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HISTORY OF ASTRONOMY 



DURING THE NINETEENTH CENTURY 



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PROBLEMS IN ASTROPHYSICS 

Containing over 100 Illustrations 
Demy 8vo., cloth 




S ' The world's a prophecy of worlds to come.' 

A POPULAR 

HISTORY OF ASTRONOMY 

DURING 

THE NINETEENTH CENTURY 

BY 

AGNES M. CLERKE 

JUPITER 1879 




SATURN 1885 



FOURTH EDITION, REVISED AND CORRECTED 



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LONDON 
ADAM AND CHARLES BLACK 

1902 



3Z 



PREFACE TO THE FOUBTH EDITION 



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

This readiness for innovation has undoubtedly its dangers and 
drawbacks. To the historian, above all, it presents frequent occa- 
sions of embarrassment. The writing of history is a strongly 
selective operation, the outcome being valuable just in so far as the 
choice what to reject and what to include has been judicious ; and 
the task is no light one of discriminating between barren specula- 
tions and ideas pregnant with coming truth. To the possession of 



vi PREFACE TO THE FOURTH EDITION 

such prescience of the future as would be needed to do this effectu- 
ally I can lay no claim ; but diligence and sobriety of thought are 
ordinarily within reach, and these I shall have exercised to good 
purpose if I have succeeded in rendering the fourth edition of A 
Popular History of Astronomy during the Nineteenth Century not wholly 
unworthy of a place in the scientific literature of the twentieth 
century. 

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



London, July, 1902. 



PEEFAGE TO THE FIRST EDITION 



The progress of astronomy during the last hundred years has been 
rapid and extraordinary. In its distinctive features, moreover, the 
nature of that progress has been such as to lend itself with facility 
to untechnical treatment. To this circumstance the present volume 
OAves its origin. It embodies an attempt to enable the ordinary 
reader to follow, with intelligent interest, the course of modern 
astronomical inquiries, and to realize (so far as it can at present be 
realized) the full effect of the comprehensive change in the whole 
aspect, purposes, and methods of celestial science introduced by the 
momentous discovery of spectrum analysis. 

Since Professor Grant's invaluable work on the History of Physical 
Astronomy was published, a third of a century has elapsed. During 
the interval a so-called "new astronomy" has grown up by the side 
of the old. One effect of its advent has been to render the science 
of the heavenly bodies more popular, both in its needs and in its 
nature, than formerly. More popular in its needs, since its progress 
now primarily depends upon the interest in, and consequent efforts 
towards its advancement of the general public; more popular in 
its nature, because the kind of knowledge it now chiefly tends to 
accumulate is more easily intelligible less remote from ordinary 
experience than that evolved by the aid of the calculus from 
materials collected by the use of the transit-instrument and chrono- 
graph. 

It has thus become practicable to describe in simple language the 
most essential parts of recent astronomical discoveries, and, being 
practicable, it could not be otherwise than desirable to do so. The 
service to astronomy itself would be not inconsiderable of enlisting 
wider sympathies on its behalf, while to help one single mind 
towards a fuller understanding of the manifold works which have 



viii PREFACE TO THE FIRST EDITION 

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 mathe- 
matical 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 funda- 
mental importance can never be diminished, and should never be 
ignored. But as the outcome of the enormous development given 
to the powers of the telescope in recent times, together with the 
swift advance of physical science, and the inclusion, by means of the 
spectroscope, of the heavenly bodies within the domain of its 
inquiries, much knowledge has been acquired regarding the nature 
and condition of those bodies, forming, it might be said, a science 
apart, and disembarrassed from immediate dependence upon in- 
tricate, and, except to the initiated, unintelligible 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 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 embellish- 
ment from literary art or high-flown phrases. Its best ornament is 
unvarnished truthfulness, and this, at least, may confidently be 
claimed to be bestowed upon it in the ensuing pages. 

In them unity of treatment is sought to be combined with a due 
regard to chronological sequence by grouping in separate chapters 
the various events relating to the several departments of descriptive 
astronomy. The whole is divided into two parts, the line between 
which is roughly drawn at the middle of the present century. 
Herschel's inquiries into the construction of the heavens strike the 
keynote of the first part ; the discoveries of sun-spot and magnetic 
periodicity and of spectrum analysis determine the character of the 
second. Where the nature of the subject required it, however, this 
arrangement has been disregarded. Clearness and consistency 
should obviously take precedence of method. Thus, in treating of 



PREFACE TO THE FIRST EDITION ix 

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 excep- 
tions, from the original authorities. The system adopted has been 
to take as little as possible at second-hand. Much pains have been 
taken to trace the origin of ideas, often obscurely enunciated long 
before they came to resound through the scientific world, and to 
give to each individual discoverer, strictly and impartially, his due. 
Prominence has also been assigned to the biographical element, as 
underlying and determining the whole course of human endeavour. 
The advance of knowledge may be called a vital process. The 
lives of men are absorbed into and assimilated by it. Inquiries 
into the kind and mode of the surrender in each separate case must 
always possess a strong interest, whether for study or for example. 

The acknowledgments of the writer are due to Professor 
Edward S. Holden, director of the Washburn Observatory, Wis- 
consin, and to Dr. Copeland, chief astronomer of Lord Crawford's 
Observatory at Dunecht, for many valuable communications. 

London, September, 1885. 



CONTENTS 



INTRODUCTION 

PAGE 

Three Kinds of Astronomy Progress of the Science during the Eighteenth 
Century Popularity and Eapid Advance during the Nineteenth 
Century ......... 1 



PART I 

PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE 
NINETEENTH CENTURY 

CHAPTER I 

FOUNDATION OF SIDEREAL ASTRONOMY 

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

CHAPTER II 

PROGRESS OF SIDEREAL ASTRONOMY 

Exact Astronomy in Germany Career of Bessel His Fundamenta Astro- 
nomice Career of Fraunhofer Parallaxes of Fixed Stars Translation 
of the Solar System Astronomy of the Invisible Struve's Researches 
in Double Stars Sir John Herschel's Exploration of the Heavens 
Fifty Years' Progress ....... 27 

CHAPTER III 

PROGRESS OF KNOWLEDGE REGARDING THE SUN 

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



xii CONTENTS 

CHAPTEE IV 

PLANETARY DISCOVERIES 

PAGE 

Bode's Law Search for a Missing Planet Its Discovery by Piazzi 
Further Discoveries of Minor Planets Unexplained Disturbance of 
Uranus Discovery of Neptune Its Satellite An Eighth Saturnian 
Moon Saturn's Dusky Ring The Uranian System . . .71 

CHAPTER V 

COMETS 

Predicted Return of Halley's Comet Career of Olbers Acceleration of 
Encke's Comet Biela's Comet Its Duplication Faye's Comet 
Comet of 1811 Electrical Theory of Cometary Emanations The 
Earth in a Comet's Tail Second Return of Halley's Comet Great 
Comet of 1843 Results to Knowledge . . . . .88 

CHAPTER VI 

INSTRUMENTAL ADVANCES 

Two Principles of Telescopic Construction Early Reflectors Three Varieties 
Herschel's Specula High Magnifying Powers Invention of the 
Achromatic Lens Guinand's Optical Glass The Great Rosse Re- 
flector Its Disclosures Mounting of Telescopes Astronomical Circles 
Personal Equation . . . . . . .108 



PART II 

RECENT PROGRESS OF ASTRONOMY 

CHAPTER I 

FOUNDATION OF ASTRONOMICAL PHYSICS 

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

CHAPTER II 

SOLAR OBSERVATIONS AND THEORIES 

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



CONTENTS xiii 

CHAPTEE III 

RECENT SOLAR ECLIPSES 

PAGR 

Expeditions to SpainGreat Indian Eclipse New Method of Viewing 
Prominences Total Eclipse Visible in North America Spectrum of 
the Corona Eclipse of 1870 Young's Reversing Layer Eclipse of 
1871 Corona of 1878 Varying Coronal Types Egyptian Eclipse 
Daylight Coronal Photography Observations at Caroline Island 
Photographs of Corona in 1886 and 1889 Eclipses of 1896, 1898, 1900, 
and 1901 Mechanical Theory of Corona Electro-Magnetic Theories- 
Nature of Corona . . . . . . .166 

CHAPTER IV 

SOLAR SPECTROSCOPY 

Chemistry of Prominences Study of their Forms Two Classes Photo- 
graphs and Spectrographs of Prominences Their Distribution Struc- 
ture of the Chromosphere Spectroscopic Measurement of Radial 
Movements Spectroscopic Determination of Solar Rotation Velocities 
of Transport in the Sun Lockyers Theory of Dissociation Solar 
Constituents Oxygen Absorption in Solar Spectrum . . .194 

CHAPTER V 

TEMPERATURE OF THE SUN 

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

CHAPTER VI 

THE SUN'S DISTANCE 

Difficulty of the Problem Oppositions of Mars Transits of Venus 
Lunar Disturbance Velocity of Light Transit of 1874 Inconclusive 
Result Opposition of Mars in 1877 Measurements of Minor Planets 
Transit of 1882 Newcomb's Determination of the Velocity of Light 
Combined Result ....... 227 

CHAPTER VII 

PLANETS AND SATELLITES 

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



xiv CONTENTS 

CHAPTER VIII 

PLANETS AND SATELLITES (continued) 

PAGE 

Analogy between Mars and the Earth Martian Snowcaps, Seas, and Con- 
tinents Climate and Atmosphere Schiaparelli's Canals Discovery 
of Two Martian Satellites Photographic Detection of Minor Planets 
Orbit of Eros Distribution of the Minor Planets Their Collective 
Mass and Estimated Diameters Condition of Jupiter His Spectrum 
Transits of his Satellites Discovery of a Fifth Satellite The Great 
Red Spot Constitution of Saturn's Rings Period of Rotation of the 
Planet Variability of Japetus Equatorial Markings on Uranus His 
Spectrum Rotation of Neptune Trans-Neptunian Planets . . 274 

CHAPTER IX 

THEORIES OF PLANETARY EVOLUTION 

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

CHAPTER X 

RECENT COMETS 

Donati's Comet The Earth again Involved in a Comet's Tail Comets of 
the August and November Meteors Star Showers Comets and 
Meteors Biela's Comet and the Andromedes Holmes's Comet De- 
flection of the Leonids Orbits of Meteorites Meteors with Stationary 
Radiants Spectroscopic Analysis of Cometary Light Comet of 1901 
Coggia's Comet ....... 323 

CHAPTER XI 

{ recent comets (continued) 

Forms of Comets' Tails Electrical Repulsion Bredikhine's Three Types 
Great Southern Comet Supposed Previous Appearances Tebbutt's 
Comet and the Comet of 1807 Successful Photographs Schaeberle's 
Comet Comet Wells Sodium Blaze in Spectrum Great Comet of 
1882 Transit across the Sun Relation to Comets of 1843 and 1880 
Cometary Systems Spectral Changes in Comet of 1882 Brooks's 
Comet of 1889 Swift's Comet of 1892 Origin of Comets . . 345 

CHAPTER XII 

STARS AND NEBULE 

Stellar Chemistry Four Orders of Stars Their Relative Ages Gaseous 
Stars Spectroscopic Star-Catalogues Stellar Chemistry Hydrogen 
Spectrum in Stars The Draper Catalogue Velocities of Stars in Line 
of Sight Spectroscopic Binaries Eclipses of Algol Catalogues of 
Variables New Stars Outbursts in Nebulae Nova Aurigae Nova 
Persei Gaseous Nebulae Variable Nebulae Movements of Nebulae 



CONTENTS xv 

PAGE 

Stellar and Nebular Photography Nebulae in the Pleiades Photo- 
graphic Star-charting Stellar Parallax Double Stars Stellar Photo- 
metry Status of Nebulse Photographs and Drawings of the Milky 
Way Star Drift . . . . . . .372 

CHAPTER XIII 

METHODS OF RESEARCH 

Development of Telescopic Power Silvered Glass Reflectors Giant Re- 
fractors Comparison with Reflectors The Yerkes Telescope Atmo- 
spheric Disturbance The Lick Observatory Mechanical Difficulties 
The Equatorial Condi The Photographic Camera Retrospect and 
Conclusion . . . . . . . 428 



APPENDIX 

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

Index ......... 471 



LIST OF ILLUSTRATIONS 



Photograph of the Great Nebula in Orion, 1883 

Photographs of Jupiter, 1879, and of Saturn, 1885 

Plate I. Photographs of the Solar Chromosphere 
and Prominences .... 

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

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

Plate IV. Photographs of Swift's Comet, 1892 

Plate V. Photographic and Visual Spectrum of 
Nova Auriga ..... 

Plate VI. Photograph of the Milky Way in Sagit- 
tarius ...... 



Frontispiece 
Vignette 

To face y. 198 
343 



359 
368 

396 

424 



HISTORY OF ASTRONOMY 

DURING THE NINETEENTH CENTURY 

INTRODUCTION 

We can distinguish three kinds of astronomy, each with a different 
origin and history, but all mutually dependent, and composing, in 
their fundamental unity, one science. First in order of time came 
the art of observing the returns, and measuring the places, of the 
heavenly bodies. This was the sole astronomy of the Chinese and 
Chaldeans ; but to it the vigorous Greek mind added a highly 
complex geometrical plan of their movements, for which Copernicus 
substituted a more harmonious system, without as yet any idea of 
a compelling cause. The planets revolved in circles because it was 
their nature to do so, just as laudanum sets to sleep because it 
possesses a virtus dormitiva. This first and oldest branch is known 
as "observational," or "practical astronomy." Its business is to 
note facts as accurately as possible ; and it is essentially unconcerned 
with schemes for connecting those facts in a manner satisfactory to 
the reason. 

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

The third and last division of celestial science may properly be 
termed "physical and descriptive astronomy." It seeks to know 
what the heavenly bodies are in themselves, leaving the How 1 and 

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

1 



2 HISTOR Y OF ASTRONOMY 

the Wherefore 1 ? of their movements to be otherwise answered. 
Now, such inquiries became possible only through the invention 
of the telescope, so that Galileo was, in point of fact, their 
originator. But Herschel first gave them a prominence which the 
whole progress of science during the nineteenth century served to 
confirm and render more exclusive. Inquisitions begun with the 
telescope have been extended and made effective in unhoped-for 
directions by the aid of the spectroscope and photographic camera ; 
and a large part of our attention in the present volume will be 
occupied with the brilliant results thus achieved. 

The unexpected development of this new physical-celestial science 
is the leading fact in recent astronomical history. It was out of the 
regular course of events. In the degree in which it has actually 
occurred it could certainly not have been foreseen. It was a seizing 
of the prize by a competitor who had hardly been thought qualified 
to enter the lists. Orthodox astronomers of the old school looked 
with a certain contempt upon observers who spent their nights in 
scrutinising the faces of the moon and planets rather than in timing 
their transits, or devoted daylight energies, not to reductions aad 
computations, but to counting and measuring spots on the sun. 
They were regarded as irregular practitioners, to be tolerated 
perhaps, but certainly not encouraged. 

The advance of astronomy in the eighteenth century ran in general 
an even and logical course. The age succeeding Newton's had for 
its special task to demonstrate the universal validity, and trace the 
complex results, of the law of gravitation. The accomplishment 
of that task occupied just one hundred years. It was virtually 
brought to a close when Laplace explained to the French Academy, 
November 19, 1787, the cause of the moon's accelerated motion. 
As a mere machine, the solar system, so far as it was then known, 
was found to be complete and intelligible in all its parts ; and in 
the Mkanique 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 



INTRODUCTION 3 

a very different kind, yet not less indispensable to the future welfare 
of astronomy, was being done in England. The Koyal Observatory 
at Greenwich is one of the few useful institutions which date their 
origin from the reign of Charles II. The leading position which it 
still occupies in the science of celestial observation was, for near a 
century and a half after its foundation, an exclusive one. Delambre 
remarked that, had all other materials of the kind been destroyed, 
the Greenwich records alone would suffice for the restoration of 
astronomy. The establishment was indeed absolutely without a 
rival. 1 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 cor- 
rection of theory, and here refinements were introduced by which 
the exquisite accuracy of modern practice in astronomy was eventually 
attained. 

The chief promoter of these improvements was James Bradley. 
Few men have possessed in an equal degree with him the power 
of seeing accurately, and reasoning on what they see. He let 
nothing pass. The slightest inconsistency between what appeared 
and what was to be expected roused his keenest attention ; and he 
never relaxed his mental grip of a subject until it had yielded to 
his persistent inquisition. It was to these qualities that he owed 
his discoveries of the aberration of light and the nutation of the 
earth's axis. The first was announced in 1729. AVhat is meant by 
it is that, owing to the circumstance of light not being instantaneously 
transmitted, the heavenly bodies appear shifted from their true 
places by an amount depending upon the ratio which the velocity of 
light bears to the speed of the earth in its orbit. Because light 
travels with enormous rapidity, the shifting is very slight ; and each 
star returns to its original position at the end of a year. 

Bradley's second great discovery was finally ascertained in 1748. 
Nutation is a real " nodding " of the terrestrial axis produced by the 
dragging of the moon at the terrestrial equatorial protuberance. 
From it results an apparent displacement of the stars, each of them 
describing a little ellipse about its true or " mean " position, in a 
period of nearly nineteen years. 

Now, an acquaintance with the fact and the laws of each of 
these minute irregularities is vital to the progress of observational 
astronomy ; for without it the places of the heavenly bodies could 
never be accurately known or compared. So that Bradley, by their 
detection, at once raised the science to a higher grade of precision. 
Nor was this the whole of his work. Appointed Astronomer-Koyal 
in 1742, he executed during the years 1750-62 a series of observations 

1 Histoire de VAstronomie au xviii e Steele, p. 267. 

12 



4 HISTORY OF ASTRONOMY 

which formed the real beginning of exact astronomy. Part of their 
superiority must, indeed, be attributed to the co-operation of John 
Bird, who provided Bradley in 1750 with a measuring instrument 
of till then unequalled excellence. For not only was the art of 
observing in the eighteenth century a peculiarly English art, but 
the means of observing were furnished almost exclusively by British 
artists. John Dollond, the son of a Spitalfields weaver, invented 
the achromatic lens in 1758, removing thereby the chief obstacle to 
the development of the powers of refracting telescopes ; James Short, 
of Edinburgh, was without a rival in the construction of reflectors ; 
the sectors, quadrants, and circles of Graham, Bird, Ramsden, and 
Cary were inimitable by Continental workmanship. 

Thus practical and theoretical astronomy advanced on parallel 
lines in England and France respectively, the improvement of 
their several tools the telescope and the quadrant on the one side, 
and the calculus on the other keeping pace. The whole future 
of the science seemed to be theirs. The cessation of interest 
through a too speedy attainment of the perfection towards which 
each spurred the other, appeared to be the only danger it held in 
store for them. When all at once, a rival stood by their side not, 
indeed, menacing their progress, but threatening to absorb their 
popularity. 

The rise of Herschel was the one conspicuous anomaly in the 
astronomical history of the eighteenth century. It proved decisive 
of the course of events in the nineteenth. It was unexplained 
by anything that had gone before ; yet all that came after hinged 
upon it. It gave a new direction to effort ; it lent a fresh impulse 
to thought. It opened a channel for the widespread public 
interest which was gathering towards astronomical subjects to 
flow in. 

Much of this interest was due to the occurrence of events 
calculated to arrest the attention and excite the wonder of the 
uninitiated. The predicted return of Halley's comet in 1759 
verified, after an unprecedented fashion, the computations of 
astronomers. It deprived such bodies for ever of their portentous 
character j it ranked them as denizens of the solar system. Again, 
the transits of Venus in 1761 and 1769 were the first occurrences 
of the kind since the awakening of science to their consequence. 
Imposing preparations, journeys to remote and hardly accessible 
regions, official expeditions, international communications, all for 
the purpose of observing them to the best advantage, brought 
their high significance vividly to the public consciousness ; a result 
aided by the facile pen of Lalande, in rendering intelligible the 
means by which these elaborate arrangements were to issue in an 



INTRODUCTION 5 

accurate knowledge of the sun's distance. Lastly, Herschel's 
discovery of Uranus, March 13, 1781, had the surprising effect of 
utter novelty. Since the human race had become acquainted with 
the company of the planets, no addition had been made to their 
number. The event thus broke with immemorial traditions, and 
seemed to show astronomy as still young and full of unlooked-for 
possibilities. 

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

This great accession of public interest gave the impulse to 
the extraordinarily rapid progress of astronomy in the nineteenth 
century. Official patronage combined with individual zeal sufficed 
for the elder branches of the science. A few well-endowed insti- 
tutions could accumulate the materials needed by a few isolated 
thinkers for the construction of theories of wonderful beauty and 
elaboration, yet precluded, by their abstract nature, from winning 
general applause. But the new physical astronomy depends for 
its prosperity upon the favour of the multitude whom its striking 
results are well fitted to attract. It is, in a special manner, the 
science of amateurs. It welcomes the most unpretending co- 
operation. There is no one "with a true eye and a faithful 
hand " but can do good work in watching the heavens. And not 
unfrequently, prizes of discovery which the most perfect appliances 
failed to grasp, have fallen to the share of ignorant or ill-provided 
assiduity. 

Observers, accordingly, have multiplied ; observatories have been 
founded in all parts of the world ; associations have been constituted 
for mutual help and counsel. A formal astronomical congress met 
in 1789 at Gotha then, under Duke Ernest II. and Von Zach, the 



6 HISTOR Y OF ASTRONOM Y 

focus of German astronomy and instituted a combined search for 
the planet suspected to revolve undiscovered between the orbits of 
Mars and Jupiter. The Astronomical Society of London was 
established in 1820, and the similar German institution in 1863. 
Both have been highly influential in promoting the interests, local 
and general, of the science they are devoted to forward; while 
functions corresponding to theirs have been discharged elsewhere 
by older or less specially constituted bodies, and new ones of a more 
popular character are springing up on all sides. 

Modern facilities of communication have helped to impress more 
deeply upon modern astronomy its associative character. The 
electric telegraph gives a certain ubiquity which is invaluable to an 
observer of the skies. With the help of a wire, a battery, and a 
code of signals, he sees whatever is visible from any portion of our 
globe, depending, however, upon other eyes than his own, and so 
entering as a unit into a widespread organisation of intelligence. 
The press, again, has been a potent agent of co-operation. It has 
mainly contributed to unite astronomers all over the world into a 
body animated by the single aim of collecting " particulars " in their 
special branch for what Bacon termed a History of Nature, eventually 
to be interpreted according to the sagacious insight of some one 
among them gifted above his fellows. The first really effective 
astronomical periodical was the Monatliche Correspondent, started by 
Von Zach in the year 1800. It was followed in 1822 by the 
Astronomische Nachrichten, later by the Memoirs and Monthly Notices 
of the Astronomical Society, and by the host of varied publications 
which now, in every civilised country, communicate the discoveries 
made in astronomy to divers classes of readers, and so incalculably 
quicken the current of its onward flow. 

Public favour brings in its train material resources. It is repre- 
sented by individual enterprise, and finds expression in an ample 
liberality. The first regular observatory in the Southern Hemi- 
sphere was founded at Paramatta by Sir Thomas Makdougall 
Brisbane in 1821. The Royal Observatory at the Cape of Good 
Hope was completed in 1829. Similar establishments were set 
to work by the East India Company at Madras, Bombay, and 
St. Helena, during the first third of the nineteenth century. The 
organisation of astronomy in the United States of America was due 
to a strong wave of popular enthusiasm. In 1825 John Quincy 
Adams vainly urged upon Congress the foundation of a National 
Observatory; but in 1843 the lectures on celestial phenomena 
of Ormsby MacKnight Mitchel stirred an impressionable audience 
to the pitch of providing him with the means of erecting at Cincin- 
nati the first astronomical establishment worthy the name in that 



INTRODUCTION 7 

great country. On the 1st of January, 1882, no less than one 
hundred and forty-four were active within its boundaries. 

The apparition of the great comet of 1843 gave an additional 
fillip to the movement. To the excitement caused by it the Harvard 
College Observatory called the " American Pulkowa " directly 
owed its origin; and the example was not ineffective elsewhere. 
The United States Naval Observatory was built in 1844, Lieutenant 
Maury being its first Director. Corporations, universities, munici- 
palities, vied with each other in the creation of such institutions ; 
private subscriptions poured in ; emissaries were sent to Europe to 
purchase instruments and to procure instruction in their use. In 
a few years the young Eepublic was, in point of astronomical 
efficiency, at least on a level with countries where the science had 
been fostered since the dawn of civilisation. 

A vast widening of the scope of astronomy has accompanied, and 
in part occasioned, the great extension of its area of cultivation 
which our age has witnessed. In the last century its purview was 
a comparatively narrow one. Problems lying beyond the range 
of the solar system were almost unheeded, because they seemed 
inscrutable. Herschel first showed the sidereal universe as accessible 
to investigation, and thereby offered to science new worlds 
majestic, manifold, "infinitely infinite" to our apprehension in 
number, variety, and extent for future conquest. Their gradual 
appropriation has absorbed, and will long continue to absorb, the 
powers which it has served to develop. 

But this is not the only direction in which astronomy has enlarged, 
or rather has levelled, its boundaries. The unification of the physical 
sciences is perhaps the greatest intellectual feat of recent times. 
The process has included astronomy j 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 Tarefied matter, performed on a huge 
scale for our benefit. The sun, stars, and nebulae form so many 
celestial laboratories, where the nature and mutual relations of the 
chemical "elements" may be tried by more stringent tests than 
sublunary conditions afford. The laws of terrestrial magnetism can 
be completely investigated only with the aid of a concurrent study 
of the face of the sun. The solar spectrum will perhaps one day, 
by its recurrent modifications, tell us something of impending 
droughts, famines, and cyclones. 

Astronomy generalises the results of the other sciences. She 
exhibits the laws of Nature working over a wider area, and under 
more varied conditions, than ordinary experience presents. Ordinary 



8 HISTORY OF ASTRONOMY 

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



PART I 

PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF 
THE NINETEENTH CENTURY 

CHAPTER I 

FOUNDATION OF SIDEREAL ASTRONOMY 

Until nearly a hundred years ago the stars were regarded by 
practical astronomers mainly as a number of convenient fixed points 
by which the motions of the various members of the solar system 
could be determined and compared. Their recognised function, in 
fact, was that of milestones on the great celestial highway traversed 
by the planets, as well as on the byeways of space occasionally 
pursued by comets. Not that curiosity as to their nature, and even 
conjecture as to their origin, were at any period absent. Both were 
from time to time powerfully stimulated by the appearance of 
startling novelties in a region described by philosophers as "incor- 
ruptible," or exempt from change. The catalogue of Hipparchus 
probably, and certainly that of Tycho Brahe, some seventeen cen- 
turies later, owed each its origin to the temporary blaze of a new 
star. The general aspect of the skies was thus (however imper- 
fectly) recorded from age to age, and with improved appliances the 
enumeration was rendered more and more accurate and complete ; 
but the secrets of the stellar sphere remained inviolate. 

In a qualified though very real sense, Sir William Herschel may 
be called the Founder of Sidereal Astronomy. Before his time 
some curious facts had been noted, and some ingenious speculations 
hazarded, regarding the condition of the stars, but not even the 
rudiments of systematic knowledge had been acquired. The facts 
ascertained can be summed up in a very few sentences. 

Giordano Bruno was the first to set the suns of space in motion ; 
but in imagination only. His daring surmise was, however, con- 
firmed in 1718, when Halley announced 1 that Sirius, AJdebaran, 
1 Phil. Trans., vol. xxx., p. 737. 



io HISTORY OF ASTRONOMY part i 

Betelgeux, and Arcturus had unmistakably shifted their quarters in 
the sky since Ptolemy assigned their places in his catalogue. A 
similar conclusion was reached by J. Cassini in 1738, from a 
comparison of his own observations with those made at Cayenne 
by Eicher in 1672; and Tobias Mayer drew up in 1756 a list 
showing the direction and amount of about fifty-seven proper 
motions, 1 founded on star-places determined by Olaus Romer fifty 
years previously. Thus the stars were no longer regarded as 
"fixed," but the question remained whether the movements 
perceived were real or only apparent; and this it was not yet 
found possible to answer. Already, in the previous century, the 
ingenious Robert Hooke had suggested an " alteration of the very 
system of the sun," 2 to account for certain suspected changes in 
stellar positions; Bradley in 1748, and Lambert in 1761, pointed 
out that such apparent displacements (by that time well ascertained) 
were in all probability a combined effect of motions both of sun 
and stars ; and Mayer actually attempted the analysis, but without 
result. 

On the 13th of August, 1596, David Fabricius, an unprofessional 
astronomer in East Friesland, saw in the neck of the Whale a star 
of the third magnitude, which by October had disappeared. It 
was, nevertheless, visible in 1603, when Bayer marked it in his 
catalogue with the Greek letter o, and was watched, in 1638-39, 
through its phases of brightening and apparent extinction by a 
Dutch professor named Holwarda. 3 From Hevelius this first-known 
periodical star received the name of " Mira," or the Wonderful, 
and Boulliaud in 1667 fixed the length of its cycle of change at 
334 days. It was not a solitary instance. A star in the Swan 
was perceived by Janson in 1600 to show fluctuations of light, 
and Montanari found in 1669 that Algol in Perseus shared the 
same peculiarity to a marked degree. Altogether the class 
embraced in 1782 half-a-dozen members. When it is added that 
a few star-couples had been noted in singularly, but it was 
supposed accidentally, close juxtaposition, and that the failure 
of repeated attempts to measure stellar parallaxes pointed to 
distances at least 400,000 times that of the earth from the sun, 4 the 

1 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. Tobicc Mayeri Op. Inedita, 
t. i., pp. 80, 81, and Herschel in Phil. Trans., vol. lxxiii., pp. 275-278. 

2 Posthumous Works, p. 701. 

3 Arago in Annuaire du Bureau des Longitudes, 1842, p. 313. 

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



chap, i SIDEREAL ASTRONOMY n 

picture of sidereal science, when the last quarter of the eighteenth 
century began, is practically complete. It included three items of 
information : that the stars have motions, real or apparent ; that 
they are immeasurably remote; and that a few shine with a 
periodically variable light. Nor were these scantily collected 
facts ordered into any promise of further development. They 
lay at once isolated and confused before the inquirer. They 
needed to be both multiplied and marshalled, and it seemed as if 
centuries of patient toil must elapse before any reliable conclu- 
sions could be derived from them. The sidereal world was thus 
the recognised domain of far-reaching speculations, which re- 
mained wholly uncramped by systematic research until Herschel 
entered upon his career as an observer of the heavens. 

The greatest of modern astronomers was born at Hanover, 
November 15, 1738. He was the fourth child of Isaac Herschel, 
a hautboy-player in the band of the Hanoverian Guard, and was 
early trained to follow his father's profession. On the termination, 
however, of the disastrous campaign of 1757, his parents removed 
him from the regiment, there is reason to believe, in a somewhat 
unceremonious manner. Technically, indeed, he incurred the 
penalties of desertion, remitted according to the Duke of Sussex's 
statement to Sir George Airy by a formal pardon handed to him 
personally by George III. on his presentation in 1782. 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 circum- 
stances that in 1760 he was engaged in training the regimental 
band of the Durham Militia, and that in 1765 he was appointed 
organist at Halifax. In the following year he removed to Bath as 
oboist in Linley's orchestra, and in October 1767 was promoted 
to the post of organist in the Octagon Chapel. The tide of 
prosperity now began to flow for him. The most brilliant and 
modish society in England was at that time to be met at Bath, 
and the young Hanoverian quickly found himself a favourite and 
the fashion in it. Engagements multiplied upon him. He became 
director of the public concerts; he conducted oratorios, engaged 
singers, organised rehearsals, composed anthems, chants, choral 
services, besides undertaking private tuitions, at times amounting 
to thirty-five or even thirty-eight lessons a week. He in fact 
personified the musical activity of a place then eminently and 
energetically musical. 

But these multifarious avocations did not take up the whole of 
his thoughts. His education, notwithstanding the poverty of his 

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



12 HISTORY OF ASTRONOMY part i 

family, had not been neglected, and he had always greedily assimi- 
lated every kind of knowledge that came in his way. Now that he 
was a busy and a prosperous man, it might have been expected that 
he would run on in the deep professional groove laid down for him. 
On the contrary, his passion for learning seemed to increase with the 
diminution of the time available for its gratification. He studied 
Italian, Greek, mathematics ; Maclaurin's Fluxions served to " un- 
bend his mind "; Smith's Harmonics and Optics and Ferguson's 
Astronomy were the nightly companions of his pillow. What he 
read stimulated without satisfying his intellect. He desired not 
only to know, but to discover. In 1772 he hired a small telescope, 
and through it caught a preliminary glimpse of the rich and varied 
fields in which for so many years he was to expatiate. Hence- 
forward the purpose of his life was fixed : it was to obtain " a 
knowledge of the construction of the heavens"; 1 and this sublime 
ambition he cherished to the end. 

A more powerful instrument was the first desideratum ; and here 
his mechanical genius came to his aid. Having purchased the 
apparatus of a Quaker optician, he set about the manufacture of 
specula with a zeal which seemed to anticipate the wonders they 
were to disclose to him. It was not until fifteen years later that his 
grinding and polishing machines were invented, so the work had at 
that time to be entirely done by hand. During this tedious and 
laborious process (which could not be interrupted without injury, 
and lasted on one occasion sixteen hours), his strength was sup- 
ported 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 5J-foot Gre- 
gorian of his own construction. A copy of his first observation 
with it, on the great Nebula in Orion an object of continual amaze- 
ment and assiduous inquiry to him is preserved by the Royal 
Society. It bears the date March 4, 1774. 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 guidance. Overwhelmed 
with professional engagements, he still contrived to snatch some 

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

3 Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in the 
same place, January 9, 1848. She came to England in 1772, and was her 
brother's devoted assistant, first in his musical undertakings, and afterwards, 
down to the end of his life, in his astronomical labours. 

3 Holden, op. cit., p. 39. 






chap, i SIDEREAL ASTRONOMY 13 

moments for the stars ; and between the acts at the theatre was 
often seen running from the harpsichord to his telescope, no doubt 
with that "uncommon precipitancy which accompanied all his 
actions." 1 He now rapidly increased the power and perfection of 
his telescopes. Mirrors of seven, ten, even twenty feet focal length, 
were successively completed, and unprecedented magnifying powers 
employed. His energy was unceasing, his perseverance indomitable. 
In the course of twenty-one years no less than 430 parabolic specula 
left his hands. He had entered upon his forty-second year when he 
sent his first paper to the Philosophical Transactions ; yet during the 
ensuing thirty-nine years his contributions many of them elaborate 
treatises numbered sixty-nine, forming a series of extraordinary 
importance to the history of astronomy. As a mere explorer of the 
heavens his labours were prodigious. He discovered 2,500 nebulae, 
806 double stars, passed the whole firmament in review four several 
times, counted the stars in 3,400 "gauge-fields," and executed a 
photometric classification of the principal stars, founded on an 
elaborate (and the first systematically conducted) investigation of 
their relative brightness. He was as careful and patient as he was 
rapid ; spared no time and omitted no precaution to secure accuracy 
in his observations ; yet in one night he would examine, singly and 
attentively, up to 400 separate objects. 

The discovery of Uranus was a mere incident of the scheme he 
had marked out for himself a fruit, gathered as it were by the way. 
It formed, nevertheless, the turning-point in his career. From a 
star-gazing musician he was at once transformed into an eminent 
astronomer ; he was relieved from the drudgery of a toilsome pro- 
fession, and installed as Eoyal Astronomer, with a modest salary of 
<200 a year ; funds were provided for the construction of the forty- 
foot reflector, from the great space-penetrating power of which he 
expected unheard-of revelations ; in fine, his future work was not 
only rendered possible, but it was stamped as authoritative. 2 On 
Whit-Sunday 1782, William and Caroline Herschel played and sang 
in public for the last time in St. Margaret's Chapel, Bath ; in August 
of the same year the household was moved to Datchet, near Windsor, 
and on April 3, 1786, to Slough. Here happiness and honours 
crowded on the fortunate discoverer. In 1788 he married Mary, 
only child of James Baldwin, a merchant of the city of London, 
and widow of Mr. John Pitt a lady whose domestic virtues were 
enhanced by the possession of a large jointure. The fruit of their 
union was one son, of whose work the worthy sequel of his father's 
we shall have to speak further on. Herschel was created a Knight 

1 Memoir of Caroline Herschel, p. 37. 

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



i 4 HISTORY OF ASTRONOMY parti 

of the Hanoverian Guelphic Order in ldl6, 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 mag- 
nitude and distribution to the lucid orbs of the constellations. 1 He 
was followed by Kant, 2 who transcended the views of his prede- 
cessor by assigning to nebulae the position they long continued to 
occupy, rather on imaginative than scientific grounds, of "island 
universes," external to, and co-equal with, the Galaxy. Johann 
Heinrich Lambert, 3 a tailor's apprentice from Miihlhausen, followed, 
but independently. The conceptions of this remarkable man were 
grandiose, his intuitions bold, his views on some points a singular 
anticipation of subsequent discoveries. The sidereal world presented 
itself to him as a hierarchy of systems, starting from the planetary 
scheme, rising to throngs of suns within the circuit of the Milky 
Way the " ecliptic of the stars," as he phrased it expanding to 
include groups of many Milky Ways; these again combining to 
form the unit of a higher order of assemblage, and so onwards and 
upwards until the mind reels and sinks before the immensity of the 
contemplated creations. 

" Thus everything revolves the earth round the sun ; the sun 
round the centre of his system ; this system round a centre common 
to it with other systems; this group, this assemblage of systems, 
round a centre which is common to it with other groups of the same 
kind ; and where shall we have done ?"* 

The stupendous problem thus speculatively attempted, Herschel 

1 An Original Theory or New Hypothesis of the Universe, London, 1750. See 
also De Morgan's summary of his views in Philosophical Magazine, April, 1848. 

2 Allgemei?ie Naturgeschichte und Theorie des Himmels, 1755. 

3 Cosmologische Brief e, Augsburg, 1761. 

4 The System of the World, p. 125, London, 1800 (a translation of Cosmologische 
Brief e). Lambert regarded nebulae as composed of stars crowded together, but 
not as external universes. In the case of the Orion nebula, indeed, he throws 
out such a conjecture, but afterwards suggests that it may form a centre for that 
one of the subordinate systems composing the Milky Way to which our sun 
belongs. 



chap, i SIDEREAL ASTRONOMY 15 

undertook to grapple with experimentally. The upshot of this 
memorable inquiry was the inclusion, for the first time, within the 
sphere of human knowledge, of a connected body of facts, and 
inferences from facts, regarding the sidereal universe; in other 
words, the foundation of what may properly be called a science of 
the stars. 

Tobias Mayer had illustrated the perspective effects which must 
ensue in the stellar sphere from a translation of the solar system, 
by comparing them to the separating in front and closing up behind 
of trees in a forest to the eye of an advancing spectator } but the 
appearances which he thus correctly described he was unable to 
detect. By a more searching analysis of a smaller collection of 
proper motions, Herschel succeeded in rendering apparent the very 
consequences foreseen by Mayer. He showed, for example, that 
Arcturus and Vega did, in fact, appear to recede from, and Sirius 
and Aldebaran to approach, each other by very minute amounts ; 
and, with a striking effort of divinatory genius, placed the "apex," 
or point of direction of the sun's motion, close to the star A in the 
constellation Hercules, 2 within a few degrees of the spot indicated 
by later and indefinitely more refined methods of research. He 
resumed the subject in 1805, 3 but though employing a more 
rigorous method, was scarcely so happy in his result. In 1806, 4 he 
made a preliminary attempt to ascertain the speed of the sun's 
journey, fixing it, by doubtless much too low an estimate, at about 
three miles a second. Yet the validity of his general conclusion as 
to the line of solar travel, though long doubted, has been triumphantly 
confirmed. The question as to the " secular parallax " of the fixed 
stars was in effect answered. 

With their annual parallax, however, the case was very different. 
The search for it had already led Bradley to the important dis- 
coveries of the aberration of light and the nutation of the earth's 
axis ; it was now about to lead Herschel to a discovery of a different, 
but even more elevated character. Yet in neither case was the object 
primarily sought attained. 

From the very first promulgation of the Copernician theory the 
seeming immobility of the stars had been urged as an argument 
against its truth ; for if the earth really travelled in a vast orbit 

1 Opera Inedita, t. i., p. 79. 

2 Phil. Trans., vol. lxxiii. (1783), p. 273. Pierre Prevost's similar investiga- 
tion, communicated to the Berlin Academy of Sciences four months later, 
July 3, 1783, was inserted in the Memoirs of that body for 1781, and thus seems 
to claim a priority not its due. Georg Simon Kliigel at Halle gave about the 
same time an analytical demonstration of Herschel's result. Wolf, Gesch.~der 
Astronomie, p. 733. 

3 Phil. Tratis., vol. xcv., p. 233. 4 Ibid., vol. xcvi., p. 205. 






i6 HISTORY OF ASTRONOMY part i 

round the sun, objects in surrounding space should appear to change 
their positions, unless their distances were on a scale which, to the 
narrow ideas of the universe then prevailing, seemed altogether 
extravagant. 1 The existence of such apparent or "parallactic" dis- 
placements was accordingly regarded as the touchstone of the new 
views, and their detection became an object of earnest desire to 
those interested in maintaining them. Copernicus himself made the 
attempt j but with his " Triquetrum," a jointed wooden rule with 
the divisions marked in ink, constructed by himself, 2 he was hardly 
able to measure angles of ten minutes, far less fractions of a second. 
Galileo, a more impassioned defender of the system, strained his 
ears, as it were, from Arcetri, in his blind and sorrowful old age, for 
news of a discovery which two more centuries had still to wait for. 
Hooke believed he had found a parallax for the bright star in the 
Head of the Dragon ; but was deceived. Bradley convinced himself 
that such effects were too minute for his instruments to measure. 
Herschel made a fresh attempt by a practically untried method. 

It is a matter of daily experience that two objects situated at 
different distances seem to a beholder in motion to move relatively 
to each other. This principle Galileo, in the third of his Dialogues 
on the Systems of the World, 3 proposed to employ for the deter- 
mination of stellar parallax; for two stars, lying apparently close 
together, but in reality separated by a great gulf of space, must shift 
their mutual positions when observed from opposite points of the 
earth's orbit ; or rather, the remoter forms a virtually fixed point, to 
which the movements of the other can be conveniently referred. By 
this means complications were abolished more numerous and per- 
plexing than Galileo himself was aware of, and the problem was 
reduced to one of simple micrometrical measurement. The "double- 
star method" was also suggested by James Gregory in 1675, and 
again by Wallis in 1693 ; 4 Huygens first, and afterwards Dr. Long 
of Cambridge (about 1750), made futile experiments with it ; and it 
eventually led, in the hands of Bessel, to the successful determination 
of the parallax of 61 Cygni. 

Its advantages were not lost upon Herschel. His attempt to 
assign definite distances to the nearest stars was no isolated effort, 
but part of the settled plan upon which his observations were con- 
ducted. He proposed to sound the heavens, and the first requisite 
was a knowledge of the length of his sounding-line. Thus it came 
about that his special attention was early directed to double stars. 

11 1 resolved," he writes, 5 " to examine every star in the heavens 

1 " Ingens bolus devorandus est," Kepler admitted to Henvart in May, 1603. 
a Described in "Praefatio Editoris" to Be Eevolutionibics, p. xix. (ed. 1854). 
3 Opere, t. i. , p. 415. 4 Phil. Trans., vol. xvii. , p. 848. 5 Ibid. , vol. lxxii., p. 97. 



chap, i SIDEREAL ASTRONOMY 17 

with the utmost attention and a very high power, that I might 
collect such materials for this research as would enable me to fix 
my observations upon those that would best answer my end. The 
subject has already proved so extensive, and still promises so rich a 
harvest to those who are inclined to be diligent in the pursuit, that 
I cannot help inviting every lover of astronomy to join with me in 
observations that must inevitably lead to new discoveries." 

The first result of these inquiries was a classed catalogue of 
269 double stars presented to the Royal Society in 1782, followed, 
after three years, by an additional list of 434. In both these 
collections the distances separating the individuals of each pair were 
carefully measured, and (with a few exceptions) the angles made 
with the hour-circle by the lines joining their centres (technically 
called "angles of position") were determined with the aid of a 
" revolving- wire micrometer," specially devised for the purpose. 
Moreover, an important novelty was introduced by the observation 
of the various colours visible in the star-couples, the singular and 
vivid contrasts of which were now for the first time described. 

Double stars were at that time supposed to be a purely optical 
phenomenon. Their components, it was thought, while in reality 
indefinitely remote from each other, were brought into fortuitous 
contiguity by the chance of lying nearly in the same line of sight 
from the earth. Yet Bradley had noticed a change of 30, between 
1718 and 1759, in the position-angle of the two stars forming 
Castor, and was thus within a hair's breadth of the discovery of 
their physical connection. 1 While the Rev. John Michell, arguing 
by the doctrine of probabilities, wrote as follows in 1767 : "It is 
highly probable in particular, and next to a certainty in general, 
that such double stars as appear to consist of two or more stars 
placed very near together, do really consist of stars placed near 
together, and under the influence of some general law." 2 And in 
1784 : 3 "It is not improbable that a few years may inform us that 
some of the great number of double, triple stars, etc., which have 
been observed by Mr. Herschel, are systems of bodies revolving 
about each other." 

This remarkable speculative anticipation had a practical counter- 
part 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. 4 But his observations 

1 Doberck, Observatory, vol. ii., p. 110. 

2 Phil. Trans., vol. lvii., p. 249. s Ibid., vol. lxxiv., p. 56. 

4 Beobachtungen von Fixdcnitrabonien, 1778 ; and Be Novis in Ccelo Sidereo 
Phoenomenis, 1779. 

2 






18 HISTORY OF ASTRONOMY part 



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 7i'a point cru a des cJwses aussi 
extraordinaires, wrote Lalande 1 within one year of a better-grounded 
announcement to the same effect. 

Herschel at first shared the general opinion as to the merely 
optical connection of double stars. Of this the purpose for which 
he made his collection is in itself sufficient evidence, since what 
may be called the differential method of parallaxes depends, as we 
have seen, for its efficacy upon disparity of distance. It was 
"much too soon," he declared in 1782, 2 "to form any theories of 
small stars revolving round large ones f while in the year follow- 
ing, 3 he remarked that the identical proper motions of the two 
stars forming, to the naked eye, the single bright orb of Castor 
could only be explained as both equally due to the "systematic 
parallax " caused by the sun's movement in space. Plainly showing 
that the notion of a physical tie, compelling the two bodies to 
travel together, had not as yet entered into his speculations. But 
he was eminently open to conviction, and had, moreover, by 
observations unparalleled in amount as well as in kind, prepared 
ample materials for convincing himself and others. In 1802 he 
was able to announce the fact of his discovery, and in the two 
ensuing years, to lay in detail before the Royal Society proofs, 
gathered from the labours of a quarter of a century, of orbital 
revolution in the case of as many as fifty double stars, henceforth, 
he declared, to be held as real binary combinations, " intimately 
held together by the bond of mutual attraction." 4 The fortunate 
preservation in Dr. Maskelyne's note-book of a remark made by 
Bradley about 1759, to the effect that the line joining the com- 
ponents of Castor was an exact prolongation of that joining Castor 
with Pollux, added eighteen years to the time during which the 
pair were under scrutiny, and confirmed the evidence of change 
afforded by more recent observations. Approximate periods were 
fixed for many of the revolving suns for Castor 342 years ; for y 
Leonis, 1200, 8 Serpentis, 375, e Bootis, 1681 years; e Lyrae was 
noted as a "double-double-star," a change of relative situation 
having been detected in each of the two pairs composing the 
group ; and the occultation was described of one star by another in 
the course of their mutual revolutions, as exemplified in 1795 by the 
rapidly circulating system of f Herculis. 

Thus, by the sagacity and perseverance of a single observer, a 
firm basis was at last provided upon which to raise the edifice of 

1 Bibliographie, p. 569. 3 Phil. Trans., vol. lxxii., p. 162. 

3 lbid. t vol. lxxiii., p. 272. * Ibid., vol. xciii., p. 340. 



chap, i SIDEREAL ASTRONOMY 19 

sidereal science. The analogy long presumed to exist between 
the mighty star of our system and the bright points of light 
spangling the firmament was shown to be no fiction of the 
imagination, but a physical reality; the fundamental quality of 
attractive power was proved to be common to matter so far as 
the telescope was capable of exploring, and law, subordination, 
and regularity to give testimony of supreme and intelligent 
design no less in those limitless regions of space than in our 
narrow terrestrial home. The discovery was emphatically (in 
Arago's phrase) " one with a future," since it introduced the 
element of precise knowledge where more or less probable con- 
jecture had previously held almost undivided sway; and precise 
knowledge tends to propagate itself and advance from point to point. 

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

The ingenious method of "star-gauging," and its issue in the 
delineation of the sidereal system as an irregular stratum of 
evenly- scattered suns, is the best-known part of his work. But 
it was, in truth, only a first rude approximation, the principle of 
which maintained its credit in the literature of astronomy a full 
half-century after its abandonment by its author. This principle 
was the general equality of star distribution. If equal portions 
of space really held equal numbers of stars, it is obvious that the 
number of stars visible in any particular direction would be strictly 
proportional to the range of the system in that direction, apparent 
accumulation being produced by real extent. The process of 
"gauging the heavens," accordingly, consisted in counting the 
stars in successive telescopic fields, and calculating thence the 
depths of space necessary to contain them. The result of 3,400 
such operations was the plan of the Galaxy familiar to every 
reader of an astronomical text-book. Widely-varying evidence 
was, as might have been expected, derived from an examination 
of different portions of the sky. Some fields of view were almost 
blank, while others (in or near the Milky Way) blazed with the 
radiance of many hundred stars compressed into an area about 
one-fourth that of the full-moon. In the most crowded parts 

22 



20 HISTORY OF ASTRONOMY parti 

116,000 were stated to have been passed in review within a 
quarter of an hour. Here the " length of his sounding-line " was 
estimated by Herschel at about 497 times the distance of Sirius 
in other words, the bounding orb, or farthest sun of the system in 
that direction, so far as could be seen with the 20-foot reflector, 
was thus inconceivably remote. But since the distance of Sirius, 
no less than of every other fixed star, was as yet an unknown 
quantity, the dimensions inferred for the Galaxy were of course 
purely relative ; a knowledge of its form and structure might 
(admitting the truth of the fundamental hypothesis) be obtained, 
but its real or absolute size remained altogether undetermined. 

Even as early as 1785, however, Herschel perceived traces of a 
tendency which completely invalidated the supposition of any 
approach to an average uniformity of distribution. This was the 
action of what he called a " clustering power " in the Milky Way. 
" Many gathering clusters " l 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 surrounded by "a magnificent collection of 
innumerable stars, called the Milky Way, which must occasion 
a very powerful balance of opposite attractions to hold the 
intermediate stars at rest. For though our sun, and all the stars 
we see, may truly be said to be in the plane of the Milky Way, 
yet I am now convinced, by a long inspection and continued 
examination of it, that the Milky Way itself consists of stars very 
differently scattered from those which are immediately about us." 
"This immense aggregation," he added, "is by no means uniform. 
Its component stars show evident signs of clustering together into 
many separate allotments." 3 

The following sentences, written in 1811, contain a definite 
retractation of the view frequently attributed to him : 

" I must freely confess," he says, " that by continuing my 
sweeps of the heavens my opinion of the arrangement of the stars 
and their magnitudes, and of some other particulars, has undergone 
a gradual change j and indeed, when the novelty of the subject is 
considered, we cannot be surprised that many things formerly taken 

1 Phil. Trans., vol. lxxv., p. 255. 2 Ibid., vol. lxxix., pp. 214, 222. 

3 Ibid., vol. xcii., pp. 479, 495. 



chap, i SIDEREAL ASTRONOMY 21 

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 calcula- 
tions ; but when we examine the Milky Way, or the closely com- 
pressed clusters of stars of which my catalogues have recorded so 
many instances, this supposed equality of scattering must be given 
up. ri 

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 
firmament, conclusions drawn from such a comparison are entirely 
worthless. 

In another branch of sidereal science besides that of stellar 
aggregation, Herschel may justly be styled a pioneer. He was 
the first to bestow serious study on the enigmatical objects known 
as "nebulae." The history of the acquaintance of our race with 
them is comparatively short. The only one recognised before the 
invention of the telescope was that in the girdle of Andromeda, 
certainly familiar in the middle of the tenth century to the Persian 
astronomer Abdurrahman Al-Sufi; and marked with dots on 
Spanish and Dutch constellation-charts of the fourteenth and 
fifteenth centuries. 3 Yet so little was it noticed that it might 
practically be said as far as Europe is concerned to have been 
discovered in 1612 by Simon Marius (Mayer of Genzenhausen), 
who aptly described its appearance as that of a "candle shining 
through horn." The first mention of the great Orion nebula is 
by a Swiss Jesuit named Cysatus, who succeeded Father Scheiner 

1 Phil. Trans., vol. ci., p. 269. * Ibid., vol. cvii., p. 311. 

8 Bullialdus, Be Nebulosd Stella* in Cingulo Andromeda:. (1667) ; see also 
G. P. Bond, Mem. Am. Ac, vol. iii., p. 75, Holden's Monograph on the Orion 
Nebula, Washington Observations, vol. xxv., 1878 (pub. 1882), and Lady 
Huggins's drawing, Atlas of Spectra, p. 119. 



22 HISTORY OF ASTRONOMY parti 

in the chair of mathematics at Ingolstadt. He used it, apparently 
without any suspicion of its novelty, as a term of comparison for 
the comet of December 1618. 1 A novelty, nevertheless, to 
astronomers it still remained in 1656, when Huygens discerned, 
"as it were, an hiatus in the sky, affording a glimpse of a more 
luminous region beyond." 2 Halley in 1716 knew of six nebulae, 
which he believed to be composed of a "lucid medium" diffused 
through the ether of space. 3 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 ; 4 and Messier (nicknamed by Louis XV. 
the " ferret of comets "), finding such objects a source of extreme 
perplexity in the pursuit of his chosen game, attempted to eliminate 
by methodising them, and drew up a catalogue comprising, in 1781, 
103 entries. 5 

These preliminary attempts shrank into insignificance when 
Herschel began to " sweep the heavens " with his giant telescopes. 
In 1786 he presented to the Royal Society a descriptive catalogue 
of 1,000 nebula? and clusters, followed, three years later, by a 
second of as many more ; to which he added in 1802 a further 
gleaning of 500. On the subject of their nature his views under- 
went a remarkable change. Finding that his potent instruments 
resolved into stars many nebulous patches in which no signs of 
such a structure had previously been discernible, he naturally 
concluded that " resolvability " was merely a question of distance 
and telescopic power. He was (as he said himself) led on by 
almost imperceptible degrees from evident clusters, such as the 
Pleiades, to spots without a trace of stellar formation, the grada- 
tions 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 nebula? variously 
arranged; large ones with small, seeming attendants; narrow, 
but much extended lucid nebula? or bright dashes; some of the 
shape of a fan, resembling an electric brush, issuing from a lucid 
point ; others of the cometic shape, with a seeming nucleus in the 
centre, or like cloudy stars surrounded with a nebulous atmosphere ; 
a different sort, again, contain a nebulosity of the milky kind, like 
that wonderful, inexplicable phenomenon about 6 Orionis; while 

1 MatJiemata Astronomica, p. 75. 2 Systema Saturnium, p. 9. 

3 Phil. Trans., vol. xxix., p. 390. * Mem. Ac. des Sciences, 1755. 

5 Conn, des Temps, 1784 (pub. 1781), p. 227. A previous list of forty-five had 
appeared in Mim. Ac. des Sciences 1771. 



chap, i SIDEREAL ASTRONOMY 23 

others shine with a fainter, mottled kind of light, which denotes 
their being resolvable into stars." 1 

" These curious objects " he considered to be " no less than whole 
sidereal systems," 2 some of which might " well outvie our Milky 
Way in grandeur." He admitted, however, a wide diversity in 
condition as well as compass. The system to which our sun belongs 
he described as "a very extensive branching congeries of many 
millions of stars, which probably owes its origin to many remarkably 
large as well as pretty closely scattered small stars, that may have 
drawn together the rest." 3 But the continued action of this same 
"clustering power" would, he supposed, eventually lead to the 
breaking-up of the original majestic Galaxy into two or three 
hundred separate groups, already visibly gathering. Such minor 
nebulae, due to the " decay " of other " branching nebulas " similar 
to our own, he recognised by the score, lying, as it were, stratified 
in certain quarters of the sky. " One of these nebulous beds," he 
informs us, "is so rich that in passing through a section of it, in 
the time of only thirty-six minutes, I detected no less than thirty- 
one 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." 4 

" This method of viewing the heavens," he concluded, " seems to 
throw them into a new kind of light. They now are seen to 
resemble a luxuriant garden which contains the greatest variety 
of productions in different flourishing beds ; and one advantage we 
may at least reap from it is, that we can, as it were, extend the 
range of our experience to an immense duration. For, to continue 
the simile which I have borrowed from the vegetable kingdom, is 
it not almost the same thing whether we live successively to 
witness the germination, blooming, foliage, fecundity, fading, 
withering, and corruption of a plant, or whether a vast number 
of specimens, selected from every stage through which the plant 

1 Phil. Trans., vol. lxxiv., p. 442. a Ibid., vol. lxxix., p. 213. 

3 Ibid., vol. lxxv., p. 254. 4 Ibid., vol. lxxix., p. 225. 



24 HISTOR Y OF ASTRONOMY part i 

passes in the course of its existence, be brought at once to our 
view T 1 

But already this supposed continuity was broken. After mature 
deliberation on the phenomena presented by nebulous stars, Herschel 
was induced, in 1791, to modify essentially his original opinion. 

" When I pursued these researches," he says, " I was in the 
situation of a natural philosopher who follows the various species 
of animals and insects from the height of their perfection down to 
the lowest ebb of life ; when, arriving at the vegetable kingdom, he 
can scarcely point out to us the precise boundary where the animal 
ceases and the plant begins ; and may even go so far as to suspect 
them not to be essentially different. But, recollecting himself, he 
compares, for instance, one of the human species to a tree, and all 
doubt upon the subject vanishes before him. In the same manner 
we pass through gentle steps from a coarse cluster of stars, such as 
the Pleiades . . . till we find ourselves brought to an object such 
as the nebula in Orion, where we are still inclined to remain in the 
once adopted idea of stars exceedingly remote and inconceivably 
crowded, as being the occasion of that remarkable appearance. It 
seems, therefore, to require a more dissimilar object to set us right 
again. A glance like that of the naturalist, who casts his eye from 
the perfect animal to the perfect vegetable, is wanting to remove 
the veil from the mind of the astronomer. The object I have 
mentioned above is the phenomenon that was wanting for this 
purpose. View, for instance, the 19th cluster of my 6th class, and 
afterwards cast your eye on this cloudy star, and the result will be 
no less decisive than that of the naturalist we have alluded to. Our 
judgment, I may venture to say, will be, that the nebulosity about the 
star is not of a starry nature.'" 2 

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 specula- 
tion. What was its nature ? Should it "be compared to the 
coruscation of the electric fluid in the aurora borealis? or to the 
more magnificent cone of the zodiacal light ?" Above all, what was 
its function in the cosmos ? And on this point he already gave a 
hint of the direction in which his mind was moving by the remark 
that this self-luminous matter seemed "more fit to produce 
a star by its condensation, than to depend on the star for its 
existence." 3 

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 

1 Phil. Trans., vol. lxxix., p. 226. 2 Ibid., vol. lxxxi., p. 72. 

3 Ibid., p. 85. 



chap, i SIDEREAL ASTRONOMY 25 

nebulous material in the Milky Way, even pointing out the space 
left dark and void by the withdrawal of the luminous stuff; and 
Kepler, theorising on a similar stellar apparition in 1604, followed 
nearly in the same track. But under Herschel's treatment the 
nebular origin of stars first acquired the consistency of a formal 
theory. He meditated upon it long and earnestly, and in two 
elaborate treatises, published respectively in 1811 and 1814, he at 
length set forth the arguments in its favour. These rested entirely 
upon the "principle of continuity." Between the successive classes 
of his assortment of developing objects there was, as he said, 
" perhaps not so much difference as would be in an annual descrip- 
tion of the human figure, were it given from the birth of a child 
till he comes to be a man in his prime." 1 From diffused nebulosity, 
barely visible in the most powerful light-gathering instruments, but 
which he estimated to cover nearly 152 square degrees of the 
heavens, 2 to planetary nebulse, supposed to be already centrally 
solid, instances were alleged 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 themselves, 
and necessarily barren of results. But they gratify an inherent 
tendency of the human mind, and, if pursued in a becoming spirit, 
should be neither reproved nor disdained. Herschel's theory still 
holds the field, the testimony of recent discoveries with regard to 
it having proved strongly confirmatory of its principle, although not 
of its details. Strangely enough, it seems to have been propounded 
in complete independence of Laplace's nebular hypothesis as to the 
origin of the solar system. Indeed, it dated, as we have seen, in its 
first inception, from 1791, while the French geometrician's view was 
not advanced until 1796. 

We may now briefly sum up the chief results of Herschel's long 
years of " watching the heavens." The apparent motions of the 
stars had been disentangled ; one portion being clearly shown to be 
due to a translation towards a point in the constellation Hercules of 
the sun and his attendant planets ; while a large balance of displace- 
ment 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 

1 Phil. Trans., vol. ci., p. 271. 2 Ibid., p. 277. 



26 HISTORY OF ASTRONOMY part i 

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, notwith- 
standing the variety and complexity seen to prevail, to an extent 
previously undreamt of, in the arrangement of that majestic system. 
The existence of a luminous fluid, diffused through enormous tracts 
of space, and intimately associated with stellar bodies, was virtually 
demonstrated, and its place and use in creation attempted to be 
divined by a bold but plausible conjecture. / Change on a stupendous 
scale was inferred or observed to be everywhere in progress. 
Periodical stars shone out and again decayed ; progressive ebbings 
or flowings of light were indicated as probable in many stars under 
no formal suspicion of variability ; forces were everywhere perceived 
to be at work, by which the very structure of the heavens them- 
selves 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 j 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," 2 the state into which the incessant action of the 
clustering power has brought the Milky Way at present, is a kind 
of chronometer that may be used to measure the time 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 J. Herschel, Phil. Trans., vol. cxvi., partiii., p. 1. 

2 His own words to the poet Campbell cited by Holden, Life and Works, 
p. 109. 

3 Phil. Trans., vol. civ., p. 283. 






CHAPTER II 

PROGRESS OF SIDEREAL ASTRONOMY 

We have now to consider labours of a totally different character 
from those of Sir William Herschel. Exploration and discovery do 
not constitute the whole business of astronomy; the less adventurous, 
though not less arduous, task of gaining a more and more complete 
mastery over the problems immemorially presented to her, may, on 
the contrary, be said to form her primary duty. A knowledge of 
the movements of the heavenly bodies has, from the earliest times, 
been demanded by the urgent needs of mankind j and science finds 
its advantage, as in many cases it has taken its origin, in condescen- 
sion to practical claims. Indeed, to bring such knowledge as near 
as possible to absolute precision has been defined by no mean 
authority 1 as the true end of astronomy. 

Several causes concurred about the beginning of the last century 
to give a fresh and powerful impulse to investigations having 
this end in view. The rapid progress of theory almost com- 
pelled a corresponding advance in observation j instrumental 
improvements rendered such an advance possible ; Herschel's 
discoveries quickened public interest in celestial inquiries ; royal, 
imperial, and grand-ducal patronage widened the scope of individual 
effort. The heart of the new movement was in Germany. Hitherto 
the observatory of Flamsteed and Bradley had been the acknowledged 
centre of practical astronomy; Greenwich observations were the 
standard of reference all over Europe ; and the art of observing 
prospered in direct proportion to the fidelity with which Greenwich 
methods were imitated. Dr. Maskelyne, who held the post of 
Astronomer Royal during forty-six years (from 1765 to 1811), was 
no unworthy successor to the eminent men who had gone before 
him. His foundation of the Nautical Almanac (in 1767) alone 
constitutes a valid title to fame ; he introduced at the Observatory 
the important innovation of the systematic publication of results ; 
and the careful and prolonged series of observations executed by 
1 Bessel, Populdre Vorlesungen, pp. 6, 408. 



28 HISTORY OF ASTRONOMY part i 

him formed the basis of the improved theories, and corrected tables 
of the celestial movements, which were rapidly being brought to 
completion abroad. His catalogue of thirty-six "fundamental" 
stars was besides excellent in its way, and most serviceable. Yet 
he was devoid of Bradley's instinct for divining the needs of the 
future. He was fitted rather to continue a tradition than to found 
a school. The old ways were dear to him; and, indefatigable as 
he was, a definite purpose was wanting to compel him, by its 
exigencies, along the path of progress. Thus, for almost fifty years 
after Bradley's death, the acquisition of a small achromatic 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 deteriorated by 
time and obsolete by the progress of invention ; and it was not 
until the very close of his career that Maskelyne, compelled by 
Pond's detection of serious errors, ordered a Troughton's circle, 
which he did not live to employ. 

Meanwhile, the heavy national disasters with which Germany was 
overwhelmed in the early part of the nineteenth century seemed 
to stimulate rather than impede the intellectual revival already for 
some years in progress there. Astronomy was amongst the first of 
the sciences to feel the new impulse. By the efforts of Bode, Olbers, 
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 received powerful aid through the foundation, in 1804, 
by a young artillery officer named Von Reichenbach, of an Optical 
and Mechanical Institute at Munich. Here the work of English 
instrumental artists was for the first time rivalled, and that of 
English opticians when Fraunhofer entered the new establishment 
far surpassed. The development given to the refracting telescope 
by this extraordinary man was indispensable to the progress of that 
fundamental part of astronomy which consists in the exact determina- 
tion of the places of the heavenly bodies. Reflectors are brilliant 
engines of discovery, but they lend themselves with difficulty to the 
prosaic work of measuring right ascensions and polar distances. A 
signal improvement in the art of making and working flint-glass 
thus most opportunely coincided with the rise of a German school of 
scientific mechanicians, to furnish the instrumental means needed 
for the reform which was at hand. Of the leader of that reform it 
is now time to speak. 

Friedrich Wilhelm Bessel was born at Minden, in Westphalia 
1 Fitted to the old transit instrument, July 11, 1772. 



chap, ii SIDEREAL ASTRONOMY 29 

July 22, 1784. A certain taste for figures, coupled with a still 
stronger distaste for the Latin accidence, directed his inclination 
and his father's choice towards a mercantile career. In his fifteenth 
year, accordingly, he entered the house of Kuhlenkamp and Sons, 
in Bremen, as an apprenticed clerk. He was now thrown completely 
upon his own resources. From his father, a struggling Government 
official, heavily weighted with a large family, he was well aware 
that he had nothing to expect ; his dormant faculties were roused 
by the necessity for self-dependence, and he set himself to push 
manfully forward along the path that lay before him. The post of 
supercargo on one of the trading expeditions sent out from the 
Hanseatic towns to China and the East Indies was the aim of his 
boyish ambition, for the attainment of which he sought to qualify 
himself by the industrious acquisition of suitable and useful know- 
ledge. He learned English in two or three months; picked up 
Spanish with the casual aid of a gunsmith's apprentice; studied 
the geography of the distant lands which he hoped to visit ; collected 
information as to their climates, inhabitants, products, and the 
courses of trade. He desired to add some acquaintance with the 
art (then much neglected) of taking observations at sea; and thus, 
led on from navigation to astronomy, and from astronomy to 
mathematics, he groped his way into a new world. 

It was characteristic of him that the practical problems of 
science should have attracted him before his mind was as yet 
sufficiently matured to feel the charm of its abstract beauties. 
His first attempt at observation was made with a sextant, 
rudely constructed under his own directions, and a common 
clock. Its object was the determination of the longitude of 
Bremen, and its success, he tells us himself, 1 filled him with a 
rapture of delight, which, by confirming his tastes, decided his 
destiny. He now eagerly studied Bode's Jahrbuch and Yon Zach's 
Monatlkhe Correspondenz, overcoming each difficulty as it arose with 
the aid of Lalande's Traite d'Astronomie, and supplying, with amazing 
rapidity, his early deficiency in mathematical training. In two 
years he was able to attack a problem which would have tasked 
the patience, if not the skill, of the most experienced astronomer. 
Amongst the Earl of Egremont's papers Von Zach had discovered 
Harriot's observations on Halley's comet at its appearance in 1607, 
and published them as a supplement to Bode's Annual. With an 
elaborate care inspired by his youthful ardour, though hardly 
merited by their loose nature, Bessel deduced from them an orbit 
for that celebrated body, and presented the work to Olbers, whose 
reputation in cometary researches gave a special fitness to the 
1 Briefwechsel vxit Olbers, p. xvi. 



3o HISTORY OF ASTRONOMY part i 

proffered homage. The benevolent physician-astronomer of Bremen 
welcomed with surprised delight such a performance emanating from 
such a source. Fifteen years previously, the French Academy had 
crowned a similar work; now its equal was produced by a youth 
of twenty, busily engaged in commercial pursuits, self-taught, and 
obliged to snatch from sleep the hours devoted to study. The paper 
was immediately sent to Von Zach for publication, with a note from 
Olbers explaining the circumstances of its author ; and the name of 
Bessel became the common property of learned Europe. 

He had, however, as yet no intention of adopting astronomy 
as his profession. For two years he continued to work in 
the counting-house by day, and to pore over the Micanigue 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 pro- 
cured for him the offer of it. It was not without a struggle that 
he resolved to exchange the desk for the telescope. His reputa- 
tion with his employers was of the highest ; he had thoroughly 
mastered the details of the business, which his keen practical intelli- 
gence followed with lively interest ; his years of apprenticeship were 
on the point of expiring, and an immediate, and not unwelcome 
prospect of comparative affluence lay before him. The love of 
science, however, prevailed; he chose poverty and the stars, and 
went to Lilienthal with a salary of a hundred thalers yearly. Look- 
ing 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 im- 
provement, not for Germany alone, but for the whole astronomical 
world. During two-and-thirty years it was the scene of Bessel's 
labours, and Bessel's labours had for their aim the reconstruction, 
on an amended and uniform plan, of the entire science of observa- 
tion. 

A knowledge of the places of the stars is the foundation of 
astronomy. 2 Their configuration lends to the skies their distinctive 
features, and marks out the shifting tracks of more mobile objects 
with relatively fixed, and generally unvarying points of light. A 
more detailed and accurate acquaintance with the stellar multitude, 
1 R. Wolf, Gesch. der Astron., p. 518. 2 Bessel, Pop. Fori, p. 22. 






chap, ii SIDEREAL ASTRONOMY 31 

regarded from a purely uranographical point of new, has accordingly- 
formed at all times a primary object of celestial science, and was, 
during the last century, cultivated with a zeal and success by which 
all previous efforts were dwarfed into insignificance. In Lalande's 
Histoire Celeste, published in 1801, the places of no less than 47,390 
stars were given, but in the rough, as it were, and consequently 
needing laborious processes of calculation to render them available 
for exact purposes. Piazzi set an example of improved methods of 
observation, resulting in the publication, in 1803 and 1814, of two 
catalogues of about 7,600 stars the second being a revision and en- 
largement of the first which for their time were models of what 
such works should be. 1 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. 2 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 grada- 
tions varying with conditions of pressure and temperature, a 
maximum at the horizon. Moreover, the points to which measure- 
ments are referred are themselves in motion, either continually in one 
direction, or periodically to and fro. The precession of the equinoxes 
is slowly progressive, or rather retrogressive ; the nutation of the pole 
oscillatory in a period of about eighteen years. Added to which, the 
non-instantaneous transmission of light, combined with the movement 
of the earth in its orbit, causes a small annual displacement known 
as aberration. 

Now it is easy to see that any uncertainty in the application of 
these corrections saps the very foundations of exact astronomy. 
Extremely minute quantities, it is true, are concerned; but the life 
and progress of modern celestial science depends upon the sure re- 
cognition of extremely minute quantities. In the early years of the 
nineteenth century, however, no uniform system of "reduction " (so 
the complete correction of observational results is termed) had been 
established. Much was left to the individual caprice of observers, 
who selected for the several "elements" of reduction such values as 

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

2 Bessel, Pop. Fori., p. 440. 



32 HISTORY OF ASTRONOMY part i 

seemed best to themselves. Hence arose much hurtful confusion, 
tending to hinder united action and mar the usefulness of laborious 
researches. For this state of things, Bessel, by the exercise of con- 
summate diligence, sagacity, and patience, provided an entirely 
satisfactory remedy. 

His first step was an elaborate investigation of the precious 
series of observations made by Bradley at Greenwich from 1750 
until his death in 1762. The catalogue of 3,222 stars which he ex- 
tracted from them gave the earliest example of the systematic 
reduction on a uniform plan of such a body of work. It is difficult, 
without entering into details out of place in a volume like the 
present, to convey an idea of the arduous nature of this task. It 
involved the formation of a theory of the errors of each of Bradley's 
instruments, and a difficult and delicate inquiry into the true value 
of each correction to be applied, before the entries in the Greenwich 
journals could be developed into a finished and authentic catalogue. 
Although completed in 1813, it was not until five years later that 
the results appeared with the proud, but not inappropriate title of 
Fundamenta Astronomies. The eminent value of the work consisted 
in this, that by providing a mass of entirely reliable information as 
to the state of the heavens at the epoch 1755, it threw back the 
beginning of exact astronomy almost half a century. By comparison 
with Piazzi's catalogues the amount of precession was more 
accurately determined, the proper motions of a considerable 
number of stars became known with certainty, and definite pre- 
diction 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 liegiomontance. They not only constituted an 
advance in accuracy, but afforded a vast increase of facility in 
application, and were at once and everywhere adopted. Thus 
astronomy became a truly universal science ; uncertainties and 
disparities were banished, and observations made at all times and 
places rendered mutually comparable. 1 

More, however, yet remained to be done. In order to verify with 
greater strictness the results drawn from the Bradley and Piazzi 
catalogues, a third term of comparison was wanted, and this Bessel 
undertook to supply. By a course of 75,011 observations, executed 
during the years 1821-33, with the utmost nicety of care, the 
number of accurately known stars was brought up to above 50,000, 
and an ample store of trustworthy facts laid up for the use of future 
astronomers. In this department Argelander, whom he attracted 
from finance to astronomy, and trained in his own methods, was his 
1 Dur&ge, BesseVs Leben und Wirken, p. 28. 



chap, ii SIDEREAL ASTRONOMY 33 

assistant and successor. The great "Bonn Durchmusterung," 1 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 improve- 
ment 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. 2 His countryman, Caland- 
relli, was similarly deluded. The celebrated controversy between 
the Astronomer Royal and Dr. Brinkley, director of the Dublin 
College Observatory, turned on the same subject. Brinkley, who 
was in possession of a first-rate meridian-circle, believed himself to 
have discovered relatively large parallaxes for four of the brightest 
stars ; Pond, relying on the testimony of the Greenwich instruments, 
asserted their nullity. The dispute, protracted for fourteen years, 
from 1810 until 1824, was brought to no definite conclusion; but 
the strong presumption on the negative side was abundantly justified 
in the event. 

There was good reason for incredulity in the matter of parallaxes. 
Announcements of their detection had become so frequent as to be 
discredited before they were disproved ; and Struve, who investi- 
gated the subject at Dorpat in 1818-21, had clearly shown that 
the quantities concerned were too small to come within the reliable 
measuring powers of any instrument then in use. Already, how- 
ever, the means Avere being prepared of giving to those powers a 
large increase. 

On the 21st July, 1801, two old houses in an alley of Munich 
tumbled down, burying in their ruins the occupants, of whom one alone 
was extricated alive, though seriously injured. This was an orphan 
lad of fourteen named Joseph Fraunhofer. The Elector Maximilian 
Joseph was witness of the scene, became interested in the survivor, 
and consoled his misfortune with a present of eighteen ducats. 
Seldom was money better bestowed. Part of it went to buy books 
and a glass-polishing machine, with the help of which young Fraun- 
hofer studied mathematics and optics, and secretly exercised himself 
in the shaping and finishing of lenses ; the remainder purchased his 

1 Bonner Beobachtungen, Bd. iii.-v., 1859-62. a Bessel, Pop. Fori. p. 238. 

3 



34 HISTORY OF ASTRONOMY part i 

release from the tyranny of one Weichselberger, a looking-glass 
maker by trade, to whom he had been bound apprentice on the 
death of his parents. A period of struggle and privation followed, 
during which, however, he rapidly extended his acquirements ; and 
was thus eminently fitted for the task awaiting him, when, in 1806, 
he entered the optical department of the establishment founded two 
years previously by Von Reichenbach and Utzschneider. He now 
zealously devoted himself to the improvement of the achromatic 
telescope ; and, after a prolonged study of the theory of lenses, and 
many toilsome experiments in the manufacture of flint-glass, he 
succeeded in perfecting, December 12, 1817, an object-glass of ex- 
quisite quality and finish, 9 \ inches in diameter, and of 14 feet 
focal length. 

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

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 ^ of a second of arc, can be measured with the utmost 
accuracy. Main, R. A. S. Mem., vol. xii., p. 53. 





chap, ii SIDEREAL ASTRONOMY 35 

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 pro- 
jected journey to Italy, he died of consumption, June 7, 1826, aged 
thirty-nine years. His tomb in Munich bears the concise eulogy r 
Approximavit sidera. 

Bessel had no sooner made himself acquainted with the exquisite 
defining powers of the Konigsberg heliometer, than he resolved to 
employ them in an attack upon the now secular problem of star- 
distances. But it was not until 1837 that he found leisure to pursue 
the inquiry. In choosing his test-star he adopted a new principle* 
It had hitherto been assumed that our nearest neighbours in space 
must be found among the brightest ornaments of our skies. The 
knowledge of stellar proper motions afforded by the critical com- 
parison of recent with earlier star-places, suggested a different 
criterion of distance. It is impossible to escape from the conclusion 
that the apparently swiftest-moving stars are, on the whole, also the 
nearest to us, however numerous the individual exceptions to the 
rule. Now, as early as 1792, 1 Piazzi had noted as an indication of 
relative vicinity to the earth, the unusually large proper motion (5*2' r 
annually) of a double star of the fifth magnitude in the constella- 
tion of the Swan. Still more emphatically in 1812 2 Bessel drew the 
attention of astronomers to the fact, and 61 Cygni became known 
as the " flying star." The seeming rate of its flight, indeed, is of so 
leisurely a kind, that in a thousand years it will have shifted its 
place by less than 3 J lunar diameters, and that a quarter of a 
million would be required to carry it round the entire circuit of 
the visible heavens. Nevertheless, it has few rivals in rapidity of 
movement, the apparent displacement of the vast majority of stars 
being, by comparison, almost insensible. 

This interesting, though inconspicuous object, then, was chosen 
by Bessel to be put to the question with his heliometer, while 
Struve made a similar and somewhat earlier trial with the bright 
gem of the Lyre, whose Arabic title of the " Falling Eagle " sur- 
vives 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- 

1 Specola Astronomica di Palermo, lib. vi., p. 10, note. 

2 Monatliche Correspondetiz, vol. xxvi., p. 162. 

32 






36 HISTORY OF ASTRONOMY part i 

Bessel made known the result of one year's observations, showing 
for 61 Cygni a parallax of about a third of a second (0-3136"). 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 in- 
dustriously thrown into celestial space, having really and indubitably 
touched bottom. It was confirmed in 1842-43 with curious exactness 
by C. A. F. Peters at Pulkowa ; but later researches showed that it 
required increase to nearly half a second. 3 

Struve's measurements inspired less confidence. They extended 
over three years (1835-38), but were comparatively few, and were 
frequently interrupted. The parallax, accordingly, of about a 
quarter of a second (0-2613") which he derived from them for a 
Lyrae, and announced in 1840, 4 has proved considerably too large. 5 

Meanwhile a result of the same kind, but of a more striking 
character than either Bessel's or Struve's, had been obtained, one 
might almost say casually, by a different method and in a distant 
region. Thomas Henderson, originally an attorney's clerk in his 
native town of Dundee, had become known for his astronomical 
attainments, and was appointed in 1831 to direct the recently 
completed observatory at the Cape of Good Hope. He began 
observing in April 1832, and, the serious shortcomings of his 
instrument notwithstanding, executed during the thirteen months 
of his tenure of office a surprising amount of first-rate work. 
With a view to correcting the declination of the lustrous double 
star a Centauri (which ranks after Sirius and Canopus as the third 
brightest orb in the heavens), he effected a number of successive 
determinations of its position, and on being informed of its very 
considerable proper motion (3*6" annually), he resolved to examine 
the observations already made for possible traces of parallactic 
displacement. This was done on his return to Scotland, where he 
filled the office of Astronomer Eoyal from 1834 until his premature 
death in 1844. The result justified his expectations. From the 

1 AstronomiscJie 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 Astr. Nach., Nos. 401-402. 

3 Sir R. Ball's measurements at Dunsink gave to 61 Cygni a parallax of 0*47" ; 
Professor Pritchard obtained, by photographic determinations, one of 0"43". 

4 Additamentum in Mensuras Micrometricas, p. 28. 

5 Elkin's corrected result (in 1897) for the parallax of Vega is 0"082*. 



chap, ii SIDEREAL ASTRONOMY 37 

declination measurements made at the Cape and duly reduced, 
a parallax of about one second'of arc clearly emerged (diminished 
by Gill's and Elkin's observations, 1882-1883, to 0*75"); but, by 
perhaps an excess of caution, was withheld from publication until 
fuller certainty was afforded by the concurrent testimony of 
Lieutenant Meadows's determinations of the same star's right 
ascension. 1 When at last, January 9, 1839, Henderson com- 
municated his discovery to the Astronomical Society, he could no 
longer claim the priority which was his due. Bessel had anticipated 
him with the parallax of 61 Cygni by just two months. 

Thus from three different quarters, three successful and almost 
simultaneous assaults were delivered upon a long-beleaguered citadel 
of celestial secrets. The same work has since been steadily pursued, 
with the general result of showing that, as regards their overwhelm- 
ing majority, the stars are far too remote to show even the slightest 
trace of optical shifting from the revolution of the earth in its orbit. 
In nearly a hundred cases, however, small parallaxes have been 
determined, some certainly (that is, within moderate limits of error), 
others more or less precariously. The list is an instructive one, 
in its omissions no less than in its contents. It includes stars of 
many degrees of brightness, from Sirius down to a nameless 
telescopic star in the Great Bear ; 2 yet the vicinity to the earth of 
this minute object is so much greater than that of the brilliant 
Vega, that the latter transported to its place would increase in 
lustre thirty-eight times. Moreover, many of the brightest stars 
are found to have no sensible parallax, while the majority of those 
ascertained to be nearest to the earth are of fifth, sixth, even ninth 
magnitudes. The obvious conclusions follow that the range of 
variety in the sidereal system is enormously greater than had been 
supposed, and that estimates of distance based upon apparent 
magnitude must be wholly futile. Thus, the splendid Canopus, 
Betelgeux, and Bigel can be inferred, from their indefinite remote- 
ness, to exceed our sun thousands of times in size and lustre ; while 
many inconspicuous objects, which prove to be in our relative 
vicinity, must be notably his inferiors. The limits of real stellar 
magnitude are then set very widely apart. At the same time, 
the so-called "optical" and "geometrical" methods of relatively 
estimating star-distances are both seen to have a foundation of fact, 
although so disguised by complicated relations as to be of very 
doubtful individual application. On the whole, the chances are in 

1 Mem. Hoy. Astr. Soc. t vol. xi., p. 61. 

2 That numbered 21,185 in Lalande's Hist. Cel., found by Argelander to have 
a proper motion of 4 "734", and by Winnecke a parallax of 0*51 1". Month. Not., 
vol. xviii., p. 289. 



38 HISTOR Y OF ASTRONOMY part 



I 



favour of the superior vicinity of a bright star over a faint one; 
and, on the whole, the stars in swiftest apparent motion are amongst 
those whose actual remoteness is least. Indeed, there is no escape 
from either conclusion, unless on the supposition of special arrange- 
ments in themselves highly improbable, and, we may confidently 
say, non-existent. 

The distances even of the few stars found to have measurable 
parallaxes are on a scale entirely beyond the powers of the human 
mind to conceive. In the attempt both to realize them distinctly, 
and to express them conveniently, a new unit of length, itself of 
bewildering magnitude, has originated. This is what we may call 
the light-journey of one year. The subtle vibrations of the ether, 
propagated on all sides from the surface of luminous bodies, travel 
at the rate of 186,300 miles a second, or (in round numbers) six 
billions of miles a year. Four and a third such measures are needed 
to span the abyss that separates us from the nearest fixed star. In 
other words, light takes four years and four months to reach the 
earth from 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 stars revolving 
in known orbits, to the determination of mass ; for the distance 
from the earth of the two bodies forming a binary system being 
ascertained, the seconds of arc apparently separating them from 
each other can be translated into millions of miles; and we only 
need to add a knowledge of their period to enable us, by an easy 
sum in proportion, to find their combined mass in terms of that of 
the sun. Thus, since according to Dr. Doberck's elements the 
components of a Centauri revolve round their common centre of 
gravity at a mean distance nearly 25 times the radius of the earth's 
orbit, in a period of 88 years, the attractive force of the two together 
must be just twice the solar. We may gather some idea of their 
relations by placing in imagination a second luminary like our sun 
in circulation between the orbits of Neptune and Uranus. But 
systems of still more majestic proportions are reduced by extreme 
remoteness to apparent insignificance. A double star of the fourth 
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed 
on the authority of O. Struve, appears to be above eight times 
as massive as the central orb of our world; while a much less 
conspicuous pair 85 Pegasi exerts, if the available data can be 
depended upon, no less than thirteen times the solar gravitating 
power. 

Further, the actual rate of proper motions, so far as regards that 
part of them which is projected upon the sphere, can be ascertained 






chap, ii SIDEREAL ASTRONOMY 39 

for stars at known distances. The annual journey, for instance, 
of 61 Cygni across the line of sight amounts to 1,000, and that of a 
; Centauri to 446 millions of miles. A small star, numbered 1,830 
in Groombridge's Circumpolar Catalogue, " devours the way " at the 
rate of at least 150 miles a second a speed, in Newcomb's opinion, 
beyond the gravitating power of the entire sidereal system to 
control; and /* Cassiopeiae possesses above two-thirds of that 
surprising velocity ; while for both objects, radial movements of 
just sixty miles a second were disclosed by Professor Campbell's 
spectroscopic measurements. 

Herschel's conclusion as to the advance of the sun among the 
stars was not admitted as valid by the most eminent of his suc- 
cessors. Bessel maintained that there was absolutely no pre- 
ponderating evidence in favour of its supposed direction towards 
a point in the constellation Hercules. 1 Biot, Burckhardt, even 
Herschel's own son, shared his incredulity. But the appearance of 
Argelander's prize-essay in 1837 2 changed the aspect of the ques- 
tion. Herschel's first memorable solution in 1783 was based upon 
the motions of thirteen stars, imperfectly known; his second, in 
1805, upon those of no more than six. Argelander now obtained 
an entirely concordant result from the large number of 390, deter- 
mined with the scrupulous accuracy characteristic of Bessel's 
work and his own. The reality of the fact thus persistently dis- 
closed 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 displacements of southern stars. In 1859 and 1863, Sir 
George Airy and Mr. Dunkin (1821-1898), 4 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 1883 5 served but 
slightly to modify. Yet astronomers were not satisfied. Dr. Auwers 
of Berlin completed in 1886 a splendid piece of work, for which he 
received in 1888 the Gold Medal of the Royal Astronomical Society. 
It consisted in reducing afresh, with the aid of the most refined 
modern data, Bradley's original stars, and comparing their places 
thus obtained for the year 1755 with those assigned to them from 
observations made at Greenwich after the lapse of ninety years. In 
the interval, as was to be anticipated, most of them were found to 

1 Fund. Astr., p. 309. 2 Mbn. Pr6s. a VAc. de St. Petersb., t. iii. 

3 Phil. Trans., vol. exxxvii., p. 79. 

4 Mem. Roy. Astr. Soc, vols, xxviii. and xxxii. 5 Ibid., vol. xlvii., p. 327. 



4 o HISTOR Y OF ASTRONOMY part i 

have travelled over some small span of the heavens, and there 
resulted a stock of nearly three thousand highly authentic proper 
motions, fhese ample materials were turned to account by M. 
Ludwig Struve 1 for a discussion of the sun's motion, of which the 
upshot was to shift its point of aim to the bordering region of the 
constellations Hercules and Lyra. And the more easterly position 
of the solar apex was fully confirmed by the experiments, with 
variously assorted lists of stars, of Lewis Boss of Albany, 2 and Oscar 
Stumpe of Bonn. 3 Fresh precautions of refinement were introduced 
into the treatment of the subject by Eistenpart of Karlsruhe, 4 by 
Kapteyn of Groningen, 5 by Newcomb 6 and Porter 7 in America, who 
ably availed themselves of the copious materials accumulated before 
the close of the century. Their results, although not more closely 
accordant than those of their predecessors, combined to show that 
the journey of our system is directed towards a point within a circle 
about ten degrees in radius, having the brilliant Vega for its centre. 
To determine its rate was a still more arduous problem. It involved 
the assumption, very much at discretion, of an average parallax for 
the stars investigated ; and Otto Struve's estimate of 154 million 
miles as the span yearly traversed was hence wholly unreliable. 
Fortunately, however, as will be seen further on, a method of 
determining the sun's velocity independently of any knowledge of 
star-distances, has now become available. 

As might have been expected, speculation has not been idle 
regarding the purpose and goal of the strange voyage of discovery 
through space upon which our system is embarked ; but altogether 
fruitlessly. The variety of the conjectures hazarded in the matter 
is in itself a measure of their futility. Long ago, before the con- 
struction of the heavens had as yet been made the subject of 
methodical inquiry, Kant was disposed to regard Sirius as the 
" central sun " of the Milky Way ; while Lambert surmised that 
the vast Orion nebula might serve as the regulating power of a 
subordinate group including our sun. Herschel threw out the 
hint that the great cluster in Hercules might prove to be the 
supreme seat of attractive force ; 8 Argelander placed his central 
body in the constellation Perseus; 9 Fomalhaut, the brilliant of 
the Southern Fish, was set in the post of honour by Boguslawski 

1 Mtmoires de St. Pitersbourg, t. xxxv,, No. 3, 1887 ; revised in Astr. Notch., 
Nos. 3,729-30, 1901. 

2 AstronomicalJournal, Nos. 213, 501. 3 Astr. Nach., Nos. 2,999, 3,000. 
4 Veroffentlichungen der Grossh. Sternwarte zu Karlsruhe, Bd. iv., 1892. 

6 Proceedings Amsterdam Acad, of Sciences, Jan. 27, 1900. 

6 Astr. Jour., No. 457. 7 Ibid., Nos. 276, 497. 

8 Phil. Trans., vol. xcvi., p. 230. 

Mem. Pres. a VAc. de St. Petersburg, t. iii., p. 603 (read Feb. 5, 1837). 



chap, ii SIDEREAL ASTRONOMY 41 

of Breslau. Madler (who succeeded Struve at Dorpat in 1839) 
concluded from a more formal inquiry that the ruling power 
in the sidereal system resided, not in any single preponderating 
mass, but in the centre of gravity of the self-controlled revolving 
multitude. 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 Madler showed that no part of the 
heavens could be indicated as a region of exceptionally swift 
movements, such as would result from the presence of a gigantic 
(though possibly obscure) ruling body j but that a community 
of extremely sluggish movements undoubtedly existed in and near 
the group of the Pleiades, where, accordingly, he placed the centre 
of gravity of the Milky Way. 3 The bright star Alcyone thus 
became the "central sun," but in a purely passive sense, its head- 
ship being determined by its situation at the point of neutralisation 
of opposing tendencies, and of consequent rest. By an avowedly 
conjectural method, the solar period of revolution round this point 
was fixed at 18,200,000 years. 

The scheme of sidereal government framed by the Dorpat 
astronomer was, it may be observed, of the most approved 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 fundamental plan 
upon which the Galaxy is organised, recent investigations tend more 
and more to exhibit it, not as monarchical (so to speak), but as 
federative. The community of proper motions detected by Madler 
in the vicinity of the Pleiades may accordingly possess a significance 
altogether different from what he imagined. 

Bessel's so-called " foundation of an Astronomy of the Invisible " 
now claims attention. 4 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 measure- 
ments with the new Eepsold circle, he announced in 1844 his 
conclusion that these irregularities were due to the presence of 

1 Die Ceniralsonne, 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, Outlines of Astronomy, p. 631, 
10th ed.) placed beyond the range of reasonable probability by its remoteness 
(fully 26) from the galactic plane. 

4 Madler in Wcstennann's Jahrbuch, 1867, p. 615. 



42 HISTORY OF ASTRONOMY part i 

obscure bodies round which the two bright Dog-stars revolved as 
they pursued their way across the sphere. 1 He even assigned to 
each an approximate period of half a century. "I adhere to the 
conviction," he wrote later to Humboldt, " that Procyon and Sirius 
form real binary systems, consisting of a visible and an invisible 
star. There is no reason to suppose luminosity an essential quality 
of cosmical bodies. The visibility of countless stars is no argument 
against the invisibility of countless others." 2 

An inference so contradictory to received ideas obtained little 
credit, until Peters found, in 1851, 3 that the apparent anomalies in 
the movements of Sirius could be completely explained by an orbital 
revolution in a period of fifty years. Bessel's prevision was destined 
to be still more triumphantly vindicated. On the 31st of January, 
1862, while in the act of trying a new 18-inch refractor, Mr. Alvan 
G. Clark (one of the celebrated firm of American opticians) actually 
discovered the hypothetical Sirian companion in the precise position 
required by theory. It has now been watched through nearly an 
entire revolution (period 49*4 years), and proves to be very slightly 
luminous in proportion to its mass. Its attractive power, in fact, 
is nearly half that of its primary, while it emits only TT7 J^^th of its 
light. Sirius itself, on the other hand, possesses a far higher radiative 
intensity than our sun. It gravitates admitting Sir David Gill's 
parallax of 0'38" to be exact like two suns, but shines like twenty. 
Possibly it is much distended by heat, and undoubtedly its atmo- 
sphere intercepts a very much smaller proportion of its light than in 
stars of the solar class. As regards Procyon, visual verification was 
awaited until November 13, 1896, when Professor Schaeberle, with 
the great Lick refractor, detected the long-sought object in the 
guise of a thirteenth-magnitude star. Dr. See's calculations 4 
showed it to possess one-fifth the mass of its primary, or rather 
more than half that of our sun. 5 Yet it gives barely Y<jJiRT tn f ^ ie 
sun's light, so that it is still nearer to total obscurity than the dusky 
satellite of Sirius. The period of forty years assigned to the system 
by Auwers in 1862 6 appears to be singularly exact. 

But Bessel was not destined to witness the recognition of 
" the invisible " as a legitimate and profitable field for astronomical 
research. He died March 17, 1846, just six months before the dis- 
covery of Neptune, of an obscure disease, eventually found to be 
occasioned by an extensive fungus-growth in the stomach. The 

1 Letter from Bessel to Sir J. Herschel, Month. Not., vol. vi., p. 139. 

2 Wolf, Gesch. d. Astr., p. 743, note. 3 Astr. Nadu, Nos. 745-748. 

4 Astr. Jour., Iso. 440. 

5 Adopting Elkin's revised parallax for Procyon of 0'325". 

6 Astr. Nach., Nos. 1371-1373. 



chap, ii SIDEREAL ASTRONOMY 43 

place which he left vacant was not one easy to fill. His life's work 
might be truly described as "epoch-making." Karely indeed shall 
we find one who reconciled with the same success the claims of 
theoretical and practical astronomy, or surveyed the science which 
he had made his own with a glance equally comprehensive, practical, 
and profound. 

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

It appears from Struve's researches that about one in forty of 
all stars down to the ninth magnitude is composite, but that the 
proportion is doubled in the brighter orders. 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, 
1 Uebcr die Doppelsterne, Bericht, 1827, p. 22. 



44 HISTORY OF ASTRONOMY part i 

because beyond that distance the chances of merely optical juxta- 
position become considerable; but the immense preponderance oj 
extremely close over (as it were) loosely yoked bodies is such as to 
demonstrate their physical connection, even if no other proof were 
forthcoming. Many stars previously believed to be single divided 
under the scrutiny of the Dorpat refractor ; while in some cases, one 
member of a supposed binary system revealed itself as double, 
thus placing the surprised observer in the unexpected presence of 
a triple group of suns. Five instances were noted of two pairs 
lying so close together as to induce a conviction of their mutual 
dependence; 1 besides which, 124 examples occurred of triple, quad- 
ruple, and multiple combinations, the reality of which was open to 
no reasonable doubt. 2 

It was first pointed out by Bessel that the fact of stars exhibit- 
ing a common proper motion might serve as an unfailing test 
of their real association into systems. This was, accordingly, one 
of the chief criteria employed by Struve to distinguish true 
binaries from merely optical couples. On this ground alone, 61 
Cygni was admitted to be a genuine double star ; and it was 
shown that, although its components appeared to follow almost 
strictly rectilinear paths, yet the probability of their forming a 
connected pair is actually greater than that of the sun rising 
to-morrow morning. 3 Moreover, this tie of an identical movement 
was discovered to unite bodies 4 far beyond the range of distance 
ordinarily separating the members of binary systems, and to 
prevail so extensively as to lead to the conclusion that single do not 
outnumber conjoined stars more than twice or thrice. 5 

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

1 Ueber die Doppelsterne, Bericht, 1827, p. 25. 2 Mensural Micr., p. xcix. 

3 Stellarum Fixarum imprimis Duplicium et Multiplicum Positiones Medics, 
pp. cxc, cciii. 

4 For instance, the southern stars, 36a Ophiuchi (itself double) and 30 
Scorpii, which are 12' 10" apart. Ibid., p. cciii. 

5 Stellarum Fixarum, etc., p. ccliii. 



chap, ii SIDEREAL ASTRONOMY 45 

illness compelled his virtual resignation in favour of his son Otto 
Struve, born at Dorpat in 1819. He died November 23, 1864. 

An inquiry into the laws of stellar distribution, undertaken 
during the early years of his residence at Pulkowa, led Struve to 
confirm in the main the inferences arrived at by Herschel as to the 
construction of the heavens. According to his view, the appearance 
known as the Milky Way is produced by a collection of irregularly 
condensed star-clusters, within which the sun is somewhat eccen- 
trically placed. The nebulous ring which thus integrates the light 
of countless worlds was supposed by him to be made up of stars 
scattered over a bent or " broken plane," or to lie in two planes 
slightly inclined to each other, our system occupying a position near 
their intersection. 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, Cheseaux 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 comprehen- 
sive type, Sir William Herschel had but one legitimate successor, 
and that successor was his son. John Frederick William Herschel 
was born at Slough, March 17, 1792, graduated with the highest 
honours from St. John's College, Cambridge, in 1813, and entered 
upon legal studies with a view to being called to the Bar. But his 
share in an early compact with Peacock and Babbage, " to do their 
best to leave the world wiser than they found it," was not thus 
to be fulfilled. The acquaintance of Dr. Wollaston decided his 
scientific vocation. Already, in 1816, we find him reviewing some 
of his father's double stars; and he completed in 1820 the 18-inch 
speculum which was to be the chief instrument of his investigations. 
Soon afterwards, he undertook, in conjunction with Mr. (later 
Sir James) South, a series of observations, issuing in the presenta- 
tion to the Royal Society of a paper 4 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 (77 Coronae), had com- 
pleted more than one entire circuit since its first discovery j another, 
t Ophiuchi, had closed up into apparent singleness ; while the motion 
of a third, f Ursas Ma j oris, in an obviously eccentric orbit, was so 

1 Etudes <V Astronomic Stellaire, 1847, p. 82. 2 Ibid., p. 86. 

3 See Encke's criticism in Astr. Nach., No. 622. 

4 Phil. Trans., vol. cxiv., part iii., 1824. 



46 HISTORY OF ASTRONOMY part i 

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, 1 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 1832 a graphical process by 
which " the aid of the eye and hand " was brought in "to guide the 
judgment in a case where judgment only, and not calculation, could 
be of any avail." 2 Improved methods of the same kind were 
published by Dr. See in 1893, 3 and by Mr. Burnham in 1894 ; 4 and 
our acquaintance with stellar orbits is steadily gaining precision, 
certainty, and extent. 

In 1825 Herschel undertook, and executed with great assiduity 
during the ensuing eight years, a general survey of the northern 
heavens, directed chiefly towards the verification of his father's 
nebular discoveries. The outcome was a catalogue of 2,306 nebulae 
and clusters, of which 525 were observed for the first time, besides 
3,347 double stars discovered almost incidentally. 5 " Strongly 
invited," as he tells us himself, "by the peculiar interest of the 
subject, and the wonderful nature of the objects which presented 
themselves," he resolved to attempt the completion of the survey in 
the southern hemisphere. With this noble object in view, he 
embarked his family and instruments on board the Mount Stewart 
Elphinstone, and, after a prosperous voyage, landed at Cape Town 
on the 16th of January, 1834. Choosing as the scene of his 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 know 
ledge 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 6 embodying them was 

1 Conn. d. Temps, 1830. 2 R. A. S. Mem., vol. v., p. 178, 1833. 

3 Astr. and Astrophysics, vol. xii., p. 581. 4 Popular Astr., vol. i., p. 213 

5 Phil. Trans., vol. cxxiii., and Results, etc., Introd. 

6 Results of Astronomical Observations made during the years 1834-8 at tht 
Cape of Good Hope. 



chap, ii SIDEREAL ASTRONOMY 47 

brought out at the expense of the Duke of Northumberland. They 
form a sequel to his father's labours such as the investigations of 
one man have rarely received from those of another. What the 
elder observer did for the northern heavens, the younger did for 
the southern, and with generally concordant results. Reviving 
the paternal method of " star-gauging," he showed, from a count of 
2,299 fields, that the Milky "Way surrounds the solar system as a 
complete annulus of minute stars ; not, however, quite symmetrically, 
since the sun was thought to lie somewhat nearer to those portions 
visible in the southern hemisphere, which display a brighter lustre 
and a more complicated structure than the northern branches. 
The singular cosmical agglomerations known as the "Magellanic 
Clouds " were now, for the first time, submitted to a detailed, though 
admittedly incomplete, examination, the almost inconceivable rich- 
ness and variety of their contents being such that a lifetime might 
with great profit be devoted to their study. In the Greater 
Nubecula, within a compass of forty-two square degrees, Herschel 
reckoned 278 distinct nebulae and clusters, besides fifty or sixty 
outliers, and a large number of stars intermixed with diffused 
nebulosity in all, 919 catalogued objects, and, for the Lesser 
Cloud, 244. Yet this was only the most conspicuous part of what 
his twenty-foot revealed. Such an extraordinary concentration of 
bodies so various led him to the inevitable conclusion that "the 
Nubecula are to be regarded as systems sui generis, and which have 
no analogues in our hemisphere." 1 He noted also the blankness of 
surrounding space, especially in the case of Nubecula Minor, " the 
access to which on all sides," he remarked, " is through a desert ;" 
as if the cosmical material in the neighbourhood had been swept up 
and garnered in these mighty groups. 2 

Of southern double stars, he discovered and gave careful measure- 
ments of 2,102, and described 1,708 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 " with 
his great reflectors a crowd of these objects into stars. His former 
somewhat hesitating belief in the existence of phosphorescent matter, 
" disseminated through extensive regions of space in the manner of 
a cloud or fog," 3 was changed into a conviction that no valid dis- 
tinction 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 

1 Results, etc., p. 147. 2 See Proctor's Universe of Stars, p. 92. 

3 A Treatise on Astronomy, 1833, p. 406. 



4 8 HISTORY OF ASTRONOMY part i 

range of possible variety in the size and mode of aggregation of the 
stellar constituents of various nebulae. Some might appear nebulous 
from the closeness of their parts ; some from their smallness. Others, 
he suggested, might be formed of " discrete luminous bodies floating 
in a non-luminous medium;" 1 while the annular 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. 2,093 in his northern catalogue as "a network or tracery of 
nebula following 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 " nebulae led him to infer their dependent condition j 
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 nebulae," in which, as he pointed 
out, " all the varieties of double stars as to distance, position, and 
relative brightness, have their counterparts." He, moreover, 
investigated the subject of nebular distribution by the simple and 
effectual method of graphic delineation or " charting," and succeeded 
in showing that while a much greater uniformity of scattering 
prevails in the southern than in the northern heavens, a condensa- 
tion is nevertheless perceptible about the constellations Pisces and 
Cetus, roughly corresponding to the "nebular region " in Virgo by 
its vicinity (within 20 or 30) to the opposite pole of the Milky 
Way. He concluded "that the nebulous system is distinct from the 
sidereal, though involving, and perhaps to a certain extent inter- 
mixed with, the latter." 4 

Towards the close of his residence at Feldhausen, Herschel was 
fortunate enough to witness one of those singular changes in the 
aspect of the firmament which occasionally challenge the attention 
even of the incurious, and excite the deepest wonder of the 
philosophical observer. Immersed apparently in the Argo nebula 
is a star denominated 77 Carinae. 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 

1 Results, etc., p. 139. 2 Ibid., pp. 24, 142. 

3 Phil. Trails., vol. cxxiii., p. 503. 4 Results, etc., p. 136. 



chap, ii SIDEREAL ASTRONOMY 49 

because he had frequently, when in Africa during the years 1811 to 
1815, noted it as of only fourth magnitude. This observation, 
however, did not become generally known until later. Herschel, 
on his arrival at Feldhausen, registered the star as a bright second, 
and had no suspicion of its unusual character until December 16, 
1837, when he suddenly perceived its light to be almost tripled. It 
then far outshone Eigel in Orion, and on the 2nd of January 
following it very nearly matched a Centauri. From that date it 
declined; but a second and even brighter maximum occurred in 
April, 1843, when Maclear, then director of the Cape Observatory, 
saw it blaze out with a splendour approaching that of Sirius. Its 
waxings and wanings were marked by curious " trepidations " of 
brightness extremely perplexing to theory. In 1863 it had sunk 
below the fifth magnitude, and in 1869 was barely visible to the 
naked eye ; yet it was not until eighteen years later that it touched 
a minimum of 7*6 magnitude. Soon afterwards a recovery of 
brightness set in, but was not carried very far ; and the star now 
shines steadily as of the seventh magnitude, its reddish light con- 
trasting effectively with the silvery rays of the surrounding nebula. 
An attempt to include its fluctuations within a cycle of seventy 
years 1 has signally failed ; the extent and character of the vicissi- 
tudes to which it is subject stamping it rather as a species of con- 
necting link between periodic and temporary stars. 2 

Among the numerous topics which engaged Herschel's attention 
at the Cape was that of relative stellar brightness. Having con- 
trived an "astrometer" in which an "artificial star," formed by the 
total reflection of moonlight from the base of a prism, served as a 
standard of comparison, he was able to estimate the lustre of the 
natural stars examined by the distances at which the artificial object 
appeared equal respectively to each. He thus constructed a table 
of 191 of the principal stars, 3 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 1 : 801,072 4 (Zollner made the ratio 1 : 618,000), 
it became possible to compare stellar with solar radiance. Hence 
was derived, in the case of the few stars at ascertained distances, 
a knowledge of real lustre. Alpha Centauri, for example, emits less 

1 Loomis, Month. Not., vol. xxix., p. 298. 

2 See the Author's System of the Stars, pp. 116-120. 

3 Outlines of A sir., App. I. 

4 Phil. Trans., vol. cxix., p. 27. 



5 o HISTORY OF ASTRONOMY part i 

than twice, Capella one hundred times as much light as our sun ; 
while Arcturus, at its enormous distance, must display the splendour 
of 1,300 such luminaries. 

Herschel returned to England in the spring of 1838, bringing 
with him a wealth of observation and discovery such as had 
perhaps never before been amassed in so short a time. Deserved 
honours awaited him. He was created a baronet on the occasion 
of the Queen's coronation (he had been knighted in 1831) ; 
universities and learned societies vied with each other in showering 
distinctions upon him; and the success of an enterprise in which 
scientific zeal was tinctured with an attractive flavour of adventurous 
romance, was justly regarded as a matter of national pride. His 
career as an observing astronomer was now virtually closed, and 
he devoted his leisure to the collection and arrangement of the 
abundant trophies of his father's and his own activity. The result- 
ing great catalogue of 5,079 nebulae (including all then certainly 
known), published in the Philosophical Transactions for 1864, is, and will 
probably long remain, the fundamental source of information on the 
subject -} but he unfortunately did not live to finish the companion 
work on double stars, for which he had accumulated a vast store of 
materials. 2 He died at Collingwood in Kent, May 11, 1871, in the 
eightieth year of his age, and was buried in Westminster Abbey, 
close beside the grave of Sir Isaac Newton. 

The consideration of Sir John Herschel's Cape observations 
brings us to the close of the period we are just now engaged in 
studying. They were given to the world, as already stated, three 
years before the middle of the century, and accurately represent the 
condition of sidereal science at that date. Looking back over the 
fifty years traversed, we can see at a glance how great was the stride 
made in the interval. Not alone was acquaintance with individual 
members of the cosmos vastly extended, but their mutual relations, 
the laws governing their movements, their distances from the 
earth, masses, and intrinsic lustre, had begun to be successfully 
investigated. Begun to be ; for only regarding a scarcely perceptible 
minority had even approximate conclusions been arrived at. Never- 
theless the whole progress of the future lay in that beginning ; it 
was the thin end of the wedge of exact knowledge. The principle 

1 Dr. Dreyer's New General Catalogue, published in 1888 as vol. xlix. of the 
Royal Astronomical Society's Memoirs, is an enlargement of Herschel's work. It 
includes 7,840 entries, and was supplemented, in 1895, by an "Index Catalogue" 
of 1,529 nebulae discovered 1888 to 1894. Mem. R. A. 8., vol. li. 

2 A list of 10,320 composite stars was drawn out by him in order of right 
ascension, and has been published in vol. xl. of Mem. R. A. S. ; but the data 
requisite for their formation into a catalogue were not forthcoming. See Main's 
and Pritchard's Pre/ace to above, and Dunkin's Obihcary Notices, p. 73. 



chap, ii SIDEREAL ASTRONOMY 51 

of measurement had been substituted for that of probability ; a basis 
had been found large and strong enough to enable calculation to 
ascend from it to the sidereal heavens ; and refinements had been 
introduced, fruitful in performance, but still more in promise. 
Thus, rather the kind than the amount of information collected was 
significant for the time to come rather the methods employed than 
the results actually secured rendered the first half of the nineteenth 
century of epochal importance in the history of our knowledge of 
the stars. 



4-2 



CHAPTER HI 

PROGRESS OF KNOWLEDGE REGARDING THE SUN 

The discovery of sun-spots in 1610 by Fabricius and Galileo 
first opened a way for inquiry into the solar constitution; but it 
was long before that way was followed with system or profit. 
The seeming irregularity of the phenomena discouraged continuous 
attention ; casual observations were made the basis of arbitrary con- 
jectures, 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 
justly celebrated Father Scheiner of Ingolstadt, and later by 
William Gascoigne, the inventor of the micrometer ; but most of 
those who were capable of thinking at all on such subjects (and they 
were but few) adhered either to the cloud theory or to the slag theory 
of sun-spots. The first was championed by Galileo, the second 
by Simon Marius, "astronomer and physician" to the brother 
Margraves of Brandenburg. The latter opinion received a further 
notable development from the fact that in 1618, a year remarkable 
for the appearance of three bright comets, the sun was almost free 
from spots ; whence it was inferred that the cindery refuse from the 
great solar conflagration, which usually appeared as dark blotches 
on its surface, was occasionally thrown off in the form of comets, 
leaving the sun, like a snuffed taper, to blaze with renewed 
brilliancy. 2 

1 Kosmos, Bd. iii. , p. 409 ; Lalande, Bibliographie Astronomique, pp. 179, 202. 

2 R. Wolf, Die Sonne und ihre Flecken, p. 9. Marius himself, however, seems 
to have held the Aristotelian terrestrial-exhalation theory of cometary origin. 



chap, in KNOWLEDGE OF THE SUN 53 

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 umbrse, and at last to 
faculse." 1 

The view, confidently upheld by Lalande, 2 that spots were rocky 
elevations uncovered by the casual ebbing of a luminous ocean, the 
surrounding penumbrse representing shoals or sandbanks, had even 
less to recommend it than Derham's volcanic theory. Both were, 
however, significant of a growing tendency to bring solar phenomena 
within the compass of terrestrial analogies. 

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

But in November, 1769, a spot of extraordinary size engaged the 
attention of Alexander Wilson, professor of astronomy in the 
University of Glasgow. He watched it day by day, and to good 
purpose. As the great globe slowly revolved, carrying the spot 
towards its western edge, he was struck with the gradual con- 
traction 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 i and resumed its 
original completeness as it returned to a central position. In other 
spots subsequently examined by him, similar perspective effects were 
visible, and he proved in I774, 3 by strict geometrical reasoning, that 
they could only arise in vast photospheric excavations. It was not, 



See his curious little tract, Astronomische und Astrologische Beschreibung de 
Cometen, Niirnberg, 1619. 

1 Phil. Trans., vol. xxvii., p. 274. Umbrae, (now called penumbrcc) are spaces 
of half-shadow which usually encircle spots. Faculce ("little torches," so named 
by Schemer) are bright streaks or patches closely associated with spots. 

2 Mtm. Ac. Sc, 1776 (pub. 1779), p. 507. D. Cassini, however, first put 
forward about 1671 the hypothesis alluded to in the text. See Delambre, Hist. 
de VAstr. Mod., t. ii., p. 694 ; and Kosmos, Bd. iii., p. 410. 

3 Phil. Trans., vol. lxiv., part i., pp. 7-11. 



54 HISTORY OF ASTRONOMY part i 

indeed, the first time that such a view had been suggested. Father 
Scheiner's later observations plainly foreshadowed it ; 1 a conjecture 
to the same effect was emitted by Leonard Rost of Nuremburg early 
in the eighteenth century; 2 both by Lahire in 1703 and by 
J. Cassini in 1719 spots had been seen as notches on the solar limb ; 
while in 1770 Pastor Schulen of Essingen, from the careful study of 
phenomena similar to those noted by Wilson, concluded their de- 
pressed nature. 8 Modern observations, nevertheless, prove those 
phenomena to be by no means universally present. 

Wilson's general theory of the sun was avowedly tentative. It 
took the modest form of an interrogatory. " Is it not reasonable to 
think," he asks, " that the great and stupendous body of the sun is 
made up of two kinds of matter, very different in their qualities ; 
that by far the greater part is solid and dark, and that this immense 
and dark globe is encompassed with a thin covering of that re- 
splendent substance from which the sun would seem to derive the 
whole of his vivifying heat and energy V 4 He further suggests that 
the excavations or spots maybe 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 con- 
stitution which held its ground until the physics of the sun were 
revolutionised by the spectroscope. 

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

" This way of considering the sun and its atmosphere," he says, 5 
"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 

1 Rosa Ursina, lib. iv., p. 507. 2 R. "Wolf, Die Sonne und ihre Flecken, p. 12. 

3 Schellen, Die Spectralanalyse, Bd. ii., p. 56 (3rd ed.). 

4 Phil. Trans., vol. lxiv., p. 20. 5 Ibid., vol. lxxxv., 1795, p. 63. 



chap, in KNOWLEDGE OF THE SUN 55 

atmosphere, and its diversified surface, the rotation upon its axis, 
and the fall of heavy bodies, leads us on to suppose that it is most 
probably also inhabited, like the rest of the planets, by beings 
whose organs are adapted to the peculiar circumstances of that 
vast globe." 

We smile at conclusions which our present knowledge condemns 
as extravagant and impossible, but such incidental flights of fancy 
in no way derogate from the high value of Herschel's contributions 
to solar science. The cloud-like character which he attributed to 
the radiant shell of the sun (first named by Schroter the "photo- 
sphere ") is borne out by all recent investigations j he observed its 
mottled or corrugated aspect, resembling, as he described it, the 
roughness on the rind of an orange ; snowed that " faculge " 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," 1 
abundantly visible as dark points on the solar disc. But should an 
uncommon quantity be formed, "it will," he maintained, "burst 
through the planetary 2 regions of clouds, and thus will produce 
great openings ; then, spreading itself above them, it will occasion 
large shallows (penumbra?), 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 radia- 
tions are maintained by some process of circulation within the solar 
mass, was reached by Herschel through prolonged study of the 
phenomena in question. The novel and important idea contained in 
it, however, it was at that time premature to attempt to develop. 
But though many of the subtler suggestions of Herschel's genius 
passed unnoticed by his contemporaries, the main result of his solar 
researches was an unmistakable one. It was nothing less than the 
definitive introduction into astronomy of the paradoxical concep- 
tion of the central fire and hearth of our system as a cold, dark, 
terrestrial mass, wrapt in a mantle of innocuous radiance an earth, 
so to speak, within a sun without. 

Let us pause for a moment to consider the value of this remark- 
able innovation. It certainly was not a step in the direction of 

1 Phil. Tram., vol. xci., 1801, p. 303. 

2 The supposed opaque or protective stratum beneath the photosphere was 
named by him "planetary," from the analogy of terrestrial clouds. 

3 Ibid., p. 305. 



56 HISTORY OF ASTRONOMY part i 

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 there- 
fore done in vain. None of Bacon's aphorisms show a clearer 
insight into the relations between the human mind and the external 
world than that which declares " Truth to emerge sooner from error 
than from confusion." 1 A definite theory (even if a false one) gives 
holding-ground to thought. Facts acquire a meaning with reference 
to it. It affords a motive for accumulating them and a means of 
co-ordinating them ; it provides a framework for their arrangement, 
and a receptacle for their preservation, until they become too strong 
and numerous to be any longer included within arbitrary limits, and 
shatter the vessel originally framed to contain them. 

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

We have called the " terrestrial " theory of the sun's nature an 
innovation, and so, as far as its general acceptance is concerned, it 
may justly be termed ; but, like all successful innovations, it was a 
long time brewing. It is extremely curious to find that Herschel 
had a predecessor in its advocacy who never looked through a 
telescope (nor, indeed, imagined the possibility of such an instru- 
ment), who knew nothing of sun-spots, was still (mistaken assertions 

1 Nomim Organum, lib. ii., aph. 20. 



chap, in KNOWLEDGE OF THE SUN 57 

to the contrary notwithstanding) in the bondage of the geocentric 
system, and regarded nature from the lofty standpoint of an idealist 
philosophy. This was the learned and enlightened Cardinal Cusa, a 
fisherman's son from the banks of the Moselle, whose distinguished 
career in the Church and in literature extended over a considerable 
part of the fifteenth century (1401-64). In his singular treatise De 
DocM 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 circumferential envelope of light and heat, and between the 
two an atmosphere of water and clouds and translucent air." The 
luminary of Herschel's fancy could scarcely be more clearly portrayed ; 
some added words, however, betray the origin of the Cardinal's idea. 
11 The earth also," he says, " would appear as a shining star to any 
one outside the fiery element." It was, in fact, an extension to the 
sun of the ancient elemental doctrine ; but an extension remarkable 
at that period, as premonitory of the tendency, so powerfully 
developed by subsequent discoveries, to assimilate the orbs of heaven 
to the model of our insignificant planet, and to extend the brother- 
hood of our system and our species to the farthest limit of the 
visible or imaginable universe. 

In later times we find Flamsteed communicating to Newton, 
March 7, 1681, his opinion "that the substance of the sun is 
terrestrial matter, his light but the liquid menstruum encompassing 
him." 1 Bode in 1776 arrived independently at the conclusion that 
"the sun is neither burning nor glowing, but in its essence a dark 
planetary body, composed like our earth of land and water, varied 
by mountains and valleys, and enveloped in a vaporous atmosphere " f 
and the learned in general applauded and acquiesced. The view, 
however, was in 1787 still so far from popular, that the holding of 
it was alleged as a proof of insanity in Dr. Elliot when accused of a 
murderous assault on Miss Boydell. His friend Dr. Simmons stated 
on his behalf that he had received from him in the preceding January 
a letter giving evidence of a deranged mind, wherein he asserted 
"that the sun is not a body of fire, as hath been hitherto supposed, 
but that its light proceeds from a dense and universal aurora, which 
may afford ample light to the inhabitants of the surface beneath, 
and yet be at such a distance aloft as not to annoy them. No 
objection, he saith, ariseth to that great luminary's being inhabited ; 
vegetation may obtain there as well as with us. There may be 
water and dry land, hills and dales, rain and fair weather ; and as 

1 Brewster's Life of Newton, vol. ii., p. 103. 

2 Beschaftigungen d. Berl. Ges. Naturforschender Freunde, Bd. ii. , p. 233. 



58 HISTORY OF ASTRONOMY part i 

the light, so the season must be eternal, consequently it may easily 
be conceived to be by far the most blissful habitation of the whole 
system !" The Recorder, nevertheless, objected that if an extrava- 
gant hypothesis were to be adduced as proof of insanity, the same 
might hold good with regard to some other speculators, and desired 
Dr. Simmons to tell the court what he thought of the theories of 
Burnet and Buffon. 1 

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

Sir John Herschel included among the results of his multifarious 
labours at the Cape of Good Hope a careful study of the sun-spots 
conspicuously visible towards the end of the year 1836 and in the 
early part of 1837. They were remarkable, he tells us, for their 
forms and arrangement, as well as for their number and size ; one 
group, measured on the 29 th of March in the latter year, covering 
(apart from what may be called its outlying dependencies) the vast 
area of five square minutes or 3,780 million square miles. 2 We have 
at present to consider, however, not so much these observations in 
themselves, as the chain of theoretical suggestions by which they 
were connected. The distribution of spots, it was pointed out, on 
two zones parallel to the equator, showed plainly their intimate 
connection with the solar rotation, and indicated as their cause fluid 
circulations analogous to those producing the terrestrial trade and 
anti-trade winds. 

" The spots, in this view of the subject," he went on to say, 3 
"would come to be assimilated to those regions on the earth's 
surface where, for the moment, hurricanes and tornadoes prevail; 
the upper stratum being temporarily carried downwards, displacing 
by its impetus the two strata of luminous matter beneath, the upper 
of course to a greater extent than the lower, and thus wholly or 
partially denuding the opaque surface of the sun below. Such 
processes cannot be unaccompanied by vorticose motions, which, left 
to themselves, die away by degrees and dissipate, with the peculiarity 
that their lower portions come to rest more speedily than their upper, 

1 Gentleman's Magazine, 1787, vol. ii., p. 636. 

2 Results, etc., p. 432. 3 Ibid., p. 434. 



chap, in KNOWLEDGE OF THE SUN 59 

by reason of the greater resistance below, as well as the remoteness 
from the point of action, which lies in a higher region, so that their 
centres (as seen in our waterspouts, which are nothing but small 
tornadoes) appear to retreat upwards. Now this agrees perfectly 
with what is observed during the obliteration of the solar spots, 
which appear as if filled in by the collapse of their sides, the 
penumbra closing in upon the spot and disappearing after it." 

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

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

The "philosopher of Newbury" was by profession a London 
stockbroker, and a highly successful one. Nevertheless, his services 
to science were numerous and invaluable, though not of the brilliant 
kind which attract popular notice. Born at Newbury in Berkshire, 
April 28, 1774, and placed in the City at the age of fourteen, he 
derived from the acquaintance of Dr. Priestley a love of science 



6o HISTORY OF ASTRONOMY part i 

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 applica- 
tion to affairs a high reputation for integrity and ability, to which 
corresponded an ample fortune. In the meantime the Astronomical 
Society (largely through his co-operation) had been founded j he had 
for three years acted as its secretary, and he now felt entitled to 
devote himself exclusively to a subject which had long occupied his 
leisure hours. He accordingly in 1825 retired from business, 
purchased a house in Tavistock Place, and fitted up there a small 
observatory. He was, however, by preference a computator rather 
than an observer. What Sir John Herschel calls the " 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 Eoyal. But he was no less 
active in meeting present needs than in revising past performances. 
The subject of the reduction of observations, then, as we have already 
explained, 1 in a state of deplorable confusion, attracted his most 
earnest attention, and he was close on the track of Bessel when made 
acquainted with the method of simplification devised at Konigsberg. 
Anticipated as an inventor, he could still be of eminent use as a 
promoter of these valuable improvements ; and, carrying them out 
on a large scale in the star-catalogue of the Astronomical Society 
(published in 1827), "he put" (in the words of Herschel) "the 
astronomical world in possession of a power which may be said, 
without exaggeration, to have changed the face of sidereal 
astronomy." 2 

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

1 See ante, p. 31. 

3 Memoir of Francis Baily, Mem. E. A. S., vol. xv., p. 524. 



chap, in KNOWLEDGE OF THE SUN 61 

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

It is now time to consider his share in the promotion of solar 
research. Eclipses of the sun, both ancient and modern, were 
a speciality with him, and he was fortunate in those which came 
under his observation. Such phenomena are of three kinds partial, 
annular, and total. In a partial eclipse, the moon, instead of passing 
directly between us and the sun, slips by, as it were, a little on one 
side, thus cutting off from our sight only a portion of his surface. 
An annular eclipse, on the other hand, takes place when the moon 
is indeed centrally interposed, but falls short of the apparent size 
required for the entire concealment of the solar disc, which conse- 
quently 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 15th of May, 1836, an annular eclipse was visible in 
the northern parts of Great Britain, and was observed by Baily at 
Inch Bonney, near Jedburgh. It was here that he saw the pheno- 
menon which obtained the name of " Baily 's Beads," from the 
notoriety conferred upon it by his vivid description. 

" When the cusps of the sun," he writes, " were about 40 asunder, 
a row of lucid points, like a string of bright beads, irregular in size 
and distance from each other, suddenly formed round that part of the 
circumference of the moon that was about to enter on the sun's disc. 
Its formation, indeed, was so rapid that it presented the appearance 
of having been caused by the ignition of a fine train of gunpowder. 



62 HISTORY OF ASTRONOMY part i 

Finally, as the moon pursued her course, the dark intervening spaces 
(which, at their origin, had the appearance of lunar mountains in 
high relief, and which still continued attached to the sun's border), 
were stretched out into long, black, thick, parallel lines, joining the 
limbs of the sun and moon ; when all at once they suddenly gave 
way, and left the circumference of the sun and moon in those points, 
as in the rest, comparatively smooth and circular, and the moon 
perceptibly advanced on the face of the sun.' 1 

These curious appearances were not an absolute novelty. Weber 
in 1791, and Von Zach in 1820, had seen the "beads"; Van 
Swinden had described the " belts " or " threads." 2 These last were, 
moreover (as Baily clearly perceived), completely analogous to the 
" black ligament " which formed so troublesome a feature in the 
transits of Venus in 1764 and 1769, and which, to the regret and 
confusion, though no longer to the surprise of observers, was renewed 
in that of 1874. The phenomenon is largely an effect of what is 
called irradiation, by which a bright object seems to encroach upon 
a dark one ; but under good atmospheric and instrumental conditions 
it becomes inconspicuous. The " Beads " must always appear when 
the projected lunar edge is serrated with mountains. In Baily 's 
observation, they were exaggerated and distorted by an irradiative 
clinging together of the limbs of sun and moon. 

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

Baily (to whose narrative we again have recourse) had set up his 
Dollond's achromatic in an upper room of the University of Pavia, 
and was eagerly engaged in noting a partial repetition of the singular 
appearances seen by him in 1836, when he was "astounded by a 
tremendous burst of applause from the streets below, and at the 
same moment was electrified at the sight of one of the most brilliant 
and splendid phenomena that can well be imagined. For at that 
instant the dark body of the moon was suddenly surrounded with 
a corona, or kind of bright glory similar in shape and relative 
magnitude to that which painters draw round the heads of saints, 
1 Mem. R. A. S., vol. x., pp. 5-6. 2 Ibid., pp. 14-17. 



chap, in KNOWLEDGE OF THE SUN 63 

and which by the French is designated an aure'ole. 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 j and 
when the total obscuration took place, which was instantaneous, there 
was a universal shout from every observer, which ' made the welkin 
ring,' and, for the moment, withdrew my attention from the object 
with which I was immediately occupied. I had indeed anticipated 
the appearance of a luminous circle round the moon during the time 
of total obscurity ; but I did not expect, from any of the accounts 
of preceding eclipses that I had read, to witness so magnificent an 
exhibition as that which took place. . . . The breadth of the 
corona, measured from the circumference of the moon, appeared 
to me to be nearly equal to half the moon's diameter. It had 
the appearance of brilliant rays. The light was most dense close 
to the border of the moon, and became gradually and uniformly 
more attenuate as its distance therefrom increased, assuming the 
form of diverging rays in a rectilinear line, which at the ex- 
tremity were more divided, and of an unequal length ; so that in 
no part of the corona could I discover the regular and well-defined 
shape of a ring at its outer margin. It appeared to me to have the 
sun for its centre, but I had no means of taking any accurate 
measures for determining this point. Its colour was quite white, 
not pearl-colour, nor yellow, nor red, and the rays had a vivid and 
flickering appearance, somewhat like that which a gaslight illumina- 
tion 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 pheno- 
menon 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 pro- 
digious 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 moun- 
tains when coloured by the rising or setting sun. They resembled 
the Alpine mountains also in another respect, inasmuch as their 



64 HISTORY OF ASTRONOMY part i 

light was perfectly steady, and had none of that flickering or spark- 
ling 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 j but they 
differed from each other in magnitude. . . . The whole of these 
three protuberances were visible even to the last moment of total 
obscuration ; at least, I never lost sight of them when looking in 
that direction ; and when the first ray of light was admitted from 
the sun, they vanished, with the corona, altogether, and daylight 
was instantaneously restored." 1 

Notwithstanding unfavourable weather, the " red flames " were 
perceived with little less clearness and no less amazement from the 
Superga than at Pavia, and were even discerned by Mr. Airy with 
the naked eye. " Their form " (the Astronomer Royal wrote) " was 
nearly that of saw-teeth in the position proper for a circular saw 
turned round in the same direction in which the hands of a watch 
turn. . . . Their colour was a full lake-red, and their brilliancy 
greater than that of any other part of the ring." 2 

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. 3 

The corona, both as to figure and extent, presented very different 
appearances at different stations. This was no doubt due to varieties 
in atmospheric conditions. At the Superga, for instance, all details 
of structure seem to have been effaced by the murky air, only a 
comparatively feeble ring of light being seen to encircle the moon. 
Elsewhere, a brilliant radiated formation was conspicuous, spreading 
at four opposite points into four vast luminous expansions, compared 
to feather-plumes or aigrettes.* Arago at Perpignan noticed con- 
siderable irregularities in the divergent rays. Some appeared curved 
and twisted, a few lay across the others, in a direction almost tan- 
gential to the moon's limb, the general effect being described as that 
of a " hank of thread in disorder." 5 At Lipeszk, where the sun 
stood much higher above the horizon than in Italy or France, the 
corona showed with surprising splendour. Its apparent extent was 
judged by Struve to be no less than twenty-five minutes (more than 

1 Mem. R. A. S., vol. xv., pp. 4-6. 2 Ibid., p. 16. 

3 Annuaire, 1846, p. 409. * Ibid., p. 317. 5 Ibid., p. 322. 



chap, in KNO WLEDGE OF THE SUN 65- 

six times Airy's estimate), while the great plumes spread their 
radiance to three or four degrees from the dark lunar edge. So* 
dazzling was the light that many well-instructed persons denied 
the totality of the eclipse. Nor was the error without precedent, 
although the appearances attending respectively a total and an 
annular eclipse are in reality wholly dissimilar. In the latter case, 
the surviving ring of sunlight becomes so much enlarged by irradia- 
tion, that the interposed dark lunar body is reduced to comparative 
insignificance, or even invisibility. Maclaurin tells us x that during 
an eclipse of this character which he observed at Edinburgh in I737 r 
" gentlemen by no means shortsighted declared themselves unable to 
discern the moon upon the sun without the aid of a smoked glass j "' 
and Baily (who, however, was shortsighted) could distinguish, in 
1836, with the naked eye, no trace of "the globe of purple velvet" 
which the telescope revealed as projected upon the face of the sun. 2 
Moreover, the diminution of light is described by him as "little 
more than might be caused by a temporary cloud passing over the 
sun " j the birds continued in full song, and " one cock in particular 
was crowing with all his might while the annulus was forming." 

Very different were the effects of the eclipse of 1842, as to which 
some interesting particulars were collected by Arago. 3 Beasts of 
burthen, he tells us, paused in their labour, and could by no amount 
of punishment be induced to move until the sun reappeared. Birds 
and beasts abandoned their food ; linnets were found dead in their 
cages; even ants suspended their toil. Diligence-horses, on the 
other hand, seemed as insensible to the phenomenon as locomotives. 
The convolvulus and some other plants closed their leaves, but those 
of the mimosa remained open. The little light that remained was 
of a livid hue. One observer described the general coloration as 
resembling the lees of wine, but human faces showed pale olive or 
greenish. We may, then, rest assured that none of the remarkable 
obscurations recorded in history were due to eclipses of the annular 
kind. 

The existence of the corona is no modern discovery. Indeed, it is 
too conspicuous an apparition to escape notice from the least atten- 
tive or least practised observer of a total eclipse. Nevertheless, 
explicit references to it are rare in early times. Plutarch, however, 
speaks of a" certain splendour " compassing round the hidden edge 
of the sun, as a regular feature of total eclipses ; 4 and the corona is 

1 Phil. Trans., vol. xl., p. 192. 2 Mem. R. A. S., vol. x., p. 17. 

3 Ann. du Bureau des Long., 1846, p. 309. 

4 Be Facie in Orbe Lunce, xix., 10. Cf. Grant, Astr. Nach., No. 1838. As 
to the phenomenon mentioned by Philostratus in his Life of Apollonius (viii. 23) ; 
see W. T. Lynn, Observatory, vol. ix., p. 128. 

5 



66 HISTORY OF ASTRONOMY part i 

expressly mentioned in a description of an eclipse visible at Corfu in 
968 a.d. 1 The first to take the phenomenon into scientific con- 
sideration was Kepler. He showed, from the orbital positions 
at the time of the sun and moon, that an eclipse observed by 
Clavius at Rome in 1567 could not have been annular, 2 as the 
dazzling coronal radiance visible during the obscuration had caused 
it to be believed. Although he himself never witnessed a total 
eclipse of the sun, he carefully collected and compared the remarks 
of those more fortunate, and concluded that the ring of " flame-like 
splendour " seen on such occasions was caused by the reflection of 
the solar rays from matter condensed in the neighbourhood either of 
the sun or moon. 3 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." 4 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; 5 but it failed to receive the 
attention which, as a step in the right direction, it undoubtedly 
merited. Nine years later we meet with Halley's comments on a 
similar event, the first which had occurred in London since March 20, 
1140. By nine in the morning of May 3, 1715, the obscuration, he 
tells us, " was about ten digits, 6 when the face and colour of the sky 
began to change from perfect serene azure blue to a more dusky 
livid colour, having an eye of purple intermixt. ... A few seconds 
before the sun was all hid, there discovered itself round the moon a 
luminous ring, about a digit or perhaps a tenth part of the moon's 
diameter in breadth. It was of a pale whiteness, or rather pearl 
colour, seeming to me a little tinged with the colours of the iris, and 
to be concentric with the moon, whence I concluded it the moon's 
atmosphere. But the great height thereof, far exceeding our earth's 
atmosphere, and the observation of some, who found the breadth 
of the ring to increase on the west side of the moon as emersion 

1 Schmidt, Astr. Nach., No. 1832. 

2 Astronomice Pars Optica, Op. omnia, t. ii., p. 317. 

3 Be Stella Nova, Op., t. ii., pp. 696, 697. 4 Astr. Pars Op., p. 320. 

5 Mem. de VAc. des Sciences, 1706, p. 119. 

6 A digit = -jV of the solar diameter. 



chap, in KNOWLEDGE OF THE SUN 67 

approached, together with the contrary sentiments of those whose 
judgment I shall always revere " (Newton is most probably referred 
to), " makes me less confident, especially in a matter whereto I 
confess I gave not all the attention requisite." He concludes by 
declining to decide whether the " enlightened atmosphere," which 
the appearance "in all respects resembled," "belonged to sun or 
moon." 1 

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, 2 and his authority carried great weight. It was, how- 
ever, much discredited by an observation made by Maraldi in 1724, 
to the effect that the luminous ring, instead o f travelling with the 
moon, was traversed by it. 3 This was in reality decisive, though, as 
usual, belief lagged far behind demonstration. In 1715 a novel ex- 
planation had been offered by Delisle and Lahire, 4 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 appre- 
ciable at our distance from the moon. 5 Don Jose Joaquim de 
Ferrer, however, who observed a total eclipse of the sun at Kinder- 
hook, in the State of New York, on June 16, 1806, ignoring this 
refined optical rationale, considered two alternative explanations of 
the phenomenon as alone 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." 6 But he 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 atten- 
tion hitherto bestowed upon it, but the most earnest study of those 

1 Phil. Trans., vol. xxix., pp. 247-249. 

1 Mem. de V Ac. des Sciences, 1715 ; Histoire, p. 49 ; Memoires, pp. 93-98. 

3 Ibid., 1724, p. 178. 4 Mem. de VAc. des Sciences, 1715, pp. 161, 166-169. 

5 Ed. Ency., art. Astronomy, p. 635. 

8 Trans. Am. Phil. Soc, vol. vi., p. 274. 



68 HISTORY OF ASTRONOMY part i 

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 appen- 
dage. 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 persist- 
ently misconstrued. Nor was it until 1869 that 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 of 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 appari- 
tion 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, by which the sun is completely covered, and from which the 
" prominences " are emanations, eruptive or flame-like. Now, con- 
tinual indications of the presence of this fire-ocean had been detected 
during eclipses in the eighteenth and nineteenth centuries. Captain 
Star ny an, describing in a letter to Flamsteed an occurrence of the 
kind witnessed by him at Berne on May 1 (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 precisely similar appearance was 
noted by both Halley and De Louville in 1715 ; during annular 
eclipses by Lord Aberdour in 1737, 3 and by Short in 1748, 4 the tint 
of the ruby border being, however, subdued to " brown " or " dusky 
red " by the surviving sunlight ; while observations identical in 
character were made at Amsterdam in 1820, 5 at Edinburgh by 
Henderson 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 professor named Vassenius, 
who observed a total eclipse at Gothenburg, May 2 (o.s.), 1733. 7 

1 Memoir of Caroline Herschel, p. 327. 

2 Phil. Travis., 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. 

7 Phil. Trans., vol. xxxviii., p. 134. Father Secchi, however, adverted to a 
distinct mention of a prominence observed in 1239 a.d. A description of a total 
eclipse of that date includes the remark, ' ' Et quoddam foramen erat ignitum in 
circulo solis ex parte inferiore " (Muratori, Per. It. Scriptores, t. xiv., col. 1097.) 
The " circulus solis " of course signifies the corona. 



chap, in KNOWLEDGE OF THE SUN 69 

His astonishment equalled his admiration when he perceived, just 
outside the edge of the lunar disc, and suspended, as it seemed, in 
the coronal atmosphere, 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. The observation was repeated 
in 1778 by a Spanish Admiral, but with no better success in direct- 
ing 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. 1 
He was satisfied that it belonged to the sun because of its fiery 
colour and growth in magnitude, and supposed that it was occa- 
sioned 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), 2 might be detailed ; but the evidence 
already adduced suffices to show that the prominences viewed with 
such amazement in 1842 were no unprecedented or even unusual 
phenomenon. 

It was more important, however, to decide what was their nature 
than whether their appearance might have been anticipated. They 
were generally, and not very incorrectly, set down as solar clouds. 
Arago believed them to shine by reflected light, 3 but the Abbe 
Peytal rightly considered them to be self-luminous. Writing in a 
Montpellier paper of July 16, 1842, he declared that we had now 
become assured of the existence of a third or outer solar envelope, com- 
posed of a glowing substance of a bright rose tint, forming mountains 
of prodigious elevation, analogous in character to the clouds piled 
above our horizons. 4 This first distinct recognition of a very 
important feature of our great luminary was probably founded on 
an observation made by Berard at Toulon during the then recent 
eclipse, "of a very fine red band, irregularly dentelated, or, as it 
were, crevassed here and there," 5 encircling a large arc of the moon's 
circumference. It can hardly, however, be said to have attracted 
general notice until July 28, 1851. On that day a total eclipse 

1 Phil. Trans., vol. lxix., p. 114. 

2 Tram. Am. Phil. Soc, vol. vi., 1809, p. 267. 3 Annuaire, 1846, p. 460. 
4 Ibid., p. 439, note. 5 Ibid., p. 416. 



70 HISTORY OF ASTRONOMY part i 

took place, which was observed with considerable success 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," x described by Dawes as " a low ridge of red 
prominences, ir resembling in outline the tops of a very irregular range 
of hills." 2 Airy termed the portion of this " rugged line of pro- 
jections " visible to him the sierra, and was struck with its brilliant 
light and " nearly scarlet " colour. 3 Its true character of a con- 
tinuous solar envelope was inferred from these data by Grant, Swan, 
and Littrow, and was by Father Secchi, after the great eclipse of 
I860, 4 formally accepted as established. 

Several; prominences of remarkable forms, especially one variously 
compared to a Turkish scimitar, a sickle, and a boomerang, were 
seen in 1851. In connection with them two highly significant 
circumstances were pointed out. First, that of the approximate 
coincidence between their positions and those of sun-spots previously 
observed. 5 Next, that " the moon passed over them, leaving them 
behind, and revealing successive portions as she advanced." 6 This 
latter perfectly well-attested fact 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; 7 Feilitsch of Greifswald published in 1852 a 
treatise for the express purpose of proving all the luminous pheno- 
mena attendant on solar eclipses corona, prominences and " sierra " 
to be purely optical appearances. 8 Happily, however, the un- 
answerable 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 fortified by fresh testimony into unexpected and over- 
whelming preponderance. 

1 Mem. R. A. S., vol. xxi., p. 82. 2 Ibid., p. 90. 3 Ibid., pp. 7, 8. 

4 Le Soleil, t. i., p. 386. 

5 By Williams and Stanistreet, Mem. R. A. S., vol. xxi., pp. 54, 56. Santini 
had made a similar observation at Padua in 1842. Grant, Hist. Astr., p. 401. 

6 Lassell in Month. Not., vol. xii., p. 53. 

7 Comptes Rendus, t. xxxiv., p. 155. 

8 Optische Untersuchungen, and Zeitsehrift fur populate Mittheilungen, Bd. i., 
1860, p. 201. 



CHAPTER IV 

PLANETARY DISCOVERIES 

In the course of his early gropings towards a law of the planetary- 
distances, Kepler tried the experiment of setting a planet, invisible 
by reason of its smallness, to revolve in the vast region of seemingly 
desert space separating Mars from Jupiter. 1 The disproportionate 
magnitude of the same interval was explained by Kant as due to the 
overweening size of Jupiter. The zone in which each planet moved 
was, according to the philosopher of Konigsberg, to be regarded as 
the empty storehouse from which its materials had been derived. A 
definite relation should thus exist between the planetary masses and 
the planetary intervals. 2 Lambert, on the other hand, sportively 
suggested that the body or bodies (for it is noticeable that he speaks 
of them in the plural) which once bridged this portentous gap in the 
solar system, might, in some remote age, have been swept away by 
a great comet, and forced to attend its wanderings through space. 3 

These speculations were destined before long to assume a more 
definite form. Johann Daniel Titius, a professor at Wittenberg 
(where he died in 1796), pointed out in 1772, in a note to a trans- 
lation 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 of numbers, increasing in 
regular progression, 5 he showed that the distances of the six known 
planets from the sun might be represented wifch a close approach to 
accuracy. But with one striking interruption. The term of the 

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 Cosmologische Brief e, No. 1 (quoted by Von Zach, Monat. Corr., vol. iii., 
p. 592). 

4 Second ed., p. 7. See Bode, Von clem neuen Hauptplanetcn, p. 43, note. 

5 The representative numbers are obtained by adding 4 to the following series 
(irregular, it will be observed, in its first member, which should be instead 
of 0) ; 0, 3, 6, 12, 24, 48, etc. The formula is a purely empirical one, and is, 
moreover, completely at fault as regards the distance of Neptune. 



72 HISTORY OF ASTRONOMY part i 

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 a hypo- 
thetical 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 Von Zach was 
actually at the pains, in 1785, to calculate what he termed 
"analogical" elements 1 for this unseen and (by any effect or in- 
fluence) 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 German 
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 accord- 
ingly divided for purposes of scrutiny into twenty-four zones ; their 
apportionment to separate observers was in part effected, and the 
association was rapidly getting into working order, when news arrived 
that the missing planet had been found, through no systematic plan 
of search, but by the diligent, though otherwise directed labours of 
a distant watcher of the skies. 

Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 
1746. He studied at various places and times under 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 founda- 
tion for which was afforded by one of the towers of the viceregal 
palace. This architecturally incongruous addition to an ancient 
Saracenic edifice once the abode of Kelbite and Zirite Emirs was 
completed in February, 1791. Piazzi, meanwhile, had devoted 
nearly three years to the assiduous study of his new profession, 
acquiring a practical knowledge of Lalande's methods at the ficole 
Militaire, and of Maskelyne's at the Royal Observatory; and 
returned to Palermo in 1789, bringing with him, in the great five- 
foot circle which he had prevailed upon Ramsden to construct, 

1 Monat. Corr., vol. iii., p. 596. 



chap, iv PLANETARY DISCOVERIES 73 

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 1 of astronomical detectives was being held in reserve for him, 
when, on the first evening of the nineteenth century, January 1, 
1801, he noted the position of an eighth-magnitude star in a part 
of the constellation Taurus to which an error of Wollaston's had 
directed his special attention. Keobserving, 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, exchanged 
retrograde for direct motion on January 14, 2 and was carefully 
watched by Piazzi until February 11, when a dangerous illness 
interrupted his observations. He had, however, not omitted to give 
notice of his discovery ; but so precarious were communications in 
those unpeaceful times, that his letter to Oriani of January 23 did 
not reach Milan until April 5, while a missive of one day later 
addressed to Bode came to hand at Berlin, March 20. The delay 
just afforded time for the publication, by a young philosopher of 
Jena named Hegel, of a " Dissertation " showing, by the clearest 
light of reason, that the number of the planets could not exceed 
seven, and exposing the folly of certain devotees of induction who 
sought a new celestial body merely to fill a gap in a numerical series. 3 

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 sensa- 
tion, 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 rediscover 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 observations afforded; 4 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 serving only to 

1 Wolf, Geschichte der Astrcmomie, p. 648. 

2 Such reversals of direction in the apparent movements of the planets are a 
consequence of the earth's revolution in its orbit. 

3 Dissertatio Philosophica de Orbitis Planctarum, 1801. See Wolf, Gesch. 
d. Astr., p. 685. 

4 Observations on Uranus, as a supposed fixed star, went back to 1690. 



74 HISTORY OF ASTRONOMY part i 

mislead as to the places where, from September, 1801, 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 tuition at 
Brunswick, with many possibilities, but no settled career before him. 
The news from Palermo may be said to have converted him from an 
arithmetician into an astronomer. He was already in possession of 
a new and more general method of computing elliptical orbits ; 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 November of his elements and 
ephemeris for the lost object revived the drooping hopes of the little 
band of eager searchers. Their patience, however, was to be still 
further tried. Clouds, mist, and sleet seemed to have 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 Virgo, nearly in the position 
assigned by Gauss to the runaway planet, a strange star was dis- 
cerned by Von Zach 1 at Gotha, and on a 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 conse- 
quence 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 inconspicuous, but just then at its maximum of bright- 
ness ; 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) revolved 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 compromised 
by the admission of many, where room could, according to old- 
fashioned rules, only be found for one. A daring hypothesis of 

1 He had caught a glimpse of it on December 7, but was prevented by bad 
weather from verifying his suspicion. Monat. Corr., vol. v., p. 171. 



chap, iv PLANETARY DISCOVERIES 75 

Olbers's 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 two bodies now known respectively as Juno and Vesta. 
The first was found near the predicted spot in Cetus by Harding, 
Seliroter's assistant at Lilienthal, September 2, 1804 ; the second by 
Olbers himself in Virgo, after three years of persistent 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 
sun, nearly the same parts of Cetus and Virgo ; while the eccen- 
tricities and inclinations of their paths departed widely from the 
planetary type that of Pallas, to take an extreme instance, 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. 3 But these values are now known to be con- 
siderably too small. 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 

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. 

3 Ibid., p. 218. In a letter to Von Zach of June 24, 1802, he speaks of Pallas 
as "almost incredibly small," and makes it only seventy English miles in 
diameter. Monat. Corr., vol. vi., pp. 89, 90. 



76 HISTORY OF ASTRONOMY part i 

speak), was added to the confirmatory evidence. 1 The strong point 
of the theory, however, lay not in what it explained, but in what it 
had predicted. It had been twice confirmed by actual exploration 
of the skies, and had produced, in the recognition of Vesta, the 
first recorded instance of the premeditated discovery of a heavenly 
body. 

The view not only commended itself to the facile imagination of 
the unlearned, but received the sanction of the highest scientific 
authority. The great Lagrange bestowed upon it his analytical 
itnpimatur, showing that the explosive forces required to produce 
the supposed catastrophe came well within the bounds of possibility ; 
since a velocity of less than twenty times that of a cannon-ball leaving 
the gun's mouth would have sufficed, according to his calculation, to 
launch the asteroidal fragments on their respective paths. Indeed, 
he was disposed to regard the hypothesis of disruption as more 
generally available than its author had designed it to be, and 
proposed to supplement with it, as explanatory of the eccentric 
orbits of comets, the nebular theory of Laplace, thereby obtaining, 
as he said, " a complete view of the origin of the planetary system 
more conformable to Nature and mechanical laws than any yet 
proposed." 2 

Nevertheless the hypothesis of Olbers has not held its ground. It 
seemed as if all the evidence available for its support had been pro- 
duced at once and spontaneously, while the unfavourable items were 
elicited slowly, and, as it were, by cross-examination. A more 
extended acquaintance with the group of bodies whose peculiarities 
it was framed to explain has shown them, after all, as recalcitrant 
to any such explanation. Coincidences at the first view significant 
and striking have been swamped by contrary examples ; and a hasty 
general conclusion has, by a not uncommon destiny, at last perished 
under the accumulation of particulars. Moreover, as has been 
remarked by Professor Newcomb, 3 mutual perturbations would 
rapidly efface all traces of a common disruptive origin, and the 
catastrophe, to be perceptible in its effects, should have been com- 
paratively 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 1 830, 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 

1 Olbers, Monat. Corr., vol. vi., p. 88. 2 Conn. d. Terns for 1814, p. 218. 

3 Popular Astronomy, p. 327. 



chap, iv PLANETARY DISCOVERIES 77 

found by him, December 8, 1845, received the name of Astraea, and 
his further prosecution of the search resulted, July 1, 1847, in the 
discovery of Hebe. A few weeks later (August 13), John Eussell 
Hind (1823-1893), after many months' exploration from Mr. Bishop's 
observatory in the Regent's Park, picked up Iris, and October 18, 
Flora. 1 The next on the list was Metis, found by Mr. Graham, 
April 25, 1848, at Markree, in Ireland. 2 At the close of the period 
to which our attention is at present limited, the number of these 
small bodies known to astronomy was thirteen ; and the course of 
discovery has since proceeded far 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-established planetary order; the 
profusion of resource, so conspicuous 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. Both theoretical and practical 
astronomy 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 J$ 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 TUemia 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 dignified 
disguise of a distant sun. Harding'? "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 

1 Month. Not., vol. vii., p. 299 ; vol. viii., p. 1. 2 Ibid., p. 146. 



78 HISTORY OF ASTRONOMY parti 

upon a score of years previously. Still more important were the 
great star-maps of the Berlin Academy, undertaken at Bessel's 
suggestion, with the same object of distinguishing errant from fixed 
stars, and 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," 1 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 have continued to 
haunt astronomical consciousness. Moreover, it was prepared by 
many, suggested as possible by not a few, and actually achieved, 
simultaneous^, independently, and completely, 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 inde- 
fatigable 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 divergencies regarded as menacing the very 
foundation of its theories never entered the range of unaided vision. 
In other words, if the theoretical and the real Uranus had been 
placed side by side in the sky, they would have seemed, to the 
sharpest eye, to form a single body. 2 

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 

1 Airy. Mem. R. A. S., vol. xvi., p. 386. 

2 See Newcomb's Pop. Astr., p. 359. The error of Uranus amounted, in 
1844, to 2' ; but even the tailor of Breslau, whose extraordinary powers of vision 
Humboldt commemorates (Kosmos, Bd. ii., p. 112), could only see Jupiter's first 
satellite at its greatest elongation, 2' 15". He might, however, possibly have 
distinguished two objects of equal lustre at a lesser interval. 



chap, iv PLANETARY DISCOVERIES 79 

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 difficulty, the 
triumphant exit from which fatal illness frustrated his hopes of 
effecting or even witnessing. 

The problem was practically untouched when, in 1841, an under- 
graduate 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 mathematician 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 on October 21, 

1845, was able to communicate to the Astronomer Royal numerical 
estimates of the elements and mass of the unknown planet, together 
with an indication of its actual place in the heavens. These results, 
it has been well said, 1 gave "the final and inexorable proof" of the 
validity of Newton's Law. The date October 21, 1845, "may there- 
fore be regarded as marking a distinct epoch in the history of 
gravitational astronomy." 

Sir George Biddell Airy had begun in 1835 his long and energetic 
administration of the Royal Observatory, and was already in posses- 
sion of data vitally important to the momentous inquiry then on 
foot. At his suggestion, and under his superintendence, the reduc- 
tion of all the planetary observations made at Greenwich from 1750 
onwards had been undertaken 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, investi- 
gators, both native and foreign, were freely supplied with the 
"places and errors," which, clearly exhibiting the discrepancies 
between observation and calculation between what was and what 
was expected formed the very groundwork of future improvements. 

Mr. Adams had no reason to complain of official discourtesy. His 
labours received due and indispensable aid ; but their purpose was 
regarded as chimerical. " I have always," Sir George Airy wrote,' 2 
" considered the correctness of a distant mathematical result to be a 
subject rather of moral than of mathematical evidence." And that 

1 J. W. L. Glaisher, Observatory ; vol. xv., p. 177. 

2 Mem. E. A. S., vol. xvi., p. 399. 



8o HISTORY OF ASTRONOMY part i 

actually before him seemed, from its very novelty, to incur a 
j suspicion of unlikelihood. No problem in planetary disturbance had 
heretofore been attacked, so to speak, from the rear. The inverse 
method was untried, and might well be deemed impracticable. For 
the difficulty of determining the perturbations 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. 

f Moreover, Adams unaccountably neglected to answer until too late 

i$ If a question regarded by Airy in the light of an experimentum crucis 

K 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 

Lobscurity. 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 (1 49') of the spot assigned to it by Adams. 

A competitor, however, equally daring and more fortunate 
audax fortund adjutm, as Gauss said of him was even then entering 
the field. Urbain Jean Joseph Leverrier, the son of a small Govern- 
ment employe' in Normandy, was born at Saint-L6, March 11, 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 mathe- 
matical 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 dutiful promptitude the 
summons of the astronomical chief of France. In his first memoir 
on the subject (communicated to the Academy, November 10, 1845), 
he proved the inadequacy of all known causes of disturbance to 
account for the vagaries of Uranus; in a second (June 1, 1848), 
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 Septem- 



chap, iv PLANETARY DISCOVERIES 81 

ber 10, Sir John Herschel declared to the British Association respect- 
ing the hypothetical new planet: "We see it as Columbus saw 
America from the coast of Spain. Its movements 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 fort- 
night later, September 23, Professor Galle, of the Berlin Observatory, 
received a letter from Leverrier requesting his aid in the telescopic 
part of the inquiry already analytically completed. He directed 
his refractor to the heavens that same night, and perceived, within 
less than a degree of the spot indicated, an object with a measur- 
able 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 direc- 
tion 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 dis- 
covery, which faithfully reflected the duplicate character 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 Adams in the previous October ; and on the 9th of July he 
wrote to Professor Challis, director of the Cambridge Observatory, 
recommending a search with the great Northumberland equatoreal. 
Had a good star-map been at hand, the process would have been 
a simple one j but of Bremiker's " Hora XXI." no news had yet 
reached England, and there was no other sufficiently comprehensive 
to be available for an inquiry which, in the absence of such aid, 
promised to be both long and laborious. As the event proved, it 
might have been neither. " After four days of observing," Challis 
wrote, October 12, 1846, to Airy, "the planet was in my grasp \t\ 
only I had examined or mapped the observations." 2 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, more- 
over, his faith in the precision of the indications furnished to him 
was, by his own confession, a very feeble one. For both reasons he 
postponed to a later stage of the proceedings the discussion and 
comparison of the data nightly furnished to him by his telescope, 
and thus allowed to lie, as it were, latent in his observations the 

1 For an account of D' Arrest's share in the detection see Copernicus, vol. ii., 
pp. 63, 96. 

2 Mem. R. A. 8., vol. xvi., p. 412. 

6 



82 HISTORY OF ASTRONOMY part i 

momentous result which his diligence had insured, but which his 
delay suffered to be anticipated. 1 

Nevertheless, it should not be forgotten that the Berlin astronomer 
had two circumstances in his favour apart from which his swift 
success could hardly have been achieved. The first was the posses- 
sion 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 23rd of September, 
but also Challis on the 29th, immediately after reading the French 
geometer's lucid and impressive treatise, picked out from among 
the stellar points strewing the zodiac, a small planetary disc, which 
eventually proved to be that of the precise body he had been in 
search of during two months. 

The controversy that ensued had its ignominious side ; but it was 
entered into by neither of the parties principally concerned. Adams 
bore the disappointment, which the dilatory proceedings at Green- 
wich and Cambridge had inflicted upon him, with quiet heroism. 
His silence on the subject of what another man would have called 
his wrongs remained unbroken to the end of his life f and he took 
every opportunity of testifying his admiration for the genius of 
Leverrier. 

Personal questions, however, vanish in the magnitude of the event 
they relate to. By it the last lingering doubts as to the absolute 
exactness of the Newtonian Law were dissipated. Eecondite 
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 our earth 
was shown to circulate, in perpetual frozen exile, at thirty times its 
distance from the sun. Nay, the possibility was made apparent 
that the limits of our system were not even thus reached, but that 
yet profounder abysses of space might shelter obedient, though little 
favoured, members of the solar family, by future astronomers to be 
recognised through the sympathetic thrillings of Neptune, even as 
Neptune himself was recognised through the tell-tale deviations of 
Uranus. 

It is curious to find that the fruit of Adams's and Leverrier's 

1 He had recorded the places of 3,150 stars (three of which were different 
positions of the planet), and was preparing to map them, when, October 1, 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. M. A. S., vol. xvi., p. 411. 

3 He died January 21, 1892, in his 71st year. 



chap, iv PLANETARY DISCOVERIES 83 

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 10th of May, 1795, Lalande noted the position 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 immortality which he would have been the 
last to despise hung in the balance ; the feather-weight of his care- 
lessness, 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 provided 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 pro- 
gression requires ; and Neptune's vast distance of 2,800 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 con- 
formity with the curious and unexplained rule which planetary 
discoveries have alternately 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 celebrated two- 
foot reflector 2 erected by Mr. Lassell at his suggestively 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 
four-fifths of the momentous period which we have undertaken to 
traverse. At the age of twenty-one, being without the means to 
purchase, he undertook to construct telescopes, and naturally 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 eventually 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 defini- 
tion, with an aperture of two, and a focal length of twenty feet, 

1 Ledger, The Sun, its Planets and their Satellites, p. 414. 

2 Presented by the Misses Lassell, after their father's death, to the Greenwich 
Observatory. 

62 



84 H1ST0R Y OF ASTRONOMY part i 

provided by a novel artifice with the equatoreal mounting, 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 -gj^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 system, are performed from east to west, in 
a plane inclined at an angle of 35 to that of the ecliptic. Their 
swiftness serves to measure the mass of the globe round which they 
are performed. For while our moon takes twenty-seven days and 
nearly eight hours to complete its circuit of the earth, the satellite 
of Neptune, at a distance not greatly inferior, sweeps round its 
primary in five days and twenty-one hours, showing (according to a 
very simple principle of computation) that it is urged by a force 
seventeen times greater than the terrestrial pull upon the lunar 
orb. Combining this result with those of Professor Barnard's 1 
and Dr. See's 2 recent measurements of the small telescopic disc of 
this farthest known planet, it is found that while in mass Neptune 
equals seventeen, in bulk it is equivalent to forty-nine earths. 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 is about twice that of water. 

We must now turn from this late-recognised member of our 
system to bestow some brief attention upon the still fruitful field 
of discovery offered by one of the immemorial five. The family of 
Saturn, unlike that of its brilliant neighbour, has been gradually 
introduced to the notice of astronomers. Titan, the sixth Saturnian 
moon in order of distance, led the way, being detected by Huygens, 
March 25, 1655; Cassini made the acquaintance of four more 
between 1671 and 1684; while Mimas and Enceladus, the two inner- 
most, were caught by Herschel in 1789, as they threaded their lucid 

1 Astr. Jour., No. 508. 2 Report of U. S. Naval Observatory for 1900, p. 15. 



chap, iv PLANETARY DISCOVERIES 85 

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 conspicuous interrup- 
tion, 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 19th 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 18th. 
On the following evening, both observers perceived that the 
problematical speck of light kept up with, instead of being left 
behind by the planet as it moved, and hence inferred its true 
character. 1 Hyperion, the seventh by distance and eighth by 
recognition of Saturn's attendant train, is of so insignificant a size 
when compared with some of its fellow-moons (Titan is but little 
inferior to the planet Mars), as to have suggested to Sir John 
Herschel 2 the idea that it might be only one of several bodies 
revolving very close together in fact, an asteroidal satellite ; but the 
conjecture has, so far, not been verified. 

The coincidence of its duplicate discovery was singularly paralleled 
two years later. Galileo's amazement when his " optic glass " re- 
vealed to him the " triple " form of Saturn planeta tergeminus has 
proved to be, like the laughter of the gods, " inextinguishable." It 
must revive in every one who contemplates anew the unique arrange- 
ments of that world apart known to us as the Saturnian system. 
The resolution of the so-called misce, or " handles," into one en- 
circling ring by Huygens in 1655, the discovery by Cassini in 1675 
of the division of that ring into two concentric ones, together with 
Laplace's investigation of the conditions of stability of such a forma- 
tion, constituted, with some minor observations, the sum of the 
knowledge obtained, up to the middle of the last 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 Cranch Bond, born in 1789 at 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 contemplated, he undertook a mission to England for the 
purpose of studying the working of similar institutions there, and on 
1 Grant, Hist, of Astr., p. 271. a Month. Not., vol. ix., p. 91. 



86 HISTORY OF ASTRONOMY part i 

his return erected a private observatory at Dorchester, where he 
worked diligently for many years. Then at last, in 1843, the long- 
postponed design of the Harvard authorities was resumed, and on 
the completion of the new establishment, Bond, who had been from 
1838 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 distinguished Transatlantic observers. 
Like the elder Struve, he left an heir to his office and to his 
eminence, but George Bond unfortunately died in 1865, at the early 
age of thirty-nine, having survived his father but six years. 

On the night of November 15, 1850 the air, remarkably enough, 
being so hazy that only the brightest stars could be perceived with the 
naked eye William Bond discerned a dusky ring, extending about 
halfway 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 
Wateringbury, 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 drawings executed by Campani in 
1664 3 and by Hooke in 1666 ; 4 while Picard (June 15, 1673), 5 Hadley 
(spring of 1720), 6 and Herschel, 7 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 

1 Month. Not., vol. xi., p. 21. 2 Astr. Nach., No. 756 (May 2, 1851). 

3 Phil. Trans., vol. i., p. 246. See H. T. Vivian, Engl. Mech., April 20, 1894. 

4 Secchi, Month. Not., vol. xiii., p. 248. 5 Hind, ibid., vol. xv., p. 32. 

6 Lynn, Observatory, Oct. 1, 1883 ; Hadley, Phil. Trans., vol. xxxii., p. 385. 

7 Proctor, Saturn and its System, p. 64. 



chap, iv PLANETARY DISCOVERIES 87 

meaning of this suspected change it was reserved for later investiga- 
tions 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 satisfactory 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 11, 1787, x two Uranian moons, since called 
Oberon and Titania, and ascertained 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 20-foot reflector, similar to that with which they had been 
originally discovered. Thenceforward they were kept fairly within 
view, but their four questionable companions, in spite of some false 
alarms of detection, remained in the dubious condition in which 
Herschel had left them. At last, on October 24, 1851, 2 after some 
years of fruitless watching, 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 perfec- 
tion. In all probability they were then for the first time seen ; for 
although Professor Holden 3 made out a plausible case in favour of 
the fitful visibility to Herschel of each of them in turn, LasseH's 
argument 4 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 impor- 
tant of the " negative results " 5 secured by LasselFs 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 quartette, not- 
withstanding the unquestioned place long assigned to them in 
astronomical text-books, to the Nirvana of non-existence. 

1 Phil. Trans., vol. lxxvii., p. 125. 2 Month. Not., vol. xi. f p. 248. 

3 Ibid., vol. xxxv., pp. 16-22. 4 Ibid., p. 26. 5 Ibid., vol. xli., p. 190. 



CHAPTER V 

COMETS 

Newton showed that the bodies known as "comets," or hirsute stars, 
obey the law of gravitation ; but it was by no means certain that the 
individual of the species observed by him in 1680 formed a permanent 
member of the solar system. The velocity, in fact, of its rush round 
the sun was quite possibly sufficient to carry it off for ever into the 
depths of space, there to wander, a celestial casual, from star to star. 
With another comet, however, which appeared two years later, the 
case was different. Edmund Halley, who afterwards succeeded 
Flamsteed as Astronomer Royal, calculated the elements of its orbit 
on Newton's principles, and found them to resemble so closely those 
similarly arrived at for comets observed by Peter Apian in 1531, 
and by Kepler in 1607, as almost to compel the inference that all 
three were apparitions of a single body. This implied its revolution 
in a period of about seventy-six years, and Halley accordingly fixed 
its return for 1758-9. So fully alive was he to the importance of 
the announcement that he appealed to a "candid posterity," in the 
event of its verification, to acknowledge that the discovery was due 
to an Englishman. The prediction was one of the test-questions 
put by Science to Nature, on the replies to which largely depend 
both the development of knowledge and the conviction of its reality. 
In the present instance, the answer afforded may be said to have 
laid the foundation of this branch of astronomy. Halley's comet 
punctually reappeared on Christmas Day, 1758, and effected its 
perihelion passage on the 12th of March following, thus proving 
beyond dispute that some at least of these erratic bodies are 
domesticated within our system, and strictly conform, if not to its 
unwritten customs (so to speak), at any rate to its fundamental 
laws. Their movements, in short, were demonstrated by the most 
unanswerable of all arguments that of verified calculation to be 
calculable, and their investigation was erected into a legitimate 
department of astronomical science. 



chap, v COMETS 89 

This notable advance was the chief result obtained in the field of 
inquiry just now under consideration during the eighteenth century. 
But before it closed, its cultivation had received a powerful stimulus 
through the invention of an improved method. The name of Olbers 
has already been brought prominently before our readers in connec- 
tion 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 heavens 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 thank- 
less 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 j moreover, all the best years of his life 
were absorbed in the assiduous exercise of a toilsome profession. 
Born in 1758 at the village of Arbergen, where his father was pastor, 
he settled in 1781 as a physician in the neighbouring town of Bremen, 
and continued in active practice there 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 pleasures. 
Yet his recreation was, as Von Zach remarked, 1 no less prolific of 
useful results than the severest work of other men. The upper part 
of his house in the Sandgasse was fitted up with such instruments 
and appliances as restrictions of space permitted, 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 bis 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 
Germany enjoyed the benefits of a frequently active correspondence 
with him, and his communications to the scientific periodicals of the 
1 Allgemeine Geographische Ephemeriden, yol. iv., p. 287. 



9 o 



HISTORY OF ASTRONOMY part i 



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 
conspired to render unique. 

Amongst the many younger men who were attracted and stimulated 
by intercourse with him was Johann Franz Encke. But while Olbers 
became a mathematician because he was an astronomer, Encke became 
an astronomer because he was a mathematician. A born geometer, 
he was naturally sent to Gottingen and placed under the tuition 
of Gauss. But geometers are men ; and the contagion of patriotic 
fervour which swept over Germany after the battle of Leipsic did 
not spare Gauss's promising pupil. He took up arms in the Hanseatic 
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, became his successor. Thence he 
was promoted in 1825 to Berlin, where he superintended the building 
of the new observatory, 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 discovered 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 M^chain in 1786, by Caroline Herschel in 1795, by Pons, 
Huth, and Bouvard in 1805, and after six laborious weeks of 
research into the disturbances 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 observa- 
tory at Paramatta was fortunately ready equipped for its recapture, 
which Riimker effected quite close to the spot indicated by Encke's 
ephemeris. 

The importance of this event can be better understood when it is 

1 Astr. Jahrbuch, 1823, p. 217. The period (1,208 days) of this body is 
considerably shorter than that of any other known comet. 



chap, v COMETS 91 

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 about thirty have been found to circulate 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 excursions north and south ; and their paths round the 
sun, although much more eccentric than the approximately circular 
planetary orbits, are far less so than the extravagantly long ellipses 
in which comets comparatively untrained (as it were) in the habits 
of the solar system ordinarily perform their revolutions. 

No great comet is of the " planetary " kind. These are, indeed, 
only by exception visible to the naked eye ; they possess extremely 
feeble tail-producing powers, and give small signs of central conden- 
sation. Thin wisps of cosmical cloud, they flit across the telescopic 
field of view without sensibly obscuring the smallest star. Their 
appearance, in short, suggests what some notable facts in their 
history will presently be shown to confirm that they are bodies 
already effete, and verging towards dissolution. If it be asked what 
possible connection can be shown to exist between the shortness of 
period by which they are essentially characterised, and what we 
may call their superannuated condition, we are not altogether at a 
loss for an answer. Kepler's remark, 1 that comets are consumed by 
their own emissions, has undoubtedly a measure of truth in it. The 
substance ejected into the tail must, in overwhelmingly large pro- 
portion, be for ever lost to the central mass from which it issues. 
True, it is of a nature inconceivably 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, subject to many adventures and vicissitudes. 
Their aphelia or the farthest points of their orbits from the sun 
are usually, if not invariably, 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 

1 "Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et 
denique mori." De Cometis, 0p. } vol. vii., p. 110. 



92 HISTORY OF ASTRONOMY part i 

all likelihood, originally fixed in their present tracks ; and by their 
influence, exerted in an opposite sense, they may, in some cases, be 
eventually ejected from them. Careers so varied, as can easily be 
imagined, are apt 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 pene- 
trates within the orbit of Mercury ; it considerably transcends at 
aphelion the farthest excursion of Pallas. Its vicinity to the former 
planet in August, 1835, offered the first convenient opportunity 
of placing that body in the astronomical balance. Its weight or 
mass had previously been assumed, not ascertained ; and the com- 
paratively slight deviation from its regular course impressed upon 
the comet by its attractive power showed that it had been assumed 
nearly twice too great. 1 That fundamental datum of planetary 
astronomy the mass of Jupiter was corrected by similar means ; 
and it was reassuring to find the correction in satisfactory accord 
with that already introduced from observations 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 conspicuous and un- 
deniable. 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 further from the 
sun than 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 ttJtttt^ P arfc ^ lts 
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, appeared to be reduced, as it drew near 
the hearth of our system, in the enormous proportion of 800,000 

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 2 ; 2 l 81 ir that of the sun (Laplace, Exposition du 
Syst. du Monde, t. ii., p. 50, ed. 1824). Von Asten deduced from the move- 
ments of Encke's comet, 1818-48, a value of T rjixTir 5 while Backlund from its 
seven returns, 1871-1891, derived i^-Hd-to {Comptes Hendus, Oct. 1, 1894). 

3 Arago, Annuaire (1832), p. 218. 3 Hind, The Comets, p. 20. 



chap, v COMETS 93 

to 1. A corresponding expansion accompanied on each occasion its 
retirement from the sphere of observation. Similar changes of 
volume, though rarely to the same astounding 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. 

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 besides 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 dis- 
closure 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 1 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 j for plainly all must in some 
degree suffer the same kind of hindrance as Encke's, if the 
cause of that hindrance be the one suggested. None of its 
congeners, however, show any trace of similar symptoms. True, 

1 Phil. Trans., vol. xlvi., p. 204. 



94 HISTOR Y OF ASTRONOMY part i 

the late Professor Oppolzer announced, 1 in 1880, that a comet, 
first seen by Pons in 1819, and rediscovered by Winnecke in 1858, 
having a period of 2,052 days (5-6 years), was accelerated at each 
revolution precisely in the manner required by Encke's theory. 
But M. von HaerdtFs subsequent investigation, the materials for 
which included numerous observations of the body in question at its 
return to the sun in 1886, decisively negatived the presence of any 
such effect. 2 Moreover, the researches of Von Asten and Backlund 8 
into the movements of Encke's comet revealed a perplexing circum- 
stance. They confirmed Encke's results for the period covered by 
them, but exhibited the acceleration as having suddenly diminished 
by nearly one-half in 1868. The reality and permanence of this 
change were fully established by observations of the ensuing 
return in March, 1885. Some physical alteration of the retarded 
body seems indicated ; but visual evidence countenances no such 
assumption. In aspect the comet is no less thin and diffuse than in 
1795 or in 1848. 

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. 4 
This cannot be a solar atmosphere, since it is mathematically certain, 
as Laplace has shown, 5 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 ; while physical evidence assures us that 
the actual depth of the solar atmosphere bears a very minute propor- 
tion to the possible depth theoretically assigned to it. That matter, 
however, not atmospheric in its nature that is, neither forming one 
body with the sun nor altogether aeriform exists in its neighbour- 
hood, can admit of no reasonable doubt. The great lens-shaped 
mass of the zodiacal light, stretching out at times far beyond the 
earth's orbit, may indeed be regarded as an extension of the corona, 
the streamers of which themselves mark the wide diffusion, all 
round the solar globe, of granular or gaseous materials. Yet comets 
have been known to penetrate the sphere occupied by them without 
perceptible loss of velocity. The hypothesis, then, of a resisting 
medium receives at present no countenance from the movements of 
comets, whether of short or of long periods. 

Although Encke's comet has made thirty-five complete rounds of 
its orbit since its first detection in 1786, it shows no certain signs 

1 Astr. Nacli., No. 2,134. 2 Comptes fiendus, t. cvii., p. 588. 

3 Mim. de St. Petersbourg, t. xxxii., No. 3, 1884 ; Astr. Nach., No. 2,727. 

4 Month. Not., vol. xix., p. 72. 5 Mecanique Cilestc, t. ii., p. 197. 



chap, v COMETS 95 

of decay. Variations in its brightness are, it is true, conspicuous, 
but they do not proceed continuously. 1 

The history of the next known planet-like 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 con- 
veying these strikingly similar inferences were printed side by side 
in the same number of the Astronomische Nachrichten (No. 94) j 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 1805, 
visible to the naked eye. Its aspect in Sir John Herschel's great 
reflector on the 23rd of September, 1832, was described by him as 
that of a " conspicuous nebula," nearly 3 minutes in diameter. No 
trace of a tail was discernible. While he was engaged in watching 
it, a small knot of minute stars was directly traversed by it, " and 
when on the cluster," he tells us, 2 it " presented the appearance of a 
nebula resolvable and partly resolved into stars, the stars of the 
cluster being visible through the comet." Yet the depth of cometary 
matter through which such faint stellar rays penetrated undimmed, 
was, near the central parts of the globe, not less than 50,000 miles. 

It is curious to find that this seemingly harmless, and we may 
perhaps add effete body, gave occasion to the first (and not the last) 
cometary " scare " of an enlightened century. Its orbit, at the 
descending node, may be said to have intersected that of the earth ; 
since, according as it bulged in or out under the disturbing influence 
of the planets, the passage of the comet was effected inside or outside 
the terrestrial track. Now, certain calculations published by Olbers 
in 1828 3 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 conse- 

1 See Berberich, Astr. Nach., Nos. 2,836-7, 3,125; Deichmuller, Ibid., 
No. 3,123. 

2 Month. Not., vol. ii., p. 117. 3 Astr. Nach., No. 128. 



96 HISTOR Y OF ASTRONOMY part i 

quences frigidly enumerated by Laplace. Nor did the panic sub- 
side until Arago formally demonstrated that the earth and the comet 
could by no possibility approach within less than fifty millions of 
miles. 1 

The return of the same body in 1845-46 was marked by an ex- 
traordinary circumstance. When first seen, November 28, it wore 
its usual aspect of a faint round patch of cosmical fog; but on 
December 19, Mr. Hind noticed that it had become distorted some- 
what into the form of a pear; and ten days later, it had divided 
into two separate objects. This singular duplication was first 
perceived at New Haven in America, December 29, 2 by Messrs. 
Herrick and Bradley, and by Lieutenant Maury at Washington, 
January 13, 1846. The earliest British observer of the phenomenon 
(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 15; then, distrustful of what his senses had told 
him, he called in his judgment to correct their improbable report by 
resolving one of the dubious objects into a hazy star. 3 On the 23rd, 
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 16th of April), 4 continued to be watched with 
equal curiosity and amazement by astronomers in every part of the 
northern hemisphere. What Seneca reproved Ephorus for suppos- 
ing 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 distance 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 show- 

1 Annuaire, 1832. p. 186. 

2 Am. Journ. of Science, vol. i. (2nd series), p. 293. Prof. Hubbard's calcula- 
tions indicated a probability that the definitive separation of the two nuclei 
occurred as early as September 30, 1884. Astronomical Journal (Gould's), 
vol. iv., p. 5. See also, on the subject of this comet, W. T. Lynn, Intellectual 
Observer, vol. xi., p. 208; E. Ledger, Observatory, August, 1883, p. 244 ; and 
H. A. Newton, Am. Journ. of Science, vol. xxxi., p. 81, February, 1886. 

3 Month. Not., vol. vii.., p. 73, 

4 Bulletin Ac. Imp. de St. Pttersbourg, t. vi., col. 77. The latest observation 
of the parent nucleus was that of Argelander, April 27, at Bonn. 






chap, v COMETS 97 

ing 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 direc- 
tion perpendicular to the line joining their centres, and each 
developed a bright nucleus, although the original comet had 
exhibited neither of these signs of cometary vitality. A singular 
interchange of brilliancy was, besides, observed to take place between 
the coupled objects, each of which alternately outshone and was out- 
shone 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 replaced 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, when Father Secchi at Kome perceived 
it to have increased its distance from the originating body to a 
million and a quarter of miles, or about eight times the average 
interval at the former appearance. Both vanished shortly after- 
wards, 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. A 
dangerously near approach to Jupiter in 1841 is believed to have 
occasioned their disruption, and the disaggregating process thus 
started was likely to continue. 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 coelos 
universos. 1 

Biela's is not the only vanished comet. Brorsen's, discovered at 
Kiel in 1846, and observed at four subsequent returns, failed un- 
accountably to become visible in 1890. 2 Yet numerous sentinels 
were on the alert to surprise its approach along a well-ascertained 
track, traversed in five and a half years. The object presented from 
the first a somewhat time-worn aspect. It was devoid of tail, or 
any other kind of appendage ; and the rapid loss of the light 
acquired during perihelion passage was accompanied by inordinate 
expansion of an already tenuous globular mass. Another lost or 
mislaid comet is one found by De Vico at Eome, August 22, 1844. 
It was expected to return early in 1 850, but did not, and has never 

1 D' Arrest, Astr. Nach., No. 1,624. 

3 Der Brorsen'sche Comet. Von Dr. E. Lamp, Kiel, 1892 ; Plimimer, Know- 
ledge, vol. xix., p. 41. 

7 



98 HISTOR Y OF ASTRONOMY part i 

since been seen; unless its re-appearance as E. Swift's comet of 1894 
should be ratified by closer inquiry. 1 

A telescopic comet with a period of 7J years, discovered Novem- 
ber 22, 1843, by M. Faye of the Paris Observatory, formed the sub- 
ject of a characteristically patient and profound inquiry on the part of 
Leverrier, designed to test its suggested identity with Lexell's comet 
of 1770. The result was decisive against the hypothesis of Valz, 
the divergences between the orbits of the two bodies being found to 
increase instead of to diminish, as the history of the new-comer was 
traced backward into the previous 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 was proved by the 
admirable researches of Professor Axel Moller, 3 director of the 
Swedish observatory of Lund, to exhibit no trace of the action of 
a resisting medium. 

Periodical comets are evidently bodies which have each lived 
through a chapter of accidents, and a significant hint as to the 
nature of their adventures can be gathered from the fact that their 
aphelia are pretty closely grouped about the tracks of the major 
planets. Halley's, and five other comets are thus related to 
Neptune ; three connect themselves with Uranus, two with Saturn, 
above a score with Jupiter. Some form of dependence is plainly 
indicated, and the researches of Tisserand, 4 Callandreau, 5 and 
Newton 6 of Yale College, leave scarcely a doubt that the "capture- 
theory " represents the essential truth in the matter. The original 
parabolic paths of these comets were then changed into ellipses 
by the backward pull of a planet, whose sphere of attraction they 
chanced to enter when approaching the sun from outer space. 
Moreover, since a body thus affected should necessarily return at 
each revolution to the scene of encounter, the same process of 
retardation may, in some cases, have been repeated many times, 
until the more restricted cometary orbits were reduced to their 
present dimensions. The prevalence, too, among periodical comets, 
of direct motion, is shown to be inevitable by M. Callandreau's 
demonstration that those travelling in a retrograde direction would, 
by planetary action, be thrown outside the probable range of 
terrestrial observation. The scarcity of hyperbolic comets can be 

1 Schulhof, Astr. Nach., No. 3,267; Observatory, vol. xviii., p. 64; F. H. 
Seares, Astr. Nach., Nos. 3,606-7 ; Plumnier, Knowledge, vol. xix., p. 156. 

2 Comptes Rendus, t. xxv., p. 570. 3 Month. Not., vol. xii., p. 248. 
* Bull. Astr., t. vi, pp. 241, 289. 

5 Etude sur la TMoriedes Cometes periodiques. Annalesde V ' Observatoire, t. xx., 
Paris, 1891. 

6 Amer. Journ. of Science, vol. xlii., pp. 183, 482, 1891. 



chap, v COMETS 99 

similarly explained. They would be created whenever the attrac- 
tive influence of the disturbing planet was exerted in a forward or 
accelerative sense, but could come only by a rare exception to our 
notice. The inner planets, including the earth, have also un- 
questionably played their parts in modifying cometary orbits ; ancl 
Mr. Plummer suggests, with some show of reason, that the capture 
of Encke's comet may be a feat due to Mercury. 1 

No great comet appeared between the " star " which presided at 
the birth of Napoleon and the "vintage" comet of 1811. The 
latter was first descried by Flaugergues at Viviers, March 26, 1811 ; 
Wisniewski, at Neu-Tscherkask in Southern Russia, caught a final 
glimpse of it, August 17, 1812. Two disappearances in the solar 
rays as the earth moved round in its orbit, and two reappearances 
after conjunction, were included in this unprecedentedly long period 
of visibility of 510 days. This relative permanence (so far as the 
inhabitants of Europe were concerned) was due to the high northern 
latitude attained near perihelion, combined with a certain leisure- 
liness 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 J degrees in length, corresponding to a real extension of 
one hundred millions of miles. Its form was described by Sir 
William Herschel as that of "an inverted hollow cone," and its 
colour as yellowish, strongly contrasting with the bluish-green tint 
of the " head," round which it was flung like a transparent veil. 
The planetary disc of the head, 127,000 miles across, appeared to 
be composed of strongly-condensed nebulous matter \ but somewhat 
eccentrically situated within it was a star-like nucleus of a reddish 
tinge, which 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 by 
Olbers, 3 the striking yet steadily reasoned-out suggestions contained 
in which there was at that time no means of following up with 
profit. Only of late has the " electrical theory," of which Zollner 4 
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. 

1 Observatory, vol. xiv., p. 194. 2 Phil. Trans., vol. cii., pp. 118-124. 

3 Ueber den Schweif des grossen Cometen von 1811, Monat. Corr., vol. xxv.. 
pp. 3-22. Reprinted by Zollner, Ueber die Natur der Cometen, pp. 3-15. 

4 Natur der Cometen, p. 148. 

72 



ioo HISTORY OF ASTRONOMY part 






The paraboloidal shape of the bright envelope separated by a dark 
interval from the head of the great comet of 1811, and constituting, 
as it were, the root of its tail, seemed to the astronomer of Bremen 
to reveal the presence of a double repulsion ; the expelled vapours 
accumulating where the two forces, solar and cometary, balanced 
each other, and being then swept backwards in a huge train. He 
accordingly distinguished 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 1807. 1 Thirdly, comets 
like that of 1811, giving evidence of action of both kinds. These 
are distinguished by a dark hoop encompassing the head and dividing 
it from the luminous envelope, as well as by an obscure caudal axis, 
resulting from the hollow, cone-like structure of the tail. 

Again, the ingenious view subsequently propounded by M. Bredikhin 
as to the connection between the form of these appendages and the 
kind of matter composing them, was very clearly anticipated by 
Olbers. The amount of tail-curvature, he pointed out, depends in 
each case upon the proportion borne by the velocity of the ascending 
particles to that of the comet in its orbit ; the swifter the outrush, 
the straighter the resulting tail. But the velocity of the ascending 
particles varies with the energy of their repulsion by the sun, and 
this again, it may be presumed, with their quality. Thus multiple 
tails are developed when the same comet throws off, as it approaches 
perihelion, specifically distinct substances. The long, straight ray 
which proceeded from the comet of 1807, for example, was doubtless 
made up of particles subject to a much more vigorous solar repulsion 
than those formed into the shorter curved emanation issuing from 
it nearly in the same direction. In the comet of 1811 he calculated 
that the particles expelled from the head travelled to the remote 
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 much more powerful than the opposing 
one of gravity. The not uncommon phenomena of multiple envelopes, 
on the other hand, he explained as due to the varying amounts of 
repulsion exercised by the nucleus itself on the different kinds of 
matter developed from it. 

The movements and perturbations of the comet of 1811 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 3,065 years for 

1 The subject of a classical memoir by Bessel, published in 1810. 



chap, v COMETS 101 

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 civilizations still deep buried among the secrets of the 
coming time. 1 

On the 26th of June, 1819, while the head of a comet passed 
across the face of the sun, the earth was in all probability involved 
in its tail. But of this remarkable double event nothing was 
known until more than a month later, when the fact of its past 
occurrence emerged from the calculations of Olbers. 2 Nor had the 
comet itself been generally visible previous to the first days of July. 
Several observers, however, on the publication of these results, 
brought forward accounts of singular spots perceived by them upon 
the sun at the time of the transit, and an original drawing of one 
of them, by Pastorff of Buchholtz, has been preserved. This un- 
doubtedly authentic delineation 3 represents a round nebulous object 
with a bright spot in the centre, of decidedly cometary aspect, and 
not in the least like an ordinary solar "macula." Mr. Hind, 4 never- 
theless, showed its position on the sun to be irreconcilable with that 
which the comet must have occupied ; and Mr. Ranyard's discovery 
of a similar smaller drawing by the same author, dated May 26, 
1828, 5 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 subject of a prize ultimately 
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. 6 

They were verified by the detection at Rome, August 6, 1835, of 

1 A fresh investigation of its orbit has been published by N. Herz of Vienna. 
See Bull. Astr., t. ix., p. 427. 

2 Astr. Jahrbuch (Bode's), 1823, p. 134. 

3 Reproduced in Webb's Celestial Objects, 4th. ed. 

4 Month. Not., vol. xxxvi., p. 309. 5 Celestial Objects, p. 40, note. 

6 See Airy's Address, Mem. . A. S., vol. x., p. 376. Rosenberger calculated 
no more, though he lived until 1890. W. T. Lynn, Observatory, vol. xiii., 
p. 125. 



io2 HISTORY OF ASTRONOMY part i 

a nearly circular misty object not far from the predicted place of 
the comet. It was not, however, until the middle of September 
that it began to throw out a tail, which by the 15th of October had 
attained a length of about 24 degrees (on the 19th, at Madras, it ex- 
tended to fully 30), J the head showing to the naked eye as a reddish 
star rather brighter than Aldebaran 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 emphatically 
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 from Herschel's observation at 
Feldhausen as if by melting into adjacent space from the excessive 
diffusion of its light. A very uncommon circumstance in its develop- 
ment was that it lost all trace of tail previous to its arrival at peri- 
helion on the 16th of November. Nor did it begin to recover 
its elongated shape for more than two months afterwards. On the 
23rd of January, Boguslawski perceived it as a star of the 
sixth magnitude, without measurable disc. 6 Only two nights later, 
Maclear, director of the Cape Observatory, found the head to be 131 
seconds across. 7 And so rapidly did the augmentation of size 
progress, that Sir John Herschel estimated the actual bulk of this 

1 Hind, The Comets, p. 47. 2 Arago, Annuaire, 1836, p. 228. 

8 Astr. Nach., No. 300. 

4 It deserves to be recorded that Robert Hooke drew a very similar inference 
from his observations of the comets of 1680 and 1682. Month. Not., vol. xiv., 
pp. 77-83. 

5 Brief wechsel zwischen Olbers und Bessel, Bd. ii., p. 390. 

8 Herschel, Results, p. 405. 7 Mem. R. A. S., vol. x., p. 92. 



chap, v COMETS 103 

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-precipita- 
tion on the nucleus." 1 A plausible, but no longer admissible, inter- 
pretation of this still unexplained phenomenon. The next return 
of this body, which will be considerably accelerated by Jupiter's 
influence, is expected to take place in 1910. 2 

By means of an instrument devised to test 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. 3 Indications of the same kind had been afforded 4 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 ex- 
periments, however, are in reality more beautiful and ingenious than 
instructive, since ignited 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 
peculiarity known as polarisation. 

The most brilliant comets of the century were suddenly rivalled 
if not surpassed by the extraordinary object which blazed out beside 
the sun, February 28, 1843. It was simultaneously perceived in 
Mexico and the United States, in Southern Europe, and at sea off 
the Cape of Good Hope, where the passengers on board the Owen 
Glendower were amazed by the sight of a " short, dagger-like object," 
closely following the sun towards the western horizon. 5 At Florence, 
Amici found its distance from the sun's centre at noon to be only 
1 23' ; and spectators at Parma were able, when sheltered from 
the direct glare of mid-day, 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 3rd of March it 
measured 25, and on the 11th, 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 remarked, " conveys an 
astounding impression of the intensity of the forces at work." " It 
is clear," he continued, "that if we have to deal here with matter, such 

1 Results, p. 401. 2 Pontecoulant, Camptes Rendus, t. lviii., p. 825. 

3 Annuaire, 1836, p. 233. 4 Cosmos, vol. i., p. 90, note (Otte's trans.). 

5 Herschel, Outlines of Astronomy, p. 399, 9th ed. 



1 04 HISTOR Y OF ASTRO NO M Y part i 

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." 1 

On the 17th of March a silver}' ray, some 40 long and slightly 
curved at its extremity, shone out above the sunset clouds in this 
country. No previous intimation had been received of the possi- 
bility 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 per- 
ceptible interval was reduced nearly one-half in the case we are now 
concerned with. The centre of the comet of 1843 approached the 
formidable luminary within 78,000 miles, leaving, it is estimated, a 
clear space of not more than 32,000 between the surfaces of the 
bodies brought into such perilous proximity. The escape of the 
wanderer was, however, secured by the extraordinary rapidity of its 
flight. It swept past perihelion at a rate 366 miles a second 
which, if continued, would have carried it right round the sun in 
two hours ; and in only eleven minutes more than that short period 
it actually described half the curvature of its orbit an arc of 180 
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 experimentum 
crucis as to the nature of tails. For clearly no fixed appendage 
many millions of miles in length could be whirled like a brandished 
sabre from one side of the sun to the other in 131 minutes. Come- 
tary trains are then, as Olbers rightly conceived them to be, emana- 
tions, not appendages inconceivably rapid outflows of highly rare- 
fied 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. 2 It was narrow, and bounded by nearly parallel and nearly 
rectilinear lines, resembling to borrow a comparison of Aristotle's 
a " road " through the constellations ; and after the 3rd of March 
showed no trace of hollowness, the axis being, in fact, rather brighter 

1 Outlines, p. 398. 

2 Boguslawski calculated that it extended on the 21st of March to 581 
millions. Report Brit. Ass., 1845, p. 89. 



chap, v COMETS 105 

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 observed to charac- 
terise 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 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 pro- 
pagated 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 -} Clausen 
looked for its return at the end of between six and seven. A recent 
discussion 2 by Professor Kreutz of all the available data gives a pro- 
bable period of 512 years for this body, and precludes its hypothetical 
identity with the comet of 1668, known as the "Spina" of Cassini. 

It may now be asked, what were the conclusions regarding the 
nature of comets drawn by astronomers from the considerable amount 
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 observa- 
tions showed that the feeblest rays of light might traverse some 
hundreds of thousands of miles of their substance, even where it 
was apparently most condensed, without being perceptibly weakened. 
Nay, instances were recorded in which stars were said to have 
gained in brightness from the process ! 3 On the 24th of June, 1825, 
Olbers 4 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 1, 1811, when the great comet of that year 
approached so close to Altair, the lucida of the Eagle, that the star 
seemed to be transformed into the nucleus of the comet. 5 Even the 

1 Comptes Eendus, t. xvi., p. 919. 

a Observatory, vol. xxiv., p. 167 ; Astr. Nach., No. 3,320. 

3 Piazzi noticed a considerable increase of lustre in a very faint star of the 
twelfth magnitude viewed through a comet. Madler, Reden, etc., p. 248, note. 

4 Astr. Jahrbuch, 1828, p. 151. 5 Madler, Gesch. d. Astr., Bd. ii., p. 412. 



106 HISTORY OF ASTRONOMY part i 

central blaze of Halley's comet in 1835 was powerless to impede the 
passage of stellar rays. Struve 1 observed at Dorpat, on September 
17, an all but central occupation ; Glaisher 2 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 11th of October, 1847, Mr. Dawes, 3 an exceptionally keen 
observer, distinctly saw a star of the tenth magnitude through the 
exact centre of a comet discovered on the first of that month by 
Maria Mitchell of Nantucket. 

Examples, on the other hand, are not wanting of the diminution 
of stellar light under similar circumstances; 4 and we meet two 
alleged instances of the vanishing of a star behind a comet. 
Wartmann of Geneva observed the first, November 28, 1828 f but 
his instrument was defective, and the eclipsing body, Encke's comet, 
has shown itself otherwise perfectly translucent. The second case 
of occultation occurred September 13, 1890, when an eleventh 
magnitude star was stated to have completely disappeared during 
the transit over it of Denning's comet. 6 

From the failure to detect any effects of refraction in the light of 
stars occulted by comets, it was inferred (though, as we know now, 
erroneously) that their composition is rather that of dust than that 
of vapour ; that they consist not of any continuous 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 5757575- of tna ^ contained in our 
globe, the effect of its attraction, on the occasion of its approach 
within 1,438,000 miles of the earth, July 1, 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's in 1846. 

But the discovery in this branch most distinctive of the period 
under review is that of " short period " comets, of which four 7 were 
known in 1850. These, by the character of their movements, serve 
as a link between the planetary and cometary worlds, and by the 

1 Hecueil de VAc. Imp. de St. PUersbourg, 1835, p. 143. 

a Guillemin's World of Comets, trans, by J. Glaisher, p. 294, note. 

3 Month. Not., vol. viii., p. 9. 

4 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. 

5 Arago, Annuaire, 1832, p. 205. 6 Ibid., 1891, p. 290. 
7 Viz., Encke's, Biela's, Faye's, and Brorsen's. 



chap, v COMETS 107 

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 demon- 
strated by the division and disappearance of one amongst their 
number, as well as by the singular and rapid changes in appearance 
undergone by many, and the seemingly irrevocable diffusion of 
their substance visible in nearly all. They might then be defined, 
according to the ideas respecting them prevalent fifty years ago, as 
bodies unconnected by origin with the solar system, but encoun- 
tered, 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 dissipa- 
tion and extinction. 



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 what has been achieved, without 
any corresponding knowledge of how it has been achieved, supplies 
food for barren wonder rather than for fruitful and profitable 
thought. Ideas advance most readily along the solid ground of 
practical reality, and often find true sublimity while laying aside 
empty marvels. Progress is the result, not so much of sudden 
flights of genius, as of sustained, patient, often commonplace en- 
deavour ; and the true lesson of scientific history lies in the close 
connection which it discloses 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 refradi&n, and that in which the same end is attained 
by reflection, The image formed is in each case viewed through a 
magnifying lens, or combination of lenses, called the eye-piece. Not 
for above a century after the " optic glasses " invented or stumbled 
upon by the spectacle-maker of Middelburg (1608) had become 
diffused over Europe, did the reflecting telescope come, even in 
England, the place of its birth, into general use. Its principle (a 
sufficiently obvious one) had indeed been suggested by Mersenne as 



chap, vi INSTRUMENTAL ADVANCES 10^ 

early as 1639 ; x James Gregory in 1663 2 described in detail a mode 
of embodying that principle in a practical shape; and Newton, 
adopting an original system of construction, actually produced in 
1668 a tiny speculum, one inch across, by means of which the 
apparent distance of objects was reduced thirty-nine times. Never- 
theless, the exorbitantly long tubeless refractors, introduced by 
Huygens, maintained their reputation until Hadley exhibited to the 
Royal Society, January 12, 1721, 3 a reflector of six inches aperture, 
and sixty-two in focal length, which rivalled in performance, and of 
course indefinitely surpassed in manageability, one of the " aerial " 
kind of 123 feet. 

The concave-mirror system now gained a decided ascendant, 
and was brought to unexampled perfection by James Short of 
Edinburgh during the years 1732-68. Its resources were, however, 
first fully developed by William Herschel. The energy and in- 
ventiveness 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- 
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 gigantic 
forty-foot, completed August 28, 1789. It was the first re- 
flector in which only a single mirror was employed. In the 
" Gregorian " form, the focussed rays are, by a second reflection 

1 Grant, Hist. Astr., p. 527. 2 Optica Promota, p. 93. 

* Phil. Trans., vol. xxxii., p. 383. 4 Ibid., vol. xc, p. 65. 



no HISTORY OF ASTRONOMY part i 

from a small concave 1 mirror, thrown straight back through a 
central aperture in the larger one, behind which the eye-piece is 
fixed. The object under examination is thus seen in the natural 
direction. The " Newtonian," on the other hand, shows the object in 
a line of sight at right angles to the true one, the light collected by 
the speculum being diverted to one side of the tube by the inter- 
position of a small plane mirror, situated at an angle of 45 to the 
axis of the instrument. Upon these two systems Herschel worked 
until 1787, when, becoming convinced of the supreme importance of 
economising light (necessarily wasted by the second reflection), he 
laid aside the small mirror of his forty-foot then in course of con- 
struction, and turned it into a "front- view" reflector. This was done 
according to the plan proposed by Lemaire in 1732 by slightly 
inclining the speculum so as to enable the image formed by it to be 
viewed with an eye-glass fixed at the upper margin of the tube. 
The observer thus stood with his back turned to the object he was 
engaged in scrutinising. 

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 telescope 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 
imaginable care, the delicate lustre of its surface could not be 
preserved longer than two years, 2 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 exceedingly high magnifying powers employed by Herschel 
constituted a novelty in optical astronomy, to which he attached 
great importance. The work of ordinary observation would, 
however, be hindered rather than helped by them. The attempt to 
increase 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 

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. 

2 Phil. Trans., vol. civ., p. 275, note. 



chap, vi INSTJl UMENTAL AD VANCE S 1 1 1 

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 con- 
tinually broken by minute fluctuations of refractive power caused 
by changes of temperature and pressure, and the currents which 
these engender. With such luminous quiverings and waverings the 
astronomer has always more or less to reckon; their absence is 
simply a question of degree ; if sufficiently magnified, they are at 
all times capable of rendering observation impossible. 

Thus, such powers as 3,000, 4,000, 5,000, even 6,652/ 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, 
experimentally, not systematically, and with the clearest discrimina- 
tion of their advantages and drawbacks. It is obvious that perfectly 
different ends are subserved by increasing the aperture and by in- 
creasing 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 in the 
image accordingly attends, cceteris paribus, upon each augmentation of 
its apparent size. For this reason, such faint objects as nebulae are 
most successfully observed with moderate powers applied to instru- 
ments 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, not merely 
" spurious," and so to distinguish 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 unpretending 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 the defect known as " chromatic 
aberration." This is due to no other cause than that which produces 

1 Phil. Traits., vol. xc, p. 70. With the forty-foot, however, only very 
moderate powers seemed to have been employed, whence Dr. Robinson argued 
a deficiency of defining power. Proc. Hoy. Irish Ac, vol. ii., p. 11. 

2 Phil. Trans., vol. lxxi., p. 492. 



ii2 HISTORY OF ASTRONOMY part i 

the rainbow and the spectrum the separation, or " dispersion " in 
their passage through a refracting medium, of the variously coloured 
rays composing a beam of white light. In an ordinary lens there 
is no common point of concentration; each colour has its own 
separate focus; and the resulting image, formed by the superposi- 
tion of as many images as there are hues in the spectrum, is in- 
definitely 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 indis- 
tinct vision in the spherical shape of lenses) ; but no attempt to 
remedy it was made until an Essex gentleman succeeded, in 1733, in 
so combining lenses of flint and crown glass as to produce refraction 
without colour. 1 Mr. Chester More Hall was, however, equally 
indifferent to fame and profit, and took no pains to make his inven- 
tion 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 vicissitudes of temperature and 
climate. They retain their efficiency with fewer precautions and 
under more trying circumstances. Above all, they co-operate more 
readily with mechanical appliances, and lend themselves with far 
greater facility to purposes of exact measurement. 

A practical difficulty, however, impeded the realisation of the 
brilliant prospects held out by Dollond's invention. It was found 
impossible to procure flint-glass, such as was needed for optical use 
that is, of perfectly homogeneous quality except in fragments of 
insignificant size. Discs of more than two or three inches in 
diameter were of extreme rarity ; and the crushing excise duty 
imposed upon the article by the financial unwisdom of the Govern- 
ment, both limited its production, and, by rendering experiments 
too costly for repetition, barred its improvement. 

Up to this time, Great Britain had left foreign competitors far 
behind in the instrumental department of astronomy. The quadrants 
and circles of Bird, Cary and Ramsden were unapproached abroad. 
The reflecting telescope came into existence and reached maturity 
on British soil. The refracting telescope was cured of its inherent 

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 Catoptriaz et IXoptrwce Sphericce Elementa, p. 98. 






chap, vi INSTRUMENTAL ADVANCES 113 

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 position of 
supremacy for one of at least partial and temporary inferiority. 

Somewhere about the time that Herschel set about polishing his 
first speculum, Pierre Louis Guinand, a Swiss artisan, living near 
Chaux-de-Fonds, in the canton of Neuchatel, began to grind spectacles 
for his own use, and was thence led on to the rude construction of 
telescopes by fixing lenses in pasteboard tubes. The sight of an 
England achromatic stirred a higher ambition, and he took the first 
opportunity of procuring some flint glass from England (then the 
only source of supply), with the design of imitating an instrument 
the full capabilities of which he was destined to be the humble 
means of developing. The English glass proving of inferior quality, 
he conceived the possibility, 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, 
constructed upon it a furnace capable of melting two quintals of 
glass, and reducing himself and his family to the barest necessaries 
of life, he poured his earnings (he at this time made bells for 
repeaters) unstintingly into his crucibles. 1 His undaunted resolution 
triumphed. In 1799 he carried to Paris and there showed to 
Lalande several discs of flawless crystal four to six inches in diameter. 
Lalande advised him to keep his secret, but in 1805 he was induced 
to remove to Munich, where he became the instructor of the im- 
mortal Fraunhofer. His return to Les Brenets in 1814 was 
signalised by the discovery 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 18 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 College, Dublin, it was employed 
at the Dunsink Observatory by Briinnow and Ball in their investi- 
gations of stellar parallax. A still larger objective (of nearly 14 
inches) made of Guinand's glass was secured in Paris, about the same 

1 Wolf, Biographien, Bd. ii., p. 301. 2 Month. Not., vol. i., p. 153, note. 

8 



ii 4 HISTORY OF ASTRONOMY part i 

time, 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. 1 It was communicated 
by Henry Guinand (a son of the inventor) to Bontemps, one of the 
directors of the glassworks at Choisy-le-Roi, and by him transmitted 
to Messrs. Chance of Birmingham, with whom he entered into 
partnership when the revolutionary troubles of 1848 obliged him to 
quit his native country. The celebrated American opticians, Alvan . 
Clark & Sons, derived from the Birmingham firm the materials for 
some of their earlier telescopes, notably the 19 -inch Chicago and 
26-inch "Washington equatoreals ; but the discs for the great Lick 
refractor, and others shaped by them in recent years, have been 
supplied by Feil of Paris. 

Two distinguished amateurs, meanwhile, were preparing to re- 
assert 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. 2 They were composed of 
an alloy of copper and tin, with a minute proportion of arsenic 
{after the example of Newton 3 ), and were remarkable for perfection 
of figure and brilliancy of surface. 

The capabilities of the Newtonian plan 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 under- 
graduate at Oxford, and continued to represent the same con- 
stituency for thirteen years (1821-34). From 1845 until his death, 
which took place, October 31, 1867, he sat, silent but assiduous, in 
the House of Lords as an Irish representative peer ; he held the not 
unlaborious post of President of the Royal Society from 1849 to 
1854 ; presided over the meeting of the British Association at Cork 
in 1843, and was elected Vice- Chancellor of Dublin University in 
1862. In addition to these extensive demands upon his time and 
thoughts, were those derived from his position as practically the 
feudal chief of a large body of tenantry in times of great and 
anxious responsibility, to say nothing of the more genial claims of 
an unstinted hospitality. Yet, while neglecting no public or private 
duty, this model nobleman found leisure to render to science services 
so conspicuous as to entitle his name to a lasting place in its annals 



1 Henrivaux, Encycloptdie Chimique, t. v., fasc. 5, p. 363. 

2 See ante, p. 83. 3 Phil. Trans., vol. vii., p. 4007. 






chap, vi INSTRUMENTAL ADVANCES 115 

He early formed the design of reaching the limits of the attainable 
in enlarging the powers of the telescope, and the qualities of his 
mind conspired with the circumstances of his fortune to render the 
design a feasible one. From 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 7 J 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, when the impost which had so long hampered 
their efforts was removed, that the optical artists of these islands 
were able to compete on equal terms with their rivals on the 
Continent. In the case of reflectors, however, there seemed no 
insurmountable obstacle to an almost unlimited increase of light- 
gathering capacity ; and it was here, after some unproductive ex- 
periments 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 f 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 opera- 
tions 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 work- 
shops. 

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 in 1828. 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, 3 can scarcely be 
conceived. It is harder than steel, yet brittle as glass, crumbling 

1 J. Herschel, The Telescope, p. 39. 2 Month. Not., vol. xxix., p. 125. 

3 A slight excess of copper renders the metal easier to work, but liable to 
tarnish. Robinson, Proc. Roy. Irish Ac, vol. ii., p. 4. 

8 



u6 HISTORY OF ASTRONOMY parti 

into fragments with the slightest inadvertence of handling or treat- 
ment -} and the precision of figure requisite to secure good definition 
is almost beyond the power of language 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 
Tl 1 3 3 of an inch ;" 2 yet upon this minute difference of form depends 
the clearness of the image, and, as a consequence, the entire efficiency 
of the instrument. " Almost infinite," indeed (in the phrase of the 
late Dr. Robinson), must be the exactitude of the operation adapted 
to bring about so delicate a result. 

At length, in 1839, 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 insure success 
were now ascertained and under control ; all that was necessary 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 
13th 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 examination of the heavens. 

The suitable mounting of this vast machine was a problem 
scarcely less difficult than its construction. The shape of a 
speculum needs to be maintained with an elaborate care equal to 
that used in imparting it. In fact, one of the most formidable 
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 composed 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. 3 Thus the delicacy of its form 
was perishable equally by the stress of its own gravity, and by the 
slightest irregularity in the means taken to counteract that stress. 
The problem of affording a perfectly equable support in all possible 
positions was solved by resting the speculum upon twenty-seven 
platforms of cast iron, felt-covered, and carefully fitted to the shape 
of the areas they were to carry, which platforms were themselves 

1 Brit. Ass., 1848, Dr. Robinson's closing Address. Athenceum, Sept. 23, 
p. 866. 

2 The Telescope, p. 82. 

3 Lord Rosse in Phil. Trans., vol. cxl., p. 302. 



chap, vi INSTRUMENTAL ADVANCES 117 

borne by a complex system of triangles and levers, ingeniously 
adapted to distribute the weight with complete uniformity. 1 

A tube which resembled, when erect, one of the ancient round 
towers of Ireland, 2 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 Pea- 
cock 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 a 
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 counter- 
poised, that the tube could with ease be raised or depressed by two 
men working a windlass. Its horizontal range was limited by the 
lofty walls erected for its support to about ten degrees on each side 
of the meridian ; but it moved vertically from near the horizon 
through the zenith as far as the pole. Its construction was of the 
Newtonian kind, the observer looking into the side of the tube near 
its upper end, which a series of galleries and sliding stages enabled 
him to reach in any position. It has also, though rarely, been used 
without a second mirror, as a " Herschelian " reflector. 

The splendour of the celestial objects as viewed with this vast 
"light-grasper" surpassed all expectation. "Never in my life," 
exclaimed Sir James South, " did I see such glorious sidereal 
pictures ; " 3 The orb of Jupiter produced an effect compared to 
that of the introduction of a coach-lamp into the telescope ; 4 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 description." But it was in the examina- 
tion of the nebulae that the superiority of the new instrument was 
most strikingly displayed. A large number of these misty objects, 
which the utmost powers of Herschel's specula had failed to resolve 
into stars, yielded at once to the Parsonstown reflector ; while many 
others showed under entirely changed forms through the disclosure 
of previously unseen details of structure. 

One extremely curious result of the increase of light was the 
abolition of any sharp 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 

1 This method is the same in principle with that applied by Grubb in 1834 to a 
15-inch speculum for the observatory of Armagh. Phil. Trans., vol. clix., p. 145. 
3 Robinson, Proc. Roy. Ir. Ac, vol. iii., p. 120. 
* Astr. Nach., No. 536. 4 Airy, Month. Not., vol. ix., p. 120. 



u8 HISTORY OF ASTRONOMY part i 

hemisphere were known to astronomers ; the}?- were now reinforced 
by five of the planetary kind, the discs of which were observed to be 
centrally perforated ; while the definite margins visible in weaker 
instruments were replaced by ragged edges or filamentous fringes. 

Still more striking was the discovery of an entirely new and 
most remarkable species of nebulae. These were termed "spiral," 
from the more or less regular convolutions, resembling the whorls of 
a shell, in which the matter composing them appeared to be dis- 
tributed. The first and most conspicuous specimen of this class was 
met with in April, 1845 j it is situated in Canes Venatici, close to 
the tail of the Great Bear, and wore, in Sir J. Herschel's instru- 
ments, the aspect of a split ring encompassing a bright nucleus, thus 
presenting, as he supposed, a complete analogue to the system of the 
Milky Way. In the Eosse mirror it shone out as a vast whirlpool 
of light a stupendous witness to the presence of cosmical activities 
on the grandest scale, yet regulated by laws as to the nature of 
which we are profoundly ignorant. Professor Stephen Alexander 
of New Jersey, however, concluded, from an investigation (necessarily 
founded on highly precarious data) of the mechanical condition of 
these extraordinary 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 j the stars 
composing which, no longer held together in a delicately adjusted 
system like that of the sun and planets, are advancing through a 
period of seeming confusion towards an appointed goal of higher 
order and more perfect and harmonious adaptation. 2 

The class of spiral nebulae included, in 1850, fourteen members, 
besides several in which the characteristic arrangement seemed 
partial or dubious. 3 A tendency in the exterior stars of other 
clusters to gather into curved branches (as in our Galaxy) was like- 
wise noted ; and the existence of unsuspected analogies was pro- 
claimed by the significant combination in the "Owl" nebula (a 
large planetary in Ursa Major) 4 of the twisted forms of a spiral with 
the perforated effect distinctive of an annular nebula. 

1 Astronomical Journal (Gould's), vol. ii., p. 97. 

2 Ibid., p. 160. 3 Lord Rosse in Phil. Trans., vol. cxl., p. 505. 

4 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. 



chap, vi INSTRUMENTAL ADVANCES 119 

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 nebulae 
had been resolved, all were resolvable, very few imitated his truly 
scientific caution ; and the results of Bond's investigations 1 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 6,000 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 obser- 
vation 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. 2 

The French opticians' rule of doubling the number of millimetres 
contained in the aperture of an instrument to find the highest 
magnifying power usefully applicable to it, would give 3,600 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 telescopic power. Its construction may ac- 
cordingly 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 obser- 
vation 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. 

Scarcely less important for the practical uses of astronomy than 
the optical qualities of the telescope is the manner of its mounting. 

1 Mem. Am. Ac, vol. iii., p. 87 ; Astr. Nach., No. 611. 

2 Pop. Astr., p. 145. 



120 HISTORY OF ASTRONOMY parti 

The most admirable performance of the optician can render but un- 
satisfactory service if its mechanical accessories are ill-arranged or 
inconvenient. 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 dis- 
coveries themselves. 

There are two chief modes of using the telescope, to which all 
others may be considered subordinate. 1 Either it may be in- 
variably 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, thus keeping the 
object viewed permanently in sight instead of simply noting the 
instant of its flitting across the telescopic field. The first plan 
is that of the "transit instrument," the second that of the 
" equatoreal." Both were, by a remarkable coincidence, intro- 
duced about 1690 2 by Olaus Romer, the brilliant Danish astronomer 
who first measured the velocity of light. 

The uses of each are entirely different. With the transit, the 
really fundamental task of astronomy the determination of the 
movements of the heavenly bodies is mainly accomplished ; while 
the investigation of their nature and peculiarities is best conducted 
with the equatoreal. One is the instrument of mathematical, the 
other of descriptive astronomy. One furnishes the materials with 
which theories are constructed and the tests by which they are 
corrected ; the other registers new facts, takes note of new appear- 
ances, sounds the depths and peers into every nook of the heavens. 
The great improvement of giving to a telescope equatoreally 
mounted an automatic movement by connecting it with clockwork, 
was proposed in 1674 by Robert Hooke. Bradley in 1721 actually 
observed Mars with a telescope " moved by a machine that made 
keep pace with the stars ;" 3 and Von Zach relates 4 that he had once 

1 This statement must be taken in the most general sense. Supplementary 
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. 

2 As early as 1620, according to R. Wolf (Ges. 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, 
existed at Pekin until 1900, when they were carried off as "loot" to Berlin. 
J. L. E. Dreyer, Copernicus, vol. i., p. 134. 

3 Miscellaneous Works, p. 350. 

* Astr. Jahrbuch, 1799 (published 1796), p. 115. 



chap, vi INSTRUMENTAL ADVANCES 121 

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 declination, thus necessarily accom- 
panying the movement of any star upon which it may be fixed. It 
accordingly forms part of the large sum of Fraunhofer's merits to 
have secured this inestimable advantage to observers. 

Sir John Herschel considered that Lassell's application of equa- 
toreal mounting to a nine-inch Newtonian in 1840 made an epoch 
in the history of " that eminently British instrument, the reflect- 
ing telescope." 1 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 different one ; he employed the crossed axes of the true 
equatoreal, and his success removed, to a great extent, the fatal 
objection of inconvenience in use, until then 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 declina- 
tion-parallels. 

The art of accurately dividing circular arcs into the minute 
equal parts which serve as the units of astronomical measurement, 
remained, during the whole of the eighteenth century, almost ex- 
clusively in English hands. It was brought to a high degree of per- 
fection 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 Romer had already found the 
advantage of employing. The five-foot vertical circle, which Piazzi 
with some difficulty induced 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 invention), by which both 
the elevation of a celestial object above the horizon and its position 
as referred to the horizon could be measured. In the same year, 
Borda invented the " repeating circle " (the principle of which had 

1 Month. ML, vol. xli., p. 189. 2 Phil. Tracts., vol. xlvi., p. 242. 



1 2 2 HISTOR Y OF ASTR ONOMY part i 

been suggested by Tobias Mayer in 1756 1 ), a device for extermi- 
nating, so far as possible, errors of graduation by repeating an observa- 
tion with different parts of the limb. This was perhaps the earliest 
systematic effort to correct the imperfections of instruments by the 
manner of their use. 

The manufacture of astronomical circles was brought to a very 
refined state of excellence early in the nineteenth century by 
Eeichenbach 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 ^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 1809, 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 instrumental errors 
might almost be said to have created a new art of observation. 
Every instrument, he declared in memorable words, 5 must be twice 
made once by the artist, and again by the observer. Knowledge 
is power. Defects that are ascertained 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, 
through skilful use, be made to yield valuable results. The Konigs- 
berg observations of which the first instalment was published in 
1815 set the example of regular "reduction" for instrumental 
errors. Since then, it has become an elementary part of an astro- 
nomer's duty to study the idiosyncrasy of each one of the mechanical 
contrivances at his disposal, in order that its inevitable, but now 
certified deviations from ideal accuracy may be included amongst 
the numerous corrections by which the pure essence of even 
approximate truth is distilled from the rude impressions of sense. 

Nor is this enough ; for the casual circumstances attending each 
observation have to be taken into account with no less care than the 
inherent or constitutional peculiarities of the instrument with which 
it is made. There is no " once for all " in astronomy. Vigilance 
can never sleep ; patience can never tire. Variable as well as con- 
stant 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, 

1 Grant, Hist, of Astr., p. 487. 2 Pop. Vorl, p. 546. 

3 Phil. Trans., vol. xcix., p. 105. * Report Brit. Ass., 1832, p. 132. 

5 Pop. Vorl., p. 432. 



chap, vi INSTRUMENTAL ADVANCES 123 

the disturbing 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 transmits 
its impressions, must all enter into his calculations, and be sifted out 
from his results. 

It was in 1823 that Bessel drew attention to discrepancies in the 
times of transits given by different astronomers. 1 The quantities 
involved were far from insignificant. He was himself nearly a 
second in advance of all his contemporaries, Argelander lagging 
behind him as much as a second and a 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 correction of observations that a special instrument 
for accurately determining its amount in each 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 acknowledged, difficult to comply with. They 
involve an unceasing struggle against the infirmities of his nature 
and the instabilities of his position. But the end is not unworthy 
the sacrifices demanded. One additional ray of light thrown on the 
marvels of creation a single, minutest encroachment upon the 
strongholds of ignorance is recompense enough for a lifetime of 
toil. Or rather, the toil is its own reward, if pursued in the lofty 
spirit which alone becomes it. For it leads through the abysses of 
space and the unending vistas of time to the very threshold of that 
infinity and eternity of which the disclosure is reserved for a life to 
come. 

1 C. T. Anger, Grundziige der neueren astroTWinischen Beobachtungs-Kutist, 
p. 3. 



PART II 

RECENT PROGRESS OF ASTRONOMY 

CHAPTER I 

FOUNDATION OF ASTRONOMICAL PHYSICS 

In the year 1826, Heinrich Schwabe of Dessau, elated with the hope 
of speedily delivering himself from the hereditary incubus of an 
apothecary's shop, 1 obtained from Munich a small telescope and 
began to observe the sun. His choice of an object for his researches 
was instigated by his friend Harding of Gottingen. It was a 
peculiarly happy one. The changes visible in the solar surface were 
then generally regarded as no less capricious than the changes in the 
skies of our 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 Herschel, profoundly as he studied them, and 
intimately as he was convinced of their importance as symptoms of 
solar activity, saw no reason to suspect that their abundance and 
scarcity were subject to orderly alternation. One man alone in the 
eighteenth 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 him might be 
investigated with success ; but this prophetic utterance was of the 
nature of a soliloquy rather than of a communication, and remained 
hidden away in an unpublished journal until 1859, when it was 
brought to light in a general ransacking of archives. 4 

1 Wolf, Gesch. der Astr., p. 655. 

3 Manuel Johnson, Mem. B. A. S., vol. xxvi., p. 197. 

3 Astronomic Thiorique et Pratique, t. iii., p. 20. 

4 Wolf, Gesch. der Astr., p. 654. 



126 HISTORY OF ASTRONOMY part ii 

Indeed, Schwabe himself was far from anticipating the discovery 
which fell to his share. He compared his fortune to that of Saul, 
who, seeking his father's asses, found a kingdom. 1 For the hope 
which inspired his early resolution lay in quite another direction. 
His patient ambush was laid for a possible intramercurial 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 " 2 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 
unvarying system. In 1843 he made his first announcement of a 
probable decennial period, 3 but it met with no general attention ; 
although Julius Schmidt of Bonn (afterwards director of the Athens 
Observatory) and Gautier of Geneva were impressed with his 
figures, and Littrow had himself, in 1836, 4 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 Humboldt published in 1851, in the third volume of his 
Kosmos, 5 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 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 
turning both to the best account. He died where he was born and 
had lived, April 11, 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 international 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 

1 Month. Not., vol. xvii., p. 241. 2 Mem. R. A. S., vol. xxvi., p. 200. 

3 Astr. Nach., No. 495. 

4 Gehler's Physikalisches Worterbuch, art. Sonnenflecken, p. 851. 

5 Zweite Abth., p. 401. 



chap, i ASTRONOMICAL PHYSICS 127 

apparatus, and a new system for its employment, issued j there, in 
1833, the first regular magnetic observatory was founded, whilst at 
Gottingen was fixed the universal time - standard for magnetic 
observations. A 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 neighbourhood of the South Pole. In 
1841, the elaborate organisation created by the disinterested efforts of 
scientific " agitators " was complete ; Gauss's " magnetometers " were 
vibrating under the view of attentive observers in five continents, 
and simultaneous results began to be recorded. 

Ten years later, in September, 1851, Dr. John Lamont, the 
Scotch director of the Munich Observatory, in reviewing the 
magnetic observations made at Gottingen and Munich from 
1835 to 1850, perceived with some surprise that they gave 
unmistakable indications of a period which he estimated at 10^ 
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 countries 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 Lamont found to increase and diminish once in every 10 J years. 

In the following winter, Sir Edward Sabine, ignorant as yet of 
Lamont's conclusion, undertook to examine a totally different set of 
observations. The materials in his hands had been collected at the 
British colonial stations of Toronto and Hobarton 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 
1 Annalen der Physik (PoggendorfTs), Bd. lxxxiv., p. 580. 



128 HISTORY OF ASTRONOMY part n 

violence and frequency. Sabine was the first to note the coincidence 
between this unlooked-for result and Schwabe's sun-spot period. 
He showed that, so far as observation had yet gone, the two cycles 
of change agreed perfectly both in duration and phase, maximum 
corresponding to maximum, minimum to minimum. What the 
nature of the connection could be that bound together by a common 
law effects so dissimilar as the rents in the luminous garment of the 
sun, and the swayings to and fro of the magnetic needle, was and 
still remains beyond the reach of well-founded theory ; 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, 1852. 1 
On the 31st of July following, Rudolf Wolf at Berne, 2 and on the 
18th of August, Alfred Gautier at Si on, 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-operation in discovery, by which 
"particulars" are amassed by one set of investigators correspond- 
ing to the " Depredators " and " Inoculators " of Solomon's House 
while inductions are drawn from them by another and a higher 
class the " Interpreters of Nature." Yet even here the conver- 
gence 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 (11*11) years and he further 
showed that this fell in with the ebb and flow of magnetic change 
even better than Lamont's lOJ-year cycle. The analogy was also 
pointed out between the " light-curve," or zig-zagged line represent- 
ing 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 maximum to minimum j while an additional point of resem- 
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 

1 Phil. Trans., vol. cxlii., p. 103. 

2 Mittheilungen der Naturforschenden Gesellschaft, 1852, p. 183. 

3 Archives des Sciences, t. xxi., p. 194. 

4 Neue Untersuchungen, Mitth. Naturf. Ges., 1852, p. 249. 



chap, i ASTRONOMICAL PHYSICS 129 

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 phenomenon, 
to connect sun-spots with weather was less successful. The first 
attempt of the kind was made by Sir William Herschel in 1801, 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 

Yet this plausible inference 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 Vogel's collection of Zurich 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 review of the subject in 1859 convinced him that no 
relation of any kind between the two kinds of effects was trace- 
able. 5 With the singular affection of our atmosphere known as the 
Aurora Borealis (more properly Aurora Polaris) the case was 
different. Here the Zurich Chronicles set Wolf on the right track 
in leading him to associate such luminous manifestations with a 
disturbed condition of the sun ; since subsequent detailed observation 
has exhibited the curve of auroral frequency as following with such 
fidelity the jagged lines figuring to the eye the fluctuations of solar 

1 Phil. Trans., vol. xci., p. 316. 

2 Evidence of an eleven-yearly fluctuation in the price of food-grains in India 
was collected some years ago by Mr. Frederick Chambers. Nature, vol. xxxiv. , 
p. 100. 

3 Bill. Un. de Gentve, t. li., p. 336. 4 Neue Untersuchungen, p. 269. 

5 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 condition amassed at the 
Paris Observatory from 1822 to 1830 were not sufficiently precise to support any 
inference. 

9 



i 3 o HISTORY OF ASTRONOMY part ii 

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 1716, 1 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 with minor corrections were fully substantiated by 
Sabine's more extended researches. It was thus 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. 2 Moreover, the lunar, like 
the solar influence (as was proved in each case by Sabine's analysis 
of the Hobarton and Toronto observations), extends to all three 
" magnetic elements," affecting not only the position of the hori- 
zontal 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 
imperceptible by distance. 

We have now to speak of the discovery and application to the 
heavenly bodies of a totally new method of investigation. Spectrum 
analysis may be shortly described as a mode of distinguishing 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 cognisance 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 refract- 

1 Phil. Trans., vol. xxix., p. 421. 

3 Ibid., vols, cxliii., p. 558, cxlvi., p. 505. 



chap, i ASTRONOMICAL PHYSICS 131 

ing prism or the combination of prisms known as the "spectro- 
scope " 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 for them, and for them only we 
should at once be in possession of a mode of identifying such sub- 
stances 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 distinc- 
tive 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 successively introduced sal am- 
moniac, potash, alum, nitre, and sea-salt, and observed the singular 
predominance, under almost all circumstances, of a particular shade 
of yellow light, perfectly definite in its degree of refrangibility 1 
in other words, taking up a perfectly definite position in the spec- 
trum. 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, redis- 
covered 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 per- 
sistent 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 

1 Observations on Light and Colours, p. 35. 2 Phil. Trans., vol. lxxv., p. 190. 

3 Denkschriften (Munich. Ac. of Sc), 1814, 1815, Bd. v., p. 197. 

4 Edinburgh Journal of Science, vol. v, , p. 77. See also Phil. Mag., Feb., 
1834, vol. iv., p. 112. 

92 



i 3 2 HISTORY OF ASTRONOMY part ii 

fundamental principle. He was inclined to believe that the presence 
in the spectrum of any individual ray told unerringly of the 
volatilisation in the flame under scrutiny of some body as whose 
badge or distinctive symbol that ray might be regarded ; but the 
continual prominence of the yellow beam staggered him. It ap- 
peared, indeed, without fail where sodium was ; but it also appeared 
where it might be thought only reasonable to conclude that sodium 
was not. Nor was it until thirty years later that William Swan, 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 (chloride of 
sodium) is, in fact, the most diffusive of solids. It floats in the air : 
it flows with water ; every grain of dust has its attendant particle ; 
its 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 180 million parts, and one alone of 
such inconceivably minute fragments be present in a source of light, 
the spectroscope 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 Wheat- 
stone. 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 
eminent chemist Robert Bunsen, took the matter in hand. By 
them the general question as to the necessary and invariable con- 
nection of certain rays in the spectrum with certain kinds of matter, 
was first resolutely confronted, and first definitely 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 under- 

1 Ed. Phil. Trans., vol. xxi., p. 411. 

2 On the Absorption of Light bij 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. 11 (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 133 

taken. 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 there- 
upon 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 telescope, made the sur- 
prising discovery that the solar spectrum is crossed, not by seven, 
but by thousands of obscure transverse 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, measuring their distances apart with a theodolite, and 
affixing to them the letters of the alphabet, by which they are still 
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 differ- 
ences 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 inter- 
rupted by three massive bars of darkness two in the blue and one 
iu the green ; 5 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 (afterwards, as we have said, 
identified with the light of glowing sodium) which he had already 

1 Phil. Mag., vol. xx., p. 93. 2 Annalen der PhysiTc, Bd. cxiii., p. 357. 

3 Phil. Trans., vol. xcii., p. 378. 4 Dcnkschriften, Bd. v., p. 202. 

5 Ibid., p. 220 ; Edin. Jour, of Science, vol. viii., p. 9. 



i 3 4 HISTORY OF ASTRONOMY part ii 

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 Fraunhoferin 1815, 
was contained the very essence of solar chemistry; but its true 
significance did not become apparent until long afterwards. Fraun- 
hofer 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. 1 

Partially and inconclusively at first this was done. The " fixed 
lines " (as they were called) of the solar spectrum took up the 
position of a standing problem, to the solution of which no approach 
seemed possible. Conjectures as to their origin were indeed rife. 
An explanation put forward by Zantedeschi 2 and others, and 
dubiously favoured by Sir David Brewster and Dr. J. H. Gladstone, 3 
was that they resulted from " 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 sup- 
posed 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. 4 These are the well-known " atmospheric lines ;" but 
the immense majority of their companions in the spectrum remain 
quite unaffected by the thickness of the stratum of air traversed by 
the sunlight containing them. They are then obviously due to 
another cause. 

There remained the true interpretation absorption in the sun's 
atmosphere ; and this, too, was extensively canvassed. But a 
remarkable observation made by Professor Forbes of Edinburgh 5 on 
the occasion of the annular eclipse of May 15, 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 

1 Denkschriften, Bd. v., p. 222. 2 Arch, des Sciences, 1849, p. 43. 

3 Phil. Trans., vol. cl., p. 159, note. 4 Ed. Phil. Trans., vol. xii., p. 528. 

5 Phil. Trans., vol. cxxvi., p. 453. "I conceive," he says, "that this result 
proves decisively that the sun's atmosphere has nothing to do with the production 
of this singular phenomenon " (p. 455). And Brewster's well-founded opinion 
that it had much to do with it was thereby, in fact, overthrown. 



chap, i ASTRONOMICAL PHYSICS 135 

strongest in light proceeding from his edges, which, cutting that 
envelope obliquely, passed through a much greater depth of it. But 
the circle of light left by the 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 inquirers, already 
sufficiently perplexed. It still remains an anomaly, of which no 
satisfactory explanation has been offered. 

Convincing evidence as to the true nature of the solar lines was 
however at length, in the autumn of 1859, brought forward at 
Heidelberg. Kirchhoff's experimentum cruris 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 principle 
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 : Substances of every kind 
are opaque to the precise rays which they emit at the same 
temperature j 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 

1 Monatsberichte, Berlin, 1859, p. 664. 

2 Abhandlungen, 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. Mag., vol. xx., p. 534) ; but his experiments wanted the 
precision of those executed at Heidelburg. 



136 HISTORY OF ASTRONOMY part 11 

not follow that cool bodies absorb the rays which they would give 
out if sufficiently heated. Hydrogen at ordinary temperatures, for 
instance, is almost perfectly transparent, but if raised to the glowing 
point as by the passage of electricity it then becomes capable of 
arresting, and at the same time of displaying in its own spectrum 
light of four distinct colours. 

This principle 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 
dark in the unrolled sheaf of sun-rays ; the meaning remains un- 
changed. 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. Eemove the dazzling solar background, by 
contrast with which they show as obscure, and they will be seen, 
and, at critical moments, actually have 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 latter branch appears to have been first entered upon 
by Dr. Thomas Young in 1803 j 1 it was pursued by the younger 
Herschel, 2 by William Allen Miller, Brewster, and Gladstone. 
Brewster indeed made, in 1833, 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 ex- 
quisite accuracy, and printed in three shades of ink to convey the 
graduated obscurity of the lines, was published in the Transactions 

1 Miscellaneous Works, vol. i., p. 189. 2 Ed. Phil. Trans., vol. ix., p. 458. 
:i Ibid., vol. xii., p. 519. 4 Quart. Jour. Chem. Soc, vol. x., p. 79. 



chap, i ASTRONOMICAL PHYSICS 137 

of the Berlin Academy for 1861 and 1862. 1 [Representations of the 
principal lines belonging to various elementary bodies formed, as 
it were, a series of marginal notes accompanying the great solar 
scroll, 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 2 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 dissenting 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 com- 
position no less reliable than any of the tests used in the labora- 
tory, was equally 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 1845. 3 Nay, more, it was performed 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 coin- 
cidence 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. 4 He was surprised to see that the D-line 

1 A facsimile accompanied Sir H. Roscoe's translation of Kirchhoff's "Re- 
searches on the Solar Spectrum" (London, 1862-63). 

Estimated by Kirchhoff at a trillion to one. Abhandl., 1861, p. 79. 

:! Phil. Mag., vol. xxvii. (3rd series), p. 90. 

4 L'Institut, Feb. 7, 1849, p. 45 ; Phil. Mag., vol. xix. (4th series), p. 193. 



138 HISTORY OF ASTRONOMY part ii 

was rendered more intensely dark by the combination of lights. 
To assure himself still further, he substituted a reflected image of 
one of the white-hot carbon-points for the sunbeam, with an identical 
result. The same ray was missing. It needed but another step to 
have generalised this result, and thus laid hold of a natural truth 
of the highest importance ; but that step was not taken. Foucault, 
keen and brilliant though he was, rested satisfied with the in- 
formation that the voltaic aire had the power of stopping the kind of 
light emitted by it ; he asked no further question, and was conse- 
quently the bearer of no further intelligence on the subject. 

The truth conveyed by this remarkable experiment was, however, 
divined by one eminent man. Professor Stokes of Cambridge stated 
to Sir William Thomson (now Lord Kelvin), shortly after it had 
been made, his conviction that an absorbing atmosphere of sodium 
surrounded the sun. And so forcibly was his hearer impressed with 
the weight of the arguments based upon the absolute agreement of the 
D-line in the solar spectrum with the yellow ray of burning sodium 
(then freshly certified by W. H. Miller), combined with Foucault's 
" reversal " of that ray, that he regularly inculcated, in his public 
lectures on natural philosophy at Glasgow, five or six years before 
Kirchhoff 's discovery, not only the fad 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 pro- 
fessors themselves among the foremost of their time in the suc- 
cessful 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 pregnant 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 statement, 
a body absorbs all the series of vibrations it is, under any 
circumstances, capable of emitting, as well as those connected with 

1 Ann. d. PJiys., vol. cxviii., p. 110. 

* Phil. Mag., vol. ix. (4th series), p. 327. 



chap, i ASTRONOMICAL PHYSICS 139 

them by simple harmonic relations. This is far too wide. To 
render it either true or useful, it had to be reduced to the cautious 
terms employed by Kirchhoff. Radiation strictly and necessarily 
corresponds with absorption only when tlie temperature is the same. 
In point of fact, Angstrom 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 appear trite to some than 
obscure even to a few. 

The spectrum, then, of a body is simply the light proceeding from 
it spread out by refraction 1 into a brilliant variegated band, passing 
from brownish-red through crimson, orange, yellow, green, and azure 
into dusky violet. The reason of this spreading-out or "dispersion" 
is that the various colours have different wave-lengths, and con- 
sequently meet with different degrees of retardation in traversing 
the denser medium of the prism. The shortest and quickest vibra- 
tions (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 refraction by a prism, so as to form a tinted riband. 
This visible spectrum is prolonged invisibly at both ends by a long 
range of vibrations, either too rapid or too sluggish to affect the eye 
as light, but recognisable through their chemical and heating effects. 

Now all incandescent solid or liquid substances, and even gases 
ignited under great pressure, give what is called a " continuous 
spectrum f that is to say, the light derived from them is of every 
conceivable hue. Sorted out with the prism, its tints merge imper- 
ceptibly 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, 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 

1 Spectra may be produced by diffraction as well as by refraction ; but we are 
here only concerned with the subject in its simplest aspect. 



140 HISTORY OF ASTRONOMY part n 

what kind of matter is concerned in producing them. We may 
suppose that the inconceivably minute particles which by their 
rapid thrilling 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 I 
is drowned, so to speak, in a 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 absmption, 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 vibra- - 
tions, and those only, when transmitted past them, or, phrasing 
it otherwise, are opaque to them, and transparent to all others. 

It should further be explained that the shape of the bright or 
dark spaces in the spectrum has nothing whatever to do with the 
nature of the phenomena. The " lines " and " bands " so frequently 
spoken of are seen as such for no other reason than because the 
light forming them is admitted through a narrow, straight opening. 
Change that opening into a fine crescent or a sinuous curve, and 
the " lines " will at once appear as crescents or curves. 

Eesuming 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 j secondly, the identity of the light absorbed with the light 
emitted by each j thirdly, the coincidences observed between rays 
missing from the solar spectrum and rays absorbed by various 
terrestrial substances. Thus, a realm of knowledge, pronounced by 
Morinus 1 in the seventeenth century, and no less dogmatically by 
Auguste Comte 2 in the nineteenth, hopelessly out of reach of the 
human intellect, was thrown freely open, and the chemistry of the 
sun and stars took at once a leading place among the experimental 
sciences. 

1 Astrologia Gallica (1661), p. 189. 

2 Pos. Phil, vol. i., pp. 114, 115 (Martineau's trans.). 



chap, i ASTRONOMICAL PHYSICS 141 

The immediate increase of knowledge was not the chief result of 
KirchhofFs labours; still more important was the change in the 
scope and methods of astronomy, which, set on foot in 1852 by the 
detection of a common period affecting at once the spots on the sun 
and the magnetism of the earth, was extended and accelerated by 
the discovery of spectrum analysis. The nature of that change is 
concisely indicated by the heading of the present chapter ; we would 
now ask our readers to endeavour to realise somewhat distinctly 
what is implied by the " foundation of astronomical physics." 

Just three centuries ago, Kepler drew a forecast of what he 
called a " physical astronomy " a science treating of the efficient 
causes of planetary motion, and holding the " key to the inner 
astronomy." 1 What Kepler dreamed of and groped after, Newton 
realized. He showed the beautiful 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 "cor- 
ruptile " 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 observa- 
tions, that the magnetism of the earth is subject to subtle influences, 
emanating, certainly from some, and presumably 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 proceeds, or of the effects which it produces 
that is to say, either by the astronomer or by the physicist, or, 
better still, by both concurrently. Their acquisitions are mutually 
profitable ; nor can either be considered as independent of the other. 
Any important accession to knowledge respecting the sun, for 
example, may be expected to cast a reflected light on the still 
obscure subject of terrestrial magnetism ; while discoveries in 
magnetism or its alter ego electricity must profoundly affect solar 
inquiries. 

The establishment of the new method of spectrum analysis drew 
1 Proem. Astronomice Pars Optica (1604), Op., t. ii. 



142 HISTORY OF ASTRONOMY part ii 

far closer this alliance between celestial and terrestrial science. 
Indeed, they have come to merge so intimately one into the other, I 
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 last century, astronomy, J 
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 supplied by the chemist, the electrician, the inquirer 
into the most recondite mysteries of light and the molecular con- I 
stitution 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 community 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 observation, 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 made 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 1 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 farther advance must be in 
the line of minute technical 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 doubt- 
less 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. 

1 Pop. Fori., pp. 14, 19, 408. 2 Pos. Phil, p. 115. 



CHAPTER II 

SOLAR OBSERVATIONS AND THEORIES 

The zeal with which solar studies have been pursued during the last 
half 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 bewilder- 
ing variety of speculations, conjectures, and more or less justifiable 
inferences. It is satisfactory to find this novel impulse not only 
shared, but to a large extent guided, by our countrymen. 

William Rutter Dawes, one of many clergymen eminent in 
astronomy, observed, in 1852, with the help of a solar eye-piece of 
his own devising, some curious details of spot-structure. 1 The umbra 
heretofore taken for the darkest part of the spot was seen to be 
suffused with a mottled, nebulous illumination, in marked contrast 
with the striated appearance of the penumbra ; while through this 
"cloudy stratum" a "black opening" permitted the eye to divine 
farther unfathomable depths beyond. The hole thus disclosed 
evidently the true nucleus was found to be present in all consider- 
able, 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 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." 2 An interpretation founded, as is easily seen, on the 
Herschelian theory, then still in full credit. 

An instance of the same kind was observed by Mr. W. R. Birt 

1 Mem. R. A. S., vol. xxi., p. 157. a Ibid., p. 160. 



i 4 4 HISTORY OF ASTRONOMY part ii 

in I860, 1 and cyclonic movements are now a recognised feature 
of sun-spots. They are, however, as Father Secchi 2 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 in- 
cidental 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 terrestrial storms would absolutely require, are 
not to be found. So that the " cyclonic theory " of sun-spots, 
suggested by Herschel in 1847, 3 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, a self-constituted astronomer, 
gifted with the courage and the instinct of thoughtful labour. 

Born at Chelsea in May, 1826, Richard Christopher Carrington 
entered Trinity College, 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 farther prospect of self-improve- 
ment. 

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 super- 
posed upon it before it had even begun to be executed. In 1852, 
while the Redhill Observatory was in course of erection, 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, 

1 Month. Not., vol. xxi., p. 144. 

3 Le Soleil, t. i., pp. 87-90 (2nd ed., 1871). s See ante, p. 58. 



chap, ii SOLAR THEORIES 145 

both written and depicted, of past solar observations. Struck 
with their fragmentary and inconsistent character, he resolved 
to " appropriate," as he said, by " close and methodical research," 
the eleven-year period next ensuing. 1 He calculated rightly 
that he should have the field pretty nearly to himself ; for many 
reasons conspire to make public observatories slow in taking up 
new subjects, and amateurs with freedom to choose, and means 
to treat them effectually, were scarcer then than they are now. 

The execution of this laborious task was commenced November 9, 
1853. It was intended to be merely a parergon a " second 
subject," upon which daylight energies might be spent, while the 
hours of night were reserved for cataloguing those stars that " are 
bereft of the baths of ocean." Its results, however, proved of the 
highest interest, although the vicissitudes of life barred the comple- 
tion, 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 bloodvessel on the 
brain, before he had completed his fiftieth year. 2 

His observations of sun-spots were of a geometrical character. 
They concerned positions and movements, leaving out of sight 
physical peculiarities. Indeed, the prudence with which he limited 
his task to what came strictly within the range of his powers to 
accomplish, was one of Carrington's most valuable qualities. The 
method of his observations, moreover, was chosen with the same 
practical sagacity as their objects. As early as 1847, Sir John 
Herschel had recommended the daily self-registration of sun-spots, 3 
and he enforced the suggestion, with more immediate prospect of 
success, in 1854. 4 The art of celestial photography, however, was 
even then in a purely tentative stage, and Carrington wisely re- 
solved to waste no time on dubious experiments, but employ the 
means of registration and measurement actually at his command. 

1 Observations at Rcdhill (1863), Introduction. 

2 Month. Not., vol. xxxvi., p. 142. 3 Gape Observations, p. 435, note, 
4 Month. Not., vol. x., p. 158. 

10 



146 HISTORY OF ASTRONOMY part ii 

These were very simple, yet very effective. To the " helioscope " 
employed by Father Schemer 1 two centuries and a quarter earlier, a 
species of micrometer was added. The image of the sun was pro- 
jected upon a screen by means of a firmly -clamped telescope, in the 
focus of which were placed two cross-wires forming angles of 45 
with the meridian. The six instants were then carefully noted at 
which these were met by the edges of the disc as it traversed the 
screen, and by the nucleus of the spot to be measured. 2 A short 
process of calculation then gave the exact position of the spot as 
referred to the sun's centre. 

From a series of 5,290 observations made in this way, together 
with a great number of accurate drawings. Carrington derived con- 
clusions 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 de- 
terminations of the sun's rotation by different observers. Galileo, 
with "comfortable generality," estimated the period at "about a 
lunar month " f Scheiner, at twenty -seven days. 4 Cassini, in 1678, 
made it 25*58; Delambre, in 1775, no more than twenty-five days. 
Later inquiries brought these divergences within no more tolerable 
limits. Laugier's result of 25 "34 days obtained in 1841 enjoyed 
the highest credit, yet it differed widely in one direction from that 
of Bohm (1852), giving 25*52 days, and in the other from that of 
Kysseus (1846), giving 25*09 days. Now the cause of these varia- 
tions was really obvious from the first, although for a long time 
strangely overlooked. 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. 5 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, 6 and utterly baffled Laugier, 7 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, 

1 Rosa Ursina, lib. iii., p. 348. 2 Observations at Redhill, p. 8. 

8 Op., t. iii., p. 402. 

* 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. 

6 Rosa Ursina, lib. iii., p. 260. 

6 ' Faye, Comptes Rendus, t. lx., p. 818. 

7 Ibid., t. xii., p. 648. 



chap, ii SOLAR THEORIES 147 

a valuable course of observations made at Capo di Monte, Naples, in 
1845-6, enabled C. H. F. Peters 1 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 revolution is shortest at the equator and 
lengthens with increase of latitude. Carrington devised a mathe- 
matical formula by which the rate or " law " of this lengthening was 
conveniently expressed ; but it was a purely empirical one. It was 
a concise statement, but implied no physical interpretation. It 
summarised, but did not explain the facts. An assumed " mean 
period" for the solar rotation of 25-38 days (twenty-five days nine 
hours, very nearly), was thus found to be actually conformed to only 
in two parallels of solar latitude (14 north and south), while the 
equatorial period was slightly less than twenty-five, and that of 
latitudes 50 rose to twenty-seven days and a half. 2 These curious 
results gave quite a new direction to ideas on solar physics. 

The other two " elements " of the sun's rotation were also ascer- 
tained 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' (for the epoch 1850 A.D.). These 
data which have scarcely yet been improved upon suffice to 
determine the position in space of the sun's equator. Its north pole 
is directed towards a star in the coils of the Dragon, midway 
between Vega and the Pole-star j 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 3rd 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 

1 Proc. Am. Ass. Adv. of Science, 1855, p. 85. 

2 Observations at Redhill, p. 221. 

102 



i 4 8 HISTORY OF ASTRONOMY part ii 

south latitude. Individual equatorial spots are not uncommon, but 
nearer to the poles than 35 they are a rare exception. Carrington 
observed as an extreme instance in July, 1858, one in south 
latitude 44 ; and Peters, in June, 1846, watched, during several 
days, a spot in 50 24' north latitude. But beyond this no true 
macula has ever been seen ; for Lahire's reported observation of one 
in latitude 70 is now believed to have had its place on the solar 
globe erroneously assigned ; and the " veiled spots " described by 
Trouvelot in 1875 1 as occurring within 10 of the pole can only be 
regarded as, at the most, the same kind of disturbance in an un- 
developed 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 approached, 
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 downwards with the development of 
the new phase of activity. Scarcely had this remark been made 
public, 2 when Wolf 3 found a confirmation of its general truth in 
Bohm's observations during the years 1833-36 ; and a perfectly 
similar behaviour was noted both by Sporer and Secchi at the 
minimum epoch of 1867. The ensuing period gave 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 16, 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 predecessor. Each series of 
sun-spots is thus, to some extent, overlapped by the succeeding one ; 
so that while the average interval from one maximum to the next 
is eleven years, the period of each distinct wave of agitation is 
twelve or fourteen. 4 Curious evidence of the retarded character of 
the maximum of 1883-4 was to be found in the unusually low 
latitude of the spot-zones when it occurred. Their movement down- 
ward having gone on regularly while the crisis was postponed, its 
final symptoms were hence displaced locally as well as in time. The 
u law of zones" was duly obeyed at the minima of 1890 5 and 1901, 
and Sporer found evidence of conformity to it so far back as 1619. 6 
His researches, however, also showed that it was in abeyance 

1 Am. Jour, of Science, vol. xi., p. 169. 2 Month. Not., vol. xix., p. 1. 

3 Vierteljahrsschrift der Naturfors. Gcscllschaft (Zurich), 1859, p. 252. 

4 Lockyer, Chemistry of the Sun, p. 428. 

6 Maunder, Knowledge, vol. xv., p. 130. 6 Month. Not., vol. 1., p. 251. 






chap, ii SOLAR THEORIES 149 

during some seventy years previously to 1716, during which period 
sun-spots remained persistently scarce, and auroral displays were 
feeble and infrequent even in high northern latitudes. An un- 
accountable suspension of solar activity is, in fact, indicated. 1 

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 assiduity and success with limited means attracted atten- 
tion, and a Government endowment was procured for his little solar 
observatory at Anclam, in Pomerania, the Crown Prince (afterwards 
Emperor Frederick), adding a five-inch refractor to its modest equip- 
ment. Unaware of Carrington's discovery (not made known until 
January, 1859), he arrived at and published, in June, 1861, 2 a 
similar conclusion as to the equatorial quickening of the sun's move- 
ment on its axis. Appointed observer in the new Astrophysical 
establishment at Potsdam in 1874, he continued his sun-spot deter- 
minations there for twenty years, and died July 7, 1895. 

The time had now evidently come for a fundamental revision of 
current notions respecting the nature of the sun. Herschel's theory 
of a cool, dark, habitable globe, surrounded by, and protected 
against, the radiations of a luminous and heat-giving envelope, was 
shattered by the first dicta of spectrum analysis. Traces of it may 
be found for a few years subsequent to 1859, 3 but they are obviously 
survivals from an earlier order of ideas, doomed to speedy extinc- 
tion. It needs only a moment's consideration of the meaning at 
last found for the Fraunhofer lines to see the incompatibility of the 
new facts with the old conceptions. They implied not only the 
presence near the sun, as glowing vapours, of bodies highly refrac- 
tory to heat, but that these glowing vapours formed the relatively 
cool envelope of a still hotter internal mass. Kirchhoff, accordingly, 
included in his great memoir " On the Solar Spectrum," read before 
the Berlin Academy of Sciences, July 11, 1861, an exposition of 
the views on the subject to which his memorable investigations had 
led him. They may be briefly summarised as follows : 

Since the body of the sun gives a continuous spectrum, it must be 
either solid or liquid, 4 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. 

1 Maunder, Knmcledge, vol. xvii., p. 173. 2 Astr. Nach., No. 1,315. 

3 As late as 1866 an elaborate treatise in its support was written by F. Coy- 
teux, entitled Qu'est-ce que le Soleil ? Peut-il itre halite ? and answering the 
question in the affirmative. 

4 The subsequent researches of Plucker, Frankland, Wiillner, and others, 
showed that gases strongly compressed give an absolutely unbroken spectrum. 



150 HISTORY OF ASTRONOMY part ii 

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 propounder, it was consistently up- 
held perhaps by no man eminent in science except Sporer ; and his 
advocacy of it proved ineffective to secure its general adoption. 

M. Faye, of the Paris Academy of Sciences, was the first to 
propose a coherent scheme of the solar constitution covering the 
whole range of new discovery. The fundamental ideas on the 
subject now in vogue here made their first connected appearance. 
Much, indeed, remained to be modified and corrected j but the 
transition was finally made from the old to the new order of 
thought. The essence of the change may be conveyed in a single 
sentence. The sun was thenceforth regarded, not as a mere heated 
body, or still more remotely from the truth as a cool body un- 
accountably spun round with a cocoon of fire, but as a vast heat- 
radiating machine. The terrestrial analogy was abandoned in one 
more particular besides that of temperature. The solar system of 
circulation, instead of being adapted, like that of the earth, to the 
distribution of heat received from without, was seen to be directed 
towards the transportation towards the surface of the heat con- 
tained within. Polar and equatorial currents, tending to a purely 
superficial equalisation of temperature, were replaced 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, I860, 1 were avowedly based on the anomalous 
mode of solar rotation discovered by Carrington. This may be 
regarded either as an acceleration increasing from the poles to the 
equator, or as a retardation increasing from the equator to the poles, 
according to the rate of 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 adop- 
tion of this expedient has been shown to involve inadmissible 
consequences. 

1 Comptes Rendus, t. lx., pp. 89, 138. '-' Ibid., t. c, p. 595. 



chap, ii SOLAR THEORIES 151 

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 Secchij 1 a year earlier, and by Sir John Herschel 
in April, 1864 ; 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 1822, 3 proving 
that, under conditions of great heat and pressure, the vaporous 
state was compatible with a very considerable density. The position 
was strengthened when Andrews showed, in 1869, 4 that above a 
fixed limit of temperature, varying for different bodies, true lique- 
faction 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 admitted is of such a 
nature as to afford the consistence rather of honey or pitch than of 
the aeriform fluids with which we are familiar. 

On another important point the course of subsequent thought 
was powerfully influenced by Faye's conclusions in 1865. Arago 
somewhat hastily inferred from experiments with the polariscope 
the wholly gaseous nature of the visible disc of the sun. Kirchhoff, 
on the contrary, believed (erroneously, 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 regarding the photosphere no longer "as a defined surface, in the 
mathematical sense, but as a limit to which, in the general fluid 
mass, ascending currents carry the physical or chemical phenomena 
of incandescence." 6 Uprushing floods of mixed 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 photospheric 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 

1 Bull. Meteor, dell Osservatorio dell Coll. Mom., Jan. 1, 1864, p. 4. 

2 Quart. Jour, of Science, vol. i., p. 222. 

3 Ann. de Chim. et de Phys., t. xxii., p. 127. 

4 Phil. Trans., vol. clix., p. 575. 5 Les Mondes, Dec. 22, 1864, p. 707. 
Rendus, t. lx., p. 147. 



152 HISTORY OF ASTRONOMY part ii 

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 was emitted by Dr. Johnstone Stoney in 1867 1 that the 
photospheric clouds are composed of carbon-particles precipitated 
from their mounting vapour just where the temperature is lowered 
by expansion and radiation to the boiling-point of that substance. 
But this view, though countenanced by Angstrom, 2 and advocated 
by Hastings of Baltimore, 8 and other authorities, 4 is open to grave 
objections. 5 

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 temperature was too high to permit 
the occurrence of the precipitations to which the photosphere is due. 
Their obscurity was attributed, as in Dr. Brester's more recent 
Tlidorie du Soleil, to deficiency of emissive power. Yet here the 
verdict of the spectroscope is adverse and irreversible. 

After every deduction, however, has been made, we still find that 
several ideas of permanent value were embodied in this compre- 
hensive 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 upward, and of comparatively cool matter 
downward ; thirdly, that the photosphere is a surface of condensa- 
tion, 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 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" 

1 Proc. Roy. Society, vol. xvi., p. 29. 

2 Recherches sur le Spectre Solaire, p. 38. 

3 Am. Jour, of Science, 1881, vol. xxi., p. 41. Hastings stipulated only for 
some member of the triad, carbon, silicon, and boron. 

4 Ranyard, Knowledge, vol. xvi., p. 190. 

5 Young, The Sun, p. 336, ed. 1897. 



chap, ii SOLAR THEORIES 153 

one inch in diameter ; l 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 
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, and died in 
London April 19, 1889. Educated at the Ecole Sainte-Barbe in 
Paris, he made a large fortune as a paper manufacturer in England, 
and thus amply and early provided the material supplies for his 
scientific campaign. Towards the end of 1853 he took some 
successful lunar photographs. They were remarkable as the first 
examples of the application to astronomical light-painting of the 
collodion process, invented by Archer in 1851 ; and also of the use 
of reflectors (De la Rue's was one of thirteen inches, constructed 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. De la Rue's new 
observatory, built in that year at Cranford, was expressly dedicated 
to celestial photography ; and there he 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 was taken at Paris, April 2, 1845, 2 by 
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 exposure indispensable, under pain of 
getting completely "burnt-up" plates. In 1857 De la Rue was 
commissioned by the Royal Society to construct an instrument 
specially adapted to the purpose for the Kew Observatory. The 
resulting " photoheliograph " may be described as a small telescope 
(of 3J 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 

1 H. Draper, Quart. Journ. of Sc, vol. i., p. 381 ; also Phil. Mag., vol. xvii., 
1840, p. 222. 
a Reproduced in Arago's Popular Astronomy, plate xii., vol. 1. 



154 HISTORY OF ASTRONOMY part ii 

condition recommended by Sir John Herschel was commenced and 
continued at Kew during fourteen years 1858-72. The work 
of photographing the sun is now carried on in every quarter of 
the globe, from 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 364 were similarly provided for in 1897 
and 1899. 

The conclusions arrived at by photographic means at Kew were 
communicated to the Eoyal Society in a series of papers drawn up 
jointly by De la Rue, Balfour Stewart, and Benjamin Loewy, in 
1865 and subsequent years. They influenced materially the pro- 
gress of thought on the subject they were concerned with. 

By its rotation the sun itself offers opportunities for bringing the 
stereoscope to bear upon it. Two pictures, taken at an interval of 
twenty-six minutes, show just the amount of difference needed to 
give, by their combination, the maximum effect of solidity. 1 De la 
Rue thus obtained, in 1861, a stereoscopic 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 faculse occupy the highest portions of the sun's photo- 
sphere, the spots appearing like holes in the penumbrse, which 
appeared lower than the regions surrounding them; in one case, 
parts of the faculse were discovered to be sailing over a spot ap- 
parently 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 
convincing kind. A careful application of Wilson's own geometrical 
test gave results only a trifle less decisive. Of 694 spots observed, 
78 per cent, showed, as they traversed the disc, the expected effects 
of perspective ; 3 and their absence in the remaining 22 per cent, 
might be 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 measure- 
ment of the "parallax of profundity" 4 that is, of apparent dis- 
placements attendant on the sun's rotation, due to depression below 
the sun's surface. He found that in every case it fell short of 
4,000 miles, and averaged not more than 1,321, corresponding, on 

1 Report Brit. Ass., 1859, p. 148. 2 Phil. Trans., vol. clii., p. 407. 

3 Researches in Solar Physics, part i. , p. 20. 

4 Both the phrase and the method were suggested by Faye, who estimated the 
average depth of the luminous sheath of spots at 2,160 miles. Comptes Rendus, 
t. lxi., p. 1082 ; t. xcvi., p. 356. 






chap, ii SOLAR THEORIES 155 

the terrestrial scale, to an excavation in the earth's crust of 1 -J- miles. 
Of late, however, the reality of even this moderate amount of de- 
pression has been denied. Mr. Howlett's persevering observations, 
extending over a third of a century, the results of which were 
presented to the Royal Astronomical Society in December, 1894, 1 
availed to shatter the consensus of opinion which had so long been 
maintained on the subject of spot-structure. 2 It has become 
impossible any longer to hold that it is uniformly cavernous ; and 
what seem like actually protruding umbrae are occasionally vouched 
for on unimpeachable authority. 3 We can only infer that the forms 
of sun-spots are really more various than had been supposed ; that 
they are peculiarly subject to disturbance ; and that the level of the 
nuclei may rise and fall during the phases of commotion, like lavas 
within volcanic craters. 

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 1,137 
instances of spots accompanied by faculse, 584 had those faculse 
chiefly or entirely on the left, 508 showed a nearly equal distri- 
bution, while 45 only had faculous appendages mainly on the 
right side. 4 Now the rotation of the sun, as we see it, is per- 
formed 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 up- 
wards 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. An attempt, it is true, made 
by M. Wilsing at Potsdam in 1888 5 to determine the solar rotation 
from photographs of faculse had an outcome inconsistent with this 
view of their origin. They unexpectedly gave a uniform period. 
No trace of the retardation poleward from the equator, shown by the 
spots, could be detected in their movements. But the experiment 
was obviously inconclusive ; 6 and M. Stratonoff's 7 repetition of it 
with ampler materials gave a full assurance that faculse rotate like 
spots in periods lengthening as latitude augments. 

The ideas of M. Faye were, on two fundamental points, con- 

1 Month. Not, vol. lv., p. 74. 

2 Sidgreaves, Ibid., p. 282 ; Cortie, Ibid., vol. lviii., p. 91. 

3 Explained by East as refraction-effects. Jour. Brit. Astr. Ass., vol. viii., 
p. 187. 

4 Proc. Hoy. Soc., vol. xiv., p. 39. 

5 Potsdam Publicationen, No. 18 ; Astr. Nach., Nos. 3,000, 3,287. 

6 Faye, Oomptes Rendus, t. cxi., p. 77 ; Belopolsky, Astr. Nach., No. 2,991. 



Ibid., Nos. 3,275, 3,344. 



156 HISTORY OF ASTRONOMY part ii 

tradicted by the Kew investigators. He held spots to be regions 
of uprush and of heightened temperature; they believed their 
obscurity to be due to a downrush of comparatively cool vapours. 
Now M. Chacornac observing at Ville-Urbanne, March 6, 1865, 
saw floods of photospheric matter visibly precipitating themselves 
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 existence 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 supposition of a central 
uprush. 

A fresh witness was by this time at hand. The application of 
the spectroscope to the direct examination of the sun's surface 
dates from March 4, 1866, when Sir Norman Lockyer (to give him 
his present title) undertook an inquiry into the cause of the darken- 
ing in spots. 3 It was made possible by the simple device of throw- 
ing 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 un- 
mistakable, and was again, in this case, adverse to the French 
theorist's view. The obscurations in question were found to be 
produced by no deficiency of emissive power, but by an increase 
of absorptive action. The background of variegated light remains 
unchanged, but more of it is stopped by the interposition of a dense 
mass of relatively cool vapours. The spectrum of a sun-spot is 
crossed by the same set of multitudinous dark lines, with some 
minor differences, visible in the ordinary solar spectrum. We must 
then conclude 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 accumulation being 
betrayed by the thickening of certain lines of absorption. But 
there is also a general absorption, extending almost continuously 
from one end of the spot-spectrum to the other. Using, however, 
a spectroscope of exceptionally high dispersive power, Professor 
Young of Princeton, New Jersey, succeeded in 1883 in " resolving" 
the supposed continuous obscurity of spot-spectra into a countless 
multitude of fine dark lines set very close together. 4 Their struc- 
ture was seen still more perfectly, about five years later, by M. 
Duner, 5 Director of the Upsala Observatory, who traced besides some 

1 Lockyer, Contributions to Solar Physics, p. 70. 2 Le Soleil, p. 87. 

8 Proc. Roy. Soc, vol. xv., p. 256. 

4 Phil. Mag., vol. xvi., p. 460. 5 Eecherches sur la Potation du Soleil, p. 12. 



chap, ii SOLAR THEORIES 157 

shadowy vestiges of the crowded doublets and triplets forming the 
array, from the spots on to the general solar surface. They cease 
to be separable in the blue part of the spectrum ; and the ultra- 
violet radiations of spots show nothing distinctive. 1 

As to the movements of the constipated vapours forming spots, 
the spectroscope is also competent to supply information. The 
principle of the method by which it is procured will be explained 
farther on. Suffice it here to say that the transport, at any con- 
siderable 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. But the result, so far, 
has been to negative the ascription to them of any systematic 
direction. Uprushes and downrushes are doubtless, as Father 
Cortie remarks, 2 " correlated phenomena in the production of a 
sun-spot " ; but neither seem to predominate as part of its regular 
internal economy. 

The same kind of spectroscopic evidence tells heavily against a 
theory of sun-spots started by Faye in 1872. He had been fore- 
most 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 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 recognised, the " solar tornado " hypothesis at once fell 
to pieces ; but M. Faye 3 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 j the spots to be such eddies developed into whirlpools. 
It only needs to thrust a stick into a stream to produce the kind of 
effect designated. And it happens that the differences of angular 
movement adverted to attain a maximum just about the latitudes 
where spots are most frequent and conspicuous. 

1 Hale, Astr. and Astrophysics, vol. xi., p. 814. 

2 Jour. Brit. Astr. Ass., vol. i., p. 177. 

3 Comptes Rendus, t. lxxv., p. 1664 ; Revue Scientiflque, t. v., p. 359 (1883). 
Mr. Herbert Spencer had already (in The Reader, Feb. 25, 1865) put forward an 
opinion that spots were of the nature of "cyclonic clouds." 



158 HISTORY OF ASTRONOMY part n 

There are, however, grave difficulties in identifying the two kinds 
of phenomena. 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 Young, 1 is that the cause is inadequate to the effect. 
The difference of movement, or relative drift, supposed to occasion 
such prodigious disturbances, amounts, at the utmost, for two 
portions of the photosphere 123 miles apart, to about five yards a 
minute. Thus the friction of contiguous sections must be quite 
insignificant. 

A view better justified by observation was urged by 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. 2 Spots are manifestly associated with violent eruptive 
action, giving rise to the faculse and prominences which usually 
garnish their borders. It is accordingly contended that upon the 
withdrawal of matter from below by the flinging up of a prominence 
must ensue a sinking-in of the surface, into which the partially 
cooled erupted vapours rush and settle, producing just the kind 
of darkening by increased absorption told of by the spectroscope. 
Round the edges of the cavity the rupture of the photospheric shell 
will form lines of weakness provocative of further eruptions, which 
will, in their turn, deepen and enlarge the cavity. The phenomenon 
thus tends 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. 

But on the earth, the solid crust forcibly represses the steam 
gathering beneath until it has accumulated strength for an 
explosion, while 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 has proved adverse to 
his assumptions. 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. 

1 The Sun, p. 174. For Faye's answer to the objection, see Comptes Rendus, 
t. xcv., p. 1310. 

2 A revised edition appeared in 1897. 



chap, ii SOLAR THEORIES 159 

If the " volcanic hypothesis " represented the truth, no spot 
could possibly appear without a precedent eruption. The real 
order of the phenomenon, however, is exceedingly difficult to 
ascertain ; nor is it perhaps invariable. Although, in most cases, 
the " opening " shows first, that may be simply because it is more 
easily seen. According to Father Sidgreaves, 1 the disturbance has 
then already passed the incipient stage. He considers it indeed 
" highly probable that the preparatory sign of a new spot is always 
a small, bright patch of facula." 

This sequence, if established, would be fatal to Lockyer's theory 
of sun-spots, communicated to the Royal Society, May 6, 1886, 2 
and further developed some months later in his work on The 
Chemistry of the Sun. Spots are represented in it as incidental to a 
vast system of solar atmospheric circulation, starting with 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 evidence is, however, 
slight that such a circulatory flow as would be needed to maintain 
this supposed cycle of occurrences really prevails in the sun's atmo- 
sphere ; and a similar objection applies to an " an ticy clonic theory " 
(so to designate it) elaborated by Egon von Oppolzer in 1893. 3 
August Schmidt's optical rationale of solar phenomena 4 was, on the 
other hand, a complete novelty, both in principle and development. 
Attractive to speculators from its recondite nature and far-reaching- 
scope, it by no means commended itself to practical observers, 
intolerant of finding the all but palpable realities of their daily 
experience dealt with as illusory products of "circular refraction." 

A singular circumstance has now to be recounted. On the 
1st of September, 1859, while Carrington was engaged in his daily 
work of measuring the positions of sun-spots, he was startled by the 
sudden appearance of two patches of peculiarly intense light within 
the area of the largest group visible. His first idea was that a ray 
of unmitigated sunshine had penetrated the screen employed to 
reduce the brilliancy of the image ; but, having quickly convinced 
himself to the contrary, he ran to summon an additional witness of 
an unmistakably remarkable occurrence. On his return he was dis- 
appointed to find the strange luminous outburst already on the 

1 Astr. and Astrophysics, vol. xii., p. 832. 2 Proc. Roy. Soc., No. 244. 

3 Astr. Nach., No. 3,146 ; Astr. and Astrophysics, vol. xii., pp. 419, 736. 

4 Sirius, Sept., 1893 ; ibid., vol. xxiii., p. 97 ; Astrophy. Jour., vol. i., p. 112 
(Wilc/.ynski), p. 178 (Keeler) ; vol. ii., p. 73 (Hale). 



160 HISTORY OF ASTRONOMY part n 

wane ; shortly afterwards the last trace vanished. Its entire dura- 
tion 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 observation 
made at Highgate. Mr. E. 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 dura- 
tion, was in progress over the entire globe. Telegraphic communica- 
tion 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 ; gorgeous aurorae draped the skies in solemn crimson over 
both hemispheres, 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* of the solar outburst witnessed by 
Carrington and Hodgson, the photographic 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 " 5 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 registered in 1882 ; and symptoms of the same kind, 
including the beautiful " Eose Aurora," marked the progress across 

1 Month. Not., vol. xx., p. 13. 2 Ibid., p. 15. 

3 Am. Jour., vol. xxix. (2nd 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. 

6 Phil. Trans., vol. cli., p. 428. 



chap, ii SOLAR THEORIES 161 

the sun of the enormous spot-group of February, 1892 the largest 
ever recorded at Greenwich. This extraordinary formation, which 
covered about ^J^- of the sun's disc, survived through five complete 
rotations. 1 It was remarkable for a persistent drift in latitude, its place 
altering progressively from 17 to 30 south of the solar equator. 

Again, the central passage of an enormous spot on September 9, 
1898, synchronised with a sharp magnetic disturbance and brilliant 
aurora ; 2 and the coincidence was substantially repeated in March, 

1899. 3 when it was emphasised by the prevalent cosmic calm. The 
theory of the connection is indeed far from clear. Lord Kelvin, in 

1892. 4 pronounced against the possibility of any direct magnetic 
action by the sun upon the earth, on the ground of its involving an 
extravagant output of energy ; but the fact is unquestionable that 
in Professor Bigelow's words " abnormal agitations affect the sun 
and the earth as a whole and at the same time." 5 

The nearest approach to the event of September 1, 1859, was 
photographically observed by Professor George E. Hale at Chicago, 
July 15, 1892. 6 An active spot in the southern hemisphere was the 
scene of this curiously sudden manifestation. During an interval of 
12m. between two successive exposures, a bridge of dazzling light 
was found to have spanned the boundary-line dividing the twin- 
nuclei of the spot; and these, after another 27m., were themselves 
almost obliterated by an overflow of far-spreading brilliancy. Yet 
two hours later, no trace of the outburst remained, the spot and its 
attendant faculse remaining just as they had been previously to its 
occurrence. Unlike that seen by Carrington, it was accompanied by 
no exceptional magnetic phenomena, although a "storm" set in next 
day. 7 Possibly a terrestrial analogue to the former might be dis- 
covered 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. 8 

Meantime M. Kudolf Wolf, transferred to the direction of the 
Ziirich Observatory, where he died, December 6, 1893, 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 

1 Maunder, Journal Brit. Astr. Ass., vol. ii., p. 386 ; Miss E. Brown, Ibid., 
p. 210 ; Month. Not., vol. lii., p. 354. 

2 Observatory, vol. xxi., p. 387 ; Maunder, Knowledge, vol. xxi., p. 228 ; Fenyi, 
Astroph. Jour., vol. x., p. 333. 

3 Ibid., p. 336 ; W. Anderson, Observatory, vol. xxii., p. 196. 

4 Proc. Roy. Society, vol. lii., p. 307; Rev. W. Sidgreaves, Mem. R. A. S., 
vol. liv., p. 85. 

5 Report on Solar and Terrestrial Magnetism, Washington, 1898, p. 27. 

6 Astr. and Astrophysics, vol. xi., p. 611. 7 Ibid., p. 819 (Sidgreaves). 
8 See J. Rand Capron, Phil. Mag., vol. xv., p. 318. 

11 



1 62 HISTORY OF ASTRONOMY part ii 

materials led him, in 1859, 1 to the conclusion that while the mean 
period differed little from that arrived at in 1852 of 11*11 years, 
very considerable fluctuations on either side of that mean were 
rather the rule than the exception. Indeed, the phrase " sun-spot 
period " must be understood as fitting very loosely the great fact it 
is taken to represent; so loosely, that the interval between two 
maxima may rise to sixteen and a half or sink below seven and a 
half years. 2 In 1861 3 Wolf showed, and the remark was fully con- 
firmed at Kew, that the shortest periods brought the most acute 
crises, and vice versd ; 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 j and Hermann Fritz showed soon afterwards that the aurora 
borealis is subject to an identical double periodicity. 4 The same 
inquirer has more recently detected both for auroras and sun-spots 
a " secular period " of 222 years, 5 and the Kew observations indicate 
for the latter, oscillations accomplished within twenty-six and 
twenty-four days, 6 depending, most likely, upon the rotation of the 
sun. This is certainly reflected in magnetic, and perhaps in auroral 
periodicity. The more closely, in fact, spot-fluctuations are looked 
into, the more complex they prove. Maxima of one order are super- 
posed upon, or in part neutralised by, maxima of another order ; 7 
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 

1 Mittheilungen iiber die Sonnenjkcken, No. ix. , Vierteljahrsschrift der Natur- 
forschenden Oesellschaft in Zurich, Jahrgang 4. 

2 Mitth., No. Hi., p. 58 (1881). 

8 Ibid., No. xii., p. 192. Baxendell, of Manchester, reached independently a 
similar conclusion. See Month. Not., vol. xxi., p. 141. 

4 "Wolf, Mitth., No. xv., p. 107, etc. Olmsted, following Hansteen, had already, 
in 1856, sought to establish an auroral period of sixty-five years. Smithsonian 
Contributions, vol. viii., p. 37. 

5 Hahn, Ueber die Eeziehungen der SonnenfieeJcenperiode zu meteorologischen 
Erscheinungen, p. 99 (1877). 

6 Report Brit. Ass., 1881, p. 518 ; 1883, p. 418. 

7 The Rev. A. Cortie {Month. Not., vol. Ix., p. 538) detects the influence of a 
short subsidiary cycle, Dr. W. J. S. Lockyer that of a thirty-five year period 
{Nature, June 20, 1901). Professor Newcomb {Astroph. Jour., vol. xiii., p. 11) 
considers that solar activity oscillates uniformly in 11*13 years, with super- 
posed periodic variations. 



chap, ii SOLAR THEORIES 163 

position of the planets occurred to Galileo in 1612. 1 It has been 
industriously sifted by a whole bevy of modern solar physicists. 
"Wolf in 1859 2 found reason to believe that the eleven-year curve is 
determined by the action of Jupiter, modified by that of Saturn, and 
diversified by influences proceeding from the earth and Venus. Its 
tempting approach to agreement 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 down- 
wards, there were approximate coincidences between Jupiter's 
aphelion passages and sun-spot maxima, the relation had been 
almost exactly reversed in the two periods preceding that date ; 3 and 
Wolf himself finally concluded that the Jovian origin must be 
abandoned. 4 M. Duponchel's 5 prediction, nevertheless, of an abnormal 
retardation of the maximum due in 1881 through certain peculi- 
arities in the positions of Uranus and Neptune about the time it fell 
due, was partially verified by the event, since, after an abortive 
phase of agitation in April, 1882, the final outburst was postponed 
to January, 1894. The interval was thus 13*5 instead of 11*1 years j 
and it is noticeable that the delay affected chiefly the southern 
hemisphere. Alternations of activity in the solar hemispheres were 
indeed a marked feature of the maximum of 1884, which, in 
M. Faye's view, 6 derived thence its indecisive character, while 
sharp, strong crises arise with the simultaneous advance of agita- 
tion north and south of the solar equator. The curve of magnetic 
disturbance followed with its usual strict fidelity the anomalous 
fluctuations of the sun-spot curve. The ensuing minimum occurred 
early in 1889, and was succeeded in 1894 by a maximum slightly 
less feeble than its predecessor. 7 

It cannot be said that much progress has been made towards 
the disclosure of the cause, or causes, of the sun-spot cycle. No 
external influence adequate to the effect has, at any rate, yet been 
pointed out. Most thinkers on this difficult subject provide a 
quasi-explanation of the periodicity in question through certain 
assumed vicissitudes affecting internal processes; 8 Sir Norman 
Lockyer and E. von Oppolzer reach the same end by establishing 
self-compensatory fluctuations in the solar atmospheric circulation 

1 Opere, t. iii., p. 412. 2 Mitth., Nos. vii. and xviii. 

3 Observations at Redhill, p. 248. 4 Comptes Rendus, t. xcv., p. 1249. 

5 Ibid., t. xciii., p. 827 ; t. xcvi., p. 1418. 

6 Ibid., t. c, p. 593. 

7 Ellis, Proc. Roy. Society, vol. lxiii., p. 70. 

8 Schultz, Astr. Nach., Nos. 2,817-18, 2,847-8; Wilsing, Ibid., No. 3,039; 
Belopolsky, Ibid., No. 2,722. 

112 



164 HISTORY OF ASTRONOMY part n 

Dr. Schuster resorts to changes in the electrical conductivity of 
space near the sun. 1 In all these theories, however, the course of 
transition is arbitrarily arranged to suit a period, which imposes 
itself as a fact peremptorily claiming admittance, while obstinately 
defying explanation. 

The question so much discussed, as to the influence of sun-spots 
on weather, does not 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 particulars may be inverted by 
situation ; and yet it is only by the collection and collocation of 
particulars that we can hope to reach any general law. There is, 
however, a good deal of evidence to support the opinion the 
grounds for which were primarily derived from the labours of 
Dr. Meldrum at 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 
recently obtained some countenance from Savelieff's measures 
showing a gain in the strength of the sun's radiation pari passu 
with increase in the number of spots visible on his surface. 2 

The examination of what we may call the texture of the sun's 
surface derived new interest from a remarkable announcement made 
by Mr. James Nasmyth in 1862. 2 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. 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 j and there is little doubt that he was right. It is, 
nevertheless, 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 untrimmed at the edges. 4 

The term "granulated," suggested by Dawes in 1864, 5 best 
describes the mottled aspect of the solar disc as shown by modern 
telescopes and cameras. The grains, or rather the " noccules," 

1 Report Brit. Ass., 1892, p. 635. 

2 A. W. Augur, Astroph. Jour., vol. xiii., p. 346. 

3 Report Brit. Ass., 1862, p. 16 (pt. ii.). 

4 Mem. R. A. S., vol. xxi., p. 161. 5 Month. Not., vol. xxiv., p. 162. 



chap, ii SOLAR THEORIES 165 

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 relatively minute 
elements, composing, jointly, about one-fifth of the visible photo- 
sphere, 1 he estimates that three-quarters of the entire light of the 
sun are derived. 2 Janssen agrees, 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 y^oVoo f a secon d ! 
By their means, also, the curious phenomenon known as the rdseau 
photosphdrique has been made evident. 4 This consists in the diffusion 
over the entire disc of fleeting blurred patches, separated by a 
reticulation of sharply -outlined and regularly-arranged granules. 
The imperfect definition in the smudged areas may be due to 
agitations in the solar or terrestrial atmosphere, unless it be as 
Dr. Scheiner thinks possible 5 merely a photographic effect. 
M. Janssen considers that the photospheric cloudlets change their 
shape and character with the progress of the sun-spot period ; 6 but 
this is as yet uncertain. 

The "grains," or more brilliant parts of the photosphere, are 
now generally held to represent the upper termination of ascending 
and condensing currents, while the darker interstices (Herschel's 
"pores") mark the positions of descending cooler ones. In the 
penumbra? 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 penumbrsB which sug- 
gested the comparison to a rude thatch. Accepting this theory as 
in the main correct, we perceive that the very same circulatory 
process which, in its spasms of activity, gives rise to spots, produces 
in its regular course the singular "marbled" appearance, for the 
recording of which we are no longer at the mercy of the fugitive 
or delusive impressions of the human retina. And precisely this 
circulatory process it is which gives to our great luminary its per- 
manence as a sun, or warming and illuminating body. 

1 Am. Jour, of Science, vol. vii., 1874, p. 92. 2 Young, The Sun, p. 103. 

3 Ann. Bur. Long., 1879, p. 679. 4 Ibid., 1878, p. 689. 

5 Himmelsphotographie, p. 273. 

6 Ranyard, Knowledge, vols, xiv., p. 14, xvi., p. 189 ; see also the accom- 
panying photographs. 



CHAPTER III 

RECENT SOLAR ECLIPSES 

By observations made during a series of five remarkable eclipses, 
comprised within a period of eleven years, knowledge of the solar 
surroundings was advanced nearly to its present stage. Each of 
these events brought with it a fresh disclosure of a definite and 
unmistakable character. We will now briefly review this orderly 
sequence of discovery. 

Photography was first systematically applied to solve the problems 
presented by the eclipsed sun, July 18, 1860. It is true that a 
daguerreotype, 1 taken by Berkowski with the Konigsberg heliometer 
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 ; 2 but the ground remained practically unbroken 
until 1860. 

In that year the track of totality crossed Spain, and thither, 
accordingly, 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 considerable an authority as M. Faye, it was 
impossible to treat it with contempt. Two crucial tests were 
available. If it could be shown that the fantastic shapes suspended 
above the edge of the dark moon were seen under an identical 
aspect from two 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 

1 Vierteljahrsschrift Astr. Ges., Jahrg. xxvi., p. 274. 

2 Astr. Jour., vol. iv., p. 33. 



chap, in RECENT ECLIPSES 167 

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 every- 
thing 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. 

De la Rue occupied a station at Rivabellosa, in the Upper Ebro 
valley ; 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 j while their solar character was demon- 
strated by the passage of the moon in front of them, indisputably 
attested by pictures taken at successive stages of the eclipse. That 
forms seeming to defy all laws of equilibrium were, 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 
some prominences invisible in the telescope. Moreover, photo- 
graphic evidence strongly confirmed the inference previously 
drawn by Grant and others, and now with fuller assurance by 
Secchi that an uninterrupted stratum of prominence-matter encom- 
passes the sun on all sides, forming a reservoir from which gigantic 
jets issue, and into which they subside. 

Thus, first-fruits of accurate knowledge regarding the solar sur- 
roundings were gathered, while the value of the brief moments of 
eclipse gained indefinite increase, by supplementing transient visual 
impressions with the faithful and lasting records of the camera. 

In the year 1868 the history of eclipse spectroscopy virtually 
began, as that of eclipse photography in 1860; that is to say, the 
respective methods then first gave definite results. On the 18th of 
August, 1868, the Indian and Malayan peninsulas were traversed by 
a lunar shadow producing total obscuration during five minutes and 
thirty-eight seconds. Two English and two French expeditions 
were despatched to the distant regions favoured by an event so 
propitious to the advance of 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, 



168 HISTORY OF ASTRONOMY part n 

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 j three bright lines red, 
orange, and blue flashed out, and the problem was solved. 1 The 
problem was solved in this general sense, that the composition out 
of glowing vapours of the objects infelicitously termed " protuber- 
ances " or " prominences " was no longer doubtful ; although further 
inquiry was needed for the determination of the particular species 
to which those vapours belonged. 

Similar, but more complete observations were made, with less 
atmospheric hindrance, by Tennant and Janssen at Guntoor, by 
Pogson at Masulipatam, and by Rayet at Wha-Tonne, on the 
coast of the Malay peninsula, the last observer counting as many as 
nine bright lines. 2 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 the spectroscope, Janssen exclaimed to those about him, "Je 
verrai ces lignes-la en dehors des eclipses !" On the following 
morning he carried into execution the plan which formed itself in 
his brain while the phenomenon which suggested it was still before 
his eyes. It rests upon an easily intelligible principle. 

The glare of our own atmosphere alone hides the appendages of 
the sun from our daily view. To a spectator on an airless planet, 
the central globe would appear attended by all its splendid retinue 
of crimson prominences, silvery corona, and far-spreading zodiacal 
light projected on the star-spangled black background of an 
absolutely unilluminated sky. Now the spectroscope offers the 
means of indefinitely weakening atmospheric glare by diffusing a 
constant amount of it over an area widened ad libitum. But mono- 
chromatic 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 

1 Proc. Roy. Soc., vol. xvii., p. 116. 2 Comptes Rendios, t. lxvii., p. 757. 



chap, in RECENT ECLIPSES 169 

every addition to the dispersive power of the spectroscope with 
which the heterogeneous mass of beams is analysed. Employ 
prisms enough, and eventually the undiminished rays of persistent 
colour will stand out from the continually fading rainbow-tinted 
band, by which they were at first effectually veiled. 

This Janssen saw by a flash of intuition while the eclipse 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 advan- 
tage of a prolonged eclipse. The intense interest with which he 
surveyed the region suddenly laid bare to his scrutiny was 
heightened by evidences of rapid and violent change. On the 
18th of August, during the eclipse, a vast spiral structure, at lead 
89,000 miles high, was perceived, planted in surprising splendour 
on the rim of the interposed moon. It was formed, as General 
Tennant judged from its appearance in his photographs, by the en- 
counter 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 spectroscope 
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 " sierra " lying at the base of the 
prominences. Everywhere at the sun's edge the same bright lines 
appeared. 

It was not until the 19 th of September that Janssen thought fit 
to send news of his discovery to Europe. It seemed little likely to 
be anticipated ; yet 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 Sir Norman Lockyer. 
There is no need to discuss the narrow and wearisome question of 
priority ; each of the competitors deserves, and has obtained, full 
credit for his invention. With noteworthy and confident prescience, 
Lockyer, in 1866, before anything was yet known regarding the 
constitution of the "red flames," ordered a strongly dispersive 
spectroscope for the express purpose of viewing, apart from eclipses, 
the bright-line spectrum which he expected them to give. Various 
delays, however, supervened, and the instrument was not in his hands 
until October 16, 1868. On the 20th he picked up the vivid rays, 
of which the presence and (approximately) the positions had in the 
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 18 only accelerated a discovery already assured. 
1 Comptes Bendus, t. bcvii., p. 839. 



170 HISTORY OF ASTRONOMY part n 

Sir William 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, 1868, 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 
survey them at any time from the necessity of studying prominences 
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 dis- 
tinctive bright line far above the summits of the flames, connected 
it, nevertheless, with those objects. On the other hand, Lieutenant 
Campbell ascertained 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 sunshine. 
But if reflected sunshine, it was objected, the chief at least of the 
dark Fraunhofer lines should be visible in it, as they are visible in 
moonbeams, sky illumination, and all other sun-derived light. The 
objection was well founded, but was 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 done in Iowa. At 
Des Moines, Professor Harkness of the Naval Observatory, Washing- 
ton, obtained from the corona an " absolutely continuous spectrum," 
slightly less bright than that of the full moon, but traversed by a 
single green ray. 3 The same green ray was seen at Burlington and 
its position measured by Professor Young of Dartmouth College. 4 
It appeared to coincide with that of a dark line of iron in the solar 
spectrum, numbered 1,474 on KirchhofFs scale. But in 1876 Young 
was able, by the use of greatly increased dispersion, to resolve the 
Fraunhofer line " 1474 " into a pair, the more refrangible member 
of which he considered to be the reversal of the green coronal ray. 5 
Scarcely called in question for over twenty years, the identification 
nevertheless broke down through the testimony of the eclipse- 
photographs of 1898. Sir Norman Lockyer derived from them a 

1 Month. Not., vol. xxvii., p. 88. Proc. Roy. Soc, vol. xvii., p. 123. 

3 Washington Observations, 1867, App. ii., Harkness's Report, p. 60. 

4 Am. Jour., vol. xlviii. (2nd series), p. 377. 

5 Am. Jour., vol. xi. (3rd series), p. 429. 



chap, in RECENT ECLIPSES 171 

position for the line in question notably higher up in the spectrum 
than that previously assigned to it. Instead of 5,317, its true 
wave-length proved to be 5,303 ten-millionths of a millimetre; 1 nor 
does it make any show by absorption in dispersed sunlight. The 
originating substance, designated " coronium," of which nothing is 
known to terrestrial chemistry, continues luminous 2 at least 300,000 
miles above the sun's surface, and is hence presumably much lighter 
even than hydrogen. 

A further trophy was carried off by American skill 3 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 him- 
self shut behind a cloud-curtain more impervious than the Prussian 
lines. Everywhere the sky was more or less overcast. 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 heights on Mount 
Etna saw absolutely nothing. Nevertheless important information 
was snatched in despite of the elements. 

The prominent event was Young's discovery of the "reversing 
layer." As the surviving solar crescent narrowed before the 
encroaching moon, " the dark lines of the spectrum," he tells us, 
"and the spectrum itself, gradually faded away, until all at once, as 
suddenly as a bursting rocket shoots out its stars, the whole field of 
view was filled with bright lines more numerous than one could 
count. The phenomenon was so sudden, so unexpected, and so 
wonderfully beautiful, as to force an involuntary exclamation." 4 Its 
duration was about two seconds, and the impression produced was 
that of a complete reversal of the Fraunhofer spectrum that is, the 
substitution of a bright for every dark line. 

Now something of the kind was theoretically necessary to account 
for the dusky rays in sunlight which have taught us so much, and 
have yet much more to teach us ; so that, although surprising from 
its transitory splendour, the appearance could not strictly be called 
" unexpected." Moreover, its premonitory symptom in the fading 
out of those rays had been actually described by Secchi in 1868, 5 

1 Campbell, Astroph. Jour., vol. x., p. 186. 

2 Keeler, Reports on Eclipse of January 1, 1889, p. 47. 

3 Everything in such observations depends upon the proper manipulation of 
the slit of the spectroscope. 

4 Mem. R. A. S., vol. xli., p. 435. 5 Comptes Revdus, t. lxvii., p. 1019. 



1 7 2 HISTOR Y OF ASTRONOMY part ii 

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 Fraun- 
hofer series appeared bright (omitting other instances) to Maclear, 
Herschel, and Fyers in 1871, at the beginning or end of totality; 
to Pogson, at the break-up of an annular eclipse, June 6, 1872 ; to 
Stone at Klipfontein, April 16, 1874, when he saw "the field full of 
bright lines." 1 But between the picture presented by the " veritable 
pluie de lignes brillantes," 2 which descended into M. Trepied's 
spectroscope for three seconds after the disappearance of the sun, 
May 17, 1882, and the familiar one of the dark-line solar spectrum, 
certain differences were perceived, showing their relation to be not 
simply that of a positive to a negative impression. 

A "reversing layer," or stratum of mixed vapours, glowing, but 
at a lower temperature than that of the actual solar surface, was an 
integral part of Kirchhoffs theory of the production of the Fraun- 
hofer lines. Here it was assumed that the missing rays were 
stopped, and here also it was assumed that the missing rays would 
be seen bright, could they be isolated from the overpowering 
splendour of their background. This isolation is effected by 
eclipses, with the result beautifully confirmatory of theory of 
reversing, or turning from dark to bright, the Fraunhofer spectrum. 
The completeness and precision of the reversal, however, could not 
be visually attested ; and a quarter of a century elapsed before a 
successful " snap-shot " provided photographic evidence on the 
subject. It was taken at Novaya Zemlya by Mr. Shackleton, a 
member of the late Sir George Baden-Powell's expedition to observe 
the eclipse of August 9, 1896 f and similar records in abundance 
were secured during the Indian eclipse of January 22, 1898, 4 and 
the Spanish- American eclipse of May 28, 1900. 5 The result of 
their leisurely examination has been to verify the existence of a 
" reversing-layer," in the literal sense of the term. It is true that 
no single " flash " photograph is an inverted transcript of the 

1 Mem. P. A. S., vol. xli., p. 43. 2 Comptes Pendus, t. xciv., p. 1640. 

3 Young, Pop. Astr., Oct., 1897, p. 333. 

4 J. Evershed, Itidian Eclipse, 1898, p. 65 ; Month. Not, vol. lviii., p. 298 ; 
Proc. Pvy. Soc, Jan. 17, 1901. 

5 Frost, Astroph. Jour., vol. xii p. 85 ; Lord Ibid., vol. xiii., p. 149. 



chap, in RECENT ECLIPSES 173 

Fraunhofer spectrum. The lines are, indeed, in each case speaking 
broadly the same ; but their relative intensities are widely different. 
Yet this need occasion no surprise when we remember that the 
Fraunhofer spectrum integrates the absorption of multitudinous 
strata, various in density and composition, while only the upper 
section of the formation comes within view of the sensitive plates 
exposed at totalities, the low-lying vaporous beds being necessarily 
covered by the moon. The total depth of this glowing envelope 
may be estimated at 500 to 600 miles, and its normal state seems to 
be one of profound tranquillity, judging from the imperturbable 
aspect of the array of dark lines due to its sifting action upon light. 
The last of the five eclipses which we have grouped together for 
separate consideration was visible in Southern India and Australia, 
December 12, 1871. Some splendid photographs were secured by 
the English parties on the Malabar coast, showing, for the first time, 
the remarkable branching forms of the coronal emanations \ but the 
most conspicuous result was Janssen's detection of some of the dark 
Fraunhofer lines, long vainly sought in the continuous spectrum 
of the corona. Chief among 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 eclwing bach of the 
distinctive notes of the Fraunhofer spectrum, that the polariscope 
had spoken the truth in asserting a large part of the coronal 
radiance to be reflected sunlight. But it is usually so drenched in 
original luminosity, that its special 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 brilliancy. 
His observation, repeated during the Caroline Island eclipse of 
1883, was photographically verified ten years later by M. de la 
Baume Pluvinel in Senegal. 1 

An instrument of great value for particular purposes was intro- 
duced into eclipse-work in 1871. The "slitless spectroscope" 
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 simultaneously viewed by Respighi at Poodacottah, and 
by Lockyer at Baikul. Their photographic registration by the 
latter in 1875 initiated the transformation of the slitless spectroscope 
into the prismatic camera. 2 Meanwhile, the use of an ordinary 

1 Covvptes llevdus, t. cxvii., No. 1 ; Jour. Brit. Astr. Ass., vol. iii., p. 532. 

2 Lockyer, Phil. Tram., vol. clvii., p. 551. 



174 HISTORY OF ASTRONOMY part ii 

spectroscope by Herschel and Tennant at Dodabetta showed the 
green ray of coronium 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 
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 sunlight. We may 
now proceed to consider the results of subsequent 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 favour- 
able. The weather was superb ; above the Rocky Mountains the sky 
was of such purity as to permit the detection of Jupiter's satellites 
with the naked eye on several successive nights. The oppor- 
tunity for 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 condi- 
tion 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 dis- 
turbance 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 0*1, as against 
135*4 for December, 1870, an epoch of maximum activity. The 
"chromosphere" 1 was, for the most part, shallow and quiescent; 

1 The rosy envelope of prominence-matter was so named by Lockyer in 1868 
{Phil. Trans., vol. clix., p. 430). 



chap, in RECENT ECLIPSES 175 

its depth, above the spot zones, had sunk from about 6,000 to 2,000 
miles; 1 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. Lockyer pronounced it ten times 
fainter than in 1871 ; Harkness estimated its light at less than 
one-seventh that derived from the mist-blotted aureola of 1870. 2 
In shape, too, it was markedly different. When sun-spots are 
numerous, the corona appears to be most fully developed above the 
spot-zones, thus offering to our eyes a rudely quadrilateral contour. 
The four great luminous sheaves forming the corners of the square 
are made up of rays curving together from each side into 
" 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 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 with which the systematic study of coronal 
structure may be said to have begun, Mr. Kanyard first generalised 
the synclinal peculiarity by a comparison of records ; but the 
symmetry of the arrangement, though frequently striking, is liable 
to be confused by secondary formations. He further pointed out, 
with the help of careful drawings from the photographs of 1871 
made by Mr. Wesley, the curved and branching shapes assumed by 
the component filaments of massive bundles of rays. Nothing of 
all this, however, was visible in 1878. Instead, there was seen, as 
the groundwork of the corona, a ring of pearly light, nebulous to 
the eye, but shown by telescopes and in photographs to have a 
fibrous texture, as if made up of tufts of fine hairs. North and 
south, a series of short, vivid, electrical-looking flame-brushes 
diverged with conspicuous regularity from each of the solar poles. 
Their direction was not towards the centre of the sun, but towards 
each summit of his axis, so that the farther rays on either side 
started almost tangentially to the surface. 

But the leading, and a truly amazing, characteristic of the 

1 According to Trouvelot {Wash. Obs., 1876, App. iii., p. 80), the subtracted 
matter was, at least to some extent, accumulated in the polar regions. 

2 Bull. Phil. Soc. Washington, vol. iii., p. 118. 

3 Mem. R.A.S., vol. xli., 1879. 



176 HISTORY OF ASTRONOMY part ii 

phenomenon was formed by two vast, faintly-luminous wings of 
light, expanded on either side of the sun in the direction of the 
ecliptic. These were missed by very few careful onlookers ; but the 
extent assigned to them varied with skill in, and facilities for seeing. 
By far the most striking observations were made by Newcomb at 
Separation (Wyoming), by Cleveland Abbe from the shoulder of 
Pike's Peak, and by Langley at its summit, an elevation of 14,100 
feet above the sea. Never before had an eclipse been viewed 
from anything approaching that altitude, or under so translucent 
a sky. A proof of the great reduction in atmospheric glare 
was afforded by the perceptibility of the corona four minutes after 
totality was over. For the 165 seconds of its duration, the remark- 
able 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 termina- 
tion having been reached ; and there were no grounds for supposing 
the other more restricted. 

The resemblance to the zodiacal light was striking ; and a com- 
munity 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. Neverthe- 
less 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 1 of the Santiago Observatory, 
sun-spots were at a minimum; while the corona of 1715, which 
appears from the record of it by Eoger Cotes 2 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, encom- 
passed with a ring of light about one-sixth of the moon's diameter 
in breadth, upon which was superposed 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 resemblance between his sketch and Cleveland Abbe's drawing 
of the corona of 1878 is extremely striking. It should, nevertheless, 
be noted that some conspicuous spots were visible on the sun's disc 

1 Astr. Nach., No. 1,737. 

2 Correspondence with Newton, pp. 181-184 ; Ranyard, Mem. Astr. Soc, 
vol. xli., p. 501. 



chap, in RECENT ECLIPSES 177 

at the time of Cotes's eclipse, and that the preceding minimum 
(according to Wolf) occurred in 1712. Thus, the coincidence of 
epochs is imperfect. 

Professor Cleveland Abbe was fully persuaded that the long rays 
carefully observed by him from Pike's Peak were nothing else than 
streams of meteorites rushing towards or from perihelion ; and it is 
quite certain that the solar neighbourhood must be crowded with 
such bodies. But the peculiar structure at the base of the streamers 
displayed in the photographs, the curved rays meeting in pointed 
arches like Gothic windows, the visible upspringing tendency, the 
filamentous texture, 1 speak unmistakably of the action of forces 
proceeding from the sun, not of extraneous matter circling round 
him. 

A further proof of sympathetic change in the corona is afforded 
by the analysis of its light. In 1878 the bright line so conspicuous 
in the coronal spectrum in 1870 and 1871 had faded to the very 
limit of visibility. Several skilled observers failed to see it at all ; 
but Young and Eastman succeeded in tracing the green "coronium" 
ray all round the sun, to a height estimated at 340,000 miles. The 
substance emitting it was thus present, though in a low state of 
incandescence. The continuous spectrum was relatively strong ; 
faint traces of the Fraunhofer lines attested for it an origin, in part 
by reflection ; 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 seemed to 
show that the corona radiates a sensible amount of heat. 

The next promising eclipse occurred May 17, 1882. The con- 
course of astronomers which has become usual on such occasions 
assembled this time at Sohag, in Upper Egypt. Earely have 
seventy-four seconds been turned to such account. To each 
observer a special task was assigned, and the advantages of a strict 
division of labour were visible in the variety and amount of the in- 
formation 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 
equatorial expansions and polar "brushes" of 1878 would be found 
replaced by the star-like structure of 1871. This expectation was 
literally fulfilled. No lateral streamers were to be seen. The 
universal failure to perceive them, after express search in a sky of 
;he most transparent purity, justifies the emphatic assertion that 
'hey were not there. Instead, the type of corona observed in India 
1 S. P. Langley, Wash. Obs., 1876, App. iii., p. 209 ; Nature, vol. lxi., p. 443. 

12 



178 HISTORY OF ASTRONOMY part n 

eleven years earlier, was reproduced with its shining aigrettes, com- 
plex texture and brilliant radiated aspect. 

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 intensified. Never- 
theless, most of the bright lines recorded as coronal 1 were really 
due, there can be no doubt, to diffused chromospheric light. On 
this occasion, the first successful attempt was made to photograph 
the coronal spectrum procured in the ordinary way with a slit and 
prisms, while the prismatic camera was also profitably employed. 
It served to bring out at least one important fact that of the un- 
common strength in chromospheric regions of the twin violet beams 
of calcium, designated "H" and "K"; and prominence-photography 
signalised its improvement by the registration, in the spectrum of 
one such object, of twenty-nine rays, including man} 7- of the ultra- 
violet hydrogen series discovered by Sir William Huggins in the 
emissions of white stars. 2 

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 transparency of the streamers 
was shown by the delineation through them of the delicate tracery 
beyond. The singular and picturesque feature was added of a 
bright comet, self-depicted in all the exquisite grace of swift move- 
ment betrayed by the fine curve of its tail, hurrying away from one 
of its rare visits 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 Sir William 
Huggins derived the idea of a possible mode of photographing the 
corona without an eclipse? As already stated, its ordinary invisibility 
is entirely due to the " glare " or reflected light diffused through 
our atmosphere. But Huggins found, on examining Schuster's 
negatives, that a large proportion of the light in the coronal 
spectrum, both continuous and interrupted, is collected in the violet 
region between the Fraunhofer lines G and H. There, then, he 
hoped that, all other rays being excluded, it might prove strong 
enough to vanquish inimical glare, and stamp on prepared plates, 
through local superiority in illuminative power, the forms of the 
appendage by which it is emitted. 

1 Schuster (Proc. Roy. Soc, vol. xxxv., p. 154) measured and photographed 
about thirty. 

2 Abney, Phil. Trans., vol. clxxv., p. 267. 

3 Proc. Roy. Soc, vol. xxxiv., p. 409. Experiments directed to the same end 
had been made by Dr. 0. Lohse at Potsdam, 1878-80. Astr. Nach., No. 2,486. 



chap, in RECENT ECLIPSES 179 

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 accomplished 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 
Plates thus sensitised received impressions which it was hardly 
possible to regard as spurious. "Not only the general features," 
Captain Abney affirmed, 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 Sir William Huggins before and after that event, 
each picture taking its proper place in a series of progressive 
modifications highly interesting in themselves, and full of promise 
for the value of the method employed to record them. 3 But experi- 
ments 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 magnificent 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 atmo- 
spheric condition of all others the most prejudicial to delicate 
researches in the solar vicinity. The filmy coronal forms, accord- 
ingly, which had been hopefully traced on the Tulse Hill plates ceased 
to appear there j nor were any substantially better results obtained 
by Mr. C. Kay Woods, in the purer air either of the Riffel or the Cape 
of Good Hope, during the three ensuing years. Moreover, attempts 
to obtain coronal photographs during the partial phases of the eclipse 
of August 29, 1886, completely failed. No part of the lunar globe 
became visible in relief against circumfluous solar radiance on any of 
the plates exposed at Grenada j and what vestiges of " structure " 
there were, came out almost better upon the moon than beside her, 
thus stamping themselves at once as of atmospheric origin. 

That the effect sought is a perfectly possible one is proved by the 
distinct appearance of the moon projected on the corona, in photo- 
graphs of the partially eclipsed sun in 1858, 1889, and 1890, and 
very notably in 1898 and 1900. 4 

In the spring of 1893, Professor Hale 5 attacked the problem of 

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. 

4 Maunder, Indian Eclipse, p. 125 ; Eclipse 0^1900, p. 143. 

5 Astr. and Astrophysics, vol. xiii., p. 662. 

122 



180 HISTORY OF ASTRONOMY part ii 

coronal daylight photography, employing the "double-slit" method 
so eminently serviceable for the delineation of prominences. 1 But 
neither at Kenwood nor at the summit of Pike's Peak, whither, 
in the course of the summer, he removed his apparatus, was any 
action of the desired kind secured. Similar ill success attended his 
and Professor Kicco's employment, on Mount Etna in July, 1894, 
of a specially designed coronagraph. Yet discouragement did not 
induce despair. The end in view is indeed too important to be 
readily abandoned ; but it can be reached only when a more par- 
ticular acquaintance with the nature of coronal light than we now 
possess indicates the appropriate device for giving it a preferential 
advantage in self-portraiture. Moreover, the effectiveness of this 
device may not improbably be enhanced, through changes in the 
coronal spectrum at epochs of sun-spot maximum. 

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 without 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 
previously to 1874, and visited only for the sake of its stores of 
guano. Seldom has a more striking proof been given of the vivid- 
ness 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 Royal Society 

of London. M. Janssen was chief of the French Academy mission ; 

1 See infra, p. 197. 



chap, in RECENT ECLIPSES 181 

he was accompanied from Meudon by Trouvelot, and joined from 
Vienna by Palisa, and from Rome by Tacchini. A large share of 
the work done was directed to assuring or negativing previous 
results. The circumstances of an eclipse favour illusion. A single 
observation by a single observer, made under unfamiliar conditions, 
and at a moment of peculiar excitement, can scarcely be regarded as 
offering more than a suggestion for future inquiry. But incredulity 
may be carried too far. Janssen, for instance, felt compelled, by the 
survival of unwise doubts, to devote some of the precious minutes 
of obscurity at Caroline Island to confirming what, in his own per- 
suasion, needed no confirmation that is, the presence of reflected 
Fraunhofer lines in the spectrum of the corona Trouvelot and 
Palisa, on the other hand, instituted an exhaustive, but fruitless 
search for the spurious " intramercurian " planets announced by Swift 
and Watson in 1878. 

New information, however, was not deficient. The corona proved 
identical in type with that of 1882, 1 agreeably to what was expected 
at an epoch of protracted solar activity. The characteristic aigrettes 
were of even greater brilliancy than in the preceding year, and the 
chemical effects of the coronal light proved unusually intense. 
Janssen's photographs, owing to the considerable 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, 2 and showed its forms as absolutely fixed and of remark- 
able complexity. 

The English pictures, taken with exposures up to sixty seconds, 
were likewise of great value. They exhibited details of structure 
from the limb to the tips of the streamers, which terminated 
definitely, and as it seemed actually, where the impressions on the 
plates ceased. The coronal spectrum was also successfully photo- 
graphed, and although the reversing layer in its entirety evaded 
record, a print was caught of some of its more prominent rays just 
before and after totality. The use of the prismatic camera was 
baffled by the anomalous scarcity of prominences. 

Using an ingenious apparatus for viewing simultaneously the 
spectrum from both sides of the sun, Professor Hastings noticed at 
Caroline Island alternations, with the advance of the moon, in the 
respective heights above the right and left solar limbs of the coronal 
green line, which were thought to imply that the corona, with its 
rifts and sheaves and " tangled hanks " of rays, is, after all, merely 
an illusive appearance produced by the diffraction of sunlight at the 

1 Abney, Phil. Trans., vol. clxxx., p. 119. 

2 Comjjtcs Bendus, t. xcvii., p. 592. 



i8 2 HISTORY OF ASTRONOMY part ii 

moon's edge. 1 But the observation was assuredly misleading or 
misinterpreted. 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 Professor Harkness, 
in 1869, 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 con- 
sequently surround the apparent places of the superposed sun and 
moon. Mr. Proctor, 3 with his usual ability, impressed this mathe- 
matically certain truth 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 relating 
to the state of things in the solar vicinity was brought into view. 
On February 11, 1869, Messrs. Frankland and Lockyer com- 
municated to the Eoyal Society a series of experiments 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 Wiillner ; 
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 
outside, and incumbent upon the chromosphere, no such vast atmo- 
sphere as the corona appeared to represent. Upon the strength of 
which conviction the " glare " theory entered, chiefly under the 
auspices of Sir Norman Lockyer, upon the second stage of its 
existence. 

The genuineness of the " inner corona " to the height of 5' or 6' 
from the limb was admitted ; but it was supposed that by the de- 
tailed reflection of its light in our air the far more extensive " outer 
corona " was optically created, the irregularities of the moon's edge 
being called in to account for the rays and rifts by which its 
structure was varied. This view received some countenance from 
Admiral Maclear's observation, during the eclipse of 1870, of bright 
lines " everywhere " even at the centre of the lunar disc. Here, 

1 Memoirs National Ac. of Sciences, vol. ii., p. 102. 

2 Wash. Obs., 1867, App. ii., p. 64. 3 The Sun, p. 357. 

4 Proc. Roy. Soc, vol. xvii., p. 289. 

5 Comptes Eendus, t. lxxiii., p. 434. 



chap, in RECENT ECLIPSES 183 

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 cirrus 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 sub- 
stantial reality of the radiated outer corona, in the appearance 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 extensive, brilliant, and 
intricate in the details of its structure the corona appears. Take 
as an example General Myer's description of the eclipse of 1869, as 
seen from the summit of White Top Mountain, Virginia, at an eleva- 
tion above the sea of 5,530 feet, in an atmosphere of peculiar 
clearness. 

" To the unaided eye," he wrote, 1 " the eclipse presented, during 
the total obscuration, a vision magnificent beyond description. As 
a centre stood the full and intensely black disc of the moon, 
surrounded by the aureola of a soft bright light, through which shot 
out, as if from the circumference of the moon, straight, massive, 
silvery rays, seeming distinct and separate from each other, to a 
distance of two or three 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 out- 
line of the corona reached at its farthest only a single semidiameter of 
the moon from the limb. The plain fact, that our atmosphere acts 
rather as a veil to hide the coronal radiance than as the medium 
through which it is visually formed, emerges from further in- 
numerable records. 

No observations of importance were made during the eclipse of 
September 9, 1885. The path of total obscurity 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 
1 Wash. Obs. t 1867, App. ii., p. 195. 



184 HISTOR Y OF ASTRONOMY part 11 

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 prominences 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 com- 
pletely extinct. The development, again from the ends of a 
diameter, of a pair of similar objects was watched, September 19 and 
20, 1893, by Father Fenyi, Director of the Kalocsa Observatory ; 
and the phenomenon has been too often repeated to be accidental. 

The eclipse of August 29, 1886, was total during about four 
minutes over tropical Atlantic regions ; and an English expedition, 
led by Sir Norman Lockyer, was accordingly despatched to 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 j and the elaborately planned 
programme of observation could only in part be carried through. 
Some good work, none the less, was done. 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 growing vaporous cores of these objects are shown by the 
spectroscope under ordinary circumstances; their upper sections, 
giving a faint continuous spectrum, and composed of presumably 
cooler materials, can only be seen 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 
prove to be ordinarily invisible, because silvery throughout 
" sheeted ghosts," as it were, met only in the dark. 

Specimens of the class had been noted as far back as 1842, but 
Tacchini first drew particular attention to them. The one observed 
by him in 1886 rose in a branching form to a height of 150,000 
miles, and gave a brilliantly continuous spectrum, with bright lines 
at H and K, but no hydrogen-lines. 3 Hence the total invisibility of 
the object before and after the eclipse. During the eclipse, it was 
seen framed, as it were, in a pointed arch of coronal light, the 

1 Stokes, Anniversary Address, Nature, vol. xxxv., p. 114, 

2 Comptes Rendus, t. ci., p. 50. 

3 Harvard Annals, vol. xviii., p. 99. 



chap, in RECENT ECLIPSES 185 

symmetrical arrangement of which with regard to it was obviously 
significant. Both its unspringing shape, and the violet rays of 
calcium strongly emitted by it, contradicted the supposition that 
" white prominences " represent a downrush of refrigerated 
materials. 

The corona of 1886, as photographed by Dr. Schuster and 
Mr. Maunder, showed neither the petals and plumes of 1871, nor 
the streamers of 1878. It might be called of a transition type. 1 
Wide polar rifts were filled in with tufted radiations, and bounded 
on either side by irregularly disposed, compound luminous masses. 
In the south-western quadrant, a triangular ray, conspicuous to the 
naked eye, represented, Mr. W. H. Pickering thought, the projec- 
tion of a huge, hollow cone. 2 Branched and recurving jets were 
curiously associated with it. The intrinsic photographic brightness 
of the corona proved, from Pickering's measures, to be about -^ that 
of the average surface of the full moon. 

The Russian eclipse of August 19, 1887, can only be remembered 
as a disastrous failure. Much was expected of it. The shadow-path 
ran overland from Leipsic to the Japanese sea, so that the solar 
appurtenances would, it was hoped, be disclosed to observers 
echeloned along a line of 6,000 miles. But the incalculable element 
of weather rendered all forecasts nugatory. The clouds never 
parted, during the critical three minutes, over Central Russia, where 
many parties were stationed, and Professor D. P. Todd was 
equally unfortunate in Japan. Some good photographs were, 
nevertheless, secured by Professor Arai, Director of the Tokio 
Observatory, as well as by MM. Belopolsky and Glasenapp at 
Petrovsk and Jurjevitch respectively. They showed a corona of 
simpler form than that of the year before, but not yet of the pro- 
nounced type first associated by Mr. Ranyard with the lowest stage 
of solar activity. 

The genuineness of the association was ratified by the duplicate 
spectacle of the next-ensuing minimum year. Two total eclipses 
of the sun distinguished 1889. The first took place on New Year's 
Day, when a narrow shadow -path crossed California, allowing 
less than two minutes for the numerous experiments prompted 
by the varied nature of modern methods of research. American 
astronomers availed themselves of the occasion to the full. The 
heavens were propitious. Photographic records were obtained 
in unprecedented abundance, and of unusual excellence. Their 
comparison and study placed it beyond reasonable doubt that 
the radiated corona belonging to periods of maximum sun-spots 

1 Wesley, Phil. Trans., vol. clxxx., p. 350. 

2 Harvard Annals, vol. xviii., p. 108. 



1 86 HISTORY OF ASTRONOMY part ii 

gives place, at periods of minimum, to the "winged "type of 1878. 
Professor Holden perceived further that the equatorial extensions 
characterising the latter tend to assume a " trumpet-shape." 1 Their 
extremities diverge, as if mutually repellent, instead of flowing 
together along a medial plane. The maximum actinic brilliancy of 
the corona of January 1, 1889, was determined at Lick to be twenty- 
one times less than that of the full moon. 2 Its colour was described 
as "of an intense luminous silver, with a bluish tinge, similar to the 
light of an electric arc." 3 Its spectrum was comparatively simple. 
Very few bright lines besides those of hydrogen and coronium, 
and apparently no dark ones, stood out from the prismatic back- 
ground. 

" The marked structural features of the corona, as presented by 
the negatives" taken by Professors Nipher and Charroppin, were 
the filaments and the streamers. The filaments issued from polar 
calottes of 20 radius. 

"The impression conveyed to the eye," Professor Pritchett wrote, 4 
" is that the equatorial stream of denser coronal matter extends across 
and through the filaments, simply obscuring them by its greater 
brightness. The effect is just as if the equatorial belt were super- 
posed upon, or passed through, the filamentary structure. There is 
nothing in the photographs to prove that the filaments do not exist 
all round the sun. 5 The testimony from negatives of different 
lengths of exposure goes to show that the equatorial streamers are 
made up of numerous interlacing parts inclined at varying angles to 
the sun's equator." 

The coronal extensions, perceptible with the naked eye to a 
distance of more than 3 from the sun, appeared barely one- 
third of that length on the best negatives. Little more could be 
seen of them either in Barnard's exquisite miniature pictures, or 
in the photographs obtained by W. H. Pickering with a thirteen- 
inch refractor the largest instrument so far used in eclipse- 
photography. 

The total eclipse of December 22, 1889, held out a prospect, 
unfortunately not realized, of removing some of the doubts and 
difficulties that impeded the progress of coronal photography. 6 
Messrs. Burnham and Schaeberle secured at Cayenne some excellent 
impressions, showing enough of the corona to prove its identical 

1 Lick Report, p. 20. 2 Ibid., p. 14. 3 Ibid., p. 155. 

4 Pub. Astr. Soc. of the Pacific, vol. iii., p. 158. 

5 Professor Holden concluded, with less qualification, " that so-called 'polar' 
rays exist at all latitudes on the sun's surface." Lick Report, p. 19. 

6 Holdeu, Report on Eclipse of December, 1889, p. 18 ; Charroppin, Pub. Astr. 
Soc. of the Pacific, vol. iii., p. 26. 



chap, in RECENT ECLIPSES 187 

character with that depicted in the beginning of the year, but not 
enough to convey additional information about its terminal forms 
or innermost structure. Any better result was indeed impossible, 
the moisture-laden air having cut down the actinic power of the 
coronal light to one-fourth its previous value. 

Two English expeditions organized by the Eoyal Astronomical 
Society fared still worse. Mr. Taylor was stationed on the West 
Coast of Africa, one hundred miles south of Loanda ; Father Perry 
chose as the scene of his operations the Salut Islands, off French 
Guiana. Each was supplied with a reflector constructed by Dr. 
Common, endowed, by its extremely short focal length of forty -five, 
combined with an aperture of twenty inches, with a light-concen- 
trating force capable, it was hoped, of compelling the very filmiest 
coronal branches to self-registration. Had things gone well two 
sets of coronal pictures, absolutely comparable in every respect, and 
taken at an interval of two hours and a half, would have been at 
the disposal of astronomers. But things went very far from well. 
Clouds altogether obscured the sun in Africa ; they only separated 
to allow of his shining through a saturated atmosphere in South 
America. Father Perry's observations were the last heroic effort of 
a dying man. Stricken with malaria, he crawled to the hospital as 
soon as the eclipse was over, and expired five days later, at sea, on 
board the Comus. He was buried at Barbados. And the sacrifice 
of his life had, after all, purchased no decisive success. Most of the 
plates exposed by him suffered deterioration from the climate, or from 
an inevitably delayed development. A drawing from the best of 
them by Miss Violet Common 1 represented a corona differing from 
its predecessor of January 1, chiefly through the oppositely 
unsymmetrical relations of its parts. Then the western wing had 
been broader at its base than the eastern ; now the inequality was 
conspicuously the other way. 2 

The next opportunity for retrieving the mischances of the past 
was offered April 16, 1893. The line of totality charted for that day 
ran from Chili to Senegambia. American parties appropriated the 
Andes; both shores of the Atlantic were in English occupation; 
French expeditions, led by Deslandres and Bigourdan, took up posts 
south of Cape Verde. A long totality of more than four minutes 
was favoured by serene skies ; hence an ample store of photographic 
data was obtained. Professor Schaeberle, of the Lick Observatory, 
took, almost without assistance, at Mina Bronces, a mining station 
6,600 feet above the Pacific, fifty-two negatives, eight of them with 
a forty -foot telescope, on a scale of four and a half inches to the 

1 Published as the Frontispiece to the Observatory, No. 160. 

2 Wesley, Ibid., p. 107. 



1 88 HISTORY OF ASTRONOMY part ii 

solar diameter. Not only the inner corona, but the array of 
prominences then conspicuous, appeared in them to be composed of 
fibrous jets and arches, held to be sections of elliptic orbits de- 
scribed by luminous particles about the sun's centre. 1 One plate 
received the impression of a curious object, 2 entangled amidst 
coronal streamers, and the belief in its cometary nature was ratified 
by the bestowal of a comet-medal in recognition of the discovery. 
Similar paraboloidal forms had, nevertheless, occasionally been seen 
to make an integral part of earlier coronas ; and it remains extremely 
doubtful whether Schaeberle's " eclipse-comet " was justly entitled to 
the character claimed for it. 

The eclipse of 1893 disclosed a radiated corona such as a year of 
spot-maximum was sure to bring. An unexpected fact about it 
was, however, ascertained. The coronal had been believed to have 
much in common with the chromospheric spectrum ; it proved, on 
investigation with a large prismatic camera, employed under Sir 
Norman Lockyer's directions by Mr. Fowler at Fundium, to be 
absolutely distinct from it. The fundamental green ray had, on 
the West African plates, seven more refrangible associates f but all 
alike are of unknown origin. They may be due to many substances, 
or to one ; future research will perhaps decide ; we can at present 
only say that the gaseous emissions of the corona include none 
from hydrogen, helium, calcium, or any other recognisable terrestrial 
element. Deslandres' attempt to determine the rotation of the 
corona through opposite displacements, east and west of the inter- 
posed moon, of the violet calcium-lines supposed to make part of 
the coronal spectrum, was thus rendered nugatory. Yet it gave an 
earnest of success, by definitely introducing the subject into the 
constantly lengthened programme of eclipse- work. There is, how- 
ever, little prospect of its being treated effectively until the green 
line is vivified by a fresh access of solar activity. 

The flight of the moon's shadow was, on August 9, 1896, dogged 
by atrocious weather. It traversed, besides, some of the most 
inhospitable regions on the earth's surface, and afforded, at the 
best, but a brief interval of obscurity. At Novaya Zemlya, how- 
ever, of all places, the conditions were tolerably favourable, and, as 
we have seen, the trophy of a " flash-spectrograph " was carried off. 
Some coronal photographs, moreover, taken by the late Sir George 
Baden-Powell 4 and by M. Hansky, a member of a Kussian party, 
were marked by features of considerable interest. They made 

1 Lick Observatory Contributions, No. 4, p. 108. 

2 Astr. and Astrophysics, vol. xiii., p. 307. 

3 Lockyer, Phil. Trans., vol. clxxxvii., p. 592. 

4 He died in London, November 20, 1898. 



chap, in RECENT ECLIPSES 189 

apparent a close connection between coronal outflows and chromo- 
spheric jets, cone-shaped beams serving as the sheaths, or envelopes, 
of prominences. M. Hansky, 1 indeed, thought that every streamer 
had a chromospheric eruption at its base. Further, dark veinings 
of singular shapes unmistakably interrupted the coronal light, and 
bordered brilliant prominences, 2 reminding us of certain "black 
lines " traced by Swift across the " anvil protuberance " August 7, 
1869. 3 In type the corona of 1896 reproduced that of 1886, as 
befitted its intermediate position in the solar cycle. 

The eclipse-track on January 22, 1898, crossed the Indian penin- 
sula from Viziadrug, on the Malabar coast, to Mount Everest in the 
Himalayas. Not a cloud obstructed the view anywhere, and an 
unprecedented harvest of photographic records was garnered. The 
flash-spectrum, in its successive phases, appeared on plates taken by 
Sir Norman Lockyer, Mr. Evershed, Professor Campbell, 4 and others ; 
Professor Turner 5 set on foot a novel mode of research by picturing 
the corona in the polarised ingredient of its light ; Mrs. Maunder 6 
practically solved the problem of photographing the faint coronal 
extensions, one ray on her plates running out to nearly six diameters 
from the moon's limb. Yet she used a Dallmeyer lens of only one 
and a half inches aperture. Her success accorded perfectly with 
Professor Wadsworth's conclusion that effectiveness in delineation 
by slight contrasts of luminosity varies inversely with aperture. 
Triple-coated plates, and a comparatively long exposure of twenty 
seconds, contributed to a result unlikely, for some time, to be sur- 
passed. The corona of 1898 presented a mixed aspect. The polar 
plumes due at minimum were combined in it with the quadrilateral 
ogives belonging to spot-maxima. A slow course of transformation, 
in fact, seemed in progress ; and it was found to be completed in 
1900, when the eclipse of May 28 revealed the typical halo of a 
quiescent sun. 

The obscurity on this occasion was short less than 100 seconds 
but was well observed east and west of the Atlantic. No striking 
gain in knowledge, however, resulted. Important experiments were 
indeed made on the heat of the corona with Langley's bolometer, 
but their upshot can scarcely be admitted as decisive. They indi- 
cated a marked deficiency of thermal radiations, implying for 
coronal light, in Professor Langley's opinion, 7 an origin analogous 
to that of the electric glow-discharge, which, at low pressures, was 

1 Bull. Acad. St. Petersbaurg, t. vi., p. 253. 

2 W. H. Wesley, Phil. Trans., vol. cxc, p. 204. 

3 Lick Reports on Eclipse of January 1, 1889, p. 204. 

4 Astroph. Jour., vol. xi., p. 226. 5 Observatory, vol. xxi., p. 157. 

6 The Indian Eclipse, 1898, p. 114 

7 Science, June 22, 1900 ; Astroph. Jour., vol. xii., p. 370. 



190 HISTORY OF ASTRONOMY part ii 

found by K. Angstrom in 1893 to have no invisible heat-spectrum. 1 The 
corona was photographed by Professor Barnard, at Wadesborough, 
North Carolina, with a 61 J-foot horizontal " coelostat." In this instru- 
ment, of a type now much employed in eclipse operations and first 
recommended by Professor Turner, a six-inch photographic objective 
preserved an invariable position, while a silvered plane mirror, revolv- 
ing by clockwork once in forty-eight hours (since the angle of move- 
ment is doubled by reflection), supplied the light it brought to a focus. 
A temporary wooden tube connected the lens with the photographic 
house where the plates were exposed. Pictures thus obtained with 
exposures of from one to fourteen seconds, were described as "remark- 
ably sharp and perfectly defined, showing the prominences and inner 
corona very beautifully. The polar fans came out magnificently." 2 

The great Sumatra eclipse left behind it manifold memories of 
foiled expectations. A totality of above six minutes drew observers 
to the Far East from several continents, each cherishing a plan of 
inquiry which few were destined to execute. All along the line of 
shadow, which, on May 18, 1901, crossed Reunion and Mauritius, 
and again met land at Sumatra and Borneo, the meteorological fore- 
cast was dubious, and the meteorological actuality in the main 
deplorable. Nevertheless, the corona was seen, and fairly well 
photographed through drifting clouds, and proved to resemble in 
essentials the appendage viewed a year previously. Negatives taken 
by members of the Lick Observatory expedition led by Mr. Perrine 3 
disclosed the unique phenomenon of a violent coronal disturbance, 
with a small compact prominence as its apparent focus. Tumbling 
masses and irregular streamers radiating from a point subsequently 
shown by the Greenwich photographs to be the seat of a conspicuous 
spot, suggested the recent occurrence of an explosion, the far- 
reaching effects of which might be traced in the confused floccular 
luminosity of a vast surrounding region. Again, photographs in 
polarised light attested the radiance of the outer corona to be in 
large measure reflected, while that of the inner ring was original ; 
and the inference was confirmed by spectrographs, recording many 
Fraunhofer lines when the slit lay far from the sun's limb, but none 
in its immediate vicinity. On plates exposed by Mr. Dyson and 
Dr. Humphrys with special apparatus, the coronal spectrum, 
continuous and linear, impressed itself more extensively in the ultra- 
violet than on any previous occasion ; and Dr. Mitchell succeeded in 
photographing the reversing layer by means of a grating spectro- 
scope. Finally, Mrs. Maunder, at Mauritius, despite mischievous 

1 Ann. der Physik, Bd. xlviii., p. 528. See also "Wood, Physical Review 
vol. iv., p. 191, 189G. 

2 Science, August 3, 1900. 3 Lick Observatory Bulletin, No. 9. 



chap, in RECENT ECLIPSES 191 

atmospheric tremors, obtained with the Newbegin telescope an 
excellent series of coronal pictures. 1 

The principles of explanation applied to the corona may be 
briefly described as eruptive and electrical. The first was adopted 
by Professor Schaeberle in his " Mechanical Theory," advanced in 
1890. 2 According to this view, the eclipse-halo consists of streams 
of matter shot out with great velocity from the spot-zones by forces 
acting perpendicularly to the sun's surface. The component 
particles return to the sun after describing sections of extremely 
elongated ellipses, unless their initial speed happen to equal or 
exceed the critical rate of 383 miles a second, in which case they 
are finally driven off into space. The perspective overlapping and 
interlacing of these incandescent outflows was supposed to occasion 
the intricacies of texture visible in the corona ; and it should be 
recorded that a virtually identical conclusion was reached by 
Mr. Perrine in 1901, 3 by a different train of reasoning, based upon 
a distinct set of facts. A theory on very much the same lines was, 
moreover, worked out by M. Belopolsky in 1897. 4 Schaeberle, 
however, had the merit of making the first adequate effort to deduce 
the real shape of the corona, as it exists in three dimensions, from 
its projection upon the surface of the sphere. He failed, indeed, to 
account for the variation in coronal types by the changes in our 
situation with regard to the sun's equator. It is only necessary to 
remark that, if this were so, they should be subject to an annual 
periodicity, of which no trace can be discerned. 

Electro -magnetic theories have the charm, and the drawback, of 
dealing largely with the unknown. But they are gradually losing 
the vague and intangible character which long clung to them ; and 
the improved definition of their outlines has not, so far, brought 
them into disaccord with truth. The most promising hypothesis of 
the kind is due to Professor Bigelow of Washington. His able 
discussion of the eclipse photographs of January 1, 1889, 5 showed 
a striking agreement between the observed coronal forms and the 
calculated effects of a repulsive influence obeying the laws of electric 
potential, also postulated by Huggins in 1885. 6 Finely subdivided 
matter, expelled from the sun along lines of force emanating from the 
neighbourhood of his poles, thus tends to accumulate at "equipotential 

1 Observatory, vol. xxiv., pp. 321, 375. 

2 Lick Report on Eclipse of December 22, 1889, p. 47 ; Month. Not., vol. 1., 
p. 372. 

3 Lick Obs. Bull., No. 9. 

4 Bull, de VAcad. St. Petersbourg, t. iv., p. 289. 

5 The Solar Corona discussed by Spherical Harmonics, Smithsonian Institution, 
1889. 

6 Bakerian Lecture, Proc. Boy. Soc, vol. xxxix. 



192 HISTORY OF ASTRONOMY part n 

surfaces." In deference, however, to a doubt more strongly felt then 
than now, whether the presence of free electricity is compatible with 
the solar temperature, he avoided any express assertion that the 
coronal structure is an electrical phenomenon, merely pointing out 
that, if it were, its details would be just what they are. 

Later, in 1892, Pupin in America, 1 and Ebert in Germany, 2 imitated 
the coronal streamers by means of electrical discharges in low vacua 
between small conducting bodies and strips of tinfoil placed on the 
outside of the containing glass receptacles. Finally, a critical experi- 
ment made by Ebert in 1895 served, as Bigelow justly said, "to 
clear up the entire subject, and put the theory on a working basis." 
Having obtained coronoidal effects in the manner described, he pro- 
ceeded to subject them to the action of a strong magnetic field, 
with the result of marshalling the scattered rays into a methodical 
and highly suggestive array. They followed the direction of the 
magnetic lines of force, and, forsaking the polar collar of the 
magnetised sphere, surrounded it like a ruffle. The obvious analogy 
with the aurora polaris and the solar corona was insisted upon by 
Ebert himself, and has been further developed by Bigelow. 3 Accord- 
ing to a recent modification of his hypothesis, the latter appendage 
is controlled by two opposing systems of forces; the magnetic 
causing the rays to diverge from the poles towards the equator, and 
the electrostatic urging their spread, through the mutual repulsion 
of the particles accumulated in the "wings," from the equator 
towards either pole. The cyclical change in the corona, he adds, is 
probably due to a variation in the balance of power thus established, 
the magnetic polar influence dominating at minima, the electrostatic 
at maxima. And he may well feel encouraged by the fortunate 
combination of many experimental details into one explanatory 
whole, no less than by the hopeful prospect of further developments, 
both practical and theoretical, along the same lines. 

What we really know about the corona can be summed up in a 
few words. It is certainly not a solar atmosphere. It does not 
gravitate upon the sun's surface and share his rotation, as our air 
gravitates upon and shares the rotation of the earth ; and this for 
the simple reason that there is no visible growth of pressure 
downwards (of which the spectroscope would infallibly give notice) 
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. 4 

1 Astr. and Astrophysics, vol. xi., p. 483. 2 Ibid., vol. xii., p. 804. 

3 Am. Journ. of Science, vol. xi., p. 253, 1901. 

4 See Huggins, Proc. Roy. Soc, vol. xxxix., p. 108 ; Young, North Am. 
Review, February, 1885, p. 179. 



chap, in RECENT ECLIPSES 193 

They are apparently in a perpetual state of efflux from, and 
influx to our great luminary, under the stress of opposing forces. 
It is not unlikely that some part, at least, of the coronal materials 
are provided by eruptions from the body of the sun ;* it is almost 
certain that they are organized and arranged round it through 
electro-magnetic action. This, however, would seem to be in- 
fluential only upon their white-hot or reflective ingredients, out of 
which the streamers and aigrettes are composed j since the coronal 
gases appear, from observations during eclipses, to form a shapeless 
envelope, with condensations above the spot-zones, or at the bases 
of equatorial extensions. The corona is undoubtedly affected both 
in shape and constitution by the periodic ebb and flow of solar 
activity, its low-tide form being winged, its high-tide form stellate ; 
while the rays emitted by the gases contained in it fade, and the 
continuous spectrum brightens, at times of minimum sun-spots. 
The appendage, as a whole, must be of inconceivable tenuity, since 
comets cut their way through it without experiencing sensible re- 
tardation. Not even Sir William 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 the sea-level, the molecules number (according to 
Dr. Johnstone Stoney) 20,000 trillions ! 

The most important lesson, however, derived from eclipses is that 
of partial independence of them. Some of its fruits in the daily 
study of prominences the next chapter will collect ; and the harvest 
has been rendered more abundant, as well as more valuable, since it 
has been found possible to enlist, in this department too, the versatile 
aid of the camera. 

1 Professor W. A. Norton, of Yale College, appears to have been the earliest 
formal advocate of the Expulsion Theory of the solar surroundings, in the second 
(1845) and later editions of his Treatise on Astronomy. 



13 



CHAPTER IV 

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, Sporer at Anclam, Young 
at Hanover, New Hampshire, Secchi and Respighi at Rome. There 
were many others, but these names stood out conspicuously. 

The first point to be cleared up was that of chemical composition. 
Leisurely measurements verified the presence above the sun's sur- 
face of hydrogen in prodigious volumes, 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 D 3 , and the unknown substance emitting it was named 
by Lockyer " helium." Its terrestrial discovery ensued after twenty- 
six years. In March, 1895, Professor Ramsay obtained from the 
rare mineral clevite a volatile gas, the spectrum of which was 
found to include the yellow prominence-ray. Helium was actually 
at hand, and available for examination. The identification cleared 
up many obscurities in chromospheric chemistry. Several bright 
lines, persistently seen at the edge of the sun, and early suspected 
by Young 1 to emanate from the same source as D 3 , were now 
derived from helium in the laboratory ; and all the complex 
emissions of that exotic substance ranged themselves into six sets 
or series, the members of which are mutually connected by numerical 
relations of a definite and simple kind. Helium is of rather more 
than twice the density of hydrogen, and has no chemical affinities. 
In almost evanescent quantities it lurks in the earth's crust, and 
is diffused through the earth's atmosphere. 

The importance of the part played in the prominence-spectrum by 
1 Phil. Mag., vol. xlii., p. 380, 1871. 



chap, iv SOLAR SPECTROSCOPY 195 

the violet lines of calcium was noticed by Professor Young in 1872, 
but since H and K lie near the limit of the visible spectrum, photo- 
graphy was needed for a thorough investigation of their appear- 
ances. Aided by its resources, Professor George E. Hale, then at 
the beginning of his career, detected in 1889 their unfailing and 
conspicuous presence. 1 The substance emitting them not only con- 
stitutes a fundamental ingredient of the chromosphere, but rises, in 
the fantastic jets thence issuing, to greater heights than hydrogen 
itself. The isolation of H and K in solar prominences from any 
other of the lines usually distinctive of calcium was experimentally 
proved by Sir William and Lady Huggins in 1897 to be due to the 
extreme tenuity of the emitting vapour. 2 

Hydrogen, helium, and calcium form, then, the chief and unvary- 
ing materials of the solar sierra and its peaks; but a number of 
metallic elements make their appearance spasmodically under the 
influence of disturbances in the layers beneath. In September, 
1871, Young 3 drew up at Dartmouth College a list of 103 lines 
significant of injections into the chromosphere of iron, titanium, 
chromium, magnesium, and many other substances. During two 
months' observation in the pure air of Mount Sherman (8,335 feet 
high) in the summer of 1872, these tell-tale lines mounted up to 
273 ; 4 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" had been suddenly 
thrust upwards into the chromosphere, and as quickly allowed to 
drop back again. The opinion would thus appear to be well- 
grounded 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 
Huggins's observation of one through an " open slit," February 13, 
1869. 5 At first it had been thought possible to examine 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 

1 Astr. Nach., No. 3,053, Amer. Jour., vol. xlii., p. 162 ; Deslandres, Comptes 
Rendus, t. cxiii., p. 307. 

2 Proe. Roy. Society, vol. lxi., p. 433. 8 Phil. Mag., vol. xlii., p. 377. 

4 Frost-Scheiner, Astr. Spectroscopy, pp. 184, 423. 

5 Proc. Roy. Soc, vol. xvii., p. 302. 

132 



196 HISTORY OF ASTRONOMY part ii 

impression of one part while others were successively presented to 
it. It was an immense gain to find that their rays had strength to 
bear so much of dilution with ordinary light as was involved in 
opening the spectroscopic shutter wide enough to exhibit the tree- 
like, or horn-like, or flame-shaped bodies rising over the sun's rim in 
their undivided proportions. Several diversely-coloured images of 
them are formed in the spectroscope ; each may be seen under a 
crimson, a yellow, a green, and a deep blue aspect. 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 1, 1869. 1 Shortly afterwards the plan was success- 
fully 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 in- 
dependently. The distinction of " cloud-prominences " from 
" flame-prominences " was announced by Lockyer, April 27 ; by 
Zollner, June 2; and by Respighi, December 4, 1870. 

The first description are tranquil and relatively permanent, some- 
times enduring without striking change for many days. Certain of 
the included species 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. The solar clouds, however, have the peculiarity of pos- 
sessing 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 occasionally seeming to fringe the sun's limb. 
But while this mode of structure suggests an actual outpouring of in- 
candescent material, certain facts require a different interpretation. 
At a distance, and quite apart from the chromosphere, prominences 
have been perceived, both by Secchi and Young, to farm, just as 
clouds form in a clear sky, condensation being replaced by ignition. 
Filaments were then thrown out downward towards the chromo- 
sphere, and finally the usual appearance of a " stemmed promi- 
nence" was assumed. Still more remarkable was an observation 
made by Trouvelot at Harvard College Observatory, June 26, 1874.* 
A gigantic comma-shaped prominence, 82,000 miles high, vanished 
1 Astr. Nach., No. 1,769. 2 Am. Jour, of Science, vol. xv., p. 85. 



chap, iv SOLAR SPECTROSCOPY 197 

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 first photograph of a prominence, as shown by the spectro- 
scope in daylight, was taken by Professor Young in 1870. 1 But 
neither his method, nor that described by Dr. Braun in 1872, 2 had 
any practical success. This was reserved to reward the efforts 
towards the same end of Professor Hale. Begun at Harvard College 
in 1889, 3 they were prosecuted soon afterwards at the Kenwood 
Observatory, Chicago. The great difficulty was to extricate the 
coloured image of the gaseous structure, spectroscopically visible at 
the sun's limb, from the encompassing glare, a very little of which 
goes a long way in fogging sensitive plates. To counteract its 
mischievous effects, a second slit, 4 besides the usual narrow one in 
front of the collimator, was placed on guard, as it were, behind the 
dispersing apparatus, so as to shut out from the sensitised surface 
all light save that of the required quality. The sun's image being 
then allowed to drift across the outer slit, while the plate-holder 
was kept moving at the same rate, the successive sectional impres- 
sions thus rapidly obtained finally " built up " a complete picture of 
the prominence. Another expedient was soon afterwards contrived. 5 
The H and K rays of calcium are always, as we have seen, bright in 
the spectrum of prominences. They are besides fine and sharp, 
while the corresponding absorption-lines in the ordinary solar 
spectrum are wide and diffuse. Hence, prominences formed by the 
spectroscope out of these particular qualities of violet light, can be 
photographed entire and at once, for the simple reason that they are 
projected upon a naturally darkened background. Atmospheric 
glare is abolished by local absorption. This beautiful method was 
first realised by Professor Hale in June, 1891. 

A " spectroheliograph," consisting of a spectroscopic and a photo- 
graphic apparatus of special type, attached to the eye-end of an 
equatoreal twelve inches in aperture, was erected at Kenwood in 
March, 1891 ; and with its aid, Professor Hale entered upon 
original researches of high promise for the advancement of solar- 
physics. Noteworthy above all is his achievement of photographing 
both prominences and f aculse on the very face of the sun. The latter 

1 Journ. Franklin Institute, vol. xl., p. 232. 

2 Pogg. Annalen, Bd. cxlvi., p. 475 ; Astr. Nach., No. 3,014. 

3 Astr. Nach., Nos. 3,006, 3,037. 

4 This device was suggested by Janssen in 1869. 

5 Astr. and Astrophysics, vol. xi., pp. 70, 407. 



198 HISTORY OF ASTRONOMY part n 

had, until then, been very imperfectly observed. They were only 
visible, in fact, when relieved by their brilliancy against the dusky 
edge of the solar disc. Their convenient emission of calcium light, 
however, makes it possible to photograph them in all positions, and 
emphasises their close relationship to prominences. The simul- 
taneous picturing, moreover, of the entire chromospheric ring, with 
whatever trees or fountains of fire chance to be at the moment 
issuing from it, has been accomplished by a very simple device. 
The disc of the sun itself having been screened with a circular 
metallic diaphragm, it is only necessary to cause the slit to traverse 
the virtually eclipsed luminary, in order to get an impression of the 
whole round of its fringing appendages. And the record can be 
extended to the disc by removing the screen, and carrying the slit 
back at a quicker rate, when an "image of the sun's surface, with 
the faculae and spots, is formed on the plate exactly within the image 
of the chromosphere formed during the first exposure. The whole 
operation," Professor Hale continues, " is completed in less than a 
minute, and the resulting photographs give the first true pictures of 
the sun, showing all of the various phenomena at its surface." 1 Most 
of these novel researches were, by a remarkable coincidence, pursued 
independently and contemporaneously by M. Deslandres, of the 
Paris Observatory. 2 

The ultra-violet prominence spectrum was photographed for the 
first time from an uneclipsed sun, in June, 1891, at Chicago. Besides 
H and K, four members of the Huggins-series of hydrogen- 
lines imprinted themselves on the plate. 3 Meanwhile M. Des- 
landres was enabled, by fitting quartz lenses to his spectroscope, 
and substituting a reflecting for a refracting telescope, to 
get rid of the obstructive action of glass upon the shorter 
light-waves, and thus to widen the scope of his inquiry into 
the peculiarities of those derived from prominences. 4 As the 
result, not only all the nine white-star lines were photographed 
from a brilliant sun-flame, but five additional ones were found 
to continue the series upward. The wave-lengths of these last 
had, moreover, been calculated beforehand with singular exact- 
ness, from a simple formula known as " Balmer's Law." 5 The new 
lines, accordingly, filled places in a manner already prepared for 
them, and were thus unmistakably associated with the hydrogen- 
spectrum. This is now known to be represented in prominences by 
twenty -seven lines, 6 forming a kind of harmonic progression, only 

1 Astr. and Astrophysics, vol. xi., p. 604. 

8 Comptes Rendus, t. cxiii. , p. 307. 

3 Astr. and Astrophysics, vol. xi., p. 50. 4 Ibid., pp. 60, 314. 

5 Wiedemann's Annalen der Physik, Bd. xxv. , p. 80. 

6 Evershed, Knowledge, vol. xxi., p. 133. 



PLATE I. 




Photographs of the Solar Chromosphere and Prominences. 

Taken with the Spectroheliograph of the Kenwood Observatory, Chicago, 
by Professor George E. Hale. 



chap, iv SOLAR SPECTROSCOPY 199 

four of which are visibly darkened in the Fraunhofer spectrum of 
the sun. 

The chemistry of "cloud-prominences" is simple. Hydrogen, 
helium, and calcium are their chief constituents. "Flame-promi- 
nences," on the other hand, show, in addition, the characteristic rays 
of a number of metals, among which iron, titanium, barium, 
strontium, sodium, and magnesium are conspicuous. They are 
intensely brilliant ; sharply denned in their varying forms of jets, 
spikes, fountains, waterspouts ; of rapid formation and speedy 
dissolution, seldom attaining to the vast dimensions of the more 
tranquil kind. Eruptive or explosive by origin, they occur in close 
connection with spots; whether causally, the materials ejected as 
" flames " cooling and settling down as dark, depressed patches of 
increased absorption; 1 or consequentially, as a reactive effect of 
falls of solidified substances from great heights in the solar atmo- 
sphere. 2 The two classes of phenomena, at any rate, stand in a 
most intimate relation ; they obey the same law of periodicity, and 
are confined 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 genera, 
follows that of faculae much more closely than that of spots. From 
Father Secchi's and Professor Respighi's observations, 1869-71, were 
derived the first clear ideas on the subject, which have been sup- 
plemented and modified by the later researches of Professors Tacchini 
and Riccd at Rome and Palermo. The results are somewhat com- 
plicated, 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. 3 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 occur- 
rence. Distribution in time is governed by the spot-cycle, but the 
maximum lasts longer for prominences than for spots. 

The structure of the chromosphere was investigated in 1869 and 
subsequent years by Professor Respighi, director of the Capitoline 
Observatory, as well as by Spbrer, and Bredikhine of the Moscow 
Observatory. They found this supposed solar envelope to be of 
the same eruptive nature as the vast protrusions from it, and to be 
made up of a congeries of minute flames 4 set close together like 

1 Secchi, Le Soleil, t. ii., p. 294. 2 Loekyer, Chemistry of the Sun, p. 418. 

3 L'Astronmnie, August, 1884, p. 292 (Ricco) ; see also Evershed, Jour. British 
Astr. Ass., vol. ii., p. 174. 

4 Averaging about 100 miles across and 300 high. Le Soleil, t. ii. , p. 35. 



200 HISTORY OF ASTRONOMY part ii 

blades of grass. "The appearance," Professor Young writes, 1 
" 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,' 2 but Tacchini 
showed in 1876 that the deflections upon which this inference was 
based ceased to be visible as the spot-minimum drew near. 3 

Another peculiarity of the chromosphere, denoting the remote- 
ness of its character from that of a true atmosphere, 4 is the irregu- 
larity of its distribution over the sun's surface. There are no signs 
of its bulging out at the equator, as the laws of fluid equilibrium in 
a rotating mass would require ; but there are some that the fluctua- 
tions in its depth are connected with the phases of solar agitation. 
At times of minimum it seems to accumulate and concentrate its 
activity at the poles ; while maxima probably bring a more equable 
general distribution, with local depressions at the base of great 
prominences and above spots. 

A low-lying stratum of carbon- vapour was, in 1897, detected in 
the chromosphere by Professor Hale with a grating-spectroscope 
attached to the 40-inch Yerkes refractor. 5 The eclipse-photographs 
of 1893 disclosed to Hartley's examination the presence there of 
gallium ; 6 and those taken by Evershed in 1898 were found by 
Jewell 7 to be crowded with ultra-violet lines of the equally rare 
metal scandium. The general rule has been laid down by Sir 
Norman Lockyer that the metallic radiations from the chromosphere 
are those " enhanced " in the electric spark. 8 Hence, the com- 
parative study of conditions prevalent in the arc and the spark has 
acquired great importance in solar physics. 

The reality of the appearance of violent disturbance presented by 
the " flaming " kind of prominence can be tested in a very remark- 
able manner. Christian Doppler, 9 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 move- 

1 The Sun, p. 192. 2 Astr. Nach., No. 1,854. 

Mem. degli Spettroscopisti Italiani, t. v., p. 4 ; Secchi, ibid., t. vi., 
p. 56. 

4 Its non-atmospheric character was early defined by Proctor, Month. Not., 
vol. xxxi., p. 196. 

5 Astroph. Jour., vol. vi., p. 412. 6 Ibid., vol. xi., p. 165. 

7 Ibid., p. 243. 8 Sun > s pi ace in Nature, pp. Ill, 288. 

Abh. d. Kim. Bohm. Ges. d. Wiss., Bd. ii., 1841-42. p. 467. 



chap, iv SOLAR SPECTROSCOPY 201 

ments 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 decreasing it. In the one case, the 
vibrating body pursues and crowds together the waves emanating 
from it; in the other, it retreats from them, and so lengthens out 
the 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, anyone 
can convince himself that the effect he predicted is a real one, by 
listening to the alternate shrilling and sinking of the steam-whistle 
when an express train rushes through a station. But in applying 
this principle to the colours of stars he went widely astray ; for he 
omitted from consideration the double range of invisible vibrations 
which partake of, and to the eye exactly compensate, changes of 
refrangibility in the visible rays. There is, then, no possibility of 
finding a criterion of velocity in the hue of bodies shining, like the 
sun and stars, with continuous light. The entire spectrum is 
slightly shifted up or down in the scale of refrangibility; certain 
rays normally visible become exalted or degraded (as the case may 
be) into invisibility, and certain other rays at the opposite end 
undergo the converse process ; but the sum-total of impressions on 
the retina continues the same. 

We are not, however, without the means of measuring this sub- 
sensible transportation of the light-gamut. Once more the wonder- 
ful Fraunhofer lines came to the rescue. They were called by the 
earlier physicists "fixed lines;" but it is just because they are not 
fixed that, in this instance, we find them useful. They share, and 
in sharing betray, the general shift of the spectrum. This aspect of 
Doppler's principle was adverted to by Fizeau in 1848, 1 and 
the first tangible results in the estimation of movements of 
approach and recession between the earth and the stars, were com- 
municated by Sir William Huggins to the Royal Society, April 23, 

1 In a paper read before the Societe Philomathique de Paris, December 23, 1848, 
and first published in extenso in Ann. de Chim. et de Phys., t. xix., p. 211 (1870). 
Hippolyte Fizeau died in September, 1896. 



202 HISTORY OF ASTRONOMY part ii 

1868. Eighteen months later, Zollner devised his " reversion- 
spectroscope "* for doubling the measurable effects of line-displace- 
ments ; aided by which ingenious instrument, and following a 
suggestion of its inventor, Professor H. C. Vogel succeeded at 
Bothkamp, June 9, 1871, 2 in detecting effects of that nature due to 
the solar rotation. This application constitutes at once the test and 
the triumph of the method. 3 

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 cor- 
responding to this velocity are very small ; so small that it seems 
hardly credible that they should have been laid bare to perception. 
They amount to but T i^th part of the interval between the two 
constituents of the D-line of sodium ; and the D-line of sodium 
itself can be separated into a pair only by a powerful spectroscope. 
Nevertheless, Professor Young 4 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, then 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 instru- 
ment rotated through 90, so as to bring its luminous supplies 
from opposite extremities of the equator, than the same rays became 
perceptibly " notched." The telluric lines, meanwhile, remained 
unaffected, so as to be " virtually mapped" by the process. 5 This 
rapid and unfailing mode of distinction was used by Cornu with 
perfect ease during his investigation of atmospheric absorption near 
Loiret in August and September, 1883. 6 

A beautiful experiment of the same kind was performed by 
M. Thollon, of M. Bischoffsheim's observatory at Nice, in the 
summer of 1880. 7 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 

1 Asir. Nach., No. 1,772. 2 Ibid., No. 1,864. 

3 A. Cornu, Sur la Mtthode Doppler-Fizeau, p. D. 23. 

4 Am. Jour, of Sc. y vol. xii., p. 321. 5 Ibid., vol. xiv., p. 140. 

6 Bull. Astronom.., February, 1884, p. 77. 

7 Comptes Eendus, t. xci., p. 368. 



chap, iv SOLAR SPECTROSCOPY 203 

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 Dunecht, December 14, 1883, 
when it was noticed that a strong iron-line in the yellow part of 
the solar spectrum is permanently double on the sun's eastern, 
but single on his western limb; 1 opposite motion-displacements 
bringing about this curious effect of coincidence with, and separation 
from, an adjacent stationary line of our own atmosphere's produc- 
tion, according as the spectrum is derived from the retreating or 
advancing 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 they already occupy an im- 
portant place among astronomical data. 

The subtle method of which they served to assure the validity 
was employed in 1887-9 by M. Duner to test and extend 
Carrington's and Sporer's conclusions as to the anomalous nature of 
the sun's axial movement. 2 His observations for the purpose, made 
with a fine diffraction-spectroscope, just then mounted at the 
observatory of Upsala, were published in 1891. 3 Their upshot was 
to confirm and widen the law of retardation with increasing latitude 
derived from the progressive motions of spots. Determinations 
made within 15 of the pole, consequently far beyond the 
region of spots, gave a rotation-period of 38, that of the 
equatorial belt being of 25J days. Spots near the equator indeed 
complete their rounds in a period shorter by at least half a day ; 
and proportionate differences were found to exist elsewhere in 
corresponding latitudes; but Duner's observations, it must be 
remembered, apply to a distinct part of the complex solar machine 
from the disturbed photospheric surface. It is amply possible that 
the absorptive strata producing the Fraunhofer lines, significant, by 
their varying displacements at either limb, of the inferred varying 
rates of rotation, may gyrate more slowly than the spot-generating 
level. Moreover, faculse appear to move at a quicker pace than 
either f so that we have, for three solar formations, three different 
periods of average rotation, the shortest of which belongs to the 
faculse, one of intermediate length to the spots, and the most 
protracted to the reversing layer. All, however, agree in lengthen- 
ing progressively from the equator towards the poles. Professor 
Holden aptly compared the sun to "a vast whirlpool where the 

1 Month. Not., vol. xliv., p. 170. 2 See ante, p. 147. 

* Recherehes sur la Rotation du Soldi, Upsal, 1891. 

4 Harder, Astr. Nach., No. 3,026 ; Stratonoff, Ibid., No. 3,344. 



2o 4 HISTORY OF ASTRONOMY part ii 

velocities of rotation depend not only on the situation of the 
rotating masses as to latitude, but also as to depth beneath the 
exterior surface." 1 

Sir Norman Lockyer 2 promptly perceived the applicability of 
the surprising discovery of line-shiftings through end-on motion to 
the study of prominences, the discontinuous light of which affords 
precisely the same means of detecting movement without seeming 
change of place, as do lines of absorption in a continuous spectrum. 
Indeed, his observations at the sun's edge almost compelled re- 
course 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 fall- 
ing 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 1 20 miles a second ; while their undisturbed sections prove the 
persistence of a " heart of peace " in the midst of that unimaginable 
fiery whirlwind. Velocities up to 250 miles a second, or 15,000 times 
that of an express train at the top of its speed, were thus observed 
by Young during his trip to Mount Sherman, August 2, 1872; and 
these were actually doubled in an extraordinary outburst observed 
by Father Jules Fenyi, on June 17, 1891, at the Haynald Observa- 
tory in Hungary, as well as by M. Trouvelot at Meudon. 3 

Motions ascertainable in this way near the limb are, of course, 
horizontal as regards the sun's surface ; the analogies 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 elucida- 
tion of sun-spot theories. Indeed, they almost certainly take 
place at a great height above the actual openings 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 

1 Publ. Astr. Pacific Soc. t vol. ii., p. 193. 

2 Proc. Roy. Society, vols, xvii., p. 415 ; xviii., p. 120. 

3 Comptes fiendus, t. cxii., p. 1421 ; t. cxiii., p. 310. 



chap, iv SOLAR SPECTROSCOPY 205 

glowing matter has, by the aid of the spectroscope, been watched in 
its ascent. 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 compared by Lockyer to that of a grove of banyans. Called 
away for a few minutes at 12.30, on returning at 12.55 the observer 
found 

"That in the meantime the whole thing had been literally blown 
to shreds by some inconceivable uprush from beneath. In place of 
the quiet cloud I had left, the air, if I may use the expression, was 
filled with flying debris a mass of detached, vertical, fusiform 
filaments, each from 10" to 30" long by 2" or 3" wide, 1 brighter and 
closer together where the pillars had formerly stood, and rapidly 
ascending. They rose, with a velocity estimated at 166 miles a 
second, to fully 200,000 miles above the sun's surface, then gradually 
faded away like a dissolving cloud, and at 1.15 only a few filmy 
wisps, with some brighter streamers low down near the photosphere, 
remained to mark the place." 2 

A velocity of projection of at least 500 miles per second was, by 
Proctor's" calculation, required to account for this extrarodinary 
display, to which the earth immediately responded by a magnetic 
disturbance, and a fine aurora. It has proved by no means an 
isolated occurrence. Young saw its main features repeated, 
October 7, 1881, 4 on a still vaster scale; for the exploded pro- 
minence attained, this time, an altitude of 350,000 miles the 
highest yet chronicled. Lockyer, moreover, has seen a prominence 
40,000 miles high shattered in ten minutes ; while uprushes 
have been witnessed by Eespighi, 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 383 miles a second would, if it encountered no resistance, escape 
for ever from his control, it is obvious that we have, in the enormous 
forces of eruption or repulsion manifested 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, 5 Faye, 6 and others have sought to do, by substi- 
tuting for the rush of matter in motion, progressive illumination 

1 At the sun's distance, one second of arc represents about 450 miles. 

2 Amer. Jour, of Sc, vol. ii., p. 468, 1871. 

3 Month. Not., vol. xxxii., p. 51. 4 Nature, vol. xxiii., p. 281. 
5 Comptes ReTidus, t. lxxxvii., p. 532. 6 Ibid., t. xcvi., p. 359. 



206 HISTORY OF ASTRONOMY part h 

through electric discharges, chemical processes, 1 or even through 
the mere reheating of gases cooled by expansion. 2 All the appear- 
ances 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 12th of December, 1878, Sir Norman Lockyer formally 
expounded before the Royal Society his hypothesis of the com- 
pound nature of the " chemical elements." 3 An hypothesis, it is 
true, over and over again propounded from the simply terrestrial 
point of view. What was novel was the supra-terrestrial evidence 
adduced in its support; and even this had been, in a general and 
speculative way, anticipated by Professor F. W. Clarke of Wash- 
ington.* Lockyer had been led to his conclusion along 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 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 substances as oxygen, nitrogen, sulphur, and 
chlorine, into simpler constituents possessing unknown spectra ; 
while metals were at that time still admitted to be capable of exist- 
ing there in a state of integrity. Three years later he shifted his 
position onward. He announced, as the result of a comparative 
study of the Fraunhofer and electric-arc spectra of calcium, that 
the " molecular grouping " of that metal, which at low 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. 5 This 
came to be regarded by him as " a truly typical case." 6 

During four years (1875-78 inclusive) this diligent observer was 
engaged in mapping a section of the more refrangible part of the 
solar spectrum (wave-lengths 3,800-4,000) 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 photography, of metallic spectra, seemed to indicate 
the existence of what he called "basic lines." These held their 
ground persistently in the spectra of two or more metals after all 

1 A. Brester, Theorie du Soleil, p. 66. 

2 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. 

3 Proc. Roy. Soc, vol. xxviii., p. 157. 

4 "Evolution and the Spectroscope," Pop. Science Monthly, January, 1873. 

5 Proc. Roy. Soc, vol. xxiv., p. 353. These are the H and K of prominences. 
H. W. Vogel discovered in 1879 a hydrogen-line nearly coincident with H 
(Monatsb. Preuss. Ak., February, 1879, p. 118). 

6 Proc. Roy. Soc, vol. xxviii., p. 444. 



chap, iv SOLAR SPECTROSCOPY 207 

possible " impurities " had been eliminated, and were therefore 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. 

Later inquiries have shown, however, that between the spectral 
lines of different substances there are probably no absolute coinci- 
dences. " 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 resisted 
Thollon's and Young's powerful spectroscopes ; and the process of 
resolution was completed by Rowland. Thus the argument from 
community of lines to community of substance has virtually 
collapsed. It was replaced by one founded on certain periodical 
changes in the spectra of sun-spots. They emerged from a series 
of observations begun at South Kensington under Sir Norman 
Lockyer's direction in 1879, and continued for fifteen years. 2 

The principle of the method employed is this. The whole range 
of Fraunhofer lines is visible when the light from a spot is examined 
with the spectroscope ; but relatively few are widened. Now these 
widened lines alone constitute (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 over-lying strata of the normal 
solar atmosphere. Here then we have the criterion that was wanted 
the means of distinguishing, spectroscopically and chemically, 
between the cavity and the absorbing layers piled up above it. By 
its persistent employment some marked peculiarities have been 
brought out, such as the unfamiliar character of numerous lines in 
spot-spectra, especially at epochs of disturbance ; and the strange 
individuality in the behaviour of every one of these darkened and 
distended rays. Each seems to act on its own account ; it comports 
itself as if it were the sole representative of the substance emitting 
it ; its appearance is unconditioned by that of any of its terrestrial 
companions in the same spectrum. 

The most curious fact, however, elicited by these inquiries was 
that of the attendance of chemical vicissitudes upon the advance of 
the sun-spot period. As the maximum approached, unknown re- 
placed known components of the spot-spectra in a most pronounced 
and unmistakable way. 3 It seemed as if the vapours emitting lines of 
iron, titanium, nickel, etc., had ceased to exist as such, and their 

1 Many of these were referred by Lockyer himself, who first sifted the 
matter, to traces of the metals concerned. 

2 Chemistry of the Sun, p. 312 ; Proc. Roy. Society, vol. lvii., p. 199. 

3 Lockyer's Chemistry of the Sun, p. 324. 



2o8 HISTORY OF ASTRONOMY part ii 

room been taken by others, total strangers in terrestrial laboratories. 
These were held by Lockyer to be simply the finer constituents of 
their predecessors, dissociation having been effected by the higher 
temperature ensuing upon increased solar activity. But Father 
Cortie's supplementary investigations at Stonyhurst 1 modified, while 
they in the main substantiated, the South Kensington results. They 
showed that the substitution of unknown for known lines char- 
acterizes disturbed spots, at all stages of the solar cycle, so that no 
systematic course of chemical change can be said to affect the sun 
as a whole. They showed further 2 from evidence independent of 
that obtained by Young in 1892 3 the remarkable conspicuousness 
in spot-spectra of vanadium lines excessively faint in the Fraunhofer 
spectrum. Lockyer's " unknown lines " may probably thus be 
accounted for. They represent absorption, not by new, but by 
scarce elements, especially, Father Cortie thinks, those with atomic 
weights of about 50. The circumstance of their development in solar 
commotions, largely to the exclusion of iron, is none the less curious ; 
but it cannot be explained by any process of dissociation. 

The theory has, however, to be considered under still another 
aspect. It frequently happens that the contortions or displace- 
ments due to motion are seen to affect a single line belonging 
to a particular substance, while the other lines of that same substance 
remain imperturbable. Now, how is this most singular fact, 
which seems at first sight to imply that a body may be at rest 
and in motion at one and the same instant, to be accounted for 1 ? 
It is accounted for, on the present 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 
penetrates, at the sun's edge, a depth of about 300,000 miles, 4 all 
the incandescent materials separately occurring along which line are 
projected into a single "flame " or "cloud," it will be perceived that 
there is ample room for diversities of behaviour. 

The alternative mode of escape from the perplexity consists in 
assuming that the vapour in motion is rendered luminous under 

1 Month. Not., vol. 1L, p. 76. 2 Ibid., vol. lviii., p. 370. 

3 Astr. and Astrophysics, vol. xi., p. 615. 

4 Thollon's estimate (Comptes Jlendus, t. xcvii., p. 902) of 300,000 kilo-metres, 
seems considerably too low. Limiting the "average prominence region" to a 
shell 54,000 miles deep (2' of arc as seen from the earth), the visual line will, 
at mid-height (27,000 miles from the sun's surface), travel through (in round 
numbers) 320,000 miles of that region. 



chap, iv SOLAR SPECTROSCOPY 209 

conditions which reduce its spectrum to a few rays, the unaffected 
lines being derived from a totally distinct mass of the same sub- 
stance shining with its ordinary emissions. 1 Thus, calcium can be 
rendered virtually monochromatic by attenuation, and analogous 
cases are not rare. 

Sir Norman Lockyer only asks us to believe that effects which 
follow certain causes on the earth are carried a stage further in the 
sun, where the same causes must be vastly intensified. 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 witlxout the range of our resources 1 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. 

There are indeed theoretical objections to it which, though 
probably not insuperable, are unquestionably 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 absolutely such to contain, that is, the " irreducible minimum " 
of material substance ; but we do certainly gather from it 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 Lockyer's results menace us, is 
at first sight startling. They may lead, we are told, to eventual 
unification, but the prospect appears remote. Their only obvious 
outcome is the disruption into several constituents of each terres- 
trial " element." The components of iron alone 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. Sir Norman Lockyer interprets 
the observed phenomena as indicating the successive combinations, 
in varying proportions, of a very few original ingredients; 2 but no 
definite sign of their existence is perceptible ; " protyle " seems 
likely long to evade recognition ; and the only intelligible under- 
lying principle for the reasonings employed that of " one line, one 
element " implies a throng beyond counting of formative material 
units. 

Thus, added complexity is substituted for that fundamental unity 
of matter which has long formed the dream of speculators. And it 

1 Liveing and Dewar, Phil. Mag., vol. xvi. (5th ser.), p. 407. 

2 Chemistry of the Sun, p. 260. 

14 



2io HISTORY OF ASTRONOMY part ii 

is extremely remarkable that Sir William Crookes, working along 
totally different lines, has been led to analogous conclusions. To 
take only one example. As the outcome of extremely delicate 
operations of sifting and testing carried on for years, he finds that the 
metal yttrium splits up into five, if not eight constituents. 1 Evi- 
dently, old notions are doomed, nor are any preconceived ones likely 
to take their place. It would seem, on the contrary, as if their com- 
plete reconstruction were at hand. Subversive facts are steadily 
accumulating; the revolutionary ideas springing from them tend, 
if we interpret them aright, towards the substitution of electrical 
for chemical theories of matter. Dissociation by the brute force of 
heat is already nearly superseded, in the thoughts of physicists, by 
the more delicate process of " ionisation." Precisely what this implies 
and involves we do not know ; but the symptoms of its occurrence 
are probably altogether different from those gathered by Sir Norman 
Lockyer from the collation of celestial spectra. 

A. J. Angstrom of Upsala takes rank after Kirchhoff as a sub- 
ordinate founder, so to speak, of solar spectroscopy. His great map 
of the "normal" solar spectrum 2 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. For more than a score of years it 
held its ground as the universal standard of reference in all spectro- 
scopic inquiries within the range of the visible emanations. Those 
that are invisible by reason of the quickness of their vibrations were 
mapped by Dr. Henry Draper, of New York, in 1873, and with 
superior accuracy by M. Cornu in 1881. The infra-red part of the 
spectrum, investigated by Langley, Abney, and Knut Angstrom, 
reaches perhaps no definite end. The radiations oscillating too 
slowly to affect the eye as light may pass by insensible gradations 
into the long Hertzian waves of electricity. 3 

Professor Rowland's photographic map of the solar spectrum, 
published in 1886, and in a second enlarged edition in 1889, opened 
fresh possibilities for its study, from far down in the red to high up 
in the ultra-violet, and the accompanying scale of absolute wave- 
lengths 4 has been, with trifling modifications, universally adopted. 

1 Nature, October 14, 1886. 

2 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 refraction spectrum (upon 
which Kirchhoff s map was founded), the relative positions of the lines vary with 
the material of the prisms. 

3 Scheiner, Die Spectralanalyse der Gestirne, p. 168. 

4 Phil. Mag., vol. xxvii., p. 479. 



chap, iv SOLAR SPECTROSCOPY 211 

His new table of standard solar lines was published in 1893. 1 
Through his work, indeed, knowledge of the solar spectrum so far 
outstripped knowledge of terrestrial spectra, that the recognition of 
their common constituents was hampered by intolerable uncertainties. 
Thousands of the solar lines charted with minute precision re- 
mained unidentified for want of a corresponding precision in the 
registration of metallic lines. Rowland himself, however, undertook 
to provide a remedy. Aided by Lewis E. Jewell, he redetermined, 
at the Johns Hopkins University, the wave-lengths of about 16,000 
solar lines, 2 photographing for comparison with them the spectra of 
all the known chemical elements except gallium, of which he could 
procure no specimen. The labour of collation was well advanced 
when he died at the age of fifty -two, April 16, 1901. Investiga- 
tions of metallic arc-spectra have also been carried out with signal 
success by Hasselberg, 3 Kayser and Runge, 0. Lohse, 4 and others. 

Another condition sine qua non of progress in this department is 
the separation of true solar lines from those produced by absorption 
in our own atmosphere. And here little remains to be done. 
Thollon's great Atlas 5 was designed for this purpose of discrimina- 
tion. Each of its thirty -three maps exhibits in quadruplicate a 
subdivision of the solar spectrum under varied conditions of weather 
and zenith-distance. Telluric effects are thus made easily legible, 
and they account wholly for 866, partly for 246, out of a total of 
3,200 lines. But the death of the artist, April 8, 1887, unfortunately 
interrupted the half-finished task of the last seven years of his life. 
A most satisfactory record, meanwhile, of selective atmospheric 
action has been supplied by the experiments and determinations of 
Janssen, Cornu and Egoroff, by Dr. Becker's drawings, 6 and Mr. 
McClean's photographs of the analysed light of the sun at high, 
low, and medium altitudes ; and the autographic pictures obtained by 
Mr. George Higgs, of Liverpool, of certain rhythmical groups in the 
red, emerging with surprising strength near sunset, excite general 
and well-deserved admiration. 7 The main interest, however, of all 
these documents resides in the information afforded by them regard- 
ing the chemistry of the sun. 

The discovery that hydrogen exists in the atmosphere of the 
sun was made by Angstrom in 1862. His list of solar elements 

1 Astr. and Astrophysics, vol. xii., p. 321 ; Frost-Scheiner, Astr. Spectr., p. 363. 

2 Published in Astroph. Jour., vols. i. to vi. 

3 Astr. and Astrophysics, vol. xi., p. 793. 

4 Astroph. Jour., vol. vi., p. 95. 

6 Annales de V Obserratoire de Nice, t. iii., 1890. 

6 Trans. Royal Society of Edinburgh, vol. xxxvi., p. 99. 

7 Rev. A. L. Cortie, Astr. and Astrophysics, vol. xi., p. 401. Specimens of his 
photographs were given by Ranyard in Knowledge, vol. xiii., p. 212. 

142 



212 HISTORY OF ASTRONOMY part n 

published in that year, 1 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 
among 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 were established j and in 
1866 a fourth hydrogen line in the extreme violet (named h) was 
detected in the solar spectrum. With Thalen, he besides added 
manganese, titanium, and cobalt 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. 2 

Thus, when Sir Norman 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 to increase the 
list provisionally to thirty-three, 3 all except hydrogen metals. This 
rapid success was due to his adoption of the test of length in lieu of 
that of strength in the comparison of lines. He measured their 
relative significance, in other words, rather by their persistence 
through a wide range of temperature, than by their brilliancy at 
any one temperature. The distinction was easily drawn. Photo- 
graphs of the electric arc, in which any given metal had been volati- 
lised, showed some of the rays emitted by it stretching across the 
axis of the light to a considerable distance on either side, while many 
others clung more or less closely to its central hottest core. The 
former "long lines," regarded as certainly representative, were 
those primarily sought in the solar spectrum ; while the attendant 
11 short lines," often, in point of fact, due to foreign admixtures, were 
set aside as likely to be misleading. 4 The criterion is a valuable 
one, and its employment has greatly helped to quicken the progress 
of solar chemistry. 

Carbon was the first non-metallic element discovered in the 
sun. Messrs. Trowbridge and Hutchins of Harvard College 
concluded in 1887, 5 on the ground of certain spectral coincidences, 
that this protean substance is vaporised in the solar atmosphere 
at a temperature approximately that of the voltaic arc. Partial 
evidence to the same effect had earlier been alleged by Lockyer, 
as well as by Liveing and Dewar; and the case was rendered 

1 Ann. d. Phys., Bd. cxvii., p. 296. 2 Comptes Rendus, t. lxiii., p. 647. 

3 Ibid., t. lxxxvi., p. 317. Some half dozen of these identifications have 
proved fallacious. 

4 Chemistry of the Sun, p. 143. 

5 Amer. Jour, of Science, vol. xxxiv., p. 348. 



chap, iv SOLAR SPECTROSCOPY 213 

tolerably complete by photographs taken by Kayser and Runge 
in 1889. 1 It was by Professor Rowland shown to be irresistible. 
Two hundred carbon-lines were, through his comparisons, sifted 
out from sunlight, and it contains others significant of the presence 
of silicon a related substance, and one as important to rock- 
building on the earth, as carbon is to the maintenance of life. 
The general result of Rowland's labours was the establishment 
among solar materials, not only of these two out of the fourteen 
metalloids, or non-metallic substances, but of thirty-three metals, 
including silver and tin. Gold, mercury, bismuth, antimony, and 
arsenic were discarded from the catalogue ; platinum and uranium, 
with six other metals, remained doubtful ; while iron was recorded 
as crowding the spectrum with over two thousand obscure rays. 2 
Gallium-absorption was detected in it by Hartley and Ramage in 
1889. 3 

Dr. Henry Draper 4 announced, in 1877, his imagined discovery, 
in the solar spectrum, of eighteen especially brilliant spaces cor- 
responding to oxygen-emissions. But the agreement proved, when 
put to the test of very high dispersion, to be wholly illusory. 5 Nor 
has it yet been found possible to identify, in analysed sunlight, any 
significant bright beams. 6 

The book of solar chemistry must be read in characters exclusively 
of absorption. Nevertheless, the whole truth is unlikely to be 
written there. That a substance displays none of its distinctive 
beams in the spectrum of the sun or of a star, affords scarcely a pre- 
sumption against its presence. For it may be situated below the 
level where absorption occurs, or under a pressure such as to efface 
lines by widening and weakening them ; it may be at a temperature 
so high that it gives out more light than it takes up, and yet its 
incandescence may be masked by the absorption of other bodies ; 
finally, it may just balance absorption by emission, with the result 
of complete spectral neutrality. An instructive example is that 
of the chromospheric element helium. Father Secchi remarked 
in 1868 7 that there is no dark line in the solar spectrum matching 
its light ; and his observation has been fully confirmed. 8 Helium- 

1 Berlin Abhandlungen, 1889. 

Amer. Jour. 0/ Science, vol. xli., p. 243. See Appendix, Table II. 

3 Astrophy. Jour., vol. ix., p. 219 ; Fowler, Knowledge, vol. xxiii., p. 11. 

4 Amer. Jour, of Science, vol. xiv., p. 89 ; Nature, vol. xvi., p. 364 ; Month. 
Not., vol. xxxix., p. 440. 

5 Month. Not., vol. xxxviii., p. 473; Trowbridge and Hutchins, Amer. Jour, 
of Science, vol. xxxiv., p. 263. 

6 Scheiner, Die Spectralanalyse, p. 180. 

7 Comptes Rendus, t. lxvii., p. 1123. 

8 Rev. A. L. Cortie, Month. Not., vol. li., p. 18. 



2i 4 HISTORY OF ASTRONOMY part ii 

absorption is, however, occasionally noticed in the penumbrse 
of spots. 1 

Our terrestrial vital element might then easily subsist unrecognis- 
ably in the sun. The inner organisation of the oxygen molecule is 
a considerably plastic one. It is readily modified by heat, and these 
modifications are reflected in its varying modes of radiating light. 
Dr. Schuster enumerated in 1879 2 four distinct oxygen spectra, 
corresponding to various stages of temperature, or phases of 
electrical excitement; and a fifth has been added by M. Egoroffs 
discovery in 1883 3 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. These persist down to 
the lowest temperatures, and even survive a change of state. They 
are produced essentially the same by liquid, as by aerial oxygen. 4 

It seemed, however, possible to M. Janssen that these bands 
owned a joint solar and terrestrial origin. Oxygen in a fit condition 
to produce them might, he considered, exist in the outer atmosphere 
of the sun; and he resolved to decide the point. No one could 
bring more skill and experience to bear upon it than he. 5 By 
observations on the summit of the Faulhorn, as well as by direct 
experiment, he demonstrated, nearly thirty years ago, the leading 
part played by water- vapour in generating the atmospheric spectrum ; 
and he had recourse to similar means for appraising the share in it 
assignable to oxygen. An electric beam, transmitted from the 
Eiffel Tower to Meudon in the summer of 1888, having passed 
through a weight of oxygen about equal to that piled above the 
surface of the earth, showed the groups A and B just as they appear 
in the high-sun spectrum. 6 Atmospheric action is then adequate to 
produce them. But M. Janssen desired to prove, in addition, that 
they diminish proportionately to its amount. His ascent of Mont 
Blanc 7 in 1890 was undertaken with this object. It was perfectly 
successful. In the solar spectrum, examined from that eminence, 
oxygen-absorption was so much enfeebled as to leave no possible 
doubt of its purely telluric origin. Under another form, neverthe- 
less, it has been detected as indubitably solar. A triplet of dark 
lines low down in the red, photographed from the sun by Higgs and 

1 Young, The Sun, p. 135; Hale, Astr. and Astrophysics, vol. xi., p. 312; 
Buss, Jour. Brit. Astr. Ass., vol. ix., p. 253. 
3 Phil. Trans., vol. clxx., p. 46. 

3 Comptes Rendus, t. xcvii., p. 555 ; t. ci., p. 1145. 

4 Liveing and Dewar, Astr. and Astrophysics, vol. xi., p. 705. 

5 Comptes fiendus, t. Ix., p. 213 ; t. lxiii., p. 289. 
Ibid., t. cviii., p. 1035. 

7 Ibid., t. cxi., p. 431. 



chap, iv SOLAR SPECTROSCOPY 215 

McClean, was clearly identified by Runge and Paschen in 1896 1 with 
the fundamental group of an oxygen series, first seen by Piazzi 
Smyth in the spectrum of a vacuum-tube in 1883. 2 The pabulum 
mtce of our earth is then to some slight extent effective in arresting 
transmitted sunlight, and oxygen must be classed as a solar element. 
The rays of the sun, besides being stopped selectively in our 
atmosphere, suffer also a marked general absorption. This tells 
chiefly upon the shortest wave-lengths ; the ultra-violet spectrum is 
in fact closed, as if by the interposition of an opaque screen. Nor 
does the screen appear very sensibly less opaque from an elevation 
of 10,000 feet. Dr. Simony's spectral photographs, taken on the 
Peak of Teneriffe, 3 extended but slightly further up than M. Cornu's, 
taken in the valley of the Loire. Could the veil be withdrawn, 
some indications as to the originating temperature of the solar 
spectrum might be gathered from its range, since the proportion 
of quick vibrations given out by a glowing body grows with the 
intensity of its incandescence. And this brings us to the subject of 
our next Chapter. 

Astroph. Jour., vols, iv., p. 317 ; vi., p. 426. 
Trans. Roy. Soc. Edin., vol. xxxii., p. 452. 
* Comptes Rendiis, t. cxi., p. 941 ; Huggins, Proc. Roy. Soc, vol. xlvi., p. 168. 



CHAPTER V 

TEMPERATURE OF THE SUN 

Newton was the first who attempted to measure the quantity of 
heat received by the earth from the sun. His object in making the 
experiment was to ascertain the temper&ture encountered by the 
comet of 1680 at its passage through perihelion. 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 2,000 times that of red-hot iron. 1 

Determinations of the sun's thermal power, made with some 
scientific exactness, date, however, from "837. A few days previous 
to the beginning of that year, Herschel began observing at the Cape 
of Good Hope with an " actinometer," and obtained results agreeing 
quite satisfactorily with those derived by Pouillet from experiments 
made in France some months later with a " pyrheliometer." 2 
Pouillet found that the vertical rays of the sun falling on each 
square centimetre of the earth's surface are competent (apart from 
atmospheric absorption) to raise the temperature of 1*7633 grammes 
of water one degree Centigrade per minute. This number (1-7633) 
he called the " solar constant " ; anc 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 ob- 
servations showed that nearly half the heat incident upon our 
atmosphere is stopped in its passage through it. 

Herschel got somewhat larger figures, though he assigned only 
a third as the spoil of the air. Taking a mean between his own and 
Pouillet's, he calculated that the ordinary expenditure 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, 
1 Principia, p. 498 (1st ed.). 3 Comptes Bendus, t. vii., p. 24. 



chap, v TEMPERATURE OF THE SUN 217 

putting it otherwise, that an ice-rod 45*3 miles across, continually- 
darted into the sun with the velocity of light, would scarcely 
consume, in dissolving, the thermal supplies now poured abroad 
into space. 1 It is nearly certain that this estimate should be 
increased by about two-thirds in order to bring it up to the 
truth. 

Nothing would, at first sight, appear simpler than to pass 
from a knowledge of solar emission a strictly measurable 
quantity to a knowledge of the solar temperature ; this being 
defined as the temperature to which a surface thickly coated with 
lamp-black (that is, of standard radiating power) should be raised to 
enable it to send us, from the sun's distance, the amount of heat 
actually received from the sun. Sir John Herschel showed that 
heat-rays at the sun's surface must be 92,000 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 
92,000 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 absorption, 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 pari passu that you have only to ascertain 
the quantity of heat received from, and the distance of a remote 
body in ofder to know how hot it is. 2 And the validity of this 
principle, known as "Newton's Law" of cooling, was never 
questioned until De la Koche pointed out, in 1812, 3 that it was 
approximately true only over a low range of temperature; while 
five years later, Dulong and Petit generalised experimental results 
into the rule, that while temperature grows by arithmetical, 
radiation increases by geometrical progression. 4 Adopting this 
formula, Pouillet derived from his observations on solar heat a solar 
temperature of somewhere between 1,461 and 1,761 C. 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 

1 Results of Astr. Observations, p. 446. 

8 ' ' Est enini calor solis ut radiorum densitas, hoc est, reciproce ut quadra turn 
distantiae locorum a sole." Principia, p. 508 (3d ed., 1726). 

3 Jour, de Physique, t. lxxv., p. 215. 

4 Ann. de Chimie, t. vii., 1817, p. 365. 



2i8 HISTORY OF ASTRONOMY part n 

diamond become rapidly vaporised, notwithstanding the great loss 
of heat by absorption, first in passing through the air, and again 
in traversing the lens. Pouillet's maximum is then manifestly too 
low, since it involves the absurdity of supposing a radiating mass 
capable of heating a distant body more than it is itself heated. 

Less demonstrably, but scarcely less surely, Mr. J. J. Waterston, 
who attacked the problem in 1860, erred in the opposite direction. 
Working up, on Newton's principle, data collected by himself in 
India and at Edinburgh, he got for the " potential temperature " of 
the sun 12,880,000 Fahr., 1 equivalent to 7,156,000 C. The phrase 
potential temperature (for which Violle substituted, in 1876, effective 
temperature) was designed to express the accumulation in a single 
surface, postulated for the sake of simplicity, of the radiations 
not improbably received from a multitude of separate solar layers 
reinforcing each other ; and might thus (it was explained) be con- 
siderably higher than the actual temperature of any one stratum. 

At Rome, in 1861, Father Secchi repeated Waterston's experi- 
ments, and reaffirmed his conclusion f while Soret's observations, 
made on the summit of Mont Blanc in 1867, 3 furnished him with 
materials for a fresh and even higher estimate of ten million degrees 
Centigrade. 4 Yet from the very same data, substituting Dulong 
and Petit's for Newton's law, Vicaire deduced in 1872 a provisional 
solar temperature of 1,398. 5 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 
reestablish the shaken credit of Newton's principle, and arrived, 
by its means, at a temperature of 4,000,000 Fahrenheit. 6 Subse- 
quently, an " underrated computation," based upon observation of the 
quantity of heat received by his " sun motor," gave him 3,000,000. 
And the result, as he insisted, followed inevitably from the principle 
that the temperature produced by radiant heat is proportional to its 
density, or inversely as its diffusion. 7 The principle, however, is 
demonstrably unsound. 

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) 

1 Phil. Mag., vol. xxiii. (4th ser.), p. 505. 

* Nuovo Cimento, t. xvi., p. 294. 3 Comptes Rendus, t. lxv., p. 526. 

4 The direct result of 5 million degrees was doubled in allowance for absorp- 
tion in the sun's own atmosphere. Comptes Rendus, t. lxxiv. , p. 26. 

5 Ibid., p. 31. 6 Nature, vols, iv., p. 204 ; v., p. 505. 
7 Ibid., vol. xxx., p. 467. 



chap, v TEMPERATURE OF THE SUN 219 

arrived at an effective temperature of 1,500 C, but considered that 
it might actually reach 2,500 C, if the emissive power of the photo- 
spheric clouds fell far short (as seemed probable) of the lamp-black 
standard. 1 Experiments made in April and May, 1881, giving a 
somewhat higher result, he raised this figure to 3,000 C. 2 

Appraisements so outrageously discordant as those of Waterston, 
Secchi, and Ericsson on the one hand, and those of the French 
savans on the other, served only to show that all were based upon a 
vicious principle. Professor F. Rosetti, 3 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 2,800 C. an estimate certainly not very far from the 
truth. But if the two systems of measurement applied to the sun 
be used to determine the heat of a solid body rendered incandescent 
in this flame, it comes out, by Newton's mode of calculation, 45,000 
C. ; by Dulong and Petit's, 870 C. 4 Both, then, are justly dis- 
carded, the first as convicted of exaggeration, the second of under- 
valuation. The formula substituted by Rosetti in 1878 was tested 
successfully up to 2,000 C. ; but since, like its predecessors, it was 
a purely empirical rule, guaranteed by no principle, and hence not 
to be trusted out of sight, it was, like them, liable to break down at 
still higher elevations. Radiation by this new prescription increases 
as the square of the absolute temperature that is, of the number of 
degrees counted from the "absolute zero" of -273 C. Its employ- 
ment gave 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. A "law of cooling," 
proposed by M. Stefan at Vienna in 1879, 5 was shown by Boltz- 
mann, many years later, to have a certain theoretical validity. 6 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 Violle's measures of solar 
radiative intensity, to be just 6,000 C. ; while M. H. Le Chatelier 7 

1 Ann. de Chim., t. x. (5th ser.), p. 361. 

2 Comptes Rendus, t. xcvi., p. 254. 3 Phil. Mag., vol. viii., p. 324, 1879. 
4 Ibid., p. 325. 5 Sitzungsberichte, Wien, Bd. lxxix., ii., p. 391. 

6 Wiedemann's Annalen, Bd. xxii., p. 291 ; Scheiner, Strahlung und Tem- 
peratur der Sonne, p. 27. 

7 Comptes Rendus, March 28, 1892 ; Aitr. and Astrophysics, toI. xi., p. 517. 



220 HISTOR Y OF ASTRONOMY part ii 

derived 7,600 from a formula, conveying an intricate and unaccount- 
able relation between the temperature of an incandescent body and 
the intensity of its red radiations. 

From a series of experiments carefully conducted at Daramona, 
Ireland, with a delicate thermal balance, of the kind invented by 
Boys and designated a "radio-micrometer," Messrs. Wilson and 
Gray arrived in 1893, with the aid of Stefan's Law, at a photo- 
spheric temperature of 7,400 C., 1 reduced by the first-named in- 
vestigator in 1901 to 6,590. 2 Dr. Paschen, of Hanover, on the 
other hand, ascribed to the sun a temperature of 5,000 from 
comparisons between solar radiative intensity and that of glowing 
platinum; 3 while F. W. Very showed in 1895 4 that a minimum 
value of 20,000 C. for the same datum resulted from Paschen's 
formula connecting temperature with the position of maximum 
spectral energy. 

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 formulae, he investigated their intensity at their source. 
He showed 5 that, taking prominences to be simple effects of the 
escape of powerfully compressed 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 development. These came out 27,700 C. for the strata 
lying immediately above, and 68,400 C. for the strata lying im- 
mediately 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, having regard to the enormous velocities of projection 
observed in the interim, fixed two million degrees Centigrade as the 
lowest internal temperature by which they could be accounted for j 
although admitting the photospheric condensations to be incom- 
patible with a higher external temperature than 50,000 to 100,000 C. 6 

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 assump- 
tion that they are nothing but phenomena of elasticity is a purely 
gratuitous one. Spectroscopic indications, again, give hope of 
eventually affording a fixed point of comparison with terrestrial 

1 Phil. Trans., vol. clxxxv., p. 361. 2 Proc. Roy. Society, December 12, 1901. 
3 Schemer, Temp, der Sonne, p. 36. ' Astroph. Jour., vol. ii., p. 318. 

8 Astr. Nach., Nos. 1,815-16. 6 L'Astronomie, September, 1884, p. 334. 



chap, v TEMPERATURE OF THE SUN 221 

heat sources; but their interpretation is still beset with uncer- 
tainties ; nor can, indeed, the expression of transcendental tempera- 
tures in degrees 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 radia- 
tive intensity than any mere estimates of temperature, was provided 
in some experiments made by Professor Langley in 1878. 1 Using 
means of unquestioned validity, he found the sun's disc to radiate 
87 times as much heat, and 5,300 times as much light as an equal 
area of metal in a Bessemer converter after the air-blast had con- 
tinued about twenty minutes. The brilliancy of the incandescent 
steel, nevertheless, was so blinding, that melted iron, flowing in a 
dazzling white-hot stream into the crucible, showed " deep brown by 
comparison, presenting a contrast like that of dark coffee poured 
into a white cup." Its temperature Avas estimated (not quite 
securely) 2 at about 2,000 C. ; and no allowances were made, in 
computing relative intensities, for atmospheric ravages on sun- 
light, for the extra impediments to its passage presented by the smoke- 
laden air of Pittsburgh, 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 telescope, 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, 161 3. 3 Father Scheiner, however, fully 
admitted it, and devoted some columns of his bulky tome to the 
attempt to find its appropriate 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, thermal, and chemical 
rays were carefully studied by Father Secchi, who in 1870 6 inferred 
the total absorption to be y^ of all radiations taken together, and 
added the important observation that the light from the limb is no 

1 Amer. Jour, of Science, vol. i. (3rd ser.), p. 653. 2 Young, The Sun, p. 310. 
3 Op., t. vi., p. 198. * Rosa Ursina, lib. iv., p. 618. 

5 Mtc. Cel., liv. x., p. 323. 6 Le Soleil (1st ed.), p. 136. 



222 HISTORY OF ASTRONOMY part ii 

longer white, but reddish-brown. Absorptive effects were thus seen 
to be unequally distributed ; and they 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 trans- 
mission. 

This was done by H. C. Vogel in 1877. 1 Using a polarising 
photometer, he found that only 13 per cent, of the violet rays 
escape at the edge of the solar disc, 16 of the blue and green, 
25 of the yellow, and 30 per cent, of the red. Midway between 
centre and limb, 88*7 of violet light and 96 -7 of red penetrate the 
absorbing envelope, the abolition of which would increase the 
intensity of the sun's visible spectrum above two and a half times 
in the most, and once and a half times in the least refrangible parts. 
The nucleus of a small spot was ascertained to be of the same 
luminous intensity as a portion of the unbroken surface about two 
and a half minutes from the limb. These experiments having been 
made during a spot-minimum when there is reason to think that 
absorption is below its average strength, Vogel suggested their 
repetition at a time of greater activity. They were extended to the 
heat-rays by Edwin B. Frost. Detailed inquiries made at Potsdam 
in 1892 2 went to show that, were the sun's atmosphere removed, 
his thermal power, as regards ourselves, would be increased 1*7 
times. They established, too, the practical uniformity in radiation 
of all parts of his disc. A confirmatory result was obtained about 
the same time by Wilson and Rambaut, who found that the unveiled 
sun would be once and a half times hotter than the actual sun. 3 

Professor Langley, now of Washington, gave to measures of the 
kind a refinement previously undreamt of. Reliable determinations 
of the " energy " of the individual spectral rays were, for the first 
time, rendered possible by his invention of the "bolometer" in 
1880. 4 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 of electricity 
is impeded at that point, and the included galvanometer records 
a disturbance of the electrical flow. Amounts of heat were thus 

1 Monatsber., Berlin, 1877, p. 104. 

3 Astr. Nach., Nos. 3,105-6 ; Astr. and Astrophysics, vol. xi., p. 720. 

* Proc. Roy. Irish Acad., vol. ii., No. 2, 1892. 

* Am. Jour, of Sc, vol. xxi., p. 187. 



chap, v TEMPERATURE OF THE SUN 223 

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; and further improvements rendered this 
marvellous instrument capable of thrilling to changes of temperature 
falling short of one ten-millionth of a degree Centigrade. 1 

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 among 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. 
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. 2 Thus the last shred of 
evidence in favour of the threefold division of solar radiations 
vanished, and it became obvious that the varying effects thermal, 
luminous, or chemical produced by them are due, not to any dis- 
tinction of quality in themselves, but to the different properties of 
the substances they impinge upon. They are simply bearers of 
energy, conveyed in shorter or longer vibrations ; the result in each 
separate case depending 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. 

A long series of experiments at Allegheny was completed in the 
summer of 1881 on the crest of Mount Whitney in the Sierra 
Nevada. Here, at an elevation of 14,887 feet, in the driest and 
purest air, perhaps, in the world, atmospheric absorptive inroads 
become less sensible, and the indications of the bolometer, conse- 
quently, 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 milli- 

1 Amer. Jour, of Science, vol. v., p. 245, 1898. 

2 For J. W. Draper's partial anticipation of this result, see Ibid., vol. iv., 
1872, p. 174. 



224 HISTORY OF ASTRONOMY part ii 

metre, or three " microns." The known extent of the solar spectrum 
was thus at once more than doubled. Its visible portion covers a 
range of about one octave; bolometric indications already in 1884 
comprised between three and four. The great importance of the 
newly explored region appears from the fact that three-fourths of 
the entire energy of sunlight reside in the infra-red, while scarcely 
more than one-hundredth part of that amount is found in the better 
known ultra-violet space. 1 These curious facts were reinforced, in 
1886, 2 by further particulars learned with the help of rock-salt 
lenses and prisms, glass being impervious to very slow, as to very 
rapid vibrations. Traces were thus detected of solar heat distri- 
buted into bands of transmission alternating with bands of atmo- 
spheric absorption, far beyond the measurable limit of 5*3 microns. 

In 1894, Langley described at the Oxford Meeting of the British 
Association 3 his new " holographic " researches, in which the sensi- 
tive plate was substituted for the eye in recording deflections of the 
galvanometer responding to variations of invisible heat. Finally, 
in 1901, 4 he embodied in a splendid map of the infra-red spectrum 
740 absorption-lines of determinate wave-lengths, ranging from 
0'76 to 5*3 microns. Their chemical origin, indeed, remains almost 
entirely unknown, no extensive investigations having yet been 
undertaken of the slower vibrations distinctive of particular sub- 
stances ; but there is evidence that seven of the nine great bands 
crossing the " new spectrum " (as Langley calls it) 5 are telluric, and 
subject to seasonal change. Here, then, he thought, might eventually 
be found a sure standing-ground for vitally important previsions of 
famines, droughts, and bonanza-crops. 

Atmospheric absorption had never before been studied with such 
precision as it was by 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 pre- 
dominantly the quicker vibrations is shared, as both Vogel and 

1 Phil. Mag., vol. xiv., p. 179, 1883. 

2 "The Solar and the Lunar Spectrum," Memoirs National Acad, of Science, 
vol. xxxii. ; "On hitherto Unrecognised Wave-lengths," Amer. Jour, of Science, 
vol. xxxii., August, 1886. 

3 Astroph. Jour., vol. i., p. 162. 

4 Annals of the Smithsonian Astroph. Observatory, vol. i. ; Comptes liendus, 
t. cxxxi., p. 734 ; Astroph. Jour., vol. iii., p. 63. 

5 Phil. Mag., July, 1901. 



chap, v TEMPERATURE OF THE SUN 225 

Langley 1 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 three or four times more brilliant, but 
in colour distinctly greenish-blue. 2 

The fact of the uncovered sun being "blue has an important 
bearing upon the question of his temperature, to afford a somewhat 
more secure answer to which was the ultimate object of Professor 
Langley's persevering researches; for it is well known that as 
bodies grow hotter, the proportionate representation in their 
spectra of the more refrangible rays becomes greater. The lowest 
stage of incandescence is the familiar one of red heat. As it gains 
intensity, the quicker vibrations come in, and an optical balance of 
sensation is established at white heat. The final term of blue heat, 
as we now know, is attained by the photosphere. On this ground 
alone, then, of the large original preponderance of blue light, we 
must raise our estimate of solar heat; and actual measurements 
show the same upward tendency. Until quite lately, Pouillet's 
figure of 1*7 calories per minute per square centimetre of terrestrial 
surface, was the received value for the "solar constant." Forbes 
had, it is true, got 2*85 from observations on the Faulhorn in 1842; 3 
but they failed to obtain the confidence they merited. Pouillet's 
result was not definitively superseded until Violle, from actinometrical 
measures at the summit and base of Mont Blanc in 1875, com- 
puted the intensity of solar radiation at 2'54, 4 and Crova, about the 
same time, at Montpellier, showed it to be above two calories. 5 
Langley went higher still. Working out the results of the Mount 
Whitney expedition, he was led to conclude atmospheric absorption 
to be fully twice as effective as had hitherto been supposed. 
Scarcely 60 per cent., in fact, of those solar radiations which strike 
perpendicularly through a seemingly translucent sky, were estimated 
to 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 in transmission, and the solar constant rose at 
once to three calories. Nor did the rise stop there. M. Savelieff 
deduced for it a value of 3*47 from actinometrical observations 
made at Kieff in 1890 ; 6 and Knut Angstrom, taking account of the 
arrestive power of carbonic acid, inferred enormous atmospheric 
absorption, and a solar constant of four calories. 7 In other words, 
the sun's heat reaching the outskirts of our atmosphere is capable 

1 Comptes Rendus, t. xcii., p. 701. 2 Nature, vol. xxvi., p. 589. 

3 Phil. Trans., vol. cxxxii., p. 273. 4 Ann. de Chim., t. x., p. 321. 

8 Ibid., t. xi., p. 505. 6 Comptes Rendus, t. cxii., p. 1200. 

7 Wied. Ann., Bd. xxxix., p. 294 ; Schemer, Temperatur der Sonne, pp. 36, 38. 

15 



226 HISTORY OF ASTRONOMY part ii 

of doing without cessation the work of an engine of four-horse 
power for each square yard of the earth's surface. Thus, modern 
inquiries tend to render more and more evident the vastness of the 
thermal stores contained in the great central reservoir of our system, 
while bringing into fair agreement the estimates of its probable tem- 
perature. This is in great measure due to the acquisition of a 
workable formula by which to connect temperature with radiation. 
Stefan's rule of a fourth-power relation, if not actually a law of 
nature, is a colourable imitation of one ; and its employment has 
afforded a practical certainty that the sun's temperature, so far 
as it is definable, neither exceeds 12,000 C, nor falls short of 
6,500 C. 



CHAPTER VI 

THE SUN'S DISTANCE 

The question of the sun's distance arises naturally from the con- 
sideration of his temperature, since the intensity of the radiations 
emitted as compared with those received and measured, depends 
upon it. But the knowledge of that distance has a value quite 
apart from its connection with solar physics. The semi-diameter of 
the earth's orbit is our standard measure for the universe. It is the 
great fundamental datum of astronomy the unit of space, any 
error in the estimation of which is multiplied and repeated in a 
thousand different ways, both in the planetary and sidereal systems. 
Hence its determination was 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 I 1 What is needed in order to attain knowledge 
of the desired exactness is no less than this : to measure an angle 
about equal to that subtended by a halfpenny 2,000 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 2,000 feet he is, to a spectator on the rotating earth, 
removed at rising and setting from his meridian place in the 
heavens. Such, in other terms, would be the magnitude of the 
terrestrial radius as viewed from the sun. If we knew this magni- 
tude with certainty and precision, we should also know with cer- 
tainty 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 pro- 
fessedly of astronomy. But this angle of parallax or apparent 

1 Airy, Month. Not., vol. xvii., p. 210. 

152 



228 HISTORY OF ASTRONOMY part ii 

displacement cannot be directly measured cannot even be per- 
ceived with the finest instruments. Not from its smallness. The 
parallactic shift of the nearest of the stars as seen from opposite 
sides of the earth's orbit, is many times smaller. But at the sun's 
limb, and close to the horizon, where the visual angle in question 
opens out to its full extent, atmospheric troubles become over- 
whelming, 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 occurs 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 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 
observations of the same occurrence a difference quite insignificant 
at that stage of the inquiry. But Picard's result was just half Flam- 
steed'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. 2 So that uncertainty 
continued to have an enormous range. 

Venus, on the other hand, comes closest to the earth when she 
passes between it and the sun. At such times of "inferior con- 
junction " she is, however, still twenty-six million miles, or (in 

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. 

2 J. D. Cassini, Hist. Abregee de la Parallaxe du Soleil, p. 122, 1772. 



chap, vi THE SUN'S DISTANCE 229 

round numbers) 109 times as distant as the moon. Moreover, 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, 1 of which the com- 
ponents are separated by eight years, recurring at intervals alter- 
nately of 105 J and 121 J 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 121 \ years, came the June couple of 1761 and 1769; and again, 
after 105 J, the two last observed, December 8, 1874, and Decem- 
ber 6, 1882. Throughout the twentieth century there will be no 
transit of Venus ; but the astronomers of the twenty -first will only 
have to wait four years for the first of a June pair. The rarity of 
these events is due to the fact that the orbits of the earth and 
Venus do not lie in the same plane. If they did, there would be a 
transit each time that our twin-planet overtakes us in her more rapid 
circling that is, on an average, every 584 days. As things are 
actually arranged, she passes above or below the sun, except when 
she happens to be very near the line of intersection of the two 
tracks. 

Such an occurrence as a transit of Venus seems, at first sight, 
full of promise for solving the problem of the sun's distance. For 
nothing would appear easier than to determine exactly either the 
duration of the passage of a small, dark orb across a large brilliant 
disc, or the instant of its entry upon or exit from it. And the 
differences in these times (which, owing to the comparative nearness 
of Venus, are quite considerable), as observed from remote parts of 
the earth, can be translated into differences of space that is, into 
apparent or parallactic displacements, whereby the distance of Venus 
becomes known, and thence, by a simple sum in proportion, the 
distance of the sun. But in that word " exactly " what snares and 
pitfalls lie hid ! It is so easy to think and to say ; so indefinitely 
hard to realise. The astronomers of the eighteenth century were 
full of hope and zeal. They confidently expected to attain, through 

1 The present period of coupled eccentric transits will, in the course of ages, 
be succeeded by a period of single, nearly central transits. The alignments by 
which transits are produced, of the earth, Venus, and the sun, close to the place 
of intersection of the two planetary orbits, now occur, the first a little in front of, 
the second, after eight years less two and a half days, a little behind the node. 
But when the first of these two meetings takes place very near the node, giving 
a nearly central transit, the second falls too far from it, and the planet escapes 
projection on the sun. The reason of the liability to an eight-yearly recurrence 
is that eight revolutions of the earth are accomplished in only a very little more 
time than thirteen revolutions of Venus. 



2 3 o HISTORY OF ASTRONOMY part ii 

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, re- 
mained at many millions. 

In 1822, however, Encke, then director of the Seeberg Observatory 
near Gotha, undertook to bring order out of the confusion of dis- 
cordant, and discordantly interpreted observations. His combined 
result for both transits (1761 and 1769) was published in 1824, 1 and 
met universal acquiescence. The parallax of the sun thereby estab- 
lished was 8*5776", corresponding to a mean distance 2 of 95 \ million 
miles. Yet this abolition of doubt was far from being so satisfactory 
as it seemed. Serenity on the point lasted exactly thirty years. It 
was disturbed in 1854 by Hansen's announcement 3 that the observed 
motions of the moon could be drawn into accord with theory only 
on the terms of bringing the sun considerably nearer to us than he 
was supposed to be. 

Dr. Matthew Stewart, professor of mathematics in the University 
of Edinburgh, had made a futile attempt in 1763 to deduce the sun's 
distance from his disturbing power over our satellite. 4 Tobias 
Mayer of Gottingen, however, whose short career was fruitful of 
suggestions, struck out the right way to the same end ; and Laplace, 
in the seventh book of the Mfoanique Cdeste, 5 gave a solar parallax 
derived from the lunar " parallactic inequality " substantially 
identical with that issuing from Encke's subsequent discussion of 
the eighteenth -century transits. Thus, two wholly independent 
methods the trigonometrical, or method by survey, and the gravita- 
tional, or method by perturbation seemed to corroborate each the 
upshot of the use of the other until the nineteenth century was 
well past its meridian. It is singular how often errors conspire to 
lead conviction astray. 

Hansen's note of alarm in 1854 was echoed by Leverrier in 1858. 6 
He found that an apparent monthly oscillation of the sun which 

1 Die Entfemung der Sonne: Fortsetzung, p. 108. Encke slightly corrected 
his result of 1824 in Berlin Abh., 1835, p. 295. 

* Owing to the ellipticity of its orbit, the earth is nearer to the sun in January 
than in June by 3,100,000 miles. The quantity to be determined, or "mean 
distance," is that lying midway between these extremes is, in other words, half 
the major axis of the ellipse in which the earth travels. 

3 Month. Not., vol. xv., p. 9. 

4 The Distance of the Sun from the Earth determined by the Theory of Gravity, 
Edinburgh, 1763. 

5 Opera, t. iii., p. 326. 

6 Oomptes Rendus, t. xlvi., p. 882. The parallax 8*95" derived by Leverrier 
from the above-described inequality in the earth's motion, was corrected by 
Stone to 8 '91". Month. Not., vol. xxviii., p. 25. 



chap, vi THE SUJSTS DISTANCE 231 

reflects a real monthly movement of the earth round its common 
centre of gravity with the moon, and which depends for its amount 
solely on the mass of the moon and the distance of the sun, 
required a diminution in the admitted value of that distance by 
fully four million miles. Three years later he pointed out that 
certain perplexing discrepancies between the observed and computed 
places both of Venus and Mars, would vanish on the adoption of a 
similar measure. 1 Moreover, a favourable opposition of Mars gave 
the opportunity 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 92J million. 2 The accumu- 
lating 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 Eomer in 1675, through observing that the eclipses of 
Jupiter's satellites invariably occurred later, when the earth was on 
the far side, than when it was on the near side of its orbit. Half 
the difference, 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, Professor Harkness adopts a safe medium value of 
498 seconds. Hence, if we had any independent means of ascer- 
taining how fast light travels, we could tell at once how far off the 
sun is. 

There is yet another way by which knowledge of the swiftness 
of light would lead us straight to the goal. The heavenly 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 (already adverted 
to) has been known since Bradley's time as "aberration." It arises 
from a combination of the two movements 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 

1 Month. Not., vol. xxxv., p. 156. 2 Wash. Obs., 1865, App. ii., p. 28. 



232 HISTORY OF ASTRONOMY part ii 

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 ascertaining the 
velocity of light except through the imperfect knowledge possessed 
as to the distance of the sun. The first successful terrestrial experi- 
ments on the point date from 1849 ; and it is certainly no slight 
triumph of human ingenuity to have taken rigorous account of the 
delay of a sunbeam in flashing from one mirror to another. Fizeau 
led the way, 1 and he was succeeded, after a few months, by Leon 
Foucault, 2 who, in 1862, had so far perfected Wheatstone's method 
of revolving mirrors, as to be able to announce with authority that 
light travelled slower, and that the sun was in consequence nearer, 
than had been supposed. 3 Thus a third line of separate research 
was found to converge to the same point with the two others. 

Such a conspiracy of proof was not to be resisted, and at the 
anniversary meeting of the Eoyal Astronomical Society in February, 
1864, the correction of the solar distance took the foremost place 
in the annals of the year. Lest, however, a sudden bound of four 
million miles nearer to the centre of our system should shake 
public faith in astronomical accuracy, it was explained that the 
change in the solar parallax corresponding to that huge leap, 
amounted to no more than the breadth of a human hair 125 feet 
from the eye ! 4 The Nautical Almanac gave from 1870 the altered 
value of 8 -9 5", for which Newcomb's result of 8*85", adopted in 
1869 in the Berlin Ephemeris, was substituted some ten years 
later. 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 improved 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 proportionately diminishing the solar distance. Once 

1 Comptes Rendus, 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 of Fizeau's experiments in 1872. 
Comptes Rendus, t. lxxvi., p. 338. 

4 Month. Not., vol. xxiv., p. 103. 



chap, vi THE SUN'S DISTANCE 233 

more in 1890, and this time with better success, the eighteenth- 
century transits were investigated by Professor Newcomb. 1 Turning 
to account the experience gained in the interim regarding the 
optical phenomena accompanying such events, he elicited from the 
mass of somewhat discordant observations at his command, a 
parallax (8 -79") in close agreement with the value given by sundry 
modes of recent research. 

Conclusions on the subject, however, were still regarded as purely 
provisional. A transit of Venus was fast approaching, and to its 
arbitrament, as to that of a court of final appeal, the pending ques- 
tion 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 indecisive a 
character as to form only a starting-point for fresh litigation ; but 
that century had not passed in vain, and it was confidently antici- 
pated that observational 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 Sir George Airy, then Astronomer-Eoyal, in 1857, 2 and 
formed the subject of eager discussion in this and other countries 
down to the very eve of the occurrence. In these Mr. Proctor took 
a leading part ; and it was due to his urgent representations that 
provision was made for the employment of the method identified 
with the name of Halley, 3 which had been too hastily assumed inap- 
plicable to the first of each transit-pair. It depends upon the 
difference in the length of time taken by the planet to cross the 
sun's disc, as seen from various points of the terrestrial surface, and 
requires, accordingly, the visibility of both entrance and exit at the 
same station. Since these were, in 1874, separated by about three 
and a half hours, and the interval may be much longer, the choice 
of posts for the successful use of the " method of durations " is a 
matter of some difficulty. 

The system described by Delisle in 1760, on the other hand, 
involves merely noting the instant of ingress or egress (according to 
situation) from opposite extremities of a terrestrial diameter; the 
disparity in time giving a measure of the planet's apparent displace- 
ment, 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. But 
this was much more sensibly felt a century ago than it is now, 

1 Astr. Papers of the American Ephemeris, vol. ii., p. 263. 

2 Month. Not., vol. xvii., p. 208. 

3 Because closely similar to that proposed by him in Phil. Trans, for 1716. 



234 HISTORY OF ASTRONOMY part ii 

the improved facility and certainty of modern determinations 
tending 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 accumulated 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 principle of the two older methods was to ascertain the 
exact times of meeting between the solar and planetary limbs ; 
that of the two modern to determine 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 pro- 
jection." 

Every country which had a reputation to keep or to gain for 
scientific zeal was forward to co-operate in the great cosmopolitan 
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 in Dutch occupation. In 
all, at a cost of nearly a quarter of a million, some fourscore distinct 
posts of observation were provided ; among them such inhospitable, 
and all but inaccessible rocks in the bleak Southern Ocean, as 
St. Paul's and Campbell Islands, swept by hurricanes, and fitted 
only for the habitation of seabirds, where the daring votaries of 
science, in the wise prevision of a long leaguer by the elements, 
were supplied with stores for many months, or even a whole year. 
Siberia and the Sandwich Islands were thickly beset with observers; 
parties of three nationalities encamped within the mists of Kerguelen 
Island, expressively termed the " Land of Desolation," in the 
sanguine, though not wholly frustrated hope of a glimpse of the 
sun at the right moment. M. Janssen narrowly escaped destruction 
from a typhoon in the China seas on his way to Nagasaki ; Lord 
Lindsay (now Earl of Crawford and Balcarres) equipped, at his 
private expense, an expedition to Mauritius, which was in itself an 
epitome of modern resource and ingenuity. 



chap, vi THE SUN'S DISTANCE 235 

During several years, the practical methods best suited to insure 
success for the impending enterprise formed a subject of European 
debate. Official commissions were appointed to receive and decide 
upon evidence ; and experiments were in progress for the purpose 
of defining the actual circumstances of contacts, the precise deter- 
mination 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 previous century, a formidable and prevalent phenomenon, 
which acquired notoriety as the "Black Drop" or "Black Liga- 
ment," had swamped all pretensions to rigid accuracy. It may be 
described as substituting adhesion for contact, the limbs of the sun 
and planet, instead of meeting and parting with the desirable clean 
definiteness, 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 ascribed this 
baffling appearance entirely to instrumental imperfections; others 
to atmospheric agitation ; others again to the optical encroachment 
of light upon darkness known as "irradiation." It is probable 
that all these causes conspired, in various measure, to produce it ; 
and it is certain that its conspicuous appearance may, by suitable 
precautions, be obviated. 

The organisation of the British forces reflected the utmost credit 
on the energy and ability of Lieutenant-Colonel Tupman, 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 obtainable, 
and were attained ; but diversity of judgment unhappily survived 
the best-directed efforts for its extirpation. 

The eventful day had no sooner passed than telegrams began to 
pour in, announcing an outcome of considerable, though not un- 
qualified success. The weather had proved generally favourable j 
the manifold arrangements had 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. Gradu- 
ally, nevertheless, 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 



236 HISTORY OF ASTRONOMY part ii 

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 j 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 adequate 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." 1 

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 documents available. 3 Yet, from 
the same, Colonel Tupman deduced 8*81", 4 implying a distance 
700,000 miles greater than Stone had obtained. The best French 
observations of contacts gave a parallax of about 8*88"; French 
micrometric measures the obviously exaggerated one of 9"05". 5 

Photography, as practised by most of the European parties, was 
a total failure. Utterly discrepant values of the microscopic dis- 
placements 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 
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 those dating from 
Vladivostock, Nagasaki, and Pekin, with others from Kerguelen 
and Chatham Islands, Professor D. P. Todd, of Amherst College, 
deduced a solar distance of about ninety -two million miles (parallax 
8-883" 0*034"), 6 and the value was much favoured by concurrent 
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 f five years after the 

1 Month. Not., vol. xxxviii., p. 447. 2 Ibid., p. 11. 3 Ibid., p. 294. 

4 Ibid., p. 334. 6 Comptes Rendus, t. xcii., p. 812. 

6 Observatory, vol. v., p. 205. 7 Transits of Venus, p. 89 (1st ed.). 



chap, vi THE SUN'S DISTANCE 237 

transit, Professor Harkncss judged it to be still 1,575,950 miles; 1 
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 applying those 
already known. A new phase of exertion was entered upon. 

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 
Sir David) 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, 2 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 performance. 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 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. 3 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 4 
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 ; 
in fact, the absence of sensible magnitude is one of their chief 
recommendations, since a point of light offers far greater facilities 
for exact measurement than a disc. The first attempt to work this 
new vein was made at the opposition of Phocaea in 1872 j and from 
observations of Flora in the following year at twelve observatories 

1 Am. Jour, of Sc, vol. xx., p. 393. * Month. Not., vol. xvii., p. 219. 

3 Mem. Roy. Astr. Soc, vol. xlvi., p. 163. 4 Astr. Nach,, No. 1,897. 



238 HISTORY OF ASTRONOMY part ii 

in the northern and southern hemispheres, Galle deduced a solar 
parallax of 8-87". 1 At Mauritius in 1874, Lord Lindsay and Sir 
David Gill applied the " diurnal method " to Juno, then conveniently 
situated for the purpose ; and the continued use of similar occasions 
affords an unexceptionable 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. Dr. Gill, however, organised a more 
complex plan of operations upon Iris in 1888, and upon Victoria 
and Sappho in 1889. A novel method was adopted. Its object 
was to secure simultaneous observations made from opposite 
sides of the globe just when the planet lay in the plane passing 
through the centre of the earth and the two observers, the same 
pair of reference-stars being used on each occasion. The displace- 
ments caused by parallax were thus in a sense doubled, since the 
star to which the planet seemed approximated in the northern 
hemisphere, showed as if slightly removed from it in the southern, 
and mce versd. As the planet pursued its course, fresh star-couples 
came into play, during the weeks that the favourable period lasted. 
In these determinations, only heliometers were employed. Dr. 
Elkin, of Yale College, co-operated throughout, and the heliometers 
of Dresden, Gottingen, Bamberg, and Leipzig, shared in the work, 
while Dr. Auwers of Berlin was Sir David Gill's personal coadjutor 
at the Cape. Voluminous data were collected ; meridian observa- 
tions of the stars of reference for Victoria occupied twenty-one 
establishments during four months ; the direct work of triangu- 
lation kept four heliometers in almost exclusive use for the best 
part of a year ; and the ensuing toilsome computations, carried 
out during three years at the Cape Observatory, filled two bulky 
tomes 2 with their details. Gill's final result, published in 1897, was 
a parallax of 8*802", equivalent to a solar distance of 92,874,000; 
and it was qualified by a probable error so small that the value 
might well have been accepted as definitive but for an unlooked- 
for discovery. The minor planet Eros, detected August 14, 
1898, was found to pursue a course rendering it an almost ideal 
intermediary in solar parallax-determinations. Once in thirty years, 
it comes within fifteen million miles of the earth ; and although the 
next of these choice epochs must be awaited for some decades, an 
opposition too favourable to be neglected occurred in 1900. At an 
International Conference, accordingly, held at Paris in July of that 
year, a plan of photographic operations was concerted between the 

1 Hilfiker. Bern Mittheilungen, 1878, p. 109. 

2 Annals of the Cape Observatory, vols, vi., vii. 



chap, vi THE SUJSTS DISTANCE 239 

representatives of no less than 58 observatories. 1 Its primary object 
was to secure a large stock of negatives showing the planet with 
the comparison-stars along the route traversed by it from October, 
1900, to March, 1901, 2 and this at least was successfully attained. 
Their measurement will in due time educe the apparent displace- 
ments of the moving object as viewed simultaneously from remote 
parts of the earth; and the upshot should be a solar parallax 
adequate in accuracy to the exigent demands of the twentieth 
century. 

The second of the nineteenth-century pair of Venus-transits 
wa^ looked forward to with much abated enthusiasm. Russia 
refused her active co-operation in observing it, on the ground that 
oppositions of the minor planets were trigonometrically more useful, 
and financially far less costly; and her example was followed by 
Austria ; while Italian astronomers limited their sphere of action to 
their own peninsula. Nevertheless, it was generally held that a 
phenomenon which the world could not again witness until it was 
four generations older should, at the price of any effort, not be 
allowed to pass in neglect. 

The persuasion of its importance justified the summoning of an 
International Conference at Paris in 1881, from which, however, 
America, preferring independent action, held aloof. 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 regula- 
tions for insuring agreement in the estimation of the critical 
moments of ingress and egress. 3 But in fact (as M. Puiseux had 
shown 4 ), contacts between the limbs of the sun and planet, so far 
from possessing the geometrical simplicity 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 determining 
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 Venus across the sun extremely slow, being but the 
excess of her real motion over that of the earth ; but three distinct 
atmospheres the solar, terrestrial, and Cytherean combine to 
deform outlines and mask the geometrical relations which it is 
desired to connect with a strict count of time. 

The result was very much what had been expected. The 

1 Rapport sur I' Mat de V ' Observatoire de Paris pour VAnnee 1900, p. 7. 

2 Observatory, vol. xxiii., p. 311 ; Newcomb, Astr. Jour., No. 480. 

3 Comptes Rcndus, t. xciii. , p. 569. 

4 Ibid., t. xcii., p. 481. 



2 4 o HISTORY OF ASTRONOMY part ii 

arrangements were excellent, and were only in a few cases discon- 
certed by bad weather. The British parties, under the experienced 
guidance of Mr. Stone, the late Eadcliffe observer, took up positions 
scattered over the globe, from Queensland to Bermuda ; the Ameri- 
cans collected a whole library of photographs; the Germans and 
Belgians trusted to the heliometer ; the French used the camera as 
an adjunct to the method of contacts. Yet little or no approach 
was made to solving the problem. Thus, from 606 measures of 
Venus on the sun, taken with a new kind of heliometer at Santiago 
in Chili, M. Houzeau, of the Brussels Observatory, derived a solar 
parallax of 8-907", and a distance of 91,727,000 miles. 1 But the 
"probable error" of this determination amounted to 0"084" either 
way: it was subject to a "more or less" of 900,000, or to a total 
uncertainty of 1,800,000 miles. The "probable error" of the 
English result, published in 1887, was less formidable, 2 yet the 
details of the discussion showed that no great confidence could be 
placed in it. The sun's distance came out 92,560,000 miles; while 
92,360,000 was given by Professor Harkness's investigation of 1,475 
American photographs. 3 Finally, Dr. Auwers deduced from the 
German heliometric measures the unsatisfactorily small value of 
92,000,000 miles. 4 The transit of 1882 had not, then, brought about 
the desired unanimity. 

The state and progress of knowledge on this important topic were 
summed up by Faye and Harkness in 1881. 5 The methods employed 
in its investigation fall (as we have seen) into three separate classes 
the trigonometrical, the gravitational, and the "phototachy- 
metrical " 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 trust- 
worthy result from celestial surveys, was at that time furnished 
by 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. 6 
The scarcely perceptible errors which still impede its application are 
of such a nature as to accumulate year by year ; eventually, then, 
they will challenge, and must receive, a more and more perfect 
correction. The light-velocity method, however, claimed, and for 
some years justified, M. Faye's preference. 

By a beautiful series of experiments on Foucault's principle, 

1 Bull, de I'Acad., t. vi., p. 842. 2 Month. Not., vol. xlviii., p. 201. 

3 Astr. Jour., No. 182. 4 Astr. Nach., No. 3,066. 

6 Comptes Rendus, t. xcii., p. 375 ; Am. Jour, of Sc, vol. xxii., p. 375. 
6 Month. Not., vol. xxxv., p. 401. 



chap, vi THE SUN'S DISTANCE 241 

Michelson fixed in 1879 the rate of luminous transmission 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 if the solar 
parallax of 8-758* deduced from it by Professor Harkness errs 
somewhat by defect, it is doubtless because Glasenapp's "light- 
equation," with which it was combined, errs slightly by excess. 
But all earlier efforts of the kind were thrown into the shade by 
Professor Newcomb's arduous operations at Washington in 1880- 
1882. 2 The scale upon which they were conducted was in itself 
impressive. Foucault's entire apparatus in 1862 had been enclosed 
in a single room ; Newcomb's revolving and fixed 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 contrivance 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. 

Just as these experiments were concluded in 1882, M. Magnus 
Nyren, of St. Petersburg, published an elaborate investigation of 
the small annular displacements of the stars due to the successive 
transmission of light, involving an increase of Struve's " constant of 
aberration" from 20-445" to 20-492". And from the new value, 
combined with Newcomb's light-velocity, was derived a valuable 
approximation to the sun's distance, concluded at 92,905,021 miles 
(parallax = 8-794"). Yet it is not quite certain that Nyren's correc- 
tion was an improvement. A differential method of determining 
the amount of aberration, struck out by M. Loewy of Paris, 3 avoids 
most of the objections to the absolute method previously in vogue ; 
and the upshot of its application in 1891 was to show that Struve's 
constant might better be retained than altered, Loewy's of 20*447" 
varying from it only to an insignificant extent. Professor Hall 
had, moreover, deduced nearly the same value (20*454") from the 
Washington observations since 1862, of a Lyrse (Vega); whence, in 
conjunction with Newcomb's rate of light transmission, he arrived 
at a solar parallax of 8-81". 4 Inverting the process, Sir David Gill 
in 1897 derived the constant from the parallax. If the earth's 
orbit have a mean radius, as found by him, of 92,874,000 miles, 
then, he calculated, the aberration of light Newcomb's measures of 

1 Am. Jour, of Sc, vol. xviii., p. 393. 

2 Nature, vol. xxxiv., p. 170 ; Astron. Papers of the American Ephemeris, 
vol. ii., p. 113. 

3 Comptes Rendus, t, cxii., p. 549. 4 Astr. Journ., Nos. 169, 170. 

16 



242 HISTORY OF ASTRONOMY part ii 

its velocity being supposed exact amounts to 20*467*. This figure 
can need very slight correction. 

Professor Harkness surveyed in 1891, 1 from an eclectic point of 
view, the general situation as regarded the sun's parallax. Con- 
vinced that no single method deserved an exclusive preference, he 
reached a plausible result through the combination, on the principle 
of least squares that is, by the mathematical rules of probability 
of all the various quantities upon which the great datum depends. 
It thus summed up and harmonised the whole of the multifarious 
evidence bearing upon the point, and, as modified in 1894, 2 falls 
very satisfactorily into line with the Cape determination. We may, 
then, at least provisionally, accept 92,870,000 miles as the length of 
our measuring-rod for space. Nor do we hazard much in fixing 
100,000 miles as the outside limit of its future correction. 

1 The Solar Parallax and its Related Constants, Washington, 1891. 

2 Astr. and Astrophysics, vol. xiii. , p. 626. 



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 intelligence, 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 Gottingen, 
and there imbibed from Kastner a life-long devotion to science. 
From the law, however, he got the means of living, and, what was 
to the full as precious to him, the means of observing. Entering 
the sphere of Hanoverian officialism in 1788, he settled a few years 
later at Lilienthal, near Bremen, as " Oberamtmann," or chief magis- 
trate. 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 superintendence 
and at the cost of the astronomical Oberamtmann, a thirteen-foot 
reflector, declared by Lalande to be the finest telescope in existence, 
and one twenty-seven feet in focal length, probably as inferior to its 
predecessor in real efficiency as it was superior in size. 

Thus, with instruments of gradually increasing power, Schroter 
studied during thirty-four years the topography of the moon and 
planets. The field was then almost untrodden; he had but few 
and casual predecessors, and has since had no equal in the sustained 
and concentrated patience of his hourly watchings. Both their 
prolixity and their enthusiasm are faithfully reflected in his various 
treatises. Yet the one may be pardoned for the sake of the other, 
especially when it is remembered that he struck out a substantially 
new line, and that one of the main lines of future advance. More- 

162 



244 HISTORY OF ASTRONOMY part ii 

over, 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 Yandamme occupied Bremen. On the night of April 20, the 
Vale of Lilies was, by their wanton destructiveness, laid waste with 
fire j the Government offices were destroyed, and with them the 
chief part of Schroter'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 sur- 
vived 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 par- 
ticulars 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 saw reason to conclude, from the gradual 
degradation of light on its partially illuminated disc, that Mercury 
possesses a tolerably dense atmosphere. 1 During the transit of 
May 7, 1799, he was, moreover, struck with the appearance of a 
ring of softened luminosity encircling the planet to an apparent 
height of three seconds, or about a quarter of its own diameter. 2 
Although a "mere thought" in texture, 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 grayish 
boundary, and reminded him, though indefinitely fainter, of the 
penumbra of a sun-spot. A similar appendage had been noticed by 
De Plantade at Montpellier, November 11, 1736, and again in 1786 
and 1789 by Prosperin and Flaugergues j but Herschel, on Novem- 
ber 9, 1802, saw the preceding limb of the planet projected on the 
sun cut the luminous solar clouds with the most perfect sharpness. 3 
The presence, however, of a "halo" was unmistakable in 1832, 

1 Neueste Beytrage zur Erweiterung der Sternkutide, Bd. iii., p. 14 (1800). 

2 Ibid., p. 24. 3 Phil. Trans., vol. xciii., p. 215. 



chap, vii PLANETS AND SATELLITES 245 

when Professor Moll, of Utrecht, described it as a " nebulous ring 
of a darker tinge approaching to the violet colour." 1 Again, 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. 2 
It was next seen by Christie and Dunkin at Greenwich, May 6, 
1878, 8 and with much precision of detail by Trouvelot at Cambridge 
(U.S.). 4 Professor Holden, on the other hand, noted at Hastings- 
on-Hudson the total absence of all anomalous appearances. 5 Nor 
could any vestige of them be perceived by Barnard at Lick on 
November 10, 1894. 6 Various effects of irradiation and diffraction 
were, however, observed by Lowell and W. H. Pickering at Flag- 
staff; 7 and Davidson was favoured at San Francisco with glimpses 
of the historic aureola, 8 as well as of a central whitish spot, which 
often accompanies it. That both are somehow of optical produc- 
tion can scarcely be doubted. 

Nothing can be learned from them regarding the planet's physical 
condition. Airy showed that refraction in a Mercurian atmosphere 
could not possibly originate the noted aureola, which must accord- 
ingly be set down as "strictly an ocular nervous phenomenon." 9 It 
is the less easy to escape from this conclusion that we find the 
virtually airless moon capable of exhibiting a like appendage. Pro- 
fessor 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 ; 10 
and photographic effects of the same kind appear in pictures of 
transits of Venus and partial solar eclipses. 

The spectroscope affords little information as to the constitution 
of Mercury. 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, 11 but, it would seem, 
on slight grounds. It is, however, certainly very poor in blue rays. 
More definite conclusions were, in 1874, 12 derived by Zollner from 
photometric observations of Mercurian phases. A similar study of 
the waxing and waning moon had afforded him the curious discovery 

1 Mem. Roy. Astr. Soc, vol. vi., p. 116. 

2 Month. Not., vol. xix., pp. 11, 25. 3 Ibid., vol. xxxviii., p. 398. 

4 Am. Jour, of Sc, vol. xvi., p. 124. 

5 Wash. Obs. for 1876, Part ii., p. 34. 

8 Pop. Astr., vol. ii., p. 168 ; Astr. Jour., No. 335. 

7 Astr. and Astrophysics, vol. xiii., p. 866. 8 Ibid., p. 867. 

9 Month. Not., vol. xxiv., p. 18. 10 Ibid., vol. xxiii., p. 234 (Challis). 

11 Untersuchungen uber die Spectra der Planeten, p. 9. 

12 Sirius, vol. vii., p. 131. 



246 HISTORY OF ASTRONOMY part ii 

that light-changes dependent upon phase vary with the nature of 
the reflecting surface, following a totally different law on a smooth 
homogeneous 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 mode of surface-formation. This conclusion 
was fully borne out by Miiller's more extended observations at 
Potsdam during the years 1 885-1 893. 1 Practical assurance was 
gained from them that the innermost planet has a rough rind of 
dusky rock, absorbing all but 17 per cent, of the light poured 
upon it by the fierce adjacent sun. Its "albedo," in other words, is 
0-1 7, 2 which is precisely that ascribed to the moon. The absence 
of any appreciable Mercurian atmosphere followed almost necessarily 
from these results. 

On March 26, 1800, Schroter, observing with his 13-foot reflector 
in a peculiarly clear sky, perceived the southern horn of Mercury's 
crescent to be quite distinctly blunted. 3 Interception of sunlight 
by a Mercurian mountain rather more than eleven English miles 
high explained the effect to his satisfaction. By carefully timing 
its recurrence, he concluded rotation on an axis in a period of 24 
hours 4 minutes. This first determination of the kind rewarded 
twenty years of unceasing vigilance. It received ostensible con- 
firmation from the successive appearances of a dusky streak and 
blotch in May and June, 1801. 4 These, however, were inferred to 
be no permanent markings on the body of the planet, but atmospheric 
formations, the streak at times drifting forwards (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. 

The rounded appearance of the southern horn seen by Schroter 
was more or less doubtfully caught by Noble (1864), Burton, and 
Franks (1877) f but was obvious to Mr. W. F. Denning at Bristol 
on the morning of November 5, 1882. 6 That the southern polar 
regions are usually less bright than the northern is well ascertained ; 
but the cause of the deficiency remains dubious. If inequalities of 

1 Potsdam Publ. y No. 30 ; Astr. Nach., No. 3,171 ; Frost, Astr. and Astro- 
physics, vol. xii., p. 619. 

2 Zbllner and Winnecke made it = 0*13, Astr. Naeh., No. 2,245. 

3 Neueste Beytrage, Bd. iii. , p. 50. 

4 Astr. Jahrbuch, 1804, pp. 97-102. 

6 Webb, Celestial Objects, p. 46 (4th ed.). 
6 V Astronomie, t. ii., p. 141. 



chap, vii PLANETS AND SATELLITES 247 

surface are in question, they must be on a considerable scale ; and 
a similar explanation might be given of the deformations of the 
11 terminator " or dividing-line between darkness and light in the 
planet's phases first remarked by Schroter, and again clearly seen 
by Trouvelot in 1878 and 1881. 1 The displacement, during four 
days, of certain brilliant and dusky spaces on the disc indicated to 
Mr. Denning in 1882 rotation in about twenty -five hours ; while the 
general aspect of the planet reminded him of that of Mars. 2 But 
the difficulties in the way of its observation are enormously enhanced 
by its constant close attendance on the sun. 

In his sustained study of the features of Mercury, Schroter had 
no imitator until Schiaparelli took up the task at Milan in 1882. 
His observations were made in daylight. It was found that much 
more could be seen, and higher magnifying powers used, high up 
in the sky near the sun, than at low altitudes, through the agitated 
air of morning or evening twilight. A notable discovery ensued. 3 
Following the planet hour by hour, instead of making necessarily 
brief inspections at intervals of about a day, as previous observers 
had done, it was found that the markings faintly visible remained 
sensibly fixed, hence, that there was no rotation in a period at all 
comparable with that of the earth. And after long and patient 
watching, the conclusion was at last reached that Mercury turns on 
his axis in the same time needed to complete a revolution in his 
orbit. One of his hemispheres, then, is always averted from the 
sun, as one of the moon's hemispheres from the earth, while the 
other never shifts from beneath his torrid rays. The " librations," 
however, of Mercury are on a larger scale than those of the moon, 
because he travels in a more eccentric path. The temporary in- 
equalities arising between his " even pacing " on an axis and his 
alternately accelerated and retarded elliptical movement occasion, in 
fact, an oscillation to and fro of the boundaries of light and darkness 
on his globe over an arc of 47 22', in the course of his year of 88 
days. Thus the regions of perpetual day and perpetual night are 
separated by two segments, amounting to one-fourth of the entire 
surface, where the sun rises and sets once in 88 days. Else there is 
no variation from the intense glare on one side of the globe, and 
the nocturnal blackness on the other. 

To Schiaparelli's scrutiny, Mercury appeared as a " spotty globe," 
enveloped in a tolerably dense atmosphere. The brownish stripes 
and streaks, discerned on his rose- tinged disc, and judged to be 
permanent, were made the basis of a chart. They were not indeed 

1 Observations sur les Planetes Venus et Mercure t p. 87. 

2 Observatory, vol. vi., p. 40. 

3 Atti delV Accad. del Lincei, t. v. ii., p. 283, 1889 ; Astr. Nach., No. 2,944. 



248 HISTORY OF ASTRONOMY part ii 

always equally well seen. They disappeared regularly near the 
limb, and were at times veiled even when centrally situated. Some 
of them had been clearly perceived by De Ball at Bothkamp in 
1882. 1 

Mr. Lowell followed Schiaparelli's example by observing Mercury 
in the full glare of noon. " The best time to study him," he re- 
marked, "is when planetary almanacs state 'Mercury invisible.'" 
A remarkable series of drawings executed, some at Flagstaff in 
1896, the remainder at Mexico in 1897, supplied grounds for the 
following, among other, conclusions. 2 Mercury rotates syn- 
chronously with its revolution that is, once in 88 days on an 
axis sensibly perpendicular to its orbital plane. No certain signs 
of a Mercurian atmosphere are visible. The globe is seamed and 
furrowed with long narrow markings, explicable as cracks in 
cooling. It is, and always was, a dead world. From micrometrical 
measures, moreover, the inferences were drawn that the planet's 
mass has a probable value about $ that of the earth, while its mean 
density falls considerably short of the terrestrial standard. 

The theory of Mercury's movements has always given trouble. 
In Lalande's, 3 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 12th of September, 1859, however, he was able to 
announce before the Academy of Sciences 4 the terms of a com- 
promise between observation and calculation. They involved 
the addition of a new member to the solar system. The hitherto 
unrecognised presence of a body about the size of Mercury itself 
revolving at somewhat less than half its mean distance from the 
sun (or, if farther, then of less mass, and vice versd), would, it 
was pointed out, produce exactly the effect required, of dis- 
placing the perihelion of the former planet 38" a century more 
than could otherwise be accounted for. The planes of the two 
orbits, however, should not lie far apart, as otherwise a nodal 
disturbance would arise not 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 achieve- 
ment, 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 

1 Adr. Nach., No. 2,479. 2 Memoirs Amer. Acad., vol. xii., No. 4, p. 464. 
3 Hist, de VAstr., p. 682. 4 Comptes Mendus, t. xlix., p. 379. 



chap, vii PLANETS AND SATELLITES 249 

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 reobserving 
the phenomenon, however, kept him silent, and it was not until 
December 22, after the news of Leverrier's prediction 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 observa- 
tion. He named the new planet " Vulcan," and computed elements 
giving it a period of revolution slightly under twenty days. 2 But 
it has never since been seen. M. Liais, director of the Brazilian 
Coast Survey, thought himself justified in asserting that it never 
had been seen. Observing the sun for twelve minutes after the 
supposed ingress recorded at Orgeres, he noted those particular 
regions of its surface as "tres uniformes d'intensite." 3 He subse- 
quently, 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 announce- 
ment of a transit witnessed, it was asserted, by "Weber at Peckeloh, 
April 4, 1876. 4 The pseudo-planet, indeed, was detected shortly 
afterwards on the Greenwich photographs, and was found to have 
been seen by M. Ventosa at Madrid in its true character of a sun- 
spot without penumbra; but Leverrier had meantime undertaken 
the investigation of a list of twenty similar dubious appearances, 
collected by Haase, and republished by Wolf in 1872. 5 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. 6 But, widespread watchfulness notwith- 
standing, no suspicious object came into view at either epoch. 

The next announcement of the discovery of "Vulcan" was 
on the occasion of the total solar eclipse of July 29, 1878. 7 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 

1 Comptes Rendus, t. 1., p. 40. 2 Ibid., p. 46. 

3 Astr. Nock., Nos. 1,248 and 1,281. 

4 Comptes Rendus, t. lxxxiii., pp. 510, 561. 

6 Handbuch der Mathematik, Bd. ii., p. 327. 

6 Comptes Rendus, t. lxxxiii., p. 721. 

7 Nature, vol. xviii., pp. 461, 495, 539. 



250 HISTORY OF ASTRONOMY part ii 

James C. Watson, stationed at Rawlins (Wyoming Territory), and 
Professor Lewis Swift at Denver (Colorado) was at first readily 
admitted. But their separate observations could, on a closer 
examination, by no possibility be brought into harmony, and, if 
valid, certainly referred to two distinct objects, if not to four; each 
astronomer eventually claiming a pair of planets. Nor could any 
one of the four be identified with Lescarbault's and Leverrier's 
Vulcan, which, if a substantial body revolving round the sun, must 
then have been found on the east side of that luminary. 1 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. 2 Nevertheless, they strenuously main- 
tained their opposite conviction. 3 

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 
Holden "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. Photographic explorations during recent eclipses 
have been equally fruitless. Belief in the presence of any consider- 
able body or bodies within the orbit of Mercury is, accordingly, at 
a low ebb. Yet the existence of the anomaly in the Mercurian 
movements indicated by Leverrier has been made only surer by 
further research. 4 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 libration of Venus 
he was not sure which in about twenty-three hours. 5 By 
Bianchini in 1726 the period was augmented to twenty-four days 
eight hours. J. J. Cassini, however, in 1740, showed that the data 
collected by both observers were consistent with rotation in twenty- 
three hours twenty minutes. 6 So the matter rested until Schroter's 

1 Oppolzer, Astr. Nach., No. 2,239. 

2 Ibid., Nos. 2,253-4 (C. H. F. Peters). 

3 Ibid., Nos. 2,263 and 2,277. See also Tisserand in Ann. Bur. des Long., 
1882, p. 729. 

4 See J. Bauschinger's Untersuchungen (1884), summarised in Bull. Astr., t. i., 
p. 506, and Astr. Nach., No. 2,594. Newcomb finds the anomalous motion of the 
perihelion to be even larger (43" instead of 38") than Leverrier made it. Month. 
Not.. February, 1884, p. 187. Harzer's attempt to account for it in Astr. Nach., 
No. 3,030, is more ingenious than successful. 

6 Jour, des Scavans, December, 1667, p. 122. 

6 El&mens d'Astr., p. 525. Cf. Chandler, Pop. Astr., February, 1897, p. 393. 



chap, vii PLANETS AND SATELLITES 251 

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 applica- 
tion of 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 23h. 21m. 1 To this only twenty-two seconds were 
added by De Vico, as the result of over 10,000 observations made 
with the Cauchoix refractor of the Collegio Eomano, 1 839-41. 2 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 11'. 

These conclusions inspired, it is true, much distrust, consequently 
there were no received ideas on the subject to be subverted. Never- 
theless, a shock of surprise was felt at Schiaparelli's announcement, 
early in 1890, 3 that Venus most probably rotates after the fashion 
just previously ascribed to Mercury. A continuous series of obser- 
vations, from November, 1877, to February, 1878, with their records 
in above a hundred drawings, supplied the chief part of the data 
upon which he rested his conclusions. They certainly appeared 
exceptionally well-grounded ; and the doubts at first qualifying 
them were removed by a fresh set of determinations in July, 1895. 4 
Most observers have depended, in their attempts to ascertain the 
rotation-period of Venus, upon evanescent shadings, most likely 
of atmospheric origin, and scarcely recognisable from day to day. 
Schiaparelli fixed his attention upon round, defined, lustrously white 
spots, the presence of which near the cusps of the illuminated 
crescent has been attested for close upon two centuries. His steady 
watch over them showed the invariability of their position with 
regard to the terminator ; and this is as much as to say that the 
regions of day and night do not shift on the surface of the planet. 
In other words, she keeps the same face always turned towards the 
sun. Moreover, since her orbit is nearly circular, libratory effects 
are very small. They amount in fact to only just one-thirtieth of 
those serving to modify the severe contrasts of climate in Mercury. 

1 Beobachtungen ilber die sehr betrdchtlichen Gebirge unci Rotation der Venus, 
1792, p. 35. Schroter's final result in 1811 was 23h. 21m. 7 "977s. Monat. 
Corr., Bd. xxv., p. 367. 

2 Astr. Nach., No. 404. 

3 Rendiconti del R. Istituto Lombardo, t. xxiii., serie ii. 

4 Astr. Nach., No. 3,304. 



252 HISTORY OF ASTRONOMY parti: 

Confirmatory evidence of Schiaparelli's result for Venus is not 
Wanting. Thus, observations irreconcilable with a swift rate oj 
rotation were made at Bothkamp in 1871 by Vogel and Lohse; 
and a drawing executed by Professor Holden with the great 
Washington refractor, December 15, 1877, showed the same mark 
ings in the positions recorded at Milan to have been occupied bj 
them eight hours previously. Further, a series of observations 
carried out by M. Perrotin at Nice, May 15 to October 4, 1890, anc 
from Mont Mounier in 1895-6, with the special aim of testing tht 
inference of synchronous rotation and revolution, proved strongly 
corroborative of it. 2 A remarkable collection of drawings made bj 
Mr. Lowell in 1896 appeared decisive in its favour; 3 Tacchini al 
Rome, 4 Mascari at Catania and Etna, 5 Cerulli at Terano, 6 obtainec 
in 1892-6 evidence similar in purport. On the other hand, Niester | 
of Brussels found reason to revert to Vico's discarded elements foi 
the planet's rotation f and Trouvelot, 8 Stanley Williams, 9 Yilliger, 1 ' 
and Leo Brenner, 11 so far agreed with him as to adopt a period oj 
approximately twenty-four hours. Finally, E. Von Oppolzer sug 
gested an appeal to the spectroscope ; 12 and Belopolsky secured ii 
1900 13 spectrograms apparently marked by the minute displace 
ments corresponding to a rapid rate of axial movement. But the^ 
were avowedly taken only as an experiment, with unsuitable 
apparatus ; and the desirable verification of their supposed impon 
is not yet forthcoming. Until it is, Schiaparelli's period of 225 days 
must be allowed to hold the field. 

Effects attributed to great differences of level in the surface ol 
Venus have struck many observers. Francesco Fontana at Naples 
in 1643 noticed irregularities along the inner edge of the crescent. 1 ' 
Lahire in 1700 considered them regard being had to difference oi 
distance to be much more strongly marked than those visible ir 
the moon. 16 Schroter's assertions to the same effect, though scoutec 
with some unnecessary vehemence by Herschel, 16 have since beer 

1 Bothkamp Beobachtungen, Heft ii., p. 120. 

2 Comptes Rendus, t. cxi., p. 542; t. cxxii., p. 395. 

3 Month. Not., vol. lvii., p. 402 ; Astr. Nach., No. 3,406. 

4 Mem. Spettroscopisti Italiani, t. xxv., p. 93 ; Nature, vol. liii., p. 306. 

5 Astr. Nach., No. 3,329. 6 Ibid. 

7 Bull, de VAcad. de Belgique, t. xxi., p. 452, 1891. 

8 Observations sur les Planetes Vinus et Mcrcure, 1892. 

9 Astr. Nach., No. 3,300. 10 Ibid., No. 3,332. 
11 Ibid., No. 3,314. 12 Ibid., No. 3,170. 

13 Ibid., No. 3,641. The velocity of a point on the equator of Venus, i 
Brenner's period of 23h. 57m. were exact, would be "28 miles per second ; bu1 
the displacements due to this rate would be doubled by reflection. 

14 Nova Observationes, p. 92. 

15 M&m. de VAc, 1700, p. 296. 16 Phil. Trans., vol. lxxxiii., p. 201. 



chap, vii PLANETS AND SATELLITES 253 

repeatedly confirmed ; amongst others by Madler, De Vico, Langdon, 
who in 1873 saw the broken line of the terminator with peculiar 
distinctness through a veil of auroral cloud ; l by Denning, 2 March 
30, 1881, despite preliminary impressions to the contrary, as well 
as by C. V. Zenger at Prague, January 8, 1883. The great mountain 
mass, presumed to occasion the periodical blunting of the southern 
horn, was precariously estimated by the Lilienthal observer to rise 
to the prodigious height of nearly twenty-seven miles, or just five 
times the elevation of Mount Everest ! Yet the phenomenon 
persists, whatever may be thought of the explanation. Moreover, 
the speck of light beyond, interpreted as the visible sign of a 
detached peak rising high enough above the encircling shadow to 
catch the first and last rays of the sun, was frequently discerned by 
Baron Van Ertborn in 1876 ; 3 while an object near the northern 
horn of the crescent, strongly resembling a lunar ring-mountain, was 
delineated both by De Vico in 1841 and by Denning forty years 
later. 

We are almost equally sure that Venus, as that the earth is 
encompassed with an atmosphere. Yet, notwithstanding luminous 
appearances plainly due to refraction during the transits both of 
1761 and 1769, Schroter, in 1792, took the initiative in coming to 
a definite conclusion on the subject. 4 It was founded, first, on the 
rapid diminution of brilliancy towards the terminator, attributed to 
atmospheric absorption ; next, on the extension beyond a semicircle 
of the horns of the crescent ; lastly, on the presence of a bluish 
gleam illuminating the early hours of the Cytherean night with what 
was taken to be genuine twilight. Even Herschel admitted that 
sunlight, by the same effect through which the heavenly bodies show 
visibly above our horizons while still geometrically below them, appeared 
to be bent round the shoulder of the globe of Venus. Ample con- 
firmation of the fact has since been afforded. At Dorpat in May, 
1849, the planet being within 3 26' of inferior conjunction, Madler 
found the arms of waning light upon the disc to embrace no less than 
240 of its extent f and in December, 1842, Mr. Guthrie, of Bervie, 
N.B., actually observed, under similar conditions, the whole circum- 
ference to be lit up with a faint nebulous glow. 6 The same curious 
phenomenon was intermittently seen by Mr. Leeson Prince at 
Uckfield in September, 1861 ; 7 but with more satisfactory distinct- 

1 Webb, Cel. Objects, p. 58. 2 Month. Not., vol. xlii., p. 111. 

3 Bull. Ac. de Bruxelles, t. xliii., p. 22. 

4 Phil. Trans., vol. lxxxii., p. 309 ; Aphroditographische Fragmente, p. 85 
(1796). 

5 Astr. Nach., No. 679. 6 Month. Not,, vol. xiv., p. 169. 
7 Ibid., vol. xxiv., p. 25. 



254 HISTORY OF ASTRONOMY part i 

ness by Mr. C. S. Lyman of Yale College, 1 before and after the con 
junction of December 11, 1866, and during nearly five hour 
previous to the transit of 1874, when the yellowish ring of refraetec 
light showed at one point an approach to interruption, possibb 
through the intervention of a bank of clouds. Again, on Decembe 
2, 1898, Venus being 1 45' from the sun's centre, Mr. H. N. Eussell 
of the Halsted Observatory, descried the coalescence of the cusps 
and founded on the observation a valuable discussion of such effects. 
Taking account of certain features in the case left unnoticed h} 
Neison 3 and Proctor, 4 he inferred from them the presence of i 
Cytherean atmosphere considerably less refractive than our own 
although possibly, in its lower strata, encumbered with dust O] 
haze. 

Similar appearances are conspicuous during transits. But while" 
the Mercurian halo is characteristically seen on the sun, the " silvei ! 
thread " round the limb of Venus commonly shows on the part qfi 
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 a dissimilarity in theii 
respective modes of production. 5 Such a dissimilarity probably 
exists, in the sense that the inner section of the ring is illusory, the 
outer, a genuine result of the bending of light in a gaseous 
envelope ; but the distinction 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' 1 
surround the whole disc when ingress was two-thirds accomplished ; 
Mr. Tornaghi, at Goulburn, perceived a halo, entire and unmistak- 
able, at half egress. 6 Similar observations were made at Sydney, 7 
and were renewed in 1882 by Lescarbault at Orgeres, by Metzger 
in Java, and by Barnard at Vanderbilt University. 8 

Spectroscopic indications of aqueous vapour as present in the 
atmosphere of Venus, were obtained in 1874 and 1882, by Tacchini 
and Eicco in Italy, and by Young in New Jersey. 9 Janssen, how- 
ever, who made a special study of the point subsequently to the 
transit of 1882, found them much less certain than he had antici- 

1 Am. Jour, of Sc, vol. xliii., p. 129 (2d ser.) ; vol. ix., p. 47 (3d ser.). 

2 Astroph. Jour., vol. ix., p. 284. 3 Month. Not., vol. xxxvi., p. 347. 
4 Old and Neio Astronomy, p. 448. 5 Hist. Phys. Astr., p. 431. 

6 Mem. Roy. Astr. Soc, vol. xlvii., pp. 77, 84. 

7 Astr. Reg., vol. xiii., p. 132. 

8 L' Astronomic, t. ii., p. 27 ; Astr. Nach., No. 2,021 ; Am. Jour, of Sc, 
vol. xxv., p. 430. 

9 Mem. Spettr. Ital., Dicembre, 1882 ; Am. Jour, of Sc, vol. xxv., p. 328. 



chap, vii PLANETS AND SATELLITES 255 

pated; 1 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 thicken- 
ing of a few water and oxygen-lines ; but so nearly evanescent as 
to induce the persuasion that most of the light we receive from 
Venus has traversed only the tenuous upper portion of its atmo- 
sphere. 2 It is reflected, at any rate, with comparatively slight 
diminution. 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. 3 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 com- 
bination of Zollner's with Miiller's results, that its albedo is but 
little inferior to that of new-fallen snow ; in other words, it gives 
back 77 per cent, of the luminous rays impinging upon it. 

This extraordinary brilliancy would be intelligible were it per- 
missible to suppose that we see nothing of the planet but a dense 
canopy of clouds. But the hypothesis is discountenanced by the 
Flagstaff observations, and is irreconcilable with the visibility of 
mountainous elevations, and permanent surface-markings. To 
Mr. Lowell these were so distinct and unchanging as to furnish data 
for a chart of the Cytherean globe, and the peculiar arrangement of 
divergent shadings exhibited in it cannot off-hand be set down as 
unreal, in view of Perrotin's earlier discernment of analogous linear 
traces. Gruithuisen's "snow-caps," 4 however it is safe to say do 
not exist as such ; although shining regions near the poles form a 
well-attested trait of the strange Cytherean landscape. 

The " secondary," or " ashen light," of Venus was first noticed 
by Riccioli in 1643; it was seen by Derham about 1715, by Kirch 
in 1721, by Schroter and Harding in 1806 f and the reality of the 
appearance has since been authenticated by numerous and trust- 
worthy 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, 6 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 

1 Comptes Rendus, t. cxvi., p. 288. 2 Vogel, Spectra der Planeten, p. 15. 

3 Nature, vol. xix., p. 23. 

* Nova Acta Acad. Natural Curiosorum, Bd. x., 239. 

5 Astr. Jahrlnwh, 1809, p. 164. 6 Month. Not., vol. xliii., p. 331. 



256 HISTORY OF ASTRONOMY part ii 

earth-light " on Venus, at its nearest, has little more than ^Js-tf its 
intensity on the moon, we see at once that the explanation is in- 
adequate. 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 O. Lohse, at Bothkamp, 
November 3 to 11, 1871, saw the dark hemisphere partially illumi- 
nated by secondary light, extending 30 from the terminator, and 
thought the effect might be produced by a very extensive twilight. 3 
Others have had recourse to the analogy of our aurorse, and J. Lamp 
suggested that the grayish gleam, visible to him at Bothkamp, 
October 21 and 26, 1887, 4 might be an accompaniment of electrical 
processes connected with the planet's meteorology. Whatever the 
origin of the phenomenon, it may serve, on a night-enwrapt hemi- . 
sphere, to dissipate some of the thick darkness otherwise encroached 
upon only by " the pale light of stars." 

Venus was once supposed to possess a satellite. But belief in its 
existence has died out. No one, indeed, has caught even a deceptive 
glimpse of such an object during the last 125 years. Yet it was 
repeatedly and, one might have thought, well observed in the seven- 
teenth and eighteenth centuries. Fontana "discovered" it in 1645 ; 
Cassini an adept in the art of seeing recognised it in 1672, and 
again in 1686 ; Short watched it for a full hour in 1740 with varied - 
instrumental means; Tobias Mayer in 1759, Montaigne in 1761; 
several astronomers at Copenhagen in March, 1764, noted what they 
considered its unmistakable presence ; as did Horrebow in 1768. 
But M. Paul Stroobant, 5 who in 1887 submitted all the available 
data on the subject to a searching examination, identified Horrebow's 
satellite with Librae, a fifth-magnitude star; and a few other 
apparitions were, by his industry, similarly explained away. Never- 
theless, several withstood all efforts to account for them, and together 
form a most curious case of illusion. For it is quite certain that 
Venus has no such conspicuous attendant. 

The third planet encountered in travelling outward from the sun 
is the abode of man. He has in consequence opportunities for 
studying its physical habitudes altogether different from the baffling 
glimpses afforded to him of the other members of the solar family. 

1 Report Brit. Ass., 1873, p. 407. The paper contains a valuable record of 
observations of the phenomenon. 

2 Photom. Untersuchungen, p. 301. 

3 Bothkamp Beobachtungen, Heft ii., p. 126. 4 Astr. Nock., No. 2,818. 
5 M&moires de V Acad. deBruxelles, t. xlix., No. 5, 4to ; Astr. Nach., No. 2,809 ; 

Cf. Schorr, Der Venusmond, 1875. 



chap, vii PLANETS AND SATELLITES 257 

Regarding the earth, then, a mass of knowledge so varied and com- 
prehensive has been accumulated as to form a science or rather 
several sciences apart. But underneath all lie astronomical rela- 
tions, 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. Among these is the ascer- 
tainment 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 1,000 miles. 1 His argument was, that if it 
were a mere shell filled with liquid, precession and nutation would 
be much larger than they are observed to be. For the shell alone 
would follow the pull of the sun and moon on its equatorial girdle, 
leaving the liquid behind ; and being thus so much the lighter, would 
move the more readily. There is, it is true, grave reason to doubt 
whether this reasoning corresponds with the actual facts of the 
case ; 2 but the conclusion to which it led has been otherwise affirmed 
and extended. 

Indications of an identical purport have been derived from another 
kind of external disturbance, affecting our globe through the same 
agencies. Lord Kelvin (then Sir William Thomson) pointed out in 
1862 3 that tidal influences are brought to bear on land as well as on 
water, although obedience to them is perceptible only in the mobile 
element. Some bodily distortion of the earth's figure must, however, 
take place, unless we suppose it of absolute or " preternatural " 
rigidity, and the amount of such distortion 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 to- 
gether. It is to the difference in the behaviour of solid and liquid 

1 Phil. Trans., 1839, 1841, 1842. 

2 Delaunay objected (Comptes Bendiis, t. lxvii., p. 65) that the viscosity of the 
contained liquid (of which Hopkins took no account) would, where the move- 
ments were so excessively slow as those of the earth's axis, almost certainly cause 
it to behave like a solid. Lord Kelvin, however {Report Brit. Ass., 1876, ii., p. l) r 
considered Hopkins's argument valid as regards the comparatively quick solar 
semi-annual and lunar fortnightly nutations. 

8 Phil. Trans., cliii., p. 573. 

17 



258 HISTORY OF ASTRONOMY part 11 

terrestrial constituents that the ebb and flow of the waters 
are due. 

Six years later, the distinguished Glasgow professor suggested 
that this criterion might, by the aid of a prolonged series of exact 
tidal observations, be practically applied to test the interior 
condition of our planet. 1 In 1882, accordingly, suitable data ex- 
tending over thirty-three years having at length become available, 
Mr. G. H. Darwin performed the laborious task of their analysis, 
with the general result that the " effective rigidity " of the earth's 
mass must be at least as great as that of steel. 2 

Ratification from an unexpected quarter has lately been brought 
to this conclusion. The question of a possible mobility in the 
earth's axis of rotation has often been mooted. Now at last it has 
received an affirmative reply. Dr. Kiistner detected, in his observa- - 
tions of 1884-85, effects apparently springing from a minute variation 
in the latitude of Berlin. The matter having been brought before 
the International Geodetic Association in 1888, special observations 
were set on foot at Berlin, Potsdam, Prague, and Strasbourg, the 
upshot of which was to bring plainly to view synchronous, and 
seemingly periodic fluctuations of latitude to the extent of half a 
second of arc. The reality of these was verified by an expedition to 
Honolulu in 1891-92, the variations there corresponding inversely to 
those simultaneously determined in Europe. 3 Their character was ] 
completely defined by Mr. S. C. Chandler's discussion in October, 
1891. 4 He showed that they could be explained by supposing the 
pole of the earth to describe a circle with a radius of thirty feet in 
a period of fourteen months. Confirmation of this hypothesis was 
found by Dr. B. A. Gould in the Cordoba observations, 5 and it was 
provided with a physical basis through the able co-operation of 
Professor Newcomb. 6 The earth, owing to its ellipsoidal shape, 
should, apart from disturbance, rotate upon its "axis of figure," or 
shortest diameter ; since thus alone can the centrifugal forces 
generated by its spinning balance each other. Temporary causes, 
however, such as heavy falls of snow or rain limited to one conti- 
nental area, the shifting of ice-masses, even the movements of winds, 
may render the globe slightly lop-sided, and thus oblige it to forsake 
its normal axis, and rotate on one somewhat divergent from it. 
This "instantaneous axis" (for it is incessantly changing) must, by 
mathematical theory, revolve round the axis of figure in a period of 
306 days. Provided, that is to say, the earth were a perfectly 
rigid body. But it is far from being so ; it yields sensibly to every 

1 Report Brit. Ass., 1868, p. 494. 2 Ibid., 1882, p. 474. 

3 Albrecht, Astr. Nach., No. 3,131. 4 Astr. Jour., Nos. 248, 249. 

5 Ibid., No. 258. e Month. Not., vol. lii., p. 336. 



chap, vii PLANETS AND SATELLITES 259 

strain put upon it ; and this yielding tends to protract the time of 
circulation of the displaced pole. The length of its period, then, 
serves as a kind of measure of the plasticity of the globe ; which, 
according to Newcomb'sand S. S. Hough's independent calculations, 1 
seems to be a little less than that of steel. In an earth compacted 
of steel, the instantaneous axis would revolve in 441 days ; in the 
actual earth, the process is accomplished in 428 days. By this new 
path, accordingly, astronomers have been led to an identical estimate 
of the consistence of our globe with that derived from tidal investi- 
gations. 

Variations of latitude are intrinsically complex. To produce 
them, an incalculable interplay of causes must be at work, each 
with its proper period and law of action. 2 All the elements of 
the phenomenon are then in a perpetual state of flux, 3 and absorb, for 
their continual redetermination, the arduous and combined labours 
of many astronomers. Nor is this trouble superfluous. Minute in 
extent though they be, the shiftings of the pole menace the very 
foundations of exact celestial science ; their neglect would leave the 
entire fabric insecure. Just at the beginning of the present century, 
they reached a predicted minimum, but are expected again to 
augment their range after the year 1902. The interesting suggestion 
has been made by Mr. J. Halm that such fluctuations are, in some 
obscure way, affected by changes in solar activity, and conform like 
them to an eleven-year cycle. 4 

In a paper read before the Geological Society, December 15, 1830, 5 
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 unten- 
able ; 6 and it was left to the late Dr. James Croll, in 1864 7 and 
subsequent years, to reoccupy and fortify it. Within restricted 
limits (as Lagrange and, more certainly and definitely, Leverrier 
proved), the path pursued by our planet round the sun alternately 
contracts, in the course of ages, into a moderate ellipse, and expands 
almost to a circle, the major axis, and consequently the mean 

1 Astr. Nach., No. 3,097 ; Phil. Trans., vol. clxxxvi., A., p. 469 ; Proc. Roy. 
Soc., vol. lix. 

2 See Chandler's searching investigations, Astr. Jour., Nos. 329, 344, 351, 392, 
402, 406, 412, 446, 489, 490, 494, 495. 

3 Rees, Pop. Astr., No. 74, 1900. 

4 Nature, vol. lxi., p. 447 ; see also A. V. Backlund, Astr. Nach., No. 3,787. 

5 Trans. Geol. Soc, vol. iii. (2d ser.), p. 293. 

6 See his Treatise on Astronomy, p. 199 (1833). 

7 Phil. Mag., vol. xxviii. (4th ser.), p. 121. 

172 



26o HISTORY OF ASTRONOMY part ii 

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. Dr. Croll brought 
together 1 a mass of evidence to support the view that, at epochs of 
considerable eccentricity, the hemisphere of which the winter, occur- 
ring at aphelion, was both intensified and prolonged, must have 
undergone extensive glaciation ; while the opposite hemisphere, with 
a short, mild winter, and long, cool summer, enjoyed an approach 
to perennial spring. These conditions were exactly reversed at the 
end of 10,500 years, through the shifting of the perihelion combined 
with the precession of the equinoxes, the frozen hemisphere blooming 
into a luxuriant garden as its seasons came round to occur at the 
opposite sides of the terrestrial orbit, and the vernal hemisphere 
subsiding simultaneously into ice-bound rigour. 2 Thus a plausible 
explanation was offered of the anomalous alternations of glacial and 
semi-tropical periods, attested, on incontrovertible geological 
evidence, as having succeeded each other in times past over what 
are now temperate regions. They succeeded each other, it is true, 
with much less frequency and regularity than the theory demanded ; 
but the discrepancy was overlooked or smoothed away. The most 
recent glacial epoch was placed by Dr. Croll about 200,000 years 
ago, when the eccentricity of the earth's orbit was 3*4 times as great 
as it is now. At present, a faint representation of such a state of 
things is afforded by the southern hemisphere. One condition of 
glaciation in the coincidence of winter with the maximum of remote- 
ness 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 cor- 
responding 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. 3 What is still 
more serious, its acceptance seems precluded by time -relations of a 
simple kind. Dr. Wright 4 has established with some approach to 
certainty that glacial conditions ceased in Canada and the United 
States about ten or twelve thousand years ago. The erosive action 
of the Falls of Niagara qualifies them to serve as a clepsydra, or 
water-clock on a grand scale ; and their chronological indications 
have been amply corroborated elsewhere and otherwise on the same 

1 Climate and Time, 1875 ; Discussions on Climate and Cosmology, 1885. 

3 See for a popular account of the theory, Sir R. Ball's The Cause of an Ice 
Age, 1892. 3 See A. Woeikof, Phil. Mag., vol. xxi., p. 223. 

4 The Ice Age in North America, London, 1890. 



chap, vii PLANETS AND SATELLITES 261 

continent. The astronomical Ice Age, however, should have been 
enormously more antique. No reconciliation of the facts with the 
theory appears possible. 

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 4J times as 
much as an equal bulk of water, 1 was not very exact. It was con- 
siderably improved upon by Cavendish, who, in 1798, brought into 
use the " torsion-balance" constructed 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 
experiments on the subject made in 1872-73 by Cornu and Bailie 
the slightly inferior value of 5-56 was derived ; and it was further 
shown that the data collected by Baily, when corrected for a syste- 
matic error, gave practically the same result (5*55). 2 M. Wilsing's 
of 5-58, obtained at Potsdam in 1889, 3 nearly agreed with it ; while 
Professor Poynting, by means of a common balance, arrived at a 
terrestrial mean density of 5*49. 4 Professor Boys next entered the 
field with an exquisite apparatus, in which a quartz fibre performed 
the functions of a torsion-rod ; and the figure 5*53, determined by 
him, and exactly confirmed by Dr. Braun's research at Mariaschein, 
Bohemia, in 1896, 5 may be called the standard value of the required 
datum. 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 were carried 
out during the last century on an unprecedented scale. The Russo- 
Scandinavian arc, of which the measurement was completed under 
the direction of the elder Struve in 1855, reached from Hammerfest 
to Ismailia on the Danube, a length of 25 20'. But little inferior 
to it was the Indian arc, begun by Lambton in the first years of the 
century, continued by Everest, revised and extended by Walker. 
Both were surpassed in compass by the Anglo-French arc, which 
embraced 28 ; and considerable segments of meridians near the 
Atlantic and Pacific shores of North America were measured under 
the auspices of the United States Coast Survey. But these operations 
shrink into insignificance by comparison with Sir David Gill's 
grandiose scheme for uniting two hemispheres by a continuous 
network of triangulation. The history of geodesy in South Africa 

1 Phil. Trans., vol. lxviii., p. 783. 2 Comptes Pendus, t. lxxvi., p. 954. 

3 Potsdam Pull., Nos. 22, 23. 

4 Phil. Trans., vol. clxxxii., p. 565 ; Adams Prize Essay for 1893. 

5 Derikschriften Akad. der Wiss. Wien, Bd. lxiv. ; quoted by Poynting, 
Nature, vol. lxii., p. 404. 



262 HISTOR Y OF ASTRONOMY part 11 

began with Lacaille's measurements in 1752. They were repeated 
and enlarged in scope by Sir Thomas Maclear in 1841-48 ; and his 
determinations prepared the way for a complete survey of Cape 
Colony and Natal, executed during the ten years 1883-92 by Colonel 
Morris, E.E., under the direction of Sir David Gill. 1 Bechuanaland 
and Ehodesia were subsequently included in the work ; and the 
Eoyal Astronomer obtained, in 1900, the support of the International 
Geodetic Association for its extension to the mouth of the Nile. 
Nor was this the limit of his design. By carrying the survey along 
the Levantine coast, connection can be established with Struve's 
system, and the magnificent amplitude of 105 will be given to the 
conjoined African and European arcs. Meantime, the French have 
undertaken the remeasurement of Bouguer's Peruvian arc, and a 
corresponding Eusso-Swedish 2 enterprise is progressing in Spitz- 
bergen ; so that abundant materials will ere long be provided for 
fresh investigations of the shape and size of our planet. The small- 
ness of the outstanding uncertainty can be judged of by comparing 
J. B. Listing's 3 with General Clarke's 4 results, published in the same 
year (1878). Listing stated the dimensions of the terrestrial spheroid 
as follows: Equatorial radius = 3,960 miles; polar radius = 3,947 
miles ; ellipticity = 5fV*T- Clarke's corresponding figures were : 
3,963 and 3,950 miles, giving an ellipticity of -ois-T- The vaRie oi 
the latter fraction at present generally adopted is -^hz ; that is to 
say, the thickness of the protuberant equatorial ring is held to be 
-2^-2- of the equatorial radius. From astronomical considerations, it 
is true, Newcomb estimated the ratio at ^-J-g- ; 5 but for obtaining 
this particular datum, geodetical methods are unquestionably to be 
preferred. 

The moon possesses for us a unique interest. She in all pro- 
bability shared the origin of the earth ; she perhaps prefigures its 
decay. She is at present its minister and companion. Her exist- 
ence, so far as we can see, serves no other purpose than to illuminate 
the darkness of terrestrial nights, and to measure, by swiftly-recur- 
ring and conspicuous changes of aspect, the long span of terrestrial 
time. Inquiries stimulated 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. 

1 Report on the Geodetic Survey of S. Africa, 1894. 

2 Nature, vol. lxii., p. 622; Hollis, Observatory, vol. xxiii., p. 337; Poincare, 
Comptes Rendus, July 23, 1900. 

3 Astr. Nach., No. 2,228. 4 Young's Gen. Astr., p. 601. 
5 Astr. Constants, p. 195. 



chap, vii PLANETS AND SATELLITES 263 

Selenography, in the modern sense, is little more than a hundred 
years old. It originated! with the publication in 1791 of Schroter's 
Selenotopographische Fragmented Not but that the lunar surface had 
already been diligently studied, chiefly by Hevelius, Cassini, Riccioli, 
and Tobias Mayer ; the idea, however, of investigating the moon's 
physical condition, and detecting symptoms of the activity there of 
natural forces through minute topographical inquiry, first obtained 
effect at Lilienthal. Schroter's delineations, accordingly, imperfect 
though they were, afforded a starting-point for a comparative study 
of the superficial features of our satellite. 

The first of the curious objects which he named "rills" was noted 
by him in 1787. Before 1801 he had found eleven; Lohrmann 
added 75; Madler 55; Schmidt published in 1866 a catalogue 
of 425, of which 278 had been detected by himself ; 2 and he 
eventually brought the number up to nearly 1,000. They are, 
then, a very persistent lunar feature, though wholly without 
terrestrial analogue. There is no difference of opinion as to their 
nature. They are quite obviously clefts in a rocky surface, 100 to 
500 yards deep, 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. 3 

On February 24, 1792, Schroter perceived what he took to be 
distinct traces of a lunar twilight, and continued to observe them 
during nine consecutive years. 4 They indicated, he thought, the 
presence of a shallow atmosphere, about 29 times more tenuous than 
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 T ^ that of terrestrial air. 5 Newcomb 
places the maximum at t Jq. Sir John Herschel concluded "the 
non-existence of any atmosphere at the moon's edge having t -^q 
part of the density of the earth's atmosphere." 6 

This decision was fully borne out by Sir William Huggins's 
spectroscopic observation of the disappearance behind the moon's 
limb of the small star s Piscium, January 4, 1865. 7 Not the slightest 

1 The second volume was published at Gottingen in 1802. 

2 Ueber Rillen aufdem Monde, p. 13. Cf. The Moon, by T. Gwyn Elger, p. 20. 
W. H. Pickering, Harvard Annals, vol. xxxii., p. 249. 

3 TJie Moon, p. 73. 4 Men. Fragm., Th. ii., p. 399. 

5 Astr. Nach., !N T o. 263 (1834) ; Pop. Vorl., pp. 615-620 (1838). 

6 Outlines of Astr., par. 431. 7 Month. Not., vol. xxv., p. 61. 



264 HISTORY OF ASTRONOMY part n 

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 observation 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 reflective 
capability of the lunar surface, to less than one-fifth its incident 
intensity, but wholly unmodified in quality. 

Nevertheless, the diameter of the moon appeared from the 
Greenwich observations discussed by Airy in 1865 1 to be 4" smaller 
than when directly measured j and the effect would be explicable 
by refraction in a lunar atmosphere 2,000 times thinner than our 
own at the sea-level. But the difference was probably illusory. It 
resulted in part, if not wholly, from the visual enlargement by irra- 
diation of the bright disc of the moon. Professor Comstock, employ- 
ing the 16-inch Clark equatoreal of the Washburn Observatory, 
found in 1897 the refractive displacements of occulted stars so 
trifling as to preclude the existence of a permanent lunar atmo- 
sphere of much more than -^Vo - tne density of the terrestrial 
envelope. 2 The possibility, however, was admitted that, on the 
illuminated side of the moon, temporary exhalations of aqueous 
vapour might arise from ice-strata evaporated by sun-heat. Mean- 
time, some renewed evidence of actual crepuscular gleams on the 
moon had been gathered by MM. Paul and Prosper Henry of the 
Paris Observatory, as well as by Mr. W. H. Pickering, in the pure 
air of Arequipa, at an altitude of 8,000 feet above the sea. 3 An 
occultation of Jupiter, too, observed by him August 12, 1892, 4 was 
attended with a slight flattening of the planet's disc through the 
effect, it was supposed, of lunar refraction but of refraction in 
an atmosphere possessing, at the most, T ^Yo- * ne density at the 
sea-level of terrestrial air, and capable of holding in equilibrium no 
more than -^^ of an inch of mercury. Yet this small barometric 
value corresponds, Mr. Pickering remarks, " to a pressure of hundreds 
of tons per square mile of the lunar surface." The compression 
downward of gaseous strata on the moon should, in any case, 
proceed very gradually, owing to the slight power of lunar gravity, 5 
and they might hence play an important part in the economy of our 
satellite while evading spectroscopic and other tests. Thus as 

1 Month. Not, vol. xxv., p. 264. 2 Astroph, Jour., vol. vi., p. 422. 

3 Harvard Annals, vol. xxxii., p, 81. 

4 Astr. and Astrophysics, voL xi., p. 778. 6 Keison, The Moon, p. 25. 



chap, vii PLANETS AND SATELLITES 265 

Mr. Ranyard remarked 1 the cliffs and pinnacles of the moon bear 
witness, by their unworn condition, to the efficiency of atmospheric 
protection against meteoric bombardment ; and Mr. Pickering shows 
that it could be afforded by such a tenuous envelope as that postu- 
lated by him. 

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 
1840, leaving materials from which the work was completed and 
published in 1878 by Dr. Julius Schmidt, late director of the Athens 
Observatory. Much had been done in the interim. Beer and 
Madler began at Berlin in 1830 their great trigonometrical survey 
of the lunar surface, as yet neither revised nor superseded. A map, 
issued in four parts, 1834-36, on nearly the same scale as Lohr- 
mann's, but more detailed and authoritative, embodied the results. 
It was succeeded, in 1837, by a descriptive volume bearing the 
imposing title, Der Mond ; ocler allgemeine vergleichende Selenographie. 
This summation of knowledge in that branch, though in truth 
leaving many questions open, had an air of finality which tended to 
discourage further inquiry. 2 It gave form to a reaction against the 
sanguine views entertained by Hevelius, Schroter, Herschel and 
Gruithuisen as to the possibilities of agreeable residence on the 
moon, and relegated the " Selenites," one of whose cities Schroter 
thought he had discovered, and of whose festal processions Gruit- 
huisen had not despaired of becoming a spectator, to the shadowy 
land of the Ivory Gate. All examples of change in lunar forma- 
tions 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 displayed 
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. Nasmyth's and 
Carpenter's beautifully illustrated volume (1874) was succeeded, 
after two years, by a still more weighty contribution to lunar 
science in Mr. Neison's well-known book, accompanied by a map, 
based on the survey of Beer and Madler, but adding some 500 

1 Knowledge, vol. xvii., p. 85. 2 Neiaon, The Moon, p. 104. 



266 HISTORY OF ASTRONOMY part 11 

measures of positions, besides the representation of several thousand 
new objects. With Schmidt's Charte der Gebirge der Mondes, 
Germany once more took the lead. This splendid delineation, 
built upon Lohrmann's foundation, embraced the detail contained 
in upwards of 3,000 original drawings, representing the labour of 
thirty-four years. No less than 32,856 craters are represented in it, 
on a scale of seventy-five inches to a diameter. An additional help 
to lunar inquiries was provided at the same time in this country by 
the establishment, through the initiative of the late Mr. W. R Birt, 
of the Selenographical Society. 

But the strongest incentive to diligence in studying the rugged | 
features of our celestial helpmate has been the idea of probable or I 
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 conformation reveals itself indirectly through 
irregularities in the distribution of light and darkness. The forms 
of its elevations and depressions can be inferred only from the 1 
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." 1 The result is, that no single observation can be 
exactly repeated by the same observer, since identical conditions 
recur only after the lapse of a great number of years. 

Local peculiarities of surface, besides, are liable to produce per- 
plexing effects. The reflection of earth-light at a particular angle 
from certain bright summits completely, though temporarily, 
deceived Herschel into the belief that he had witnessed, in 1783 
and 1787, volcanic outbursts on the dark side of the moon. The 
persistent recurrence, indeed, of similar appearances under circum- 
stances less amenable to explanation inclined Webb to the view 

1 The combination of a uniform rotational with an unequal orbital movement 
causes a slight swaying of the moon's globe, now east, now west, by which we 
are able 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 produced 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 T 2 T of the invisible side come into view. 



chap, vii PLANETS AND SATELLITES 267 

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 crater "Linne," in the 
Mare Serenitatis, 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, 
scarcely two miles across, emerges into view. 3 The crateral char- 
acter of this comparatively minute depression was detected by 
Father Secchi, February 11, 1867. 

This is not all. Schroter's description of Linne, as seen by 
him November 5, 1788, tallies quite closely with modern observa- 
tion; 4 while its inconspicuousness in 1797 is shown by its omission 
from Russell's lunar globe and maps. 5 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 the century, and has since passed away, perhaps again 
to recur. The latter hypothesis seems the more probable ; and its 
probability is strengthened by much evidence of actual obscuration 
or variation of tint in other parts of the lunar surface, more 
especially on the floor of the great "walled plain" named "Plato." 6 
From a re-examination with a 13-inch refractor at Arequipa in 
1891-92, of this region, and of the Mare Serenitatis, Mr. W. H. 
Pickering inclines to the belief that lunar volcanic action, once 
apparently so potent, is not yet wholly extinct. 7 

An instance of an opposite kind of change was alleged by 
Dr. Hermann J. Klein of Cologne in March, 1878. 8 In Linne the 

1 Cel. Objects, p. 58 (4th ed.). 2 Astr. Nach., No. 1,631. 

3 Of. Leo Brenner, Naturvnss. Wochenschrift, January 13, 1895 ; Jour. Brit. 
Astr. Ass., vol. v., pp. 29, 222. 

4 Respighi, LesMondes, t. xiv., p. 294 ; Huggins, Month. Not., vol. xxvii., p. 298. 

6 Birt, Ibid., p. 95. 6 Report Brit. Ass., 1872, p. 245. 

7 Observatory, vol. xv., p. 250. 

8 Astr. Reg., vol. xvi., p. 265 ; Astr. Nach., No. 2,275. 



268 HISTORY OF ASTRONOMY part n 

obliteration of an old crater had been assumed ; in " Hyginus N.," 
the formation of a new crater was asserted. Yet, quite possibly, I 
the same cause may have produced the effects thought to be I 
apparent in both. It is, however, far from certain that any real I 
change has affected the neighbourhood 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 fluctua- 
tions of light and shade. 1 Moreover, it seems to be certain, from 
Messrs. Pratt and Capron's attentive study, that " Hyginus N." is ! 
no true crater, but a shallow, saucer-like depression, difficult of clear 
discernment. 2 Under suitable illumination, nevertheless, it contains, 
and is marked by, an ample shadow. 3 

In both these controverted instances of change, lunar photography 
was invoked as a witness ; but, notwithstanding the great advances 
made in the art by De la Rue in this country, by Draper, and, 
above all, by Rutherfurd in America, without decisive results. 
Investigations of the kind began to assume a new aspect in 
1890, when Professor Holden organised them at the Lick Observa- 
tory. 4 Autographic moon-pictures were no longer taken casually, 
but on system; and Dr. Weinek's elaborate study, and skilful re- 
productions of them at Prague, 5 gave them universal value. They 
were designed to provide materials for an atlas on the scale of 
Beer and Madler's, of which some beautiful specimen-plates have 
been issued. At Paris, in 1894, with the aid of a large "equatoreal 
coude," a work of similar character was set on foot by MM. Loewy 
and Puiseux. Its progress has been marked by the successive 
publication of five instalments of a splendid atlas, on a scale of 
about eight feet to the lunar diameter, accompanied by theoretical 
dissertations, designed to establish a science of "selenology." The 
moon's formations are thus not only delineated under every variety 
of light-incidence, but their meaning is sought to be elicited, and 
their history and mutual relations interpreted. 6 Henceforth, at any 
rate, the lunar volcanoes can scarcely, without notice taken, breathe 
hard in their age-long sleep. 

1 Lindsay and Copeland, Month. Not., vol. xxxix., p. 195. 

2 Observatory, vols, ii., p. 296 ; iv., p. 373. N. E. Green (Astr. Reg., vol. xvii., 
p. 144) concluded the object a mere "spot of colour," dark under oblique light. 

3 Webb, Cel. Objects, p. 101. 

4 Publ. Lick Observatory, vol. iii., p. 7. 

5 Ibid., p. 21 ; Mee, Knowledge, vol. xviii., p. 135. 

6 Comptes Rcndus, t. cxxii., p. 967 ; Bull. Astr., August, 1899 ; Ann. Bureau 
des Long., 1898 ; Nature, vols. Iii., p. 439 ; lvi., p. 280 ; lix., p. 304 ; lx., p. 491 ; 
Astroph. Jour., vol. vi., p. 51. 



chap, vii PLANETS AND SATELLITES 269 

Melloni was the first to get undeniable heating effects from 
moonlight. His experiments, made on Mount Vesuvius early in 
1846, 1 were repeated with like result by Zantedeschi at Venice four 
years later. A rough measure of the intensity of those effects was 
arrived at by Piazzi Smyth at Guajara, on the Peak of Teneriffe, in 
1856. At a distance of fifteen feet from the thermomultiplier, a 
Price's candle was found to radiate just twice as much heat as the 
full moon. 2 Then, after thirteen years, in 1869-72, an exact and ex- 
tensive series of observations on the subject were made by the present 
Earl of Eosse. The lunar radiations, from the first to the last 
quarter, displayed, 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. 3 (corrected later to 
197), 4 the moon thus acting as a direct radiator no less than as a 
reflector of heat. But the 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, 1884. 5 This 
initial opportunity of measuring the heat-phases of an eclipsed moon 
was used with the remarkable result of showing that the heat dis- 
appeared 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 ob- 
servations at Allegheny of the partial eclipse of September 23, 1885. 6 
Yet it is certain that the moon sends us a perceptible quantity of 
heat on its own account, besides simply throwing back solar radiations. 
For 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 is made up 
of two superposed spectra, one due to reflection, the other, with a 
maximum far down in the infra-red, to radiation. 7 The correspond- 
ing temperature of the moon's sunlit surface Professor Langley 

1 Comptes Rendus, t. xxii., p. 541. 

2 Phil. Trans., vol. cxlviii., p. 502. 

3 Proc. Roy. Soc, vol. xvii., p. 443. 4 Phil. Trans., vol. clxiii., p. 623. 
3 Trans. R. Dublin Soc, vol. iii., p. 321. 6 Science, vol. vii., p. 9. 
7 Amer. Jour. 0/ Science, vol. xxxviii., p. 428. 



2 7 o HISTORY OF ASTRONOMY part ii 

considers to be about that of freezing water. 1 Kepeated experi- 
ments having failed to get any thermal effects from the dark part of 
the moon, it was inferred that our satellite " has no internal heat 
sensible at the surface " ; so that the radiations from the lunar soil 
giving the low maximum in the heat-spectrum, "must be due purely 
to solar heat which has been absorbed and almost immediately re- 
radiated." Professor Langley's explorations of the terra incognita 
of immensely long wave-lengths where lie the unseen heat-emissions 
from the earth into space, led him to the discovery that these, 
contrary to the received opinion, are in good part transmissible by 
our atmosphere, although they are completely intercepted by glass. 
Another important result of the Allegheny work was the abolition 
of the anomalous notion of the "temperature of space," fixed by 
Pouillet at -140 C. For space in itself can have no temperature, 
and stellar radiation is a negligible quantity. Thus, it is safe to 
assume "that a perfect thermometer suspended in space at the 
distance of the earth or moon from the sun, but shielded from its 
rays, would sensibly indicate the absolute zero," 2 ordinarily placed 
at-273C. 

A " Prize Essay on the Distribution of the Moon's Heat " (The 
Hague), 1891, by Mr. Frank W. Very, who had taken an active 
part in Professor Langley's long-sustained inquiry, embodies the 
fruits of its continuation. They show the lunar disc to be tolerably 
uniform in thermal power. The brighter parts are also indeed 
hotter, but not much. The traces perceived of a slight retention 
of heat by the substances forming the lunar surface, agreed well 
with the Parsonstown observations of the total eclipse of the moon, 
January 28, 1888. 3 For they brought out an unmistakable diver- 
gence between the heat and light phases. A curious decrease of heat 
previous to the first touch of the earth's shadow upon the lunar 
globe remains unexplained, unless it be admissible to suppose the 
terrestrial atmosphere capable of absorbing heat at an elevation of 
190 miles. The probable range of temperature on the moon was 
discussed by Professor Very in 1898. 4 He concluded it to be very 
wide. Hotter than boiling water under the sun's vertical rays, the 
arid surface of our dependent globe must, he found, cool in the 
14-day lunar night to about the temperature of liquid air. 

Although that fundamental part of astronomy known as "celestial 

1 "The Temperature of the Moon," Memoirs National Acad, of Sciences, 
vol. iv., p. 193, 1889. 

2 Temperature of the Moon, p. iii. ; see also App. ii., p. 206. 

3 Trans. R. Dublin Soc., vol. iv., p. 481, 1891 ; Rosse, Proc. Roy. Institution, 
May 31, 1895. 

4 Astroph. Jour., vol. viii., pp. 199, 265. 



chap, vii PLANETS AND SATELLITES 271 

mechanics" lies outside the scope of this work, and we therefore 
pass over in silence the immense labours of Plana, Damoiseau, 
Hansen, Delaunay, G. W. Hill, and Airy in reconciling the observed 
and calculated motions of the moon, there is one slight but sig- 
nificant 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 2,000 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 proportionately quickens her pace. Many thousands of years 
hence the process will be reversed j the terrestrial orbit will close in 
at the sides, the lunar orbit will open out under the growing stress 
of solar gravity, and our celestial chronometer will lose instead of 
gaining time. 

This is all quite true as Laplace put it ; but it is not enough. 
Adams, the virtual discoverer of Neptune, found with surprise in 
1853 that the received account of the matter was "essentially in- 
complete," and explained, when the requisite correction was intro- 
duced, only half the observed acceleration. 2 What was to be done 
with the remaining half % Here Delaunay, the eminent French 
mathematical astronomer, unhappily drowned at Cherbourg in 1872 
by the capsizing of a pleasure-boat, came to the rescue. 3 

It is obvious to anyone who considers the subject a little atten- 
tively, 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 move- 
ments of the moon. The moon, accordingly, holds them against the 
whirling earth, which revolves like a shaft in a fixed collar, slowly 

1 Airy, Observatory, vol. iii., p. 420. 

3 Phil. Tram., vol. cxliii., p. 397 ; Proc. Roy. Soc, vol. vi., p. 321. 

Comptes Rendus, t. lxi. , p. 1023. 



272 HISTORY OF ASTRONOMY part ii 

losing motion and gaining heat, eventually dissipated through space. 1 
This must go on (so far as we can see) until the periods of the 
earth's rotation and of the moon's revolution coincide. Nay, the 
process will be continued should our oceans survive so long by 
the feebler tide-raising power of the sun, ceasing only when day and 
night cease to alternate, when one side of our planet is plunged in 
perpetual darkness and the other seared by unchanging light. 

Here, then, we have the secret of the moon's turning always the 
same face towards the earth. It is that in primeval times, when the 
moon was liquid or plastic, an earth-raised tidal wave rapidly and 
forcibly reduced her rotation to its present exact agreement with 
her period of revolution. This was divined by Kant 2 nearly a 
century before the necessity for such a mode of action presented 
itself to any other thinker. In a weekly paper published at Konigs- 
berg 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 consum- 
mated on the rotation of the moon the whole forming a pre- 
liminary 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 ; 3 while similar, and probably original, con- 
clusions were reached by William Ferrel of Allensville, Kentucky, 
in 1858. 4 

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, completely neutralised by 
the contraction of the globe through cooling, was a fact to be 
reckoned with in computing the movements, as well as in specu- 
lating on the history, of the heavenly bodies. The outstanding 
acceleration of the moon was thus at once explained. 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 in- 
vestigating the errors of Hansen's Lunar Tables as compared with 

1 Professor Darwin calculated 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. 

2 Sdmmtl. Werke (ed. 1839), Th. vi., pp. 5-12. See also C. J. Monro's useful 
indications in Nature, vol. vii., p. 241. 

3 Dynamik des Himmels, p. 40. 4 Gould's Astr. Jour., vol. iii., p. 138. 



chap, vii PLANETS AND SATELLITES 273 

observations prior to 1750. The results, published in 1878, 1 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 diminu- 
tion, and at the same time conciliate Alexandrian and Arabian 
observations, it is necessary to reject as total the ancient solar 
eclipses known as those of Thales and Larissa. This may be a neces- 
sary, but it must be admitted to be a hazardous expedient. Its 
upshot was to indicate a possibility that the observed and calculated 
values of the moon's acceleration might after all prove to be iden- 
tical ; and the small outstanding discrepancy was still further 
diminished by Tisserand's investigation, differently conducted, of the 
same Arabian eclipses discussed by Newcomb. 2 The necessity of 
having recourse to a lengthening day is then less pressing than it 
seemed some time ago ; and the effect, if perceptible in the moon's 
motion, should, M. Tisserand remarked, be proportionately so in the 
motions of all the other heavenly bodies. The presence of the 
apparent general acceleration that should ensue can be tested with 
most promise of success, according to the same authority, by delicate 
comparisons of past and future transits of Mercury. 

Newcomb further showed 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. 3 If set down to the 
account of imperfections in the "time-keeping" of the earth, it 
could only be on the arbitrary supposition of fluctuations in its rate 
of going themselves needing explanation. This, it is true, might be 
found in very slight changes of figure, 4 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 prerogative of assured prediction. 

1 Wash. Obs. for 1875, vol. xxii., App. ii. 

2 Comptes fiendus, t. cxiii., p. 669 ; Annuaire, Paris, 1892. 

3 Newcomb, Pop. Astr. (4th ed.), p. 101. 

4 Sir W. Thomson, Report Brit. Ass., 1876, p. 12. 



18 



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 1783, 1 
and so we may safely say to-day, after six score further years of 
scrutiny. The circumstance lends a particular interest to inquiries 
into the physical habitudes of our exterior planetary neighbour. 

Fontana first caught glimpses, at Naples in 1636 and 1638, 2 of 
dusky stains on the ruddy disc of Mars. They were next seen by 
Hooke and Cassini in 1666, and this time with sufficient distinctness 
to serve as indexes to the planet's rotation, determined by the 
latter as taking place in a period of twenty-four hours forty 
minutes. 3 Increased confidence was given to this result through 
Maraldi's precise verification of it in 1719. 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. The inference of frozen pre- 
cipitations could scarcely be resisted when once it was clearly 
perceived that the shining polar zones did actually by turns 
diminish 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 
was accompanied by a steady assertion, on Herschel's part, of 
permanence in the dark markings, notwithstanding partial obscura- 
tions by clouds and vapours floating in a " considerable but moderate 
atmosphere." Hence the presumed inhabitants of the planet were 

1 Phil. Trans., vol. lxxiv., p. 260. 2 Novce Observationes, p. 105. 

3 Phil. Trans., vol. i., p. 243. * M6m. de VAc, 1720, p. 146. 



chap, vni PLANETS AND SATELLITES 275 

inferred to " probably enjoy a situation in many respects similar to 



ours. 



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 condensation of 
subjacent but unseen areographical features; 2 and his opinion 
prevailed with his contemporaries. It was, however, rejected by 
Kunowsky in 1822, and finally overthrown by Beer and Madler's 
careful studies during five consecutive oppositions, 1830-39. They 
identified at each the same dark spots, frequently blurred with 
mists, especially when the local winter prevailed, but fundamentally 
unchanged. 3 In 1862 Lockyer established a "marvellous agreement" 
with Beer and Madler's results of 1830, leaving no doubt as to the 
complete 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 1672 with a 
writing-pen in his diary. 5 From these data the Leyden observer 
arrived at a period of rotation of 24h. 37m. 22*62s., being just one 
second shorter than that deduced, exclusively from their own 
observations, by Beer and Madler. The exactness of this result was 
practically confirmed by the inquiries of Professor Bakhuyzen of 
Leyden. 6 Using for a middle term of comparison the disinterred 
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 (24h. 37m. 22-735s.) ascribed to Mars by Mr. Proctor 7 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 j for, 
owing to the comparative 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 

1 Phil. Trans., vol. lxxiv., p. 273. 

2 A large work, entitled Areographische 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 Beitrage, p. 124. 4 Mem. JR. A. Soc., vol. xxxii., p. 183. 
5 Astr. Nach., No. 1,468. 6 Observatory, vol. viii., p. 437. 
7 Month. Not., vols, xxviii., p. 37; xxix., p. 232; xxxiii., p. 552. 

182 



276 HISTORY OF ASTRONOMY part n 

of 24h. 37m. 22*66s. is undoubtedly of a precision unapproached 
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 general permanence of certain dark gray or 
greenish patches, perceived with the telescope as standing out from 
the deep yellow ground of the disc. That these varieties of tint 
correspond to the real diversities of a terraqueous globe, the " ripe 
cornfield ' n sections representing land, the dusky spots and streaks, 
oceans and straits, has long been the prevalent opinion. Sir J. Herschel 
in 1830 led the way in ascribing the redness of the planet's light 
to an inherent peculiarity of soil. 2 Previously it had been assimi- 
lated 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 seventeenth century, vanished before the sharp 
occultation of a small star in Leo, witnessed by Sir James South 
in 1822 ; 3 and Dawes's observation in 1865, 4 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 Sir William 
Huggins in 1867. 5 

All recent observations tend to show that the atmosphere of Mars 
is much thinner than our own. This was to have been expected 
a priori, since the same proportionate mass of air would on his 
smaller globe form a relatively sparse covering. 6 Besides, gravity 
there possesses less than four-tenths its force here, so that this 
sparser covering would weigh less, and be less condensed, 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 7 led him to conclude that fully twice as much light 

1 Flammarion, V 'Astronomic, t. i., p. 266. 
3 Smyth, Gel. Cycle, vol, L, p. 148 (1st ed.). 

3 Phil. Trans., vol. cxxi., p. 417. 

4 Month. Not., vol. xxv., p. 227. 5 Phil. Mag., vol. xxxiv., p. 75. 

6 Proctor, Quart. Jour, of Science, vol. x., p. 185; Maunder, Sunday Mag., 
January, February, March, 1882 ; Campbell, Publ. Astr. Pac. Soc, vol. vi., p. 273. 

7 Am. Jour, of Sc, vol. xxviii., p. 163. 



chap, viii PLANETS AND SATELLITES 277 

is absorbed by our air as had previously been supposed say 40 
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 sand- 
stone ; 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. 

This consideration at once shows us how much more tenuous the 
Martian air must be, since it admits of topographical delineations 
of the Martian globe. The clouds, too, that form in it seem in 
general to be rather of the nature of ground-mists than of heavy 
cumulus. 1 Occasionally, indeed, durable and extensive strata 
become visible. During the latter half of October, 1894, for instance, 
a region as large as Europe remained apparently cloud-covered. Yet 
most recent observers are unable to detect the traces of aqueous 
absorption in the Martian spectrum noted by Huggins in 1867 2 and 
by Vogel in 1873. 3 Campbell vainly looked for them, 4 visually in 
1894, spectrographically in 1896 ; Keeler was equally unsuccessful ; 5 
Jewell 6 holds that they could, with present appliances, only be 
perceived if the atmosphere of Mars were much richer in water- 
vapour than that of the earth. There can be little doubt, however, 
that its supply is about the minimum adequate to the needs of a 
living, and perhaps a life-nurturing planet. 

The climate of Mars seems to be unexpectedly mild. Its 
theoretical mean temperature, taking into account both distance 
from the sun and albedo, is 34 C. below freezing. 7 Yet its polar 
snows are both less extensive and less permanent than those on 
the earth. The southern white hood, noticed by Schiaparelli in 1877 
to have survived the summer only as a small lateral patch, melted 
completely in 1894. Moreover, Mr. W. H. Pickering observed 
with astonishment the disappearance, in the course of thirty-three 
days of June and July, 1892, of 1,600,000 square miles of southern 

1 Burton, Trans. Boy. Dublin Soc, vol. i., 1880, p. 169. 

2 Month. Not., vol. xxvii., p. 179 ; Astroph. Journ., vol. i., p. 193. 

3 Untersuchungen iiber die Spectra der Planeten, p. 20 ; Astroph. Journ., vol. i., 
p. 203. 

4 Pull. Astr. Pac. Soc., vols, vi., p. 228 ; ix., p. 109 ; Astr. and Astroph., 
vol. xiii., p. 752 ; Astroph. Jour., vol. ii., p. 28. 

5 Ibid., vol. v., p. 328. 6 Ibid., vols, i., p. 311 ; iii., p. 254. 
7 C. Christiansen, Beibldtter, 1886, p. 532. 



278 HISTORY OF ASTRONOMY part n 

snow. 1 Curiously enough, the initial stage of shrinkage in the 
white calotte was marked by its division into two unequal parts, as 
if in obedience to the mysterious principle of duplication governing 
so many Martian phenomena. 2 Changes of the hues associated 
respectively with land and water accompanied in lower latitudes, 
and were thought to be occasioned by floods ensuing upon this rapid 
antarctic thaw. It is true that scarcity of moisture would account 
for the scantiness and transitoriness of snowy deposits easily liquefied 
because thinly spread. But we might expect to see the whole 
wintry hemisphere, at any rate, frost-bound, 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 nowadays 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 -g-^Vii 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 con- 
firm. 

The first really good map of Mars was constructed in 1869 by 
Proctor from drawings by Dawes. Kaiser of Leyden followed in 
1872 with a representation founded upon data of his own providing 
in 1862-64 ; and Terby, in his valuable ArSographie, presented to the 
Brussels Academy in 1873 3 a careful discussion of all important ob- 
servations from the time of Fontana downwards, thus virtually 
adding to knowledge by summarising and digesting it. The 
memorable opposition of September 5, 1877, marked a fresh epoch 
in the study of Mars. While executing a trigonometrical survey 
(the first attempted) of the disc, then of the unusual size of 25" 
across, G. V. Schiaparelli, director of the Milan Observatory, 
detected a novel and curious feature. What had been taken for 
Martian continents were found to be, in point of fact, agglomerations 
of islands, separated from each other by a network of so-called 
" canals " (more properly channels)* These are obviously extensions 
of the " seas," originating and terminating in them, and sharing 
their gray-green hue, but running sometimes to a length of three or 

1 Astr. and Astrophysics, vol. xi., p. 671. 

2 Flammarion, La Planete Mars, p. 574. 

3 Memoires Couronnds, t. xxxix. 4 Lockyer, Nature, vol. xlvi., p. 447. 



chap, vni PLANETS AND SATELLITES 279 

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 drawings in his possession 
showed M. Terby that they had been seen, though not distinctively 
recognised, by Dawes, Secchi, and Holden; several were in- 
dependently traced out by Burton at the opposition of 1879; all 
were recovered by Schiaparelli himself in 1879 and 1881-82 ; and 
their indefinite multiplication resulted from Lovell's observations in 
1894 and 1896. 

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, 1881, and February, 1882. The canals were all again 
there, but this time they were in as many as twenty cases seen 
in duplicate. That is to say, a twin-canal ran parallel to the 
original one at an interval of 200 to 400 miles. 1 

We are here brought face to face with an apparently insoluble 
enigma. Schiaparelli regards the " germination " of his canals as a 
periodical phenomenon depending on the Martian seasons. It is, 
assuredly, not an illusory one, since it was plainly apparent, during 
the opposition of 1886, to MM. Perrotin and Thollon at Nice, 2 and 
to the former, using the new 30-inch refractor of that observatory, 
in 1888 ; Mr. A. Stanley Williams, with the help of only a 6|-inch 
reflector, distinctly perceived in 1890 seven of the duplicate objects 
noted at Milan, 3 and the Lick observations, both of 1890 and of 
1892, together with the drawings made at Flagstaff" and Mexico 
during the last favourable oppositions of the nineteenth century, 
brought unequivocal confirmation to the accuracy of Schiaparelli's 
impressions. 4 Various conjectures have been hazarded in explanation 
of this bizarre appearance. The difficulty of conceiving a physical 
reality corresponding to it has suggested recourse to an optical 
rationale. Proctor regarded it as an effect of diffraction f Stanislas 

1 Mem. Spettr. Italiani, t. xi., p. 28. 2 Bull. Astr., t. iii., p. 324. 

3 Journ. Brit. Astr. Ass., vol. i., p. 88. 

4 Publ. Pac. Astr. Soc, vol. ii., p. 299 ; Percival Lowell, Mars, 1896 ; Annals 
of the Lowell Observatory, vol. ii., 1900. 

5 Old and New Astr., p. 545. 



2 8o HISTORY OF ASTRONOMY part ii 

Meunier, of oblique reflection from overlying mist-banks j 1 Flam- 
marion considers it possible that companion-canals might, under 
special circumstances, be evoked by refraction as a kind of mirage. 2 
But none of these speculations are really admissible, when all the 
facts are taken into account. The view that the canals of Mars are 
vast rifts due to the cooling of the globe, is recommended by the 
circumstance that they tend to follow great circles; nevertheless, 
it would break down if, as Schiaparelli holds, the fluctuations in 
their visibility depend upon actual obliterations and re-emergences. 
Fantastic though the theory of their artificial origin appear, it is 
held by serious astronomers. Its vogue is largely due to Mr. Lowell's 
ingenious advocacy. He considers the Martian globe to be every- 
where intersected by an elaborate system of irrigation-works, rendered 
necessary by a perennial water-famine, relieved periodically by the 
melting of the polar snows. Nor does he admit the existence of 
oceans, or lakes. What have been taken for such are really tracts 
covered with vegetation, the bright areas intermixed with them re- 
presenting sandy deserts. And it is noteworthy in this connection 
that Professor Barnard obtained in 1894, 3 with the great Lick 
refractor, " suggestive and impressive views " disclosing details of 
light and shade on the gray-green patches so intricate and minute 
as almost to preclude the supposition of their aqueous nature. 

The closeness of the terrestrial analogy has thus of late been much 
impaired. Even if the surface of Mars be composed of land and 
water, their distribution must 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 ; 4 and Schiaparelli carries most observers with him in 
holding that their outlines are not absolutely constant, encroach- 
ments of dusky upon bright tints suggesting extensive inundations. 5 
The late N. E. Green's observations at Madeira in 1877 indicated, 
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. 6 Still more elevated, if 
similarly explained, must be the " ice island " first seen in a com- 
paratively low latitude by Dawes in January, 1865. 

On August 4, 1892, Mars stood opposite to the sun at a distance 
of only 34,865,000 miles from the earth. In point of vicinity, then, 
its situation was scarcely less favourable than in 1877. The low 

1 L'Astronomie, t. xi., p. 445. 2 La Planete Mars, p. 588. 

3 Month. Notices, vol. lvi., p. 166. 4 L'Astronomie, t. viii. 

5 Astr. Nach., No. 3,271 ; Astr. and Astrophysics, vol. xiii., p. 716. 

6 Month. Not., vol. xxxviii., p. 41 ; Mem. Roy. Astr. Soc, vol. xliv., p. 123. 



chap, vni PLANETS AND SATELLITES 281 

altitude of the planet, however, practically neutralised this advan- 
tage for northern observers, and public expectation, which had been 
raised to the highest pitch by the announcements of sensation- 
mongers, was somewhat disappointed at the "meagreness" of the 
news authentically received from Mars. Valuable series of observa- 
tions were, nevertheless, made at Lick and Arequipa; and they 
unite in testifying to the genuine prevalence of surface-variability, 
especially in certain regions of intermediate tint, and perhaps of the 
" crude consistence " of " boggy Syrtes, neither sea, nor good dry 
land." Professor Holden insisted on the " enormous difficulties in 
the way of completely explaining the recorded phenomena by 
terrestrial analogies"; 1 Mr. W. H. Pickering spoke of "conspicuous 
and startling changes." They, however, merely overlaid, and 
partially disguised, a general stability. Among the novelties 
detected by Mr. Pickering were a number of "lakes," or "oases" 
(in Lowell's phraseology), under the aspect of black dots at the 
junctions of two or more canals ; 2 and he, no less than the Lick 
astronomers and M. Perrotin at Nice, 3 observed brilliant clouds 
projecting beyond the terminator, or above the limb, while carried 
round by the planet's rotation. They seemed to float at an altitude 
of at least twenty miles, or about four times the height of terrestrial 
cirrus; but this was not wonderful, considering the low power of 
gravity acting upon them. Great capital was made in the journal- 
istic interest out of these imaginary signals from intelligent 
Martians, desirous of opening communications with (to them) 
problematical terrestrial beings. Similar effects had, however, been 
seen before by Mr. Knobel in 1873, by M. Terby in 1888, and at the 
Lick Observatory in 1890; and they were discerned again with 
particular distinctness by Professor Hussey at Lick, August 27, 
1896.* 

The first photograph of Mars was taken by Gould at Cordoba in 
1879. Little real service in planetary delineation has, it is true, 
been so far rendered by the art, yet one achievement must be 
recorded to its credit. A set of photographs obtained by Mr. 
W. H. Pickering on Wilson's Peak, California, April 9, 1890, 
showed the southern polar cap of Mars as of moderate dimensions, 
but with a large dim adjacent area. Twenty -four hours later, on 
a corresponding set, the dim area was brilliantly white. The polar 
cap had become enlarged in the interim, apparently through a 
wide-spreading snow-fall, by the annexation of a territory equal to 

1 Astr. and Astrophysics, vol. xi., p. 668. - Ibid,, p. 850. 

3 Comptes Rendus, t. cxv., p. 379. 

4 Astr. Jour., No. 384 ; Publ. Astr. Pae. Soc, vol. vL, p. 109. Cf. Observatory, 
vol. xvii., pp. 295-336. 



282 HISTORY OF ASTRONOMY part n 

that of the United States. The season was towards the close of 
winter in Mars. Never until then had the process of glacial 
extension been actually (it might be said) superintended in that 
distant globe. 

Mars was gratuitously supplied with a pair of satellites long 
before he was found actually to possess them. Kepler interpreted 
Galileo's anagram of the "triple" Saturn in this sense; they 
were perceived by Micromegas on his long voyage through space ; 
and the Laputan astronomers had even arrived at a knowledge, 
curiously accurate under the circumstances, of their distances 
and periods. But terrestrial observers could see nothing of them 
until the night of August 11, 1877. The planet was then within 
one month of its second nearest approach to the earth during 
the last century ; and in 1845 the Washington 26-inch refractor 
was not in existence. 1 Professor Asaph Hall, accordingly, deter- 
mined 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 11. Bad weather, however, intervened, and it 
was not until the 16th 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 passages 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 comparatively small instru- 
ments. At the opposition of 1884, indeed, the distance of the 
planet was too great to permit of the detection of both elsewhere 
than at Washington. But the Lick equatoreal showed them, 
July 18, 1888, when their brightness was only 0*12 its amount at 
the time of their discovery j so that they can now be followed for 
a considerable time before and after the least favourable opposi- 
tions. 

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 interest- 
ing and remarkable bodies. As to size, they may be said to stand 
midway between meteorites and satellites. From careful photo- 
metric measures executed at Harvard in 1877 and 1879, Professor 
Pickering concluded their diameters to be respectively six and 
seven miles. 3 This is on the assumption that they reflect the same 

1 See Mr. Wentworth Erck's remarks in Trans. Roy. Dublin Soc, vol. i., p. 29. 

2 Month. Not., vol. xxxviii., p. 206. 

8 Annals Harvard Coll. Obs., vol. xi., pt. ii., p. 217. 



chap, vni PLANETS AND SATELLITES 283 

proportion of the light incident upon them that their primary does. 
But it may very well be that they are less reflective, in which case 
they would be more extensive. The albedo of Mars is put by 
M tiller at 027 ; his surface, in other words, returns 27 per cent, of 
the rays striking it. If we put the albedo of his satellites equal to 
that of our moon, 0*17, their diameters will be increased from 6 and 
7 to 7J and 9 miles, Phobos, the inner one, being the larger. 
Mr. Lowell, however, formed a considerably larger estimate of their 
dimensions. 1 It is interesting to note that Deimos, according to 
Professor Pickering's very distinct perception, does not share the 
reddish tint of Mars. 

Deimos completes its nearly circular revolutions in thirty hours 
eighteen minutes, at a distance from the surface of its ruling body 
of 12,500 miles; Phobos traverses an elliptical orbit 2 in seven hours 
thirty -nine minutes twenty -two seconds, at a distance of only 
3,760 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 
curvature 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 thronging stars amid 
which they seem to move; 1875 brought seventeen such recogni- 
tions; their number touched a minimum of one in 1881 ; it rose in 
1882, and again in 1886, to eleven ; dropped to six in 1889, and 
sprang up with the aid of photography to twenty-seven in 1892. 
That high level has since, on an average, been maintained ; and on 
January 1, 1902, nearly 500 asteroids were recognised as revolving 
between the orbits of Mars and Jupiter. Of these, considerably 
more than one hundred are claimed by one investigator alone 
Dr. Max Wolf of Heidelberg; M. Charlois of Nice comes second 
with 102 ; while among the earlier observers Palisa of Vienna con- 
tributed 86, and C. H. F. Peters of Clinton (N.Y.), whose varied 
and useful career terminated July 19, 1890, 52 to the grand total. 

1 Young, Gen. Astr., p. 366. 

2 Campbell, Pull. Pae. Astr. Soc, vol. vi., p. 270. 



284 HISTORY OF ASTRONOMY part ii 

The construction by Chacornac and his successors at Paris, and 
more recently by Peters at Clinton, of ecliptical charts showing all 
stars down to the thirteenth and fourteenth magnitudes respectively, 
rendered the picking out of moving objects above that brightness 
a mere question of time and diligence. Both, however, are vastly 
economised by the photographic method. Tedious comparisons of 
the sky with charts are no longer needed for the identification of 
unrecorded, because simulated stars. Planetary bodies declare 
themselves by appearing upon the plate, not in circular, but in linear 
form. Their motion converts their images into trails, long or short 
according to the time of exposure. The first asteroid (No. 323) thus 
detected was by Max Wolf, December 22, 1891. 1 Eighteen others 
were similarly discovered in 1892, by the same skilful operator ; and 
ten more through Charlois's adoption at Nice of the novel plan now 
in exclusive use for picking up errant light-specks. Far more 
onerous than the task of their discovery is that of keeping them in 
view once discovered 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 cui bono ? however, began to be agitated. Was it worth 
while to maintain a staff of astronomers for the sole purpose of 
keeping hold over the identity of the innumerable component 
particles of a cosmical ring 1 The prospect, indeed, of all but a select 
few of the asteroids being thrown back by their contemptuous 
captors into the sea of space seemed so imminent that Professor 
Watson provided by will against the dereliction of the twenty-two 
discovered by himself. But the fortunes of the whole family im- 
proved through the distinction obtained by one of them. On 
August 14, 1898, the trail of a rapidly-moving, star-like object of 
the eleventh magnitude imprinted itself on a plate exposed by Herr 
Witt at the Urania Observatory, Berlin. Its originator proved to 
bo unique among asteriods. " Eros " is, in sober fact, 

' one of those mysterious stars 
"Which hide themselves between the Earth and Mars, ' 

divined or imagined by Shelley.' 2 True, several of its congeners 
invade the Martian sphere at intervals ; but the proper habitat of 
Eros is within that limit, although its excursions transcend it. In 
other words, its mean distance from the sun is about 135, as 

1 Astr. Nach., No. 3,319. 

2 Witch of Atlas, stanza iii. I am indebted to Dr. Garnett for the reference. 



chap, vni PLANETS AND SATELLITES 285 

compared with the Martian distance of 141 million miles. Further, 
its orbit being so fortunately circumstanced as to bring it once in 
sixty-seven years within some 15 millions of miles of the earth, it is of 
extraordinary value to celestial surveyors. The calculation of its 
movements was much facilitated by detections, through a retrospec- 
tive search, 1 of many of its linear images among the star-dots on the 
Harvard plates. 2 The little body which can scarcely be more than 
twenty miles in diameter shows peculiarities of behaviour as well 
as of position. Dr. von Oppolzer, in February, 1901, 3 announced 
it to be extensively and rapidly variable. Once in 2 hours 38 minutes 
it lost about three-fourths of its light, 4 but these fluctuations 
quickly diminished in range, and in the beginning of May ceased 
altogether. 5 Evidently, then, they depend upon the situation of the 
asteroid relatively to ourselves ; and, so far, events lent countenance 
to M. Andre's eclipse hypothesis, since mutual occultations of the 
supposed planetary twins could only take place when the plane of 
their revolutions passed through the earth, and this condition would 
be transitory. Yet the recognition in Eros of an "Algol asteroid " 
seems on other grounds inadmissible ; 6 nor until the phenomenon 
is conspicuously renewed as it probably will be at the opposition 
of 1903 can there be much hope of finding its appropriate 
rationale. 

The crowd of orbits disclosed by asteroidal detections invites atten- 
tive study. D'Arrest remarked in 1851, 7 when only thirteen minor 
planets were known, that supposing their paths to be represented by 
solid hoops, not one of the thirteen could be lifted from its place 
without bringing the others with it. The complexity of interwoven 
tracks thus illustrated has grown almost in the numerical proportion 
of discovery. Yet no two actually intersect, 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. s 

The maze threaded by the 500 asteroids contrasts singularly with 
the harmoniously ordered and rhythmically separated orbits of the 
larger planets. Yet the seeming confusion is not without a plan. 

1 Recommended by Chandler, Astr. Jour., No. 452. 

9 Harvard Circulars, Nos. 36, 37, 51. 3 Astr. Nach., No. 3,687. 

4 Montangerand, Comptes llendus, March 11, 1901. 

5 Pickering, Astroph. Jour., vol. xiii., p. 277. 

6 Harvard Circular, No. 58. 7 Astr. Nach., No. 752. 
8 L. Niesten, Annuaire, Bruxjelles, 1881, p. 269. 



286 HISTORY OF ASTRONOMY part ii 

The established rules of our system are far from being totally dis- 
regarded by its minor members. The orbit of Pallas, with its 
inclination of 34 42', touches the limit of departure from the 
ecliptic level ; the average obliquity of the asteroidal paths is some- 
what less than that of the sun's equator ; x 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 dovetails into the sphere of 
Mars. 

Their distribution is very unequal. They are most densely 
congregated about the place where a single planet ought, by Bode's 
Law, to revolve ; it may indeed be said that only stragglers from 
the main body are found more than fifty million miles within or 
without a mean distance from the sun 2*8 times that of the earth. 
Significant gaps, too, occur where some force prohibitive of their 
presence would seem to be at work. The probable nature of that 
force was suggested by the late Professor Kirk wood, first in 1866, 
when the number of known asteroids was only eighty-eight, and 
again with more confidence in 1876, from the study of a list then 
run up to 172. 2 It appears that these bare spaces are found just 
where a revolving body would have a period connected by a simple 
relation with that of Jupiter. It would perform two or three 
circuits to his one, five to his two, nine to his five, and so on. 
Kirkwood's inference was that the gaps in question were cleared of 
asteroids by the attractive influence of Jupiter. For disturbances 
recurring time after time owing to commensurability of periods 
nearly at the same part of the orbit, would have accumulated until 
the shape of that orbit was notably changed. The body thus 
displaced would have come in contact with other cosmical particles 
of the same family with itself then, it may be assumed, more 
evenly scattered 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. But its 
effects must have been produced while they were still in an un- 
formed, perhaps a nebular condition. In a system giving room for 

1 According to Svedstrup (Astr. Nach., Nos. 2,240-41), the inclination to the 
ecliptic of the "mean asteroid's " orbit is = 6. 

" Smiths. Report, 1876, p. 358 ; The Asteroids (Kirkwood), p. 42, 1888. 



chap, vin PLANETS AND SATELLITES 287 

considerable modification through disturbance, the recurrence of 
conjunctions with a dominating mass at the same orbital point need 
not involve instability. 1 On the whole, the correspondence of facts 
with Kirk wood's hypothesis has not been impaired by their more 
copious collection. 2 Some chasms of secondary importance have 
indeed been bridged; but the principal stand out more conspicuously 
through the denser scattering of orbits near their margins. Nor is 
it doubtful that the influence of Jupiter in some way produced 
them. M. de Freycinet's study of the problem they present 3 has, 
however, led him to the conclusion that they existed ab origine, thus 
testifying rather to the preventive than to the perturbing power of 
the giant planet. 

'The existence, too, of numerous asteroidal pairs travelling in 
approximately coincident tracks, must date from a remote antiquity. 
They result, Professor Kirkwood 4 believed, from the divellent action 
of Jupiter upon embryo pigmy planets, just as comets moving in 
pursuit of one another are a consequence of the sundering influence 
of the sun. 

Leverrier fixed, in 1853, 5 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. Berberich 6 held that the 
moon would more than outweigh the whole of them, a million of 
the lesser bodies shining like stars of the twelfth magnitude being 
needed, according to his judgment, to constitute her mass. And 
M. Niesten estimated that the whole of the 216 asteroids discovered 
up to August, 1880, amounted in volume to only T <yVo- f our globe, 7 
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. A fairly concordant result was published in 1895 
by Mr. B. M. Roszel. 8 He found that the lunar globe probably 
contains forty times, the terrestrial globe 3,240 times the quantity 
of matter parcelled out among the first 311 minor planets. The 
actual size of a few of them may now be said to be known. 
Professor Pickering, from determinations of light-intensity, assigned 
to Vesta a diameter of 319 miles, to Pallas 167, to Juno 94, down 

1 Tisserand, Annuaire, Paris, 1891, p. B. 15; Newcomb, Astr. Jour., No. 477; 
Backlund, Bull. Astr., t. xvii., p. 81 ; Parmentier, Bull. Soc. Astr. de France, 
March, 1896 ; Observatory, vol. xviii., p. 207. 

2 Berberich, Astr. Nach., No. 3,088. 3 Bull. Astr., t. xviii., p. 39. 

4 The Asteroids, p. 48 ; Publ. Astr. Pac. Soc., vols, ii., p. 48 ; iii., p. 95. 

5 Comptes Rendus, t. xxxvii., p. 797. 6 Bull. Astr., t. v., p. 180. 

7 Annuaire, Bruxelles, 1881, p. 243. 

8 Johns Hopkins Un. Circular, January, 1895 ; Observatory, vol. xviii., p. 127. 



288 HISTORY OF ASTRONOMY part ii 

to twelve and fourteen for the smaller members of the group. 1 An 
albedo equal to that of Mars was assumed as the basis of the calcula- 
tion. Moreover, Professor G. Miiller 2 of Potsdam examined photo- 
metrically the phases of seven among them, of which four namely, 
Vesta, Iris, Massalia, and Amphitrite were found to conform 
precisely to the behaviour of Mars as regards light-change from 
position, while Ceres, Pallas, and 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 j the second to be moon-like bodies. 

Finally, Professor Barnard, directly measuring with the Yerkes 
refractor the minute discs presented by the original quartette, 
obtained the following authentic data concerning them : 3 Diameter 
of Ceres, 477 miles, albedo = 0*18; diameter of Pallas, 304 miles, 
albedo = 0'23 j diameter of Vesta, 239 miles, albedo = 0*74 ; diameter 
of Juno, 120 miles, albedo = - 45. Thus, the rank of premier 
asteroid proves to belong to Ceres, and to have been erroneously 
assigned to Vesta in consequence of its deceptive brilliancy. What 
kind of surface this indicates, it is hard to say. The dazzling 
whiteness of snow can hardly be attributed to bare rock j yet the 
dynamical theory of gases as Dr. Johnstone Stoney pointed out 
in 1867 4 prohibits the supposition that bodies of insignificant 
gravitative power can possess aerial envelopes. Even our moon, it 
is calculated, could not permanently hold back the particles of 
oxygen, nitrogen, or water-gas from escaping into infinite space ; 
still less, a globe one thousand times smaller. Vogel's suspicion of 
an air-line in the spectrum of Vesta 5 has, accordingly, not been 
confirmed. 

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 dissimilar physical constitu- 
tion. 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 Harvard Annals, vol. xi., part ii., p. 294. 2 Astr. Nach., Nos. 2,724-5. 

* Month. Not., vol. lxi., p. 69. 4 Astroph. Jour., vol. vii., p. 25. 

5 Spectra der Planeten, p. 24. 



chap, vni PLANETS AND SATELLITES 289 

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, 
impressed upon modern readers by Mr. Proctor's popular works, was 
anticipated in the last century. Buffon wrote in his Epoques de la 
Nature (1778) r 1 "La surface de Jupiter est, comme Ton sait, 
sujette a des changemens 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 has 
not yet had time to cool. Kant thought similarly in 1785 ; 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 1853. 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 swift- 
ness 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 photographic 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. Bond, however, rightly considered his data too un- 
certain for the support of so bold an assumption as that of original 
luminosity, 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 1865, 5 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 atmo- 

1 Tome i., p. 93. 2 Berlinische Monatsschrift, 1785, p. 211. 

3 Month. Not., vol. xiii., p. 40. 4 Mem. Am. Ac, vol. viii., p. 221. 

6 Phot&m. Unters., p. 303. 

19 



2 9 o HISTORY OF ASTRONOMY part ii 

spheric movements on the earth are sun-heat transformed into 
motion. But sun-heat at the distance of Jupiter possesses but -} T , 
at that of Saturn t Jq 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-pervading fires, not yet banked in with a thick solid crust. 

The same acute investigator dwelt, in 1871, l on the similarity 
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 2 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. 3 It is now well ascertained that, as a rule (not without 
exceptions), equatorial spots give a period some 5J minutes shorter 
than those in latitudes of about 30. But, as Mr. Denning has 
pointed out, 4 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. 

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 inventiveness, yet with- 
out bringing to it a categorical reply. Zollner in 1865, Muller in 
1893, estimated his albedo at 0*62 and 0*75 respectively, that of 
fresh-fallen snow being - 78, and of white paper 0"70. 5 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 suffused with chocolate -browns and 
rosy hues. It is occasionally seen ruled from pole to pole with 
dusky bars, and is never wholly free from obscure markings. The 
reflection, then, by it, as a whole, of about 70 per cent, of the 
rays impinging upon it, might well suggest some original reinforce- 
ment. 

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 
Vogel, 1871-73, shows the familiar Fraunhofer rays belonging to 
reflected sunlight. But it also shows lines of native absorption. 

1 Astr. Nach., No. 1,851. 2 Mem. de VAc, t. x., p. 514. 

3 Ibid., 1692, p. 7. 4 Month. Not., vol. xliv., p. 63. 

6 Photom. Untcrs., pp. 165, 273 ; Potsdam Publ, No. 30. 



chap, vin PLANETS AND SATELLITES 291 

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 among the latter a strong band in the red agrees in position 
with a dark line in the spectra of some ruddy stars. 1 There is, 
besides, a general absorption of blue rays, intensified as Le Sueur 
observed at Melbourne in 1869 2 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 atmosphere. One 
spectrograph, it is true, taken by Dr. Henry Draper, September 27, 
1879, 3 seemed to attest the action of intrinsic light; but the peculi- 
arity was referred by Dr. Vogel, with convincing clearness, to a flaw 
in the film. 4 So far, then, native emissions from any part of 
Jupiter's diversified surface have not been detected; and, indeed, 
the blackness of the shadows cast by his satellites on his disc suffi- 
ciently proves that he sends out virtually none but reflected light. 5 
This conclusion, however, by no means invalidates 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 several times 
less luminous than those near the centre, they invariably show as 
bright spots, then usually vanish as the background gains lustre, to 
reappear, after crossing the disc, thrown into relief, as before, against 
the dusky limb. But instances are not rare, more especially of the 
third and fourth satellites standing out, during the entire middle 
part of their course, in such inky darkness as to be mistaken for 
their own shadows. The earliest witness of a " black transit " was 
Cassini, September 2, 1665; Eomer in 1677, and Maraldi in 1707 
and 1713, made similar observations, which have been multiplied 
in recent years. In some cases the process of darkening has 
been visibly attended by the formation, or emergence into view, 
of spots on the transiting body, as noted by the two Bonds at 
Harvard, March 18, 1848. 6 The third satellite was seen by Dawes, 
half dark, half bright, when crossing Jupiter's disc, August 21, 

1 Vogel, Sp. der Planeten, p. 33, note. 2 Proc. Roy. Soc, vol. xviii., p. 250. 
3 Month. Not., vol. xl., p. 433. 4 Sitzungsberichte, Berlin, 1895, ii., p. 15. 

5 The anomalous shadow-effects recorded by "Webb (Cel. Objects, p. 170, 
4th ed.) are obviously of atmospheric and optical origin. 

6 Engelmann, Ueber die Helligkeitsverhaltnisse der Jupiterstrabanten, p. 59. 

192 



292 HISTORY OF ASTRONOMY part n 

1867 j 1 one-third dark by Davidson of California, January 15, 1884, 
under the same circumstances; 2 and unmistakably spotted, both 
on and off the planet, by Schroter, Secchi, Dawes, and Lassell. 

The first satellite sometimes looks dusky, but never absolutely 
black, in travelling over the disc of Jupiter. The second appears 
uniformly white a circumstance attributed by Dr. Spitta 3 to its 
high albedo. The singularly different aspects, even during successive 
transits, of the third and fourth satellites, are connected by Professor 
Holden 4 with the varied luminosity of the segments of the planetary 
surface they are projected upon, and W. H. Pickering inclines to 
the same opinion ; but fluctuations in their own brightness 5 may be 
a concurrent cause. Herschel concluded in 1797 that, like our moon, 
they always turn the same face towards their primary, and as 
regards the outer satellite, Engelmann's researches in 1871, and 
C. E. Burton's in 1873, made this almost certain; while both for 
the third and fourth Jovian moons it was completely assured by 
W. H. Pickering's and A. E. Douglass's observations at Arequipa in 
1892, 6 and at Flagstaff in 1894-95. 7 Strangely enough, however, the 
interior members of the system have preserved a relatively swift 
rotation, notwithstanding the enormous checking influence upon it 
of Jove-raised tides. 

All the satellites are stated, on good authority, to be more or less 
egg-shaped. On September 8, 1890, Barnard saw the first elongated, 
and bisected by a bright equatorial belt, during one of its dark 
transits; 8 and his observation, repeated August 3, 1891, was com- 
pletely verified by Schaeberle and Campbell, who ascertained, more- 
over, that the longer axis of the prolate body was directed towards 
Jupiter's centre. 9 The ellipticity of its companions was determined 
by Pickering and Douglass ; indeed, that of No. 3 had long previously 
been noticed by Secchi. 10 No. 3 also shows equatorial stripes, per- 
ceived in 1891 by Schaeberle and Campbell, 11 and evident later to 
Pickering and Douglass ; 12 nor need we hesitate to admit as authentic ; 
their records of similar, though less conspicuous markings on the 1 

1 Month. Not., vol. xxviii., p. 11. 8 Observatory, vol. vii., p. 175. 

3 Month. Not., vol. xlviii., p. 43. 4 Pull. Astr. Pac. Soc, vol. ii., p. 296. j 

Pickering failed to obtain any photometric evidence of their variability. 
Harvard Annals, vol. xi., p. 245. 

6 Astr. and Astroph., vol. xii., pp. 194, 481. 

7 Annals Lowell Obs., vol. ii., pt. i. 

8 Astr. Nach., Nos. 2,995, 3,206 ; Month. Not., vols. Ii., p. 556 ; liv., p. 134. 
Barnard remains convinced that the oval forms attributed to Jupiter's satellites 
are illusory effects of their markings. Astr. Nach., Nos. 3,206, 3,453 ; Astr. and 
Astroph., vol. xiii., p. 272. 

9 Publ. Astr. Pac. Soc, vol. iii., p. 355. 10 Astr. Nach., No. 1,017. 
n Publ. Astr. Pac. Soc, vol. iii., p. 359. ]a Astr. Nach., No. 3,432. 



chap, vni PLANETS AND SATELLITES 293 

other satellites. A constitution analogous to that of Jupiter himself 
was thus unexpectedly suggested ; and Vogel's detection of lines or 
traces of lines in their spectra, agreeing with absorption-rays derived 
from their primary, lends support to the conjecture that they possess 
gaseous envelopes similar to his. 

The system of Jupiter, as it was discovered by Galileo, and inves- 
tigated by Laplace, appeared in its outward aspect so symmetrical, 
and displayed in its inner mechanism such harmonious dynamical 
relations, that it might well have been deemed complete. Neverthe- 
less, a new member has been added to it. Near midnight on 
September 9, 1892, Professor Barnard discerned with the Lick 
36-inch "a tiny speck of light," closely following the planet. 1 He 
instantly divined its nature, watched its hurried disappearance in 
the adjacent glare, and made sure of the reality of his discovery on 
the ensuing night. It was a delicate business throughout, the Lili- 
putian luminary subsiding into invisibility before the slightest glint 
of Jovian light, and tarrying, only for brief intervals, far enough 
from the disc to admit of its exclusion by means of an occulting 
plate. The new satellite is estimated to be of the thirteenth stellar 
magnitude, and, if equally reflective of light with its next neighbour, 
Io (satellite No. 1), its diameter must be about one hundred miles. 
It revolves at a distance of 112,500 miles from Jupiter's centre, and 
of 68,400 from his bulging equatorial surface. Its period of llh. 
57m. 23s. is just two hours longer than Jupiter's period of rotation, 
so that Phobos still remains a unique example of a secondary body 
revolving faster than its primary rotates. Jupiter's innermost moon 
conforms in its motions strictly, indeed inevitably, to the plane of 
his equatorial protuberance, following, however, a sensibly elliptical 
path the major axis of which is in rapid revolution. 2 Its very insig- 
nificance raises the suspicion that it may not prove solitary. Possibly 
it belongs to a zone peopled by asteroidal satellites. More than 
iifteen thousand such small bodies could be furnished out of the 
materials of a single full- sized satellite spoiled in the making. But 
we must be content for the present to register the fact without 
seeking to penetrate the meaning of its existence. Very high and 
very fine telescopic power is needed for its perception. Outside the 
United States, it has been very little observed. The only instru- 
ments in this country successfully employed for its detection are, we 
believe, Dr. Common's 5-foot reflector and Mr. Ne wall's 2 5 -inch 
refractor. 

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 

1 Astr. Jour., Nos. 275, 325, 367, 472 ; Observatory, vol. xv., p. 425. 

2 Tisserand, Comptes Rendus, October 8, 1894 ; Cohn, Astr. Nach., No. 3,404. 



294 HISTORY OF ASTRONOMY part ii 

beneath the dark southern equatorial band. 1 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 some- 
thing short of 7,000 miles. The earliest record of its appearance 
seems to be by Professor Pritchett, director of the Morrison Obser- 
vatory (U.S.), who figured and described it July 9, 1878. 2 It was 
again delineated August 9, by Tempel at Florence. 3 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 into 
a full brick-red, and was set off by contrast with a white equatorial 
spot of unusual brilliancy. During three ensuing years these re- 
markable objects continued to offer a visible and striking illustration 
of the compound nature of the planet's rotation. The red spot com- 
pleted a circuit in nine hours fifty-five minutes thirty-six seconds ; 
the white spot in about five and a half minutes less. Their relative 
motion was thus no less than 260 miles an hour, bringing them 
together in the same meridian at intervals of forty-four days ten 
hours forty-two minutes. Neither, however, preserved continuously 
the same uniform rate of travel. The period of each had lengthened 
by some seconds in 1883, while sudden displacements, associated 
with the recovery of lustre after recurrent fadings, were observed in 
the position of the white spot, 4 recalling the leap forward of a re- 
viving sun-spot. Just the opposite effect attended the rekindling of 
the companion object. While semi-extinct, in 1882-84, it lost little 
motion j but a fresh access of retardation was observed by Professor 
Young 5 in connection with its brightening in 1886. This suggests 
very strongly that the red spot is fed from below. A shining aureola 
of " faculse," described by Bredichin at Moscow, and by Lohse at 
Potsdam, as encircling it in September, 1879, 6 was held to strengthen 
the solar analogy. 

The conspicuous visibility of this astonishing object lasted three 
years. When the planet returned to opposition in 1882-83, it had 
faded so considerably that Riccd'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 presented to Mr. Denning in the 
beginning of 1886 much the same aspect as in October, 1882. 7 
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 " descried in the same latitude by Mr. Gledhill in 

1 Bull. Ac. R. Bruxelles, t. xlviii., p. 607. 2 Astr. Nach., No. 2,294. 

3 Ibid., No. 2,284. 

4 Denning, Month. Not., vol. xliv., pp. 64, 66 ; Nature, vol. xxv., p. 226. 

5 Sidereal Mess., December, 1886, p. 289. 

6 Astr. Nach., Nos. 2,280, 2,282. 7 Month. Not., vol. xlvi., p. 117. 



chap, vin PLANETS AND SATELLITES 295 

1869-70. This, indeed, might be called the preliminary sketch for 
the famous object brought to perfection ten years later, but which 
Mr. H. C. Russell of Sydney saw and drew still unfinished in June, 
1876, 1 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 southern- 
most 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. 3 

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. It was, 
indeed, certainly not self-luminous, a satellite projected upon it in 
transit having been seen to show as bright as upon the dusky 
equatorial bands. A fundamental objection to all three hypotheses 
is that the rotation of the spot was variable. It did not then ride 
at anchor, but floated free. Some held that its surface was depressed 
below the average cloud-level, and that the cavity was filled with 
vapours. Professor Wilson, on the other hand, observing with the 
16-inch equatoreal of the Goodsell Observatory in Minnesota, 
received a persistent impression of the object " being at a higher 
level than the other markings." 4 A crucial experiment on this 
point was proposed by Mr. Stanley Williams in 1890. 5 A dark spot 
moving faster along the same parallel was timed to overtake the red 
spot towards the end of July. A unique opportunity hence appeared 
to be at hand of determining the relative vertical depths of the 
two formations, one of which must inevitably, it was thought, pass 
above the other. No forecast included a third alternative, which 
was nevertheless adopted by the dark spot. It evaded the obstacle 
in its path by skirting round its southern edge. 6 Nothing, then, 
was gained by the conjunction, beyond an additional proof of the 
singular repellent influence exerted by the red spot over the 

1 Proc. Boy. Soc. N. S. Wales, vol. xiv., p. 68. 

2 Phil. Tram., vol. i., p. 143. 

3 For indications relative to the early history of the red spot, see Holden 
Publ. Astr. Pac. Soc, vol. ii., p. 77; Noble, Month. Not., vol. xlvii., p. 515; 
A. S. Williams, Observatory, vol. xiii., p. 338. 

4 Astr. and Astrophysics, vol. xi., p. 192. 

5 Month. Not., vol. 1., p. 520. 6 Observatory, vol. xiii., pp. 297, 326. 



296 HISTORY OF ASTRONOMY part ii 

markings in its vicinity. It has, for example, gradually carved out 
a deep bay for its accommodation in the gray belt just north of it. 
The effect was not at first steadily present. A premonitory excavation 
was drawn by Schwabe at Dessau, September 5, 1831, and again by 
Trouvelot, Barnard, and Elvins in 1879; yet there was no sign of 
it in the following year. Its development can be traced in Dr. 
Boeddicker's beautiful delineations of Jupiter, made with the Par- 
sonstown 3-foot reflector, from 1881 to 1886. 1 They record the 
belt as straight in 1881, but as strongly indented from January, 1883 ; 
and the cavity now promises to outlast the spot. So long as it 
survives, however, the forces at work in the spot can have lost little 
of their activity. For it must be remembered that the belt has a 
shorter rotation-period than the red spot, which, accordingly (as 
Mr. Elvins of Toronto has pointed out), breasts and diverts, by its 
interior energy, a current of flowing matter, ever ready to fill up its 
natural bed, and override the barrier of obstruction. 

The famous spot was described by Keeler in 1889, as " of a pale 
pink colour, slightly lighter in the middle. Its outline was a fairly 
true ellipse, framed in by bright white clouds." 2 The fading con- 
tinuously in progress from 1887, was temporarily interrupted in 
1891. The revival, indeed, was brief. Professor Barnard wrote 
in August, 1892 : " The great red spot is still visible, but it has just 
passed through a crisis that seemingly threatened its very existence. 
For the past month it has been all but impossible to catch the 
feeblest trace of the spot, though the ever-persistent bay in the 
equatorial belt close north of it, and which has been so intimately 
connected with the history of the red spot, has been as conspicuous 
as ever. It is now, however, possible to detect traces of the entire 
spot. An obscuring medium seems to have been passing over it, 
and has now drifted somewhat preceding the spot." 3 

The object is now always inconspicuous, and often practically 
invisible, and may be said to float passively in the environing 
medium. 4 Yet there are sparks beneath the ashes. A rosy tinge 
faintly suftused it in April, 1900, 5 and its absolute end may still be 
remote. 

The extreme complexity of the planet's surface -movements has 
been strikingly evinced by Mr. Stanley Williams's detailed investi- 
gations. He enumerated in 1896 6 nine principal currents, all flowing 

1 Trans. R. Dublin Soc, vol. iv., p. 271, 1889. 

2 Publ. Astr. Pac Soc, vol. ii., p. 289. 

3 Astr. and Astrophysics, vol. xi., p. 686. 

4 Denning, Knowledge, vol. xxiii., p. 200; Observatory, vol. xxiv., p. 312; 
Pop. Astr., vol. ix., p. 448 ; Nature, vol. Iv., p. 89. 

5 Williams, Observatory, vol. xxiii., p. 282. 

6 Month. Not., vol. lvi., p. 143. 



chap, vin PLANETS AND SATELLITES 297 

parallel to the equator, but unsymmetrically placed north and south 
of it, and showing scant signs of conformity to the solar rule of 
retardation with increase of latitude. The linear rate of the planet's 
equatorial rotation was spectroscopically determined by Belopolsky 
and Deslandres in 1895. Both found it to fall short of the cal- 
culated speed, whence an enlargement, by self-refraction, of the 
apparent disc was inferred. 1 

Jupiter was systematically photographed with the Lick 36 -inch 
telescope during the oppositions of 1890, 1891, and 1892, the 
image thrown on the plates (after eightfold direct enlargement) 
being one inch in diameter. Mr. Stanley Williams's measurements 
and discussion of the set for 1891 showed the high value of the 
materials thus collected, although much more minute details can be 
seen than can at present be photographed. The red spot shows as 
"very distinctly annular" in several of these pictures. 2 Eecently, 
the planet has been portrayed by Deslandres with the 62-foot 
Meudon refractor. 3 The extreme actinic feebleness of the 
equatorial bands was strikingly apparent on his plates. 

In 1870, Mr. Kanyard 4 whose death, December 14, 1894, was a 
serious loss to astronomy acting upon an earlier suggestion of 
Sir William 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 equatorial " 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 5 went to strengthen the coincidence, which had been 
anticipated a priori by Zollner in 1871. 6 Moreover, separate and 
distinct evidence was alleged by Mr. Denning in 1899 of decennial 
outbreaks of disturbance in north temperate regions. 7 It may, 
indeed, be taken for granted that what Hahn terms the universal 
pulse of the solar system 8 affects the vicissitudes of Jupiter; but 
the law of those vicissitudes is far from being so obviously sub- 
ordinate to the rhythmical flow of central disturbance as are certain 
terrestrial phenomena. The great planet, being in fact himself a 
" semi-sun," may be regarded as an originator, no less than a 
recipient, of agitating influences, the combined effects of which may 
well appear insubordinate to any obvious law. 

1 Belopolsky, Astr. Nach., No. 3,326. 

2 Publ. Astr. Pac. Soc., vol. iv., p. 176. 3 Bull. Astr., 1900, p. 70. 
4 Month. Not., vol. xxxi., p. 34. 5 Beobachtungen, Heft ii., p. 99. 

* Ber. Sachs. Ges. der Wins., 1871, p. 553. 7 Month. Not., vol. lix., p. 76. 

8 Beziehungen der Sonnenfleckenperiode, p. 175. 



298 HISTORY OF ASTRONOMY part n 

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, 1 
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 temperature 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, 73,000 miles in diameter, 
is composed of heated vapours, kept in active and agitated circulation 
by the process of cooling. 

His unique set of appendages has, since the middle of the last 
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 postulated ; 
and the question remained in abeyance until once more brought 
prominently forward by the discovery of the 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 1851,- inconsistent with such a hypo- 
thesis. 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-solidity. 3 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 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 revolving independently in a period 

1 A. Hall, Astr. Nach., No. 2,269. 

2 Astr. Jour. (Gould's), vol. ii., p. 17. 3 Ibid., p. 5. 



chap, vin PLANETS AND SATELLITES 299 

corresponding to its distance from the planet. 1 This idea of a 
satellite-formation had been, remarkably enough, several times 
entertained and lost sight of. It was first put forward by Roberval 
in the seventeenth century, again by Jacques Cassini in 1715, and 
with perfect definiteness by Wright of Durham in 1750. 2 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 disappear- 
ance 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 3 show the effects of waves 
of disturbance traversing a moving mass of gravitating particles; 4 
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. The whole ring-system, too, appears to be somewhat 
elliptical. 6 

The satellite-theory has derived unlooked-for support from 
photometric inquiries. Professor Seeliger pointed out in 1888 6 
that the unvarying brilliancy of the outer rings under all angles of 
illumination, from to 30, can be explained from no other point 
of view. Nor does the constitution of the obscure inner ring offer 
any difficulty. For it is doubtless 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, indeed, at first 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 has been urged by 
Professor Hastings of Baltimore. The brightest parts of these 
appendages, he remarked, 7 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. Professor Seeliger, however, replied 8 that 
the darkening is due to the never-ending swarms of their separate 

1 On the Stability of the Motion of Saturn's Rings, p. 67. 

2 M6m. de VAc., 1715, p. 47 ; Montucla, Hist. des. Math., t. iv., p. 19 ; An 
Original Theory of the Universe, p. 115. 

3 Comptes Rendus, t. xcviii., p. 718. 

4 Proctor, Saturn and its System (1865), p. 125. 

5 Perrotin, Comptes Rendus, t. cvi., p. 1716. 

6 Abhandl. Akad. der Wiss., Munich, Bd. xvi., p. 407. 

7 Smiths. Report, 1880 (Holden). 

8 Quoted by Dr. E. Anding, Astr. Nach., No. 2,881. 



300 HISTORY OF ASTRONOMY part ii 

shadows transiting the planet's disc. Sunlight is not, indeed, 
wholly excluded. Many rays come and go between the open ranks 
of the meteorites. For the dusky ring is transparent. The planet 
it encloses shows through it, as if veiled with a strip of crape. A 
beautiful illustration of its quality in this respect was derived by 
Professor Barnard from an eclipse of Japetus, November 1, 1889. 1 
The eighth moon remained steadily visible during its passage through 
the shadow of the inner ring, but with a progressive loss of lustre in 
approaching its bright neighbour. There was no breach of continuity. 
The satellite met no gap, corresponding to that between the dusky 
ring and the body of Saturn, through which it could shine with 
undiminished light, but was slowly lost sight of as it plunged into 
deeper and deeper gloom. The important facts were thus established, 
that the brilliant and obscure rings merge into each other, and that 
the latter thins out towards the Saturnian globe. 

The meteoric constitution of these appendages was beautifully 
demonstrated in 1895 by Professor Keeler, 2 then director of the 
Alleghany Observatory, Pittsburgh. From spectrographs taken 
with the slit adjusted to coincidence with the equatorial plane of 
the system, he determined the comparative radial velocities of its 
different parts. And these supply a crucial test of Clerk Maxwell's 
theory. For if the rings were solid, the swiftest rates of rotation 
should be at their outer edges, corresponding to wider circles 
described in the same period ; while, if they are pulverulent, the 
inverse relation must hold good. This proved to be actually the 
case. The motion slowed off outward, in agreement with the 
diminishing speed of particles travelling freely, each in its own 
orbit. Keeler's result was promptly confirmed by Campbell, 3 as well 
as by Deslandres and Belopolsky. 

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 permanent part in the 
economy of the system j 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 attempted to give a definite 
answer to this important query. 

A study of early and later records of observations disclosed to 
him, in 1851, an apparent progressive approach of the inner edge of 
the bright ring to the planet. The rate of approach he estimated at 
about fifty-seven English miles a year, or 11,000 miles during the 

1 Astr. and Astrojjhysics, vol. xi., p. 119 ; Month. Not., vol. L, p. 108. 

2 Astroph. Jour., vol. i., p. 416. 3 Ibid., vol. ii., p. 127. 



chap, viii PLANETS AND SATELLITES 301 

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 1 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 comparison j 
and he was enabled to renew them, under closely similar circum- 
stances, in 1882. 2 But the expected diminution of the space 
between Saturn's globe and his rings had not taken place. A slight 
extension in the width of the system, both outward and inward, 
was, indeed, hinted at; and it is worth notice that just such a 
separation of the rings was indicated by Clerk Maxwell's theor}^, so 
that there is an h priori likelihood of its being in progress. Yet 
Hall's measures in 1884-87 3 failed to supply evidence of alteration 
with time; and Barnard's, executed at Lick in 1894-95, 4 showed no 
sensible divergence from them. Hence, much weight cannot be laid 
upon Huygens's drawings and descriptions, which had been held to 
prove conclusively a partial filling up, since 1657, of the interval 
between the ring and the planet. 5 

The rings of Saturn replace, in Professor G-. H. Darwin's view, 6 
an abortive satellite, scattered by tidal action into annular form. 
For they lie closer to the planet than is consistent with the 
integrity of a revolving body of reasonable bulk. The limit of 
possible existence for such a mass was fixed by Roche of Mont- 
pellier, in 1848, 7 at 2*44 mean radii of its primary; while the outer 
edge of the ring-system is distant 2*38 radii of Saturn from his 
centre. The virtual discovery of its pulverulent condition dates, 
then, according to Professor Darwin, from 1848. He conjectures 
that the appendage will eventually disappear, partly through the 
dispersal of its constituent particles inward, and their subsidence 
upon the planet's surface, partly by their dispersal outward, to a 
region beyond "Roche's limit," where coalescence might proceed 
unhindered by the strain of unequal attractions. One modest 
satellite, revolving inside Mimas, would then be all that was left of 
the singular appurtenances we now contemplate with admiration. 

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 them. Here the influential 

1 Mem. de V Ac. Imp. (St Petersl).), t. vii., 1853, p. 464. 

2 Asir. Nach., No. 2,49S. 3 Washington Observations, App. ii., p. 22. 

4 Month. Not., vol. lvi., p. 163. 

5 T. Lewis, Observatory, vol. xviii., p. 379. 6 Harper's Magazine, June 1889. 
7 Mim. de VAcad. de Montpellier, t. viii., p. 296, 1873. 



3 o2 HISTORY OF ASTRONOMY part ii 

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, 1 that a body circulating in the chasm between the 
bright rings known as "Cassini's division," would have a period 
nearly commensurable with those of four out of the eight moons ; 
and Meyer of Geneva subsequently calculated all such combinations, 
with the result of bringing out coincidences between regions of 
maximum perturbation and the limiting and dividing lines of the 
system. 2 This is in itself a strong confirmation of the view that the 
rings are made up of independently revolving small bodies. 

On December 7, 1876, Professor Asaph Hall discovered at 
Washington a bright equatorial spot on Saturn, which he followed 
and measured through above sixty rotations, each performed in ten 
hours fourteen minutes twenty-four seconds. 3 This, he was careful 
to add, represented the period, not necessarily of the planet, but only 
of the individual spot. The only previous determination of Saturn's 
axial movement (setting aside some insecure estimates by Schroter) 
was Herschel's in 1794, giving a period of ten hours sixteen minutes. 
The substantial accuracy of Hall's result was verified by Mr. Den- 
ning in 1891. 4 In May and June of that year, ten vague bright 
markings near the equator were watched by Mr. Stanley Williams, 
who derived from them a rotation period only two seconds shorter 
than that determined at Washington. Nevertheless, similarly 
placed spots gave in 1892 and 1893 notably quicker rates; 5 so that 
the task of timing the general drift of the Saturnian surface by the 
displacements of such objects is hampered, to an indefinite extent, 
by their individual proper motions. 

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 conspicuous telescopic 
object when west, a scarcely discernible one when east of the 
planet. Herschel's inference 6 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 remarked further that the dusky and 
brilliant hemispheres must be so posited as to divide the disc, viewed 
from Saturn, into nearly equal parts ; so that this Saturnian moon, 

1 Meteoric Astronomy, chap. xii. He carried the subject somewhat farther in 
1871. See Observatory, vol. vi., p. 335. 

3 Astr. Nach., No. 2,527. 5 Amer. Jour, of Sc, vol. xiv., p. 325. 

4 Observatory, vol. xiv., p. 369. 6 Month. Not, vol. liv., p. 297. 
6 Phil. Trans., vol. lxxxii., p. 14. 



chap, vin PLANETS AND SATELLITES 303 

even when " full," appears very imperfectly illuminated over one-half 
of its surface. 1 

Zollner estimated the albedo of Saturn at 0*51, Miiller at 0*88, a 
value impossibly high, considering that the spectrum includes no 
vestige of original emissions. Closely similar to that of Jupiter, it 
shows the distinctive dark line in the red (wave-length 618), which 
we may call the " red-star line " ; and Janssen, from the summit of 
Etna in 1867 2 found traces in it of aqueous absorption. The light 
from the ring appears to be pure reflected sunshine unmodified by 
original atmospheric action. 3 

Uranus, when favourably situated, can easily be seen with the 
naked eye as a star between the fifth and sixth magnitudes. There 
is, indeed, some reason to suppose that he had been detected as a 
wandering orb by savage " watchers of the skies " in the Pacific long 
before he swam into Herschel's ken. Nevertheless, inquiries into 
his physical habitudes are still in an early stage. They are ex- 
ceedingly difficult of execution, even with the best and largest modern 
telescopes ; and their results remain clouded with uncertainty. 

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 might be 
presumed, from the analogy of the other planets, to be executed in 
the same tilted plane. Yet this is far from being certainly the case. 
Mr. Buffham in 1870-72 caught traces of bright markings on the 
Uranian disc, doubtfully suggesting a rotation in about twelve hours 
in a plane not coincident with that in which his satellites circulate. 4 
Dusky bands resembling those of Jupiter, but very faint, were 
barely perceptible to Professor Young at Princeton in 1883. Yet, 
though almost necessarily inferred to be equatorial, they made a 
considerable angle with the trend of the satellites' orbits. 5 More 
distinctly by the brothers Henry, with the aid of their fine refractor, 
two gray parallel rulings, separated by a brilliant zone, were discerned 
every clear night at Paris from January to June, 1884. 6 What were 
taken to be the polar regions appeared comparatively dusky. The 
direction of the equatorial rulings (for so we may safely call them) 
made an angle of 40 with the satellites' line of travel. Similar ob- 
servations were made at Nice by MM. Perrotin and Thollon, March 
to June, 1884, a lucid spot near the equator, in addition, indicating 

1 Smiths. Report, 1880. 2 Comptes Rendus, t. lxiv., p. 1304. 

3 Huggins, Proc. R. Soc, vol. xlvi., p. 231 ; Keeler, Astr. Nach., No. 2,927 
Vogel, Astroph. Jour., vol. i., p. 278. 

4 Month. Not., vol. xxxiii., p. 164. 5 Astr. Nach., No. 2,545. 
6 Comptes Rendus, t. xcviii., p. 1419. 



3o 4 HISTORY OF ASTRONOMY part ii 

rotation in a period of about ten hours. 1 The discrepancy was, 
however, considerably reduced by Perrotin's study of the planet in 
1889 with the new 30-inch equatoreal. 2 The dark bands, thus 
viewed to better advantage than in 1884, appeared to deviate no 
more than 10 from the satellites' orbit-plane. No definitive results, 
on the other hand, were derived by Professors Holden, Schaeberle, 
and Keeler from their observations of Uranus in 1889-90 with the 
potent instrument on Mount Hamilton. Shadings, it is true, were 
almost always, though faintly, seen ; but they appeared under an 
anomalous, possibly an illusory aspect. They consisted, not of parallel, 
but of forked bands. 3 

Measurements of the little sea-green disc which represents to 
us the massive bulk of Uranus, by Young, Schiaparelli, 4 Safarik, 
H. C. Wilson 5 and Perrotin, prove it to be quite distinctly bulged. 
The compression at once caught Barnard's trained eye in 1894, 6 
when he undertook at Lick a micrometrical investigation of the 
system; and he was surprised to perceive that the major axis 
of the elliptical surface made an angle of about 28 with the line 
of travel pursued by the satellites. Nothing more can be learned 
on this curious subject for some years, since the pole of the planet 
is just now turned nearly towards the earth; but Barnard's I 
conclusion is unlikely to be seriously modified. He fixed the mean 
diameter of Uranus at 34,900 miles. But this estimate was 
materially reduced through Dr. See's elimination of irradiative 
effects by means of daylight measures, executed at Washington in 
1901. 7 

The visual spectrum of this planet was first examined by Father 
Secchi in 1869, and later, with more advantages for accuracy, by 
Huggins, Vogel, 8 and Keeler. 9 It is a very remarkable one. In lieu 
of the reflected Fraunhofer lines, imperceptible perhaps through 
feebleness of light, six broad bands of original absorption appear, 
one corresponding to the blue-green ray of hydrogen (F), another 
to the "red-star line" of Jupiter and Saturn, the rest as yet 
unidentified. The hydrogen band seems much too strong and diffuse 
to be the mere echo of a solar line, and might accordingly be held 
to imply the presence of free hydrogen in the Uranian atmosphere. 
This, however, would be difficult of reconcilement with Keeler's iden- 
tification of an absorption-group in the yellow with a telluric water- 

1 Comptes Rendus, t. xcviii., pp. 718, 967. 

2 V. J. S. Astr. Ges., Jahrg. xxiv., p. 267. 

3 Fubl. Astr. Pox. Soc, vol. iii., p. 287. 4 Astr. Nach., No. 2,526. 
5 Ibid., No. 2,730. 6 Astr. Jour., Nos. 370, 374. 

7 Astr. Nach. No. 3,768. 

8 Ann. der Phys., Bd. clviii., p. 470 ; Astroph. Jour., vol. i., p. 280. 

9 Astr. Nach., No. 2,927. 



chap, vin PLANETS AND SATELLITES 305 

band. Notwithstanding its high albedo 0-62, according to Zollner 
proof is wanting that any of the light of Uranus is inherent. Mr. 
Albert Taylor announced, indeed, in 1 889, his detection, with Common's 
giant reflector, of bright flutings in its spectrum; 1 but Professor 
Keeler's examination proved them to be merely contrast effects. 2 
Sir William and Lady Huggins, moreover, obtained about the same 
time a photograph purely solar in character. The spectrum it re- 
presented was crossed by numerous Fraunhof er lines, and by no others. 
It was, then, presumably composed entirely of reflected light. 

Judging from the indications of an almost evanescent spectrum, 
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 ; 3 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 ob- 
servations were not precisely contemporaneous with those of 
Mr. Hall, 4 who believes the partial obscurations recorded by himself 
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. 

It was ascertained at first by indirect means that the orbit of Nep- 
tune's satellite is inclined about 20 to his equator. Mr. Marth 5 
having drawn attention to the rapid shifting of its plane of motion, 
M. Tisserand and Professor Newcomb 6 independently published the 
conclusion that such shifting necessarily results from Neptune's 
ellipsoidal shape. The movement is of the kind exemplified 
although with inverted relations in the precession of the equinoxes. 
The pole of the satellite, owing to the pull of Neptune's equatorial 
protuberance, describes a circle round the pole of his equator in a 
retrograde direction, and in a period of over five hundred years. 
The amount of compression indicated for the primary body is, at the 
outside, g^5 ; whence it can be inferred that Neptune possesses a 
lower rotatory velocity than the other giant planets. Direct 

1 Month. Not., vol. xlix., p. 405. 

2 Astr. Nach., No. 2,927 ; Schemer's Spectralanalyse, p. 221. 

3 Month. Not, vol. xliv., p. 257. 4 Observatory, vol. vii., pp. 134, 221, 264. 

5 Month. Not., vol. xlvi., p. 507. 

6 Comptes Rendus, t. cvii., p. 804 ; Astr. and Astroplu, vol. xiii., p. 291 ; Astr. 
Jour., No. 186. 

20 



3 o6 HISTORY OF ASTRONOMY part ii 

verification of the trend theoretically inferred for the satellite's move- 
ment was obtained by Dr. See in 1899. The Washington 26-inch 
refractor disclosed to him, under exceptionally favourable conditions, 
a set of equatorial belts on the disc of Neptune, and they took just 
the direction prescribed by theory. Their objective reality cannot 
be doubted, although Barnard was unable, either with the Lick or 
the Yerkes telescope, 1 to detect any definite markings on this planet. 
Its diameter was found by him to be 32,900 miles. 

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 his orbital verge has been approximately 
fixed by two separate investigators. 

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, 
1879. 2 It depends upon the movements of comets. It is well known 
that those of moderately short periods are, for a reason already 
explained, connected with the larger planets in such a way that the 
cometary aphelia fall near some planetary orbit. Jupiter claims a 
large retinue of such partial dependents, 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 1,000, 
the other of 5,000 years, Professor Forbes maintains that an un- 
seen planet circulates. He even computed elements for the nearer 
of the two, and fixed its place on the celestial sphere; 3 but the 
photographic searches made for it by Dr. Roberts at Crowborough 
and by Mr. Wilson at Daramona proved unavailing. Undeterred 
by Deichmiiller's discouraging opinion that cometary orbits extend- 
ing beyond the recognised bounds of the solar system are too j 
imperfectly known to serve as the basis of trustworthy conclusions, 4 
the Edinburgh Professor returned to the attack in 1901. 5 He now 
sought to prove that the lost comet of 1556 actually returned in 
1844, but with elements so transformed by ultra-Neptunian pertur- 
bations as to have escaped immediate identification. If so, the 
" wanted " planet has just entered the sign Libra, and, being larger 
than Jupiter, should be possible to find. 

Almost simultaneously with Forbes, Professor Todd set about 

1 Astr. Jour., Nos. 342, 436, 508. 

a Astr. Pop., p. 661 ; La Nature, January 3, 1880. 

8 Proc. Roy. Soc. Edinb., vols, x., p. 429 ; xi., p. 89. 

4 Vierteljahrsschrift. Astr. Ges., Jahrg. xxi., p. 206. 

5 Proc. Boy. Soc. Edinb., vol. xxiii., p. 370 ; Nature, vol. lxiv., p. 524. 



chap, vin PLANETS AND SATELLITES 307 

groping for the same object by the help of a totally different set of 
indications. Adams's approved method commended itself to him 
but the hypothetical divagations of Neptune having scarcely yet 
had time to develop, he was thrown back upon the "residual 
errors " of Uranus. They gave him a virtually identical situation 
for the new planet with that derived from the clustering of cometary 
aphelia. 1 Yet its assigned distance was little more than half that of 
the nearer of Professor Forbes's remote pair, and it completed a 
revolution in 375 instead of 1,000 years. The agreement in them 
between the positions determined, on separate grounds, for the 
ultra -Neptunian traveller was merely an odd coincidence; nor can 
we be certain, until it is seen, that we have really got into touch 
with it. 

1 Amer. Jour, of Science, vol. xx., p. 225. 



202 



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 Omni- 
potent 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 satisfactory. 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 formless mass of evenly diffused 
particles, and the uniformity of their movements was sought to be 
accounted for by the unvarying 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 appear- 
ance in 1796. 1 It was presented by Laplace with diffidence, as a 
speculation unfortified by numerical buttresses of any kind, yet 
with visible exultation at having, as he thought, penetrated the 
hirth-secret of our system. He demanded, indeed, more in the way 
of postulates than Kant had done. He started with a sun ready 
made, 2 and surrounded with a vast glowing atmosphere, extending 

1 Exposition du Systeme du Monde, t. ii., p. 295. 

2 In later editions a retrospective clause was added admitting a prior condition 
.of all but evanescent nebulosity. 



chap, ix PLANETARY EVOLUTION 309 

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 contraction, or not, according to 
circumstances. Saturn's ring, it was added, afforded a striking con- 
firmation of the theory of annular separation, 1 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 appropriate to the ideas of 
a time of renovation, when the complexity of nature was little 
accounted of in comparison with the imperious orderliness of the 
thoughts of man. Since its promulgation, 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. The march of events 
certainly did not everywhere even if it did anywhere follow 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 principles 
of science. This means that, under the ordinary circumstances 
1 M4c. Cel., lib. xiv., ch. iii. 



3 io HISTORY OF ASTRONOMY part ii 

of observation, the old maxim ex nihilo nihil 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 I 
of the maintenance of the sun's heat was thus inevitably raised; I 
and with the question of maintenance that of origin is indissolubly I 
connected. 

Dr. Julius Robert Mayer, a physician residing at Heilbronn, I 
was the first to apply the new light to the investigation of what I 
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 1 
such a rate as to supply solar light and heat during the interim, I 
only a few cinders would now remain in lieu of our undiminished I 
glorious orb. Mayer looked round for an alternative. He found I 
it in the " meteoric hypothesis " of solar conservation. 1 The I 
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 in countless swarms round the | 
sun; that the earth daily encountered 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 supporting 
the sun's radiations. He proved that, by the stoppage of their 
motion through falling into the sun, bodies would evolve from 
4,600 to 9,200 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 revolutions 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 unceasing waste 
of substance, such as Newton had supposed must accompany 
emission of the material corpuscles of light, to neutralise continual 
reinforcement. 

1 Beitrage zur Dynamik des Himmels, p. 12. 



chap, ix PLANETARY EVOLUTION 311 

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 sustentation was expounded 
by him before the British Association in 1853. It was developed 
with his usual ability by Lord Kelvin, 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." 1 We know it to exist, but we now also 
know it to be entirely insufficient. The supplies presumed to 
be contained in the zodiacal light would be quickly exhausted ; a 
constant inflow from space would be needed to meet the demand. 
But if moving bodies were drawn into the sun at anything like the 
required rate, the air, even out here at ninety-three millions of miles 
distance, would be thick with them ; the earth would be red-hot 
from their impacts f geological deposits would be largely meteoric f 
to say nothing of the effects on the mechanism of the heavens. 
Lord Kelvin 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 mainte- 
nance 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 
hailstorm by which in old times it was fashioned and warmed. Mr. 
E. W. Brayley supported this view of planetary production in 1864, 4 
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 equivalence of heat and 
motion which had suggested the meteoric hypothesis. But here the 
movement surrendered and transformed belongs to the particles, not 

1 Trans. Boy. Soc. of Edinburgh, vol. xxi., p. 66. 

2 Newcomb, Pop. Astr., p. 521 (2nd ed.). 

3 M. Williams, Nature, vol. iii., p. 26. 4 Comp. Brit. Almanac, p. 94. 



3 1 2 HIS TOR Y OF ASTR ONOMY part ii 

of any foreign bodies, but of the sun itself. Drawn together from a 
wide ambit by the force of their own gravity, their fall towards the 
sun's centre must have engendered a vast thermal store, of which |4J 
are computed to be already spent. Presumably, however, this 
stream of reinforcement is still flowing. In the very act of parting 
with heat, the sun develops a fresh stock. His radiations, in short, 
are the direct result of shrinkage through cooling. A diminution of 
the solar diameter by 380 feet yearly would just suffice to cover the 
present rate of emission, and would for ages remain imperceptible 
with our means of observation, since, after the lapse of 6,000 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 contracted to half 
its present bulk. In seven million more, it will be as dense as the 
earth. It is difficult to believe that it will then be a luminous body. 2 
Nor can an unlimited past duration be admitted. Helmholtz con- 
sidered 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 allow- 
ances, to double the latter figure. But this is far from meeting the 
demands of geologists and biologists. 

An attempt was made in 1881 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 inordinate "waste" of energy, which 
shocks our thrifty ideas, was simultaneously abolished. The earth 
stops and turns variously to account one 2, 2 50- 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 
purpose we know not. Now, on the late Sir William Siemens's 
plan, this reckless expenditure would cease ; the solar incomings 
and outgoings would be regulated on approved economic principles, 
and the inevitable final bankruptcy would be staved off to remote 



But there was a fatal flaw in its construction. He imagined a 
perpetual circulation of combustible materials, alternately surrender- 
ing and regaining chemical energy, the round being kept going by 
the motive force of the sun's rotation. 3 This, however, was merely to 
perch the globe upon a tortoise, while leaving the tortoise in the air. 
The sun's rotation contains a certain definite amount of mechanical 
power enough, according to Lord Kelvin, if directly converted into 

1 Radau, Bull. Astr., t. ii., p. 316. 2 Newcomb, Pop. Astr., pp. 521-525. 

3 Proc. Roy. Soc, vol. xxxiii., p. 393. 



chap, ix PLANETARY EVOLUTION 313 

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, nevertheless, 
under the most favourable circumstances, ensue in a comparatively 
short period. 1 Many other objections equally unanswerable have 
been urged to the " regenerative " hypothesis, but this one suffices. 

Dr. Croll's collision-hypothesis 2 is less demonstrably unsound, but 
scarcely less unsatisfactory. By the mutual impact of two dark 
masses rushing together with tremendous speed, he sought to 
provide the solar nebula with an immense original stock of heat for 
the reinforcement of that subsequently evolved in "the course of its 
progressive contraction. The sun, while still living on its capital, 
would thus have a larger capital to live on, and the time-demands of 
the less exacting geologists and biologists might be successfully met. 
But the primitive event, assumed for the purpose of dispensing them 
from the inconvenience of "hurrying up their phenomena," is not 
one that a sane judgment can readily admit to have ever, in point of 
actual fact, happened. 

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 con- 
densed to their present volume with development of heat and light, 
and, it may plausibly be added, with 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 anomalous conditions of 
the Uranian world were due to no extraordinary 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 move- 

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. 

2 Stellar Evolution, and its Relations to Geological Time, 1889. 



3 i4 HISTORY OF ASTRONOMY part ii 

ment, upon the probabilities 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 circu- 
lation 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 on ; therefore, the older time 
of axial rotation should invariably be the longer. This obstacle 
can, however, as we shall presently see, be turned. 

More serious is one connected with the planetary periods, pointed 
out by Babinet in 1861. 1 In order to make them fit in with the 
hypothesis of successive separation from a rotating and contracting 
body, certain arbitrary assumptions have to be made of fluctuations 
in the distribution of the matter forming that body at the various 
epochs of separation. 2 Such expedients usually merit the distrust 
which they inspire. Primitive and permanent irregularities of 
density in the solar nebula, such as Miss Young's calculations 
suggest, 3 do not, on the other hand, appear intrinsically improbable. 

Again, it was objected by Professor Kirkwood in 1869 4 that there 
could be no sufficient cohesion in such an enormously diffused mass 
as the planets are supposed to have sprung from to account for the 
wide intervals between them. The matter separated through the 
growing excess of centrifugal speed would have been cast off, not by 
rarely recurring efforts, but continually, fragmentarily, pari passu 
with condensation and acceleration. Each wisp of nebula, as it 
found itself unduly hurried, would have declared its independence, 
and set about revolving and condensing on its own account. The 
result would have been a meteoric, not a planetary system. 

Moreover, it is a question whether the relative ages of the planets 
do not follow an order just the reverse of that concluded by Laplace. 
Professor Newcomb holds the opinion that the rings which eventu- 
ally 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 f while in M. Faye's cos- 
mogony, 6 the retrograde motion of the systems formed by the two 
outer planets is ascribed on grounds, it is true, of dubious validity 
to their comparatively late origin. 

This ingenious scheme was designed, not merely to complete, but 

1 Comptes Rendus, t. Hi., p. 481. See also Kirkwood, Observatory, vol. iii., 
p. 409. 

2 Fouche, Comptes Rendus, t. xcix., p. 903. 

3 Astroph. Jour., vol. xiii., p. 338. 4 Month. Not., vol. xxix., p. 96. 
5 Pop. Astr., p. 257. 6 Sur VOrigine du Monde, 1884. 



chap, ix PLANETARY EVOLUTION 315 

to supersede that of Laplace, which, undoubtedly, through the 
inclusion by our system of oppositely directed 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 circumstances 
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 hypothesi after the sun ; central condensation had 
reached an advanced stage when the rings they were derived from 
separated j the principle 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 develop- 
ment 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 " Who first, 
who last?" 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 2 and Professor Darwin, 3 are 
grave. Not the least so is his omission to take account of an 
agency of change presently to be noticed. 

A further valuable discussion of the matter was published by 

1 Kirkwood adverted to it in 1864, Am. Jour., vol. xxxviii., p. 1. 

2 Bull. Astr., t. ii. 3 Nature, vol. xxxi., p. 506. 



3 1 6 HISTOR Y OF ASTRONOMY part ii 

M. du Ligondes in 1897. 1 His views are those of Faye, modified 
to disarm the criticisms they had encountered j and special atten- 
tion may be claimed for his weighty remark that each planet has 
a life-history of its own, essentially distinct from those of the 
others, and, despite original unity, not to be confounded with them. 
The drift of recent investigations seems, indeed, to be to find the 
embryonic solar system already potentially complete in the parent 
nebula, like the oak in an acorn, and to relegate detailed explana- 
tions of its peculiarities to the dim pre-nebular fore-time. 

We now come to a most remarkable investigation one, indeed, 
unique in its profession to lead us back with mathematical certainty 
towards the origin of a heavenly body. We refer to Professor 
Darwin's inquiries into the former relations of the earth and 
moon. 2 

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, how- 
ever, 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 ; in 
other words, the moon is, as a consequence of tidal friction, very 
slowly receding from the earth. This will go on (other circum- 
stances remaining unchanged) until the lengthening day overtakes 
the more tardily lengthening month, when each will be of about 
1,400 hours. 3 A position of what we may call tidal equilibrium 
between earth and moon will (apart from disturbance by other 
bodies) then be attained. 

If, however, it be true that, in the time to come, the moon will 
be much farther from us, it follows that in the time past she was 
much nearer to us than she now is. Tracing back her history by 
the aid of Professor Darwin's clue, we at length find her revolving 
in a period of somewhere between two and four hours, almost in 

1 Formation Mecanique du SysUme du Monde; Bull. Astr., t. xiv., p. 313 
(0. Callandreau). See also, Le Probleme Solaire, by l'Abbe Th. Moreux, 1900. 

2 Phil. Trans., vol. clxxi., p. 713. 

3 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 1,240 hours by the effects of 
solar tidal fric ion. 



chap, ix PLANETAR Y E VOLUTION 3 1 7 

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 uncompensated 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 equilibrium, is two hours and twenty minutes. Quicken 
the movement but by one second and the globe must fly asunder. 
Hence the inference that the earth actually did fly asunder through 
over-fast spinning, the ensuing disruption representing the birth- 
throes of the moon. It is likely that the event was hastened or 
helped by solar tidal disturbance. 

To recapitulate. Analysis tracks backward the two bodies until 
it leaves them in very close contiguity, one rotating and the other 
revolving in approximately the same time, and that time certainly 
not far different from, and quite possibly identical with, the critical 
period of instability for the terrestrial spheroid. "Is this," Professor 
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 rotation V* l 

We are tempted, but are not allowed to give an unqualified assent. 
Mr. James Nolan of Victoria has 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 Professor Darwin, 
admitting the objection, concedes to our satellite, in its initial stage, 
the alternative form of a flock of meteorites. 2 But such a congrega- 

1 Phil. Trans., vol. clxxi., p. 835. 

2 Nature, vol. xxxiii., p. 368 ; see also Nolan, Ibid., vol. xxxiv., p. 286. 



3 i8 HISTORY OF ASTRONOMY part ii 

tion must have been quickly dispersed, by tidal action, into a 
meteoric ring. The same investigator subsequently fixed 6,500 miles 
from centre to centre as the minimum distance at which the moon 
could have revolved in its entirety j and he concluded it " necessary 
to suppose that, after the birth of a satellite, if it takes place at all 
in this way, a series of changes occur which are quite unknown." 1 
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 inter- 
dependent 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 inclination and eccentricity of the lunar 
orbit. All this can scarcely be accidental. 

Professor Darwin's first researches on this subject were communi- 
cated to the Royal Society, December 18, 1879. They were 
followed, January 20, 1881, 2 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 develop- 
ment, 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 dis- 
turbing 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 our present authority) very greatly from what they now are 
when her solitary offspring came, somehow, into existence. This is 
found not to have been the case with any other of the planets. It 
is unlikely that the satellites of Jupiter, Saturn, or Mars (we may 
safely add, of Uranus or Neptune) ever revolved in much narrower 
orbits than those they now traverse ; it is practically certain that 
they did not, like our moon, originate very near the present surfaces 
of their primaries. 3 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 

1 Phil. Trans., vol. clxxviii., p. 422. 

2 Ibid., vol. clxxii., p. 491. 3 Ibid., p. 530. 



chap, ix PLANETARY EVOLUTION 319 

the moon. We conclude once more that tidal friction had an 
influence here very different from its influence elsewhere. Quite 
possibly, however, that influence may be more nearly spent than in 
less advanced combinations of revolving globes. Mr. Nolan con- 
cluded in 1895 1 that it still retains appreciable efficacy in the several 
domains of the outer planets. The moons of Jupiter and Saturn are, 
by his calculations, in course of sensible retreat, under compulsion of 
the perennial ripples raised by them on the surfaces of their gigantic 
primaries. He thus connects the interior position of the fifth Jovian 
satellite with its small mass. The feebleness of its tide-raising power 
obliged it to remain behind its companions j for there is no sign of 
its being more juvenile than the Galilean quartette. 

The yielding of plastic bodies to the strain of unequal attractions 
is a phenomenon of far-reaching consequence. 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 neg- 
ligible % To this, too, Professor 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 viscous 
and glowing mass, pushing them then gradually backward from its 
surface into their present places. For the utmost possible increase 
in the length of the year through tidal friction is one hour ; and five 
minutes is a more probable estimate. 2 So far as the pull of tide- 
waves raised on the sun by the planets is concerned, then, the 
distances of the latter have never been notably different from what 
they now are; though that cause may have converted the paths 
traversed by them from circles into ellipses. 

Over their physical history, however, it was probably in a large 
measure influential. The first vital issue for each of them was 
satellites or no satellites 1 Were they to be governors as well as 
governed, or should they revolve in sterile isolation throughout the 
seons of their future existence? Here there is strong reason to 
believe that solar tidal friction was the overruling power. It is 
remarkable that planetary fecundity increases at least so far out- 
ward as Saturn with distance from the sun. Can these two facts 
be in any way related 1 In other words, is there any conceivable 
way by which tidal influence could prevent or impede the throwing- 

1 Satellite Evolution, Melbourne, 1895 ; Knowledge, vol. xviii., p. 205. 

2 Phil. Trans., vol. clxxii., p. 533. 



320 HISTORY OF ASTRONOMY part n 

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. 1 

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 con- 
tracting 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 complete 
destruction of their relative axial movement has been rendered 
probable by recent observations ; and that the process went on 
rapidly is a reasonable further inference. The earth barely escaped 
the fate of loneliness incurred by her neighbours. Her first and 
only epoch of instability was retarded until she had nearly reached 
maturity. The late appearance of the moon accounts for its large 
relative size through the increased cohesion of an already strongly 
condensed parent mass and for the distinctive peculiarities of its 
history and influence on the producing globe. 

Solar tidal friction, although it did not hinder the formation of 
two minute dependents 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 origin- 
ally nearly equal. The difference, it is alleged, was brought about 
by tidal waves raised by the sun on the semi-fluid spheroid of Mars. 
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 progres- 
sively to diminish. So that Phobos, unlike our moon, was in the 
beginning farther from its primary than now. 

But here again Mr. Nolan entered a caveat Applying the simple 
test of numerical evaluation, he showed that before solar tidal fric- 
tion could lengthen the rotation-period of Mars by so much as one 
minute, Phobos should have been precipitated upon its surface. 2 
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 

1 This was perceived by M. Ed. Roche in 1872. Mem. de VAcad. des Sciences 
dc Montpcllier, t. viii., p. 247. 

2 Nature, vol. xxxiv., p. 287. 



chap, ix PLANETAR Y E VOL UTION 32 1 

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 only a very feeble reactive pull com- 
pared with the friction which the sun should apply in order to 
protract the Martian day by one minute. And from the propor- 
tionate 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 rota- 
tional 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 
to possess at present any sensible power. But it is far from certain 
that this was always so. It seems not unlikely 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, 

1 Bull. Astr., t. ii., p. 223. 2 Montpellier Mtms., t. viii., p. 242. 

3 Amer. Jour., vol. xxxviii. (1864), p. 1. 

21 



322 HISTORY OF ASTRONOMY part ii 

by solar tidal friction, quickly rendered direct. 1 For it is scarcely 
necessary to point out that a planet turning an invariable face to \ 
the sun rotates in the same direction in which it revolves, and in 
the same period. As, with the progress 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 movements 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 to the extent, at least, of 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. 

1 Wolf, Bull. Astr., t. ii., p. 76. 



CHAPTER X 

RECENT COMETS 

On the 2nd 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 
surprisingly rapid, as it swept with accelerated 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 Septem- 
ber 12, when the nucleus shone with about the brightness of the 
pole-star, and the tail, notwithstanding large foreshortening, 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 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 memories retain the impress. Donati's comet was, 

212 



3 24 HISTOR Y OF ASTR ONOMY part ii 

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 
impressive to the spectator. At Pulkowa, on the 16th Septem- 
ber, Winnecke, 2 the first director of the Strasburg Observatory, 
observed a faint outer envelope resembling a veil of almost evan- 
escent texture flung somewhat 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 
some 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 repulsive forces was never more beautifully 
illustrated. The triple tail seemed 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, estimated 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 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 apparently as black, quite close up to the nucleus, as 
the sky indicated for the tail a hollow, cone -like structure ; 3 while 
the repetition of certain spots and rays in the same corresponding 
situation on one envelope after another served to show that the 
nucleus to some local peculiarity of which they were doubtless due 
had no proper rotation, but merely shifted sufficiently on an axis 
to preserve the same aspect towards the sun as it moved round it." 1 
This observation of Bond's was strongly confirmatory of Bessel's 
hypothesis of opposite polarities in such bodies' opposite sides. 

The protrusion towards the sun, on September 25, of a brilliant 
luminous fan-shaped sector completed the resemblance to Halley'f 

1 Month. Not., vol. xix., p. 27. 2 Mem. de VAc. Imp., t. ii., 1859, p. 46. 

:i Harvard Annals, vol. iii., p. 368. 4 Ibid., p. 371. 



:hap. x RECENT COMETS 325 

;omet. The appearance of the head was now somewhat that of 
1 "bat's-wing" gaslight. There were, however, no oscillations to 
ind fro, such as Bessel had seen and speculated upon in 1835. As 
ihe size of the nucleus contracted with approach to perihelion, its 
ntensity augmented. On October 2, it outshone Arcturus, and for 
% week or ten days was a conspicuous object half an hour after 
sunset. Its lustre setting aside the light derived from the tail 
was, at that date, 6,300 times what it had been on June 15, though 
theoretically taking into account, that is, only the differences of 
listance from sun and earth it should have been only ^ of that 
imount. Here, it might be thought, was convincing evidence of 
}he comet itself becoming ignited under the growing intensity of 
;he solar radiations. Yet experiments with the polariscope were 
interpreted in an adverse sense, and Bond's conclusion that the 
3omet sent us virtually unmixed reflected sunshine was generally 
acquiesced in. It was, nevertheless, negatived by the first applica- 
tion of the spectroscope to these bodies. 

Very few comets have been so well or so long observed as 
;Donati's. It was visible to the naked eye during 112 days; it was 
:elescopically discernible for 275, the last observation having been 
nade by Mr. William Mann at the Cape of Good Hope, March 4, 
11859. Its coarse through the heavens combined singularly with 
/he orbital place of the earth to favour curious inspection. The 
sail, when near its greatest development, lost next to nothing by 
)he effects of perspective, and at the same time lay in a plane 
sufficiently inclined to the line of sight to enable it to display its 
exquisite curves to the greatest advantage. Even the weather was, 
>n both sides of the Atlantic, propitious during the period of 
greatest interest, and the moon as little troublesome as possible, 
fhe volume compiled by the younger Bond is a monument to 
;he care and skill with which these advantages were turned to 
account. Yet this stately apparition marked no turning-point in 
:he history of cometary science. By its study knowledge was 
ndeed materially advanced, but along the old lines. No quick and 
vivid illumination broke upon its path.* Quite insignificant objects 
as we have already partly seen have often proved more vitally 
nstructive. 

Donati's comet has been identified with no other. Its path is an 
mmensely elongated ellipse, lying in a plane far apart from that of the 
planetary movements, carrying it at perihelion considerably within 
;he orbit of Venus, and at aphelion out into space to 5J times the 
listance from the sun of Neptune. The entire circuit occupies over 
2,000 years, and is performed in a retrograde direction, or against the 
)rder of the Signs. Before its next return, about the year 4000 A.D., 



326 HISTORY OF ASTRONOMY part i: 

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 the 
century, through the tail of a great comet. Some of our readers 
may 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 surrounding the head produced, according to the 
late Mr. Webb, 1 a magnificent effect. Portions of six distinct emana- 
tions were traceable. " It was as though a number of light, hazy 
clouds were floating round a miniature full moon." As the sky 
darkened the tail emerged to view. 2 Although in brightness and 
sharpness of definition it could not compete with the display of 1858, 
its dimensions proved to be extraordinary. It reached upwards 
beyond the zenith when the head had already set. By some 
authorities its extreme length was stated at 118, and it showed no 
trace of curvature. Most remarkable, however, was the appearance 
of two widely divergent rays, each pointing towards the head, though 
cut off from it by sky-illumination, of which one was seen by Mr. 
Webb, and both by Mr. Williams at Liverpool, a quarter of an hour 
before midnight. There seems no doubt that Webb's interpretation 
was the true one, and that these beams were, in fact, " the perspec- 
tive representation of a conical or cylindrical tail, hanging closely 
above our heads, and probably just being lifted up out of our atmo- 
sphere." 3 The cometary train was then rapidly receding from the 
earth, so that the sides of the " outspread fan " of light shown by it 
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 
11 to 14, and Mr. Tebbutt, \>y whom the comet was discovered in 
New South Wales on May 13, had anticipated such an encounter, 
while the former 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 outward just along the line of inter- 

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. lxi., p. 953. 



chap, x RECENT COMETS 327 

section of its own with the terrestrial orbit to an extent of fifteen 
million miles; so that our globe, happening to pass at the time, 
found itself during some hours involved in the flimsy appendage. 

No perceptible effects were produced by the meeting ; it was 
known to have occurred by theory alone. A peculiar glare in the 
sky, thought by some to have distinguished the evening of June 30, 
was, at best, inconspicuous. Nor were there any symptoms of 
unusual electric excitement. The Greenwich instruments were, 
indeed, disturbed on the following night, but it would be rash to 
infer that the comet had art or part in their agitation. 

The perihelion-passage of this body occurred June 11, 1861 ; and 
its orbit has been shown by M. Kreutz of Bonn, from a very com- 
plete investigation founded on observations extending over nearly 
a year, to be an ellipse traversed in a period of 409 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 11, without pomp of train or 
other appendages, and might have seemed hardly worth the trouble 
of pursuing. Fortunately, 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. 

The idea prevalent in the eighteenth century as to the nature of 
shooting stars was that they were mere aerial ignesfatui inflammable 
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. 2 Acting on his suggestion, Brandes 

1 Smiths. Report, 1881 (Holden) ; Nature, vol. xxv., p. 94 ; Observatory, 
vol. xxi., p. 378 (W. T. Lynn). 

2 Ueber den Ursprung der von Pallas gcfandenen Eisenmassen, p. 24. 



328 HISTORY OF ASTRONOMY part n 

and Benzenberg, two students at the University of Gottingen, 
began in 1798 to determine the heights of falling stars by simul- 
taneous observations at a distance. They soon found that they 
move with planetary velocities in the most elevated regions of our 
atmosphere, 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 ignorance. It needed one of those amazing 
spectacles, at which man assists, no longer in abject terror for his 
own frail fortunes, but with keen curiosity and the vivid expectation 
of new knowledge, to bring about a clarification. 

On the night of November 12-13, 1833, 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 count- 
ing; but as it waned, a reckoning was attempted, from which it 
was computed, on the basis of that much diminished rate, that 
240,000 must have been visible during the nine hours they continued 
to fall. 1 

Now there was one very remarkable feature common to the 
innumerable small bodies which traversed, or were consumed in our 
atmosphere that night. They all seemed to come from the same part 
of the sky. Traced 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 realize. Denison Olmsted was then Professor of 
Mathematics in Yale College. He showed early in 1834 3 that the 
emanation of the showering meteors from a fixed "radiant " proved 

1 Arago, Annuaire, 1836, p. 294. 

2 Humboldt had noticed the emanation of the shooting stars of 1799 from a 
single point, or "radiant," as Greg long afterwards termed it ; but no reasoning 
was founded on the observation. 

8 Am. Journ. of Sc, vol. xxvi., p. 132. 



chap, x RECENT COMETS 329 

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 appearances 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 astronomy. Their 
membership of the solar system was no longer a theory or a con- 
jecture it was an established fact. The discovery might be com- 
pared 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 a nous." 

Evidences of periodicity continued to accumulate. It was re- 
membered that Humboldt and Bonpland had been the spectators at 
Cumana, after midnight on November 12, 1799, of a fiery shower 
little inferior to that of 1833, and reported to have been visible from 
the equator to Greenland. Moreover, 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 pheno- 
menon 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 traditionally associated 
with August 10 by the popular phrase in which they figured as " the 
tears of St. Lawrence." But the association 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, 1836. 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 agglomeration a period of an 
exact year, or sub-multiple of a year, it would be plainly absurd to 

1 Annuaire, 1836, p. 297. 2 Ann. de VObserv., Bruxelles, 1839, p. 248. 

8 Ibid., 1837, p. 272. 



330 HISTORY OF ASTRONOMY part ii 

suppose the movements of two or more regulated by such highly 
artificial conditions. An alternative was proposed by Adolf Erman 
of Berlin in 1839. 1 No longer in clouds, but in closed rings, he 
supposed meteoric matter to revolve round the sun. Thus the mere 
circumstance of intersection by a meteoric of the terrestrial orbit, 
without any coincidence of period, would account for the earth 
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. 

Hubert A. Newton took up, in 1864, 2 the dropped thread of 
inquiry. The son of a mathematical mother, he attained, at the 
age of twenty-five, to the dignity of Professor of Mathematics in 
Yale University, and occupied the post until his death in 1896. 
The diversion of his powers, however, from purely abstract studies 
stimulated their effective exercise, and constituted him one of the 
founders of meteoric astronomy. 

A search through old records carried the November phenomenon 
back to the year 902 A.D., long distinguished as " the year of the 
stars." For in the same night in which Taormina was captured by 
the Saracens, and the cruel Aghlabite tyrant Ibrahim ibn Ahmed 
died "by the judgment of God" before Cosenza, stars fell from 
heaven in such abundance as to amaze and terrify beholders far 
and near. This was on October 13, and recurrences were traced 
down through the subsequent centuries, always with a day's delay 
in about seventy years. It was easy, too, to derive from the dates 
a cycle of 33 years, so that Professor Newton did not hesitate to 
predict the exhibition of an unusually striking meteoric spectacle on 
November 13-14, 1866. 3 

For the astronomical explanation of the phenomena, recourse 
was had to a method introduced by Erman of computing meteoric 
orbits. It was found, however, that conspicuous recurrences every 
thirty -three or thirty-four years could be explained on the supposi- 
tion 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 354J days, but indicated 
the means of deciding with certainty upon the true one. It was 
furnished by the advancing motion of the node, or that day's 
delay of the November shower every seventy years, which the old 
chronicles had supplied data for detecting. For this is a strictly 

1 Astr. JSFach., Nos. 385, 390. 

2 Am. Jour, of Sc, vol. xxxvii. (2nd ser.), p. 377. 

3 Ibid., vol. xxxviii., p. 61. 



chap, x RECENT COMETS 331 

measurable effect of gravitational disturbance by the various planets, 
the amount of which naturally depends upon the course pursued by 
the disturbed bodies. Here the great mathematical resources of 
Professor Adams were brought to bear. By laborious processes of 
calculation, he ascertained that four out of Newton's five possible 
periods were entirely incompatible with the observed nodal displace- 
ment, while for the fifth that of 33J years a perfectly harmonious 
result was obtained. 1 This was the last link in the chain of evidence 
proving that the November meteors or " Leonids," as they had by 
that time come to be called revolve round the sun in a period of 
33*27 years, in an ellipse spanning the vast gulf between the orbits 
of the earth and Uranus, the group being so extended as to occupy 
nearly three 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. Baxendell's memory, 2 inferior to that of 1833, was of extra- 
ordinary impressiveness. Dense crowds of meteors, equal in lustre 
to the brightest stars, and some rivalling Venus at her best, 3 darted 
from east to west across the sky with enormous apparent velocities, 
and with a certain determinateness of aim, as if let fly with a 
purpose, and 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 1,1 20. 5 Before daylight the earth 
had fairly cut her way through the star-bearing stratum; the 
" ethereal rockets " had ceased to fly. 

This event brought the subject of shooting stars once more vividly 
to the notice of astronomers. Schiaparelli had, 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, and published in the Bulletino of the Roman 
Observatory. 6 Their upshot was to show, in the first place, that 

1 Month. Not, vol. xxvii., p. 247. 

2 Am. Jour, of Sc, vol. xliii. (2nd ser.), p. 87. 

3 Grant, Month. Not., vol. xxvii., p. 29. 4 P. Smyth, Ibid., p. 256. 
5 Hind, Ibid., p. 49. 6 Reproduced in Les Mondes, t. xiii. 



332 HISTORY OF ASTRONOMY part ii 

meteors possess a real velocity considerably greater than that of the 
earth, and travel, accordingly, 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 occasionally 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 
named "Perseids" 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, 1 elements for the November 
swarm, founded on the most recent and authentic observations ; at 
once identified by Dr. C. F. W. Peters of Altona with Oppolzer's 
elements for Tempel's comet of 1866. 2 A few days later, Schiaparelli, 
having recalculated 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 
(Lyraids), as well as between those of Biela's comet and certain 
conspicuous meteors of November 28. 3 

These instances do not seem to be exceptional. The number of 
known or suspected accordances of cometary tracks with meteor 
streams contained in a list drawn up in 18 78 4 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 only absolutely sure examples 
of a relation as significant as it was, to most astronomers, unex- 
pected. 

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, 1774, 5 need count for much. But Chladni, 
in 1819, 6 considered both to be fragments or particles of the same 
primitive matter, irregularly scattered through space as nebulae ; 
and Morstadt of Prague suggested about 1837 7 that the meteors of 

1 Comptes Rendus, t. lxiv., p. 96. 2 Astr. Nach., No. 1,626. 

3 Ibid., No. 1,632. 4 Month. Not., vol. xxxviii., p. 369. 

5 Schiaparelli, Le Stelle Cadenti, p. 54. 6 Ueber Feuer-Meteorc, p. 406. 

7 Astr. Nach., No. 347 (Madler) ; see also Roguslawski, Die Kometen, p. 98, 
1857. 



chap, x RECENT COMETS 333 

November might be dispersed atoms from the tail of Biela's comet, 
the path of which is cut across by the earth near that epoch. 
Professor Kirkwood, however, by a luminous intuition, penetrated 
the whole secret, so far as it has yet been made known. In an 
article published, or rather buried, in the Danville Quarterly Review 
for December, 1861, he argued, from the observed division of Biela, 
and other less noted instances of the same 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 debris of ancient but 
now disintegrated comets, whose matter has become distributed 
round their orbits V 1 

The gist of Schiaparelli's discovery could not be more clearly 
conveyed. For it must be borne in mind that with the ultimate 
destiny of comets' tails this had nothing to do. The tenuous 
matter composing them is, no doubt, permanently lost to the body 
from which it emanated ; but science does not pretend to track its 
further wanderings through space. It can, however, state categori- 
cally that these will no longer be conducted along the paths 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 cometary nuclei 
most likely are) 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. In 126 a.d. a 6lose 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 care- 
1 Nature, vol. vi., p. 148. 



334 HISTORY OF ASTRONOMY part ii 

fully watched for with the best telescopes ; its path was accurately 
known j every perturbation it could suffer was scrupulously taken 
into account. Under these circumstances, 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 were 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 Andromedse. 1 Now this is precisely the direction in which the 
orbit of Biela's comet would seem to lie, as it runs down to cut the 
terrestrial track very near the place of the earth at the above dates. 
The inference was, then, an easy one, that the meteors were pursuing 
the same path with the comet ; and it was separately arrived at, 
early in 1867, by Weiss, D' Arrest, and Galle. 2 But Biela travels in 
the opposite direction to Tempel's comet and its attendant 
" Leonids "; its motion is direct, or from west to east, while theirs is 
retrograde. Consequently, the motion of its node is in the opposite 
direction too. In other words, the meeting-place of its orbit with 
that of the earth retreats (and very rapidly) along 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 antici- 
pate the times of their recurrence by as much as a week in half a 
century. All doubt as to the fact may be said to have been 
removed by Signor Zezioli's observation of the annual shower in 
more than usual abundance at Bergamo, November 30, 1867. 

The missing comet was next due at perihelion in the year 1872, 
and the probability was contemplated by both Weiss and Galle of its 
being replaced by a copious discharge of falling stars. The precise 
date of the occurrence was not easily determinable, but Galle thought 
the chances in favour of November 28. The event anticipated the 
prediction by twenty-four hours. Scarcely had the sun set in Western 
Europe on November 27, when it became evident that Biela's comet 
was shedding over us the pulverised products of its disintegration. 
The meteors came in volleys from the foot of the Chained Lady, 
their numbers at times baffling the attempt to keep a reckoning. 
At Moncalieri, about 8 p.m., they constituted (as Father Denza said 3 ) 
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, 

1 A. S. Herschel, Month. Not., vol. xxxii., p. 355. 

2 Astr. Nach., Nos. 1,632, 1,633, 1,635. 3 Nature, vol. vii., p. 122. 



chap, x RECENT COMETS 335 

the stars of 1872, though about equally numerous, were less brilliant 
than those of 1866 j the phosphorescent tracks marking their passage 
were comparatively evanescent and their movements sluggish. This 
is easily understood when we remember that the Andromedes over- 
take 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, never- 
theless, 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. 1 

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 accompaniment 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 f and 
an enhancement of electrical disturbance may well be associated with 
the flittings of such cosmical atoms. 

A singular incident connected with the meteors of 1872 has now 
to be recounted. The late Professor Klinkerfues, who had observed 
them very completely at Gottingen, was led to believe that not 
merely the debris strewn along its path, but the comet itself must 
have been in immediate proximity to the earth during their appear- 
ance. 3 If so, it might be possible, he thought, to descry it as it re- 
treated 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 remains 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 

1 A. S. Herschel, Report Brit. Ass., 1873, p. 390. 
a Humboldt, Cosmos, vol. i., p. 114 (Otte's trans.). 
8 Month. Not., vol. xxxiii., p. 128. 



336 HISTORY OF ASTRONOMY part n 

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 1 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 shower 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 dis- 
appointed. 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 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 ! 2 Thus the 
dazzling effect of a luminous throng was produced without jostling 
or overcrowding, 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, 3 "traversed 
short paths with very slow motions, and became extinct in evolved 
streams of yellowish sparks." The conclusion seemed obvious "that 
these meteors are formed of very soft materials, which expand while 
incalescent, and are immediately crumbled and dissipated into 
exiguous dust." 

The Biela meteors of 1885 did not merely gratify astronomers 
with a fulfilled prediction, but were the means of communicating 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 1. 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 

1 Even this was denied by Bruhns, Astr. Nach., No. 2,054. 

2 Am. Jour., vol. xxxi., p. 425. 3 Month. Not., vol. xlvi., p. 69. 



chap, x RECENT COMETS 337 

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 entering it. Thus, their luminous tracks did not 
always represent (even apart from the effects of the earth's attrac- 
tion) 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 encounter with its 
meteoric fragments. These are now probably scattered over more 
than five 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 experi- 
enced, as they seem to have done, the same kind and amount of 
disturbance. The rapidity of cometary disintegration is thus 
curiously illustrated. 

A short-lived persuasion that the missing heavenly body itself had 
been recovered, was created by Mr. Edwin Holmes's discovery, at 
London, November 6, 1892, of a tolerably bright, tailless comet, just 
in a spot which Biela's comet must have traversed in approaching 
the intersection of its orbit with that of the earth. A hasty 
calculation by Berberich assigned elements to the newcomer 
seeming not only to ratify the identity, but to promise a quasi- 
encounter with the earth on November 21. The only effect of the 
prediction, however, was to raise a panic among the negroes of the 
Southern States of America. The comet quietly ignored it, and 
moved away from instead of towards the appointed meeting-place. 
Its projection, then, on the night of its discovery, upon a point of 
the Biela-orbit was by a mere caprice of chance. North America, 
nevertheless, was visited on November 23 by a genuine Andromede 
shower. Although the meteors were less numerous than in 1885, 
Professor Young estimated that 30,000, at the least, of their orange 
fire-streaks came, during five hours, within the range of view at 
Princeton. 2 Bredikhine estimated the width of the space containing 
them at about 2,700,000 miles. 3 The anticipation of their due 
time by four days implied if they were a prolongation of the main 
Biela group, the nucleus of which passed the spot of encounter five 

1 In Schiaparelli's opinion, centuries must have elapsed while the observed 
amount of scattering was being produced. Le Stelle Cadervti, 1886, p. 112. 
a Astr. and Astroph., vol. xi., p. 943. 
3 Bull, de VAcad. St. Fetersbourg, t. xxxv., p. 598, 1894. 

22 



338 HISTORY OF ASTRONOMY part ii 

months previously a recession of the node since 1885 by no less 
than three degrees. Unless, indeed, Mr. Denning were right in 
supposing the display to have proceeded from "an associated branch 
of the main swarm through which we passed in 1872 and 1885." 1 
The existence of separated detachments of Biela meteors, due to 
disturbing planetary action, was contemplated as highly probable 
by Schiaparelli. 2 Such may have been the belated nights met with 
in 1830, 1838, 1841, and 1847, and such the advance flight plunged 
through in 1892. A shower looked for November 23, 1899, did not 
fall, and no further display from this quarter is probable until 
November 17, 1905, although one is possible a year earlier. 3 

The Leonids, through the adverse influence of Jupiter and Saturn, 
inflicted upon multitudes of eager watchers a still more poignant 
disappointment. A dense part of the swarm, having nearly com- 
pleted a revolution since 1866, should, travelling normally, have 
met the earth November 15, 1899 ; in point of fact, it swerved sun- 
ward, and the millions of meteorites which would otherwise have 
been sacrificed for the illumination of our skies escaped a fiery 
doom. The contingency had been forecast in the able calculations 
of Dr. Johnstone Stoney and Dr. A. M. W. Downing, 4 super- 
intendent of the Nautical Almanac Office; but the verification 
scarcely compensated the failure. Nor was the situation retrieved 
in the following years. Only ragged fringes of the great tempest- 
cloud here and there touched our globe. As the same investigators 
warned us to expect, the course of the meteorites had been not 
only rendered sinuous by perturbation, but also broken and 
irregular. We can no longer count upon the Leonids. Their 
glory, for scenic purposes, is departed. The comet associated with 
them also evaded observation. Although it doubtless kept its 
tryst with the sun in the spring of 1899, the attendant circum- 
stances were too unfavourable to allow it to be seen from the earth. 5 
By an almost fantastic coincidence, nevertheless, a faint comet was 
photographed, November 14, 1898, 6 by Dr. Chase, of the Yale 
College Observatory, close to the Leonid radiant, whither a 
" meteorograph " was directed with a view to recording trails left 
by precursors of the main Leonid body. A promising start, too, 
was made on the same occasion with meteoric researches from 

1 Observatory, vol. xvi., p. 55. 

2 Le Stelle Cadenti, p. 133 ; Bendiconti dclV Istituto Lombardo, t. iii., ser. ii., 
p. 23. 

3 Denning, Memoirs Boy. Astr. Soc, vol. liii., p. 214 ; Abelmann, Astr. Nach., 
No. 3,516. 

4 Proc. Boy. Soc, March 2, 1899 ; Nature, November 9, 1899. 

5 Berberich, Astr. Nach., No. 3,526. 

6 Elkin, Astroph. Jour., vol. ix., p. 22. 



chap, x RECENT COMETS 339 

sensitive plates. 1 Indeed, Schaeberle and Colton 2 had already, in 
1896, determined the height of a Leonid by means of photographs 
taken at stations on different ridges of Mount Hamilton; and 
Professor Pickering has prosecuted similar work at Harvard, with 
encouraging results. Everything in this branch of science depends 
upon how far they can be carried. Without the meteorograph, 
rigid accuracy in the observation of shooting stars is unattainable, 
and rigid accuracy is the sine qua non for obtaining exact knowledge. 
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. 3 But a division once established, 
separation must continually progress. The periodic times of the 
fragments will never be identical ; one must drop a little behind the 
other at each revolution, until at length they come to travel in 
remote parts of nearly the same orbit. Thus the comet predicted 
by Klinkerfues and discovered by Pogson had already lagged to the 
extent of twelve weeks, and we shall meet instances farther on 
where the retardation is counted, not by weeks, but by years. 
Here original identity emerges only from calculation and comparison 
of orbits. 

Comets, then, die, as Kepler wrote long ago, sicut bombyces filo 
fundendo. This certainty, anticipated by Kirkwood in 1861, we 
have at least acquired from the discovery of their generative con- 
nection with meteors. Nay, their actual materials become, in 
smaller or larger proportions, incorporated with our globe. It is 
not, indeed, universally admitted that the ponderous masses of which, 
according to Daubree's estimate, 4 at least 600 fall annually from 
space upon the earth, ever formed part of the bodies known to us as 
comets. Some follow Tschermak 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 remarkable 
that a meteoric fusillade may be kept up for hours without a single 
solid projectile reaching its destination. It would seem as if the 
celestial army had been supplied with blank cartridges. Yet, since 
a few detonating meteors have been found to proceed from ascer- 
tained radiants of shooting-stars, it is difficult to suppose that any 
generic difference separates them. 

Their assimilation is further urged though not with any demon- 

1 Elkin, Astroph. Jour., vol. x., p. 24. 

a Pop. Astr., September, 1897, p. 232. 8 Month. Not, vol. xx., p. 336. 

4 Revue des deux Mondes, December 15, 1885, p. 889. 

222 



340 HISTORY OF ASTRONOMY part ii 

strative force by two instances, the only two on record, of the 
tangible descent of an aerolite during the progress of a star-shower. 
On April 4, 1095, the Saxon Chronicle informs us that stars fell "so 
thickly that no man could count them," and adds that one of them 
having struck the ground in France, a bystander " cast water upon 
it, which was raised in steam with a great noise of boiling." 1 And 
again, on November 27, 1885, while the skirts of the Andromede- 
tempest were trailing over Mexico, "a ball of fire " was precipitated 
from the sky at Mazapil, within view of a ranchman. 2 Scientific 
examination proved it to be a " siderite," or mass of " nickel-iron " ; 
its weight exceeded eight pounds, and it contained many nodules of 
graphite. We are not, however, authorised by the circumstances of 
its arrival to regard the Mazapil fragment of cosmic metal as a 
specimen torn from Biela's comet. In this, as in the preceding case, 
the coincidence of the fall with the shower may have been purely 
casual, since no hint is given of any sort of agreement between the 
tracks followed by the sample provided for curious study, and the 
swarming meteors consumed in the upper air. 

Professor Newton's inquiries into the tracks pursued by meteorites 
previous to their collisions with the earth tend to distinguish them, 
at least specifically, from shooting-stars. He found that nearly all 
had been travelling with a direct movement in orbits the perihelia 
of which lay in the outer half of the space separating the earth from 
the sun. 3 Shooting-stars, on the contrary, are entirely exempt from 
such limitations. The Yale Professor concluded " that the larger 
meteorites moving in our solar system are allied much more closely 
with the group of comets of short period than with the comets 
whose orbits are nearly parabolic." They would thus seem to be 
more at home than might have been expected amid the planetary 
family. Father Carbon elle has, moreover, shown 4 that meteorites, if 
explosion-products of the earth or moon, should, with rare excep- 
tions, follow just the kind of paths assigned to them, from data of 
observation, by Professor Newton. Yet it is altogether improbable 
that projectiles from terrestrial volcanoes should, at any geological 
epoch, have received impulses powerful enough to enable them, 
not only to surmount the earth's gravity, but to penetrate its atmo- 
sphere. 

A striking indeed, an almost startling peculiarity, on the other 
hand, divides from their congeners a class of meteors identified by 

1 Palgrave, Phil. Trans. , vol. cxxv., p. 175. 

2 W. E. Hidden, Century Mag., vol. xxxiv., p. 534. 

3 Amer. Jour, of Science, vol. xxxvi., p. i., 1888. 

4 Revue des Questions Scientifiques, January, 1889, p. 194 ; Tisserand, Bull. 
Astr., t. viii., p. 460. 



chap, x RECENT COMETS 341 

Mr. Denning during ten years' patient watching of such phenomena 
at Bristol. 1 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, analogous 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 
many 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. Ranyard's calculation, at least 880 miles a second. 2 
But the aspect of the meteors justifies no such extravagant assump- 
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 the " incredible and unaccountable " 3 fact of 
the existence of these " long radiants," although doubted by 
Tisserand 4 because of its theoretical refractoriness, must apparently 
be admitted. The first plausible explanation of them was offered 
by Professor Turner in 1899. 5 They represent, in his view, the 
cumulative effects of the earth's attraction. The validity of his 
reasoning is, however, denied by M. Bredikhine, 6 who prefers to 
regard them as a congeries of separate streams. The enigma they 
present has evidently not yet received its definitive solution. 

The Perseids afford, on the contrary, a remarkable instance of a 
" shifting radiant." Mr. Denning's observations of these yellowish, 
leisurely meteors extend over nearly six weeks, from July 8 to 
August 16; the point of radiation meantime progressing no less 
than 57 in right ascension. Doubts as to their common origin 
were hence freely expressed, especially by Mr. Monck of Dublin. 7 
But the late Dr. Kleiber 8 showed, by strict geometrical reasoning, 
that the forty-nine radiants successively determined for the shower 
were all, in fact, comprised within one narrowly limited region of 
space. In other words, the application of the proper corrections for 
the terrestrial movement, and the effects of attraction by which each 
individual shooting-star is compelled to describe a hyperbola round 
the earth's centre, reduces the extended line of radiants to a compact 

1 Month. Not., vol. xlv., p. 93. 

2 Observatory, vol. viii., p. 4. 8 Denning, Month. Not., vol. xxxviii., p. 114. 
4 Comptes Rendus, t. cix., p. 344. 5 Month. Not., vol. lix., p. 140. 

6 Bull, de VAcad. St. Petersb., t. xii., p. 95. 

7 Publ. Astr. Pox. Soc, vol. iii., p. 114. 8 Month. Not., vol. lii., p. 341. 



342 HISTORY OF ASTRONOMY part n 

group, with the cometary radiant for its central point ; the cometary 
radiant being the spot in the sky met by a tangent to the orbit of 
the Perseid comet of 1862 at its intersection with the orbit of the 
earth. The reality of the connection between the comet and the 
meteors could scarcely be more clearly proved ; while the vast 
dimensions of the stream into which the latter are found to be 
diffused cannot but excite astonishment not unmixed with per- 
plexity. 

The first successful application of the spectroscope to comets was 
by Donati in 1864. 1 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. 2 On August 5 Donati admitted its light through 
his train of prisms, and found it, thus analysed, to consist of three 
bright bands yellow, green, and blue separated by wider dark 
intervals. This implied a good deal. Comets had previously been 
considered, 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 j and this was taken by Sir William Huggins in 1868. 3 

A comet of subordinate brilliancy, known as comet 1868 ii., or 
sometimes as Winnecke's, was the subject of his experiment. 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 
tested by light -analysis, between 1868 and 1880, showed the typical 
hydro-carbon spectrum 4 common to the whole group of those com- 
pounds, but probably due immediately to the presence of acetylene. 
Some minor deviations from the laboratory pattern, in the shifting 
of the maxima of light from the edge towards the middle of the 
yellow and blue bands, have been experimentally reproduced by 
Vogel and Hasselberg in tubes containing a mixture of carbonic 
oxide with olefiant gas. 5 Their illumination by disruptive electric 
discharges was, however, a condition sine qud non for the exhibition 
of the cometary type of spectrum. When a continuous current was 

1 Astr. Nach., No. 1,488. 2 Annuaire, Paris, 1883, p. 185. 

3 Phil. Trans., vol. clviii., p. 556. 

4 Hasselberg, Mim. de VAc. Imp. de St. Pttersbourg, t. xxviii. (7th ser.), No. 2, 
p. 66. 

5 Schemer, Die Spectralanalyse der Gestirne, p. 234. Kayser (Asir. and 
Astroph., vol. xiii., p. 368) refers the anomalies of the carbon-spectrum in 
comets wholly to instrumental sources. 



PLATE II. 




Great Comet. 



Photographed, May 5, 1901, with.the thirteen-inch Astrographic Refractor of the 
Royal Observatory, Cape of Good Hope. 



chap, x RECENT COMETS 343 

employed, the carbonic oxide bands asserted themselves to the 
exclusion of the hydro-carbons. The distinction has great signifi- 
cance as regards the nature of comets. Of particular interest in this 
connection is the circumstance that carbonic oxide is one of the 
gases evolved by meteoric stones and irons under stress of heat. 1 
For it must apparently have formed part of an aeriform mass in 
which they were immersed at an earlier stage of their history. 

In a few exceptional comets the usual carbon-bands have been 
missed. Two such were observed by Sir William Huggins in 1866 
and 1867 respectively. 2 In each a green ray, approximating in 
position to the fundamental nebular line, crossed an otherwise 
unbroken spectrum. And Holmes's comet of 1892 displayed only 
a faint prismatic band devoid of any characteristic feature. 3 Now 
these three might well be set down as partially effete bodies ; but a 
brilliant comet, visible in southern latitudes in April and May, 1901, 
so far resembled them in the quality of its light as to give a 
spectrum mainly, if not purely, continuous. This, accordingly, is no 
symptom of decay. 

The earliest comet of first-class lustre to present itself for spectro- 
scopic examination was that discovered by Coggia at Marseilles, 
April 17, 1874. Invisible to the naked eye till 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. Bredikhine, Vogel, and Huggins 4 were 
unanimous in pronouncing its spectrum to be that of marsh or 
olefiant gas. Father Secchi, in the clear sky of Kome, was able to 
push the identification even closer than had heretofore been done. 
The complete hydro-carbon spectrum consists of five zones of 
variously coloured light. Three of these only the three central 
ones had till then been obtained from comets; owing, it was 
supposed, to their temperature not being high enough to develop 
the others. The light of Coggia's comet, however, was found to 
contain all five, traces of the violet band emerging June 4, of the 
red, July 2. 5 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, largely made up of 
a compound of hydrogen with carbon. Other materials are also 

1 Dewar, Proc. Roy. Inst., vol. xi., p. 541. 

2 Proc. R. Soc, vol. xv., p. 5 ; Month. Not., vol. xxvii., p. 288. 

3 Keeler, Astr. and Astrophysics, vol. xi., p. 929 ; Vogel, Astr. Nach., 
No. 3,142. 

4 Proc. Roy. Soc, vol. xxiii., p. 154. 5 Hasselberg, loc. cit., p. 58. 



344 HISTORY OF ASTRONOMY part ii 

present ; but the hydro-carbon element is probably unfailing and 
predominant. Its luminosity is, there is little doubt, an effect of 
electrical excitement. Zollner showed in 1872 1 that, owing to 
evaporation and other changes produced by rapid approach to the 
sun, electrical processes of considerable intensity must take place in 
comets j and that their original light is immediately connected with 
these, and depends upon solar radiation, rather through its direct or 
indirect electrifying effects, than through its more obvious thermal 
power, may be considered a truth permanently acquired to science. 2 
They are not, it thus seems, bodies incandescent through heat, but 
glowing by electricity ; and this is compatible, under certain 
circumstances, 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 j in 
part, most likely, to the ignition of minute solid particles. 

1 Ueber die Natur der Cometen, p. 112. 2 Hasselberg, loc. cit., p. 38. 



CHAPTER XI 

RECENT COMETS {continued) 

The mystery of comets' tails has been to some extent penetrated ; 
so far, at least, that, by making certain assumptions strongly 
recommended by the facts of the case, their forms can be, with 
very approximate precision, calculated beforehand. We have, then, 
the assurance that these extraordinary appendages are composed of 
no ethereal or supersensual stuff, but of matter such as we know 
it, and subject to the ordinary laws of motion, though in a state of 
extreme tenuity. 

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 appearances presented by Halley's 
comet in 1835. He, moreover, provided a formula for computing 
the movement of a particle under the influence of a repulsive force 
of any given intensity, and thus laid firmly the foundation of a 
mathematical theory of cometary emanations. Professor W. A. 
Norton, of Yale 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. 

The 'physical theory of repulsion, however, was, it might be said, 
still in the air. Nor did it even begin to assume consistency until 
Zollner took it in hand in 187 1. 2 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 repulsion 
relatively to solar gravitational attraction grows, consequently, as 
the size of the particle diminishes. Make this small enough, and it 

1 Astr. Naeh., Nos. 1,172-4. 2 Berichte Sachs. Ges., 1871, p. 174. 



346 HISTORY OF ASTRONOMY part ii 

will virtually cease to gravitate, and will unconditionally obey the 
impulse to recession. 

This principle Zollner was the first to realise in its application 
to comets. It gives the key to their constitution. Admitting that 
the sun and they are similarly 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 1 that it need not be higher than that attributed by the 
best authorities to the terrestrial surface. 

Forty years have elapsed since M. Bredikhine, director successively 
of the Moscow and of the Pulkowa Observatories, turned 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." 2 This idea was confirmed on fuller investigation. In 1882 
the appendages of thirty-six well-observed comets had been recon- 
structed theoretically, without a single exception being met with 
to the rule of the three types. A further study of forty comets led, 
in 1885, only to a modification of the numerical results previously 
arrived at. 

In the first of these, the repellent energy of the sun is fourteen 
times stronger than his attractive energy f 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 kilo- 
metres 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. Less narrow limits were assigned to the values of the 
repulsive force employed to produce the second type. For the 
axis of the tail, it exceeds by one-tenth ( = 1*1) the power of solar 

1 Natur der C&meten, p. 124 ; Astr. Nach., No. 2,086. 

2 Annales de VObs. de Moscou, t. iii., pt. i., p. 37. 

3 Bull. Astr., 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. 



chap, xi RECENT COMETS 347 

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 1,500 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 con- 
structed 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 
\>3 only found in combination with tails of the higher classes. 
Multiple tails, indeed that is, tails of different types emitted simul- 
taneously by one comet are perceived, as experience advances and 
observation becomes closer, to be rather the rule than the exception. 1 
Now what is the meaning of these three types 1 Is any transla- 
tion of them into physical fact possible 1 To this question Bredikhine 
supplied, in 1879, a plausible answer. 2 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 emana- 
tions from the comet of 1807 ; Norton had applied it to the faint 
light-tracks proceeding from that of Donati; 3 Winnecke to the 
varying deviations of its more brilliant plumage. Bredikhine 
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 
proportion as the repulsive forces employed in producing the 
appendages they are supposed to form ; and Zollner had pointed out 
in 1875 that the " heliofugal " power by which comets' tails are 
developed would, in fact, be effective just in that ratio. 4 Hydrogen, 
as the lightest known element that is, the least under the influence 
of gravity was naturally selected as that which yielded most 
readily to the counter-persuasions of electricity. Hydro-carbons 
had been shown by the spectroscope to be present in comets, and 
were fitted by their specific weight, as compared with that of 
hydrogen, to form tails of the second type ; while the atoms of iron 
were just heavy enough to compose those of the third, and, from 
the plentifulness of their presence in meteorites, might be presumed 
to enter, in no inconsiderable proportion, into the mass of comets. 

1 Faye, Comptes Rendus, t. xciii., p. 13. 2 Annales, t. v., pt. ii., p. 137. 

3 Am. Jour, of Sc, rol. xxxii. (2nd ser.), p. 57. 4 Astr. JSfach., No. 2,082. 



348 HISTORY OF ASTRONOMY part ii 

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; 1 
while the expanded plume of Donati was shown to be, in reality, a 
whole system of tails, made up of many substances, each spreading 
into a separate hollow cone, more or less deviating from, and partially 
superposed upon the others. 

Yet these felicities of explanation must not make us forget that 
the chemical composition attributed to the first type of cometary 
trains has, so far, received no countenance from the spectro- 
scope. The emission lines of free, incandescent hydrogen have 
never been derived from any part of these bodies. Dissentient 
opinions, accordingly, were expressed as to the cause of their 
structural peculiarities. Ranyard, 2 Zenker, and others advocated 
the agency of heat repulsion in producing them ; Kiaer somewhat 
obscurely explains them through the evolution of gases by colliding 
particles; 3 Herz of Vienna concludes tails to be mere illusory 
appendages produced by electrical discharges through the rare 
medium assumed to fill space. 4 But Him 5 conclusively showed 
that no such medium could possibly exist without promptly bring- 
ing ruin upon our " daedal earth " and its revolving companions. 

On the whole, modern researches tend to render superfluous the 
chemical diversities postulated by Bredikhine. Electricity alone 
seems competent to produce the varieties of cometary emanation 
they were designed to account for. The distinction of types rests 
on a solid basis of fact, but probably depends upon differences 
rather in the mode of action than in the kind of substance acted 
upon. Suggestive sketches of electrical and " light-pressure " theories 
of comets have been published respectively by Mr. Fessenden of 
Alleghany, 6 and by M. Arrhenius at Stockholm. 7 Although evi- 
dently of a tentative character, they possess great interest. 

Bredikhine's hypothesis was promptly and profusely illustrated. 
Within three years of its promulgation, five bright comets made 
their appearance, each presenting some distinctive peculiarity by 
which knowledge of these curious objects was materially helped for- 
ward. The first of these is remembered as the "Great Southern 

1 Annales de VObs. de Moscou, t. vi., pt. i., p. 60.; 2 Astr. Register, March, 1833. 

3 Astr. Nach., No. 3,018. 4 Ibid., No. 3,093. 

Constitution de VEspace Celeste, p. 224. 6 Astroph. Jour., vol. iii., p. 36. 

7 Phynkalische Zeitschrift, November 10 and 17, 1900 ; Astroph. Jour., 
vol. xiii., p. 344. Cf. Schwarzschild, Sitzungsb., Munclien, 1901, Heft iii.; 
J. Hahn, Nature, vols, lxv., p. 415 ; lxvi., p. 55. 



chap, xi RECENT COMETS 349 

Comet." It was never visible in these latitudes, but made a short 
though stately progress through southern skies. Its earliest detec- 
tion was at Cordoba on the last evening of January, 1880; and it 
was seen on February 1, 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, then 
director of the National Observatory of the Argentine Eepublic at 
Cordoba, caught a glimpse of it very low in the west ; and on the 
following evening, Mr. Eddie, at Graham's Town, discovered a faint 
nucleus, of a straw-coloured tinge, about the size of the annular 
nebula in Lyra. Its condensation, however, was very imperfect, 
and the whole apparition showed an exceedingly filmy texture. The 
tail was enormously long. On February 5 it extended large per- 
spective retrenchment notwithstanding over an arc of 50 ; but its 
brightness nowhere exceeded that of the Milky Way in Taurus. 
There was little curvature perceptible ; the edges of the appendage 
ran parallel, forming a nebulous causeway from star to star ; and the 
comparison to an auroral beam was appropriately used. The aspect 
of the famous comet of 1843 was forcibly recalled to the memory of 
Mr. Janisch, Governor of St. Helena j and the resemblance proved 
not merely superficial. But the comet of 1880 was less brilliant, 
and even more evanescent. After only eight days of visibility, it 
had faded so much as no longer to strike, though still discoverable 
by the unaided eye ; and on February 20 it was invisible with the 
great Cordoba equatoreal pointed to its known place. 

But the most astonishing circumstance connected with this body 
is the identity of its path with that of its predecessor in 1843. This 
is undeniable. Dr. Gould, 1 Mr. Hind, and Dr. Copeland, 2 each com- 
puted a separate set of elements from the first rough observations, 
and each was struck with an agreement 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 extraordinary 
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 

1 Astr. Nock., No. 2,307. * Ibid., No. 2,304. 

3 Observatory, vol. iii., p. 390. 



35o HISTORY OF ASTRONOMY part ii 

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 
dubiously supposed to have affected Encke's comet, the result in 
shortening the period ought to be of the most marked kind. It was 
proved by Oppolzer 1 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 about ten. But such restricted observations 
as were available on either occasion of its visibility gave no sign of 
such a rapid progress towards engulfment. 

Another form of the theory was advocated by Klinkerfues. 2 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 2,039 years 
being successively reduced by the withdrawal at each perihelion 
passage of ywyu of the velocity acquired by falling from the far 
extremity of its orbit towards the sun, to 175 and 37 years. A 
continuance of the process would bring the comet of 1880 back in 
1897. 

Unfortunately, the earliest of these apparitions cannot be identified 
with the recent ones unless by doing violence to the plain meaning 
of Aristotle's words in describing it. He states that the comet was 
first seen " during the frosts and in the clear skies of winter," setting 
due west nearly at the same time as the sun. 3 This implies some 
considerable north latitude. But the objects lately observed had 
practically no north latitude. 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." 

1 Astr. Nach., No. 2,319. 

8 Uebcr die Kometen von 371 v. Chr., 1668, 1843, I. und 1880 I. Gottingen, 1880. 

3 Meteor.^ lib. i., cap. 6. 



chap, xi RECENT COMETS 351 

With the comet of 1668 the case seemed 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 returned in less than thirty- 
seven, and whether the comet of 1880 was to be looked for at the 
end of 17J years. But the truth is that both these objects were 
observed over so small an arc 8 and 3 respectively that their 
periods remained virtually undetermined. For while the shape and 
position of their orbits could be and were fixed with a very close 
approach to accuracy, the length of those orbits might vary enor- 
mously without any very sensible difference being produced in the 
small part of the curves traced out near the sun. Dr. Wilhelm 
Meyer, however, arrived, by an elaborate discussion, at a period of 
thirty-seven years for the comet of 1880, 1 while the observations of 
1843 were admittedly best fitted by Hubbard's ellipse of 533 years ; 
but these Dr. Meyer supposed 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 inferred finally that, 
in spite of previous non-appearances, the two comets represented a 
single 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 circulation 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 maintained when tried by exact methods of inquiry. M. 
Bredikhine found that the gigantic ray emitted in 1843 belonged to 
his type No. 1 ; 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 1, and at Madras, March 11, 1843; 
and the conjecture was accordingly hazarded that the materials 
composing on that occasion the principal appendage having become 
exhausted, those of the secondary one remained predominant, and 

1 Mim. Soc. Phys. de Geneve, t. xxviii., p. 23. 

8 Annates de VObs. de Moscou, t. vii., pt. i., p. 60. 



352 HISTORY OF ASTRONOMY part ii 

reappeared alone in the " hydro-carbon " train of 1880. But the one 
known instance in point is against such a supposition. Halley's 
comet, the only great comet of which the returns have been securely 
authenticated and carefully observed, has preserved its "type" 
unchanged through many successive revolutions. The dilemma 
presented to astronomers by the Great Southern Comet of 1880 was 
unexpectedly renewed in the following year. 

On the 22nd 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 1, 
announcing the unexpected return of that body. So unexpected 
that theoretically it was not possible before the year 3346 ; and 
Bessel's investigation was one which inspired and eminently deserved 
confidence. Here, then, once more the perplexing choice had to be 
made between a premature and unaccountable reappearance 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 constellation Auriga on its ddbut 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 j while a curious dorsal spine 
of strong illumination formed the axis of the tail, which extended 
in clear skies over an arc of 20. It belonged to the same "type " 
as Donati's great plume; the particles composing it being driven 
from the sun by a force twice as powerful as that urging them 
towards it. 1 But the appendage was, for a few nights, and by two 
observers, perceived to be double. Tempel, on June 27, and Lewis 
Boss, at Albany (N.Y.), June 26 and 28, saw a long straight ray 
corresponding to a far higher rate of emission than the curved train, 
and shown by Bredikhine to be a member of the (so-called) hydrogen 
class. It had vanished by July 1, but made a temporary reappear- 
ance July 22. 2 

The appendages of this comet were of remarkable transparency. 

1 Bredikhine, Annates, t. viii., p. 68. 

2 Am. Jour, of Sc, vol. xxii., p. 305. 



chap, xi RECENT COMETS 353 

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. 1 Dr. Meyer, 
indeed, at the Geneva Observatory, detected apparent signs of 
refractive action upon rays thus transmitted ; 2 but his observations 
remain isolated, and were presumably illusory. 

The track pursued by this comet gave peculiar advantages for its 
observation. Ascending from Auriga through Camelopardus, it 
stood, July 19, on a line between the Pointers and the Pole, within 
8 of the latter, and thus remained for a lengthened 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 two and a half millenniums. 3 Nevertheless, the twin 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 con- 
jecture 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. 

Imperfect photographs were taken of Donati's comet both in 
England and America ; 4 but Tebbutt's comet was the first to which 
the process was satisfactorily applied. The difficulties to be overcome 
were very great. The chemical intensity of cometary light is, to 
begin with, extraordinarily small. Janssen estimated it at 13 ooVoo - 
of moonlight. 5 Hence, if the ordinary process by which lunar photo- 
graphs are taken had been applied to the comet of 1881, an ex- 
posure 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 

1 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. 

2 Archives des Sciences, t. viii., p. 535. Of. Perrine's negative results for 
Swift's comet in 1899, Astr. Nach., No. 3,602. 

3 Riem concluded in 1896 for a definitive period of 2,429 years ; Observatory, 
vol. xix., p. 282. 4 Holden, Publ. Astr. Pac. Soc, vol. ix., p. 89. 

6 Annuaire, Paris, 1882, p. 781. 

23 



354 HISTORY OF ASTRONOMY part ii 

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 emploj^ment of a reflector 
specially adapted to give an image illuminated eight or ten times as 
strongly as that produced in the focus of an ordinary telescope. 1 

The photographic feebleness of cometary rays was not the only 
obstacle in the way of success. The proper motion of these bodies 
is so rapid as to render the usual devices for keeping a heavenly 
body steadily in view quite inapplicable. The machinery by which 
the diurnal movement of the sphere is followed, must be especially 
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 chemically recorded. Both 
Huggins and Draper were successful in this respect, but Huggins 
was more completely 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 unmis- 
takably 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 
Sir William 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 con- 
tinuous, that the characteristic cometary bands could scarcely be 
detached from their bright background. But as the nucleus faded 
towards the end of June, they came out strongly, and were more 
and more clearly seen, both at Greenwich and at Princeton, to agree, 
not with the spectrum of hydro-carbons glowing in a vacuum tube, 
but with that of the same substances burning in a Bunsen flame. 4 

1 Annuaire, 1882, p. 766. 2 Am. Jour, of Sc, vol. xxii., p. 134. 

3 Report Brit. Assoc, 1881, p. 520. 

4 Month. Not, vol. xlii., p. 14 ; Am. Jour, of Sc., vol. xxii., p. 136. 



chap, xi RECENT COMETS 355 

It need not, however, be inferred that cometary materials are really 
in a state of combustion. This, from all that we know, may be 
called an impossibility. The additional clue furnished was rather 
to the manner of their electrical illumination. 1 

The spectrum of the tail was, in this comet, found to be not essen- 
tially different from that of the head. Professor Wright of Yale 
College ascertained a large percentage of its light to be polarized in 
a plane passing through the sun, and hence to be reflected sunlight. 2 
A faint continuous spectrum corresponded to this portion of its 
radiance ; but gaseous emissions were also present. At Potsdam, on 
June 30, the hydro-carbon bands were indeed traced by Vogel to the 
very end of the tail f 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 spectra is subject to fluctuations. 

The comet of 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. Schaeberle, which, as his claim to priority is un- 
disputed, is often allowed to bear his name, although designated, 
in strict scientific parlance, comet 1881 iv. It was observed in 
Europe after three days, became just discernible 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 newcomer, 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 proceeding from it to 
a distance of 10. This was easily shown by Bredikhine to belong to 
the hydrogen type of tails f while a " strange, faint second tail, or 
bifurcation of the first one," observed by Captain Noble, August 24, 5 
fell into the hydro-carbon class of emanations. It was seen, August 
22 and 24, by Dr. F. Terby of Lou vain, 6 as a short nebulous brush, 

1 Piazzi Smyth, Nature, vol. xxiv., p. 430. 

2 Astr. Nach., No. 2,395. 3 Ibid. 

4 Astr. Nach., No. 2,411. 5 Month. Not., vol. xlii., p. 49. 

6 Astr. Nach., No. 2,414. 

232 



356 HISTORY OF ASTRONOMY part n 

like the abortive beginning of a congeries of curving trains ; but 
appeared no more. Its well-attested presence was significant of the 
complex constitution of such bodies, and the manifold kinds of action 
progressing in them. 

The only peculiarity in the spectrum of Schaeberle'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 sixteen 
months, was discovered by Mr. C. S. Wells at the Dudley Observa- 
tory, Albany, March 17, 1882. Two days later it was described by 
Mr. Lewis Boss as "a great comet in miniature," so well defined 
and regularly developed were its various parts and appendages. Dis- 
cernible with optical aid early in May, it was on June 5 observed 
on the meridian at Albany just before noon an astronomical event 
of extreme rarity. Comet Wells, however, never became an object 
so conspicuous 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 (with 
the two transiently observed exceptions already mentioned) to 
conform to one invariable type of luminous emission. Individual 
distinctions there had been, but no specific differences. Now all 
these bodies had kept at a respectful distance from the sun ; for of 
the great comet of 1880 no spectroscopic 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 the vicinity had 
the effect of developing a novel feature in its incandescence. 

During the first half of April its spectrum was of the normal 
type, though the carbon bands were unusually weak ; but with 
approach 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 



chap, xi RECENT COMETS 357 

the yellow ray of sodium. 1 By June 1, this had kindled into a 
blaze overpowering all other emissions. The light of the comet was 
practically monochromatic j 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. 

At Potsdam, the bright yellow line was perceived with astonish- 
ment by Vogel on May 31, and was next evening identified with 
Fraunhofer's " D." Its character led him to infer a very consider- 
able density in the glowing vapour emitting it. 2 Hasselberg 
founded an additional argument in favour of the electrical origin of 
cometary light on the changes in the spectrum of comet Wells. 3 
For they were closely paralleled by some earlier experiments of 
Wiedemann, in which the gaseous spectra of vacuum tubes were at 
once effaced on the introduction of metallic vapours. It seemed as 
if the metal had no sooner been rendered volatile by heat, than 
usurped the entire office of carrying the discharge, the resulting 
light being thus exclusively of its production. Had simple in- 
candescence by heat been in question, the effect would have been 
different; the two spectra would have been superposed without 
prejudice to either. Similarly, the replacement 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. 

Sir William Huggins succeeded in photographing the spectrum of 
comet Wells by an exposure of one hour and a quarter. 4 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 retreat- 
ing towards, vastly remote depths of space. An American computer 5 
found a period indicated for it of no less than 400,000 years; 
A. Thraen of Dingelstadt arrived at one of 361 7. 6 Both are perhaps 
equally insecure. 

1 Copernicus, vol. ii., p. 229. 2 Astr. Nach., Nos. 2,434, 2,437. 

8 Ibid., No. 2,441. 4 Report Brit. Assoc, 1882, p. 442. 

6 J. J. Parsons, Am. Jour, of Science, vol. xxvii., p. 34. 
6 Astr. Nach., No. 2,441. 



358 HISTORY OF ASTRONOMY part ii 

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 Eio Janeiro, September 11, 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 of the 
Cape Observatory, and at Auckland as early as September 3. A 
later, but very singularly conditioned detection, quite unconnected 
with any of the preceding, was effected by Dr. 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 
photographs, entangled, one might almost say, in the outer rays of 
the corona, he had scrutinized 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 mid-day was long past. Mr. Finlay at the Cape was more 
completely fortunate. Divided from his fellow-observer by half the 
world, he unconsciously finished, under a clearer sky, his interrupted 
observation. The comet, of which the silvery radiance contrasted 
strikingly with the reddish-yellow glare of the sun's margin it drew 
near to, was followed "continuously right into the boiling 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. 50m. 58s., Cape mean time), that the 
comet was at first believed to 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 

1 Observatory, vol. v., p. s 355. The transit had been foreseen by Mr. Tebbutt, 
but it occurred after sunset in New South Wales. 



PLATE III. 




The Great Comet of September, 1882. 
Photographed at the Royal Observatory. Cape of Good Hope. 



chap, xi RECENT COMETS 359 

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 admissible 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 noticeable part of the solar 
rays a piece of information worth remembering. 

On the following morning, the object of this unique observation 
showed (in Sir David Grill's words) " an astonishing brilliancy as it 
rose behind the mountains on the east of Table Bay, and seemed in 
no way diminished in brightness when the sun rose a few minutes 
afterward. It was only necessary to shade the eye from direct 
sunlight with the hand at arm's length, to see the comet, with its 
brilliant white nucleus and dense white, sharply bordered tail of 
quite half a degree in length. 1 All over the world, wherever the 
sky was clear during that day, September 18, it was obvious to 
ordinary vision. Since 1843 nothing had been seen like it. From 
Spain, Italy, Algeria, Southern France, despatches came in an- 
nouncing the extraordinary appearance. At Cordoba, in South 
America, the " blazing star near the sun " was the one topic of dis- 
course. 2 Moreover and this is altogether extraordinary the 
records of its daylight visibility to the naked eye extend over three 
days. At Eeus, near Tarragona, it showed bright enough to be seen 
through a passing cloud when only three of the sun's diameters from 
his limb, just before its final rush past perihelion on September 17 ; 
while at Carthagena in Spain, on September 19, it was kept in view 
during two hours before and two hours after noon, and was similarly 
visible in Algeria on the same day. 3 

But still more surprising than the appearance of the body itself 
were the nature and relations of the path it moved in. The first 
rough elements computed for it by Mr. Tebbutt, Dr. Chandler, 
and 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 j but it bore a strong and unmistakable stamp, 
broadly challenging explanation. 

Two hypotheses only were really available. Either the comet of 

1 Observatory, vol. v., p. 354. a Gould, Astr. Kach., No. 2,481. 

3 Flammarion, Comptes Bendus, t. xcv., p. 558. 



360 HISTOR Y OF ASTRONOMY part ii 

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 of its efficacy was for some time 
both prevalent and popular, and formed the basis, moreover, of some- 
thing 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 licence of imagination than countenance from 
science, as likely to be of catastrophic import to the inhabitants of 
our little planet. 

But there was a test available in 1882 which it had not been 
possible to apply either in 1843 or in 1880. The two bodies visible 
in those years had been observed only after they had already passed 
perihelion ; x 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. Finlay's and Elkin's observation of its disap- 
pearance 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 approximate 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 harmoniously 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 grdce to Encke's theory 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, Avas neverthe- 
less 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 2,000 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 

1 Captain Ray's sextant observation of the comet of 1843, a few hours before 
perihelion, was too rough to be of use. 



chap, xi RECENT COMETS 361 

was discerned with the naked eye at Cordoba as late as March 7, 
1883, and still showed in the field of the great equatoreal on June 1 
as an " excessively faint whiteness." 1 It was then about 480 millions 
of miles from the earth a distance to which no other comet not 
even excepting the peculiar one of 1729 had been pursued. 2 More- 
over, 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. That its movement is in a very 
eccentric ellipse, traversed in several hundred years, was ascertained. 3 
The later inquiries of Dr. Kreutz, 4 completed in a volume published 
in 1901, 5 demonstrated the period to be of about 800 years, while that 
of its predecessor in 1843 might possibly agree with it, but is much 
more probably estimated at 512 years. The hypothesis that they, 
or any of the comets associated with them, were returns of an indi- 
vidual body is peremptorily excluded. They may all, however, have 
been separated from one original mass by the divellent action of the 
sun at close quarters. Each has doubtless its own period, since each 
has most likely suffered retardations or accelerations special to itself, 
which, though trifling in amount, would avail materially to alter the 
length of the major axis, while leaving the remaining elements of 
the common orbit virtually unchanged. 6 

A fifth member was added to the family in 1887. On the 18th 
of January in that year, 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, contracting 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 distinct 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. The observations secured were indeed, from the nature 
of the apparition, neither numerous nor over-reliable ; and the earliest 
of them dated from a week after perihelion, passed, almost by a 
touch-and-go escape, January 11. On January 27, this mysterious 
object could barely be discerned telescopically at Cordoba. 7 That 
it belonged to the series of "southern comets" can scarcely be 

1 Astr. Nach., No. 2,538. 2 Nature, vol. xxix., p. 135. 

3 Astr. Nach., No. 2,482. 

4 Vierteljahrsschrift Astr. Ges., Jahrg. xxiv., p. 308; Bull. Astr., t. vii., 
p. 513. 

5 Observatory, vol. xxiv., p. 167. 

6 The attention of the author was kindly directed to this point by Professor 
Young of Princeton (N.J.). Cf. Rebeur-Paschwitz, Sirius, Bd. xvi., p. 233. 

7 Oppenheim, Astr. Nach., No. 2,902. 



362 HISTOR Y OF ASTRONOMY part ii 

doubted ; but the inference that it was an actual return of the comet 
of 1880, improbable in itself, was negatived by its non-appearance in 
1894. Meyer's incorporation with this extraordinary group of the 
"eclipse-comet" of 1882 1 has been approved by Kreutz, after search- 
ing examination. 

The idea of cometary systems was first suggested by Thomas 
Clausen in 1831. 2 It was developed by the late M. Hoek, director 
of the Utrecht Observatory, in 1865 and some following years. 3 He 
found that in quite a considerable 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 con- 
sisted, 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 similar conditions of motion betray a pre-existent physical 
connection. Never before, however, had geometrical relationship 
been so notorious as between the comets now under consideration ; 
and never before, in a comet still, it might be said, in the prime of 
life, had physical peculiarities tending to account for that affinity 
been so obvious as in the chief member 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, besides 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 Professor Holden, October 13 and 
17. 4 A few days later, M. Tempel found the head to consist of four 
lucid aggregations, ranged nearly along the prolongation of the 
caudal axis; 5 and Dr. Common, January 27, 1883, saw five nuclei 
in a line " like pearls on a string." 6 This remarkable character was 
preserved to the last moment of the comet's distinct visibility. It 

1 Astr. Nach., No. 2,717. 2 Gruithuisen's Analekten, Heft 7, p. 48. 

3 Month. Not., vols, xxv., xxvi., xxviii. Gf. Plummer, Observatory, vol. xiii., 
Ii. 263. 

4 Nature, vol. xxvii., p. 246. 5 Astr. Nach., No. 2,468. 
6 Athenceum, February 3, 1883. 



chap, xi RECENT COMETS 363 

was a consequence, according to Dr. Kreutz, of violent interior 
action in the comet itself while close to the sun. 

There were, however, other curious proofs of a disaggregative 
tendency in this body. 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. 1 This is rendered more probable by the unexampled 
spectacle offered, October 14, to Prof essor Barnard, then 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. 2 A week later, however, one similar object 
was discerned by Mr. W. R. Brooks, in the opposite direction from 
the comet. Thus space appeared to be strewn with the filmy debris 
of this beautiful but fragile structure 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 200 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 Observatory, Eddie at Grahamstown, and Cruls 
at Rio Janeiro. It diverged at a low angle from the denser curved 
train, and was produced, according to Bredikhine, 3 by the action of 
a repulsive force twelve times as strong as the counter-pull of gravity. 
It belonged, that is, to type 1 ; while the great bifurcate 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 outpourings of attenuated matter. (See Plate III.) 
Its diameter was computed by Schmidt to be, October 15, no less 

1 Astr. Nach., Nos. 2,462, 2,466. 2 Ibid., No. 2,489. 

3 Annales, Moscow, t. ix., pt. ii., p. 52. 



364 HISTORY OF ASTRONOMY part ii 

than four million miles, and it was described by Cruls as a " truncated 
cone of nebulosity," stretching 3 or 4 sunwards. 1 This, and the 
entire anterior part of the comet, were again surrounded by a thin, 
but enormously voluminous paraboloidal envelope, observed by 
Schiaparelli for a full month from October 19. 2 There can be little 
doubt that these abnormal effluxes were a consequence of the 
tremendous physical disturbance suffered at perihelion; and it is 
worth remembering that something analogous was observed in the 
comet of 1680 (Newton's), also noted for its excessively close 
approach to the sun, and possibly moving in a related orbit. The 
only plausible hypothesis as to the mode of their 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; 3 a very significant addition to our 
knowledge of cometary constitution, and one which lent counte- 
nance to Bredikhine'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 a 
recession from the earth of the radiating body at the rate of 37 to 
46 miles a second. A similar observation, made by M. Thollon at 
Nice on the same day, gave emphatic sanction to 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 increasing its distance from our 
planet by from 61 to 76 kilometres per second. 4 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 Kicc6 was, 
in fact, able to infer, from the sequence of prismatic phenomena, 

1 Comptes Rendus, t. xcvii., p. 797. 2 Astr. Nach., No. 2,966. 

3 Copernicus, vol. ii., p. 235. 4 Comptes Rendus, t. xcvi., p. 371. 



chap, xi RECENT COMETS 365 

that the comet had already passed the sun ; thus establishing a 
novel criterion for determining the position of a comet in its orbit 
by the varying quality of its radiations. 

Recapitulating what was learnt from the five conspicuous comets 
of 1880-82, we find that the leading facts acquired to science 
were these three. First, that comets may be met with pursuing 
each other, after intervals of many years, in the same, or nearly the 
same, track ; so that identity of orbit can no longer be regarded as 
a sure test of individual identity. Secondly, that at least the outer 
corona may be traversed by such bodies with perfect apparent 
impunity. Finally, that their chemical constitution is highly com- 
plex, and that they possess, in some cases at least, a metallic core 
resembling the meteoric masses which occasionally reach the earth 
from planetary space. 

A group of five 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 71 years. It 
was not, however, until September 1, 1883, that Mr. Brooks 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, Dr. Chandler 1 observed it at 
Harvard as a very faint, diffused nebulosity, with slight central 
condensation. 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 2 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 on January 1, when they were witnessed with amazement, 
1 Astr. Naeh., No. 2,553. a Ibid. 



366 HISTORY OF ASTRONOMY part ii 

and photometrically studied by Miiller of Potsdam. 1 The entire 
cycle this time was run through in less than four hours the 
comet having, in that brief space, condensed, with a vivid outburst 
of light, into a seeming star, and the seeming star having expanded 
back again into a comet. Scarcely less transient, though not 
altogether similar, changes of aspect were noted by M. Perrotin, 2 
January 13 and 19, 1884. On the latter date, the continuous 
spectrum given by a reddish-yellow disc surrounding the true 
nucleus seemed intensified by bright knots corresponding to the 
rays of sodium. 

A comet discovered by Mr. Sawerthal at the Royal Observatory, 
Cape of Good Hope, February 19, 1888, distinguished itself by 
blazing up, on May 19, to four or five times its normal brilliancy, 
at the same time throwing out from the head two lustrous lateral 
branches. 3 These had, on June 1, spread backward so as to join 
the tail, with an effect like the playing of a fountain ; ten or eleven 
days later, they had completely disappeared, leaving the comet in 
its former shape and insignificance. Its abrupt display of vitality 
occurred two full months after perihelion. 

On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, 
New York, eminent as a successful comet-hunter, secured one 
of his customary trophies. The faint object in question was 
moving through the constellation Cetus, and turned out to be 
a member of Jupiter's numerous family of comets, revolving 
round the sun in a period of seven years. Its past history 
came then, to a certain extent, within the scope of investiga- 
tion, and proved to have been singularly eventful; nor had the 
body escaped scatheless from the vicissitudes to which it had 
been exposed. Observing from Mount Hamilton, August 2 and 5, 
Professor Barnard noticed this comet (1889, v.) to be attended in its 
progress through space by four outriders^ "The two brighter com- 
panions " (the fainter pair survived a very short time) " were perfect 
miniatures," Professor Barnard tells us, 4 "of the larger comet, each 
having a small, fairly defined head and nucleus, with a faint, hazy 
tail, the more distant one being the larger and less developed. The 
three comets were in a straight line, nearly east and west, their 
tails lying along this line. There was no connecting nebulosity 
between these objects, the tails of the two smaller not reaching 
each other, or the large comet. To all appearance they were 
absolutely independent comets." Nevertheless, Spitaler, at Vienna, 

1 Astr. Nach., No. 2,568. 

2 Annates de V ' Observatoire de Nice, t. ii., c. 53. 

3 Fenyi, Astr. Nach., No. 2,844 ; Kammermann, Ibid., No. 2,849. 

4 Publ. Astr. Pac. Soc, vol. i., p. 72. 



chap, xi RECENT COMETS 367 

in the early days of August, perceived, as it were, a thin cocoon of 
nebulosity woven round the entire trio. 1 One of them faded from 
view September 5 ; the other actually outshone the original comet 
on August 31, but was plainly of inferior vitality. It was last seen 
by Barnard on November 25, with the thirty-six inch refractor, 
while its primary afforded an observation for position with the 
twelve-inch, March 20, 1890. 2 A cause for the disruption it had 
presumably undergone had, before then, been plausibly assigned. 

The adventures of Lexell's comet have long served to exemplify 
the effects of Jupiter's despotic sway over such bodies. Although 
bright enough in 1770 to be seen with the naked eye, and ascer- 
tained to be circulating in five and a half years, it had never 
previously been seen, and failed subsequently to present itself. 
The explanation of this anomaly, suggested by Lexell, and fully 
confirmed by the analytical inquiries both of Laplace and Leverrier, 3 
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. Yet the possibility was not 
lost sight of that the great planet, by inverting its mode of action, 
might undo its own work, and fling the comet once more into the 
inner part of the solar system. This possibility seemed to be 
realized by Chandler's identification of Brooks's and Lexell's comet. 4 
An exceedingly close approach to Jupiter in 1886 had, he found 
reason to believe, produced such extensive alterations in the 
elements of its motion as to bring the errant body back to our 
neighbourhood in 1889. But his inference, though ratified by 
Mr. Charles Lane Poor's preliminary calculations, proved dubious 
on closer inquiry, and was rendered wholly inadmissible by the 
circumstances attending the return of Brooks's comet in 1896. 5 
The companion-objects watched by Barnard in 1889 had by that 
time, perhaps