six months before the discovery
of Neptune, of an obscure disease, eventually found to be occasioned by
an extensive fungus-growth in the stomach. The place which he left
vacant was not one easy to fill. His life's work might be truly
described as "epoch-making." Rarely indeed shall we find one who
reconciled with the same success the claims of theoretical and practical
astronomy, or surveyed the science which he had made his own with a
glance equally comprehensive, practical, and profound.

The career of Friedrich Georg Wilhelm Struve illustrates the maxim that
science _differentiates_ as it develops. He was, while much besides, a
specialist in double stars. His earliest recorded use of the telescope
was to verify Herschel's conclusion as to the revolving movement of
Castor, and he never varied from the predilection which this first
observation at once indicated and determined. He was born at Altona, of
a respectable yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He then turned to
science, was appointed in 1813 to a professorship of astronomy and
mathematics, and began regular work in the Dorpat Observatory just
erected by Parrot for Alexander I. It was not, however, until 1819 that
the acquisition of a 5-foot refractor by Troughton enabled him to take
the position-angles of double stars with regularity and tolerable
precision. The resulting catalogue of 795 stellar systems gave the
signal for a general resumption of the Herschelian labours in this
branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him to
undertake, February 11, 1825, a review of the entire heavens down to 15°
south of the celestial equator, which occupied more than two years, and
yielded, from an examination of above 120,000 stars, a harvest of about
2,200 previously unnoticed composite objects. The ensuing ten years were
devoted to delicate and patient measurements, the results of which were
embodied in _Mensuræ Micrometricæ_, published at St. Petersburg in 1837.
This monumental work gives the places, angles of position, distances,
colours, and relative brightness of 3,112 double and multiple stars, all
determined with the utmost skill and care. The record is one which gains
in value with the process of time, and will for ages serve as a standard
of reference by which to detect change or confirm discovery.

It appears from Struve's researches that about one in forty of all stars
down to the ninth magnitude is composite, but that the proportion is
doubled in the brighter orders.[102] This he attributed to the
difficulty of detecting the faint companions of very remote orbs. It was
also noticed, both by him and Bessel, that double stars are in general
remarkable for large proper motions. Struve's catalogue included no star
of which the components were more than 32" apart, because beyond that
distance the chances of merely optical juxtaposition become
considerable; but the immense preponderance of extremely close over (as
it were) loosely yoked bodies is such as to demonstrate their physical
connection, even if no other proof were forthcoming. Many stars
previously believed to be single divided under the scrutiny of the
Dorpat refractor; while in some cases, one member of a supposed binary
system revealed itself as double, thus placing the surprised observer in
the unexpected presence of a triple group of suns. Five instances were
noted of two pairs lying so close together as to induce a conviction of
their mutual dependence;[103] besides which, 124 examples occurred of
triple, quadruple, and multiple combinations, the reality of which was
open to no reasonable doubt.[104]

It was first pointed out by Bessel that the fact of stars exhibiting a
common proper motion might serve as an unfailing test of their real
association into systems. This was, accordingly, one of the chief
criteria employed by Struve to distinguish true binaries from merely
optical couples. On this ground alone, 61 Cygni was admitted to be a
genuine double star; and it was shown that, although its components
appeared to follow almost strictly rectilinear paths, yet the
probability of their forming a connected pair is actually greater than
that of the sun rising to-morrow morning.[105] Moreover, this tie of an
identical movement was discovered to unite bodies[106] far beyond the
range of distance ordinarily separating the members of binary systems,
and to prevail so extensively as to lead to the conclusion that single
do not outnumber conjoined stars more than twice or thrice.[107]

In 1835 Struve was summoned by the Emperor Nicholas to superintend the
erection of a new observatory at Pulkowa, near St. Petersburg, destined
for the special cultivation of sidereal astronomy. Boundless resources
were placed at his disposal, and the institution created by him was
acknowledged to surpass all others of its kind in splendour, efficiency,
and completeness. Its chief instrumental glory was a refractor of
fifteen inches aperture by Merz and Mahler (Fraunhofer's successors),
which left the famous Dorpat telescope far behind, and remained long
without a rival. On the completion of this model establishment, August
19, 1839, Struve was installed as its director, and continued to fulfil
the important duties of the post with his accustomed vigour until 1858,
when illness compelled his virtual resignation in favour of his son Otto
Struve, born at Dorpat in 1819. He died November 23, 1864.

An inquiry into the laws of stellar distribution, undertaken during the
early years of his residence at Pulkowa, led Struve to confirm in the
main the inferences arrived at by Herschel as to the construction of the
heavens. According to his view, the appearance known as the Milky Way is
produced by a collection of irregularly condensed star-clusters, within
which the sun is somewhat eccentrically placed. The nebulous ring which
thus integrates the light of countless worlds was supposed by him to be
made up of stars scattered over a bent or "broken plane," or to lie in
two planes slightly inclined to each other, our system occupying a
position near their intersection.[108] He further attempted to show that
the limits of this vast assemblage must remain for ever shrouded from
human discernment, owing to the gradual extinction of light in its
passage through space,[109] and sought to confer upon this celebrated
hypothesis a definiteness and certainty far beyond the aspirations of
its earlier advocates, Chéseaux and Olbers; but arbitrary assumptions
vitiated his reasonings on this, as well as on some other points.[110]

In his special line as a celestial explorer of the most comprehensive
type, Sir William Herschel had but one legitimate successor, and that
successor was his son. John Frederick William Herschel was born at
Slough, March 17, 1792, graduated with the highest honours from St.
John's College, Cambridge, in 1813, and entered upon legal studies with
a view to being called to the Bar. But his share in an early compact
with Peacock and Babbage, "to do their best to leave the world wiser
than they found it," was not thus to be fulfilled. The acquaintance of
Dr. Wollaston decided his scientific vocation. Already, in 1816, we find
him reviewing some of his father's double stars; and he completed in
1820 the 18-inch speculum which was to be the chief instrument of his
investigations. Soon afterwards, he undertook, in conjunction with Mr.
(later Sir James) South, a series of observations, issuing in the
presentation to the Royal Society of a paper[111] containing
micrometrical measurements of 380 binary stars, by which the elder
Herschel's inferences of orbital motion were, in many cases, strikingly
confirmed. A star in the Northern Crown, for instance (Eta Coronæ),
had completed more than one entire circuit since its first discovery;
another, Tau Ophiuchi, had _closed up_ into apparent singleness; while
the motion of a third, Xi Ursæ Majoris, in an obviously eccentric orbit,
was so rapid as to admit of being traced and measured from month to month.

It was from the first confidently believed that the force retaining
double stars in curvilinear paths was identical with that governing the
planetary revolutions. But that identity was not ascertained until
Savary of Paris showed, in 1827,[112] that the movements of the
above-named binary in the Great Bear could be represented with all
attainable accuracy by an ellipse calculated on orthodox gravitational
principles with a period of 58-1/4 years. Encke followed at Berlin with
a still more elegant method; and Sir John Herschel, pointing out the
uselessness of analytical refinements where the data were necessarily so
imperfect, described in 1832 a graphical process by which "the aid of
the eye and hand" was brought in "to guide the judgment in a case where
judgment only, and not calculation, could be of any avail."[113]
Improved methods of the same kind were published by Dr. See in
1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with
stellar orbits is steadily gaining precision, certainty, and extent.

In 1825 Herschel undertook, and executed with great assiduity during the
ensuing eight years, a general survey of the northern heavens, directed
chiefly towards the verification of his father's nebular discoveries.
The outcome was a catalogue of 2,306 nebulæ and clusters, of which 525
were observed for the first time, besides 3,347 double stars discovered
almost incidentally.[116] "Strongly invited," as he tells us himself,
"by the peculiar interest of the subject, and the wonderful nature of
the objects which presented themselves," he resolved to attempt the
completion of the survey in the southern hemisphere. With this noble
object in view, he embarked his family and instruments on board the
_Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at
Cape Town on the 16th of January, 1834. Choosing as the scene of his
observations a rural spot under the shelter of Table Mountain, he began
regular "sweeping" on the 5th of March. The site of his great reflector
is now marked with an obelisk, and the name of Feldhausen has become
memorable in the history of science; for the four years' work done there
may truly be said to open the chapter of our knowledge as regards the
southern skies.

The full results of Herschel's journey to the Cape were not made public
until 1847, when a splendid volume[117] embodying them was brought out
at the expense of the Duke of Northumberland. They form a sequel to his
father's labours such as the investigations of one man have rarely
received from those of another. What the elder observer did for the
northern heavens, the younger did for the southern, and with generally
concordant results. Reviving the paternal method of "star-gauging," he
showed, from a count of 2,299 fields, that the Milky Way surrounds the
solar system as a complete annulus of minute stars; not, however, quite
symmetrically, since the sun was thought to lie somewhat nearer to those
portions visible in the southern hemisphere, which display a brighter
lustre and a more complicated structure than the northern branches. The
singular cosmical agglomerations known as the "Magellanic Clouds" were
now, for the first time, submitted to a detailed, though admittedly
incomplete, examination, the almost inconceivable richness and variety
of their contents being such that a lifetime might with great profit be
devoted to their study. In the Greater Nubecula, within a compass of
forty-two square degrees, Herschel reckoned 278 distinct nebulæ and
clusters, besides fifty or sixty outliers, and a large number of stars
intermixed with diffused nebulosity--in all, 919 catalogued objects,
and, for the Lesser Cloud, 244. Yet this was only the most conspicuous
part of what his twenty-foot revealed. Such an extraordinary
concentration of bodies so various led him to the inevitable conclusion
that "the Nubeculæ are to be regarded as systems _sui generis_, and
which have no analogues in our hemisphere."[118] He noted also the
blankness of surrounding space, especially in the case of Nubecula
Minor, "the access to which on all sides," he remarked, "is through a
desert;" as if the cosmical material in the neighbourhood had been swept
up and garnered in these mighty groups.[119]

Of southern double stars, he discovered and gave careful measurements of
2,102, and described 1,708 nebulæ, of which at least 300 were new. The
list was illustrated with a number of drawings, some of them extremely
beautiful and elaborate.

Sir John Herschel's views as to the nature of nebulæ were considerably
modified by Lord Rosse's success in "resolving" with his great
reflectors a crowd of these objects into stars. His former somewhat
hesitating belief in the existence of phosphorescent matter,
"disseminated through extensive regions of space in the manner of a
cloud or fog,"[120] was changed into a conviction that no valid
distinction could be established between the faintest wisp of cosmical
vapour just discernible in a powerful telescope, and the most brilliant
and obvious cluster. He admitted, however, an immense range of possible
variety in the size and mode of aggregation of the stellar constituents
of various nebulæ. Some might appear nebulous from the closeness of
their parts; some from their smallness. Others, he suggested, might be
formed of "discrete luminous bodies floating in a non-luminous
medium;"[121] while the annular kind probably consisted of "hollow
shells of stars."[122] That a physical, and not merely an optical,
connection unites nebulæ with the _embroidery_ (so to speak) of small
stars with which they are in many instances profusely decorated, was
evident to him, as it must be to all who look as closely and see as
clearly as he did. His description of No. 2,093 in his northern
catalogue as "a network or tracery of nebula following the lines of a
similar network of stars,"[123] would alone suffice to dispel the idea
of accidental scattering; and many other examples of a like import might
be quoted. The remarkably frequent occurrence of one or more minute
stars in the close vicinity of "planetary" nebulæ led him to infer their
dependent condition; and he advised the maintenance of a strict watch
for evidences of circulatory movements, not only over these supposed
stellar satellites, but also over the numerous "double nebulæ," in
which, as he pointed out, "all the varieties of double stars as to
distance, position, and relative brightness, have their counterparts."
He, moreover, investigated the subject of nebular distribution by the
simple and effectual method of graphic delineation or "charting," and
succeeded in showing that while a much greater uniformity of scattering
prevails in the southern than in the northern heavens, a condensation is
nevertheless perceptible about the constellations Pisces and Cetus,
roughly corresponding to the "nebular region" in Virgo by its vicinity
(within 20° or 30°) to the opposite pole of the Milky Way. He concluded
"that the nebulous system is distinct from the sidereal, though
involving, and perhaps to a certain extent intermixed with, the
latter."[124]

Towards the close of his residence at Feldhausen, Herschel was fortunate
enough to witness one of those singular changes in the aspect of the
firmament which occasionally challenge the attention even of the
incurious, and excite the deepest wonder of the philosophical observer.
Immersed apparently in the Argo nebula is a star denominated Eta
Carinæ. When Halley visited St. Helena in 1677, it seemed of the fourth
magnitude; but Lacaille in the middle of the following century, and
others after him, classed it as of the second. In 1827 the traveller
Burchell, being then at St. Paul, near Rio Janeiro, remarked that it had
unexpectedly assumed the first rank--a circumstance the more surprising
to him because he had frequently, when in Africa during the years 1811
to 1815, noted it as of only fourth magnitude. This observation,
however, did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second, and had
no suspicion of its unusual character until December 16, 1837, when he
suddenly perceived its light to be almost tripled. It then far outshone
Rigel in Orion, and on the 2nd of January following it very nearly
matched Alpha Centauri. From that date it declined; but a second
and even brighter maximum occurred in April, 1843, when Maclear, then
director of the Cape Observatory, saw it blaze out with a splendour
approaching that of Sirius. Its waxings and wanings were marked by
curious "trepidations" of brightness extremely perplexing to theory. In
1863 it had sunk below the fifth magnitude, and in 1869 was barely
visible to the naked eye; yet it was not until eighteen years later that
it touched a minimum of 7·6 magnitude. Soon afterwards a recovery of
brightness set in, but was not carried very far; and the star now shines
steadily as of the seventh magnitude, its reddish light contrasting
effectively with the silvery rays of the surrounding nebula. An attempt
to include its fluctuations within a cycle of seventy years[125] has
signally failed; the extent and character of the vicissitudes to which
it is subject stamping it rather as a species of connecting link between
periodic and temporary stars.[126]

Among the numerous topics which engaged Herschel's attention at the Cape
was that of relative stellar brightness. Having contrived an
"astrometer" in which an "artificial star," formed by the total
reflection of moonlight from the base of a prism, served as a standard
of comparison, he was able to estimate the lustre of the _natural_ stars
examined by the distances at which the artificial object appeared equal
respectively to each. He thus constructed a table of 191 of the
principal stars,[127] both in the northern and southern hemispheres,
setting forth the numerical values of their apparent brightness
relatively to that of Alpha Centauri, which he selected as a unit
of measurement. Further, the light of the full moon being found by him
to exceed that of his standard star 27,408 times, and Dr. Wollaston
having shown that the light of the full moon is to that of the sun as
1:801,072[128] (Zöllner made the ratio 1:618,000), it became possible to
compare stellar with solar radiance. Hence was derived, in the case of
the few stars at ascertained distances, a knowledge of real lustre.
Alpha Centauri, for example, emits less than twice, Capella one hundred
times as much light as our sun; while Arcturus, at its enormous
distance, must display the splendour of 1,300 such luminaries.

Herschel returned to England in the spring of 1838, bringing with him a
wealth of observation and discovery such as had perhaps never before
been amassed in so short a time. Deserved honours awaited him. He was
created a baronet on the occasion of the Queen's coronation (he had been
knighted in 1831); universities and learned societies vied with each
other in showering distinctions upon him; and the success of an
enterprise in which scientific zeal was tinctured with an attractive
flavour of adventurous romance, was justly regarded as a matter of
national pride. His career as an observing astronomer was now virtually
closed, and he devoted his leisure to the collection and arrangement of
the abundant trophies of his father's and his own activity. The
resulting great catalogue of 5,079 nebulæ (including all then certainly
known), published in the _Philosophical Transactions_ for 1864, is, and
will probably long remain, the fundamental source of information on the
subject;[129] but he unfortunately did not live to finish the companion
work on double stars, for which he had accumulated a vast store of
materials.[130] He died at Collingwood in Kent, May 11, 1871, in the
eightieth year of his age, and was buried in Westminster Abbey, close
beside the grave of Sir Isaac Newton.

The consideration of Sir John Herschel's Cape observations brings us to
the close of the period we are just now engaged in studying. They were
given to the world, as already stated, three years before the middle of
the century, and accurately represent the condition of sidereal science
at that date. Looking back over the fifty years traversed, we can see at
a glance how great was the stride made in the interval. Not alone was
acquaintance with individual members of the cosmos vastly extended, but
their mutual relations, the laws governing their movements, their
distances from the earth, masses, and intrinsic lustre, had begun to be
successfully investigated. _Begun to be_; for only regarding a scarcely
perceptible minority had even approximate conclusions been arrived at.
Nevertheless the whole progress of the future lay in that beginning; it
was the thin end of the wedge of exact knowledge. The principle of
measurement had been substituted for that of probability; a basis had
been found large and strong enough to enable calculation to ascend from
it to the sidereal heavens; and refinements had been introduced,
fruitful in performance, but still more in promise. Thus, rather the
kind than the amount of information collected was significant for the
time to come--rather the methods employed than the results actually
secured rendered the first half of the nineteenth century of epochal
importance in the history of our knowledge of the stars.


FOOTNOTES:

[Footnote 58: Bessel, _Populäre Vorlesungen_, pp. 6, 408.]

[Footnote 59: Fitted to the old transit instrument, July 11, 1772.]

[Footnote 60: _Briefwechsel mit Olbers_, p. xvi.]

[Footnote 61: R. Wolf, _Gesch. der Astron._, p. 518.]

[Footnote 62: Bessel, _Pop. Vorl._, p. 22.]

[Footnote 63: A new reduction of the observations upon which they were
founded was undertaken in 1896 by Herman S. Davis, of the U.S. Coast
Survey.]

[Footnote 64: Bessel, _Pop. Vorl._, p. 440.]

[Footnote 65: Durège, _Bessel's Leben und Wirken_, p. 28.]

[Footnote 66: _Bonner Beobachtungen_, Bd. iii.-v., 1859-62.]

[Footnote 67: Bessel, _Pop. Vorl._, p. 238.]

[Footnote 68: The heads of the screws applied to move the halves of the
object-glass in the Königsberg heliometer are of so considerable a size
that a thousandth part of a revolution, equivalent to 1/20 of a second
of arc, can be measured with the utmost accuracy. Main, _R. A. S. Mem._,
vol. xii., p. 53.]

[Footnote 69: _Specola Astronomica di Palermo_, lib. vi., p. 10,
_note_.]

[Footnote 70: _Monatliche Correspondenz_, vol. xxvi., p. 162.]

[Footnote 71: _Astronomische Nachrichten_, Nos. 365-366. It should be
explained that what is called the "annual parallax" of a star is only
half its apparent displacement. In other words, it is the angle
subtended at the distance of that particular star by the _radius_ of the
earth's orbit.]

[Footnote 72: _Astr. Nach._, Nos. 401-402.]

[Footnote 73: Sir R. Ball's measurements at Dunsink gave to 61 Cygni a
parallax of 0·47"; Professor Pritchard obtained, by photographic
determinations, one of 0·43".]

[Footnote 74: _Additamentum in Mensuras Micrometricas_, p. 28.]

[Footnote 75: Elkin's corrected result (in 1897) for the parallax of
Vega is 0·082".]

[Footnote 76: _Mem. Roy. Astr. Soc._, vol. xi., p. 61.]

[Footnote 77: That numbered 21,185 in Lalande's _Hist. Cél._, found by
Argelander to have a proper motion of 4·734", and by Winnecke a parallax
of O·511". _Month. Not._, vol. xviii., p. 289.]

[Footnote 78: _Fund. Astr._, p. 309.]

[Footnote 79: _Mém. Prés. à l'Ac. de St. Pétersb._, t. iii.]

[Footnote 80: _Phil. Trans._, vol. cxxxvii., p. 79.]

[Footnote 81: _Mem. Roy. Astr. Soc._, vols. xxviii. and xxxii.]

[Footnote 82: _Ibid._, vol. xlvii., p. 327.]

[Footnote 83: _Mémoires de St. Pétersbourg_, t. xxxv., No. 3, 1887;
revised in _Astr. Nach._, Nos. 3,729-30, 1901.]

[Footnote 84: _Astronomical Journal_, Nos. 213, 501.]

[Footnote 85: _Astr. Nach._, Nos. 2,999, 3,000.]

[Footnote 86: _Veröffentlichungen der Grossh. Sternwarte zu Karlsruhe_,
Bd. iv., 1892.]

[Footnote 87: _Proceedings Amsterdam Acad. of Sciences_, Jan. 27, 1900.]

[Footnote 88: _Astr. Jour._, No. 457.]

[Footnote 89: _Ibid._, Nos. 276, 497.]

[Footnote 90: _Phil. Trans._, vol. xcvi., p. 230.]

[Footnote 91: _Mém. Prés. à l'Ac. de St. Pétersbourg_, t. iii., p. 603
(read Feb. 5, 1837).]

[Footnote 92: _Die Centralsonne, Astr. Nach._, Nos. 566-567, 1846.]

[Footnote 93: Sir J. Herschel, note to _Treatise on Astronomy_, and
_Phil. Trans._, vol. cxxiii., part ii., p. 502.]

[Footnote 94: The position is (as Sir J. Herschel pointed out, _Outlines
of Astronomy_, p. 631, 10th ed.) placed beyond the range of reasonable
probability by its remoteness (fully 26°) from the galactic plane.]

[Footnote 95: Mädler in _Westermann's Jahrbuch_, 1867, p. 615.]

[Footnote 96: Letter from Bessel to Sir J. Herschel, _Month. Not._, vol.
vi., p. 139.]

[Footnote 97: Wolf, _Gesch. d. Astr._, p. 743, _note_.]

[Footnote 98: _Astr. Nach._, Nos. 745-748.]

[Footnote 99: _Astr. Jour._, No. 440.]

[Footnote 100: Adopting Elkin's revised parallax for Procyon of 0·325".]

[Footnote 101: _Astr. Nach._, Nos. 1371-1373.]

[Footnote 102: _Ueber die Doppelsterne_, Bericht, 1827, p. 22.]

[Footnote 103: _Ueber die Doppelsterne_, Bericht, 1827, p. 25.]

[Footnote 104: _Mensuræ Micr._, p. xcix.]

[Footnote 105: _Stellarum Fixarum imprimis Duplicium et Multiplicum
Positiones Mediæ_, pp. cxc., cciii.]

[Footnote 106: For instance, the southern stars, 36A Ophiuchi (itself
double) and 30 Scorpii, which are 12' 10" apart. _Ibid._, p. cciii.]

[Footnote 107: _Stellarum Fixarum_, etc., p. ccliii.]

[Footnote 108: _Études d'Astronomie Stellaire_, 1847, p. 82.]

[Footnote 109: _Ibid._, p. 86.]

[Footnote 110: See Encke's criticism in _Astr. Nach._, No. 622.]

[Footnote 111: _Phil. Trans._, vol. cxiv., part iii., 1824.]

[Footnote 112: _Conn. d. Temps_, 1830.]

[Footnote 113: _R. A. S. Mem._, vol. v., p. 178, 1833.]

[Footnote 114: _Astr. and Astrophysics_, vol. xii., p. 581.]

[Footnote 115: _Popular Astr._, vol. i., p. 243.]

[Footnote 116: _Phil. Trans._, vol. cxxiii., and _Results_, etc.,
Introd.]

[Footnote 117: _Results of Astronomical Observations made during the
years 1834-8 at the Cape of Good Hope._]

[Footnote 118: _Results_, etc., p. 147.]

[Footnote 119: See Proctor's _Universe of Stars_, p. 92.]

[Footnote 120: _A Treatise on Astronomy_, 1833, p. 406.]

[Footnote 121: _Results_, etc., p. 139.]

[Footnote 122: _Ibid._, pp. 24, 142.]

[Footnote 123: _Phil. Trans._, vol. cxxiii., p. 503.]

[Footnote 124: _Results_, etc., p. 136.]

[Footnote 125: Loomis, _Month. Not._, vol. xxix., p. 298.]

[Footnote 126: See the Author's _System of the Stars_, pp. 116-120.]

[Footnote 127: _Outlines of Astr._, App. I.]

[Footnote 128: _Phil. Trans._, vol. cxix., p. 27.]

[Footnote 129: Dr. Dreyer's New General Catalogue, published in 1888 as
vol. xlix. of the Royal Astronomical Society's _Memoirs_, is an
enlargement of Herschel's work. It includes 7,840 entries, and was
supplemented, in 1895, by an "Index Catalogue" of 1,529 nebulæ
discovered 1888 to 1894. _Mem. R. A. S._, vol. li.]

[Footnote 130: A list of 10,320 composite stars was drawn out by him in
order of right ascension, and has been published in vol. xl. of _Mem. R.
A. S._; but the data requisite for their formation into a catalogue were
not forthcoming. See Main's and Pritchard's _Preface_ to above, and
Dunkin's _Obituary Notices_, p. 73.]




                                CHAPTER III

                _PROGRESS OF KNOWLEDGE REGARDING THE SUN_


The discovery of sun-spots in 1610 by Fabricius and Galileo first opened
a way for inquiry into the solar constitution; but it was long before
that way was followed with system or profit. The seeming irregularity of
the phenomena discouraged continuous attention; casual observations were
made the basis of arbitrary conjectures, and real knowledge received
little or no increase. In 1620 we find Jean Tarde, Canon of Sarlat,
arguing that because the sun is "the eye of the world," and the eye of
the world _cannot suffer from ophthalmia_, therefore the appearances in
question must be due, not to actual specks or stains on the bright solar
disc, but to the transits of a number of small planets across it! To
this new group of heavenly bodies he gave the name of "Borbonia Sidera,"
and they were claimed in 1633 for the House of Hapsburg, under the title
of "Austriaca Sidera" by Father Malapertius, a Belgian Jesuit.[131] A
similar view was temporarily maintained against Galileo by the justly
celebrated Father Scheiner of Ingolstadt, and later by William
Gascoigne, the inventor of the micrometer; but most of those who were
capable of thinking at all on such subjects (and they were but few)
adhered either to the _cloud theory_ or to the _slag theory_ of
sun-spots. The first was championed by Galileo, the second by Simon
Marius, "astronomer and physician" to the brother Margraves of
Brandenburg. The latter opinion received a further notable development
from the fact that in 1618, a year remarkable for the appearance of
three bright comets, the sun was almost free from spots; whence it was
inferred that the cindery refuse from the great solar conflagration,
which usually appeared as dark blotches on its surface, was occasionally
thrown off in the form of comets, leaving the sun, like a snuffed taper,
to blaze with renewed brilliancy.[132]

In the following century, Derham gathered from observations carried on
during the years 1703-11, "That the spots on the sun are caused by the
eruption of some new volcano therein, which at first pouring out a
prodigious quantity of smoke and other opacous matter, causeth the
spots; and as that fuliginous matter decayeth and spendeth itself, and
the volcano at last becomes more torrid and flaming, so the spots decay,
and grow to umbræ, and at last to faculæ."[133]

The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbræ representing shoals or sandbanks, had even less to
recommend it than Derham's volcanic theory. Both were, however,
significant of a growing tendency to bring solar phenomena within the
compass of terrestrial analogies.

For 164 years, then, after Galileo first levelled his telescope at the
setting sun, next to nothing was learned as to its nature; and the facts
immediately ascertained, of its rotation on an axis nearly erect to the
plane of the ecliptic, in a period of between twenty-five and twenty-six
days, and of the virtual limitation of the spots to a so-called "royal"
zone extending some thirty degrees north and south of the solar equator,
gained little either in precision or development from five generations
of astronomers.

But in November, 1769, a spot of extraordinary size engaged the
attention of Alexander Wilson, professor of astronomy in the University
of Glasgow. He watched it day by day, and to good purpose. As the great
globe slowly revolved, carrying the spot towards its western edge, he
was struck with the gradual contraction and final disappearance of the
penumbra _on the side next the centre of the disc_; and when on the 6th
of December the same spot re-emerged on the eastern limb, he perceived,
as he had anticipated, that the shady zone was now deficient _on the
opposite side_, and resumed its original completeness as it returned to
a central position. In other spots subsequently examined by him, similar
perspective effects were visible, and he proved in 1774,[135] by strict
geometrical reasoning, that they could only arise in vast photospheric
excavations. It was not, indeed, the first time that such a view had
been suggested. Father Scheiner's later observations plainly
foreshadowed it;[136] a conjecture to the same effect was emitted by
Leonard Rost of Nuremburg early in the eighteenth century;[137] both by
Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches
on the solar limb; while in 1770 Pastor Schülen of Essingen, from the
careful study of phenomena similar to those noted by Wilson, concluded
their depressed nature.[138] Modern observations, nevertheless, prove
those phenomena to be by no means universally present.

Wilson's general theory of the sun was avowedly tentative. It took the
modest form of an interrogatory. "Is it not reasonable to think," he
asks, "that the great and stupendous body of the sun is made up of two
kinds of matter, very different in their qualities; that by far the
greater part is solid and dark, and that this immense and dark globe is
encompassed with a thin covering of that resplendent substance from
which the sun would seem to derive the whole of his vivifying heat and
energy?"[139] He further suggests that the excavations or spots may be
occasioned "by the working of some sort of elastic vapour which is
generated within the dark globe," and that the luminous matter, being in
some degree fluid, and being acted upon by gravity, tends to flow down
and cover the nucleus. From these hints, supplemented by his own
diligent observations and sagacious reasonings, Herschel elaborated a
scheme of solar constitution which held its ground until the physics of
the sun were revolutionised by the spectroscope.

A cool, dark, solid globe, its surface diversified with mountains and
valleys, clothed in luxuriant vegetation, and "richly stored with
inhabitants," protected by a heavy cloud-canopy from the intolerable
glare of the upper luminous region, where the dazzling coruscations of a
solar aurora some thousands of miles in depth evolved the stores of
light and heat which vivify our world--such was the central luminary
which Herschel constructed with his wonted ingenuity, and described with
his wonted eloquence.

"This way of considering the sun and its atmosphere," he says,[140]
"removes the great dissimilarity we have hitherto been used to find
between its condition and that of the rest of the great bodies of the
solar system. The sun, viewed in this light, appears to be nothing else
than a very eminent, large, and lucid planet, evidently the first, or,
in strictness of speaking, the only primary one of our system; all
others being truly secondary to it. Its similarity to the other globes
of the solar system with regard to its solidity, its atmosphere, and its
diversified surface, the rotation upon its axis, and the fall of heavy
bodies, leads us on to suppose that it is most probably also inhabited,
like the rest of the planets, by beings whose organs are adapted to the
peculiar circumstances of that vast globe."

We smile at conclusions which our present knowledge condemns as
extravagant and impossible, but such incidental flights of fancy in no
way derogate from the high value of Herschel's contributions to solar
science. The cloud-like character which he attributed to the radiant
shell of the sun (first named by Schröter the "photosphere") is borne
out by all recent investigations; he observed its mottled or corrugated
aspect, resembling, as he described it, the roughness on the rind of an
orange; showed that "faculæ" are elevations or heaped-up ridges of the
disturbed photospheric matter; and threw out the idea that spots may
ensue from an excess of the ordinary luminous emissions. A certain
"empyreal" gas was, he supposed (very much as Wilson had done),
generated in the body of the sun, and rising everywhere by reason of its
lightness, made for itself, when in moderate quantities, small openings
or "pores,"[141] abundantly visible as dark points on the solar disc.
But should an uncommon quantity be formed, "it will," he maintained,
"burst through the planetary[142] regions of clouds, and thus will
produce great openings; then, spreading itself above them, it will
occasion large shallows (penumbræ), and mixing afterwards gradually with
other superior gases, it will promote the increase, and assist in the
maintenance, of the general luminous phenomena."[143]

This partial anticipation of the modern view that the solar radiations
are maintained by some process of circulation within the solar mass, was
reached by Herschel through prolonged study of the phenomena in
question. The novel and important idea contained in it, however, it was
at that time premature to attempt to develop. But though many of the
subtler suggestions of Herschel's genius passed unnoticed by his
contemporaries, the main result of his solar researches was an
unmistakable one. It was nothing less than the definitive introduction
into astronomy of the paradoxical conception of the central fire and
hearth of our system as a cold, dark, terrestrial mass, wrapt in a
mantle of innocuous radiance--an earth, so to speak, within--a sun
without.

Let us pause for a moment to consider the value of this remarkable
innovation. It certainly was not a step in the direction of truth. On
the contrary, the crude notions of Anaxagoras and Xeno approached more
nearly to what we now know of the sun, than the complicated structure
devised for the happiness of a nobler race of beings than our own by the
benevolence of eighteenth-century astronomers. And yet it undoubtedly
constituted a very important advance in science. It was the first
earnest attempt to bring solar phenomena within the compass of a
rational system; to put together into a consistent whole the facts
ascertained; to fabricate, in short, a solar machine that would in some
fashion work. It is true that the materials were inadequate and the
design faulty. The resulting construction has not proved strong enough
to stand the wear and tear of time and discovery, but has had to be
taken to pieces and remodelled on a totally different plan. But the work
was not therefore done in vain. None of Bacon's aphorisms show a clearer
insight into the relations between the human mind and the external world
than that which declares "Truth to emerge sooner from error than from
confusion."[144] A definite theory (even if a false one) gives
holding-ground to thought. Facts acquire a meaning with reference to it.
It affords a motive for accumulating them and a means of co-ordinating
them; it provides a framework for their arrangement, and a receptacle
for their preservation, until they become too strong and numerous to be
any longer included within arbitrary limits, and shatter the vessel
originally framed to contain them.

Such was the purpose subserved by Herschel's theory of the sun. It
helped to _clarify_ ideas on the subject. The turbid sense of groping
and viewless ignorance gave place to the lucidity of a possible scheme.
The persuasion of knowledge is a keen incentive to its increase. Few men
care to investigate what they are obliged to admit themselves entirely
ignorant of; but once started on the road of knowledge, real or
supposed, they are eager to pursue it. By the promulgation of a
confident and consistent view regarding the nature of the sun,
accordingly, research was encouraged, because it was rendered hopeful,
and inquirers were shown a path leading indefinitely onwards where an
impassable thicket had before seemed to bar the way.

We have called the "terrestrial" theory of the sun's nature an
innovation, and so, as far as its general acceptance is concerned, it
may justly be termed; but, like all successful innovations, it was a
long time brewing. It is extremely curious to find that Herschel had a
predecessor in its advocacy who never looked through a telescope (nor,
indeed, imagined the possibility of such an instrument), who knew
nothing of sun-spots, was still (mistaken assertions to the contrary
notwithstanding) in the bondage of the geocentric system, and regarded
nature from the lofty standpoint of an idealist philosophy. This was the
learned and enlightened Cardinal Cusa, a fisherman's son from the banks
of the Moselle, whose distinguished career in the Church and in
literature extended over a considerable part of the fifteenth century
(1401-64). In his singular treatise _De Doctâ Ignorantiâ_, one of the
most notable literary monuments of the early Renaissance, the following
passage occurs:--"To a spectator on the surface of the sun, the
splendour which appears to us would be invisible, since it contains, as
it were, an earth for its central mass, with a circumferential envelope
of light and heat, and between the two an atmosphere of water and clouds
and translucent air." The luminary of Herschel's fancy could scarcely be
more clearly portrayed; some added words, however, betray the origin of
the Cardinal's idea. "The earth also," he says, "would appear as a
shining star to any one outside the fiery element." It was, in fact, an
extension to the sun of the ancient elemental doctrine; but an extension
remarkable at that period, as premonitory of the tendency, so powerfully
developed by subsequent discoveries, to assimilate the orbs of heaven to
the model of our insignificant planet, and to extend the brotherhood of
our system and our species to the farthest limit of the visible or
imaginable universe.

In later times we find Flamsteed communicating to Newton, March 7, 1681,
his opinion "that the substance of the sun is terrestrial matter, his
light but the liquid menstruum encompassing him."[145] Bode in 1776
arrived independently at the conclusion that "the sun is neither burning
nor glowing, but in its essence a dark planetary body, composed like our
earth of land and water, varied by mountains and valleys, and enveloped
in a vaporous atmosphere";[146] and the learned in general applauded and
acquiesced. The view, however, was in 1787 still so far from popular,
that the holding of it was alleged as a proof of insanity in Dr. Elliot
when accused of a murderous assault on Miss Boydell. His friend Dr.
Simmons stated on his behalf that he had received from him in the
preceding January a letter giving evidence of a deranged mind, wherein
he asserted "that the sun is not a body of fire, as hath been hitherto
supposed, but that its light proceeds from a dense and universal aurora,
which may afford ample light to the inhabitants of the surface beneath,
and yet be at such a distance aloft as not to annoy them. No objection,
he saith, ariseth to that great luminary's being inhabited; vegetation
may obtain there as well as with us. There may be water and dry land,
hills and dales, rain and fair weather; and as the light, so the season
must be eternal, consequently it may easily be conceived to be by far
the most blissful habitation of the whole system!" The Recorder,
nevertheless, objected that if an extravagant hypothesis were to be
adduced as proof of insanity, the same might hold good with regard to
some other speculators, and desired Dr. Simmons to tell the court what
he thought of the theories of Burnet and Buffon.[147]

Eight years later, this same "extravagant hypothesis," backed by the
powerful recommendation of Sir William Herschel, obtained admittance to
the venerable halls of science, there to abide undisturbed for nearly
seven decades. Individual objectors, it is true, made themselves heard,
but their arguments had little effect on the general body of opinion.
Ruder blows were required to shatter an hypothesis flattering to human
pride of invention in its completeness, in the plausible detail of
observations by which it seemed to be supported, and in its
condescension to the natural pleasure in discovering resemblance under
all but total dissimilarity.

Sir John Herschel included among the results of his multifarious labours
at the Cape of Good Hope a careful study of the sun-spots conspicuously
visible towards the end of the year 1836 and in the early part of 1837.
They were remarkable, he tells us, for their forms and arrangement, as
well as for their number and size; one group, measured on the 29th of
March in the latter year, covering (apart from what may be called its
outlying dependencies) the vast area of five square minutes or 3,780
million square miles.[148] We have at present to consider, however, not
so much these observations in themselves, as the chain of theoretical
suggestions by which they were connected. The distribution of spots, it
was pointed out, on two zones parallel to the equator, showed plainly
their intimate connection with the solar rotation, and indicated as
their cause fluid circulations analogous to those producing the
terrestrial trade and anti-trade winds.

"The spots, in this view of the subject," he went on to say,[149] "would
come to be assimilated to those regions on the earth's surface where,
for the moment, hurricanes and tornadoes prevail; the upper stratum
being temporarily carried downwards, displacing by its impetus the two
strata of luminous matter beneath, the upper of course to a greater
extent than the lower, and thus wholly or partially denuding the opaque
surface of the sun below. Such processes cannot be unaccompanied by
vorticose motions, which, left to themselves, die away by degrees and
dissipate, with the peculiarity that their lower portions come to rest
more speedily than their upper, by reason of the greater resistance
below, as well as the remoteness from the point of action, which lies in
a higher region, so that their centres (as seen in our waterspouts,
which are nothing but small tornadoes) appear to retreat upwards. Now
this agrees perfectly with what is observed during the obliteration of
the solar spots, which appear as if filled in by the collapse of their
sides, the penumbra closing in upon the spot and disappearing after it."

But when it comes to be asked whether a cause can be found by which a
diversity of solar temperature might be produced corresponding with that
which sets the currents of the terrestrial atmosphere in motion, we are
forced to reply that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equator, due
to the slightly spheroidal or bulging form of its outer atmospheric
envelope, assuredly gives no sufficient account of such circulatory
movements as he supposed to exist. Nevertheless, the view that the sun's
rotation is intimately connected with the formation of spots is so
obviously correct, that we can only wonder it was not thought of sooner,
while we are even now unable to explain with any certainty _how_ it is
so connected.

Mere scrutiny of the solar surface, however, is not the only means of
solar observation. We have a satellite, and that satellite from time to
time acts most opportunely as a screen, cutting off a part or the whole
of those dazzling rays in which the master-orb of our system veils
himself from over-curious regards. The importance of eclipses to the
study of the solar surroundings is of comparatively recent recognition;
nevertheless, much of what we know concerning them has been snatched, as
it were, by surprise under favour of the moon. In former times, the sole
astronomical use of such incidents was the correction of the received
theories of the solar and lunar movements; the precise time of their
occurrence was the main fact to be noted, and subsidiary phenomena
received but casual attention. Now, their significance as a geometrical
test of tabular accuracy is altogether overshadowed by the interest
attaching to the physical observations for which they afford propitious
occasions. This change may be said to date, in its pronounced form, from
the great eclipse of 1842. Although a necessary consequence of the
general direction taken by scientific progress, it remains associated in
a special manner with the name of Francis Baily.

The "philosopher of Newbury" was by profession a London stockbroker, and
a highly successful one. Nevertheless, his services to science were
numerous and invaluable, though not of the brilliant kind which attract
popular notice. Born at Newbury in Berkshire, April 28, 1774, and placed
in the City at the age of fourteen, he derived from the acquaintance of
Dr. Priestley a love of science which never afterwards left him. It was,
however, no passion such as flames up in the brain of the destined
discoverer, but a regulated inclination, kept well within the bounds of
an actively pursued commercial career. After travelling for a year or
two in what were then the wilds of North America, he went on the Stock
Exchange in 1799, and earned during twenty-four years of assiduous
application to affairs a high reputation for integrity and ability, to
which corresponded an ample fortune. In the meantime the Astronomical
Society (largely through his co-operation) had been founded; he had for
three years acted as its secretary, and he now felt entitled to devote
himself exclusively to a subject which had long occupied his leisure
hours. He accordingly in 1825 retired from business, purchased a house
in Tavistock Place, and fitted up there a small observatory. He was,
however, by preference a computator rather than an observer. What Sir
John Herschel calls the "archæology of practical astronomy" found in him
an especially zealous student. He re-edited the star-catalogues of
Ptolemy, Ulugh Beigh, Tycho Brahe, Hevelius, Halley, Flamsteed,
Lacaille, and Mayer; calculated the eclipse of Thales and the eclipse of
Agathocles, and vindicated the memory of the first Astronomer Royal. But
he was no less active in meeting present needs than in revising past
performances. The subject of the reduction of observations, then, as we
have already explained,[150] in a state of deplorable confusion,
attracted his most earnest attention, and he was close on the track of
Bessel when made acquainted with the method of simplification devised at
Königsberg. Anticipated as an inventor, he could still be of eminent use
as a promoter of these valuable improvements; and, carrying them out on
a large scale in the star-catalogue of the Astronomical Society
(published in 1827), "he put" (in the words of Herschel) "the
astronomical world in possession of a power which may be said, without
exaggeration, to have changed the face of sidereal astronomy."[151]

His reputation was still further enhanced by his renewal, with vastly
improved apparatus, of the method, first used by Henry Cavendish in
1797-98, for determining the density of the earth. From a series of no
less than 2,153 delicate and difficult experiments, conducted at
Tavistock Place during the years 1838-42, he concluded our planet to
weigh 5·66 as much as a globe of water of the same bulk; and this result
slightly corrected is still accepted as a very close approximation of
the truth.

What we have thus glanced at is but a fragment of the truly surprising
mass of work accomplished by Baily in the course of a variously occupied
life. A rare combination of qualities fitted him for his task. Unvarying
health, undisturbed equanimity, methodical habits, the power of directed
and sustained thought, combined to form in him an intellectual toiler of
the surest, though not perhaps of the highest quality. He was in harness
almost to the end. He was destined scarcely to know the miseries of
enforced idleness or of consciously failing powers. In 1842 he completed
the laborious reduction of Lalande's great catalogue, undertaken at the
request of the British Association, and was still engaged in seeing it
through the press when he was attacked with what proved his last, as it
was probably his first serious illness. He, however, recovered
sufficiently to attend the Oxford Commemoration of July 2, 1844, where
an honorary degree of D.C.L. was conferred upon him in company with Airy
and Struve; but sank rapidly after the effort, and died on the 30th of
August following, at the age of seventy, lamented and esteemed by all
who knew him.

It is now time to consider his share in the promotion of solar research.
Eclipses of the sun, both ancient and modern, were a speciality with
him, and he was fortunate in those which came under his observation.
Such phenomena are of three kinds--partial, annular, and total. In a
partial eclipse, the moon, instead of passing directly between us and
the sun, slips by, as it were, a little on one side, thus cutting off
from our sight only a portion of his surface. An annular eclipse, on the
other hand, takes place when the moon is indeed centrally interposed,
but falls short of the apparent size required for the entire concealment
of the solar disc, which consequently remains visible as a bright ring
or annulus, even when the obscuration is at its height. In a total
eclipse, on the contrary, the sun completely disappears behind the dark
body of the moon. The difference of the two latter varieties is due to
the fact that the apparent diameter of the sun and moon are so nearly
equal as to gain alternate preponderance one over the other through the
slight periodical changes in their respective distances from the earth.

Now, on the 15th of May, 1836, an annular eclipse was visible in the
northern parts of Great Britain, and was observed by Baily at Inch
Bonney, near Jedburgh. It was here that he saw the phenomenon which
obtained the name of "Baily's Beads," from the notoriety conferred upon
it by his vivid description.

"When the cusps of the sun," he writes, "were about 40° asunder, a row
of lucid points, like a string of bright beads, irregular in size and
distance from each other, _suddenly_ formed round that part of the
circumference of the moon that was about to enter on the sun's disc. Its
formation, indeed, was so rapid that it presented the appearance of
having been caused by the ignition of a fine train of gunpowder.
Finally, as the moon pursued her course, the dark intervening spaces
(which, at their origin, had the appearance of lunar mountains in high
relief, and which still continued attached to the sun's border) were
stretched out into long, black, thick, parallel lines, joining the limbs
of the sun and moon; when all at once they _suddenly_ gave way, and left
the circumference of the sun and moon in those points, as in the rest,
comparatively smooth and circular, and the moon perceptibly advanced on
the face of the sun."[152]

These curious appearances were not an absolute novelty. Weber in 1791,
and Von Zach in 1820, had seen the "beads"; Van Swinden had described
the "belts" or "threads."[153] These last were, moreover (as Baily
clearly perceived), completely analogous to the "black ligament" which
formed so troublesome a feature in the transits of Venus in 1764 and
1769, and which, to the regret and confusion, though no longer to the
surprise of observers, was renewed in that of 1874. The phenomenon is
largely an effect of what is called _irradiation_, by which a bright
object seems to encroach upon a dark one; but under good atmospheric and
instrumental conditions it becomes inconspicuous. The "Beads" must
always appear when the projected lunar edge is serrated with mountains.
In Baily's observation, they were exaggerated and distorted by an
irradiative _clinging together_ of the limbs of sun and moon.

The immediate result, however, was powerfully to stimulate attention to
solar eclipses in their _physical_ aspect. Never before had an
occurrence of the kind been expected so eagerly or prepared for so
actively as that which was total over Central and Southern Europe on the
8th of July, 1842. Astronomers hastened from all quarters to the
favoured region. The Astronomer Royal (Airy) repaired to Turin; Baily to
Pavia; Otto Struve threw aside his work amidst the stars at Pulkowa, and
went south as far as Lipeszk; Schumacher travelled from Altona to
Vienna; Arago from Paris to Perpignan. Nor did their trouble go
unrewarded. The expectations of the most sanguine were outdone by the
wonders disclosed.

Baily (to whose narrative we again have recourse) had set up his
Dollond's achromatic in an upper room of the University of Pavia, and
was eagerly engaged in noting a partial repetition of the singular
appearances seen by him in 1836, when he was "astounded by a tremendous
burst of applause from the streets below, and at the same moment was
electrified at the sight of one of the most brilliant and splendid
phenomena that can well be imagined. For at that instant the dark body
of the moon was suddenly surrounded with a corona, or kind of bright
glory similar in shape and relative magnitude to that which painters
draw round the heads of saints, and which by the French is designated an
_auréole_. Pavia contains many thousand inhabitants, the major part of
whom were, at this early hour, walking about the streets and squares or
looking out of windows, in order to witness this long-talked-of
phenomenon; and when the total obscuration took place, which was
_instantaneous_, there was a universal shout from every observer, which
'made the welkin ring,' and, for the moment, withdrew my attention from
the object with which I was immediately occupied. I had indeed
anticipated the appearance of a luminous circle round the moon during
the time of total obscurity; but I did not expect, from any of the
accounts of preceding eclipses that I had read, to witness so
magnificent an exhibition as that which took place.... The breadth of
the corona, measured from the circumference of the moon, appeared to me
to be nearly equal to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense close to the border of the
moon, and became gradually and uniformly more attenuate as its distance
therefrom increased, assuming the form of diverging rays in a
rectilinear line, which at the extremity were more divided, and of an
unequal length; so that in no part of the corona could I discover the
regular and well-defined shape of a ring at its _outer_ margin. It
appeared to me to have the sun for its centre, but I had no means of
taking any accurate measures for determining this point. Its colour was
quite white, not pearl-colour, nor yellow, nor red, and the rays had a
vivid and flickering appearance, somewhat like that which a gaslight
illumination might be supposed to assume if formed into a similar
shape.... Splendid and astonishing, however, as this remarkable
phenomenon really was, and although it could not fail to call forth the
admiration and applause of every beholder, yet I must confess that there
was at the same time something in its singular and wonderful appearance
that was appalling; and I can readily imagine that uncivilised nations
may occasionally have become alarmed and terrified at such an object,
more especially at times when the true cause of the occurrence may have
been but faintly understood, and the phenomenon itself wholly
unexpected.

"But the most remarkable circumstance attending the phenomenon was the
appearance of _three large protuberances_ apparently emanating from the
circumference of the moon, but evidently forming a portion of the
corona. They had the appearance of mountains of a prodigious elevation;
their colour was red, tinged with lilac or purple; perhaps the colour of
the peach-blossom would more nearly represent it. They somewhat
resembled the snowy tops of the Alpine mountains when coloured by the
rising or setting sun. They resembled the Alpine mountains also in
another respect, inasmuch as their light was perfectly steady, and had
none of that flickering or sparkling motion so visible in other parts of
the corona. All the three projections were of the same roseate cast of
colour, and very different from the brilliant vivid white light that
formed the corona; but they differed from each other in magnitude....
The whole of these three protuberances were visible even to the last
moment of total obscuration; at least, I never lost sight of them when
looking in that direction; and when the first ray of light was admitted
from the sun, they vanished, with the corona, altogether, and daylight
was instantaneously restored."[154]

Notwithstanding unfavourable weather, the "red flames" were perceived
with little less clearness and no less amazement from the Superga than
at Pavia, and were even discerned by Mr. Airy with the naked eye. "Their
form" (the Astronomer Royal wrote) "was nearly that of saw-teeth in the
position proper for a circular saw turned round in the same direction in
which the hands of a watch turn.... Their colour was a full lake-red,
and their brilliancy greater than that of any other part of the
ring."[155]

The height of these extraordinary objects was estimated by Arago at two
minutes of arc, representing, at the sun's distance, an actual elevation
of 54,000 miles. When carefully watched, the rose-flush of their
illumination was perceived to fade through violet to white as the light
returned, the same changes in a reversed order having accompanied their
first appearance. Their forms, however, during about three minutes of
visibility, showed no change, although of so apparently unstable a
character as to suggest to Arago "mountains on the point of crumbling
into ruins" through topheaviness.[156]

The corona, both as to figure and extent, presented very different
appearances at different stations. This was no doubt due to varieties in
atmospheric conditions. At the Superga, for instance, all details of
structure seem to have been effaced by the murky air, only a
comparatively feeble ring of light being seen to encircle the moon.
Elsewhere, a brilliant radiated formation was conspicuous, spreading at
four opposite points into four vast luminous expansions, compared to
feather-plumes or _aigrettes_.[157] Arago at Perpignan noticed
considerable irregularities in the divergent rays. Some appeared curved
and twisted, a few lay _across_ the others, in a direction almost
tangential to the moon's limb, the general effect being described as
that of a "hank of thread in disorder."[158] At Lipeszk, where the sun
stood much higher above the horizon than in Italy or France, the corona
showed with surprising splendour. Its apparent extent was judged by
Struve to be no less than twenty-five minutes (more than six times
Airy's estimate), while the great plumes spread their radiance to three
or four degrees from the dark lunar edge. So dazzling was the light that
many well-instructed persons denied the totality of the eclipse. Nor was
the error without precedent, although the appearances attending
respectively a total and an annular eclipse are in reality wholly
dissimilar. In the latter case, the surviving ring of sunlight becomes
so much enlarged by irradiation, that the interposed dark lunar body is
reduced to comparative insignificance, or even invisibility. Maclaurin
tells us[159] that during an eclipse of this character which he observed
at Edinburgh in 1737, "gentlemen by no means shortsighted declared
themselves unable to discern the moon upon the sun without the aid of a
smoked glass;" and Baily (who, however, _was_ shortsighted) could
distinguish, in 1836, with the naked eye, no trace of "the globe of
purple velvet" which the telescope revealed as projected upon the face
of the sun.[160] Moreover, the diminution of light is described by him
as "little more than might be caused by a temporary cloud passing over
the sun"; the birds continued in full song, and "one cock in particular
was crowing with all his might while the annulus was forming."

Very different were the effects of the eclipse of 1842, as to which some
interesting particulars were collected by Arago.[161] Beasts of burthen,
he tells us, paused in their labour, and could by no amount of
punishment be induced to move until the sun reappeared. Birds and beasts
abandoned their food; linnets were found dead in their cages; even ants
suspended their toil. Diligence-horses, on the other hand, seemed as
insensible to the phenomenon as locomotives. The convolvulus and some
other plants closed their leaves, but those of the mimosa remained open.
The little light that remained was of a livid hue. One observer
described the general coloration as resembling the lees of wine, but
human faces showed pale olive or greenish. We may, then, rest assured
that none of the remarkable obscurations recorded in history were due to
eclipses of the annular kind.

The existence of the corona is no modern discovery. Indeed, it is too
conspicuous an apparition to escape notice from the least attentive or
least practised observer of a total eclipse. Nevertheless, explicit
references to it are rare in early times. Plutarch, however, speaks of a
"certain splendour" compassing round the hidden edge of the sun, as a
regular feature of total eclipses;[162] and the corona is expressly
mentioned in a description of an eclipse visible at Corfu in 968
A.D.[163] The first to take the phenomenon into scientific consideration
was Kepler. He showed, from the orbital positions at the time of the sun
and moon, that an eclipse observed by Clavius at Rome in 1567 could not
have been annular,[164] as the dazzling coronal radiance visible during
the obscuration had caused it to be believed. Although he himself never
witnessed a total eclipse of the sun, he carefully collected and
compared the remarks of those more fortunate, and concluded that the
ring of "flame-like splendour" seen on such occasions was caused by the
reflection of the solar rays from matter condensed in the neighbourhood
either of the sun or moon.[165] To the solar explanation he gave his own
decided preference; but, with one of those curious flashes of
half-prophetic insight characteristic of his genius, declared that "it
should be laid by ready for use, not brought into immediate
requisition."[166] So literally was his advice acted upon, that the
theory, which we now know to be (broadly speaking) the correct one, only
emerged from the repository of anticipated truths after 236 years of
almost complete retirement, and even then timorously and with
hesitation.

The first eclipse of which the attendant phenomena were observed with
tolerable exactness was that which was central in the South of France,
May 12, 1706. Cassini then put forward the view that the "crown of pale
light" seen round the lunar disc was caused by the illumination of the
zodiacal light;[167] but it failed to receive the attention which, as a
step in the right direction, it undoubtedly merited. Nine years later we
meet with Halley's comments on a similar event, the first which had
occurred in London since March 20, 1140. By nine in the morning of May
3, 1715, the obscuration, he tells us, "was about ten digits,[168] when
the face and colour of the sky began to change from perfect serene azure
blue to a more dusky livid colour, having an eye of purple intermixt....
A few seconds before the sun was all hid, there discovered itself round
the moon a luminous ring, about a digit or perhaps a tenth part of the
moon's diameter in breadth. It was of a pale whiteness, or rather pearl
colour, seeming to be a little tinged with the colours of the iris, and
to be concentric with the moon, whence I concluded it the moon's
atmosphere. But the great height thereof, far exceeding our earth's
atmosphere, and the observation of some, who found the breadth of the
ring to increase on the west side of the moon as emersion approached,
together with the contrary sentiments of those whose judgment I shall
always revere" (Newton is most probably referred to), "makes me less
confident, especially in a matter whereto I confess I gave not all the
attention requisite." He concludes by declining to decide whether the
"enlightened atmosphere," which the appearance "in all respects
resembled," "belonged to sun or moon."[169]

A French Academician, who happened to be in London at the time, was less
guarded in expressing an opinion. The Chevalier de Louv

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