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THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
BEQUEST OF
PROFESSOR JOHN S. P. TATLOCK
HISTORY OF ASTRONOMY.
All rights reserved.
A POPULAR
HISTORY OF ASTRONOMY
DURING
THE NINETEENTH CENTURY
BY
AGNES M. CLERKE
JUPITER 1879
SATURN 1885
SECOND EDITION
EDINBURGH: A. & C. BLACK
NEW YORK: MACMILLAN & CO.
MDCCOLXXXVII
LIBRARY
PREFACE TO THE SECOND EDITION.
IN preparing the Second Edition, much advantage has been
derived from criticisms upon the first, some through the press,
others most gratefully received by the author from persons
of eminence and authority in science. Omissions have been
supplied, errors have been corrected, and pains taken to render
the work in every way more worthy of the cordial reception
granted to it on its first appearance. Astronomical research
has, in the short interval, made such rapid progress as to
demand the addition of a considerable amount of new matter
to the later Chapters, their growth illustrating, however
imperfectly, the vitality of the science. The Index has also
been enlarged, and a Chronological Table added for convenience
of reference. By the great kindness of Mr. Common and the
MM. Henry, to whom the author tenders her best acknow-
ledgments for their courteous assistance, some striking speci-
mens of celestial photography are presented in the form of a
Frontispiece and Vignette. Beyond any mere ornamental
effect, these beautiful autograph pictures have a scientific
value and significance which time will but enhance. The
process used in their reproduction is that of the Messrs.
Woodbury.
LONDON, March 1887.
PREFACE TO THE FIRST EDITION.
THE progress of astronomy during the last hundred years has
been rapid and extraordinary. In its distinctive features,
moreover, the nature of that progress has been such as to lend
itself with facility to untechnical treatment. To this circum-
stance the present volume owes its origin. It embodies an
attempt to enable the ordinary reader to follow, with intelli-
gent interest, the course of modern astronomical inquiries,
and to realise (so far as it can at present be realised) the full
effect of the comprehensive change in the whole aspect, pur-
poses, and methods of celestial science introduced by the
momentous discovery of spectrum analysis.
Since Professor Grant's invaluable work on the History of
Physical Astronomy was published, a third of a century has
elapsed. During the interval, a so-called " new astronomy "
has grown up by the side of the old. One effect of its advent
has been to render the science of the heavenly bodies more
popular, both in its needs and in its nature, than formerly.
More popular in its needs, since its progress now primarily
depends upon the interest in, and consequent efforts towards
its advancement of the general public ; more popular in its
nature, because the kind of knowledge it now chiefly tends to
accumulate is more easily intelligible less remote from ordi-
nary experience than that evolved by the aid of the calculus
viii PREFACE.
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 incon-
siderable of enlisting wider sympathies on its behalf ; while
to help one single mind towards a fuller understanding of the
manifold works which have, in all ages, irresistibly spoken to
man of the glory of God, might well be an object of no ignoble
ambition.
The present volume does not profess to be a complete or
exhaustive History of Astronomy during the period covered
by it. Its design is to present a view of the progress of
celestial science, on its most characteristic side, since the time
of Herschel. Abstruse mathematical theories, unless in some
of their more striking results, are excluded from consideration.
These, during the eighteenth century, constituted the sum and
substance of astronomy ; and their fundamental importance
can never be diminished, and should never be ignored. But,
as the outcome of the enormous development given to the
powers of the telescope in recent times, together with the
swift advances of physical science, and the inclusion, by means
of the spectroscope, of the heavenly bodies within the domain
of its inquiries, much knowledge has been acquired regarding
the nature and condition of those bodies, forming, it might be
said, a science apart, and disembarrassed from immediate de-
pendence upon intricate, and, except to the initiated, unin-
telligible formulae. This kind of knowledge forms the main
subject of the book now offered to the public.
There are many reasons for preferring a history to a formal
treatise on astronomy. In a treatise, what we know is set
PREFACE. ix
forth. A history tells us, in addition, how we came to know
it. It thus places facts before us in the natural order of their
ascertainment, and narrates instead of enumerating. The
story to be told leaves the marvels of imagination far behind,
and requires no embellishment from literary art or high-flown
phrases. Its best ornament is unvarnished truthfulness, and
this at least may confidently be claimed to be bestowed upon
it in the ensuing pages.
In them unity of treatment is sought to be combined with
a due regard to chronological sequence by grouping in sepa-
rate chapters the various events relating to the several depart-
ments of descriptive astronomy. The whole is divided into
two parts, the line between which is roughly drawn at the
middle of the present century. Herschel's inquiries into the
construction of the heavens strike the keynote of the first
part ; the discovery of sun-spot and magnetic periodicity and
of spectrum analysis, determine the character of the second.
Where the nature of the subject required it, however, this
arrangement has been disregarded. Clearness and consist-
ency should obviously take precedence of method. Thus,
in treating of the telescopic scrutiny of the various planets,
the whole of the related facts have been collected into an
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
x PREFACE.
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 deter-
mining 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 com-
munications.
LONDON, September 1885.
CONTENTS.
INTRODUCTION.
]
Three Kinds of Astronomy Progress of the Science during the
Eighteenth Century Popularity and Rapid Advance during
the Nineteenth Century
Ipart I.
PROGRESS OF ASTRONOMY DURING THE FIRST
HALF OF THE NINETEENTH CENTURY.
CHAPTER I.
FOUNDATION OF SIDEREAL ASTEONOMY.
State of Knowledge regarding the Stars in the Eighteenth Cen-
tury Career of Sir William Herschel Constitution of the
Stellar System Double Stars Herschel's Discovery of their
Revolutions His Method of Star-Gauging Discoveries of
Nebulae Theory of their Condensation into Stars Summary
of Results 1 1
CHAPTER II.
PROGRESS OF SIDEREAL ASTRONOMY.
Exact Astronomy in Germany Career of Bessel His Funda-
menta Astronomies Career of Fraunhofer Parallaxes of
Fixed Stars Translation of the Solar System Astronomy
of the Invisible Struve's Researches in Double Stars Sir
John Herschel's Exploration of the Heavens Character of
Fifty Years' Progress 34
xii CONTENTS.
CHAPTER III.
PROGRESS OF KNOWLEDGE REGARDING THE SUN.
PAGE
Early Views as to the Nature of Sun- Spots Wilson's Observations
and Reasonings 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 64
CHAPTER IV.
PLANETARY DISCOVERIES.
Bode's Law Search for a Missing Planet Its Discovery by Piazzi
Further Discoveries of Minor Planets Unexplained Dis-
turbance of Uranus Discovery of Neptune Its Satellite
An Eighth Saturnian Moon Saturn's Dusky Pang The
Uranian System 89
CHAPTER V.
COMETS.
Predicted Return of Halley's Comet Career of Olbers Accelera-
tion 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 Know-
ledge in
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 135
CONTENTS. xiii
part JEJE.
RECENT PROGRESS OF ASTRONOMY.
CHAPTER I.
FOUNDATION OF ASTRONOMICAL PHYSICS.
PAGE
Schwabe's Discovery of a Decennial Sun-Spot Period Coincid-
ence with Period of Magnetic Disturbance Sun- Spots and
Weather Spectrum Analysis Preliminary Inquiries
Fraunhofer Lines KirchhofFs Principle Anticipations
Elementary Principles of Spectrum Analysis Unity of
Nature 157
CHAPTER II.
SOLAR OBSERVATIONS AND THEORIES.
Black Openings in Spots Carrington's Observations Rotation of
the Sun Kirchhoff's Theory of the Solar Constitution
Faye's Views Solar Photography Kew Observations
Whirlpool Theory of Sun-Spots Volcanic Hypothesis
Lockyer's Theory A Solar Outburst Sun-Spot Periodicity
Planetary Influence Nasmyth's Willow Leaves . .181
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 Egyptian
Eclipse Daylight Coronal Photography Eclipse Observed
at Caroline Island Glare Theory of Corona Eclipses Visible
in New Zealand and the West Indies Nature of the Corona 210
CHAPTER IV.
SOLAR SPECTROSCOPY.
Chemistry of Prominences Study of their Forms Two Classes
Distribution of Prominences Structure of the Chromo-
sphere Spectroscopic Measurement of Movements in Line
xiv CONTENTS.
PAGE
of Sight Velocities of Transport in the Sun Lockyer's
Theory of Dissociation Spectra of Sun- Spots Hydrogen a
Solar Constituent Oxygen in the Snn 239
CHAPTER V.
TEMPERATURE OF THE SUN.
Thermal Power of the Sun Radiation and Temperature Esti-
mates of Solar Temperature Rosetti's Result Zollner's
Method Langley's Experiment at Pittsburg The Sun's
Atmosphere Selective Absorption by our Air The Sun
Blue . 260
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 Measure-
ments of Minor Planets Transit of 1882 Newcomb's Deter-
mination of the Velocity of Light The Problem Provisionally
Solved 272
CHAPTER VII.
PLANETS AND SATELLITES.
Schroter's Life and Work Luminous Appearances during Transits
of Mercury Mountains of Mercury Intra-Mercurian Planets
Rotation of Venus Mountains and Atmosphere Ashen
Light Solidity of the Earth Secular Changes of Climate
Figure of the Globe Study of the Moon's Surface Lunar
Atmosphere New Craters Thermal Effects of Moonlight
Tidal Friction 291
'CHAPTER VIII.
PLANETS AND SATELLITES (continued).
Analogy between Mars and the Earth Martian Snowcaps, Seas,
and Continents Climate and Atmosphere Schiaparelli's
Canals Discovery of Two Martian Satellites Distribution
of the Minor Planets Their Collective Mass and Estimated
CONTENTS. iv
PAGE
Diameters Condition of Jupiter His Spectrum Transits of
his Satellites The Great Red Spot Constitution of Saturn's
Rings Period of Rotation of the Planet Variability of
Japetus Equatorial Markings on Uranus His Spectrum
Rotation of Neptune Trans- Neptunian Planets . . . 322
CHAPTER IX.
THEORIES OF PLANETARY EVOLUTION.
Origin of the World according to Kant Laplace's Nebular Hypo-
thesis Maintenance of the Sun's Heat Meteoric Hypothesis
Radiation the Result of Contraction Regenerative Theory
Faye's Scheme of Planetary Development Origin of the
Moon Effects of Tidal Friction 351
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
Meteors with Stationary Radiants Spectroscopic Analysis
of Cometary Light 369
CHAPTER XI.
RECENT COMETS (continued.)
Forms of Comets' Tails Electrical Repulsion Bredichin's Three
Types Great Southern Comet Supposed Previous Appear-
ances Tebbutt's Comet and the Comet of 1807 Successful
Photographs Schaberle's Comet Comet Wells Sodium
Blaze in Spectrum Great Comet of 1882 Transit Across the
Sun Relation to Comets of 1843, 1880, and 1887 Cometary
Systems Origin of Comets 392
CHAPTER XII.
STARS AND NEBULAE.
Stellar Chemistry Four Orders of Stars Their Relative Ages
Variable Stars New Stars Stars with Bright-Line Spectra
Discovery of Gaseous Nebulae Variable Nebulre Velo-
CONTENTS.
cities of Stars in Line of Sight Stellar and Nebular Photo-
graphy Construction of the Heavens Investigations of
Stellar ParaUax Double Stars Stellar Photometry Status
of Nebula? Star Drift 420
CHAPTER XIII.
METHODS OF RESEARCH.
Development of Telescopic Power Silvered Glass Reflectors-
Giant Refractors Difficulty of Further Improvement
Atmospheric Disturbance Mountain Observatories The
Equatoreal Coude The Photographic Camera Rowland's
Concave Gratings Retrospect and Conclusion . . .461
CHRONOLOGICAL TABLE 1774-1887 477
INDEX ' 489
ERRATUM.
Page 114, line 15 from bottom, for "Riimker," read "Dunlop."
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." * It is based on the
idea of cause; and the whole of its elaborate structure is
1 The denomination "physical astronomy," first used by Kepler, and
long appropriated to this branch of the science, has of late been otherwise
applied.
A
2 HISTORY OF ASTRONOMY.
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 with the invention of the telescope, so that Galileo was,
in point of fact, their originator. But Herschel was the first
to give them a prominence which the whole progress of science
during the nineteenth century has served to confirm and
render more exclusive. Inquisitions begun with the telescope
have been extended and made effective in unhoped-for direc-
tions by the aid of the spectroscope and photographic camera ;
and a large part of our attention in the present volume will
be occupied with the brilliant results thus achieved.
The unexpected development of this new physical-celestial
science is the leading fact in recent astronomical history.
It was out of the regular course of events. In the degree in
which it has actually occurred it could certainly not have been
foreseen. It was a seizing of the prize by a competitor who
had hardly been thought qualified to enter the lists. Orthodox
astronomers of the old school looked with a certain contempt
upon observers who spent their nights in scrutinising the faces
of the moon and planets rather than in timing their transits,
or devoted daylight energies, not to reductions and computa-
tions, but to counting and measuring spots on the sun. They
were regarded as irregular practitioners, to be tolerated per-
haps, 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 gravita-
tion. 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,
INTRODUCTION. 3
the cause of the moon's accelerated motion. As a mere
machine, the solar system, so far as it was then known, was
found to be complete and intelligible in all its parts ; and in
the Mecanique Celeste its mechanical perfections were displayed
under a form of majestic unity which fitly commemorated
the successive triumphs of analytical genius over problems
amongst the most arduous ever dealt with by the mind
of man.
Theory, however, demands a practical test. All its data are
derived from observation; and their insecurity becomes less
tolerable as it advances nearer to perfection. Observation, on
the other hand, is the pitiless critic of theory ; it detects weak
points, and provokes reforms which may be the 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. It was absolutely without a
rival. Systematic observations of sun, moon, stars, and planets
were during the whole of the eighteenth century made only
at Greenwich. Here materials were accumulated for the
secure correction of theory, and here refinements were intro-
duced by which the exquisite accuracy of modern practice in
astronomy was eventually attained.
The chief promoter of these improvements was James
Bradley. Few men have possessed in an equal degree with
him the power of seeing accurately, and reasoning on what
they see. He let nothing pass. The slightest inconsistency
between what appeared and what was to be expected roused
his keenest attention ; and he never relaxed his mental grip
4 HISTORY OF ASTRONOMY.
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. It means 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 equa-
torial protuberance. From it results an apparent displacement
of the stars, each of them describing a little ellipse about its
true, or " mean " position, in a period of eighteen years and
about seven months.
Now an acquaintance with the fact and the laws of each of
these minute irregularities is vital to the progress of obser-
vational astronomy ; for without it the places of the heavenly
bodies could never be accurately known or compared. So that
Bradley, by their detection, at once raised the science to a
higher grade of precision. Nor was this the whole of his
work. Appointed Astronomer- Royal in 1742, he executed
during the years 1750-62 a series of observations which
formed the real beginning of exact astronomy. Part of their
superiority must, indeed, be attributed to the co-operation of
John Bird, who provided Bradley in 1750 with a measuring
instrument of till then unequalled excellence. For not only
was the art of observing in the eighteenth century a peculiarly
English art, but the means of observing were furnished almost
exclusively by British artists. John. Dollond, the son of a
Spitalfields weaver, invented the achromatic lens in 1758,
removing thereby tl^e 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
INTRODUCTION. 5
sectors, quadrants, and circles of Graham, Bird, Ramsden, and
Gary were inimitable by Continental workmanship.
Thus practical and theoretical astronomy advanced on
parallel lines in England and France respectively, the im-
provement of their several tools the telescope and the
quadrant on the one side, and the calculus on the other
keeping pace. The whole future of the science seemed to be
theirs. The cessation of interest through a too speedy attain-
ment of the perfection towards which each spurred the other,
appeared to be the only danger it held in store for them.
When, all at once, a rival stood by their side not, indeed,
menacing their progress, but threatening to absorb their
popularity.
The rise of Herschel was the one conspicuous anomaly in
the astronomical history of the eighteenth century. It proved
decisive of the course of events in the nineteenth. It was
unexplained by anything that had gone before ; yet all that
came after hinged upon it. It gave a new direction to effort ;
it lent a fresh impulse to thought. It opened a channel for
the widespread public interest which was gathering towards
astronomical subjects, to flow in.
Much of this interest was due to the occurrence of events
calculated to arrest the attention and excite the wonder of the
uninitiated. The predicted return of Halley's comet in 1759
verified, after an unprecedented fashion, the computations of
astronomers. It deprived such bodies for ever of their por-
tentous character; it ranked them as denizens of the solar
system. Again, the transits of Yenus in 1761 and 1769 were
the first occurrences of the kind since the awakening of science
to their consequence. Imposing preparations, journeys to
remote and hardly accessible regions, official expeditions, inter-
national communications, all for the purpose of observing them
to the best advantage, brpught 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,
6 HISTORY OF ASTRONOMY.
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 con-
veyed formed a combination powerfully effective to those
least susceptible of new impressions. Nor was the evoked
enthusiasm limited to the British Isles. In Germany, Schroter
followed longo intervallo in Herschel's track. Yon Zach
set on foot from Gotha that general communication of ideas
which gives life to a forward movement. Bode wrote much
and well for unlearned readers. Lalande, by his popular lec-
tures and treatises, helped to form an audience which Laplace
himself did not disdain to address in the Exposition du
Systems du Monde.
This great accession of popularity gave the impulse to the
extraordinarily rapid progress of astronomy in the nineteenth
century. Official patronage combined with individual zeal
sufficed for the elder branches of the science. A few well-
endowed institutions could accumulate the materials needed
by a few isolated thinkers for the construction of theories of
wonderful beauty and elaboration, yet precluded, by their
abstract nature, from winning general applause. But the new
physical astronomy depends for its prosperity upon the favour
of the multitude whom its striking results are well fitted to
attract. It is, in a special manner, the science of amateurs.
It welcomes the most unpretending co-operation. There is no
one " with a true eye and a faithful hand " but can do good
INTRODUCTION. 7
work in watching the heavens. And not unfrequently prizes
of discovery which the most perfect appliances failed to grasp
have fallen to the share of ignorant or ill-provided assiduity.
Observers, accordingly, have multiplied ; observatories have
been founded in all parts of the world ; associations have been
constituted for mutual help and counsel. A formal astro-
nomical congress met in 1798 at Gotha then, under Duke
Ernest II. and Yon Zach, the focus of German astronomy
and instituted a combined search for the planet suspected to
revolve undiscovered between the orbits of Mars and Jupiter.
The Astronomical Society of London was established in 1820,
and the similar German institution in 1863. Both have been
highly influential in promoting the interests, local and general,
of the science they are devoted to forward ; while functions
corresponding to theirs have been discharged elsewhere by
older or less specially constituted bodies, and new ones are
springing up on all sides.
Modern facilities of communication have helped to impress
more deeply upon modern astronomy its associative character.
The electric telegraph gives a certain ubiquity which is invalu-
able to an observer of the skies. With the help of a wire, a
battery, and a code of signals, he sees whatever is visible from
any portion of our globe, depending, however, upon other
eyes than his own, and so entering as a unit into a widespread
organisation of intelligence. The press, again, has been a
potent agent of co-operation. It has mainly contributed to
unite astronomers all over the world into a body animated by
the single aim of collecting " particulars " in their special
branch for what Bacon termed a History of Nature, eventually
to be interpreted according to the sagacious insight of some
one among them gifted above his fellows. The first really
effective astronomical periodical was the Monatliche Corres-
pondent, started by Von Zach in the year 1800. It was
followed in 1822 by the Astronomisclie Nachricliten, 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 astro-
8 HISTORY OF ASTRONOMY.
nomy to divers classes of readers, and so incalculably quicken
the current of its onward flow.
Public favour ^brings in its train material resources. It is
represented by individual enterprise, and finds expression in
an ample liberality. The first regular observatory in the
southern hemisphere was founded at Paramatta by Sir Thomas
Makdougall Brisbane in 1821. The Royal Observatory at the
Cape of Good Hope was completed in 1829. Similar estab-
lishments were set to work by the East India Company at
Madras, Bombay, and St. Helena, during the first third of the
nineteenth century. The organisation of astronomy in the
United States of America was due to a strong wave of popular
enthusiasm. In 1825 John Quincy Adams vainly urged upon
Congress the foundation of a National Observatory; but in
1843 the lectures of Ormsby MacKnight Mitchel on celestial
phenomena stirred an impressionable audience to the pitch of
providing him with the means of erecting at Cincinnati the
first astronomical establishment worthy the name in that
great country. On the ist of January 1882 no less than one
hundred and forty-four were active within its boundaries.
The apparition of the great comet of 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. 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 procure instruction in their use.
In a few years the young Republic was, in point of astro-
nomical efficiency, at least on a level with countries where the
science had been fostered since the dawn of civilisation.
A vast widening of the scope of astronomy has accompanied,
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
INTRODUCTION. 9
sidereal universe as accessible to investigation, and thereby
offered to science new worlds majestic, manifold, " infinitely
infinite " to our apprehension in number, variety, and extent
for future conquest. Their gradual appropriation has absorbed,
and will long continue to absorb, the powers which it has
served to develop.
But this is not the only direction in which astronomy has
enlarged, or rather has levelled, its boundaries. The unification
of the physical sciences is perhaps the greatest intellectual feat
of recent times. The process has included astronomy ; so
that, like Bacon, she may now be said to have "taken all
knowledge" (of that kind) "for her province." In return,
she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale
for our benefit. The sun, stars, and nebulae form so many
celestial laboratories, where the nature and mutual relations of
the chemical " elements " may be tried by more stringent tests
than sublunary conditions afford, and where discoveries of
far-reaching importance in molecular physics may be confirmed
or originated. The laws of terrestrial magnetism can be
completely investigated only with the aid of a concurrent study
of the face of the sun. The positions of the planets will
perhaps one day tell us something of impending droughts,
famines, and cyclones. Nay, with certain qualities of our
own atmosphere, no less essential to the existence of the
human race than its power of supporting respiration, we have
only quite recently become acquainted through the detailed
study of its " selective " action upon solar radiations.
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. The starry sphere no longer, as of
old, absorbs the whole of her attention. 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
io HISTORY OP ASTRONOMY.
seem to lengthen oat 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.
( II )
part
PROGRESS OF ASTRONOMY DURING THE FIRST HALF
OF THE NINETEENTH CENTURY.
CHAPTER I.
FOUNDATION OF SIDEREAL ASTRONOMY,
UNTIL nearly a hundred years ago the stars were regarded by
practical astronomers mainly as a number of convenient fixed
points by which the motions of the various members of the
solar system could be determined and compared. Their
recognised function, in fact, was that of milestones on the
great celestial highway traversed by the planets, as well as
on the byeways of space occasionally pursued by comets.
Not that curiosity as to their nature, and even conjecture as
to their origin, were at any period absent Both were from
time to time powerfully stimulated by the appearance of
startling novelties in a region described by philosophers as
"incorruptible," or exempt from change. The catalogue of
Hipparchus probably, and certainly that of Tycho Brahe,
some seventeen centuries later, owed each its origin to the
temporary blaze of a new star. The general aspect of the
skies was thus (however imperfectly) recorded from age to
age, and with improved appliances the enumeration was ren-
dered more and more accurate and complete ; but the secrets
of the stellar sphere remained inviolate.
In a qualified, though very real sense, Sir William Herschel
12 HISTORY OF ASTRONOMY. PARTI.
may be called the Founder of Sidereal Astronomy. Before
his time some curious facts had been noted, and some inge-
nious speculations hazarded, regarding the condition of the
stars, but not even the rudiments of systematic knowledge
had been acquired. The facts ascertained can be summed up
in a very few sentences.
Giordano Bruno was the first to set the suns of space in
motion, but in imagination only. His daring surmise was,
however, confirmed in 1718, when Halley announced 1 that
Sirius, Aldebaran, Betelgeux, and Arcturus had unmistakably
shifted their quarters in the sky since Ptolemy assigned jtheir
places in his catalogue. A similar conclusion was reached by
J. Cassini in 1738, from a comparison of his own observations
with those made at Cayenne by Richer in 1672 ; and Tobias
Mayer drew up in 1756 a list showing the direction and
amount of about fifty-seven proper motions, 2 founded on
star-places determined by Olaus Homer fifty years previously.
Thus the stars were 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," 3 to account for certain suspected changes
in stellar positions ; Bradley in 1748, and Lambert in 1761,
pointed out that such apparent displacements (by that time
well ascertained) were in all probability a combined effect of
motions, both of sun and stars ; and Mayer actually attempted
the analysis, but without result.
On the 1 3th of August 1596, David Fabricius, an unpro-
fessional astronomer in East Friesland, saw in the neck of the
Whale a star of the third magnitude, which by October had
disappeared. It was, however, visible in 1603, when Bayer
1 Phil. Trans., vol. xxx. p. 737. 2 Out of eighty stars compared, fifty-
seven were found to have changed their places by more than 10". Lesser
discrepancies were at that time regarded as falling within the limits of
observational error. Tobice Mayeri Op. Inedita, t. i. pp. 80-8 1, and
Herschel in Phil. Trans., vol. Ixxiii. pp. 275-278. 3 Posthumous Works,
p. 506.
CHAP. i. SIDEREAL ASTRONOMY. 13
marked it in his catalogue with the Greek letter o, and was
watched through its phases of brightening and apparent ex-
tinction by a German professor named Holwarda in 1 638-39. 1
From Hevelius this first known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud, in 1667,
fixed the length of its cycle of change at 334 days. It was
not a solitary instance. A star in the Swan was perceived by
Janson in 1600 to show fluctuations of light, and Montanari
found in 1669 that Algol in Perseus shared the same peculiarity
to a marked degree. Altogether the class embraced in 1782
half a dozen members. When it is added that a few star-
couples had been noted in singularly, but it was supposed
accidentally, close juxtaposition, and that the failure of repeated
attempts to find an annual parallax pointed to distances at
least 400,000 times that of the earth from the sun, 2 the picture
of sidereal science, when the last quarter of the eighteenth
century began, is practically complete. It included three items
of information that the stars have motions, real or apparent ;
that they are immeasurably remote ; and that a few shine with
a periodically variable light. Nor were the facts thus scantily
collected ordered into any promise of further development.
They lay at once isolated and confused before the inquirer.
They needed to be both multiplied and marshalled, and it
seemed as if centuries of patient toil must elapse before any
reliable conclusions could be derived from them. The side-
real world was thus the recognised domain of venturesome
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.
1 Arago in A nnuaire du Bureau des Longitudes, 1 842, p. 3 1 3. 2 Bradley
to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His observations were
directly applicable to only two stars, 7 Draconis and 77 Ursse Majoris, but
some lesser ones were included in the same result.
I 4 HISTORY OF ASTRONOMY. PARTI.
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. 1 At the age of
nineteen, then, his military service having lasted four years,
he came to England to seek his fortune. Of the life of
struggle and privation which ensued little is known beyond
the circumstances that in 1760 he was engaged in training the
regimental band of the Durham Militia, and that in 1765 he
was appointed organist at Halifax. This post he exchanged
a year later for the more distinguished one of organist at the
Octagon Chapel in Bath. The tide of prosperity now began
to flow for him. The most brilliant and modish society in
England was at that time to be met at Bath, and the young
Hanoverian quickly found himself a favourite and the fashion
in it. Engagements multiplied upon him. He became
director of the public concerts; he conducted oratorios, en-
gaged singers, organised rehearsals, composed anthems, chants,
choral services, besides undertaking private tuitions, at times
amounting to thirty-five or even thirty- eight lessons a week.
He in fact personified the musical activity of a place then
eminently and energetically musical.
But these multifarious avocations did not take up the whole
of his thoughts. His education, notwithstanding the poverty
of his family, had not been neglected, and he had always
greedily assimilated every kind of knowledge that came in his
way. Now that he was a busy and a prosperous man, it might
have been expected that he would run on in the deep pro-
fessional groove laid down for him. On the contrary, his
passion for learning seemed to increase with the diminution of
the time available for its gratification. He studied Italian,
Greek, mathematics ; Maclaurin's Fluxions served to " unbend
his mind;" Smith's Harmonics and Optics and Ferguson's
1 Holden, Sir William HerscM, his Life and Works, p. 17.
CHAP. i. SIDEREAL ASTRONOMY. 15
Astronomy were the nightly companions of his pillow. What
he read stimulated without satisfying his intellect. He desired
not only to know, but to discover. In 1773 he hired a
small telescope, and through it caught a preliminary glimpse
of the rich and varied fields in which, for so many years, he
was to expatiate. Henceforward the purpose of his life was
fixed. It was to obtain " a knowledge of the construction of
the heavens ; " l and to this sublime ambition he remained
true until the end.
A more powerful instrument was the first desideratum ; and
here his mechanical genius came to his aid. Having purchased
the apparatus of a Quaker optician, he set about the manu-
facture of specula with a zeal which seemed to anticipate the
wonders they were to disclose to him. It was not until fifteen
years later that his grinding and polishing machines were
invented, so the work had at that time to be entirely done by
hand. During this tedious and laborious process (which could
not be interrupted without injury, and lasted on one occasion
sixteen hours), his strength was supported by morsels of food
put into his mouth by his sister, 2 and his mind amused by
her reading aloud to him the Arabian Nights, Don Quixote,
or other light works. At length, after repeated failures, he
found himself provided with a reflecting telescope a 5 1 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 pre-
served by the Royal Society. It bears the date March 4,
I774. 3
In the following year he executed his first " review of the
heavens," memorable chiefly as an evidence of the grand and
novel conceptions which already inspired him, and of the
enthusiasm with which he delivered himself up to their guid-
1 Phil. Trans., voL ci. p. 269. 2 Caroline Lucretia Herschel, born at
Hanover, March 1 6, 1750, died in the same place, January 9, 1848. She
came to England in 1772, and was her brother's devoted assistant, first
in his musical undertakings, and afterwards, down to the end of his life,
in his astronomical labours. 3 Holden, Op. cit., p. 39.
16 HISTORY OF ASTRONOMY. PARTI.
ance. 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 harpsi-
chord to his telescope, no doubt with that " uncommon precipi-
tancy which accompanied all his actions." l He now rapidly
increased the power and perfection of his telescopes. Mirrors
of seven, ten, even twenty feet focal length were successively
completed, and unprecedented magnifying powers employed.
His energy was unceasing, his perseverance indomitable. In
the course of twenty-one years no less than 430 parabolic
specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the Philosophical Trans-
actions ; yet during the ensuing thirty-nine years his contribu-
tions many of them elaborate treatises numbered sixty-nine,
forming a series of extraordinary importance to the history of
astronomy. As a mere explorer of the heavens his labours
were prodigious. He discovered 2500 nebulae, 806 double stars,
passed the whole firmament in review four several times, counted
the stars in 3400 "gauge-fields," and executed a photometric
classification of the principal stars, founded on an elaborate
(and the first systematically conducted) investigation of their
relative brightness. He was as careful and patient as he
was rapid; spared no time and omitted no precaution to
secure accuracy in his observations; yet in one night
he would examine, singly and carefully, up to 400 separate
objects.
The discovery of Uranus was a mere incident of the scheme
he had marked out for himself a fruit gathered, as it were,
by the way. It formed, nevertheless, the turning-point in his
career. From a star-gazing musician he was at once trans-
formed into an eminent astronomer ; he was relieved from
the drudgery of a toilsome profession, and installed as royal
astronomer, with a modest salary of ^200 a year ; funds were
provided for the construction of the forty-foot reflector, from
the great space-penetrating power of which he expected as yet
unheard-of revelations ; in fine, his future work was not only
1 Memoir of Caroline Iferschel, p. 37.
CHAP. i. SIDEREAL ASTRONOMY. 17
rendered possible, but it was stamped as authoritative. 1 On
"Whit- Sunday 1782, William and Caroline Herschel played
and sang in public for the last time in St. Margaret's Chapel,
Bath ; in August of the same year the household was moved
to Datchet, near Windsor, and on April 3, 1786, to Slough.
Here happiness and honours crowded on the fortunate dis-
coverer. 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. 2 He was followed by Kant, 3
who transcended the views of his predecessor by assigning to
nebulae the position they long continued to occupy, rather on
imaginative than on scientific grounds, of " island universes,"
external to, and co-equal with the Galaxy. Johann Heinrich
1 See Holden's Sir William Herschel, p. 54. 2 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. 3 Allye-
meine Naturgeschichte und Theorie des Himmels, 1755.
B
18 HISTORY OF ASTRONOMY. PARTI.
Lambert, 1 the tailor's apprentice of Miihlhausen, followed, but
independently. The conceptions of this remarkable man were
grandiose, his intuitions bold, his views on some points a
singular anticipation of subsequent discoveries. The sidereal
world presented itself to him as a hierarchy of systems, starting
from the planetary scheme, rising to throngs of suns within
the circuit of the Milky Way the "ecliptic of the stars," as
he phrased it expanding to include groups of many Milky
"Ways ; these again combining to form the unit of a higher
order of assemblage, and so onwards and upwards until the
mind reels and sinks before the immensity of the contemplated
creations.
" Thus everything revolves the earth round the sun ; the
sun round the centre of his system ; this system round a
centre in common to it with other systems ; this group, this
assemblage of systems, round a centre which is common to it
with other groups of the same kind ; and where shall we have
done ? " 2
The stupendous problem thus speculatively attempted, Her-
schel undertook to grapple with experimentally. The upshot
of this memorable inquiry was the inclusion for the first time
within the sphere of human knowledge, of a connected body of
facts and inferences from facts regarding the sidereal universe
in other words, the foundation of what may properly be
called a science of the stars.
Tobias Mayer had illustrated the perspective effects which
must ensue in the stellar sphere from a translation of the solar
system, by comparing them to the separating in front and
closing up behind of trees in a forest to the eye of an advanc-
ing spectator ; 3 but the appearances which he thus correctly
described he was unable to detect. By a more searching
1 Cosmologische Brief e, Augsburg, 1761. 2 The System of the World,
p. 125, London, 1800 (a translation of the above). Lambert regarded
nebulae as composed of stars crowded together, but not as external
universes. In the ca.se 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. 3 Op. In., t. i. p. 79.
CHAP. i. SIDEREAL ASTRONOMY. 19
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 Yega
did, in fact, appear to recede from, and Sirius and Aldebaran
to approach, each other by very minute amounts ; and, with a
striking effort of divinatory genius, placed the " apex," or
point of direction of the sun's motion, close to the star X in
the constellation Hercules, 1 within a few degrees of the spot
indicated by the latest and most refined methods of research.
The validity of this conclusion was long doubted ; but it has
been triumphantly confirmed, and scarcely corrected. The
question as to the " secular parallax " of the fixed stars was
in effect answered.
With their annual parallax, however, the case was very
different. The search for it had already led Bradley to the
important discoveries of the aberration of light and the nuta-
tion of the earth's axis ; it was now about to lead Herschel
to a discovery of a different, but even more elevated char-
acter. Yet in neither case was the object primarily sought
attained.
From the very first promulgation of the Copernican theory
the seeming immobility of the stars had been urged as an
argument against its truth ; for if the earth really travelled
in a vast orbit round the sun, objects in surrounding space
should appear to change their positions, unless their distances
were on a scale which, to the narrow ideas of the universe then
prevailing, seemed altogether extravagant. 2 The existence of
such apparent, or " parallactic " displacements was accordingly
regarded as the touchstone of the new views, and their de-
tection became an object of earnest desire to those interested
in maintaining them. Copernicus himself made the attempt ;
1 Phil. Trans., vol. Ixxiii. (1783), p. 273. He resumed the subject in
1805 (Phil. Trans., vols. xcv. and xcvi.), but, though employing a more
rigorous method, was scarcely so happy in his result. It is worthy of
remark that Provost, almost simultaneously with Herschel, executed an
investigation similar to his with very considerable success. Klugel con-
firmed Herschel's result by an analytical inquiry in 1789. 2 " Ingens
bolus devorandus est," Kepler admitted to Herwart in May 1603.
20 HISTORY OF ASTRONOMY. PARTI.
but with his " Triquetrum," a jointed wooden rule with the
divisions marked in ink, constructed by himself, he was hardly
able to measure angles of ten minutes, far less fractions
of a second. Galileo, a more impassioned defender of the
system, strained his ears, as it were, from Arcetri, in his
blind and sorrowful old age, for news of a discovery which
two more centuries had still to wait for. Hooke believed
he had found a parallax for the bright star in the Head
of the Dragon ; but was deceived. Bradley convinced him-
self that such effects were too minute for his instruments to
measure. Herschel made a fresh attempt ^ by a practically
untried method.
It is a matter of daily experience that two objects 'situated
at different distances, seem to a beholder in motion to move
relatively to each other. This principle Galileo, in the third
of his Dialogues on the Systems of the World, 1 proposed to
employ for the determination of stellar parallax ; for two stars,
lying apparently close together, but in reality separated by a
great gulf of space, must shift their mutual positions when
observed from opposite points of the earth's orbit ; or rather,
the remoter forms a virtually fixed point, to which the move-
ments of the other can be conveniently referred. By this
means complications were abolished more numerous and per-
plexing than Galileo was himself aware of, and the problem
was reduced to one of simple micrometrical measurement.
The " double-star method " was also suggested by James Gre-
gory in 1675$ an d again by Wallis in 1693 ; 2 Huygens first,
and afterwards Dr. Long of Cambridge (about 1750), made
futile experiments with it ; and it eventually led, in the hands
of Bessel, to the successful determination of the parallax of
6 1 Cygni.
Its advantages were not lost upon Herschel. His attempt
to assign definite distances to the nearest stars was no isolated
effort, but part of the settled plan upon which his observations
were conducted. He proposed to sound the heavens, and the
first requisite was a knowledge of the length of his sounding-
1 Opere, t. i. p. 415. 2 Phil. Trans., vol. xvii. p. 848.
CHAP. i. SIDEREAL ASTRONOMY. 21
line. Thus it came about that his special attention was early
directed to double stars.
"I resolved," he writes, 1 "to examine every star in the
heavens with the utmost attention and a very high power,
that I might collect such materials for this research as would
enable me to fix my observations upon those that would best
answer my end. The subject has already proved so extensive,
and still promises so rich a harvest to those who are inclined
to be diligent in the pursuit, that I cannot help inviting every
lover of astronomy to join with me in observations that must
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 excep-
tions) the directions with reference to an invariable line, of
the lines joining their centres (technically called "angles of
position ") were determined with the aid of a " revolving- wire
micrometer," specially devised for the purpose. Moreover,
an important novelty was introduced by the observation of
the various colours visible in the star-couples, the singular
and vivid contrasts of which were now for the first time
described.
Double stars were at that time supposed to be a purely
optical phenomenon. Their components, it was thought,
while in reality indefinitely remote from each other, were
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. 2 Moreover, 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
1 Phil. Trans., vol. Ixxii. p. 97. 2 Doberck, Observatory, vol. ii. p. 1 10.
22 HISTORY OF ASTRONOMY. PARTI.
or more stars placed very near together, do really consist of
stars placed near together, and under the influence of some
general law." 1 And in 1784:2 "It is not improbable that
a few years may inform us that some of the great number of
double, triple stars, &c., which have been observed by Mr.
Herschel, are systems of bodies revolving about each other."
This remarkable speculative anticipation had a practical
counterpart in Germany. Father Christian Mayer, a Jesuit
astronomer at Mannheim, set himself, in January 1776, to
collect examples of stellar pairs, and shortly after published
the supposed discovery of " satellites " to many of the principal
stars. 3 His observations, however, were neither exact nor
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 d, des clwses aussi
extraordinaires, wrote Lalande 4 within one year of a better-
grounded announcement to the same effect.
Herschel at first shared the general opinion as to the merely
optical connection of double stars. Of this the purpose for
which he compiled his lists 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^ "to
form any theories of small stars revolving round large ones j "
while in the year following, 6 he remarked that the identical
proper motion of the two stars forming, to the naked eye, the
single bright orb of Castor could only be explained as both
equally due to the " systematic parallax " caused by the sun's
movement in space. Plainly showing that the notion of a
physical tie compelling the two bodies to travel together, had
not as yet entered into his speculations. But he was eminently
open to conviction, and had, moreover, by observations un-
paralleled in amount as well as in kind, prepared ample
1 Phil. Trans., vol. Ivii. p. 249. 2 Ibid., vol. Ixxiv. p. 56. 3 Beolach-
tungen von Fixsterntrabantcn, 1778, and De Novis in Ccelo Sidereo Phccno-
menis, 1779. 4 Bibliographic, p. 569. 5 Phil. Trans., vol. Ixxii. p. 162.
6 Ibid. , vol. Ixxiii. p. 272.
CHAP. i. SIDEREAL ASTRONOMY. 23
materials for convincing himself and others. In 1802 he was
able to announce the fact of his discovery, and in the two
ensuing years to lay in detail before the Royal Society, proofs,
gathered from the labours of a quarter of a century, of orbital
revolution in the case of as many as fifty double stars, hence-
forth, he declared, to be held as real binary combinations,
" intimately held together by the bond of mutual attraction." 1
The fortunate preservation in Dr. Maskelyne's notebook of a
remark made by Bradley about 1759, to the effect that the
line joining the components of Castor was precisely coin-
cident with that joining Castor with Pollux, added eighteen
years to the time during which the pair were under scrutiny,
and confirmed the evidence of change afforded by more recent
observations. Approximate periods were fixed for many of
the revolving suns for Castor, 342 years; for y Leonis, 1200,
3 Serpentis, 375, s Bootis, 1681 years; s Lyrse was noted as
a " double- double star," a change of situation having been
detected in each of the two pairs composing the group;
and the occultation of one star by another in the course
of their mutual revolutions, of which curious phenomenon
two examples (in d Cygni and Herculis) occurred in 1802,
was described.
Thus, by the sagacity and perseverance of a single observer,
a firm basis was at last provided upon which to raise the edi-
fice 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 em-
phatically (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
1 Phil. Trans., vol. xciii. p. 340.
24 HISTORY OF ASTRONOMY. PARTI.
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 Jife, 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 pos-
sess on this sublime subject was prepared, and the greater
part of it anticipated, by him.
The ingenious method of " star-gauging," and its issue in
the delineation of the sidereal system as an irregular stratum
of evenly scattered suns, is the best known part of his work.
But it was, in truth, only a first rude approximation, the prin-
ciple of which maintained its credit in the literature of astro-
nomy a full half -century after its abandonment by its author.
This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars,
it is obvious that the number of stars visible in any particular
direction would be strictly proportional to the range of the
system in that direction, apparent accumulation being pro-
duced by real extent. The process of " gauging the heavens,"
accordingly, consisted in counting the stars in successive tele-
scopic fields, and calculating thence the depths of space neces-
sary to contain them. The result of 3400 such operations was
the plan of the Galaxy familiar to every reader of an astro-
nomical 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 "
CHAP. i. SIDEREAL ASTRONOMY. 25
was estimated by Herschel at about 497 times the distance of
Sirius in other words, the bounding orb, or farthest sun of
the system in that direction, so far as was revealed by the
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 funda-
mental hypothesis) be obtained, but its real or absolute size
remained altogether undetermined.
Even as early as 1785, however, Herschel perceived traces
of a tendency which completely invalidated the supposition
of any approach to an average uniformity of distribution.
This was the action of what he called a " clustering power " in
the Milky "Way. " Many gathering clusters " x were already
discernible to him even while he endeavoured to obtain a " true
mean result " on the assumption that each star in space was
separated from its neighbours as widely as the sun from Sirius.
"It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems ; " and
in certain assemblages he was able to trace "a course or tide
of stars setting towards a centre," denoting, not doubtfully,
the presence of attractive forces. 2 Thirteen years later, he
described our sun and his constellated companions as sur-
rounded by " a magnificent collection of innumerable stars,
called the Milky Way, which must occasion a very powerful
balance of opposite attractions to hold the intermediate stars
at rest. For though our sun, and all the stars we see, may
truly be said to be in the plane of the Milky Way, yet I am
now convinced, by a long inspection and continued examination
of it, that the Milky Way itself consists of stars very differently
scattered from those which are immediately about us." " This
immense aggregation," he added, " is by no means uniform.
Its component stars show evident signs of clustering together
into many separate allotments." 3
1 Phil. Trans., vol. Ixxv. p. 255. 2 Ibid., vol. Ixxix. pp. 214, 222.
* Ibid., vol. xcii. pp. 479, 495.
26 HISTORY OF ASTRONOMY. PARTI.
The following sentences, written in 1 8 1 1 , 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." i
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," 2 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 firma-
ment, 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
1 Phil. Trans., vol. ci. p. 269. 2 Ibid., vol. cvii. p. 311.
CHAP. i. SIDEREAL ASTRONOMY. 27
known as " nebulae." The history of the acquaintance of our
race with them is comparatively short. The only one recog-
nised before the invention of the telescope was that in the
girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sufi ;
and marked with dots on an old Dutch chart of the constella-
tion, presumably about 1500 A.D. 1 Yet so little was it noticed,
that it might practically be said as far as Europe is con-
cerned 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 men-
tion of the great Orion nebula is by a Swiss Jesuit named
Cysatus, who succeeded Father Scheiner in the chair of
mathematics at Ingolstadt. He used it, apparently without
any suspicion of its novelty, as a term of comparison for the
comet of December i6i8. 2 A novelty, nevertheless, to astro-
nomers it still remained in 1656, when Huygens discerned,
"as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond." 3 Halley in 1714 knew of six
nebulae, which he believed to be composed of a " lucid medium "
diffused through the ether of space. 4 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; 5 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 me-
thodising them, and drew up a catalogue comprising, in 1781,
103 entries. 6
These preliminary attempts shrank into insignificance when
1 Bullialdus, De Nebulosd Stella in Cingulo Andromedce (1667) ; see also
G. P. Bond, Mem. Am. Ac., vol. iii. p. 75, and Holden's Monograph on
the Orion Nebula, Washington Observations, vol. xxv., 1878 (pub. 1882).
2 Mathemata Astronomica, p. 75. 3 Systema Saturnium, p. 9. 4 Phil.
Trans., vol. xxix. p. 390. 5 Mem. Ac. des Sciences, 1755. 6 C nn - des
Temps, 1784 (pub. 1781), p. 227. A previous list of forty-five had appeared
in Mem. Ac. d. Sc., 1771.
23 HISTORY OF ASTRONOMY. PART i.
Herschel began to " sweep the heavens " with his giant tele-
scopes. In 1786 he presented to the Royal Society a descrip-
tive catalogue of 1000 nebuke and clusters, followed, three
years later, by a second of an equal number; to which he
added in 1802 a further gleaning of 500. On the subject
of their nature his views underwent a remarkable change.
Finding that his potent instruments resolved into stars many
nebulous patches in which no signs of such a structure had
previously been discernible, he naturally concluded that " re-
solvability " was merely a question of distance and telescopic
power. He was (as he said himself) led on by almost imper-
ceptible degrees from evident clusters, such as the Pleiades,
to spots without a trace of stellar formation, the gradations
being so well connected as to leave no doubt that all these
phenomena were equally stellar. The singular variety of their
appearance was thus described by him :
" I have seen," he says, ''double and treble nebulae variously
arranged ; large ones with small, seeming attendants ; narrow,
but much extended lucid nebulae or bright dashes; some of
the shape of a fan, resembling an electric brush, issuing from
a lucid point; others of the cometic shape, with a seeming
nucleus in the centre, or like cloudy stars surrounded with a
nebulous atmosphere, a different sort, again, contain a nebu-
losity of the milky kind, like that wonderful, inexplicable phe-
nomenon about & Orionis ; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable
into stars." l
" These curious objects " he considered to be " no less than
whole sidereal systems," 2 some of which might " well outvie
our Milky Way in grandeur." He admitted, however, a wide
diversity in condition as well as compass. The system to which
our sun belongs he described as " a very extensive branching
congeries of many millions of stars, which most probably owes
its origin to many remarkably large, as well as pretty closely
scattered small stars, that may have drawn together the rest." 3
1 Phil. Trans., vol. Ixxiv. p. 442. 2 Hid., vol. Ixxix. p. 213. 3 Ibid.,
vol. Ixxv. p. 254.
CHAP. i. SIDEREAL ASTRONOMY. 29
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 nebulae, due
to the " decay " of other " branching nebulae " similar to our
own, he recognised by the score, lying, as it were, stratified in
certain quarters of the sky. " One of these 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 nebulae, all distinctly visible upon a fine blue
sky." The stratum of Coma Berenices he judged to be the
nearest to our system of such layers; nor did the marked
aggregation of nebulae towards both poles of the circle of the
Milky Way escape his notice.
By a continuation of the same process of reasoning, he was
enabled (as he thought) to trace the life-history of nebulae
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." l
"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 im-
mense 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 ger-
mination, 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 ? " 2 ,
But already this supposed continuity was broken. After
mature deliberation on the phenomena presented by nebulous
1 Phil. Trans., vol. Ixxix. p. 225. 2 Ibid., p. 226.
30 HISTORY OF ASTRONOMY. PARTI.
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 per-
fection 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 in-
conceivably crowded, as being the occasion of that remark-
able appearance. It seems, therefore, to require a more dis-
similar object to set us right again. A glance like that of the
naturalist, who casts his eye from the perfect animal to the
perfect vegetable, is wanting to remove the veil from the mind
of the astronomer. The object I have mentioned above is the
phenomenon that was wanting for this purpose. View, for
instance, the iQth cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less
decisive than that of the naturalist we have alluded to. Our
judgment, I may venture to say, will be that the nebulosity
about the star is not of a starry nature" 1
The conviction thus arrived at of the existence in space of a
widely diffused "shining fluid" (a conviction long afterwards
fully justified by the spectroscope), led him into a field of
endless speculation. What was its nature ? Should it "be
compared to the coruscation of the electric fluid in the aurora
borealis? or to the more magnificent cone of the zodiacal
light ? " Above all, what was its function in the cosmos ?
And on this point he already gave a hint of the direction
1 Pldl. Trans., vol. Ixxxi. p. 72.
CHAP. i. SIDEREAL ASTRONOMY. 31
in which his mind was moving by the remark that this
self-luminous matter seemed "more fit to produce a star
by its condensation, than to depend on the star for its exist-
ence." l
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 progressive assortment of objects, there was, as
he said, " perhaps not so much difference as would be in an
annual description of the human figure, were it given from the
birth of a child till he comes to be a man in his prime." 2
From diffused nebulosity, barely visible in the most power-
ful light-gathering instruments, but which he estimated to
cover nearly 152 square degrees of the heavens, 3 to planetary
nebulae, supposed to be already centrally solid, instances
were alleged by him of every stage and phase of condensa-
tion. The validity of his reasoning, however, was evidently
impaired by his confessed inability to distinguish between the
dim rays of remote clusters and the milky light of true
gaseous nebulae.
It may be said that such speculations are futile in them-
selves, and necessarily barren of results. But they gratify an
inherent tendency of the human mind, and, if pursued in a
becoming spirit, should be neither reproved nor disdained.
Herschel's theory still exercises men's thoughts, and not
unworthily, although the testimony of recent discoveries with
1 Phil. Trans., vol. Ixxxi. p. 85. 2 Ibid., vol. ci. p. 271.
. Ibid., p. 277.
32 HISTORY OF ASTRONOMY. PARTI.
regard to it is, at the best, hesitating and inconclusive. It
should be added, that it seems to have been propounded in
complete independence of Laplace's nebular hypothesis as to
the origin of the solar system. Indeed, it dated, as we have
seen, in its first inception, from 1791, while the French geo-
metrician's view was not advanced until 1796.
We may now briefly sum up the chief results of Herschel's
long years of " watching the heavens." The apparent motions
of the stars had been disentangled ; one portion being clearly
shown to be due to a translation towards a point in the con-
stellation Hercules of the sun and his attendant planets ;
while a large balance of displacement was left to be accounted
for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to,
if not identical with gravity, suns of every degree of size and
splendour, and sometimes brilliantly contrasted in colour, were
seen to be held together in systems, consisting of two, three,
four, even six members, whose revolutions exhibited a wide
range of variety both in period and in orbital form. A new
department of physical astronomy was thus created, 1 and rigid
calculation for the first time made possible within the astral
region. The vast problem of the arrangement and relations
of the millions of stars forming the Milky Way was shown to
be capable of experimental treatment, and of at least partial
solution, notwithstanding the variety and complexity seen to
prevail, to an extent previously undreamt of, in the arrange-
ment of that majestic system. The existence of a luminous
fluid, diffused through enormous tracts of space, and intimately
associated with stellar bodies, was virtually demonstrated, and
its place and use in creation attempted to be divined by a bold
but plausible conjecture. Change on a stupendous scale was
observed to be everywhere in progress. One star 55 Her-
culis vanished, it might be said, under the very eye of the
astronomer, and other disappearances were more than sur-
mised; progressive ebbings or flowings of light were indi-
cated as probable in many stars under no formal suspicion of
1 Sir J. Herschel, Phil. Trans., vol. cxiv. part iii. p. i.
CHAP. i. SIDEREAL ASTRONOMY. 33
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," l the state
into which the incessant action of the clustering power has
brought the Milky Way at present, is a kind of chronometer
that may be used to measure the time of its past and future
existence ; and although we do not know the rate of going of
this mysterious chronometer, it is nevertheless certain that
since the breaking up of the parts of the Milky Way affords
a proof that it cannot last for ever, it equally bears witness
that its past duration cannot be admitted to be infinite." 2
1 His own words to the poet Campbell, cited by Holden, Life and
Works, p. 109. 2 Phil. Trans., vol. civ. p. 283.
{ 34 )
CHAPTER II.
PROGRESS OF SIDEREAL ASTRONOMY.
WE have now to consider labours of a totally different character
from those of Sir William Herschel. Exploration and discovery
do not constitute the whole business of astronomy ; the less
adventurous, though not less arduous, task of gaining a more
and more complete mastery over the problems immemorially
presented to her, may, on the contrary, be said to form her
primary duty. A knowledge of the movements of the heavenly
bodies has, from the earliest times, been demanded by the
urgent needs of mankind ; and science finds prosperity, as in
many cases it has taken its origin, in condescension to practical
claims. Indeed, to bring such knowledge as near as possible
to absolute precision has been denned by no mean authority l
as the true end of astronomy.
Several causes concurred about the beginning of the present
century to give a fresh and powerful impulse to investigations
having this end in view. The rapid progress of theory almost
compelled a corresponding advance in observation ; instru-
mental improvements rendered such an advance possible;
Herschel's discoveries quickened public interest in celestial
inquiries ; royal, imperial, and grand-ducal patronage widened
the scope of individual effort. The heart of the new move-
ment 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
1 Bessel, Populdre Vorlesungen, pp. 6, 408.
CHAP. ii. SIDEREAL ASTRONOMY. 35
in direct proportion to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of
Astronomer Royal during forty-six years (from 1765 to 1811),
was no unworthy successor to the eminent men who had gone
before him. His foundation of the Nautical Almanac (in
1767) alone constitutes a valid title to fame; he introduced at
the Observatory the important innovation of the systematic
publication of results ; and the careful and prolonged series of
observations executed by him formed the basis of the improved
theories, and corrected tables of the celestial movements, which
were rapidly being brought to completion abroad. But he had
in him no stirrings of the future. He was fitted rather to
continue a tradition than to found a school. The old ways
were dear to him; and, indefatigable as he was, a definite
purpose was wanting to compel him, by its exigencies, along
the path of progress. Thus, for almost fifty years after
Bradley's death, the acquisition of a small achromatic 1 was
the only notable change made in the instrumental equipment
of the Observatory. The transit, the zenith sector, and the
mural quadrant, with which Bradley had done his incomparable
work, retained their places long after they had become dete-
riorated by time, and obsolete by the progress of invention ;
and it was not until the very close of his career that Maskelyne,
compelled by Pond's detection of serious errors, ordered a
Troughton's circle, which he did not live to employ.
Meanwhile, the heavy national disasters with which Germany
was overwhelmed in the early part of the present century,
seemed to stimulate, rather than impede, the intellectual revival
already for some years in progress there. Astronomy was
amongst the first of the sciences to feel the new impulse. By
the efforts of Bode, Olbers, Schroter, and Von Zach, just and
elevated ideas on the subject were propagated, intelligence was
diffused, and a firm ground prepared for common action in
mutual sympathy and disinterested zeal. They were powerfully
seconded by the foundation, in 1804, by a young artillery
officer named Yon Reichenbach, of an Optical and Mechanical
1 Fitted to the old transit instrument, July II, 1772.
38 HISTORY OF ASTRONOMY. PARTI.
He had, however, as yet no intention of adopting astronomy
as his profession. For two years he continued to work in the
counting-house by day, and to pore over the Mecanique Celeste
and the Differential Calculus by night. But the post of
assistant in Schroter's Observatory at Lilienthal having become
vacant by the removal of Harding to Gottingen in 1805, Olbers
procured for him the offer of it. It was not without a struggle
that he resolved to exchange the desk for the telescope. His
reputation with his employers was of the highest ; he had
thoroughly mastered the details of the business, which his keen
practical intelligence followed with lively interest; his years
of apprenticeship were on the point of expiring, and an imme-
diate, and not unwelcome prospect of comparative affluence lay
before him. The love of science, however, prevailed ; he chose
poverty and the stars, and went to Lilienthal with a salary of
a hundred thalers yearly. Looking back over his life's work,
Olbers long afterwards declared that the greatest service which
he had rendered to astronomy was that of having discerned,
directed, and promoted the genius of Bessel. 1
For four years he continued in Schroter's employment. At
the end of that time the Prussian Government chose him to
superintend the erection of a new observatory at Konigsberg,
which, after many vexatious delays caused by the prostrate
condition of the country, was finished towards the end of 1813.
Konigsberg was the first really efficient German Observatory.
It became, moreover, a centre of improvement, not for Ger-
many alone, but for the whole astronomical world. During
two-and-thirty years it was the scene of Bessel's labours, and
Bessel's labours had for their aim the reconstruction, on an
amended and uniform plan, of the entire science of observation.
A knowledge of the places of the stars is the foundation of
astronomy. 2 Their configuration lends to the skies their dis-
tinctive features, and marks out the shifting tracks of more
mobile objects with relatively fixed, and generally unvarying
points of light. A more detailed and accurate acquaintance
with the stellar multitude, regarded from a purely uranogra-
1 R. Wolf, Gesck. der Astron., p. 518. 2 Bessel ; Pop. VwL, p. 22.
CHAP. ii. SIDEREAL ASTRONOMY. 39
phical point of view, has accordingly formed at all times a
primary object of celestial science, and has, during the present
century, been cultivated with a zeal and success by which all
previous efforts are dwarfed into insignificance. In Lalande's
Histoire Celeste, published in 1801, the places of no less than
47,390 stars were given, but in the rough, as it were, and con-
sequently needing laborious processes of calculation to render
them available for exact purposes. Piazzi set an example of
improved methods of observation, resulting in the publication,
in 1803 and 1814, of two catalogues of about 7000 stars, which
for their time were models of what such works should be.
Stephen Groombridge at Blackheath was similarly and most
beneficially active. But something more was needed than the
diligence of individual observers. A systematic reform was
called for ; and it was this which Bessel undertook and carried
through.
Direct observation furnishes only what has been called the
"raw material" of the positions of the heavenly bodies. 1 A
number of highly complex corrections have to be applied before
their mean can be disengaged from their apparent places on
the sphere. Of these, the most considerable and familiar is
atmospheric refraction, by which objects seem to stand higher
in the sky than they in reality do, the effect being evanescent
at the zenith, and attaining, by gradations varying with condi-
tions of pressure and temperature, a maximum at the horizon.
Moreover, the points 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 successive transmission of light, combined with the
movement of the earth in its orbit, causes a minute displace-
ment known as aberration.
Now it is easy to see that any uncertainty in the application
of these corrections saps the very foundations of exact astro-
nomy. Extremely minute quantities, it is true, are concerned ;
1 Bessel, Pop. VorL, p. 440.
3o HISTORY OF ASTRONOMY. PARTI.
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 tele-
scope by this extraordinary man was indispensable to the pro-
gress 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 oppor-
tunely 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 "West-
phalia, July 22, 1784. A certain taste for figures, coupled
with a still stronger distaste for the Latin accidence, directed
his inclination and his father's choice towards a mercantile
career. In his fifteenth year, accordingly, he entered the
house of Kuhlenkamp & Sons, in Bremen, as an apprenticed
clerk. He was now thrown completely upon his own re-
sources. 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 acquisi-
tion 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
CHAP. ii. SIDEREAL ASTRONOMY. 37
(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, 1 filled him with a
rapture of delight, which, by confirming his tastes, decided
his destiny. He now eagerly studied Bode's Jahrbuch and
Von Zach's Monatliche Correspondenz, overcoming each diffi-
culty as it arose with the aid of Lalande's Traite d' Aslronomie,
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 Yon 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
before, the French Academy had crowned a similar perform-
ance ; 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.
1 Brief wechsel mit Olbers, p. xvi.
38 HISTORY OF ASTRONOMY. PARTI.
He had, however, as yet no intention of adopting astronomy
as his profession. For two years he continued to work in the
counting-house by day, and to pore over the Mecanique Celeste
and the Differential Calculus by night. But the post of
assistant in Schroter's Observatory at Lilienthal having become
vacant by the removal of Harding to Gottingen in 1805, Olbers
procured for him the offer of it. It was not without a struggle
that he resolved to exchange the desk for the telescope. His
reputation with his employers was of the highest ; he had
thoroughly mastered the details of the business, which his keen
practical intelligence followed with lively interest; his years
of apprenticeship were on the point of expiring, and an imme-
diate, and not unwelcome prospect of comparative affluence lay
before him. The love of science, however, prevailed ; he chose
poverty and the stars, and went to Lilienthal with a salary of
a hundred thalers yearly. Looking back over his life's work,
Olbers long afterwards declared that the greatest service which
he had rendered to astronomy was that of having discerned,
directed, and promoted the genius of Bessel. 1
For four years he continued in Schroter's employment. At
the end of that time the Prussian Government chose him to
superintend the erection of a new observatory at Konigsberg,
which, after many vexatious delays caused by the prostrate
condition of the country, was finished towards the end of 1813.
Konigsberg was the first really efficient German Observatory.
It became, moreover, a centre of improvement, not for Ger-
many alone, but for the whole astronomical world. During
two-and-thirty years it was the scene of Bessel's labours, and
Bessel's labours had for their aim the reconstruction, on an
amended and uniform plan, of the entire science of observation.
A knowledge of the places of the stars is the foundation of
astronomy. 2 Their configuration lends to the skies their dis-
tinctive features, and marks out the shifting tracks of more
mobile objects with relatively fixed, and generally unvarying
points of light. A more detailed and accurate acquaintance
with the stellar multitude, regarded from a purely uranogra-
1 R. Wolf, Gesch. der Astron., p. 518. 2 Besselj Pop. VorL, p. 22.
CHAP. ii. SIDEREAL ASTRONOMY. 39
phical point of view, has accordingly formed at all times a
primary object of celestial science, and has, during the present
century, been cultivated with a zeal and success by which all
previous efforts are dwarfed into insignificance. In Lalande's
Histoire Celeste, published in 1801, the places of no less than
47,390 stars were given, but in the rough, as it were, and con-
sequently needing laborious processes of calculation to render
them available for exact purposes. Piazzi set an example of
improved methods of observation, resulting in the publication,
in 1803 and 1814, of two catalogues of about 7000 stars, which
for their time were models of what such works should be.
Stephen Groombridge at Blackheath was similarly and most
beneficially active. But something more was needed than the
diligence of individual observers. A systematic reform was
called for ; and it was this which Bessel undertook and carried
through.
Direct observation furnishes only what has been called the
" raw material" of the positions of the heavenly bodies. 1 A
number of highly complex corrections have to be applied before
their mean can be disengaged from their apparent places on
the sphere. Of these, the most considerable and familiar is
atmospheric refraction, by which objects seem to stand higher
in the sky than they in reality do, the effect being evanescent
at the zenith, and attaining, by gradations varying with condi-
tions of pressure and temperature, a maximum at the horizon.
Moreover, the points 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 successive transmission of light, combined with the
movement of the earth in its orbit, causes a minute displace-
ment known as aberration.
Now it is easy to see that any uncertainty in the application
of these corrections saps the very foundations of exact astro-
nomy. Extremely minute quantities, it is true, are concerned ;
1 Bessel, Pop. VorL, p. 440.
40 HISTORY OF ASTRONOMY. PARTI.
but the life and progress of modern celestial science depends
upon the sure recognition of extremely minute quantities. In
the early years of this century, however, no uniform system
of " reduction " (so the complete correction of observational
results is termed) had been established. Much was left to the
individual caprice of observers, who selected for the several
" elements " of reduction such values as seemed best to them-
selves. 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 satisfac-
tory remedy.
His first step was an elaborate investigation of the precious
series of observations made by Bradley at Greenwich from 1750
until his death in 1762. The catalogue of 3222 stars which
he extracted from them, gave the earliest example of the sys-
tematic reduction on a uniform plan of such a body of work.
It is difiicult, 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 difiicult 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 Astronomice. The eminent value of the work con-
sisted 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 Tabulce
CHAP. ii. SIDEREAL ASTRONOMY. 41
RegiomontancB. They not only constituted an advance in accu-
racy, but afforded a vast increase of facility in application,
and were at once and everywhere adopted. Thus astronomy
became a truly universal science ; uncertainties and disparities
were banished, and observations made at all times and places
rendered mutually comparable. 1
More, however, yet remained to be done. In order to verify
with greater strictness the results drawn from the Bradley
and Piazzi catalogues, a third term of comparison was wanted,
and this Bessel undertook to supply. By a course of 75,011
observations, executed during the years 1821-33, with the
utmost nicety of care, the number of accurately known stars
was brought up to above 50,000, and an ample store of trust-
worthy facts laid up for the use of future astronomers. In
this department Argelander, whom he attracted from finance
to astronomy, and trained in his own methods, was his assist-
ant and successor. The great Bonn Durchmusterung," 2 in
which 324,198 stars visible in the northern hemisphere are
enumerated, and the corresponding " Atlas " published in
1857-63, constituting a picture of our sidereal surroundings
of heretofore unapproached completeness, may be justly said
to owe their origin to Bessel's initiative, and to form a sequel
to what he commenced.
But his activity was not solely occupied with the promotion
of a comprehensive reform in astronomy ; it embraced special
problems as well. The long-baffled search for a parallax of
the fixed stars was resumed with fresh zeal as each mechanical
or optical improvement held out fresh hopes of a successful
issue. Illusory results abounded. Piazzi in 1805 perceived,
as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon; the truth being that his
instruments were worn out with constant use, and could no
longer be depended upon. 3 His countryman, Calandrelli, was
similarly deluded. The celebrated controversy between the
Astronomer Royal and Dr. Brinkley, director of the Dublin
1 Durege, PesseVs Leben und Wirlcen, p. 28. 2 Banner Beotachtunyen,
Bd. iii.-v. 1859-62. 3 Bessel, Pop. VorL, p. 238.
42 HISTORY OF ASTRONOMY. PARTI.
College Observatory, turned on the same subject. Brinkley,
who was in possession of a first-rate meridian circle, believed
himself to have discovered relatively large parallaxes for four
of the brightest stars ; Pond, relying on the testimony of the
Greenwich instruments, asserted their nullity. The dispute
was protracted for fourteen years, from 1810 to 1824, and
was brought to no definite conclusion; but the strong pre-
sumption on the negative side was abundantly justified in
the event.
There was good reason for incredulity in the matter of par-
allaxes. Announcements of their detection had become so
frequent as to be discredited before they were disproved ; and
Struve, who made an investigation of the subject at Dorpat
in 1818-21, had clearly shown that the quantities con-
cerned were so small as to lie beyond the reliable measuring
powers of any instrument then in use. Already, however,
the means were being prepared of giving to those powers a
large increase.
On the 2 ist 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 be-
stowed. 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 priva-
tion 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 depart-
ment of the establishment founded two years previously
by Yon Reichenbach and Utzschneider. He now zealously
CHAP. ii. SIDEREAL ASTRONOMY. 43
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 suc-
ceeded in perfecting, December 12, 1817, an object-glass of
exquisite quality and finish, 9 J inches in diameter, and of four-
teen feet focal length.
This (as it was then considered) gigantic lense 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 unprece-
dented facilities both for the examination of double stars (in
which Struve chiefly employed it), and for such subtle measure-
ments as might serve to reveal or disprove the existence of a
sensible stellar parallax. Fraunhofer, moreover, constructed
for the Observatory of Konigsberg the first really available
heliometer. The principle of this instrument (termed with
more propriety a " divided object-glass micrometer ") is the
separation, by a strictly measurable amount, of two distinct
images of the same object. If a double star, for instance, be
under examination, the two half-lenses into which the object-
glass is divided are shifted until the upper star (say) in one
image is brought into coincidence with the lower star in the
other, when their distance apart becomes known by the amount
of motion employed. 1
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,
1 The heads of the screws applied to move the halves of the object-glass
in the Konigsberg heliometer are of so considerable a size that a thousandth
part of a revolution, equivalent to ^th of a second of arc, can be measured
with the utmost accuracy. Main, R. A. S. Mem., vol. xii. p. 53.
44 HISTORY OF ASTRONOMY. PART i.
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 con-
sumption, 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 denning powers of the Konigsberg heliometer, than
he resolved to employ them in an attack upon the now secular
problem of star- distances. But it was not until 1837 that he
found leisure to pursue the inquiry. In choosing his test-star
he adopted a new principle. It had hitherto been assumed
that our nearest neighbours in space must be found amongst
the brightest ornaments of our skies. The knowledge of stellar
proper motions afforded by the attentive comparison of recent
with earlier star-places, suggested a different criterion of dis-
tance. It is impossible to escape from the conclusion that the
apparently swiftest -moving stars are, on the whole, also the
nearest to us, however numerous the individual exceptions to
the rule. Now, as early as I7Q2, 1 Piazzi had noted, as an
indication of relative vicinity to the earth, the unusually large
proper motion (5.2" annually) of a double star of the fifth
magnitude in the constellation of the Swan. Still more
emphatically in 1 8 1 2 2 Bessel drew the attention of astro-
nomers to the fact, and 61 Cygni became known as the
" flying star." The seeming rate of its flight, indeed, is of so
leisurely a kind, that in a thousand years it will have shifted
its place by less than 3^ lunar diameters, and that a quarter
1 Specola Astronomica di Palermo, lib. vi. p. 10, note. z Monatliche
Correspondenz, Bd. xxvi. p. 162.
CHAP. ir. SIDEREAL ASTRONOMY. 45
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 (O.3I36"). 1 He then
had his heliometer taken down and repaired, after which he
resumed the inquiry, and finally terminated a series of 402
measures in March 1840.2 The resulting parallax of 0.3483"
(corresponding to a distance about 600,000 times that of the
earth from the sun), seemed to be ascertained beyond the
possibility of cavil, and is memorable as the first published
instance of the fathom-line, so industriously thrown into celes-
tial 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 just half a second. 3
Struve's measurements inspired less confidence. They ex-
tended over three years (1835-38), but were comparatively few,
and were frequently interrupted. Nevertheless the parallax
of about a quarter of a second (0.2613") which he derived from
1 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. 2 Ibid., Nos.
401-402. 3 Sir R. Ball's measurements at Dunsink give to 6 1 Cygni
a parallax of 0.47" ; almost exactly confirmed by Professor Hall at Wash-
ington in 1 88 1. But a very careful determination by the latter, of which
the result was published in 1886, reduced the value to 0.27".
48 HISTORY OF ASTRONOMY. PARTI.
selves highly improbable, and, we may confidently say, non-
existent.
The distances even of the few stars found to have measure-
able parallaxes are on a scale entirely beyond the powers of
the human mind to conceive. In the attempt both to realise
them distinctly, and to express them conveniently, a new unit
of length, itself of bewildering magnitude, has originated. This
is what we may call the light-journey of one year. The subtle
vibrations of the ether, propagated on all sides from the surface
of luminous bodies, travel at the rate of 186,300 miles a second,
or (in round numbers) six billions of miles a year. Four and
a third such measures are needed to span the abyss that
separates us from the nearest fixed star. In other words,
light takes four years and four months to reach the earth
from a Centauri ; yet a Centauri lies some ten billions of miles
nearer to us (so far as is yet known) than any other member
of the sidereal system !
The determination of parallax leads, in the case of binary
systems, to the determination of mass ; for the distance from
the earth of the two bodies forming such a system being
ascertained, the seconds of arc apparently separating them
from each other can be translated into millions of miles ; and
we only need to add a knowledge of their period to enable us,
by an easy sum in proportion, to find their combined mass in
terms of that of the sun. Thus, since according to Mr.
Hind's elements the two stars forming a Centauri revolve
round their common centre of gravity at a mean distance 29
times the radius of the earth's orbit, in a period of 85 years,
the attractive force of the two together must be fully 3 4 times
the solar. We may gather some idea of their relations by
placing in imagination a second luminary like our sun in
circulation just within the orbit of Neptune. But systems of
still more majestic proportions are reduced by extreme re-
moteness to apparent insignificance. A double star of the
fourth magnitude in Cassiopeia (Eta) to which a small
parallax is ascribed on the authority of 0. Struve, appears to
be more than ten times as massive as the central orb of our
CHAP. ii. SIDEREAL ASTRONOMY. 49
world, while 6 1 Cygni affords an instance of a binary combina-
tion in which only a fractional part of the solar gravitating
power resides.
Further, the actual rate of proper motions, so far as regards
that part of them which is projected upon the sphere, can be
ascertained for stars at known distances. The annual journey,
for instance, of 61 Cygni across the line of sight amounts to
1000, and that of a Centauri to 446 millions of miles. A
small star, numbered 1830 in Groombridge's Circumpolar .Cata-
logue, "devours the way " at the rate of 230 miles a second
a speed, in Newcomb's opinion, beyond the gravitating power
of the entire sidereal system to control ; and Toucanee pos-
sesses, according to Dr. Gill, nearly half that amazing velocity,
besides whatever movement each may have towards or from
the earth, of which the spectroscope may eventually give an
account.
Herschel's conclusion as to the movement of the sun among
the stars was not admitted as valid by the most eminent of his
successors. Bessel maintained that there was absolutely no
preponderating evidence in favour of its supposed direction
towards a point in the constellation Hercules. 1 Biot, Burck-
hardt, even Herschel's own son, shared his incredulity. But
the appearance of Argelander's prize-essay in 1837 2 changed
the aspect of the question. Herschel's first memorable solution
in 1783 was based upon the motions of thirteen stars, im-
perfectly 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 3 by Galloway's
investigation, founded exclusively on the apparent displace-
ments of southern stars. In 1859 and 1863, Mr. (now Sir
1 Fund. Astr., p. 309. 2 Mem. Pres. cl I' Ac. de St. Petersb., t. iii.
3 Phil. Trans., vol. cxxxvii. p. 79.
D
50 HISTORY OF ASTRONOMY. PARTI.
George) Airy and Mr. Dunkin, 1 employing all the resources
of modern science, and commanding the wealth of material
furnished by 1167 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 18832 served but slightly to modify. The general
direction of the solar movement may thus be regarded as
known ; but as to its rate, the grounds of inference are much
less satisfactory. Otto Struve's estimate of 154 million miles
a year is based upon the assumption of an average annual
parallax, for stars of the first magnitude, of about a quarter
of a second ; and since only five out of twenty stars of the
first magnitude appear to have any measurable parallax, it is
obvious that it merits a very restricted confidence.
As might have been expected, speculation has not been idle
regarding the purpose and goal of the strange voyage of
discovery through space upon which our system is embarked ;
but altogether fruitlessly. The variety of the conjectures
hazarded in the matter is in itself a measure of their futility.
Long ago, before the construction Jof 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 (estimated to include 14,000 stars) might
prove to be the supreme seat of attractive force ; 3 Argelander
placed his central body in the constellation Perseus ; 4 Fomal-
haut, the brilliant of the Southern Fish, was set in the post
of honour by Boguslawski of Breslau. Ma'dler (who succeeded
Struve at Dorpat in 1839) concluded from a more formal
inquiry that the ruling power in the sidereal system resided,
not in any single preponderating mass, but in the centre of
1 Mem. Roy. Astr. Soc., vols. xxviii. and xxxii. 2 Ibid., vol. xlvii. p. 327.
3 Phil. Trans., vol. xcvi. p. 230. 4 Mem. Prts. a VAc. de St. Peter s-
bourg, t. iii. p. 603 (read Feb. 5, 1837).
CHAP. ii. SIDEREAL ASTRONOMY. 51
gravity of the self-controlled revolving multitude. 1 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. 2 IMadler 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 in the neighbourhood of the group of the
Pleiades, where, accordingly, he placed the centre of gravity
of the Milky Way. 3 The bright star Alcyone thus became
the " central sun," but in a purely passive sense, its headship
being determined by its situation at the point of neutralisation
of opposing tendencies, and of consequent rest. The solar
period of revolution round this point was, by an avowedly
conjectural method, fixed at 18,200,000 years, implying, on
the extremely hazardous supposition that the distance of
Alcyone is thirty-four million times that of the earth from
the sun, a velocity for our system of about thirty miles a
second.
The scheme of sidereal government framed by the Dorpat
astronomer was, it may be observed, of the most approved con-
stitutional type ; deprivation, rather than increase of influence,
accompanying the office of chief dignitary. But while we are
still ignorant, and shall perhaps ever remain so, of the funda-
mental plan upon which the Galaxy is organised, recent investi-
gations tend more and more to exhibit it, not as monarchical
(so to speak), but as federative. The community of proper
motions detected by Ma'dler 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
i Die CentraUonne, Astr. Nach., Nos. 566-567, 1846. 2 Sir J. Herschel,
note to Treatise on Astronomy, and Phil. Trans., vol. cxxiii. part ii. p. 502.
3 The position is (as Sir J. Herschel pointed out, Otitlines of Astronomy,
p. 631, loth ed.) placed beyond the range of reasonable probability by its
remoteness (fully 26) from the galactic plane.
52 HISTORY OP ASTRONOMY. PARTI.
Invisible " now claims attention. 1 His prediction regarding
the planet Neptune does not belong to the present division
of our subject ; a strictly analogous discovery in the sidereal
system was, however, also very clearly foreshadowed by him.
His earliest suspicions of non-uniformity in the proper motion
of Sirius dated from 1834 ; they extended to Procyon in 1840 ;
and after a series of refined measurements with the new
Repsold circle, he announced in 1844 his conclusion that
these irregularities were due to the presence of obscure bodies
round which the two bright Dog-stars revolved as they pursued
their way across the sphere. 2 He even assigned to each an
approximate period of half a century. " I adhere to the con-
viction," he wrote later to Humboldt, " that Procyon and
Sirius form real binary systems, consisting of a visible and an
invisible star. There is no reason to suppose luminosity an
essential quality of cosmical bodies. The visibility of count-
less stars is no argument against the invisibility of countless
others." 3
An inference so contradictory to received " ideas * obtained
little credit, until Peters found, in i85i, 4 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 3ist of January 1862, while in the act of
trying a new 1 8-inch refractor, Alvan Clark, jun. (one of the
celebrated firm of American opticians), actually discovered
the hypothetical Sirian companion in the precise position re-
quired by theory. It has now been watched through just half a
revolution (period 49.4 years), and unless there should prove
to be other bodies concerned in disturbing the motion of Sirius
must be very slightly luminous in proportion to its mass.
Its attractive power, in fact, is about half that of its primary,
while it emits only T^outh ^ ^ s light. Sirius itself, on the
other hand, possesses a far higher radiative intensity than our
1 Madler in Wcstermann's JahrbucJi, 1867, p. 615. 2 Letter fromBessel
to Sir J. Herschel, Month. Not., vol. vi. p. 139. 3 Wolf, Gesch. <L Astr.,
p. 743, note. 4 Astr. Nach,, Nos. 745-748.
CHAP. ii. SIDEREAL ASTRONOMY. 53
sun. It gravitates admitting Dr. Gill's parallax of 0.38" to
be exact like three suns, but shines like seventy. Possibly
it is enormously distended by heat, and undoubtedly its atmos-
phere intercepts a very much smaller proportion of its light than
in stars of the solar class. As regards Procyon, visual verifi-
cation is still wanting, but to the mental eye the presence of a
considerable disturbing mass is fully assured by the inquiry
instituted by Auwers in 1862.! A period of forty years is
assigned by him to the system.
But Bessel was not destined to witness the recognition of
"the invisible" as a legitimate and profitable field for astro-
nomical research. He died March 17, 1846, just six months
before the discovery of Neptune, of an obscure disease,
eventually found to be occasioned by an extensive fungus-
growth in the stomach. The place which he left vacant was
not one easy to fill. Rarely indeed shall we find one who
reconciled with the same success the claims of theoretical and
practical astronomy, or surveyed the science which he had
made his own with a glance equally comprehensive, practical,
and profound.
The career of Friedrich Georg Wilhelm Struve illustrates
the maxim that science differentiates as it develops. He
might be called 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 indi-
cated and determined. He was born at Altona, of a respect-
able yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He
then turned to science, was appointed in 1813 to a professor-
ship of astronomy and mathematics, and began regular work in
the Dorpat Observatory just erected by Parrot for Alexander I.
It was not, however, until 1819 that the acquisition of a 5-foot
refractor by Troughton enabled him to take the position-angles
of double stars with regularity and tolerable precision. The
resulting catalogue of 795 stellar systems gave the signal
1 Astr. Nach., Nos. 1371-1373.
54 HISTORY OF ASTRONOMY. PARTI.
for a general resumption of the Herschelian labours in this
branch. The extraordinary facilities for observation afforded
by the Fraunhofer achromatic encouraged him to undertake,
February 1 1, 1825, a review of the entire heavens down to 15
south of the celestial equator, which occupied more than two
years, and yielded, from an examination of above 120,000 stars,
a harvest of about 2200 previously unnoticed composite objects.
The ensuing ten years were devoted to delicate and patient
measurements, the results of which were embodied in Mensurce
Micrometricce, published at St. Petersburg in 1837. This monu-
mental work gives the places, positions, distances, colours,
and relative brightness of 3112 double and multiple stars, all
determined with the utmost skill and care. The record is one
which gains in value with the process of time, and will for
ages serve as a standard of reference by which to detect change
or confirm discovery.
It appears from Struve's researches that about one in forty
of all stars down to the ninth magnitude is composite, but
that the proportion is doubled in the brighter orders. 1 This
he attributed to the difficulty of detecting the faint companions
of very remote orbs. It was also noticed, both by him and
Bessel, that double stars are in general remarkable for large
proper motions. Struve's catalogue included no star of which
the components were more than 32" apart, because beyond
that distance the chances of merely optical juxtaposition
become appreciable; but the immense preponderance of
extremely close over (as it were) loosely yoked bodies is such
as to demonstrate their physical connection, even if no other
proof were forthcoming. Many stars previously believed to
be single divided under the scrutiny of the Dorpat refrac-
tor ; while in some cases, one member of a (supposed) binary
system revealed itself as double, thus placing the surprised
observer in the unexpected presence of a triple group of suns.
Five instances were noted of two pairs lying so close together
as to induce a conviction of their mutual dependence ; 2 besides
Veber die Doppelsterne, Bericht, 1827, p. 22. 2 lUd., p. 25.
CHAP. ii. SIDEREAL ASTRONOMY. 55
which, 124 examples occurred of triple, quadruple, and multiple
combinations, the reality of which was open to no reasonable
doubt. 1
It was first pointed out by Bessel that the fact of stars exhi-
biting a common proper motion might serve as an unfailing
test of their real association into systems. This was, accord-
ingly, one of the chief criteria employed by Struve to distin-
guish true binaries from merely optical couples. On this
ground alone, 61 Cygni was admitted to be a genuine double
star ; and it was shown that, although its components 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. 2 Moreover,
this tie of an identical movement was discovered to unite
bodies 3 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. 4
In 1835 Struve was summoned by the Emperor Nicholas
to superintend the erection of a new observatory at Pulkowa,
near L 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
1 Mensurce Micr., p. xcix. 2 Stellarum Fixarum imprimis Duplicium
et Multiplicium Positiones Medice, pp. cxc., cciii. 3 For instance, the
southern stars, 36A Ophiuchi (itself double) and 30 Scorpii, which are
12' 10" apart. Ibid., p. cciii. Recent investigations have vastly enlarged
the area over which this species of connection extends. 4 Stellarum
Fixarum, d'C., p. ccliii.
56 HISTORY OF ASTRONOMY. PARTI.
Struve, born at Dorpat in 1819. He died November 23,
1864.
An inquiry into the laws of stellar distribution, undertaken
during the early years of his residence at Pulkowa, led Struve
to confirm, in the main, the inferences arrived at by Herschel
as to the construction of the heavens. According to his view,
the appearance known as the Milky Way is produced by a
collection of (for the most part) irregularly condensed star-
clusters, within which the sun is somewhat eccentrically placed.
The nebulous ring which thus integrates the light of countless
worlds he found to differ slightly from the form of a great
circle, and he accounted for this deviation from symmetry by
supposing the stars composing it to be scattered over a bent
or "broken plane," or to lie in two planes slightly inclined
to each other, our system occupying a position near their
intersection. 1 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, 2 and sought to confer upon this
celebrated hypothesis a definiteness and certainty far beyond
the aspirations of its earlier advocates, Ch^seaux and Olbers ;
but arbitrary assumptions vitiated his reasonings on this, as
well as on some other points. 3
In his special line as a celestial explorer of the most
comprehensive type, Sir William Herschel had but one legiti-
mate successor, and that successor was his son. John Fre-
derick 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 scien-
tific vocation. Already, in 1816, we find him reviewing some
of his father's double stars; and he completed in 1820 the
1 fitudes dAstronomie Stdlaire, 1847, P- 2 - 2 /&"?, P- 86. 3 See
Encke's criticism in Astr. Nach., No. 622.
CHAP. ii. SIDEREAL ASTRONOMY. 57
1 8-inch speculum which was to be the chief instrument of his
investigations. Soon after he undertook, in conjunction with
Mr. (afterwards Sir James) South, a series of observations,
issuing in the presentation to the Royal Society of a paper 1
containing micrometrical measurements of 380 binary stars, by
which the elder Herschel's inferences of orbital motion were,
in many cases, strikingly confirmed. A star in the Northern
Crown, for instance (n Coronas), had completed more than one
entire circuit since its first discovery ; another, r Serpentarii,
had closed up into apparent singleness ; while the motion of a
third, J Ursse 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 1827,2
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^ years. Encke followed at Berlin with a still
more elegant method ; and Sir John Herschel, pointing out
the uselessness of analytical refinements where the data were
necessarily so imperfect, described in 1831 a graphical process
by which " the aid of the eye and hand " was brought in "to
guide the judgment in a case where judgment only, and not
calculation, could be of any avail." 3 The subject has since
been cultivated with diligence, and not without success ; but
our acquaintance with stellar orbits can hardly yet be said to
have emerged from the tentative stage.
In 1825 Herschel undertook, and executed with great assi-
duity during the ensuing eight years, a general survey of the
northern heavens, directed chiefly towards the verification of
his father's nebular discoveries. The outcome was a catalogue
of 2306 nebulae and clusters, of which 525 were observed for
1 Phil. Trans., vol. cxiv. part iii. 1824. 2 Conn. d. Temps. 1830.
3 11. A. S. Mem., vol. v. 1833, p. 178.
58 HISTORY OF ASTRONOMY. PARTI.
the first time, besides 3347 double stars discovered almost
incidentally. 1 " Strongly invited," as he tells us himself, " by
the peculiar interest of the subject, and the wonderful nature
of the objects which presented themselves," he resolved to
attempt the completion of the survey in the southern hemi-
sphere. With this noble object in view, he embarked his family
and instruments on board the Mount Stewart Elpliinstone,
and, after a prosperous voyage, landed at Cape Town on the
1 6th of January 1834. Choosing as the scene of his observa-
tions a rural spot under the shelter of Table Mountain, he
began regular "sweeping" on the 5th of March. The site of
his great reflector is now marked with an obelisk, and the
name of Feldhausen has become memorable in the history of
science ; for the four years' work done there may truly be said to
open the chapter of our knowledge as regards the southern skies.
The full results of Herschel's journey to the Cape were not
made public until 1847, when a splendid volume 2 embodying
them was brought out at the expense of the Duke of Nor-
thumberland. They form a sequel to his father's labours such
as the investigations of one man have rarely received from
those of another. What the elder observer did for the
northern heavens, the younger did for the southern, and
with generally concordant results. Reviving the paternal
method of "star- gauging," he showed, from a count of 2299
fields, that the Milky Way surrounds the solar system as a
complete annulus of minute stars; not, however, quite sym-
metrically, since it appears that the sun lies somewhat above
its medial plane, as well as somewhat nearer to those por-
tions visible in the southern hemisphere, which accordingly
display a brighter lustre and a more complicated structure
than the northern branches. The singular cosmical agglo-
merations known as the " Magellanic Clouds " were now, for
the first time, submitted to a detailed, though admittedly in-
complete, examination, the almost inconceivable richness and
1 Phil. Trans., vol. cxxiii., and Results, &c., Introd. 2 Results of Astro-
nomical Observations made during the years 1834-8 at the Cape of Good
Hope.
CHAP. ii. SIDEREAL ASTRONOMY. 59
variety of their contents being such that a lifetime might
with great profit be devoted to their study. In the Greater
Nubecula, within a compass of forty-two square degrees,
Herschel reckoned 278 distinct nebulae and clusters, besides
fifty or sixty outliers, and a large number of stars intermixed
with diffused nebulosity, in all, 919 catalogued objects, and,
for the Lesser Cloud, 244. Yet this was only the most con-
spicuous part of what his twenty-foot revealed. Such an
extraordinary concentration of bodies so various led him to
the inevitable conclusion that "the Nubeculse are to be
regarded as systems sui generis, and which have no analogues
in our hemisphere." l He noted also the blankness of sur-
rounding space, especially in the case of Nubecula Minor,
"the access to which on all sides," he remarked, "is
through a desert ; " as if the cosmical material in the neigh-
bourhood had been swept up and garnered in these mighty
groups. 2
Of southern double stars, he discovered and gave careful
measurements of 2102, and described 1708 nebulae, of which
at least 300 were new. The list was illustrated with a number
of drawings, some of them extremely beautiful and elaborate.
Sir John Herschel's views as to the nature of nebulae were
considerably modified by Lord Rosse's success in "resolving"
a crowd of these objects into stars with his great reflectors.
His former somewhat hesitating belief in the existence of phos-
phorescent matter, "disseminated through extensive regions
of space in the manner of a cloud or fog," 3 was changed into
a conviction that no valid distinction could be established
between the faintest wisp of cosmical vapour just discernible
in a powerful telescope, and the most brilliant and obvious
cluster. He admitted, however, an immense range of possible
variety in the size and mode of aggregation of the stellar con-
stituents of various nebulae. Some might appear nebulous
from the closeness of their parts ; some from their smallness.
Others, he suggested, might be formed of " discrete luminous
1 Results, <&c., p. 147. a See Proctor's Universe of Stars, p. 92.
3 A Treatise of Astronomy, 1833, p. 406.
60 HISTORY OF ASTRONOMY. PARTI.
bodies floating in a non-luminous medium ; " 1 while the an-
nular kind probably consisted of " hollow shells of stars." 2
That a physical, and not merely an optical, connection unites
nebulae with the embroidery (so to speak) of small stars with
which they are in many instances profusely decorated, was
evident to him, as it must be to all who look as closely and
see as clearly as he did. His description of No. 2093 in his
northern catalogue as " a network or tracery of nebula fol-
lowing the lines of a similar network of stars," 3 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 " nebulas 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 nebulas," in which, as he pointed out, " all
the varieties of double stars as to distance, position, and
relative brightness, have their counterparts." He, moreover,
investigated the subject of nebular distribution by the simple
and effectual method of graphic delineation or " charting," and
succeeded in showing that while a much greater uniformity
of scattering prevails in the southern heavens than in the
northern, a condensation is nevertheless perceptible about the
constellations Pisces and Cetus, roughly corresponding to the
"nebular region" in Yirgo 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." *
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
1 Results, tfcc., p. 139. 2 Ibid., pp. 24, 142. 3 Phil. Trans., vol.
cxxiii. p. 503. 4 Results, &c. t p. 136.
CHAP. ii. SIDEREAL ASTRONOMY. 61
in the Argo nebula is a large star denominated j Argus. When
Halley visited St. Helena in 1677, it seemed of the fourth
magnitude ; but Lacaille in the middle of the following century,
and others after him, classed it as of the second. In 1827 the
traveller Burchell, being then at St. Paul, near Rio Janeiro,
remarked that it had unexpectedly assumed the first rank, a
circumstance the more surprising to him because he had fre-
quently, when in Africa during the years 1811 to 1815, noted
it as of only fourth magnitude. This observation, however,
did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second,
and had no suspicion of its unusual character until December
1 6, 1837, when he suddenly perceived it with its light almost
tripled. It then far outshone Rigel in Orion, and on the 2d
of January following it very nearly matched a Centauri. From
that date it declined ; but a second and even brighter maxi-
mum occurred in April 1843, when Maclear, then director of
the Cape Observatory, saw it blaze out with a splendour ap-
proaching that of Sirius. Its waxings and wanings were marked
by curious " trepidations " of brightness extremely perplexing
to theory. In 1863 it had sunk below the fifth magnitude,
and in 1869 was barely visible to the naked eye ; since when,
a slow and somewhat fluctuating loss of light has carried it
down nearly to the eighth magnitude. There is some reason
to believe that its variations are included in a cycle of about
seventy years, 1 confused probably by the superposition of more
than one secondary period ; but the extent and character of
the vicissitudes to which it is subject stamp it as a species
of connecting link between regularly periodic and (so-called)
"temporary stars."
Among the numerous topics which engaged Herschel's atten-
tion at the Cape was that of relative stellar brightness. Having
contrived an "astrometer " in which an "artificial star," formed
by the total reflection of moonlight from the base of a prism,
served as a standard of comparison, he was able to estimate
the lustre of the natural stars examined by the distances at
1 Loomis, Month. Not., vol. xxix. p. 298.
62 HISTORY OF ASTRONOMY. PARTI.
which the artificial object appeared equal respectively to each.
He thus constructed a table of 191 of the principal stars, 1 both
in the northern and southern hemispheres, setting forth the
numerical values of their apparent brightness relatively to that
of a Centauri, which he selected as a unit of measurement.
Further, the light of the full moon being found by him to
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 i : 8oi,o72 2 (Zollner made the ratio i : 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,
is found to emit four times, Yega nearly forty times as much
light as our sun ; while Arcturus (if its measured parallax of
0.13" can be depended upon) displays the splendour of fully
200 such luminaries.
Herschel returned to England in the spring of 1838, bring-
ing with him a wealth of observation and discovery such as
had perhaps never before been amassed in so short a time.
Deserved honours awaited him. He was created a baronet
on the occasion of the Queen's coronation (he had been
knighted in 1831) ; universities and learned societies vied with
each other in showering distinctions upon him ; and the suc-
cess 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 5079 nebulae (including all then certainly
known), published in the Philosophical Transactions for 1864,
is, and will probably long remain, the leading source of infor-
mation on the subject ; 3 but he unfortunately did not live to
1 Outlines of Astr., App. I. 2 Phil. Trans., vol. cxix. p. 27. 3 Dr.
Dreyer communicated to the Royal Irish Academy in 1877 (Trans., vol.
xx vi. p. 381) a supplement to the work, bringing the number of catalogued
nebulae up to 625 1 ; and a second supplement is now in preparation by him,
CHAP. ii. SIDEREAL ASTRONOMY. 63
finish the companion work on double stars, for which he had
accumulated a vast store of materials. 1 He died at Colling-
wood in Kent, May n, 1871, in the eightieth year of his age,
and was buried in Westminster Abbey, close beside the grave
of Sir Isaac Newton.
The consideration of Sir John Herschel's Cape observations
brings us to the close of the period we are just now engaged
in studying. They were given to the world, as already stated,
three years before the middle of the century, and accurately
represent the condition of sidereal science at that date. Look-
ing back over the fifty years traversed, we can see at a glance
how great was the stride made in the interval. Not alone was
acquaintance with individual members of the cosmos vastly
extended, but their mutual relations, the laws governing their
movements, their distances from the earth, masses, and intrinsic
lustre, had begun to be successfully investigated. Begun to be ;
for only regarding a scarcely perceptible minority had even
approximate conclusions been arrived at. Nevertheless the
whole progress of the future lay in that beginning; it was
the thin end of the wedge of exact knowledge. The principle
of measurement had been substituted for that of probability ;
a basis has 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.
1 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.
64
CHAPTER III.
PROGRESS OF KNOWLEDGE REGARDING THE SUN.
THE discovery of sun-spots in 1610 by Fabricius and Galileo
first opened a way for inquiry into the solar constitution ; but
it was long before that way was followed with system or profit.
The seeming irregularity of the phenomena discouraged con-
tinuous attention; casual observations were made the basis
of arbitrary conjectures, and real knowledge received little or
no increase. In 1620 we find Jean Tarde, canon of Sarlat,
arguing that because the sun is " the eye of the world," and
the eye of the world cannot suffer from ophthalmia, therefore
the appearances in question must be due, not to actual specks
or stains on the bright solar disc, but to the transits of a
number of small planets across it ! To this new group of
heavenly bodies he gave the name of " Borbonia Sidera," and
they were claimed in 1633 for the House of Hapsburg, under
the title of "Austriaca Sidera," by Father Malapertius, a
Belgian Jesuit. 1 A similar view was temporarily maintained
against Galileo by the celebrated Father Scheiner of Ingolstadt,
and later by William Gascoigne, the inventor of the micro-
meter ; but most of those who were capable of thinking at all
on such subjects (and they were but few) adhered either to the
cloud theory or to the slag theory of sun-spots. The first was
championed by Galileo, the second by Simon Marius, " astro-
nomer and physician " to the brother Margraves of Branden-
burg. The latter opinion received a further notable develop-
ment from the fact that in 1618, a year remarkable for the
1 Kosmos, Bd. iii. p. 409 ; Lalande, Bibliographic Astronomique, pp.
1 79, 202.
CHAP. in. KNOWLEDGE OF THE SUN. 65
appearance of three bright comets, the sun was almost free
from spots; whence it was inferred that the cindery refuse
from the great solar conflagration, which usually appeared as
dark blotches on its surface, was occasionally thrown off in
the form of comets, leaving the sun, like a snuffed taper, to
blaze with renewed brilliancy. 1
In the following century, Derham gathered from observations
carried on during the years 1703-11, "That the spots on the
sun are caused by the eruption of some new volcano therein,
which at first pouring out a prodigious quantity of smoke and
other opacous matter, causeth the spots ; and as that fuliginous
matter decayeth and spendeth itself, and the volcano at last
becomes more torrid and flaming, so the spots decay, and grow
to umbrae, and at last to faculse." 2
The view, confidently upheld by Lalande, 3 that spots were
rocky elevations uncovered by the casual ebbing of a luminous
ocean, the surrounding penumbras representing shoals or sand-
banks, had even less to recommend it than Derham's volcanic
theory. Both were, however, significant of a growing tendency
to bring solar phenomena within the compass of terrestrial
analogies.
For 164 years, then, after Galileo first levelled his telescope
at the setting sun, next to nothing was learned as to its
nature ; and the facts immediately ascertained of its rotation
on an 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,
1 R. Wolf, Die Sonne und ihre Flecken, p. 9. Marius himself, however,
seems to have held the Aristotelian terrestrial-exhalation theory of come-
tary origin. See his curious little tract. Astronomisehe und Astrologische
Beschreibung des Cometen, Niirnberg, 1619. 2 Phil. Trans., vol. xxvii. p.
274. Umbrce (now called penumbra) are spaces of half-shadow which
usually encircle spots. Faculce ("little torches," so named by Scheiner)
are bright streaks or patches closely associated with spots. 3 Mem. Ac.
Sc., 1776 (pub. 1779), p. 507. The merit, however (if merit it be), of
having first put forward (about 1671) the hypothesis alluded to in the
text, belongs to D. Cassini. See Delambre, Hist, de VAstr. Mod., t. ii. p.
694, and Kosmos, Bd. iii. p. 410.
66 HISTORY OF ASTRONOMY. PARTI.
gained little either in precision or development from five gene-
rations of astronomers.
But in November 1769 a spot of extraordinary size engaged
the attention of Alexander Wilson, professor of astronomy in
the University of Glasgow. He watched it day by day, and
to good purpose. As the great globe slowly revolved, carry-
ing 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 complete-
ness as it returned to a central position. Similar perspective
effects were visible in numerous other spots subsequently
examined by him, and he was thus in 1774 l able to prove by
strict geometrical reasoning that such appearances were, as a
matter of fact, produced by vast excavations in the sun's sub-
stance. It was not, indeed, the first time that such a view
had been suggested. Father Schemer's later observations
plainly foreshadowed it ; 2 a conjecture to the same effect was
emitted by Leonhard Host of Nuremberg early in the eighteenth
century; 3 both by Lahire in 1703 and by J. Cassini in 1719
spots had been seen to form actual notches on the solar limb ;
while Pastor Schiilen of Essingen convinced himself in 1770,
by the careful study of appearances similar to those noted by
Wilson, of the fact detected by him* 4 Nevertheless Wilson's
demonstration came with all the surprise of novelty, as well
as with all the force of truth.
The general theory by which it was accompanied rested on
a very different footing. It was avowedly tentative, and was
set forth in the modest shape of an interrogatory. " Is it not
reasonable to think," he asks, " that the great and stupendous
body of the sun is made up of two kinds of matter, very
different in their qualities ; that by far the greater part is solid
1 Phil. Trans., vol. Ixiv. pp. 7-11. 2 Rosa Ursina, lib. iv. p. 507.
3 R. Wolf, Die Sonne und ihre Hecken, p. 12. 4 Schellen, Die Spectral-
analyse, Bd. ii. p. 56 (3d ed.)
CHAP. in. KNOWLEDGE OF THE SUN. 67
and dark, and that this immense and dark globe is encom-
passed with a thin covering of that resplendent substance
from which the sun would seem to derive the whole of his
vivifying heat and energy ? " l 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 spectro-
scope.
A cool, dark, solid globe, its surface diversified with moun-
tains and valleys, clothed with luxuriant vegetation, and " richly
stored with inhabitants," protected by a heavy cloud-canopy
from the intolerable glare of the upper luminous region, where
the dazzling coruscations of a solar aurora some thousands of
miles in depth evolved the stores of light and heat which
vivify our world such was the central luminary which Her-
schel constructed with his wonted ingenuity, and described
with his wonted eloquence.
"This way of considering the sun and its atmosphere," he
says, 2 " removes the great dissimilarity we have hitherto been
used to find between its condition and that of the rest of the
great bodies of the solar system. The sun, viewed in this
light, appears to be nothing else than a very eminent, large,
and lucid planet, evidently the first, or, in strictness of speaking,
the only primary one of our system; all others being truly
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."
1 Phil. Trans., vol. Ixiv. p. 2O. 2 Ibid., vol. Ixxxv. 1795, P- 63.
68 HISTORY OF ASTRONOMY. PARTI.
We smile at conclusions which our present knowledge con-
demns as extravagant and impossible, but such incidental
nights of fancy in no way derogate from the high value of
Herschel's contributions to solar science. The cloud-like char-
acter which he attributed to the radiant shell of the sun (first
named by Schroter the " photosphere ") is borne out by all
recent investigations; he observed its mottled or corrugated
aspect, resembling, as he described it, the roughness on the
rind of an orange; showed that "faculse" 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," l abundantly visible as dark points on
the solar disc. But should an uncommon quantity be formed,
" it will," he maintained, " burst through the planetary 2 regions
of clouds, and thus will produce great openings ; then, spread-
ing itself above them, it will occasion large shallows (penumbrse),
and, mixing afterwards gradually with other superior gases, it
will promote the increase, and assist in the maintenance of the
general luminous phenomena." 3
This partial anticipation of the modern view that the solar
radiations are maintained by some process of circulation
within the solar mass, was reached by Herschel through pro-
longed study of the phenomena in question. The novel and
important idea contained in it, however, it was at that time
premature to attempt to develop. But though many of the
subtler suggestions of Herschel's genius passed unnoticed by
his contemporaries, the main result of his solar researches was
an unmistakable one. It was nothing less than the definitive
introduction into astronomy of the paradoxical conception of
the central fire and hearth of our system as a cold, dark,
1 Phil. Trans., vol. xci. 1801, p. 303. 2 The supposed opaque or pro-
tective stratum was named by him "planetary," from the analogy of
terrestrial clouds. 3 Ibid., p. 305.
CHAP. in. KNOWLEDGE OF THE SUN. 69
terrestrial mass, wrapt in a mantle of innocuous radiance an
earth, so to speak, within a sun without.
Let us pause for a moment to consider the value of this
remarkable innovation. It certainly was not a step in the
direction of truth. On the contrary, the crude notions of
Anaxagoras and Xeno approached more nearly to what we
now know of the sun, than the complicated structure devised
for the happiness of a nobler race of beings than our own, by
the benevolence of eighteenth-century astronomers. And yet
it undoubtedly constituted a very important advance in science.
It was the first earnest attempt to bring solar phenomena
within the compass of a rational system ; to put together into
a consistent whole the facts ascertained ; to fabricate, in short,
a solar machine that would in some fashion work. It is true
that the materials were inadequate and the design faulty.
The resulting construction has not proved strong enough to
stand the wear and tear of time and discovery, but has had to
be taken to pieces and remodelled on a totally different plan.
But the work was not therefore done in vain. None of Bacon's
aphorisms show a clearer insight into the relations between
the human mind and the external world than that which
declares " Truth to emerge sooner from error than from con-
fusion." * 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 plausible scheme. The persuasion of knowledge
is a keen incentive to its increase. Few men care to investigate
what they are obliged to admit themselves entirely ignorant
1 Novum Organum, lib. ii. aph. 20.
70 HISTORY OF ASTRONOMY. PARTI.
of ; but once started on the road of knowledge (real or sup-
posed), they are eager to pursue it. By the promulgation of
a confident and consistent view regarding the nature of the
sun, accordingly, research was encouraged, because it was
rendered hopeful, and inquirers were shown a path leading
indefinitely onwards where an impassable thicket had before
seemed to bar the way.
We have called the " terrestrial " theory of the sun's nature
an innovation, and so, as far as its general acceptance is con-
cerned, it may justly be termed ; but, like all successful inno-
vations, it was a long time brewing. It is extremely curious
to find that Herschel had a predecessor in its advocacy who
never looked through a telescope (nor, indeed, imagined the
possibility of such an instrument), who knew nothing of sun-
spots, was still (mistaken assertions to the contrary notwith-
standing) in the bondage of the geocentric system, and regarded
Nature from the lofty standpoint of an idealist philosophy.
This was the learned and enlightened Cardinal Cusa, a fisher-
man's son from the banks of the Moselle, whose distinguished
career in the Church and in literature extended over a con-
siderable part of the fifteenth century (1401-64). In his
singular treatise De Doctd Ignorantid, 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 cir-
cumferential 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 pre-
monitory of the tendency, so powerfully developed by subse-
quent discoveries, to assimilate the orbs of heaven to the model
of our insignificant planet, and to extend the brotherhood of
CHAP. in. KNOWLEDGE OF THE SUN. 71
our system and our species to the farthest limit of the visible
or imaginable universe.
In later times we find Flamsteed communicating to Newton,
March 7, 1681, his opinion "that the substance of the sun is
terrestrial matter, his light but the liquid menstruum encom-
passing him." l 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 enve-
loped in a vaporous atmosphere ; " 2 and the learned in general
applauded and acquiesced. The view, however, was in 1787
still so far from popular, that the holding of it was alleged as
a proof of insanity in Dr. Elliot when accused of a murder-
ous assault on Miss Boydell. His friend Dr. Simmons stated
on his behalf that he had received from him in the preceding
January a letter giving evidence of a deranged mind, wherein
he asserted " that the sun is not a body of fire, as hath been
hitherto supposed, but that its light proceeds from a dense
and universal aurora, which may afford ample light to the
inhabitants of the surface beneath, and yet be at such a distance
aloft as not to annoy them. No objection, he saith, ariseth to
that great luminary's being inhabited ; vegetation may obtain
there as well as with us. There may be water and dry land,
hills and dales, rain and fair weather ; and as the light, so the
season must be eternal, consequently it may easily be con-
ceived to be by far the most blissful habitation of the whole
system ! " The Recorder, however, we are told, objected that
if an extravagant hypothesis were to be adduced as proof of
insanity, the same might hold good with regard to some other
speculators, and desired Dr. Simmons to tell the court what
he thought of the theories of Burnet and Buff on. 3
Eight years later, this same " extravagant hypothesis,"
backed by the powerful recommendation of Sir William
Herschel, obtained admittance to the venerable halls of science,
i Brewster's Life of Newton, vol. ii. p. 103. 2 Besch'dftignngen d. Berl.
Ges. Naturforschender Freunde, Bd. ii. p. 233. 3 Gentleman's Magazine,
1787, vol. ii. p. 636.
72 HISTORY OF ASTRONOMY. PARTI.
there to abide undisturbed for nearly seven decades. It is
true there were individual objectors, but their arguments made
little impression on the general body of opinion. Ruder blows
were required to shatter an hypothesis nattering to human
pride of invention in its completeness, in the plausible detail
of observations by which it seemed to be supported, and in its
condescension to the natural pleasure in discovering resem-
blance under all but total dissimilarity.
Sir John Herschel included among the results of his multi-
farious labours at the Cape of Good Hope a careful study of
the sun-spots conspicuously visible towards the end of the
year 1836 and in the early part of 1837. They were remark-
able, he tells us, for their forms and arrangement, as well as
for their number and size; one group, measured on the 2Qth
of March in the latter year, covering (apart from what may be
called its outlying dependencies) the vast area of five square
minutes or 3780 million square miles. 1 We have at present
to consider, however, not so much these observations in them-
selves, as the chain of theoretical suggestions by which they
were connected. The distribution of spots, it was pointed out,
on two zones parallel to the equator, showed plainly their
intimate connection with the solar rotation, and indicated as
their cause fluid circulations analogous to those producing the
terrestrial trade and anti-trade winds.
" The spots, in this view of the subject," he went on to say, 2
" would come to be assimilated to those regions on the earth's
surface where, for the moment, hurricanes and tornadoes pre-
vail ; the upper stratum being temporarily carried downwards,
displacing by its impetus the two strata of luminous matter
beneath, the upper of course to a greater extent than the lower,
and thus wholly or partially denuding the opaque surface of
the sun below. Such processes cannot be unaccompanied by
vorticose motions, which, left to themselves, die away by degrees
and dissipate, with the peculiarity that their lower portions
come to rest more speedily than their upper, by reason of the
greater resistance below, as well as the remoteness from the
i Results, Ac., p. 432. 2 Ibid., &c., 434.
CHAP. in. KNOWLEDGE OF THE SUN. 73
point of action, which lies in a higher region, so that their
centres (as seen in our waterspouts, which are nothing but small
tornadoes) appear to retreat upwards. Now this agrees per-
fectly with what is observed during the obliteration of the solar
spots, which appear as if filled in by the collapse of their
sides, the penumbra closing in upon the spot and disappearing
after it."
When, however, it comes to be asked whether a cause can
be found by which a diversity of solar temperature might be
produced corresponding with that which sets the currents of
the terrestrial atmosphere in motion, we are forced to reply
that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equa-
tor, due to the slightly spheroidal or bulging form of its outer
atmospheric envelope, assuredly gives no sufficient account of
such circulatory movements as he supposed to exist. Never-
theless, the view that the sun's rotation is intimately connected
with the formation of spots is so obviously correct, that we can
only wonder it was not thought of sooner, while we are even
now unable to explain with any certainty how it is so con-
nected.
Mere scrutiny of the solar surface, however, is not the only
means of solar observation. We have a satellite, and that
satellite from time to time acts most opportunely as a screen,
cutting off a part or the whole of those dazzling rays in which
the master-orb of our system veils himself from over-curious
regards. The importance of eclipses to the study of the solar
surroundings is of comparatively recent recognition ; neverthe-
less, much of what we know concerning them has been snatched,
as it were, by surprise under favour of the moon. In former
times, the sole astronomical use of such incidents was the cor-
rection of the received theories of the solar and lunar move-
ments ; the precise time of their occurrence was the main fact
to be noted, and subsidiary phenomena received but casual
attention. Now, their significance as a geometrical test of
tabular accuracy is altogether overshadowed by the interest
attaching to the physical observations for which they afford
74 HISTORY OF ASTRONOMY. PARTI.
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 Astro-
nomical 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, how-
ever, by preference a computator rather than an observer.
What Sir John Herschel calls the " archaeology of practical
astronomy" found in him an especially zealous student. He
re-edited the star-catalogues of Ptolemy, Ulugh Beigh, Tycho
Brahe, Hevelius, Halley, Flamsteed, Lacaille, and Mayer;
calculated the eclipse of Thales and the eclipse of Agathocles,
and vindicated the memory of the first Astronomer Royal. But
he was no less active in meeting present needs than in revising
past performances. The subject of the reduction of observa-
tions, then, as we have already explained, 1 in a state of deplor-
1 See ante, p. 40.
CHAP. in. KNOWLEDGE OF THE SUN. 75
able confusion, attracted his most earnest attention, and he was
close on the track of Bessel when made acquainted with the
method of simplification devised at Konigsberg. Anticipated
as an inventor, he could still be of eminent use as a promoter
of these valuable improvements ; and, carrying them out on a
large scale in the star-catalogue of the Astronomical Society
(published in 1827), " he put " (in the words of Herschel) " the
astronomical world in possession of a power which may be said,
without exaggeration, to have changed the face of sidereal
astronomy." l
His reputation was still further enhanced by his renewal,
with vastly improved apparatus, of the method, first used by
Henry Cavendish in 1797-98, for determining the density of
the earth. From a series of no less than 2153 delicate and
difficult experiments, conducted at Tavistock Place during
the years 1838-42, he concluded our planet to weigh 5.66 as
much as a globe of water of the same bulk ; and this result
(slightly corrected) is still accepted as a very close approxima-
tion to the truth.
What we have thus glanced at is but a fragment of the truly
surprising mass of work accomplished by Baily in the course
of a variously occupied life. A rare combination of qualities
fitted him for his task. Unvarying health, undisturbed equa-
nimity, methodical habits, the power of directed and sustained
thought, joined to form in him an intellectual toiler of the
surest, though not perhaps of the very highest quality. He
was in harness almost to the end. He was destined scarcely
to know the miseries of enforced idleness or of consciously
failing powers. In 1842 he completed the laborious reduction
of Lalande's great catalogue, undertaken at the request of the
British Association, and was still engaged in seeing it through
the press when he was attacked with what proved his last, as it
was probably his first serious illness. He, however, recovered
sufficiently to attend the Oxford Commemoration of July 2,
1844, where an honorary degree of D.C.L. was conferred upon
him in company with Airy and Struve ; but sank rapidly after
1 Memoir of Francis Baily, Mem, R. A. S., vol. xv. p. 324.
76 HISTORY OF ASTRONOMY. PARTI.
the effort, and died on the 3oth of August following, at the
age of seventy, lamented and esteemed by all who knew him.
It is now time to consider his share in the promotion of
solar research. Eclipses of the sun, both ancient and modern,
were a speciality with him, and he was fortunate in those
which came under his observation. Such phenomena are of
three kinds partial, annular, and total. In a partial eclipse,
the moon, instead of passing directly between us and the sun,
slips by, as it were, a little on one side, thus cutting off from
our sight only a portion of his surface. An annular eclipse,
on the other hand, takes place when the moon is indeed
centrally interposed, but falls short of the apparent size re-
quired for the entire concealment of the solar disc, which con-
sequently 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 diameters 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 I5th of May 1836, an annular eclipse was visible
in the northern parts of Great Britain, and was observed
by Baily at Inch Bonney near Jedburgh. It was here that he
saw the 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, irre-
gular in size and distance from each other, suddenly formed
round that part of the circumference of the moon that was about
to enter, or which might be considered as having just entered
on the sun's disc. Its formation indeed was so rapid that it
presented the appearance of having been caused by the ignition
of a fine train of gunpowder. . . . 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
CHAP. in. KNOWLEDGE OF THE SUN. 77
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." 1
These curious appearances were not an absolute novelty.
Weber in 1791, and Yon Zach in 1820, had seen the "beads; "
Van Swinden had described the "belts" or " threads." 2
These last were, moreover (as Baily clearly perceived), com-
pletely 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 irradia-
tion, by which a bright object seems to encroach upon a dark
one j but under good atmospheric and instrumental conditions
it becomes inconspicuous. The " Beads " must indeed always
appear when the projected lunar edge is serrated with moun-
tains ; but in Baily's observation, they were exaggerated and
distorted by an irradiative dinging 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 pre-
pared 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 Astro-
nomer Royal (Airy) repaired to Turin ; Baily to Pa via j Otto
Struve threw aside his work amidst the stars at Pulkowa,
and went south as far as Lipeszk ; Schumacher travelled from
Altona to Vienna ; Arago from Paris to Perpignan. Nor did
their trouble go unrewarded. The expectations of the most
sanguine were outdone by the wonders disclosed.
Baily (whose narrative we again have recourse to) had set up
his Dollond's achromatic (3^ feet focal length) in an upper
1 Mem. K A. S., vol. x. pp. 5-6. 2 Ibid., pp. 14-17.
78 HISTORY OF ASTRONOMY. PARTI.
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
aureole. Pavia contains many thousand inhabitants, the major
part of whom were, at this early hour, walking about the streets
and squares or looking out of windows, in order to witness
this long-talked-of phenomenon; and when the total obscu-
ration took place, which was instantaneous, there was an uni-
versal shout from every observer, which 'made the welkin
ring,' and, for the moment, withdrew my attention from the
object with which I was immediately occupied. I had indeed
anticipated the appearance of a luminous circle round the
moon during the time of total obscurity ; but I did not expect,
from any of the accounts of preceding eclipses that I had read,
to witness so magnificent an exhibition as that which took
place. . . . The breadth of the corona, measured from the
circumference of the moon, appeared to me to be nearly equal
to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense (indeed I may say
quite dense) close to the border of the moon, and became
gradually and uniformly more attenuate as its distance there-
from increased, assuming the form of diverging rays in a
rectilinear line, which at the extremity were more divided, and
of an unequal length ; so that in no part of the corona could I
discover the regular and well-defined shape of a ring at its
outer margin. It appeared to me to have the sun for its
centre, but I had no means of taking any accurate measures
for determining this point. Its colour was quite white, not
pearl-colour, nor yellow, nor red, and the rays had a vivid and
nickering appearance, somewhat like that which a gaslight
CHAP. in. KNOWLEDGE OF THE SUN. 79
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 mag-
nitude. . . . The whole of these three protuberances were
visible even to the last moment of total obscuration ; at least,
I never lost sight of them when looking in that direction ; and
when the first ray of light was admitted from the sun, they
vanished, with the corona, altogether, and daylight was instan-
taneously restored." *
Notwithstanding unfavourable weather, the "red flames"
were perceived with little less clearness and no less amazement
from the Superga than at Pavia, and were even discerned by
Mr. Airy with the naked eye. " Their form " (the Astronomer
1 Mem. R. A. S., vol. xv. pp. 4-6.
8o HISTORY OF ASTRONOMY. PARTI.
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." 1
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. 2
The corona, both as to figure and extent, presented very
different appearances at different stations. This was no doubt
due to varieties in atmospheric conditions. At the Superga,
for instance, all details of structure seem to have been effaced
by the murky air, only a comparatively feeble ring of light
being seen to encircle the moon. Elsewhere, a brilliant radi-
ated formation was conspicuous, spreading, at four opposite
points, into four vast luminous expansions, compared to feather-
plumes or aigrettes. 3 Arago at Perpignan noticed consider-
able irregularities in the divergent rays j some appeared
curved and twisted ; a few lay across the others, in a direction
almost tangential to the moon's limb; the general effect
being described as that of a "hank of thread in disorder." 4
At Lipeszk, where the sun stood much higher above the
horizon than in Italy or France, the corona showed with sur-
prising 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
1 Mem. R. A. , vol. xv. p. 16. 2 Annuaire, 1846, p. 409. 3 Ibid.,
p. 317. 4 Ibid., p. 322.
CHAP. in. KNOWLEDGE OF THE SUN. 81
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. Maclauriri
tells us, 1 that during an eclipse of this character which he
observed at Edinburgh in 1737, "gentlemen by no means
shortsighted declared themselves unable to discern the moon
upon the sun without the aid of a smoked glass ; " and Baily
(who, however, tvas shortsighted) could distinguish, in 1836,
with the naked eye no trace of " the globe of purple velvet "
which the telescope revealed as projected upon the face of the
sun. 2 Moreover, the diminution of light is described by him
as " little more than might be caused by a temporary cloud
passing over the sun ; " birds continued in full song ; and
" one cock in particular was crowing with all his might while
the annulus was forming."
Very different were the effects of the eclipse of 1842, as to
which some interesting particulars were collected by Arago. 3
Beasts of burthen, he tells us, paused in their labour, and
could by no amount of punishment be induced to move until
the sun reappeared. Birds and beasts abandoned their food ;
linnets were found dead in their cages ; even ants suspended
their toil. Diligence-horses, on the other hand, seemed as
insensible to the phenomenon as locomotives. The convol-
vulus 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 color-
ation 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. In-
deed, it is too conspicuous an apparition to escape notice from
1 Pldl Trans., vol. xl. p. 192. 2 Mem. R. A. S., vol. x. p. 17.
3 Ann. du, Bureau des Lony., 1846, p. 309.
F
82 HISTORY OF ASTRONOMY. PARTI.
the least attentive, or least practised observer of a total eclipse.
Nevertheless, explicit references to it are rare in early times.
Both Plutarch, 1 however, and Philostratus in his Life of
Apollonius of Tyana, 2 are unmistakable in their allusions, the
latter describing a " crown," or garland similar to the iris, by
which the sun was encompassed and obscured during an eclipse.
The first to take the phenomenon into scientific consideration-
was Kepler. He showed, from the positions in their orbits at
the time of the sun and moon, that an eclipse observed by
Clavius at Rome in 1567 could not have been annular, 3 as the
dazzling 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 con-
densed in the neighbourhood either of the sun or moon. 4 To
the solar explanation he gave his own decided preference, but,
with one of those curious flashes of half-prophetic insight
characteristic of his genius, declared that " it should be laid
by ready for use, not brought into immediate requisition." !
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 ; 6 but it failed to receive the attention which, as a step
in the right direction, it undoubtedly merited. Nine years
1 Op. Mor. et Phil., vol. ix. p. 682, edit. Lips. 1778. 2 Book viii. chap,
xxiii. Both references are due to R. Grant. A sir. Nach., No. 1838.
3 Astronomice Pars Optica, Op. omnia, t. ii. p. 317. 4 De Stella Nova,
Op., t. ii. pp. 696-697. 5 Astr. Pars. Op., p. 320. 6 Mem. de I'Ac. des
Sciences, 1715, p. 119.
CHAP. m. KNOWLEDGE OF THE SUN. 83
later we meet with Halley's comments on a similar event, the
first which had occurred in London since March 20, 1140.
By nine in the morning of April 22 (O.S.), 1715, the obscura-
tion, he tells us, "was about ten digits, 1 when the face and
colour of the sky began to change from perfect serene azure
blue to a more dusky livid colour, having an eye of purple
'intermixt. ... A few seconds before the sun was all hid,
there discovered itself round the moon a luminous ring, about
a digit or perhaps a tenth part of the moon's diameter in
breadth. It was of a pale whiteness or rather pearl colour,
seeming to me a little tinged with the colours of the iris, and
to be concentric with the moon, whence I concluded it the
moon's atmosphere. But the great height thereof, far exceed-
ing 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 con-
trary 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." 2
A French Academician, who happened to be in London at the
time, was less guarded in expressing an opinion. The Chevalier
de Louville declared emphatically for the lunar atmospheric
theory of the corona, 3 and his authority carried great weight.
It was, however, much discredited by an observation made by
Maraldi in 1724, to the effect that the luminous ring, instead
of travelling with the moon, was traversed by it. 4 This was
in reality decisive, though, as usual, belief lagged far behind
demonstration. Moreover, the advantage accruing from this
fresh testimony was adjudged to the wrong claimant. In 1 7 1 5
a novel explanation had been offered by Delisle and Lahire, 5
1 A digit = T \th of the solar diameter. 2 Phil. Trans., vol. xxix. pp.
247-249. 3 Mem. de VAc. des Sciences, 1715 ; Histoire. p. 49 ; Memoires,
pp. 93-98. 4 Ibid., 1724, p. 178. 5 Ibid., 1715, pp. 161, 166-169.
84 HISTORY OF ASTRONOMY. PARTI.
supported by experiments regarded at the time as perfectly
satisfactory. The aureola round the eclipsed sun, they argued,
is simply a result of the diffraction or apparent bending of the
sunbeams that graze the surface of the lunar globe an effect
of the same kind as the coloured fringes of shadows. And
this view prevailed amongst men of science until (and even
after) Brewster showed, with clear and simple decisiveness,
that such an effect could by no possibility be appreciable at
our distance from the moon. 1 Don Jose Joaquim de Ferrer,
who observed a total eclipse of the sun at Kinderhook, in the
State of New York, on June 16, 1806, seems to have been
ignorant that such a refined optical rationale of the phenomenon
was current in the learned world. Two alternative explanations
alone presented themselves to his mind as possible. The
bright ring round the moon must be due to the illumination
either of a lunar or of a solar atmosphere. If the former, he
calculated that it should have a height fifty times that of the
earth's gaseous envelope. "Such an atmosphere," he rightly
concluded, "cannot belong to the moon, but must without
any doubt belong to the sun." 2 He, however, stood alone in
this unhesitating assertion.
The importance of the problem was first brought fully home
to astronomers by the eclipse of 1842. The brilliant and
complex appearance which, on that occasion, challenged the
attention of so many observers, demanded and received, no
longer the casual attention hitherto bestowed upon it, but the
most earnest study of those interested in the progress of
science. Nevertheless, it was only by degrees and through a
process of " exclusions " (to use a Baconian phrase) that the
corona was put in its right place as a solar appendage. As
every other available explanation proved inadmissible and
dropped out of sight, the broad presentation of fact remained,
which, though of sufficiently obvious interpretation, was long
and persistently misconstrued. Nor was it until 1869 that
1 Ed. Ency., art. Astronomy, p. 635. 2 Trans. Am. Phil. Soc., vol. vi.
p. 274.
CHAP. in. KNOWLEDGE OF THE SUN. 85
absolutely decisive evidence on the subject was forthcoming,
as we shall see further on.
Sir John Herschel, writing to his venerable aunt, relates that
when the brilliant red flames burst into view behind the dark
moon on the morning of the 8th July 1842, the populace of
Milan, with the usual inconsequence of a crowd, raised the
shout, " Es leben die Astronomen ! " l In reality, none were
less prepared for their apparition than the class to whom the
applause due to the magnificent spectacle was thus adjudged.
And in some measure through their own fault; for many partial
hints and some distinct statements from earlier observers had
given unheeded notice that some such phenomenon might be
expected to attend a solar eclipse.
What we now call the " chromosphere " is an envelope of
glowing gases, principally hydrogen, by which the sun is
completely covered, and from which the " prominences " are
emanations, eruptive or otherwise. Now, continual indications
of the presence of this fire- ocean had been detected during
eclipses in the eighteenth and nineteenth centuries. Captain
Stannyan, describing in a letter to Flamsteed an occurrence of
the kind witnessed by him at Berne on May i (O.S.), 1706,
says that the sun's " getting out of the eclipse was preceded
by a blood-red streak of light from its left limb." 2 A pre-
cisely similar appearance was noted by both Halley and De
Louville in 1715; during annular eclipses by Lord Aberdour
in I737, 3 and by Short in 1748,* the tint of the ruby border
being, however, subdued to " brown " or " dusky red " by the
surviving sunlight j while observations identical in character
were made at Amsterdam in i82o, 5 at Edinburgh (by Hender-
son) in 1836, and at New York in 1838. 6
"Flames" or "prominences," if more conspicuous, are less
constant in their presence than the glowing stratum from
which they spring. The first to describe them was a Swedish
1 Memoir of Caroline Herschel, p. 327. 2 Phil. Trans., vol. xxv.
p. 2240. 3 Ibid., vol. xl. p. 182. 4 Ibid., vol. xlv. p. 586. 5 Mem.
R. A. S., vol. i. pp. 145, 148. 6 American Journal of Science, vol. xlii.
P- 396.
86 HISTORY OF ASTRONOMY. PARTI.
professor named Yassenius, who observed a total eclipse at
Gottenburg, May 2 (O.S.), 1733.* His astonishment equalled
his admiration when he perceived, just outside the edge of the
lunar disc, and suspended, as it seemed, in the coronal atmos-
phere, three or four reddish spots or clouds, one of which was
so large as to be detected with the naked eye. As to their
nature, he did not even offer a speculation, further than by
tacitly referring them to the moon, in which position they
appear to have remained so long as the observation was held
in mind. It was repeated in 1778 by a Spanish admiral, but
with no better success in directing efficacious attention to the
phenomenon. Don Antonio Ulloa was on board his ship the
Espagne in passage from the Azores to Cape St. Vincent on
the 24th of June in that year, when a total eclipse of the
sun occurred, of which he has left a valuable description.
His notices of the corona are full of interest ; but what just
now concerns us is the appearance of "a red luminous point "
"near the edge of the moon," which gradually increased in
size as the moon moved away from it, and was visible during
about a minute and a quarter. 2 He was satisfied that it
belonged to the sun because of its fiery colour and growth
in magnitude, and supposed that it was occasioned by some
crevice or inequality in the moon's limb, through which the
solar light penetrated.
Allusions less precise, both prior and subsequent, which it
is now easy to refer to similar objects (such as the " slender
columns of smoke " seen by Ferrer), 3 might be detailed ; but the
evidence already adduced suffices to show that the prominences
viewed with such amazement in 1842 were no unprecedented,
or even unusual phenomenon.
It was more important, however, to decide what was their
1 Phil. Trans., vol. xxxviii. p. 134. Father Secchi has, however,
pointed out a tolerably distinct mention of a prominence so far back as
1239 A.D. In a description of a total eclipse of that date it is added, "Et
quoddam foramen erat ignitum in circulo solis ex parte inferiori " (Mura-
tori, Rer. It. Scriptores, t. xiv. col. 1097). The "circulus solis" of course
signifies the corona. 2 Phil. Trans., vol. Ixix. p. 114. 3 Trans. Am.
Phil. Soc., vol. vi. 1809, p. 267.
CHAP. HI. KNOWLEDGE OF THE SUN. 87
nature than whether their appearance might have been anti-
cipated. They were generally, and not very incorrectly, set
down as solar clouds. Arago believed them to shine by re-
flected light, 1 but the Abbe Peytal rightly considered them to
be self-luminous. Writing in a Montpellier paper of July
1 6, 1842, he declared that we had now become assured of
the existence of a third or outer solar envelope, composed
of a glowing substance of a bright rose tint, forming moun-
tains of prodigious elevation, analogous in character to the
clouds piled above our horizons. 2 This first extant descrip-
tion of a very important feature of our great luminary was
probably founded on an observation made by BeYard at
Toulon during the then recent eclipse, "of a very fine red
band, irregularly dentelated, or, as it were, crevassed here and
there," 3 encircling a large arc of the moon's circumference.
It can hardly, however, be said to have obtained distinct
recognition until the 28th of July 1851. On that day a total
eclipse took place, which was observed with considerable suc-
cess in various parts of Sweden and Norway by a number of
English astronomers. Mr. Hind saw, on the south limb of
the moon, " a long range of rose-coloured flames," 4 described
by Mr. Dawes as " a low ridge of red prominences, resembling
in outline the tops of a very irregular range of hills." 5 Mr.
Airy termed the portion of this "rugged line of projections "
visible to him the sienna, and was struck with its brilliant
light and " nearly scarlet " colour. 6 Its true character of a
continuous solar envelope was inferred from these data by
(amongst others) Grant, Swan, and Littrow; and was by
Father Secchi formally accepted as established after the
great eclipse of i86o. 7
Several prominences of remarkable forms, especially one
variously compared to a Turkish scimitar, a sickle, and a
boomerang, were seen in 1851. In connection with them
two highly significant circumstances were pointed out. First,
1 Annuaire, 1846, p. 460. 2 Ibid., p. 439, note. 3 Ibid., p. 416.
4 Mem. R. A. S., vol. xxi. p. 82. 5 Ibid., p. 90. 6 Ibid., pp. 7-8. 7 Le
Soleil, t. i. p. 386.
88 HISTORY OF ASTRONOMY. PARTI.
that of the approximate coincidence between their positions
and those of sun-spots previously observed. 1 Next, that "the
moon passed over them, leaving them behind, and revealing
successive portions as she advanced." 2 This latter fact (as to
which there could be no doubt, since it was separately noted
by at least four first-rate observers), was justly considered
by the Astronomer Royal and others as affording absolute
certainty of the solar dependence of these singular objects.
Nevertheless sceptics were still found. M. Faye of the
French Academy inclined to a lunar origin for them ; 3
Professor von Feilitsch of Greifswald published in 1852 a
treatise for the express purpose of proving all the luminous
phenomena attendant on solar eclipses corona, prominences,
and " sierra " to be purely optical appearances. 4 Happily,
however, the unanswerable arguments of the photographic
camera were soon to be made available against such hardy
incredulity.
Thus, the virtual discovery of the solar appendages, both
coronal and chromospheric, may be said to have been begun
in 1842, and completed in 1851. The current Herschelian
theory of the solar constitution remained, however, for the
time, intact. Difficulties, indeed, were thickening around it ;
but their discussion was perhaps felt to be premature, and
they were permitted to accumulate without debate, until for-
tified by fresh testimony into unexpected and overwhelming
preponderance.
1 By Williams and Stanistreet, Mem. R. A. S. t vol. xxi. pp. 54, 56.
Santini had made a similar observation at Padua in 1842. Grant, Hist.
Astr., p. 401. 2 Lassell, Month. Not., vol. xii. p. 53. 3 Comptes Een-
dus, t. xxxiv. p. 155. 4 Optische Untersuchungen, and Zeitschriftfurpopu-
Idre Mittheilungen, Bd. i. 1860, p. 2OI.
CHAPTER IY.
PLANETARY DISCOVERIES.
IN the course of his early gropings towards a law of the
planetary distances, Kepler tried the experiment of setting a
planet, invisible by reason of its smallness, to revolve in the
vast region of (seemingly) desert space separating Mars from
Jupiter. 1 The disproportionate magnitude of the same in-
terval was explained by Kant as due to the overweening size
of Jupiter. The zone in which each planet moved was,, accord-
ing to the philosopher of Konigsberg, to be regarded as the
empty storehouse from which its materials had been derived.
A definite relation should thus exist between the planetary
masses and the planetary intervals. 2 Lambert, on the other
hand, sportively suggested that the body or bodies (for it is
noticeable that he speaks of them in the plural) which once
bridged this portentous gap in the solar system, might, in
some remote age, have been swept away by a great comet, and
forced to attend its wanderings through space. 3
These speculations were destined before long to assume a
more definite form. Johann Daniel Titius, a professor at
Wittenberg (where he died in 1796), pointed out in 1772, in a
note to a translation of Bonnet's Contemplation de la Nature,*
the existence of a remarkable symmetry in the disposition of
the bodies constituting the solar system. By a certain series
1 Op., t. i. p. 107. He interposed, but tentatively only, another similar
body between Mercury and Venus. 2 Allgemeine Naturgeschichte (ed.
1798), pp. 118-119. 3 Cosmologiscke Briefe, No. I (quoted by Von Zach,
Monat. Corr., Bd. iii. p. 592). 4 Second ed., p. 7. See Bode, Von dem.
neuen Hauptplaneten, p. 43, note.
9 o HISTORY OF ASTRONOMY. PARTI.
of numbers, increasing in regular progression, 1 he showed that
the distances of the six known planets from the sun might
be represented with a close approach to accuracy. But with
one striking interruption. The term of the series succeeding
that which corresponded to the orbit of Mars was without a
celestial representative. The orderly flow of the sequence was
thus singularly broken. The space where a planet should in
fulfilment of the " Law " have revolved, was, it appeared,
untenanted. Johann Elert Bode, then just about to begin his
long career as leader of astronomical thought and work at
Berlin, marked at once the anomaly, and filled the vacant
interval with an hypothetical planet. The discovery of Uranus
at a distance falling but slightly short of perfect conformity
with the law of Titius, lent weight to a seemingly hazardous
prediction, and Yon Zach was actually at the pains, in 1785,
to calculate what he termed " analogical " elements 2 for this
unseen and (by any effect or influence) unfelt body. The
search for it, though confessedly scarcely less chimerical than
that of alchemists for the philosopher's stone, he kept steadily
in view for fifteen years, and at length, September 21, 1800,
succeeded in organising, in combination with five other Ger-
man astronomers assembled at Lilienthal, a force of what he
jocularly termed celestial police, for the express purpose of
tracking and intercepting the fugitive subject of the sun. The
zodiac was accordingly divided for purposes of scrutiny into
twenty -four zones ; their apportionment to separate observers
was in part effected, aud the association was rapidly getting
into working order, when news arrived that the missing planet
had been found, through no systematic plan of search, but by
the diligent, though otherwise directed labours of a distant
watcher of the skies.
Giuseppe Piazzi was born at Ponte in the Yaltelline, July
1 6, 1746. He studied at various places and times under
1 The representative numbers are obtained by adding 4 to the following
series (irregular, it will be observed, in its first member, which should be
i instead of o) ; o, 3, 6, 12, 24, 48, &c. The formula is a purely em-
pirical one, and is, moreover, completely at fault as regards the distance of
Neptune. 3 Monat. Corr., Bd. iii. p. 596.
CHAP. iv. PLANETARY DISCOVERIES. 91
Tiraboschi, Beccaria, Jacquier, and Le Sueur; and having
entered the Theatine order of monks at the age of eighteen,
he taught philosophy, science, and theology in several of the
Italian cities as well as in Malta until 1780, when the chair of
mathematics in the University of Palermo was offered to and
accepted by him. Prince Caramanico, then viceroy of Sicily,
had scientific leanings, and was easily won over to the project
of building an observatory, a commodious foundation for which
was afforded by one of the towers of the viceregal palace.
This architecturally incongruous addition to an ancient Sara-
cenic edifice once the abode of Kelbite and Zirite Emirs
was completed in February 1791. Piazzi, meanwhile, had
devoted nearly three years to the assiduous study of his
new profession, acquiring a practical knowledge of Lalande's
methods at the Ecole Militaire, and of Maskelyne's at the
Royal Observatory; and returned to Palermo in 1789, bring-
ing with him, in the great five-foot circle which he had pre-
vailed upon Ramsden to construct, the most perfect measuring
instrument hitherto employed by an astronomer.
He had been above nine years at work on his star-catalogue,
and was still profoundly unconscious that a place amongst the
Lilienthal band of astronomical detectives was being held in
reserve for him, when, on the first evening of the nineteenth
century, January i, 1801, he noted the position of an eighth-
magnitude star in a part of the constellation Taurus, towards
which an error of Wollaston's had directed his special atten-
tion. Re-observing, according to his custom, the same set of
fifty stars on four consecutive nights, it seemed to him, on the
2nd, that the one in question had slightly shifted its position
to the west ; on the 3rd, he assured himself of the fact, and
believed that he had chanced upon a new kind of comet without
tail or coma. The wandering body (whatever its nature) ex-
changed retrograde for direct motion on January I3, 1 and was
carefully watched by Piazzi until February n, when a danger-
ous illness interrupted his observations. He had, however,
1 Such reversals of direction in the apparent movements of the planets
are a consequence of the earth's revolution in its orbit.
9 2 HISTORY OF ASTRONOMY. PARTI.
not omitted to give notice of his discovery, but so precarious
were communications in those unpeaceful times, that his letter
to Oriani of January 23 did not reach Milan until April 5,
while a missive of one day later addressed to Bode came to
hand at Berlin, March 20. The delay just afforded time for
the publication, by a young philosopher of Jena named Hegel,
of a " Dissertation " showing, by the clearest light of reason,
that the number of the planets could not exceed seven, and
exposing the folly of certain devotees of induction who
sought a new celestial body merely to fill a gap in a numerical
series.
Unabashed by speculative scorn, Bode had scarcely read
Piazzi's letter when he concluded that it referred to the precise
body in question. The news spread rapidly, and created a
profound sensation not unmixed with alarm lest this latest
addition to the solar family should have been found only to
be again lost. For by that time, Piazzi's moving star was too
near the sun to be any longer visible, and in order to redis-
cover it after conjunction a tolerably accurate knowledge of
its path was indispensable. But a planetary orbit had never
before been calculated ' from such scanty data as Piazzi's ob-
servations afforded ; 2 and the attempts made by nearly every
astronomer of note in Germany to compass the problem were
manifestly inadequate, failing even to account for the positions
in which the body had been actually seen, and a fortiori serv-
ing only to mislead as to the places where, from September
1 80 1, it ought once more to have become discernible. It was
in this extremity that the celebrated mathematician Gauss
came to the rescue. He was then in his twenty-fifth year, and
was earning his bread by tuitions at Brunswick, with many
possibilities, but no settled career before him. The news from
Palermo may be said to have converted him from an arithme-
tician into an astronomer. He was already in possession of a
new and more general method of computing elliptical orbits,
1 Dissertatio Philosophica de Orlitis Planetarum, 1801. See Wolf,
Gcsch. d. Astr., p. 685. 2 Observations on Uranus, as a supposed fixed
star, reached back to 1690.
CHAP. iv. PLANETARY DISCOVERIES. 93
and the system of "least squares," which he had devised
though not published, enabled him to extract the most probable
result from a given set of observations. Armed with these
novel powers, he set to work, and the communication in Nov-
ember of his elements and ephemeris for the lost object revived
the drooping hopes of the little band of eager searchers. Their
patience, however, was to be still further tried. Clouds, mist,
and sleet seemed to have conspired to cover the retreat of the
fugitive ; but on the last night of the year the sky cleared
unexpectedly with the setting in of a hard frost, and there, in
the north-western part of Yirgo, nearly in the position assigned
by Gauss to the runaway planet, a strange star was discerned
by Yon Zach l at Gotha, and on the subsequent evening the
anniversary of the original discovery by Olbers at Bremen.
The name of Ceres (as the tutelary goddess of Sicily) was,
by Piazzi's request, bestowed upon this first known of the
numerous, and probably all but innumerable family of the
minor planets.
The recognition of the second followed as the immediate
consequence of the detection of the first. Olbers had made
himself so familiar with the positions of the small stars along
the track of the long-missing body, that he was at once struck,
March 28, 1802, with the presence of an intruder near the
spot where he had recently identified Ceres. He at first
believed the newcomer to be a variable star usually incon-
spicuous, but just then at its maximum of brightness ; but
within two hours he had convinced himself that it was no fixed
star, but a rapidly moving object. The aid of Gauss was
again invoked, and his prompt calculations showed that this
fresh celestial acquaintance (named " Pallas " by Olbers) re-
volved round the sun at nearly the same mean distance
as Ceres, and was beyond question of a strictly analogous
character.
This result was perplexing in the extreme. The symmetry
and simplicity of the planetary scheme appeared fatally com-
1 He had caught a glimpse of it on December 7, but was prevented by
bad weather from verifying his suspicion. Monat. Corr., Bd. v. p. 171.
94 HISTORY OF ASTRONOMY. PARTI.
promised by the admission of many, where room could,
according to old-fashioned rules, only be found for one. A
daring hypothesis of Gibers' invention provided an exit from
the difficulty. He supposed that both Ceres and Pallas were
fragments of a primitive trans-Martian planet, blown to pieces
in the remote past, either by the action of internal forces or by
the impact of a comet ; and predicted that many more such
fragments would be found to circulate in the same region. He,
moreover, pointed out that these numerous orbits, however
much they might differ in other respects, must all have a
common line of intersection, 1 and that the bodies moving in
them must consequently pass, at each revolution, through two
opposite points of the heavens, one situated in the Whale, the
other in the constellation of the Virgin, where already Pallas
had been found, and Ceres recaptured. The intimation that
fresh discoveries might be expected in those particular regions
was singularly justified by the detection of the two bodies now
known respectively as Juno and Yesta. The first was found
near the predicted spot in Cetus by Harding, Schrb'ter's
assistant at Lilienthal, September 2, 1804; the second by
Olbers himself in Virgo, after three years of patient scrutiny,
March 29, 1807.
The theory of an exploded planet now seemed to have
everything in its favour. It required that the mean or average
distances of the newly discovered bodies should be nearly the
same, but admitted a wide range of variety in the shapes and
positions of their orbits, provided always that they preserved
common points of intersection. These conditions were fulfilled
with a striking approach to exactness. Three of the four
" asteroids " (a designation introduced by Sir W. Herschel 2 )
conformed with very approximate precision to "Bode's law"
of distances; they all traversed, in their circuits round the
1 Planetary fragments, hurled in any direction, and with any velocity
short of that which would for ever release them from the solar sway, would
continue to describe elliptic orbits round the sun, all passing through the
scene of the explosion, and thus possessing a common line of intersection.
2 Phil. Trans., vol. xcii. part ii. p. 228.
CHAP. iv. PLANETARY DISCOVERIES. 95
sun, nearly the same parts of Cetus and Virgo ; while the
eccentricities and inclinations of their paths departed widely
from the planetary type that of Pallas, for example, making
with the ecliptic an angle of nearly 35. The minuteness of
these bodies appeared further to strengthen the imputation of
a fragmentary character. Herschel estimated the diameter of
Ceres at 162, that of Pallas at 147 miles. 1 Juno is smaller
than either ; and even Vesta, which surpasses all the minor
planets in size, and may, under favourable circumstances, be
seen with the naked eye, has a diameter probably under 350
miles. A suspected variability of brightness in some of the
asteroids, somewhat hazardously explained as due to the
irregularities of figure to be expected in cosmical potsherds (so
to speak), was added to the confirmatory evidence. 2 The
strong point of the theory, however, lay not in what it ex-
plained, but in what it had predicted. It had been twice
confirmed by actual exploration of the skies, and had pro-
duced, in the recognition of Vesta, the first recorded instance
of the premeditated discovery of a heavenly body.
The view not only commended itself to the facile imagina-
tion of the unlearned, but received the sanction of the highest
scientific authority. The great Lagrange bestowed upon it his
analytical imprimatur, showing that the explosive forces re-
quired to produce the supposed catastrophe came well within
the bounds of possibility a velocity of less than twenty times
that of a cannon-ball leaving the gun's mouth sufficing, ac-
cording to his calculation, to have launched the asteroidal
fragments on their respective paths. Indeed, he was disposed
to regard the hypothesis of disruption as more generally
available than its author had designed it to be, and proposed
to supplement with it, as explanatory of the eccentric orbits of
comets, the nebular theory of Laplace, thereby obtaining, as
he said, " a complete view of the origin of the planetary
1 Hid., p. 218. In a letter to Von Zach of June 24, 1802, he speaks
of Pallas as " almost incredibly small," and makes it only seventy Eng-
lish miles in diameter. Monat. Corr., Bd. vi. pp. 89-90. 2 Ibid,,
p. 88.
96 HISTORY OF ASTRONOMY. PARTI.
system more conformable to Nature and mechanical laws than
any yet proposed." l
Nevertheless the hypothesis of Olbers has not held its
ground. It seemed as if all the evidence available for its
support had been produced at once and spontaneously, while
the unfavourable items were elicited slowly, and, as it were, by
cross-examination. A more extended acquaintance with the
group of bodies whose peculiarities it was framed to explain,
has shown them, after all, as recalcitrant to any such explana-
tion. Coincidences at the first view significant and striking
have been swamped by contrary examples ; and a hasty
general conclusion has, by a not uncommon destiny, at last
perished under the accumulation of particulars. Moreover,
as has been remarked by Professor Newcomb, 2 mutual per-
turbations would rapidly efface all traces of a common disrup-
tive origin, and the catastrophe, to be perceptible in its effects,
should have been comparatively recent.
A new generation of astronomers had arisen before any
additions were made to the little family of the minor planets.
Piazzi died in 1826, Harding in 1834, Olbers in 1840; all
those who had prepared, or participated in the first discoveries,
passed away without witnessing their resumption. In 1830,
however, a certain Hencke, ex-postmaster in the Prussian
town of Driessen, set himself to watch for new planets, and
after fifteen long years his patience was rewarded. The
asteroid found by him, December 8, 1845, received the name
of Astrsea, and his further prosecution of the search resulted,
July i, 1847, in the discovery of Hebe. A few weeks later,
August 13, Mr. Hind, after many months' exploration from
Mr. Bishop's observatory in the Regent's Park, picked up
Iris, and, October 18, Flora. 3 The next on the list was Metis,
found by Mr. Graham, April 25, 1848, at Markree in Ireland. 4
At the close of the period to which our attention is at present
limited, the number of these small bodies known to astronomy
1 Conn. d. Terns, for 1814, p. 218. 2 Popular Astronomy, p. 327.
3 Month. Not., vol. vii. p. 299 ; vol. viii. p. I. 4 Ibid., vol. viii. p.
146.
CHAP. iv. PLANETARY DISCOVERIES. 97
was thirteen ; and the course of discovery has since proceeded
still more rapidly and with less interruption.
Both in itself and in its consequences the recognition of the
minor planets was of the highest importance to science. The
traditional ideas regarding the constitution of the solar system
were enlarged by the admission of a new class of bodies,
strongly contrasted, yet strictly co-ordinate with the old-estab-
lished planetary order ; the profusion of resource, so conspi-
cuous in the living kingdoms of Nature, was seen to prevail
no less in the celestial spaces ; and some faint preliminary
notion was afforded of the indefinite complexity of relations
underlying the apparent simplicity of the majestic scheme to
which our world belongs. Theoretical and practical astronomy
both- derived profit from the admission of these apparently
insignificant strangers to the rights of citizenship of the solar
system. The disturbance of their motions by their giant
neighbour afforded a more accurate knowledge of the Jovian
mass, which Laplace had taken about ^yth too small; the
anomalous character of their orbits presented geometers with
highly stimulating problems in the theory of perturbations ;
while the exigencies of the first discovery had produced the
Theoria Motus, and won Gauss over to the ranks of calculating
astronomy. Moreover, the sure prospect of further detections
powerfully incited to the exploration of the skies ; observers
became more numerous and more zealous in view of the prizes
held out to them ; star-maps were diligently constructed, and
the sidereal multitude strewn along the great zodiacal belt
acquired a fresh interest when it was perceived that its least
conspicuous member might be a planetary shred or projectile
in the majestic disguise of a distant sun. Harding's " Celestial
Atlas," designed for the special purpose of facilitating asteroidal
research, was the first systematic attempt to represent to the
eye the telescopic aspect of the heavens. It was while engaged
on its construction that the Lilienthal observer successfully
intercepted Juno on her passage through the Whale in 1804 ;
whereupon promoted to Gottingen, he there completed, in
1822, the arduous task so opportunely entered upon a score of
G
98 HISTORY OF ASTRONOMY. PARTI.
years previously. Still more important were the great star-
maps of the Berlin Academy, undertaken at Bessel's sug-
gestion, with the same object of distinguishing errant from
fixed stars, and executed, under Encke's supervision, during
the years 1830-59. They have played a noteworthy part
in the history of planetary discovery, nor of the minor kind
alone.
We have now to recount an event unique in scientific history.
The discovery of Neptune has been characterised as the result
of a " movement of the age," l and with some justice. It had
become necessary to the integrity of planetary theory. Until
it was accomplished, the phantom of an unexplained anomaly
in the orderly movements of the solar system must for ever
have haunted the brains of astronomers. Moreover, it was
prepared by many, suggested as possible by not a few, and
actually achieved, simultaneously, independently, and com-
pletely, by two investigators.
The position of the planet Uranus was recorded as that of
a fixed star no less than twenty times between 1690 and
the epoch of its final detection by Herschel. But these early
observations, far from affording the expected facilities for the
calculation of its orbit, proved a source of grievous perplexity.
The utmost ingenuity of geometers failed to combine them
satisfactorily with the later Uranian places, and it became
evident, either that they were widely erroneous, or that the
revolving body was wandering from its ancient track. The
simplest course was to reject them altogether, and this was
done in the new Tables published in 1821 by Alexis Bouvard,
the indefatigable computating partner of Laplace. But the
trouble was not thus to be got rid of. After a few years fresh
irregularities began to appear, and continued to increase until
absolutely "intolerable." It may be stated, as illustrative of
the perfection to which astronomy had been brought, that
divergences regarded as menacing the very foundation of its
theories never entered the range of unaided vision. In other
words, if the theoretical and the real Uranus had been placed
1 Airy, Mem. R. A. S., vol. xvi. p. 386.
CHAP. iv. PLANETARY DISCOVERIES. 99
side by side in the sky, they would have seemed, to the sharpest
eyes, to form a single body. 1
The idea that these enigmatical disturbances were due to
the attraction of an unknown exterior body was a tolerably
obvious one; and we accordingly find it suggested in many
different quarters. Bouvard himself was perhaps the first to
conceive it. He kept the possibility continually in view, and
bequeathed to his nephew's diligence the inquiry into its reality
when he felt that his own span was drawing to a close ; but
before any progress had been made with it, he had already
(June 7, 1843) "ceased to breathe and to calculate." The
Rev. T. J. Hussey actually entertained in 1834 the notion, but
found his powers inadequate to the task, of assigning an
approximate place to the disturbing body ; and Bessel, in 1840,
laid his plans for an assault in form upon the Uranian diffi-
culty, the triumphant exit from which fatal illness frustrated
his hopes of effecting or even witnessing.
The problem was practically untouched when, in 1841, an
undergraduate of St. John's College, Cambridge, formed the
resolution of grappling with it. The projected task was an
arduous one. There were no guiding precedents for its
conduct. Analytical obstacles had to be encountered so
formidable as to appear invincible even to such a mathema-
tician as Airy. John Couch Adams, however, had no sooner
taken his degree, which he did as senior wrangler in January
1843, than he set resolutely to work, and by October 1845
was able to communicate to the Astronomer Royal numeri-
cal estimates of the elements and mass of the unknown
planet, together with an indication of its actual place in the
heavens.
Sir George Biddell Airy had begun in 1835 his long and
energetic administration of Greenwich Observatory, and was
1 See Newcomb's Pop. A sir., p. 359. The error of Uranus amounted,
in 1 844, to 2' ; but even the tailor of Breslau, whose extraordinary powers
of vision Humboldt commemorates (Kosmos, Bd. iii. p. 112), could only
see Jupiter's first satellite at its greatest elongation, 2' 15". He might, how-
ever, possibly have distinguished two objects of equal lustre at a lesser
interval.
loo HISTORY OF ASTRONOMY. PARTI.
already in possession of data vitally important to the momen-
tous inquiry then on foot. At his suggestion, and under his
superintendence, the reduction of all the planetary observations
made at Greenwich from 1750 downwards had been under-
taken in 1833. The results, published in 1846, constituted a
permanent and universal stock of materials for the correction
of planetary theory. But in the meantime, investigators, both
native and foreign, were freely supplied with the "places and
errors," which, clearly exhibiting the discrepancies between
observation and calculation between what icas and what
was expected formed the very groundwork of future improve-
ments.
Mr. Adams had no reason to complain of official discourtesy.
His labours were aided and encouraged; but they were not
fully believed in. " I have always," Sir George Airy wrote, 1
" considered the correctness of a distant mathematical result
to be a subject rather of moral than of mathematical evidence."
But that actually before him seemed, from its very novelty, to
incur a suspicion of unlikelihood. No problem in planetary
disturbance had heretofore been attacked, so to speak, from
the rear. And the difficulty of determining the perturba-
tions produced by a given planet is small compared with the
difficulty of finding a planet by its resulting perturbations.
Laplace might have quailed before it ; yet it was now grappled
with as a first essay in celestial dynamics. Moreover, Mr.
Adams unaccountably neglected to answer (until too late) a
question regarded by Sir George Airy in the light of an experi-
mentum crucis as to the soundness of the new theory. Nor
did he himself take any steps to obtain a publicity which he
was more anxious to merit than to secure. The investigation
consequently remained buried in obscurity. It is now known
that had a search been instituted in the autumn of 1845 for
the remote body whose existence had been so marvellously
foretold, it would have been found within three and a half
lunar diameters (i 49') of the spot assigned to it by Mr.
Adams.
1 Mem. R. A. S., vol. xvi. p. 399.
CHAP. iv. PLANETARY DISCOVERIES. 101
A competitor, however, equally daring and more fortunate
audax fortund adjutus, as Gauss said of him was even then
entering the field. Urbain Jean Joseph Leverrier, the son of
a small Government employe in Normandy, was born at Saint-
L6, March n, 1811. He studied with brilliant success at the
Ecole Polytechnique, accepted the post of astronomical teacher
there in 1837, and, "docile to circumstance," immediately
concentrated the whole of his vast, though as yet undeveloped
powers upon the formidable problems of celestial mechanics.
He lost no time in proving to the mathematical world that the
race of giants was not extinct. Two papers on the stability
of the solar system, presented to the Academy of Sciences,
September 16 and October 14, 1839, showed him to be the
worthy successor of Lagrange and Laplace, and encouraged
hopes destined to be abundantly realised. His attention was
directed by Arago to the Uranian difficulty in 1845, when he
cheerfully put aside certain intricate cometary researches upon
which he happened to be engaged, in order to obey with duti-
ful promptitude the summons of the astronomical chief of
France. In his first memoir on the subject (communicated
to the Academy, November 10, 1845), ne proved the inadequacy
of all known causes of disturbance to account for the vagaries
of Uranus; in a second (June i, 1846), he demonstrated that
only an exterior body occupying at a certain date a determinate
position in the zodiac could produce the observed effects ; in
a third (August 31, 1846), he assigned the orbit of the
disturbing body, and announced its visibility as an object
with a sensible disc about as bright as a star of the eighth
magnitude.
The question was now visibly approaching an issue. On
September 10, Sir John Herschel declared to the British Asso-
ciation respecting the hypothetical new planet : " We see it
as Columbus saw America from the coast of Spain. Its move-
ments have been felt, trembling along the far-reaching line
of our analysis with a certainty hardly inferior to that of ocular
demonstration." Less than a fortnight later, September 23,
Professor Galle, of the Berlin Observatory, received a letter
102 HISTORY OF ASTRONOMY. PARTI.
from Leverrier requesting his aid in the telescopic part of the
inquiry already analytically completed. He directed his re-
fractor to the heavens that same night, and perceived within
less than a degree of the spot indicated, an object with a
measurable disc nearly three seconds in diameter. Its absence
from Bremiker's recently completed map of that region of the
sky showed it to be no star, and its movement in the predicted
direction confirmed without delay the strong persuasion of its
planetary nature. 1
In this remarkable manner the existence of the remote
member of our system known as " Neptune " was ascertained.
But the discovery, which faithfully reflected the duplicate char-
acter of the investigation which led to it, had been already
secured at Cambridge before it was announced from Berlin.
Sir George Airy's incredulity vanished in the face of the
striking coincidence between the position assigned by Leverrier
to the unknown planet in June, and that laid down by Mr.
Adams in the previous October ; and on the gth of July he
wrote to Professor Challis, director of the Cambridge Observa-
tory, recommending a search with the great Northumberland
equatoreal. Had a good star-map been at hand, the process
would have been a simple one ; but of Bremiker's "HoraXXI."
no news had yet reached England, and there was no other
sufficiently comprehensive to be available for an inquiry which,
in the absence of such aid, promised to be both long and
laborious. As the event proved, it might have been neither.
''After four days of observing," Professsor Challis wrote,
October 12, 1846, to Sir George Airy, "the planet was in my
grasp if only I had examined or mapped the observations." :
Had he done so, the first honours in the discovery, both
theoretical and optical, would have fallen to the University of
Cambridge. But Professor Challis had other astronomical
avocations to attend to, and, moreover, his faith in the precision
of the indications furnished to him was, by his own confes-
sion, a very feeble one. For both reasons he postponed to a
1 For an account of D'Arrest's share in the detection see Copernicus, vol.
ji. pp. 63, 96. * Mem. . A. , vol. xvi. p. 412.
CHAP. iv. PLANETARY DISCOVERIES. 103
later stage of the proceedings the discussion and comparison
of the data nightly furnished to him by his telescope, and
thus allowed to lie, as it were, latent in his observations the
momentous result which his diligence had ensured, but which
his delay suffered to be anticipated. 1
Nevertheless it should not be forgotten that the Berlin as-
tronomer had two circumstances in his favour apart from which
his swift success could hardly have been achieved. The first
was the possession of a good star-map ; the second was the
clear and confident nature of Leverrier's instructions. " Look
where I tell you," he seemed authoritatively to say, " and you
will see an object such as I describe." 2 And in fact, not only
Galle on the 23d of September, but also Challis on the 2Qth,
immediately after reading the French geometer's lucid and
impressive treatise, picked out from among the stellar points
strewing the zodiac, a small planetary disc, which eventually
proved to be that of the precise body he had been in search of
during two months.
Personal questions, however, vanish in the magnitude of the
event they relate to. By it the last lingering doubts as to the
universal validity of the Newtonian Law were dissipated.
Recondite analytical methods received a confirmation brilliant
and intelligible even to the minds of the vulgar, and emerged
from the patient solitude of the study to enjoy an hour of
clamorous triumph. For ever invisible to the unaided eye of
man, a sister-globe to the earth was shown to circulate, in
perpetual frozen exile, at thirty times its distance from the sun.
Nay, the possibility was made apparent that the limits of our
system were not even thus reached, but that yet profounder
abysses of space might shelter obedient, though little favoured
members of the solar family, by future astronomers to be re-
cognised through the sympathetic thrillings of Neptune, even
1 He had recorded the places of 3150 stars (three of which were different
positions of the planet), and was preparing to map them, when, October
I, news of the discovery arrived from Berlin. Prof. Challis' s Report, quoted
in Obituary Notice, Month. Not., Feb. 1883, p. 170. 2 See Airy in Mem.
R. A. S., vol. xvi. p. 411.
104 HISTORY OF ASTRONOMY. PARTI.
as Neptune himself was recognised through the tell-tale devia-
tions of Uranus.
It is curious to find that the fruit of Adams's and Leverrier's
laborious investigations had been accidentally all but snatched
half a century before it was ripe to be gathered. On the 8th,
and again on the loth of May 1795, Lalande noted the posi-
tion of Neptune as that of a fixed star, but perceiving that the
two observations did not agree, he suppressed the first as
erroneous, and pursued the inquiry no further. An immor-
tality which he would have been the last to despise hung in
the balance ; the feather-weight of his carelessness, however,
kicked the beam, and the discovery was reserved to be more
hardly won by later comers.
Bode's Law did good service in the quest for a trans-Uranian
planet by affording ground for a probable assumption as to
its distance. A starting-point for approximation was pro-
vided by it ; but it was soon found to be considerably at
fault. Even Uranus is about 36 millions of miles nearer to
the sun than the order of progression requires ; and Neptune's
vast distance of 2800 million should be increased by no less
than 800 million miles, and its period of 165 lengthened out to
225 years ?1 in order to bring it into conformity with the curious
and unexplained rule which planetary discoveries have alter-
nately tended to confirm and to invalidate.
Within seventeen days of its identification with the Berlin
achromatic, Neptune was found to be attended by a satellite.
This discovery was the first notable performance of the cele-
brated two-foot reflector 2 erected by Mr. Lassell at his sugges-
tively named residence of Starfield, near Liverpool. William
Lassell was a brewer by profession, but by inclination an
astronomer. Born at Bolton in Lancashire, June 18, 1799,
he closed a life of eminent usefulness to science, October 5,
1880, thus spanning with his well-spent years almost the
entire of the momentous period which we have undertaken to
traverse. At the age of twenty-one, being without the means
1 Ledger, The Sun, its Planets and their Satellites, p. 414. 2 Lately
presented by the Misses Lassell to the Greenwich Observatory.
CHAP. iv. PLANETARY DISCOVERIES. 105
to purchase, he undertook to construct telescopes, and natu-
rally turned his attention to the reflecting sort, as favouring
amateur efforts by the comparative simplicity of its structure.
His native ingenuity was remarkable, and was developed by
the hourly exigencies of his successive enterprises. Their
uniform success encouraged him. to enlarge his aims, and in
1844 he visited Birr Castle for the purpose of inspecting the
machine used in polishing the giant speculum of Parsonstown.
In the construction of his new instrument, however, he even-
tually discarded the model there obtained, and worked on a
method of his own, assisted by the supreme mechanical skill
of James Nasmyth. The result was a Newtonian of exquisite
definition, with an aperture of two, and a focal length of twenty
feet, provided by a novel artifice with the equatoreal mount-
ing, previously regarded as available only for refractors.
This beautiful instrument afforded to its maker, October
10, 1846, a cursory view of a Neptunian attendant. But the
planet was then approaching the sun, and it was not until the
following July that the observation could be verified, which it
was completely, first by Lassell himself, and somewhat later
by Otto Struve and Bond of Cambridge (U.S.). When it
is considered that this remote object shines by reflecting
sunlight reduced by distance to -yj^th of the intensity with
which it illuminates our moon, the fact of its visibility, even
in the most perfect telescopes, is a somewhat surprising one.
It can only, indeed, be accounted for by attributing to it
dimensions very considerable for a body of the secondary
order. It shares with the moons of Uranus the peculiarity
of retrograde motion ; that is to say, its revolutions, running
counter to the grand current of movement in the solar sys-
tem, are performed from east to west, in a plane ^inclined
at an angle of 35 to that of the ecliptic. Their swiftness
serves to measure the mass of the globe round which they are
performed. For while our moon takes twenty-seven days
and nearly eight hours to complete its circuit of the earth, the
satellite of Neptune, at a distance not greatly inferior, sweeps
round its primary in five days and twenty-one hours, showing
io6 HISTORY OF ASTRONOMY. PARTI.
(according to a very simple principle of computation) that it
is urged by a force seventeen times greater than the terrestrial
pull upon the lunar orb. Combining this result with that of
measurements of the small telescopic disc of this farthest
known planet, it is found that while in mass Neptune equals
seventeen earths, in bulk it is equivalent to eighty-four. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal
heat, as yet unwasted by radiation into space, to about five
times the volume they would occupy in the interior of our
globe. The fact, at any rate, is fairly well ascertained that
the average density of Neptune differs little from that of
water.
We must now turn from this late-recognised member of our
system to bestow some brief attention upon the still fruitful
field of discovery offered by one of the immemorial five. The
family of Saturn, unlike that of its brilliant neighbour, has been
gradually introduced to the notice of astronomers. Titan, the
sixth Saturian moon in order of distance, led the way, being
detected by Huygens, March 25, 1655 ; Cassini made the
acquaintance of four more between 1671 and 1684; while
Mimas and Enceladus, the two innermost, were caught by
Herschel in 1789, as they threaded their lucid way along the
edge of the almost vanished ring. In the distances of these
seven revolving bodies from their primary, an order of pro-
gression analogous to that pointed out by Titius in the
planetary intervals was found to prevail ; but with one con-
spicuous interruption, similar to that which had first suggested
the search for new members of the solar system. Between
Titan and Japetus the sixth and seventh reckoning outwards
there was obviously room for another satellite. It was
discovered, on both sides of the Atlantic simultaneously, on
the 1 9th of September 1848. Mr. W. C. Bond, employing
the splendid 15 -inch refractor of the Harvard Observatory,
noticed, September 16, a minute star situated in the plane
of Saturn's rings. The same object was discerned by Mr.
Lassell on the i8th. On the following evening, both observers
CHAP. iv. PLANETARY DISCOVERIES. 107
perceived that the problematical speck of light kept up with,
instead of being left behind by, the planet as it moved, and
hence inferred its true character. 1 Hyperion, the seventh by
distance and eighth by recognition of Saturn's attendant train,
is of so insignificant a size when compared with some of its
fellow moons (Titan is but little inferior to the planet Mars),
as to have suggested to Sir John Herschel 2 the idea that it
might be only one of several bodies revolving very close
together in fact, an asteroidal satellite; but the conjecture
has, so far, not been verified.
The coincidence of its duplicate discovery was singularly
paralleled two years later. Galileo's amazement when his
" optic glass " revealed to him the " triple" form of Saturn
planet a tergeminus has proved to be, like the laughter of the
gods, "inextinguishable." It must revive in every one who
contemplates anew the unique arrangements of that world apart
known to us as the Saturnian system. The resolution of the
so-called ansce, or " handles," into one encircling ring by
Huygens in 1655; the discovery by Cassini in 1675 of the
division of that ring into two concentric ones ; the closely con-
cordant determination, theoretically by Laplace and optically
by Herschel, of their period of rotation, 3 constituted, with
some minor observations, the sum of the knowledge obtained,
up to the middle of the present century, on the subject of
this remarkable formation. The first place in the discovery
now about to be related belongs to an American astronomer.
William Craiich Bond, born in 1789 at Falmouth (now
Portland), in the State of Maine, was a watchmaker whom the
solar eclipse of 1806 attracted to study the wonders of the
heavens. When, in 1815, the erection of an observatory in
connection with Harvard College, Cambridge, was first con-
templated, he undertook a mission to England for the purpose
of studying the working of similar institutions there, and, on
his return, erected a private observatory at Dorchester, where
he worked diligently for many years. Meanwhile, the time
1 Grant, Hist, of Astr., p. 271. 2 Month. Not., vol. ix. p. 91. 3 The
computed period was loh. 33m. 368. ; the observed period, loh. 32m. 153.
loS HISTORY OF ASTRONOMY. PARTI.
was approaching for the resumption of the long-postponed
design of the Harvard authorities ; and on the completion of
the new establishment in 1844, Bond, who had for some time
been officially connected with the College, and had carried on
his scientific labours within its precincts, was offered and
accepted the post of its director. Placed in 1847 in possession
of one of the finest instruments in the world a masterpiece
of Merz and Mahler he headed the now long list of dis-
tinguished Transatlantic observers. Like the elder Struve,
he left an heir to his office and to his eminence j but George
Bond unfortunately died in 1865, at the early age of thirty-
nine, having survived his father but six years.
On the night of November 15, 1850 the air, remarkably
enough, being so hazy that only the brightest stars could be
perceived with the naked eye William Bond discovered a
third dusky ring, extending about half-way between the inner
brighter one and the globe of Saturn. A fortnight later, but
before the observation had been announced in England, the
same appearance was seen by the Rev. W. R. Dawes with the
comparatively small refractor of his observatory at Watering-
bury, and on December 3 was described by Mr. Lassell (then
on a visit to him) as " something like a crape veil covering
a part of the sky within the inner ring." 1 Next morning the
Times containing the report of Bond's discovery reached
Wateringbury. The most surprising circumstance in the
matter was that the novel appendage had remained so long
unrecognised. As the rings opened out to their full extent,
it became obvious with very moderate optical assistance ; yet
some of the most acute observers who have ever lived, using
instruments of vast power, had heretofore failed to detect its
presence. It soon appeared, however, that Galle of Berlin 2
had noticed, June 10, 1838, a veil-like extension of the lucid
ring across half the dark space separating it from the planet ;
but the observation, although communicated at the time
to the Berlin Academy of Sciences, had remained barren.
Traces of the dark ring, moreover, were found in a drawing
1 Month. Not., vol. xi. p. 21. ~ Astr. Nach., No. 756 (May 2, 1851).
CHAP. iv. PLANETARY DISCOVERIES. 109
executed by Campani in 1664; l and Picard (June 15, 1673), 2
Hadley (spring of iy2o), 3 and Herschel, 4 had all undoubtedly
seen it under the aspect of a dark bar or belt crossing the
Saturnian globe. It was, then, of no recent origin ; but there
seemed reason to think that it had lately gained considerably
in brightness. The full meaning of this remarkable fact it was
reserved for later investigations to develop.
What we may, in a certain sense, call the closing result of
the race for discovery, in which several observers seemed at
that time to be engaged, was the establishment, on a satis-
factory footing, of our acquaintance with the dependent system
of Uranus. Sir William Herschel, whose researches formed,
in so many distinct lines of astronomical inquiry, the starting-
points of future knowledge, detected, January n, lySy, 5 two
"[Iranian moons, since called Oberon and Titariia, and ascer-
tained the curious circumstance of their motion in a plane
almost at right angles to the ecliptic, in a direction contrary
to that of all previously known denizens (other than cometary)
of the solar kingdom. He believed that he caught occasional
glimpses of four more, but never succeeded in assuring himself
of their substantial existence. Even the two first remained
unseen save by himself until 1828, when his son re-observed
them with a 2o-foot reflector, similar to that with which they
had been originally discovered. Thenceforward they were kept
fairly within view, but their four questionable companions,
in spite of some false alarms of detection, remained in the
dubious condition in which Herschel had left them. At last,
on October 24, 185 1, 6 after some years of fruitless watching,
Mr. Lassell espied "Ariel" and "Umbriel," two Uranian
attendants, interior to Oberon and Titania, and of about half
their brightness ; so that their disclosure is still reckoned
amongst the very highest proofs of instrumental power and
perfection. Tn all probability they were then for the first
1 Secchi, Month. Not., vol. xiii. p. 248. 2 Hind, Ibid., vol. xv. p.
32. 3 Lynn, Observatory, Oct. I, 1883 ; Hadley, Phil. Trans., vol. xxxii.
p. 385. 4 Proctor, Saturn and its System, p. 64. 5 Phil. Trans., vol.
Ixxvii. p. 125. 6 Month. Not., vol. xi. p. 248.
no HISTORY OF ASTRONOMY. PARTI.
time seen ; for although Professor Holden, 1 director of the
Lick Observatory, has attempted to identify them with
two of Herschel's doubtful quartette, Mr. Lassell's argument a
that the glare of the planet in Herschel's great specula must
have rendered almost impossible the perception of objects so
minute and so close to its disc, appears tolerably decisive to
the contrary. Uranus is thus attended by four moons, and
so far as present knowledge extends by no more. Among
the most important of the "negative results" 3 secured by
Mr. Lassell's observations at Malta during the years 1852-53
and 1861-65, were the convincing evidence afforded by them
that, without great increase of optical power, no further
Neptunian or Uranian satellites can be perceived, and the
consequent relegation of Herschel's baffling four notwith-
standing the unquestioned place long assigned to them in
astronomical text-books to the repose of unreality.
i Month. Not., vol. xxxv. pp. 16-22. 2 Ibid., p. 26. 3 Ibid., vol. xli.
p. 190.
( III )
CHAPTER V.
COMETS.
NEWTON showed that the bodies known as " comets," or
hirsute stars, obey the law of gravitation ; but it was by no
means certain that the individual of the species observed by
him in 1680 formed a permanent member of the solar system.
The velocity, in fact, of its rush round the sun was quite
possibly sufficient to carry it off for ever into the depths of
space, there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years
later, the case was different. Edmund Halley, who afterwards
succeeded Flamsteed as Astronomer Royal, calculated its orbit
on Newton's principles, and found it such as to give a period
of revolution of about seventy-six years. He accordingly
announced its probable identity with the comets observed by
Peter Apian in 1531 and by Kepler in 1607, and fixed its
return for 1758-59. The prediction was one of the test-ques-
tions put by Science to Nature, on the replies to which largely
depend both the development of knowledge and the conviction
of its reality. In the present instance, the answer afforded
may be said to have laid the foundation of this branch of
astronomy. Halley 's comet punctually reappeared on Christ-
mas Day, 1758, and effected its perihelion passage on the i2th
of March following, thus proving beyond dispute that some at
least of these erratic bodies are domesticated within our system,
and strictly conform, if not to its unwritten customs (so to
speak), at any rate to its fundamental laws. Their move-
ments, in short, were demonstrated by the most unanswerable
112 HISTORY OF ASTRONOMY. PARTI.
of all arguments that of verified calculation to be calculable,
and their investigation was erected into a legitimate depart-
ment of astronomical science.
This notable advance was the chief result obtained in the
field of inquiry just now under consideration during the
eighteenth century. But before it closed, its cultivation had
received a powerful stimulus through the invention of an im-
proved method. The name of Olbers has already been brought
prominently before our readers in connection with asteroidal
discoveries ; these, however, were but chance excursions from
the path of cometary research which he steadily pursued
through life. An early predilection for the stars was fixed in
this particular direction by one of the happy inspirations of
genius. As he was watching, one night in the year 1779, by
the sick-bed of a fellow-student in medicine at Gottingen, an
important simplification in the mode of computing the paths
of comets occurred to him. Although not made public until
1797, " Olbers's method" was then universally adopted, and is
still regarded as the most expeditious and convenient in cases
where absolute rigour is not required. By its introduction,
not only many a toilsome and thankless hour was spared, but
workers were multiplied, and encouraged in the prosecution of
labours more useful than attractive.
The career of Heinrich Olbers is a brilliant example of what
may be done by an amateur in astronomy. He at no time
did regular work in an observatory ; he was never the possessor
of a transit or any other fixed instrument ; moreover, all the
best years of his life were absorbed in the assiduous exercise
of a toilsome profession. In 1781 he settled as a physician in
his native town of Bremen (he was born in 1758 at Arbergen,
a neighbouring village, of which his father was pastor), and
continued in active practice for over forty years. It was thus
only the hours which his robust constitution enabled him to
spare from sleep that were available for his intellectual plea-
sures. Yet his recreation was, as Von Zach remarked, 1 no less
prolific of useful results than the severest work of other men.
1 Allyemeine Geojraphische Ephemeriden, Bd. iv. p. 287.
CHAP. v. COMETS. 113
The upper part of his house in the Sandgasse was fitted up with
such instruments and appliances as restrictions of space per-
mitted, and there, night after night during half a century and
upwards, he discovered, calculated, or observed the cometary
visitants of northern skies. Almost as effective in promoting
the interests of science as the valuable work actually done by
him, was the influence of his genial personality. He engaged
confidence by his ready and discerning sympathy ; he inspired
affection by his benevolent disinterestedness ; he quickened
thought and awakened zeal by the suggestions of a lively and
inventive spirit, animated with the warmest enthusiasm for the
advancement of knowledge. Nearly every astronomer in Ger-
many enjoyed the benefits of a (frequently active) correspon-
dence with him, and his communications to the scientific
periodicals of the time were numerous and striking. The
motive power of his mind was thus widely felt and continually
in 'action. Nor did it wholly cease to be exerted even when
the advance of age and the progress of infirmity rendered him
incapable of active occupation. He was, in fact, alive even to
the last day of his long life of eighty-one years ; and his death,
which occurred March 2, 1840, left vacant a position which a
rare combination of moral and intellectual qualities had con-
spired to render unique.
Amongst the many younger men who were attracted and
stimulated by intercourse with him was Johann Franz Encke.
But while Olbers became a mathematician because he was an
astronomer, Encke became an astronomer because he was
a mathematician. A born geometer, he was naturally sent
to Gb'ttingen and placed under the tuition of Gauss. But
geometers are men ; and the contagion of patriotic fervour
which swept over Germany after the battle of Leipsic did not
spare Gauss's promising pupil. He took up arms in the Han-
seatic Legion, and marched and fought until the oppressor of
his country was safely ensconced behind the ocean- walls of St.
Helena. In the course of his campaigning he met Lindenau,
the militant director of the Seeberg Observatory, and by his
influence was appointed his assistant, and eventually, in 1822,
H
114 HISTORY OF ASTRONOMY. PARTI.
became his successor. Thence he was promoted in 1825 to
Berlin, where he superintended the building of the new obser-
vatory, so actively promoted by Humboldt, and remained at
its head until within some eighteen months of his death in
August 1865.
On the 26th of November 1818, Pons of Marseilles dis-
covered a comet, whose inconspicuous appearance gave little
promise of its becoming one of the most interesting objects in
our system. Encke at once took the calculation of its elements
in hand, and brought out the unexpected result that it revolved
round the sun in a period of about 3j years. 1 He moreover
detected its identity with comets seen by Mechain in 1786, by
Caroline Herschel in 1795, by Pons, Huth, and Bouvard in
1805, and after six laborious weeks of research into the dis-
turbances experienced by it from the planets during the entire
interval since its first ascertained appearance, he fixed May 24,
1822, as the date of its next return to perihelion. Although
on that occasion, owing to the position of the earth, invisible
in the northern hemisphere, Sir Thomas Brisbane's observatory
at Paramatta was fortunately ready equipped for its recapture,
which Biimker effected quite close to the spot indicated by
Encke's ephemeris.
The importance of this event can be better understood when
it is remembered that it was only the second instance of the
recognised return of a comet (that of Halley's, sixty-three years
previously, having, as already stated, been the first) ; and that
it moreover established the existence of a new class of celestial
objects, somewhat loosely distinguished as " comets of short
period." These bodies (of which a dozen are known to cir-
culate within the orbit of Saturn) are remarkable as showing
certain planetary affinities in the manner of their motions not
at all perceptible in the wider travelling members of their order.
They revolve, without exception, in the same direction as the
planets from west to east ; they exhibit a marked tendency
to conform to the zodiacal track which limits planetary excur-
1 Astr. JaTirlucJi, 1823, p. 217. The period (1208 days) of this body
is considerably shorter than that of any other known comet.
CHAP. v. COMETS. 115
sions north and south ; and their paths round the sun, although
much more eccentric than the approximately circular planetary
orbits, are far less so than the extravagantly long ellipses in
which comets comparatively untrained (as it were) in the habits
of the solar system ordinarily perform their revolutions.
No great comet is of the ''planetary" kind. These are,
indeed, only by exception visible to the naked eye ; they dis-
play extremely feeble tail-producing powers, and give small
signs of central condensation. Thin wisps of cosmical cloud,
they flit across the telescopic field of view without sensibly
obscuring the smallest star. Their appearance, in short, sug-
gests what some notable facts in their history will presently
be shown to confirm that they are bodies already effete, and
verging towards dissolution. If it be asked what possible
connection can be shown to exist between the shortness of
period by which they are essentially characterised, and what
we may call their superannuated condition, we are not altogether
at a loss for an answer. Kepler's remark, 1 that comets are
consumed by their own emissions, has undoubtedly a measure
of truth in it. The substance ejected into the tail must, in
overwhelmingly large proportion, be for ever lost to the central
mass from which it issues. True, it is of a nature inconceiv-
ably tenuous ; but unrepaired waste, however small in amount,
cannot be persisted in with impunity. The incitement to such
self-spoliation proceeds from the sun ; it accordingly progresses
more rapidly the more numerous are the returns to the solar
vicinity. Comets of short period may thus reasonably be
expected to wear out quickly.
They are, moreover, bodies subject to many adventures and
vicissitudes. Their aphelia or the farthest points of their
orbits from the sun are all situated so near to the path either
of Jupiter or of Saturn, as to permit these giant planets to act
as secondary rulers of their destinies. By their influence they
were, in all probability, originally fixed in their present tracks ;
and by their influence, exerted in an opposite sense, they may,
1 " Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi
et denique mori." De Cometis, Op., t. vii. p. no.
ii6 HISTORY OF ASTRONOMY. PARTI.
in some cases, be eventually ejected from them. A curious
instance of such capricious dealing on the part of Jupiter, was
afforded by the comet of 1770, found by Lexell of St. Peters-
burg to perform its circuit of the sun in 5^ years, but which
had never previously, and has never since been seen. The
explanation of this anomaly, suggested by Lexell, and fully con-
firmed by the analytical inquiries both of Laplace and Lever-
rier, was that a very close approach to Jupiter in 1767 had
completely changed the character of its orbit, and brought it
within the range of terrestrial observation ; while in 1779, after
having only twice traversed its new path (at its second return
it was so circumstanced as to be invisible from the earth), it
was, by a fresh encounter, diverted into one entirely different. 1
It can easily be imagined that careers so varied are likely to
prove instructive, and astronomers have not been backward
in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much
curious information, and it may be hoped that its function in
that respect is by no means at an end. The great extent of
the solar system traversed by its eccentric path makes it
peculiarly useful for the determination of the planetary masses.
At perihelion it penetrates within the orbit of Mercury ; it
considerably transcends at aphelion the farthest excursion of
Pallas. Its vicinity to the first-named planet in August 1835
offered the first convenient opportunity of placing that body
in the astronomical balance. Its weight or mass had pre-
viously been assumed, not ascertained ; and the comparatively
slight deviation from its regular course impressed upon the
1 Leverrier showed (Comptes Rendus, t. xxv. 1847, P- 5^4) that the problem
of the disturbances suffered by Lexell's comet was a far less determinate
one than it had been made to appear in the Mecanique Celeste. It is
possible that this body may, in 1779, have been finally thrust out of our
system ; it is also possible (as Laplace concluded) that it may be revolving
too far from the sun to be accessible to our view ; but it is much more
probable that its orbit still bears a recognisable likeness to the one tem-
porarily assigned ;to it by Jovian influence in 1767, in which case Lever -
rier's calculations afford criteria for its eventual re-identification.
CHAP. v. COMETS. 117
comet by its attractive power showed that it had been assumed
nearly twice too great. 1 That fundamental datum of planetary
astronomy the mass of Jupiter was corrected by similar
means ; and it was reassuring to find the correction in satis-
factory accord with that already introduced from observation
of the asteroidal movements.
The fact that comets contract in approaching the sun had
been noticed by Hevelius ; Pingre" admitted it with hesitating
perplexity ; 2 the example of Encke's comet rendered it con-
spicuous and undeniable. On the 28th of October 1828, the
diameter of the nebulous matter composing this body was
estimated at 312,000 miles. It was then about one and a half
times as remote from the sun as the earth is at the time of the
equinox. On the 24th of December following, its distance
being reduced by nearly two-thirds, it was found to be only
14,000 miles across. 3 That is to say, it had shrunk during
those two months of approach to TT,^iro~th P ar ^ ^ i* s original
volume ! Yet it had still seventeen days' journey to make
before reaching perihelion. The same curious circumstance
was even more markedly apparent at its return in 1838. Its
bulk, or the actual space occupied by it, was reduced, as it
drew near the hearth of our system (so far at least as could be
inferred from optical evidence), in the enormous proportion of
800,000 to i. A corresponding expansion on each occasion
accompanied its retirement from the sphere of observation.
Similar changes of volume, though rarely to the same astound-
ing extent, have been perceived in other comets. They still
remain unexplained ; but it can scarcely be doubted that they
are due to the action of the same energetic internal forces
which reveal themselves in so many splendid and surprising
cometary phenomena.
1 Considerable uncertainty, however, still prevails on the point. The
inverse relation assumed by Lagrange to exist between distance from the
sun and density brought out the Mercurian mass ^.^.^TT tnat of the
sun (Laplace, Exposition du, Syst. du Monde, t. ii. p. 50, ed. 1824). Von
Asten deduced from the movements of Encke's comet, 1818-48, a value
of T -g-j^ -^-3 ; while Backlund derives f.-g-g-f.yiro- from the close approach of
the same body to the planet in August 1878. Bull. Astr., t. iii. p. 473.
2 Arago, Annuaire, 1832, p. 218. 3 Hind, The Comets, p. 20.
iiS HISTORY OF ASTRONOMY. PARTI.
Another question of singular interest was raised by Encke's
acute inquiries into the movements and disturbances of the
first known " comet of short period." He found from the
first that its revolutions were subject to some influence be-
sides that of gravity. After every possible allowance had
been made for the pulls, now backward, now forward, exerted
upon it by the several planets, there was still a surplus of
acceleration left unaccounted for. Each return to perihelion
took place about two and a half hours sooner than received
theories warranted. Here then was a " residual phenomenon "
of the utmost promise for the disclosure of novel truths.
Encke (in accordance with the opinion of Olbers) explained
it as due to the presence in space of some such "subtle
matter " as was long ago invoked by Euler l to be the agent of
eventual destruction for the fair scheme of planetary creation.
The apparent anomaly of accounting for an accelerative effect
by a retarding cause disappears when it is considered that
any check to the motion of bodies revolving round a centre of
attraction causes them to draw closer to it, thus shortening
their periods and quickening their circulation. If space were
filled with a resisting medium capable of impeding, even in
the most infinitesimal degree, the swift course of the planets,
their orbits should necessarily be, not ellipses, but very close
elliptical spirals, along which they would slowly, but inevitably,
descend into the burning lap of the sun. The circumstance
that no such tendency can be traced in their revolutions by no
means sets the question at rest. For it might well be that an
effect totally imperceptible until after the lapse of countless
ages, as regards the solid orbs of our system, might be obvious
in the movements of bodies like comets of small mass and
great bulk ; just as a feather or a gauze veil at once yields its
motion to the resistance of the air, while a cannon-ball cuts its
way through with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous
character hang on the time-keeping of comets ; for plainly all
must in some degree suffer the same kind of hindrance as
1 Phil. Trans., vol. xlvL p. 204.
CHAP. v. COMETS. 119
Encke's, if the cause of that hindrance be the one suggested.
More than half a century, however, elapsed before the slightest
trace of similar symptoms could be detected in any of its
congeners. At length, in 1880, the late Professor Oppolzer
announced 1 that a comet, first seen by Pons in 1819, and
rediscovered by Winnecke in 1858, having a period of 2052
days (5.6 years), was accelerated at each revolution precisely
in the manner required by Encke's theory. The " resisting
medium " was thereby generally admitted to have made good
its footing. But Backlund's latest researches 2 (in continua-
tion of those of Yon Asten, cut short by his premature death)
into the movements of Encke's comet have revealed a per-
plexing circumstance. They confirm Encke's results for the
period covered by them, but exhibit the acceleration as having
suddenly diminished by nearly one-half in 1868. The reality
and permanence of this change have been fully established by
observations of the latest return in March 1885.
Some physical alteration of the retarded body seems indi-
cated ; but visual evidence countenances no such assumption.
In aspect the comet is no less thin and diffuse than in 1795
or in 1848. The investigation of the orbit of Winnecke's
comet thus acquires an added interest ; and it may be hoped
that the observations of 1886 (when it duly reappeared) will
help to clear up the mystery of impeded cometary motion.
The character of the supposed resistance in inter-planetary
space has, it may be remarked, been often misapprehended.
What Encke stipulated for was not a medium equally diffused
throughout the visible universe, such as the ethereal vehicle
of the vibrations of light, but a rare fluid, rapidly increasing
in density towards the sun. 3 This cannot be a solar atmos-
phere, since it is mathematically certain, as Laplace has
shown, 4 that no envelope partaking of the sun's axial rotation
can extend farther from his surface than nine-tenths of the
mean distance of Mercury. Within such an envelope Encke's
1 Astr. Nach., No. 2314. a M6m. de St. Petersbourg, t. xxxii. No. 3,
1884 ; Astr. Nach., No. 2727. 3 Month. Not., vol. xix. p. 72. 4 Meca-
nique Cdeste, t. ii. p. 197.
i-o HISTORY OF ASTRONOMY. PARTI.
comet can never penetrate. There is, besides, strong evidence
of a physical kind that the actual depth of the solar atmosphere
bears a very minute proportion to the possible depth theoreti-
cally assigned to it. That matter, however, not atmospheric
in its nature that is, neither forming one body with the sun
nor altogether aeriform exists in its neighbourhood, can
admit of no reasonable doubt. The great lens-shaped mass
of the zodiacal light, reaching out at times far beyond the
earth's orbit, may be regarded as an extension of the corona,
and, like the corona, varies perhaps in constitution, as it
certainly varies in shape and brightness. It is difficult to
believe that its condition is altogether without influence on
vaporous bodies penetrating it as they approach the sun.
The history of the next known " planetary " comet has
proved of even more curious interest than that of the first.
It was discovered by an Austrian officer named Wilhelm von
Biela at Josephstadt in Bohemia, February 27, 1826, and ten
days later by the French astronomer Gambart at Marseilles.
Both observers computed its orbit, showed its remarkable
similarity to that traversed by comets visible in 1772 and
1805, and connected them together as previous appearances
of the body just detected by assigning to its revolutions a
period of between six and seven years. The two brief letters
conveying these strikingly similar inferences were printed
side by side in the same number of the Astronomische Nacli-
richten (No. 94) ; but Biela's priority in the discovery of the
comet was justly recognised by the bestowal upon it of his
name.
The object in question was at no time (subsequently to its
appearance in 1805) visible to the naked eye. Its aspect in
Sir John Herschel's great reflector on the 23d of September
1832 was described by him as that of a "conspicuous nebula,"
about 2\ to 3 minutes in diameter. No trace of a tail was
discernible. While he was engaged in watching it, a small
knot of minute stars (i6th or i7th magnitude) was directly
traversed by it, "and when on the cluster," he tells us, 1 it
i Month. Not., vol. ii. p. 117.
CHAP. v. COMETS. 121
" presented the appearance of a nebula resolvable, and partly
resolved into stars, the stars of the cluster being visible
through the comet." Yet the depth of cometary matter
through which such faint stellar rays penetrated undimmed,
was, near the central parts of the globe,- not less than 50,000
miles.
It is curious to find that this seemingly harmless, and we
may perhaps add effete body, gave occasion to the first (and
not the last) cometary " scare " of this enlightened century.
Its orbit, at the descending node, may be said to have inter-
sected that of the earth ; since, according as it bulged in
or out under the disturbing influence of the planets, the pas-
sage of the comet was effected inside or outside the terrestrial
track. Now certain calculations published by Olbers in 1828 l
showed that, on October 29, 1832, a considerable portion of
its nebulous surroundings would actually sweep over the spot
which, a month later, would be occupied by our planet. It
needed no more to set the popular imagination in a ferment.
Astronomers after all could not, by an alarmed public, be held
to be infallible. Their computations, it was averred, which
a trifling oversight would suffice to vitiate, exhibited clearly
enough the danger, but afforded no guarantee of safety from a
collision, with all the terrific consequences frigidly enumerated
by Laplace. Nor did the panic subside until Arago formally
demonstrated that the earth and comet could by no possibility
approach within less than fifty millions of miles. 2
The return of the same body in 1845-46 was marked by
an extraordinary circumstance. When first seen, November
28, it wore its usual aspect of a faint round patch of cosmi-
cal fog ; but on December 1 9, Mr. Hind noticed that it had
become distorted somewhat into the form of a pear ; and ten
days later it had divided into two separate objects. This
singular duplication was first perceived at New Haven in
America, December 29, 3 by Messrs. Herrick and Bradley,
1 Astr. Nack., No. 128. 2 Annuaire, 1832, p. 186. 3 Am. Journ. of
Science, vol. i. (2d series), p. 293. Prof. Hubbard's calculations indicated
a probability that the definitive separation of the two nuclei occurred as
122 HISTORY OF ASTRONOMY. PARTI.
and by Lieutenant Maury at Washington, January 13, 1846.
The earliest British observer of the phenomenon (noticed by
Wichmann the same evening at Konigsberg) was Professor
Challis. " I see two comets ! " he exclaimed, putting his eye
to the great equatoreal of the Cambridge Observatory on the
night of January 1 5 ; then, distrustful of what his senses
had told him, he called in his judgment to correct their
improbable report by resolving one of the dubious objects
into a hazy star. 1 On the 23d, however, both were again
seen by him in unmistakable cometary shape, and until far
on in March (Otto Struve caught a final glimpse of the pair
on the i6th of April), 2 continued to be watched with equal
curiosity and amazement by astronomers in every part of the
northern hemisphere. What Seneca reproved Ephorus for
supposing to have taken place in 373 B.C. what Pingre
blamed Kepler for conjecturing in 1618, had then actually
occurred under the attentive eyes of science in the middle of
the nineteenth century !
At a distance from each other of about two-thirds the dis-
tance of the moon from the earth, the twin comets meantime
moved on tranquilly, so far, at least, as their course through
the heavens was concerned. Their extreme lightness, or the
small amount of matter contained in each, could not have
received a more signal illustration than by the fact that their
revolutions round the sun were performed independently ; that
is to say, they travelled side by side without experiencing any
appreciable mutual disturbance, thus plainly showing that at
an interval of only 157,250 miles, their attractive power was
virtually inoperative. Signs of internal agitation, however,
were not wanting. Each fragment threw out a short tail in a
direction perpendicular to the line joining their centres, and
early as September 30, 1844. AstronomicalJournal (Gould's), vol. iv. p. 5.
See also, on the subject of this comet, W. T. Lynn, Intellectual Observer,
vol. xi. p. 208; E. Ledger, Observatory, August 1883, p. 244, and H. A.
Newton, Am. Journ. of Science, vol. xxxi. p. 81, February 1886.
1 Month. Not., vol. vii. p. 73. 2 Bulletin Ac. Imp. de St. Petersbourg,
t. vi. col. 77. The latest observation of the parent nucleus was that of
Argelander, April 27, at Bonn.
CHAP. v. COMETS. 123
each developed a bright nucleus, although the original comet
had exhibited neither of these signs of cometary vitality. A
singular interchange of brilliancy was, besides, observed to
take place between these small objects, each of which alter-
nately outshone and was outshone by the other, while an arc
of light, apparently proceeding from the more lustrous, at times
bridged the intervening space. Obviously, the gravitational
tie, rendered powerless by exiguity of matter, was here re-
placed by some other form of mutual action, the nature of
which can as yet be dealt with only by conjecture.
Once more, in August 1852, the double comet returned to
the neighbourhood of the sun, but under circumstances not
the most advantageous for observation. Indeed, the companion
was not detected until September 16, by Father Secchi at
Rome, and was then perceived to have increased its distance
from the originating body to a million and a quarter of miles,
or about eight times the average interval at the former appear-
ance. Both vanished shortly afterwards, and have never since
been seen, notwithstanding the eager watch kept for objects
of such singular interest, and the accurate knowledge of their
track supplied by Santini's investigations. We can scarcely
doubt that the fate has overtaken them which Newton assigned
as the end of all cometary existence. Diffundi tandem et spargi
per ccelos universes. 1
A telescopic comet with a period of 7^ years, discovered
November 22, 1843, by M. Faye of the Paris Observatory,
formed the subject of a characteristically patient and profound
inquiry on the part of Leverrier, designed to test its suggested
identity with Lexell's lost comet. The result was decisive
against the hypothesis of Yalz, the divergences between the
orbits of the two bodies being found to increase instead of to
diminish, as the history of the newcomer was traced backwards
into the last century. 2 Faye's comet pursues the most nearly
circular path of any similar known object ; even at its nearest
approach to the sun it remains farther off than Mars when he
is most distant from it ; and it has been proved by the admir-
1 D'Arrest, Astr. Nach., No. 1624. 2 Comptes Rcndus, t. xxv. p. 570.
124 HISTORY OF ASTRONOMY. PARTI.
able researches of Professor Axel Mb'ller, 1 director of the
Swedish observatory of Lund, to exhibit no trace of the action
of a resisting medium.
No great comet appeared between the " star " which presided
at the birth of Napoleon and the "vintage" comet of 1811.
The latter was first descried by Flaugergues at Viviers, March
26, 1 8 1 1 ; Wisniewski, at Neu-Tscherkask in Southern Russia,
caught the last glimpse of it, August 17, 1812. Two dis-
appearances in the solar rays as the earth moved round in
its orbit, and two reappearances after conjunction, were in-
cluded in this unprecedentedly long period of visibility of 5 1 o
days. This relative permanence (so far as the inhabitants of
Europe were concerned) was due to the high northern latitude
attained near perihelion, combined with a certain leisureliness
of movement along a path everywhere external to that of
the earth. The magnificent luminous train of this body, on
October 15, the day of its nearest terrestrial approach, covered
an arc of the heavens 23^- degrees in length, corresponding to
a real extension of one hundred millions of miles. Its form
was described by Sir William Herschel as that of "an inverted
hollow cone," and its colour as yellowish, strongly contrasting
with the bluish-green tint of the "head," round which it was
flung like a transparent veil. The planetary disc of the head,
127,000 miles across, appeared to be composed of strongly
condensed nebulous matter j but somewhat eccentrically situ-
ated within it was a star-like nucleus of a reddish tinge,
which Herschel presumed to be solid, and ascertained, with his
usual care, to have a diameter of 428 miles. From the total
absence of phases, as well as from the vivacity of its radiance,
he confidently inferred that its light was not borrowed, but
inherent. 2
This remarkable apparition formed the subject of a memoir 3
by Olbers, the striking, yet steadily reasoned-out suggestions
contained in which there was at that time no means of follow-
1 Month. Not., vol. xii. p. 248. 2 Phil. Trans., vol. cii. pp. 118-124.
3 Ueber den Schweif des grossen Cometen von 1811, Monat. Corr., Bd. xxv.
pp. 3-22. Reprinted by Zollner, Ueber die Natur der Cometen, pp. 3-15.
CHAP. v. COMETS. 125
ing up with profit. Only of late has the " electrical theory,"
of which Zollner l regarded Olbers as the founder, assumed a
definite and measurable form, capable of being tested by the
touchstone of fact, as knowledge makes its slow inroads on the
fundamental mystery of the physical universe.
The paraboloid al shape of the bright envelope separated by
a dark interval from the head of the great comet of 1811, and
constituting, as it were, the root of its tail, seemed to the
astronomer of Bremen to reveal the presence of a double
repulsion ; the expelled vapours accumulating where the two
forces, solar and cometary, balanced each other, and being
then swept backwards in a huge train. He accordingly dis-
tinguished three classes of these bodies : First, comets which
develop no matter subject to solar repulsion. These have no
tails, and are probably mere nebulosities, without solid nuclei.
Secondly, comets which are acted upon by solar repulsion only,
and consequently throw out no emanations towards the sun.
Of this kind was a bright comet visible in i8oy. 2 Thirdly,
comets like that of 1811, giving evidence of action of both
kinds. These are distinguished by a dark hoop encompassing
the head and dividing it from the luminous envelope, as well
as by an obscure caudal axis, resulting from the hollow, cone-
like structure of the tail.
Again, the ingenious view recently put forward by M. Bre-
dichin of Moscow as to the connection between the form of
these appendages and the kind of matter composing them, was
very clearly anticipated by Olbers. The amount of tail-curva-
ture, he pointed out, depends in each case upon the proportion
borne by the velocity of the ascending particles to that of the
comet in its orbit ; the swifter the outrush, the straighter the
resulting tail. But the velocity of the ascending particles
varies with the energy of their repulsion by the sun, and this
again, it may be presumed, with their quality. Thus multiple
tails are developed, when the same comet throws off, as it
approaches perihelion, specifically distinct substances. The
1 Natur der Cometen, p. 148. 2 The subject of a classical memoir by
Bessel, published in iSio.
126 HISTORY OF ASTRONOMY. PARTI.
long, straight ray which proceeded from the comet of 1807,
for example, was doubtless made up of particles subject to a
much more vigorous solar repulsion than those formed into
the shorter, curved emanation issuing from it nearly in the
same direction. In the comet of 1811, he calculated that the
particles expelled from the head travelled to the remote
extremity of the tail in eleven minutes, indicating by this
enormous rapidity of movement (comparable to that of the
transmission of light) the action of a force greatly more power-
ful than the opposing one of gravity. The not uncommon
phenomena of multiple envelopes, on the other hand, he ex-
plained as due to the varying amounts of repulsion exercised
by the nucleus itself on the different kinds of matter developed
from it.
The movements and perturbations of the comet of 1811
were no less profoundly studied by Argelander than its physical
constitution by Olbers. The orbit which he assigned to it is of
such vast dimensions as to require no less than 3065 years for
the completion of its circuit ; and to carry the body describing
it at each revolution to fourteen times the distance from the
sun of the frigid Neptune. Thus, when it last visited our
neighbourhood, Achilles may have gazed on its imposing train
as he lay on the sands all night bewailing the loss of Patroclus ;
and when it returns, it will perhaps be to shine upon the ruins
of empires and civilisations still deep buried among the secrets
of the coming time.
On the 26th of June 1819, while the head of a comet
passed across the face of the sun, the earth was (in all pro-
bability) involved in its tail. But of this remarkable double
event nothing was known until more than a month later, when
the fact of its past occurrence emerged from the calculations
of Olbers. 1 Nor had the comet itself been generally visible
previous to the first days of July. Several observers, however,
on the publication of these results, brought forward accounts
of singular spots perceived by them upon the sun at the time
of the transit, and the original drawing of one of them, by
1 Astr. Jalirluch (Bode's), 1823, p. 134.
CHAP. v. COMETS. 127
Pastorff of Bucliholtz, has been preserved. This undoubtedly
authentic delineation 1 represents a round nebulous object with
a bright spot in the centre, of decidedly cometary aspect, and
not in the least like an ordinary solar "macula." Mr. Hind, 2
nevertheless, has shown that its position on the sun is irrecon-
cilable with that which the comet must have occupied ; and
Mr. Ran yard's discovery of a similar smaller drawing by the
same author, dated May 26, i828, 3 reduces to evanescence
the probability of its connection with that body. Indeed,
recent experience renders very doubtful the possibility of such
an observation.
The return of Halley's comet in 1835 was looked forward
to as an opportunity for testing the truth of floating cometary
theories, and did not altogether disappoint expectation. As
early as 1817, its movements and disturbances since 1759
were proposed by the Turin Academy of Sciences as the sub-
ject of a prize awarded to Baron Damoiseau. Pontecoulant
was adjudged a similar distinction by the Paris Academy in
1829; while Rosenberger's calculations were rewarded with
the gold medal of the Royal Astronomical Society. 4 The
result entirely disproved the hypothesis (designed to explain
the invariability of the planetary periods) of what may be
described as a vortex of attenuated matter moving with the
planets, and offering, consequently, no resistance to their
motion. For since Halley's comet revolves in the opposite
direction in other words, has a "retrograde" movement it
is plain that if compelled to make head against an ethereal
current, it would rapidly be deprived of the tangential velocity
which enables it to keep at its proper distance from the sun,
and would thus gradually but conspicuously approach, and
eventually be precipitated upon it. No such effect, however,
has in this crucial instance been detected.
On the 6th of August 1835, a nearly circular misty object
was seen at Rome not far from the predicted place of the
1 Reproduced in Webb's Celestial Objects, 4th ed. 2 Month. Not.,
vol. xxxvi. p. 309. 3 Celestial Objects, p. 40, note. 4 See Airy's
Address, Mem, R. A. S. t vol. x. p. 376.
128 HISTORY OF ASTRONOMY. PARTI.
comet. It was not, however, until the middle of September
that it began to throw out a tail, which, by the i5th of
October had attained a length of about 24 degrees (on the
1 9th, at Madras, it extended to full} 7 so), 1 the head showing
to the naked eye as a reddish star rather brighter than Alde-
baran or Antares. 2 Some curious phenomena accompanied
the process of tail-formation. An outrush of luminous matter,
resembling in shape a partially opened fan, issued from the
nucleus towards the sun, and at a certain point, like smoke
driven before a high wind, was vehemently swept backwards
in a prolonged train. The appearance of the comet at this
time was compared by Bessel, 3 who watched it with minute
attention, to that of a blazing rocket. He made the singular
observation that this fan of light, which seemed the source
of supply for the tail, oscillated like a pendulum to and fro
across a line joining the sun and nucleus, in a period of 4
days ; and he was unable to escape from the conclusion 4 that
a repulsive force, about twice as powerful as the attractive
force of gravity, was concerned in the production of these
remarkable effects. Nor did he hesitate to recur to the
analogy of magnetic polarity, or to declare, still more empha-
tically than Olbers, " the emission of the tail to be a purely
electrical phenomenon." 5
The transformations undergone by this body were almost as
strange and complete as those which affected the brigands in
Dante's " Inferno" When first seen, it wore the aspect of a
nebula; later it put on the distinctive garb of a comet; it
next appeared as a star ; finally it dilated, first in a spherical,
then in a paraboloidal form, until May 5, 1836, when it
vanished, as if by melting into adjacent space from the exces-
sive diffusion of its light. A very uncommon circumstance in
its development was that it lost (it would appear) all trace
1 Hind, The Comets, p. 47. 2 Arago, Annuaire, 1836, p. 228. 3 A sir.
Kach., No. 300. 4 It deserves to be recorded that Robert Hooke drew
a very similar inference from his observations of the comets of 1 680 and
1682. Month. Not., vol. xiv. pp. 77-83. 5 Briefwechsd zwischen Olbers
und Bessel, Bd. ii. p. 390.
CHAP. v. COMETS. 129
of tail previous to its arrival at perihelion on the i6th of
November. Nor did it begin to recover its elongated shape for
more than two months afterwards. On the 23d of January
Boguslawski perceived it as a star of the sixth magnitude,
without measurable disc. 1 Only two nights later, Maclear,
director of the Cape Observatory, found the head to be 131
seconds across. 2 And so rapidly did the augmentation of size
progress, that Sir John Herschel, who was then observing at
Feldhausen, estimated the actual bulk of this singular object to
have increased forty-fold in the ensuing week. " I can hardly
doubt," he remarks, " that the comet was fairly evaporated in
perihelio by the heat, and resolved into transparent vapour,
and is now in process of rapid condensation and re-precipi-
tation on the nucleus." 3 A plausible, but no longer admissible
interpretation of this still unexplained phenomenon.
By means of an instrument devised by himself for testing
the quality of light, Arago obtained decisive evidence that
some at least of the radiance proceeding from Halley's comet
was derived by reflection from the sun. 4 Indications of the
same kind had been afforded 5 by the comet which suddenly
appeared above the north-western horizon of Paris, July 3,
1819, after having enveloped (as already stated) our terrestrial
abode in its filmy appendages; but the " polariscope" had not
then reached the perfection subsequently given to it, and its
testimony was accordingly far less reliable than in 1835.
Such experiments, however, are in reality more beautiful
and ingenious than instructive, since incandescent as well
as obscure bodies possess the power of throwing back light
incident upon them, and will consequently transmit to us
from the neighbourhood of the sun rays partly direct, partly
reflected, of which a certain proportion will exhibit the peculi-
arity known as polarisation.
The most brilliant comets of the century were suddenly
rivalled if not surpassed by the extraordinary object which
i Herschel, Results, p. 405. 2 Mem. R. A. S., vol. x. p. 92. 3 Results,
p. 401. 4 Annuaire, 1836, p. 233. 5 Cosmos, vol. i. p. 90, note (Otte's
trans.)
130 HISTORY OF ASTRONOMY. PART i.
blazed out beside the sun, February 28, 1843. It was simul-
taneously perceived in Mexico and the United States, in
Southern Europe, and at sea off the Cape of Good Hope,
where the passengers on board the Owen Glendoiver were
amazed by the sight of a " short, dagger- like object," closely
following the sun towards the western horizon. 1 At Florence
Amici found its distance from the sun's centre at noon to be
only i 23' ; and spectators at Parma were able, when sheltered
from the direct glare of midday, to trace the tail to a length of
four or five degrees. The full dimensions of this astonishing
appurtenance began to be disclosed a few days later. On the
3d of March it measured 25, and on the nth, at Calcutta,
Mr. Clerihew observed a second streamer, nearly twice as long
as the first, and making an angle with it of 18, to have been
emitted in a single day. This rapidity of projection, Sir John
Herschel remarks, " conveys an astounding impression of the
intensity of the forces at work." "It is clear," he continues,
" that if we have to deal here with matter, such as we conceive it
viz., possessing inertia at all, it must be under the dominion
of forces incomparably more energetic than gravitation, and
quite of a different nature." 2
On the i yth of March a silvery ray, some 40 degrees long
and slightly curved at its extremity, shone out above the sun-
set clouds in this country. No previous intimation had been
received of the possibility of such an apparition, and even
astronomers no lightning messages across the seas being as
yet possible were perplexed. The nature of the phenomenon,
indeed, soon became evident, but the wonder of it did not
diminish with the study of its attendant circumstances.
Never before, within astronomical memory, had our system
been traversed by a body pursuing such an adventurous career.
The closest analogy was offered by the great comet of 1680
(Newton's), which rushed past the sun at a distance of only
144,000 miles; but even this on the cosmical scale scarcely
perceptible interval was reduced nearly one- half in the case
we are now concerned with. The centre of the comet of
1 Herschel, Outlines, p. 399 (9th ed.) 2 Ibid., p. 398.
CHAP. v. COMETS. 131
1843 approached the formidable luminary within 7 8,000 miles,
leaving, it is estimated, a clear space of not more than 32,000
between the surfaces of the bodies thus brought into such
perilous proximity. The escape of the wanderer was, how-
ever, secured by the extraordinary rapidity of its flight. Jt
swept past perihelion at a rate 366 miles a second which,
if continued, would have carried it right round the sun in two
hours ; and in only eleven minutes more than that short period
it actually described half the curvature of its orbit an arc of
1 80 although in travelling over the remaining half many
hundreds of sluggish years will doubtless be consumed.
The behaviour of this comet may be regarded as an experi-
mentum crucis as to the nature of tails. For clearly no fixed
appendage many millions of miles in length could be whirled
like a brandished sabre from one side of the sun to the other
in 1 3 1 minutes. Cometary trains are then, as Olbers rightly
conceived them to be, emanations, not appendages incon-
ceivably rapid outflows of highly rarefied matter, the greater
part, if not all of which becomes permanently detached from
the nucleus.
That of the comet of 1843 reached, about the time that it
became visible in this country, the extravagant length of 200
millions of miles. 1 It was narrow, and bounded by nearly
parallel and nearly rectilinear lines, resembling to borrow a
comparison of Aristotle's a " road " through the constella-
tions ; and after the 3d of March showed no trace of hollow-
ness, the axis being, in fact, rather brighter than the edges.
Distinctly perceptible in it were those singular aurora-like
coruscations which gave to the "tresses" of Charles V.'s
comet the appearance as Cardan described it of " a torch
agitated by the wind," and have not unfrequently been ob-
served to characterise other similar objects. A consideration
first adverted to by Olbers proves these to originate in our
own atmosphere. For owing to the great difference in the
distances from the earth of the origin and extremity of such
1 Bognslawski calculated that it extended on the 2ist of March to 581
millions. Report Brit. Ass., 1845, P- ^9-
132 HISTORY OF ASTRONOMY. PARTI.
vast effluxes, the light proceeding from their various parts is
transmitted to our eyes in notably different intervals of time.
Consequently a luminous undulation, even though propagated
instantaneously from end to end of a comet's tail, would
appear to us to occupy many minutes in its progress. But the
coruscations in question pass as swiftly as a falling star. They
are, then, of terrestrial production.
Periods of the utmost variety were by different computators
assigned to the body, which arrived at perihelion, February 27,
1843, at 9.47 P.M. Professor Hubbard of Washington found
that it required 533 years to complete a revolution; MM.
Laugier and Mauvais of Paris considered the true term to be
35 ; 1 Clausen looked for its return at the end of between six and
seven. All these estimates were indeed admittedly uncertain,
the available data affording no sure means of determining the
value of this element; yet there seems no doubt that they
fitted in more naturally with a period counted by centuries
than with one reckoned by decades. Nor could any previous
appearance be satisfactorily made out, although the similarity
of the course pursued by a brilliant comet in 1668, known as
the " Spina " of Cassini, made an identification not impossible.
This would imply a period of 175 years, and it was somewhat
hastily assumed that a number of earlier celestial visitants
might thus be connected as returns of the same body.
It may now be asked what were the conclusions regarding
the nature of comets drawn by astronomers from the consider-
able mass of novel experience accumulated during the first
half of this century? The first and best assured was that
the matter composing them is in a state of extreme tenuity.
Numerous and trustworthy observations showed that the
feeblest rays of light might traverse some hundreds of thou-
sands of miles of their substance, even where it was appa-
rently most condensed, without being perceptibly weakened.
Nay, instances were recorded in which stars were said to
have gained in brightness from the process ! 2 On the 24th
1 Comptes Rendus, t. xvi. p. 919. 2 Piazzi noticed a considerable in-
crease of lustre in a very faint star of the twelfth magnitude viewed
through a comet. Madler, Reden und Abhandlungen, p. 248, note.
CHAP. v. COMETS. 133
of June 1825, Olbers 1 saw the comet then visible all but
obliterated by the central passage of a star too small to be
distinguished with the naked eye, its own light remaining
wholly unchanged. A similar effect was noted December i,
i8ii, when the great comet of that year approached so close
to Atair, the lucida of the Eagle, that the star seemed to be
transformed into the nucleus of the comet. 2 Even the central
blaze of Halley's comet in 1835 was powerless to impede the
passage of stellar rays. Struve 3 observed at Dorpat, on
September 17, an all but central occultation ; Glaisher 4 one
(so far as he could ascertain) absolutely so eight days later
at Cambridge. In neither case was there any appreciable
diminution of the star's light. Again, on the nth of October
1847, Mr. Dawes, 5 an exceptionally keen observer, distinctly
saw a star of the tenth magnitude through the exact centre
of a comet discovered on the ist of that month by Maria
Mitchell of Nantucket.
Examples, on the other hand, were not wanting of the
diminution of stellar light under similar circumstances ; but
probably in general not more than would be accounted for by
the illumination of the background with diffused nebulous
radiance. 6 In one solitary instance, however, on the 28th of
November 1828, a star was alleged to have actually vanished
behind a comet. 7 The observer of this unique phenomenon
was Wartmann of Geneva ; but his instrument was so defective
as to leave its reality open to grave doubt, especially when
it is considered that the eclipsing body was Encke's comet,
which better equipped astronomers have, on various occasions,
found to be perfectly translucent.
From the failure to detect any effects of refraction in the
light of stars occulted by comets, it was inferred (though, as
1 Astr. Jahrbuch, 1828, p. 151. 2 Madler, Gesch. d. Astr., Bd. ii. p.
412. 3 Recudl de I'Ac. Imp. de St. Petersbourg, 1835, P- I 43- 4 Gruille-
min's World of Comets, trans, by J. Glaisher, p. 294, note. 5 Month. Not.,
vol. viii. p. 9. 6 A real, though only partial stoppage of light seems
indicated by Herschel's observations on the comet of 1807. Stars seen
through the tail, October 18, lost much of their lustre. One near the head
was only faintly visible by glimpses. Phil. Trans., vol. xcvii. p. 153.
7 Arago, Annuaire, 1832, p. 205.
I 3 4 HISTORY OF ASTRONOMY. PARTI.
we now know, erroneously) that their composition is rather that
of dust than that of vapour ; that they consist not of any con-
tinuous substance, but of discrete solid particles, very finely
divided and widely scattered. In conformity with this view
was the known smallness of their masses. Laplace had shown
that if the amount of matter forming Lexell's comet had been
as much as -5^0^ f ^ a ^ contained in our globe, the effect of
its attraction, on the occasion of its approach within 1,438,000
miles of the earth, July i, 1770, must have been apparent in
the lengthening of the year. And that some comets, at any
rate, possess masses immeasurably below this maximum value,
was clearly proved by the undisturbed parallel march of the
two fragments of Biela in 1846.
But the discovery in this branch most distinctive of the
period under review is that of " short period " comets, of
which four 1 were known in 1850. These, by the character of
their movements, serve as a link between the planetary and
cometary worlds, and by the nature of their construction,
seem to mark a stage in cometary decay. For that comets
are rather transitory agglomerations, than permanent products
of cosmical manufacture, appeared to be demonstrated by the
division and disappearance of one amongst their number, as
well as by the singular and rapid changes in appearance under-
gone by many, and the (seemingly) irrevocable diffusion of
their substance visible in nearly all. They might then be
defined, according to the ideas respecting them prevalent
forty years ago, as bodies unconnected by origin with the solar
system, but encountered, and to some extent appropriated by it
in its progress through space, owing their visibility in great
part, if not altogether, to light reflected from the sun, and their
singular and striking forms to the action of repulsive forces
emanating from him, the penalty of their evanescent splendour
being paid in gradual waste and final dissipation and extinction.
1 Viz., Encke's, Biela's, Faye's, and Brorsen's. A comet with a sup-
posed period of 5^ years, detected by De Vico at Rome, August 22, 1844,
has made no ascertained return to perihelion, unless its suggested identity
with Finlay's comet of 1886 be real.
( 135 )
CHAPTER VI.
INSTRUMENTAL ADVANCES.
IT is impossible to follow with intelligent interest the course
of astronomical discovery without feeling some curiosity as to
the means by which such surpassing results have been secured.
Indeed, the bare acquaintance with wliat has been achieved,
without any corresponding knowledge of hoiv it has been
achieved, supplies food for barren wonder rather than for
fruitful and profitable thought. Ideas advance most readily
along the solid ground of practical reality, and often find true
sublimity while laying aside empty marvels. Progress is the
result, not so much of sudden flights of genius, as of sustained,
patient, often commonplace endeavour; and the true lesson
of scientific history lies in the close connection which it dis-
closes between the most brilliant developments of knowledge
and the faithful accomplishment of his daily task by each
individual thinker and worker.
It would be easy to fill a volume with the detailed account
of the long succession of optical and mechanical improvements
by means of which the observation of the heavens has been
brought to its present degree of perfection ; but we must here
content ourselves with a summary sketch of the chief amongst
them. The first place in our consideration is naturally claimed
by the telescope.
This marvellous instrument, we need hardly remind our
readers, is of two distinct kinds that in which light is gathered
together into a focus by refraction, and that in which the same
end is attained by reflection. The image formed is in each
136 HISTORY OF ASTRONOMY. PARTI.
case viewed through a magnifying lens, or combination of
lenses, called the eye-piece. Not for above a century after
the "optic glasses" invented or stumbled upon by the spec-
tacle-maker of Middleburg (1608) had become diffused over
Europe, did the reflecting telescope come, even in England,
the place of its birth, into general use. Its principle (a suffi-
ciently obvious one) had indeed been suggested by Mersenne
as early as I639; 1 James Gregory in 16632 described in
detail a mode of embodying that principle in a practical
shape; and Newton, adopting an original system of con-
struction, actually produced in 1668 a tiny speculum, one
inch across, by means of which the apparent distance of
objects was reduced thirty-nine times. Nevertheless, the
exorbitantly long tubeless refractors, introduced by Huygens,
maintained their reputation until Hadley exhibited to the
Royal Society in I723 3 a reflector sixty-two inches in focal
length, which rivalled in performance, and of course inde-
finitely surpassed in manageability, one of the "aerial"
kind of 123 feet.
The concave-mirror system now gained a decided ascendant,
and was brought to unexampled perfection by James Short
of Edinburgh during the years 1732-68. Its capabilities were,
however, first fully developed by William Herschel. The
energy and inventiveness of this extraordinary man marked
an epoch wherever they were applied. His ardent desire to
measure and gauge the stupendous array of worlds which his
specula revealed to him, made him continually intent upon
adding to their " space-penetrating power " by increasing their
light-gathering surface. These, as he was the first to explain, 4
are in a constant proportion one to the other. For a telescope
with twice the linear aperture of another will collect four
times as much light, and will consequently disclose -an object
four times as faint as could be seen with the first, or, what
comes to the same, an object equally bright at twice the
distance. In other words, it will possess double the space-
1 Grant, Hist. A sir., p. 527. z Optica Promota, p. 93. 3 Phil. Trans.,
vol. xxxii. p. 383. 4 Ibid., vol. xc. p. 65.
CHAP. vi. INSTRUMENTAL ADVANCES. 137
penetrating power of the smaller instrument. Herschel's
great mirrors the first examples of the giant telescopes of
modern times were then primarily engines for extending
the bounds of the visible universe; and from the sublimity
of this " final cause " was derived the vivid enthusiasm which
animated his efforts to success.
It seems probable that the seven-foot telescope constructed
by him in 1775 that is, within little more than a year after
his experiments in shaping and polishing metal had begun
already exceeded in effective power any work by an earlier
optician ; and both his skill and his ambition rapidly developed.
His efforts culminated, after mirrors of ten, twenty, and thirty
feet focal length had successively left his hands, in the gigan-
tic forty-foot, completed August 28, 1789. It was the first
reflector in which only a single mirror was employed. In the
" Gregorian " form, the focussed rays are, by a second reflec-
tion from a small concave l mirror, thrown straight back
through a central aperture in the larger one, behind which
the eye-piece is fixed. The object under examination is thus
seen in the natural direction. The " Newtonian," on the
other hand, shows the object in a line of sight at right angles
to the true one, the light collected by the speculum being
diverted to one side of the tube by the interposition of a small
plane mirror, fixed at an angle of 45 to the axis of the
instrument. Upon these two systems Herschel worked until
1787, when, becoming convinced of the supreme importance
of economising light (necessarily wasted by the second reflec-
tion), he laid aside the small mirror of his forty-foot then
in course of construction, and turned it into a " front-view "
reflector. This was done according to the plan proposed by
Lemaire in 1732 by slightly inclining the speculum so as to
enable the image formed by it to be viewed with an eye-glass
fixed at the upper margin of the tube. The observer thus
1 Cassegrain, a Frenchman, substituted in 1672 a convex for a concave
secondary speculum. The tube was thereby enabled to be shortened by
twice the focal length of the mirror in question. The great Melbourne
reflector (four feet aperture, by Grubb) is constructed upon this plan.
138 HISTORY OF ASTRONOMY. PARTI.
stood with his back turned to the object he was engaged in
scrutinising.
The advantages of the increased brilliancy afforded by
this modification were strikingly illustrated by the discovery,
August 28 and September 17, 1789, of the two Saturnian
satellites nearest the ring. Nevertheless, the monster tele-
scope of Slough cannot be said to have realised the sanguine
expectations of its constructor. The occasions on which it
could be usefully employed were found to be extremely rare.
It was injuriously affected by every change of temperature.
The great weight (25 cwt.) of a speculum four feet in diameter
rendered it peculiarly liable to distortion. With all imagin-
able care, the delicate lustre of its surface could not be pre-
served longer than two years, 1 when the difficult process of
repolishing had to be undertaken. It was accordingly never
used after 1811, when, having gone blind from damp, it lapsed
by degrees into the condition of a museum inmate.
The extraordinarily high magnifying powers employed by
Herschel constituted a novelty in optical astronomy scarcely
less striking than the gigantic size of his specula. They had
never previously been approached ; they have never since been
surpassed ; and they seem to mark, for these latitudes at least,
the very outside limit of practicability. The attempt to in-
crease in this manner the efficacy of the telescope is speedily
checked by atmospheric, to say nothing of other difficulties.
Precisely in the same proportion as an object is magnified,
the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights,
the air is never for a moment really still. The rays of light
traversing it are continually broken by minute fluctuations of
refractive power caused by changes of temperature and pres-
sure, and the currents which these engender. With such
luminous quiverings and waverings the astronomer has always
more or less to reckon ; their absence is simply a question of
degree ; if sufficiently magnified, they are at all times capable
of rendering observation impossible.
1 Phil. Trans., vol. civ. p. 275, note.
CHAP. vi. INSTRUMENTAL ADVANCES. 139
Thus, such vast powers as 3000, 4000, 5000, even 6652,*
which Herschel now and again applied to his great telescopes,
must, save on the rarest occasions, prove an impediment rather
than an aid to vision. They were, however, used by him only
for special purposes, and with the clearest discrimination of
their advantages and drawbacks. It is obvious that perfectly
different ends are subserved by increasing the aperture and
by increasing the power of a telescope. In the one case, a
larger quantity of light is captured and concentrated ; in
the other, the same amount is distributed over a wider area.
A diminution of brilliancy accordingly attends upon each
augmentation of apparent size. For this reason, such faint
objects as nebulae are most successfully observed with moderate
powers applied to instruments of a great capacity for light,
the details of their structure actually disappearing when
highly magnified. With stellar groups the reverse is the case.
Stars cannot be magnified, simply because they are too remote
to have any sensible dimensions ; but the space between them
can. It was thus for the purpose of dividing very close double
stars that Herschel increased to such an unprecedented extent
the magnifying capabilities of his instruments ; and to this
improvement incidentally the discovery of Uranus, March 13,
1781,2 was due. For by the examination with strong lenses
of an object which, even with a power of 227, presented a
suspicious appearance, he was able at once to pronounce its
disc to be real, and not merely " spurious," and so to dis-
tinguish it unerringly from the crowd of stars amidst which it
was moving.
While the reflecting telescope was astonishing the world by
its rapid development in the hands of Herschel, its unpretend-
ing rival was slowly making its way towards the position
which the future had in store for it. The great obstacle which
long stood in the way of the improvement of refractors was
1 Phil. Trans., vol. xc. p. 70. With the forty-foot, however, only very
moderate powers seem to have been employed, whence Dr. Robinson
argued a deficiency of defining power. Proc. Roy. Irish Ac., vol. ii. p. u.
2 Phil. Trans., vol. Ixxi. p. 492.
140 HISTORY OF ASTRONOMY. PARTI.
the defect known as " chromatic aberration." This is due to
no other cause than that which produces the rainbow and the
spectrum the separation, or " dispersion " in their passage
through a refracting medium, of the variously coloured rays
composing a beam of white light. In an ordinary lens there
is no common point of concentration ; each colour has it own
separate focus ; and the resulting image, formed by the super-
position of as many images as there are hues in the spectrum,
is indistinctly terminated with a tinted border, eminently
baffling to exactness of observation.
The extravagantly long telescopes of the seventeenth century
were designed to avoid this evil (as well as another source
of indistinct vision in the spherical shape of lenses) ; but no
attempt to remedy it was made until an Essex gentleman
succeeded, in 1733, ^ n so combining lenses of flint and crown
glass as to produce refraction without colour. 1 Mr. Chester
More Hall was, however, equally indifferent to fame and
profit, and took no pains to make his invention public. The
effective discovery of the achromatic telescope was, accordingly,
reserved for John Dollond, whose method of correcting at the
same time chromatic and spherical aberration was laid before
the Royal Society in 1758. Modern astronomy may be said to
have been thereby rendered possible. Refractors have always
been found better suited than reflectors to the ordinary work
of observatories. They are, so to speak, of a more robust, as
well as of a more plastic nature. They suffer less from vicissi-
tudes of temperature and climate. They retain their efficiency
with fewer precautions and under more trying circumstances.
Above all, they co-operate more readily with mechanical appli-
ances, and lend themselves with far greater facility to purposes
of exact measurement.
A practical difficulty, however, impeded the realisation of
the brilliant prospects held out by Dollond's invention. It
was found impossible to procure flint-glass, such as was needed
1 It is remarkable that, as early as 1695, the possibility of an achromatic
combination was inferred by David Gregory from the structure of the
human eye. See his Catoptricce et Dioptricce Sphericce Elementa, p. 98.
CHAP. vi. INSTRUMENTAL ADVANCES. 141
for optical use that is, of perfectly homogeneous quality
except in fragments of insignificant size. Discs of more than
two or three inches in diameter were of extreme rarity ; and
the crushing excise duty imposed upon the article by the
financial unwisdom of the Government, both limited its pro-
duction, and, by rendering experiments too costly for repeti-
tion, barred its improvement.
Up to this time, Great Britain had left foreign competitors
far behind in the instrumental department of astronomy.
The quadrants and circles of Bird, Gary, and Ramsden were
un approached abroad. The reflecting telescope came into
existence and reached "maturity on British soil. The re-
fracting telescope was cured of its inherent vices by British
ingenuity. But with the opening of the nineteenth century,
the almost unbroken monopoly of skill and contrivance which
our countrymen had succeeded in establishing was invaded,
and British workmen had to be content to exchange a posi-
tion of supremacy for one of at least partial and temporary
inferiority.
Somewhere about the time that Herschel set about polish-
ing his first speculum, Pierre Louis Guinand, a Swiss artisan,
living near Chaux-de-Fonds, in [the canton of Neuchatel,
began to grind spectacles for his own use, and was thence
led on to the rude construction of telescopes by fixing lenses
in pasteboard tubes. The sight of an English achromatic,
however, stirred a higher ambition, and he took the first
opportunity of procuring some flint-glass from England (then
the only source of supply), with the design of imitating an
instrument the full capabilities of which he was destined to
be the humble means of developing. The English glass prov-
ing of inferior quality, he conceived the possibility, unaided
and ignorant of the art as he was, of himself making better,
and spent seven years (1784-90) in fruitless experiments
directed to that end. Failure only stimulated him to enlarge
their scale. He bought some land near Les Brenets, con-
structed upon it a furnace capable of melting two quintals
of glass, and reducing himself and his family to the barest
I 4 2 HISTORY OF ASTRONOMY. PARTI.
necessaries of life, he poured his earnings (he at this time
made bells for repeaters) unstintingly into his crucibles. 1 His
undaunted resolution triumphed. In 1799 he carried to Paris
and there showed to Lalande several discs of flawless crystal
four to six inches in diameter. Lalande advised him to keep
his secret, but in 1805 he was induced to remove to Munich,
where he became the instructor of the immortal Fraunhofer.
His return to Les Brenets in 1814 was signalised by the dis-
covery of an ingenious mode of removing striated portions of
glass by breaking and re-soldering the product of each melting,
and he eventually attained to the manufacture of perfect discs
up to 1 8 inches in diameter. An object-glass for which he had
furnished the material to Cauchoix, procured him, in 1823, a
royal invitation to settle in Paris ; but he was no longer equal
to the change, and died at the scene of his labours, February
13 following.
This same lens (12 inches across) was afterwards purchased
by Sir James South, and the first observation made with it,
February 13, 1830, disclosed to Sir John Herschel the sixth
minute star in the central group of the Orion nebula, known
as the " trapezium." 2 Bequeathed by South to Trinity Col-
lege, Dublin, it has been employed at the Dunsink Obser-
vatory by Briinnow and Ball in their investigations of stellar
parallax. A still larger objective (of nearly 14 inches) made
of Guinand's glass was secured about the same time in Paris,
by Mr. Edward Cooper of Markree Castle, Ireland. The
peculiarity of the method discovered at Les Brenets resided
in the manipulation, not in the quality of the ingredients ;
the secret, that is to say, was not chemical, but mechanical. 3
It was communicated by Henry Guinand (a son of the in-
ventor) to Bontemps, one of the directors of the glassworks
at Choisy-le-Roi, and by him transmitted to Messrs. Chance
of Birmingham, with whom he entered into partnership when
the revolutionary troubles of 1848 obliged him to quit his
native country. The celebrated American opticians, Alvan
1 Wolf, Biographien, Bd. ii. p. 301. 2 Month. Not., vol. i. p. 153, note.
3 Henri vaux, Encyclopedic Chimique, t. v. fasc. 5, p. 363.
CHAP. vi. INSTRUMENTAL ADVANCES. 143
Clark & Sons, have derived from the Birmingham firm the
materials for some of their finest telescopes, notably the iQ-inch
Chicago and 26-inch Washington equatoreals.
Two distinguished amateurs, meanwhile, were preparing to
reassert on behalf of reflecting instruments their claim to the
place of honour in the van of astronomical discovery. Of Mr.
Lassell's specula something has already been said. 1 They
were composed of an alloy of copper and tin, with a minute
proportion of arsenic (after the example of Newton 2 ), and
were remarkable for perfection of figure and brilliancy of
surface.
The resources of the Newtonian system were developed still
more fully it might almost be said to the uttermost by the
enterprise of an Irish nobleman. William Parsons, known as
Lord Oxmantown until 1841, when, on his father's death, he
succeeded to the title of Earl of Rosse, was born at York, June
17, 1800. His public duties began before his education was
completed. He was returned to Parliament as member for
King's County while still an undergraduate at Oxford, and
continued to represent the same constituency for thirteen years
(1821-34). From 1845 until his death, which took place at
Birr Castle, Parsonstown, October 31, 1867, he sat, silent but
assiduous, in the House of Lords as an Irish representative
peer; he held the not unlaborious post of President of the
Royal Society from 1849 * l %54> presided over the meeting
of the British Association at Cork in 1843, and was elected
Yice- Chancellor of Dublin University in 1862. In addition
to these extensive demands upon his time and thoughts, were
those derived from his position as (practically) the feudal chief
of a large body of tenantry in times of great and anxious
responsibility, to say nothing of the more genial claims of an
unstinted hospitality. Yet, while neglecting no public or
private duty, this model nobleman found leisure to render to
science services so conspicuous as to entitle his name to a
lasting place in its annals.
He early formed the design of reaching the limits of the
1 See ante, p. 104. 2 Phil. Trans., vol. vii. p. 4007.
144 HISTORY OF ASTRONOMY. PARTI.
attainable in enlarging the powers of the telescope, and the
qualities of his mind conspired with the circumstances of his
fortune to render the design a feasible one. From refractors
it was obvious that no such vast and rapid advance could be
expected. English glass-manufacture was still in a backward
state. So late as 1839, Simms (successor to the distinguished
instrumentalist Edward Troughton) reported a specimen of
crystal scarcely yj inches in diameter, and perfect only over
six, to be unique in the history of English glass-making. 1
Yet at that time the fifteen-inch achromatic of Pulkowa
had already left the workshop of Fraunhofer's successors at
Munich. It was not indeed until 1845, w ^ en the impost
which had so long hampered their efforts was removed, that
the optical artists of these islands were able to compete on
equal terms with their rivals on the Continent. In the case
of reflectors, however, there seemed no insurmountable obstacle
to an almost unlimited increase of light-gathering capacity ;
and it was here, after some unproductive experiments with
fluid lenses, that Lord Oxmantown concentrated his energies.
He had to rely entirely on his own invention, and to earn
his own experience. James Short had solved the problem of
giving to metallic surfaces a perfect parabolic figure (the only
one by which parallel incident rays can be brought to an exact
focus) ; but so jealous was he of his secret, that he caused all
his tools to be burnt before his death ; 2 nor was anything
known of the processes by which Herschel had achieved
his astonishing results. Moreover, Lord Oxmantown had no
skilled workmen to assist him. His implements, both animate
and inanimate, had to be formed by himself. Peasants taken
from the plough were educated by him into efficient mechanics
and engineers. The delicate and complex machinery needed
in operations of such hairbreadth nicety as his enterprise
involved, the steam-engine which was to set it in motion,
at times the very crucibles in which his specula were cast,
issued from his own workshops. 3
1 J. Herschel, The Telescope, p. 39. 2 Month. Not., vol. xxix. p. 125.
3 Ibid., p. 129.
CHAP. vi. INSTRUMENTAL ADVANCES. 145
In 1827 experiments on the composition of speculum-metal
were set on foot, and the first polishing-machine ever driven
by steam-power was contrived. But twelve arduous years of
struggle with recurring difficulties passed before success began
to dawn. A material less tractable than the alloy selected of
four chemical equivalents of copper to one of tin 1 can scarcely
be conceived. It is harder than steel, yet brittle as glass,
crumbling into fragments with the slightest inadvertence of
handling or treatment ; 2 and the precision of figure requisite
to secure good definition is almost beyond the power of lan-
guage to convey. The quantities involved are so small as
not alone to elude sight, but to confound imagination. Sir
John Herschel tells us that " the total thickness to be abraded
from the edge of a spherical speculum 48 inches in diameter
and 40 feet focus, to convert it into a paraboloid, is only
-2ik^s f an i nc k j " 3 J et u P on this minute difference of form
depends the clearness of the image, and, as a consequence,
the entire efficiency of the instrument. " Almost infinite,"
indeed (in the phrase of the late Dr. Robinson), must be the
exactitude of the operation adapted to bring about so delicate
a result.
At length, in 1840, two specula, each three feet in diameter,
were turned out in such perfection as to prompt a still bolder
experiment. The various processes needed to ensure success
were now ascertained and under control ; all that was neces-
sary was to repeat them on a larger scale. A gigantic mirror,
six feet across and fifty-four in focal length, was accordingly
cast on the i3th of April 1842 ; in two months it was ground
down to figure by abrasion with emery and water, and daintily
polished with rouge; and by the month of February 1845
the " leviathan of Parsonstown " was available for the exami-
nation of the heavens.
The suitable mounting of this vast machine was a problem
1 A slight excess of copper renders the metal easier to work, but liable
to tarnish. Robinson, Proc. Roy. Irish Ac., vol. ii. p. 4. 2 Brit. Ass.,
1843, ^ r ' Robinson's closing Address. Athenceum, Sept. 23, p. 866. 3 The
Telescope, p. 82.
K
146 HISTORY O& ASTRONOMY. PARTI.
scarcely less difficult than its construction. The shape of a
speculum needs to be maintained with an elaborate care equal
to that used in imparting it. In fact, one of the most for-
midable obstacles to increasing the size of such reflecting
surfaces consists in their liability to bend under their own
weight. That of the great Rosse speculum was no less than
four tons. Yet, although six inches in thickness, and com-
posed of a material only a degree inferior in rigidity to wrought
iron, the strong pressure of a man's hand at its back produced
sufficient flexure to distort perceptibly the image of a star
reflected in it. 1 Thus the delicacy of its form was perishable
equally by the stress of its own gravity, and by the slightest
irregularity in the means taken to counteract that stress.
The problem of affording a perfectly equable support in all
possible positions was solved by resting the speculum upon
twenty-seven platforms of cast iron, felt-covered, and carefully
fitted to the shape of the areas they were to carry, which plat-
forms were themselves borne by a complex system of triangles
and levers, ingeniously adapted to distribute the weight with
complete uniformity. 2
A tube which resembled, when erect, one of the ancient
round towers of Ireland, 3 served as the habitation of the great
mirror. It was constructed of deal staves bound together with
iron hoops, was fifty-eight feet long (including the speculum-
box), and seven in diameter. A reasonably tall man may
walk through it (as Dean Peacock once did) with umbrella
uplifted. Two piers of solid masonry, about fifty feet high,
seventy long, and twenty-three apart, flanked the huge engine
on either side. Its lower extremity rested on an universal
joint of cast iron ; above, it was slung in chains, and even in
a gale of wind remained perfectly steady. The weight of the
entire, although amounting to fifteen tons, was so skilfully
counterpoised, that the tube could with ease be raised or
1 Lord Rosse, Phil. Trans., vol. cxl. p. 302. 2 This method is
the same in principle with that applied by Grubb in 1834 to a 1 5-inch
speculum for the observatory of Armagh. Phil. Trans., vol. clix. p. 145.
3 Robinson, Proc. Roy. Ir. Ac., voL iii. p. 120.
CHAP. vi. INSTRUMENTAL ADVANCES. 147
depressed by two men working a windlass. Its horizontal
range was limited by the lofty walls erected for its support to
about ten degrees on each side of the meridan ; but it moved
vertically from near the horizon through the zenith as far as
the pole. Its construction was of the Newtonian kind, the
observer looking into the side of the tube near its upper end,
which a series of galleries and sliding stages enabled him to
reach in any position. It has also, though rarely, been used
without a second mirror, as a " Herschelian " reflector.
The splendour of the celestial objects as viewed with this
vast "light-grasper" surpassed all expectation. "Never in
my life," exclaims Sir James South, " did I see such glorious
sidereal pictures I" 1 The orb of Jupiter produced an effect
compared to that of the introduction of a coach-lamp into the
telescope ; 2 and certain star-clusters exhibited an appearance
(we again quote Sir James South) " such as man before had
never seen, and which for its magnificence baffles all descrip-
tion." But it was in the examination of the nebulae that the
superiority of the new instrument was most strikingly dis-
played. A large number of these misty objects, which the
utmost powers of Herschel's specula had failed to resolve into
stars, yielded at once to the Parsonstown reflector; while
many others showed under entirely changed forms through the
disclosure of previously unseen details of structure.
One extremely curious result of the increase of light was the
abolition of the distinction between the two classes of "annular"
and "planetary" nebulae. Up to that time, only four ring-
shaped systems two in the northern and two in the southern
hemisphere were known to astronomers ; they were now
reinforced by five of the planetary kind, the discs of which
were observed to be centrally perforated; while the sharp
marginal definition visible in weaker instruments was replaced
by ragged edges or filamentous fringes.
Still more striking was the discovery of an entirely new
and highly remarkable species of nebulae. These were termed
"spiral," from the more or less regular convolutions, resem-
1 Astr. Xach., No. 536. 2 Airy, Month. Not., vol. ix. p. 1 20.
I 4 8 HISTORY OF ASTRONOMY. PARTI.
bling the whorls of a shell, in which the matter composing
them appeared to be distributed. The first and most con-
spicuous specimen of this class was met with in April 1845 ;
it is situated in Canes Venatici, close to the tail of the Great
Bear, and wore, in Sir J. Herschel's instruments, the aspect
of a split ring encompassing a bright nucleus, thus present-
ing, as he supposed, a complete analogue to the system of the
Milky Way. In the Rosse mirror it shone out as a vast
whirlpool of light a stupendous witness to the presence of
cosmical activities on the grandest scale, yet regulated by
laws as to the nature of which we are profoundly ignorant.
Professor Stephen Alexander of New Jersey, however, con-
cluded, from an investigation (necessarily founded on highly
precarious data) of the mechanical condition of these extra-
ordinary agglomerations, that we see in them " the partially
scattered fragments of enormous masses once rotating in a
state of dynamical equilibrium." He further suggested
"that the separation of these fragments may still be in
progress," 1 and traced back their origin to the disruption,
through its own continually accelerated rotation, of a
" primitive spheroid " of inconceivably vast dimensions.
Such also, it was added (the curvilinear form of certain
outliers of the Milky Way, giving evidence of a spiral
structure), is probably the history of our own cluster; the
stars composing which, no longer held together in a deli-
cately adjusted system like that of the sun and planets, are
advancing through a period of seeming confusion towards
an appointed goal of higher order and more perfect and har-
monious adaptation. 2
The class of spiral nebulae included, in 1850, fourteen mem-
bers, besides several in which the characteristic arrange-
ment seemed partial or dubious. 3 A tendency in the exterior
stars of other clusters to gather into curved branches (as in
our Galaxy) was likewise noted ; and the existence of unsus-
pected analogies was proclaimed by the significant combination
1 Astronomical Journal (Gould's), vol. ii. p. 97. 2 Ibid., p. 160.
3 Lord Rosse, Phil. Trans., vol. cxl. p. 505.
CHAP. vi. INSTRUMENTAL ADVANCES. 149
in the " Owl " nebula (a large planetary in Ursa Major) l of
the twisted forms of a spiral with the perforation distinctive
of an annular nebula.
Once more, by the achievements of the Parsonstown
reflector, the supposition of a " shining fluid " filling vast
regions of space was brought into (as it has since proved)
undeserved discredit. Although Lord Rosse himself rejected
the inference that because many nebulas had been resolved,
all were resolvable, very few imitated his truly scientific
caution; and the results of Bond's investigations 2 with the
Harvard College refractor quickened and strengthened the
current of prevalent opinion. It is now certain that the
evidence furnished on both sides of the Atlantic as to the
stellar composition of some conspicuous objects of this class
(notably the Orion and "Dumb-bell" nebulae) was delusive;
but the spectroscope alone was capable of meeting it with a
categorical denial. Meanwhile there seemed good ground for
the persuasion, which now, for the last time, gained the upper
hand, that nebulae are, without exception, true " island-
universes," or assemblages of distant suns.
Lord Rosse's telescope possesses a nominal power of 6000
that is, it shows the moon as if viewed with the naked eye at
a distance of forty miles. But this seeming advantage is
neutralised by the weakening of the available light through
excessive diffusion, as well as by the troubles of the surging
sea of air through which the observation must necessarily be
made. Professor Newcomb, in fact, doubts whether with
any telescope our satellite has ever been seen to such
advantage as it would be if brought within 500 miles of the
unarmed eye. 3
The French opticians' rule of doubling the number of milli-
metres contained in the aperture of an instrument to find the
highest magnifying power usefully applicable to it, would give
1 No. 2343 of Herschel's (1864) Catalogue. Before 1850 a star was
visible in each of the two larger openings by which it is pierced ; since
then one only. Webb, Celestial Objects (4th ed.), p. 409. 2 Mem. Am.
Ac., voL iii. p. 87 j and Astr. Nach., No. 611. 3 Pop. Astr., p. 145.
150 HISTORY OF ASTRONOMY. PARTI.
3600 as the maximum for the leviathan of Birr Castle ; but
in a climate like that of Ireland the occasions must be rare
when even that limit can be reached. Indeed, the experience
acquired by its use plainly shows that atmospheric rather than
mechanical difficulties impede a still further increase of tele-
scopic power. Its construction may accordingly be said to
mark the ne plus ultra of effort in one direction, and the
beginning of its conversion towards another. It became
thenceforward more and more obvious that the conditions of
observation must be ameliorated before any added efficacy
could be given to it. The full effect of an uncertain climate
in nullifying optical improvements was recognised, and the
attention of astronomers began to be turned towards the
advantages offered by more tranquil and more translucent
skies.
Even more important for the practical uses of astronomy
than the optical qualities of the telescope, is the manner of its
mounting. There is a far greater likelihood of getting good
work done with an imperfect instrument skilfully mounted,
than with the most admirable performance of the optician of
which the mechanical accessories are ill-arranged or incon-
venient. Thus the astronomer is ultimately dependent upon
the mechanician; and so excellently have his needs been
served, that the history of the ingenious contrivances by which
discoveries have been prepared would supply a subject (here
barely glanced at) not far inferior in extent and instruction
to the history of those discoveries themselves.
There are two chief modes of using the telescope, to which
all others may be considered subordinate. 1 Either it may be
invariably directed towards the south, with no motion save
in the plane of the meridian, so as to intercept the heavenly
bodies at the moment of transit across that plane ; or it may
be arranged so as to follow the daily revolution of the sky,
1 This statement must be taken in the most general sense. Supplemen-
tary observations of great value are now made at Greenwich with the
altitude and azimuth instrument, which likewise served Piazzi to determine
the places of his stars ; while a " prime vertical instrument " is prominent
at Pulkowa.
CHAP. vi. INSTR UMENTA L ADVA NCES. 1 5 r
thus keeping the object viewed permanently in sight, instead
of simply noting the instant of its flitting across the telescopic
field. The first plan is that of the " transit instrument," the
second that of the " equatoreal. " Both were, by a remark-
able coincidence, introduced about I69O 1 by Olaus Homer, the
brilliant Danish astronomer who first measured the velocity
of light.
The uses of each are entirely different. With the transit,
the really fundamental task of astronomy the determination
of the movements of the heavenly bodies is mainly accom-
plished; while the investigation of their nature and peculi-
arities is best conducted with the equatoreal. One is the
instrument of mathematical, the other of descriptive astro-
nomy. One furnishes the materials with which theories are
constructed, and the tests by which they are corrected ; the
other registers new facts, takes note of new appearances,
sounds the depths, and pries into every nook of the heavens.
The great improvement of giving to a telescope equatoreally
mounted an automatic movement by connecting it with clock-
work, was proposed in 1674 by Robert Hooke. Bradley in
1721 actually observed Mars with a telescope "moved by a
machine that made it keep pace with the stars; " 2 and Yon
Zach relates 3 that he had once followed Sirius for twelve
hours with a "heliostat" of Ramsden's construction. But
these eighteenth-century attempts were of no practical effect.
Movement by clockwork was virtually a complete novelty
when it was adapted by Fraunhofer in 1824 to the Dorpat
refractor. By simply giving to an axis unvaryingly directed
towards the celestial pole an equable rotation with a period of
twenty-four hours, a telescope attached to it, and pointed in
any direction, will trace out on the sky a parallel of declina-
tion, thus necessarily accompanying the movement of any star
1 As early as 1620, according to R. Wolf (Gesch. der Astr., p. 587),
Father Scheiner made the experiment of connecting a telescope with an
axis directed to the pole, while Chinese "equatoreal armillae," dating
from the thirteenth century, still exist at Pekin. J. L. E. Dreyer, Coper-
nicus, vol. i. p. 134. 2 Miscellaneous Works, p. 350. 3 Astr. Jahrbuch,
1799 (published 1796), p. 115.
152 HISTORY OF ASTRONOMY. PARTI.
upon which it may be fixed. It thus forms part of the large
sum of Fraunhofer's merits to have secured this inestimable
advantage to observers.
It was considered by Sir John Herschel that Lassell's appli-
cation of equatoreal mounting to a nine-inch Newtonian in
1840 made an epoch in the history of " that eminently British
instrument, the reflecting telescope." l Nearly a century
earlier, 2 it is true, Short had fitted one of his Gregorians to a
complicated system of circles in such a manner that, by
moving a handle, it could be made to follow the revolution
of the sky ; but the arrangement did not obtain, nor did it
deserve, general adoption. Lassell's plan was a totally dif-
ferent one ; he employed the crossed axes of the true equa-
toreal, and his success removed, to a great extent, the fatal
objection of inconvenience in use, until then unanswerably
urged against reflectors. The very largest of these can now
be mounted equatoreally ; even the Rosse within its limited
range has been for some years provided with a movement by
clockwork along declination-parallels.
The art of accurately dividing circular arcs into the minute
equal parts which serve as the units of astronomical measure-
ment, remained, during the whole of the eighteenth century,
almost exclusively in English hands. It was brought to a
high degree of perfection by Graham, Bird, and Ramsden, all
of whom, however, gave the preference to the old-fashioned
mural quadrant and zenith sector over the entire circle, which
Homer had already found the advantage of employing. The
five-foot vertical circle, which Piazzi with some difficulty in-
duced Ramsden to complete for him in 1789, was the first
divided instrument constructed in what may be called the
modern style. It was provided with magnifiers for reading
off the divisions (one of the neglected improvements of Romer),
and was set up above a smaller horizontal circle, forming an
" altitude and azimuth " combination (again Romer's inven-
tion), by which both the elevation of a celestial object above
the horizon and its position as referred to the horizon could
1 Month. Not., vol. xli. p. 189. 2 Phil. Trans., vol. xlvi. p. 242.
CHAP. vi. INSTRUMENTAL ADVANCES. 153
be measured. In the same year Borda invented the " repeat-
ing circle " (the principle of which had been suggested by
Tobias Mayer in I756 1 ), a device for exterminating, so far
as possible, errors of graduation by repeating an observation
with different parts of the limb. This was perhaps the
earliest systematic effort to correct the imperfections of in-
struments by the manner of their use.
The manufacture of astronomical circles was brought to a
very refined state of excellence early in the present century
by Reichenbach at Munich, and (after 1818) by Repsold at
Hamburg. Bessel states 2 that the " reading-off " on an
instrument of the kind by the latter artist was accurate to
about f^th of a human hair. Meanwhile the traditional
reputation of the English school was fully sustained ; and Sir
George Airy did not hesitate to express his opinion that the
new method of graduating circles, published by Troughton in
iSoQ, 3 was the " greatest improvement ever made in the art
of instrument-making." 4 But a more secure road to improve-
ment than that of mere mechanical exactness was pointed out
by Bessel. His introduction of a regular theory of instru-
mental errors might almost be said to have created a new art
of observation. Every instrument, he declared in memorable
words, 5 must be twice made once by the artist, and again by
the observer. Knowledge is power. Defects that are ascer-
tained and can be allowed for are as good as non-existent.
Thus the truism that the best instrument is worthless in the
hands of a careless or clumsy observer, became supplemented
by the converse maxim that defective appliances may, by
skilful use, be made to yield valuable results. The Konigsberg
observations of which the first instalment was published
in 1815 set the example of regular "reduction " for instru-
mental errors. Since then, it has become an elementary part
of an astronomer's duty to study the idiosyncrasy of each one
of the mechanical contrivances at his disposal, in order that
1 Grant, Hist, of Astr., p. 487. 2 Pop. Vorl., p. 546. 3 Phil. Trans.,
vol. xcix. p. 105. 4 Report Brit. Ass., 1832, p. 132. 5 Pop. Vorl,
p. 432.
154 HISTORY OF ASTRONOMY. PARTI.
its inevitable, but now certified deviations from ideal accuracy
may be included amongst the numerous corrections by which
the pure essence of (even approximate) truth is distilled from
the rude impressions of sense.
Nor is this enough ; for the casual circumstances attending
each observation have to be taken into account with no less
care than the inherent or constitutional peculiarities of the
instrument with which it is made. There is no " once for all "
in astronomy. Vigilance can never sleep ; patience can never
tire. Variable as well as constant sources of error must
be anxiously heeded; one infinitesimal inaccuracy must be
weighed against another; all the forces and vicissitudes of
nature frosts, dews, winds, the interchanges of heat, the dis-
turbing effects of gravity, the shiverings of the air, the tremors
of the earth, the weight and vital warmth of the observer's
own body, nay, the rate at which his brain receives and trans-
mits its impressions, must all enter into his calculations, and
be sifted out from his results.
It was in 1823 that Bessel drew attention to discrepancies
in the times of transits given by different astronomers. 1 The
quantities involved were far from insignificant. He was him-
self nearly a second in advance of all his contemporaries,
Argelander lagging behind him as much as a second and a
quarter. Each individual, in fact, was found to have a certain
definite rate of perception, which, under the name of " personal
equation," now forms so important an element in the correc-
tion of observations that a special instrument for accurately
determining its amount in each individual case is in actual use
at Greenwich.
Such are the refinements upon which modern astronomy
depends for its progress. It is a science of hairbreadths and
fractions of a second. It exists only by the rigid enforcement
of arduous accuracy and unwearying diligence. Whatever
secrets the universe still has in store for man will only be
communicated on these terms. They are, it must be acknow-
1 C. T. Anger, Grundzilge. der neueren astronomischen Bedbachtungs-
Kunst, p. 3.
CHAP. vi. INSTR UMENTA L ADVA NCES. 1 5 5
ledged, difficult to comply with. They involve an unceasing
struggle against the infirmities of his nature and the insta-
bilities of his position. But the end is not unworthy the
sacrifices demanded. One additional ray of light thrown on
the marvels of creation a single, minutest encroachment upon
the strongholds of ignorance, is recompense enough for a life-
time of toil. Or rather, the toil is its own reward, if pursued
in the lofty spirit which alone becomes it. For it leads
through the abysses of space and the unending vistas of time
to the very threshold of that infinity and eternity of which the
disclosure is reserved for a life to come.
( 157 )
part a
EECENT PROGRESS OF ASTRONOMY.
CHAPTER I.
FOUNDATION OF ASTRONOMICAL PHYSICS.
IN the year 1826, Heinrich Schwabe of Dessau, elated with
the hope of speedily delivering himself from his hereditary
incubus of an apothecary's shop, 1 obtained from Munich a
small telescope and began to observe the sun. His choice
of an object for his researches was instigated by his friend
Harding of Gottingen. It was a peculiarly happy one. The
changes visible in the solar surface were then generally re-
garded as no less capricious than the changes in the skies
of our \temperate regions. Consequently, the reckoning and
registering of sun-spots was a task hardly more inviting to
an astronomer than the reckoning and registering of summer
clouds. Cassini, Keill, Lemonnier, Lalande, were unanimous
in declaring that no trace of regularity could be detected in
their appearances or effacements. 2 Delambre pronounced
them " more curious than really useful." 3 Even HerscheJ,
profoundly as he studied them, and intimately as he was con-
vinced of their importance as symptoms of solar activity, saw
no reason to suspect that their abundance and scarcity were
subject to orderly alternation. One man alone in the eighteenth
1 Wolf, Gesch. der Astr., p. 655. 2 Manuel Johnson, Mem. R. A. Soc.,
vol. xxvi. p. 197. 3 Astronomic Theorique et Pratique, t. iii. p. 20.
158 HISTORY OF ASTRONOMY. PART n.
century, Christian Horrebow of Copenhagen, divined their
periodical character, and foresaw the time when the effects of
the sun's vicissitudes upon the globes revolving round hfon
might be investigated with success ; but this prophetic utter-
ance was of the nature of a soliloquy rather than of a communi-
cation, and remained hidden away in an unpublished journal
until 1859, when it was brought to light in a general ransacking
of archives. 1
Indeed, Schwabe himself was far from anticipating the dis-
covery which fell to his share. He compared his fortune to
that of Saul, who, seeking his father's asses, found a kingdom. 2
For the hope which inspired his early resolution lay in quite
another direction. His patient ambush was laid for a possible
intra-mercurian planet, which, he thought, must sooner or later
betray its existence in crossing the face of the sun. He took,
however, the most effectual measures to secure whatever new
knowledge might be accessible. During forty-three years his
" imperturbable telescope " 3 never failed (weather and health
permitting), to bring in its daily report as to how many, or
if any, spots were visible on the sun's disc, the information
obtained being day by day recorded on a simple and unvary-
ing system. In 1843 he made his first announcement of a
probable decennial period, 4 but it met with no general atten-
tion ; although Julius Schmidt of Bonn (afterwards director of
the Athens Observatory) and Gautier of Geneva were im-
pressed with his figures, and Littrow had himself, in iS>^6 } 5
hinted at the likelihood of some kind of regular recurrence.
Schwabe, however, worked on, gathering each year fresh
evidence of a law such as he had indicated ; and when Hum-
boldt published in 1851, in the third volume of his Kosmos, 6
a table of the sun-spot statistics collected by him from 1826
downwards, the strength of his case was perceived with, so to
speak, a start of surprise ; the reality and importance of the
1 Wolf, Ge&ch. der Astr., p. 654. 2 Month. Not., vol. xvii. p. 241.
3 Mem. R. A. Soc., vol. xxvi. p. 200. 4 Astr. Nath., No. 495. 5 Gehler's
Pkysikalisches Worterbuch, art. Sonncnflecken, p. 851. 6 Zweite Abth., p.
401.
CHAP. i. ASTRONOMICAL PHYSICS. 159
discovery were simultaneously recognised, and the persevering
Hofrath of Dessau found himself famous among astronomers.
His merit recognised by the bestowal of the Astronomical
Society's Gold Medal in 1857 consisted in his choice of an
original and appropriate line of work, and in the admirable
tenacity of purpose with which he pursued it. His resources
and acquirements were those of an ordinary amateur ; he was
distinguished solely by the (unfortunately rare) power of turn-
ing both to the best account. He died where he was born
and had lived, April n, 1875, at the ripe age of eighty-six.
Meanwhile an investigation of a totally different character,
and conducted by totally different means, had been prosecuted
to a very similar conclusion. Two years after Schwabe began
his solitary observations, Humboldt gave the first impulse, at
the Scientific Congress of Berlin in 1828, to a great inter-
national movement for attacking simultaneously, in various
parts of the globe, the complex problem of terrestrial magnetism.
Through the genius and energy of Gauss, Gottingen became
its centre. Thence new apparatus and a new system for its
employment issued; there, in 1833, the first regular magnetic
observatory was founded, while Gottingen mean time was made
the universal standard for magnetic observations. The letter
addressed by Humboldt in April 1836 to the Duke of Sussex
as President of the Royal Society, enlisted the co-operation of
England. A network of magnetic stations was spread all over
the British dominions, from Canada to Van Diemen's Land ;
measures were concerted with foreign authorities, and an
expedition was fitted out, under the able command of Captain
(afterwards Sir James) Clark Ross, for the special purpose of
bringing intelligence on the subject from the dismal neigh-
bourhood of the South Pole. In 1841, the elaborate organisa-
tion created by the disinterested efforts of scientific " agitators "
was complete ; Gauss's " magnetometers " were vibrating under
the view of attentive observers in five continents, and simul-
taneous results began to be recorded.
Ten years later, in September 1851, Dr. John Lamont, the
Scotch director of the Munich Observatory, in reviewing the
160 HISTORY OF ASTRONOMY. PART u.
magnetic observations made at Gb'ttingen and Munich from
1835 to 1850, perceived with some surprise that they gave
unmistakable indications of a period which he estimated at
10 J years. 1 The manner in which this periodicity manifested
itself requires a word of explanation. The observations in
question referred to what is called the " declination " of the
magnetic needle that is, to the position assumed by it with
reference to the points of the compass when moving freely
in a horizontal plane. Now this position as was discovered
by Graham in 1722 is subject to a small daily fluctuation,
attaining its maximum towards the east about 8 A.M., and its
maximum towards the west shortly before 2 P.M. In other
words, the direction of the needle approaches (in these coun-
tries at the present time) nearest to the true north some four
hours before noon, and departs farthest from it between one
and two hours after noon. It was the range of this daily
variation that Lament found to increase and diminish once in
every icj years.
In the following winter, Sir Edward Sabine, ignorant as yet
of Lament's conclusion, undertook to examine a totally diffe-
rent set of observations. The materials in his hands had been
collected at the British colonial stations of Toronto and Hobar-
ton from 1843 to 1848, and had reference, not to the regular
diurnal swing of the needle, but to those curious spasmodic
vibrations, the inquiry into the laws of which was the primary
object of the vast organisation set on foot by Humboldt and
Gauss. Yet the upshot was practically the same. Once in
about ten years magnetic disturbances (termed by Humboldt
" storms ") were perceived to reach a maximum of violence and
frequency. Sabine was the first to note the coincidence be-
tween this unlooked-for result and Schwabe's sun-spot period.
He showed that, so far as observation had yet gone, the two
cycles of change agreed perfectly both in duration and phase,
maximum corresponding to maximum, minimun to minimum.
What the nature of the connection could be that bound to-
gether by a common law effects so dissimilar as the rents in
1 Annalen der Physik (Poggendorff's), Bd. Ixxxiv. p. 580.
CHAP. i. ASTRONOMICAL PHYSICS. 161
the luminous garment of the sun, and the swayings to and fro
of the magnetic needle, was and still remains beyond the reach
of well-founded conjecture; but the fact was from the first
undeniable.
The memoir containing this remarkable disclosure was pre-
sented to the Royal Society, March 18, and read May 6, I852. 1
On the 3ist of July following, Rudolf Wolf at Berne, 2 and
on the 1 8th of August Alfred Gautier at Sion, 3 announced,
separately and independently, perfectly similar conclusions.
This triple event is perhaps the most striking instance of the
successful employment of the Baconian method of co-opera-
tion in discovery, by which ''particulars" are amassed by
one set of investigators corresponding to the " Depredators "
and " Inoculators " of Solomon's House while inductions are
drawn from them by another and a higher class the "Inter-
preters of Nature." Yet even here the convergence of two
distinct lines of research was wholly fortuitous, and skilful
combination owed the most brilliant part of its success to the
unsought bounty of what we call Fortune.
The exactness of the coincidence thus brought to light
was fully confirmed by further inquiries. A diligent search
through the scattered records of sun-spot observations, from
the time of Galileo and Scheiner onwards, put Wolf 4 in
possession of materials by which he was enabled to correct
Schwabe's loosely indicated decennial period to one of slightly
over eleven (ii.n) years; and he further showed that this
fell in with the ebb and flow of magnetic change even better
than Lament's loj year cycle. For the first time, also, the
analogy was pointed out between the " light-curve," or zig-
zagged line representing on paper the varying intensity in
the lustre of certain stars, and the similar delineation of
spot-frequency ; the ascent from minimum to maximum
being, in both cases, usually steeper than the descent from
maximun to minimum ; while an additional point of resem-
1 Phil. Trans., vol. cxlii. p. 103. 2 Mittheilungen der Naturforschen-
den Gescllschaft, 1852, p. 183. 3 Archives des Sciences, t. xxi. p. 194.
4 Neue Untcrsuchungen, Mitth. Naturf. Ges., 1852, p. 249.
L
162 HISTORY OF ASTRONOMY. PART n.
blance was furnished by the irregularities in height of the
various maxima. In other words, both the number of spots
on the sun and the brightness of variable stars increase, as a
rule, more rapidly than they decrease ; nor does the amount
of that increase, in either instance, show any approach to
uniformity.
The endeavour, suggested by the very nature of the pheno-
menon, to connect sun-spots with weather was less successful.
The first attempt of the kind was made by Sir William
Herschel in the first year of the present century, and a very
notable one it was. Meteorological statistics, save of the
scantiest and most casual kind, did not then exist ; but the
price of corn from year to year was on record, and this, with
full recognition of its inadequacy, he adopted as his criterion.
Nor was he much better off for information respecting the
solar condition. What little he could obtain, however, served,
as he believed, to confirm his surmise that a copious emission
of light and heat accompanies an abundant formation of
" openings " in the dazzling substance whence our supply of
those indispensable commodities is derived. 1 He gathered,
in short, from his inquiries very much what he had expected
to gather, namely, that the price of wheat was high when the
sun showed an unsullied surface, and that food and spots
became plentiful together. 2
This plausible inference, however, was scarcely borne out
by a more exact collocation of facts. Schwabe failed to detect
any reflection of the sun-spot period in his meteorological
register. Gautier 3 reached a provisional conclusion the
reverse though not markedly the reverse of Herschel's.
Wolf, in 1852, derived from an examination of Yogel's collec-
tion of Ziirich Chronicles (1000-1800 A.D.) evidence showing
(as he thought) that minimum years were usually wet and
stormy, maximum years dry and genial ; 4 but a subsequent
1 Phil. Trans., vol. xci. p. 316. 2 Evidence of an eleven-yearly fluc-
tuation in the price of food-grains in India has lately been alleged by
Mr. Frederick Chambers in Nature, vol. xxxiv. p. 100. 3 BiU. Un. de
Gen&ve, t. li. p. 336. 4 Neue Untersuckungen, p .269.
CHAP. i. ASTRONOMICAL PHYSICS. 163
review of the subject in 1859 convinced him that no relation
of any kind between the two kinds of effects was traceable. 1
With the singular affection of our atmosphere known as the
Aurora Borealis (more properly Aurora Polaris) the case was
different. Here the Ziirich Chronicles set Wolf on the right
track in leading him to associate such luminous manifestations
with a disturbed condition of the sun ; since subsequent
detailed observation has exhibited the curve of auroral fre-
quency as following with such fidelity the jagged lines figuring
to the eye the fluctuations of solar and magnetic activity, as to
leave no reasonable doubt that all three rise and sink together
under the influence of a common cause. As long ago as
1 7 1 6, 2 Halley had conjectured that the Northern Lights were
due to magnetic " effluvia," but there was no evidence on the
subject forthcoming until Hiorter observed at Upsala in 1741
their agitating influence upon the magnetic needle. That the
effect was no casual one was made superabundantly clear by
Arago's researches in 1819 and subsequent years. Now both
were perceived to be swayed by the same obscure power of
cosmical disturbance.
The sun is not the only one of the heavenly bodies by which
the magnetism of the earth is affected. Proofs of a similar
kind of lunar action were laid by Kreil in 1841 before the
Bohemian Society of Sciences, and were fully substantiated,
though with minor corrections, by Sabine's more extended
researches. It has thus been ascertained that each lunar day,
or the interval of twenty-four hours and about fifty-four
minutes between two successive meridian passages of our
satellite, is marked by a perceptible, though very small double
oscillation of the needle two progressive movements from
east to west, and two returns from west to east. 3 Moreover,
the lunar, like the solar influence (as was proved in each case
1 Die Sonne und ihre Flecken, p. 30. Arago was the first who attempted
to decide the question by keeping, through a series of years, a parallel
register of sun-spots and weather ; but the data regarding the solar con-
dition collected at the Paris Observatory from 1822 to 1830 were not
sufficiently precise to found any inference upon. 2 Phil. Trans., vol.
xxix. p. 421. 3 Ibid. } vols. cxliii. p. 558, cxlvi. p. 505.
1 64 HISTORY OF ASTRONOMY. PART n.
by Sabine's analysis of the Hobarton and Toronto observa-
tions), extends to all three " magnetic elements," affecting
not only the position of the horizontal or declination needle,
but also the dip and intensity. It seems not unreasonable to
attribute some portion of the same subtle power to the planets,
and even to the stars, though with effects rendered impercep-
tible by distance.
We have now to speak of the discovery and application to
the heavenly bodies of a totally new method of investigation.
Spectrum analysis may be shortly described as a mode of dis-
tinguishing the various species of matter by the kind of light
proceeding from each. This definition at once explains how
it is that, unlike every other system of chemical analysis, it
has proved available in astronomy. Light, so far as quality
is concerned, ignores distance. No intrinsic change (that we
yet know of) is produced in it by a journey from the farthest
bounds of the visible universe ; so that, provided only that in
quantity it remain sufficient for the purpose, its peculiarities
can be equally well studied whether the source of its vibrations
be one foot or a hundred billion miles distant. Now the most
obvious distinction between one kind of light and another
resides in colour. But of this distinction the eye takes cog-
nisance in an aesthetic, not in a scientific sense. It finds
gladness in the " thousand tints " of nature, but can neither
analyse nor define them. Here the refracting prism or the
combination of prisms known as the "spectroscope" comes
to its aid, teaching it to measure as well as to perceive. It
furnishes, in a word, an accurate scale of colour. The various
rays which, entering the eye together in a confused crowd,
produce a compound impression made up of undistinguishable
elements, are, by the mere passage through a triangular piece
of glass, separated one from the other, and ranged side by side
in orderly succession, so that it becomes possible to tell at
a glance what kinds of light are present, and what absent.
Thus, if we could only be assured that the various chemical
substances, when made to glow by heat, emit characteristic
rays rays, that is, occupying a place in the spectrum reserved
CHAP. i. ASTRONOMICAL PHYSICS. 165
for them, and for them only we should at once be in posses-
sion of a mode of identifying such substances with the utmost
readiness and certainty. This assurance, which forms the
solid basis of spectrum -analysis, was obtained slowly and with
difficulty.
The first to employ the prism in the examination of various
flames (for it is only in a state of vapour that matter emits
distinctive light) was a young Scotchman named Thomas
Melvill, who died in 1753, at the age of twenty-seven. He
studied the spectrum of burning spirits, into which were
introduced successively sal ammoniac, potash, alum, nitre, and
sea-salt, and observed the singular predominance, under almost
all circumstances, of a particular shade of yellow light, per-
fectly definite in its degree of refrangibility * in other words,
taking up a perfectly definite position in the spectrum. His
experiments were repeated by Morgan, 2 Wollaston, and with
far superior precision and diligence by Fraunhofer. 3 The
great Munich optician, whose work was completely original,
rediscovered Melvill's deep yellow ray and measured its place
in the colour-scale. It has since become well known as the
"sodium line," and has played a very important part in the
history of spectrum analysis. Nevertheless, its ubiquity and
conspicuousness long impeded progress. It was elicited by
the combustion of a surprising variety of substances sulphur,
alcohol, ivory, wood, paper ; its persistent visibility suggesting
the accomplishment of some universal process of nature rather
than the presence of one individual kind of matter. But if
spectrum analysis were to exist as a science at all, it could
only be by attaining certainty as to the unvarying association
of one special substance with each special quality of light.
Thus perplexed, Fox Talbot 4 hesitated in 1826 to enounce
this fundamental principle. He was inclined to believe that
the presence in the spectrum of any individual ray told
1 Observations on Light and Colours, p. 35. 2 Phil. Trans., vol. Ixxv.
p. 190. 3 DenJcscfiriften (Munich Ac. of Sc.), 1814-15, Bd. v. p. 197.
4 Edinburgh Journal of Science, vol. v. p. 77. See also Phil. Mag., Feb.
1834, vol. iv. p. 112.
1 66 HISTORY OF ASTRONOMY. PART n.
unerringly of the volatilisation in the flame under scrutiny
of some body as whose badge or distinctive symbol that ray
might be regarded ; but the continual prominence of the yellow
beam staggered him. It appeared, indeed, without fail where
sodium ivas ; but it also appeared where it might be thought
only reasonable to conclude that sodium was not. Nor was it
until thirty years later that William Swan, 1 by pointing out
the extreme delicacy of the spectral test, and the singularly
wide dispersion of sodium, made it appear probable (but even
then only probable) that the questionable yellow line was
really due invariably to that substance. Common salt (chlo-
ride of sodium) is, in fact, the most diffusive of solids. It
floats in the air ; it flows with water ; every grain of dust has
its attendant particle ; its absolute exclusion approaches the
impossible. And withal, the light that it gives in burning is
so intense and concentrated, that if a single grain be divided
into 1 80 million parts, and one alone of such inconceivably
minute fragments be present in a source of light, the spectro-
scope will show unmistakably its characteristic beam.
Amongst the pioneers of knowledge in this direction were
Sir John Herschel 2 who, however, applied himself to the
subject in the interests of optics, not of chemistry W. A.
Miller, 3 and Wheatstone. The last especially made a notable
advance when, in the course of his studies on the " prismatic
decomposition " of the electric light, he reached the significant
conclusion that the rays visible in its spectrum were different
for each kind of metal employed as "electrodes." 4 Thus
indications of a wider principle were to be found in several
quarters, but no positive certainty on any single point was
obtained, until, in 1859, Gustav Kirchhoff, professor of physics
in the University of Heidelberg, and his colleague, the emi-
1 Ed. Phil. Trans., vol. xxi. p. 411. 2 On the Absorption of Light ly
Coloured Media, Ed. Phil. Trans., vol. ix/p. 445 (1823). 3 Phil. Mag.
vol. xxvii. (ser. iii.), p. 81. 4 Report Brit. Ass., 1835, p. II (pt. ii.).
Electrodes are the terminals from one to the other of which the electric spark
passes, volatilising and rendering incandescent in its transit some particles
of their substance, the characteristic light of which accordingly flashes out
in the spectrum.
CHAP. i. ASTRONOMICAL PHYSICS. 167
nent- chemist Robert Bunsen, took the matter in hand. By
them the general question as to the necessary and invariable
connection of certain rays in the spectrum with certain kinds
of matter, was first resolutely confronted, and first definitively
answered. It was answered affirmatively else there could
have been no science of spectrum analysis as the result of
experiments more numerous, more stringent, and more precise
than had previously been undertaken. 1 And the assurance of
their conclusion was rendered doubly sure by the discovery,
through the peculiarities of their light alone, of two new
metals, named, from the blue and red rays by which they
were respectively distinguished, "Caesium," and "Rubidium." 2
Both were immediately afterwards actually obtained in small
quantities by evaporation of the Durckheim mineral waters.
The link connecting this important result with astronomy
may now be indicated. In the year 1802 it occurred to
William Hyde Wollaston to substitute for the round hole
used by Newton and his successors for the admittance of light
to be examined with the prism, an elongated " crevice " ^th
of an inch in width. He thereupon perceived that the
spectrum, thus formed of light, as it were, purified by the
abolition of overlapping images, was traversed by seven dark
lines. These he took to be natural boundaries of the various
colours, 3 and, satisfied with this quasi-explanation, allowed
the subject to drop. It was independently taken up after
twelve years by a man of higher genius. In the course of
experiments on light, directed towards the perfecting of his
achromatic lenses, Fraunhofer, by means of a slit and a tele-
scope, made the surprising discovery that the solar spectrum
is crossed, not by seven, but by thousands of obscure trans-
verse streaks. 4 Of these he counted some 600, and carefully
mapped 324 ; while a few of the most conspicuous he set up
(if we may be permitted the expression) as landmarks, mea-
suring their distances apart with a theodolite, and affixing
to them the letters of the alphabet by which they are still
1 Phil. Mag., vol. xx. p. 93. 2 Annalen der Physik, Bd. cxiii. p. 357.
3 Phil. Trans., vol. xcii. p. 378. 4 Dcnkschriften, Bd. v. p. 202.
168 HISTORY OF ASTRONOMY. PART n.
universally known. Nor did he stop here. The same system
of examination applied to the rest of the heavenly bodies
showed the mild effulgence of the moon and planets to be
deficient in precisely the same rays as sunlight ; while in the
stars it disclosed the differences in likeness which are always
an earnest of increased knowledge. The spectra of Sirius and
Castor, instead of being delicately ruled crosswise throughout,
like that of the sun, were seen to be interrupted by three
massive bars of darkness two in the blue and one in the
green ; l the light of Pollux, on the other hand, seemed pre-
cisely similar to sunlight attenuated by distance or reflection,
and that of Capella, Betelgeux, and Procyon to share some
of its peculiarities. One solar line especially that marked
in his map with the letter D proved common to all the four
last-mentioned stars ; and it was remarkable that it exactly
coincided in position with the conspicuous yellow beam (after-
wards, as we have said, identified with the light of glowing
sodium) which he had already found to accompany most kinds
of combustion. Moreover, both the dark solar and the bright
terrestrial " D lines " were displayed by the refined Munich
appliances as double.
In this striking correspondence, discovered by Fraunhofer
in 1815, was contained the very essence of solar chemistry;
but its true significance did not become apparent until long
afterwards. Fraunhofer was by profession, not a physicist,
but a practical optician. Time pressed; he could not and
would not deviate from his appointed track; all that was
possible to him was to indicate the road to discovery, and
exhort others to follow it. 2
Partially and inconclusively at first this was done. The
" fixed lines " (as they were called) of the solar spectrum took
up the position of a standing problem, to the solution of
which no approach seemed possible. Conjectures as to their
origin were indeed rife. An explanation put forward by
Zantedeschi 3 and others, and dubiously favoured by Sir
1 DenJcschriften, Bd. v. p. 220 ; Edin. Jour, of Science, vol. viii. p. 9.
- DenJcschriften, Bd. v. p. 222. 3 Arch, des Sciences, 1849, p. 43.
CHAP. i. ASTRONOMICAL PHYSICS. 169
David Brewster and Dr. J. H. Gladstone, 1 was that they
resulted from " interference " that is, a destruction of the
motion producing in our eyes the sensation of light, by the
superposition of two light-waves in such a manner that the
crests of one exactly fill up the hollows of the other. This
effect was supposed to be brought about by imperfections in
the optical apparatus employed.
A more plausible view was that the atmosphere of the earth
was the agent by which sunlight was deprived of its missing
beams. For a few of them this is actually the case. Brewster
found in 1832 that certain dark lines, which were invisible
when the sun stood high in the heavens, became increasingly
conspicuous as he approached the horizon. 2 These are the
well-known " atmospheric lines ; " but the immense majority
of their companions in the spectrum remain quite unaffected
by the thickness of the stratum of air traversed by the sun-
light containing them. They are then obviously due to another
cause.
There remained the true interpretation absorption in the
sun's atmosphere; and this, too, was extensively canvassed.
But a remarkable observation made by Professor Forbes of
Edinburgh 3 on the occasion of the annular eclipse of May 15,
1836, appeared to throw discredit upon it. If the problematical
dark lines were really occasioned by the stoppage of certain
rays through the action of a vaporous envelope surrounding
the sun, they ought, it seemed, to be strongest in light pro-
ceeding from his edges, which, cutting that envelope obliquely,
passed through a much greater depth of it. But the circle
of light left by the interposing moon, and of course derived
entirely from the rim of the solar disc, yielded to Forbes's
examination precisely the same spectrum as light coming
from its central parts. This circumstance helped to baffle
1 Phil. Trans., vol. cl. p. 159, note. 2 Ed. Phil. Trans., vol. xii. p. 528,
3 Phil. Trans., vol. cxxvi. p. 453. " I conceive," he says, " that this result
proves decisively that the sun's atmosphere has nothing to do with the
production of this singular phenomenon " (p. 455). And Brewster's \vell-
foxmded opinion that it had much to do with it was thereby, in fact, over-
thrown.
170 HISTORY OF ASTRONOMY. PART n.
inquirers, already sufficiently perplexed. It still remains an
anomaly, of which no completely satisfactory explanation has
been offered.
Convincing evidence as to the true nature of the solar lines
was however at length, in the autumn of 1859, brought for-
ward at Heidelberg. Kirchhoff' s experimentum crucis in the
matter was a very simple one. He threw bright sunshine
across a space occupied by vapour of sodium, and perceived
with astonishment that the dark Fraunhofer line D, instead
of being effaced by flame giving a luminous ray of the same
refrangibility, was deepened and thickened by the superposition.
He tried the same experiment, substituting for sunbeams
light from a Drummond lamp, and with similar result. A
dark furrow, corresponding in every respect to the solar D
line, was instantly seen to interrupt the otherwise unbroken
radiance of its spectrum. The inference was irresistible, that
the effect thus produced artificially was brought about naturally
in the same way, and that sodium formed an ingredient in the
glowing atmosphere of the sun. 1 This first discovery was
quickly followed up by the identification of numerous bright
rays in the spectra of other metallic bodies with others of the
hitherto mysterious Fraunhofer lines. Kirchhoff was thus
led to the conclusion that (besides sodium) iron, magnesium,
calcium, and chromium, are certainly solar constituents,
and that copper, zinc, barium, and nickel are also present,
though in smaller quantities. 2 As to cobalt, he hesitated
to pronounce, but its existence in the sun has since been
established.
These memorable results were founded upon a general prin-
ciple first enunciated by Kirchhoff in a communication to the
Berlin Academy, December 15, 1859, and afterwards more
fully developed by him. 3 It may be expressed as follows :
1 Monatsberichte, Berlin, 1859, p. 664. 2 Alhandlungen, Berlin, 1861,
pp. 80, 81. 3 Ibid., 1861, p. 77 ; Annalen der Physik, Bd. cxix. p. 275. A
similar conclusion, reached by Balfour Stewart in 1858 for heat-rays (Ed.
Phil. Trans., vol. xxii. p. 13), was, in 1860, without previous knowledge of
Kirchhoff 's work, extended to light (Phil. May., vol. xx. p. 534) ; but his
experiments wanted the precision of those executed at Heidelberg.
CHAP. i. ASTRONOMICAL PHYSICS. 171
Substances of every kind are opaque to the precise rays which
they emit at the same temperature ; that is to say, they stop
the kinds of light or heat which they are then actually in a
condition to radiate. But it does not follow that cool bodies
absorb the rays which they would give out if sufficiently heated.
Hydrogen at ordinary temperatures, for instance, is almost
perfectly transparent, but if raised to the glowing point as
by the passage of electricity it then becomes capable of arrest-
ing, and at the same time of displaying in its own spectrum
light of four distinct colours.
This principle is fundamental to solar chemistry. It gives the
key to the hieroglyphics of the Fraunhofer lines. The identical
characters which are written bright in terrestrial spectra are
written darJc in the unrolled sheaf of sun-rays ; the meaning
remains unchanged. It must, however, be remembered that
they are only relatively dark. The substances stopping those
particular tints in the neighbourhood of the sun are at the
same time vividly glowing with the very same. Remove the
dazzling solar background, by contrast with which they show
as obscure, and they will be seen, and have, under certain
circumstances, actually been seen, in all their native splendour.
It is because the atmosphere of the sun is cooler than the globe
it envelops that the different kinds of vapour constituting
that atmosphere take more than they give, absorb more light
than they are capable of emitting ; raise them to the same
temperature as the sun itself, and their powers of emission
and absorption being brought exactly to the same level, the
thousands of dusky rays in the solar spectrum will be at once
obliterated.
The establishment of the terrestrial science of spectrum
analysis was due, as we have seen, equally to Kirchhoff and
Bunsen, but its celestial application to Kirchhoff alone. He
effected this object of the aspirations, more or less dim, of
many other thinkers and workers, by the union of two separate
though closely related lines of research the study of the
different kinds of light emitted by various bodies, and the
study of the different kinds of light absorbed by them. The
i;2 HISTORY OP ASTRONOMY. PART n.
latter branch appears to have been first entered upon by Dr.
Thomas Young in I8O3J 1 it was pursued by the younger
Herschel, 2 by William Allen Miller, Brewster, and Gladstone.
Brewster indeed made, in i833, 3 a formal attempt to found
what might be called an inverse system of analysis with the
prism based upon absorption ; and his efforts were repeated,
just a quarter of a century later, by Gladstone. 4 But no
general point of view was attained ; nor, it may be added, was
it by this path attainable.
Kirchhoff's map of the solar spectrum, drawn to scale with
exquisite accuracy, and printed in three shades of ink to
convey the graduated obscurity of the lines, was published in
the Transactions of the Berlin Academy for 1861 and i862. 5
Representations of the principal lines belonging to various
elementary bodies formed, as it were, a series of marginal
notes accompanying the great solar scroll, and enabling the
veriest tiro in the new science to decipher its meaning at a
glance. Where the dark solar and bright metallic rays agreed
in position, it might safely be inferred that the metal emitting
them was a solar constituent ; and such coincidences were
numerous. In the case of iron alone, no less than sixty
occurred in one-half of the spectral area, rendering the chances 6
absolutely overwhelming against mere casual conjunction.
The preparation of this elaborate picture proved so trying to
the eyes that Kirchhoff was compelled by failing vision to
resign the latter half of the task to his pupil Hofmann.
The complete map measured nearly eight feet in length.
The conclusions reached by Kirchhoff were no sooner
announced than they took their place, with scarcely a dis-
senting voice, among the established truths of science. The
broad result, that the dark lines in the spectrum of the sun
afford an index to its chemical composition no less reliable
than any of the tests used in the laboratory, was equally
1 Miscellaneous Works, vol. i. p. 189. 2 Ed. Phil. Trans., vol. ix. p. 458.
3 Ibid., vol. xii. p. 519. 4 Quart. Jour. Chem. Soc., vol. x. p. 79. 5 A
facsimile accompanied Sir H. Roscoe's translation of Kirchhoff's " He-
searches on the Solar Spectrum" (London, 1862-63). 6 Estimated by
Kirchhoff at a trillion to one. AbhandL, 1861, p. 79.
CHAP. i. ASTRONOMICAL PHYSICS. 173
captivating to the imagination of the vulgar, and authentic
in the judgment of the learned ; and, like all genuine advances
in the knowledge of Nature, it stimulated curiosity far more
than it gratified it. Now the history of how discoveries were
missed is often quite as instructive as the history of how
they were made ; it may then be worth while to expend a few
words on the thoughts and trials by which, in the present
case, the actual event was heralded.
Three times it seemed on the verge of being anticipated.
The experiment, which in Kirchhoff's hands proved decisive,
of passing sunlight through glowing vapours and examining
the superposed spectra, was performed by Professor W. A.
Miller of King's College in I845. 1 Nay, more, it was per-
formed with express reference to the question, then already
(as has been noted) in debate, of the possible production of
Fraunhofer's lines by absorption in a solar atmosphere. Yet
it led to nothing.
Again, at Paris in 1849, with a view to testing the asserted
coincidence between the solar D line and the bright yellow
beam in the spectrum of the electric arc (really due to the
unsuspected presence of sodium), Leon Foucault threw a ray
of sunshine across the arc and observed its spectrum. 2 He was
surprised to see that the D line was rendered more intensely
dark by the combination of lights. To assure himself still
further, he substituted a reflected image of one of the white-
hot carbon-points for the sunbeam, with an identical result.
The same ray was missing. It needed but another step to
have generalised this result, and thus laid hold of a natural
truth of the highest importance j but that step was not taken.
Foucault, keen and brilliant though he was, rested satisfied
with the information that the voltaic arc had the power of
stopping the kind of light emitted by it ; he asked no further
question, and was consequently the bearer of no further intel-
ligence on the subject.
The truth conveyed by this remarkable experiment was,
1 Phil Mag., vol. xxvii. (3d series), p. 90. 2 L'Institut, Feb. 7, 1849,
p. 45 ; Phil. Mag., vol. xix. (4th series), p. 193.
174 HISTORY OF ASTRONOMY. PART n.
however, divined by one eminent man. Professor Stokes of
Cambridge stated to Sir William Thomson, shortly after it
had been made, his conviction that an absorbing atmosphere
of sodium surrounded the sun. And so forcibly was his
hearer impressed with the weight of the arguments based
upon the absolute agreement of the D line in the solar
spectrum with the yellow ray of burning sodium (then freshly
certified by "W. H. Miller), combined with Foucault's " rever-
sal " of that ray, that he regularly inculcated, in his pub-
lic lectures on natural philosophy at Glasgow, five or six
years before Kirchhoff's discovery, not only the fact of the
presence of sodium in the solar neighbourhood, but also the
principle of the study of solar and stellar chemistry in the
spectra of flames. 1 Yet it does not appear to have occurred
to either of these two distinguished professors themselves
amongst the foremost of their time in the successful search
for new truths to verify practically a sagacious conjecture in
which was contained the possibility of a scientific revolution.
It is just to add, that Kirchhoff was unacquainted, when he
undertook his investigation, either with the experiment of
Foucault or the speculation of Stokes.
For C. J. Angstrom, on the other hand, perhaps somewhat
too much has been claimed in the way of anticipation. His
Optical Researches appeared at Upsala in 1853, and in their
English garb two years later. 2 They were undoubtedly preg-
nant with suggestion, yet made no epoch in discovery. The
old perplexities continued to prevail after, as before their
publication. To Angstrom, indeed, belongs the great merit
of having revived Euler's principle of the equivalence of
emission and absorption; but he revived it in its original
crude form, and without the qualifying proviso which alone
gave it value as a clue to new truths. According to his state-
ment, a body absorbs all the series of vibrations it is, under
any circumstances, capable of emitting, as well as those con-
nected with them by simple harmonic relations. This is far
1 Ann. d. Phys,, vol. cxviii. p. HO. 2 Phil. Mag., vol. ix. (4th series),
P- 327-
CHAP. i. ASTRONOMICAL PHYSICS. 175
too wide. To render it either true or useful, it had to be
reduced to the cautious terms employed by Kirchhoff. Radia-
tion strictly and necessarily corresponds with absorption only
when the temperature is the same. In point of fact, Angstrom,
though convinced that their explanation embraced that of the
luminous lines in the spectrum of the electric arc, was still,
in 1853, divided between absorption and interference as the
mode of origin of the Fraunhofer dark rays. Very important,
however, was his demonstration of the compound nature of
the spark-spectrum, which he showed to be made up of the
spectrum of the metallic electrodes superposed upon that of
the gas or gases across which the discharge passed.
It may here be useful since without some clear ideas on
the subject no proper understanding of recent astronomical
progress is possible to take a cursory view of the elementary
principles of spectrum analysis. To many of our readers they
are doubtless already familiar ; but it is better to Jbe trite in
repetition than obscure from lack of explanation.
The spectrum, then, of a body is simply the light proceeding
from it spread out by refraction l into a brilliant variegated
band, passing from brownish-red through crimson, orange,
yellow, green, and azure into dusky violet. The reason of
this spreading-out or " dispersion " is that the various colours
have different wave-lengths, and consequently meet with
different degrees of retardation in traversing the denser
medium of the prism. The shortest and quickest vibrations
(producing the sensation we call " violet ") are thrown farthest
away from their original path in other words, suffer the
widest "deviation;" the longest and slowest (the red)
travel much nearer to it. Thus the sheaf of rays which
would otherwise combine into a patch of white light are
separated through the divergence of their tracks after refrac-
tion by a prism, so as to form a tinted riband. This visible
spectrum is prolonged invisibly at both ends by a long range
of vibrations, either too rapid or too sluggish to affect the eye
1 Spectra may be produced by diffraction as well as by refraction; but
we are here only concerned with the matter in its simplest aspect.
176 HISTORY OF ASTRONOMY. PART n.
as light, but recognisable through their chemical and heating
effects.
Now all incandescent solid or liquid substances, and even
gases ignited under great pressure, give what is called a " con-
tinuous spectrum ; " that is to say, the light derived from them
is of every imaginable hue. Sorted out with the prism, its
tints merge imperceptibly one into the other, uninterrupted
by any dark spaces. No colours, in short, are missing. But
gases and vapours rendered luminous by heat emit rays of
only a few tints sometimes of one only which accordingly
form an interrupted spectrum, usually designated as one of
lines or bands. And since these rays are perfectly definite
and characteristic not being the same for any two substances
it is easy to tell what kind of matter is concerned in pro-
ducing them. We may suppose that the inconceivably
minute particles which by their rapid thrillings agitate the
ethereal medium so as to produce light, are free to give out
their peculiar tone of vibration only when floating apart from
each other in gaseous form ; but when crowded together into
a condensed mass, the clear ring of the distinctive note is
drowned, so to speak, in an universal molecular clang. Thus
prismatic analysis has no power to identify individual kinds
of matter except when they present themselves as glowing
vapours.
A spectrum is said to be "reversed" when lines previously
seen bright on a dark background appear dark on a bright
background. In this form it is equally characteristic of
chemical composition with the "direct" spectrum, being due
to absorption, as the latter is to emission. And absorption and
emission are, by Kirchhoff's law, strictly correlative. This is
easily understood by the analogy of sound. For just as a
tuning-fork responds to sound-waves of its own pitch, but
remains indifferent to those of any other, so those particles
of matter whose nature it is, when set swinging by heat, to
vibrate a certain number of times in a second, thus giving
rise to light of a particular shade of colour, appropriate those
same vibrations, and those only, when transmitted past them,
CHAP. i. ASTRONOMICAL PHYSICS. 177
or, phrasing it otherwise, are opaque to them, and trans-
parent to all others.
It should further be explained that the shape of the bright
or dark spaces in the spectrum has nothing whatever to
do with the nature of the phenomena. The " lines " and
"bands" so frequently spoken of are seen as such for no
other reason than because the light forming them is admitted
through a narrow, straight opening. Change that opening
into a fine crescent or a sinuous curve, and the "lines" will
at once appear as crescents or curves.
Resuming in a sentence what has been already explained,
we find that the prismatic analysis of the heavenly bodies was
founded upon three classes of facts : First, the unmistakable
character of the light given by each different kind of glowing
vapour ; secondly, the identity of the light absorbed with the
light emitted by each ; thirdly, the coincidences observed be-
tween rays missing from the solar spectrum and rays absorbed
by various terrestrial substances. Thus, a realm of knowledge,
pronounced by Morinus l in the seventeenth century, and no
less dogmatically by Auguste Comte 2 in the nineteenth, hope-
lessly out of reach of the human intellect, was thrown freely
open, and the chemistry of the sun and stars took its place
amongst the foremost of the experimental sciences.
The immediate increase of knowledge was not the chief
result of Kirchhoff's labours; still more important was the
change in the scope and methods of astronomy, which, set on
foot in 1852 by the detection of a common period affecting at
once the spots on the sun and the magnetism of the earth, was
extended and accelerated by the discovery of spectrum analysis.
The nature of that change is concisely indicated by the head-
ing of the present chapter ; we would now ask our readers to
endeavour to realise somewhat distinctly what is implied by
the "foundation of astronomical physics."
Some two centuries and fourscore years ago, Kepler drew
a forecast of what he called a " physical astronomy " a science
1 Astrologia Gallica (1661), p. 189. 2 Pos. Phil, vol. i. pp. 114-115
(Martineau's trans.)
M
i;8 HISTORY OF ASTRONOMY. PART n.
treating of the efficient causes of planetary motion, and holding
the "key to the inner astronomy." 1 What Kepler dreamed
of and groped after, Newton realised. He showed the beau-
tiful and symmetrical revolutions of the solar system to be
governed by a uniformly acting cause, and that cause no other
than the familiar force of gravity, which gives stability to all
our terrestrial surroundings. The world under our feet was
thus for the first time brought into physical connection with
the worlds peopling space, and a very tangible relationship
was demonstrated as existing between what used to be called
the " corruptible " matter of the earth and the " incorruptible "
matter of the heavens.
This process of unification of the cosmos this levelling of
the celestial with the sublunary was carried no farther until
the fact unexpectedly emerged from a vast and complicated
mass of observations, that the magnetism of the earth is
subject to subtle influences, emanating, certainly from some,
and presumably (were their amount sufficient to be percep-
tible) from all of the heavenly bodies; the inference being
thus rendered at least plausible, that a force not less universal
than gravity itself, but with whose modes of action we are as
yet unacquainted, pervades the universe, and forms, it might
be said, an intangible bond of sympathy between its parts.
Now for the investigation of this influence two roads are open.
It may be pursued by observation either of the bodies from
which it emanates, or of the effects which it produces that
is to say, either by the astronomer or by the physicist, or,
better still, by both concurrently. Their acquisitions are
mutually profitable ; nor can either be considered as indepen-
dent of the other. Any important accession to knowledge
respecting the sun, for example, may be expected to cast a
reflected light on the still obscure subject of terrestrial mag-
netism ; while discoveries in magnetism or its alter ego electri-
city must profoundly affect solar inquiries.
The establishment of the new method of spectrum analysis
drew far closer this alliance between celestial and terrestrial
1 Proem. Astronomies Pars Optica (1604), Op., t. ii.
CHAP. r. ASTRONOMICAL PHYSICS. 179
science. Indeed, they have come to merge so intimately one
into the other, that it is no easier to trace their respective
boundaries than it is to draw a clear dividing-line between the
animal and vegetable kingdoms. Yet up to the middle of the
present century, astronomy, while maintaining her strict union
with mathematics, looked with indifference on the rest of the
sciences ; it was enough that she possessed the telescope and
the calculus. Now the materials for her inductions are sup-
plied by the chemist, the electrician, the inquirer into the most
recondite mysteries of light and the molecular constitution
of matter. She is concerned with what the geologist, the
meteorologist, even the biologist, has to say ; she can afford
to close her ears to no new truth of the physical order. Her
position of lofty isolation has been exchanged for one of com-
munity and mutual aid. The astronomer has become, in the
highest sense of the term, a physicist ; while the physicist is
bound to be something of an astronomer.
This, then, is what is designed to be conveyed by the
" foundation of astronomical or cosmical physics." It means
the establishment of a science of Nature whose conclusions are
not only presumed by analogy, but are ascertained by obser-
vation, to be valid wherever light can travel and gravity is
obeyed a science by which the nature of the stars can be
studied upon the earth, and the nature of the earth can be
better known by study of the stars a science, in a word,
which is, or aims at being, one and universal, even as Nature
the visible reflection of the invisible highest Unity is one
and universal.
It is not too much to say that a new birth of knowledge has
ensued. The astronomy so signally promoted by Bessel l the
astronomy placed by Comte 2 at the head of the hierarchy of
the physical sciences was the science of the movements of the
heavenly bodies. And there were those who began to regard
it as a science which, from its very perfection, had ceased to
be interesting whose tale of discoveries was told, and whose
further advance must be in the line of minute technical
1 Pop. VorL, pp. 14, 19, 408. 2 Pos. Phil., p. 115.
i.8o HISTORY OF ASTRONOMY. PART n.
improvements, not of novel and stirring disclosures. But
the science of the nature of the heavenly bodies is one only
in the beginning of its career. It is full of the audacities,
the inconsistencies, the imperfections, the possibilities of
youth. It promises everything; it has already performed
much : it will doubtless perform much more. The means
at its disposal are vast, and are being daily augmented. What
has so far been secured by them it must now be our task to
extricate from more doubtful surroundings and place in due
order before our readers.
CHAPTER II.
SOLAR OBSERVATIONS AND THEORIES.
THE zeal with which solar studies have been pursued during
the last quarter of a century has already gone far to redeem
the neglect of the two preceding ones. Since Schwabe's
discovery was published in 1851, observers have multiplied,
new facts have been rapidly accumulated, and the previous
comparative quiescence of thought on the great subject of the
constitution of the sun, has been replaced by a bewildering
variety of speculations, conjectures, and more or less justifiable
inferences. It is satisfactory to find this novel impulse not
only shared, but to a large extent guided, by our country-
men.
William Rutter Dawes, one of many clergymen eminent in
astronomy, observed, in 1852, with the help of a solar eye-
piece of his own devising, some curious details of spot-
structure. 1 The umbra heretofore taken for the darkest part
of the spot was seen to be suffused with a mottled, nebulous
illumination, in marked contrast with the striated appearance
of the penumbra; while through this "cloudy stratum" a
" black opening " permitted the eye to divine further unfathom-
able depths beyond. The hole thus disclosed evidently the
true nucleus was found to be present in all considerable, as
well as in many small maculae.
Again, the whirling motions of some of these objects were
noticed by him. The remarkable form of one sketched at
Wateringbury, in Kent, January 17, 1852, gave him the means
1 Mem. R. A. S., vol. xxi. p. 157.
182 HISTORY OF ASTRONOMY. PART n.
of detecting and measuring a rotatory movement of the whole
spot round the black nucleus at the rate of 100 degrees in
six days. "It appeared," he said, "as if some prodigious
ascending force of a whirlwind character, in bursting through
the cloudy stratum and the two higher and luminous strata,
had given to the whole a movement resembling its own." :L An
interpretation founded, as is easily seen, on the Herscheliaii
theory, then still in full credit.
An instance of the same kind was observed by the late
Mr. W. R. Birt in i86o, 2 and cyclonic movements are now a
recognised feature of sun-spots. They are, however, as Father
Secchi 3 concluded from his long experience, but temporary
and casual. Scarcely three per cent, of all spots visible exhibit
the spiral structure which should invariably result if a conflict
of opposing, or the friction of unequal, currents were essential,
and not merely incidental to their origin. A whirlpool phase
not unfrequently accompanies their formation, and may be
renewed at periods of recrudescence or dissolution ; but it is
both partial and inconstant, sometimes affecting only one side
of a spot, sometimes slackening gradually its movement in one
direction, to resume it, after a brief pause, in the opposite.
Persistent and uniform motions, such as the analogy of terres-
trial storms would absolutely require, are not to be found.
So that the " cyclonic theory " of sun-spots, suggested by
Herschel in 1847^ and urged, from a different point of view,
by Faye in 1872, may be said to have completely broken
down.
The drift of spots over the sun's surface was first systemati-
cally investigated by Carrington. Before narrating what he
did, it is worth while to pause for a moment to consider who
he was. Nor will it take long to tell. Richard Christopher
Carrington was a self-constituted astronomer, with the will
and the courage and the instinct of thoughtful labour in
him.
Born at Chelsea in May 1826, he entered Trinity College,
1 Mem. R. A. S., vol. xxi. p. 160. 2 Month. Not., vol. xxi. p. 144.
3 Le Soldi, t. i. pp. 87-90 (2d ed. 1875). 4 See ante, p. 72.
CHAP. ir. SOLAR THEORIES. 183
Cambridge, in 1844. He was intended for the Church, but
Professor Challis's lectures diverted him to astronomy, and he
resolved, as soon as he had taken his degree, to prepare, with
all possible diligence, to follow his new vocation. His father,
who was a brewer on a large scale at Brentford, offered no
opposition j ample means were at his disposal ; nevertheless,
he chose to serve an apprenticeship of three years as observer in
the University of Durham, as though his sole object had been to
earn a livelihood. He quitted the post only when he found
that its restricted opportunities offered no further prospect of
self-improvement.
He now built an observatory of his own at Redhill in Surrey,
with the design of completing Bessel's and Argelander's survey
of the northern heavens by adding to it the circumpolar stars
omitted from their view. This project, successfully carried
out between 1854 and 1857, had another and still larger one
superposed upon it before it had even begun to be executed.
In 1852, while the Redhill Observatory was in course of erec-
tion, the discovery of the coincidence between the sun-spot and
magnetic periods was announced. Carrington was profoundly
interested, and devoted his enforced leisure to the examination
of records, both written and depicted, of past solar obser-
vations. Struck with their fragmentary and inconsistent
character, he resolved to " appropriate," as he said, by " close
and methodical research," the eleven-year period next ensuing. 1
He calculated rightly that he should have the field pretty
nearly to himself ; for many reasons conspire to make public
observatories slow in taking up new subjects, and amateurs
with freedom to choose, and means to treat them effectually,
were even scarcer then than they are now.
The execution of this laborious task was commenced Nov-
ember 9, 1853. It was intended to be merely a par ergon a
" second subject," upon which daylight energies might be
spent, while the hours of night were reserved for cataloguing
those stars that " are bereft of the baths of ocean." Its
results, however, proved of the highest interest, although the
1 Observations at Redhill (1863), Introduction.
184 HISTORY OF ASTRONOMY. PART n.
vicissitudes of life barred the completion, in its full integrity,
of the original design. By the death, in 1858, of the elder
Carrington, the charge of the brewery devolved upon his son ;
and eventually absorbed so much of his care that it was found
advisable to bring the solar observations to a premature close,
March 24, 1861.
His scientific life may be said to have closed with them.
Attacked four years later with severe, and, in its results,
permanent illness, he disposed of the Brentford business, and
withdrew to Churt, near Farnham, in Surrey. There, in a
lonely spot, on the top of a detached conical hill known as the
" Devil's Jump," he built a second observatory, and erected
an instrument which he was no longer able to use with pristine
effectiveness; and there, November 27, 1875, he died of the
rupture of a blood-vessel on the brain, before he had com-
pleted his fiftieth year. 1
His observations of sun-spots were of a geometrical character.
They concerned positions and movements, leaving out of sight
physical peculiarities. Indeed, the prudence with which he
limited his task to what came strictly within the range of
his powers to accomplish, was one of Carrington's most valu-
able qualities. The method of his observations, moreover, was
chosen with the same practical sagacity as their objects. As
early as 1847, Sir John Herschel had recommended the daily
self -registration of sun-spots, 2 and he enforced the suggestion,
with more immediate prospect of success, in i854. 3 The art
of celestial photography, however, was even then in a purely
tentative stage, and Carrington wisely resolved to waste no
time on dubious experiments, but employ the means of regis-
tration and measurement actually at his command. These
were very simple, yet very effective. To the " helioscope "
employed by Father Scheiner 4 two centuries and a quarter
earlier a species of micrometer was added. The image of the
sun was projected upon a screen by means of a firmly clamped
telescope, in the focus of which were placed two cross wires
1 Month. Not., vol. xxxvi. p. 142. 2 Cape Observations, p. 435, note.
3 Month. Not., vol. x. p. 158. 4 Rosa Ursina, lib. iii. p. 348.
CHAP. ii. SOLAR THEORIES. 185
forming angles of 45 with the meridian. The six instants
were then carefully noted at which these were met by the
edges of the disc as it traversed the screen, and by the nucleus
of the spot to be measured. 1 A short process of calculation
then gave the exact position of the spot as referred to the
sun's centre.
From a series of 5290 observations made in this way, to-
gether with a great number of accurate drawings, Carrington
derived conclusions of great importance on each of the three
points which he had proposed to himself to investigate. These
were : the law of the sun's rotation, the existence and direction
of systematic currents, and the distribution of spots on the
solar surface.
Grave discrepancies were early perceived to exist between
the determinations of the sun's rotation by different observers.
Galileo, with " comfortable generality," estimated the period
at " about a lunar month ; " 2 Scheiner, at twenty-seven days. 3
Cassini, in 1678, made it 25.58 ; Delambre, in 1775, no more
than twenty-five days. Later inquiries brought these diver-
gences within no more tolerable limits. Laugier's result of
25.34 days obtained in 1841 enjoyed the highest credit, yet
it differed widely in one direction from that of Bb'hm (1852),
giving 25.52 days, and in the other from that of KysaBus (1846),
giving 25.09 days. Now the cause of these variations was
really obvious from the first, although for a long time strangely
overlooked. Father Scheiner pointed out in 1630 that different
spots gave different periods, adding the significant remark
that one at a distance from the solar equator revolved more
slowly than those nearer to it.* But the hint was wasted. For
upwards of two centuries ideas on the subject were either
retrograde or stationary. What were called the " proper
motions " of spots were, however, recognised by Schroter, 5
1 Observations at Redhill, p. 8. z Op. , t. iii. p. 402. 3 Rosa Ursina,
lib. iv. p. 601. Both Galileo and Scheiner spoke of the apparent or
" synodical " period, which is about one and a third days longer than the
true or " sidereal " one. The difference is caused by the revolution of the
earth in its orbit in the same direction with the sun's rotation on its axis.
4 Rosa Ursina, lib. iil p. 260. 5 Faye, Comptes Rendus, t. Ix. p. 818.
186. HISTORY OF ASTRONOMY. PART ir.
and utterly baffled Laugier, 1 who despaired of obtaining any
concordant result as to the sun's rotation except by taking the
mean of a number of discordant ones. At last, in 1855, a
valuable course of observations made at Capo di Monte, Naples,
in 1845-46, enabled C. H. F. Peters 2 (now of Hamilton Col-
lege, Clinton, N.Y.) to set in the clearest light the insecurity
of determinations based on the assumption of fixity in objects
plainly affected by movements uncertain both in amount and
direction.
Such was the state of affairs when Carrington entered upon
his task. Everything was in confusion ; the most that could be
said was that the confusion had come to be distinctly admitted
and referred to its true source. What he discovered was this :
that the sun, or at least the outer shell of the sun visible to
us, has no single period of rotation, but drifts round, carrying
the spots with it, at a rate continually accelerated from the
poles to the equator. In other words, the time of axial revolu-
tion is shortest at the equator and lengthens with increase of
latitude. Carrington devised a mathematical formula by
which the rate or " law " of this lengthening was conveniently
expressed ; but it was a purely empirical one. It was a con-
cise statement, but implied no physical interpretation. It
summarised, but did not explain the facts. An assumed
"mean period" for the solar rotation of 25.38 days (twenty-
five days nine hours, very nearly), was thus found to be
actually conformed to only in two parallels of solar latitude
(14 north and south), while the equatorial period was slightly
less than twenty-five, and that of latitudes 50 rose to twenty-
seven days and a half. 3 These curious results gave quite a new
direction to ideas on solar physics.
The other two "elements " of the sun's rotation were also
ascertained by Carrington with hitherto unattained precision.
He fixed the inclination of its axis to the ecliptic at 82 45' ;
the longitude of the ascending node at 73 40' (both for the
epoch 1850 A.D.) These data which have scarcely yet been
1 Comptes Rendus, t. xii. p. 648. 2 Proc. Am. Ass. Adv. of Science, 1855,
p. 85. 3 Observations at Redhill, p. 221.
CHAP. ii. SOLAR THEORIES. 187
improved upon suffice to determine the position in space of
the sun's equator. Its north pole is directed towards a star
in the coils of the Dragon, midway between Yega and the
Pole-star; its plane intersects that of the earth's orbit in such
a way that our planet finds itself in the same level on or about
the 3d of June and the 5th of December, when any spots
visible on the disc cross it in apparently straight lines. At
other times, the paths pursued by them seem curved down-
ward (to an observer in the northern hemisphere) between
June and December, upward between December and June.
A singular peculiarity in the distribution of sun-spots
emerged from Carrington's studies at the time of the minimum
of 1856. Two broad belts of the solar surface, as we have
seen, are frequented by them, of which the limits may be put
at 6 and 35 of north and south latitude. Individual equa-
torial spots are not uncommon, but nearer to the poles than
35 they are a rare exception. Carrington observed as an
extreme instancein July 1858, one in south latitude 44;
and Peters, in June 1846, watched, during several days, a
spot in 50 24' north latitude. But beyond this no true
macula has ever been seen ; for Lahire's reported observation
of one in latitude 70 is now believed to have had its place on
the solar globe erroneously assigned ; and the " veiled spots "
described byTrouvelot in 1875 J as occurring within ioof the
pole can only be regarded as, at the most, the same kind of
disturbance in an undeveloped form.
But the novelty of Carrington's observations consisted in the
detection of certain changes in distribution concurrent with
the progress of the eleven-year period. As the minimum ap-
proached, the spot-zones contracted towards the equator, and
there finally vanished ; then, as if by a fresh impulse, spots
suddenly reappeared in high latitudes, and spread downward
with the development of the new phase of activity. Scarcely
had this remark been made public, 2 when Wolf 3 found a con-
firmation of its general truth in Bb'hm's observations during
1 Am. Jour, of Science, vol. xi. p. 169. 2 Month. Not., vol. xix. p. I.
3 Vierteljahrsschrift der Naturfors. GtseUschaft (Zurich), 1859, p. 252.
188 HISTORY OF ASTRONOMY. PART n.
the years 1 833-36 ; and a perfectly similar behaviour was noted
both by Sporer and Secchi at the minimum epoch of 1867,
The ensuing period gave corresponding indications; and it
may now be looked upon as established that the spot-zones
close in towards the equator with the advance of each cycle,
their activity culminating, as a rule, in a mean latitude of
about 1 6, and expiring when it is reduced to 6. Before
this happens, however, a completely new disturbance will have
manifested itself some 35 north and south of the equator, and
will have begun to travel over the same course as its prede-
cessor. Each series of sun-spots is thus, to some extent, over-
lapped by the ensuing one; so that while the average in-
terval from one maximum to the next is eleven years, the
period of each distinct wave of agitation is twelve or fourteen. 1
Curious evidence of the retarded character of the last maxi-
mum was to be found in the unusually low latitude of the
spot-zones when it occurred. Their movement inwards hav-
ing gone on regularly while the crisis was postponed, its
final symptoms were hence displaced locally as well as in
time.
Gustav Sporer, born at Berlin in 1822, began to observe
sun-spots with the view of assigning the law of solar rotation
in December 1860. His means were at first very limited, but
his assiduity and success attracted attention, and a Government
endowment was procured for the little solar observatory organ-
ised by him at Anclam, in Pomerania. Unaware of Carring-
ton's discovery (first made known in January 1859), he arrived
at and published, in June i86i, 2 a similar conclusion as to the
equatorial quickening of the sun's movement on its axis. His
sun-spot observations were continued at Anclam until the end
of 1873, and have since been pursued at the "astro-physical"
observatory of Potsdam.
The time had now evidently come for a fundamental revision
of current notions respecting the nature of the sun. Herschel's
theory of a " cool, dark, habitable globe, surrounded by, and
protected against the radiations of a luminous and heat-giving
1 Lockyer, Chemistry of tie Sun, p. 430. 2 Astr. Nach., No. 1315.
CHAP. ii. SOLAR THEORIES. 189
envelope, was shattered by the first dicta of spectrum analysis.
Traces of it may be found for a few years subsequent to I859, 1
but they are obviously survivals from an earlier order of ideas,
doomed to speedy extinction. It needs only a moment's
consideration of what was implied in the discovery of the
origin of the Fraunhofer lines to see the incompatibility of the
new facts with the old conceptions. It implied, not only the
presence near the sun, as glowing vapours, of bodies highly
refractory to heat, but that these glowing vapours formed the
relatively cool envelope of a still hotter internal mass. Kirch -
hoff, accordingly, included in his great memoir " On the Solar
Spectrum," read before the Berlin Academy of Sciences, July
n, 1 86 1, an exposition of the views on the subject to which
his memorable investigations had led him. They may be briefly
summarised as follows.
Since the body of the sun gives a constant spectrum, it
must be either solid or liquid, 2 while the interruptions in its
light prove it to be surrounded by a complex atmosphere of
metallic vapours, somewhat cooler than itself. Spots are simply
clouds due to local depressions of temperature, differing in no
respect from terrestrial clouds except as regards the kinds of
matter composing them. These sun-clouds take their origin in
the zones of encounter between polar and equatorial currents
in the solar atmosphere.
This explanation was liable to all the objections urged
against the "cumulus theory" on the one hand, and the
"trade-wind theory" on the other. Setting aside its pro-
pounder, it was consistently upheld perhaps by no man eminent
in science except Sporer ; and his advocacy of it tended rather
to delay the recognition of his own merits than to promote its
general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to
1 As late as 1866 an elaborate treatise in its support was written by M. F.
Coyteux, entitled Quest ce que le Soleil? Peut-il fare habite? and answer-
ing the question in the affirmative. 2 The subsequent researches of
Plucker, Frankland, Wullner, and others showed that gases strongly
compressed give an absolutely unbroken spectrum.
190 HISTORY OF ASTRONOMY. PART n.
propose a coherent scheme of the solar constitution covering
the whole range of new discovery. The fundamental ideas
on the subject now in vogue here made their first connected
appearance^S Much, indeed, remained to be modified and cor-
rected ; but the transition was finally made from the old to the
new order of conceptions. The essence of the change of view
thus effected may be conveyed in a single sentence. 2 The sun
was thenceforth regarded, not as a mere heated body, or still
more remotely from the truth as a cool body unaccountably
spun round with a cocoon of fire, out as a vast heat-radiating
machine. The terrestrial analogy was abandoned in one more
particular besides that of temperature. The solar system of
circulation, instead of being adapted, like that of the earth, to
the distribution of heat received from without, was seen to be
-'/directed towards the transportation towards the surface of the
heat contained within. Polar and equatorial currents, tending
to a purely superficial equalisation of temperature were re-
placed by vertical currents bringing up successive portions of
the intensely heated interior mass, to contribute their share in
turn to the radiation into space which might be called the
proper function of a sun.
Faye's views, which were communicated to the Academy
of Sciences, January 16, 1865,* were avowedly based on the
anomalous mode of solar rotation discovered by Carrington.
This may be regarded either as an acceleration increasing from
the poles to the equator, or as a retardation increasing from
the equator to the poles, according to the rate of revolution
we choose to assume for the unseen nucleus. Faye preferred
to consider it as a retardation produced by ascending currents
continually left behind as the sphere widened in which the
matter composing them was forced to travel. He further
supposed that the depth from which these vertical currents
rose, and consequently the amount of retardation effected by
their ascent to the surface, became 'progressively greater as
the poles were approached, owing to the considerable flattening
of the spheroidal surface from which they started ; 2 but the
1 Comptes Rcndus, t. Ix. pp. 89, 138. 2 Ibid., t. c. p. $9$.
CHAP. ii. SOLAR THEORIES. 191
adoption of this expedient has been shown to involve inadmis-
sible consequences.
The extreme internal mobility betrayed by Carrington's and
Sporer's observations led to the inference that the matter
composing the sun was mainly or wholly gaseous. This had
already been suggested by Father Secchi l a year earlier, and
by Sir John Herschel in April i864; 2 but it first obtained
general currency through Faye's more elaborate presentation.
A physical basis was afforded for the view by Cagniard de la
Tour's experiments in i822, 3 proving that, under conditions
of great heat and pressure, the vaporous state was compatible
with a very considerable density. The position was strength-
ened when Andrews showed, in i86p, 4 that above a fixed limit
of temperature, varying for different bodies, true liquefaction
is impossible, even though the pressure be so tremendous
as to retain the gas within the same space that enclosed the
liquid. The opinion that the mass of the sun is gaseous
now commands a very general assent ; although the gaseity ad-
mitted is of such a nature as to afford the consistence rather
of honey or pitch than of the aeriform fluids with which we
are familiar.
On another important point the course of subsequent
thought was powerfully influenced by Faye's conclusions in
1865. Arago somewhat hastily inferred from experiments
with the polariscope the wholly gaseous nature of the visible
disc of the sun. Kirchhoff, on the contrary, believed (errone-
ously, as we now know) that the brilliant continuous spectrum
derived from it proved it to be a white-hot solid or liquid.
Herschel and Secchi 5 indicated a cloud-like structure as that
which would best harmonise the whole of the evidence at
command. The novelty introduced by Faye consisted in re-
garding the photosphere no longer "as a defined surface, in
the mathematical sense, but as a limit to which, in the general
1 Butt. Meteor, dell Osservatorio dell Coll. Rom., Jan. I, 1864. p. 4.
2 Quart. Jour, of Science, vol. i. p. 222. 3 Ann. de Chim. et de Phys.,
t. xxii. p. 127. 4 PhU. Trans., vol. clix. p. 575. 5 Les Mondes, Dec.
22, 1864, P- 707.
192 HISTORY OF ASTRONOMY. PART n.
fluid mass, ascending currents carry the physical or chemical
phenomena of incandescence." x Uprushing floods of mixed
vapours with strong affinities say of calcium or sodium and
oxygen at last attain a region cool enough to permit their
combination ; a fine dust of solid or liquid compound particles
(of lime or soda, for example) there collects into the photo-
spheric clouds, and descending by its own weight in torrents
of incandescent rain, is dissociated by the fierce heat below,
and replaced by ascending and combining currents of similar
constitution.
This first attempt to assign the part played in cosmical
physics by chemical affinities, was marked by the importation
into the theory of the sun of the now familiar phrase dissociation.
It is indeed tolerably certain that no such combinations as
those contemplated by Faye occur at the photospheric level,
since the temperature there must be enormously higher than
would be needed to reduce all metallic earths and oxides ; but
molecular changes of some kind, dependent perhaps in part
upon electrical conditions, in part upon the effects of radiation
into space, most likely replace them. The conjecture (originally
thrown out, it would seem, by Faye himself) was countenanced
by Angstrom, 2 and has recently been advocated by Professor
Hastings of Baltimore, 3 that the photospheric clouds are com-
posed of particles of some member of the carbon-triad 4 pre-
cipitated from its mounting vapour just where the temperature
is lowered by expansion and radiation to the boiling-point of
that substance. But the question is one which must for the
present remain within the sphere of interesting and admissible
speculation.
In Faye's theory, sun-spots were regarded as simply breaks
in the photospheric clouds, where the rising currents had
strength to tear them asunder. It followed that they were
regions of increased heat regions, in fact, where the tempera-
ture was too high to permit the occurrence of the precipitations
1 Comptes Rendus, t. Ix. p. 147. 2 RechercJies sur le Spectre Solaire,
p. 38. 3 Am. Jour, of Science, 1881, vol. xxi. p. 41. 4 Carbon, silicon,
and boron.
CHAP. ii. SOLAR THEORIES. 193
to which the photosphere is due. Their obscurity was attri-
buted to deficiency of emissive power. But here it was irre-
sistibly objected by Professors Balfour Stewart and Kirch-
hoff that emissive and absorptive power being strictly cor-
relative, the supposed defect of radiation would be exactly
compensated by an increase of transparency. The light from
the farther photosphere would then, shining across the whole
body of the sun, completely fill up, to the eye, the gap in the
hither photosphere, and no macula at all would remain visible.
Besides, we now know that ignited gases under a pressure far
less than that which must exist at even a small distance below
the solar surface, give light equally brilliant and uninterrupted
with that derived from solid bodies.
After every deduction, however, has been made, we still find
that several ideas of permanent value were embodied in this
comprehensive sketch of the solar constitution. The principal
of these were : first, that the sun is a mainly gaseous body ;
secondly, that its stores of heat are rendered available at the
surface by means of vertical convection-currents by the bodily
transport, that is to say, of intensely hot matter upwards, and
of comparatively cool matter downwards; thirdly, that the
photosphere is a surface of condensation, forming the limit set
by the cold of space to this circulating process, and that a
similar formation must attend, at a certain stage, the cooling
of every cosmical body.
To Mr. Warren De la Rue belongs the honour of having
obtained the earliest results of substantial value in celestial
photography. What had been done previously was interesting
in the way of promise, but much could not be claimed for it
as actual performance. Some " pioneering experiments " were
made by Dr. J. W. Draper of New York in 1840, resulting
in the production of a few " moon-pictures " one-inch in
diameter ; * but slight encouragement was derived from them,
either to himself or others. Bond of Cambridge (U.S.), how-
ever, secured in 1850 with the Harvard 15 -inch refractor that
1 H. Draper, Quart. Journ. of Se. t vol. i. p. 381 ; also Phil. Mag., vol.
xvii. 1840, p. 222.
N
I 9 4 HISTORY OF ASTRONOMY. PART n.
daguerreotype of the moon with which the career of extra-
terrestrial photography may be said to have formally opened.
It was shown in London at the Great Exhibition of 1851, and
determined the direction of De la Rue's efforts. Yet it did
little more than prove the art to be a possible one.
Warren De la Rue was born in Guernsey in 1815, was
educated at the Ecole Sainte-Barbe in Paris, and made a large
fortune as a paper manufacturer in England. The material
supplies for his scientific campaign were thus amply and early
provided. Towards the end of 1853 he took some successful
lunar photographs. They were remarkable as the first examples
of the application to astronomical light-painting of the collo-
dion process, invented by Archer in 1851; and also of the use
of reflectors (Mr. De la Rue's was one of thirteen inches, con-
structed by himself) for that kind of work. The absence of
a driving apparatus was, however, very sensibly felt; the
difficulty of moving the instrument by hand so as accurately
to follow the moon's apparent motion being such as to cause
the discontinuance of the experiments until 1857, when the
want was supplied. Mr. De la Rue's new observatory, built
in that year at Cranford, twelve miles west of Hyde Park, was
expressly dedicated to celestial photography j and there he
immediately applied to the heavenly bodies the stereoscopic
method of obtaining relief, and turned his attention to the
delicate business of photographing the sun.
A solar daguerreotype l was taken at Paris, April 2, 1845, ^Y
MM. Foucault and Fizeau, acting on a suggestion from Arago.
But the attempt, though far from being unsuccessful, does not,
at that time, seem to have been repeated. Its great difficulty
consisted in the enormous light-power of the object to be
represented, rendering an inconceivably short period of expo-
sure indispensable, under pain of getting completely " burnt-
up" plates. In 1857 Mr. De la Rue was commissioned by
the Royal Society to construct an instrument specially adapted
to the purpose for the Kew Observatory. The resulting " photo-
heliograph " may be described as a small telescope (of 3 J
1 Reproduced in Arago's Popular Astronomy, vol. i. plate xii.
CHAP. ii. SOLAR THEORIES. 195
inches aperture and 50 focus), with a plate-holder at the eye-
end, guarded in front by a spring-slide, the rapid movement of
which across the field of view secured for the sensitive plate a
virtually instantaneous exposure. By its means the first solar
light-pictures of real value were taken, and the autographic
record of the solar condition recommended by Sir John Her-
schel was begun, and continued at Kew during fourteen
years 1858-72. The work of photographing the sun is now
carried on in every quarter of the globe, from the Mauritius
to Massachusetts, and the days are few indeed on which the
self-betrayal of the camera can be evaded by our chief luminary.
In the year 1883, the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on
340 days; and 360 were similarly provided for in 1885.
The conclusions arrived at by photographic means at Kew
were communicated to the Royal Society in a series of papers
drawn up jointly by Messrs. De la Rue, Balfour Stewart,
and Benjamin Loewy, in 1865 and subsequent years. They
influenced materially the progress of thought on the subject
they were concerned with.
By its rotation the sun itself offers opportunities for bringing
the stereoscope to bear upon it. Two pictures, taken at an
interval of twenty-six minutes, show just the amount of differ-
ence needed to give, by their combination, the maximum effect
of solidity. 1 Mr. De la Rue thus obtained, in 1861, a stereo-
scopic view of a sun-spot and surrounding faculse, representing
the various parts in their true mutual relations. "I have
ascertained in this way," he wrote, 2 " that the faculas occupy
the highest portions of the sun's photosphere, the spots appear-
ing like holes in the penumbrse, which appeared lower than
the regions surrounding them ; in one case, parts of the f aculae
were discovered to be sailing over a spot apparently at some
considerable height above it." Thus Wilson's inference as to
the depressed nature of spots received, after the lapse of not
far from a century, proof of the most simple, direct, and con-
vincing kind. A careful application of Wilson's own geometri-
1 Report Brit. Ass., 1859, p. 148. 2 Phil. Trans., vol. clii. p. 407.
196 HISTORY OF ASTRONOMY. PART n.
cal test gave results only a trifle less decisive. Of 694 spots
observed, 78 per cent, showed, as they traversed the disc, the
expected effects of perspective ; l and their absence in the
remaining 22 per cent, might be easily explained by internal
commotions producing irregularities of structure. The absolute
depth of spot-cavities at least of their sloping sides was
determined by Father Secchi through measurement of the
" parallax of profundity " 2 that is, of apparent displacements
attendant on the sun's rotation, due to depression below the
sun's surface. He found that in every case it fell short of
4000 miles, and averaged not more than 1321, corresponding,
on the terrestrial scale, to an excavation in the earth's crust
of i J miles. There may be, however, and probably are, depths
below this depth, of which the eye takes not even indirect
cognisance ; so that it would be hasty to pronounce spots to
be a merely superficial phenomenon.
The opinion of the Kew observers as to the nature of such
disturbances was strongly swayed by another curious result of
the " statistical method " of inquiry. They found that of 1137
instances of spots accompanied by faculse, 584 had those
faculae chiefly or entirely on the left, 508 showed a nearly
equal distribution, while 45 only had faculous appendages,
mainly on the right side. 3 Now, the rotation of the sun, as we
see it, is performed from left to right; so that the marked
tendency of the faculse was a lagging one. This was easily
accounted for by supposing the matter composing them to have
been flung upward from a considerable depth, whence it would
reach the surface with the lesser absolute velocity belonging to
a smaller circle of revolution, and would consequently fall
behind the cavities or " spots " formed by its abstraction.
The ideas of M. Faye were, on two fundamental points,
contradicted by the Kew investigators. He held spots to
be regions of uprusli and of heightened temperature; they
1 Researches in Solar Physics, part i. p. 20. 2 Both the phrase and
the method were suggested by Faye, who estimates the average depth of
the luminous sheath of spots at 2160 miles. Comptes Rendus, t. Ixi. p.
1082 ; t. xcvi. p. 356. 3 Proc. Roy. Soc., vol. xiv. p. 39.
CHAP. ii. SOLAR THEORIES. 197
believed their obscurity to be due to a doivnrush of compara-
tively cool vapours. On which side does the truth lie ?
Observing at Ville-Urbanne, March 6, 1865, M. Chacornac
saw floods of photospheric matter visibly precipitating them-
selves into the abyss opened by a great spot, and carrying with
them small neighbouring maculae. 1 Similar instances were
repeatedly noted by Father Secchi, who considered the exist-
ence of a kind of suction in spots to be quite beyond question. 2
The tendency in their vicinity, to put it otherwise, is centripetal,
not centrifugal ; and this alone seems to negative the supposi-
tion of a central uprush.
A fresh witness was now at hand. The application of the
spectroscope to the direct examination of the sun's surface
dates from March 4, 1866, when Mr. Norman Lockyer began
his researches into the cause of the darkening in spots. 3 They
were made possible by the simple device of throwing upon the
slit of the spectroscope an image of the sun, any part of which
could be subjected to special scrutiny, instead of (as had
hitherto been done) admitting rays from every portion of his
surface indiscriminately. The answer to the inquiry was
prompt and unmistakable, and was again, in this case,
adverse to the French theorist's view. The obscurations in
question were found to be produced by no deficiency of emissive
power, but by an increase of absorptive action. The back-
ground of variegated light remained unchanged, but more of it
was stopped by the interposition of a dense mass of relatively
cool vapours.
The spectrum of a sun-spot is crossed by the same set
of multitudinous dark lines, with some minor differences,
visible in the ordinary solar spectrum. We must then con-
clude that the same vapours (speaking generally) which are
dispersed over the unbroken solar surface are accumulated in
the umbral cavity, the compression incident to such accumu-
lation being betrayed by the thickening of certain lines of
absorption. But there is also a general absorption, extending
1 Lockyer, Contributions to Solar Physics, p. 70. 2 Le Soleil, p. 87.
3 Proc. Roy. Soc. } vol. xv. p. 256.
198 HISTORY OF ASTRONOMY. PART n.
almost continuously from one end of the spot-spectrum to the
other. And this is explained in Professor Hastings's ingenious
speculation by a deposition of soot, or something analogous
in other words, by the presence, as a slowly settling fine dust,
of cold, dark particles of carbon or silicon. 1
An inquiry, however, prosecuted by Professor Young of
Princeton, New Jersey, during the latter half of 1883, has set
the matter in a new light. Using a spectroscope of excep-
tionally high dispersive power, he succeeded to a considerable
extent in " resolving " the supposed continuous obscurity of
spot-spectra into a countless army of fine dark lines set very
close together. 2 The substances producing this darkening or
absorption are then in a gaseous state, and the " soot " theory
As to the movements of the constipated vapours forming
spots, the spectroscope is also competent to supply information.
The principle of the method by which it is procured will be
explained farther on. Suffice it here to say that the trans-
port, at any considerable velocity, to or from the eye of the
gaseous material giving bright or dark lines, can be measured
by the displacement of such lines from their previously known
normal positions. In this way movements have been detected
in or above spots of enormous rapidity, ranging up to 320
miles per second. 3 But the result, so far, has been to negative
conjectures either of uprushes or downrushes as part of the
regular internal economy of spots.
A new theory of sun-spots, started by Faye in 1872, and
still advocated by him, is sufficiently plausible to merit some
brief attention. He had been foremost in pointing out that
the observations of Carrington and Sporer absolutely forbade
the supposition that any phenomenon at all resembling our
trade-winds exists in the sun. They showed, indeed, that
beyond the parallels of 20 there is a general tendency in
spots to a slow poleward displacement, while within that zone
they incline to approach the equator; but their "proper
1 Am. Jour., vol. xxi. p. 42. 2 Phil. Mag., vol. xvi. p. 460. 3 Young,
The Sun, p. 99.
CHAP. ii. SOLAR THEORIES. 199
movements " gave no evidence of uniformly flowing currents
in latitude. The systematic drift of the photosphere is
strictly a drift in longitude ; its direction is everywhere
parallel to the equator. This fact being once clearly recog-
nised, the " solar tornado " hypothesis at once fell to pieces ;
but M. Faye perceived another source of vorticose motion in
the unequal rotating velocities of contiguous portions of the
photosphere. The " pores " with which the whole surface of
the sun is studded he took to be the smaller eddies resulting
from these inequalities ; the spots to be such eddies developed
into whirlpools. It only needs to thrust a stick into a stream
to produce the kind of effect designated. And it happens that
the differences of angular movement adverted to attain a maxi-
mum just about the latitudes where spots are most frequent
and conspicuous.
There are, however, two fatal objections. One (already
mentioned) is the total absence of the regular swirling motion
in a direction contrary to that of the hands of a watch north
of the solar equator, in the opposite sense south of it which
should impress itself upon every lineament of a sun-spot if the
cause assigned were a primary producing, and not merely (as
it possibly may be) a secondary determining one. The other,
pointed out by Professor Young, 2 is that the cause is inade-
quate to the effect. The difference of movement, or rela-
tive drift, supposed to occasion such prodigious disturbances,
amounts, at the utmost, for two portions of the photosphere
123 miles apart, to about five yards a minute. Thus the fric-
tion of contiguous sections must be quite insignificant.
A view better justified by observation was urged by Father
Secchi in and after the year 1872, and was presented in an
improved form by Professor Young in his excellent little book
on The Sun, published in 1882. Spots are manifestly asso-
ciated with violent eruptive action, giving rise to the faculas
and prominences which usually garnish their borders. It is
accordingly contended that upon the withdrawal of matter
1 Comptes Rendus, t. Ixxv. p. 1664. 2 The Sun, p. 174. For Faye'a
answer to the objection, see Comptes Kendus, t. xcv. p. 1310.
200 HISTORY OF ASTRONOMY. PART n.
from below by the flinging up of a prominence must ensue
a^sinking-in of the surface, into which the partially cooled
erupted vapours rush and settle, producing just the kind of
darkening by increased absorption told of by the spectroscope.
Round the edges of the cavity the rupture of the photospheric
shell will form lines of weakness provocative of further erup-
tions, which will, in their turn, deepen and enlarge the cavity.
The phenomenon will thus tend to perpetuate itself, until
equilibrium is at last restored by internal processes. A sun-
spot might then be described as an inverted terrestrial volcano,
in which the outbursts of heated matter take place on the
borders instead of at the centre of the crater, while the cooled
products gather in the centre instead of at the borders.
There is, however, a difficulty. On the earth, the solid crust
forcibly represses the steam gathering beneath until it has
accumulated strength for an explosion. But there is no such
restraining power that we know of in the sun. Zollner, indeed,
adapted his theory of the solar constitution to the special
purpose of procuring it ; yet with very partial success, since
almost every new fact ascertained has proved adverse to his as-
sumptions. Volcanic action is essentially spasmodic. It implies
habitual constraint varied by temporary outbreaks, incon-
ceivable in a gaseous globe such as we believe the sun to be.
Another objection to the " volcanic hypothesis " is in the
order it ascribes to the phenomena. If it represent the truth,
no spot could possibly appear without a precedent eruption ;
but there is strong evidence that a spot begins the visible
disturbance. Faculse, at any rate, always show subsequently
to the opening of a rent in the photosphere.
This sequence forms an integral part of Mr. Lockyer's
theory of sun-spots, communicated to the Royal Society, May
6, I886, 1 and further developed some months later in his work
on The Chemistry of the Sun. A number of hitherto unex-
plained facts are, it must be admitted, remarkably correlated
in this new scheme. Spots are represented in it as incidental
to a vast system of solar atmospheric circulation, starting with
1 Proc. Roy. Soc., vol. xl. p. 347.
CHAP. ii. SOLAR THEORIES. 201
the polar out- and up-flows indicated by observations during
some total eclipses, and eventuating in the plunge downward
from great heights upon the photosphere of prodigious masses
of condensed materials. From these falls result, primarily,
spots ; secondarily, through the answering uprushes in which
chemical and mechanical forces co-operate, their girdles of
flame-prominences. The limitation of spots to middle lati-
tudes is accounted for as follows. Above the equator, the
height of fall is so great that the products of condensation are
volatilised before reaching the photosphere. Near the poles,
on the contrary, the cooled substances have attained too small
an elevation to give the requisite velocity. Hence their
gentler fall can give rise to nothing more conspicuous than
pores and "veiled spots." It is especially noticeable that
this augmented height of descent with diminishing latitude
gives perhaps the best explanation yet offered of Carrington's
law of solar rotation. For the nearer a spot produced in
this way lies to the equator, the faster it must travel, be-
cause its materials, falling from a greater elevation, bring
down with them the increased velocity belonging to a wider
circumference. Local changes of temperature and extent,
inevitably produced in the sun's atmosphere by the violent
disturbances to which it is subject, are shown, moreover, on
this hypothesis, to occasion periodical fluctuations in spot-
frequency, as well as the observed oscillations of the spot-
zones. Thus it accounts, more simply and naturally than had
been done before, for the peculiarities of spot- distribution
both in time and space. Nevertheless, the evidence is not as
clear as could be wished that a circulatory system such as it
postulates is really in operation in the sun's atmosphere.
A singular circumstance has now to be recounted. On the
ist of September 1859, while Carrington was engaged in his
daily work of measuring the positions of sun-spots, he was
startled by the sudden appearance of two patches of peculiarly
intense light within the area of the largest group visible. His
first idea was that a ray of unmitigated sunshine had penetrated
the screen employed to reduce the brilliancy of the image;
202 HISTORY OF ASTRONOMY. PART n.
but, having quickly convinced himself to the contrary, he ran
to summon an additional witness of an unmistakably remark-
able occurrence. On his return he was disappointed to find the
strange luminous outburst already on the wane ; shortly after-
wards the last trace vanished. Its entire duration was five
minutes from 11.18 to 11.23 A.M., Greenwich time; and
during those five minutes it had traversed a space estimated at
35,000 miles ! No perceptible change took place in the details
of the group of spots visited by this transitory conflagration,
which, it was accordingly inferred, took place at a considerable
height above it. 1
Carrington's account was precisely confirmed by an obser-
vation made at Highgate. Mr. R. Hodgson described the
appearance seen by him as that " of a very brilliant star of
light, much brighter than the sun's surface, most dazzling to
the protected eye, illuminating the upper edges of the adjacent
spots and streaks, not unlike in effect the edging of the clouds
at sunset." 2
This unique phenomenon seemed as if specially designed to
accentuate the inference of a sympathetic relation between the
earth and the sun. From the 28th of August to the 4th of
September 1859, a magnetic storm of unparalleled intensity,
extent, and duration, was in progress over the entire globe.
Telegraphic communication was everywhere interrupted
except, indeed, that it was, in some cases, found practicable to
work the lines without batteries, by the agency of the earth-
currents alone ; 3 sparks issued from the wires j gorgeous
aurorae draped the skies in solemn crimson over both hemi-
spheres, and even within the tropics ; the magnetic needle lost
all trace of continuity in its movements, and darted to and
fro as if stricken with inexplicable panic. The coincidence
was drawn even closer. At the very instant 4 of the solar out-
burst witnessed by Carrington and Hodgson, the photographic
1 Month. Not., vol. xx. p. 13. 2 Ibid., p. 15. 3 Am. Jour., vol.
xxix. (2d series'), pp. 94-95. 4 The magnetic disturbance took place at
11.15 A.M., three minutes before the solar blaze compelled the attention of
Carrington.
CHAP. ii. SOLAR THEORIES. 203
apparatus at Kew registered a marked disturbance of all
the three magnetic elements ; while, shortly after the ensuing
midnight, the electric agitation culminated, thrilling the earth
with subtle vibrations, and lighting up the atmosphere from
pole to pole with the coruscating splendours which, perhaps,
dimly recall the times when our ancient planet itself shone
as a star.
Here then, at least, the sun was in Professor Balfour
Stewart's phrase " taken in the act " 1 of stirring up terrestrial
commotions. Nor have instances since been wanting of an
indubitable connection between outbreaks of individual spots
and magnetic disturbances four such were recorded in 1882
although the peculiar features of the event of September i,
1859, have not recurred. An attempt was made to explain
them by Professor Piazzi Smyth, 2 who suggested that the
flying luminous objects seen on that occasion were nothing
else than a pair of unusually large meteors ignited through
retardation in the solar atmosphere. But the inadequacy of
the conjecture hardly needs to be pointed out. The sudden de-
velopment of light was certainly no accidental occurrence, but
marked the climax of some systematic commotion already for
some days in progress. If we were to look for its terrestrial
analogue, we should rather find it in the "auroral beam"
which traversed the heavens during a vivid display of polar
lights, November 17, 1882, and shared, there is every reason
to believe, their electrical origin and character. 3
Meantime M. Rudolf Wolf, transferred to the direction of
the Ziirich Observatory, had relaxed none of his zeal in the
investigation of sun-spot periodicity. A laborious revision of the
entire subject with the aid of fresh materials led him, in 1859,*
to the conclusion that while the mean period differed little
from that arrived at in 1852 of ii.n years, very considerable
fluctuations on either side of that mean were rather the rule
1 Phil. Trans., vol. cli. p. 428. 2 Month. Not., vol. xx. p. 88. 3 See
J. Rand Capron, Phil. Mag., vol. xv. p. 318. 4 Mittheilungen uber die
Sonnenflecken, No. ix., Vierteljahrsschrift der Naturforschenden Gesell-
schaft in Zurich, Jahrgang 4.
204 HISTORY OF ASTRONOMY. PARTII.
than the exception. Indeed, the phrase " sun-spot period "
must be understood as fitting very loosely the great fact it is
taken to represent ; so loosely, that the interval between two
maxima may rise to sixteen and a half or sink below seven and
a half years. 1 In 1 86 1 2 Wolf showed, and the remark was
fully confirmed by the Kew observations, that the shortest
periods brought the most acute crises, and vice versa ; as if for
each wave of disturbance a strictly equal amount of energy
were available, which might spend itself lavishly and rapidly, or
slowly and parsimoniously, but could in no case be exceeded.
The further inclusion of recurring solar commotions within a
cycle of fifty-five and a half years was simultaneously pointed
out ; and Hermann Fritz showed soon after that the aurora
borealis is subject to an identical double periodicity. 3 The
same inquirer has more recently detected both for aurorae
and sun-spots a " secular period "of 222 years, 4 and the Kew
observations indicate for the latter, oscillations accomplished
within twenty- six and twenty-four days. 5 The more closely
spot-fluctuations are looked into, indeed, the more complex
they prove. Maxima of one order are superposed upon, or
in part neutralised by, maxima of another order ; originating
causes are masked by modifying causes ; the larger waves of
the commotion are indented with minor undulations, and these
again crisped with tiny ripples, while the whole rises and falls
with the swell of the great secular wave, scarcely perceptible
in its progress because so vast in scale.
The idea that solar maculation depends in some way upon
the position of the planets occurred to Galileo in i6i2. 6 It
has been industriously sifted by a whole bevy of modern solar
physicists. Wolf in 1859 7 found reason to believe that the
1 Mitth., No. Hi. p. 58 (1881). 2 Ibid., No. xii. p. 192. Mr. Joseph
Baxendell, of Manchester, reached independently a similar conclusion. See
Month. Not., vol. xxi. p. 141. 3 Wolf, Mitth., No. xv. p. 107, &c.
Olmsted, following Hansteen, had already, in 1856, sought to establish an
auroral period of sixty-five years. Smithsonian Contributions, vol. viii. p. 37-
4 Hahn, Ueber die Beziehungen der Sonnenfleckenperiode zu meteorolo-
gischen Erscheinungen, p. 99(1877). 5 Report Brit. Ass., 1881, p. 518;
1883, p. 418. 6 Opere, t. iii. p. 412. 7 Mitth., Nos. viii. and xviii.
CHAP. ii. SOLAR THEORIES. 205
eleven-year curve is determined by the action of Jupiter, modi-
fied by that of Saturn, and diversified by influences proceeding
from the earth and Venus. Its tempting approach to agree-
ment with Jupiter's period of revolution round the sun, indeed,
irresistibly suggested a causal connection ; yet it does not seem
that the most skilful " coaxing " of figures can bring about a
fundamental harmony. Carrington pointed out in 1863 that
while, during eight successive periods, from 1770 downwards,
there were approximate coincidences between Jupiter's aphelion
passages and sun-spot maxima, the relation had been almost
exactly reversed in the two periods preceding that date ; l and
the latest conclusion of M. Wolf himself is that the Jovian
origin must be abandoned. 2 Nevertheless it is still held by
M. Duponchel 3 of Paris, who accommodates discrepancies with
the help of perturbations by the large exterior planets ; and it
deserves notice that his prediction of an abnormal lengthen-
ing of the last maximum, through certain peculiarities in the
positions of Uranus and Neptune about the time it fell due,
has been partially verified by the event. The previous maximum
having occurred in June 1870, the next phase of agitation
should, if punctual, have culminated about August 1881 ;
whereas, after an abortive effort at completion in April 1882,
the final outburst was postponed to November 1883. The
interval was thus 13.3 instead of n.i years; and it is notice-
able that the delay affected chiefly the southern hemisphere.
Alternations of activity in the solar hemispheres have indeed
been a marked feature of the recent maximum, which, in M.
Faye's view, 4 derived thence its indecisive character, while sharp,
strong crises arise with the simultaneous advance of agitation
north and south of the solar equator. The curve of magnetic
disturbance, it should be added, followed with its usual strict
fidelity the late anomalous fluctuations of the sun-spot curve.
That outbreaks of solar activity are modified by influences
depending upon planetary configuration has been tolerably
well ascertained by the Kew observations. This no less signi-
1 Observations at JRedhill, p. 248. 2 Comptcs Rcndus, t. xcv. p. 1249.
3 Ibid., t. xciii. p. 827 j t. xcvi. p. 1418. 4 Ibid., t. c. p. 593.
2 o6 HISTORY OF ASTRONOMY. PART n.
ficant than surprising result was imparted by Professor Balfour
Stewart to the Royal Society of Edinburgh, April 18, I864. 1
The method of research by which it was arrived at (said to
have been privately recommended by Galileo 2 ) consisted in
studying the " behaviour " of each spot as it crossed the disc.
This, it was found, was almost always marked, about the same
epochs, with a common character. If one rent in the photo-
sphere widened as the central meridian of the sun was
approached, those in its train were pretty sure to do likewise ;
if it closed up, its successors followed suit. Moreover, the
controlling power was perceived to travel onward at a rate
quicker than that of the earth's annual revolution. It was
endowed, in short, with about the orbital velocity of Yenus.
Its nature is of such a kind as to assuage outbreaks on the
side of the sun turned towards the planet, and to aggravate
them on the opposite hemisphere. 3 The action both of Jupiter
and Mercury is, it would seem, the same in kind though less
in degree. That of the earth is more difficult to determine,
but it can scarcely be doubted that it is similarly exercised.
It has even been attempted to invert the process, and arrive
at the period of an unknown planet through the observation of
sun-spots. Professor Balfour Stewart has shown that inequali-
ties in their development exist corresponding severally to the
revolution of such a body round the sun in twenty-four days,
and to its " synodical periods " or successive meetings with
Jupiter, Yenus, and Mercury. 4 But the prediction still awaits
fulfilment.
The question so much discussed, as to the influence of sun-
spots on weather, does not yet admit of a satisfactory answer.
The facts of meteorology are too complex for easy or certain
classification. Effects owning dependence on one cause often
wear the livery of another ; the meaning of observed parti-
culars may be inverted by situation ; and yet it is only by the
1 Ed. Phil. Trans., vol. xxiii. p. 499. 2 Researches in Solar Physics,
ser. ii. p. 46 (privately printed). The Rev. Mr. Selwyn is responsible for
the statement, for which he gives no authority. 3 Proc. Roy. Soc., vol.
xiv. p. 59 ; xx. p. 210. 4 Report Brit. Ass., 1881, p. 518.
CHAP. ii. SOLAR THEORIES. 207
collection and collocation of particulars that we can hope to
reach any general law. There is, however, a good deal of
evidence to support the opinion the grounds for which were
primarily derived from the labours of Mr. Meldrum at the
Mauritius that increased rainfall and atmospheric agitation
attend spot-maxima; while Herschel's conjecture of a more
copious emission of light and heat about the same epochs has
received little support from direct investigations.
The examination of what we may call the texture of the
sun's surface derived new interest from a remarkable an-
nouncement by Mr. James Nasmyth in I862. 1 He had
made (as he supposed) the discovery that the entire luminous
stratum of the sun is composed of a multitude of elongated
shining objects on a darker background, shaped much like
willow-leaves, of vast size, crossing each other in all possible
directions, and possessed of unceasing relative motions. A
lively controversy ensued. In England and abroad, the most
powerful telescopes were directed to a scrutiny encompassed
with varied difficulties. The results, on the whole, were such
as to invalidate the precision of the disclosures made by the
Hammerfield reflector. Mr. Dawes was especially emphatic
in declaring that Nasmyth's " willow-leaves " were nothing
more than the " nodules " of Sir William Herschel seen under
a misleading aspect of uniformity ; and there is little doubt
that he was right. It is, however, admitted that something
of the kind may be seen in the penumbrse and " bridges " of
spots, presenting an appearance compared by Dawes himself
in 1852 to that of a piece of coarse straw-thatching left un-
trimmed at the edges. 2
The term " granulated," suggested by Dawes in i864, 3 best
describes the mottled aspect of the solar disc as shown by
modern telescopes and cameras. The grains, or rather the
" floccules," with which it is thickly strewn, have been resolved
by Langley, under exceptionally favourable conditions, into
" granules " not above 100 miles in diameter; and from these
1 Report Brit. Ass., 1862, p. 16 (pt. ii.) 2 Mem. R. A, Soc., vol. xxi.
p. 161. 8 Month. Not., vol. xxiv. p. 162.
2c8 HISTORY OF ASTRONOMY. PART n.
relatively minute elements, composing, jointly, about one-fifth
of the visible photosphere, 1 he estimates that three quarters
of the entire^light of the sun are derived. 2 Janssen goes so
far as to say that if the whole surface were as bright as its
brightest parts, its luminous emission would be ten to twenty
times greater than it actually is. 3
The rapid changes in the forms of these solar cloud-summits
are beautifully shown in the marvellous photographs taken
by Janssen at Meudon, with exposures reduced at times to
-i-g-^ of a second ! By their means, also, the curious phe-
nomenon known as the reseau photospherique has been made
evident. 4 This consists in the diffusion over the entire disc of
fleeting blurred patches, as if of imperfect definition, due,
doubtless, to agitations in the intervening solar atmosphere.
The same cause may perhaps account for the evanescent
obscurations described by Father Perry of Stonyhurst before
the Royal Astronomical Society, May 9, iSS/j.. 5
The " grains," or more brilliant parts of the photosphere,
are now generally held to represent the upper terminations of
ascending and condensing currents, while the darker inter-
stices (Herschel's "pores") mark the positions of descending
cooler ones. In the penumbrse of spots, the glowing streams
rushing up from the tremendous sub-solar furnace are bent
sideways by the powerful indraught, so as to change their
vertical for a nearly horizontal motion, and are thus taken, as
it were, in flank by the eye, instead of being seen end-on in
mamelon form. This gives a plausible explanation of the
channelled structure of penumbrse which suggested the com-
parison to a rude thatch. Accepting this theory as in the
main correct, we perceive that the very same circulatory
process which, in its spasms of activity, gives rise to spots,
produces in its regular course the singular " marbled " appear-
1 Am. Jour, of Science, vol. vii. 1874, p. 92. 2 Young, The Sun, p.
103. 3 Ann. Bur. Long., 1879,?. 679. 4 Ibid., 1878, p. 689. 5 Obser-
vatory, vol. vii. p. 154. Father Perry sought to identify the objects
observed by him with Trouvelot's "veiled spots;" Mr. Ranyard sug-
gested the more probable analogy of the reseau photospherique.
CHAP. ii. SOLAR THEORIES. 209
ance, for the recording of which we are no longer at the mercy
of the fugitive or delusive impressions of the human retina.
And precisely this circulatory process it is which gives to our
great luminary its permanence as a sun, or warming and
illuminating body.
( 210 )
CHAPTER III.
RECENT SOLAR ECLIPSES.
BY observations made during a series of five remarkable
eclipses, comprised within a period of eleven years, knowledge
of the solar surroundings was advanced nearly to its present
stage. Each of these events brought with it a fresh dis-
closure of a definite and unmistakable character. We will
now briefly review this orderly sequence of discovery.
Photography was first systematically applied to solve the
problems presented by the eclipsed sun, July 18, 1860. It
is true that a very creditable daguerreotype, taken by Busch
at Konigsberg during the eclipse of 1851, is still valuable as
a record of the corona of that year; and some subsequent
attempts were made to register partial phases of solar
occultation, notably by Professor Bartlett at West Point in
1854^ but the ground remained practically unbroken until
1860.
In that year the track of totality crossed Spain, and thither,
accordingly, Mr. Warren de la Rue transported his photo-
heliograph, and Father Secchi his six-inch Cauchoix refractor.
The question then primarily at issue was that relating to the
nature of the red protuberances. Although, as already stated,
the evidence collected in 1851 gave a reasonable certainty of
their connection with the sun, objectors were not silenced ;
and when the side of incredulity was supported by so con-
siderable an authority as M. Faye, it was impossible to treat
it with contempt. Two crucial tests were available. If it
1 Aatr. Jour., vol. iv. p. 33.
CHAP. in. RECENT ECLIPSES. 211
could be shown that the fantastic shapes suspended above the
edge of the dark moon were seen under an identical aspect
from two distant stations, that fact alone would annihilate
the theory of optical illusion or " mirage ; " while the certainty
that they were progressively concealed by the advancing moon
on one side, and uncovered on the other, would effectually
detach them from dependence on our satellite, and establish
them as solar appendages.
Now both these tests were eminently capable of being
applied by photography. But the difficulty arose that nothing
was known as to the chemical power of the rosy prominence-
light, while everything depended on its right estimation. A
shot had to be fired, as it were, in the dark. It was a matter
of some surprise, and of no small congratulation, that, in both
cases, the shot took effect.
Mr. De la Rue occupied a station at Kivabellosa, in the
Upper Ebro valley ; Father Secchi set up his instrument at
Desierto de las Palmas, about 250 miles to the south-east,
overlooking the Mediterranean. From the totally eclipsed
sun, with its strange garland of flames, each observer derived
several perfectly successful impressions, which were found, on
comparison, to agree in the most minute details. This at
once settled the fundamental question as to the substantial
reality of these objects ; while their solar character was
demonstrated by the passage of the moon in front of them,
indisputably attested by pictures taken at successive stages of
the eclipse. That forms seeming to defy all laws of equili-
brium were, nevertheless, not wholly evanescent, appeared
from their identity at an interval of seven minutes, during
which the lunar shadow was in transit from one station to
the other; and the singular energy of their " actinic" rays
was shown by the record on the sensitive plates of isome
prominences invisible in the telescope. Moreover, photo-
graphic evidence strongly confirmed the inference previously
drawn by Grant and others, and now repeated with fuller
assurance by Secchi that an uninterrupted stratum of
prominence-matter encompasses the sun on all sides, forming
212 HISTORY OF ASTRONOMY. PART 11.
a reservoir from which gigantic jets issue, and into which
they subside.
Thus a first-fruits of accurate knowledge regarding the solar
surroundings was gathered, and the value of the brief moments
of eclipse indefinitely increased by supplementing transient
visual impressions with the faithful and lasting records of
the camera.
In the year 1868 the history of eclipse spectroscopy vir-
tually began, as that of eclipse photography in 1860; that
is to say, the respective methods then first gave definite
results. On the iSth of August 1868, the Indian and Mal-
ayan peninsulas were traversed by a lunar shadow produc-
ing total obscuration during five minutes and thirty-eight
seconds. Two English and two French expeditions were
despatched to the distant regions favoured by an event so
propitious to the advance of knowledge, chiefly to obtain the
verdict of the prism as to the composition of prominences.
Nor were they despatched in vain. An identical discovery
was made by nearly all the observers. At Jamkandi, in the
Western Ghauts, where Lieutenant (now Colonel) Herschel
was posted, unremitting bad weather threatened to baffle his
eager expectations; but during the lapse of the critical five
and a half minutes the clouds broke, and across the driving
wrack a " long, finger-like projection " jutted out over the
margin of the dark lunar globe. In another moment the
spectroscope was pointed towards it ; three bright lines red,
orange, and blue flashed out, and the problem was solved. 1
The problem was solved in this general sense, that the com-
position out of glowing vapours of the objects infelicitously
termed " protuberances " or " prominences " was no longer
doubtful ; although further inquiry was needed for the deter-
mination of the particular species to which those vapours
belonged.
Similar, but more complete observations were made, with
less atmospheric hindrance, by Tennant and Janssen at
Guntoor, by Pogson at Masulipatam, and by Bayet at Wha-
1 Proc. Roy. Soc., vol. xvii. p. 116.
CHAP. in. RECENT ECLIPSES. 213
Tonne, on the coast of the Malay peninsula, the last observer
counting as many as nine bright lines. 1 Among them it
was not difficult to recognise the characteristic light of
hydrogen ; and it was generally, though over-hastily, assumed
that the orange ray matched the luminous emissions of sodium,
But fuller opportunities were at hand.
The eclipse of 1868 is chiefly memorable for having taught
astronomers to do without eclipses, so far, at least, as one
particular branch of solar inquiry is concerned. Inspired by
the beauty and brilliancy of the variously tinted prominence-
lines revealed to him by his spectroscope, Janssen exclaimed
to those about him, " Je verrai ces lignes-la en dehors des
eclipses ! " On the following morning he carried into execu-
tion the plan which formed itself in his brain while the
phenomenon which suggested it was still before his eyes. It
rests upon an easily intelligible principle.
The glare of our own atmosphere alone hides the appen-
dages of the sun from our daily view. To a spectator on an
airless planet, the central globe would appear attended by all
its splendid retinue of crimson prominences, silvery corona,
and far-spreading zodiacal light, projected on the star-spangled
black background of an absolutely unilluminated sky. Now
the spectroscope offers the means of indefinitely weakening
atmospheric glare by diffusing a constant amount of it over
an indefinitely widened area. But monochromatic or " bright-
line " light is, by its nature, incapable of being so diffused. It
can, of course, be deviated by refraction to any extent desired ;
but it always remains equally concentrated, in whatever direction
it may be thrown. Hence, when it is mixed up with continuous
light as in the case of the solar flames shining through our
atmosphere it derives a relative gain in intensity from every
addition to the dispersive power of the spectroscope with which
the heterogeneous mass of beams is analysed. Employ prisms
enough, and eventually the undiminished rays of persistent
colour will stand out from the continually fading rainbow-
tinted band, by which they were at first effectually veiled.
1 Comptes Rendus, t. Ixvii. p. 757.
214 HISTORY OF ASTRONOMY. PART 11.
This Janssen saw by a flash of intuition while the eclipse
was in progress ; and this he realised at 10 A.M. next morning,
August 19, 1868 the date of the beginning of spectroscopic
work at the margin of the unobscured sun. During the whole
of that day and many subsequent ones, he enjoyed, as he
said, the advantage of a prolonged eclipse. The intense inte-
rest with which he surveyed the region suddenly laid bare
to his scrutiny was heightened by evidences of rapid and
violent change. On the i8th of August, during the eclipse,
a vast spiral structure, at least 89,000 miles high, was per-
ceived, planted in surprising splendour on the rim of the
interposed moon. It was formed, as Major Tennant judged
from its appearance in his photographs, by the encounter of
two mounting torrents of flame, and was distinguished as the
" Great Horn." Next day it was in ruins ; hardly a trace
remained to show where it had been. 1 Janssen's spectro-
scope furnished him besides with the strongest confirmation
of what had already been reported by the telescope and the
camera as to the continuous nature of the scarlet " sierras "
lying at the base of the prominences. Everywhere at the
sun's edge the same bright lines appeared.
It was not until the 1 9th of September that Janssen thought
fit to send news of his discovery to Europe. He little dreamed
of being anticipated ; nor did he indeed grudge that science
should advance at the expense of his own undivided fame. A
few minutes before his despatch was handed to the Secretary
of the Paris Academy of Sciences, a communication similar
in purport had been received from Mr. Norman Lockyer.
There is no need to discuss the narrow and wearisome ques-
tion of priority; each of the competitors deserves, and has
obtained, full credit for his invention. With noteworthy and
confident prescience, Mr. Lockyer, in 1866, before anything
was yet known regarding the constitution of the " red flames,"
ordered a strongly dispersive spectroscope for the express pur-
pose of viewing, apart from eclipses, the bright-line spectrum
which he expected them to give. Various delays, however,
1 Comptes Rendus, t. Ixvii. p. 839.
CHAP. in. RECENT ECLIPSES. 215
supervened, and the instrument was not in his hands until
October 16, 1868. On the 2oth he picked up the vivid rays,
of which the presence and (approximately) the positions had
in the interim become known. But there is little doubt that,
even without that previous knowledge, they would have been
found ; and that the eclipse of August 1 8 only accelerated a
discovery already assured.
Dr. Huggins, meanwhile, had been tending towards the
same goal during two and a half years in his observatory at
Tulse Hill. The principle of the spectroscopic visibility of
prominence -lines at the edge of an uneclipsed sun was quite
explicitly stated by him in February I868, 1 and he devised
various apparatus for bringing them into actual view; but
not until he knew where to look did he succeed in seeing
them.
Astronomers, thus liberated, by the acquisition of power to
view them at any time, from the necessity of studying pro-
minences during eclipses, were able to concentrate the whole
of their attention on the corona. The first thing to be done
was to ascertain the character of its spectrum. This was seen
in 1868 only as a faintly continuous one; for Rayet, who
seems to have perceived its characteristic bright line far above
the summits of the flames, connected it, nevertheless, with
those objects. On the other hand, Lieutenant Campbell ascer-
tained on the same occasion the polarisation of the coronal
light in planes passing through the sun's centre, 2 thereby
showing that light to be, in whole or in part, reflected sun-
shine. But if reflected sunshine, it was objected, the chief at
least of the dark Fraunhofer lines should be visible in it, as
they are visible in moonbeams, sky illumination, and all other
sun-derived light. The objection was well founded, but was
prematurely urged, as we shall see.
On the 7th of August 1869, a track of total eclipse crossed
the continent of North America diagonally, entering at
Behring's Straits, and issuing on the coast of North Carolina.
It was beset with observers ; but the most effective work was
1 Month. Not., vol. xxviii. p. 88. 2 Proc. Roy. Soc., vol. xvii. p. 123.
216 HISTORY OF ASTRONOMY. PART n.
done in Iowa. At Des Moines, Professor Harkness of the
Naval Observatory, Washington, obtained from the corona an
"absolutely continuous spectrum," slightly less bright than
that of the full moon, but traversed by a single green ray. 1
The same green ray was seen at Burlington and its position
measured by Professor Young of Dartmouth College. 2 It
was found to coincide with that of a dark line of iron in the
solar spectrum, numbered 1474 on KirehhofTs scale. This
was perplexing, since it seemed, at first sight, to compel the
inference that the corona was actually composed of vapour of
iron, 3 so attenuated as to give only one line of secondary im-
portance out of the many hundreds belonging to it. But in
1876 Young was able, by the use of greatly increased disper-
sion, to resolve the Fraunhofer line " 1474 " into a pair, of
which one component is due to iron, the other (the more
refrangible) to the coronal gas. 4 This substance, of which
nothing is known to terrestrial chemistry, [is luminous at
least half a million of miles above the sun's surface, and must
be considerably lighter even than hydrogen.
A further trophy was carried off by American skill 5 sixteen
months after the determination due to it of the distinctive
spectrum of the corona. The eclipse of December 22, 1870,
though lasting only two minutes and ten seconds, drew
observers from the New, as well as from the Old World to the
shores of the Mediterranean. Janssen issued from beleaguered
Paris in a balloon, carrying with him the vital parts of a
reflector specially constructed to collect evidence about the
corona. But he reached Oran only to find himself shut be-
hind a cloud-curtain more impervious than the Prussian lines.
Everywhere the sky was more or less overcast. Mr. Lockyer's
journey from England to Sicily, and shipwreck in the Psyche,
were recompensed with a glimpse of the solar aureola during
one second and a half! Three parties stationed at various
1 Washington Observations, 1867, App. ii. Harkness's Report, p. 60.
2 Am. Jour., vol. xlviii. (2d series), p. 377. 3 This view was never
assented to by either Young or Lockyer. 4 Am. Jour., vol. xL (3d
series), p. 429. 5 Everything in such observations depends upon the
proper manipulation of the slit of the spectroscope.
CHAP. in. RECENT ECLIPSES. 217
heights on Mount Etna saw absolutely nothing. Neverthe-
less, important information was snatched in despite of the
elements.
The prominent event was Young's discovery of the "re-
versing layer." As the surviving solar crescent narrowed
before the encroaching moon, " the dark lines of the spectrum,"
he tells us, " and the spectrum itself, gradually faded away,
until all at once, as suddenly as a bursting rocket shoots out
its stars, the whole field of view was filled with bright lines
more numerous than one could count. The phenomenon was
so sudden, so unexpected, and so wonderfully beautiful, as to
force an involuntary exclamation." 1 Its duration was about
two seconds, and the impression produced was that of a com-
plete reversal of the Fraunhofer spectrum that is, the sub-
stitution of a bright for every dark line.
Now something of the kind was theoretically necessary to
account for the dusky rays in sunlight which have taught
us so much, and have yet much more to teach us ; so that,
although surprising from its transitory splendour, the appear-
ance could not strictly be called "unexpected." Moreover, its
premonitory symptom in the fading out of those rays had been
actually described by Father Secchi in i868, 2 and looked for
by Young as the moon covered the sun in August 1869. But
with the slit of his spectroscope placed normally to the sun's
limb, the bright lines gave a flash too thin to catch the eye.
In 1870 the position of the slit was tangential it ran along
the shallow bed of incandescent vapours, instead of cutting
across it : hence his success.
The same observation was made at Xerez de la Frontera
by Mr. Pye, a member of Young's party ; and, although an
exceedingly delicate one, has since frequently been repeated.
The whole Fraunhofer series appeared bright (omitting other
instances) to Maclear, Herschel, and Fyers in 1871, at the
beginning or end of totality ; to Pogson during a period (per-
haps erroneously estimated) of from five to seven seconds, at
the break up of an annular eclipse, June 6, 1872 ; to Stone at
1 Mem. R. A. Soc., vol. xli. p. 435. 2 Comptes Kendus, t. Ixvii. p. 1019.
2i8 HISTORY OF ASTRONOMY. PART n.
Klipfontein, April 16, 1874, when he saw "the field full of
bright lines." 1 But between the picture presented by the
"veritable pluie de lignes brillantes," 2 which descended into
M. Trepied's spectroscope for three seconds after the disap-
pearance of the sun, May 17, 1882, and the familiar one of
the dark-line solar spectrum, certain differences were per-
ceived, showing their relation to be not simply that of a posi-
tive to a negative impression.
A "reversing layer," or stratum of mixed vapours, glowing,
but at a lower temperature than that of the actual solar
surface, was an integral part of Kirchhoff's theory of the pro-
duction of the Fraunhofer lines. Here it was assumed that
the missing rays were stopped, and here also it was assumed
that the missing rays would be seen bright, could they be
isolated from the overpowering splendour of their background.
This isolation is effected by eclipses, with the result beau-
tifully confirmatory of theory of reversing, or turning from
dark to bright, the Fraunhofer spectrum. But there is a
difficulty. If absorption be in truth thus localised, it should
appear greatly strengthened near the edges of the solar disc.
This, however, is not the case. Kirchhoff met the objection
by giving a great depth to the reversing stratum, whereby the
difference in length of the paths across that stratum traversed
by rays from the sun's limb and centre became relatively
insignificant. In other words, he supposed that the chief
part of the light absent from the spectrum was arrested in the
region of the corona.
Faye, on the other hand, abolished the reversing layer
altogether (there was at that time no ocular demonstration of
its existence) ; or rather, sunk it out of sight below the visible
level of the photosphere, and got the necessary absorption done
in the interstices of the photospheric clouds by the vapours
in which they float, and from which they condense. It was,
however, at once seen that the lines thus produced would be
bright, not dark, since the brilliant cumuli would be cooled,
by their greater power of radiation, below the temperature of
1 Mem. R. A. Soc., voL xli. p. 43. 2 Comptes Rendus, t xciv. p. 1640.
CHAP. in. RECENT ECLIPSES. 219
the surrounding medium. Less obviously out of accord with
facts was an explanation offered by Professor Hastings of
Baltimore in iSSi. 1 Young's stratum, with its estimated
thickness of 600 miles, represented in his view only the upper
margin of a reversing ocean, in which the granules of the pho-
tosphere float at various depths. The necessary difference of
temperature is derived from the coolness of the descending
vapours, which bathe the radiating particles and rob them
of certain characteristic beams. Some observations, however,
made by Mr. Lockyer during the total eclipse of 1882, and by
Mr. Turner of the Greenwich Observatory during that of 1886,
threw a new light upon the matter. They seem to prove the
brief prismatic display seen at the beginning and end of
totality to be only a part of a varied and extensive phenome-
non. By careful watching, it was found to be preceded (in the
advancing phase) by the stealing out, first of short, vivid lines
close to the limb, corresponding to the highest known tempera-
tures, then of long, faint lines telling of absorption high up in
the coronal regions. Just such effects had been predicted by
Mr. Lockyer, 2 and are required by his theory of the origin of
the Fraunhofer spectrum through the combined and varying
absorption of all the successive layers of the sun's atmosphere,
each at a lower temperature, and with a higher molecular
complexity than those beneath. Thus a strict correspondence
between the bright rays of the so-called " reversing layer"
and the solar dusky rays is not to be expected, and would, in
fact, prove somewhat embarrassing. M. Tre"pied's detection
of differences is, for this reason, especially valuable, and we
may hope that, before long, an instantaneous photograph of
the complete " rainbow-flash " accompanying totality will afford
a more stable support to theory on the subject than it can yet
claim.
The last of the five eclipses which we have grouped together
for separate consideration was visible in Southern India and
Australia, December 12, 1871. Some splendid photographs
were secured by the English parties on the Malabar coast,
1 Am. Jour, of Science, vol. xxi. p. 33. 2 Chemist, of Sun, p. 359.
220 HISTORY OF ASTRONOMY. PART n.
showing, for the first time, the remarkable branching forms of
the coronal emanations ; but the most conspicuous result was
Janssen's detection of some of the dark Fraunhofer lines, long
vainly sought in the continuous spectrum of the corona. Chief
amongst these was the D line of sodium, the original index,
it might be said, to solar chemistry. No proof could be
afforded more decisive than this faint echoing batik of the
distinctive notes of the Fraunhofer spectrum, that the polari-
scope had spoken the truth in asserting a large part of the
coronal radiance to be reflected sunlight. But it is (especi-
ally at certain epochs) so drenched in original luminous emis-
sions, that its characteristic features are almost obliterated.
Janssen's success in seizing them was due in part to the
extreme purity of the air at Sholoor, in the Neilgherries,
where he was stationed ; in part to the use of an instrument
adapted by its large aperture and short focus to give an image
of the utmost possible luminosity.
His observations further "peremptorily demonstrated" the
presence of hydrogen far outside the region of prominences,
and forming an integral constituent of the corona. This im-
portant fact was simultaneously attested by Lockyer at Baikul,
and by Respighi at Poodacottah, each making separate trial
of a " slitless spectroscope " devised for the occasion. This
consists simply of a prism placed outside the object-glass of a
telescope or the lens of a camera, whereby the radiance encom-
passing the eclipsed sun is separated into as many differently
tinted rings as it contains different kinds of light. These
tinted rings were viewed by Respighi through a telescope,
and were photographed by Lockyer, with the same result of
showing hydrogen to ascend uniformly from the sun's surface
to a height of fully 200,006 miles. Another notable observa-
tion made by Herschel and Tennant at Dodabetta showed the
green ray " 1474 " to be just as bright in a "rift " as in the
adjacent streamer. The visible structure of the corona was
thus seen to be independent of the distribution of the gases
which enter into its composition.
By means, then, of the five great eclipses of 1860-71 it was
CHAP. in. RECENT ECLIPSES. 221
ascertained : first, that the prominences, and at least the
lower part of the corona, are genuine solar appurtenances ;
secondly, that the prominences are composed of hydrogen and
other gases in a state of incandescence, and rise, as irregular
outliers, from a continuous envelope of the same materials,
some thousands of miles in thickness ; thirdly, that the corona
is of a highly complex constitution, being made up in part of
glowing vapours, in part of matter capable of reflecting sun-
light. We may now proceed to consider the results of sub-
sequent eclipses.
These have raised, and have helped to solve, some very
curious questions. Indeed, every carefully watched total
eclipse of the sun stimulates as well as appeases curiosity,
and leaves a legacy of outstanding doubt, continually, as time
and inquiry go on, removed, but continually replaced. It
cannot be denied that the corona is a perplexing phenomenon,
and that it does not become less perplexing as we know more
about it. It presented itself under quite a new and strange
aspect on the occasion of the eclipse which visited the Western
States of North America, July 29, 1878. The conditions of
observation were peculiarly favourable. The weather was
superb; above the Rocky Mountains the sky was of such
purity as to permit the detection, with the naked eye, of
Jupiter's satellites on several successive nights. The opportu-
nity of advancing knowledge was made the most of. Nearly a
hundred astronomers (including several Englishmen) occupied
twelve separate posts, and prepared for an attack in force.
The question had often suggested itself, and was a natural
one to ask, whether the corona sympathises with the general
condition of the sun ? whether, either in shape or brilliancy,
it varies with the progress of the sun-spot period 1 A more
propitious moment for getting this question answered could
hardly have been chosen than that at which the eclipse
occurred. Solar disturbance was just then at its lowest ebb.
The development of spots for the month of July 1878 was
represented on Wolf's system of "relative numbers" by the
fraction o.i, as against 135.4 for December 1870, an epoch of
222 HISTORY OF ASTRONOMY. PART n.
maximum activity. The " chromosphere " l was, for the most
part, shallow and quiescent ; its depth, above the spot- zones,
had sunk from about 6000 to 2000 miles ; prominences were
few and faint. Obviously, if a type of corona corresponding
to a minimum of sun-spots existed, it should be seen then
or never. It was seen ; but while, in some respects, it
agreed with anticipation, in others it completely set it at
naught.
The corona of 1878, as compared with those of 1869, 1870,
and 1871, was generally admitted to be shrunken in its main
outlines, and much reduced in brilliancy. Mr. Lockyer pro-
nounced it ten times fainter than in 1871 ; Professor Hark-
ness estimated its light at less than one- seventh that derived
from the mist-blotted aureola of 1 870.2 In shape, too, it
was markedly different. When sun-spots are numerous,
the corona appears to be most fully developed above the spot-
zones, thus offering to our eyes a rudely quadrilateral con-
tour. The four great luminous sheaves forming the corners
of the square are made up of rays curving together from each
side into "synclinal" or ogival groups, each of which may
be compared to the petal of a flower. To Janssen, in 1871,
the eclipsing moon seemed like the dark heart of a gigantic
dahlia, painted in light on the sky ; and the similitude to the
ornament on a compass-card, used by Sir George Airy in 1851,
well conveys the decorative effect of the beamy, radiated kind
of aureola, never, it would appear, absent when solar activity
is at a tolerably high pitch. In his splendid volume on
eclipses, 3 Mr. Ranyard first generalised the peculiarity of the
synclinal structures by a comparison of records ; but the
symmetry of their arrangement, though frequently striking,
is liable to be confused by secondary formations. Nothing
of all this, however, was visible in 1878. Instead, there was
seen, as the groundwork of the corona, a ring of pearly light,
1 The rosy envelope of prominence-matter was so named by Lockyer in
1868 (Phil. Trans., vol. clix. p. 430) ; and the appellation, its defiance of
Greek grammar notwithstanding, has had vitality to survive and prevail.
2 Bull. Phil. Soc. Washington, vol. iii. p. 118. 3 Mem. R. A. Soc., vol.
xli. 1879.
CHAP. in. RECENT ECLIPSES. 223
nebulous to the eye, but shown by telescopes and in photo-
graphs to have a fibrous texture, as if made up of bundles of
fine hairs. North and south a series of short, vivid, electrical-
looking flame-brushes diverged with conspicuous regularity
from each of the solar poles. Their direction was not towards
the centre of the sun, but towards each summit of his axis,
so that the farther rays on either side started almost tan-
gentially to the surface. It is difficult not to connect this
unusual display of polar activity 1 with the great relative
depth of the chromosphere in those regions, noticed by Trou-
velot previous to the eclipse. 2
But the leading, and a truly amazing, characteristic of the
phenomenon was formed by two vast, faintly-luminous wings
of light, expanded on either side of the sun in the direction of
the ecliptic. These were missed by very few careful onlookers ;
but- the extent assigned to them varied with skill in, and
facilities for seeing. By far the most striking observations
were made by Newcomb at Separation (Wyoming), by Cleve-
land Abbe from the shoulder of Pike's Peak, and by Lang-
ley at its summit, an elevation of 14,100 feet above the
sea. Never before had an eclipse been viewed from anything
approaching that altitude, or under so translucent a sky. A
proof of the great reduction in atmospheric glare was afforded
by the perceptibility of the corona for above four minutes after
totality was over. For the 165 seconds of its duration, the
remarkable streamers above alluded to continued " persistently
visible," stretching away right and left of the sun to a distance
of at least ten million miles ! One branch was traced over
an apparent extent of fully twelve lunar diameters, without
sign of a definite termination having been reached ; and there
were no grounds for supposing the other more restricted.
The axis of the longest ray was found to coincide exactly, so
1 Professor W. A. Norton observed a similar phenomenon in 1869,
accompanied by some symptoms of equatorial emission. This is the more
remarkable as 1869 was a year of many sun-spots. His evidence, though
unsupported, and adverse to the theory of varying types, should not be
overlooked. See Am. Jour, of Sc., vol. i. (3d ser.), p. I. 2 Wash. Obs.
1876, App. iii. p. 80.
224 HISTORY OF ASTRONOMY. PART n.
far as could be judged, with the ecliptic. 1 Pale cross-beams
were seen by Young and Abbe.
The resemblance to the zodiacal light was striking ; and a
community of origin between that enigmatical member of our
system and the corona was irresistibly suggested. We should,
indeed, expect to see, under such exceptionally favourable
atmospheric conditions as Professor Langley enjoyed on Pike's
Peak, the roots of the zodiacal light presenting near the sun
just such an appearance as he witnessed ; but we can imagine
no reason why their visibility should be associated with a low
state of solar activity. Nevertheless this seems to be the case
with the streamers which astonished astronomers in 1878.
For in August 1867, when similar equatorial emanations,
accompanied by similar symptoms of polar excitement, were
described and depicted by Grosch 2 of the Santiago Observa-
tory, sun-spots were at a minimum; while the corona of 1715,
which appears from the record of it by Roger Cotes 3 to have
been of the same type, preceded by three years the ensuing
maximum. The eclipsed sun was seen by him at Cambridge,
May 2, 1715, encompassed with a ring of light about one-sixth
of the moon's diameter in breadth, upon which was super-
posed a luminous cross formed of long bright branches lying
very nearly in the plane of the ecliptic, and shorter polar
arms so faint as to be only intermittently visible. The re-
semblance between his sketch and Cleveland Abbe's drawing
of the corona of 1878 is extremely striking. It should, never-
theless, be noted that some conspicuous spots were visible on
the sun's disc at the time of Cotes 's eclipse, and that the pre-
ceding minimum (according to Wolf) occurred in 1712. Thus,
the coincidence of epochs is imperfect. Should, however, the
corona of 1878 reappear in 1889, the presumed connection will
be solidly established.
Professor Cleveland Abbe was fully persuaded that the long
rays carefully observed by him from Pike's Peak were nothing
1 Wash. Obs., 1876, App. iii. p. 209. 2 Astr. Nach., No. 1737. 3 Cor-
respondence with. Newton, pp. 181-184; Ranyard, Mem. R. Astr. Soc.,
vol. xli. p. 501.
CHAP. in. RECENT ECLIPSES. 225
else than streams of meteorites rushing towards or from peri-
helion ; and it is quite certain that the solar neighbourhood
must be crowded with such bodies. But there are no grounds
for supposing that they affect the ecliptic more than any other
of the infinite number of planes passing through the sun's
centre. On the contrary, everything we know leads us to
believe that meteorites, like their cometary allies, yield no
obedience to the rules of the road which bind the planets, but
travel in either direction indifferently, and in paths inclined
at any angle to the fundamental plane of our system. Be-
sides, the peculiar structure at the base of the streamers dis-
played in the photographs, the curved rays meeting in pointed
arches like Gothic windows, the visible upspringing tendency,
the filamentous texture, speak unmistakably of the action of
forces proceeding from the sun, not of extraneous matter
circling round him.
Again, it may be asked what possible relation can exist
between the zodiacal plane and the sun's internal activity ?
For it is a remarkable fact that to this approximately, and
not to the level of the solar equator, the streamers conformed.
We are acquainted with no such relation ; but it may be re-
marked that the coronal axis of symmetry has frequently been
observed during eclipses to be inclined at an appreciable angle
to the solar axis of rotation, and the corresponding " magnetic
equator " might quite conceivably be the scene of emanations
induced by some form of electrical repulsion.
The surest, though not the most striking, proof of sympa-
thetic change in the corona is afforded by the analysis of its
light. In 1878 the bright lines so conspicuous in the coronal
spectrum in 1870 and 1871 were discovered to have faded to
the very limits of visibility. Several skilled observers failed
to see them at all; but Young and Eastman succeeded in
tracing both the hydrogen and the green " 1474 " rays all
round the sun, to a height estimated at 340,000 miles. The
substances emitting them were thus present, though in a low
state of incandescence. The continuous spectrum was rela-
tively strong ; a faint reflection of the Fraunhofer lines was
P
226 HISTORY OF ASTRONOMY. PART n.
traced in it ; and polarisation was undoubted, increasing
towards the limb, whereas in 1870 it reached a maximum at
a considerable distance from it. Experiments with Edison's
tasimeter showed that the corona radiates a sensible amount
of heat.
The next promising eclipse occurred May 17, 1882. The
concourse of astronomers which has become usual on such
occasions assembled this time at Sohag, in Upper Egypt.
Rarely have seventy-four seconds been turned to such account.
To each observer a special task was assigned, and the advan-
tages of a strict division of labour were visible in the variety
and amount of the information gained.
The year 1882 was one of numerous sun-spots. On the eve
of the eclipse twenty-three separate maculae were counted. If
there were any truth in the theory which connected coronal
forms with fluctuations in solar activity, it might be anticipated
that the vast ecliptical expansions and polar " brushes " of
1878 would be found replaced by the star-like structure of
1871. This expectation was literally fulfilled. No zodiacal
streamers were to be seen. The universal failure to perceive
them, after express search in a sky of the most transparent
purity, justifies the emphatic assertion that they were not there.
Instead, the type of corona observed in India eleven years
earlier was reproduced, with its shining aigrettes, complex
texture, and brilliant decorative effect.
Concordant testimony was given by the spectroscope. The
reflected light derived from the corona was weaker than in
1878, while its original emissions were proportionately in-
tensified. A number of new bright lines were discovered.
Tacchini determined four in the red end of the spectrum;
Thollon perceived several in the violet ; and Dr. Schuster
measured and photographed about thirty. 1 The Fraunhofer
lines autographically recorded in the continuous spectrum
were not less numerous. This was the first successful attempt
to photograph the spectrum of the corona as seen with an
ordinary slit-spectroscope. The slitless spectroscope, or "pris-
1 Proc. Roy. Soc., vol. xxxv. p. 154.
CHAP. in. RECENT ECLIPSES. 227
matic camera," although its statements are necessarily of a far
looser character, was, however, also profitably employed. The
uncommon strength in the chromospheric regions of the violet
light 'concentrated in the two lines H and K, attributed to
calcium, was strikingly brought out by it ; as well as the fact
that the substance emitting the line "1474" (which might
conveniently be termed coronium) belongs especially, if not
exclusively, to the corona as distinguished from the promi-
nences. Mr. Lockyer observed the continuous part of the
coronal spectrum to be curiously ribbed and fluted; and in
the spectrum of one prominence twenty-nine rays were photo-
graphed, including the hydrogen ultra-violet series, discovered
by Dr. Huggins in the emissions of white stars. 1
Dr. Schuster's photographs of the corona itself were the
most extensive, as well as the most detailed, of any yet
secured. One rift imprinted itself on the plates to a distance
of nearly a diameter and a half from the limb ; and the trans-
parency of the streamers was shown by the delineation through
them of the delicate tracery beyond. The singular and pic-
turesque feature was added of a bright comet, self -depicted
in all the exquisite grace of swift movement betrayed by the
fine curve of its tail, hurrying away from, possibly, its only
visit to our sun, and rendered momentarily visible by the
withdrawal of the splendour in which it had been, and was
again quickly veiled.
From a careful study of these valuable records Dr. Huggins
derived the idea of a possible mode of photographing the
corona without an eclipse. 2 As already stated, its ordinary
invisibility is entirely due to the " glare " or reflected light
diffused through our atmosphere. But Dr. Huggins found, on
examining Schuster's negatives, that a large proportion of the
light in the coronal spectrum, both continuous and interrupted,
is collected in the violet region between the Fraunhofer lines
1 Abney, Phil. Trans., vol. clxxv. p. 267. 2 Proc. Poy. Soc., vol. xxxiv.
p. 409. Experiments directed to the same end had been made by Dr. 0.
Lohse at Potsdam, 1878-80 ; not without some faint promise of ultimate
success. Astr. Nach., No. 2486.
22 8 HISTORY OF ASTRONOMY. PART n.
G and H. There, then, he hoped that, all other rays being
excluded, it might prove strong enough to vanquish inimical
glare, and stamp on prepared plates, through local superiority
in illuminative power, the forms of the appendage by which it
is emitted.
His experiments were begun towards the end of May 1882,
and by September 28 he had obtained a fair earnest of success.
The exclusion of all other qualities of light save that with
which he desired to operate, was at first effected by the inter-
position of screens of purple glass, or other similarly absorbing
media; later, however, his purpose was more simply and
efficaciously realised by using chloride of silver as his sensitive
material, that substance being chemically inert to all other but
those precise rays in which the corona has the advantage. 1
The genuineness of the impressions left upon his plates was
strongly attested. " Not only the general features," Captain
Abney bore them witness, 2 " are the same, but details, such as
rifts and streamers, have the same position and form." It
was found, moreover, that the corona photographed during
the total eclipse of May 6, 1883, was intermediate in shape
between the coronas photographed by Dr. Huggins before and
after that event, each picture taking its proper place in a
series of progressive modifications highly interesting in them-
selves, and full of promise for the value of the method
employed to record them. 3 But experiments on the subject
were singularly interrupted. The volcanic explosion in the
Straits of Sunda in August 1883 brought to astronomers a
peculiarly unwelcome addition to their difficulties. The mag-
nificent sunglows due to the diffractive effects on light of the
vapours and fine dust flung in vast volumes into the air,
and rapidly diffused all round the globe, betokened an atmos-
pheric condition of all others the most prejudicial to delicate
researches in the solar vicinity. The filmy coronal forms,
1 The sensitiveness of chloride of silver extends from h to H ; that is,
over the upper or more refrangible half of the space in which the main
part of the coronal light is concentrated. 2 Proc. Roy. Soc., vol. xxxiv.
p. 414. 3 Report Brit. Assoc., 1883, p. 351.
CHAP. in. RECENT ECLIPSES. 229
accordingly, which had been hopefully traced on Dr. Huggins's
plates, ceased to appear there; nor were any substantially
better results obtained by Mr. C. Ray Woods, in the purer
air either of the Eiffel or the Cape of Good Hope, during the
three ensuing years. Nay, doubts were expressed as to the
genuineness of what had at first seemed to be accomplished ;
and the eclipse of the sun on August 29, 1886, was antici-
pated as an opportunity for resolving them in one sense or
the other. For, evidently, in a true coronal photograph taken
during the partial phases, the contour of the moon off the
sun must stand out against the faint radiance beyond ; while
deceptive appearances caused by air-glare would take no
notice of the moon, for the simple reason that they would
originate in front of her. No trace of the lunar globe, how-
ever, was visible on any of the plates exposed on August 29, at
Grenada; and what vestiges of " structure " there were, came
out almost better upon the moon than beside her, thus stamp-
ing themselves at once as spurious. It is hence quite certain
that they had not been appreciably affected by coronal light.
This was discouraging ; but when all the circumstances are
taken into account, scarcely surprising. The exceptional
darkness of the sky during totality showed that the corona
was, on that occasion, of small intrinsic splendour; it shone,
moreover, through air heavily laden with moisture ; the sun,
when the observations were made, had less than nineteen
degrees of altitude, so that a large proportion of the highly
refrangible rays selected by silver chloride must have been
cut off by atmospheric absorption ; finally, the eruptive pro-
ducts from Krakatao were still far from having wholly sub-
sided. That the effect sought is a possible one is proved by
the distinct appearance of the moon projected on the corona,
in Liais' photographs of the partially eclipsed sun in I858. 1
And his plates were of course not specially prepared to seize
differential effects of light.
1 Comptes Rendus, t. xlvii. p. 789. For instances of the same visual
appearance, see Trouvelot, Observatory, vol. ix. p. 395, and G. Dollond's
observation of Nov. 29, 1826, Month. Notices, vol. i. p. 26.
230
HISTORY OF ASTRONOMY. PART n.
For the effective success of Dr. Huggins's method, never-
theless, rarely favourable conditions are undoubtedly requisite.
Results of substantial value from it can hardly be hoped for
in this climate, and at] the sea-level. Every yard of ascent,
however, towards the void of space tells in its favour; and
coronal photography may perhaps in no distant future find
a place among the special branches of research pursued at
mountain observatories.
The prosperous result of the' Sohag observations stimulated
the desire to repeat them on the first favourable opportunity.
This offered itself one year later, May 6, 1883, yet not with-
out the drawbacks incident to terrestrial conditions. The
eclipse promised was of rare length, giving no less than five
minutes and twenty-three seconds of total obscurity, but its
path was almost exclusively a " water- track." It touched
land only on the outskirts of the Marquesas group in the
Southern Pacific, and presented, as the one available foothold
for observers, a coral reef named Caroline Island, seven and
a half miles long by one and a half wide, unknown previous
to 1874, and visited only for the sake of its stores of guano.
Seldom has a more striking proof been given of the vividness
of human curiosity as to the condition of the worlds outside
our own, than in the assemblage of a group of distinguished
men from the chief centres of civilisation, on a barren ridge,
isolated in a vast and tempestuous ocean, at a distance, in
many cases, of 11,000 miles and upwards from the ordinary
scene of their labours. And all these sacrifices the cost and
care of preparation, the transport and readjustment of delicate
instruments, the contrivance of new and more subtle means
of investigating phenomena on the precarious chance of a
clear sky during one particular five minutes ! The event,
though fortunate, emphasised the hazard of the venture. The
observation of the eclipse was made possible only by the happy
accident of a serene interval between two storms.
The American expedition was led by Professor Edward S.
Holden, and to it were courteously permitted to be attached
Messrs. Lawrance and Woods, photographers, sent out by the
CHAP. in. RECENT ECLIPSES. 231
Royal Society of London. M. Janssen was chief of the
French Academy mission ; he was accompanied from Meudon
by Trouvelot, and joined from Vienna by Palisa, and from
Rome by Tacchini. A large share of the work done was
directed to assuring or negativing previous results. The
circumstances of an eclipse favour illusion. A single obser-
vation by a single observer, made under unfamiliar conditions,
and at a moment of peculiar excitement, can scarcely be re-
garded as offering more than a suggestion for future inquiry.
But incredulity may be carried too far. Janssen, for instance,
felt compelled by the survival of unwise doubts, to devote
some of the precious minutes of obscurity at Caroline Island
to confirming what, in his own persuasion, needed no confir-
mation that is, the presence of reflected Fraunhofer lines
in the spectrum of the corona. Trouvelot and Palisa, on the
other hand, instituted an exhaustive, but fruitless search for
the spurious " intramercurian " planet announced by Swift
and Watson in 1878.
New information, however, was not deficient. The corona
proved identical in type with that of 1882, agreeably to what
was expected at an epoch of protracted solar activity. The
characteristic aigrettes (of which five appeared in Mr. Dixon's
sketch) were of even greater brilliancy than in the preceding
year, and the chemical intensity of the coronal light then
first measured with some precision was found to exceed that
of full moonlight. Janssen's photographs, owing to the con-
siderable apertures (six and eight inches) of his object-glasses,
and the long exposures permitted by the duration of totality,
were singularly perfect ; they gave a greater extension to the
corona than could be traced with the telescope, 1 and showed
its forms as absolutely fixed and of remarkable complexity.
The English pictures, taken with exposures up to sixty
seconds, were likewise of great value. They exhibited details
of structure from the limb to the tips of the streamers, which
terminated definitely, and as it seemed actually, where the
impressions on the plates ceased. The coronal spectrum
1 Comptes JRendus, t. xcvii. p. 59 2 -
232 HISTORY OF ASTRONOMY. PART n.
was also successfully photographed, with a number of bright
and dark lines ; and a print was caught of some of the more
prominent rays of the reversing layer just before and after
totality. The use of the prismatic camera was baffled by the
anomalous scarcity of prominences.
A highly suggestive observation was made during this eclipse
by Professor Tacchini. One of the aigrettes of the corona
displayed in his spectroscope, on a feebly continuous back-
ground, two of the bright bands familiar in the hydrocarbon
spectrum of comets. 1 This requires confirmation which the
latest eclipse has failed to supply ; nevertheless, the analogy
which it hints at is a tempting one. The resemblance of the
silvery sheaves of the corona to the tails of comets had already
given rise to much fruitless speculation ; and the exertion of
a repulsive force, such as is obviously at work in comets, by
the sun on his surroundings, has been considered, by some
solar physicists, absolutely necessary to explain the lowness
of atmospheric pressure at his surface. The presence of
carbon in the sun's atmosphere was inferred by Mr. Lockyer
in 1878 from a comparison of photographs of the solar and
electric-arc spectra; 2 Dr. Schuster obtained indications of
the same kind with the prismatic camera at Sohag; and
Captain Abney finds hydrocarbon bands in the invisible or
infra-red part of the Fraunhofer spectrum. 3 But the subject
is still obscure.
Another of the observations made at Caroline Island,
although probably through some unexplained cause delusive,
merits some brief notice. Using an ingenious apparatus for
viewing simultaneously the spectrum from both sides of the
sun, Professor Hastings saw (as he supposed) certain alter-
nations, with the advance of the moon, in the respective
heights above the right and left solar limbs of the coronal
line " 1474," which were thought to imply that the old
exploded idea was after all a true one, and that the corona,
with its rifts and sheaves and " tangled hanks " of rays, is an
1 Comptcs Rendus, t. xcvii. p. 594. 2 Proc. Roy. Soc., vol. xxvii. p. 308.
3 Report Brit. Ass., 1881, p. 524.
CHAP. HI. RECENT ECLIPSES. 233
illusive appearance produced by the diffraction of sunlight at
the moon's edge. 1 But the whole course of recent research is
against such a supposition, even were the validity of Professor
Hastings's arguments in favour of its optical possibility ad-
mitted. Atmospheric diffusion may indeed, under favouring
circumstances, be effective in deceptively enlarging solar
appendages ; but always to a very limited extent.
The controversy is an old one as to the part played by our
air in producing the radiance visible round the eclipsed sun.
In its original form, it is true, it came to an end when Pro-
fessor Harkness, in i869, 2 pointed out that the shadow of the
moon falls equally over the air and on the earth, and that if
the sun had no luminous appendages, a circular space of
almost absolute darkness would consequently surround the
apparent places of the superposed sun and moon. Mr. Proc-
tor, 3 with his usual ability, impressed this mathematically
certain truth (the precise opposite of the popular notion) upon
public attention ; and Sir John Herschel calculated that the
diameter of the " negative halo" thus produced would be, in
general, no less than 23.
But about the same time a noteworthy circumstance relat-
ing to the state of things in the solar vicinity was brought
into view. On February n, 1869, Messrs. Frankland and
Lockyer communicated to the Royal Society a series of experi-
ments on gaseous spectra under varying conditions of heat and
density, leading them to the conclusion that the higher solar
prominences exist in a medium of excessive tenuity, and that
even at the base of the chromosphere the pressure is far below
that at the earth's surface. 4 This inference was fully borne
out by the researches of Wullner ; and Janssen expressed the
opinion that the chromospheric gases are rarefied almost to
the degree of an air-pump vacuum. 5 Hence was derived a
general and fully justified conviction that there could be out-
side, and incumbent upon the chromosphere, no such vast
1 Memoirs National Ac. of Sciences, vol. ii. p. 102. 2 Wash. Obs. t
1867, App. ii. p. 64. 3 The Sun, p. 357. 4 Proc. Roy. Soc., vol. xvii. p.
289. 5 Comptes Rendus, t. Ixxiii. p. 434.
234 HISTORY OF ASTRONOMY. PART n.
atmosphere as the corona appeared to represent. Upon the
strength of which conviction the " glare " theory entered,
chiefly under the auspices of Mr. Lockyer, upon the second
stage of its existence.
The genuineness of the " inner corona " to a height of 5'
or 6' from the limb was admitted ; but it was supposed that
by the detailed reflection of its light in our air the far more
extensive " outer corona" was optically created, the irregu-
larities of the moon's edge being called in to account for the
rays and rifts by which its structure was varied. This view
received some countenance from Maclear's observation, during
the eclipse of 1870, of bright lines "everywhere" even at
the centre of the lunar disc. Here, indeed, was an undoubted
case of atmospheric diffusion ; but here, also, was a safe index
to the extent of its occurrence. Light scatters equally in all
directions; so that when the moon's face at the time of an
eclipse shows (as is the common case) a blank in the spectro-
scope, it is quite certain that the corona is not noticeably
enlarged by atmospheric causes. A sky drifted over with
thin cirrous clouds and air charged with aqueous vapour
amply accounted for the abnormal amount of scattering in
1870.
But even in 1870 positive evidence was obtained of the
substantial reality of the radiated outer corona, in the appear-
ance on the photographic plates exposed by Willard in Spain
and by Brothers in Sicily, of identical dark rifts. The truth
is, that far from being developed by misty air, it is peculiarly
liable to be effaced by it. The purer the sky, the more exten-
sive, brilliant, and intricate in the details of its structure the
corona appears. Take as an example General Myer's descrip-
tion of the eclipse of 1869, as seen from the summit of White
Top Mountain, Virginia, at an elevation above the sea of 5530
feet, in an atmosphere of peculiar clearness.
"To the unaided eye," he wrote, 1 "the eclipse presented,
during the total obscuration, a vision magnificent beyond
description. As a centre stood the full and intensely black
1 Wash. Obs., 1867, App. ii. p. 195.
CHAP. HI. RECENT ECLIPSES. 235
disc of the moon, surrounded by the aureola of a soft bright
light, through which shot out, as if from the circumference of
the moon, straight, massive, silvery rays, seeming distinct
and separate from each other, to a distance of two or three
diameters of the solar disc ; the whole spectacle showing as
on a background of diffused rose-coloured light."
On the same day, at Des Moines, Newcomb could perceive,
through somewhat hazy air, no long rays, and the four-pointed
outline of the corona reached at its farthest only a single semi-
diameter of the moon from the limb. The plain fact, that
our atmosphere acts rather as a veil to hide the coronal radi-
ance than as the medium through which it is visually formed,
emerges from the records of innumerable other observations.
No observations of importance were made during the eclipse
of September 9, 1885. The path of total obscuration touched
land only on the shores of New Zealand, and two minutes
was the outside limit of available time. Hence local observers
had the phenomenon to themselves ; nor were they even
favoured by the weather in their efforts to make the most of
it. One striking appearance was, however, disclosed. It was
that of two "white" prominences of unusual brilliancy, shining
like a pair of electric lamps hung one at each end of a solar
diameter, right above the places of two large spots. 1 This
coincidence of diametrically opposite disturbances is of too
frequent occurrence to be accidental. M. Trouvelot observed
at Meudon, June 26, 1885, two active and evanescent pro-
minences thus situated, each rising to the enormous height of
300,000 miles, and on August 16, one scarcely less remark-
able, balanced by an antipodal spot -group. 2 It towered
upward, as if by a process of unrolling, to a quarter of a
million of miles ; after which, in two minutes, the light died
out of it ; it had become completely extinct.
The eclipse of August 29, 1886, was total during about four
minutes over tropical Atlantic regions ; and an English expe-
dition, led by Mr. Lockyer, was accordingly despatched to
1 Stokes, Anniversary Address, Nature, vol. xxxv. p. 114. 2 Comptes
Jtendus, t. ci. p. 50.
236 HISTORY OF ASTRONOMY. PART n.
Grenada in the "West Indies, for the purpose of using the-
opportunity it offered. But the rainy season was just then
at its height ; clouds and squalls were the order of the day ;
and the elaborately planned programme of observation could
only in part be carried through. Some good work, none the
less, was done. A number of photographs were taken, the
precise value of which cannot yet be appraised ; and observa-
tions of great theoretical importance were made, tending (as
already stated) to widen the interpretation given to the
fugitive shower of bright lines marking the beginning and
end of totality. Professor Tacchini, who had been invited to
accompany the party, ascertained besides some significant
facts about prominences. From a comparison of their forms
and sizes during and after the eclipse, it appeared that only
the glowing vaporous cores of those objects are shown by the
spectroscope under ordinary circumstances; their upper sec-
tions, giving a faint continuous spectrum, and composed of
presumably cooler materials, remain hidden save when the veil
of scattered light usually drawn over them is removed by an
eclipse. Thus all moderately tall prominences have silvery
summits ; but all do not appear to possess the " red heart of
flame," by which alone they can be rendered perceptible to
daylight observation. Some are now found to be ordinarily
invisible, because silvery throughout " sheeted ghosts," as it
were, met only in the dark. Specimens of this class were noted
by Tacchini both in 1883 and 1886; and they are thought to
embody condensations and downrushes, such as were assumed,
but had not hitherto been seen to be in progress.
The stellate form of corona displayed at Caroline Island
was still preserved in 1885 and 1886. Nothing could be seen
of equatorial streamers, notwithstanding that at Grenada care-
ful precautions were taken against missing them. It is now
tolerably certain that they belong to a special coronal type,
and disappear with it.
Summing up what we have learned about the corona during
some forty-five minutes of scrutiny in as many years, we may
state, to begin with, that it is not a solar atmosphere. It does
CHAP. in. RECENT ECLIPSES. 237
not gravitate upon the sun's surface and share his rotation,
as our air gravitates upon and shares the rotation of the
earth ; and this for the simple reason that there is no visible
growth of pressure downward (such as the spectroscope
would infallibly give notice of) in its gaseous constituents;
whereas under the sole influence of the sun's attractive
power, their density should be multiplied many million
times in the descent through a mere fraction of their actual
depth. 1
The corona is properly described as a solar appendage ; and
may be conjecturally defined as matter in a perpetual state of
efflux from, and influx to our great luminary, under the stress
of electrical repulsion in one direction and of gravity in the
other. 2 Its constitution is of a composite character. It is
partly made up of self-luminous gases, chiefly hydrogen, and
the unknown substance giving the green ray " 1474; " partly
of white-hot solid or liquid particles, shining with continuous
light, both reflected and original. There is a strong probability
that it is affected by the periodic ebb and flow of solar activity,
the rays emitted by the gases contained in it fading, and the
continuous spectrum brightening, at times of minimum sun-
spots, as if by a fall of temperature producing, on the one
hand, a decline in luminosity of the incandescent materials
existing near the sun, and, on the other, a condensation of
vapours previously invisible into compact particles of some
reflective capacity. The coronal materials must be of incon-
ceivable tenuity, since comets cut their way through them
without experiencing sensible retardation. Not even Mr.
Crookes's vacua can give an idea of the rarefaction which this
fact implies. Yet the observed luminous effects may not in
reality bear witness contradictory of it. One solitary molecule
in each cubic inch of space, might, in Professor Young's opinion,
produce them ; while in the same volume of ordinary air at
1 See Huggins, Proc. Roy. Soc., vol. xxxix, p. 108 ; and Young, North
Am. Review, Feb. 1885, p. 179. 2 Professor W. A. Norton, of Yale
College, appears to have been the earliest formal advocate of the Expulsion
Theory of the solar surroundings, in the second (1845) an d later editions of
his Treatise on Astronomy.
238 HISTORY OF ASTRONOMY. PART n.
the sea level, the molecules number (according to Mr. John-
ston Stoney) 20,000 trillions !
The most important lesson, however, derived from eclipses
is that of independence of them. Some of its fruits in the
daily study of prominences the next chapter will collect ; while
the possibility of attaining, through Dr. Huggins's photo-
graphic method, a corresponding power as regards the corona,
opens a hopeful future to the investigation of that still pro-
blematical phenomenon.
239
CHAPTER IY.
SOLAR SPECTROSCOPY.
THE new way struck out by Janssen and Lockyer was at once
and eagerly followed. In every part of Europe, as well as in
North America, observers devoted themselves to the daily
study of the chromosphere and prominences. Foremost among
these were Lockyer in England, Zollner at Leipzig, Spb'rer at
Anclam, Young at Hanover, New Hampshire, Secchi and
Respighi at Rome. There were many others, but these names
are conspicuous from the outset.
The first point to be cleared up was that of chemical com-
position. Leisurely measurements verified the presence above
the sun's surface of hydrogen in prodigious masses, but showed
that sodium had nothing to do with the orange-yellow ray
identified with it in the haste of the eclipse. From its vicinity
to the D pair (than which it is slightly more refrangible), the
prominence-line was, however, designated D3, and the unknown
substance emitting it was named by Frankland " helium."
Young is inclined to associate with it two other faint but per-
sistent lines in the spectrum of the chromosphere; 1 and Messrs.
Liveing and Dewar pointed out, in 1879,2 that the wave-lengths
of all three are bound together with that of the coronal ray
"1474" by numerical ratios virtually the same with those
underlying the vibrations of hydrogen, and also conformed to
by certain lines of lithium and magnesium. This obscure but
interesting subject deserves further inquiry. It should be
1 Phil. Mag. vol. xlii. 1871, p. 380. 2 Proc. Roy. Soc., vol. xxviii. p.
475-
2 4 o HISTORY OF ASTRONOMY. PART n.
added that Mr. Lockyer attributes both the D3 and " 1474 "
lines to a modification of hydrogen ; the actual relation, how-
ever, would seem to be one of analogy rather than of identity.
Hydrogen and helium form the chief and unvarying materials
of the solar sierra and its peaks ; but a number of metallic
elements make their appearance spasmodically under the in-
fluence of disturbances in the layers beneath. In September
1871, Young 1 drew up at Dartmouth College a list of 103
lines significant of injections into the chromosphere of iron,
titanium, calcium, magnesium, and many other substances.
During two months' observation in the pure air of Mount
Sherman (8335 feet high) in the summer of 1872, these tell-
tale lines mounted up to 273 ; and he believes their number
might still be doubled by steady watching. Indeed, both
Young and Lockyer have more than once seen the whole field
of the spectroscope momentarily inundated with bright rays,
as if the " reversing layer " seen at the beginning and end of
eclipses had been suddenly thrust upward into the chromo-
sphere, and as quickly allowed to drop back again. It would
thus appear that the two form one continuous region, of which
the lower parts are habitually occupied by the heaviest vapours,
but where orderly arrangement is continually overturned by
violent eruptive disturbances.
The study of the forms of prominences practically began
with Dr. Huggins's observation of one through an " open slit,"
February 13, I86Q. 2 At first it had been thought possible to
study them only in sections that is, by admitting mere narrow
strips or " lines " of their various kinds of light ; while the
actual shape of the objects emitting those lines had been
arrived at by such imperfect devices as that of giving to the
slit of the spectroscope a vibratory movement rapid enough
to enable the eye to retain the impression of one part while
others were successively presented to it. It was an immense
gain to find their rays strong enough to bear so much of dilu-
tion with ordinary light as was involved in opening the spectro-
scopic shutter wide enough to exhibit the tree- like, or horn-
1 Phil. Mag., vol. xlii. p. 377. 2 Proc. Roy. Soc., vol. xvii. p. 302.
CHAP. iv. SOLAR SPECTROSCOPY. 241
like, or flame-shaped bodies rising over the sun's rim in their
undivided proportions. Three images of each prominence
are formed in the spectroscope a crimson, a green, and a
deep blue. The crimson, however (built up out of the C line
of hydrogen), is the most intense, and is commonly used for
purposes of observation and illustration.
Friedrich Zollner was, by a few days, beforehand with
Huggins in describing the open-slit method, but was somewhat
less prompt in applying it. His first survey of a complete
prominence, pictured in, and not simply intersected by, the
slit of his spectroscope, was obtained July i, I869. 1 Shortly
afterwards the plan was successfully adopted by the whole
band of investigators.
A difference in kind was very soon perceived to separate
these objects into two well-marked classes. Its natural and
obvious character was shown by its having struck several
observers independently. The distinction of " cloud-promin-
ences " from " flame-prominences " was announced by Lockyer,
April 2 7 ; by Zollner, June 2 ; and by Respighi, December 4,
1870.
The first description is tranquil and relatively permanent,
sometimes enduring without striking change for many days.
They mimic terrestrial cloud-scenery now appearing like
fleecy cirrus transpenetrated with the red glow of sunset
now like prodigious masses of cumulo-stratus hanging heavily
above the horizon. These solar clouds, however, have the
peculiarity of possessing stems. Slender columns can ordinarily
be seen to connect the surface of the chromosphere with its
outlying portions. Hence the fantastic likeness to forest
scenery presented by the long ranges of fiery trunks and
foliage at times seeming to fringe the sun's limb. But while
this formation suggests an actual outpouring of incandescent
material, certain facts require a different interpretation. At
a distance, and quite apart from the chromosphere, promin-
ences have been perceived, both by Secchi and Young, to form,
just as clouds form in a clear sky, condensation being replaced
1 Astr. Nach., No. 1769.
Q
242 HISTORY OF ASTRONOMY. PART n.
by ignition. Filaments were then thrown out downward
towards the chromosphere, and finally the usual appearance
of a " stemmed prominence " was assumed. Still more remark-
able was an observation made by Trouvelot at Harvard College
Observatory, June 26, iSy^.. 1 A gigantic comma-shaped pro-
minence, 82,000 miles high, vanished from before his eyes by
a withdrawal of light as sudden as the passage of a flash of
lightning. The same observer has frequently witnessed a
gradual illumination or gradual extinction of such objects,
testifying to changes in the thermal or electrical condition of
matter already in situ.
The chemistry of " cloud-prominences " is very simple.
Hydrogen and helium are their only constituents. " Flame-
prominences," on the other hand, show, in addition, the
characteristic rays of a number of metals, amongst which iron,
titanium, barium, sodium, and magnesium are conspicuous.
They are intensely brilliant ; sharply defined in their varying
forms of jets, spikes, fountains, waterspouts ; of rapid forma-
tion and speedy dissolution, seldom attaining to the vast dimen-
sions of the more tranquil kind. They are of eruptive or
explosive origin, and are closely connected with spots ; whether
causally, the materials ejected as " flames " cooling and settling
down as dark, depressed patches of increased absorption ; 2 or
consequentially, as a reactive effect of falls of solidified sub-
stances from great heights in the solar atmosphere. 3 The two
classes of phenomena, at any rate, stand in a most intimate
relation ; they obey the same law of periodicity, and are con-
fined to the same portions of the sun's surface, while quiescent
prominences may be found right up to the poles and close to
the equator.
The general distribution of prominences, including both
species, follows that of faculaB much more closely than that of
spots. From Father Secchi's and Professor Respighi's obser-
vations, 1869-71, were derived the first clear ideas on the sub-
ject, which have been supplemented and modified by the later
1 Am. Jour, of Sc., vol. xv. p. 85. 2 Secchi, Le Soleil, t. ii. p. 294.
3 Lockyer, Chemistry of the Sun, p. 418.
CHAP. iv. SOLAR SPECTROSCOPY. 243
researches of Professors Tacchini and Ricc6 at Rome and
Palermo. The results are somewhat complicated, but may be
stated broadly as follows. The district of greatest prominence-
frequency covers and overlaps by several degrees that of greatest
spot -frequency. That is to say, it extends to about 40 north
and south of the equator. 1 There is a visible tendency to a
second pair of maxima nearer the poles. The poles themselves,
as well as the equator, are regions of minimum occurrence.
Distribution in time is governed by the spot-cycle, but the
maximum lasts longer for prominences than for spots.
The structure of the chromosphere was investigated in 1869
and subsequent years by Professor Respighi, director of the
Capitoline Observatory, also by Sporer, and by Bredichin
of the Moscow Observatory. They found this supposed solar
envelope to be of the same eruptive nature as the vast pro-
trusions from it, and to be made up of a congeries of minute
flames 2 set close together like blades of grass. " The appear-
ance," Professor Young writes, 3 " which probably indicates a
fact, is as if countless jets of heated gas were issuing through
vents and spiracles over the whole surface, thus clothing it with
flame which heaves and tosses like the blaze of a conflagration."
The summits of these filaments of fire are commonly inclined,
as if by a wind sweeping over them, when the sun's activity
is near its height, but erect during his phase of tranquillity.
Sporer, in 1871, inferred the influence of permanent polar
currents, 4 but Tacchini showed in 1876 that the deflections
upon which this inference was based ceased to be visible as
the spot-minimum drew near. 5
Another peculiarity of the chromosphere, denoting the
remoteness of its character from that of a true atmosphere, 6
is the irregularity of its distribution over the sun's surface.
There are no signs of its bulging out at the equator, as the
laws of fluid equilibrium in a rotating mass would require ; but
1 ISAstronomie, t. iii. p. 292 (Ricco). 2 Averaging about loo miles
across and 300 high. Le Soleil, t. ii. p. 35. 3 The Sun, p. 180. * Astr.
Nach., No. 1854. 5 Mem. degli Spettroscopisti Italiani, t. v. p. 4. Restated
by Secchi, Ibid., t. vi. p. 56. 6 Its non-atmospheric character was early
defined by Proctor, Month. Not., vol. xxxi. p. 196.
244 HISTORY OF ASTRONOMY. PART ir.
there are some that the fluctuations in its depth are connected
with the phases of solar agitation. At times of minimum it
seems to accumulate and concentrate its activity at the poles ;
while maxima probably bring a more equable general distribu-
tion, with local depressions at the base of great prominences
and above spots.
The reality of the appearance of violent disturbance pre-
sented by the " flaming " kind of prominence can be tested in
a very remarkable manner. Christian Doppler, 1 professor of
mathematics at Prague, enounced in 1842 the theorem that
the colour of a luminous body, like the pitch of a sonorous
body, must be changed by movements of approach or recession.
The reason is this. Both colour and pitch are physiological
effects, depending, not upon absolute wave-length, but upon
the number of waves entering the eye or ear in a given interval
of time. And this number, it is easy to see, must be increased
if the source of light or sound is diminishing its distance, and
diminished if it is increasing it. In the one case, the vibrat-
ing body pursues and crowds together the waves emanating
from it ; in the other, it retreats from them, and so lengthens
out the space covered by an identical number. The principle
may be thus illustrated. Suppose shots to be fired at a target
at fixed intervals of time. If the marksman advances, say
twenty paces between each discharge of his rifle, it is evident
that the shots will fall faster on the target than if he stood
still ; if,- on the contrary, he retires by the same amount, they
will strike at correspondingly longer intervals. The result
will of course be the same whether the target or the marksman
be in movement.
So far Doppler was altogether right. As regards sound,
any one can convince himself that the effect he predicted is a
real one, by listening to the alternate shrilling and sinking of
the steam-whistle when an express train rushes through a
station. But in applying this principle to the colours of starg
he went widely astray ; for he omitted from consideration the
double range of invisible vibrations which partake of, and to
1 Abh. d. Kon. Bohm. Ges. d. Wiss., Bd. ii. 1841-42, p. 467.
CHAP. iv. SOLAR SPECTROSCOPY. 245
the eye exactly compensate, changes of refrangibility in the
visible rays. There is, then, no possibility of finding a criterion
of velocity in the hue of bodies shining, like the sun and stars,
with continuous light. There is a slight shift of the entire
spectrum up or down in the scale of refrangibility ; certain
rays normally visible become exalted or degraded (as the case
may be) into invisibility, and certain other rays at the opposite
end undergo the converse process ; but the sum total of im-
pressions on the retina continues the same.
We are not, however, without the means of measuring this
sub-sensible transportation of the light-gamut. Once more
the wonderful Fraunhofer lines came to the rescue. They
were called by the earlier physicists "fixed lines ; " but it is
just because they are not fixed that, in this instance, we find
them useful. They share, and in sharing betray, the general
shift of the spectrum. This aspect of Doppler's principle was
adverted to bj> Fizeau in I848, 1 and the first tangible results
in the estimation of movements of approach and recession
between the earth and the stars, were communicated by Dr.
Huggins to the Royal Society, April 23, 1868. Eighteen
months later, Zollner devised his "reversion-spectroscope" 2
for doubling the measurable effects of line-displacements ;
aided by which ingenious instrument, and following a sugges-
tion of its inventor, Professor H. C. Yogel succeeded at Both-
kamp, June 9, iSyi, 3 in detecting effects of that nature due to
the solar rotation. This application constitutes at once the
test and the triumph of the method.
The eastern edge of the sun is continually moving towards
us with an equatorial speed of about a mile and a quarter per
second, the western edge retreating at the same rate. The
displacements towards the violet on the east, towards the
red on the west corresponding to this velocity are very small ;
so small that it seems hardly credible that they should have
been laid bare to perception. They amount to but ji^th part
1 In a paper read before the Socie'te' Philomathique de Paris, December
23, 1848, and first published in extenso in Ann. de Chim. et de Phys. t t. xix.
p. 211 (1870). 2 Astr.Nach., No. 1772. 3 Ibid., No. 1864.
246 HISTORY OF ASTRONOMY. PART n.
of the interval between the two constituents of the D line of
sodium ; and the D line of sodium itself can be separated into
a pair only by a powerful spectroscope. Nevertheless, Pro-
fessor Young 1 was able to show quite satisfactorily, in 1876,
not only deviations in the solar lines from their proper places
indicating a velocity of rotation (1.42 miles per second) slightly
in excess of that given by observations of spots, but the
exemption of terrestrial lines (those produced by absorption in
the earth's atmosphere) from the general push upwards or
downwards. Shortly afterwards, Professor Langley, director
of the Allegheny Observatory, having devised a means of
comparing with great accuracy light from different portions
of the sun's disc, found that while the obscure rays in two
juxtaposed spectra derived from the solar poles were absolutely
continuous, no sooner was the instrument rotated through
ninety degrees, so as to bring its luminous supplies from
opposite extremities of the equator, than the same rays became
perceptibly "notched." The telluric lines, meanwhile, re-
mained unaffected, so as to be "virtually mapped" by the
process. 2 This rapid and unfailing mode of distinction was
used by Cornu with perfect ease during his investigation of
atmospheric absorption near Loiret in August and Sep-
tember 1 883. 3
A beautiful experiment of the same kind was performed by
M. Thollon, of M. Bischoffsheim's observatory at Nice, in the
summer of i88o. 4 He confined his attention to one delicately
defined group of four lines in the orange, of which the inner
pair are solar (iron) and the outer terrestrial. At the centre
of the sun the intervals separating them were sensibly equal ;
but when the light was taken alternately from the right and
left limbs, a relative shift in alternate directions of the
solar, towards and from the stationary telluric rays became
apparent. A parallel observation was made at Lord Craw-
ford's observatory of Dunecht, December 14, 1883, when
it was noticed that a strong iron-line in the yellow part
1 Am. Jour, of Sc., vol. xii. p. 321. 2 Ibid., vol. xiv. p. 140. 3 Butt.
Astronom., t. i. p. 77. * Comptes Eendus, t. xci. p. 368..
CHAP. iv. SOLAR SPECTROSCOPY. 247
of the solar spectrum is permanently double on the sun's
eastern, but single on his western limb ; l opposite motion-
displacements bringing about this curious effect of separation
from, and coincidence with, an adjacent stationary line of our
own atmosphere's production, according as the spectrum is
derived from the advancing or retreating margin of the solar
globe. Statements of fact so precise and authoritative amount
to a demonstration that results of this kind are worthy of
confidence ; and since they are, in certain cases, such as to
startle it, it is important to make sure of their foundations.
Mr. Lockyer 2 was the first to perceive the applicability of
this subtle and surprising discovery to the study of promi-
nences, the discontinuous light of which affords precisely the
same means of detecting movement without seeming change
of place, as do lines of absorption in a continuous spectrum.
Indeed, his observations at the sun's edge almost compelled
him to have recourse to an explanation made available just
when the need of it began to be felt. He saw bright lines,
not merely pushed aside from their normal places by a barely
perceptible amount, but bent, torn, broken, as if by the stress
of some tremendous violence. These remarkable appearances
were quite simply interpreted as the effects of movements
varying in amount and direction in the different parts of the
extensive mass of incandescent vapours falling within a single
field of view. Very commonly they are of a cyclonic character.
The opposite distortions of the same coloured rays betray the
fury of " counter-gales " rushing along at the rate of 120 miles
a second ; while their undisturbed sections prove the persis-
tence of a " heart of peace " in the midst of that unimaginable
fiery whirlwind. Velocities up to 250 miles a second, or
15,000 times that of an express train at the top of its speed,
were thus observed by Young during his trip to Mount
Sherman, August 3, 1872.
Motions ascertainable in this way near the limb are, of
course, horizontal as regards the sun's surface ; the analogies
1 Month. Not., vol. xliv. p. 170. 2 Proc. Roy. Soc., vols. xvii. p. 415 ;
xviii. p. 1 20.
248 HISTORY OF ASTRONOMY. PART n.
they present might, accordingly, be styled meteorological rather
than volcanic. But vertical displacements on a scale no less
stupendous can also be shown to exist. Observations of the
spectra of spots centrally situated (where motions in the line
of sight are vertical) disclose the progress of violent uprushes
and downrushes of ignited gases, for the most part in the
penumbral or outlying districts. They appear to be occasioned
by fitful and irregular disturbances, and have none of the
systematic quality which would be required for the elucidation
of sun-spot theories. Indeed, they almost certainly take place
at a great height above the actual opening in the photosphere.
As to vertical motions above the limb, on the other hand,
we have direct visual evidence of a truly amazing kind. The
projected glowing matter has, by the aid of the spectroscope,
been watched in transit. On September 7, 1871, Young
examined at noon a vast hydrogen cloud, 100,000 miles
long, as it showed to the eye, and 54,000 high. It floated
tranquilly above the chromosphere at an elevation of some
15,000 miles, and was connected with it by three or four
upright columns, presenting the not uncommon aspect com-
pared by Lockyer to that of a grove of banyans. Called
away for a few minutes at 12.30, on returning at 12.55 the
observer found
"That in the meantime the whole thing had been literally
blown to shreds by some inconceivable uprush from beneath.
In place of the quiet cloud I had left, the air, if I may use the
expression, was filled with flying debris a mass of detached,
vertical, fusiform filaments, each from 10" to 30" long by 2"
or 3" wide, 1 brighter and closer together where the pillars
had formerly stood, and rapidly ascending. They rose, with
a velocity estimated at 166 miles a second, to fully 200,000
miles above the sun's surface, then gradually faded away like
a dissolving cloud, and at 1.15 only a few filmy wisps, with
some brighter streamers low down near the photosphere,
remained to mark the place." 2
1 At the sun's distance, one second of arc represents about 450 miles.
2 Am. Jour, of Sc., vol. ii. 1871, p. 468.
CHAP. iv. SOLAR SPECTROSCOPY. 249
A velocity of projection of at least 500 miles per second has
been calculated by Proctor x to be necessary in order to account
for this extraordinary display. It was marked by the simul-
taneous record at Greenwich of a magnetic disturbance, and
was succeeded, the same evening, by a fine aurora. It has
proved by no means an isolated occurrence. Young saw its
main features repeated, October 7, i88i, 2 on a still vaster
scale; for the exploded prominence attained, this time, an
altitude of 350,000 miles the highest yet chronicled. Mr.
Lockyer, moreover, has seen a prominence 40,000 miles high
blown to pieces in ten minutes; while uprushes have been
witnessed by Respighi, of which the initial velocities were
judged by him to be 400 or 500 miles a second. When it is
remembered that a body starting from the sun's surface at the
rate of 379 miles a second would, if it encountered no resist-
ance, escape for ever from his control, it is obvious that we
have, in the enormous forces of eruption or repulsion mani-
fested in the outbursts just described, the means of accounting
for the vast diffusion of matter in the solar neighbourhood.
Nor is it possible to explain them away, as Cornu, 3 Faye, 4 and
others have sought to do, by substituting for the rush of matter
in motion, progressive illumination through electric discharges,
or even through the mere reheating of gases cooled by expan-
sion. 5 All the appearances are against such evasions of the
difficulty presented by velocities stigmatised as " fabulous "
and " improbable," but which, there is the strongest reason
to believe, really exist.
On the 1 2th of December 1878, Mr. Lockyer formally
expounded before the Royal Society his now famous hypo-
thesis of the compound nature of the " chemical elements." 6
He was led to it by several converging lines of research. In
a letter to M. Dumas, dated December 3, 1873, he had sketched
out the successive stages of " celestial dissociation " which he
1 Month. Not., vol. xxxii. p. 51. 2 Nature, vol. xxiii. p. 281. 3 Comptes
JRendus, t. Ixxxvi. p. 532. 4 Ibid., t. xcvi. p. 359. 5 Such prominences
as have been seen to grow by the spread of incandescence are of the quiescent
kind, and present no deceptive appearance of violent motion. 6 Proc.
Roy. Soc., vol. xxviii. p. 157.
250 HISTORY OF ASTRONOMY. PART n.
conceived to be represented in the sun and stars. The absence
from the solar spectrum of metalloidal absorption he explained
by the separation, in the fierce solar furnace, of such sub-
stances as oxygen, nitrogen, sulphur, carbon, &c., into simpler
constituents possessing unknown spectra ; while metals were
at that time still admitted to be capable of existing there in a
state of integrity. Three years later he made a further step.
He announced, as the result of a comparative study of the
Fraunhofer and electric- arc spectra of calcium, that the " mole-
cular grouping" of that metal, which at low temperatures
gives a spectrum with its chief line in the blue, is nearly
broken up in the sun into another or others with lines in
the violet. 1 The further progress of his work showed him
this discrepancy between solar and terrestrial spectra as no
exception, but " a truly typical case." :
During four years (1875-78 inclusive) this unwearied student
of nature was engaged in mapping a section of the more refran-
gible part of the solar spectrum (wave-lengths 3800-4000)
on a scale of magnitude such that, if completed down to the
infra-red, its length would have been about half a furlong.
The attendant laborious investigation, by the aid of photo-
graphy, of metallic spectra, afforded him the discovery of
what he called "basic lines." These are lines occurring in
the spectra of two or more metals after all possible " impuri-
ties" have been eliminated, and were held to attest the
presence of a common substratum of matter in a simpler
state of aggregation than any with which we are ordinarily
acquainted. Now it is a singular fact that these " basic lines "
are precisely those which appear, with a persistence altogether
out of proportion to their actual numbers, in the spectrum of
the chromosphere when agitated by eruptive injections. The
presence of iron, for example, instead of being signified by the
1 Proc. Roy. Soc., vol. xxiv. p. 353. The remarkable pair of lines
(H and K) at the violet end of the calcium-spectrum challenge continued
attention. Vogel discovered in 1879 a hydrogen line coincident with H
(Monatsb. Preuss. Ak., Feb. 1879, p. 115). Young attributes both H and
K to that substance, on the ground of their anomalous behaviour in pro-
minences (Nature, vol. xxiii. p. 281). 2 Proc. Hoy. Soc., vol. xxviii. p. 444.
CHAP. iv. SOLAR SPECTROSCOPY. 251
flashing out of some of the strong representative lines which
are the first to appear and the last to disappear in its labora-
tory-spectrum, makes itself known by the brightening of some
inconspicuous ray, claimed, moreover, with an equal title, by
(say) calcium or titanium. What more natural than to con-
clude the erupted substance to be not really iron at all, but
some more elementary form of matter entering into the com-
position of iron as well as of calcium and titanium, the reduc-
tion having been brought about by the inconceivable heat of
the sub-photospheric regions ?
Nevertheless the foundation of fact upon which this tempt-
ing view was made to rest is not unshaken. Between the
spectral lines of different substances there are probably no
absolute coincidences. " Basic " lines are really formed of
doublets or triplets merged together by insufficient dispersion.
Of Thalen's original list of seventy rays common to several
spectra, 1 very few have so far resisted Thollon's and Young's
powerful spectroscopes ; the process of resolution may in fact
be regarded as practically complete. Thus the argument
from community of lines to community of substance requires
modification. Yet none of its significance has been lost by
the circumstance that these twin-lines these spots of ren-
dezvous, it might be said, for different sets of vibrations are
specially selected for display in solar disturbances are pre-
dominantly brightened in flames and thickened in spots.
This last point has been accentuated by observations carried
on at South Kensington under Mr. Lockyer's direction since
November 1879. Down to August 1885 the spectra of 700
spots had been studied on a fixed plan, 2 and the results tabu-
lated with rigid impartiality. It is not too much to say that
the chemistry of sun-spots has thereby been established on a
completely new basis. The principle of the method employed
is this.
1 Many of these were shown by Mr. Lockyer, who was the first to sift
the matter, to be due to very slight admixtures of the several metals con-
cerned. The results of his latest investigations of " basic lines," and hints
for the explanation of their " shifts," will be found in his Chemistry of the
Sun, p. 368 et scq. 2 Ibid., p. 312.
252 HISTORY OF ASTRONOMY. PART n.
The whole range of Fraunhofer lines is visible when the
light from a spot is examined with the spectroscope ; but rela-
tively few are widened. Now these widened lines alone con-
stitute (presumably) the true spot-spectrum; they, and they
alone, tell what kinds of vapour are thrust down into the
strange dusky pit of the nucleus, the unaffected lines taking
their accustomed origin from the overlying strata of the
normal solar atmosphere. Here then we have the criterion
that was wanted the means of distinguishing, spectroscopi-
cally and chemically, between the cavity and the absorbing
layers piled up above it. By its persistent employment some
marked peculiarities have been brought out qualified, however,
where negative conclusions are in question, by the necessary
limitations of the method of research. Positive results, mean-
while, of an indubitable kind, are not wanting. Such are the
singular prominence of u basic " lines in spot-spectra ; the
unfamiliar character of numerous others, especially at epochs
of disturbance; above all, the strange individuality in the
behaviour of every one of these darkened and distended
rays. 1 Each seemed to act on its own account ; it comported
itself as if it were the sole representative of the substance
emitting it ; its appearance was not conditional upon that of
any of its terrestrial companions in the same spectrum.
For some metals, as cobalt, chromium, and calcium, the
lines widened in spots are the same with those brightened
in the uprushing flames at the sun's edge; for a good many
others, they are, as a rule, totally dissimilar, the spot-spectrum
of iron, for example, having only a remote relationship to its
" storm "-spectrum. It is, moreover, abundantly clear, from
the character of the lines severally affected, that the change
is connected with a difference of temperature, and that the
prominences are much hotter than the spots. Hence, two
well-defined heat-levels are placed, as it were, at the disposal
of the solar chemist, while a third, lower than either, charac-
terised by the ordinary Fraunhofer spectrum, is found at the
photospheric surface.
1 Chemistry of the Sun, p. 314.
CHAP. iv. SOLAR SPECTROSCOPY. 253
By far the most curious fact, however, elicited by these
inquiries, is that of the attendance of chemical vicissitudes
upon the advance of the sunspot period. As the maximum
approached, unknown replaced known lines in the spot-spectra
with a rapid and emphatic progression. 1 It would really
seem as if the vapours emitting rays characteristic of iron,
titanium, nickel, &c., had ceased to exist as such, and their
room been taken by others, total strangers in terrestrial
laboratories. These, in Mr. Lockyer's view, are simply the
finer constituents of their predecessors, dissociation having
been effected by the higher temperature ensuing upon increased
solar activity. And for the present there is at hand no other
explanation of the striking facts collected by him.
But the strongest point of the " dissociation theory " has yet
to be mentioned. It is that the contortions or displacements
due to motion are frequently seen to affect a single line belong-
ing to a particular substance, while the other lines of that
same substance remain imperturbable. Now, how is this most
singular fact, which seems at first sight to imply that a body
may be at rest and in motion at one and the same instant,
to be accounted for ? It is accounted for, on Mr. Lockyer's
hypothesis, easily enough, by supposing that the rays thus
discrepant in their testimony, do not belong to one kind of
matter, but to several, combined, at ordinary temperatures, to
form a body in appearance "elementary." Of these different
vapours, one or more may of course be rushing rapidly towards
or from the observer, while the others remain still ; and since
the line of sight across the average prominence region pene-
trates, at the sun's edge, a depth of about 300,000 miles, 2 all
the incandescent materials separately occurring along which line
are projected into a single " flame " or "cloud," it will be per-
ceived that there is ample room for diversities of behaviour.
1 Chemistry of the Sun, p. 324. 2 Thollon's estimate (Comptes Rendus,
t. xcvii. p. 902) of 300,000 kilometres seems considerably too low. Limit-
ing the " average prominence region " to a shell 54,000 miles deep (2' of
arc as seen from the earth), the visual line will, at mid-height (27,000
miles from the sun's surface), travel through (in round numbers) 320,000
miles of that region.
254 HISTORY OF ASTRONOMY. PART n.
The alternative mode of escape from the perplexity consists
in assuming that the vapour in motion is rendered luminous
under conditions which reduce its spectrum to a few rays, the
unaffected lines being derived from a totally distinct mass of
the same substance shining with its ordinary emissions. 1 But
these conditions are assuredly not those prevailing in solar
cyclones ; while the seemingly capricious choice of lines associ-
ated to indicate rest or motion, negative a supposition imply-
ing orderly and invariable sequence. It is thus difficult to
resist the conclusion that distinct kinds of matter are really
aligned before the eye, embarrassing us with the contradictory
testimony of their integrated light.
Mr. Lockyer's view has the argument from continuity in
its favour. It only asks us to believe that processes which we
know to take place on the earth under certain conditions, are
carried further in the sun, where the same conditions are,
it may be presumed, vastly exalted. We find that the bodies
we call " compound " split asunder at fixed degrees of heat
within the range of our resources. Why should we hesitate
to admit that the bodies we call " simple " do likewise at
degrees of heat' without the range of our resources ? The
term " element " simply expresses terrestrial incapability of
reduction. That, in celestial laboratories, the means and their
effect here absent should be present, would be an inference
challenging, in itself, no expression of incredulity.
Yet it is, in point of fact, a revolutionary one, and its
acceptance will involve the reconstruction of more than one
fair edifice of scientific thought. It appears, none the less,
likely to become inevitable. There are indeed theoretical
objections, which, though probably not insuperable, are un-
questionably grave. Our seventy chemical " elements," for
instance, are placed by the law of specific heats on a separate
footing from their known compounds. We are not, it is true,
compelled by it to believe their atoms to be really and abso-
lutely such to contain, that is, the "irreducible minimum"
of material substance ; but we do certainly gather from it
1 Liveing and Dewar, Phil. Mag., vol. xvi (5th ser.), p. 407.
CHAP. iv. SOLAR SPECTROSCOPY. 255
that they are composed on a different principle from the salts
and oxides made and unmade at pleasure by chemists. Then
the multiplication of the species of matter with which Mr.
Lockyer's results menace us, is at first sight startling. They
may lead, we are told, to eventual unification, but the prospect
seems remote. For the present each terrestrial "element"
is asserted by them to be broken up in the sun into several,
and the existence of even a single common constituent is
uncertain. The components of iron alone, for instance, should
be counted by the dozen. And there are other metals, such
as cerium, which, giving a still more complex spectrum, would
doubtless be still more numerously resolved. It is true Mr.
Lockyer interprets the observed phenomena as indicating the
successive combinations, in varying proportions, of a very
few original ingredients; 1 but if the emission, at exalted
temperatures, of a single quality of light be admitted as the
criterion of a truly elementary body, then the independent
behaviour of a considerable number of lines in the spectrum
of each metal seems to assert that its formative units are
numerous.
Thus, added complexity is substituted for that fundamental
unity of matter which has long formed the dream of specu-
lators. And it is extremely remarkable that Mr. Crookes,
working along totally different lines, has been led to analogous
conclusions. To take only one example. As the outcome of
processes of sifting and testing of extreme delicacy carried on
for years, he finds that the metal yttrium splits up into five,
if not eight constituents. 2 Evidently, old notions are doomed,
nor are any preconceived ones likely to take their place. But,
whatever comes of it, we have no choice but to admit facts.
There can be no doubt that the spectra of bodies are an
index to changes in their molecular constitution of every kind
and degree, from a complete disruption of the molecule into
atoms, homogeneous or heterogeneous, to some unspeakably
minute, yet orderly and harmonious rearrangement of parts
in the complex little system of which the movements are the
1 Chemistry of the Sun, p. 260. 2 Nature, Oct. 14, 1886.
256 HISTORY OF ASTRONOMY. PART n.
source of light. Mr. Lockyer's " working hypothesis " thus
raises questions which science is not yet prepared to answer.
It brings us face to face with the mysteries of the ultimate
constitution of matter, and of its relations to the vibrating
medium filling space. It makes our ignorance on the subject
seem at once more dense and more definite. Nevertheless,
this in itself (though the saying appear paradoxical) consti-
tutes an advance. Unfelt ignorance persists. Ignorance that
is stricken with uneasy self -consciousness is already on the
way to be turned into knowledge.
Professor A. J. Angstrom of Upsala takes rank after
Kirchhoff as a subordinate founder, so to speak, of solar
spectroscopy. His great map of the "normal " solar spectrum 1
was published in 1868, two years before he died. Robert
Thalen was his coadjutor in its execution, and the immense
labour which it cost was amply repaid by its eminent and
lasting usefulness. It is still the universal standard of
reference in all spectroscopic inquiries within the range of
the visible emanations.
The discovery that hydrogen exists in the atmosphere of the
sun was made by Angstrom in 1862. His list of solar elements
published in that year, 2 the result of an investigation separate
from, though conducted on the same principle as Kirchhoff s,
included the substance which we now know to be predominant
amongst them. Dr. Pliicker of Bonn had identified in 1859
the Fraunhofer line F with the green ray of hydrogen, but
drew no inference from his observation. The agreement was
verified by Angstrom ; two further coincidences w r ere estab-
lished; and in 1866 a fourth hydrogen line in the extreme
violet (named 7^) was detected in the solar spectrum. With
Thalen, he besides added manganese, aluminium, and titanium
1 The normal spectrum is that depending exclusively upon wave-length
the fundamental constant given by nature as regards light. It is obtained
by the interference of rays, in the manner first exemplified by Fraunhofer,
and affords the only unvarying standard for measurement. In the refrac-
tion-spectrum (upon which Kirchhoff's map was founded), the relative
positions of the lines vary with the material of the prisms. 2 Ann. d.
Phys., Bd. cxvii. p. 296.
CHAP. iv. SOLAR SPECTROSCOPY. 257
to the constituents of the sun enumerated by Kirchhoff, and
raised the number of identical rays in the solar and terrestrial
spectra of iron to no less than 460. l
Thus, when Mr. Lockyer entered on that branch of inquiry
in 1872, fourteen substances were recognised as common to
the earth and sun. Early in 1878 he was able, by applying
the test of length in lieu of that of strength in the comparison
of lines (looking, that is, rather to their persistence through a
wide range of temperature, than to their brilliancy at any one
temperature), to increase the list provisionally to thirty- three. 2
All these are metals ; for there is strong reason to believe that
hydrogen presents a solitary instance of an ordinarily gaseous
metal, just as mercury does of an ordinarily liquid one. Up
to 1877 the fourteen metalloids (non-metallic elements) were
conspicuous by absence.
But in that year the late Dr. Henry Draper of New York
announced a discovery of very wide significance. As the up-
shot of an investigation lasting several years, he found oxygen
to be revealed in the sun, not, like the metals, by the reversal
of its spectral rays, but by their direct presence. Each one of
eighteen bright lines in its photographed spectrum was seen
to be represented by a strictly corresponding brilliant band in
the analysed light of the sun. 3 The reality of these coincidences
having been doubted, Dr. Draper set to work afresh, and on
the 1 3th of June 1879* laid before the Royal Astronomical
Society photographs on a scale four times that of the original
ones, in which the solar counterpart of the laboratory-spectrum
of oxygen was no less apparent than before. Mr. Ranyard
remarked that, by this fourfold dispersion, the evidential value
of the eighteen observed coincidences was increased 4 18 , or (in
round numbers) 68,719 million times ; but the rigid numerical
test of probability does not in this case carry its full weight
of conviction. The discrimination of bright lines from a very
slightly less lucid background must, it is plain, be always a
1 Comptes Rendus, t. Ixiii. p. 647. 2 Ibid., t. Ixxxvi. p. 317. 3 ^m.
Jour, of Sc., vol. xiv. p. 89; Nature, vol. xvi. p. 364. 4 Month. Not.,
vol. xxxix. p. 440.
R
258 HISTORY OF ASTRONOMY. PART n.-
matter of much delicacy and some uncertainty, especially when
the lines to be discriminated are not sharp, but more or less
blurred and widened. An investigation by Mr. Christie, who
succeeded Sir George Airy as Astronomer Royal in 1881,
proved unfavourable to the genuineness of the correspondences
in Dr. Draper's photographs ; * and they are rejected by such
eminent spectroscopists as Professor Yogel of Potsdam.
Nevertheless, evidence is at hand tending to ratify the con-
clusion (recommended, besides, by our innate tendency to
complete an analogy) that the most widely prevalent super-
ficial constituent of the earth is not missing from the sun.
The possibility of its showing bright, instead of dark lines,
illustrates the endless variety in nature's modes of proceeding,
and accentuates the danger of negative inferences. That a
substance displays none of its distinctive beams in the spec-
trum of the sun or of a star, no longer affords even a presump-
tion against its presence there. For it may be situated below
the level where absorption occurs, or under a pressure such as
to efface lines by continuous lustre ; it may be at a tempera-
ture so high that it gives out more light than it takes up, and
yet its incandescence may be masked by the absorption of
other bodies ; finally, it may just balance absorption by emis-
sion, with the result of complete spectral neutrality. An
instructive example is that of helium, the enigmatical chromo-
spheric element. Father Secchi remarked in 1868 2 that there
is no dark line in the solar spectrum matching its light ; and
the faint traces of D3 absorption since detected would pro-
bably never have been observed, had not the substance pro-
ducing them been otherwise known to exist.
An explanation might easily be found of the sun's oxygen
attesting its presence anomalously. The inner organisation
of the oxygen molecule is a considerably plastic one. It is
readily modified by heat, and these modifications are reflected
in its varying modes of radiating light. Dr. Schuster enu-
merated in i8y9 3 four distinct oxygen spectra, corresponding
1 Month. Not., vol. xxxviii. p. 473. 2 Comptes Rendus, t. Ixvii. p. 1123.
3 Phil. Trans., vol. clxx. p. 46.
CHAP. iv. SOLAR SPECTROSCOPY. 259
to various stages of temperature, or phases of electrical excite-
ment ; and a fifth has been added by M. Egoroff 's discovery
in 1883 l that certain well-known groups of dark lines in the
red end of the solar spectrum (Fraunhofer's A and B) are due
to absorption by the cool oxygen of our air.
Now, of these five different systems of luminous emission,
three are claimed to be represented one, as just stated, through
terrestrial, the others through solar action in analysed sun-
light. The brilliant range of lines identified by Dr. Draper be-
long to the maximum heat developed by high-tension electricity.
The question whether they are bond fide bright lines, or mere
delusive bright background, is still, as we have seen, sub judice ;
but if incandescent oxygen produce them, it must certainly lie
at a low level in the sun, since its lines never appear in the
spectrum of the chromosphere j and we may conclude that it
forms part of the hottest layers of which we receive the radia-
tions. The next, or " compound-line spectrum," produced at
a considerably lower stage of thermal excitement, Dr. Schuster
has found, with evidence "little short of absolute certainty,"
to be dark in the sun. 2 And here some prospect seemed to
him to open of meeting with a definite criterion of the solar
temperature. For evidently the degree of heat (whatever that
may be) at which spectrum No. i changes to spectrum No. 2
occurs somewhere between the stratum giving Draper's bright
lines and the stratum giving Schuster's dusky lines. This
brings us to the subject of the next chapter.
1 Comptes Rendus, t. xcvii. p. 555 ; t. ci. p. 1145. 2 Nature, vol. xvii.
p. 148.
( 260 )
CHAPTER Y.
TEMPERATURE OF THE SUN.
NEWTON was the first who attempted to measure the quantity
of heat received by the earth from the sun. His object in
making the experiment was to ascertain the temperature en-
countered by the comet of 1680 at its passage through peri-
helion. He found it, by multiplying the observed heating
effects of direct sunshine according to the familiar rule of the
" inverse squares of the distances," to be about 2000 times
that of red-hot iron." *
Determinations of the sun's thermal power made with some
scientific exactness, date, however, from 1837. A few days
previous to the beginning of that year, Herschel began ob-
serving at the Cape of Good Hope with an " actinometer,"
and obtained results agreeing quite satisfactorily with those
derived by Pouillet from experiments made in France some
months later with a " pyrheliometer." 5 Pouillet found that
the vertical rays of the sun falling on each square centimetre
of the earth's surface are competent (apart from atmospheric
absorption) to raise the temperature of 1.7633 grammes of
water one degree centigrade per minute. This number (i. 7633)
he called the "solar constant;" and the unit of heat chosen is
known as the " calorie." Hence it was computed that the
total amount of solar heat received during a year would suffice
to melt a layer of ice covering the entire earth to a depth of
30.89 metres, or 100 feet; while the heat emitted would melt,
at the sun's surface, a stratum 11.80 metres thick each minute.
A careful series of observations showed that nearly half the
1 Principia, p. 498 (isted.)- 2 Comptes Rcndus, t. vii. p. 24.
CHAP. v. TEMPERATURE OF THE SUN. 261
heat incident upon our atmosphere is stopped in its passage
through it.
Herschel got somewhat larger figures, though he assigned
only a third as the spoil of the air. Taking a mean between
his own and Pouillet's, he calculated that the ordinary expen-
diture of the sun per minute would have power to melt a
cylinder of ice 184 feet in diameter, reaching from his surface
to that of a Centauri ; or, putting it otherwise, that an ice-rod
45.3 miles across, continually darted into the sun with the
velocity of light, would scarcely consume, in dissolving, the
thermal supplies now poured abroad into space. 1 It is nearly
certain that this estimate should be increased by about two-
thirds in order to bring it up to the truth.
Nothing would, at first sight, appear simpler than to pass
from a knowledge of solar emission a strictly measurable
quantity to a knowledge of the solar temperature j this being
defined as the temperature to which a surface thickly coated
with lamp-black (that is, of standard radiating power) should
be raised to enable it to send us, from the sun's distance, the
amount of heat actually received from the sun. Sir John
Herschel showed that heat-rays at the sun's surface must be
192,000 times as dense as when they reach the earth; but it
by no means follows that either the surface emitting, or a body
absorbing those heat-rays must be 192,000 times hotter than a
body exposed here to the full power of the sun. The reason
is, that the rate of emission consequently the rate of absorp-
tion, which is its correlative increases very much faster than
the temperature. In other words, a body radiates or cools at
a continually accelerated pace as it becomes more and more
intensely heated above its surroundings.
Newton, however, took it for granted that radiation and
temperature advance paripassu that you have only to ascertain
the quantity of heat received from, and the distance of a remote
body in order to know how hot it is. 2 And this principle,
1 Results of Astr. Observations, p. 446. 2 " Est enim calor solis ut
radiorum densitas, hoc est, reciproce ut qyadratum distantiae locoruni a
sole." Principia, p. 508 (3d ed. 1726).
262 HISTORY OF ASTRONOMY. PART n.
which is known as " Newton's Law " of cooling, has still a
limited number of adherents. Its validity was never questioned
until De la Roche pointed out, in I8I2, 1 that it was approxi-
mately true only over a low range of temperature ; and five
years later, Dulong and Petit generalised experimental results
into the rule, that while temperature grows by arithmetical,
radiation increases by geometrical progression. 2 Adopting
this formula, Pouillet derived from his observations on solar
heat a solar temperature of somewhere between 1461 and
1761 Cent. Now, the higher of these points which is nearly
that of melting platinum is undoubtedly surpassed at the
focus of certain burning-glasses which have been constructed
of such power as virtually to bring objects placed there within
a quarter of a million of miles of the photosphere. In the
rays thus concentrated, platinum and diamond become rapidly
vaporised, notwithstanding the great loss of heat by absorp-
tion, first in passing through the air, and again in traversing
the lens. Pouillet's maximum is then manifestly too low,
since it involves the absurdity of supposing a radiating
mass capable of heating a distant body more than it is itself
heated.
Less demonstrably, but scarcely less surely, Mr. J. J.
Waterston, who attacked the problem in 1860, erred in the
opposite direction. Working up, on Newton's principle, data
collected by himself in India and at Edinburgh, he got for
the "potential temperature" of the sun 12,880,000 Fahr., 3
equivalent to 7,156,093 Cent. The phrase potential tempera-
ture (for which Yiolle substituted, in 1876, effective temperature)
was designed to express the accumulation in a single surface,
postulated for the sake of simplicity, of the radiations not
improbably received from a multitude of separate solar layers
reinforcing each other ; and might thus (it was explained) be
considerably higher than the actual temperature of any one
stratum.
At Rome, in 1861, Father Secchi repeated Waterston's
1 Jour, de Physique, t. Ixxv. p. 215. 2 Ann. de Chimie, t. vii. 1817,
p. 365. 3 Phil. Mag., vol. xxiii. (4th ser.), p. 505.
CHAP. v. TEMPERATURE OF THE SUN. 263
experiments, and reaffirmed his conclusion ; l while Soret's
observations, made on the summit of Mont Blanc in 1867,2
furnished him with materials for a fresh and even higher
estimate of ten million degrees centigrade. 3 Yet from the
very same data, substituting Dulong and Pe tit's for Newton's
law, Vicaire deduced in 1872 a provisional solar temperature
of 1398.* This is below that at which iron melts, and we
know that iron- vapour exists high up in the sun's atmosphere.
The matter was taken into consideration on the other side of
the Atlantic by Ericsson in 1871. He attempted to re-estab-
lish the shaken credit of Newton's principle, and arrived, by
its means, at a temperature of four million degrees of Fahren-
heit. 5 More recently, what he considers an " underrated compu-
tation," based upon observation of the quantity of heat received
by his " sun motor," has given him three million degrees.
This, he rightly thinks, must be accepted, if it be granted that
the temperature produced by radiant heat is proportional to its
density, or inversely as its diffusion. 6 Could this be granted,
the question would be much simplified j but there is little doubt
that the case is far otherwise when heat becomes intensified.
In 1876 the sun's temperature was proposed as the subject
of a prize by the Paris Academy of Sciences ; but although
the essay of M. Jules Violle was crowned, the problem was
declared to remain unsolved. Violle (who adhered to Dulong
and Petit's formula) arrived at an effective temperature of
1500 C., but considered that it might actually reach 2500
C., if the emissive power of the photospheric clouds fell far
short (as seemed probable) of the lamp-black standard. 7 Ex-
periments made in April and May 1881 giving a somewhat
higher result, he raised this figure to 3000 C. 8
Appraisements so outrageously discordant as those of Water-
ston, Secchi, and Ericsson on the one hand, and those of the
1 Nuovo Oimento, t. xvi. p. 294. 2 Comptes Rendus, t. Ixv. p. 526.
3 The direct result of 5^ million degrees was doubled in allowance for
absorption in the sun's own atmosphere. Ibid., t. Ixxiv. p. 26. 4 Ibid.,
p. 31. 5 Nature, vols. iv. p. 204 ; v. p. 505. 6 Ibid., vol. xxx. p. 467.
7 Ann. de Chim., t. x. (5th ser.), p. 361. 8 Comptes Rendus, t. xcvi.
P- 254-
264 HISTORY OF ASTRONOMY. PART n.
French savans on the other, served only to show that all were
based upon a vicious principle. The late Professor F. Rosetti, 1
accordingly, of the Paduan University, at last perceived the
necessity for getting out of the groove of " laws " plainly in
contradiction with facts. The temperature, for instance, of
the oxy-hydrogen flame was fixed by Bunsen at 2800 C. an
estimate certainly not very far from the truth. But if the two
systems of measurement applied to the sun be used to deter-
mine the heat of a solid body rendered incandescent in this
flame, it comes out, by Newton's mode of calculation, 45,000
C. ; by Dulong and Petit's, 870 C. 2 Both, then, are justly
discarded, the first as convicted of exaggeration, the second
of undervaluation. The formula substituted by Rosetti was
tested successfully up to 2000 C. ; but since it is, like its
predecessors, a purely empirical rule, is guaranteed by no
principle, and can, in consequence, not be trusted out of sight,
it may, like them, break down at still higher elevations. All
that can be said is that it gives the most plausible results.
Radiation, so far as it obeys this new prescription, increases
as the square of the absolute temperature that is, of the
number of degrees counted from the " absolute zero" of 273
C. Its employment gives for the sun's radiating surface an
effective temperature of 20,380 C. (including a supposed loss
of one-half in the solar atmosphere) ; and setting a probable
deficiency in emission (as compared with lamp-black) against a
probable mutual reinforcement of superposed strata, Professor
Rosetti considered " effective " as nearly equivalent to " actual "
temperature.
Yet one more "law of cooling" was proposed by M. Stefan
at Vienna in 1879. 3 It is that^ emission grows as the fourth
power of absolute temperature. Hence the temperature of
the photosphere would be proportional to the square root of
the square root of its heating effects at a distance, and
appeared, by Stefan's calculations from Yiolle's measures of
solar radiative intensity, to be just 6000 C.
1 Phil. Mag., vol. viii. 1879, p. 324. 2 Hid., p. 325. 3 Sitzungsler.,
Wien, Bd. Ixxix. ii. p. 391.
CHAP. v. TEMPERATURE OF THE SUN. 265
A new line of inquiry was struck out by Zollner in 1870.
Instead of tracking the solar radiations backward with the
dubious guide of empirical formulas, he investigated their
intensity at their source. He showed 1 that, taking promin-
ences to be simple effects of the escape of powerfully com-
pressed gases, it was possible, from the known mechanical laws
of heat and gaseous constitution, to deduce minimum values
for the temperatures prevailing in the area of their develop-
ment. These came out 27,700 C. for the strata lying imme-
diately above, and 68,400 C. for the strata lying immediately
below the photosphere, the former being regarded as the
region into which, and the latter as the region from which
the eruptions took place. In this calculation, no prominences
exceeding 40,000 miles (1.5') in height were included. But in
1884, G. A. Hirn of Colmar, taking into consideration the enor-
mous velocities of projection observed in the interim, fixed two
million degrees centigrade as the lowest internal temperature
by which they could be accounted for; although of opinion
that the condensations, presumed to give origin to the photo -
spheric clouds, were incompatible with a higher external tem-
perature than 50,000 to 100,000 C. 2
This method of going straight to the sun itself, observing
what goes on there, and inferring conditions, has much to
recommend it ; but its profitable use demands knowledge we
are still very far from possessing. We are quite ignorant, for
instance, of the actual circumstances attending the birth of
the solar flames. The assumption that they are nothing but
phenomena of elasticity is a purely gratuitous one. Spectro-
scopic indications, again, give hope of eventually affording a
fixed point of comparison with terrestrial heat-sources; but
their interpretation is still beset with uncertainties ; nor can,
indeed, the expression of transcendental temperatures in de-
grees of impossible thermometers be, at the best, other than
a futile attempt to convey notions respecting a state of things
altogether outside the range of our experience.
A more tangible, as well as a less disputable proof of solar
1 A&tr. Nack., Nos. 1815-16. 2 V Astronomic, t. iii. p. 334.
266 HISTORY OF ASTRONOMY. PART n.
radiative power than any mere estimates of temperature, was
provided in some experiments made by Professor Langley
in 1 8 78.* Using means of unquestioned validity, he found
the sun's disc to radiate 87 times as much heat, and 5300
times as much light as an equal area of metal in a Bessemer
converter after the air-blast had continued about twenty
minutes. The brilliancy of the incandescent steel, neverthe-
less, was so blinding, that melted iron, flowing in a dazzling
white-hot stream into the crucible, showed "deep brown by
comparison, presenting a contrast like that of dark coffee
poured into a white cup." Its temperature was estimated
(not quite securely, as Young has pointed out) 2 at 1800 to
2000 C. ; and no allowances were made, in computing relative
intensities, for atmospheric ravages on sunlight, for the extra
impediments to its passage presented by the smoke-laden air
of Pittsburg, or for the obliquity of its incidence. Thus a
very large balance of advantage lay on the side of the metal.
A further element of uncertainty in estimating the intrinsic
strength of the sun's rays has still to be considered. From
the time that his disc first began to be studied with the tele-
scope, it was perceived to be less brilliant near the edges.
Lucas Valerius of the Lyncean Academy seems to have been
the first to note this fact, which, strangely enough, was denied
by Galileo in a letter to Prince Cesi of January 25, i6i3. 3
Father Scheiner, however, fully admitted it, and devoted some
columns of his bulky tome to the attempt to find an appro-
priate explanation. 4 In 1729, Bouguer measured, with much
accuracy, the amount of this darkening; and from his data,
Laplace, adopting a principle of emission now known to be
erroneous, concluded that the sun loses eleven-twelfths of his
light through absorption in his own atmosphere. 5 The real
existence of this atmosphere, which is totally distinct from
the beds of ignited vapours producing the Fraunhofer lines, is
not open to doubt, although its nature is still a matter of
conjecture. The separate effects of its action on luminous,
1 Jour, of Sc., vol. i. (3d ser.), p. 653. 2 The Sun, p. 269. * Op.,
t. vi. p. 198. 4 Hosa Ursina, lib. iv. p. 6 1 8. 6 Mec. Gel., liv. x. p. 323.
CHAP. v. TEMPERATURE OF THE SUN. 267
thermal, and chemical rays were carefully studied by Father
Secchi, who in 1870 l inferred the total absorption to be -f^j of
all radiations taken together, and added the important obser-
vation that the light from the limb is no longer white, but
reddish-brown. Selective absorption was thus seen to be at
work ; and this could evidently be studied to advantage only
by taking the various rays of the spectrum separately, and
finding out how much each had suffered in transmission.
This was done by H. C. Vogel in 1 877.2 Using a polaris-
ing photometer, he found that only 13 per cent, of the violet
rays escape at the edge of the solar disc, 16 of the blue and
green, 25 of the yellow, and 30 per cent, of the red. Midway
between centre and limb, 88.7 of violet light and 96.7 of red
penetrate the absorbing envelope, the removal of which would
leave the sun's visible spectrum of just three times its present
intensity in the most, and once and a half times in the least
refrangible parts. The nucleus of a small spot was ascertained
to be of the same luminous intensity as a portion of the un-
broken surface about two and a half minutes from the limb.
These experiments having been made during a spot-minimum,
when there is reason to think that absorption is below its
average strength, Yogel suggested their repetition at a time
of greater activity.
Professor Langley pursued the subject still further. Reli-
able determinations of the " energy " of the individual spectral
rays were, for the first time, rendered possible by his inven-
tion of the "bolometer" in i88o. 3 This exquisitely sensitive
instrument affords the means of measuring heat, not directly,
like the thermopile, but in its effects upon the conduction of
electricity. It represents, in the phrase of the inventor, the
finger laid upon the throttle-valve of a steam-engine. A
minute force becomes the modulator of a much greater force,
and thus from imperceptible becomes conspicuous. By locally
raising the temperature of an inconceivably fine strip of
platinum serving as the conducting -wire in a circuit, the flow
1 Le Soldi (ist. ed.), p. 136. 2 Monatsber., Berlin, 1877, p. 104.
3 Am. Jour, of Sc. t vol. xxi. p. 187.
268 HISTORY OF ASTRONOMY. PART n.
of electricity is impeded at that point, and the included
galvanometer records a disturbance of the electrical flow.
Amounts of heat have, in this way, been detected in less than
ten seconds, which, expended during a thousand years on the
melting of a kilogramme of ice, would leave a part of the
work still undone.
The heat contained in the diffraction spectrum is, with
equal dispersions, barely one-tenth of that in the prismatic
spectrum. It had, accordingly, never previously been found
possible to measure it in detail that is, ray by ray. But it
is only from the diffraction, or normal spectrum that any
true idea can be gained as to the real distribution of energy
amongst the various constituents, visible and invisible, of a
sunbeam. The effect of passage through a prism is to crowd
together the red rays very much more than the blue. To this
prismatic distortion was owing the establishment of a pseudo-
maximum of heat in the infra-red, which disappeared when
the natural arrangement by wave-length was allowed free play.
Professor Langley's bolometer has shown that the hottest part
of the normal spectrum virtually coincides with its most
luminous part, both lying in the orange, close to the D line. 1
Thus the last shred of evidence in favour of the threefold divi-
sion of solar radiations vanished, and it became obvious that
the varying effects thermal, luminous, or chemical pro-
duced by them are due, not to any distinction of quality in
themselves, but to the different properties of the substances
they impinge upon. They are simply bearers' of energy, con-
veyed in shorter or longer vibrations ; upon the capacity of
the material particles meeting them for taking up those
shorter or longer vibrations, and turning them variously to
account in their inner economy, depends the result in each
separate case.
A long series of experiments at Allegheny was completed
in the summer of 1881 on the crest of Mount Whitney in the
Sierra Nevada. Here, at an elevation of 14,887 feet, in the
1 For J. W. Draper's partial anticipation of this result, see Am. Jour,
of Sc., vol. iv. 1872, p. 174.
CHAP. v. TEMPERATURE OF THE SUN. 269
driest and purest air, perhaps, in the world, atmospheric
absorptive inroads become less sensible, and the indications
of the bolometer, consequently, surer and stronger. An
enormous expansion was at once given to the invisible region
in the solar spectrum below the red. Captain Abney had got
chemical effects from undulations twelve ten-thousandths of a
millimetre in length. These were the longest recognised as,
or indeed believed, on theoretical grounds, to be capable of
existing. Professor Langley now got heating effects from
rays of above twice that wave length, his delicate thread of
platinum groping its way down nearly to thirty ten-
thousandths of a millimetre, or three "microms," where
sun-heat seems to be abruptly cut off, as if by the interposi-
tion of an absorbing screen. The known extent of the solar
spectrum was thus at once more than doubled. Its visible
portion covers a range of about one octave ; bolometric indi-
cations comprise between three and four. The great import-
ance of the newly explored region appears from the fact that
three-fifths of the entire energy of sunlight reside in the
infra-red, while scarcely more than one-hundreth part of that
amount is found in the better known ultra-violet space. 1
Atmospheric absorption had never before been studied with
such precision as it was by Professor Langley on Mount
Whitney. Aided by simultaneous observations from Lone
Pine, at the foot of the Sierra, he was able to calculate the
intensity belonging to each ray before entering the earth's
gaseous envelope in other words, to construct an extra-
atmospheric curve of energy in the spectrum. The result
showed that the blue end suffered far more than the red,
absorption varying inversely as wave-length. This property
of stopping predominantly the quicker vibrations is shared,
as both Vogel and Langley 2 have conclusively shown, by the
solar atmosphere. The effect of this double absorption is as
if two plates of reddish glass were interposed between us and
the sun, the withdrawal of which would leave his orb, not only
1 Phil. Mag., vol. xiv. p. 179 (March 1883). 2 Comptcs Rendus, t. xcii.
p. 701.
270 HISTORY OF ASTRONOMY. PART n.
three or four times more brilliant, but in colour of a distinct
greenish-blue, not very different from the tint of the second
(F) line of hydrogen. 1
The fact of the unveiled sun being blue has an important
bearing upon the question of his temperature, to afford a
somewhat more secure answer to which was the ultimate
object of Professor Langley's persevering researches; for it
is well known that, as bodies grow hotter, the proportionate
representation in their spectra of the more refrangible rays
becomes greater. The lowest stage of incandescence is the
familiar one of red heat. As it gains intensity, the quicker
vibrations come in, and an optical balance of sensation is
established at white heat. The final term of blue heat, as we
now know, is attained by the photosphere. On this ground
alone, then, of the large original preponderance of blue light,
we must raise our estimate of solar heat j and actual measure-
ments show the same upward tendency. Until quite lately,
Pouillet's figure of 1.7 calories per minute per square centi-
metre of terrestrial surface, was the received value for the
" solar constant." Forbes had, it is true, got. 2.85 from
observations on the Faulhorn in i842; 2 but they failed to
obtain the confidence they merited. Pouillet's result was
not definitively superseded until Yiolle, from actinometrical
measures at the summit and base of Mont Blanc in 1875,
computed the intensity of solar radiation at 2.54, 3 and Crova,
about the same time, at Montpellier, showed it to be above
two calories. 4 Langley went higher still. Working out the
results of the Mount Whitney expedition, he was led to con-
clude atmospheric absorption to be fully twice as effective as
had hitherto been supposed. 5 Scarcely sixty per cent., in fact,
of those solar radiations which strike perpendicularly through
a seemingly translucent sky, attain the sea-level. The rest
are reflected, dispersed, or absorbed. This discovery involved
a large addition to the original supply so mercilessly cut down
1 Nature^ vol. xxvi. p. 589. 2 Phil. Trans., vol. cxxxii. p. 273. 3 Ann.
de Chim., t. x. p. 321. 4 Ibid., t. xi. p. 505. 5 Am. Jour, of Sc.
vol..xxviii. p. 163 ; Observatory, vol. vii. p. 309.
CHAP. v. TEMPERATURE OF THE SUN. 271
in transmission, and the solar constant rose at once to three
calories its actual standard value. Phrased otherwise, this
means that the sun's heat reaching the outskirts of our atmos-
phere is capable of doing the work of an engine of one-horse
power for each square yard of the earth's surface. Thus,
modern inquiries, though they give no signs of agreement,
within any tolerable limits of error, as to the probable tem-
perature of the sun, tend, with growing certainty, to render
more and more evident the vastness of the thermal stores
contained in the great central reservoir of our system.
CHAPTER VI.
THE SUN'S DISTANCE.
THE question of the sun's distance arises naturally from the
consideration of his temperature, since the intensity of the
radiations emitted as compared with those received and
measured, depends upon it. But the knowledge of that
distance has a value quite apart from its connection with
solar physics. The semi-diameter of the earth's orbit is our
standard measure for the universe. It is the great funda-
mental datum of astronomy the unit of space, any error in
the estimation of which is multiplied and repeated in a thou-
sand different ways, both in the planetary and sidereal
systems. Hence its determination has been called by Airy
" the noblest problem in astronomy." It is also one of the
most difficult. The quantities dealt with are so minute that
their sure grasp tasks all the resources of modern science. An
observational inaccuracy which would set the moon nearer to,
or farther from us than she really is by one hundred miles,
would vitiate an estimate of the sun's distance to the extent of
sixteen million ! l What is needed in order to attain knowledge
of the desired exactness is no less than this : to measure an
angle about equal to that subtended by a halfpenny 2000 feet
from the eye, within a little more than a thousandth part of
its value.
The angle thus represented is what is called the " horizontal
parallax " of the sun. By this amount the breadth of a half-
penny at 2000 feet he is, to a spectator on the rotating
earth, removed at rising and setting from his meridian place
1 Airy, Month. Not., vol. xvii. p. 210.
CHAP. vi. THE SUN'S DISTANCE. 273
in the heavens. Such, in other terms, would be the magnitude
of the terrestrial radius as viewed from the sun. If we knew
this magnitude with certainty and precision, we should also
know with certainty and precision the dimensions of the
earth being, as they are, well ascertained the distance of
the sun. In fact, the one quantity commonly stands for the
other in works treating professedly of astronomy. But this
angle of parallax or apparent displacement cannot be directly
measured cannot even be perceived with the finest instru-
ments. Not from its smallness. The parallactic shift of the
nearest of the stars as seen from opposite sides of the earth's
orbit, is many times smaller. But at the sun's limb, and close to
the horizon, where the visual angle in question opens out to its
full extent, atmospheric troubles become overwhelming, and
altogether swamp the far more minute effects of parallax.
There remain indirect methods. Astronomers are well
acquainted with the proportions which the various planetary
orbits bear to each other. They are so connected, in the
manner expressed by Kepler's Third Law, that the periods
being known, it only needs to find the interval between any
two of them in order to infer at once the distances separating
them all from one another and from the sun. The plan is given ;
what we want to discover is the scale upon which it is drawn ;
so that, if we can get a reliable measure of the distance of a
single planet from the earth, our problem is solved.
Now some of our fellow-travellers in our unending journey
round the sun, come at times well within the scope of celestial
trigonometry. The orbit of Mars lies at one point not more
than thirty-five million miles outside that of the earth, and
when the two bodies happen to arrive together in or near the
favourable spot a conjuncture which recurs every fifteen years
the desired opportunity is granted. Mars is then "in
opposition," or on the opposite side of us from the sun, crossing
the meridian consequently at midnight. 1 It was from an
1 Mars comes into opposition once in about 780 days ; but owing to the
eccentricity of both orbits, his distance from the earth at those epochs
varies from thirty-five to sixty-two million miles.
S
274 HISTORY OF ASTRONOMY. PART n.
opposition of Mars, observed in 1672 by Richer at Cayenne in
concert with Cassini in Paris, that the first scientific estimate
of the sun's distance was derived. It appeared to be nearly
eighty- seven millions of miles (parallax 9.5") ; while Flamsteed
deduced 81,700,000 (parallax 10") from his independent obser-
vations of the same occurrence a difference quite insigni-
ficant at that stage of the inquiry. But Picard's result was
just half Flamsteed's (parallax 20"; distance forty-one million
miles) ; and Lahire considered that we must be separated from
the hearth of our system by an interval of at least 136 million
miles. 1 So that uncertainty continued to be on a gigantic
scale.
Yenus, on the other hand, comes closest to the earth when
she passes between it and the sun. At such times of " inferior
conjunction " she is, however, still twenty-six million miles, or
(in round numbers) 109 times as distant as the moon. More-
over, she is so immersed in the sun's rays that it is only when
her path lies across his disc that the requisite facilities for
measurement are afforded. These " partial eclipses of the sun
by Venus " (as Encke termed them) are coupled together in
pairs, 2 of which the components are separated by eight years,
recurring at intervals alternately of 105^ and I2ij years.
Thus, the first calculated transit took place in December 1631,
and its companion (observed by Horrocks) in the same month
(N".S.) 1639. Then, after the lapse of i2ij years, came the
June couple of 1761 and 1769; and again, after 105^, the two
recently observed, December 8, 1874, and December 6, 1882.
1 J. D. Cassini, Hist. Abrigie de la Parallaxe du Soleil, p. 122, 1772.
2 The present period of coupled eccentric transits will, in the course of ages,
be succeeded by a period of single, nearly central transits. The align-
ments by which transits are produced, of the earth, Venus, and the sun,
close to the place of intersection of the two planetary orbits, now occur,
the first a little in front of, the second, after eight years less two and a
half days, a little behind the node. But when the first of these two
meetings takes place very near the node, giving a nearly central transit,
the second falls too far from it, and the planet escapes projection on the
sun. The reason of the liability to an eight-yearly recurrence is that eight
revolutions of the earth are accomplished in only a very little more time
than thirteen revolutions of Venus.
CHAP. vi. THE SUN'S DISTANCE. 275
Throughout the twentieth century there will be no transit of
Venus ; but the astronomers of the twenty-first will only have
to wait four years for the first of a June pair. The rarity of
these events is due to the fact that the orbits of the earth and
Yenus do not lie in the same plane. If they did, there would
be a transit each time that our twin-planet overtakes us in her
more rapid circling that is, on an average, every 584 days.
As things are actually arranged, she passes above or below the
sun, except when she happens to be very near the line of inter-
section of the two tracks.
Such an occurrence as a transit of Yenus seems, at first
sight, full of promise for solving the problem of the sun's
distance. For nothing would appear easier than to determine
exactly either the duration of the passage of a small, dark orb
across a large brilliant disc, or the instant of its entry upon or
exit from it. And the differences in these times (which, owing
to the comparative nearness of Yenus, are quite considerable),
as observed from remote parts of the earth, can be translated
into differences of space that is, into apparent or parallactic
displacements, whereby the distance of Yenus becomes known,
and thence, by a simple sum in proportion, the distance of the
sun. But in that word " exactly " what snares and pitfalls lie
hid ! It is so easy to think and to say ; so indefinitely hard
to realise. The astronomers of the eighteenth century were
full of hope and zeal. They confidently expected to attain,
through the double opportunity offered them, to something
like a permanent settlement of the statistics of our system.
They were grievously disappointed. The uncertainty as to
the sun's distance, which they had counted upon reducing to
a few hundred thousand miles, remained at many millions.
In 1822, however, Encke, then director of the Seeberg
Observatory near Gotha, undertook to bring order out of the
confusion of discordant, and discordantly interpreted observa-
tions. His combined result for both transits (1761 and 1769)
was published in 1824,* and met universal acquiescence.
1 Die Entfernuny der Sonne: Fortsetzung, p. 108. Encke slightly cor-
rected his result of 1824 in Berlin Abh., 1835, p. 295.
276 HISTORY OF ASTRONOMY. PART n.
The parallax of the sun thereby established was 8.5776",
corresponding to a mean distance 1 of 95^- million miles. Yet
this abolition of doubt was far from being so satisfactory as
it seemed. Serenity on the point lasted exactly thirty years.
It was disturbed in 1854 by Hansen's announcement 2 that the
observed motions of the moon could be drawn into accord with
theory only on the terms of bringing the sun considerably
nearer to us than he was supposed to be.
Dr. Matthew Stewart, professor of mathematics in the
University of Edinburgh, had made a futile attempt in 1763
to deduce the sun's distance from his disturbing power over
our satellite. 3 Tobias Mayer of Gottingen, however, whose
short career was so fruitful of suggestions, struck out the
right way to the same end ; and Laplace, in the seventh book
of the Mecanique Celeste* gave a solar parallax derived from
the lunar " parallatic inequality " substantially identical with
that issuing from Encke's subsequent discussion of the
eighteenth century transits. Thus two wholly independent
methods the trigonometrical, or method by survey, and the
gravitational, or method by perturbation seemed to corro-
borate each the upshot of the use of the other until the nine-
teenth century was well past its meridian. It is singular
how often errors conspire to lead conviction astray.
Hansen's note of alarm in 1854 was echoed by Leverrier in
i858. 5 He found that an apparent monthly oscillation of
the sun which reflects a real monthly movement of the earth
round its common centre of gravity with the moon, and which
depends for its amount solely on the mass of the moon and
the distance of the sun, required a diminution in the admitted
value of that distance by fully four million miles. Three
1 Owing to the ellipticity of its orbit, the earth is nearer to the sun in
January than in June by 3,100,000 miles. The quantity to be determined,
or " mean distance," is that lying midway between these extremes is, in
other words, half the major axis of the ellipse in which the earth travels.
2 Month. Not., vol. xv. p. 9. 3 TJie Distance of the Sun from the Earth
determined by the Theory of Gravity, Edinburgh, 1763. 4 Opera, t. iii. p.
326. 5 Comptes Rendus, t. xlvi. p. 882. The parallax 8.95" derived by
Leverrier from the " parallactic inequality " in the earth's motion, was
corrected by Stone to 8.91". Month. J\'ot., vol. xxviii. p. 25.
CHAP. vi. THE SUN'S DISTANCE. 277
years later he pointed out that certain perplexing discrepancies
between the observed and computed places both of Venus and
Mars, would vanish on the adoption of a similar measure. 1
Moreover, a favourable opposition of Mars gave the oppor-
tunity in 1862 for fresh observations, which, separately
worked out by Stone and Winnecke, agreed with all the
newer investigations in fixing the great unit at slightly over
91 million miles. In Newcomb's hands they gave 92 \ million. 2
The accumulating evidence in favour of a large reduction in
the sun's distance was just then reinforced by an auxiliary
result of a totally different and unexpected kind.
The discovery that light does not travel instantaneously
from point to point, but takes some short time in transmission,
was made by Olaus Romer in 1675, through observing that
the eclipses of Jupiter's satellites invariably occurred later,
by a considerable interval, when the earth was on the far side,
than when it was on the near side of its orbit. Half this
interval, or the time spent by a luminous vibration in crossing
the " mean radius " of the earth's orbit, is called the " light-
equation;" and the determination of its precise value has
claimed the minute care distinctive of modern astronomy.
Delambre in 1792 made it 493 seconds. Glasenapp, a Russian
astronomer, raised the estimate in 1874 to 501 seconds, which,
from the extreme care employed, can hardly, at the outside,
be more than a couple of seconds in error; and even this
remaining uncertainty we may expect to see very much
reduced as the upshot of a twelve years' series of photometric
observations of Jupiter's satellites, begun at Harvard College
observatory in June 1878. Hence, if we had any independent
means of ascertaining how fast light travels, we could tell at
once how far off the sun is.
There is yet another way by which knowledge of the swift-
ness of light would lead us straight to the goal. The heavenly
bodies are perceived, when carefully watched and measured, to
be pushed forward out of their true places, in the direction of
the earth's motion, by a very minute quantity. This effect
1 Month. Not., vol. xxxv. p. 156. 2 Wash. Obs., 1865, App. ii. p. 28.
278 HISTORY OF ASTRONOMY. PART n.
(already adverted to) has been known since Bradley's time as
" aberration." It arises from a combination of the two move-
ments of the earth round the sun and of the light-waves
through the ether. If the earth stood still, or if light spent
no time on the road from the stars, such an effect would not
exist. Its amount represents the proportion between the
velocities with which the earth and the light-rays pursue their
respective journeys. This proportion is, roughly, one to ten
thousand. So that here again, if we knew the rate per second
of luminous transmission, we should also know the rate per
second of the earth's movement, consequently the size of its
orbit and the distance of the sun.
But, until lately, instead of finding the distance of the sun
from the velocity of light, there has been no means of ascer-
taining the velocity of light except through the imperfect
knowledge possessed as to the distance of the sun. The first
successful terrestrial experiments on the point date from
1 849 ; and it is certainly no slight triumph of human ingenuity
to have taken rigorous account of the delay of a sunbeam in
flashing from one mirror to another. Fizeau led the way, 1
and he was succeeded, after a few months, by Lon Foucault, 2
who, in 1862, had so far perfected Wheatstone's method of
revolving mirrors, as to be able to announce with authority
that light travelled slower, and that the sun was in consequence
nearer, than had been supposed. 3 Thus a third line of separate
research was found to converge to the same point with the
two others.
Such a conspiracy of proof was not to be resisted, and at
the anniversary meeting of the Royal Astronomical Society in
February 1864, the correction of the solar distance took the
foremost place in the annals of the year. Lest, however, a
sudden bound of four million miles nearer to the centre of our
1 Comptes JRendus, t. xxix. p. 90. 2 Ibid., t. xxx. p. 551. 3 Ibid., t.
lv. p. 501. The previously admitted velocity was 308 million metres per
second ; Foucault reduced it to 298 million. Combined with Struve's
"constant of aberration" this gave 8.86" for the solar parallax, which
exactly agreed with Cornu's result from a repetition in 1872 of Fizeau's ex-
periments. Comptes Rendus, t. Ixxvi. p. 338.
CHAP. vi. THE SUN'S DISTANCE. 279
system should shake public faith in astronomical accuracy, it
was explained that the change in the solar parallax correspond-
ing to that huge leap, amounted to no more than the breadth
of a human hair 125 feet from the eye! 1 From 1866 the
improved value of 8.90" was adopted in the Nautical Almanac,
while Newcomb's result of 8.85" has appeared since 1869 in
the Berlin Ephemeris. In astronomical literature the change
was initiated by Sir Edmund Beckett in the first edition
(1865) of his Astronomy without Mathematics.
If any doubt remained as to the misleading character of
Encke's deduction, so long implicitly trusted in, it was removed
by Powalky's and Stone's rediscussions, in 1864 and 1868
respectively, of the transit observations of 1769. Using im-
proved determinations of the longitude of the various stations,
and a selective judgment in dealing with their materials, which,
however indispensable, did not escape adverse criticism, they
brought out results confirmatory of the no longer disputed
necessity for largely increasing the solar parallax, and propor-
tionately diminishing the solar distance.
Conclusions on the subject, however, were still regarded as
purely provisional. A transit of Yenus was fast approaching,
and to its arbitrament, as to that of a court of final appeal,
the pending question was to be referred. It is true that the
verdict in the same case by the same tribunal a century earlier
had proved of so indistinct a character as to form only a
starting-point for fresh litigation ; but that century had not
passed in vain, and it was confidently anticipated that observa-
tional difficulties, then equally unexpected and insuperable,
would yield to the elaborate care and skill of forewarned
modern preparation.
The conditions of the transit of December 8, 1874, were
sketched out by the then Astronomer-Royal (Sir George Airy)
in 1857,2 and formed the subject of eager discussions in this
and other countries down to the very eve of the occurrence.
In these Mr. Proctor took a leading part, supplying official
omissions, and working out, with geometrical accuracy, the
1 Month. Not., vol. xxiv. p. 103. 2 Ibid., vol. xvii. p. 208.
2So HISTORY OF ASTRONOMY. PART n.
details of the relations between the different parts of the earth
and Yenus's shadow-cone ; and it was due to his urgent repre-
sentations that provision was made for the employment of the
method identified with the name of Halley, 1 which had been
too hastily assumed inapplicable to the first of each transit-
pair. It depends upon the difference in the length of time
taken by the planet to cross the sun's disc, as seen from
various points of the terrestrial surface, and requires, accord-
ingly, the visibility of both entrance and exit at the same
station. Since these were, in 1874, separated by about three
and a half hours, and a much longer interval is possible, the
choice of posts for the successful use of the "method of dura-
tions " is a matter of some difficulty.
The system described by Delisle in 1760, on the other hand,
involves merely noting the instant of ingress or egress (accord-
ing to situation) from opposite extremities of a terrestrial dia-
meter ; the disparity in time giving a measure of the planet's
apparent displacement, hence of its actual rate of travel in
miles per minute, from which its distances severally from
earth and sun are immediately deducible. Its chief attendant
difficulty is the necessity for accurately fixing the longitudes
of the points of observation. This, however, was much more
sensibly felt a century ago than it is now, and the improved
facility and certainty of modern determinations have tended to
give the Delislean plan a decided superiority over its rival.
These two traditional methods were supplemented in 1874
by the camera and the heliometer. From photography, above
all, much was expected. Observations made by its means
would have the advantages of impartiality, multitude, and
permanence. Peculiarities of vision and bias of judgment
would be eliminated; the slow progress of the phenomenon
would permit an indefinite number of pictures to be taken,
their epochs fixed to a fraction of a second ; while subsequent
leisurely comparison and measurement could hardly fail, it was
thought, to educe approximate truth from the mass of accumu-
1 Because closely similar to that proposed by him in Phil. Trans, for
1716.
CHAP. vi. THE SUN'S DISTANCE. 281
lated evidence. The use of the heliometer (much relied on by
German observers) was so far similar to that of the camera
that the object aimed at by both was the determination of the
relative positions of the centres of the sun and Venus viewed,
at the same absolute instant, from opposite sides of the globe.
So that the two older methods seek to ascertain the exact times
of meeting between the solar and planetary limbs ; while the
two modern methods work by measurement of the position of
the dark body already thrown into complete relief by its shining
background. The former are " methods by contact," the latter,
" methods by projection."
Every country which had a reputation to keep or to gain for
scientific zeal was forward to co-operate in the great cosmo-
politan enterprise of the transit. France and Germany each
sent out six expeditions ; twenty-six stations were in Russian,
twelve in English, eight in American, three in Italian, one
(equipped with especial care) in Dutch occupation. In all, at
a cost of nearly a quarter of a million, some fourscore dis-
tinct posts of observation were provided ; amongst them such
inhospitable, and all but inaccessible rocks in the bleak
Southern Ocean, as St. Paul's and Campbell Islands, swept
by hurricanes, and fitted only for the habitation of seabirds,
where the daring votaries of science, in the wise prevision of
a long leaguer by the elements, were supplied with stores for
many months, or even a whole year. Siberia and the Sand-
wich Islands were thickly beset with observers; parties of
three nationalities encamped within the mists of Kerguelen
Island, expressively termed the " Land of Desolation," in the
sanguine, though not wholly frustrated hope of a glimpse of
the sun at the right moment. M. Janssen narrowly escaped
destruction from a typhoon in the China seas on his way to
Nagasaki ; Lord Lindsay (now Earl of Crawford and Bal-
carres) equipped, at his private expense, an expedition to the
Mauritius, which was in itself an epitome of modern resource
and ingenuity.
During several years the practical methods best suited to
ensure success for the impending enterprise, formed a subject
282 HISTORY OF ASTRONOMY. PART n.
of " European debate." Official commissions were appointed
to receive and decide upon evidence ; and experiments were
in progress for the purpose of defining the actual circumstances
of the contacts, the accurate determination of which constituted
the only tried, though by no means an assuredly safe road to
the end in view. In England, America, France, and Germany,
artificial transits were mounted, and the members of the various
expeditions were carefully trained to unanimity in estimating
the phases of junction and separation between a moving dark
circular body and a broad illuminated disc. In the last cen-
tury, a formidable and prevalent phenomenon had swamped
all pretensions to rigid accuracy. This was an effect analogous
to "Baily's Beads," which acquired notoriety as the "Black
Drop "or " Black Ligament." It may be described as substi-
tuting adhesion for contact, the limbs of the sun and planet,
instead of meeting and parting with the desirable clean definite-
ness, clinging together as if made of some glutinous material,
and prolonging their connection by means of a dark band
or dark threads stretched between them. Some astronomers
ascribe this baffling appearance entirely to instrumental im-
perfections ; others to atmospheric agitation ; others again to
the optical encroachment of light upon darkness known as
" irradiation." It is probable that all these causes conspire,
in various measure, to produce it ; and it is certain that by
suitable precautions, combined with skill in the observer and
a reasonably tranquil air, its conspicuous appearance may, in
most cases, be obviated.
The organisation of the British forces reflected the utmost
credit on the energy and ability of Lieutenant-Colonel Tupman,
of the Royal Marine Artillery, who was responsible for the
whole. No useful measure was neglected. Each observer
went out ticketed with his "personal equation," his senses
drilled into a species of martial discipline, his powers absorbed,
so far as possible, in the action of a cosmopolitan observing
machine. Instrumental uniformity and uniformity of method
were attainable, and were attained ; but diversity of judgment
unhappily survived the best- directed efforts for its extirpation.
CHAP. vi. THE SUN'S DISTANCE. 283
The eventful day had no sooner passed than telegrams
began to pour in, announcing an outcome of considerable,
though not unqualified success. The weather had proved
generally favourable ; all the manifold arrangements had (save
for some casual mishaps) worked well; contacts had been
plentifully observed; photographs in lavish abundance had
been secured ; a store of materials, in short, had been laid up,
of which it would take years to work out the full results by
calculation. Gradually, however, it came to be known that
the hope of a definitive issue must be abandoned. Unanimity
was found to be as remote as ever. The dreaded " black
ligament " gave, indeed, less trouble than was expected ; but
another appearance supervened which took most observers by
surprise. This was the illumination due to the atmosphere of
Venus. Astronomers, it is true, were not ignorant that the
planet had, on previous occasions, been seen girdled with a
lucid ring ; but its power to mar observations by the distorting
effect of refraction had scarcely been reckoned with. It
proved, however, to be very great. Such was the difficulty of
determining the critical instant of internal contact, that (in
Colonel Tupman's words) " observers side by side, with ade-
quate optical means, differed as much as twenty or thirty
seconds in the times they recorded for phenomena which they
have described in almost identical language." l
Such uncertainties in the data admitted of a corresponding
variety in the results. From the British observations of ingress
and egress Sir George Airy 2 derived, in 1877, a solar parallax
of 8.76" (corrected to 8.754"), indicating a mean distance of
93,375,000 miles. Mr. Stone obtained a value of ninety-two
millions (parallax 8.88"), and held any parallax less than 8.84"
or more than 8. 93" to be " absolutely negatived " by the docu-
ments available. 3 Yet, from the same, Colonel Tupman de-
duced 8.8i", 4 implying a distance 700,000 miles greater than
Stone had obtained. The French observations of contacts
gave (the best being selected) a parallax of about 8.88";
1 Month. Not., vol. xxxviii. p. 447. 2 llil., p. II. 3 Ibid., p. 294.
* Ibid., p. 334.
284 HISTORY OF ASTRONOMY. PART n.
French micrometric measures the obviously exaggerated one
of 9-Q5". 1
Photography, as practised by most of the European parties,
was a total failure. Utterly discrepant values of the micro-
scopic displacements designed to serve as sounding-lines for
the solar system, issued from attempts to measure even the
most promising pictures. " You might as well try to measure
the zodiacal light," it was remarked to Sir George Airy.
Those taken on the American plan (adopted by Lord Lindsay),
of using telescopes of so great focal length as to afford, without
further enlargement, an image of the requisite size, gave notably
better results. From an elaborate comparison of these (some
dating from Vladivostock, Nagasaki, and Pekin, others from
Kerguelen and Chatham Islands), Professor D. P. Todd,
director of the Amherst College Observatory, deduced a solar
distance of about ninety-two million miles (parallax 8.883" +
O.O34/'), 2 a value, as Mr. Stone has pointed out, favoured by
a considerable accumulation of independent evidence.
On the whole, estimates of the great spatial unit cannot be
said to have gained any security from the combined effort of
1874. A few months before the transit, Mr. Proctor considered
that the uncertainty then amounted to 1,448,000 miles; 3 five
years after the transit, Professor Harkness judged it to be still
1,575,950 miles ; 4 yet it had been hoped that it would have
been brought down to 100,000. As regards the end for which
it had been undertaken, the grand campaign had come to
nothing. Nevertheless, no sign of discouragement was
apparent. There was a change of view, but no relaxation of
purpose. The problem, it was seen, could be solved by no
single heroic effort, but by the patient approximation of
gradual improvements. Astronomers, accordingly, looked
round for fresh means, or more refined expedients for apply-
ing those already known. A new phase of exertion was
entered upon.
1 Comptes Eendus, t. xcii. p. 8 1 2. 2 Observatory, No. 51, p. 205.
3 Transits of Venus, p. 89 (ist ed.). 4 Am. Journal of Sc., vol. xx.
P- 393-
CHAP. -vi. THE SUN'S DISTANCE. 285
On September 5, 1877, Mars came into opposition near the
part of his orbit which lies nearest to that of the earth, and Dr.
Gill (now Her Majesty's astronomer at the Cape of Good
Hope) took advantage of the circumstance to appeal once more
to him for a decision on the qucestio vexata of the sun's distance.
He chose, as the scene of his labours, the Island of Ascension,
and for their plan a method recommended by Airy in 1857,*
but never before fairly tried. This is known as the " diurnal
method of parallaxes." Its principle consists in substituting
successive morning and evening observations from the same
spot, for simultaneous observations from remote spots, the
rotation of the earth supplying the necessary difference in the
points of view. Its great advantage is that of unity in per-
formance. A single mind, looking through the same pair of
eyes, reinforced with the same optical appliances, is employed
throughout, and the errors inseparable from the combination
of data collected under different conditions are avoided. There
are many cases in which one man can do the work of two
better than two men can do the work of one. The result
of Dr. Gill's skilful determinations (made with Lord Lindsay's
heliometer) was a solar parallax of 8. 78", corresponding to a
distance of 93.080,000 miles. 2 The bestowal of the Royal
Astronomical Society's gold medal stamped the merit of this
distinguished service.
But there are other subjects for this kind of inquiry besides
Mars and Venus. Professor Galle of Breslau suggested in
1872 3 that some of the minor planets might be got to repay
astronomers for much disinterested toil spent in unravelling
their motions, by lending aid to their efforts towards a correct
celestial survey. Ten or twelve come near enough, and are
bright enough for the purpose ; and, in fact, the absence of
sensible magnitude is one of their chief recommendations,
since a point of light offers far greater facilities for exact
measurement than a disc. The first attempt to work this new
vein was made at the opposition of Phoca3a in 1872; and
1 Month. Not., vol. xvii. p. 219. 2 Mem. Roy. Astr. Soc., vol. xlvi.
p. 163. * Astr. Nach., No. 1897.
286 HISTORY OF ASTRONOMY. PART 11.
from observations of Flora in the following year at twelve
observatories in the northern and southern hemispheres, Galle
deduced a solar parallax of S.Sy". 1 At the Mauritius in 1874,
Lord Lindsay and Dr. Gill applied the "diurnal method " to
Juno, then conveniently situated for the purpose; and the
continued use of similar occasions affords, in the opinion of
the latter, the best available means for improving knowledge
of the sun's distance. They frequently recur ; they need no
elaborate preparation; a single astronomer armed with a
heliometer can do all the requisite work. The recommenda-
tion of Dr. Gill was accordingly acted upon in 1882, when
favourable oppositions of both Victoria and Sappho took
place ; and it is probable that each future event of the kind
will be made to serve as a step towards the desired level of
accuracy.
The second of the nineteenth-century pair of Yenus-transits
was looked forward to with much abated enthusiasm. Russia
refused her active co-operation in observing it, on the ground
that oppositions of the minor planets were trigonornetrically
more useful, and financially far less costly ; and her example
was followed by Austria, while Italian astronomers limited
their sphere of action to their own peninsula. Nevertheless,
it was generally held that a phenomenon which the world
could not again witness until it was four generations older
should, at the price of any effort, not be allowed to pass in
neglect.
An International Conference, accordingly, met at Paris in
1 88 1 with a view to concerting a plan of operations. America,
however, preferring independent action, sent no representative;
and the European break-down of photography in recording
transit-phases was admitted by its official abandonment. It
was decided to give Delisle's method another trial ; and the
ambiguities attending and marring its use were sought to be
obviated by careful regulations for ensuring agreement in the
estimation of the critical moments of ingress and egress. 2 But,
1 Hilfiker, Bern Mittkeilungen, 1878, p. 109. 2 Comptes JRendus, t. xciii.
p. 569.
CHAP. vi. THE SUN'S DISTANCE. 287
in fact (as M. Puiseux had shown 1 ), contacts between the
limbs of the sun and planet, so far from possessing the geo-
metrical simplicity long attributed to them, are really made
up of a prolonged succession of various and varying phases,
impossible either to predict or identify with anything like
rigid exactitude. Sir Robert Ball compared the task of de-
termining the precise instant of their meeting or parting, to
that of telling the hour with accuracy on a watch without a
minute-hand ; and the comparison is admittedly inadequate.
For not only is the apparent movement of Yenus across the
sun extremely slow, being but the excess of her real motion
over that of the earth ; but three distinct atmospheres the
solar, terrestrial, and cytherean combine to deform outlines
and mask the geometrical relations which it is desired to con-
nect with a strict count of time.
The result was very much what had been expected. The
arrangements were excellent, and were only in a few cases
disconcerted by bad weather. The British parties, under the
experienced guidance of Mr. Stone, the Radcliffe observer,
took up positions scattered (not at random) over the globe
from Queensland to Bermuda, and accumulated an ample
supply of skilful observations ; the Americans gathered in a
whole library of photographs, amongst them a fine series taken
at the new Lick Observatory on Mount Hamilton; the Germans
and Belgians trusted to the heliometer ; the French used the
camera as an adjunct to the method by contacts. Yet little
or no approach was made to solving the problem. The range
of doubt as to the sun's distance remained as wide as before.
The value published, in i884, 2 by M. Houzeau, late director
of the Brussels Observatory, forcibly illustrates this un-
welcome conclusion. From 606 measures of Yenus on the
sun, taken with a new kind of heliometer at Santiago in
Chili, he derived a solar parallax of 8.907", and a distance
of 91,727,000 miles. But the "probable error" of this deter-
mination amounts to 0.084" either way; that is, it is subject
1 Comptes Rendus, t. xcii. 481. * Bull, de VAcad., t. vi. p. 842.
288 HISTORY OF ASTRONOMY. PART n.
to a " more or less " of 900,000 miles, or to a total uncertainty
of 1,800,000.
The state and progress of knowledge on this important
subject were summed up by Faye and Harkness in iSSi. 1
The methods employed in its investigation fall (as we have
seen) into three separate classes the trigonometrical, the
gravitational, and the " phototachometrical " an ungainly
adjective used to describe the method by the velocity of light.
Each has its special difficulties and sources of error ; each has
counter-balancing advantages. The only distinct and trust-
worthy result, so far, from celestial surveys, was furnished by
Dr. Gill's observations of Mars in 1877. But the method by
lunar and planetary disturbances is unlike all the others in
having time on its side. It is this which Leverrier declared
with emphasis must inevitably prevail, because its accuracy is
continually growing. 2 The scarcely perceptible errors which
still impede its application are of such a nature as to accumu-
late year by year ; eventually, then, they will challenge, and
must receive, a more and more perfect correction.
For the present, however, the light-velocity method justifies
M. Faye's preference. By a beautiful series of experiments
on Foucault's principle, Master A. A. Michelson, of the
United States Navy, fixed in 1879 *^ e ra ^ e f luminous trans-
mission at 299,930 (corrected later to 299,910) kilometres a
second. 1 This determination was held by Professor Todd to
be entitled to four times as much confidence as any previous
one; and the solar parallax of 8.758" deduced by Professor
Harkness from its combination with Glasenapp's " light-
equation," was of corresponding weight. But all earlier
efforts of the kind were thrown into the shade by Professor
Newcomb's arduous operations at Washington in 1880-1882. 4
The scale upon which they were conducted was in itself im-
pressive. Foucault's entire apparatus in 1862 had been
enclosed in a single room ; Newcomb's revolving and fixed
1 ComptesRendus, t. xcii. p. 375 ; Am. Jour, of Sc., vol. xxii. p. 375.
2 Month. Not., vol. xxxv. p. 401. 3 Am. Jour, of Sc., vol. xviii. p. 393.
4 Nature, vol. xxxiv. p. 170.
CHAP. vi. THE SUN'S DISTANCE. 289
mirrors, between which the rays of light were to run their
timed course, were set up on opposite shores of the Potomac,
at a distance of nearly four kilometres. This advantage was
turned to the utmost account by ingenuity and skill in con-
trivance and execution; and the deduced velocity of 299,860
kilometres = 186,328 miles a second, had an estimated error
(30 kilometres) only one-tenth that ascribed by Cornu to his
own result in 1874.
Now it fortunately happened that this growth of exactitude
in one direction was matched by a corresponding advance in
another. M. Magnus Nyre"n, of St. Petersburg, published in
1882 an elaborate investigation of the small annual displace-
ments of the stars due to the successive transmission of light,
involving an increase of Struve's ''constant of aberration"
from 20.445" t 20.492". And there can be no doubt that
the correction is an improvement. The new value combined
with Newcomb's light-velocity, yielded probably the closest
approximation yet made to the sun's distance, concluded at
92,905,021 miles (parallax = 8. 794").
We have then two perfectly independent and unmistakably
authentic statements regarding this most important datum to
place side by side ; and it is reassuring to find them mutually
confirmatory. Apart, each claims a high degree of confidence ;
taken together, they afford something like certainty that the
margin of doubt has been at last, in a satisfactory degree,
straitened. Between the length of the " mean radius " of the
earth's orbit deduced by Dr. Gill from the Opposition of Mars,
and that reached by Professor Newcomb through his experi-
ments on the velocity of light, there is a difference of no more
than 175,000 miles. The intermediate round number of 93
million miles is unlikely to be above 100,000 astray an error
quite insignificant in comparison with the enormous uncer-
tainties of a few years past.
The problem of the sun's distance is then provisionally
solved, and without the aid of the long-anticipated transits of
Yenus. A new method, undreamt-of thirty years ago, has
stepped in to help astronomers out of their difficulty; and
T
290 HISTORY OF ASTRONOMY. PART n.
light has lent them its wings to span the vast gap lying
between them and its source. Further research on the sub-
ject is indeed indispensable, and will no doubt be diligently
pursued ; but we may confidently hope that it will issue, not
in the subversion of what we now seem to know, but in giving
to it greater stability and accuracy.
( 291 )
CHAPTER VII.
PLANETS AND SATELLITES.
JOHANN HIERONYMUS ScHROTER was the Herschel of Germany.
He did not, it is true, possess the more brilliant gifts of his
rival. Herschel's piercing discernment, comprehensive in-
telligence, and inventive splendour were wanting to him. He
was, nevertheless, the founder of descriptive astronomy in
Germany, as Herschel was in England.
Born at Erfurt in 1745, he prosecuted legal studies at Gb't-
tingen, and there imbibed from Kastner a life-long devotion
to science. From the law, however, he got the means of living,
and, what was to the full as precious to him, the means of
observing. Entering the sphere of Hanoverian officialism in
1 788, he settled a few years later at Lilienthal, near Bremen, as
" Oberamtmann," or chief magistrate. Here he built a small
observatory, enriched in 1785 with a seven-foot reflector by
Herschel, then one of the most powerful instruments to be
found anywhere out of England. It was soon surpassed,
through his exertions, by the first-fruits of native industry in
that branch. Schrader of Kiel transferred his workshops to
Lilienthal in 1792, and constructed there, under the super-
intendence and at the cost of the astronomical Oberamtmann,
a thirteen-foot reflector, declared by Lalande to be the finest
telescope in existence, and one twenty-seven feet in focal
length, probably as inferior to its predeccessor in real efficiency
as it was superior in size.
Thus, with instruments of gradually increasing power,
Schroter studied during thirty-four years the topography of the
moon and planets. The field was then almost untrodden ; he
292 HISTORY OF ASTRONOMY. PART 11.
had but few and casual predecessors, and has since had no
equal in the sustained and concentrated patience of his hourly
watchings. Both their prolixity and their enthusiasm are faith-
fully reflected in his various treatises. Yet the one may be
pardoned for the sake of the other, especially when it is re-
membered that he struck out a substantially new line, and
that one of the main lines of future advance. Moreover, his
infectious zeal communicated itself; he set the example of
observing when there was scarcely an observer in Germany ;
and under his roof Harding and Bessel received their training
as practical astronomers.
But he was reserved to see evil days. Early in 1813 the
French under Vandamme occupied Bremen. On the night of
April 20, the Yale of Lilies was, by their wanton destructive-
ness, laid waste with fire; the Government offices were de-
stroyed, and with them the chief part of Schrb'ter's property,
including the whole stock of his books and writings. There
was worse behind. A few days later, his observatory, which
had escaped the conflagration, was broken into, pillaged, and
ruined. His life was wrecked with it. He survived the
catastrophe three years without the means to repair, or the
power to forget it, and gradually sank from disappointment
into decay, terminated by death, August 29, 1816. He had,
indeed, done all the work he was capable of ; and though not
of the first quality, it was far from contemptible. He laid the
foundation of the comparative study of the moon's surface, and
the descriptive particulars of the planets laboriously collected
by him constituted a store of more or less reliable information
hardly added to during the ensuing half century. They rested,
it is true, under some shadow of doubt ; but the most recent
observations have tended on several points to rehabilitate the
discredited authority of the Lilienthal astronomer. "We may
now briefly resume, and pursue in its further progress the
course of his studies, taking the planets in the order of their
distances from the sun.
In April 1792 Schroter first saw reason to conclude, from
the gradual degradation of light on its partially illuminated
CHAP. vir. PLANETS AND SATELLITES. 293
disc, that Mercury possesses a tolerably dense atmosphere. 1
During the transit of May 7, 1799, he was, moreover, struck
with the appearance of a ring of softened luminosity encircling
the planet to an apparent height of three seconds, or about a
quarter of its own diameter. 2 Although a "mere thought "
in texture, yet it remained persistently visible both with the
seven-foot and the thirteen-foot reflectors, armed with powers
up to 288. It had a well-marked greyish boundary, and
reminded him, though indefinitely fainter, of the penumbra of
a sun-spot. A similar appendage, but more distinctly bright,
had been noticed by De Plantade at Montpellier, November
u, 1736, and again in 1786 and 1789 by Prosperin and
Flaugergues. Mercury projected on the sun, November 9,
1802, appeared to Ljunberg at Copenhagen surrounded with a
dark zone ; but Herschel, on the same day, saw its "preceding
limb cut the luminous solar clouds with the most perfect
sharpness." 3 The presence, however, of a " halo," appearing
to some observers a little darker, to others a little brighter than
the solar surface, was unmistakable in 1832. Professor Moll
of Utrecht described it as "a nebulous ring of a darker tinge,
approaching to the violet colour." To Huggins and Stone,
November 5, 1868, it showed as lucid and most distinct. No
change in the colour of the glasses used, or the powers applied,
could get rid of it, and it lasted throughout the transit. 6 It was
again well seen by Christie and Dunkin at Greenwich, May 6,
i878, 6 and with much precision of detail by Trouvelot at Cam-
bridge (U.S.). 7 Professor Holden, on the other hand, noted
at Hastings-on-Hudson the total absence of all anomalous
appearances. 8 No observations of much interest were made
during the transit of November 8, 1881. Dr. Little, at
Shanghai, perceived an unvarying " darkish halo," of which,
however, neither Mr. Ellery at Melbourne, nor Mr. Tebbutt
at Windsor, New South Wales, saw any trace. 9 They, how-
1 Neueste Beytrage zur Erweiteruny der SternTcunde, Bd. iii. p. 14 (1800).
2 Ibid., p. 24. 3 Phil. Trans., vol. xciii. p. 215. 4 Mem. Roy. Astr.
Soc., vol. vi. p. 1 1 6. 5 Month. Not., vol. xxix. pp. II, 25. 6 Ibid., vol.
xxxviii. p. 398. 7 Am. Jour, of Sc., vol. xvi. p. 124. 8 Wash. Obs.,
1876, Pt. ii. p. 34. 9 Month. Not., vol. xlii. pp. 101-104.
294 HISTORY OF ASTRONOMY. PART n.
ever, took note of a certain whitish spot on the planet's disc,
which, ever since 1 697, when it was detected by Wurzelbauer at
Erfurt, has been one of the most frequent attendant phenomena
of a transit of Mercury. It is not always centrally situated,
and is sometimes seen in duplicate, so that Powell's explana-
tion by diffraction is obviously insufficient. Nevertheless there
can scarcely be a doubt that it is an optical effect of some
kind.
As to the " halo," it is less easy to decide. That Mercury
possesses a considerable refractive atmosphere is certified by
the observation of De Plantade in I736, 1 and the still more
definite observation of Simms in i832, 2 of a luminous edge to
the part of the disc outside the sun at ingress or egress. The
natural complement to this appearance would be a dusky
annulus round the planet on the sun precisely such as was
seen by Moll and Little due to the imperfect transparency of
its gaseous envelope. But the brilliant ring vouched for by
others is not so readily explicable. Airy has shown that it
cannot possibly be caused by refraction, and must accordingly
be set down as " strictly an ocular nervous phenomenon." 3 It
is the less easy to escape from this conclusion that we find the
virtually airless moon capable of exhibiting a like appendage.
Professor Stephen Alexander of the United States Survey,
with two other observers, perceived, during the eclipse of the
sun of July 18, 1860, the advancing lunar limb to be bordered
with a bright band; 4 and photographic effects of the same
kind appear in pictures of transits of Yenus and partial solar
eclipses.
In the case of Mercury, a real effect is perhaps complicated
with an illusory one. Different eyes are very differently
sensitive to degrees of light and shade. Absorption by a
Mercurian atmosphere is doubtless in some degree present,
and it may be that the faintly shadowed ring produced by it
impresses some observers, by contrast with the ink-black disc
of the planet, as bright. The further investigation of this
1 Mim. de VAc., 1736, p. 440. 2 Month. Not., vol. ii. p. 103. 3 Ibid.,
vol. xxiv. p. 1 8. 4 Ibid., vol. xxiii. p. 234 (Challis).
CHAP. vii. PLANETS AND SATELLITES. 295
curious subject must wait for the next transit of Mercury,
May 9, 1891.
As to the constitution of this planet, the spectroscope has
little to tell. Its light is of course that of the sun reflected,
and its spectrum is consequently a faint echo of the Fraunhofer
spectrum. Dr. H. C. Vogel, who first examined it in April
1871, suspected traces of the action of an atmosphere like ours, 1
but, it would seem, on slight grounds. It is, however, certainly
very poor in blue rays. More definite conclusions were, in
1874,2 derived by Zollner from photometric observations of
Mercurian phases. A similar study of the waxing and waning
moon had afforded him the curious discovery that light-changes
dependent upon phase vary with the nature of the reflecting
surface, following a totally different law on a smooth homo-
geneous globe and on a rugged and mountainous one. Now
the phases of Mercury so far as could be determined from
only two sets of observations correspond with the latter
kind of structure. Strictly analogous to those of the moon,
they seem to indicate an analogous superficial conformation.
It is at any rate certain that the reflective capacity of Mercury
does not differ much from the lunar standard. The measure-
ments of Zollner and Winnecke 3 concur in assigning to it an
" albedo " represented by the fraction 0.126 (that of the moon
being = 0.119), signifying that it absorbs all but 126 thou-
sandths of the light with which the fierce near sun inundates
it. The inferred absence of an atmosphere is indeed scarcely
reconcilable with some of the transit-phenomena just adverted
to ; but heights and hollows in abundance seem to exist.
On March 26, 1800, Schroter, observing with his 1 3-foot
reflector in a peculiarly clear sky, perceived the southern horn
of Mercury's crescent to be quite distinctly blunted. 4 Inter-
ception of sunlight by a Mercurian mountain rather more than
eleven English miles high, explained the effect to his satis-
faction. By carefully timing its recurrence, he concluded
rotation on an axis in a period of 24 hours 4 minutes. This
1 Untersuchungen uber die Spectra der Planeten, p. 9. 2 Sirius, vol. vii.
p. 131. 3 Astr. Nach., No. 2245. 4 Neueste Beytrage, Bd. iii. p. 50.
296 HISTORY OF ASTRONOMY. PART n.
was the first determination of the kind, and was the reward of
twenty years' unceasing vigilance. It was confirmed by watch-
ing the successive appearances of a dusky streak and blotch in
May and June iSoi. 1 These, however, were inferred to be no
permanent markings on the body of the planet, but atmos-
pheric formations, the streak at times drifting forward (it was
thought) under the fluctuating influence of Mercurian breezes.
From a rediscussion of these somewhat doubtful observations
Bessel inferred that Mercury rotates on an axis inclined 70
to the plane of its orbit in 24 hours 53 seconds. A close
analogy would thus exist between the alternations of its seasons
and those of the earth, save that their effects must (except
within the polar circles) be well-nigh swallowed up in the
larger vicissitudes produced by the considerable eccentricity of
its path, causing its distance from the sun to vary from 29 to
43 million miles, and the light and heat received, from four
to ten times the amount reaching our planet.
The rounded appearance of the southern horn seen by
Schroter was more or less doubtfully caught by Noble (1864),
Burton, and Franks (iS;;); 2 but was obvious to Mr. W. F.
Denning at Bristol on the morning of November 5, i882. 3
He also discerned brilliant and dusky spaces, the displace-
ments of which, during four days, indicated rotation in about
twenty-five hours. The general aspect of the planet reminded
him of that of Mars ; 4 but the difficulties in the way of its
observation are enormously enhanced by its constant close
attendance on the sun.
The theory of Mercury's movements has always given
trouble. In Lalande's, 5 as in Mastlin's time, the planet
seemed to exist for no other purpose than to throw discredit
on astronomers ; and even to Leverrier's powerful analysis it
long proved recalcitrant. On the i2th of September 1859,
however, he was able to announce before the Academy of
Sciences 6 the terms of a compromise between observation and
1 Astr. Jahrbuch, 1804, pp. 97-102. 2 Webb, Celestial Objects, p. 46
(4th ed.). 8 L 1 Astronomic, t. ii. p. 141. 4 Observatory, No. 82, p. 40.
6 Hist.de I' Astr., p. 682. 6 Comptes Rendus, t. xlix. p. 379.
CHAP. vii. PLANETS AND SATELLITES. 297
calculation. They involved the addition of a new member to
the solar system. The hitherto unrecognised presence of a
body about the size of Mercury itself, revolving at somewhat
less than half its mean distance from the sun (or, if farther,
then of less mass, and vice versa), would, it was pointed out,
produce exactly the effect required, of displacing the perihelion
of the former planet 38 seconds a century more than could
otherwise be accounted for. The planes of the two orbits,
however, should not lie far apart, as otherwise a nodal distur-
bance would arise not perceived to exist. It was added that
a ring of asteroids similarly placed would answer the purpose
equally well, and was more likely to have escaped notice.
Upon the heels of this forecast followed promptly a seeming
verification. Dr. Lescarbault, a physician residing at Orgeres,
whose slender opportunities had not blunted his hopes of
achievement, had, ever since 1845, when he witnessed a transit
of Mercury, cherished the idea that an unknown planet might
be caught thus projected on the solar background. Unable to
observe continuously until 1858, he, on March 26, 1859, saw
what he had expected a small perfectly round object slowly
traversing the sun's disc. The fruitless expectation of re-
observing the phenomenon, however, kept him silent, and it
was not until December 22, after the news of Leverrier's pre-
diction had reached him, that he wrote to acquaint him with
his supposed discovery. 1 The Imperial Astronomer thereupon
hurried down to Orgeres, and by personal inspection of the
simple apparatus used, by searching cross-examination and
local inquiry, convinced himself of the genuine character and
substantial accuracy of the reported observation. He named
the new planet " Vulcan," and computed elements giving it a
period of revolution slightly under twenty days. 2 But it has
never since been seen. M. Liais, director of the Brazilian
Coast Survey, thought himself justified in asserting that it
never had been seen. Observing the sun for twelve minutes
after the supposed ingress recorded at Orgeres, he noted those
particular regions of its surface as "tres uniformes cTin-
1 Comptes Rendus, t. 1. p. 40. 2 Ibid., p. 46.
298 HISTORY OF ASTRONOMY. PART n.
tensit^." 1 He subsequently, however, admitted Lescarbault's
good faith, at first rashly questioned. The planet- seeking
doctor was, in truth, only one among many victims of similar
illusions.
Waning interest in the subject was revived by a fresh an-
nouncement of a transit witnessed, it was asserted, by Weber
at Peckeloh, April 4, i8y6. 2 The pseudo-planet, indeed, was
detected shortly afterwards on the Greenwich photographs,
and was found to have been seen by M. Ventosa at Madrid
in its true character of a sun-spot without penumbra; but
Leverrier had meantime undertaken the investigation of a list
of twenty similar dubious appearances, collected by Haase,
and republished by Wolf in 1 8 7 2. 3 From these five were picked
out as referring in all likelihood to the same body, the reality
of whose existence was now confidently asserted, and of which
more or less probable transits were fixed for March 22, 1877,
and October 15, 1882.* But, widespread watchfulness notwith-
standing, no suspicious object came into view at either epoch.
The next announcement of the discovery of " Yulcan " was
on the occasion of the total solar eclipse of July 29, i878. 5
This time it was stated to have been seen at some distance
south-west of the obscured sun, as a ruddy star with a minute
planetary disc ; and its simultaneous detection by two observers
the late Professor James C. Watson, stationed at Rawlins
(Wyoming Territory), and Professor Lewis Swift at Denver
(Colorado) was at first readily admitted. But their separate
observations could, on a closer examination, by no possibility
be brought into harmony, and, if valid, certainly referred to
two distinct objects, if not to four j each astronomer eventually
claiming a pair of planets. Nor could any one of the four be
identified with Lescarbault's and Leverrier's Yulcan, which,
if a substantial body revolving round the sun, must then (as
Oppolzer showed) 6 have been found on the east side of that
i A sir. Nock,, Nos. 1248 and 1281. 2 Comptes Rendus, t. Ixxxiii. pp.
510, 561. 3 tfandbuch der MathematiJc, Bd. ii. p. 327. 4 Comptes Ren-
dus, t. Ixxxiii. p. 721. 5 Nature, vol. xviii. pp. 461, 495, 539. 6 Astr.
., No. 2239.
CHAP. vii. PLANETS AND SATELLITES. 299
luminary. The most feasible explanation of the puzzle seems
to be that Watson and Swift merely saw each the same two
stars in Cancer : haste and excitement doing the rest. 1 Never-
theless they strenuously maintained their opposite conviction. 2
Intra-Mercurian planets have since been diligently searched
for when the opportunity of a total eclipse offered, especially
during the long obscuration at Caroline Island. Not only did
Professor Hold en " sweep " in the solar vicinity, but Palisa
and Trouvelot agreed to divide the field of exploration, and
thus make sure of whatever planetary prey there might be
within reach ; yet with only negative results. Belief in the
presence of any considerable body or bodies within the orbit
of Mercury is, accordingly, now at a low ebb. Yet the exist-
ence of the anomaly in the Mercurian movements indicated by
Leverrier has been made only surer by further research. 3 Its
elucidation constitutes one of the " pending problems " of
astronomy.
From the observation at Bologna in 1666-67 of some very
faint spots, Domenico Cassini concluded a rotation or libra-
tion of Venus he was not sure which in about twenty- three
hours. 4 By Bianchini in 1726 the period was augmented to
twenty-four days eight hours. J. J. Cassini, however, in 1 740,
showed that the data collected by both observers were con-
sistent with rotation in twenty-three hours twenty minutes. 5
So the matter rested until Schroter's time. After watching
nine years in vain, he at last, February 28, 1788, perceived the
ordinarily uniform brightness of the planet's disc to be marbled
with a filmy streak, which returned periodically to the same
position in about twenty-three hours twenty-eight minutes.
This approximate estimate was corrected by the application of
1 A str. Nach., Nos. 2253-2254 (C. H. F. Peters). 2 Ibid., Nos. 2263 and
2277. See also Tisserand in Ann. Bur. des Long., 1882, p. 729. 3 See J.
Bauschinger's Untersuchungen (1884), summarised in Bull. Astr., t. i. p. 506,
and Astr. Nach., No. 2594. Newcomb finds the anomalous motion of the
perihelion to be even larger (43" instead of 38") than Leverrier made it.
Month. Not., Feb. 1884, p. 187. 4 Jour, des Spawns, Dec. 1667, p. 122.
5 Clemens d'Astr., p. 525.
300 HISTORY OF ASTRONOMY. PART n.
a more definite criterion. On December 28, 1789, the southern
horn of the crescent Venus was seen truncated, an outlying
lucid point interrupting the darkness beyond. Precisely the
same appearance recurred two years later, giving for the planet's
rotation a period of twenty-three hours twenty-one minutes. 1
To this only twenty-two seconds were added by De Yico, as
the result of over 10,000 observations made with the Cauchoix
refractor of the Collegio Romano, 1839-41. The axis of
rotation was found to be much more bowed towards the orbital
plane than that of the earth, the equator making with it an
angle of 53 n' 26''. If dependence be placed on De Yico's
identification of individual markings drawn by Bianchini 113
years earlier, 2 they cannot possess the evanescent atmospheric
character attributed to them by Schrb'ter, but must be inherent
peculiarities of surface. The point, however, remains doubtful.
Of the frequently mountainous nature of that surface there
appears to be no reasonable doubt. Francesco Eontana at
Naples in 1643 noticed irregularities along the inner edge of
the crescent. 3 Lahire in 1700 considered them regard being
had to difference of distance to be much more strongly
marked than those visible in the moon. 4 Schroter's assertions
to the same effect, though scouted with some unnecessary
vehemence by Herschel, 5 have since been repeatedly confirmed ;
amongst others by Madler, De Yico, Langdon, who in 1873
saw the broken line of the " terminator " (the boundary between
light and darkness) with peculiar distinctness through a veil of
auroral cloud; 6 by Denning, 7 March 30, 1881, despite prelimi-
nary impressions to the contrary, as well as by C. Y. Zenger at
Prague, January 8, 1883. The great mountain mass, presumed
to occasion the periodical blunting of the southern horn, was
precariously estimated by the Lilienthal observer to rise to the
prodigious height of nearly twenty- seven miles, or just five
1 Beobachtungen uber die sehr betrachtlichen Gebirge und Rotation der
Venus, 1793, P- 45- Schroter's final result in 1811 was 2$h. 2im. 7.9773.
Monat. Corr., Bd. xxv. p. 367. 2 A sir. Nach., No. 404. 3 Nova Obser-
rationes, p. 92. 4 Mem. deVAc., 1700, p. 296. 5 Phil. Trans., vol.
Ixxxiii. p. 201. 6 Webb, Cel. Objects, p. 58. 7 Month. Not., vol. xlii. p.
in.
CHAP. vii. PLANETS AND SATELLITES. 301
times the elevation of Mount Everest ! Yet the phenomenon
persists, whatever may be thought of the explanation. More-
over, the speck of light beyond, interpreted as the visible sign
of a detached peak rising high enough above the encircling
shadow to catch the first and last rays of the sun, was frequently
discerned by Baron von Ertborn in 1876 Awhile an object
near the northern horn of the crescent, strongly resembling a
lunar ring-mountain, was delineated both by De Vico in 1841
and by Denning forty years later.
We are almost equally sure that Yenus, as that the earth
is encompassed with an atmosphere. Yet, notwithstanding
luminous appearances plainly due to refraction during the
transits both of 1761 and 1769, Schroter, in 1792, took the
initiative in coming to a definite conclusion on the subject. 2 It
was founded, first, on the rapid diminution of brilliancy towards
the terminator, attributed to atmospheric absorption ; next, on
the extension beyond a semicircle of the horns of the crescent;
lastly, on the presence of a bluish gleam illuminating the early
hours of the Cytherean night with what was taken to be
genuine twilight. Even Herschel admitted that sunlight, by
the same effect through which the heavenly bodies show visibly
above our horizons while still geometrically below them, appeared
to be bent round the shoulder of the globe of Yenus. Ample
confirmation of the fact has since been afforded. At Dorpat
in May 1849, the planet being within 3 26' of inferior con-
junction, Madler found the arms of waning light upon the disc
to embrace no less than 240 of its extent ; 3 and in December
1842, Mr. Guthrie, of Bervie, N.B., actually observed, under
similar conditions, the whole circumference to be lit up with a
faint nebulous glow. 4 Here the solar rays evidently pierced
the planet's atmosphere from behind, pursuing a curved path,
as if through a lens. The same curious phenomenon was
intermittently seen by Mr. Leeson Prince at Uckfield in
September 1863 ; 5 but with more satisfactory distinctness by
i Bull. Ac. de Bruxelles, t. xliii. p. 22. 2 Phil Trans., vol. Ixxxii. p. 309 ;
Aphroditographische Fragmente, p. 85 (1796). 3 Astr. Nach., No. 679.
4 Month. Not., vol. xiv. p. 169. 5 Ibid., vol. xxiv. p. 25.
302 HISTORY OF ASTRONOMY. PART n.
Mr. C. S. Lyman of Yale College, 1 before and after the con-
junction of December n, 1866, and during nearly five hours
previous to the transit of 1874, when the yellowish ring of
refracted light showed at one point an approach to interruption,
it might be presumed through the intervention of a bank of
clouds. These effects can be accounted for, as Mr. Neison
pointed out, 2 only by supposing the atmosphere of Yenus to
be nearly twice as dense at the surface of its globe, and to
possess nearly twice as much refractive power as that of the
earth.
Similar appearances are conspicuous during transits. But
while the Mercurian halo is characteristically seen on the sun,
the " silver thread " round the limb of Yenus commonly shows
on the part off the sun. There are, however, instances of each
description in both cases. Mr. Grant, in collecting the records
of physical phenomena accompanying the transits of 1761 and
1769, remarks that no one person saw both kinds of annulus,
and argues thus a dissimilarity in their respective modes of
production. 3 Such a dissimilarity probably exists, in the sense
that the inner section of the ring is due to absorption, the outer
to refraction by the same planetary atmosphere ; but the dis-
tinction of separate visibility has not been borne out by recent
experience. Several of the Australian observers during the
transit of 1874 witnessed the complete phenomenon. Mr. J.
Macdonnell, at Eden, saw a " shadowy nebulous ring " sur-
round the whole disc when ingress was two-thirds accomplished;
Mr. Tornaghi, at Goulburn, perceived a halo, entire and un-
mistakable, at half egress. 4 Similar observations were made
at Sydney, 5 and were renewed in 1882 by Lescarbault at
Orgeres, by Metzger in Java, and by Barnard at Yanderbilt
University. 6
Spectroscopic indications of aqueous vapour as present in
i Am. Jour, of Sc.,\vol. xliii. p. 129 (2d ser.) ; vol. ix. p. 47 ^dser.).
2 Month. Not., vol. xxxvi. p. 347. 8 Hist. Phys. Astr., p. 431. 4 Mem.
Roy. Astr. Soc., vol. xlvii. pp. 77, 84. 5 Astr. Reg., vol. xiii. p. 132.
6 IS Astronomic, t. ii.'p. 27 ; Astr. Nach., No 2O2I ; Am. Jour, of Sc., vol.
xxv. p. 430.
CHAP. vii. PLANETS AND SATELLITES. 303
the atmosphere of Venus, were obtained in 1874 and 1882, by
Tacchini and Bicc6 in Italy, and by Young in New Jersey. 1
Janssen, however, who made a special study of the point
subsequently to the transit of 1882, found them much less
certain than his earlier expectations led him to expect ; 2 and
Vogel, by repeated examinations, 1871-73, could detect only
the very slightest variations from the pattern of the solar
spectrum. Some additions there indeed seem to be in the
thickening of certain water-lines, and also of a group (B) since
shown by Egoroff to be developed through the absorptive
action of cool oxygen ; but so nearly evanescent as to induce
the persuasion that the light we receive from Venus is reflected
from a heavy cloud-stratum, and has traversed, consequently,
only the rarer upper portion of its atmosphere. 3 This would
also account for the extreme brilliancy of the planet. On the '
26th and 27th of September 1878, a close conjunction gave
Mr. James Nasmyth the rare opportunity of watching Venus
and Mercury for several hours side by side in the field of his
reflector ; when the former appeared to him like clean silver,
the latter as dull as lead or zinc. 4 Yet the light incident upon
Mercury is, on an average, three and a half times as strong as
the light reaching Venus. Thus, the reflective power of Venus
must be singularly strong. And we find accordingly, from
a combination of Zollner's with Pickering's results, that its
albedo is but little inferior to that of new-fallen snow; in
other words, it gives back 72 J per cent, of the luminous rays
impinging upon it.
This view, that we see only the cloud-canopy of Venus, is
manifestly inconsistent with the supposed permanency of its
spots, or with the perception of shadow effects on a rugged
crust. It is, however, with some reservation, shared by Mr.
E. L. Trouvelot, who in 1875 and some subsequent years
pursued a diligent telescopic study of the planet at Cambridge
(U.S.). Not the least surprising fact about this sister-globe is
1 Mem. Spettr. Ital., Dicembre 1882 ; Am. Jour, of Sc., vol. xxv. p. 328.
2 Comptes Rendus, t. xcvi. p. 288. 3 Vogel, Unters. uber die Spectra der
Planeten, p. 15. 4 Nature, vol. xix. p. 23.
304 HISTORY OF ASTRONOMY. PART ir.
that the axis on which it rotates is hooded at each end with
some shining substance. These polar appendages were dis-
covered in 1813 by Gruithuisen, 1 who set them down as polar
snow-caps like those of Mars. Nor is it altogether certain
that he was wrong. Trouvelot, indeed, in January 1878,
perceived (or thought that he perceived) the southern one to
be composed of isolated peaks thrown into relief against the
sky, and hence concluded both to represent lofty groups of
mountains penetrating the vapour-stratum supposed to form
the greater part of the visible disc. He pointed out, more-
over, that the place of the southern spot might be called
identical with that of a projection above the limb detected by
MM. Bouquet de la Grye and Arago in measuring photographs
of Venus in transit taken at Puebla and Port-au-Prince in
i882. 2 This projection corresponded to a real elevation of
about sixty-five miles. But it was more probably due to
" photographic irradiation " from a local excess of brilliancy,
the result according to the French investigator's conjecture
of accumulations of ice and snow, or the continuous forma-
tion of vast cloud-masses.
The same photographs seem to show that in figure Venus
very closely resembles our earth, the estimated equatorial
bulging produced by rotation being 3^3- of the radius.
The "secondary," or "ashen light," of Venus was first
noticed by Biccioli in 1 643 \ it was seen by Derham about
1715, by Kirch in 1721, by Schroter and Harding in i8o6; 3
and the reality of the appearance has since been authenticated
by numerous and trustworthy observations. It is precisely
similar to that of the "old moon in the new moon's arms; "
and Zenger, who witnessed it with unusual distinctness,
January 8, 1883,* supposes it due to the same cause namely,
to the faint gleam of reflected earth-light from the night-side
of the planet. When we remember, however, that " full earth-
light " on Venus, at its nearest, has little more than T ^,J^^ its
1 Nova Ada Acad. Natures Curiosorum, Bd. x. p. 239. 2 Observatory,
vols. iii. p. 416, vii. p. 239. 3 Astr. Jakrbuch, 1809, p. 164. 4 Month.
Not., vol. xliii. p. 331.
CHAP. vii. PLANETS AND SATELLITES. 305
intensity on the moon, we see at once that the explanation is
inadequate. Nor can Professor Safarik's, 1 by phosphorescence
of the warm and teeming oceans with which Zollner 2 regarded
the globe of Venus as mainly covered, be seriously entertained.
Vogel's suggestion is more plausible. He and 0. Lohse, at
Bothkamp, November 3-11, 1871, saw the dark hemisphere
partially illuminated by secondary light, extending 30 from
the terminator, and thought the effect might be produced by
a very extensive twilight. 3 An atmospheric diffusion of sun-
light seems, in fact, the best answer to the riddle. It involves
difficulties, but probably none that are insuperable.
The third planet encountered in travelling outward from the
sun is the abode of man. He has in consequence opportunities
of studying its physical habitudes altogether different from the
baffling glimpses afforded to him of the other members of the
solar family. Regarding the earth, then, a mass of knowledge
so varied and comprehensive has been accumulated as to form
a science or rather several sciences apart. But underneath
all lie astronomical relations, the recognition and investigation
of which constitute one of the most significant intellectual
events of the present century.
It is indeed far from easy to draw a line of logical distinction
between items of knowledge which have their proper place
here, and those which should be left to the historian of geology.
There are some, however, of which the cosmical connections
are so close that it is impossible to overlook them. Amongst
these is the ascertainment of the solidity of the globe. At first
sight it seems difficult to conceive what the apparent positions
of the stars can have to do with subterranean conditions ; yet
it was from star measurements alone that Hopkins, in 1839,
concluded the earth to be solid to a depth of at least 800 or
1000 miles. 4 His argument was, that if it were a mere shell
1 Report Brit. Ass., 1873, p. 407. The paper contains a valuable record
of observations of the phenomenon. 2 Photom. Untersuchungen, p. 301.
3 Beobaclitungen zu Bothkamp, Heft ii. p. 126. 4 Phil. Trans., 1839,
1841, 1842.
U
306 HISTORY OF ASTRONOMY. PART ir.
filled with liquid, precession and nutation would be much
larger than they are observed to be. For the shell alone would
follow the pull of the sun and moon on its equatorial girdle,
leaving the liquid behind ; and being thus so much the lighter,
would move the more readily. There is, it is true, grave
reason to doubt whether this reasoning corresponds with the
actual facts of the case ; l but the conclusion to which it led
has been otherwise affirmed and extended.
Indications to an identical effect have been derived from
another kind of external disturbance, affecting our globe
through the same agencies. Sir William Thomson pointed
out in 1862 2 that tidal influences are brought to bear on land
as well as on water, although obedience to them is perceptible
only in the mobile element. Some bodily distortion of the
earth's figure must however take place, unless we suppose it of
absolute or "preternatural" rigidity, and the amount of such
distortion can be determined from its effect in diminishing
oceanic tides below their calculated value. For if the earth
were perfectly plastic to the stresses of solar and lunar gravity,
tides in the ordinary sense would not exist. Continents
and oceans would swell and subside together. It is to the
difference in the behaviour of solid and liquid terrestrial con-
stituents that the ebb and flow of the waters are due.
Six years later, the distinguished Glasgow professor sug-
gested that this criterion might, by the aid of a prolonged
series of exact tidal observations, be practically applied to test
the interior condition of our planet. 3 In 1882, accordingly,
suitable data extending over thirty-three years having at
length become available, Mr. G. H. Darwin performed the
laborious task of their analysis, with the general result that
1 Delaunay objected (Comphs Rcndus, t. Ixvii. p. 65) that the viscosity
of the contained liquid (of which Hopkins took no account) would, where
the movements were so excessively slow as those of the earth's axis, almost
certainly cause it to behave like a solid. Sir W. Thomson, however (Report
Brit. Ass., 1876, ii. p. l), considers Hopkins's argument valid as regards the
comparatively quick solar semi-annual and lunar fortnightly nutations.
2 Phil Trans., vol. cliii. p. 573. 3 Report Brit. Ass., 1868, p. 494.
CHAP. vii. PLANETS AND SATELLITES. 307
the " effective rigidity " of the earth's mass must be at least as
great as that of steel. 1
In a paper read before the Geological Society, December 15,
1 830,2 Sir John Herschel threw out the idea that the perplexing
changes of climate revealed by the geological record might be
explained through certain slow fluctuations in the eccentricity
of the earth's orbit, produced by the disturbing action of the
other planets. Shortly afterwards, however, he abandoned the
position as untenable ; 3 and it was left to Mr. James Croll, in
i864 4 an d subsequent years, to reoccupy and convert it into a
strong, if not an impregnable one. Within restricted limits
(as Lagrange, and, more certainly and definitely, Leverrier
proved), the path pursued by our planet round the sun alter-
nately contracts, in the course of ages, into a moderate ellipse,
and expands almost to a circle, the major axis, and conse-
quently the mean distance, remaining invariable. Even at
present, when the eccentricity approaches a minimum, the sun
is nearer to us in January than in July by above three million
miles, and some 850,000 years ago this difference was more
than four times as great. Mr. Groll has brought together 5 a
mass of evidence to support the view that, at epochs of con-
siderable eccentricity, the hemisphere of which the winter,
occurring at aphelion, was both intensified and prolonged, must
have undergone extensive glaciation ; while the opposite hemi-
sphere, with a short, mild winter, and long, cool summer,
enjoyed an approach to perennial spring. These conditions
were exactly reversed at the end of 10,500 years, through the
shifting of the perihelion combined with the precession of the
equinoxes, the frozen hemisphere blooming into a luxuriant
garden as its seasons came round to occur at the opposite sides
of the terrestrial orbit, and the vernal hemisphere subsiding
simultaneously into ice-bound rigour. Thus a plausible ex-
planation was offered of the anomalous alternations of glacial
1 Report Brit. Ass., 1882, p. 474. 2 Trans. Geol. Soc., vol. iii. (2d ser.),
p. 293. 3 See his Treatise on Astronomy, p. 199 (1833). 4 Phil. Mag.,
vol. xxviii. (4th ser.), p. 121. 5 Climate and Time, 1875 ; Discussions on
Climate and Cosmology, 1885.
308 HISTORY OF ASTRONOMY. PART 11.
and seini-tropical periods, attested, on incontrovertible geo-
logical evidence, as having succeeded each other in times past
over what are now temperate regions. The most recent glacial
epoch is placed by Mr. Croll about 200,000 years ago, when
the eccentricity of the earth's orbit was 3.4 times as great as
it now is. At present, a faint representation of such a state
of things is afforded by the southern hemisphere. One con-
dition of glaciation in the coincidence of winter with the
maximum of remoteness from the sun, is present ; the other
a high eccentricity is deficient. Yet the ring of ice-bound
territory hemming in the southern pole is well known to be
far more extensive than the corresponding region in the north.
The verification of this ingenious hypothesis depends upon
a variety of intricate meteorological conditions, some of which
have been adversely interpreted by competent authorities. 1
It has, however, certainly made good its footing among the
better- warranted speculations of science. The precise nature
of the connection between geological and astronomical events
indicated by it may be questioned, but there can no longer be
any doubt that, in some form, such a relation exists. Its
ascertainment marks one further step in that process of uni-
fication between things celestial and things terrestrial which
forms, it might be said, the vast presiding idea of astronomical
history during the last three centuries.
The first attempt at an experimental estimate of the " mean
density" of the earth was Maskelyne's observation in 1774
of the deflection of a plumb-line through the attraction of
Schehallien. The conclusion thence derived, that our globe
weighs 4-J times as much as an equal bulk of water, 1 was not
very exact. It was considerably improved upon by Cavendish,
who, in 1798, brought into use the "torsion-balance" con-
structed for the same purpose by John Michell. The resulting
estimate of 5.48 was raised to 5.66 by Francis Baily's elaborate
repetition of the process in 1838-42. From the latest ex-
periments on the subject those made in 1872-73 by Cornu
1 See A. Woeikof, Phil. Mag., vol. xxi. p. 223.
2 Phil. Trans., vol. Ixviii. p. 783.
CHAP. vii. PLANETS AND SATELLITES. 309
and Bailie the slightly inferior value of 5.56 was derived ; and
it was further shown that the data collected by Baily, when
corrected for a systematic error, gave a practically identical
result (5-55). 1 Newton's guess at the average weight of the
earth as five or six times that of water has thus been curiously
verified.
Operations for determining the figure of the earth have been
carried out during the present century on an unprecedented
scale. The Russo-Scandinavian arc, of which the measure-
ment was completed under the direction of the elder Struve
in 1855, Cached from Hammerfest to Ismailia on the Danube,
a length of 25 20'. But little inferior to it was the Indian
arc, begun by Lambton in the first years of the century,
continued by Everest, revised and extended by Walker. The
general upshot is to show that the polar compression of the
earth is somewhat greater than had been supposed. The
admitted fraction until lately was ^-^ ; that is to say, the
thickness of the protuberant equatorial ring was taken to be
^_ of the equatorial radius. But Sabine's pendulum experi-
ments, discussed by Airy in 1826, gave -$ ', 2 and arc measure-
ments tend more and more towards agreement with this figure.
A fresh investigation led the late J. B. Listing in i8y8 3 to
state the dimensions of the terrestrial spheroid as follows : equa-
torial radius = 6,377,377 metres; polar radius = 6,355,270
metres ; ellipticity = ^J-. j^ The fraction at present adopted
It is, however, far from certain that the figure of the earth
is one of strict geometrical regularity. Nay, it is by no means
clear that even its main outlines are best represented by what
is called an " ellipsoid of revolution " in other words, by a
globe flattened at top and bottom, but symmetrical on every
side. From a survey of geodetical results all over the world,
Colonel Clarke concludes that different meridians possess
different amounts of curvature ; 4 so that the equator, instead
of being a circle, as it should be apart from perturbing causes
1 Comptes Rendus, t. Ixxvi. p. 954. ' 2 Phil. Trans., vol. cxvi. p. 548.
3 Astr. Nach., No. 2228. * Phil. Mag., vol. vi. (5th ser.), p. 92.
3 io HISTORY OF ASTRONOMY. PART n.
in a rotating body, must, on this view, be itself an ellipse,
and our planet be correctly described as in shape " an ellip-
soid of three unequal axes." But the point is still sub judice.
Operations towards its decision are in active progress both in
Europe and India.
The moon possesses for us an unique interest. She in all
probability shared the origin of the earth ; she perhaps pre-
figures its decay. She is at present its minister and com-
panion. Her existence, so far as we can see, serves no other
purpose than to illuminate the darkness of terrestrial nights,
and to measure, by swiftly-recurring and conspicuous changes
of aspect, the long span of terrestrial time. Inquiries stimu-
lated by visible dependence, and aided by relatively close
vicinity, have resulted in a wonderfully minute acquaintance
with the features of the single lunar hemisphere open to our
inspection.
Selenography, in the modern sense, is not yet a hundred
years old. It originated with the publication in 1791 of
Schroter's Selenotopograpliische Fragmented Not but that the
lunar surface had already been diligently studied, chiefly by
Hevelius, Cassini, and Tobias Mayer ; the idea, however, of
investigating the moon's physical condition, and detecting
symptoms of the activity there of natural forces through
minute topographical inquiry, first obtained effect at Lilien-
thal. Schroter's delineations, accordingly, imperfect though
they were, afforded a starting-point for a comparative study of
the superficial features of our satellite.
The first of the curious objects which he named " rills "
was noted by him in 1787. Before 1801 he had found eleven ;
Lohrrnann added 75 ; Ma'dler 55 ; Schmidt published in 1866
a catalogue of 425, of which 278 had been detected by himself;^
and he eventually brought the number up to nearly 1000.
They are, then, a very persistent lunar feature, though wholly
without terrestrial analogue. There is no difference of opinion
1 The second volume was published at Gb'ttingen in 1802.
2 Ueber Rillen auf dem Monde, p. 13.
CHAP. vii. PLANETS AND SATELLITES. 311
as to their nature. They are quite obviously clefts in a rocky
surface, 100 to 500 yards deep (the depression of the great
rill near Aristarchus was estimated by Schmidt at 554 yards),
usually a couple of miles across, and pursuing straight, curved,
or branching tracks up to 150 miles in length. As regards
their origin, the most probable view is that they are fissures
produced in cooling; but Neison inclines to consider them
rather as dried watercourses. 1
On February 24, 1792, Schroter perceived what he took to
be distinct traces of a lunar twilight, and continued to observe
them during nine ensuing years. 2 They indicated, he thought,
the presence of a shallow atmosphere (not reaching a height
of more than 8400 feet), about -o^th as dense as our own.
Bessel, on the other hand, considered that the only way of
" saving " a lunar atmosphere was to deny it any refractive
power, the sharpness and suddenness of star-occultations
negativing the possibility of gaseous surroundings of greater
density (admitting an extreme supposition) than -g J^ that of
terrestrial air. 3 Newcomb places the maximum at ^Q. Sir
John Herschel concluded " the non-existence of any atmo-
sphere at the moon's edge having one-igSoth part of the
density of the earth's atmosphere." 4
This decision was fully borne out by Dr. Huggins's spectro-
scopic observation of the disappearance behind the moon's
limb of the small star t Piscium, January 4, iB>6^. 5 Not the
slightest sign of selective absorption, or unequal refraction
was discernible. The entire spectrum went out at once, as
if a slide had suddenly dropped over it. The spectroscope
has uniformly told the same tale ; for M. Thollon's observa-
tion during the total solar eclipse at Sohag of a supposed
thickening at the moon's rim, of certain dark lines in the
solar spectrum, is now acknowledged to have been illusory.
Moonlight, analysed with the prism, is found to be pure
reflected sunlight, diminished in quantity, owing to the low
1 The Moon, p. 73. z Selen.Fragm., Th. ii. p. 399. 3 Astr. Nach., No.
263 (1834) ; Pop. Vorl., pp. 615-620 (1838). ^ Outlines of Astr., par. 431.
5 Month. Not., vol. xxv. p. 61.
3 I2 HISTORY OF ASTRONOMY. PART n.
reflective capability of the lunar surface, to about one-sixth its
incident intensity, but wholly unmodified in quality.
Yet there is little or no doubt that the diameter of the moon,
as determined from occultations, is 4" smaller than it appears
by direct measurement. This fact, which emerged from Sir
George Airy's discussion, in I865, 1 of an extensive series of
Greenwich and Cambridge observations, would naturally result
from lunar atmospheric refraction. He showed, however, that
even if the entire effect were thus produced (a certain share
is claimed by irradiation) the atmosphere involved would be
2000 times thinner than our own air at the sea-level. A
gaseous stratum of such extreme tenuity could scarcely pro-
duce any spectroscopic effect. It is certain (as Mr. Neison
has shown 2 ) that a lunar atmosphere of very great extent
and of no inconsiderable mass, would possess, owing to the low
power of lunar gravity, a very small surface density, and might
thus escape direct observation while playing a very important
part in the economy of our satellite. Some renewed evidence
of actual crepuscular gleams on the moon has, besides, been
lately furnished to MM. Paul and Prosper Henry of the Paris
Observatory by their skilful use of a powerful telescope. 3
The first to emulate Schroter's selenographical zeal was
Wilhelm Gotthelf Lohrmann, a land-surveyor of Dresden, who,
in 1824, published four out of twenty-five sections of the first
scientifically executed lunar chart, on a scale of 37^- inches
to a lunar diameter. His sight, however, began to fail three
years later, and he died in 1 840, leaving materials from which
the work was completed and published in 1878 by Dr. Julius
Schmidt, late director of the Athens Observatory. Much had
been done in the interim. Beer and Madler began at Berlin
in 1830 their great trigonometrical survey of the lunar surface,
as yet neither revised nor superseded. A map, issued in four
parts, 1834-36, on nearly the same scale as Lohrmann's, but
more detailed and authoritative, embodied the results. It was
succeeded, in 1837, by a descriptive volume bearing the im-
1 Month. Not., vol. xxv. p. 264. 2 The Moon, p. 25.
Webb, Cd. Objects, p. 79.
CHAP. vii. PLANETS AND SATELLITES. 313
posing title, Der Mond ; oder allgemeine vcrgleicliende Seleno-
graphie. This summation of knowledge in that branch, though
in truth leaving many questions open, had an air of finality
which tended to discourage further inquiry. 1 It gave form to
a reaction against the sanguine views entertained by Hevelius,
Schroter, Herschel, and Gruithuisen as to the possibilities of
agreeable residence on the moon, and relegated the " Selenites,"
one of whose cities Schroter thought he had discovered, and
of whose festal processions Gruithuisen had not despaired of
becoming a spectator, to the shadowy land of the Ivory Gate.
All examples of change in lunar formations were, moreover,
dismissed as illusory. The light contained in the work was, in
short, a " dry light," not stimulating to the imagination. " A
mixture of a lie," Bacon shrewdly remarks, " doth ever add
pleasure." For many years, accordingly, Schmidt had the
field of selenography almost to himself.
Reviving interest in the subject was at once excited and dis-
played by the appointment, in 1864, of a Lunar Committee of
the British Association. The indirect were of greater value
than the direct fruits of its labours. An English school of
selenography rose into importance. Popularity was gained for
the subject by the diffusion of works conspicuous for ingenuity
and research. Messrs. Nasmyth's and Carpenter's beautifully
illustrated volume (1874) was succeeded, after two years, by a
still more weighty contribution to lunar science. Mr. Neison's
book was accompanied by a map, based on the survey of Beer
and Madler, but adding some 500 measures of positions, besides
the representation of several thousand new objects. With
Schmidt's Cliarte der Gebirge des Mondes, Germany once more
took the lead. This splendid delineation the result of thirty-
two years' labour was built upon Lohrmann's foundation, but
embraces the detail contained in upwards of 3000 original
drawings. No less than 32,856 craters are represented in it.
The scale is seventy-five inches to a diameter. An additional
help to lunar inquires was provided at the same time in this
1 Neison, The Moon, p. 104.
314 HISTORY OF ASTRONOMY. PART n.
country by the establishment, through the initiative of the
late Mr. W. R. Birt, of the Selenographical Society.
But the strongest incentive to diligence in studying the
rugged features of our celestial helpmate has been the idea of
probable or actual variation in them. A change always seems
to the inquisitive intellect of man like a breach in the defences
of Nature's secrets, through which it may hope to make its
way to the citadel. What is desirable easily becomes credible ;
and thus statements and rumours of lunar convulsions have
successively, during the last hundred years, obtained credence,
and successively, on closer investigation, been rejected. The
subject is one as to which illusion is peculiarly easy. Our
view of the moon's surface is a bird's-eye view. Its conforma-
tion reveals itself indirectly through irregularities in the dis-
tribution of light and darkness. The forms of its elevations
and depressions can be inferred only from the shapes of the
black, unmitigated shadows cast by them. But these shapes
are in a state of perpetual and bewildering fluctuation, partly
through changes in the angle of illumination, partly through
changes in our point of view, caused by what are called
the moon's " librations." l The result is, that no single obser-
vation can be exactly repeated by the same observer, since
identical conditions recur only after the lapse of a great num-
ber of years.
Local peculiarities of surface, besides, are liable to produce
perplexing effects. The reflection of earth-light at a particular
angle from certain bright summits completely, though tem-
porarily deceived Herschel into the belief that he had wit-
nessed, in 1783 and 1787, volcanic outbursts on the dark side
of the moon. The persistent recurrence, indeed, of similar
1 The combination of a uniform rotational, with an unequal orbital move-
ment causes a slight swaying of the moon's globe, now east, now west, by
which we are enabled to see round the edges of the averted hemisphere.
There is also a " parallactic " libration, depending on the earth's rotation ;
and a species of nodding movement the " libration in latitude " is pro-
duced by the inclination of the moon's axis to her orbit, and by her changes
of position with regard to the terrestrial equator. Altogether, about y r
of the invisible side come into view.
CHAP. vii. PLANETS AND SATELLITES. 315
appearances under circumstances less amenable to explanation
inclined Webb to the view that effusions of native light actually
occur. 1 More cogent proofs must, however, be adduced before
a fact so intrinsically improbable can be admitted as true.
But from the publication of Beer and Madler's work until
1866, the received opinion was that no genuine sign of activity
had ever been seen, or was likely to be seen, on our satellite ;
that her face was a stereotyped page, a fixed and irrevisable
record of the past. A profound sensation, accordingly, was
produced by Schmidt's announcement, in October 1866, that
the well-known crater " Linn6 " had disappeared, 2 effaced, as
it was supposed, by an igneous outflow. The case seemed
undeniable, and is still dubious. Linne" had been known to
Lohrmann and Madler, 1822-32, as a deep crater, five or six
miles in diameter, the third largest in the dusky plain known
as the " Mare Serenitatis ; " and Schmidt had observed and
drawn it, 1840-43, under a practically identical aspect. Now
it appears under high light as a whitish spot, in the centre of
which, as the rays begin to fall obliquely, a pit, probably under
two miles across, emerges into view. The crateral character
of this comparatively minute depression was detected by
Father Secchi, February u, 1867.
This, however, is not all. Schroter's description of Linne",
as seen by him November 5, 1788, tallies quite closely with
modern observation; 3 while its inconspicuousness iri 1797 is
shown by its omission from Russell's lunar globe and maps. 4
We are thus driven to adopt one of two suppositions : either
Lohrmann, Madler, and Schmidt were entirely mistaken in the
size and importance of Linne", or a real change in its outward
semblance supervened during the first half of this century, and
has since passed away, perhaps again to recur. The latter
hypothesis seems the more probable; and its probability is
strengthened by much evidence of actual obscuration or
variation of tint in other parts of the lunar surface, more
1 Cel. Objects, p. 58 (4th ed.) 2 Astr. Nach., No. 1631. 3 Respighi,
Les Mondes, t. xiv. p. 294 ; Huggins, Month. Not., vol. xxvii. p. 298.
4 Birt, Ibid., p. 95.
3 i6 HISTORY OF ASTRONOMY. PART n.
especially on the floor of the great " walled plain ?5 named
"Plato." 1
An instance of an opposite kind of change was alleged
by Dr. Hermann J. Klein of Cologne in March 187 8. 2 In
Linne, the obliteration of an old crater had been assumed ; in
" Hyginus N.," the formation of a new crater was asserted.
Yet, quite possibly, the same cause may have produced the
effects thought to be apparent in both. It is, however, far
from certain that any real change has affected the neighbour-
hood of Hyginus. The novelty of Klein's observation of May
19, 1877, may have consisted simply in the detection of a
hitherto unrecognised feature. The region is one of complex
formation, consequently of more than ordinary liability to
deceptive variations in aspect under rapid and entangled
fluctuations of light and shade. 3 Moreover, it seems to be
certain, from Messrs. Pratt and Capron's attentive study, that
''Hyginus N." is no true crater, but a shallow, saucer-like
depression, difficult of clear discernment. 4 Under suitable
illumination, nevertheless, it contains, and is marked by, an
ample shadow. 5
In both these controverted instances of change, lunar photo-
graphy was invoked as a witness ; but, notwithstanding the
great advances made in the art by Mr. De la Rue in this
country, by Dr. Henry Draper, and above all by Mr. Lewis
M. Rutherfurd, in America, without decisive results. Auto-
graphic records, it may be expected, will gain increasing
authority on such points in the future.
Melloni was the first to get undeniable heating effects from
moonlight. His experiments were made at Naples early in
i846, 6 and were repeated with like result by Zantedeschi at
Venice four years later. A rough measure of the intensity of
those effects was arrived at by Piazzi Smyth at Guajara, on the
1 Report Brit. Ass., 1872, p. 245. 2 Astr. Reg., vol. xvi. p. 265 ; Astr.
Nach., No. 2275. 3 See Lord Lindsay and Dr. Copeland \i\Month. Not.,
vol. xxxix. p. 195. 4 Observatory, vols. ii. p. 296 ; iv. p. 373. Mr. N.
E. Green (Astr. Reg., vol. xvii. p. 144) concludes the object a mere "spot
of colour," dark under oblique light. f Webb, Cel. Objects, p. 101.
6 Comptes Rendus, t. xxif. p. 541.
CHAP. vii. PLANETS AND SATELLITES.
317
Peak of Teneriffe, in 1856. At a distance of fifteen feet from
the thermomultiplier, a Price's candle was found to radiate
just twice as much heat as the full moon. 1 But by far the
most exact and extensive series of observations till then made
on the subject were those by the present Earl of Rosse, 1869-7 2.
The lunar radiations, from the first to the last quarter, dis-
played, when concentrated with the Parsonstown three-foot
mirror, appreciable thermal energy, increasing with the phase,
and largely due to " dark heat," distinguished from the quicker-
vibrating sort by inability to traverse a plate of glass. This was
supposed to indicate an actual heating of the surface, during the
long lunar day of 300 hours, to about 500 F., 2 the moon thus
acting as a direct radiator no less than as a reflector of heat.
But this conclusion was very imperfectly borne out by Dr.
Boeddicker's observations with the same instrument and
apparatus during the total lunar eclipse of October 4, i884. 3
This was the first opportunity of measuring the heat phases
of an eclipsed moon, and was used with the remarkable result
of showing that the heat disappeared almost completely, though
not quite simultaneously, with the light. Confirmatory evidence
of the extraordinary promptitude with which our satellite parts
with heat already to some extent appropriated, was afforded
by Professor Langley's bolometric observations at Allegheny
of the partial eclipse of September 23, i885. 4 Yet it is cer-
tain that the moon sends us a perceptible quantity of heat on
its own account, besides simply throwing back solar radiations.
In February 1885, Professor Langley succeeded, after many
fruitless attempts, in getting measures of a " lunar heat-
spectrum." The incredible delicacy of the operation may
be judged of from the statement that the sum- total of
the thermal energy dispersed by his rock-salt prisms was
insufficient to raise a thermometer fully exposed to it one-
thousandth of a degree centigrade ! The singular fact was,
however, elicited that this almost evanescent spectrum con-
tains rays of greater wave-length than any coming direct
1 Phil. Trans., vol. cxlviii. p. 502. 2 Proc. Roy. Soc., vol. xvii. p. 443.
3 Trans. R. Dublin Soc., vol. iii. p. 321. 4 Science, vol. vii. p. 9.
318 HISTORY OF ASTRONOMY. PART n.
from the sun is, in short, made up of two superposed spectra,
one due to reflection, the other, with a maximum far down in
the infra-red, to radiation. 1 The corresponding temperature
of the moon's surface Professor Langley considers to be about
that of freezing water. Yet, since his previous researches
had shown that, under the fiercest sunshine, mercury could
never liquefy on an airless moon, 2 even this moderate degree
of heat implies some kind of atmospheric clothing. In imme-
diate contact with the cold of space, our attendant luminary
would be a still more frigid body than it appears actually to be.
Although that fundamental part of astronomy known as
" celestial mechanics " lies outside the scope of this work, and
we must therefore pass over in silence the immense labours of
Plana, Damoiseau, Hansen, Delaunay, and Airy in reconciling
the observed and calculated motions of the moon, there is one
slight, but significant discrepancy which is of such importance
to the physical history of the solar system, that some brief
mention must be made of it.
Halley discovered in 1693, by examining the records of
ancient eclipses, that the moon was going faster then than
2000 years previously so much faster, as to have got ahead
of the place in the sky she would otherwise have occupied, by
about two of her own diameters. It was one of Laplace's
highest triumphs to have found an explanation of this puzzling
fact. He showed, in 1787, that it was due to a very slow
change in the ovalness of the earth's orbit, tending, during the
present age of the world, to render it more nearly circular.
The pull of the sun upon the moon is thereby lessened ; the
counter-pull of the earth gets the upper hand ; and our satellite,
drawn nearer to us by something less than an inch each year, 1
proportionally quickens her pace. Many thousands of years
hence the process will be reversed ; the terrestrial orbit will
close in at the sides, the lunar orbit will open out under the
growing stress of solar gravity, and our celestial chronometer
will lose instead of gaining time.
1 Nature, vol. xxxiii. p. 211. 2 Ibid., vol. xxvi. p. 316.
3 Airy, Observatory, vol. iii. p. 420.
CHAP. vii. PLANETS AND SATELLITES. 319
This is all quite true as Laplace put it ; but it is not enough.
Adams, the virtual discoverer of Neptune, found with surprise
in 1853 that the received account of the matter was "essen-
cially incomplete," and explained, when the requisite correction
was introduced, only half the observed acceleration. 1 What
was to be done with the remaining half 1 Here Delaunay,
the eminent French mathematical astronomer, unhappily
drowned at Cherbourg in 1872 by the capsizing of a pleasure-
boat, came to the rescue. 2
It is obvious to any one who considers the subject a little
attentively, that the tides must act to some extent as a friction-
brake upon the rotating earth. In other words, they must
bring about an almost infinitely slow lengthening of the day.
For the two masses of water piled up by lunar influence on
the hither and farther sides of our globe, strive, as it were, to
detach themselves from the unity of the terrestrial spheroid,
and to follow the movements of the moon. The moon, accord-
ingly, holds them against the whirling earth, which revolves
like a shaft in a fixed collar, slowly losing motion and gaining
heat, eventually dissipated through space. 3 This must go on
(so far as we can see) until the periods of the earth's rotation
and of the moon's revolution coincide. Nay, the process will
be continued should our oceans survive so long by the
feebler tide-raising power of the sun, ceasing only when day
and night cease to alternate, when one side of our planet is
plunged in perpetual darkness and the other seared by un-
changing light.
Here, then, we have the secret of the moon's turning always
the same face towards the earth. It is that in primeval times,
when the moon was liquid or plastic, an earth-raised tidal
wave rapidly and forcibly reduced her rotation to its present
exact agreement with her period of revolution. This was
1 Phil. Trans., vol. cxliii. p. 397 ; Proc. Roy. Soc., vol. vi. p. 321.
2 Comptes Rendus, t. Ixi. p. 1023. 3 Mr. G. H. Darwin calculates that
the heat generated by tidal friction in the course of lengthening the earth's
period of rotation from 23 to 24 hours, equalled 23 million times the
amount of its present annual loss by cooling. Nature, vol. xxxiv. p. 422.
320 HISTORY OF ASTRONOMY. PART n.
divined by Kant l nearly a century before the necessity for
such a mode of action presented itself to any other thinker.
In a weekly paper published at Konigsberg in 1754, the modern
doctrine of " tidal friction " was clearly outlined by him, both
as regards its effects actually in progress on the rotation of the
earth, and as regards its effects already consummated on the
rotation of the moon the whole forming a preliminary attempt
at what he called a " natural history " of the heavens. His
sagacious suggestion, 'however, remained entirely unnoticed
until revived it would seem independently by Julius Robert
Mayer in 1848 ; 2 while similar, and probably original conclu-
sions were reached by William Ferrel of Allensville, Kentucky,
in 1858.3
Delaunay was not then the inventor or discoverer of tidal
friction ; he merely displayed it as an effective cause of change.
He showed reason for believing that its action in checking the
earth's rotation, far from being, as Ferrel had supposed, com-
pletely neutralised by its contraction through cooling, was a
fact to be reckoned with in computing the movements, as well
as in speculating on the history of the heavenly bodies. The
outstanding acceleration of the moon was thus at once ex-
plained. It was explained as apparent only the reflection of
a real lengthening, by one second in 100,000 years, of the
day. But on this point the last word has not yet been
spoken.
Professor Newcomb undertook in 1870 the onerous task of
investigating the errors of Hansen's Lunar Tables as com-
pared with observations prior to 1750. The results, published
in 1878,* have proved somewhat perplexing. They tend, in
general, to reduce the amount of acceleration left unaccounted
for by Laplace's gravitational theory, and proportionately to
diminish the importance of the part played by tidal friction.
But, in order to bring about this diminution, and at the same
1 Sdmmtl. Werke (ed. 1839), Th. vi. pp. 5-12. See also Mr. C. J.
Monro's useful indications in Nature, vol. vii. p. 241. 2 Dynamik dcs
Jlimmds, p. 40. 3 Gould's Astr. Jour., vol. iii. p. 138. 4 Wash. Obs. for
1875, v l- xx
CHAP. viz. PLANETS AND SATELLITES. 321
time conciliate Alexandrian and Arabian observations, it is
necessary to reject as total the ancient solar eclipses known as
those of Thales and Larissa. This may be a necessary, but it
must be admitted to be a hazardous expedient.
It was further shown that small residual irregularities are
still found in the movements of our satellite, inexplicable either
by any known gravitational influence, or by any uniform value
that could be assigned to secular acceleration. 1 If set down
to the account of imperfections in the " time-keeping " of the
earth, it could only be on the arbitrary supposition of fluc-
tuations in its rate of going themselves needing explanation.
This, it is true, might be found, as Sir W. Thomson pointed
out in i876, 2 in very slight changes of figure, not altogether
unlikely to occur. But into this cloudy and speculative region
astronomers for the present decline to penetrate. They prefer,
if possible, to deal only with calculable causes, and thus to
preserve for their "most perfect of sciences " its special prero-
gative of assured prediction.
i Newcomb, Pop. Astr. (4th ed.), p. 101. 2 Report Brit. Ass., 1876,
p. 12.
( 322 )
CHAPTER VIII.
PLANETS AND SATELLITES (continued}.
" THE analogy between Mars and the earth is perhaps by far
the greatest in the whole solar system." So Herschel wrote
in I783, 1 and so it may safely be repeated to-day, after an ad-
ditional hundred years of scrutiny. This circumstance lends
a particular interest to inquiries into the physical habitudes of
our exterior planetary neighbour.
Fontana was the first to catch glimpses, at Naples in 1636
and i638, 2 of dusky stains on the ruddy disc of Mars. They
were next seen by Hooke and Cassini in 1666, and this time
with sufficient distinctness to serve as indexes to the planet's
rotation, determined by the latter as taking place in a period
of twenty-four hours forty minutes. 3 Increased confidence was
given to this result through Maraldi's precise verification of it in
lyig. 4 Among the spots observed by him, he distinguished
two as stable in position, though variable in size. They were
of a peculiar character, showing as bright patches round the
poles, and had already been noticed during sixty years back.
A current conjecture of their snowy nature obtained validity
when Herschel connected their fluctuations in extent with the
progress of the Martian seasons. It was hard to resist the
inference of frozen precipitations when once it was clearly per-
ceived that the shining polar zones did actually diminish alter-
nately and grow with the alternations of summer and winter
in the corresponding hemisphere.
This, it may be said, was the opening of our acquaintance
with the state of things prevailing on the surface of Mars. It
1 Phil. Trans., vol. Ixxiv. p. 260. 2 Novce Observations, p. 105.
3 Phil. Trans., vol. i. p. 243. 4 Mem. de I' Ac., 1720, p. 146.
CHAP. viii. PLANETS AND SATELLITES. 323
was accompanied by a steady assertion, on Herschel's part, of
permanence in the dark markings, notwithstanding partial
obscurations by clouds and vapours floating in a "considerable
but moderate atmosphere." Hence the presumed inhabitants
of the planet were inferred to " probably en joy a situation in
many respects similar to ours." 1
Schroter, on the other hand, went altogether wide of the
truth as regards Mars. He held that the surface visible to us
is a mere shell of drifting cloud, deriving a certain amount of
apparent stability from the influence on evaporation and con-
densation of subjacent but unseen areographical feaures ; 2
and his opinion prevailed with his contemporaries. It was,
however, rejected by Kunowsky in 1822, and finally overthrown
by Beer and Madler's careful studies during five consecutive
oppositions, 1830-39. They identified at each the same dark
spots, frequently blurred with mists, especially when the local
winter prevailed, but fundamentally unchanged. 3 In 1862
Mr. Lockyer established a " marvellous agreement " with Beer
and Madler's results of 1830, leaving no doubt as to the com-
plete fixity of the main features, amid "daily, nay, hourly,"
variations of detail through transits of clouds. 4 On seventeen
nights of the same opposition, F. Kaiser of Leyden obtained
drawings in which nearly all the markings noted in 1830 at
Berlin reappeared, besides spots frequently seen respectively
by Arago in 1813, by Herschel in 1783, and one sketched by
Huygens in 1 6 7 2 with a writing-pen in his diary. 5 From these
data the Leyden observer arrived at a period of rotation of
24h. 37m. 22.625., being just one second shorter than that
deduced, exclusively from their own observations, by Beer
and Madler. The exactness of this result has been practically
confirmed by the late inquiries of Professor Bakhuyzen of
Leyden. 6 Using for a middle term of comparison the dis-
1 Phil. Trans., vol. Ixxiv. p. 273. 2 A large work, entitled Areograph-
ische Fragmente, in which Schroter embodied the results of his labours on
Mars, 1785-1803, narrowly escaped the conflagration of 1813, and was
published at Leyden in 1881. 3 Beitrdge, p. 124. 4 Mem. R. A. Soc.,
vol. xxxii. p. 183. 5 Astr. Nach., No. 1468. 6 Observatory, vol. viii.
P- 437-
324 HISTORY OF ASTRONOMY. PART n.
interred observations of Schroter, with those of Huygens at
one and of Schiaparelli at the other end of an interval of 220
years, he was enabled to show, with something like certainty,
that the time of rotation (24!!. 37m. 22.7353.) ascribed to
Mars by Mr. Proctor l in reliance on a drawing executed by
Hooke in 1666, was too long by nearly one-tenth of a second.
The minuteness of the correction indicates the nicety of care
employed. Nor employed vainly; for, owing to the com-
parative antiquity of the records available in this case, an
almost infinitesimal error becomes so multiplied by frequent
repetition as to produce palpable discrepancies in the positions
of the markings at distant dates. Hence Bakhuyzen's period
of 24h. 37m. 22.66s. is undoubtedly of a precision unap-
proached as regards any other heavenly body save the earth
itself.
Two facts bearing on the state of things at the surface of
Mars were, then, fully acquired to science in or before the year
1862. The first was that of the seasonal fluctuations of the
polar spots; the second, that of the permanence of certain
dark grey or greenish patches, perceived with the telescope as
standing out from the deep yellow ground of the disc. The
opinion has steadily gained consistency during the last half-
century that these varieties of tint correspond to the real
diversities of a terraqueous globe, the " ripe cornfield " 2 sections
representing land, the dusky spots and streaks, oceans and
straits. Sir J. Herschel in 1830 led the way in ascribing the
redness of the planet's light to an inherent peculiarity of soil. 3
Previously it had been assimilated to our sunset glows rather
than to our red sandstone formations set down, that is, to an
atmospheric stoppage of blue rays. But the extensive Martian
atmosphere, implicitly believed in on the strength of some
erroneous observations by Cassini and Romer in the seven-
teenth century, vanished before the sharp occultation of a
small star in Leo, witnessed by Sir James South in 1822 j 1
1 Month. Not., veils, xxviii. p. 37 ; xxix. p. 232 ; xxxiii. p. 552.
2 Flammarion, L 'Astronomic, t. i. p. 266. 3 Smyth, Cel. Cycle, vol. i.
p. 148 (ist ed.). 4 Phil. Trans., vol. cxxi. p. 417.
CHAP. viii. PLANETS AND SATELLITES. 325
and Dawes's observation in I865, 1 that the ruddy tinge is
deepest near the central parts of the disc, certified its non-
atmospheric origin. The absolute whiteness of the polar
snow-caps was alleged in support of the same inference by
Dr. Huggins in i867. 2
All recent observations tend to show that the atmosphere of
Mars is much thinner than our own. This was to have been
expected ct priori, since the same proportionate mass of air
would, owing to the small size and inferior specific gravity of
Mars, as compared with the earth, form a very much sparser
covering over each square mile of his surface. 3 Besides,
gravity there possesses much less than half its force here, so
that this sparser covering would weigh less, and be less con-
densed, than if it enveloped the earth. Atmospheric pressure
would accordingly be of about two and a quarter, instead of
fifteen terrestrial pounds per square inch. This corresponds
with what the telescope shows us. It is extremely doubtful
whether any features of the earth's actual surface could be
distinguished by a planetary spectator, however well provided
with optical assistance. Professor Langley's inquiries 4 have
led him to conclude that fully twice as much light is absorbed
by our air as had previously been supposed say forty per
cent, of vertical rays in a clear sky. Of the sixty reaching
the earth, less than a quarter would be reflected even from
white sandstone ; and this quarter would again pay heavy toll
in escaping back to space. Thus not more than perhaps ten
or twelve out of the original hundred sent by the sun would,
under the most favourable circumstances, and from the very
centre of the earth's disc, reach the eye of a Martian or lunar
observer. The light by which he views our world is, there
is little doubt, light reflected from the various strata of our
atmosphere, cloud- or mist-laden or serene, as the case may
be, with an occasional snow-mountain figuring as a permanent
white spot.
1 Month. Not., vol. xxv. p. 227. 2 Phil. Mag., vol. xxxiv. p. 75.
2 Proctor, Quart. Jour, of Science, vol. x. p. 185 ; Maunder, Sunday
Mag., Jan., Feb., March, 1882. 4 Am. Jour, of Sc. } vol. xxviii. p. 163.
326 HISTORY OF ASTRONOMY. PART n.
This consideration at once shows us how much more tenu-
ous the Martian air must be, since it admits of topographical
delineations of the Martian globe. The clouds, too, that form
in it seem to be rather of the nature of ground- mists than of
heavy cumulus. 1 There is, indeed, plenty of aqueous vapour
present. A characteristic group of dark rays, due to its
absorptive action, was detected by Dr. Huggins in the analysed
light of the planet in 1867, 2 and serves to raise the conjecture
of " snowy poles " to a verisimilitude scarcely to be distin-
guished from certainty.
The climate of Mars seems to be unexpectedly mild. Its
theoretical mean temperature, taking into account both distance
from the sun and albedo, is thirty-four centigrade degrees be-
low freezing. 3 Yet its polar snows are both less extensive and
less permanent than those on the earth. The southern white
hood, always eccentrically situated, was noticed by Schiaparelli
in 1877 to have survived the summer only as a small lateral
patch, the pole itself being quite free from snow. But we
might expect to see the whole wintry hemisphere, at any rate,
frostbound, since the sun radiates less than half as much heat
on Mars as on [the earth. Water seems, nevertheless, to
remain, as a rule, uncongealed everywhere outside the polar
regions. We are at a loss to imagine by what beneficent
arrangement the rigorous conditions naturally to be looked for,
can be modified into a climate which might be found tolerable
by creatures constituted like ourselves.
Martian topography may be said to form now-a-days a
separate sub -department of descriptive astronomy. The
amount of detail become legible by close scrutiny on a little
disc which, once in fifteen years, attains a maximum of about
soVo^ the area of the full moon, must excite surprise and
might provoke incredulity. Spurious discoveries, however,
have little chance of holding their own where there are so
many competitors quite as ready to dispute as to confirm.
The first really good map of Mars was constructed in 1869 by
1 Burton, Trans. Roy. Dublin Soc., vol. i. 1880, p. 169. 2 Month.
Kot. t vol. xxvii. p. 179. 3 C. Christiansen, Beiblatter, 1886, p. 532.
CHAP. viii. PLANETS AND SATELLITES. 327
Mr. Proctor from drawings by Dawes. Kaiser of Leyclen fol-
lowed in 1872 with a representation founded upon data of his
own providing in 1862-64; and M. Terby, in his valuable
Areographie, presented to the Brussels Academy in I874 1 a
careful discussion of all important observations from the time
of Fontana downward, thus virtually adding to knowledge by
summarising and digesting it. The memorable opposition of
September 5, 1877, marked a fresh epoch in the study of Mars.
While executing a trigonometrical survey (the first attempted)
of the disc, then of the unusual size of 25" across, Signer G.
V. Schiaparelli, director of the Milan Observatory, detected a
novel and curious feature. What had been taken for Martian
continents were found to be, in point of fact, agglomerations
of islands, separated from each other by a net-work of so-called
"canals." These are obviously extensions of the "seas,"
originating and terminating in them, and sharing their grey-
green hue, but running sometimes to a length of three or four
thousand miles in a straight line, and preserving throughout a
nearly uniform breadth of about sixty miles. Further inquiries
have fully substantiated the discovery made at the Brera
Observatory. The " canals " of Mars are an actually existent
and permanent phenomenon. An examination of the draw-
ings in his possession showed M. Terby that they had been
seen, though not distinctively recognised, by Dawes, Secchi,
and Holden ; several were independently traced out by Burton
at the opposition of 1879; and all were recovered by Schia-
parelli himself in 1879 and 1881-82.
When the planet culminated at midnight, and was therefore
in opposition, December 26, 1881, its distance was greater,
and its apparent diameter less than in 1877, in the proportion
of sixteen to twenty -five. Its atmosphere was, however, more
transparent, and ours of less impediment to northern observers,
the object of scrutiny standing considerably higher in northern
skies. Never before, at any rate, had the true aspect of Mars
come out so clearly as at Milan, with the 8f -inch Merz refractor
of the observatory, between December i88[ and February
1 Memoires Couronnes, t. xxxix.
328 HISTORY OF ASTRONOMY. PART n.
1882. The canals were all again there, but this time they were
in as many as twenty cases seen in duplicate. That is to
say, a twin-canal ran parallel to the original one at an interval
of 200 to 400 miles. 1
We are here brought face to face with an apparently in-
soluble enigma. Schiaparelli regards the "gemination" of
liis canals as a periodical phenomenon depending on the
Martian seasons. It is, at any rate, not an illusory one, since
it was plainly apparent, during the opposition of 1886, to
MM. Perrotin and Thollon using the 15 -inch Nice equa-
toreal ; 2 but it is too soon to form an opinion as to its cause.
Fresh evidence will, it is to be hoped, become available during
the next favourable opposition in 1892.
Meanwhile, the closeness of the terrestrial analogy remains
somewhat impaired. The distribution of land and water on
Mars, at any rate, appears to be of a completely original type.
The interlacing everywhere of continents with arms of the sea
(if that be the correct interpretation of the visual effects)
implies that their levels scarcely differ ; 3 and it is held by
Schiaparelli, Bakhuyzen, and others that their outlines are not
absolutely constant, encroachments of dusky upon bright tints
suggesting the possibility of extensive inundations. Mr. N. E.
Green's noteworthy observations at Madeira in 1877 seem to
indicate, on the other hand, a rugged south polar region. The
contour of the snow-cap not only appeared indented, as if by
valleys and promontories, but brilliant points were discerned
outside the white area, attributed to isolated snow-peaks. 4
Still more elevated, if similarly explained, must be the " ice
island " first seen in a comparatively low latitude by Dawes in
January 1865.
Mars was gratuitously supplied with a pair of satellites long
before he was found actually to possess them. Kepler inter-
preted Galileo's anagram of the " triple " Saturn in this sense ;
they were perceived by Microme"gas on his long voyage through
space ; and the Laputan astronomers had even arrived at a
i Mem. Spettr. I(aliani,t. xi. p. 28. 2 Bull. A sir., t. iii. p. 324. 3 Flam-
mariou, L 1 Astronomic, t. i. p. 206. 4 Month. Not., vol. xxxviii. p. 41.
CHAP. viir... PLANETS AND SATELLITES. 329
knowledge, curiously accurate under the circumstances, of
their distances and periods. But terrestrial observers could
see nothing of them until the night of August u, 1877. The
planet was then within one month of its second nearest
approach to the earth during this century; and in 1845 the
Washington 26-inch refractor was not in existence. 1 Professor
Asaph Hall, accordingly, determined to turn the conjuncture
to account for an exhaustive inquiry into the surroundings of
Mars. Keeping his glaring disc just outside the field of view,
a minute attendant speck of light was " glimpsed " August n.
Bad weather, however, intervened, and it was not until the
1 6th that it was ascertained to be what it appeared a satellite.
On the following evening a second, still nearer to the primary,
was discovered, which, by the bewildering rapidity of its pas-
sages hither and thither, produced at first the effect of quite
a crowd of little moons. 2
Both these delicate objects have since been repeatedly
observed, both in Europe and America, even with compara-
tively small instruments. But at each opposition since that of
1877 the distance of the planet has been increasing, and in
1884 was too great to permit of the detection of both elsewhere
than at Washington. It is unlikely that they will be again
seen before 1888 or 1890.'
The names chosen for them were taken from the Iliad, where
"Deimos" and "Phobos" (Fear and Panic) are represented
as the companions in battle of Ares. In several respects, they
are interesting and remarkable bodies. As to size, they may
be said to stand midway between meteorites and satellites.
From careful photometric measures executed at Harvard in
1877 and 1879, Professor Pickering concluded their diame-
ters to be respectively six and seven miles. 3 This is on
the assumption that they reflect the same proportion of the
light incident upon them that their primary does. But it
may very well be that they are less reflective, in which case
1 See Mr. Wentworth Erck's remarks in Trans. Roy. Dublin Soc,, vol.
i. p. 29. 2 Month. Not., vol. xxxviii. p.. 206. 3 Annals Harvard CoU.
Obs., vol. xi. pt. ii. p. 317.
330 HISTORY OF ASTRONOMY. PART n.
they would be more extensive. The albedo of Mars, accord-
ing to Zollner, is 0.2762 ; his surface, in other words, returns
27.62 per cent, of the rays striking it. If we put the albedo of
his satellites equal to that of our moon, 0.1195, their diameters
will be increased from 6 and 7 to 14 and 16 miles, Phobos,
the inner one, being the larger. Their actual dimensions can
scarcely exceed this estimate. It is interesting to note that
Deimos, according to Professor Pickering's very distinct per-
ception, does not share the reddish tint of Mars.
Both satellites move quickly in small orbits. Deirnos com-
pletes a revolution in thirty hours eighteen minutes, at a dis-
tance from the surface of its ruling body of 12,500 miles;
Phobos in seven hours thirty-nine minutes twenty-two seconds,
at a distance of only 3760 miles. This is the only known
instance of a satellite circulating faster than its primary rotates,
and is a circumstance of some importance as regards theories
of planetary development. To a Martian spectator the curious
effect would ensue of a celestial object, seemingly exempt from
the general motion of the sphere, rising in the west, setting
in the east, and culminating twice, or even thrice a day; which,
moreover, in latitudes above 69 north or south, would be
permanently and altogether hidden by the intervening curva-
ture of the globe.
The detection of new members of the solar system has come
to be one of the most ordinary of astronomical events. Since
1846 no single year has passed without bringing its tribute of
asteroidal discovery. In the last of the seventies alone, a full
score of miniature planets were distinguished from the throng-
ing stars amid which they seem to move; 1875 brought seven-
teen such recognitions ; their number touched a minimum of
one in 1881 ; it rose in 1882 to eleven, dropped to four in
1883, remounted as far as nine both in 1884 and 1885, and
again reached eleven in 1886. On January i, 1887, 264
asteroids were recognised as revolving between the orbits
of Mars and Jupiter. Of these, no less than fifty-seven are
claimed by a single observer Professor J. Palisa of Vienna ;
CHAP. vin. PLANETS AND SATELLITES. 331
Dr. C. H. F. Peters of Clinton, N.Y., comes in a good second
with forty-six ; Watson, Borrelly, Luther, Hind, Goldschmidt,
Tempel, Paul and Prosper Henry, and many others, have each
contributed numerously to swell the sum-total. The construc-
tion by Chacornac and his successors at Paris, and more
recently by Peters at Clinton, of ecliptical charts showing all
stars down to the thirteenth and fourteenth magnitudes re-
spectively, renders the picking out of moving objects above
that brightness a mere question of time and diligence. Far
more onerous is the task of keeping them in view once dis-
covered of tracking out their paths, fixing their places, and
calculating the disturbing effects upon them of the mighty
Jovian mass. These complex operations have come to be
centralised at Berlin under the superintendence of Professor
Tietjen, and their results are given to the public through the
medium of the Berliner Astronomisches Jahrbuch.
The crowd of orbits thus disclosed invites attentive study.
D' Arrest remarked in 185 1, 1 when only thirteen minor planets
were known, that supposing their paths to be represented by
solid hoops, not one of the thirteen could be lifted from its
place without bringing the others with it. The complexity of
interwoven tracks thus illustrated has grown almost in the
numerical proportion of discovery. Yet no tw^o actually in-
tersect, because no two lie exactly in the same plane, so that
the chances of collision are at present nil. There is only one
case, indeed, in which it seems to be eventually possible.
M. Lespiault has pointed out that the curves traversed by
"Fides" and "Maia" approach so closely that a time may
arrive when the bodies in question will either coalesce or
unite to form a binary system. 2
The maze threaded by the 2 64 asteroids contrasts singularly
with the harmoniously ordered and rhythmically separated
orbits of the larger planets. Yet the seeming confusion is not
without a plan. The established rules of our system are far
from being totally diregarded by its minor members. The
1 Astr. Nach., No. 752. 2 L. Niesten. Annuaire, Bruxelles, 1881, p.
269.
33 2 HISTORY OF ASTRONOMY. PART 11.
orbit of Pallas, with its inclination of 34 42', touches the limit
of departure from the ecliptic level ; the average plane of the
asteroidal paths differs by only about one degree from that of
the sun's equator ; their mean eccentricity is below that of the
curve traced out by Mercury, and all without exception are
pursued in the planetary direction from west to east.
The zone in which these small bodies travel is about three
times as wide as the interval separating the earth from the
sun. It extends perilously near to Jupiter, and actually en-
croaches upon the sphere of Mars. In one of his lectures at
Gresham College in 1879,* Mr. Ledger remarked that the
minor planet Aethra, when in perihelion, gets inside Mars in
aphelion by as much as five millions of miles, though at so
different a level in space that there is no close approach.
The distribution of the asteroids over the zone frequented
by them is very unequal They are most densely congregated
about the place where a single planet ought, by Bode's Law,
to revolve ; it may indeed be said that only stragglers from
the main body are found more than fifty million miles within
or without a mean distance from the sun 2.8 times that of the
earth. Significant gaps, too, occur where some force prohibi-
tive of their presence would seem to be at work. What the
nature of that force may be, Professor Daniel Kirkwood of the
Indiana University indicated, first in 1866 when the number
of known asteroids was only eighty-eight, and again with more
confidence in 1876 from the study of a list then run up to 172.2
It appears that these bare spaces are found just where a re-
volving body would have a period connected by a simple
relation with that of Jupiter. It would perform two or three
circuits to his one, five to his two, nine to his five, and so on.
Kirkwood's inference is that the gaps in question were cleared
of asteroids by the attractive influence of Jupiter. For dis-
turbances recurring time after time owing to commensura-
bility of periods nearly at the same part of the orbit, would
have accumulated until the shape of that orbit was notably
changed. The body thus displaced would have come in
1 Sun and Planets, p. 267. 2 Smiths. Report, 1876, p. 358.
CHAP. viii. PLANETS AND SATELLITES. 333
contact with other cosmical particles of the same family with
itself then, it may be assumed, more evenly distributed than
now would have coalesced with them, and permanently left
its original track. In this way the regions of maximum
perturbation would gradually have become denuded of their
occupants.
We can scarcely doubt that this law of commensurability
has largely influenced the present distribution of the asteroids.
The correspondence of the facts with the hypothesis is in
general striking. At the same time it is not perfect. The
minor planet Menippe, for example, revolves almost exactly
five times while Jupiter revolves once; and (as Professor
Newcomb has pointed out 1 ) several of its companions have
periods nearly three-eighths that of the disturbing planet.
The clue offered by Professor Kirkwood is not therefore to be
rejected ; but further inquiry, here as elsewhere, is needed.
Leverrier fixed, in 1 853,2 one-fourth of the earth's mass as
the outside limit for the combined masses of all the bodies
circulating between Mars and Jupiter; but it is far from
probable that this maximum is at all nearly approached. M.
Niesten estimated that the whole of the 216 asteroids dis-
covered up to August 1880 amounted in volume to only j^Vo"
of our globe, 3 and we may safely add since they are tolerably
certain to be lighter, bulk for bulk, than the earth that their
proportionate mass is smaller still. Professor Pickering, from
determinations of light intensity, assigns to Yesta a diameter
of 319 miles, to Pallas 167, to Juno 94, down to twelve and
fourteen for the smaller members of the group. 4 An albedo
equal to that of Mars is assumed as the basis of the calculation.
Still more recently, Professor G. Miiller 5 of Potsdam has
examined photometrically the phases of seven minor planets,
of which four namely, Yesta, Iris, Massalia, and Amphitrite
were found to conform precisely to the behaviour of Mars as
regards light-change from position, while Ceres, Pallas, and
i Pop. Astr., p. 338 (2d ed.) 2 Comptes Rendus, t. xxxvii. p. 797.
3 Annuaire, Bruxelles, 1881, p. 243. 4 Harvard Annals, voL xi. part
ii. p. 294. 5 Astr. Nach., Nos. 2724-5.
334 HISTORY OF ASTRONOMY. PART 11.
Irene varied after the manner of the moon and Mercury. The
first group were hence inferred to resemble Mars in physical
constitution, nature of atmosphere, and reflective capacity;
the second to be moon-like bodies. The low albedo of these
last rendering them less conspicuous proportionately to their
extent of surface than the others, the values of their diameters
arrived at by Pickering probably require some increase. Yesta,
however (of which the Harvard estimate of size has been still
further assured at Potsdam), still retains its pre-eminence as
the largest minor planet. The rapid and regular changes of
its brightness, observed by Professor M. W. Harrington 1 at
Ann Arbor, and supposed by him to be those of a swiftly
rotating and unequally reflective globe, are unconfirmed and
disbelieved in by Miiller.
There is no direct evidence that any of the minor planets
possess atmospheres. The aureolse seen by Schroter to sur-
round Ceres and Pallas have been dissipated by optical
improvements. Yogel in 1872 thought he had detected an
air-line in the spectrum of Yesta; 2 but admitted that its
presence required confirmation, which has not been forth-
coming.
Crossing the zone of asteroids on our journey outward from
the sun, we meet with a group of bodies widely different from
the " inferior " or terrestrial planets. Their gigantic size, low
specific gravity, and rapid rotation, obviously from the first
threw the " superior " planets into a class apart ; and modern
research has added qualities still more significant of a dis-
similar physical constitution. Jupiter, a huge globe 86,000
miles in diameter, stands pre-eminent among them. He is,
however, only primus inter pares ; all the wider inferences
regarding his condition may be extended, with little risk of
error, to his fellows ; and inferences in his case rest on surer
grounds than in the case of the others, from the advantages
offered for telescopic scrutiny by his comparative nearness.
1 Am. Jour, nf Sc., vol. xxvi. (3d ser.), p. 464. 2 Spectra der Plane-
ten, p. 24.
CHAP. viii. PLANETS AND SATELLITES. 335
' Now the characteristic modern discovery concerning Jupiter
is that he is a body midway between the solar and terrestrial
stages of cosmical existence a decaying sun or a developing
earth, as we choose to put it whose vast unexpended stores
of internal heat are mainly, if not solely, efficient in producing
the interior agitations betrayed by the changing features of his
visible disc. This view was anticipated in the last century.
Buff on wrote in his jSpoquts de la Nature (1778) : J "La
surface de Jupiter est, comme Ton sait, sujette a des change-
mens sensibles, qui semblent indiquer que cette grosse planete
est encore dans un etat d'inconstance et de bouillonnement."
Primitive incandescence, attendant, in his fantastic view, on
planetary origin by cometary impacts with the sun, combined,
he concluded, with vast bulk to bring about this result. Jupiter
had not yet had time to cool. Kant thought similarly in 1 785 ; 2
but the idea did not commend itself to the astronomers of the
time, and dropped out of sight until Mr. Nasmyth arrived at it
afresh in 1 85 3 3 Even still, however, terrestrial analogies held
their ground. The dark belts running parallel to the equator,
first seen at Naples in 1630, continued to be associated as
Herschel had associated them in 1781 with Jovian trade-
winds,' in raising which the deficient power of the sun was
supposed to be compensated by added swiftness of rotation.
But opinion was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some
remarkable indications from experiments on the light of
Jupiter. 4 They showed that fourteen times more of the photo-
graphic rays striking it are reflected by the planet than by our
moon, and that, unlike the moon, which sends its densest rays
from the margin, Jupiter is brightest near the centre. But the
most perplexing part of his results was that Jupiter actually
seemed to give out more light than he received. The question
of original luminosity thus definitely raised can assuredly not
be answered with an unqualified negative. Bond, however,
considered his data too uncertain for the support of so bold an
1 Tom. i. p. 93. 2 Berlinische Monatsschrift, 1785, p. 211.
8 Month. Not., vol. xiii. p. 40. 4 Mem. Am. Ac., vol. viii. p. 221.
336 HISTORY OF ASTRONOMY. PART n.
assumption, and, even if the presence of native light were
proved, thought that it might emanate from auroral clouds of
the terrestrial kind. The conception of a sun-like planet was
still a remote, and seemed an extravagant one.
Only since it was adopted and enforced by Zollner in I865, 1
can it be regarded as permanently acquired to science. The
rapid changes in the cloud -belts both of Jupiter and Saturn,
he remarked, attest a high internal temperature. For we
know that all atmospheric movements on the earth are sun-
heat transformed into motion. But sun-heat at the distance
of Jupiter possesses but ^ , at that of Saturn yj-g- of its force
here. The large amount of energy, then, obviously exerted
in those remote firmaments must have some other source, to
be found nowhere else than in their own active and all-pervad-
ing fires, not yet banked in with a thick solid crust.
The same acute investigator dwelt, in 1871,2 on the simi-
larity between the modes of rotation of the great planets and
of the sun, applying the same principles of explanation to each
case. The fact of this similarity is undoubted. Cassini 3 and
Schroter. both noticed that markings on Jupiter travelled
quicker the nearer they were to his equator ; and Cassini even
hinted at their possible assimilation to sun-spots. 4 It is now
well ascertained that, as a rule (not without exceptions),
equatorial spots give a period some 5^ minutes shorter than
those in latitudes of about 30. But, as Mr. Denning has
pointed out, 5 no single period will satisfy the observations
either of different markings at the same epoch, or of the same
markings at different epochs. Accelerations and retardations,
depending upon processes of growth or change, take place in
very much the same kind of way as in solar maculae, inevitably
suggesting similarity of origin.
Amongst popular writers, Mr. Proctor has been foremost in
realising the highly primitive condition of these giant orbs, and
in impressing the facts and their logical consequences upon
the public mind. The inertia of ideas on the subject has been
1 Photom. Vnters., p. 303. 2 A sir. Nach., No. 1851. 3 Mem. del' Ac.,
t. x. p. 514. 4 Ibid., 1692. p. 7. 5 Month. Not., vol. xliv. p. 63.
CHAP. viii. PLANETS AND SATELLITES. 337
overcome largely through the arguments reiterated in the
various and well-known works published by him since 1870.
It should be added that Mr. Mattieu Williams in his Fuel of
the Sun adopted, equally early, similar views.
The interesting query as to Jupiter's surface-incandescence
has been studied since Bond's time with the aid of all the
appliances furnished to physical inquirers by modern inven-
tiveness, yet without bringing to it a categorical reply. Zollner
in 1865 estimated his albedo at 0.62, that of fresh-fallen snow
being 0.78, and of white paper 0.70. 1 But the disc of Jupiter
is by no means purely white. The general ground is tinged
with ochre, the polar zones are leaden or fawn-coloured, large
spaces are at times stained or diffused with chocolate-browns
and rosy hues. It is occasionally seen ruled from pole to pole
with dusky bars, and is never wholly free from obscure mark-
ings. The reflection then by it, as a whole, of 62 per cent, of
the rays impinging upon it, might well suggest some original
reinforcement.
Nevertheless, the spectroscope gives little countenance to
the supposition of any considerable permanent light-emission.
The spectrum of Jupiter, as examined by Huggins, 1862-64,
and by Yogel, 1871-73, shows the familiar Fraunhofer rays
belonging to reflected sunlight. But it also shows lines of
native absorption. Some of these are identical with those
produced by the action of our own atmosphere, especially one
or more groups due to aqueous vapour ; others are of unknown
origin, and it is remarkable that one amongst the latter a
strong band in the red agrees in position with a dark line in
the spectra of some ruddy stars. 2 There is, besides, a general
absorption of blue rays, intensified as Le Sueur observed at
Melbourne in 1869 3 in the dusky markings, evidently through
an increase of depth in the atmospheric strata traversed by
the light proceeding from them.
All these observations, however (setting aside the stellar
line as of doubtful significance), point to a cool planetary
1 Fhotom. Unters., pp. 165, 273. 2 Vogel, Sp. d. Planeten, p. 33, note.
3 Proc. Roy. Soc., vol. xviii. p. 250.
Y
338 HISTORY OF ASTRONOMY. PART n.
atmosphere. There is, we believe, only one on record evincing
unmistakably the presence of intrinsic light. On September
27, 1879, Dr. Henry Draper obtained a photograph of Jupiter's
spectrum, in which a strengthening of the impression was
visible in the parts corresponding to the planet's equatorial
regions. 1 This is just the right sort of evidence, but it is
altogether exceptional. We are driven then to conclude that
native emissions from Jupiter's visible surface are local and
fitful, not permanent and general. Indeed, the total dis-
appearance of his satellites on entering his shadow-cone,
sufficiently proves that they receive from him no sensible
illumination. This conclusion, however, by no means invali-
dates that of his high internal temperature.
The curious phenomena attending Jovian satellite-transits
may be explained, partly as effects of contrast, partly as due
to temporary obscurations of the small discs projected on the
large disc of Jupiter. At their first entry upon its marginal
parts, which are two or three times less luminous than those
near the centre, they usually show as bright spots, then vanish,
and re-emerge dusky against the more lustrous background
met in their gradual advance. But they sometimes appear
bright throughout ; while, on the other hand, instances are not
rare, more especially of the third and fourth satellites stand-
ing out in such inky darkness as to be mistaken for their
own shadows. The earliest witness of a " black transit " was
Cassini, September 2, 1665; Homer in 1677, and Maraldi in
1707 and 1713, made similar observations, which have been
multiplied during the present century. In some cases, the
process of darkening has been visibly attended by the forma-
tion, or emergence into view, of spots on the transiting body, as
noted by the two Bonds at Harvard, March 18, 1848.2 The
third satellite was seen by Dawes, half dark, half bright, when
crossing Jupiter's disc, August 21, i867; 3 one-third dark by
Davidson of California, January 15, 1884, under the same
1 Month. Not., vol. xl. p. 433. 2 Engelmann, TJeber die HettigTceitsver-
hdltnisse der Jupiterstrabanten, p. 59, 3 Month. Not., voL xxviii.
p. II.
CHAP. vin. PLANETS AND SATELLITES. 339
circumstances; 1 and unmistakably spotted, both on and off
the planet, by Schroter, Secchi, Dawes, and Lassell.
The different effects produced by Jupiter's satellites in
transit result then intelligibly from the marked variability of
their light ; 2 and their variability seems, in some degree, to
depend upon their orbital positions. This amounts to saying
that, as Herschel concluded in 1797, they always, like our
moon, turn the same face towards their primary, thus always
presenting to us when in the same relative situations, the same
obscure or brilliant sections of their globes. As regards the
outer satellite, Engelmann's researches in 1871, and the late
C. E. Burton's in 1873, make this almost certain ; and there
is a strong probability that it also applies to the other three.
The phenomena, however, are quite too irregular to be com-
pletely rationalised on so simple and obvious a principle. We
must also admit changes in the power of reflecting light of the
satellites themselves, which Yogel's detection of lines in their
spectra or traces of such indicative of gaseous envelopes
similar to that of Jupiter, entitle us to regard as possibly of
atmospheric production.
In the course of his observations on Jupiter at Brussels in
1878, M. Niesten was struck with a rosy cloud attached to a
whitish zone beneath the dark southern equatorial band. 3 Its
size was enormous. At the distance of Jupiter, its measured
dimensions of 13" by 3" implied a real extension in longitude
of 30,000, in latitude of something short of 7000 miles. The
earliest record of its appearance seems to be by Professor
Pritchett, director of the Morrison Observatory (U.S.), who
figured and described it July 9, 1 878.4 It was again de-
lineated August 9, by Tempel at Florence. 5 In the follow-
ing year it attracted the wonder and attention of almost every
possessor of a telescope. Its colour had by that time deepened
1 Observatory, vol. vii. p. 175. 2 There is a consensus among observers
as to the variability of all Jupiter's satellites, though Pickering strangely
finds no trace of it in his exact measures of their light. See Harvard
Annals, vol. xi. pt. ii. p. 245. 3 Bull. Ac. R. Eruxelles, t. xlviii. p. 607.
4 Astr. Nach., No. 2294. 5 Ibid., No, 2284.
340 HISTORY OF ASTRONOMY. PART 11.
into a full brick-red, and was set off by contrast with a white
equatorial spot of unusual brilliancy. During three ensuing
years these remarkable objects continued to offer a visible
and striking illustration of the compound nature of the planet's
rotation. The red spot completed a circuit in nine hours fifty -
five minutes thirty-six seconds ; the white spot in about five
and a half minutes less. Their relative motion was thus no
less than 260 miles an hour, bringing them together in the
same meridian at intervals of forty-four days ten hours forty-
two minutes. - Neither, however, preserved continuously the
same uniform rate of travel. The period of each had length-
ened by some seconds in 1883, while sudden displacements,
associated with the recovery of lustre after recurrent fadings,
were observed in the position of the white spot, 1 recalling the
leap forward of a reviving sun-spot. Just the opposite effect
has attended the late rekindling of the companion object.
While semi-extinct in 1882-4, it lost little motion; but a
fresh access of retardation has been observed by Professor
Young 2 in connection with its brightening in 1886. This
suggests very strongly that the red spot is fed from below.
A shining aureola of " faculae," described by Bredichin at
Moscow, and by Lohse at Potsdam, as encircling it in Sep-
tember i879, 3 was ne ld t strengthen the solar analogy.
The conspicuous visibility of this astonishing object lasted
three years. When the planet returned to opposition in
1882-83, ft na -d faded so considerably that Bicco's uncertain
glimpse of it at Palermo, May 31, 1883, was expected to be
the last. It had, nevertheless, begun to recover in December,
and in the beginning of 1886 presented to Mr. Denning much
the same aspect as in October 1882.* Observed by him in an
intermediate stage, February 25, 1885, when "a mere skeleton
of its former self," it bore a striking likeness to an
" elliptical ring " observed in the same latitude by Mr. Gledhill
at Halifax in 1869-70. This, indeed, might be called the
1 Denning, Month. Not., vol. xliv. pp. 64, 66 ; Nature, voL xxv. p. 226.
2 Sidereal Mess., Dec. 1886, p. 289. 3 Astr. Nach., Nos. 2280, 2282.
4 Month. Not., vol. xlvi. p. 117.
CHAP. vin. PLANETS AND SATELLITES. 341
preliminary sketch for the famous object brought to perfection
ten years later, but which Mr. H. C. Russell of Sydney saw
and drew in June i876, 3 in what might be called an unfinished
condition, before it had separated from its matrix, the dusky
south-tropical belt. In earlier times, too, a marking " at once
fixed and transient " had been repeatedly perceived attached to
the southernmost of the central belts. It gave Cassini in 1665
a rotation-period of nine hours fifty-six minutes, 2 reappeared
and vanished eight times during the next forty-three years,
and was last seen by Maraldi in 1713. It was, however, very
much smaller than the recent object, and showed no unusual
colour.
The assiduous observations made on the " Great Red Spot "
by Mr. Denning at Bristol and by Professor Hough at Chicago
afforded grounds only for negative conclusions as to its nature.
It certainly did not represent the outpourings of a Jovian
volcano ; it was in no sense attached to the Jovian soil if the
phrase have any application to that planet ; it was not a mere
disclosure of a glowing mass elsewhere seethed over by rolling
vapours. There is some reason to suppose that its surface
was depressed below the average cloud level, and that the
cavity was filled with vapours. But it was almost certainly
not self-luminous, a satellite projected upon it in transit having
been seen to show as bright as upon the dusky equatorial
bands.
In 1870, Mr. Ranyard, 3 acting upon an earlier suggestion
of Dr. Huggins, collected records of unusual appearances on
the disc of Jupiter, with a view to investigate the question of
their recurrence at regular intervals. He concluded that the
development of the deeper tinges of colour, and of the equa-
torial "port-hole" markings girdling the globe in regular
alternations of bright and dusky, agreed, so far as could be
ascertained, with epochs of sun-spot maximum. The further
inquiries of Dr. Lohse at Bothkamp in 1873 4 went to strengthen
1 Proc. Roy. Soc. N. S. Wales, .vol. xiv. p. 68. 2 Phil. Trans., vol. i.
p. 143. 3 Month. Not., voL xxxi. p. 34. 4 Beolachtungen, Heft ii.
p. 99.
342 HISTORY OF ASTRONOMY. PART n.
the coincidence, which had been anticipated d priori by
Zollner in 187 1. 1 Yet subsequent experience has rather added
to than removed doubts as to the validity of that first con-
clusion. It may, indeed, be taken for granted that what
Hahn terms the universal pulse of the solar system 2 affects
the vicissitudes of Jupiter ; but the law of those vicissitudes
is far from being so obviously subordinate to the rhythmical
flow of central disturbance as are certain terrestrial pheno-
mena. The fundamental agreement which probably exists is
confused in its display by secondary causes.
It is likely that Saturn is in a still earlier stage of planetary
development than Jupiter. He is the lightest for his size of
all the planets. In fact, he would float in water. And since
his density is shown, by the amount of his equatorial bulging,
to increase centrally, 3 it follows that his superficial materials
must be of a specific gravity so low as to be inconsistent, on
any probable supposition, with the solid or liquid states.
Moreover, the chief arguments in favour of the high tempera-
ture of Jupiter apply, with increased force, to Saturn ; so
that it may be concluded, without much risk of error, that a
large proportion of his bulky globe, 72,000 miles in diameter,
is composed of heated vapours, kept in active aiid agitated
circulation by the process of cooling.
His unique set of appendages has, since the middle of the
century, formed the subject of searching and fruitful inquiries,
both theoretical and telescopic. The mechanical problem of
the stability of Saturn's rings was left by Laplace in a very
unsatisfactory condition. Considering them as rotating solid
bodies, he pointed out that they could not maintain their
position unless their weight were in some way unsymmetrically
distributed ; but made no attempt to determine the kind or
amount of irregularity needed to secure this end. Some
observations by Herschel gave astronomers an excuse for
taking for granted the fulfilment of the condition thus vaguely
1 Ber. Sachs. Ges. der Wiss., 1871, p. 553. 2 Beziehungen der Sonnen-
fleckenperiode, p. 175. 3 A. Hall, Astr. Nach., No. 2269.
CHAP. vin. PLANETS AND SATELLITES. 343
postulated ; and the question remained in abeyance until once
more brought prominently forward by the discovery of the
inner dusky ring in 1850.
The younger Bond led the way, among modern observers,
in denying the solidity of the structure. The fluctuations in
its aspect were, he asserted in I85I, 1 inconsistent with such
an hypothesis. The fine dark lines of division, frequently
detected in both bright rings, and as frequently relapsing into
imperceptibility, were due, in his opinion, to the real mobility
of their particles, and indicated a fluid formation. Professor
Benjamin Peirce of Harvard University immediately followed
with a demonstration, on abstract grounds, of their non-soli-
dity. 2 Streams of some fluid denser than water were, he
maintained, the physical reality giving rise to the anomalous
appearance first disclosed by Galileo's telescope.
The mechanism of Saturn's rings, proposed as the subject
of the Adams Prize, was dealt with by the late James Clerk
Maxwell in 1857. His investigation forms the groundwork of
all that is at present known in the matter. Its upshot was to
show that neither solid nor fluid rings could continue to exist,
and that the only possible composition of the system was by
an aggregated multitude of unconnected particles, each revolv-
ing independently in a period corresponding to its distance
from the planet. 3 This idea of a satellite-formation had been,
remarkably enough, several times entertained and lost sight of.
It was first put forward by Roberval in the seventeenth century,
again by Jacques Cassini in 1715, and with perfect definiteness
by Wright of Durham in 1 750.* Little heed, however, was taken
of these casual anticipations of a truth which reappeared, a
virtual novelty, as the legitimate outcome of the most refined
modern methods.
The details of telescopic observation accord, on the whole,
admirably with this hypothesis. The displacements or dis-
1 Astr. Jour. (Gould's), vol. ii. p. 17. 2 Ibid., p. 5. 3 On the Stability
of the Motion of Saturn's Rings, p. 67. 4 Mem. de I' Ac., 1715, p. 47 ;
Montucla, Hist, des Math., t. iv. p. 19 ; An Original Theory of the Uni-
verse, p. 115.
344 HISTORY OF ASTRONOMY. PART n.
appearance of secondary dividing-lines the singular striated
appearance, first remarked by Short in the eighteenth century,
last by Perrotin and Lockyer at Nice, March 18, 1884,! show
the effects of waves of disturbance traversing a moving mass
of gravitating particles ; 2 the broken and changing line of the
planet's shadow on the ring gives evidence of variety in the
planes of the orbits described by those particles. There is but
one serious discrepancy.
On the satellite -theory, the obscure inner ring is formed of
similar small bodies to those aggregated in the lucid members
of the system, only much more thinly strewn, and reflecting,
consequently, much less light. It is not, however, easy to see
why these sparser flights should show as a dense dark shading
on the body of Saturn. Yet this is invariably the case. The
objection, long felt, has recently been urged by Professor
Hastings of Baltimore. The brightest parts of these ap-
pendages, he remarks, 3 are more lustrous than the globe they
encircle ; but if the inner ring consists of identical materials,
possessing presumably an equal reflective capacity, the mere
fact of their scanty distribution would not cause them to show
as dark against the same globe. The conclusion seems in-
evitable, that the bright and dark rings are not composed of
identical materials.
A question of singular interest, and one which we cannot
refrain from putting to ourselves, is whether we see in the
rings of Saturn a finished structure, destined to play a per-
manent part in the economy of the system ; or whether they
represent merely a stage in the process of development out of
the chaotic state in which it is impossible to doubt that the
materials of all planets were originally merged. M. Otto
Struve has attempted to give a definite answer to this im-
portant query.
A study of early and later records of observations disclosed
to him, in 1851, an apparent progressive approach of the inner
edge of the bright ring to the planet. The rate of approach he
1 Comptes Rendus, t. xcviii. p. 718. - Proctor, Saturn and his System
(1865), p. 125. 3 Smiths. Report, 1880 (Holden).
CHAP. vni. PLANETS AND SATELLITES. 345
estimated at about fifty-seven English miles a year, or 11,000
miles during the 194 years elapsed since the time of Huygens. 1
Were it to continue, a collapse of the system must be far
advanced within three centuries. But was the change real or
illusory a plausible, but deceptive inference from insecure
data ? M. Struve resolved to put it to the test. A set of
elaborately careful micrometrical measures of the dimensions
of Saturn's rings, executed by himself at Pulkowa in the
autumn of 1851, was provided as a standard of future com-
parison; and he was enabled to renew them, under closely
similar circumstances, in 1882. 2 But the expected diminution
of the space between Saturn's globe and his rings had not
taken place. There was, indeed, a slight extension in the
width of the system, both outward and inward ; but so slight
that it could hardly be considered to lie outside the limits
of probable error. Still it is worth notice that just such a
separation of the rings was indicated by Clerk Maxwell's theory,
so that there is an & priori likelihood of its being in progress.
Moreover, since 1657, when Huygens described the interval
between the ring and the planet as rather exceeding the width
of the ring, it is all but certain that a growth inward has
actually occurred. For the two bright rings together, instead
of being narrower than the interval, are now more than once
and a half times as broad. Hence the expressions used by
Huygens, no less than most of the old drawings, are glaringly
inconsistent with the planet's present appearance.
There seems reason to admit that Kirkwood's law of com-
mensurability has had some effect in bringing about the
present distribution of the matter composing these appendages.
Here the disturbing bodies are Saturn's moons, while the
divisions and boundaries of the rings represent the spaces
where their disturbing action conspires to eliminate revolving
particles. Kirkwood, in fact, showed, in 1867, 3 that a body
circulating in the chasm between the bright rings known as
1 Mim. de I' Ac. Imp. (St. Petersb.), t. vii. 1853, p. 464. 2 Astr. Nach.,
No. 2498. 3 Meteoric Astronomy, chap. xii. He carried the subject
somewhat further in 1871. See Observatory, vol. vi. p. 335.
346 HISTORY OF ASTRONOMY. PART ir.
" Cassini's division," would have a period nearly commensurable
with those of four out of the eight moons ; and Dr. Meyer of
Geneva has recently calculated all such combinations with the
result of bringing out coincidences between regions of maximum
perturbation and the limiting and dividing lines of the system. 1
This is in itself a strong confirmation of the view that the rings
are made up of independently revolving small bodies.
On December 7, 1876, Professor Asaph Hall discovered at
Washington a bright equatorial spot on Saturn, which he
followed and measured through above sixty rotations, each
performed in ten hours fourteen minutes twenty-four seconds. 2
He is careful to add that this represents the period, not
necessarily of the planet, but only of the individual spot. The
only previous determination of Saturn's axial movement (set-
ting aside some insecure estimates by Schroter) was Herschel's
in 1794, giving a period of ten hours sixteen minutes.
Saturn's outermost satellite, Japetus, is markedly variable
so variable that it sends us, when brightest, just 4^ times as
much light as when faintest. Moreover, its fluctuations depend
upon its orbital position in such a way as to make it a con-
spicuous telescopic object when west, a scarcely discernible
one when east of the planet. Herschel's inference 3 of a
partially obscured globe turning always the same face towards
its primary, seems the only admissible one, and is confirmed
by Pickering's measurements of the varying intensity of its
light. He remarks further that the dusky and brilliant hemi-
spheres must be so posited as to divide the disc, viewed from
Saturn, into nearly equal parts ; so that this Saturnian moon,
even when " full," appears very imperfectly illuminated over
one-half of its surface. 4
The spectrum of Saturn is closely similar to that of Jupiter.
It shows the distinctive dark line in the red, which we may
call the "red-star line;" and Janssen examining it from the
summit of Etna in i867, 5 found unmistakable traces of aqueous
1 Astr. Nach., No. 2527. z Am. Jour, of Sc., vol. xiv. p. 325. 3 Phil.
Trans., vol. Ixxxii. p. 14. 4 Smiths. Report, 1880. 5 Comptes Rendus,
t. Ixiv. p. 1304.
CHAP. viii. PLANETS AND SATELLITES. 347
absorption. The light from the ring is much less modified by
original atmospheric action.
Uranus, when favourably situated, can now easily be seen
with the naked eye as a star somewhat below the fifth magni-
tude. He thus appears considerably brighter than when dis-
covered 1 06 years ago. Not, however, through any intrinsic
change. He is at present conspicuous simply because he has
but lately passed perihelion. 1 This circumstance has enabled
astronomers, provided with the powerful telescopes of modern
times, to make some highly interesting observations on this
remote planet.
It will be remembered that Uranus presents the unusual
spectacle of a system of satellites travelling nearly at right
angles to the plane of the ecliptic. The existence of this
anomaly gives a special interest to investigations of his axial
movement, which the analogy of the other planets might lead us
to presume to be executed in the same tilted plane. Yet this,
strange to say, does not seem to be the case.
Mr. Buffham in 1870-72 caught traces of bright markings
on the Uranian disc, suggesting, with much uncertainty, a
rotation in about twelve hours in a plane not coincident with
that in which his satellites circulate. 2 Dusky bands resembling
those of Jupiter, but very faint, were barely perceptible to
Professor Young at Princeton in 1883. Yet, though inevitably
inferred to be equatorial, they made a considerable angle with
the trend of the satellites' orbits. 3 More distinctly by the
brothers Henry, with the aid of their fine refractor, two grey
parallel rulings, separated by a brilliant zone, were discerned
every clear night at Paris from January to June 1884.* What
were taken to be the polar regions appeared comparatively
dusky. The direction of the equatorial belts (for so we must
for the present call them) made an angle of 40 with the
satellites' line of travel. Similar observations were made at Nice
1 Tebbutt, Trans. Roy. Soc. N. S. Wales, vol. xiv. p. 23. 2 Month.
Not., vol. xxxiii. p. 164. 3 Astr. Nach., No. 2545. 4 Comptes Rendus,
t. xcviii. p. 1419.
348 HISTORY OF ASTRONOMY. PART n.
by MM. Perrotin and Thollon, March to June 1884, a lucid
spot near the equator, in addition, indicating rotation in a
period of about ten hours.
Measurements of the little sea-green disc which represents
to us the massive bulk of Uranus, give, however, a different
result. Young, Schiaparelli, 2 Safarik, and quite lately, H. C.
Wilson of Cincinnati, 3 have each found it to be quite distinctly
bulged; and all agree that the bulging lies just in the plane
of the satellites' orbits. If this be so, there can be no ques-
tion but that the same plane is that of the planet's rotation,
the spheroidal shape of a rotating globe being the necessary
consequence of the greater equatorial velocity of its particles.
But the " equatorial " markings visibly assert a rapid whirling
in a direction removed by nearly half a right angle from that
plane. Which are we to believe ? Where such minute quan-
tities are concerned as the differences between the various
diameters of a disc about four seconds across, conclusions might
well appear precarious. Yet they come to us so well authen-
ticated that it is scarcely possible to doubt their substantial
correctness. On the other hand, the parallel bands seen at
Princeton, no less than at Nice and Paris, seem to convey a
positive assurance that Uranus now rotates in a plane widely
removed from that in which the bodies dependent upon him
circulate. The discrepancy is glaring. Accommodation be-
tween the two sets of observations appears impossible. One
or other is bound to give way. Neither, however, appears at
present likely to do so. The puzzle adds to the interest long
excited by the anomalous rotation of Uranus.
The spectrum of this planet was first examined by Father
Secchi in 1869, and later, though with more advantages for
accuracy, by Huggins and Yogel. It is a very remarkable
one. In lieu of the reflected Fraunhofer lines, imperceptible
perhaps through feebleness of light, six broad bands of origi-
nal absorption appear, 4 one corresponding to the blue-green
ray of hydrogen (F), another to the "red-star line" of Jupiter
1 Comptes Rendus, t. xcviii. pp. 718, 967. 2 Astr. Nach., No. 2526.
3 Ibid., No. 2730. 4 Vogel, Annalen der Phys., voL clviii. p. 470.
CHAP. vin. PLANETS AND SATELLITES. 349
and Saturn, the rest as yet unidentified. The hydrogen band
seems much too strong and diffuse to be the mere echo of a
solar line, and implies accordingly the presence of free hydro-
gen in the Uranian atmosphere, where a temperature must
thus prevail sufficiently high to reduce water to its constituent
elements.
Judging from the indications of an almost evanescent spec-
trum, Neptune, as regards physical condition, is the twin of
Uranus, as Saturn of Jupiter. Of the circumstances of his
rotation we are as good as completely ignorant. Mr. Maxwell
Hall, indeed, noticed at Jamaica, in November and December
1883, certain rhythmical fluctuations of brightness, suggesting
revolution on an axis in slightly less than eight hours ; l but
Professor Pickering reduces the supposed variability to an
amount altogether too small for certain perception, and Dr.
G. Miiller denies its existence in toto. It is true their observa-
tions were not precisely contemporaneous with those of Mr.
Hall, 2 who believes the partial obscurations recorded by him-
self to have been of a passing kind, and to have suddenly
ceased after a fortnight of prevalence. Their less conspicuous
renewal was visible to him in November 1884, confirming a
rotation period of 7.92 hours.
The possibility that Neptune may not be the most remote
body circling round the sun has been contemplated ever since
he has been known to exist. Within the last few years the
position at a given epoch of a planet far beyond him has been
approximately fixed by two separate investigators. Its actual
discovery is perhaps one of the prizes reserved for the astrono-
mers of the future.
Professor George Forbes of Edinburgh adopted in 1880 a
novel plan of search for unknown members of the solar system,
the first idea of which was thrown out by M. Flammarion in
November iSyg. 3 It depends upon the movements of comets.
It is well known that those of moderately short periods are,
1 Month. Not., vol. xliv. p. 257. 2 Observatory, vol. vii. pp. 134, 221,
264. 3 Astr. Pop., p. 66 1 ; La Nature, Jan. 3, 1880.
350 HISTORY OF ASTRONOMY. PARTII.
for some reason, connected with the larger planets in such a
way that the cometary aphelia fall near some planetary orbit.
Jupiter claims above a dozen of such partial dependants,
Neptune owns six, and there are two considerable groups, the
farthest distances of which from the sun lie respectively near
100 and 300 times that of the earth. At each of these vast
intervals, one involving a period of 1000, the other of 5000
3^ears, Professor Forbes maintains that an unseen planet cir-
culates. He has even computed elements for the nearer of
the two, and fixed its place on the celestial sphere. 1
In the meantime, Professor Todd had been searching for the
same object by the help of a totally different set of indications.
The old approved method of perturbations was that adopted
by him ; but those of Neptune have scarcely yet had time to
develop, so that he was thrown back upon the " residual
errors " of Uranus. They gave him a virtually identical
situation for the new planet with that arrived at by Professor
Forbes. 2 If this be a coincidence, it is a very remarkable one,
the more so as each inquirer worked in complete ignorance of
the results of the other.
1 Proc. Roy. Soc. Edirib., vol. x. p. 429 ; Observatory, vol. iii. p. 439.
2 Am. Jour, of Sc. t vol. xx. p. 225.
CHAPTER IX.
THEORIES OF PLANETARY EVOLUTION.
WE cannot doubt that the solar system, as we see it, is the
result of some process of growth that, during innumerable
ages, the forces of Nature were at work upon its materials,
blindly modelling them into the shape appointed for them
from the beginning by Omnipotent Wisdom. To set ourselves
to inquire what that process was, may be an audacity, but it
is a legitimate, nay, an inevitable one. For man's implanted
instinct to "look before and after" does not apply to his own
little life alone, but regards the whole history of creation,
from the highest to the lowest from the microscopic germ of
an alga or a fungus to the visible frame and furniture of the
heavens.
Kant considered that the inquiry into the mode of origin of
the world was one of the easiest problems set by Nature ; but
it cannot be said that his own solution of it was a satisfactory
one. He, however, struck out in 1755 a track which thought
still pursues. In his Allgemeine Naturgeschichte the growth of
sun and planets was traced from the cradle of a vast and form-
less mass of evenly diffused particles, and the uniformity of
their movements was sought to be accounted for by the uniform
action of attractive and repulsive forces, under the dominion
of which their development was carried forward.
In its modern form, the " Nebular Hypothesis " made its
appearance in I796. 1 It was presented by Laplace with
diffidence, as a speculation unfortified by numerical buttresses
of any kind, yet with visible exultation in having, as he
1 Exposition du Systtme du Monde, t. ii. p. 295.
352 HISTORY OF ASTRONOMY. PART n.
thought, penetrated the birth- secret of our system. He de-
manded, indeed, more in the way of postulates than Kant had
done. He started with a sun ready-made, 1 and surrounded
with a vast glowing atmosphere, extending into space out
beyond the orbit of the farthest planet, and endowed with a
slow rotatory motion. As this atmosphere or nebula cooled,
it contracted; and as it contracted, its rotation, by a well-
known mechanical law, became accelerated. At last, a point
arrived when tangential velocity at the equator increased
beyond the power of gravity to control, and equilibrium was
restored by the separation of a nebulous ring revolving in the
same period as the generating mass. After a time, the ring
broke up into fragments, all eventually reunited in a single
revolving and rotating body. This was the first and farthest
planet.
Meanwhile the parent nebula continued to shrink and whirl
quicker and quicker, passing, as it did so, through successive
crises of instability, each resulting in, and terminated by, the
formation of a planet, at a smaller distance from the centre,
and with a shorter period of revolution than its predecessor.
In these secondary bodies the same process was repeated on
a reduced scale, the birth of satellites ensuing upon their con-
traction, or not, according to circumstances. Saturn's ring,
it was added, afforded a striking confirmation of the theory of
annular separation, 2 and appeared to have survived in its
original form in order to throw light on the genesis of the
whole solar system ; while the four first discovered asteroids
offered an example in which the debris of a shattered ring had
failed to coalesce into a single globe.
This scheme of cosmical evolution was a characteristic
bequest from the eighteenth century to the nineteenth. It
possessed the self-sufficing symmetry and entireness appro-
priate to the ideas of a time of renovation, when the com-
plexity of nature was little accounted of in comparison with
the imperious orderliness of the thoughts of man. Since it
1 In later editions a retrospective clause was added admitting a prior
condition of all but evanescent nebulosity. 2 Mec. Cel., lib. xiv. ch. iii.
CHAP. ix. PLANETARY EVOLUTION. 353
was propounded, however, knowledge has transgressed many
boundaries, and set at naught much ingenious theorising.
How has it fared with Laplace's sketch of the origin of the
world ? It has at least not been discarded as effete. The
groundwork of "speculation on the subject is still furnished by
it. It is, nevertheless, admittedly inadequate. Of much that
exists it gives no account, or an erroneous one. It is certain
that the march of events did not everywhere it is doubtful
whether it anywhere followed the exact path prescribed for
it. Yet modern science attempts to supplement, but scarcely
ventures to supersede it.
Thought has, in many directions, been profoundly modified
by Mayer's and Joule's discovery, in 1842, of the equivalence
between heat and motion. Its corollary was the grand idea of
the " conservation of energy," now one of the cardinal prin-
ciples of science. This means that, under the ordinary circum-
stances of observation, the old maxim ex niliilo niliil Jit applies
to force as well as to matter. The supplies of heat, light,
electricity, must be kept up, or the stream will cease to flow.
The question of the maintenance of the sun's heat was thus
inevitably raised ; and with the question of maintenance that
of origin is indissolubly connected. .
Dr. Julius Robert Mayer, a physician residing at Heilbronn,
was the first to apply the new light to the investigation of what
Sir John Herschel had termed the " great secret." He showed
that if the sun were a body either simply cooling or in a state
of combustion, it must long since have "gone out." Had an
equal mass of coal been set alight, four or five centuries after
the building of the Pyramid of Cheops, and kept burning
at such a rate as to supply solar light and heat during the
interim, only a few cinders would now remain in lieu of our
undiminished glorious orb. Mayer looked round for an alter-
native. He found it in the " meteoric hypothesis " of solar
conservation. 1 The importance in the economy of our system
of the bodies known as falling stars was then (in 1848)
beginning to be recognised. It was known that they revolved
1 Btiirdje zur Dynamik dcs Illmmels, p. 12.
Z
354 HISTORY OF ASTRONOMY. PART n.
in countless swarms round the sun ; that the earth daily en-
countered millions of them ; and it was surmised that the cone
of the zodiacal light represented their visible condensation
towards the attractive centre. From the zodiacal light, then,
Mayer derived the store needed for maintaining the sun's
radiations. He proved that, by the stoppage of their motion
through falling into the sun, bodies would evolve from 4600
to 9200 times as much heat (according to their ultimate
velocity) as would result from the burning of equal masses of
coal, their precipitation upon the sun's surface being brought
about by the resisting medium observed to affect the revolu-
tions of Encke's comet. There was, however, a difficulty.
The quantity of matter needed to keep, by the sacrifice of its
movement, the hearth of our system warm and bright, would
be very considerable. Mayer's lowest estimate put it at
94,000 billion kilogrammes per second, or a mass equal to
that of our moon bi-annually. But so large an addition to the
gravitating power of the sun would quickly become sensible
in the movement of the bodies dependent upon him. Their
revolutions would be notably accelerated. Mayer admitted
that each year would be shorter than the previous one by a
not insignificant fraction of a second, and postulated an un-
ceasing waste of substance, such as Newton had supposed
must accompany emission of the material corpuscles of light,
to neutralise continual reinforcement.
Mayer's views obtained a very small share of publicity, and
owned Mr. Waterston as their independent author in this
country. The meteoric, or " dynamical " theory of solar sus-
tentation was expounded by him before the British Associa-
tion in 1853. It was developed with his usual ability by Sir
William Thomson in the following year. The inflow of
meteorites, he remarked, "is the only one of all conceivable
causes of solar heat which we know to exist from independent
evidence." l We know it to exist, but we now also know it to
be entirely insufficient. The supplies presumed to be con-
tained in the zodiacal light would be quickly exhausted ; a
1 Trans. Roy. Soc. of Edinburgh, vol. xxi. p. 66.
CHAP. ix. PLANETARY EVOLUTION. 355
constant inflow from space would be needed to meet the
demand. But if moving bodies were drawn into the sun at
anything like the required rate, the air, even out here at
ninety-three millions of miles distance, would be thick with
them ; the earth would be red-hot from their impacts ; 1 geo-
logical deposits would be largely meteoric; 2 to say nothing
of the effects on the mechanism of the heavens. Sir William
Thomson himself urged the inadmissibility of the "extra-
planetary " theory of meteoric supply on the very tangible
ground that, if it were true, the year would be shorter now,
actually by six weeks, than at the opening of the Christian era.
The " intra-planetary " supply, however, is too scanty to be
anything more than a temporary makeshift.
The meteoric hypothesis was naturally extended from the
maintenance of the sun's heat to the formation of the bodies
circling round him. The earth no less doubtless than the
other planets is still growing. Cosmical matter in the shape
of falling stars and aerolites, to the amount, adopting Professor
Newton's estimate, of 100 tons daily, is swept up by it as it
pursues its orbital round. Inevitably the idea suggested itself
that this process of appropriation gives the key to the life-
history of our globe, and that the momentary streak of fire in
the summer sky represents a feeble survival of the glowing
hail-storm by which, in old times, it was fashioned and warmed.
Mr. E. W. Brayley supported this view of planetary produc-
tion in i864, 3 and it has recommended itself to Haidinger,
Helmholtz, Proctor, and Faye. But the negative evidence of
geological deposits appears fatal to it.
The theory of solar energy now generally regarded as the
true one, was enounced by Helmholtz in a popular lecture
in 1854. It depends upon the same principle of the equiva-
lence of heat and motion which had suggested the meteoric
hypothesis. But here the movement surrendered and trans-
formed belongs to the particles, not of any foreign bodies, but
of the sun itself. Drawn together by the force of their own
1 Newcomb, Pop. Astr.,p. 521 (2d ed.). M. Williams, Nature, vol.
iii. p. 26. 3 Comp. Brit. Almanac, p. 94.
356 HISTORY OF ASTRONOMY. PART n.
gravity from a wide ambit, their fall towards the sun's centre
must have engendered a vast thermal store, of which |4f are
computed to be already spent. Presumably, however, this
stream of reinforcement is still flowing. In the very act of
parting with heat, the sun develops a fresh stock. His radia-
tions, in short, are the direct result of shrinkage through
cooling. A diminution of the solar diameter by 380 feet
yearly would just sufiice to cover the present rate of emission,
and would for ages remain imperceptible with our means of
observation, since, after the lapse of 6000 years, the lessening
of angular size would scarcely amount to one second. 1 But
the process, though not terminated, is strictly a terminable
one. In less than five million years, the sun will have con-
tracted to half its present bulk. In seven million more, it
will be as dense as the earth. It is difficult to believe that it
will then be a luminous body. 2 Nor can an unlimited past
duration be admitted. Helmholtz considered that radiation
might have gone on with its actual intensity for twenty- two,
Langley allows only eighteen million years. The period can
scarcely be stretched, by the most generous allowances, to
double the latter figure. But this is far from meeting the
demands of geologists and biologists.
An ingenious attempt has lately been made to supply the
sun with machinery analogous to that of a regenerative furnace,
enabling it to consume the same fuel over and over again, and
so to prolong indefinitely its beneficent existence. The inor-
dinate " waste " of energy, which shocks our thrifty ideas, was
simultaneously abolished. The earth stops and turns variously
to account one 2250-millionth part of the solar radiations;
each of the other planets and satellites takes a proportionate
share ; the rest, being all but an infinitesimal fraction of the
whole, is dissipated through endless space, to serve what pur-
pose we know not. Now, on the late Sir William Siemens's
plan, this reckless expenditure would cease ; the solar incom-
ings and outgoings would be regulated on approved economic
1 Radau, Bull. Astr., t. ii. p. 316.
- Newcomb, Pop. Astr., pp. 521-525.
CHAP. ix. PLANETARY EVOLUTION. 357
principles, and the inevitable final bankruptcy would be staved
off to remote ages. Let us see how it is to be done.
We must first imagine space to be filled with combustible
substances hydrogen, hydro-carbons, and oxygen in an ex-
cessively rarefied state. Next, that the sun keeps up, by its
rotation, a fan-like action on this floating matter, drawing it
inward at the polar surfaces, and projecting it outward at the
equator " in a continuous disc-like stream." l But it will not
travel from the sun unchanged. Combustion will have inter-
vened. In other words, the particles sucked in will have
surrendered their stored-up energy in the shape of heat and
light, and they will depart, no longer combustible, but the
mere inert products of combustion. By the very power of
the radiations they had contributed to supply, however, they
may be restored to activity. Sir W. Siemens obtained some
experimental evidence that carbonic acid and water may
possibly be dissociated in space, as they undoubtedly are in
the leaves of plants, by the power of direct sunshine. Their
particles, thus compulsorily separated, and by the act restocked
with energy, are ready to rush together again with fresh
evolution of heat and light. A mechanical circulation is,
in this way, combined with a "pendulum-swing of chemical
change, and the round might go on for ever, if only one
condition were granted. That one condition is an unlimited
supply of motive power. It is, however, an inexorable law
of nature that there is no work without waste. Ex niliilo
nildl fit.
In this case, the heart-throb of the circulating system resides
in the rotation of the sun. Therein is contained a certain
definite amount of mechanical power enough, according to
Sir W. Thomson, if directly converted into heat, to keep up
the sun's emission during 116 years and six days a mere
moment in cosmical time. More economically applied, it
would no doubt go farther. Its exhaustion would neverthe-
less, under the most favourable circumstances, ensue in a
1 Proc. Roy. Soc., vol. xxxiii. p. 393.
358 HISTORY OF ASTRONOMY. PART n.
comparatively short period. 1 Many other objections equally
unanswerable have been urged to the " regenerative " hypo-
thesis, but this one suffices.
There remains, then, as the only intelligible rationale of solar
sustentation, Helmholtz's shrinkage theory. And this has a
very important bearing upon the nebular view of planetary
formation ; it may, in fact, be termed its complement. For
it involves the idea that the sun's materials, once enormously
diffused, gradually condensed to their present volume with
development of heat and light, and, it may plausibly be added,
the separation of dependent globes. The data furnished by
spectrum analysis, too, favour the supposition of a common
origin for sun and planets by showing their community of
substance; while gaseous nebulae present examples of vast
masses of tenuous vapour, such as our system may plausibly
be conjectured to have primitively sprung from.
But recent science raises many objections to the details, if
it supplies some degree of confirmation to the fundamental
idea of Laplace's cosmogony. The detection of the retrograde
movement of Neptune's satellite made it plain that the anoma-
lous conditions of the Uranian world were due to no extra-
ordinary disturbance, but to a systematic variety of arrangement
at the outskirts of the solar domain. So that, were a trans-
Neptunian planet discovered, we should be fully prepared to
find it rotating, and surrounded by satellites circulating from
east to west. The uniformity of movement, upon the proba-
bilities connected with which the French geometer mainly
based his scheme, thus at once vanishes.
The excessively rapid revolution of the inner Martian moon
is a further stumbling-block. On Laplace's view, no satellite
can revolve in a shorter time than its primary rotates ; for in
its period of circulation survives the period of rotation of the
parent mass which filled the sphere of its orbit at the time of
giving it birth. And rotation quickens as contraction goes
1 To this hostile argument, as urged by Mr. E. Douglas Archibald, Sir
W. Siemens opposed the increase of rotative velocity through contraction
(Nature, vol. xxv. p. 505). But contraction cannot restore lost momentum.
CHAP. ix. PLANETARY EVOLUTION. 359
on ; therefore, the older time of axial rotation should invariably
be the longer. This obstacle can, however, it seems, be turned.
More serious is one connected with the planetary periods,
pointed out by Babinet in I86I. 1 In order to make them fit
in with the hypothesis of successive separation from a rotat-
ing and contracting body, certain arbitrary assumptions have
to be made of fluctuations in the distribution of the matter
forming that body at the various epochs of separation. 2
Such expedients usually merit the distrust which they
inspire.
Again, it was objected by Professor Kirkwood in 1869 3 that
there could be no sufficient cohesion in such an enormously
diffused mass as the planets are supposed to have sprung from,
to account for the wide intervals between them. The matter
separated through the growing excess of centrifugal speed,
would have been cast of, not by rarely recurring efforts, but
continually, fragmentarily, pari passu with condensation and
acceleration. Each wisp of nebula, as it found itself unduly
hurried, would have declared its independence, and set about
revolving and condensing on its own account. The result
would have been a meteoric, not a planetary system.
Moreover, it is a question whether the relative ages of the
planets do not follow an order just the reverse of that con-
cluded by Laplace. Professor Newcomb holds the opinion
that the rings which eventually constituted the planets, divided
from the main body of the nebula almost simultaneously,
priority, if there were any, being on the side of the inner and
smaller ones ; 4 while in M. Faye's new cosmogony, 5 the retro-
grade motion of the systems formed by the two outer planets
is ascribed on grounds, it is true, of dubious validity to
their comparatively late origin.
This ingenious scheme is designed, not merely to com-
plete, but to supersede that of Laplace, which, undoubtedly,
through the inclusion by our system of oppositely directed
1 Comptes JRcndus, t. lii. p. 481. See also Kirkwood, Observatory, vol.
iii. p. 409. 2 Pouche", Comptes Rendus, t. xcix. p. 903. 3 Month. Not .
vol. xxix. p. 96. 4 Pop. Astr., p. 257. 5 Sur VOrigine du Monde, 1884.
360 HISTORY OF ASTRONOMY. PART n.
rotations, forfeits its claim, simply and singly to account
for the fundamental peculiarities of its structure.
M. Faye's leading contention is that, under the circum-
stances assumed by Laplace, not the two outer planets alone,
but the whole company must have been possessed of retrograde
rotation. For they were formed ex Ttypotliesi after the
sun; central condensation had reached an advanced stage
when the rings they were derived from separated ; the prin-
ciple of inverse squares consequently held good, and Kepler's
Laws were in full operation. Now particles circulating in.
obedience to these laws can only since their velocity decreases
outward from the centre of attraction coalesce into a globe
with a backward axial movement. Nor was Laplace blind to
this flaw in his theory ; but his effort to remove it, though it
passed muster for the best part of a century, 1 was scarcely
successful. His planet-forming rings were made to rotate
. all in one piece, their outer parts thus necessarily travelling
at a swifter linear rate than their inner parts, and eventually
uniting, equally of necessity, into a forward-spinning body.
The strength of cohesion involved may, however, safely be
called impossible, especially when it is considered that nebulous
materials were in question.
The reform proposed by M. Faye consists in admitting that
all the planets inside Uranus are of pre-solar origin that
they took globular form in the bosom of a nearly homogeneous
nebula, revolving in a single period, with motion accelerated
from centre to circumference, and hence agglomerating into
masses with a direct rotation. Uranus and Neptune owe
their exceptional characteristics to their later birth. When
they came into existence, the development of the sun was
already far advanced, central force had acquired virtually its
present strength, unity of period had been abolished by its
predominance, and motion was retarded outward.
Thus, what we may call the relative chronology of the solar
system is thrown once more into confusion. The order of
seniority of the planets is now no easier to determine than the
1 Kirkwood adverted to it in 1864, Am. Jour., vol. xxxviii. p. I.
CHAP. ix. PLANETARY EVOLUTION. 361
" Who first, who last 1 " among the victims of Hector's spear.
For M. Faye's arrangements, notwithstanding the skill with
which he has presented them, cannot be unreservedly accepted.
The objections to them, thoughtfully urged by M. C. Wolf, 1
and Mr. G. H. Darwin, 2 are grave. Not the least so is his
omission to take account of an agency of change presently
to be noticed.
We now come to a most remarkable investigation one,
indeed, unique in its profession to lead us back with mathe-
matical certainty towards the origin of a heavenly body. We
refer to Mr. G. H. Darwin's inquiries into the former rela-
tions of the earth and moon. 3
They deal exclusively with the effects of tidal friction, and
primarily with those resulting, not from oceanic, but from
" bodily" tides, such as the sun and moon must have raised
in past ages on a liquid or viscous earth. The immediate
effect of either is, as already explained, to destroy the rotation
of the body on which the tide is raised, as regards the tide-
raising body, bringing it to turn always the same face towards
its disturber. This, we can see, has been completely brought
about in the case of the moon. There is, however, a secondary
or reactive effect. Action is always mutual. Precisely as
much as the moon pulls the terrestrial tidal wave backward,
the tidal wave pulls the moon forward. But pulling a body
forward in its orbit implies the enlargement of that orbit;
that is to say, the moon is, as a consequence of tidal friction,
very slowly receding from the earth. This will go on (other
circumstances remaining unchanged) until the lengthening day
overtakes the more tardily lengthening month, when each will
be of about 1400 hours. 4 A position of what we may call tidal
equilibrium between earth and moon will (apart from disturb-
ance by other bodies) then be attained.
If, however, it be true that, in the time to come, the moon
1 Butt. Astr. t. ii. 2 Nature, vol. xxxi. p. 506. . 3 Phil. Trans., vol.
clxxi. p. 713. 4 Mr. J. Nolan has pointed out (Nature, vol. xxxiv. p.
287) that the length of the equal day and month will be reduced to about
1240 hours by the effects of solar tidal friction.
362 HISTORY OF ASTRONOMY. PART n.
will be much farther from us, it follows that in the time past
she was much nearer to us than she now is. Tracing back
her history by the aid of Mr. Darwin's clue, we at length find
her revolving in a period of somewhere between two and four
hours, almost in contact with an earth rotating just at the
same rate. This was before tidal friction had begun its work
of grinding down axial velocity and expanding orbital range.
But the position was not one of stable equilibrium. The
slightest inequality must have set on foot a series of uncom-
pensated changes. If the moon had whirled the least iota
faster than the earth spun she must have been precipitated
upon it. Her actual existence shows that the trembling
balance inclined the other way. By a second or two to begin
with, the month exceeded the day ; the tidal wave crept ahead
of the moon ; tidal friction came into play, and our satellite
started on its long spiral journey outward from the parent
globe. This must have occurred, it is computed, at least fifty-
four million years ago.
That this kind of tidal reactive effect played its part in
bringing the moon into its present position, and is Still, to
some slight extent, at work in changing it, there can be no
doubt whatever. An irresistible conjecture carried the explorer
of its rigidly deducible consequences one step beyond them.
The moon's time of revolution, when so near the earth as barely
to escape contact with it, must have been, by Kepler's Law,
more than two, and less than two and a half hours. Now it
happens that the most rapid rate of rotation of a fluid mass of
the earth's average density, consistent with spheroidal equili-
brium, is two hours and twenty minutes. Quicken the move-
ment but by one second and the globe must fly asunder.
Hence the inference that the earth actually did fly asunder
through over-fast spinning, the ensuing disruption repre-
senting the birth-throes of the moon. It is likely that the
event was hastened or helped by solar tidal disturbance.
To recapitulate. Analysis tracks backward the two bodies
until it leaves them in very close contiguity, one rotating and
the other revolving in approximately the same time, and that
CHAP. ix. PLANETARY EVOLUTION. 363
time certainly not far different from, and quite possibly iden-
tical with, the critical period of instability for the terrestrial
spheroid. " Is this," Mr. Darwin asks, " a mere coincidence,
or does it not rather point to the break-up of the primeval
planet into two masses in consequence of a too rapid rota-
tion 1 " i
We are tempted, but are not allowed to give an unqualified
assent. Mr. James Nolan of Victoria has lately made it clear
that the moon could not have subsisted as a continuous mass
under the powerful disruptive strain which would have acted
upon it when revolving almost in contact with the present
surface of the earth ; and Mr. Darwin, admitting the objection,
concedes to our satellite, in its initial stage, the alternative
form of a flock of meteorites. 2 But such a congregation must
have been quickly dispersed, by tidal action, into a meteoric
ring; and the moon remains unexplained. The evidence,
however, for the efficiency of tidal friction in bringing about
the actual configuration of the lunar-terrestrial system, is not
invalidated by this failure to penetrate its natal mystery.
Under its influence the principal elements of that system fall
into interdependent mutual relations. It connects, causally
and quantitatively, the periods of the moon's revolution and
of the earth's rotation, the obliquity of the ecliptic, the inclina-
tion and eccentricity of the lunar orbit. All this can scarcely
be accidental. .
Mr. Darwin's first researches on this subject were com-
municated to the Royal Society, December 18, 1879. They
were followed, January 20, i88i, 3 by an inquiry on the same
principles into the earlier condition of the entire solar system.
The results were a warning against hasty generalisation.
They showed that the lunar-terrestrial system, far from being
a pattern for their development, was a singular exception
among the bodies swayed by the sun. Its peculiarity resides
in the fact that the moon is proportionately by far the most
massive attendant upon any known -planet. Its disturbing
1 Phil. Trans., vol. clxxi. p. 835. 2 Nature, vol. xxxiii. p. 368 ; see
also Nolan, Ibid., vol. xxxiv. p. 286. 3 Phil. Trans., vol. clxxii. p. 491.
364 HISTORY OF ASTRONOMY. PART 11.
power over its primary is thus abnormally great, and tidal
friction has, in consequence, played a predominant part in
bringing their mutual relations into their present state.
The comparatively late birth of the moon tends to ratify
this inference. The dimensions of the earth did not differ
(according to Mr. Darwin) very greatly from what they now
are when her solitary offspring came, somehow, into exist-
ence. This is found not to have been the case with any other
of the planets. It is unlikely that the satellites of Jupiter,
Saturn, or Mars (we may safely add, of Uranus or Neptune)
ever revolved in much narrower orbits than those they now
traverse ; it is practically certain that they did not originate
very near the present surfaces of their primaries, like our
moon. 1 What follows ? The tide-raising power of a body
grows with vicinity in a rapidly accelerated ratio. Lunar
tides must then have been on an enormous scale when the
moon swung round at a fraction of its actual distance from the
earth. But no other satellite with which we are acquainted
occupied at any time a corresponding position. Hence no
other satellite ever possessed tide-raising capabilities in the
least comparable to those of the moon. We conclude once
more that tidal friction had an influence here quite different
from its influence elsewhere.
There is, however, another branch of the same subject.
We know that the sun as well as the moon causes tides in
our oceans. There must then be solar, no less than lunar
tidal friction. The question at once arises : What part has it
played in the development of the solar system ? Has it ever
been one of leading importance, or has its influence always
been, as it now is, subordinate, almost negligeable ? To this,
too, Mr. Darwin supplies an answer.
It can be stated without hesitation that the sun did not
give birth to the planets, as the earth has been supposed to
have given birth to the moon, by the disruption of its already
condensed, though plastic and glowing mass, pushing them
then gradually backward from its surface into their present
1 Phil. Trans., vol. clxxii. p. 530.
CHAP. ix. PLANETARY EVOLUTION. 365
places. For the utmost possible increase in the length of the
year through tidal friction is one hour ; and five minutes is a
more probable estimate. 1 So far as the pull of tide- waves
raised on the sun by the planets is concerned, then, the dis-
tances of the latter have never been notably different from
what they now are ; though that cause may have converted
the paths traversed by them from circles into ellipses.
Over their physical history, however, it was probably in a
large measure influential. The first vital issue for each of
them was satellites or no satellites ? Were they to be gover-
nors as well as governed, or should they revolve in sterile
isolation throughout the seoris of their future existence ?
Here there is strong reason to believe that solar tidal fric-
tion was the overruling power. It is remarkable that plane-
tary fecundity increases at least so far outward as Saturn
with distance from the sun. Can these two facts be in
any way related ? In other words, is there any conceivable
way by which tidal influence could prevent or impede the
throwing-off of secondary bodies ] We have only to think for
a moment in order to see that this is precisely one of its direct
results. 2
Tidal friction, whether solar or lunar, tends to reduce the
axial movement of the body it acts upon. But the separation
of satellites depends according to the received view upon
the attainment of a disruptive rate of rotation. Hence, if
solar tidal friction were strong enough to keep down the pace
below this critical point, the contracting mass would remain
intact there would be no satellite-production. This, in all
probability, actually occurred in the case both of Mercury
and Venus. They cooled without dividing, because the solar
friction-brake applied to them was too strong to permit
acceleration to pass the limit of equilibrium. The earth
barely escaped the same fate of loneliness. Her first and
only epoch of instability was retarded until she had nearly
reached maturity. The late appearance of the moon accounts
1 Phil. Trans., vol. clxxii. p. 533. 2 This was perceived by M. d.
Roche in 1872. Mem. de VAcad. des Sciences de Montpellier, t. viii. p. 247.
366 HISTORY OF ASTRONOMY. PART n.
for its large relative size through the increased cohesion of
an already strongly condensed parent mass and for the dis-
tinctive peculiarities of its history and influence on the pro-
ducing globe.
Solar tidal friction, although it did not hinder the forma-
tion of two minute dependants of Mars, has been invoked
to explain the anomalously rapid revolution of one of them.
Phobos, we have seen, completes more than three revolutions
while Mars rotates once. But this was probably not always
so. The two periods were originally nearly equal. The dif-
ference, it is alleged, was brought about by tidal waves raised
on the viscous spheroid of Mars by the sun. Rotatory velocity
was thereby destroyed, the Martian day slowly lengthened, and,
as a secondary consequence, the period of the inner satellite,
become shorter than the augmented day, began progressively
to diminish. So that Phobos, unlike our moon, was in the
beginning farther from its primary than now.
But here again Mr. Nolan enters a caveat. Applying the
simple test of numerical evaluation, he shows that before solar
tidal friction can lengthen the rotation-period of Mars by so
much as one minute, Phobos must already have been precipitated
upon its surface. 1 For the enormous disparity of mass between
it and the sun is so far neutralised by the enormous disparity
in their respective distances from Mars, that solar tidal force
there is only fifty times that of the little satellite. But the
tidal effects of a satellite circulating quicker than its primary
rotates, exactly reverse those of one moving, like our moon,
comparatively slowly, so that the tides raised by Phobos tend
to shorten both periods. Its orbital momentum, however, is
so extremely small in proportion to the rotational momentum
of Mars, that any perceptible inroad upon the latter is attended
by a lavish and ruinous expenditure of the former. It is as
if a man owning a single five-pound note were to play for equal
stakes with a man possessing a million. The bankruptcy sure
to ensue is typified by the coming fate of the Martian inner
satellite. The catastrophe of its fall needs to bring it about
1 Nature, vol. xxxiv. p. 287.
CHAP. ix. PLANETARY EVOLUTION. 367
only a very feeble reactive pull compared with the friction
which should be applied in order to protract the Martian day
by one minute. And from the proportionate strength of the
forces at work, it is quite certain that one result cannot take
place without the other. Nor can things have been materially
different in the past ; hence the idea must be abandoned that
the primitive time of rotation of Mars survives in the period
of its inner satellite.
The anomalous shortness of the latter may, however, in M.
Wolf's opinion, 1 be explained by the " trainees elliptiques "
with which Roche supplemented nebular annulation. 2 These
are traced back to the descent of separating strata from the
shoulders of the great nebulous spheroid towards its equatorial
plane. Their rotational velocity being thus relatively small,
they formed "inner rings," very much nearer to the centre of
condensation than would have been possible on the unmodified
theory of Laplace. Phobos might, in this view, be called a
polar offset of Mars ; and the rings of Saturn are thought to
own a similar origin.
Outside the orbit of Mars, solar tidal friction can scarcely
be said at present to possess any sensible power. But it is
far from certain that this was always so. It seems not un-
likely that its influence was the overruling one in determining
the direction of planetary rotation. M. Faye, as we have seen,
objected to Laplace's scheme that only retrograde secondary
systems could be produced by it. In this he was anticipated
by Kirkwood, who, however, supplied an answer to his own
objection. 3
Sun-raised tides must have acted with great power on the
diffused masses of the embryo planets. By their means they
doubtless very soon came to turn (in lunar fashion) the same
hemisphere always towards their centre of motion. This
amounts to saying that even if they started with retrograde
rotation, it was, by solar tidal friction, quickly rendered direct.
For it is scarcely necessary to point out that a planet turning
1 Butt. Astr., t. ii. p. 223. z Montpellier Mems., t. viii. p. 242.
3 Anc. Jour., vol. xxxviii. (1864), p. I.
3 68 HISTORY OF ASTRONOMY. PART n.
an invariable face to the sun, rotates in the same direction in
which it revolves, and in the same period. As, with the pro-
gress of condensation, tides became feebler, and rotation more
rapid, the accelerated spinning necessarily proceeded in the
sense thus prescribed for it. Hence the backward axial move-
ments of Uranus and Neptune may very well be a' survival,
due to the inefficiency of solar tides at their great distance, of
a state of things originally prevailing universally throughout
the system.
The general outcome of Mr. Darwin's researches has been to
leave Laplace's cosmogony untouched. He concludes nothing
against it, and, what perhaps tells with more weight in the
long run, has nothing to substitute for it. In one form or the
other, if we speculate at all on the development of the planetary
system, our speculations are driven into conformity with the
broad lines of the Nebular Hypothesis so far, at least, as
admitting an original material unity and motive uniformity.
But we can see now, better than formerly, that these supply a
bare and imperfect sketch of the truth. We should err gravely
were we to suppose it possible to reconstruct, with the help of
any knowledge our race is ever likely to possess, the real and
complete history of our admirable system. " The subtlety of
nature," Bacon says, "transcends in many ways the subtlety
of the intellect and senses of man. " By no mere barren formula
of evolution, indiscriminately applied all round, the results we
marvel at, and by a fragment of which our life is conditioned,
were brought forth, but by the manifold play of interacting
forces, variously modified and variously prevailing, according
to the local requirements of the design they were appointed to
execute.
CHAPTER X.
RECENT COMETS.
ON the 2d of June 1858, Giambattista Donati discovered at
Florence a feeble round nebulosity in the constellation Leo,
about one-tenth the diameter of the full moon. It proved to
be a comet approaching the sun. But it changed little in
apparent place or brightness for some weeks. The gradual
development of a central condensation of light was the first
symptom of coming splendour. At Harvard, in the middle of
July, a strong stellar nucleus was seen; on August 14 a tail
began to be thrown out. As the comet wanted still over six
weeks of the time of its perihelion-passage, it was obvious that
great things might be expected of it. They did not fail of
realisation.
Not before the early days of September was it generally
recognised with the naked eye, though it had been detected
without a glass at Pulkowa, August 19. But its growth was
thenceforward a surprisingly rapid one, as it swept with ac-
celerated motion under the hindmost foot of the Great Bear,
and past the starry locks of Berenice. A sudden leap upward
in lustre was noticed on September 12, when the nucleus
shone with about the brightness of the pole-star, and the tail,
notwithstanding large fore-shortening, could be traced with
the lowest telescopic power over six degrees of the sphere.
The appendage, however, attained its full development only
after perihelion, September 30, by which time, too, it lay nearly
square to the line of sight from the earth. On October 10
it stretched in a magnificent scimitar -like curve over a third
and upwards of the visible hemisphere, representing a real
2 A
370 HISTORY OF ASTRONOMY. PART n.
extension in space of fifty-four million miles. But the most
striking view was presented on October 5, when the brilliant
star Arcturus became involved in the brightest part of the tail,
and during many hours contributed, its lustre undiminished
by the interposed nebulous screen, to heighten the grandeur
of the most majestic celestial object of which living memo-
ries retain the impress. Donati's comet was, according to
Admiral Smyth's testimony, 1 outdone " as a mere sight- object "
by the great comet of 1811 ; but what it lacked in splendour,
it surely made up in grace, and variety of what we may call
"scenic" effects.
Some of these were no less interesting to the student than
they were impressive to the spectator. At Pulkowa, on the
1 6th September, Winnecke 2 observed a faint outer envelope
resembling a veil of almost evanescent texture flung some-
what widely over the head. Next evening, the first of the
" secondary" tails appeared, possibly as part of the same
phenomenon. This was a narrow, straight ray, forming a
tangent to the strong curve of the primary tail, and reaching
to a still greater distance from the nucleus. It continued
faintly visible for about three weeks, during part of which time
it was seen in duplicate. For from the chief train itself, at a
point where its curvature abruptly changed, issued, as if through
the rejection of part of its materials, a second beam nearly
parallel to the first, the rigid line of which contrasted singularly
with the softly diffused, and waving aspect of the plume of
light from which it sprang. Olbers's theory of unequal repul-
sive forces was never more beautifully illustrated. The triple
tail was a visible solar analysis of cometary matter.
The processes of luminous emanation going on in this body
forcibly recalled the observations made on the comets of 1744
and 1835. From the middle of September, the nucleus, esti-
mated by^ Bond to be'under five hundred miles in diameter,
was the centre of action of the most energetic kind. Seven
distinct "envelopes" were detached in succession from the
nebulosity surrounding the head, and after rising towards the
1 Month. Not., vol. xix. p. 27. 2 Mem. de VAc. Imp., t. ii. 1859, p. 46.
CHAP. x. RECENT COMETS. 371
sun during periods of from four to seven days, finally cast their
material backward to form the right and left branches of the
great train. The separation of these by an obscure axis ap-
parently as black, quite close up to the nucleus, as the sky
indicated for the tail a hollow, cone-like structure ; l while the
repetition of certain spots and rays in the same corresponding
situation on one envelope after another, served to show that the
mucleus to some local peculiarity of which they were doubt-
less due had no proper rotation, but merely shifted sufficiently
on an axis to preserve the same aspect towards the sun as it
moved round it. 2 This observation of Bond's was strongly
confirmatory of BesseFs hypothesis of opposite polarities in
such bodies' opposite sides.
The protrusion towards the sun, on September 25, of a
brilliant luminous, fan-shaped sector completed the resem-
blance to Halley's comet. The appearance of the head was
now somewhat that of a " bat's-wing " gaslight. There were,
however, no oscillations to and- fro, such as Bessel had seen
and speculated upon in 1835. As the s i ze f the nucleus con-
tracted with approach to perihelion, its intensity augmented.
On October 2, it outshone Arcturus, and for a week or ten
days was a conspicuous object half an hour after sunset. Its
lustre setting aside the light emitted from the tail was, at
that date, 6300 times what it had been in June 15, though
theoretically taking into account, that is, only the differences
of distance from sun and earth it should have been only ^
of that amount. Here, it might be thought, was convincing
evidence of the comet itself becoming ignited under the
growing intensity of the solar radiations. Experiments with
the polariscope were, however, interpreted in an adverse sense,
and Bond's conclusion that the comet sent us virtually un-
mixed reflected sunshine was generally acquiesced in. It did
not, nevertheless, survive the first application of the spectro-
scope to these bodies.
Very few comets have been so well or so long observed as
Donati's. It was visible to the naked eye during 112 days ;
1 Harvard Annals, vol. iii. p. 368. 2 Ibid., p. 371.
372 HISTORY OF ASTRONOMY. PART n.
it was telescopically discernible for 275, the last observation
having been made by Dr. Mann at the Cape of Good Hope,
March 4, 1859. Its course through the heavens combined
singularly with the orbital place of the earth to favour curious
inspection. The tail, when near its greatest development,
lost next to nothing by the effects of perspective, and at the
same time lay in a plane sufficiently inclined to the line of
sight to enable it to display its exquisite curves to the great-
est advantage. Even the weather was, on both sides of the
Atlantic, propitious during the period of greatest interest, and
the moon as little troublesome as possible. The splendid
volume compiled by the younger Bond is a monument to the
care and skill with which these advantages were turned to
account. Yet this stately apparition marked no turning-point
in the history of cometary science. By its study knowledge
was indeed materially advanced, but along the old lines.
]STo quick and vivid illumination broke upon its path. Quite
insignificant objects as we have already partly seen have
often proved more vitally instructive.
Donati's comet has been identified with no other. Its path
is an immensely elongated ellipse, lying in a plane far apart
from that of the planetary movements, carrying it at perihelion
considerably within the orbit of Venus, and at aphelion out into
space to 5 J times the distance from the sun of Neptune. The
entire circuit occupies over 2000 years, and is performed
in a retrograde direction, or against the order of the Signs.
Before its next return, about the year 4000 A.D., the enigma
of its presence and its purpose may have been to some extent
though we may be sure not completely penetrated.
On June 30, 1861, the earth passed, for the second time in
this century, through the tail of a great comet. Many of our
readers must remember the unexpected disclosure, on the
withdrawal of the sun below the horizon on that evening, of an
object so remarkable as to challenge universal attention. A
golden-yellow planetary disc, wrapt in dense nebulosity, shone
out while the June twilight of these latitudes was still in its first
strength. The number and complexity of the envelopes sur-
CHAP. x. RECENT COMETS. 373
rounding the head produced, according to the late Mr. Webb, 1
a magnificent effect. Portions of six distinct emanations were
traceable. " It was as though a number of light, hazy clouds
were floating round a miniature full moon." As the light faded,
the tail came out. 2 Although in brightness and sharpness of
definition it could not compete with the display of 1858, its
dimensions proved to be extraordinary. It reached upwards
beyond the zenith when the head had already set. By some
authorities its extreme length was stated at 1 18, and it showed
no trace of curvature. Most remarkable, however, was the
appearance of two widely divergent rays, each pointing towards
the head, though cut off from it by sky-illumination, of which
one was seen by Mr. Webb, and both by Mr. Williams at
Liverpool, a quarter of an hour before midnight. There seems
no doubt that Mr. Webb's interpretation was the true one,
and that these beams were, in fact, "the perspective repre-
sentation of a conical or cylindrical tail, hanging closely above
our heads, and probably just being lifted up out of our atmos-
phere." : The cometary train was then rapidly receding from
the earth, so that the sides of the " outspread fan " of light
which it showed when we were right in the line of its axis,
must have appeared (as they did) to close up in departure.
The swiftness with which the visually opened fan shut, proved
its vicinity; and indeed Mr. Hind's calculations showed that
we were not so much near, as actually within its folds at that
very time.
Already M. Liais, from his observations at Rio de Janeiro,
June ii to 14, had anticipated, as a probability, such an
encounter, and had subsequently proved that it must have
occurred in such a way as to cause an immersion of the earth
in cometary matter to a depth of 300,000 miles. 4 The comet
then lay between the earth and the sun at a distance of about
fourteen million miles from the former ; its tail stretched out-
ward just along the line of intersection of its own with the
terrestrial orbit to an extent of fifteen million miles ; so that
1 Month. Not, vol. xxii. p. 306. 2 Stothard in Ibid., vol. xxi. p. 243.
3 Intell Observer, vol. i. p. 65. 4 Comptes Rendus, t. Ixi. p. 953.
374 HISTORY OF ASTRONOMY. PART n.
our globe, happening to pass at the time, found itself during
some hours involved in the flimsy appendage.
No perceptible effects were produced by the meeting ; it was
known to have occurred by theory alone. A peculiar glare in
the sky, thought by some to have distinguished the evening
of June 30, was, at best, inconspicuous. Nor were there any
symptoms of unusual electric excitement. The Greenwich
instruments were, indeed, disturbed on the following night ;
but it would be rash to infer that the comet had art or part in
their agitation.
The perihelion-passage of this body occurred June u,
1 86 1 ; and its orbit has been shown by M. Kreutz of Bonn,
from a very complete investigation founded on observations
extending over nearly a year, to be an ellipse traversed in a
period of 49j years. 1
Towards the end of August 1862, a comet became visible
to the naked eye high up in the northern hemisphere, with a
nucleus equalling in brightness the lesser stars of the Plough
and a feeble tail 20 in length. It thus occupied quite a
secondary position among the members of its class. It was,
nevertheless, a splendid object in comparison with a telescopic
nebulosity discovered by Tempel at Marseilles, December 19,
1865. This, the sole comet of 1866, slipped past perihelion,
January n, without pomp of train or other appendages, and
might have seemed hardly worth the trouble of pursuing.
Fortunately, however, this was not the view entertained by
observers and computers ; since upon the knowledge acquired
of the movements of these two bodies has been founded one
of the most significant discoveries of modern times. The first
of them is now styled the comet (1862 iii.) of the August
meteors, the second (1866 i.) that of the November meteors.
The steps by which this curious connection came to be
ascertained were many, and were taken in succession by a
number of individuals. But the final result was reached by
Schiaparelli of Milan, and remains deservedly associated with
his name.
1 Smiths. Report, 1881 (Holden).
CHAP. x. RECENT COMETS. 375
The idea prevalent in the last century as to the nature of
shooting stars was that they were mere aerial ignes fatui in-
flammable vapours accidentally kindled in our atmosphere.
But Halley had already entertained the opinion of their
cosmical origin; and Chladni in 1794 formally broached the
theory that space is filled with minute circulating atoms, which,
drawn by the earth's attraction, and ignited by friction in its
gaseous envelope, produce the luminous effects so frequently
witnessed. 1 Acting on his suggestion, Brandes and Benzenberg,
two students at the University of Got tin gen, began in 1798 to
determine the heights of falling stars by simultaneous obser-
vations at a distance. They soon found that they move with
planetary velocities in the most elevated regions of our atmos-
phere, and by the ascertainment of this fact laid a foundation
of distinct knowledge regarding them. Some of the data
collected, however, served only to perplex opinion, and even
caused Chladni temporarily to renounce his. Many high
authorities, headed by Laplace in 1802, declared for the lunar-
volcanic origin of meteorites ; but thought on the subject was
turbid, and inquiry seemed only to stir up the mud of ignor-
ance. It needed one of those amazing spectacles, at which
man assists, no longer in abject terror for his own frail for-
tunes, but with keen curiosity and the vivid expectation of
new knowledge, to bring about a clarification.
On the night of November 12-13, I ^33> a tempest of falling
stars broke over the earth. North America bore the brunt
of its pelting. From the Gulf of Mexico to Halifax, until
daylight with some difficulty put an end to the display, the
sky was scored in every direction with shining tracks and
illuminated with majestic fireballs. At Boston, the frequency
of meteors was estimated to be about half that of flakes of
snow in an average snowstorm. Their numbers, while the
first fury of their coming lasted, were quite beyond counting ;
but as it waned, a reckoning was attempted, from which it was
computed, on the basis of that much-diminished rate, that
1 Ueber den Ursprung der von Pallas gefundenen Eisenmassen, p. 24.
376 HISTORY OF ASTRONOMY. PART n.
240,000 must have been visible during the nine hours they
continued to fall. 1
Now there was one very remarkable feature common to the
innumerable small bodies which traversed, or were consumed
in our atmosphere that night. They all seemed to come from
the same part of the sky. Traced backward, their paths were
invariably found to converge to a point in the constellation
Leo. Moreover, that point travelled with the stars in their
nightly round. In other words, it was entirely independent of
the earth and its rotation. It was a point in inter-planetary
space.
The effective perception of this fact 2 amounted to a discovery,
as Olmsted and Twining, who had " simultaneous ideas " on
the subject, were the first to realise. Denison Olmsted was
then professor of mathematics in Yale College. He showed
early in i834 3 that the emanation of the showering meteors
from a fixed "radiant" proved their approach to the earth
along nearly parallel lines, appearing to diverge by an effect of
perspective ; and that those parallel lines must be sections of
orbits described by them round the sun and intersecting that
of the earth. For the November phenomenon was now seen
to be a periodical one. On the same night of the year 1832,
although with less dazzling and universal splendour than in
America in 1833, it had been witnessed over great part of
Europe and in Arabia. Olmsted accordingly assigned to the
cloud of cosmical particles (or " comet," as he chose to call
it), by terrestrial encounters with which he supposed the ap-
pearances in question to be produced, a period of about 182
days ; its path a narrow ellipse, meeting, near its farthest end
from the sun, the place occupied by the earth on November 12.
Once for all, then, as the result of the star-fall of 1833, the
study of luminous meteors became an integral part of astro-
nomy. Their membership of the solar system was no longer
1 Arago, Annuaire, 1836, p. 294. z Humboldt had noticed the emana-
tion of the shooting stars of 1799 from a single point, or "radiant," as
Greg long afterwards termed it ; but no reasoning was founded on the
observation. 2 Am. Jour, of Sc., vol. xxvi. p. 132.
CHAP. x. RECENT COMETS. 377
a theory or a conjecture it was an established fact. The
discovery might be compared to, if it did not transcend in
importance, that of the asteroidal group. " C'est un nouveau
monde planetaire," Arago wrote, 1 "qui commence a se reveler
& nous."
Evidences of periodicity continued to accumulate. It was
remembered that Humboldt and Bonpland had been the spec-
tators at Cumana, after midnight of November 12, 1799, of
a fiery shower little inferior to that of 1833, and reported
to have been visible from the equator to Greenland. More-
over, in 1834 and some subsequent years, there were waning
repetitions of the display, as if through the gradual thinning-
out of the meteoric supply. The extreme irregularity of its
distribution was noted by Olbers in 1837, who conjectured
that we might have to wait until 1867 to see the phenomenon
renewed on its former scale of magnificence. 2 This was the
first hint of a thirty-three or thirty-four year period.
The falling stars of November did not alone attract the
attention of the learned. Similar appearances were tradition-
ally associated with August 10 by the popular phrase in which
they figured as " the tears of St. Lawrence." But the asso-
ciation could not be taken on trust from mediaeval authority.
It had to be proved scientifically, and this Quetelet of Brussels
succeeded in doing in December i836. 3
A second meteoric revolving system was thus shown to
exist. But its establishment was at once perceived to be fatal
to the " cosmical cloud " hypothesis of Olmsted. For if it be
a violation of probability to attribute to one such agglomera-
tion a period of an exact year, or sub-multiple of a year, it
would be plainly absurd to suppose the movements of two
or more regulated by such highly artificial conditions. An
alternative was proposed by Adolf Erman of Berlin in 1839.*
No longer in clouds, but in closed rings, he supposed meteoric
matter to revolve round the sun. Thus the mere circumstance
of intersection by a meteoric, of the terrestrial orbit, without
1 Annuaire, 1836, p. 297. 2 Ann. de l'0lsen>., Bruxelles, 1839, p. 248.
3 Ibid., 1837, p. 272. 4 Astr. Nach., Nos. 385, 390.
378 HISTORY OF ASTRONOMY. PART n.
any coincidence of period, would account for the earth meeting
some members of the system at each annual passage through
the " node " or point of intersection. This was an important
step in advance, yet it decided nothing as to the forms of the
orbits of such annular assemblages ; nor was it followed up in
any direction for a quarter of a century.
Professor Hubert A. Newton, of Yale College, took up,
however, the dropped thread of inquiry in 1864. 1 A search
through old records carried the November phenomenon back
to the year 902 A.D., long distinguished as " the year of the
stars." For in the same night in which Taormina was cap-
tured by the Saracens, and the cruel Aghlabite tyrant Ibrahim
ibn Ahmed died " by the judgment of God " before Cosenza,
stars fell from heaven in such abundance as to amaze and
terrify beholders far and near. This was on October 13, and
recurrences were traced down through the subsequent centu-
ries, always with a day's delay in about seventy years. It
was easy, too, to derive from the dates a cycle of 33 J years, so
that Professor Newton did not hesitate to predict the exhibi-
tion of an unusually striking meteoric spectacle on November
13-14, i866. 2
For the astronomical explanation of the phenomena, recourse
was had to a method introduced by Erman of computing
meteoric orbits. It was found, however, that conspicuous
recurrences every thirty-three or thirty-four years could be
explained on the supposition of five widely different periods,
combined with varying degrees of extension in the revolving
group. Professor Newton himself gave the preference to the
shortest of the five of 354^ days, but indicated the means
of deciding with certainty upon the true one. It was fur-
nished by the advancing motion of the node, or that day's
delay of the November shower every seventy years, which the
old chronicles had supplied data for detecting. For this is
a strictly measurable effect of gravitational disturbance by
the various planets, the amount of which naturally depends
1 Am. Jour, of Sc., vol. xxxvii. (2dser.), p. 377.
z Ibid., vol. xxxviii., p. 61.
CHAP. x. RECENT COMETS. 379
upon the course pursued by the disturbed bodies. Here
the great mathematical resources of Professor Adams were
brought to bear. By laborious processes of calculation, he
ascertained that four out of Newton's five possible periods
were entirely incompatible with the observed nodal displace-
ment, while for the fifth that of 33 J years a perfectly har-
monious result was obtained. 1 This was the last link in the
chain of evidence proving that the November meteors or
"Leonids," as they had by that time come to be called revolve
round the sun in a period of 33*27 years, in an ellipse span-
ning the vast gulf between the orbits of the earth and Uranus,
the group being so extended as to occupy six or eight years in
defiling past the scene of terrestrial encounters. But before it
was completed in March 1867 the subject had assumed a new
aspect and importance.
Professor Newton's prediction of a remarkable star-shower
in November 1866 was punctually fulfilled. This time, Europe
served as the main target of the celestial projectiles, and
observers were numerous and forewarned. The display,
although, according to Mr. BaxendelPs memory, 2 inferior to
that of 1833, was of extraordinary impressiveness. Dense
crowds of meteors, equal in lustre to the brightest stars, and
some rivalling Yenus at her best, 3 darted from east to west
across the sky with enormous apparent velocities, and with a
certain determinateness of aim, as if let fly with a purpose,
and at some definite object. 4 Nearly all left behind them
trains of emerald green or clear blue light, which occasionally
lasted many minutes, before they shrivelled, and curled up
out of sight. The maximum rush occurred a little after one
o'clock on the morning of November 14, when attempts to
count were overpowered by frequency. But during a previous
interval of seven minutes five seconds, four observers at Mr.
Bishop's observatory at Twickenham reckoned 514, and during
an hour ii2o. 5 Before daylight the earth had fairly cut her
1 Month. Not., vol. xxvii. p 247. 8 Am. Jour, of Sc. vol. xliii. (2d
ser.), p. 87. 3 Grant, Month. Not., vol. xxvii. p. 29. 4 P. Smyth,
Ibid., p. 256. 5 Hind, Ibid., p. 49.
380 HISTORY OF ASTRONOMY. PART 11.
way through the star-bearing stratum; the "ethereal rockets"
had ceased to fly.
This event brought the subject of shooting stars once more
vividly to the notice of astronomers. Schiaparelli had, indeed,
been already attracted by it. The results of his studies were
made known in four remarkable letters, addressed, before the
close of the year 1866, to Father Secchi, aid published in the
Bullettino of the Roman Observatory. 1 Their upshot was to
show, in the first place, that meteors possess a real velocity
considerably greater than that of the earth, and travel, accord-
ingly, to enormously greater distances from the sun along
tracks resembling those of comets in being very eccentric, in
lying at all levels indifferently, and in being pursued in either
direction. It was next inferred that comets and meteors
equally have an origin foreign to the solar system, but are
drawn into it temporarily by the sun's attraction, and occa-
sionally fixed in it by the backward pull of some planet. But
the crowning fact was reserved for the last. It was the
astonishing one that the August meteors move in the same
orbit with the bright comet of 1862 that the comet, in fact,
is but a larger member of the family of Perseids (so named
because their radiant point is situated in the constellation
Perseus).
This discovery was quickly capped by others of the same
kind. Leverrier published, January 21, 1867, 2 elements for
the November swarm, founded on the most recent and authen-
tic observations; at once identified by Dr. C. F. W. Peters
of Altona, with Oppolzer's elements for Tempel's comet of
i866. 3 A few days later, Schiaparelli, having re-calculated
the orbit of the meteors from improved data, arrived at the
same conclusion; while Professor Weiss of Vienna pointed
to the agreement between the orbits of a comet which had
appeared in 1861 and of a star- shower found to recur on April
20 (LyraTds), as well as between those of Biela's comet and
certain conspicuous meteors of November 28. 4
1 Reproduced in Les Mondes, t. xiii. 2 Comptes Rendus, t. Ixiv. p. 96.
3 Astr. Nach., No. 1626. 4 Ibid., No. 1632.
CHAP. x. RECENT COMETS. 381
These instances do not seem to be exceptional. The num-
ber of known or suspected accordances of cometary tracks
with meteor streams contained in a list drawn up in 1878 l by
Professor Alexander S. Herschel (who has made the subject
peculiarly his own), amounts to seventy-six ; although the four
first detected still remain the most conspicuous, and perhaps
the sole absolutely sure examples of a relation as significant
as it was, to most astronomers, unexpected.
There had, indeed, been anticipatory ideas. Not that
Kepler's comparison of shooting stars to "minute comets,"
or Maskelyne's "forse risultera che essi sono comete," in a
letter to the Abate Cesaris, December 12, 1783,2 need count
for much. But Chladni, in i8i9, 3 considered both to be frag-
ments or particles of the same primitive matter, irregularly
dispersed through space as nebulae ; and Morstadt of Prague
suggested about 1837 4 that the November meteors might be
dispersed atoms from the tail of Biela's comet, the path of
which is cut across by the earth near that epoch. Professor
Kirk wood, however, by a luminous intuition, penetrated the
whole secret, so far as it has yet been made known. In an
article published, or rather buried, in the Danville Quarterly
Review for December 1861, he argued from the observed
division of Biela, and other less noted instances of the same
kind, that the sun exercises a " divellent influence " on the
nuclei of comets, which may be presumed to continue its
action until their corporate existence (so to speak) ends in
complete pulverisation. " May not," he continued, " our
periodic meteors be the de"bris of ancient, but now disinte-
grated comets, whose matter has become distributed round
*heir orbits 1 " 5
The gist of Schiaparellf s discovery could not be more clearly
conveyed. For it must be borne in mind that with the ulti-
mate destiny of comets' tails this had nothing to do. The
tenuous matter composing them is, no doubt, permanently
1 Month. Not., vol. xxxviii. p. 369. 2 Schiaparelli, Le Stelle Cadenti,
p. 54. 3 Ueber Feuer- Meteor e, p. 406. 4 Astr. Nach., No. 347 (Madler);
see also Boguslawski, Die Kometen, p. 98, 1857. 5 Nature, vol. vi. p. 148.
382 HISTORY OF ASTRONOMY. PART ir.
lost to the body from which it emanated ; but science does
not pretend to track its further wanderings through space.
It can, however, state categorically that these will no longer
be conducted along the path forsaken under solar compulsion.
From the central, and probably solid parts of comets, on the
other hand, are derived the granules by the swift passage of
which our skies are seamed with periodic fires. It is certain
that a loosely agglomerated mass (such as there is every
reason to believe cometary nuclei to be) must gradually
separate through the unequal action of gravity on its various
parts through, in short, solar tidal influence. Thenceforward
its fragments will revolve independently in parallel orbits, at
first as a swarm, finally when time has been given for the
full effects of the lagging of the slower moving particles to
develop as a closed ring. The first condition is still, more
or less, that of the November meteors ; those of August have
already arrived at the second. For this reason, Leverrier
pronounced, in 1867, the Perseid to be of older formation
than the Leonid system. He even assigned a date at which
the introduction of the last-named bodies into their present
orbit was probably effected through the influence of Uranus. 1
In 126 A.D. a close approach must have taken place between
the planet and the parent comet of the November stars, after
which its regular returns to perihelion, and the consequent
process of its disintegration, set in. Though not complete, it
is already far advanced.
The view that meteorites are the dust of decaying comets
was now to be put to a definite test of prediction. Biela's
comet had not been seen since its duplicate return in 1852.
Yet it had been carefully watched for with the best telescopes ;
its path was accurately known ; every perturbation it could
suffer was scrupulously taken into account. Under these cir-
1 Mr. Proctor's recent inquiries have shown that the effect of no single
planetary encounter can suffice (as the " capture theory " of comets re-
quires that it should) to compel a body approaching the sun from an
indefinite distance to revolve thenceforth in an orbit having its aphelion
near the meeting-place. Several successive encounters, ^however, may
have done the work.
CHAP. x. RECENT COMETS. 383
cumstances, its repeated failure to come up to time might fairly
be thought to imply a cessation from visible existence. Might
it not, however, be possible that it would appear under another
form that a star-shower might have sprung from and would
commemorate its dissolution ?
An unusually large number of falling stars was seen by
Brandes, December 6, 1798. Similar displays were noticed
in the years 1830, 1838, and 1847, and the point from which
they emanated was shown by Heis at Aix-la-Chapelle to be
situated near the bright star y Andromedae. 1 Now this is
precisely the direction in which the orbit of Biela's comet
would seem to lie, as it runs down to cut the terrestrial track
very near the place of the earth at the above dates. The in-
ference was then an easy one that the meteors were pursuing
the same path with the comet ; and it was separately arrived
at, early in 1867, by Weiss, D'Arrest, and Galle. 2 But Biela
travels in the opposite direction to Tempel's comet and its
attendant " Leonids j " its motion is direct, or from west to
east, while theirs is retrograde. Consequently, the motion of
its node is in the opposite direction too. In other words, the
meeting-place of its orbit with that of the earth retreats (and
very rapidly) along the ecliptic instead of advancing. So that
if the " Andromedes " stood in the supposed intimate relation
to Biela's comet, they might be expected to anticipate the
times of their recurrence by as much as a week (or there-
abouts) in half a century. All doubt as to the fact may be
said to have been removed by Signer Zezioli's observation of
the annual shower in more than usual abundance at Bergamo,
November 30, 1867.
The missing comet was next due at perihelion in the year
1872, and the probability was contemplated by both Weiss and
Galle of its being replaced by a somewhat dense drift of falling
stars. The precise date of the occurrence was not easily de-
terminable, but Galle thought the chances in favour of No-
vember 28. The event anticipated the prediction by twenty -
1 A, S. Herschel, Month. Not., vol. xxxii. p. 355. 2 Astr. Nach., Nos.
1632, 1633, 1635.
384 HISTORY OF ASTRONOMY. PART n.
four hours. Scarcely had the sun set in Western Europe on
November 27, when it became evident that Biela's comet was
shedding over us the pulverised products of its disintegration.
The meteors came in volleys from the foot of the Chained
Lady, their numbers at times baffling the attempt to keep a
reckoning. At Moncalieri, about 8 P.M., they constituted (as
Father Denza said 1 ) a "real rain of fire." Four observers
counted, on an average, four hundred each minute and a half ;
and not a few fireballs, equalling the moon in diameter, traversed
the sky. On the whole, however, the stars of 1872, though
about equally numerous, were less brilliant than those of 1866 ;
the phosphorescent tracks marking their passage were com-
paratively evanescent and their movements sluggish. This is
easily understood when we remember that the Andromedes
overtake the earth, while the Leonids rush to meet it ; the
velocity of encounter for the first class of bodies being under
twelve, for the second above forty-four miles a second. The
spectacle was, nevertheless, magnificent. It presented itself
successively to various parts of the earth, from Bombay and
the Mauritius to New Brunswick and Venezuela, and was
most diligently and extensively observed. Here it had well-
nigh terminated by midnight. 2
It was attended by a slight aurora, and although Tacchini
had telegraphed that the state of the sun rendered some show
of polar lights probable, it has too often figured as an accom-
paniment of star-showers to permit the coincidence to rank
as fortuitous. Admiral Wrangel was accustomed to describe
how, during the prevalence of an aurora on the Siberian coast,
the passage of a meteor never failed to extend the luminosity
to parts of the sky previously dark ; 3 and the power of excit-
ing electrical disturbance seems to belong to all such flitting
cosmical atoms.
A singular incident connected with the meteors of 1872 has
now to be recounted. The late Professor Klinkerfues, who
had observed them very completely at Gottingen, was led to
1 Nature, vol. vii. p. 122. 2 A. S. Herschel, Report Brit. Ass., 1873,
p. 390. 3 Humboldt, Cosmos, vol. i. p. 114 (Otte's trans.)
CHAP. x. RECENT COMETS. 385
believe that not merely the debris strewn along its path, but
the comet itself must have been in the closest proximity to
the earth during their appearance. 1 If so, it might be possible,
he thought, to descry it as it retreated in the diametrically
opposite direction from that in which it had approached. On
November 30, accordingly, he telegraphed to Mr. Pogson, the
Madras astronomer, " Biela touched earth November 27; search
near Theta Centauri" the "anti-radiant," as it is called,
being situated close to that star. Bad weather prohibited
observation during thirty-six hours, but when the rain-clouds
broke on the morning of December 2, there a comet was, just
in the indicated position. In appearance it might have passed
well enough for one of the Biela twins. It had no tail, but a
decided nucleus, and was about 45 seconds across, being thus
altogether below the range of naked -eye discernment. It was
again observed December 3, when a short tail was perceptible ;
but overcast skies supervened, and it has never since been
seen. Its identity accordingly remained in doubt. It seems
tolerably certain, however, that it was not the lost comet, which
ought to have passed that spot twelve weeks earlier, and was
subject to no conceivable disturbance capable of delaying to
that extent its revolution. On the other hand, there is the
strongest likelihood that it belonged to the same system 2
that it was a third fragment, torn from the parent-body of
the Andromedes at a period anterior to our first observations
of it.
In thirteen years, Biela's comet (or its relics) travels nearly
twice round its orbit, so that a renewal of the meteoric show
of 1872 was looked for on the same day of the year 1885, the
probability being emphasised by an admonitory circular from
Dunecht. Astronomers were accordingly on the alert, and
were not disappointed. In England, observation was partially
impeded by clouds ; but at Malta, Palermo, Beyrout, and other
southern stations, the scene was most striking. The meteors
were both larger and more numerous than in 1872. Their
Month. Not., vol. xxxiii. p. 128. 2 Even this was denied by Bruhns,
Astr. NacTi., No. 2054.
2 B
3 86 HISTORY OF ASTRONOMY. PART n.
numbers in the densest part of the drift were estimated by
Professor Newton at 75,000 per hour, visible from one spot to
so large a group of spectators that practically none could be
missed. Yet each of these multitudinous little bodies was
found by him to travel in a clear cubical space of which the
edge measured twenty miles ! l Thus the dazzling effect of a
luminous throng was produced without jostling or confusion,
by particles, it might almost be said, isolated in the void.
Their aspect was strongly characteristic of the Andromede
family of meteors. " They invariably," Mr. Denning wrote, 2
" traversed short paths with very slow motions, and became
extinct in evolved streams of yellowish sparks." The conclu-
sion seemed obvious " that these meteors are formed of very
soft materials, which expand while incalescent, and are imme-
diately crumbled and dissipated into exiguous dust."
The Biela meteors of 1885 did not merely gratify astrono-
mers with a fulfilled prediction, but were the means of com-
municating to them some valuable information. Although
their main body was cut through by the moving earth in six
hours, and was not more than 100,000 miles across, skirmishers
were thrown out to nearly a million miles on either side of the
compact central battalions. Members of the system were, on
the 26th of November, recorded by Mr. Denning at the hourly
rate of about 130; and they did not wholly cease to be visible
until December i. They afforded besides a particularly well-
marked example of that diffuseness of radiation, previously
observed in some less conspicuous displays. Their paths
seemed to diverge from an area, rather than from a point in
the sky. They came so ill to focus, that divergences of
several degrees were found between the most authentically
determined radiants. These incongruities are attributed by
Professor Newton to the irregular shape of the meteoroids,
producing unsymmetrical resistance from the air, and hence
causing them to glance from their original direction on enter-
ing it. Thus their luminous tracks did not always represent
1 Am. Jour., vol. xxxi. p. 425. " Month. Not., vol. xlvi. p. 69.
CHAP. x. RECENT COMETS. 387
(even apart from the effects of the earth's attraction) the true
prolongation of their course through space.
The Andromedes of 1872 were laggards behind the comet
from which they sprang : those of 1885 were its avant-couriers.
That wasted and disrupted body was not due at the node until
January 26, 1886, sixty days, that is, after the earth's encoun-
ter with its meteoric fragments. These are accordingly now
scattered over more than three hundred million miles of its
orbit; yet Professor Newton considers that all must have
formed one compact group with Biela at the time of its close
approach to Jupiter about the middle of 1841. For otherwise,
both comet and meteorites could not have experienced, as they
seem to have done, the same kind and amount of disturbance.
The rapidity of cometary disintegration is thus curiously illus-
trated.
Biela does not offer the only example of cometary disruption.
Setting aside the unauthentic reports of early chroniclers,
we meet the " double comet " discovered by Liais at Olinda
(Brazil), February 27, 1860, of which the division appeared
recent, and about to be carried farther. 1 But a division once
established, separation must continually progress. The periodic
times of the fragments will never be identical ; one must drop
a little behind the other at each revolution, until at length
they come to travel in remote parts of nearly the same orbit.
Thus the comet predicted by Klinkerfues and discovered
by Pogson had already lagged to the extent of twelve weeks,
and we shall meet instances farther on where the retarda-
tion is counted, not by weeks, but by years. Here, original
identity emerges only from calculation and comparison of
orbits.
Comets then die, as Kepler wrote long ago, sicut bombyces
filo fundendo. This certainty, anticipated by Kirkwood in
1 86 1, we have at least acquired from the discovery of their
generative connection with meteors. Nay, their actual mate-
rials become, in smaller or larger proportions, incorporated
with our globe. It is not indeed universally admitted that
1 Month. Not., vol. xx. p. 336.
3 88 HISTORY OF ASTRONOMY. PART n.
the ponderous masses of which, according to Daubree's esti-
mate, 1 at least 600 fall annually from space upon the earth,
ever formed part of the bodies known to us as comets. Some
follow Tscherrnak in attributing to aerolites a totally different
origin from that of periodical shooting-stars. That no clear
line of demarcation can be drawn is no valid reason for assert-
ing that no real distinction exists ; and it is certainly remark-
able that a meteoric fusillade may be kept up for hours without
a single solid projectile reaching its destination. It would
seem as if the celestial army had been supplied with blank
cartridges. There is indeed much probability that the com-
ponents of the recent brilliant showers surpassed only by
exception the size of a canary-seed. Yet, since a few detonat-
ing meteors have been found to proceed from ascertained
radiants of shooting-stars, 2 the probability is small that any
generic difference separates them.
A striking indeed, an almost startling peculiarity, on
the other hand, divides from their congeners a class of meteors
identified by Mr. Denning during ten years' patient watch-
ing of such phenomena at Bristol. 3 These are described as
" meteors with stationary radiants," since for months together
they seem to come from the same fixed points in the sky.
Now this implies quite a portentous velocity. The direction
of meteor-radiants is affected by a kind of aberration, analo-
gous to the aberration of light. It results from a composition
of terrestrial with meteoric motion. Hence, unless that of
the earth in its orbit be by comparison insignificant, the visual
line of encounter must shift, if not perceptibly from day to
day, at any rate conspicuously from month to month. The
fixity, then, of half-a-dozen or more systems observed by Mr.
Denning seems to demand the admission that their members
travel so fast as to throw the earth's movement completely
out of the account. The required velocity would be, by Mr.
Kanyard's calculation, at least 880 miles a second. 4 But the
aspect cf the meteors justifies no such extravagant assump-
1 Revue d. d. Mondes, Dec. 15, 1885, p. 889. 2 Report Brit. A$s., 1880,
p. 40. 3 Month. Not., vol. xlv. p. 93. 4 Observatory, vol. viii. p. 4.
CHAP. x. RECENT COMETS. 389
tion. Their seeming swiftness is very various, and what is
highly significant it is notably less when they pursue than
when they meet the earth. Yet, " incredible and unaccount-
able " 1 though it be, the fact of the existence of these " t long
radiants " appears unquestionable.
The first successful application of the spectroscope to comets
was by Donati in i864. 2 A comet discovered by Tempel,
July 4, brightened until it appeared like a star somewhat
below the second magnitude, with a feeble tail 30 in length.
It was remarkable as having, on August 7, almost totally
eclipsed a small star a very rare occurrence. 3 On August 5
Donati admitted its light through his train of prisms, and
found it, thus analysed, to consist of three bright bands
yellow, green, and blue separated by wider dark intervals.
This implied a good deal. Comets had previously been con-
sidered, as we have seen, to shine mainly, if not wholly, by
reflected sunlight. They were now perceived to be self-
luminous, and to be formed, to a large extent, of glowing
gas. The next step was to determine what kind of gas it
was that was thus glowing in them ; and this was taken by
Dr. Huggins in 1868.*
A comet of subordinate brilliancy, known as comet 1868 ii.,
or sometimes as Winnecke's, was the subject of his experi-
ment. On comparing its spectrum with that of an olefiant-
gas " vacuum tube " rendered luminous by electricity, he
found the agreement exact. It has since been abundantly
confirmed. All the eighteen comets of which the light had
been analysed down to 1880, showed the typical hydro-carbon
spectrum 5 common to the whole group of those compounds,
but probably due immediately to the presence of acetylene.
Some slight apparent anomalies have been almost certainly
caused, not by any real differences of constitution, but by
deficient light-power, rendering observations difficult and in-
1 Denning, Month. Not., vol. xxxviii. p. 114. 2 Astr. Nach., No.
1488. 3 Annuaire, Paris, 1883, p. 185. 4 Phil Trans., vol. clviii. p.
556. 5 Hasselberg. Mem. de VAc. Imp. de St. Petersbourg, t. xxviii. (7th
ser.), No. 2, p. 66.
390 HISTORY OF ASTRONOMY. PART n.
secure. The brighter the comet, the more perfect proved its
conformity to the type.
The earliest comet of first-class lustre to present itself for
spectroscopic examination was that discovered by Coggia at
Marseilles, April 17, 1874. Invisible to the naked eye until
June, it blazed out in July a splendid ornament of our
northern skies, with a just perceptibly curved tail, reaching
more than half-way from the horizon to the zenith, and a
nucleus surpassing in brilliancy the brightest stars in the
Swan. Bredichin, Vogel, and Huggins * were unanimous in
pronouncing its spectrum to be that of marsh, or olefiant gas.
Father Secchi, in the clear sky of Rome, was able to push the
identification even closer than had heretofore been done. The
complete hydro-carbon spectrum consists of five zones of vari-
ously coloured light. Three of these only the three central
ones had till then been obtained from comets ; it was sup-
posed, because their temperature was not high enough to de-
velop the others. The light of Coggia's comet, however, was
found to contain all five, traces of the violet band emerging
June 4, of the red July 2. 2 Presumably, all five would show
universally in cometary spectra, were the dispersed rays strong
enough to enable them to be seen.
The gaseous surroundings of comets are then made up of
a compound of hydrogen with carbon. Other materials are
also present, as will be seen by and by ; but the hydro-carbon
element is probably unfailing and predominant. Its lumino-
sity is, there is little doubt, an effect of electrical excitement.
Zollner showed in 1872 3 that, owing to evaporation and other
changes produced by rapid approach to the sun, electrical
processes of considerable intensity must take place in comets ;
and that their original light is immediately connected with
these, and is only an indirect result of solar radiation, may be
considered a truth permanently acquired to science. 4 They
are not, it thus seems, bodies incandescent through heat, but
1 Proc. Roy. Sbc., vol. xxiii. p. 154. 2 Hasselberg, loc. cit. t p. 58.
s Ueler die Natw\ der Cometen, p. 112. 4 Hasselberg, loc. cit.> p. 38.
CHAP. x. RECENT COMETS. 391
glowing by electricity ; and this is compatible with a relatively
low temperature.
The gaseous spectrum of comets is accompanied, in varying
degrees, by a continuous spectrum. This is usually derived
most strongly from the nucleus, but extends, more or less,
to the nebulous appendages. In part, it is certainly due to
reflected sunlight ; in part, it is likely, to the ignition of minute
solid particles.
( 392 )
CHAPTER XI.
RECENT COMETS (continued).
THE mystery of comets' tails has been to some extent pene-
trated ; so far, at least, that, by making certain assumptions
strongly recommended by the facts of the case, their forms can,
with very approximate precision, be calculated beforehand.
We have, then, the assurance that these extraordinary appen-
dages are composed of no ethereal or super-sensual stuff,
but of matter such as we know it, and subject to the ordinary
laws of motion, though in a state of extreme tenuity. This
is unquestionably one of the most remarkable discoveries of
our time.
Olbers, as already stated, originated in 1812 the view that
the tails of comets are made up of particles subject to a force
of electrical repulsion proceeding from the sun. It was
developed and enforced by Bessel's discussion of the appear-
ances presented by Halley's comet in 18.35. He, moreover,
provided a formula for computing the movement of a particle
under the influence of a repulsive force of any given intensity,
and thus laid firmly the foundation of a mathematical theory
of cometary emanations. Professor W. A. Norton of Yale
College considerably improved this by inquiries begun in
1844, and resumed on the apparition of Donati's comet; and
Dr. C. F. Pape at Altona 1 gave numerical values for the
impulses outward from the sun, which must have actuated
the materials respectively of the curved and straight tails
adorning the same beautiful and surprising object.
1 Astr. Nach., Nos. 1172-74.
CHAP. xi. RECENT COMETS. 393
The physical theory of repulsion, however, was, it might be
said, still in the air. Nor did it assume an aspect of even
moderate plausibility until Zollner took it in hands in iSyi. 1
It is perfectly well ascertained that the energy of the push
or pull produced by electricity depends (other things being
the same) upon the surface of the body acted on; that of
gravity, upon its mass. The efficacy of solar electrical repul-
sion relatively to solar gravitational attraction grows, conse-
quently, as the size of the particle diminishes. Make this
small enough, and it will virtually cease to gravitate, and will
unconditionally obey the impulse to recession.
This principle Zollner was the first to realise in its applica-
tion to comets. It gives the key to their constitution. Admit-
ting (as we seem bound to do) that the sun and they are simi-
larly electrified, their more substantially aggregated parts will
still follow the solicitations of his gravity, while the finely-
divided particles escaping from them will, simply by reason of
their minuteness, fall under the sway of his repellent electric
power. They will, in other words, form "tails." Nor is any
extravagant assumption called for as to the intensity of the
electrical charge concerned in producing these effects. Zollner,
in fact, showed 2 that it need not be higher than that attributed
by the best authorities to the terrestrial surface.
It is now nearly a quarter of a century since M. Bredichin,
director of the Moscow Observatory, directed his attention to
these curious phenomena. His persistent inquiries on the
subject, however, date from the appearance of Coggia's comet
in 1874. On computing the value of the repulsive force
exerted in the formation of its tail, and comparing it with
values of the same force arrived at by him in 1862 for some
other conspicuous comets, it struck him that the numbers
representing them fell into three well-defined classes. "I
suspect," he wrote in 1877, "that comets are divisible into
groups, for each of which the repulsive force is perhaps the
same." 3 This idea was confirmed on fuller investigation. In
1 BericTite Sachs. Ges., 1871, p. 174. 2 Natur der Cometen, p. 124 ;
Astr. Nock., No. 2086. 3 Annales de VOls. de Moscou, t. iii. pt. i. p. 37.
394 HISTORY OF ASTRONOMY. PART n.
1882 the appendages of thirty- six well-observed comets had
been re-constructed theoretically, without a single exception
being met with to the rule of the three types. A further
study of forty comets led, however, in 1885, to a modification
of the numerical results previously arrived at.
In the first of these, the repellent energy of the sun, is four-
teen times as strong as his attractive energy ; l the particles
forming the enormously long, straight rays projected outward
from this kind of comet, leave the nucleus with a mean velocity
of just seven kilometres per second, which, becoming constantly
accelerated, carries them in a few days to the limit of visibility.
The great comets of 1811, 1843, and 1861, that of 1744 (so far
as its principal tail was concerned), and Halley's comet at its
various apparitions, belonged to this class. For the second
type, the value of the repulsive force employed is less narrowly
limited. For the axis of the tail, it exceeds by one tenth ( = i i )
the power of solar gravity ; for the anterior edge, it is more
than twice (2*2), for the posterior only half as strong. The
corresponding initial velocity (for the axis) is 1500 metres a
second, and the resulting appendage a scimitar-like or plumy
tail, such as Donati's and Coggia's comets furnished splendid
examples of. Tails of the third type are constructed with
forces of repulsion from the sun ranging from one-tenth to
three-tenths that of his gravity, producing an accelerated
movement of attenuated matter from the nucleus, beginning
at the leisurely rate of 300 to 600 metres a second. They are
short, strongly bent, brush-like emanations, and in bright
comets seem to be only found in combination with tails of the
higher classes. Multiple tails, indeed that is, tails of different
types emitted simultaneously by one comet are perceived, as
experience advances and observation becomes closer, to be
rather the rule than the exception. 2
Now what is the meaning of these three types? Is any
translation of them into physical fact possible? To this
1 Bull Astr. t t. iii. p. 598. The value of the repellent force for the
comet of 1811 (which offered peculiar facilities for its determination) was
found = 17-5. 2 Faye, Comptes Rendus, t. xciii. p. 13.
CHAP. xi. RECENT COMETS. 395
question Bredichin supplied in 1879 a plausible answer. 1 It
was already a current surmise that multiple tails are composed
of different kinds of matter, differently acted on by the sun.
Both Olbers and Bessel had suggested this explanation of the
straight and curved emanations from the comet of 1807; Nor-
ton had applied it to the faint light-tracks proceeding from
that of Donati ; 2 Winnecke, to the varying deviations of its
more brilliant plumage. Bredichin defined and ratified the
conjecture. He undertook to determine (provisionally as yet)
the several kinds of matter appropriated severally to the three
classes of tails. These he found to be hydrogen for the first,
hydro-carbons for the second, and iron for the third. The
ground of this apportionment is that the atomic weights of
these substances bear to each other the same inverse propor-
tion as the repulsive forces employed in producing the append-
ages they are supposed to form ; and Zollner had pointed out
in 1875 that the " heliof ugal " power by which comets' tails
are developed, would, in fact, be effective just in that ratio. 3
Hydrogen, as the lightest known element that is, the least
under the influence of gravity was naturally selected as that
which yielded most readily to the counter-persuasions of elec-
tricity. Hydro-carbons had been shown by the spectroscope
to be present in comets, and were fitted by their specific weight,
as compared with that of hydrogen, to form tails of the second
type ; while the atoms of iron were just heavy enough to com-
pose those of the third, and, from the plentifulness of their
presence in meteorites, might be presumed to enter, in no in-
considerable proportion, into the mass of comets. These three
substances, however, were by no means supposed to be the sole
constituents of the appendages in question. On the contrary,
the great breadth of what, for the present, were taken to be
characteristically " iron " tails, was attributed to the presence
of many kinds of matter of high, and slightly different specific
weights ; 4 while the expanded plume of Donati was shown to
1 Annettes, t. v. pt. ii. p. 137. 2 Am. Jour, of Sc., vol. xxxii. (2d ser.),
p. 57. 3 Astr. Nach., No. 2082. Bredichin has, however, lately modified
(as we have just seen) his original values for the repulsive force in the
three types. 4 Annales, t. vi. pt. i. p. 60.
396 HISTORY OF ASTRONOMY. PART n.
be, in reality, a whole system of tails, made up of many sub-
stances, each spreading into a separate hollow cone, more or
less deviating from, and partially superposed upon the others.
Never was a theory more promptly or profusely illustrated
than this of Bredichin. Within three years of its promulga-
tion, five bright comets made their appearance, each present-
ing some distinctive peculiarity by which knowledge of these
curious objects was materially helped forward. The first of
these is remembered as the " Great Southern Comet." Tt was
never visible in these latitudes, but made a short, though
stately progress through southern skies. Its earliest detection
was at Cordoba on the last evening of January, 1880; and it
was seen on February i, as a luminous streak, extending just
after sunset from the south-west horizon towards the pole, in
New South Wales, at Monte Video, and the Cape of Good
Hope. The head was lost in the solar rays until February 4,
when Dr. Gould, director of the National Observatory of the
Argentine Republic at Cordoba, caught a glimpse of it very
low in the west ; and on the following evening, Mr. Eddie, at
Graham's Town, discovered a faint nucleus, of a straw-coloured
tinge, about the size of the annular nebula in Lyra. Its con-
densation, however, was very imperfect, and the whole appari-
tion was of an exceedingly filmy texture. The tail was enor-
mously long. On February 5 it extended large perspective
retrenchment notwithstanding over an arc of 50; but its
brightness nowhere exceeded that of the Milky Way in Taurus.
There was little curvature perceptible ; the edges of the ap-
pendage ran parallel, forming a nebulous causeway from star
to star ; and the comparison to an auroral beam was appropri-
ately used. The aspect of the famous comet of 1843 was for-
cibly recalled to the memory of Mr. Janisch, governor of St.
Helena ; and the resemblance proved not merely superficial.
But the comet of 1880 was less brilliant, and even more evan-
escent. After only eight days of visibility, it had faded so
much as no longer to strike, though still discoverable by the
unaided eye ; and on February 20 it was invisible with the
great Cordoba equatoreal pointed to its known place.
CHAP. xi. RECENT COMETS. 397
But the most astonishing circumstance connected with this
body is the identity of its path with that of its predecessor in
1843. This is undeniable. Dr. Gould, 1 Mr. Hind, and Dr.
Copeland, 2 each computed a separate set of elements from the
first rough observations, and each was struck with an agree-
ment between the two orbits so close as to render them
virtually indistinguishable. " Can it be possible," Mr. Hind
wrote to Sir George Airy, " that there is such a comet in the
system, almost grazing the sun's surface in perihelion, and
revolving in less than thirty-seven years ? I confess I feel a
difficulty in admitting it, notwithstanding the above extra-
ordinary resemblance of orbits." 3
Mr. Hind's difficulty was shared by other astronomers. It
would, indeed, be a violation of common sense to suppose
that a celestial visitant so striking in appearance had been for
centuries back an unnoticed frequenter of our skies. Various
expedients accordingly were resorted to for getting rid of the
anomaly. The most promising at first sight was that of the
"resisting medium." It was hard to believe that a body,
largely vaporous, shooting past the sun at a distance of less
than a hundred thousand miles from his surface, should have
escaped powerful retardation. It must have passed through the
very midst of the corona. It might easily have had an actual
encounter with a prominence. Escape from such proximity
might, indeed, very well have been judged beforehand to be
impossible. Even admitting no other kind of opposition than
that met with by Encke's and Winnecke's comets, the effect
in shortening the period ought to be of the most marked kind.
It was proved by Oppolzer 4 that if the comet of 1843 had
entered our system from stellar space with parabolic velocity,
it would, by the action of a medium such as Encke postulated
(varying in density inversely as the square of the distance
from the sun), have been brought down, by its first perihelion
passage, to elliptic movement in a period of twenty-four years,
with such rapid diminution that its next return would be in
1 Astr. Nach., No. 2307. 2 Ibid., No. 2304. 3 Observatory, vol. iii. p.
390. 4 Astr. Nach., No. 2319.
398 HISTORY OF ASTRONOMY. PART n.
about ten. But such restricted observations as were on either
occasion of its visibility available, gave no sign of such a rapid
progress towards engulfment.
Another form of the theory was advocated by Klinkerfues.
He supposed that four returns of the same body had been
witnessed within historical memory the first in 371 B.C., the
next in 1668, besides those of 1843 and 1880; an original
period of 2039 years being successively reduced by the with-
drawal at each perihelion passage of y-gVo" ^ ^ e velocity
acquired by falling from the far extremity of its orbit towards
the sun, to 175 and 37 years. A continuance of the process
would bring the comet of 1880 back in 1897.
Unfortunately, the earliest of these apparitions cannot be
identified with the recent ones unless by doing violence to the
plain meaning of Aristotle's words in describing it. He states
that the comet was first seen " during the frosts and in the
clear skies of winter," setting due west nearly at the same time
as the sun. 1 This implies some considerable north latitude.
But the objects lately observed had practically no north lati-
tude. They accomplished their entire course above the ecliptic
in two hours and a quarter, during which space they were
barely separated a hand's-breadth (one might say) from the
sun's surface. For the purposes of the desired assimilation,
Aristotle's comet should have appeared in March. It is not
credible, however, that even a native of Thrace should have
termed March " winter."
With the comet of 1668 the case is more dubious. The
circumstances of its appearance are barely reconcilable with
the identity attributed to it, although too vaguely known to
render certainty one way or the other attainable. It might,
however, be expected that recent observations would at least
decide the questions whether the comet of 1843 could have re-
turned in less than thirty-seven years, and whether the comet
of 1880 was to be looked for at the end of 17^. But the
truth is that both these objects were observed over so small
an arc 8 and 3 respectively that their periods remained
1 Meteor., lib. i. cap. 6.
CHAP. xi. RECENT COMETS. 399
virtually undetermined. For while the shape and position of
their orbits could be and were fixed with a very close approach
to accuracy, the length of those orbits might vary enormously
without any very sensible difference being produced in the
small part of the curves traced out near the sun. It is, how-
ever, remarkable that Dr. Wilhelm Meyer arrived, by an
elaborate discussion, at a period of thirty-seven years for the
comet of iSSo, 1 while the observations of 1843 are admittedly
best fitted by Hubbard's ellipse of 533 years ; but these Dr.
Meyer supposes to be affected by some constant source of
error, such as would be produced by a mistaken estimate of
the position of the comet's centre of gravity. He infers finally
that, in spite of previous non-appearances, we really have to do
with a regular denizen of our system, returning once in thirty-
seven years along an orbit of such extreme eccentricity that its
movement might be described as one of precipitation towards,
and rapid escape from the sun, rather than of sedate circula-
tion round it.
The geometrical test of identity has hitherto been the only
one which it was possible to apply to comets, and in the case
before us it may fairly be said to have broken down. We
may, then, tentatively, and with much hesitation, try a physical
test, though scarcely yet, properly speaking, available. We
have seen that the comets of 1843 and 1880 were strikingly
alike in general appearance, though the absence of a formed
nucleus in the latter, and its inferior brilliancy, detracted from
the convincing effect of the resemblance. Nor was it main-
tained when tried by exact methods of inquiry. M. Bredichin
found that the gigantic ray emitted in 1 843 belonged to his type
No. i; that of 1880 to type No. 2. 2 The particles forming the
one were actuated by a repulsive force ten times as powerful
as those forming the other. It is true that a second noticeably
curved tail was seen in Chili, March i, and at Madras, March
1 1, 1843 ; and M. Bredichin, accordingly, thinks the conjecture
justified that the materials composing on that occasion the
Soc. Phys. de Gentve, t. xxviii. p. 23.
2 Annales, t. vii. pt. i, p. 60.
400 HISTORY OF ASTRONOMY. PART n.
principal appendage having become exhausted, those of the
secondary one remained predominant, and reappeared alone
in the " hydron-carbon " train of 1880. But the one known
instance in point is against such a supposition. Halley's
comet, the only great comet of which the returns have been
securely authenticated and carefully observed, has preserved
its "type" unchanged through many successive revolutions.
The dilemma presented to astronomers by the Great Southern
Comet of 1880 was unexpectedly renewed in the following year.
On the 22d of May 1881, Mr. John Tebbutt of Windsor,
New South Wales, scanning the western sky, discerned a hazy-
looking object which he felt sure was a strange one. A marine
telescope at once resolved it into two small stars and a comet,
the latter of which quickly attracted the keen attention of
astronomers; for Dr. Gould, computing its orbit from his
first observations at Cordoba, found it to agree so closely with
that arrived at by Bessel for the comet of 1807, that he
telegraphed to Europe, June i, announcing the unexpected
return of that body. So unexpected, that theoretically it
was not possible before the year 3346 ; and Bessel's in-
vestigation was one which inspired, and eminently deserved
confidence. Here then once more the perplexing choice had
to be made between a premature and unaccountable re-appear-
ance, and the admission of a plurality of comets moving nearly
in the same path. But in this case facts proved decisive.
Tebbutt's comet passed the sun June 16, at a distance of
sixty-eight millions of miles, and became visible in Europe
six days later. It was, in the opinion of some, the finest
object of the kind since 1861. In traversing the constella-
tion Auriga, on its debut in these latitudes, it outshone
Capella. On June 24 and some subsequent nights, it was
unmatched in brilliancy by any star in the heavens. In the
telescope, the "two interlacing arcs of light" which had
adorned the head of Coggia's comet were reproduced ; while
a curious dorsal spine of strong illumination formed the axis
of the tail, which extended in clear skies over an arc of 20.
It belonged to the same " type " as Donati's great plume ;
CHAP. XL RECENT COMETS. 401
the particles composing it being driven from the sun by a
force twice as powerful as that urging them towards it. 1 But
the appendage was, for a few nights, and by two observers,
perceived to be double. Tempel, on June 27, and Lewis
Boss, at Albany (N.Y.), June 26 and 28, saw a long straight
ray corresponding to a far higher rate of emission than the
curved train, and shown by Bredichin to be a member of the
(so-called) hydrogen class. It had vanished by July i, but
made a temporary reappearance July 22. 2
The appendages of this comet were of remarkable trans-
parency. Small stars shone wholly undimmed across the
tail, and a very nearly central transit of the head over one of
the seventh magnitude on the night of June 29, produced if
any change an increase of brilliancy in the object of this
spontaneous experiment. 3 Yet Dr. Meyer, at the Geneva
Observatory, found distinct evidence of refraction suffered by
stellar rays under these circumstances. Three times he pur-
sued with micrometric measurements the course of a star across
the cometary surroundings ; and on each occasion the unifor-
mity of its progress was disturbed in a manner corresponding
to the optical action of a gaseous mass increasing in density
and refractive power as the square of the distance from the
nucleus diminished. Supposing olefiant gas to be in question,
its density, 102,000 kilometres from the nucleus, was estimated
to be xoV^ that of our atmosphere at the sea-level. 4 This was
the first successful attempt to measure the effects of cometary
refraction, and will doubtless be renewed on a favourable
opportunity.
The track pursued by this comet gave peculiar advantages
for its observation. Ascending from Auriga through Camelo-
pardus, it stood, July 1 9, on a line between the Pointers and
the Pole, within 8 of the latter, thus remaining for a con-
1 Bredichin, Annales, t. viii. p. 68. 2 Am. Jour, of Sc., vol. xxii. p.
305. 3 Messrs. Burton and Green observed a dilatation of the stellar
image into a nebulous patch by the transmission of its rays through a
nuclear jet of the comet. Am. Jour, of Sc., vol. xxii. p. 163. 4 Archives
des Sciences, t. viii. p. 535. Meyer founded his conclusions on the theory
of M. Gustave Celle"rier.
2 C
402 HISTORY OF ASTRONOMY. PART n.
siderable period constantly above the horizon of northern
observers. Its brightness, too, was no transient blaze, but
had a lasting quality which enabled it to be kept steadily in
view during nearly nine months. Visible to the naked eye
until the end of August, the last telescopic observation of it
was made February 14, 1882, when its distance from the
earth considerably exceeded 300 million miles. Under these
circumstances, the knowledge acquired of its orbit was of more
than usual accuracy, and showed conclusively that the comet
was not a simple return of Bessel's ; for this would involve a
period of seventy-four years, whereas Tebbutt's comet cannot
revisit the sun until after the lapse of close upon three millen-
niums. Nevertheless, the two bodies move so nearly in the
same path that an original connection of some kind is obvious ;
and the recent example of Biela readily suggested a conjecture
as to what the nature of that connection might have been.
The comets of 1807 and 1881 are then regarded with much
probability as fragments of a primitive disrupted body, one
following in the wake of the other at an interval of seventy-
four years.
Tebbutt's comet was the first of which a satisfactory photo-
graph was obtained. The difficulties to be overcome were
very great. The chemical intensity of cometary light is,
to begin with, extraordinarily small. Janssen estimates it at
STroYooTT * moonlight. 1 So that, if the ordinary process by
which lunar photographs are taken had been applied to the
comet of 1 88 1, an exposure of at least three days would have
been required in order to get an impression of the head with
about a tenth part of the tail. But by that time a new
method of vastly increased sensitiveness had been rendered
available, by which dry gelatine-plates were substituted for the
wet collodion-plates hitherto in use; and this improvement
alone reduced the necessary time of exposure to two hours.
It was brought down to half an hour by Janssen's employment
of a reflector specially adapted to give an image illuminated
3 Annuairc, Paris, 1882, p. 781.
CHAP. xi. RECENT COMETS. 403
eight or ten times as strongly as that produced in the focus of
an ordinary telescope. 1
The photographic feebleness of cometary rays was not the
only obstacle in the way of success. The proper motion of
these bodies is so rapid as to render the usual devices for
keeping a heavenly body steadily in view quite inapplicable.
The machinery by which the diurnal movement of the sphere
is followed, must be specially modified to suit each eccentric
career. This too was done, and on June 30, 1881, Janssen
secured a perfect photograph of the brilliant object then visible,
showing the structure of the tail with beautiful distinctness
to a distance of 2^ from the head. An impression to nearly
10 was obtained about the same time by Dr. Henry Draper
at New York, with an exposure of 162 minutes. 2
Tebbutt's (or comet 1881 iii.) was also the first comet of
which the spectrum was so much as attempted to be chemi-
cally recorded. Both Dr. Huggins and Dr. Draper were
successful in this respect, but Dr. Huggins the more com-
pletely so. 3 The importance of the feat consisted in its
throwing open to investigation a part of the spectrum invisible
to the eye, and so affording an additional test of cometary
constitution. The result was fully to confirm the origin from
carbon-compounds assigned to the visible rays, by disclosing
additional bands belonging to the same series in the ultra-
violet ; as well as to establish unmistakably the presence of a
not inconsiderable proportion of reflected solar light by the
clear impression of some of the principal Fraunhofer lines.
Thus the polariscope was found to have told the truth, though
not the whole truth.
The photograph so satisfactorily communicative was taken
by Dr. Huggins on the night of June 24 ; and on the 29th,
at Greenwich, the tell-tale Fraunhofer lines were perceived to
interrupt the visible range of the spectrum. This was at first
so vividly continuous, that the characteristic cometary bands
could scarcely be detached from their bright background.
1 Annuaire, p. 776. 2 Am. Jour, of Sc., vol. xxii. p. 134. 3 Report
Brit. Ass., 1 88 1, p. 520.
404. HISTORY OF ASTRONOMY. PART n.
But as the nucleus faded towards the end of June, they came
out strongly, and were more and more clearly seen, both at
Greenwich and at Princeton, to agree, not with the spectrum
of hydro-carbons lit up in a vacuum tube by an electric dis-
charge, but with that of the same substances burning in a
Bunsen flame. 1 Here we have an additional clue to the mole-
cular condition of cometary materials. It need not, however,
be inferred that they are really in a state of combustion. This,
from all that we know, may be called an impossibility. The
truth pointed to seems rather to be that the electricity by
which comets are rendered luminous is of very low intensity. 2
The spectrum of the tail was, in this comet, found to be not
essentially different from that of the head. Professor Wright
of Yale College ascertained a large, but probably variable per-
centage of its light to be polarised in a plane passing through
the sun, and hence to be reflected sunlight. 3 A faint con-
tinuous spectrum corresponded to this portion of its radiance ;
but gaseous emissions were also present. At Potsdam, on
June 30, the hydro-carbon bands were traced by Yogel to the
very end of the tail ; 4 and they were kept in sight by Young at
a greater distance from the nucleus than the more equably
dispersed light. There seems little doubt that, as in the
solar corona, the relative strength of the two orders of spec-
trum is subject to fluctuations.
The comet 1881 iii. was thus of signal service to science.
It afforded, when compared with the comet of 1807, the first
undeniable example of two such bodies travelling so nearly in
the same orbit as to leave absolutely no doubt of the existence
of a genetic tie between them. Cometary photography came
to its earliest fruition with it ; and cometary spectroscopy made
a notable advance by means of it. Before it was yet out of
sight, it was provided with a successor.
At Ann Arbor Observatory, Michigan, on July 14, a comet
was discovered by Dr. Schaberle, which, as his claim to priority
1 Month. Not., vol. xlii. p. 14 ; Am. Jour, of Sc., vol. xxii. p. 136.
2 Piazzi Smyth, Nature, vol. xxiv. p. 430. 3 Astr. Nach., No. 2395.
4 Ibid.
CHAP. xi. RECENT COMETS. 405
is undisputed, is often allowed to bear his name. In strict
scientific parlance, however, it is designated comet 1881 iv.
It was observed in Europe after three days, became just dis-
cernible by the naked eye at the end of July, and brightened
consistently up to its perihelion passage, August 22, when it was
still about fifty million miles from the sun. During many days
of that month, the uncommon spectacle was presented of two
bright comets circling together, though at widely different
distances, round the north pole of the heavens. The new-
comer, however, never approached the pristine lustre of its
predecessor. Its nucleus, when brightest, was comparable to
the star Cor Caroli, a narrow, perfectly straight ray proceed-
ing from it to a distance of 10. This was easily shown by
Bredichin to belong to the hydrogen type of tails ; x while a
"strange, faint second tail, or bifurcation of the first one,"
observed by Captain Noble, August 24,2 fell into the hydro-
carbon class of emanations. It was seen, August 2 2 and 24, by
Dr. F. Terby of Louvain, 3 as a -short nebulous brush, like the
abortive beginnings of a congeries of curving trains; but
appeared no more. Its well-attested presence was, however,
significant of the complex constitution of such bodies, and the
manifold kinds of action progressing in them.
The only peculiarity in the spectrum of Schaberle's comet
consisted in the almost total absence of continuous light. The
carbon-bands were nearly isolated and very bright. Barely
from the nucleus proceeded a rainbow-tinted streak, indicative
of solid or liquid matter, which, in this comet, must have been
of very scanty amount. Its visit to the sun in 1881 was, so
far as is known, the first. The elements of its orbit showed no
resemblance to those of any previous comet, nor any marked
signs of periodicity. So that, although it may be considered
probable, we do not know that it is moving in a closed curve, or
will ever again penetrate the precincts of the solar system. It
was last seen from the southern hemisphere, October 19, 1881.
The third of a quartette of lucid comets visible within six-
1 Astr. Nach,, No. 2411. 2 Month. Not., vol. xlii. p. 49. 3 Astr.
.y No. 2414.
4 o6 HISTORY OF ASTRONOMY. PART n.
teen months, was discovered by Mr. C. S. Wells at the Dudley
Observatory, Albany, March 17, 1882. Two days later it was
described by Mr. Lewis Boss as " a great comet in miniature,"
so well denned and regularly developed were its various parts
and appendages. It was discernible without optical aid early
in May ; and on June 5 it was observed on the meridian at
Albany just before noon an astronomical event of extreme
rarity. Comet Wells, however, never became an object so con-
spicuous as to attract general attention, owing to its immersion
in the evening twilight of our northern June.
But the study of its spectrum revealed new facts of the
utmost interest. All the comets till then examined had been
found to conform to one invariable type of luminous emission.
Individual distinctions there had been, but no specific dif-
ferences. Now all these bodies had kept at a respectful dis-
tance from the sun; for of the great comet of 1880 no
spectroscopic inquiries had been made. Comet Wells, on the
other hand, approached its surface within little more than
five million miles on June 10, 1882; and it is not doubtful
that to this circumstance the novel feature in its incandes-
cence was due.
During the first half of April its spectrum was of the normal
type, though the carbon-bands were unusually weak ; but with
increasing vicinity to the sun they died out, and the entire
light seemed to become concentrated into a narrow, unbroken,
brilliant streak, hardly to be distinguished from the spectrum
of a star. This unusual behaviour excited attention, and a
strict watch was kept. It was rewarded at the Dunecht
Observatory, May 27, by the discernment of what had never
before been seen in a comet the yellow ray of sodium. 1 By
June i, this had kindled into a blaze overpowering all other
emissions. The light of the comet was practically mono-
chromatic ; and the image of the entire head, with the root
of the tail, could be observed, like a solar prominence, depicted,
in its new saffron vesture of vivid illumination, within the
jaws of an open slit.
1 Copernicus, vol. ii. p. 229.
CHAP. xi. RECENT COMETS. 407
At Potsdam, the bright yellow line was perceived with
astonishment by Vogel on May 31, and was next evening
identified with Fraunhofer's " D." Its character led him to
infer a very considerable density in the glowing vapour
emitting it. 1 Hasselberg founded an additional argument in
favour of the electrical origin of cometary light on the changes
in the spectrum of comet Wells. 2 For they were closely
paralleled by some earlier experiments of Wiedemann, in
which the gaseous spectra of vacuum tubes were at once
effaced on the introduction of metallic vapours. It seemed
as if the metal had no sooner been rendered volatile by heat,
than it usurped the entire office of carrying the discharge, the
resulting light being thus exclusively of its production. Had
simple incandescence by heat been in question, the effect
would have been different ; the two spectra would have been
superposed without prejudice to either. Similarly, the replace-
ment of the hydro-carbon bands in the spectrum of the comet
by the sodium line, proved electricity to be the exciting agent.
For the increasing thermal power of the sun might, indeed,
have ignited the sodium, but it could not have extinguished
the hydro-carbons.
Dr. Huggins succeeded in photographing the spectrum of
comet Wells by an exposure of one hour and a quarter. 3 The
result was to confirm the novelty of its character. None of
the ultra-violet carbon-groups were apparent ; but certain
bright rays, as yet unidentified, had imprinted themselves.
Otherwise the spectrum was strongly continuous, uninterrupted
even by the Fraunhofer lines detected in the spectrum of
Tebbutt's comet. Hence it was concluded that a smaller
proportion of reflected light was mingled with the native
emissions of the later arrival.
All that is certainly known about the extent of the orbit
traversed by the first comet of 1882 is that it came from, and
is now retreating towards, vastly remote depths of space. An
American computer 4 found a period indicated for it of no less
i Astr. NacJt., Nos. 2434, 2437. 2 Ibid., No. 2441. 3 Report Brit.
Ass., 1882, p. 442. 4 J. J. Parsons, Am. Jour, of Science, vol. xxvii. p. 34.
4o8 HISTORY OF ASTRONOMY. PART n.
than 400,000 years ; A. Thraen of Dingelstadt arrived at one
of 36I7- 1 Both are perhaps equally insecure.
"We have now to give some brief account of one of the
most remarkable cometary apparitions on record, and with
the single exception of that identified with the name of Halley
the most instructive to astronomers. The lessons learned
from it were as varied and significant as its aspect was
splendid ; although from the circumstance of its being visible
in general only before sunrise, the spectators of its splendour
were comparatively few.
The discovery of a great comet at Rio Janeiro, September
n, 1882, became known in Europe through a telegram from
M. Cruls, director of the observatory at that place. It had,
however (as appeared subsequently), been already seen on the
8th by Mr. Finlay, assistant at the Cape Observatory, and at
Auckland as early as September 3. A later, but very singu-
larly conditioned detection, quite unconnected with any of the
preceding, was effected by Mr. Common at Ealing. Since the
eclipse of May 17, when a comet named " Tewfik " in honour
of the Khedive of Egypt was caught on Dr. Schuster's photo-
graphs, entangled, one might almost say, in the outer rays of
the corona, he had scrutinised the neighbourhood of the sun
on the infinitesimal chance of intercepting another such body
on its rapid journey thence or thither. We record with wonder
that, after an interval of exactly four months, that infinitesimal
chance turned up in his favour.
On the forenoon of Sunday, September 17, he saw a great
comet close to, and rapidly approaching the sun. It was, in
fact, then within a few hours of perihelion. Some measures of
position were promptly taken; but a cloud-veil covered the
interesting spectacle before midday was long past. Mr. Finlay
at the Cape was more completely fortunate. Divided from
his fellow-observer by half the world, he unconsciously finished,
under a clearer sky, his interrupted observation. The comet,
of which the silvery radiance contrasted strikingly with the
reddish-yellow glare of the sun's margin it drew near to, was
1 Astr. Nach., No. 2441.
CHAP. xi. RECENT COMETS. 409
followed " continuously right into the boiling of the limb "
a circumstance without precedent in cometary history. 1 Dr.
Elkin, who watched the progress of the event with another
instrument, thought the intrinsic brilliancy of the nucleus
scarcely surpassed by that of the sun's surface. Nevertheless
it had no sooner touched it than it vanished as if annihilated.
So sudden was the disappearance (at 4h. 5om. 583. Cape mean
time), that it was at first thought that the comet must have
passed behind the sun. But this proved not to have been the
case. The observers at the Cape had witnessed a genuine
transit. Nor could non-visibility be explained by equality of
lustre. For the gradations of light on the sun's disc are amply
sufficient to bring out against the dusky background of the
limb any object matching the brilliancy of the centre ; while
an object just equally luminous with the limb must inevitably
show dark at the centre. The only practicable view, then, is
that the bulk of the comet was of too filmy a texture, and its
presumably solid nucleus too small, to intercept any notice-
able part of the solar rays a piece of information worth re-
membering.
On the following morning, the object of this unique ob-
servation showed (in Dr. Gill's words) " an astonishing brilli-
ancy as it rose behind the mountains on the east of Table
Bay, and seemed in no way diminished in brightness when the
sun rose a few minutes afterward. It was only necessary to
shade the eye from direct sunlight with the hand at arm's
length, to see the comet with its brilliant white nucleus and
dense, white, sharply-bordered tail of quite half a degree in
length." 5 All over the world, wherever the sky was clear
during that day, September 18, it was obvious to ordinary
vision. Since 1843 nothing had been seen like it. From Spain,
Italy, Algeria, Southern France, despatches came in announc-
ing the extraordinary appearance. At Cordoba, in South
America, the " blazing star near the sun " was the one topic
of discourse. 3 Moreover, and this is altogether extraordinary,
1 Observatory, vol. v. p. 355. 2 Ibid., p. 354. 3 Gould, Astr.
Nach., No. 2481.
410 HISTORY OF ASTRONOMY. PART n.
the records of its daylight visibility to the naked eye extend
over three days. At Reus, near Tarragona, it showed bright
enough to be seen through a passing cloud when only three
of the sun's diameters from his limb, just before its final rush
past perihelion on September 1 7 ; while at Carthagena in
Spain, on September 19, it was kept in view during two hours
before, and two hours after noon, and was similarly visible in
Algeria on the same day. 1
But still more surprising than the appearance of the body
itself, were the nature and relations of the path it moved in.
The first rough elements computed for it by Mr. S. C. Chandler
of Harvard, and by Mr. White, assistant at the Melbourne
Observatory, showed at once a striking resemblance to those
of the twin comets of 1843 and 1880. This suggestive fact
became known in this country, September 27, through the
medium of a Dunecht Circular. It was fully confirmed by
subsequent inquiries, for which ample opportunities were
luckily provided. The likeness was not, indeed, so absolutely
perfect as in the previous case ; it included some slight, though
real differences ; but it bore a strong and unmistakable stamp,
broadly challenging explanation.
Two hypotheses only were really available. Either ] the
comet of 1882 was an accelerated return of those of 1843 and
1880, or it was a fragment of an original mass to which they
also had belonged. For the purposes of the first view the
"resisting medium" was brought into full play; the opinion
invoking it was, for some time, both prevalent and popular,
and formed the basis, moreover, of something of a sensational
panic. For a comet which, at a single passage through the
sun's atmosphere, encountered sufficient resistance to shorten
its period from thirty-seven, to two years and eight months,
must, in the immediate future, be brought to rest on his surface ;
and the solar conflagration thence ensuing was represented in
some quarters, with more license of imagination than coun-
tenance from science, as likely to be of catastrophic import to
the inhabitants of our little planet.
1 Flammarion, Comptes Rendus, t. xcv. p. 558.
CHAP. xi. RECENT COMETS. 411
But there was a test available in 1882 which it had not been
possible to apply either in 1843 or in 1880. The two bodies
visible in those years had been observed only after they had
already passed perihelion ; 1 the third member of the group,
on the other hand, was accurately followed for a week before
that event, as well as during many months after it. Mr.
Finlay's and Dr. Elkin's observation of its disappearance at
the sun's edge formed, besides, a peculiarly delicate test of its
motion. The opportunity was thus afforded, by directly com-
paring the comet's velocity before and after its critical plunge
through the solar surroundings, of ascertaining with some
approach to certainty whether any considerable retardation
had been experienced in the course of that plunge. The
answer distinctly given was that there had not. The computed
and observed places on both sides of the sun, fitted harmo-
niously together. The effect, if any were produced, was too
small to be perceptible.
This result is, in itself, a memorable one. It seems to give
the coup de grace to Encke's theory somewhat discredited, in
addition, by Backlund's investigation of a resisting medium
growing rapidly denser inwards. For the perihelion distance
of the comet of 1882, though somewhat greater than that of
its predecessors, was nevertheless extremely small. It passed
at less than 300,000 miles of the sun's surface. But the
ethereal substance long supposed to obstruct the movement of
Encke's comet, would there be nearly 2000 times denser than
at the perihelion of the smaller body, and must have exerted
a conspicuous retarding influence. That none such could be
detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the
question of identity. The " Great September Comet "of 1882
was in no hurry to withdraw itself from curious terrestrial
scrutiny. It was discerned with the naked eye at Cordoba
as late as March 7, 1883, and still showed in the field of the
1 Captain Ray's sextant-observation of the comet of 1843 a ^ ew hours
before perihelion, was too rough to be of use
4 I2 HISTORY OF ASTRONOMY. PART n.
great equatoreal on June i as an " excessively faint whiteness." l
It was then about 470 millions of miles from the earth a
distance to which no other comet save the exceptional one
of 1729 has been pursued. 2 Moreover an arc of 340 out of
the entire 360 degrees of its circuit had been described under
the eyes of astronomers ; so that its course came to be very
well known. It may then be taken as ascertained that its
movement is in a very eccentric ellipse, traversed in several
hundred years. The lowest estimate of period, founded on
sufficiently extensive data, is of 652^ years (Morrison); the
highest deserving any confidence that by Kreutz of 843. 3
There is reason to believe that this last is not very far from
the truth.
This conclusion of a period to be counted by many centuries
assures us positively that the comet of 1882 was not a return
of either of the bodies so singularly connected with it. But
it has little or no further application to them. 4 For very
slightly differing disturbances of each would suffice to develop
a marked variety in their periods. A loss of velocity at peri-
helion, for instance, of only 49 metres a second (less than
i^J^ of the whole), would bring a comet revolving in 175
years back in 37. 5 Yet the orbit would remain otherwise
almost unchanged. A body moving in it would perceptibly
diverge from its former track only at a considerable distance
from the sun. Hence each of these three comets has doubt-
less a period of its own.
The unexpected presentation to our acquaintance of a fourth
member of the family complicates still further the problem
they confront us with. On the i8th of January 1887, M.
Thome discovered at Cordoba a comet reproducing with curious
fidelity the lineaments of that observed in the same latitudes
seven years previously. The narrow ribbon of light, con-
1 Astr. Nach.,No. 2538. 2 Nature, vol. xxix. p. 135. 3 Astr. Nach.,
No. 2482. 4 The attention of the author was kindly directed to this
point by Professor Young of Princeton (N.J.). 5 Rebeur-Paschwitz,
Sirius, Bd. xvi. p. 233. This reasoning was designed to show the possible
identity of the comets of 1668, 1843, and I 88o.
CHAP. xi. RECENT COMETS. 413
tracting towards the sun, and running outward from it to
a distance of thirty- five degrees; the unsubstantial head
a veiled nothingness, as it appeared, since no trace of nucleus
could be made out; the quick fading into invisibility, were
all accordant peculiarities, and they were confirmed by some
rough calculations of its orbit, showing geometrical affinity
to be no less unmistakable than physical likeness. There
might indeed appear to be strong grounds for the surmise
that we have here, not merely relationship, but identity,
and that " Thome's comet " was really an accelerated return
of the comet of 1880. If this were so, its engulfment in
the sun should be imminent; but the probability, when all
the facts are considered, seems to be the other way. Con-
clusive evidence on the point may, perhaps, never be forth-
coming. Very few, and not very reliable observations, can, from
the nature of the recent apparition, have been secured ; and
none until long after perihelion, which was passed on January
ii. It is worth noting that M. Meyer appends the "eclipse-
comet " of 1882 to this extraordinary group, and has lately
adduced some evidence in support of his contention. 1
The idea of cometary systems was first suggested by Thomas
Clausen in 1 831.2 It was developed by the acute inquiries of
the late M. Hoek, director of the Utrecht Observatory, in 1865
and some following years. 3 He found that in quite a consider-
able number of cases, the paths of two or three comets had a
common point of intersection far out in space, indicating with
much likelihood a community of origin. This consisted,
according to his surmise, in the disruption of a parent mass
during its sweep round the star latest visited. Be this as it
may, the fact is undoubted that numerous comets fall into
groups, in which affinity of geometrical relations betrays a pre-
existent physical connection. Never before, however, had
geometrical affinity been so notorious as between the four
comets now under consideration ; and never before, in a comet
still, it might be said, in the prime of life, had physical peculi-
1 Astr. Nach., No. 2717. 2 Gruithuisen's Analekten, Heft vii. p. 48.
3 Month. Not., vols. xxv., xxvi., xxviii.
4 i4 HISTORY OF ASTRONOMY. PART n.
arities tending to account for that affinity been so obvious as
in the second- last -comer of the group.
Observation of a granular structure in cometary nuclei dates
far back into the seventeenth century, when Cysatus and
Hevelius described the central parts of the comets of 1618 and
1652 respectively, as made up of a congeries of minute stars.
Analogous symptoms of a loose state of aggregation have of
late been not unfrequently detected in telescopic comets, be-
sides the instances of actual division offered by those connected
with the names of Biela and Liais. The forces concerned in
producing these effects seem to have been peculiarly energetic
in the great comet of 1882.
The segmentation of the nucleus was first noticed in the
United States and at the Cape of Good Hope, September 30.
It proceeded rapidly. At Kiel, on October 5 and 7, Professor
Kriiger perceived two centres of condensation. A definite and
progressive separation into three masses was observed by Pro-
fessor Holden, October 13 and ly. 1 A few days later, M.
Tempel found the head to consist of four lucid aggregations,
ranged nearly along the prolongation of the caudal axis ; 2 and
Mr. Common, January 27, 1883, saw five nuclei in a line "like
pearls on a string." 3 This remarkable character was preserved
to the last moment of the comet's distinct visibility.
There were, however, other curious proofs of a marked
tendency in this body to disaggregation. On October 9,
Schmidt discovered at Athens a nebulous object 4 south-west
of the great comet, and travelling in the same direction. It
remained visible for a few days, and, from Oppenheim's and
Hind's calculations, there can be little doubt that it was really
the offspring by fission of the body it accompanied. 4 This is
rendered more probable by the unexampled spectacle offered,
October 14, to Mr. E. E. Barnard of Nashville, Tennessee, of
six or eight distinct cometary masses within 6 south by west
of the comet's head, none of which reappeared on the next
opportunity for a search. 5 A week later, however, one similar
1 Nature, vol. xxvii. p. 246. 2 Astr. Nach., No. 2468. 3 Athenaum,
Feb. 3, 1883. 4 Astr. Nach., Nos. 2462, 2466. 5 Ibid., No. 2489.
CHAP. xi. RECENT COMETS. 415
object was discerned by Mr. Brooks, of Phelps, N.Y., in the
opposite direction from the comet. Thus, space appeared to
be strewn with the filmy debris of this extraordinary body all
along the track of its retreat from the sun.
Its tail was only equalled (if it were equalled) in length by
that of the comet of 1843. It extended in space to the vast
distance of two hundred millions of miles from the head ; but,
so imperfectly were its proportions displayed to terrestrial
observers, that it at no time covered an arc of the sky of more
than 30. This apparent extent was attained, during a few
days previous to September 25, by a faint, thin, rigid streak,
noticed only by a few observers by Elkin at the Cape Ob-
servatory, Eddie at Grahamstown, and Cruls at Rio Janeiro.
It diverged at a low angle from the denser curved train, and
was produced, according to Bredichin, 1 by the action of a
repulsive force twelve times as strong as the counter-pull of
gravity. It belonged, that is, to type i ; while the great forked
appendage, obvious to all eyes, corresponded to the lower rate
of emission characteristic of type 2.' "This was remarkable for
the perfect definiteness of its termination, for its strongly forked
shape, and for its unusual permanence. Down to the end of
January 1883, its length, according to Schmidt's observations,
was still 93 million miles ; and a week later it remained visible
to the naked eye, without notable abridgment.
Most singular of all was an anomalous extension of the
appendage towards the sun. During the greater part of
October and November, a luminous "tube" or "sheath"
of prodigious dimensions, seemed to surround the head, and
project in a direction nearly opposite to that of the usual out-
pourings of attenuated matter. Its diameter was computed
by Schmidt to be, October 15, no less than four million miles,
and it was described by Cruls as a " truncated cone of nebu-
losity," stretching 3 or 4 sunwards. 2 There can be little
doubt that this abnormal kind of efflux was a consequence of
the tremendous physical disturbance suffered at perihelion;
and it is worth remembering that something analogous was
1 Annales, Moscow, t. ix. pt. ii. p. 52. 2 Comptes Eendus, t. xcvii. p. 797.
416 HISTORY OP ASTRONOMY. PART n.
observed in the comet of 1680 (Newton's), also noted for its
excessively close approach to the sun. The only plausible
hypothesis as to the mode of its production is that of an
opposite state of electrification in the particles composing the
ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a
repetition of that of its immediate predecessor, thus confirming
the inference that the previously unexampled sodium-blaze
was in both a direct result of the intense solar action to which
they were exposed. But the D line was, this time, not seen
alone. At Dunecht, on the morning of September 18, Drs.
Copeland and J. G. Lohse succeeded in identifying six brilliant
rays in the green and yellow with as many prominent iron-
lines ; l a very significant addition to our knowledge of come-
tary constitution, and one which goes far to justify Bredichin's
assumption of various kinds of matter issuing from the nucleus
with velocities inversely as their atomic weights. All the
lines equally showed a slight displacement, indicating recession
from the earth at the rate of 37 to 46 miles a second. A
similar observation made by M. Thollon at Nice on the same
day, supplied a highly satisfactory test of the accuracy of the
spectroscopic method of estimating movement in the line of
sight. Before anything was as yet known of the comet's path
or velocity, he announced, from the position of the double
sodium line alone, that at 3 P.M. on September 18 it was in-
creasing its distance from our planet by from 61 to 76 kilo-
metres per second. 2 M. Bigourdan's subsequent calculations
showed that its actual swiftness of recession was at that
moment 73 kilometres.
Changes in the inverse order to those seen in the spectrum
of comet Wells, soon became apparent. In the earlier body,
carbon bands had died out with approach to perihelion, and
had been replaced by sodium-emissions; in its successor,
sodium emissions became weakened and disappeared with
retreat from perihelion, and found their substitute in carbon-
bands. Professor Bicco was, in fact, able to infer, from the
1 Copernicus, vol. ii. p. 235. z Comptes Rendus, t. xcvi. p. 371.
CHAP. xi. RECENT COMETS. 417
sequence of prismatic phenomena, that the comet had already
passed the sun ; thus establishing a novel criterion for deter-
mining the position of a comet in its orbit by the varying
quality of its radiations.
Recapitulating what has been learnt from the five con-
spicuous comets of 1880-82, we find that the leading facts
acquired to science were these three. First, that groups of
comets may be met with pursuing each other, after intervals
of many years, in the same, or nearly the same track ; so that
identity of orbit can no longer be regarded as a sure test of indi-
vidual identity. Secondly, that at least the outer corona may be
traversed by such bodies with perfect apparent impunity, and
without appreciable retardation of their movements. Finally,
that their chemical constitution is a highly complex one, and
that they possess, in some cases at any rate, a metallic core
resembling the meteoric masses which occasionally reach the
earth from planetary space.
A group of six comets, including Halley's, own a sort of
cliental dependence upon the planet Neptune. They travel
out from the sun just to about his distance from it, as if to
pay homage to a powerful protector who gets the credit of
their establishment as periodical visitors to the solar system.
The second of these bodies to effect a looked-for return was a
comet the sixteenth within ten years discovered by Pons,
July 20, 1812, and found by Encke to revolve in an elliptic
orbit, with a period of nearly 7 1 years. It was not, however,
until September i, 1883, that Mr. Brooks of Phelps (N.Y.)
caught its reappearance; it passed perihelion January 25,
and was last seen June 2, 1884. At its brightest, it had the
appearance of a second magnitude star furnished with a poorly
developed double tail, and was fairly conspicuous to the naked
eye in southern Europe, from December to March. One
exceptional feature distinguished it. Its fluctuations in form
and luminosity were unprecedented in rapidity and extent. On
September 21, Mr. C. Chandler 1 observed it at Harvard as a
very faint, diffused nebulosity, with slight central condensation.
1 Astr. Nach., No. 2553.
2 D
418 HISTORY OF ASTRONOMY. PART n.
On the next night, there was found in its place a bright star
of the eighth magnitude, scarcely marked out, by a bare trace of
environing haze, from the genuine stars it counterfeited. The
change was attended by an eight-fold augmentation of light,
and was proved by Schiaparelli's confirmatory observations l
to have been accomplished within a few hours. The stellar
disguise was quickly cast aside. The comet appeared on
September 23, as a wide nebulous disc, and soon after faded
down to its original dimness. Its distance from the sun was
then no less than 200 million miles, and its spectrum showed
nothing unusual. These strange variations recurred slightly
on October 15, and with marked emphasis January i, when
they were witnessed with amazement and photometrically
studied by Dr. Miiller of Potsdam. 2 The entire cycle this
time was run through in less than four hours the comet had,
with a vivid outburst of light, condensed into a seeming star,
and the seeming star had expanded back again into a comet. ;
A third member of the Neptunian group Olbers's comet of
1815 is now due at perihelion; but it has not been found
possible to fix within very narrow limits the epoch of its return.
As to the origin of comets, there has been of late years much
speculation, ingenious or inane, which, however, it were quite
superfluous to review. Yet we are not wholly without the
guidance of ascertained fact on the subject. Laplace assumed
that the fundamental shape of comets' orbits, when unmodified
by planetary perturbations, is that of a hyperbola a circum-
stance which, if true, would imply their total disconnection
from our system, save by fortuitous encounter. But Gauss
and Schiaparelli separately proved, on the contrary, that these
bodies move naturally in prodigiously long ellipses, 3 the hyper-
bolic form, in the extremely rare cases where it may exist,
being a result of disturbance. This being so, it follows that
their condition previous to being attracted by the sun was
one of relative repose. 4 In other words, they shared the
movement of translation through space of the solar system.
1 Astr. Nach., No. 2553. 2 Ibid., No. 2568. 3 Thury and Meyer, Arch.
des Sciences, t. vi. (3d ser.), p. 187. 4 W. Forster, Pop. Mitth., 1879, p. 7.
CHAP. xi. RECENT COMETS. 419
This significant conclusion had been indicated, on other
grounds, as the upshot of researches undertaken independently
by Carrington 1 and Mohn 2 in 1860, with a view to ascertain-
ing the anticipated existence of a relationship between the
general lie of the paths of comets, and the direction of the
sun's journey. It is tolerably obvious that, if they wander at
haphazard through the interstellar regions, a preponderance
of their apparitions should seem to arrive from the vicinity of
the constellation Hercules; that is to say, we should meet
considerably more comets than would overtake us. Just for
the very same reason that falling stars are more numerous
after than before midnight. Moreover, the comets met by
us should be apparently swifter-moving objects than those
coming up with us from behind ; because, in the one case, our
own real movement would be added to, in the other, subtracted
from theirs. But nothing of all this can be detected. Comets
approach the sun indifferently from all quarters, and with
velocities quite independent of direction.
We conclude then, with Schiaparelli and Forster, that the
" cosmical current " which bears the solar system towards its
unknown goal, carries also with it nebulous masses of undefined
extent, and at an undefined remoteness, fragments detached
from which, continually entering the sphere of the sun's attrac-
tion, flit across our skies under the form of comets. These
are, however, almost certainly so far strangers to our system
that they had no part in the long processes of development by
which its present condition was attained. They are, perhaps,
survivals of an earlier, and by us scarcely and dimly con-
ceivable state of things, when the chaos from which sun and
planets were, by a supreme edict, to emerge, had not as yet
separately begun to be.
1 Mem. R. A. Soc., vol. xxix. p. 335. 2 Month. Not., vol. xxiii. p. 203.
( 420 )
CHAPTER XII.
STARS AND NEBULA.
THAT a science of stellar chemistry should not only have
become possible, but should already have made material
advances, is assuredly one of the most amazing features in the
swift progress of knowledge our age has witnessed. Custom
can never blunt the wonder with which we must regard the
achievement of compelling rays emanating from a source
devoid of sensible magnitude through immeasurable distance,
to reveal, by its peculiarities, the composition of that source.
The discovery of revolving double stars assured us that the
great governing force of the planetary movements, and of our
own material existence, sways equally the courses of the
farthest suns in space j the application of prismatic analysis
certified to the presence in the stars of the familiar materials,
no less of the earth we tread, than of the human bodies built
up out of its dust and circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained
the generic participation of stellar light in the peculiarity by
which sunlight, spread out by transmission through a prism,
shows numerous transverse rulings of interrupting darkness.
No sooner had Kirchhoff supplied the key to the hidden mean-
ing of those ciphered characters, than it was eagerly turned to
the interpretation of the dim scrolls unfolded in the spectra of
the stars. Donati made at Florence, in 1860, the first efforts
in this direction ; but with little result, owing to the imper-
fections of the instrumental means at his command. His
comparative failure, however, was a prelude to others' success.
Almost simultaneously, in 1862, the novel line of investigation
CHAP. xir. -STARS AND NEBULA. 421
was entered upon by Huggins and Miller near London, by
Father Secchi at Rome, and by Lewis M. Rutherfurd in New
York. Fraunhofer's device of using a cylindrical lens for the
purpose of giving a second dimension to stellar spectra, was
adopted by all, and was indeed indispensable. For a lumi-
nous point, such as a star appears, becomes, when viewed
through a prism, a variegated line, which, until broadened
into a band by the intervention of a cylindrical lens, is all but
useless for purposes of research. This process of rolling out
involves, it is true, much loss of light a scanty and precious
commodity, as coming from the stars ; but the loss is an inevi-
table one. And so fully is it compensated by the great light-
grasping power of modern telescopes, that important informa-
tion can now be gained from the spectroscopic examination of
stars far below the range of the unarmed eye.
The effective founders of stellar spectroscopy, then (since
Rutherfurd shortly turned his efforts elsewhither), were Father
Secchi, the eminent Jesuit astronomer of the Collegio Romano,
where he died, February 26, 1878, and Dr. Huggins, with
whom the late Professor W. A. Miller was associated. The
work of each was happily directed so as to supplement that of
the other. With less perfect appliances, the Roman astronomer
sought to render his extensive rather than precise ; at Upper
Tulse Hill, searching accuracy over a narrower range was aimed
at and attained. To Father Secchi is due the merit of having
executed the first spectroscopic survey of the heavens. Above
4000 stars were in all passed in review by him, and classified
according to the varying qualities of their light. His provi-
sional establishment (1863-67) of four types of stellar spectra x
has proved a genuine aid to knowledge through the facilities
afforded by it for the arrangement and comparison of rapidly
accumulating facts. Moreover, it is scarcely doubtful that
these spectral distinctions correspond to differences in physical
condition of a marked kind.
1 Report Brit. Ass., 1868, p. 1 66. Rutherfurd gave a rudimentary
sketch of a classification of the kind in December 1862, but based on
imperfect observations. See Am. Jour, of Se., vol. xxxv. p. 77.
422 HISTORY OF ASTRONOMY. PART n.
The first order comprises more than half the visible stars,
and a still larger proportion of those eminently lustrous.
Sirius, Yega, Regulus, Altair, are amongst its leading members.
Their spectra are distinguished by the breadth and intensity of
the four dark bars due to the absorption of hydrogen, and by
the extreme faintness of the metallic lines, of which, never-
theless, hundreds are disclosed by careful examination. The
light of these " Sirian " orbs is white or bluish ; and it is found
to be rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of
stars, which is about one-sixth less numerously represented
than the first. Their spectra are quite closely similar to that
of sunlight, in being ruled throughout by innumerable fine
dark lines ; and they share its yellowish tinge.
The third class includes most red and variable stars (com-
monly synonymous), of which Betelgeux in the shoulder of
Orion, and " Mira " in the Whale are noted examples. Their
characteristic spectrum is of the " fluted " description. It
shows like a strongly illuminated range of seven or eight
variously tinted columns seen in perspective, the light falling
from the red end towards the violet. This kind of absorption
is produced by the vapours of metalloids or of compound
substances.
To the fourth order of stars belongs also a colonnaded spec-
trum, but reversed ; the light is thrown the other way. The
three broad zones of absorption which interrupt it are sharp
towards the red, insensibly gradated towards the violet end.
The individuals composing Class IY. are few, and apparently
insignificant, the brightest of them not exceeding the fifth
magnitude. They are commonly distinguished by a deep red
tint, and gleam like rubies in the field of the telescope.
Father Secchi, who detected the peculiarity of their analysed
light, ascribed it to the presence of carbon in some form in
their atmospheres ; and this has been confirmed by the latest
researches of H. C. Yogel, 1 now director of the astro-physical
observatory at Potsdam. The hydro-carbon bands, in fact,
1 Pullicationen, Potsdam, No. 14, 1884, p. 31.
CHAP. xii. STARS AND NEBULA. 423
seen bright in comets, are dark in these singular objects the
only ones in the heavens (save, perhaps, a coronal streamer
or a rare meteor) l which display a cometary analogy of the
fundamental sort revealed by the spectroscope.
The members of all four orders are, however, emphatically
suns. They possess, it would appear, photospheres radiating
all kinds of light, and differ from each other (so far as we are
able to judge) solely in the varying qualities of their absorp-
tive atmospheres. The principle that the colours of stars
depend, not on the intrinsic nature of their light, but on the
kinds of vapours surrounding them, and stopping out certain
portions of that light, was laid down by Huggins in i864/ 2
The phenomena of double stars seem to indicate a connection
between the state of the investing atmospheres by the action
of which their often brilliantly contrasted tints are produced,
and their mutual physical relations. A remarkable tabular
statement put forward by Professor Holden in June i88o 3
made it, at any rate, clear that inequality of magnitude be-
tween the components of binary systems accompanies unlike-
ness in colour, and that stars more equally matched in one
respect, are pretty sure to be so in the other. Besides, blue
and green stars of a decided tinge are never (so far as is cer-
tainly known) solitary ; they invariably form part of systems.
So that association has undoubtedly a predominant influence
upon colour.
Nevertheless, the crude notion thrown out by Zollner in
1 86 5, 4 that yellow and red stars are simply white stars in various
stages of cooling, has obtained undeserved currency. D' Arrest,
it is true, protested against it, but Yogel adopted it in 1874 as
the "rational" basis of his classification. 5 This differs from
Father Secchi's only in presenting his third and fourth types
as subdivisions of the same order ; but the seductive, though
possibly misleading idea of progressive development is added.
1 Von Konkoly once derived from a slow-moving meteor a hydro-carbon
spectrum. A. S. Herschel, Nature, vol. xxiv. p. 507. 2 Phil. Trans.,
"vol. cliv. p. 429. 3 Am. Jour, of Sc., vol. xix. p. 467. 4 Photom. Unters.,
p. 243. 5 A sir. Nacli., No. 2000.
424 HISTORY OF ASTRONOMY. PART n.
Thus, the white Sirian stars are represented as the youngest,
because the hottest of the sidereal family ; those of the solar
pattern as having already wasted much of their store by
radiation, and being well advanced in middle life ; while the
red stars with banded spectra figure as effete suns, hastening
rapidly down the road to final extinction. 1
Now the truth is, that we are just as ignorant of the relative,
as of the absolute ages of the stars. The essential difference
between one order and another so far as the spectroscope
informs us resides in the strength and quality of absorption
exercised by their atmospheres. There is no valid reason to
suppose that the absolute quantity of heat contained in a
white star is greater cceteris paribus than that contained in
a red. All that we are entitled to assert is that its gaseous
surroundings are vastly more translucent, and, in all probability,
much more restricted.
Yogel's scheme, moreover, is incomplete. It traces the
downward curve of decay, but gives no account of the slow
ascent to maturity. The present splendour of Vega, for
instance, was prepared, according to all creative analogy, by
almost endless processes of gradual change. What was its
antecedent condition 1 If its growth was by condensation, it
is likely to have possessed formerly a denser and more extensive
atmosphere than it now does. Its actual unveiled incand-
escence must, in that case, have emerged from the comparative
obscurity imposed upon it by the surging of turbulent and
heterogeneous vapours to great heights above its photosphere.
It would then have given a "banded spectrum." So that red
may after all be " younger " than white stars. The possibility,
however, should not be lost sight of, that development has
nothing to do with stellar types. Quite conceivably, they are
the badge of species distinct ab origine, never to be merged or
transformed.
1 Mr. J. Birmingham, in the Introduction to his valuable Catalogue of
Red Stars, comments upon this " singular conceit," and alleges various
instances of change of colour in a direction the opposite of that which it
supposes to be the inevitable result of time. Trans. R. Irish Ac., vol.
xxvi. pp. 251-253.
CHAP. xii. STARS AND NEBULA. 425
A spectroscopic star-catalogue (the first attempted) is now
in course of preparation at Potsdam and Lund by Drs. Yogel
and Duner. It will include all stars down to magnitude 7^
situated between the north pole of the heavens and one degree
south of the equator. The first part, giving the results of obser-
vations upon the spectra of 4051 stars (12,000 were incidentally
examined), was published in I883, 1 and a further instalment
will shortly follow. The provision of such a vast and accurate
store of data for future reference is a duty, in Vogel's estima-
tion, which the present generation owes to posterity, and may
prove of inestimable importance to the progress of discovery.
Meanwhile, M. Duner issued at Stockholm in 1884, a special
work of striking interest. This is a catalogue of 352 stars
with banded spectra, of which 297 belong to Secchi's third,
55 to his fourth class (Yogel's classes Ilia, and III?;.). For
the latter kind of very rare objects, the record is complete, so
far as detection has yet gone ; while 476 stars in all are known
to belong to the family of Mira and Betelgeux. One fact well
ascertained as regards both species, is the invariability of the
type. The prismatic flutings of the one, and the broader
zones of the other, are as if stereotyped they undergo, in
their fundamental outlines, no modification in passing from
star to star. They are always accompanied by, or superposed
upon, a spectrum of dark lines, in producing which sodium
and iron have an obvious share ; but detailed examination is
encompassed with difficulties. Betelgeux alone among stars of
the "banded " sort, has been submitted to a strict analysis.
A fairly complete answer to the question, "What are the
stars made of? was given by Dr. Huggins in 1864.2 By
laborious processes of comparison between stellar dark lines
and the bright rays emitted by terrestrial substances, he made
quite sure of his conclusions, though at much cost of time
and pains. He assures us, indeed, that taking into account
restrictions by weather and position the thorough investiga-
1 Publicationen, No. II, Potsdam, 1883. 2 Phil. Trans., vol. cliv. p.
413. Some preliminary results were embodied in a "note " communicated
to the Royal Society, February 19, 1863 (Proc. Roy. Soc., vol. xii. p. 444).
426 HISTORY OF ASTRONOMY. PART n.
tion of a single star-spectrum would be the work of some years.
Of two, however those of Betelgeux and Aldebaran he was
able to furnish detailed and accurate drawings. The dusky
flutings in the prismatic light of the first of these stars have
not been identified with the absorption of any particular sub-
stance ; but associated with them are dark lines telling of the
presence of sodium, iron, calcium, magnesium, and bismuth.
Hydrogen rays are also inconspicuously present. That an
exalted temperature reigns, at least in the lower strata of the
atmosphere, is certified by the vaporisation there of matter so
refractory to heat as iron. 1
Nine elements those identified in Betelgeux, with the addi-
tion of tellurium, antimony, and mercury were recognised as
having stamped their signature on the spectrum of Aldebaran ;
while the existence in Sirius, and nearly all the other stars
inspected, of hydrogen, sodium, iron, and magnesium was
rendered certain or highly probable. This was admitted to be
a bare gleaning of results ; nor is there reason to suppose any
of his congeners inferior to our sun in complexity of consti-
tution.
The evidence given by the spectroscope of fluctuations in
quality as well as quantity, in the light of variable stars,
suggests a rationale of the surprising appearances presented
by them, which may eventually be expected to supersede all
others. Speaking generally, stellar variability is an accom-
paniment of a ruddy tint and a banded spectrum. In other
words, it prevails in stars surrounded by powerfully absorptive
atmospheres. Moreover, the strength of their absorption in-
creases as the light diminishes; perhaps we might say, the
light diminishes because it increases.
Heretofore the explanations of variability chiefly entertained
had been these two : rotation on an axis, showing alternately
a darker and a brighter side, and the interposition of a non-
luminous body, revolving round a star periodically eclipsed by
it. But in truth, the facts, for the most part, fitted very ill
with either, and were, not unfrequently, in glaring disaccord
1 Phil. Trans., vol. cliv. p. 429, note.
CHAP. xii. STARS AND NEBULJS. 427
with both. There are, however, a few exceptional cases to
which the " eclipse " theory has been thought to be peculiarly
applicable ; and assuredly, if it fail in them, it will succeed
nowhere else. The leading member of this small group is the
star called Algol in the head of Medusa.
It stands apart in several respects from most other variables.
In the first place, it is a white star, and shows a spectrum of
the Sirian pattern two circumstances highly favourable to
stability in lustre. Further, the diminution of its light is by
a strictly impartial process \ no individual rays are attacked
more than others ; it remains unchanged in kind even when
reduced to a sixth of its original amount. Finally, the accom-
plishment of its decline and revival, instead of being distributed,
with more or less of irregularity, over its entire period, is
restricted to a perfectly definite fractional part of it. During
somewhat more than two days and a half it shines quite steadily
as a star of the second magnitude ; its fall to, and recovery
from the fourth are hurried over in about seven hours.
This manner of procedure suggested to Goodricke, who dis-
covered in 1782 the periodical variability of Algol, the inter-
position of a large satellite ; and the explanation has been
generally accepted. The conditions under which it must be
available were, however, first seriously investigated by Pro-
fessor Pickering in iSSo. 1 He found that the appearances in
question could be quite satisfactorily accounted for by admitting
the revolution round the star of an obscure body 0.764 of its
own diameter, in a period of two days twenty hours forty-nine
minutes. It needs, indeed, a mind trained to the docile adoption
of views authoritatively recommended, to contemplate without
some measure of incredulity a system in which a satellite of
the same relative magnitude that 466 Jupiters would bear to
our sun, circulates in a relative contiguity to its primary only
a little less close than that of his inner satellite to Mars.
But, as Professor Pickering remarks in a similar connection,
1 Proc. Am. Ac. Sc., vol. xvi. p. 17 ; Observatory, vol. iv. p. 116. For a
preliminary essay by T. S. Aldis iu 1870, see Phil. Mag. vol. xxxix. p.
363.
428 HISTORY OF ASTRONOMY. PART n.
" what could be more improbable than the phenomenon itself,
were it not verified by observation 1 " l
The Algol class of variables includes as yet only eight
members. If the hypothesis of an eclipsing body represent
the truth in one case, it must be capable of adaptation to all.
But the attempt to fit it to a remarkable star in the constella-
tion Cepheus, discovered by M. Ceraski at Moscow, June 23,
1880, may be said to have broken down. Its phases are of
the same rapid and well-defined description as those of Algol,
and recur in the still shorter period of two and a half days.
Its bluish white rays, however, turn ruddy at minimum,
which implies, not mere stoppage, but selective absorption.
We are, indeed, asked to admit that, in this case, the eclipse
is a total one by a partially luminous body, the light of
which might of course be supposed redder than that of its
primary. But this is not the only difficulty. Irregularities
and complexities have been detected, completely subversive
of the proposed rationale. Not alone have deviations of ten
to thirteen minutes from the computed times of minimum
been disclosed by Mr. Knott's observations, i88o-4, 2 one
anticipation noted by Mr. Baxendell even exceeding forty
minutes, but high and low minima are found to alternate,
showing the period and curve of luminous change to be double.
Dr. Wilsing of Potsdam discovered further, by means of exact
photometric comparisons, that the two curves are geometrically
unlike, the light increasing faster than it diminished in No. I.,
and diminishing faster than it increased in No. II. 3 No
occulting arrangements, however skilfully devised, can unravel
such a tangled skein of vicissitudes.
Mr. Gore's " Catalogue of Known Variables " 4 included, in
1884, 190 entries, and the number is continually increasing.
Indeed, Dr. Gould is of opinion that most stars fluctuate
slightly in brightness through surface-alternations similar to,
but on a larger scale than those of the sun. The solar analogy
might, perhaps, be pushed somewhat further. It may be
1 Proc. Am. Ac., vol. xvi. p. 259. 2 Astr. Nach. No. 2596. 3 Ibid.
4 Proc. P. Irish Ac., July 1884.
CHAP. xii. STARS AND NEBULAE. 429
found to contain a clue to much that is perplexing in stellar
behaviour. Wolf pointed out in 1852 the striking resemblance
in character between curves representing sun-spot frequency,
and curves representing the changing luminous intensity of
many variable stars. There were the same steep ascent to
maximum and more gradual decline to minimum, the same
irregularities in heights and hollows, and, it may be added, the
same tendency to a double maximum, and complexity of super-
posed periods. It is impossible to compare the two sets of
phenomena thus graphically portrayed, without reaching the
conclusion that they are of closely related origin that our
sun, in fact, is of the kindred of variable stars, though the
family peculiarities have, for some reason, remained compara-
tively undeveloped.
Every kind and degree of variability is exemplified in the
heavens. At the bottom of the scale are stars like the sun,
of which the lustre is tried by our instrumental means
sensibly steady. At the other extreme are ranged the astound-
ing apparitions of "new," or "temporary" stars. Within
the last score of years five of these stellar guests (as the
Chinese call them) have presented themselves, and we meet
with a sixth no farther back than April 27, 1848. But of the
" new star " in Ophiuchus found by Mr. Hind on that night,
little more could be learnt than of the brilliant objects of the
same kind observed by Tycho and Kepler. The spectroscope
had not then been invented. Let us hear what it had to tell
of later arrivals.
Between thirty and fifteen minutes before midnight of May
12, 1866, Mr. John Birmingham of Millbrook, near Tuam, in
Ireland, saw with astonishment a bright star of the second
magnitude unfamiliarly situated in the constellation of the
Northern Crown. Four hours earlier, Schmidt of Athens had
been surveying the same part of the heavens, and was able to
testify that it was not visibly there. That is to say, a few
hours, or possibly a few minutes, sufficed to bring about a
conflagration, the news of which may have occupied hundreds
of years in travelling to us across space. The rays which
430 HISTORY OF ASTRONOMY. PART n.
were its messengers, admitted within the slit of Dr. Huggins's
spectroscope, May 16, proved to be of a composition highly
significant as to the nature of the catastrophe. The star
which had already declined below the third magnitude
showed what was described as a double spectrum. To the
dusky flutings of Secchi's third type four brilliant rays were
added. 1 The chief of these agreed in position with lines of
hydrogen ; so that the immediate cause of the outburst was
plainly perceived to have been the eruption, or ignition, of
vast masses of that subtle kind of matter, the universal im-
portance of which throughout the cosmos is one of the most
curious facts revealed by the spectroscope.
T Coronae (as the new star was called) quickly lost its adven-
titious splendour. Nine days after its discovery it was again
invisible to the naked eye. It is now a pale yellow, slightly
variable star near the tenth magnitude, and finds a place as
such in Argelander's charts. It was thus obscurely known
before it made its sudden leap into notoriety.
The mantle of gaseous incandescence in which it was tem-
porarily wrapt, is a permanent or recurrent feature in some
other stars. Two of these /3 Lyrse, a white star variable
(by a rare exception) in a period of twelve days and nearly
twenty-two hours, and y Cassiopeiae were noticed by Father
Secchi at the outset of his spectroscopic inquiries. Both show
bright lines of hydrogen and helium, so that the peculiarity
of their condition probably consists in the unusual extent, and
intense ignition of their chromospheric surroundings. But
this condition is subject to change. The brilliant rays indi-
cative of it fade and flash out again with very singular alter-
nations. Dr. Yogel's observations at Bothkamp in 1871-2
already afforded him a suspicion of such vicissitudes ; 2 but
their ascertainment is due to M. Eugen von Gothard. After
the completion of his new astrophysical observatory at Hereny
in the autumn of 1881, he repeatedly observed the spectra of
both stars without perceiving a trace of bright lines ; and was
1 Proc. Roy. Soc., vol. xv. p. 146. 2 Bothkamp Beobaclitungen, Heft
ii. p. 29.
CHAP. xii. STARS AND NEBULA. 431
thus taken quite by surprise when he caught a twinkling of
the crimson C in 7 Cassiopeia, August 13, I883. 1 A few
days later, the whole range (including D 3 ) was lustrous.
Duly apprised of the recurrence of a phenomenon he had
himself vainly looked for during some years, M. von Konkoly
took the opportunity of the great Vienna refractor being placed
at his disposal to examine with it the relighted spectrum on
August 27.2 In its wealth of light C was dazzling; D 3 , and
the green and blue hydrogen rays, shone somewhat less vividly ;
D and the group b showed faintly dark; while three broad
absorption- bands, sharply terminated towards the red, diffuse
towards the violet, shaded the spectrum near its opposite
extremities. They thus agreed with the zones of "carbon-
stars " in the plan of their structure, though not at all in
position; but proved significantly what the spectrum of T
Coronse had already rendered apparent the compatibility in
stellar atmospheres of fluted absorption with a high state of
incandescence.
The previous absence of bright lines from the spectrum of
this star was, however, by no means so protracted or complete
as M. von Gothard supposed. At Dunecht, C was " superbly
visible " to Lord Lindsay, Drs. Copeland and J. G. Lohse,
December 20, 1879 ; 3 F was seen bright on October 28 of the
same year, and frequently at Greenwich in 1880-1, indeed
much more conspicuously so than three years later. The
curious fact has moreover been adverted to by Dr. Copeland,
that C is much more variable^than F. To Vogel, June 18,
1872, the first was invisible, while the second was bright; at
Dunecht, January n, 1887, the conditions were so far in-
verted that C was resplendent, F comparatively dim.
The spectrum of /3 Lyras is subject to analogous transi-
tions. Perfectly continuous, as observed at Here"ny, June 1 7
and July 24, 1882, it was interrupted by dark lines of
hydrogen, September 5.* These were first seen bright by
Von Gothard, August 26, 1883. The helium-ray was, how-
l 'Astr. Nach. No. 2539. 2 Ibid., No. 2548. 3 Observatory, vol. x. p.
82. 4 Astr. Nach. 2581.
432 HISTORY OF ASTRONOMY. PART n.
ever, found to vary independently of, and even more strikingly,
than the hydrogen lines. During 1884 it was followed through
several complete cycles from dazzling brilliancy to total ex-
tinction, in a period provisionally estimated at seven days. 1
No connection has yet been made out between these unac-
countable changes and the fluctuations in the general light of
the star ; and similar emissive alternations are in 7 Cassiopeise
unattended by any perceptible variability in brightness. Both
stars should be carefully and continuously watched, if possible,
in a better climate than our own.
These two luminaries formed the nucleus of what is now
generally regarded as a distinct stellar class. To it belong the
extraordinary variable j Argus, with 7 in the same constella-
tion, examined by Respighi in 1871; and it includes three
small stars in the Swan, the peculiar character of which was
discovered in 1867 by MM. Wolf and Rayet of the Paris
Observatory. 2 Their light betrayed a mainly gaseous origin,
separating into about five lines or bands, identical in each
star, and connected by an almost evanescent continuous
spectrum. One of these was found by Vogel, using the
27-inch Vienna equatoreal in the summer of 1883, to corre-
spond with the green line of hydrogen. The others are
unidentified. He could perceive no sign of change since his
last observations of these remarkable spectra ten years pre-
viously. 3 Seven analogous objects have since been discovered
by Professor Pickering, the four last by photographic means ; 4
and five more were found by Dr. Copeland in 1883 in the
course of an excursion exploratory of visual possibilities in the
Andes, 5 besides (at Dunecht in 1884) a fine additional northern
example in Cygnus.
Now the question arises, have we here to do with stars in
the ordinary sense at all 1 that is, with suns like our -own,
reduced by the immensity of their distance to sparkling points
of light? We should reply in the negative were the above
1 Astr. Nach., No. 2651-2. 2 Comptes Rendus, t. Ixv. p. 292. 3
Bd. xvii. p. 135. 4 Nature, vol. xxxiv. p. 440. 5 Copernicus, vol. iii. p.
207.
CHAP. xii. STARS AND NEBULAE. 433
definition to be adopted ; but our readers will have already
gathered that it requires much extension and qualification.
How far we may yet be led to extend it, and how profoundly
our ideas of what constitutes a "star" may eventually have
to be modified, a recent noteworthy event has gone a great
way to indicate.
On the 24th of November 1876, at Athens, Dr. Schmidt
discovered a new star in the constellation Cygnus. It was
then nearly of the third magnitude, and in its previous state
must have been below the ninth, since Argelander had made
no record of its existence. Its spectrum was examined
December 2, by Cornu at Paris, 1 and a few days later by
Vogel and O. Lohse at Potsdam. 2 It proved of a closely
similar character to that of T Coronae. A range of bright
lines, including those of hydrogen, helium, and perhaps of the
coronal gas (1474), stood out from a continuous background
strongly " fluted " by absorption. It may be presumed that
in reality the gaseous substances, which, by their sudden
incandescence had produced the apparent conflagration, lay
comparatively near the surface of the star, while the screen
of cooler materials rhythmically intercepting large portions
of its light, was situated at a considerable elevation in its
atmosphere.
The object, meanwhile, steadily faded. By the end of the
year it was of no more than seventh magnitude. After the
second week of March 1877, strengthening twilight combined
with the decline of its radiance to arrest further observation.
It was resumed, September 2, at Dunecht, with a strange
result. Practically the whole of its scanty light (it had then
sunk below the tenth magnitude) was perceived to be gathered
into a single bright line in the green, and that the most
characteristic line of gaseous nebulae. 3 The star had, in fact, so
far as outward appearance was concerned, become transformed
into a planetary nebula, many of which are so minute as to
be distinguishable from small stars only by the quality of
1 Comptes Rendus, t. Ixxxiii. p. 1172. 2 Monatsb., Berlin, 1877, pp. 241,
826. 3 Copernicus, vol. ii. p. 101.
2 E
434 HISTORY OF ASTRONOMY. PART n.
their radiations. The nebular phase, however, seems to have
been transient. In the course of 1880, Professor Pickering
found that Nova Cygni gave an ordinary stellar spectrum of
barely perceptible continuous light ; 1 and his observation was
negatively confirmed at Dunecht, February i, 1881.
This enigmatical object has now dropt to about the sixteenth
magnitude, being thus entirely beyond the reach of spectro-
scopic scrutiny. The lesson learnt from its changes appears
to be no less than this : That no clear dividing-line can be
drawn between stars and nebulae ; but that in what are called
" planetary nebulae " on the one side, and in " gaseous stars "
(those giving a spectrum of bright lines) on the other, we
meet with transitional forms, serving to bridge the gap between
such vast and highly finished orbs if we may be permitted
the expression as Sirius, and the inchoate, faintly-lucent
stuff which curdles round the trapezium of Orion.
A supposed new star in Orion's Club (near ^ Orionis)
observed by Mr. J. E. Gore, December 13, 1885, proved to
be a till then unknown and highly interesting variable,
closely akin to Mira. By the end of April it had declined
from nearly the sixth to below the ninth magnitude ; 2 but
rose to a second maximum (determined by Dr. Miiller) on
December 12, 1886. Its period of 364 days is just one
calendar month longer than that of the " wonderful " star in
Cetus ; and it shows a precisely similar colonnaded spectrum
of great vividness and beauty. 3
Perhaps none of the marvellous changes witnessed in the
heavens has given a more significant hint as to their con-
struction, than the stellar blaze kindled in the heart of the
great Andromeda nebula some undetermined number of years
or centuries before its rays reached the earth in the month
of August 1885. The first published discovery was by Dr.
Hartwig at Dorpat on August 3 1 ; but it was found to have
been already seen, on the iQth, by Mr. Isaac W. Ward of
Belfast, and on the iyth by M. Ludovic Gully of Rouen.
1 Annual Report, 1880, p. 7. 2 Miiller, Astr. Nacli., No. 2734.
3 C. Wolf, Comptes Rendus, t. ci. p. 1444.
CHAP. xir. STARS AND NEBULA. 435
The negative observations, on the 1 6th, of Tempel 1 and Max
Wolf, limited very narrowly the epoch of the apparition.
Nevertheless, it did not, like most temporaries, attain its
maximum brightness all at once. When first detected, it
was of the ninth, by September i, it had risen to the seventh
magnitude, from which it so rapidly fell off that in March it-
touched the limit of visibility (sixteenth magnitude) with the
Washington 26-inch. Its light bleached very perceptibly as
it faded. 2 During the earlier stages of its decline, the con-
trast was striking between the sharply defined, ruddy disc of
the star, and the hazy, greenish-white background upon which
it was projected, 3 and with which it was inevitably suggested
to be in some sort of physical connection.
Let us consider what evidence was really available on this
point. To begin with, the position of the star was not exactly
central. It lay sixteen seconds of arc to the south-west of
the true nebular nucleus. Its appearance did not then signify
a sudden advance of the nebula towards condensation, nor
was it attended by any visible change in it save the transient
effect of partial effacement through superior brightness.
Equally indecisive information was derived from the spec-
troscope. To Yogel, Hasselberg, and Young, the light of the
" Nova " seemed perfectly continuous ; but Dr. Huggins
caught traces of bright lines on September 3, confirmed on
the gth ; 4 and Dr. Copeland succeeded, on September 30, in
measuring three bright bands with a special acute-angled
prism constructed for the purpose. 5 A shimmer of F was
suspected, and had also been perceived by Mr. O. T. Sherman
of Yale College. Still the effect was widely different from
that of the characteristic blazing spectrum of a temporary
star, and prompted the surmise that here too a variable might
be under scrutiny. The star, however, was certainly so far
" new," that its rays, until their sudden accession of strength,
1 Astr. Nach., No. 2682. 3 A. Hall, Am. Jour, of Sc., vol. xxxi. p.
301. 3 Young, Sid. Messenger, vol. iv. p. 282 ; Hasselberg, Astr. Nach.,
No. 2690. 4 Report Brit. Ass. 1885, p. 935. 5 Month. Notices, vol.
xlvii. p. 54.
436 HISTORY OF ASTRONOMY. PART n.
were too feeble to affect even our reinforced senses. Not one
of the 1283 small stars recorded in charts of the nebula could
be identified with it ; and a photograph taken by Mr. Common,
August 1 6, 1884, on which a multitude of stars down to the
fifteenth magnitude had imprinted themselves, showed the
uniform, soft gradation of nebulous light to be absolutely
unbroken by a stellar indication in the spot reserved for the
future occupation of the " Nova." l
So far then the view that its relation to the nebula was a
merely optical one, might be justified; but it became alto-
gether untenable when it was found that what was taken
to be a chance coincidence had repeated itself within living
memory. On the 2ist of May 1860, M. Auwers perceived
at Kbnigsberg a seventh magnitude star shining close to the
centre of a nebula in Scorpio, numbered 80 in Messier's
catalogue. 2 Three days earlier it certainly was not there,
and three weeks later it had vanished. The effect to Mr.
Pogsori (who independently discovered the change, May 28) 3
was as if the nebula had been replaced by a star, so entirely
were its dim rays overpowered by the concentrated blaze in
their midst. Now it is simply incredible that two outbursts
of so uncommon a character should have accidentally occurred
just on the line of sight between us and the central portions
of two nebulae ; we must then conclude that they showed on
these objects because they took place in them. The most
favoured explanation is that they were what might be called
effects of over-crowding that some of the numerous small
bodies, presumably composing the nebulae, jostled together in
their intricate circlings, and obtained compensation in heat
for their sacrifice of motion. But this is scarcely more than
a plausible makeshift of perplexed thought. Mr. W. H. S.
Monck, on the other hand, has suggested that new stars appear
when dark bodies are rendered luminous by rushing through
the gaseous fields of space, 4 just as meteors kindle in our
atmosphere. The idea is ingenious, but does not (nor was it
1 Nature, vol. xxxii. p. 522. 2 Astr. Nach., Nos. 1267, 2715.
9 Month. Notices, vol. xxi. p. 32. ib 4 Observatory, vol. via. p. 335.
CHAP. xn. STARS AND NEBULA. 437
designed to) apply to our present case. Neither of the objects
distinguished by the striking variations just described, is of
gaseous constitution. That in Scorpio appears under high
magnifying powers as a " compressed cluster ; " that in Andro-
meda is perhaps, as Sir J. Herschel suggested, "optically
nebulous through the smallness of its constituent stars " l if
stars they deserve to be called.
We have been compelled somewhat to anticipate our narra-
tive as regards inquiries into the nature of nebulae. The ex-
cursions of opinion on the point were abruptly restricted and
defined by the application to them of the spectroscope. On
August 29, 1864, Dr. Huggins sifted through his prisms
the rays of a bright planetary nebula in Draco. 2 To his
infinite surprise, they proved to be mainly of one colour. In
other words, they avowed their origin from a mass of glowing
vapour. As to what kind of vapour it might be by which
Herschel's conjecture of a "shining fluid" variously diffused
throughout the cosmos, was thus unexpectedly verified, an
answer was also at hand. The conspicuous bright line of the
Draco nebula was found to belong very probably to nitrogen ;
of its two fainter companions, one was unmistakably the F line
of hydrogen, while the other, in position intermediate between
the two, still remains unidentified. The extreme faintness of
nebular light was experimentally shown to be reason sufficient
for the solitariness in its spectrum of the lines emanating re-
spectively from nitrogen and hydrogen ; the surviving nebular
rays being precisely those which resist extinction longest.
By 1868, Dr. Huggins had satisfactorily examined the
spectra of about seventy nebulae, of which one-third dis-
played a gaseous character. 3 In all of these (and the rule has
hitherto proved without exception) the nitrogen line appeared ;
though in some cases as "the Dumb-bell" nebula in Yul-
pecula it appeared alone. On the other hand, a fourth line,
the dark blue of hydrogen in addition to the normal three,
was subsequently detected in the light of the great Orion
1 Observatory, vol. viii. p. 325 (Maunder). z Phil. Trans., vol. cliv. p.
437. 3 Ibid., vol. clviii. p. 540.
438 HISTORY OF ASTRONOMY. PART n.
nebula. But, fundamentally, the composition of all bodies of
this class may be assumed the same. The differences in their
radiations seem to be of intensity, not of kind. All planetary
and annular nebulae belong to it, as well as those termed
" irregular ?) which frequent the region of the Milky Way.
Thus the signs of resolvability noted at Parsonstown and
Cambridge (U.S.) in Orion and the " Dumb-bell," were proved
fallacious, so far, at least, as they had been taken to indicate
a stellar constitution ; though they may have quite faithfully
corresponded to the existence in discrete masses of the glow-
ing vapours elsewhere more equably diffused.
The well-known nebula in Andromeda, and the great spiral
in Canes Venatici are among the more remarkable of those
giving a continuous spectrum j and, as a general rule, the
emissions of all such nebulae as present the appearance of star-
clusters grown misty through excessive distance, are of the
same kind. It would, however, be eminently rash to conclude
thence that they are really aggregations of sun-like bodies.
The improbability of such an inference has been greatly en-
hanced by the occurrence, at an interval of a quarter of a cen-
tury, of stellar outbursts in the midst of two of them. For it
is practically certain that, however distant the nebulae, the
stars were equally remote ; hence, if the constituent particles
of the former be suns, the incomparably vaster orbs by which
their feeble light was well-nigh obliterated must, as was argued
by Mr. Proctor, have been on a scale of magnitude such as the
imagination recoils from contemplating.
Among the ascertained analogies between the stellar and
nebular systems is that of variability of light. On October
n, 1852, Mr. Hind discovered a small nebula in Taurus.
Chacornac observed it at Marseilles in 1854, but was con-
founded four years later to find it vanished. D'Arrest missed
it October 3, and re-detected it December 29, 1861. It was
easily seen in 1865-66, but invisible in the most powerful
instruments iSyy-So. 1 This was the first undisputed instance
1 Chambers. Descriptive Astronomy (3d ed.), p. 543 ; Flammarion,
L'Univers Sideral, p. 8 1 8.
CHAP. xii. STARS AND NEBULA. 439
of nebular variability. Brought to the notice of astronomers by
D' Arrest in I862, 1 it has since been confirmed by others of the
same nature. Two such, exhibiting probably periodical changes,
were adduced by Winnecke in 1879 ; 2 and Professor Holden,
having co-ordinated in his admirable " Monograph of the Nebula
of Orion " 3 the results of all the more prominent inquiries into
the structure of that marvellous object since 1758, reached
the conclusion that, while the figure of its various parts has
(with only one possible exception) remained the same, their
brightness has been and is in a state of continual fluctuation.
This accords precisely with the conviction expressed by O.
Struve in 1857,* and awaits only the ratification which it
may be expected to obtain from a comparison of photographs
taken at some years' interval, to be accepted as an ascertained
fact.
More dubious is the case of the " trifid " nebula in Sagit-
tarius, investigated by Professor Holden in 187 7. 5 That
change of some kind has occurred, is indeed established by a
comparison of his own and others' observations with those of
the two Herschels ; but he inclines to the view that motion
with or without an accompanying variation in light, is here the
agent of change. What is certain is, that a remarkable triple
star which, during the years 1784-1833, was centrally situated
in a dark space between the three great lobes of the nebula,
has since become involved in one of them ; and since the
star gives no sign of sensible displacement, the movement if
movement there should prove to be must be thrown upon
the nebula.
A similar example was in 1885 alleged by Mr. H. Sadler, 6
but the evidence upon which it rests is disputed. The ascer-
tainment of true proper motion in a nebula would be the
more interesting from its absolute novelty. Hitherto this
class of bodies have shown no sign of sharing the busy journey-
1 Astr. Nach.,No. 1366. 2 Month. Not., vol. xxxviii. p. 105; Astr.
Hack., No. 2293. 3 Wash. Obs., vol. xxv. App. I. 4 Month. Not., vol.
xvii. p. 230. 5 Am. Jour, of Sc., vol. xiv. p. 433. G Observatory, vol.
viii. p. 127. For Dreyer's rebutting evidence, see Ibid., p. 175.
440 HISTORY OF ASTRONOMY. PART 11.
ings of the stars. 1 They have remained as seemingly fixed in
their places as if exempt from all relation with the multitu-
dinous hosts of the galactic world. This singular immobility
might, on a casual view, be set down to the account of enor-
mous distance, no single nebula having, so far, exhibited the
faintest trace of parallactic displacement. But there is a
method of estimating motion independent of distance, and to
this also nebulae have hitherto proved unresponsive.
The principle upon which " motion in the line of sight " can
be detected and measured with the spectroscope, has already
been explained. 2 It depends, as our readers will remember,
upon the removal of certain lines, dark or bright (it matters
not which), from their normal places by almost infinitesimal
amounts. The whole spectrum of the moving object, in fact,
is very slightly shoved hither or thither, according as it is
travelling towards or from the eye j but, for convenience of
measurement, one line is usually picked out from the rest,
and attention concentrated upon it. The application of this
method to the stars, however, is encompassed with difficulties.
It needs a powerfully dispersive spectroscope to show line-
displacements of the minute order in question; and powerful
dispersion involves a strictly proportionate enfeeblement of
light. This, where the supply is already to a deplorable
extent niggardly, can ill be afforded ; and it ensues that the
operation of determining a star's approach or recession is,
even apart from atmospheric obstacles, an excessively deli-
cate one.
It was first successfully executed by Dr. Huggins early in
i868. 3 The brightest star in the heavens was selected as the
most promising subject of experiment, and proved amenable.
In the spectrum of Sirius, the F line was perceived to be just
so much displaced towards the red as to indicate (the orbital
motion of the earth being deducted) recession at the rate of
twenty-nine miles a second. Of this an undetermined propor-
1 Some instances of supposed orbital movement in " double " nebulae are
given by Flammarion, Comptes Rendus, t. Ixxxviii. p. 27. 2 See ante,
p. 245. 3 Phil. Trans., vol. clviii. p. 529.
CHAP. xii. STARS AND NEBULA. 441
tion was no doubt attributable to the advance through space
of the solar system, for which Struve's estimate of four miles a
second was almost certainly too small. Still there remained a
large surplus of Sirian proper motion. Its reality and direction
were placed beyond doubt by Yogel and Lohse's observation,
March 22, 1871, of a similar, but even more considerable dis-
placement. 1 The inquiry was resumed by Dr. Huggins with
improved apparatus in the following year, when the movements
of thirty stars were approximately determined. 2 The retreat
of Sirius was now diminished in estimated velocity to about
twenty miles per second, and it was discovered to be shared, at
rates varying from twelve to twenty-nine miles, by Betelgeux,
Bigel, Castor, Regulus, and five of the principal stars in the
Plough. Arcturus, on the contrary, gave signs of rapid ap-
proach (fifty-five miles a second), as well as Pollux, Vega,
Deneb in the Swan, and the brightest of the Pointers.
The realisation of this method of investigating stellar
motions has an importance far beyond that of which the idea
is conveyed by the bare enumeration of its preliminary results.
It may confidently be expected to play a leading part in the
unravelment of the vast and complex relations which we can
dimly detect as prevailing amongst the innumerable orbs of
the sidereal world ; for it supplements the means which we
possess of measuring by direct observation movements trans-
verse to the line of sight, and thus completes our knowledge
of the courses and velocities of stars at ascertained distances,
while supplying for all a valuable index to the amount of
perspective foreshortening of apparent movement. Thus some,
even if an imperfect, knowledge may at length be gained of
the revolutions of the stars of the systems they unite to form,
of the paths they respectively pursue, and of the forces under
the compulsion of which they travel.
Already, though the method can scarcely be said to have
passed the tentative stage, a most curious fact has been
brought to light. Since 1874, spectroscopic measures of the
1 Schellen, Die Spectralanalyse, Bd. ii. p. 326 (ed. 1883). 2 Proc. Roy.
Soc., vol. xx. p. 386.
442 HISTORY OF ASTRONOMY. PART n.
visual component of stellar motions have been made part of
the regular work at the Royal Observatory, Greenwich. The
results have proved, on the whole, strongly confirmatory of
Dr. Huggins's. But in the movement of Sirius a perplexing
change has taken place. In March 1876 it was estimated
to be adding to its distance from the earth by twenty-seven
miles each second. In 1877 a slackening was perceived ; and
this progressively advanced, until, in 1882, the rate of reces-
sion was diminished to, or below, seven miles a second. A
reversal of direction was even anticipated, and shortly occurred.
The spectrum was markedly shifted towards the blue end,
November 16, 1883;* and a series of forty-five measures exe-
cuted by Mr. Maunder on thirteen nights in 1884, gave to
the star a mean motion of approach of twenty-two miles a
second. 2 It does not appear that the known elliptic revolution
of Sirius round the centre of gravity of its system will account for
these vicissitudes, although it is remarkable that they are sus-
pected also to affect, in some degree, the course of Procyon, a
star similarly circumstanced to Sirius in its vicinity to a com-
paratively obscure source of disturbance. The further develop-
ment of these significant changes will be of the highest interest.
Knowledge of the velocities of the stars will eventually lead
to a knowledge of the relative velocity of the sun. Already
M. Homann has made a preliminary attempt to derive from the
spectroscopically determined movements of 49 stars some in-
formation as to the drift through space of our system ; with
the result (to be taken for what it is worth) of shifting its
apex from the constellation Hercules to a point in the Milky
Way, near the tail of the Swan. 3 The corresponding solar
velocity is about fifteen miles a second.
None of the nebulae hitherto examined show the slightest
trace of displacement in the line of sight. 4 And this con-
1 Month. Not., vol. xliv. p. 91. 2 Observatory, vol. viii. p. 109. Dr.
Huggins in 1872 anticipated as a possible consequence of its circulation in
an orbit the occurrence of such changes in the movement of Sirius as have
actually been observed. Proc. Roy. Soc., vol. xx. p. 387. 3 Viertel-
jahrsschrift Astr. Ges., Jahrg. xxi. p. 59. 4 Huggins, Proc. Hoy. Soc., vol.
xxii. p. 251.
CHAP. xii. STARS AND NEBULJE. 443
elusion, unlike estimates of apparent movement across the sky,
has absolutely no connection with their greater or less remote-
ness. So that we seem compelled to draw an inference which
must largely affect our ideas of the whole structure of the
heavens ; namely, that nebulae, as a class, are very much
slower-moving bodies than stars.
The uses of photography in celestial investigations become
every year more manifold and more apparent. The earliest
chemical star-pictures were those of Castor and Vega, obtained
with the Cambridge refractor in 1850 by Whipple of Boston
under the direction of W. C. Bond. Double-star photography
was inaugurated under the auspices of G-. P. Bond, April 27,
1857, with an impression, obtained in eight seconds, of Mizar,
the middle star in the handle of the Plough. A series of
measures from sixty-two similar images, gave the distance
and position - angle of its companion with about the same
accuracy attainable by ordinary micrometrical operations ; and
the method and upshot of these novel experiments were de-
scribed in three papers remarkably forecasting the purposes
to be served by stellar photography. 1 The matter next fell
into the able hands of Rutherfurd, who completed in 1864 a
fine object glass (of n-J inches) corrected for the ultra-violet
rays, consequently useless for visual purposes. The sacrifice
was recompensed by conspicuous success. A set of measure-
ments from his photographs of nearly fifty stars in the
Pleiades, enabled Dr. Gould in 1866 to ascertain, by com-
parison with Bessel's places for the same stars, that during the
intervening quarter of a century no changes of importance
had occurred in their relative positions. 2
The construction of photographic star-maps of real and per-
manent value was thus demonstrated to be a possibility, and is
rapidly being converted into a reality of the utmost moment
to the future of science. In carrying on the work of ecliptical
1 Astr. Nach, Bande xlvii. p. I, xlviii. p. I, xlix. p. 8 1. Pickering,
Mem. Am. Ac., vol. xi. p. 180. 2 Gould on Celestial Photography, Obser-
vatory, vol. ii. p. 1 6. Professor Pritchard communicated to the Roy. Astr.
Soc., May 9, 1884, his detection of some small movements inter se of
members of the Pleiades group. Observatory, vol. vii. p. 163.
444 HISTORY OF ASTRONOMY. PART n.
charting, left half completed by Chacornac, the MM. Henry
encountered sections of the Milky Way which defied the
enumerating efforts of eye and hand, and resolved in conse-
quence to have recourse to the camera. The perfect success
of some preliminary trials made with an instrument constructed
expressly for the purpose, was announced to the Academy of
Sciences at Paris, May n, 1885. By its means, stars down
to the sixteenth magnitude clearly record their presence and
their places ; and we are hence doubtless on the eve of seeing
the co-operative photographic survey of the heavens, recom-
mended by Dr. Gill, carried into execution. It is scarcely pos-
sible, however, that this magnificent project can be accomplished,
on the proposed scale, within less than ten years. About
14,000 plates will first have to be exposed, during one hour
each, to a perfectly clear, dark sky under conditions, that is
to say, not too common at ordinary sea-level observatories.
The object in view, however, is of such sublime importance,
and the enthusiasm excited by it so strong and widespread,
that there can be little doubt of its eventual realisation. The
places, at a given date, of at least twenty million stars, many
of them beyond the optical range of the telescopes employed
will then be securely ascertained. The enormous increase of
knowledge involved may be judged of from the fact that in a
space of the Milky Way in Cygnus 2 15' by 3, where 170
stars had been mapped by the old laborious method, about five
thousand have stamped their images on a single Henry plate.
Not only will the detailed and comprehensive Celestial
Atlas thus obtained serve future astronomers as a priceless
criterion of change, but the progress of the work cannot fail
to be attended by disclosures of great interest. Even in the
course of the experiments preliminary to it, novelties have
revealed themselves. A splendid photograph of 1421 stars
in the Pleiades, taken by the MM. Henry with three hours'
exposure, November 16, 1885, showed one of the brightest of
them to have a small spiral nebula, somewhat resembling a
strongly curved comet's tail, attached to it. The reappearance
of this strange appurtenance on three subsequent plates left
CHAP. xn. STARS AND NEBULA. 445
no doubt of its real existence, visually attested at Pulkowa,
February 5, 1886, by one of the first observations made with
the 30-inch equatoreal. 1 Much smaller apertures, however,
suffice to disclose the " Maia nebula," now it is known to be
there. Not only does it appear greatly extended in the Vienna
2 7 -inch, 2 but MM. Perrotin and Thollon have seen it with
the Nice 15 -inch, and M. Kammermann of Geneva, employ-
ing special precautions, with a refractor of only ten inches
aperture. 3 The advantage derived by him for bringing it
into view, from the insertion into the eye-piece of an uranium
film, gives, with its photographic intensity, cogent proof that
a large proportion of the light of this remarkable object is of
the ultra-violet kind.
This is not the only nebula in the Pleiades. On October
19, 1859, Wilhelm Tempel, who, drawn by an overmastering
impulse, had then recently exchanged his graver's tools for a
small telescope, discovered an elliptical nebulosity, originating
and stretching far to the southward from the star Merope.
It has since been pretty constantly observed, though its
extreme susceptibility to unfavourable aerial influences has
led to a perhaps unfounded suspicion of variability. Nothing
corresponding to this delicate object appeared on any of the
Henry plates ; but instead some streaky nebulous patches in
the same vicinity, never seen except by Mr. Common with his
great reflector, February 8, i88o. 4 A further mass near
Alcyone, perceived on the same occasion, received photo-
graphic confirmation elsewhere.
It is now about four years since Mr. Isaac Roberts of
Liverpool first turned his attention to celestial photography ;
and in March 1885, Mr. Howard Grubb mounted for him,
expressly for the purpose, a 2o-inch silver-on-glass reflector, in
which, to avoid loss of light by a second reflection, the image
is thrown from the large mirror directly upon the sensi-
tive plates exposed in its focus. 5 A picture of the Pleiades
thus procured in eighty-nine minutes, October 23, 1886,
1 Astr. Nach., No. 2719. 2 Ibid., No. 2726. 3 Ibid., No. 2730.
4 Month. Not., vol. xl. p. 376. 5 Ibid. vol. xlvi. p. 99.
446 HISTORY OF ASTRONOMY. PART n.
revealed nebulous surroundings to no less than four leading
stars of the group, namely, Alcyone, Electra, Merope, and
Maia ; and a second impression, taken in three hours on the
following night, showed further " that the nebulosity extends
in streamers and fleecy masses, till it seems almost to fill the
spaces between the stars, and to extend far beyond them." l
There can be little hesitation as to which of Mr. Roberts's
alternative explanations to adopt. That the principal stars
in the Pleiades are involved in one vast nebula, is a truth
made palpable by his striking photographs. That they are
involved in it " directly," and are not merely " in sight
alignment " with it, the visibly close relations of the stars to
the nebulous structure around them renders scarcely less
obvious. Thus Goldschmidt's notion that all the clustered
Pleiades constitute, as it were, a second Orion trapezium in
the midst of a huge formation of which Tempel's nebula is
but a fragment, 2 has been to some extent verified. Yet it
seemed fantastic enough in 1863.
Some progress, meanwhile, has been made in both hemi-
spheres with a more expeditious star survey than that about
to be organised at the approaching Astronomical Congress in
Paris. Mr. Roberts began May i, 1885, the work of con-
structing a photographic chart of the northern heavens on a
scale twice that of Argelander's "Atlas," and containing a
much larger number of stars. The size of field adopted is
2 of declination by about i J of right ascension, and each plate
is exposed fifteen minutes. 3 The southern " Durchmusterung,"
in progress at the Cape of Good Hope under Dr. Gill and Mr.
C. Ray Woods, will include the first nine or ten magnitudes,
and will probably be completed in a couple of years. Each
plate covers an area of about thirty-six square degrees, and a
thousand of them will be taken. An exposure of one hour is
allowed with a six-inch lens. The catalogue, for the construc-
tion of which from these materials arrangements have already
been made, will be both fuller and more accurate than Arge-
1 Month. Not., vol. xlvl p. 24. 2 Les Mondes, t. iii. p. 529. 3 Month.
Not., vol. xlvi. p. 99.
CHAP. xii. STARS AND NEBULA. 447
lander's the faint stars included in it will be more numerous,
and their places reliable within one second of arc.
In the use of photography as an engine of research into the
physical condition of the stars, Dr. Huggins led the way. In
March 1863 he obtained with his coadjutor, Dr. Miller, micro-
scopic prints of the spectra of Sirius and Capella. 1 But they
told nothing. No lines were visible in them. They were mere
characterless streaks of light. He tried again in 1876, when
the 1 8-inch speculum of the Royal Society had come into
his possession, using prisms of Iceland spar, and lenses of
quartz ; and this time with better success. A photograph
of the spectrum of Vega showed seven strong lines. 2 Still
he was not satisfied. He waited and worked for three years
longer. At length, on December 18, 1879, he was able to
communicate to the Royal Society 3 results answering to his
expectations. The delicacy of eye and hand needed to attain
them may be estimated from the single fact, that the image
of a star had to be kept, by continual minute adjustments,
exactly projected upon a slit ^4^ of an inch in width during
nearly an hour, in order to give it time to imprint the char-
acters of its analysed light upon a gelatine plate raised to the
highest pitch of sensitiveness.
The ultra-violet spectrum of the white stars of which Vega
was taken as the type was by this means shown to be a very
remarkable one. Twelve strong lines, arranged at intervals
diminishing regularly upwards, intersected it. They belonged
presumably to one substance ; and since the two least refran-
gible were known hydrogen rays, that substance could scarcely
be any other than hydrogen. This was rendered certain by
direct photographs of the hydrogen-spectrum taken by H. W.
Vogel at Berlin a few months earlier. 4 In them seven of the
white-star series of grouped lines were visible ; and the full
complement of twelve appeared on Cornu's plates in i886. 5
1 Month. Not., vol. xxiii. p. 180. 2 Proc. Roy. Soc., vol. xxv. p. 446.
H. Draper had succeeded, in 1872, in getting an impression of four lines
in the spectrum of the same star. 3 Phil. Trans., vol. clxxi. p. 669.
4 Astr. Nach., No. 2301. 5 Jour, de Physique, t. v. p. 98.
44 8 HISTORY OF ASTRONOMY. PART n.
In yellow stars, such as Capella and Arcturus, the same
rhythmical series was partially represented, but associated
with a great number of other lines; their state, as regards
ultra-violet absorption, thus approximating to that of the
sun ; while the redder stars betrayed so marked a deficiency
in actinic rays, that from Betelgeux, with an exposure forty
times that required for Sirius, only a faint spectral impression
could be obtained, and from Aldebaran, in the strictly invisible
region, almost none at all.
The same process was successfully applied to the Orion
nebula, March 7, I882. 1 Five lines in all stamped them-
selves upon the plate during forty-five minutes of exposure.
Of these, four were the known visible rays, and the fifth
seemed to agree with one of the hydrogen set displayed by
Yega. Almost simultaneously, this notable feat in celestial
photography was emulated by Dr. Draper at New York, 2
though his result was so far incomplete that the significant
fifth line failed to appear. On both the English and American
plates, the bright rays were connected by a faint continuous
spectrum derived from the nebulous "knots" near the trapezium.
This was thought to indicate an advance of central condensation
possibly even the beginning of the long birth-process of an
orderly revolving system, reserved for the future habitation of
rational beings. It may be so ; the ways of creative power
are dark. Yet we cannot help remarking that the presence
of so many stars fully formed, yet seemingly wrapt up arid
involved in the prodigious masses of nebulosity filling that
portion of the sky, appears in some degree to discount the
expectation of stellar development from them.
A photographic investigation, on a novel system, of the
spectra of all the brighter northern stars, is now being prose-
cuted at Harvard College observatory, under the form of a
memorial to the late Dr. H. Draper. The spectra are taken,
as it were, wholesale. A large prism placed in front of the
object-glass (a device originally suggested by Father Secchi)
1 Proc. Roy. Soc., vol. xxxiii. p. 425 ; Report. Brit. Ass., 1882, p. 444.
3 Comptes liendus, t. xciv. p. 1243.
CHAP. xii. STARS AND NEBULAE. 449
analyses at once, with slight loss of light, the rays of all the
stars in the field. As many as one hundred spectra of stars
down to the eighth magnitude, may thus be printed on a
single plate with a single exposure. No cylindrical lens is
used. The movement of the stars themselves is turned to
account for giving the desirable width to their spectra. The
star is allowed by disconnecting, or suitably regulating the
clock to travel slowly across the line of its own dispersed
light, so broadening it gradually into a band. Excellent
results have been secured in this way. About fifty lines
appear in the photographed spectrum of Aldebaran, and
eight in that of Yega. On January 26, 1886, with an
exposure of thirty- four minutes, a simultaneous impression
was obtained of the spectra (among many others) of close
upon forty Pleiades. With few and doubtful exceptions,
they all proved to belong to the same type. The hydrogen-
lines were predominant in all, alone in most. An additional
argument for the common origin of the stars forming this
beautiful group is thus provided. 1
The first promising photograph of the Orion nebula was
obtained by Draper, September 30, i88o. 2 The marked
approach towards a still more perfectly satisfactory result
shown by his plates of March 1881 and 1882, was unhappily
cut short by his premature death. Meanwhile, M. Janssen
was at work in the same field from 1881, with his accustomed
success. 3 But Mr. A. Ainslie Common left all competitors
far behind with a splendid picture, taken January 30, 1883,
by means of an exposure of thirty-seven minutes in the focus
of his three-foot silver-on-glass mirror. 4 Photography may
thereby be said to have definitively assumed the office of his-
toriographer to the nebulae ; since this one impression embodies
a mass of facts hardly to be compassed by months of labour with
the pencil, and affords a record of shape and relative bright-
ness in the various parts of the stupendous object it delineates,
1 Pickering, Mem. Am. Ac., vol. xi. p. 215. " Wash, Obs,, vol. xxv.
App. i. p. 226. 3 Comptes Rendus, t. xcii. p. 261. 4 Month. Not., vol.
xliii. p. 255.
2 F
450 HISTORY OF ASTRONOMY. PART n.
which must prove invaluable to the students of its future con-
dition. Its beauty and merit were officially recognised by
the award of the Astronomical Society's Gold Medal in 1884.
A second picture of equal merit, obtained by the same means,
February 28, 1883, with an exposure of one hour, is repro-
duced in our frontispiece. The vignette includes two specimens
of planetary photography. The Jupiter, with the great red
spot conspicuous in the southern hemisphere, is by Mr.
Common. It dates from September 3, 1879, an( ^ was accord-
ingly one of the earliest results with his 36-inch, the direct
image in which imprinted itself in a fraction of a second,
and was subsequently enlarged on paper about twelve times.
The exquisite little picture of Saturn was taken at Paris
by MM. Paul and Prosper Henry, December 21, 1885,
with their i3j-inch photographic refractor. The telescopic
image was in this case magnified eleven times previous to
being photographed with an exposure of about five seconds ;
and the total enlargement, as it now appears, is nineteen
times. The perfect distinctness (especially in the negative)
of the division between the rings an interval of less than
half a second gives promise of success in measuring the photo-
graphic images of close double stars. A trace of the dusky
ring perceptible on the original negative, is lost in the print.
A photograph of the Orion nebula taken by Mr. Roberts, in
67 minutes, November 30, 1886, makes a remarkable disclosure
of the extent of that prodigious object. More than six times
the nebulous area depicted on Mr. Common's plates is covered
by it, and it plainly shows an adjacent nebula, separately
catalogued by Messier, to belong to the same vast formation. 1
The sublime problem of the construction of the heavens
has not been neglected amid the multiplicity of tasks imposed
upon the cultivators of astronomy by its rapid development.
But data of a far higher order of precision, and indefinitely
greater in amount, than those at the disposal of Herschel or
Struve, must be accumulated before any definite conclusions
on the subject are possible. The first organised effort towards
1 R. C. Johnson, Observatory, vol. x. p. 99.
CHAP. xii. STARS AND NEBULA. 451
realising this desideratum, was made by the German Astro-
nomical Society in 1865, two years after its foundation at
Heidelberg. The scheme, as originally proposed, consisted in
the exact determination of the places of about 100,000 stars,
from the re-observation of which, say, in the year 1950,
astronomers of two or three generations hence may gather a
vast store of knowledge directly of the apparent motions,
indirectly of the mutual relations binding together the suns
and systems of space. Fourteen observatories in Europe and
America joined in the work, which is now far advanced. Its
scope, however, has, since its inception, been widened so as to
include southern zones as far as the Tropic of Capricorn ; and
a preliminary survey of the new region on Argelander's plan
has lately been made by Schonfeld at Bonn.
Through Dr. Gould's unceasing labours, during his fifteen
years' residence at Cordoba, a detailed acquaintance with
southern stars has at length been brought about. His Urano-
metria Argentina (1879) enumerates the magnitudes of 8198
out of 10,649 stars visible to the naked eye under those trans-
parent skies; 73,160 down to 9^ magnitude are embraced in
his " zones ; " besides which, he has brought back with him
to Boston materials for a catalogue including 35,000 entries.
Valuable work of the same kind is being done at Virginia by
Professor O. Stone ; while the present Radcliffe observer's
"Cape Catalogue for 1880" affords an aid to the practical
astronomer south of the line, of which it would be difficult to
over-estimate the importance. Moreover, the gigantic task
entered upon in 1860 by Dr. C. H. F. Peters, director of the
Litchfield Observatory, Clinton (N.Y.), and of which a large
instalment was finished in 1882, deserves honourable mention.
It was nothing less than to map all stars down to, and even
below, the fourteenth magnitude, situated within 30 on either
side of the ecliptic, and so to afford " a sure basis for drawing
conclusions with respect to the changes going on in the starry
heavens." l
It is tolerably safe to predict that no work of its kind and
1 Gilbert, Sidereal Messenger, vol. i. p. 288.
452 HISTORY OF ASTRONOMY. PART 11.
for its purpose, will ever again be undertaken. In a small
part of one night, more stars can now be got to register them-
selves, and more accurately, than the eye and hand of the
most skilled observer could accomplish the record of in a year.
Fundamental catalogues, constructed by the old time-honoured
method, will continue to furnish indispensable starting-points
for measurement ; l but the relative places of the small crowded
stars the sidereal o/ -roXXo/ will henceforth be derived from
their autographic statements on the sensitive plate. Even
the secondary purpose that of asteroidal discovery served
by detailed stellar enumeration, will be more surely attained
by photography than by laborious visual comparison. For
planetary movement betrays itself in a comparatively short
time by turning the imprinted image of the object affected by
it from a dot into a trail.
In the arduous matter of determining star-distances, too,
progress has been made. Together, yet independently, Drs.
Gill and Elkin carried out, at the Cape Observatory in 1882-
83, an investigation of remarkable accuracy into the parallaxes
of nine southern stars. One of these was the famous a Cen-
tauri, the distance of which from the earth was ascertained to
be just one-third greater than Henderson had made it. The
parallax of Sirius, on the other hand, was doubled, or its
distance halved; while Canopus was discovered to be quite
immeasurably remote a circumstance which, considering
that, amongst all the stellar multitude, it is outshone only
by the radiant Dog-star, gives a stupendous idea of its real
splendour and dimensions.
Inquiries of this kind have been, during the last score
of years, successfully pursued at the observatory of Dunsink,
near Dublin. Annual perspective displacements were by
Dr. Briinnow detected in several stars, and in others re-
ineasured with a care which inspired just confidence. His
parallax for a Lyra? (0.13") was confirmed by Hall in 1886
from an extended series of observations (giving cr = 0.13 4") j
and the received value (^=0.09") for the parallax of the
1 Mouchez, Comptes Rendus, t. cii. p. 151.
CHAP. xii. STARS AND NEBULAE. 453
remarkable star " Groombridge 1830 " the swiftest traveller,
so far known, in the sidereal heavens is that arrived at by
him in 1871.
His successor as Astronomer- Royal for Ireland, Sir Robert
Stawell Ball, has devoted much attention to the same subject.
Besides approximately confirming Struve's parallax of half
a second of arc for 61 Cygni, he refuted, in 1881, by a sweep-
ing search for (so-called) " large " parallaxes, certain baseless
conjectures of comparative nearness to the earth, in the case
of red and temporary stars. 1 Of 300 thus cursorily examined,
only one star of the seventh magnitude, numbered 1618 in
Groombridge's Circumpolar Catalogue, gave signs of measure-
able vicinity. Noteworthy also are Otto Struve's detection of
a parallax of half a second for Aldebaran, and Professor A.
Hall's measures of 61 Cygni, Vega, and 40 (o 2 ) Eridani the
central star of a ternary group with the great Washington
refractor, 1880-86.
A fresh impulse will doubtless be given to such researches by
the possible availability of a means for enormously expedit-
ing them. Hence the importance of Professor Pritchard's
photographic determination of the parallax of 61 Cygni. 2
From measurements of 200 negatives taken between May and
December 1886, it was provisionally fixed at 0.438", a value
agreeing very satisfactorily with Ball's of 0.468." A detailed
examination convinced the Astronomer-Royal (Mr. Christie)
that this result is more accurate than Bessel's with the
heliometer. Protests, indeed, are audible ; and it would
certainly be unwise, at this early stage of the investigation,
to overlook the chances of illusion which its progress may tend
very sensibly to diminish.
Observers of double stars are among the most meritorious,
and need to be among the most patient and painstaking
workers in sidereal astronomy. They are scarcely as numerous
as could be wished. Dr. Doberck, distinguished as a computer
of stellar orbits, complained recently 3 that data sufficient for
1 Nature, vol. xxiv. p. 91. 2 Observatory, vol. x. p. 84.
3 Nature, vol. xxvi. p. 177.
454 HISTORY OF ASTRONOMY. PART n.
the purpose had not been collected for above 30 or 40 binaries
out of between five and six hundred certainly or probably
existing. Few have done more to supply this deficiency than
the late Baron Ercole Dembowski of Milan. He devoted the
last thirty years of his life, which came to an end January
19, 1 88 1, to the revision of the Dorpat Catalogue, and left
behind him a store of micrometrical measures as numerous
as they are precise.
Of living observers in this branch, Mr. S. W. Burnham of
Chicago is beyond question the foremost, notwithstanding
that his legal avocations left him but scanty leisure for
exploring the skies. His discoveries of mostly very close
double stars (some intensely difficult of separation) numbered
one thousand in May 1882, when he brought his regular
astronomical work to a close. 1 The curious phenomenon of
one star revolving round another in a period shorter than that
in which Jupiter circulates round the sun, came to his notice
in 1 883.2 The very close pair in question, discovered by Otto
Struve in 1852, is known as d Equulei, and the period pro-
bably assigned to it is of 10.8 years by far the shortest
attributable to any member of a stellar system.
Another fact of interest in this connection is that 6 1 Cygni
at length gives signs of yielding up its secret. The seemingly
parallel tracks followed by its components during a century
and a quarter of observation, were found by Struve in 1875
to exhibit deviations countenancing the inference of mutual
revolution; for which, in 1880, a period of about eleven
hundred years was arrived at as a first approximation. 3 From
a fresh discussion three years later, Mr. N. M. Mann of
Rochester (N.Y.) concluded it 1159 years, giving (with a
parallax taken at 0.55") a value for the combined mass of the
connected bodies only one-seventh the solar mass. 4 The orbit,
however, assigned to the pair by Dr. C. F. W. Peters of Kiel
in i885, 5 claims a higher degree of confidence. It was founded
1 Mem. R. A. Soc., vol. xlvii. p. 178. 2 Observatory, vol. vii. p. 13.
3 Mim, de I' Ac., St. P^tersbourg, t. xxvii. p. 16. 4 Sidereal Messenger,
vol. ii. p. 22. 5 Astr. Nach. Nos. 2708-9.
CHAP. xii. STARS AND NEBULA. 455
on a series of fifty years' observations at Dorpat and Pulkowa,
and is traversed in 783 years at a mean distance seventy times
that of the earth from the ruler of its motion. Assuming
a parallax midway between Bessel's and Struve's, the mass
of the system comes out, from these data, just half that of
the sun.
Stellar photometry, initiated by the elder Herschel, and
provided with exact methods by his son, has of late years
assumed the importance of a separate department of astrono-
mical research. Two monumental works on the subject lately
completed on opposite sides of the Atlantic, were thus ap-
propriately coupled in the bestowal of the Royal Astronomical
Society's Gold Medal in 1886. Harvard College observatory
led the way under the able direction of Professor E. C. Picker-
ing. His photometric catalogue of 4260 stars, 1 constructed
from nearly 95,000 observations of light- intensity during the
years 1879-82, constitutes a record of incalculable value for
the detection and estimation of stellar variability. It was
succeeded in 1885 by Professor Pritchard's " Uranometria
Nova Oxoniensis," including photometric determinations of
the magnitudes of all naked eye stars from the pole to ten
degrees south of the equator, to the number of 2784. The
instrument employed was the "wedge photometer," which
measures brightness by resistance to extinction. A wedge of
neutral-tint glass, accurately divided to scale, is placed in the
path of the stellar rays, when the thickness of it they have
power to traverse furnishes a criterion of their intensity.
Professor Pickering's "meridian photometer," on the other
hand, is based upon the principle of equalisation effected by a
polarising apparatus. After all, however, as Professor Pritchard
observed, "the eye is the real photometer," and its judgment
can only be valid over a limited range. 2 Absolute uniformity
then, in estimates made by various means, under varying con-
ditions, and by different observers, is not to be looked for ;
and it is satisfactory to find substantial agreement attain-
able and attained. Only in an insignificant fraction of the
1 Harvard Annals, vol. xiv. pt. i., 1884. 2 Observatory, vol. viii. p 309.
456 HISTORY OF ASTRONOMY. PART ir.
stars common to the Harvard and Oxford catalogues, dis-
cordances are found exceeding one third of a magnitude; a
large proportion (71 per. cent) agree within one fourth, a
considerable minority (31 per cent) within one tenth, of a
magnitude. 1
Meanwhile, thought cannot be held aloof from the great
subject upon the future illustration of which so much patient
industry is being expended. Nor are partial glimpses denied
to us of relations fully discoverable perhaps only by the slow
efflux of time. Some important points in cosmical economy
have, indeed, become quite clear within the last thirty years,
and scarcely any longer admit of a difference of opinion. One
of these is that of the true status of nebulae.
This was virtually settled by Sir J. Herschel's description
in 1847 of the structure of the Magellanic clouds; but it was
not until Whewell in 1853, and Herbert Spencer in 1858,2
enforced the conclusions necessarily to be derived therefrom,
that the conception of the nebulae as remote galaxies, which
Lord Rosse's resolution of many into stellar points had ap-
peared to support, began to withdraw into the region of dis-
carded and half-forgotten speculations. In the Nubeculae, as
Whewell insisted, 3 " there co-exist, in a limited compass, and
in indiscriminate position, stars, clusters of stars, nebulae, regular
and irregular, and nebulous streaks and patches. These, then,
are different kinds of things in themselves, not merely different
to us. There are such things as nebulae side by side with stars
and with clusters of stars. Nebulous matter resolvable occurs
close to nebulous matter irresolvable."
This argument from co-existence in nearly the same region
of space, was reiterated and reinforced with others by Mr.
Spencer, and has more lately been urged with his accustomed
force and freshness by Mr. Proctor. It is unanswerable.
There is no maintaining nebulae to be simply remote worlds
of stars in the face of an agglomeration like the Nubecula
Major, containing in its (certainly capacious) bosom both stars
1 Month. Not., vol. xlvi. p. 277. 2 Essays (2d ser.), The Nebular
Hypothesis. 3 On the Plurality of Worlds, p. 214 (2d ed.).
CHAP. xii. STARS AND NEBULA. 457
and nebulae. Add the evidence of the spectroscope to the
effect that a large proportion of these perplexing objects are
gaseous, with the facts of their distribution telling of an inti-
mate relation between the mode of their scattering and the
lie of the Milky Way, and it becomes impossible to resist the
conclusion that both nebular and stellar systems are parts of
a single scheme. 1
As to the stars themselves, the presumption of their approxi-
mate uniformity in size and brightness has been effectually
dissipated. Differences of distance can no longer be invoked
to account for dissimilarity in lustre. Minute orbs, altogether
invisible without optical aid, are found to be indefinitely
nearer to us than such radiant objects as Capella, Regulus, or
Procyon. Moreover, intensity of light is perceived to be a
very imperfect index to real magnitude. Brilliant suns are
swayed from their courses by the attractive power of massive,
yet faintly luminous companions, and are suspected of suf-
fering eclipse from obscure interpositions. Besides, effective
lustre is now known to depend no less upon the qualities of
the investing atmosphere, than upon the extent and radiative
power of the stellar surface. Red stars must be far larger in
proportion to the light diffused by them than white stars. 2
It is highly probable that our sun would at least double its
brightness were the absorption suffered by its rays to be
reduced to the Sirian standard ; and, on the other hand, that
it would lose half its present efficiency as a light-source, if the
atmosphere partially veiling its splendours were rendered as
dense as that of Aldebaran.
Thus, variety of all kinds is seen to abound in the heavens ;
and it must be admitted that the inevitable abolition of all
hypotheses as to the relative distances of the stars singularly
complicates the question of their allocation in space. Never-
theless, something has been learnt even on that point; and
the tendency of modern research is, on the whole, strongly
confirmatory of the views expressed by Herschel in 1802. He
1 Proctor, Month. Not., vol. xxix. p. 342. 2 This remark is due to the
late Mr. J. Birmingham.
458 HISTORY OF ASTRONOMY. PART n.
then no longer regarded the Milky Way as the mere visual
effect of an enormously extended stratum of stars, but as an
actual aggregation, highly irregular in structure, made up of
stellar clouds and groups and nodosities. All the facts since
ascertained fit in with this conception; and to them Mr.
Proctor has added, what we may almost call the discovery
that the stars forming the galactic stream are not only situated
more closely together, but are also really, as well as apparently,
of smaller dimensions than the lucid orbs studding our skies.
By the laborious process of isographically charting the whole
of Argelander's 324,000 stars, he made it clear, in 187 1, 1
that the brighter stars show, in their distribution, a detailed
relationship to the complex branchings of the Milky Way,
avoiding, to a marked extent, its vacuities, and thronging its
denser convolutions. It follows that they must be actually
intermingled with them. So that, for every triton sun there
are doubtless swarms of minnows bodies not perhaps larger
than our own little planet, yet self-luminous and diffusive of
beneficent influences according to the inscrutable design of
the Creator.
The first step towards the unravelment of the tangled web
of stellar movements was taken when Herschel established
the reality, and indicated the direction of the sun's journey.
But the gradual shifting backward of the whole of the celestial
scenery amid which we advance, accounts for only a part of
the observed displacements. The stars have motions of their
own besides those reflected upon them from ours. All attempts,
however, to grasp the general scheme of these motions, have
hitherto failed. Yet they have not remained wholly fruitless.
The community of slow movement in Taurus, upon which
Madler based his famous theory, has proved to be a fact, and
one of very extended significance.
In 1870 Mr. Proctor undertook to chart down the direc-
tions and proportionate amounts of about 1600 proper motions,
as determined by Messrs. Stone and Main, with the result of
bringing to light the remarkable phenomenon termed by him
1 Month. Not., vols. xxxi. p. 175 ; xxxii. p. I.
CHAP. xii. STARS AND NEBULA. 459
" stardrift." 1 Quite unmistakably, large groups of stars,
otherwise apparently disconnected, were seen to be in pro-
gress together, in the same direction, and at the same rate,
across the sky. A striking instance of this kind of unanimity
is afforded by the five intermediate stars of the Plough. So
clearly were they marked out from their companions in the
same asterism, that Mr. Proctor ventured to invite the appli-
cation of the spectroscope as a sure means of ratifying the
distinction. And so indeed it proved. The five associated
stars were discerned by Dr. Huggins in 1872 2 to be in rapid
retreat from the earth, while the brightest of the Pointers, and
the last star in the tail of the Great Bear, verified their sur-
mised independence by displaying, the one a diametrically
opposite, the other a widely different rate of motion.
Here then we have a system on a scale so vast that the
imagination shrinks from the effort to conceive it. None of
the stars forming it have any sensible parallax, so that they
certainly surpass our sun many, perhaps thousands of times
in dimensions and splendour. Moreover, the distances separat-
ing them one from the other must be enormous to be reckoned
by billions of miles, or years of light-travel. Yet a special tie
unites them; they are subject to the stress of an identical
force, swaying their movements into harmonious accord ; and
they doubtless shed one upon the other mutual influences
apart from which their function in the cosmos would be im-
perfectly fulfilled.
And this is by no means a solitary example. Particular
association, indeed as was surmised by Michell six- score
years ago appears to be the rule, rather than an exception
in the sidereal scheme. Stars are bound together by twos,
by threes, by dozens, by hundreds. Our own sun is perhaps
not exempt from this gregarious tendency. Dr. Gould con-
jectures that it belongs to a group of about four hundred
of the brightest visible stars, forming a subordinate system
within the confines of the Milky Way. 3 Such another would
1 Proc. Roy. Soc., vol. xviii. p. 169. 2 Ibid., vol. xx. p. 392.
3 Month. Not., vol. xl. p. 249.
460 HISTORY OF ASTRONOMY. PART n.
be the Pleiades. The laws and revolutions of such majestic
communities lie, for the present, far beyond the range of
possible knowledge ; centuries may elapse before even a rudi-
mentary acquaintance with them begins to develop ; while
the economy of the higher order of association, which we
must reasonably believe that they unite to compose, will
possibly continue to stimulate and baffle human curiosity to
the end of time.
461
CHAPTER XIII.
METHODS OF RESEARCH.
COMPARING the methods now available for astronomical in-
quiries with those in use thirty years ago, we are at once
struck with the fact that they have multiplied. The telescope
has been supplemented by the spectroscope and the photo-
graphic camera. Now this really involves a whole world of
change. It means that astronomy has left the place where
she dwelt apart in rapt union with mathematics, indifferent
to all things on earth save only to those mechanical improve-
ments which should aid her to penetrate further into the
heavens, and has descended into the forum of human know-
ledge, at once a suppliant and a patron, alternately invoking
help from, and promising it to each of the sciences, and
patiently waiting upon the advance of all. The science of
the heavenly bodies has, in a word, become a branch of ter-
restrial physics, or rather a higher kind of integration of all
their results. It has, however, this leading peculiarity, that
the materials for the whole of its inquiries are telescopically
furnished. They are such as the unarmed eye takes no, or a
very imperfect cognisance of.
Spectroscopic and photographic apparatus are simply addi-
tions to the telescope. They do not supersede, or render it of
less importance. On the contrary, the efficacy of their action
depends primarily upon the optical qualities of the instrument
they are attached to. Hence the development, to their fullest
extent, of the powers of the telescope is of vital moment to
the progress of modern physical astronomy, while the older
mathematical astronomy could afford to remain comparatively
indifferent to it.
462 HISTORY OF ASTRONOMY. PART 11.
The colossal Rosse reflector still marks, as to size, the ne
plus ultra of performance in that line. No existing mirror
comes nearer to it than that, four feet in diameter, sent out
to Melbourne by the late Thomas Grubb of Dublin in 1870.
This is mounted in the Cassegrainian manner; so that the
observer looks straight through it towards the object viewed,
of which he really sees a twice-reflected image. It is of
excellent definition and rare convenience in management ; but
the dust-laden atmosphere of Melbourne is said to impede very
seriously its usefulness.
It may be doubted whether so large a speculum will ever
again be constructed. A new material for the mirrors of re-
flecting telescopes was introduced by L4on Foucault in I857, 1
which has already in a great measure superseded the use of
a metallic alloy. This is glass upon which a thin film of silver
has been deposited by a process known as Drayton's. It gives
a peculiarly brilliant reflective surface, throwing back more
light than a metallic mirror of the same area, in the proportion
of about sixteen to nine. Liability to tarnish in part coun-
teracts this great advantage. The largest instrument as yet
successfully turned out on this plan is Mr. Common's 36-inch
reflector, finished in 1879. To its excellent qualities his
triumphs in celestial photography are largely due. One in
course of construction by him on the same principle, but with
an aperture of five feet, will concentrate more light than any
telescope yet built. Like its predecessor, it will be devoted
chiefly to the chemical delineation of the heavenly bodies.
It is, however, in the construction of refracting telescopes
that the most conspicuous advances have recently been made.
The Harvard College 15 -inch achromatic was mounted and
ready for work in June 1847. A similar instrument had
already for some years been in its place at Pulkowa. It was
long before the possibility of surpassing these masterpieces of
German skill presented itself to any optician. For fifteen
years it seemed as if a line had been drawn just there. It
was first transgressed in America. A portrait- painter of
1 Comptes Rendus, t. xliv. p. 339.
CHAP. xin. METHODS OF RESEARCH. 463
Cambridge-port, Massachusetts, named Alvan Clark, had for
some time amused his leisure with grinding lenses, the
singular excellence of which was discovered in England by
Mr. Dawes in I853- 1 Seven years passed, and then an order
came from the University of Mississippi for an object-glass
of the unexampled size of eighteen inches. An experimental
glance through it to test its definition resulted, as we have
seen, in the detection of the companion of Sirius, January 31,
1862. It never reached its destination in the South. War
troubles supervened ; and it was eventually sent to Chicago,
where it has served Professor Hough in his investigations of
Jupiter, and Mr. Burnham in his scrutiny of double stars.
The next step was an even longer one, and it was again
taken by a self-taught optician, Thomas Cooke, the son of a
shoemaker at Allerthorpe, in the East Riding of Yorkshire.
Mr. Newall of Gateshead ordered from him in 1863 a 25-inch
object-glass. It was finished early in 1868, but at the cost of
shortening the life of its maker, who died October 19, 1869,
before the giant refractor he had toiled at for five years, was
completely mounted. Although believed to be still the finest
telescope in England, its high qualities have been largely
neutralised by an unfavourable situation.
Close upon its construction followed that of the Washington
2 6 -inch, for which twenty thousand dollars were paid to Alvan
Clark. Set to work in 1873, the most illustrious point in its
career, so far, has been the discovery of the satellites of Mars.
Once ascertained as present, these were, indeed, found to be
perceptible with very moderate optical means (Mr. Wentworth
Erck saw Deimos with a 7j-inch Clark); but the first detec-
tion of such minute objects is a feat of a very different order
from their subsequent observation.
For a little over eight years the Washington refractor held
the primacy. It had to yield the place of honour in December
1880 to a giant achromatic, twenty-seven inches in aperture,
built by Howard Grubb (son and successor of Thomas Grubb)
for the Vienna Observatory. This, in its turn, has been sur-
1 Newcomb, Pop. Astr. p. 137.
464 HISTORY OF ASTRONOMY. PART n.
passed by two of respectively 29 J and thirty inches, sent by
Gautier of Paris to Nice, and by Alvan Clark to Pulkowa ;
and an object-glass, fully three feet in diameter, has just been
successfully executed by the latter firm for the Lick Obser-
vatory in California. The difficulties, however, encountered
in procuring discs of glass of the size and purity required for
this last venture, seem to indicate that a- term to progress in
this direction is near at hand. The flint was indeed cast with
comparative ease in the workshops of M. Feil at Paris. The
flawless mass weighed 170 kilogrammes, was over 38 inches
across, and cost 2000 pounds. But with the crown part
of the designed achromatic combination, things went less
smoothly. The production of a perfect disc was only achieved
after nineteen failures, involving a delay of more than two
years ; and the glass for a third lens, designed to render the
telescope available at pleasure for photographic purposes,
proved to be strained, and consequently went to pieces in the
process of grinding.
Nor is the difficulty in obtaining suitable material almost
overwhelming though it be the only obstacle to increasing
the size of refractors. Colour-fringes also step in and bar the
way, their complete, or approximately complete correction
demanding, in the case of such vast apertures as have recently
been attempted, a focal length so exorbitant as to be practi-
cally, under the ordinary conditions of mounting, out of the
question. Besides, a refracting telescope loses one of its chief
advantages over a reflector when its size is increased beyond
a certain limit. That advantage is the greater luminosity of
the images given by it. Considerably more light is trans-
mitted through a glass lens than is reflected from an equal
metallic surface. But only so long as both are of moderate
dimensions. For the glass necessarily grows in thickness as
its area augments, and consequently stops a larger percentage
of the rays it refracts. So that a point at length arrives
fixed by the late Dr. Robinson at a diameter a little short of
three feet l where the glass and the metal are, in this respect,
1 H. Grubb, Trans. Roy. Dub. Soc., vol. i. (new ser.). p. 2.
CHAP. xin. METHODS OF RESEARCH. 465
on an equality ; while above it, the metal has the advantage.
And since silvered glass gives back considerably more light
than speculum metal, the stage of equalisation with lenses
is reached proportionately sooner where this material is em-
ployed
It will thus probably be long before the light-grasp of Mr.
Common's three-foot mirror is surpassed by a refractor. But
in the inquiries for which the great telescopes of modern
times are more especially designed, light-grasp is everything.
For the spectroscopic examination of stars, for the measure-
ment of their motions in the line of sight, for the study of
nebulae, for stellar and nebular photography, the cry con-
tinually is, " more light." Apart from the exigencies of these,
and a few other enticing branches of research, there would be
little to be gained in adding to the power of optical apparatus.
And there is much lost. The penalties of bigness are heavy.
Perfect definition becomes, with increasing size, more and
more difficult to attain ; once attained, it becomes more and
more difficult to keep. For the huge masses of material
employed to form great object-glasses or specula, tend, with
every movement, to become deformed by their own weight.
Gravity exacts the further tax of unwieldiness. Each glance
through a large instrument is highly paid for in time and
trouble. Nor is the glance thus paid for often a satisfactory
one. Atmospheric troubles intervene.
These are the worst plagues of all those that afflict the
astronomer. No mechanical skill avails to neutralise or allevi-
ate them. They augment, in a rapidly increasing ratio, with
each addition to the aperture of the telescope, or of the mag-
nifying powers applied to it. To them chiefly is due the
growing discontent with the performance of the colossal
instruments of modern times. It is admitted on all hands
that, for the ordinary work of an observatory, an aperture of
ten or twelve inches is the outside limit of usefulness. But
it is also found, with disappointment, that even in the field of
descriptive research, where it might be expected that lumi-
nosity and magnification would be all- important, results fall
2 G
4 66 HISTORY OF ASTRONOMY. PART n.
far short of anticipation. Schiaparelli, with an 8-inch achro-
matic, obtained views of Mars such as were never vouch-
safed to Harkness or Hall, though using the Washington
26-inch ; and, according to Mr. Denning, 1 details of the Jovian
surface are shown by an insignificant 4^-inch, which remain
invisible with the majestic refractor of the Dearborn Obser-
vatory, Chicago.
Now this is due to no imperfections inherent in the instru-
ments themselves ; it is due to the conditions of our habitation
on an air-wrapt globe. It is not only that much less than half
the light incident upon the surface of the atmospheric ocean
penetrates to the bottom of it. That loss might, in some
measure, be repaired ; but what no optical contrivance can get
rid of, is the disturbance suffered by the rays that reach us.
The twinkling of stars to the naked eye is but a faint symptom
of their behaviour in the telescope ; while the images of sun,
moon, and planets " boil " at the edges, or are suffused and
distorted by waves of agitation caused by the magnified surg-
ings of the turbulent vapours we see through. The mischief,
Dr. Robinson estimated in the case of reflectors, grows with
the cubes of their diameters ; and it is frequently found, in
practice, that the " seeing " will be perfectly good with a small
telescope, but altogether intolerable with a large one standing
beside it. Under such skies as ours, in fact, there are not
more than three or four nights in the year when an aperture
of as much as eighteen inches can be used to real advantage ;
and Mr. Newall remarked in 1885 that during fifteen years
he had known but one fine night 2 fine, that is, in the sense
of availability for observation with his great refractor. It
is true that voices have been raised and raised with great
authority in defence of great telescopic capacity. Professor
Young gives it as his opinion that in planetary observation
"one can always see with the larger aperture everything
shown by the smaller, and see it more easily." 3 Professor
Hough protests that he gets better views, under any atmos-
1 Observatory ', vols. viii. p. 79, ix. p. 277. 2 Ibid., vol. viii. p. 80.
3 Ibid., vol. ix. p. 92.
CHAP. xiii. METHODS OF RESEARCH. 467
pheric conditions, with his i8J-inch than with any smaller
instrument ; l and Mr. Burnham takes the same side, while
fully admitting that superior light is, in reflectors, counter-
balanced by inferior definition. 2
Nevertheless it seems clear that we have reached a turning-
point in the history of telescopic improvement. Not alone
have the material obstacles to any further increase of size
become all but insuperable, but the conviction is forced upon
us that, were instruments of greater power than any now
possessed by astronomers actually in their hands, they must
remain almost useless save on one condition that of an
improved climate.
Ever since the Parsonstown telescope was built, it has been
obvious that the limit of profitable augmentation of aperture
had been reached, if not overpassed ; and Lord Rosse himself
was foremost to discern the need of pausing to look round the
world for a clearer and stiller air than was to be found within
the bounds of the United Kingdom. With this express object
Mr. Lassell transported his two- foot Newtonian to Malta in
1852, and mounted there, in 1860, a similar instrument of
four- fold capacity, with which in the course of about two years
600 new nebulae were discovered. Professor Piazzi Smyth's
experiences during a trip to the Peak of Teneriffe in 1856 in
search of astronomical opportunities, 3 gave countenance to the
most sanguine hopes of deliverance, at suitably elevated sta-
tions, from some of the oppressive conditions of low-level star-
gazing ; yet for a number of years nothing effectual was done
for their realisation. Now at last, however, mountain observa-
tories are not only an admitted necessity, but an accomplished
fact ; and Newton's long forecast of a time when astronomers
would be compelled, by the developed powers of their tele-
scopes, to mount high above the " grosser clouds " in order to
use them, 4 has been justified by the event.
Mr. James Lick, a millionaire of San Francisco, had already
chosen when he died, October i, 1876, a site for the new
1 Observatory, vol. viii. p. 275. 2 Ibid., p. 342. 3 Phil. Trans., vol.
cxlviii. p. 465. 4 Optice, p. 107 (2d ed., 1719).
468 HISTORY OF ASTRONOMY. PART n.
observatory, to the building and endowment of which he had
devoted a part of his large fortune. The establishment only
awaits the completion (now close at hand) of the 36-inch
refractor and its great sheltering dome, to be in a state of
perfect efficiency. Indeed, its present instrumental outfit
including a .12 -inch Clark's achromatic is one of high
excellence. The situation of the " Lick " Observatory is ex-
ceptional and splendid. Planted on one of the three peaks of
Mount Hamilton, a crowning summit of the Calif ornian Coast
Range, at an elevation of 4200 feet above the sea, in a climate
scarce rivalled throughout the world, it commands views both
celestial and terrestrial which the lover of nature and astronomy
may alike rejoice in. Impediments to observation are there
found to be most materially reduced. Professor Holden, who
was appointed in 1885 President of the University of Cali-
fornia, and director of the new observatory affiliated to it, tells
us that during six or seven months of the year an unbroken
serenity prevails, and that half the remaining nights are
clear. 1 The power of continuous work thus afforded is of
itself an inestimable advantage ; and when combined with the
high visual excellences testified to by Mr. Burnham's dis-
covery, during a two months' trip to Mount Hamilton in the
autumn of 1879, of forty-two new double stars with a 6-inch
achromatic, it gives hopes of a brilliant future for the Lick
establishment. Its advantages, according to the generous
design of Professor Holden, will be shared by the whole
astronomical world. The great equatoreal, in its unmatched
position, will be made, so far as possible, of cosmopolitan use.
Specialists from either continent will be allowed welcome
opportunities of getting their difficulties elucidated, their
questions to the stars answered, with its aid. 2
A higher altitude than the comparatively modest one at
which it is placed, would hardly prove suitable to a great per-
manent observatory ; but considerably more elevated posts
for temporary astronomical occupation are being provided,
1 Observatory, vol. viii. p. 85. 2 Holden on Celestial Photography,
Overland Monthly, Nov. 1886.
CHAP. xiii. METHODS OF RESEARCH. 469
and will shortly be looked upon as indispensable. One such
was fitted up near the summit of Mount Etna in 1882. The
building is the highest in Europe, standing 9655 feet above
the sea, and includes within its walls the " Casa Inglese," in
which travellers were used to seek repose before attempting
the final ascent of the cone. Splendid telescopic opportunities
are indicated by Professor Langley's experimental observa-
tions, carried through under every disadvantage in the winter
of 1879-80 ; and the Merz equator eal of nearly fourteen inches
aperture, provided for the Etnean establishment, may be
expected, freed from the impeding mists and restless currents
of the lower atmosphere, to prove of singular efficiency.
The Pic du Midi, too, is destined for an astronomical out-
post. A meteorological observatory was in 1881, thanks to
the enterprise of General de Nansouty and M. Vaussenat,
opened on its summit, at an altitude of 9600 feet ; and the
glowing account given by MM. Thollon and Trepied in 1883 l
of the advantages offered by the dark translucency of its sky,
determined Admiral Mouchez upon founding there a species
of su 
