CHAPTER I.

PROBLEMS IN ASTROPHYSICS

[Illustration:

PLATE I.

Photograph of the η Argûs Nebula. Taken by Sir David Gill in March

1892.

(_Knowledge_, vol. xvi. p. 70).

[_Frontispiece._

]

PROBLEMS

IN

ASTROPHYSICS

BY

AGNES M. CLERKE

AUTHOR OF

‘A HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY,’ ‘THE SYSTEM OF

THE STARS,’ AND OTHER WORKS

“_In solidis et veris aspiramus ad ultima et summa._”

Novum Organum, ii. 51.

CONTAINING 81 ILLUSTRATIONS

LONDON

ADAM & CHARLES BLACK

1903

THIS WORK IS DEDICATED

BY PERMISSION

TO

SIR DAVID GILL, K.C.B.

WHOSE SUGGESTION AND ENCOURAGEMENT PROMPTED ITS COMPOSITION AND

ANIMATED ITS PROGRESS

PREFACE

The object of the present work is not so much to instruct as to suggest.

It represents a sort of reconnaissance, and embodies the information

collected by scouts and skirmishers regarding practicable lines of

advance and accessible points of attack, with a view to annexing for the

realm of knowledge some further strips and corners from the territory of

ignorance. Its inspiring motive, in short, is the desire for a

rectification of the frontier in the interests of science. Material

resources for the furtherance of such encroachments are not lacking. The

globe is studded with observatories, variously and admirably equipped.

Yet innumerable objects in the sidereal heavens remain neglected, mainly

through inadvertence to the extraordinary interest of the questions

pending with respect to them. In the following pages it has been sought

to indicate some of these individually, and in their relations to the

larger meanings of cosmical research. But this could be done effectually

only from the vantage-ground of our actual acquirements; hence the book,

although primarily designed to stimulate the progress of astrophysics,

necessarily includes an account of its present state. Before attempting

to add to our store of learning, we must realise what is already

possessed.

The unknown, it is true, is indefinitely vast, and the rays of light

which we can project into its darkness penetrate but a short way. Our

programme of inquiry must accordingly be limited to what is now

practicable, or promises to become so in the immediate future. The

unforeseen, too, will have something, perhaps much, to do with

prescribing directions for fresh researches. Queries, in the coming

years, will be put to the skies very different from those here

propounded; and answers of a surprising kind will doubtless be afforded

to our present interrogatory. The keen delight of such revelations will

reward those who, loving truth for its own sake, have laboured for its

promotion; and if these pages should, in any degree, help to quicken and

guide their noble enthusiasm, they will have amply served their purpose.

They do not, however, cover the whole field of astrophysics. Planetary

and cometary astronomy are deliberately, although for different reasons,

excluded from treatment in them. The moon and planets still belong to

the theoretical and descriptive departments of the elder celestial

science. Nearly all that is known about their condition has been learned

by direct telescopic observation. The items of information added through

the aid of the camera and the spectroscope, though valuable, are very

few. The orbital characteristics, on the other hand, of comets and

meteors are too prominent to be set aside in any profitable discussion

of their nature. They are of the very essence of the phenomena; yet they

would be out of place in a book strictly limited to the consideration of

the heavenly bodies under their physical aspect.

The writer has received much courteous help from various quarters in

preparing the illustrations, and desires in particular to acknowledge

her obligations to Sir William and Lady Huggins, to Sir David Gill, to

Dr. Roberts, F.R.S., to the Rev. W. Sidgreaves, S.J., Mr. W. E. Wilson,

F.R.S., M. Deslandres, Professor Hale, Professor Barnard, to Professors

E. C. and W. H. Pickering, and to Professor Campbell.

LONDON, _December 1902_.

CONTENTS

INTRODUCTION

PAGE

RISE AND SCOPE OF ASTROPHYSICS 1

PART I

PROBLEMS IN SOLAR PHYSICS

CHAP.

1. PROGRESS OF SOLAR PHYSICS 13

2. THE CHEMISTRY OF THE SUN 21

3. PECULIARITIES OF THE SOLAR SPECTRUM 36

4. THE REVERSING LAYER 44

5. HYDROGEN, HELIUM, AND CORONIUM 52

6. THE PHOTOSPHERE AND ITS DUSKY VEIL 62

7. STRUCTURE AND MOVEMENTS OF SUN-SPOTS 73

8. THE SPECTRUM OF SUN-SPOTS 88

9. FACULÆ AND PROMINENCES 98

10. THE CHROMOSPHERIC SPECTRUM 112

11. THE CORONA 123

12. THE SUN’S ROTATION 142

13. THE SOLAR CYCLE 150

14. THE SUN AS A WHOLE 161

PART II

PROBLEMS IN SIDEREAL PHYSICS

1. PROGRESS OF SIDEREAL PHYSICS 171

2. THE CLASSIFICATION OF STELLAR SPECTRA 179

3. HELIUM STARS 189

4. HYDROGEN STARS 197

5. SOLAR STARS 203

6. STARS WITH FLUTED SPECTRA 209

7. CARBON STARS 215

8. STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES 222

9. HELIUM STARS WITH BRIGHT LINES 229

10. WOLF-RAYET STARS 237

11. THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA 243

12. ANOMALOUS AND VARIABLE SPECTRA 246

13. COLOUR VARIABILITY 253

14. THE SPECTRA OF DOUBLE STARS 263

15. THE EVOLUTION OF THE STARS 271

16. ROTATION OF THE STARS 280

17. SPECTROSCOPIC BINARIES 286

18. ECLIPSING STARS 299

19. SHORT-PERIOD VARIABLES 319

20. SHORT-PERIOD VARIABLES—_continued_ 328

21. THE PROBLEM OF BETA LYRÆ 337

22. STARS VARIABLE IN LONG PERIODS 347

23. PECULIAR AND IRREGULAR VARIABLES 363

24. TEMPORARY STARS 375

25. DARK STARS 399

26. THE GENERAL QUESTION OF STELLAR VARIABILITY 404

27. IRREGULAR STAR CLUSTERS 409

28. NEBULOUS CLUSTERS—THE PLEIADES 415

29. NEBULOUS CLUSTERS—_continued_ 423

30. GLOBULAR CLUSTERS 428

31. WHITE NEBULÆ 439

32. DOUBLE NEBULÆ 452

33. NEBULOUS STARS 460

34. PLANETARY NEBULÆ 469

35. ANNULAR NEBULÆ 484

36. THE ORION NEBULA 495

37. OTHER IRREGULAR NEBULÆ 506

38. NONDESCRIPT NEBULÆ 516

39. VARIABLE NEBULÆ 522

40. THE NATURE OF NEBULÆ 531

41. THE PHYSICS OF THE MILKY WAY 538

APPENDIX 547

INDEX 553

LIST OF ILLUSTRATIONS

PRINTED SEPARATELY

PLATE

I. Photograph of the Argo Nebula _Frontispiece_

II. Photograph of A-Band in Solar Spectrum

(McClean) _to face page_ 22

III. Flash and Cusp Spectra compared „ 46

IV. The Sun portrayed in Calcium Light (1);

Photograph of the Chromosphere, 31st May

1894 (2) „ 100

V. Types of the Corona (Hansky) „ 128

VI. The Corona of 1900 (Jewell) (1); Reversal and

Distortion of the C-line (Hale) (2) „ 96

VII. The Corona of 1896 (Morin) (1); the Corona of

1898 (Wesley) (2) „ 134

VIII. The Corona of 1900 (Wesley) „ 136

IX. Photographs of Typical Star-Spectra (Huggins) „ 180

X. Photographed Spectrum of ε Canis Majoris

(Lunt) (1); Photographed Spectrum of α_{2}

Centauri (Gill) (2) „ 182

XI. Spectrum of α Herculis (Espin) (1);

Photographed Spectrum of 152 Schjellerup

(Hale) (2) „ 184

XII. Photographed Spectra of Carbon Stars (Hale

and Ellerman) „ 186

XIII. Photographed Spectra of Mira and of α

Herculis (Sidgreaves) „ 188

XIV. Spectrum of γ Cassiopeiæ. Photographed by the

Rev. W. Sidgreaves „ 190

XV. Photographed Spectra of Types II., III., and

IV. compared „ 220

XVI. Sections of the Spectra of ß Lyræ and Nova

Persei „ 344

XVII. Visible Spectrum of Nova Aurigæ (Campbell) „ 376

XVIII. Nebulosity round Nova Persei (Ritchey) (1);

Spectrum of Nova Persei, 11th August 1901,

with corresponding Intensity-Curve (2) „ 394

XIX. Photograph of Messier 11 (Roberts) „ 410

XX. Photograph of a Spiral Nebula in Cetus

(Roberts) „ 442

XXI. Photograph and Drawings of a Spiral Nebula in

Pegasus (Keeler) „ 444

XXII. Photograph of Whirlpool Nebula (W. E. Wilson) „ 446

XXIII. Fan Nebulæ (Barnard) (1); Planetary Nebulæ

with their Spectra (Keeler) (2, 3, 4) „ 448

XXIV. Photograph of Owl Nebula (Roberts) (1);

Photograph of the Orion Trapezium

(Pickering) (2) „ 472

XXV. Photograph of the Ring Nebula in Lyra (W. E.

Wilson) „ 484

XXVI. Photograph of the Orion Nebula (Pickering) „ 494

XXVII. Nebulous Formation in Orion. Photographed by

W. H. Pickering „ 496

XXVIII. Photograph of the Trifid Nebula (Roberts)

(1); Photograph of Messier 77 (Roberts) (2) „ 510

XXIX. Photograph of a Nebula in Perseus (Barnard) „ 514

XXX. Photograph of the Dumb-Bell Nebula (W. E.

Wilson) „ 516

XXXI. Photograph of the Milky Way in Ophiuchus

(Barnard) „ 540

PRINTED IN THE TEXT

FIG. PAGE

1. General View of the Atmospheric Spectrum 24

2. The Infra-Red Spectrum (Langley) 24

3. Oxygen-Triplet in the Solar Spectrum 29

4. Curves representing the Distribution of Light

in Fraunhofer and Arc Lines (Jewell) 41

5. Diagram of the Helium Spectrum 57

6. Sun-spot photographed by Janssen, 1st April

1894 73

7. Photograph of a Bridged Sun-spot (Janssen) 76

8. Sun-spot drawn by J. de M. Pereira, 18th June

1894 78

9. Group of Sun-spots drawn by Miss E. Brown 78

10. Portion of a Sun-spot Spectrum photographed

by Professor Young 89

11. Reversal of the D-Lines in the Spectrum of a

Sun-spot 94

12. Prominence observed at Kalocsa, 19th

September 1893 104

13. Prominence observed at Kalocsa, 3rd October

1892 107

14. Forms of a Prominence in Hydrogen, Helium,

and Pseudo-Coronium (Fényi) 117

15. Dark Markings in the Corona of 1871 (Wesley) 134

16. Dark Markings in the Corona of 1896 (Wesley) 134

17. Spoerer’s Curves of Sun-spot Latitude 152

18. Curves of Sun-spot Frequency and Magnetic

Agitation (Ellis) 155

19. Intensity-Curves of Ηγ in Spectrum of Mira

(Campbell) 225

20. Hydrogen-Envelope of a Wolf-Rayet Star

(Campbell) 241

21. Photometric Curves of Algol Variables

(Pickering) 309

22. Light-Curve of η Aquilæ (Schur) 322

23. Orbit of η Aquilæ (Wright) 323

24. Light-Curve of S Sagittæ (Yendell) 326

25. Light and Velocity Curves of ζ Geminorum

(Campbell) 329

26. Orbit of ζ Geminorum (Campbell) 330

27. Photometric Curve of U Pegasi (Pickering) 332

28. Light-Curves of Cluster-Variables (Bailey) 336

29. Light-Curve of β Lyræ (Argelander) 337

30. System of β Lyræ (Myers) 341

31. Distribution of the Periods of 208 Variables 347

32. Light-Curve of U Orionis (Porro) 351

33. Light-Curve of S Ursæ Majoris in 1894 354

34. Mean Light-Curve of S Ursæ Majoris 355

35. Mean Light-Curve of T Ursæ Majoris 355

36. Mean Light-Curve of T Cassiopeiæ 355

37. Light-Curve of T Andromedæ 357

38. Light-Curve of S Cephei 359

39. Light-Curve of R Scuti 364

40. Long and Short Maxima of U Geminorum 365

41. Light-Curve of S^2 Cygni 367

42. Light-Curve of Nova Aurigæ 380

43. Photographs before and after the Apparition

of Nova Persei 389

44. Light-Curve of Nova Persei, 22nd February to

6th March 391

45. Light-Curve of Nova Persei, 17th March to 3rd

May 1901 391

46. Exterior Nebulosities of the Pleiades 419

47. Photograph of ω Centauri (Bailey) 430

48. Drawing of a Pair of Spindle Nebulæ

(Spitaler) 457

49. Sketch of T Tauri and Hind’s Nebula (Barnard) 524

50. Drawing from Photographs of T Tauri and

Hind’s Nebula (Keeler) 525

INTRODUCTION.

RISE AND SCOPE OF ASTROPHYSICS.

The astronomy of the ancients was purely formal. It did not profess to

look beyond appearances. Its aim was reached, provided that phenomena

were—in the old Greek phrase—mathematically “saved,” at whatever cost of

material impossibility. Kepler first speculated on the causes of

celestial movements, and introduced the term “physical astronomy” with a

full sense of what it implied. Its establishment as an effective branch

of knowledge was a prime desideratum with Francis Bacon. While rejecting

the Copernican system and ignoring Kepler’s laws, he aspired towards a

science of the skies that should be no simulacrum, like the “stuffed ox

of Prometheus,” but should have in it the breath of life and the

instinct of progress.[1] Anticipating with imaginative insight the

prosecution of researches which Comte, two centuries later, declared to

be, in the nature of things, futile, he broadly laid down the lines of a

new astronomy, indistinguishable from modern astrophysics. As the

province of this “philosophy” of the heavenly bodies, he assigned

inquiries into the nature of their substance, of their qualities,

properties, and influences, as well as into the source of the motive

power acting upon them. Vitalised, as it were, by contact with

motherearth, it was destined in his prevision to a community of advance

with terrestrial science, one imparting to the other novel truths

tending to mutual profit and simultaneous development. Thus the

long-divorced sublunary and translunary worlds were conjoined, and their

material unity—the essential principle of astrophysics—was definitively

proclaimed. This daring forecast transcended the scope even of Newton’s

discovery, and left Kepler’s prescience far behind. For Kepler, as an

astronomer _ex professo_, took full account of apparent impossibilities,

while Bacon’s was “the golden guess”—

That’s morning star to the full round of truth.

Yet it might have been derided as that of an uninformed amateur. It rose

indeed out of sight of ordinary minds.

The establishment of the law of gravity was the first step towards its

realisation. Thereby the terrestrial regimen was, in part, extended to

the skies. The moon assumed the aspect of a projectile in perpetual

flight, tangentially launched _ab initio_, and hence keeping its

distance from the earth, while the planets, including our own globe,

proved to be similarly related to the sun. Thus celestial movements lost

the mystical character long ignorantly attributed to them, and were

found to own a common cause with movements at the surface of the earth.

They became predictable, since the cause acts uniformly and simply;

theoretical astronomy, with practical astronomy at hand to provide its

raw materials and test its results, took rank as the most perfect of the

sciences; the idea of a definable force put to the rout the old vague

notions of “tendencies,” “appetites,” “passions,” or “potencies,” and a

dynamical was substituted for a merely kinematical system.

Gravity, however, is a force of the utmost generality in the way it

affects matter. It takes no notice of distinctions of kind or quality.

The substances acted upon may be hot or cold, dense or rare, elementary

or compound; they may be of any imaginable chemical or mineralogical

constitution; they may be in any state of aggregation; they may be

organic or inorganic; no difference is perceptible; gravity is concerned

solely with mass, and is measured strictly by movement; and from

gravitational inquiries, accordingly, mass and movement can alone be

learned. So far, then, only one principle of unification was introduced.

One fundamental property of matter was known from 1687 to belong equally

to the earth and planets; and Herschel’s discovery in 1802 of mutually

revolving stars virtually made the “writ to run” throughout the sidereal

world as well. The universality of an apparent mass-attraction was a

great fact, but seemed destined to remain isolated; for Olbers’s

“electrical theory” of comets amounted to no more than a suggestive

speculation. Then in 1852 the triple identification by Sabine, Wolf, and

Gautier of the sun-spot and terrestrial magnetic periods showed the

reality of solar influences exercised in a manner not easy to apprehend,

but capable of being brought to the test of experimental investigation.

Cosmical physics began to separate out and take recognisable shape. The

spring of its most rapid growth, however, lay in another direction.

The discovery (in Professor Keeler’s words[2]) “that the light which

reveals to us the existence of the heavenly bodies also bears the secret

of their constitution and physical condition,” afforded a solid basis

for a science of far-reaching import. “The spectroscope placed new and

hitherto undreamtof powers in the hands of men. It is to the

astrophysicist what the graduated circle and the telescope are to the

astronomer.” Observations of the heavenly bodies by means of their

_analysed_ light came to the aid of observations through their

_integrated_ light. Their radiations, visible and invisible, were

brought within the range of detailed study.

Of study, not only visual, but photographic. The sensitive plate has

three leading prerogatives. It _sees_ where the eye is blind; its

impressions are cumulative to an indefinite extent; they are permanent;

they constitute documentary evidence of incontestable validity, which

can be produced or referred to at pleasure. Spectroscopic photography,

or “spectrography,” dates from Sir William Huggins’s adoption of the dry

gelatine process in 1876; and his discovery, three years later, of the

ultra-violet series of hydrogen-lines in stellar spectra started it on

its career amid acclamations. Nor has the promise been belied. The

efficiency of the camera is of so high an order that direct visual

observations of prismatic light are now only by exception made. This is

perhaps unfortunate, since the two kinds of results are, to some extent,

supplementary, and can often be most usefully compared and correlated.

The superiority of the chemical method, however, is nowhere more

conspicuous than in the motion department of the new astronomy. Its

powers in this direction were tested with striking success by Vogel in

1888, and their development, rapid as it has been, does not seem to be

near its term. The determination of radial velocities through changes in

the refrangibility of light emanating from the bodies actuated by them,

has made astrophysicists free of a territory which belongs equally to

the domain of traditionally equipped astronomers. Here we get back to

elementary facts of mass and motion, ascertained, however, not

_frontally_ by measures of position, but _strategically_ by inference

from radiative modifications. They are, indeed, of a nature unaffected

by position, and hence undiscernible with the micrometer. A body shown

spectroscopically to be in swift movement might be absolutely immobile

telescopically; or the conditions might be inverted, each method taking

cognisance of only one component of the total velocity. An immense

extension was accordingly given to the field of research in sidereal

dynamics by the application, through Sir William Huggins’s initiative,

of “Doppler’s principle.” It supplied not alone the means of completing

investigations which could otherwise be pursued only in a one-sided

manner, but of setting on foot entirely new ones of incalculable

significance. Thus the rate of the solar translation through space,

valued little better than conjecturally from the _proper_, or thwartwise

motions of the stars, can be derived securely and at once from their

_radial_ motions. Of the movements of nebulæ nothing is known—and very

little is likely to become known for some centuries yet to come—except

through spectroscopic measurements; for they are so remote that their

positions change with extreme slowness, while the evidence of radial

speed is tendered immediately, without regard either to time or

distance. But the most curious discoveries afforded by it are of double

stars revolving in such close contiguity as to be permanently

inaccessible to telescopic observation. And these are precisely the

systems of highest cosmogonic interest, as being, most likely, at the

outset of their evolutionary careers. They are surprisingly numerous,

and will doubtless prove to be linked on to telescopic binaries by an

uninterrupted succession of couples farther and farther apart.

This common ground of the two astronomies, where motion in the line of

sight is the object of research, has already proved fruitful of varied

novelties, and its yield is not within view of being exhausted. It

could, however, never have been worked to advantage but for the timely

assistance of photography. The living retina is here conspicuously

inferior to the chemical retina; for aerial disturbances are eminently

baffling to eyeestimates of line-shiftings, while the sensitive plate,

ignoring momentary fluctuations, records true mean positions. Visual

measures are hence rarely trustworthy; advantageous occasions for

securing them are few; so that they must always be either poor in

quality or scant in quantity; and accumulated data are needed as the

bases of systematic inquiries. The use of the camera is accordingly

indispensable, and has become all but exclusive.

Astrophysics widens in scope year by year, and as it wins _extension_ it

gains _intension_, each advance carrying it deeper into the secrets of

nature. Towards this result the alliance with photography has

contributed with signal effectiveness. The impersonal method confers a

certainty and power in dealing with obscure phenomena which can only in

special cases be claimed for the eye. Moreover, it is of larger

application. It can be employed on an expanded scale both of time and

space. It is thus the fittest means for collecting statistics of the

heavens; and statistics are urgently in demand for the ultimate purposes

of celestial science. The whole future of astronomy has indeed come to

depend upon the validity of photographic evidence, and specialties of

manipulation and development, the idiosyncrasies of variously prepared

plates, the shrinkage of gelatine films, the effects of graduated

exposures, have to be studied no less diligently than the theories and

errors of brass and glass instruments where immediate determinations of

celestial situations are in question. Astrophotography is an art, and

has a technique of its own needing labour for its mastery.

The ramifications of astrophysics are numerous and intricate. To trace

them out in detail would be to unroll an elaborate chart of the

sciences. Celestial chemistry is in itself an all but limitless

department. It includes terrestrial chemistry, thermotics,

thermo-electricity, and slides inevitably into the wonderland of

molecular physics and ethereal powers and qualities. For the

interpretation of spectra demands acquaintance with the nature of the

vibrating systems originating them, with their relations to

“imponderable” agencies, with their perturbations, modifications, and

disruptions under the stress of circumstances at present scarcely

definable. Here there are worlds to conquer. One phase of these

inquiries is marked by the recognition of harmonic line-series in the

spectra of the chemical elements. Another by the discovery that

wave-length is a function of density, that an increase of pressure

slightly shifts the rays emitted by a glowing vapour downward towards

the red. A third, still more significantly, by the “Zeeman effect,” with

its barely conjectured implications. It consists in the distension and

subdivision of lines normally slender and single, when the radiation

takes place in a strong magnetic field; and the specification of the

laws of its production, whether close at hand under controlled

conditions, or far out of reach at the surface of celestial spheres,

allures the imagination with possibilities of far-reaching consequence.

Above all, there seems to be a reasonable chance of learning from it

something about the electrical state of the stars. The relative strength

and brilliancy, moreover, of spectral lines afford criteria of

temperature, density, and modes of electrical action, but not with

satisfactory explicitness. There is much difficulty in duly apportioning

the effects. Thermal and electrical conditions are rarely separable;

degrees of density and of temperature again need very careful

discrimination. Electricity is the indispensable agent for exciting

luminosity; precisely, however, what part it reserves for itself in the

matter—whether heat, as generally assumed, is its plenipotentiary, or

merely a delegate with limited powers—is, so far, unknown. Nor is it

easy to define what takes place in the path of the discharge, yet it is

from the carrying molecules only that the light examined is derived, and

it is their state only that is indicated by its peculiarities. Still,

beginnings have been made in the experimental disentanglement of this

web of interdependent operations, and specific inferences of value

regarding the heavenly bodies have already been drawn from some

preliminary ordering of the various classes of facts.

The rotation of the heavenly bodies is a department annexed, while their

chemistry has been created by the new astronomy. No longer treated as a

simple geometrical datum, it is studied as an index to their physical

constitution. Spectroscopic observations of axial movements in the sun

and planets are among the most delicate and curious that have been made.

They may possibly be extended to stars, nebulæ, and comets, but the

prospects here are dubious. Nor has the old direct mode of determining

rotation been superseded by the novel method. Its employment, in some

cases supplementary, is rendered in others, by the force of

circumstances, exclusive.

Moreover, nearly the whole “descriptive” section of astronomy is

embraced by astrophysics. It is now extensively yet not altogether

worked by photographic means. The camera has so far succeeded very

imperfectly in depicting planetary surfaces; but the required special

conditions are being carefully studied, and will perhaps before long be

realised. The difficulties attending lunar photography have of late

been, in the main, overcome, as the magnificent Paris and Lick Atlases

of the moon testify. They nevertheless record essentially what was known

before; they elucidate no perplexity; selenology has been adorned and

illustrated, but not greatly promoted by their compilation. The

self-portrayal of recent comets, on the other hand, has been accompanied

by remarkable disclosures. They need, however, skilled interpretation,

and experts in this branch are to seek. The pictures are there, full of

rapidly changing and significant detail; yet patience must be exercised

before we can read in explicit terms what they implicitly convey

regarding the constitution of the bodies they represent.

The photographic study of the Milky Way—pursued systematically by

Professor Barnard—has been more definitely and distinctly communicative.

For his plates not only bring clearly to view the mixed stellar and

nebulous nature of that gigantic assemblage, but also afford grounds for

inferences of great moment as to the general distribution of the stars.

This indeed is a subject which might seem expressly reserved for

treatment by the older astronomy. Yet the all-pervasive physics of the

skies has a lien upon it. Spectroscopic considerations come into play.

The modes of stellar scattering in space are different for the various

stellar types, and the connection suggests queries, not readily

answered, regarding the origin of those types, and the very genesis of

the sidereal system itself. Abysses of speculation open before us as we

contemplate the surging galactic cloud-forms depicted through the simple

instrumentality of a portrait-lens and a sensitive-plate.

In the photometric branch of astronomy there is a similar concurrence of

claims. The arrangement of the stars in light-ranks serves primarily as

a test of their arrangement in space; the test, however, is illusory

unless the nature of their spectra be taken into account. Again, while

measurements of the brightness of individual stars are essentially

physical in their import, they are also carried out for the geometrical

purpose of determining occultation-phases. The photometric observation

of the eclipses of Jupiter’s satellites is a corresponding example in

the solar system. Otherwise, in its varied applications to the sun and

moon, to planets, asteroids, and comets, photometry may be said to have

purely physical aims. These have to do, not only with integral, but also

with analysed light. The “spectrometric” division of photometry consists

in the comparative estimation of ray-intensities, in balancing one

against another the differently refrangible beams from a given source of

luminosity, in constructing, that is to say, its spectral energy-curve.

In both departments the camera proves an invaluable ally. Photographic

photometry occupies, indeed, a place apart among the arts and crafts of

astronomy. It has its own laws, its own problems, its own difficulties,

and it furnishes data which can be interpreted on principles valid for

them alone.

The specialties of solar physics are too numerous to be particularised.

Among stars, perhaps an insignificant star, the sun is nevertheless by

its comparative vicinity to ourselves brought within range of a whole

series of observations impracticable elsewhere. In solar research,

accordingly, novel devices abound; such as the “double-slit method,” so

happily availed of by Hale and Deslandres for the spectrographic

portrayal of “flames,” facular and chromospheric. The complex operations

conducted under shelter of eclipse are equally peculiar in their objects

and in their system; by them only is the unique problem of the corona at

present accessible to attack; that of the “reversing layer” is even more

elusive in its momentary presentations. Sun-spots, on the contrary, are

open to leisurely daylight inspection; yet the perplexities connected

with their structure and spectra grow rather more than less acute as

facilities for their scrutiny are increased. But this is no uncommon

experience in the arduous walks of science.

The pliancy and generality of astrophysics contrasts singularly with the

austere exclusiveness of gravitational astronomy. The new mode of

celestial inquiry follows every indication, lays hold of every clue; it

promises much, it often performs more; yet its advance is at times

hampered by the very circumstances which make it brilliant and

surprising. For it “deals,” as Professor Mendenhall said in 1892,[3]

“with a matter of many properties, some of which are but little

understood. While its conclusions are of vital importance and of intense

interest, they result from deductions in which the premisses are

insufficient, and are proportionately uncertain. The new astronomy must

for a long time abound in contradictions and controversies, until, and

largely through its development, we shall possess a knowledge of the

properties of matter when subjected to conditions differing enormously

from those with which we are now quite familiar.”

Here indeed lies the fundamental peril, and at the same time the

essential prerogative of astrophysics. Its concern is with phenomena

falling partly within, partly without the range of ordinary experience.

It has to do with matter in transcendental states. Hence the necessity

for having recourse to the risky expedient of “extrapolation”—that is,

of applying unrestrictedly to the unknown, rules gathered from

observation over a comparatively narrow area. The indefinite continuity

of natural laws is assumed by it, but certainly on no sufficient

warrant. There is indeed no help; no other means are available; the line

and plummet that have proved serviceable for sounding the estuary must

be used likewise for the ocean. The upshot, however, is merely a “first

approximation,” to be subsequently corrected and controlled. And it may

be of immense importance as an index to consequences or possibilities

which could not have been foretold, and defy even imaginative

realisation. But just here resides the exploring faculty of

astrophysics. It often acts as the pioneer of terrestrial science. “The

discovery of unknown laws” (in Professor Keeler’s words), “as well as

the explanation of phenomena by laws already known, is one of its most

important objects.”

A great future is reserved for it. It postulates a law of order, the

same always and everywhere, and its primary function is to verify that

postulate, step by step, point by point, under continually widening

horizons of knowledge. There is no such thing as chaos, it tacitly

asserts, in the sidereal world or outside of it. For chaos is the

negation of law, and law is the expression of the Will of God.

PART I

PROBLEMS IN SOLAR PHYSICS

CHAPTER I.

PROGRESS OF SOLAR PHYSICS.

Solar Physics is the science of the sun as an individual body. It is not

concerned with the sun as the ruler of the planetary system, or as a

member of the sidereal system. The questions which it seeks to answer

relate exclusively to the “thing in itself.” And these questions,

through the effectiveness of modern methods, have become _answerable_.

Few of them, it is true, have yet been _answered_, and all can never be

set at rest, since each reply marks only the starting-point for a fresh

interrogatory. This must be so; the prospect inevitably widens with the

attainment of a higher point of view. Nor is it likely that the ascent

will soon terminate. It is indeed towards a summit cloud-wrapt and

self-withdrawn. The essential point, however, is that stagnation has

given way to progress, surmise to inquiry, and barren wonder to

stimulating curiosity.

Solar research made a threefold start about the middle of the last

century. First came Schwabe’s discovery of a decennial sun-spot

period, followed up by Sabine’s announcement of a coincident

terrestrial-magnetic period. Then, in 1860, Kirchhoff published his

momentous chemical interpretation of the Fraunhofer lines, showing the

presence of familiar metals as glowing vapours in the sun’s

atmosphere. Finally, on 18th July of the same year, the “red

prominences” were photographically referred to their true location,

and the whole marvellous eclipse-garniture was at once annexed to the

domain of solar physics. The investigations corresponding to these

three beginnings were pursued at very unequal rates of advance, and

with considerable disparity of success. The chief triumphs were those

of the prismatic method. From the spectroscope single-handed, the old

order of ideas received its death-blow. Glaring incongruities

notwithstanding, it might have survived a couple of decades longer had

it not been for the reading of the strange Fraunhofer inscription. But

the subversive effect of the attack delivered in 1860 was too palpable

to be ignored. At last, unmistakably, the Herschelian theory of the

sun was in ruins, and it only remained to clear away the rubbish of

the structure preparatory to erecting a modern edifice on new

foundations.

Its corner-stone was the principle of the conservation of energy. This

obtained its first solar application from Helmholtz in 1853. His

gravitational hypothesis explained the enormous outflow of heat from the

focal hearth of the planetary household with a directness and simplicity

that compelled conviction of its truth. Energy of position is, in this

view, the store drawn upon by radiation; and it is a store so vast that

millenniums of thermal expenditure will make no perceptible encroachment

upon it. As the great globe cools, it contracts; and each one of its

constituent particles falls, day by day, infinitesimally nearer to the

centre, heat being thus mechanically evolved. Potential energy is in

this way converted into actual energy, and we are warmed and lighted

because the sun shrinks, and is raised by shrinkage to a surpassing

pitch of incandescence. His constitution must then be such as to meet

these requirements. For an ideal body, endowed at pleasure with fanciful

properties, a machine has to be substituted, definitely adapted to the

fulfilment of a recognised function. This change in the point of view is

characteristic of astrophysical aims. It makes all the difference

between antique and modern science. A great deal is involved in it. The

demands of the novel situation are multitudinous, and can be met, not by

speculative efforts, but only by toilsome experimental comparisons.

These will need time and much patience, and can never be wholly

satisfactory in view of the contrast between terrestrial and solar

conditions. Yet the efforts towards their assimilation, unremittingly

prompted by the new astronomy, lead to a continual growth of knowledge,

and are unlikely to be relaxed until the torch has finally dropped from

human hands.

The absence, then, of a satisfactory all-round theory of the sun need

not be taken as an implication of failure. On the contrary, progress is

necessarily attended by incompleteness. Facts, when research is most

active and successful, accumulate too rapidly to be at once collocated.

Finality means stagnation. Compare the map of the world drawn by

Hecataeus with that by Herodotus. The earlier is by far the more

finished production. Neat and trim, with its circumfluent Ocean Stream,

it pictured the earth mainly from ideas of what it ought to be. The

later chart, on the other hand, as the upshot of wider experience,

admitted ignorance by abolishing limits and leaving room for the

unknown. A true theory must always be somewhat expansible. It must be

capable of accommodating new facts. Otherwise their intrusion will

speedily rive it asunder.

Only the broad lines of solar theory can then at present be laid down.

Details must be filled in gradually with the progress of research. The

preliminary ideas, however, already acquired are unlikely to be

subverted; we can represent to ourselves a sun which is a reality, and

no figment of the brain.

Our luminary is neither solid nor liquid. It is mainly, perhaps

entirely, gaseous; but its gaseity is of the “critical” kind, due to the

combination of intense heat with enormous pressure. The thermal supplies

needed to meet its vast emissive expenditure must be continually and

rapidly brought from the central parts to the surface; and this can only

be accomplished by the actual transport of the heated materials,

conductive processes being much too slow to meet the exigencies of the

situation. We thus recognise in the sun a globe riddled with

convection-currents, of which the shining cloud-shell of the photosphere

constitutes the limit. At the photospheric level the uprushing torrents

deliver their cargo of radiative energy, and from the photospheric level

the corresponding subsidence of cooled matter starts for the

unimaginable furnace below. This course of exchange, however, must be

greatly complicated by the rotation of the plastic mass in which it

progresses. A true vertical circulation is rendered by it impossible;

the ascending and descending currents must be variously and incalculably

deflected. Incalculably, since the state of the sun’s interior lies, in

some respects, beyond the range even of conjecture.

The very remarkable circumstance has been emphasised by recent inquiries

that the photosphere fixes a boundary between two solar regions scarcely

less strongly contrasted—to speak illustratively—than the terraqueous

globe and its encompassing atmosphere. The sun has several distinct

envelopes, but none, apparently, in the condition of atmospheric

equilibrium. There is first a shallow, veil-like covering by which the

disc is reddened and darkened. Next comes the “reversing layer,” a bed

of mixed incandescent vapours, some hundreds of miles in thickness, the

absorptive action of which mainly produces the dusky lines in the

Fraunhofer spectrum. It is overlaid, to a depth of four or five thousand

miles, by the chromosphere, a gaseous ocean incarnadined by the crimson

blaze of hydrogen. The irregularities of its outline develop, locally

and temporarily, into “prominences,” often of gigantic size, but

belonging to the chromosphere as essentially as mounting waves and

tossed spray do to the ocean. Finally, we reach the far-spreading

corona, a mere lustrous phantom, approaching the absolute zero of

density, yet of astounding decorative effect during total eclipses.

Between the corona and the chromosphere there seems to be absolutely no

material continuity, although structural relationships have been traced.

These appendages are distinguished by two peculiarities, rendered more

obvious at each step forward in research. The first is that they contain

an extremely small quantity of matter. The second, that the effect of

the sun’s gravity upon them is, in some way, neutralised. We have only

to consider that at an elevation of three and a half miles air is

reduced to one-half its sea-level density, while the corresponding

height at the surface of the sun—where gravity is twenty-eight times

more powerful than it is on the earth—is but one-eighth of a mile. Into

the compass of a shell just one furlong thick, accordingly, half the

substance of the reversing layer, chromosphere, and corona should be

compressed, if the sway of gravity over them were undisputed. Its

comparative impotence is attested not only by their vast extent and

excessively slow rate of luminous degradation, but still more

emphatically by the almost total absence from them of spectroscopic

symptoms of internal compression.

The various “claims” into which the wide field of solar physics has

inevitably come to be divided are marked by curious differences of

productiveness. Some are thickly sown with “pockets” of bright ore;

others have hitherto yielded little beyond the “sparkle of golden

splendour” on the surface. Thus the geometrical relations of sun-spots

are not now more surely known than in the days of Derham and Cassini. A

consensus of opinion that lasted a full century has given way to

notorious disagreement. The elementary question as to whether spot-umbræ

are elevations or excavations, is once more actively debated. True, the

overthrow of an artificial unanimity often preludes a forward movement;

yet it might have been expected that the immense mass of photographic

records accumulated during thirty years would have amply sufficed to

settle this matter once for all. It must indeed be admitted that direct

sun-pictures, notwithstanding the exquisite perfection to which the art

of taking them has been brought, and the striking nature of the details

they often exhibit, have contributed only in a minor degree to the

promotion of definite knowledge. Super-eminent among them are the long

series due to M. Janssen’s skill; yet after twenty years the _réseau

photosphérique_, a phenomenon of “blurring” manifested by their means,

continues enigmatic as to its nature, and open to doubt even as to its

solar origin.

The swiftest advances in solar physics have been along the various

routes opened by light-analysis. Four of these are broadly separated by

differences of aim and method. The inquiries they have made practicable

relate to the chemistry of the sun itself, the daylight study of

prominences and faculæ through a selected element of their emissions, to

radial movements in the sun, and to spectroscopic disclosures during

eclipses. The Fraunhofer spectrum has been studied year by year with

minuter accuracy, and similar refinements in the treatment of the

arc-spectra compared with it have assured real, and annulled fictitious

correspondences, largely, as may readily be imagined, through

photographic agency. Spectra, to be exactly collated, must be durably

imprinted. The fine measurements now executed upon them would be an

impossible task for the eye. Professor Rowland’s invention of concave

gratings in 1883 has also contributed very notably to the development of

solar chemistry. They simultaneously focus impinging rays and disperse

by diffracting them. No lens needs to be interposed, and thus the

disturbing effects of selective absorption and unequal deviation are

avoided. Improvements in the technical processes of photography—the

substitution of gelatine for collodion as the vehicle of the

decomposable salt; the intensification of sensitiveness in plates;

modifications in their colour-susceptibility—have been equally

essential. Mechanical contrivance has not been behindhand. Without

faultless screws, for instance, there could be no perfectly ruled

gratings. The chemistry of the sun has indeed drawn upon many and

unexpected resources for its promotion.

Spectroscopic work at the sun’s edge was carried on steadily for

twenty-three years after its initiation in 1868. Its outcome was the

collection of a mass of valuable information regarding the chromosphere

and its jutting eminences. Their forms, movements, and duration were

registered, the law of their distribution was ascertained, the mode of

their conformity to the spot-cycle inferred. So rich a harvest was, in

fact, gathered at once that the soil began to show signs of exhaustion;

the prospect seemed dim of detecting any further essential novelties in

this direction; the routine task of daily promenading the slit of the

spectroscope round the limb lost its zest. Then in 1891 a novel

commencement was made, and made in duplicate by Professor Hale at

Chicago and M. Deslandres in Paris. They transferred the business from

the eye to the sensitive plate, definitively and with splendid success.

The photography of prominences, although tried on a correct principle by

Professor Young in 1870, remained in an abortive experimental stage

until recourse was had to the device of isolating the K-line of calcium,

and depicting them in this single element of their light. It proved

applicable to faculæ as well, and in one minute a complete picture of

the disc and its appurtenances, as shown in the violet ray profusely

emitted by them, can be secured with the spectroheliograph whenever

the sun shines on either side of the Atlantic. The very

name—“spectroheliograph”—of the instrument invented for the purpose

comprises a history of changing methods—of the supersession by

photography of eye-and-hand delineation, and of the replacement in turn

of direct photographic portrayal by impressions of spectral images. And

there has been a corresponding modification of ideas. New conceptions

are gaining ground, not through the broaching of startling theories, but

under the steady guidance of undeniable, and often surprising facts.

Doppler’s principle was applied by Sir Norman Lockyer about 1870 to

_meteorological_ investigations (as they might be called) in the sun.

They disclosed the not infrequent occurrence there of portentous

cyclonic agitations. Distortions and displacements of the hydrogen lines

attested the rushing of incandescent whirlwinds at speeds up to 250

miles a second. Much has yet to be learned regarding these extraordinary

phenomena, their relationships having scarcely received the detailed and

particular attention that they deserve. Professor Young’s use of the

same method in 1876 to measure the solar rate of rotation served both as

a test of its validity, which it established beyond cavil, and as a

prelude to important refinements in the treatment of that intricate

subject. Notwithstanding its anomalous retardation north and south from

the equator, M. Dunér obtained in 1887–9 spectroscopic evidence of axial

movement up to fifteen degrees from either pole, and thereby brought a

widened range of its complexities under observational control.

Line-displacements, too, similarly produced, have become the standard

criterion for discriminating between the genuine solar, and the merely

telluric constituents of the Fraunhofer spectrum; and it need scarcely

be pointed out that to set them decisively apart is a pre-requisite to

solar chemical progress.

Each favourable eclipse since 1842 has furnished to science its quota of

new facts and inspiring suggestions. In 1851, the solar status of the

“sierra” and “red protuberances,” demonstrated in 1860, was recognised

by all except a few obstinate sceptics. In 1868, the hydrogen and helium

spectrum of these wonderful objects came into view; in 1869, the green

coronal ray was detected. Then on 22nd December 1870, Young’s tangential

slit was momentarily lit up by the “flash spectrum” of the reversing

layer, which, after twenty-six years, was photographically captured by

Shackleton, and so became the subject of definite and critical

investigation. This was greatly promoted by the multiplied records of it

obtained along the line of totality which crossed India, 22nd January

1898. The cyclical variation of coronal types, indicated by the

substitution of luminous “wings” for the more familiar “glory,” during

the Rocky Mountains eclipse of 29th July 1878, was verified by a

splendid series of coronal photographs taken in Egypt in 1882, at the

Caroline Islands and Grenada in 1883 and 1886, in California and at

Cayenne during the January and December totalities of 1889, at Novaya

Zemlya in 1896, in the Deccan in 1898, in Sumatra and Mauritius in 1901.

Sir William Huggins’s device for photographing the corona in daylight,

invented under the stimulus of the Nile-eclipse disclosures,

unfortunately remains in abeyance. Its realisation is a prime

desideratum in solar physics.

The progress of science in this branch might with substantial accuracy

be described in the condensed statement that a fabulous luminary has

made way for a working machine—a machine, it is true, of infinite

complexity, yet in touch with, although transcending, the common order

of things. Just here reside the extreme interest and value of such

inquiries. They deal with what is concrete; they can be pushed on by

experiment, but by experiment always straining to widen its resources.

Limits are accordingly pushed back little by little—limits of

temperature, of rarefaction, of ethereal stress as manifested by

electric and magnetic intensity. The end of the process is not within

view. The way, arduous though it be, lies open, and is securely

travelled by those who, relying on the unity and continuity of nature,

confidently hope to attain by it to the knowledge of higher truths.

CHAPTER II.

THE CHEMISTRY OF THE SUN.

Knowledge of solar chemistry is based exclusively upon the analysis of

solar light. It advances _pari passu_ with the interpretation of the

Fraunhofer lines. And by their interpretation is signified the process

of identifying them, one by one, with the rays of known substances, made

to glow artificially in the laboratory. They are the characters of a

script in the main decipherable, and already, to a satisfactory extent,

deciphered. Their reversal from bright to dark simply implies that the

prismatic background upon which they are projected represents a hotter

source of radiation than theirs. In other words, the temperature of the

photosphere is above that of the ignited vapours through which its light

is sifted, and by which it is selectively absorbed.

Fraunhofer’s survey of the solar spectrum was necessarily confined to

its visible section, and was executed with very imperfect appliances.

Yet the lines laid down in his map had the importance of permanent

landmarks. The following is a list of the chief among them, their

wave-lengths on Rowland’s scale, and the chemical origins ascertained

for them, being added:—

Designation. Wave-length in ten-millionths of a Origin.

millimetre.

A 7594·059 (upper edge of a band) Terrestrial oxygen

B 6867·461 „ „ „ „

C 6563·054 Hydrogen

D_{1} 5896·156 Sodium

D_{2} 5890·182 „

E_{1} 5270·495 Iron

_b__{1} 5183·792 Magnesium

F 4861·496 Hydrogen

G 4308·034 Iron

H 3968·620 Calcium

K 3933·809 „

The first and last lines in this table approximately define the range of

dispersed sunlight visible to ordinary eyes. It extends over nearly an

octave; and a higher half-octave in the ultra-violet is disclosed

photographically. The infra-red, however, offers a far vaster scope for

exploration. Using a “bolographic” method, in which the camera registers

what the bolometer[4] _feels_, Professor Langley has surveyed a stretch

of dark radiations eight times longer than the bright strip mapped by

Fraunhofer; nor was his advance downward in the spectrum checked by any

insurmountable barrier. There is, indeed, much probability that long

heat-waves and short “Hertzian” waves are really indistinguishable, and

that the luminous spectrum passes without a break through the thermal

into the electric spectrum.

In the ultra-violet region, on the other hand, a peremptory stop is put

to research by the interposition of the air. It excludes by absorbing

ether-waves shorter than about λ 2950. Cornu found sunlight to be

arrested just at this point. Sir William Huggins fixed the limit for the

photographic spectrum of Vega (α Lyræ) at λ 2970.[5] It does not

fluctuate with meteorological conditions. Dampness and drought are

equally ineffective in shifting the atmospheric barrier against the

entry of quick vibrations. Cornu ascertained in 1881[6] that it is

affected only by the height of the barometer. Nor is the reduction of

impermeability through ascent above the earth’s surface nearly as great

as it would be if aqueous vapour were the producing agent. That oxygen

is chiefly concerned is rendered certain by converging proofs. Nitrogen

seems to be, in this respect as in others, nearly inert.

[Illustration:

PLATE II.

Fraunhofer’s A-Band in the Solar Spectrum. (_Photographed by Frank

McClean, F.R.S._)

]

The general enfeeblement, by transmission through our atmosphere, of the

violet and blue sections of sunlight becomes obvious in the redness of

the sinking sun. On the lower radiations telluric absorption acts more

specifically. They are interrupted by a multitude of dark bands and

lines certainly referable to it. The question as to the terrestrial or

solar origin of such effects is evidently of fundamental importance to

solar chemistry. It can be answered in two distinct ways. The earlier

and simpler method is by comparing the spectra of the high and low sun.

The groovings that gain strength with approach to the horizon stand

self-declared as atmospheric, while lines unaffected by altitude tell

plainly of exotic conditions. The latter class are much the more

numerous. Of 3200 lines mapped by Thollon, 2090 are purely solar, 866

telluric, and 246 of compound production.[7] And the proportion is not

very different in Dr. Becker’s catalogue of 3637 spectral lines,

published in 1890.[8] Among them 928 came out blackened in “low-sun”

observations, and proved in the main due to the selective absorption of

water-vapour. A considerable proportion, indeed, belonged to the

“rain-band,” and varied hygroscopically. Dry-air absorption is almost

exclusively an oxygen product. It takes effect chiefly in three wide

bands, Fraunhofer’s “A” and “B” and Ångström’s “_a_,” all relieved

against a crimson background. They are characteristic of cool oxygen.

The molecules, whose vibrations they in a manner reflect, are broken up

at high temperatures. They survive, however, the liquefaction of the gas

at −181° C. Professors Liveing and Dewar observed the atmospheric A and

B in light that had been transmitted through three inches of this frigid

fluid.[9] The rhythmical flutings composing the former are shown in

Plate II., from a photograph by Mr. McClean. The work, of which it is a

specimen, portrays the solar spectrum in seven sections, from D to below

A (λ 5800 to λ 7700), the dispersion having been effected by means of a

Rutherfurd grating of 17,296 lines to the inch.

Fig. 1 gives a general view of the atmospheric spectrum, so far as it

can be seen, but it is largely invisible. Langley found the immense

tract of the heat spectrum, down to wave-lengths of nearly six

“microns,”[10] thronged with “cold” rays, 652 of which[11] were

accurately determined from “bolographs,” but remain, with few

exceptions, chemically unidentified.

[Illustration:

FIG. 1.—General View of Atmospheric Spectrum (Schemer’s _Astronomical

Spectroscopy_, translated by Frost).

]

[Illustration:

FIG. 2.—The Infra-Red Spectrum (Langley).

]

Fig. 2 reproduces Langley’s drawing of part of the infra-red spectrum.

The blank strip to the left shows the comparative brevity of the visible

part of the scroll. The invisible part includes the distinctive

signature of one other atmospheric constituent besides oxygen and

water-vapour. Two strong bands in the infra-red are assigned by Knut

Ångström to the absorption of carbon dioxide,[12] a substance of which

four volumes are present in ten thousand of air at sea-level. The huge

nitrogen envelope of our globe, together with its argon-ingredient,

appears to be perfectly transparent to rays of all refrangibilities. The

unexplained fact of its spectral nullity emphasises the inadmissibility

of negative conclusions regarding the chemistry of the heavenly bodies.

The second peculiarity which distinguishes telluric lines is a negative

one. They do not shift as the sun rotates. But lines genuinely emanating

from the equatorial edges of the sun are displaced towards the blue by

the advancing movement of the left or eastern limb, towards the red by

the recession of the western limb. The juxtaposition, accordingly, of

spectra from these two opposite sources serves as an unfailing test of

the origin of their constituent markings, those claimed by the sun being

perceptibly notched at the points of junction, while their telluric

associates run on continuously.

Little has been added to knowledge of the sun’s constitution by

researches in the infra-red part of the spectrum. They are as yet

crippled by the lack of metallic comparison-lines. Sir William Abney

obtained in 1879 a modification of bromide of silver sensitive to slow

heat-vibrations, and thus succeeded in directly photographing the solar

spectrum between the wave-lengths λ 7600 and λ 10,750. Of 590

absorption-lines measured by him in this region in 1886,[13] only an

insignificant fraction have been identified. All these belong to metals

with low melting-points.[14] Further, certain bands which Becquerel

succeeded in rendering visible by phosphorescence proved assignable to

magnesium, calcium, sodium, and potassium.[15] This confirmation of the

presence in the sun of potassium was far from superfluous, as only one

line due to it can ordinarily be seen.

The recent era in solar chemistry may be said to date from Rowland’s

production of a perfect screw in 1882. This minor feat of ingenuity

opened the way for vital improvements. Through its means, gratings ruled

with almost ideal regularity became widely available, and the

difficulties impeding the diffractive mode of light-analysis were

removed or diminished. Now observations are mutually comparable only

when the _absolute_ wave-lengths of the observed rays are known; and

they are derivable immediately from the diffraction spectrum, while in

the refraction spectrum several complicating circumstances come into

play. Hence the supreme value of gratings. For in the spectra afforded

by them the positions of rays depend simply and solely upon the distance

from crest to crest of the minute ethereal undulations they represent.

Rowland’s photographic map of the solar spectrum[16] was a document in

advance of the time. The amount of detail shown in it may be exemplified

by the statement that 150 lines of absorption could be separately

reckoned between H and K, the great calcium pair in the violet. No

comparable delineations of terrestrial spectra (apart from that of iron)

were, however, then extant; coincidences between the rays in them and

Fraunhofer lines might, accordingly, be often apparent only, and devoid

of chemical significance. Dr. Scheiner gave expression to a general

sense of discouragement when he wrote in 1890: “It is unfortunately the

case that less is known to-day as to the meaning of the Fraunhofer lines

than was supposed to be known ten years ago.”[17]

The need for fresh efforts was, however, promptly met. Photographic

investigations of metallic spectra, fully coming up to the new standard

of accuracy, were set on foot, among others, by Kayser and Runge at

Hanover, by Hasselberg at Stockholm, above all, by Rowland and his

coadjutors at the Johns Hopkins University. Here the spectra of nearly

all the chemical elements have been photographed with high dispersion

for the purpose of solar comparisons. And the end of the process is well

within view. Measurements have already been carried far enough to give a

multitude of identifications. Between 1895 and 1897 Professor Rowland

published in the _Astrophysical Journal_ a “Preliminary Table of Solar

Wave-Lengths,” extending from λ 7331 to λ 2976—that is, from dusky

crimson up to the highest ultra-violet ray capable of penetrating the

aerial barrier. He unhappily did not live to make the list definitive;

but it comprises, as he left it, nearly 20,000 lines, about a third of

which, by a rough estimate, may be confidently referred to absorption by

various terrestrial substances. These are enumerated below, according to

the number of lines associated with them in the sun. The corresponding

atomic weights are given in a second column.

ROWLAND’S TABLE OF SOLAR ELEMENTS.

Element. Atomic Weight.

Iron (about 2750 line-coincidences) 56

Nickel 58

Titanium 48

Manganese 55

Chromium 52

Cobalt 59

Carbon (about 240) 12

Vanadium 51

Zirconium 65

Cerium 140

Calcium (over 75) 40

Scandium 44

Neodymium 140

Lanthanum 139

Yttrium 89

Niobium 94

Molybdenum 96

Palladium 106

Magnesium (about 24 coincident lines) 24

Sodium (13) 23

Silicon 32

Hydrogen 1

Strontium 87

Barium 137

Aluminium 27

Cadmium 112

Rhodium 103

Erbium 166

Zinc 65

Copper (2) 63

Silver (2) 108

Glucinum (2) 9

Germanium 72

Tin 117

Lead (1) 207

Potassium (1) 39

Of the following substances no traces could be found the solar

spectrum:—

Element. Atomic Weight.

Antimony 120

Arsenic 75

Bismuth 208

Boron 11

Cæsium 133

Gold 197

Indium 113

Lithium 7

Mercury 200

Nitrogen (vacuum tube) 14

Phosphorus 31

Praseodymium 144

Rubidium 85

Selenium 79

Sulphur 32

Thallium 232

Argon may now be included among the non-apparent elements, and the

presence in the sun of platinum and the metals cognate with it is still

an open question. A metal belonging to a very different class was added

to the number of solar ingredients by Messrs. Hartley and Ramage in

1897.[18] They convincingly identified two blue rays of gallium with

Fraunhofer lines, pointing out that the proportion to iron of the new

metal indicated as existing in the reversing stratum was by weight only

one to thirty thousand. This accords well with its terrestrial

relations. Gallium, discovered by Boisbaudrin in 1875, seems to be

widely, but very minutely, diffused throughout the earth’s crust. It

occurs also in meteorites. It has an atomic weight of 70, is singularly

volatile, and melts almost as readily as butter.

No substance has been more eagerly looked for in the sun than oxygen.

But the search was long in vain. Henry Draper’s recognition, in 1877, of

_bright_ lines of oxygen in the solar spectrum created a sensation, but

proved illusory. J. C. Draper’s _dark_ lines were a still less plausible

personation. Eisig ascertained in 1894 that none of the eighty-one

emission-lines measured by himself occur in the solar spectrum.[19]

Janssen demonstrated, by observations from the summit of Mont Blanc,

that the _cool_ oxygen-absorption conspicuous in it is of purely

telluric origin;[20] and his conclusion was ratified by Dunér’s

application of the motion-displacement test.[21] Oxygen, however, is a

substance of most complex, and perhaps unstable molecular structure. No

less than six distinct spectra characterise it, two of them produced at

low temperatures, and known through their absorptive effects alone; four

derived from vacuum tubes, under varying degrees of electrical

excitement. Moreover, one of these forms of emission is a series

spectrum of the most intricate kind, comprising six different sets of

harmonic vibrations, three made up of triple, three of single lines.[22]

And here at last a significant coincidence was found. A triplet in the

red part of the Fraunhofer spectrum, photographed by Higgs and McClean,

was in 1897 clearly identified by Runge and Paschen as a fundamental

oxygen group[23] (see Fig. 3). The representation of the element,

although certain and authentic, is reduced to a minimum.

[Illustration:

FIG. 3.—Oxygen-Triplet in the Solar Spectrum (_Astroph. Journ._ vol.

iv. p. 318).

]

The spectrum of helium, which is analogous to the “series spectrum” of

oxygen, makes no show in analysed sunlight, but appears bright above the

limb. In the case of this substance, moreover, the usual order of

detection was reversed. Its recognition as a chromospheric material

preceded by nearly a quarter of a century the expulsion in Professor

Ramsay’s laboratory of an identical gas from clevite. But about helium

more will be said presently.

So far, then, thirty-nine of the chemical elements are known to be

common to the earth and sun, and the remaining forty may very well be so

likewise. The absence from among the solar ingredients of any single

terrestrial species of matter is unproved, and perhaps unprovable. The

Fraunhofer spectrum sums up the combined absorption of a heterogeneous

mixture of vapours. But the aggregate is widely different from what

would be obtained by simply adding together the separate effects. For it

is the outcome, so to speak, of struggle and survival. In a medley of

ignited substances, the rays of certain among them predominate, while

those of others are effaced. Thus non-metals, as a rule, make a poor

figure in the spectral competition with metals, and this is doubtless

one reason for their inconspicuousness in the sun. Apart from hydrogen,

the properties of which are exceptional, only three metalloids, silicon,

carbon, and oxygen, contribute to produce the Fraunhofer lines, and

their contributions are feeble and fragmentary. That other similar

substances—selenium, sulphur, nitrogen, argon, and the rest—may be

there, yet exercise no perceptible absorption, is amply possible.

The metals themselves, too, differ widely as regards conditions of

visibility. Some are rich in strong lines, favourably situated for

observation. Iron is an example. It emits thousands of rays, widely

distributed over the spectrum, although most crowded in its higher

sections; and they hold their own vigorously against the adverse

influences of dilution. Again, the rare metal cerium is extraordinarily

prolific of blue rays. No less than 400 were measured by O. Lohse in

1897 in the comparatively narrow region between λ 4000 and λ 4600.[24]

Most of them, however, are quite feeble, and only twenty-nine have yet

been identified as Fraunhofer lines. Those of bismuth, on the other

hand, being all compound, are too diffuse (as Rowland observes) to be

detected in sunlight. And most of the radiations of lithium are so

highly refrangible as to fall under the ban of atmospheric exclusion.

Their reversal in the solar spectrum can thus only be a matter of

inference. That the inference should be negative is suggested by the

absence of a strongly characteristic line in the carmine red. It _ought_

to be readily seen, if lithium be a solar constituent. Its visibility

should be promoted by the small atomic weight—only seven times that of

hydrogen—and low fusing-point of the metal; and the persistence of the

carmine beam is actually shown by its emergence in the spectrum of the

Bessemer flame. Yet it is unlikely that lithium is, in fact, missing

from the sun. The case deserves particular attention.

Heavy substances are obviously at a disadvantage as regards the

production of absorptive effects. Their vapours must tend to lie low,

like carbonic acid in the earth’s atmosphere. Hence the average

lightness of the solar elements is only what we should expect. The mean

atomic weight of the thirty-five metals represented in the Fraunhofer

spectrum is, in fact, just 72, while that of the non-apparent metals is

159. Atomic weight, however, is only one of many conditions affecting

this result. The inclusion of lead-absorption in the scroll, and the

exclusion from it of lithium, sufficiently prove that vapour-density is

far from being alone concerned.

The detection of carbon in the sun was difficult and noteworthy.

Originating with Sir Norman Lockyer in 1878,[25] it was ratified by

Trowbridge and Hutchins in 1887,[26] and still more decisively ten

years later by Rowland. It was, and could only have been made,

photographically. The visible carbon bands are barely discernible in

the Fraunhofer spectrum. One, however, in the ultra-violet (beginning

at λ 3883) comes out unmistakably on sensitive plates. It is due,

according to the best authorities,[27] not to elementary carbon, but

to cyanogen—that is, to carbon in its combination with nitrogen. The

fact is not easy to explain. No other compound body is known to exist

in the sun; and it might have been judged _à priori_ impossible that

any could prove capable of resisting the enormous temperature reigning

near the photosphere. Sir Norman Lockyer attempted to get rid of the

anomaly by locating the absorbing cyanogen in coronal regions, where

relative coolness must prevail; but recent observations point rather

to its presence as a shallow, deeplying stratum. It is certain,

moreover, that the line-spectrum derived from free carbon through the

exciting influence of a powerful electric spark has no counterpart in

the sun. We are then bound to admit, at least provisionally, that the

ultra-violet solar band genuinely indicates absorption by cyanogen.

There is a further complication. The green fluting, a few _shreds_ of

which were measured by Rowland among the Fraunhofer lines, makes part

of the typical hydro-carbon spectrum given primarily by acetylene. The

whole subject is indeed thick-set with embarrassing considerations;

they need careful sifting out. Carbon molecules are remarkably

sensitive in their modes of vibration, four of which have been

separately distinguished.[28] The conditions, however, prescribing the

replacement of one by another are still in large measure obscure.

Temperature is concerned, but it is not alone concerned; density,

admixture with foreign substances, perhaps variations of electrical

state, come into play. Yet the broad certainty has been gained that

carbon, in one, if not in several of its many forms, exists in the

photospheric neighbourhood. And this has an important bearing, not

only upon theories of the solar constitution, but also upon questions

of great interest regarding solar relationships with the stars.

Professor Rowland disbelieves in any fundamental difference between

solar and terrestrial chemistry. The earth, heated to the solar pitch,

would give, he affirms, a spectrum virtually identical with that of the

sun. Yet we cannot well ignore evidences, apparently valid, of some real

diversity. Even if all our “elements,” without exception, are found in

the sun, they are unlikely to occur in the same proportions there as

here; _quantitative_, if not _qualitative_ dissimilarity must be

recognised. Thus certain metals, so scarce that their ores rank as

mineralogical curiosities, produce marked effects of absorption in the

sun. Zirconium, yttrium, cerium, lanthanum may be instanced. Titanium

and vanadium are multitudinously represented in the solar spectrum.

Hasselberg ascertained for the former substance in 1896 no less than 562

coincidences with Fraunhofer lines out of a total of 718 photographed by

him from the metal.[29] On the exclusion of the feeblest rays on his

plates as being of quite uncertain origin, the percentage of agreement

rose to 88 per cent. The Swedish spectroscopist might well claim that

“the presence of titanium in the solar atmosphere is confirmed, with

even superfluous evidence, by these investigations.”

Vanadium—first registered as a solar constituent by Sir Norman

Lockyer—might be called the satellite of titanium. Where one is, the

other is sure to be not far off. Hasselberg’s recent discovery of

vanadium in the Scandinavian mineral rutile[30]—a form of titanic

acid—accentuates the relationship. Both occur, too, although very

scantily, in lead and iron ores, and just traceably in trap and basalt.

This close association may be accounted for by inherent resemblance. The

atomic weight of titanium is 48, that of vanadium 51. Both are eminently

infusible. They share the unusual peculiarity of exhibiting a strong

high-temperature affinity for nitrogen.[31] They exemplify, moreover, a

transition-stage from metals to metalloids, titanium approximating to

silicon, vanadium to phosphorus. Both, it may be added, have been

detected in meteorites. A large proportion of the numerous rays emitted

by vanadium are reversed in the sun, but somewhat faintly reversed,

except in cases of special disturbance, and with these we are not at

present concerned. It is worth remarking that titanium and vanadium,

notwithstanding their near kinship, physical and chemical, show no

coincident spectral lines. In these twin elements, if in any, a common

material substratum might be looked for. There is not the slightest

sign, however, that it exists.

The new metal germanium is an obvious solar constituent; yet it only

_lurks_ in one uncommon terrestrial product. Winkler recognised it in

the mineral argyrodite in 1886. And the sun appears to be much richer

than the earth in Sir William Crookes’s “meta-elements.”[32] The number

of these is almost indefinite; their individualisation, resting upon the

dubious principle, “one band, one element,” is often imperfect or

misleading; and many of the evasive substances, ranked for a time as

separate entities, have failed to make good their footing, and relapsed

into the condition of “sub-aggregates of atoms.” The chemistry of “rare

earths” has of late assumed a kind of departmental importance. It began

in 1794 with the extraction by the Finnish chemist Gadolin of “yttria”

from a jet-black material picked up at Ytterby, near Stockholm. “Ceria,”

detected in 1803 in the “heavy stone” from Bastnäs, was named after the

first asteroid; “lanthana,” obscurely associated with it, came to light

in 1839; “didymia,” “terbia,” “erbia,” successively followed.[33] The

opening, in 1878, of a fresh and fairly abundant source of supply in the

American mineral samarskite started vigorous inquiries into the nature

of these remarkable bodies; and Cleve enumerated in 1895 nine fully

characterised metallic bases, most of them emitting, under electrical

excitation, a brilliant array of spectral beams. The nine “rare” metals

are scandium, yttrium, lanthanum, cerium, erbium, praseodymium,

samarium, gadolinium, and ytterbium. The first four absorb strongly in

the sun, where the presence of erbium and of neodymium, a constituent of

the original didymium, is also evident. Twenty-two additional

meta-elements swell Sir William Crookes’s “suspense account,” but only a

minority are at all likely to obtain ultimate recognition as substantive

forms of matter. Perhaps the surest test of their quality will be found

in the appearance among the Fraunhofer lines of their characteristic

emissions. Those, at any rate, of terbium, holmium, and thulium should

be carefully looked for.

The light of glowing metallic vapours tends, as we have seen, to

suppress or efface the rays of non-metals. Professor Trowbridge made

some experiments in 1896 with a view to determining the conditions of

obliteration. Photographing on a single plate the spectra of pure carbon

and of an electric arc between carbons containing 28 per cent of iron,

he found that the iron ingredient sufficed very nearly to wipe out the

carbon bands in the arc.[34] “This proportion, therefore, of iron,” he

remarked,[35] “in the atmosphere of the sun, were there no other vapours

of metals present, would be sufficient to prevent our seeing the full

spectrum of carbon.” An interesting illustration was thus afforded of a

fundamental principle in solar interpretations. The principle, indeed,

has scarcely yet begun to be applied. Hitherto the absorptive effects of

each of the forty substances vaporised above the photosphere have been

considered apart. But they are not independently produced. They are

often profoundly modified by extraneous action. A systematic

investigation of the various modes in which it comes into play is

desirable, although likely to prove arduous. “There is at the present

time,” Dr. Ames wrote in 1895,[36] “no more fruitful field open to

research than that of the study of the influence of the presence of one

substance upon the spectrum of another.” And Mr. Percival Lewis’s recent

treatment of the subject has had the preliminary result of showing that

“very small traces of an impurity in a gas may cause considerable

changes in its spectrum, whether this impurity be chemically active or

not.”[37] The changes, too, are in many ways perplexing. They are

governed by no traceable rules, depending apparently, in each case, upon

intimate, and to us unknown, molecular relations with electricity.[38]

The explanation of their anomalies is evidently needed for the

satisfactory future progress of solar chemistry.

CHAPTER III.

PECULIARITIES OF THE SOLAR SPECTRUM.

The solar spectrum is densely thronged with unidentified lines. Of these

upwards of twelve thousand have been measured and registered, but lack

chemical interpretation. They are, however, on the way to receive it.

Their recognition will doubtless attend the gradual progress of

acquaintance with metallic spectra. Thus cerium, scandium, and other

bases of “rare earths” may satisfactorily account for a considerable

proportion of them, these substances emitting crowds of rays, as yet

only in part recorded. But besides, say, twelve thousand catalogued

“unknown” lines, an inestimable number remain unnoticed.[39] They are

still, as it were, “in the street”; they have not been admitted even to

the antechamber of science; the preliminary steps to their

identification have not been taken. They will of course be taken in due

time, little by little, as the photography of the Fraunhofer spectrum is

brought nearer to perfection; and to many of them chemical meanings full

of interest will certainly be assigned. Nevertheless, it can scarcely be

expected that the significance of all can ever be made plain. To the

very end, probably, a residuum will keep the secret of an origin due to

forms or conditions of matter strange to terrestrial experience.

That the _type_ of the sun’s spectrum becomes modified in the course of

ages—that it has been, and will again be different from what it now

is—may be admitted without hesitation. But this evolutionary change is

effected imperceptibly at more than millennial leisure. It might,

however, have been expected that transient alterations would manifest

themselves—alterations caused by tumultuous movements in the “reversing

layer,” or connected, possibly, with periodical outbreaks of spots and

prominences. Yet almost none of this definite and obvious character have

been noticed. Only a few lines may be set down as somewhat vaguely and

indeterminately variable. To take a few examples. In 1891 Father

Sidgreaves of Stonyhurst obtained several photographs of the group b

(magnesium and iron) in the green part of the spectrum. They showed with

excellent definition more faint lines than are contained in Rowland’s or

Thollon’s maps; yet one relatively strong in them—“Winlock’s No. 17”—was

barely discernible.[40] This indication of change does not seem to have

been followed up. Again, Sir Norman Lockyer noted in 1873 the

disappearance of a zinc line in the red (Ångström λ 6361·16), which,

nevertheless, was seen as usual in 1878, and has not since been

missed.[41] The most recent instance of the kind was vouched for from

Baltimore. To a faint, slightly nebulous line of unknown origin in the

ultra-violet (λ 3719·796) Rowland attached the note, “Variable, though

not atmospheric.” Jewell[42] describes it as situated within the shading

of a strong iron line, and as “quite distinct upon some plates, while

not visible upon others showing lines closer to the iron line, and much

weaker than the variable line,” as it originally appeared. It has also

been photographed in an intermediate condition, so that its fluctuations

of intensity may be said to be ascertained, although their law and cause

remain wholly obscure. The only hope of learning anything about these is

by continuous and minute observation, which should extend to other

suspicious cases of the same kind. Certain interesting questions might

thus be answered. For instance, are the alleged alterations connected

effects of some general disturbance, or do they occur sporadically, each

on its own account? Can they, in any way, be brought into relation with

the spot cycle? Are they visible in light taken indiscriminately from

all parts of the sun, or are they confined to special localities? These

may serve as specimens of the inquiries suggested by phenomena, perhaps

none the less significant for being inconspicuous. With the camera at

hand the task of daily comparison becomes easy and simple. As Professor

Hale wrote in 1896,[43] “Every photograph of the solar spectrum taken

with high dispersion must now be regarded as a document of great value,

which may ultimately reveal irregular or periodic changes in the

condition of the gases and vapours of the solar atmosphere.”

The Fraunhofer lines have of late forfeited their early reputation

as “constants of nature.” They are not really “fixed”; their

positions in the spectrum are affected by several minutely modifying

causes, and they cannot, accordingly, be depended upon as standards

for the most refined measurements. It is true that only the extreme

accuracy of modern methods has caused them to “step down” from the

high level of invariability, for their deviations are very small,

and might, superficially regarded, appear negligible. They are of

two kinds, physical and kinematical, the former being produced in

the very act of emission, the latter in the course of transmission.

Pressure-displacements and motion-displacements are, in fact,

respectively concerned.

Symptoms of a persistent shift of the Fraunhofer lines towards the red

were first detected in 1890 by Professor Lewis E. Jewell of the Johns

Hopkins University. Persistent, although unequal. It is not the same for

the lines of different elements; it is not even the same for all the

lines of the same element. Motion, then, is not its cause. Fortunately,

a clue was supplied by laboratory-experiments. Attentive study of the

behaviour of metallic lines under varying conditions showed that “with

an increase in the amount of material in the arc there was increasing

displacement towards the red.” “Considering the subject carefully,”

Professor Jewell adds, “there seemed no reason to doubt that the

wave-length of a line depended, to a certain extent, upon the conditions

under which the material producing the line was present in the electric

arc, the vacuum tube, or the solar atmosphere; or, in other words, the

vibration period of an atom depends to some extent upon its environment.

An increase of the density of the material, and presumably an increase

of pressure, seemed to produce a damping effect upon the vibration

period.”[44] Confirmatory results were obtained by Messrs. Humphreys and

Mohler,[45] and the assumption of a constant vibration-frequency as an

essential attribute of the ultimate particles of matter had to be

finally abandoned.

The observed changes are clearly distinguishable from ordinary

temperature effects. Lines are often broadened; under peculiar

circumstances they may be unsymmetrically broadened by thermal

influences; but simple displacements are never due to heat. Moreover,

they can be produced artificially by condensing the air about an

electric arc, so that their immediate cause is not doubtful. “It was

often easy,” according to the Baltimore investigators, “to observe a

line gradually change its position while the pressure was being let off

without alteration in width or other appearance.”

The general upshot of their inquiries[46] was to show that the spectral

shifts in question, far from being an isolated phenomenon, stand in

close relationship to all the most intimate properties of matter. Their

amount, _cœteris paribus_, is proportional to the pressure and to the

wave-lengths of the shifted lines. It differs, however, for each series

in a given spectrum. For different substances it is usually large or

small in the inverse ratio of the absolute temperatures of their

melting-points. Again, it is largest for those substances which expand

most readily with heat. Finally, and most significantly, line

displacements are, in the same group of elements, proportional to the

cube roots of their atomic weights. Or, as Mr. Humphreys expresses it,

“The shift of similar lines is a periodic function of atomic weight, and

consequently may be compared with any other property of the elements

which itself is a periodic function of their atomic weights”—that is to

say, the measured displacements show recurring maxima and minima in

passing from one to the next of Mendeléef’s elemental families. Their

gradations thus correspond with those of other physical attributes of

material species, and plainly imply that the retarded vibrations are

executed by “ultimate” atoms. The confirmatory fact should be noted that

band-spectra, universally associated with aggregates of atoms, display

no sensitiveness to pressure. And by pressure in this connection is to

be understood, not the separate density of the vapour emitting the

damped rays, but the total pressure of all the substances promiscuously

diffused throughout the stratum or enclosure.

Its complex effects add, in some respects, to the difficulty of

interpreting spectral appearances; but they lend to them, on the other

hand, new and unlooked-for significance. In solar inquiries more

particularly, they have started a fresh lead, sure to be followed up.

Thus indications may be gathered from them as to the relative altitudes

in the sun’s atmosphere at which different Fraunhofer lines originate,

no less than as to the absolute pressures to which they correspond.

These are lower than might have been anticipated. They range, according

to Professor Jewell, “from little more than zero to only two or three

atmospheres, though the shading of the stronger lines may be produced at

a greater pressure.”[47] The subject, however, has not got beyond the

stage of inception. One important branch of it, the discrimination of

lines belonging to the same series, is barely sketched. The phenomena of

displacement through pressure evidently involve much more than is yet

apparent. They must be present in stars and nebulæ, and may afford

curious disclosures regarding their states of density and rarity.

The Fraunhofer lines are, as a rule, narrow and sharp; but minute

photographic study reveals, in a certain proportion of them, singular

complexities of structure. These are illustrated from Professor Jewell’s

observations in Fig. 4, which shows graphically, by four typical

examples, the comparative distribution of light in corresponding solar

and arc lines.

[Illustration:

FIG. 4.—Curves representing the Distribution of Light in Fraunhofer

and Arc Lines (_Astroph. Journ._ vol. iii. p. 100).

]

No. i., a green ray of iron, is bordered in the sun (where it is of

course reversed) by a filmy illumination, “the remains of an emission

line, either produced at the photosphere, or lower down in the solar

atmosphere than the absorption line.”[48] The notch at the summit of the

same line gives evidence of radiation at a high level. It is, in fact,

an _abortive_ bright iron line, superposed upon a strong absorption

line, itself superposed upon a faint effusion of light of identical

quality from underlying vapour. Thus this single line is built in three

stages, although the foundation and coping are barely discernible as

traces of luminosity. In Nos. ii. and iii., “shaded lines” are depicted

in the same manner as the “sharp line” in No. i. They belong

respectively to iron and magnesium (λ 5183·8 = _b__{1}). Their

characteristic feature is the outlying obscurity, which deepens from the

edges towards the central shaft. Professor Jewell remarks that the gas

producing these shadings “extends through a much greater range of

pressure” than that giving rise to the green iron line (No. i.), while

the clean-cut line in the middle must be due to absorption “much higher

up in the solar atmosphere, where the pressure is very much less.”

Similar appearances are conspicuous in the great calcium pair H and K

(see Fig. 4, No. iv.). Here the abnormal breadth of their wing-like

appendages proves that the “absorption must persist through an extreme

range of pressure, or that the amount of calcium gas varies enormously

in the solar atmosphere where this absorption is produced.” These lines

are obviously twice reversed. A stratum of radiative calcium is

apparently interposed, in the sun’s neighbourhood, between two

absorptive strata of the same material. That their arrangement is,

however, subject to some kind of disturbance is indicated by the

irregularity of the diagram; nor is it always disturbed to the same

extent. “Upon some plates,” Professor Jewell says, “the central

absorption line is almost symmetrical with respect to the emission line,

while upon other plates its unsymmetrical character is very marked, the

central line being displaced considerably towards the red, and the part

of the emission line on the violet side of the central line being much

the strongest.” Motion-displacements due to ascending and descending

currents are thought to be in question,[49] but there are obstacles to

be removed before this explanation can be unreservedly accepted. The

extreme difference of velocity suggested by the observed dissymmetry of

the calcium lines amounts to no more than 75 miles a minute, but is

notably variable. All the “shaded lines” in the spectrum appear to be

similarly affected, though in a minor degree. Thus the descending motion

corresponding to the narrow central components of the sodium “D” are at

the rate of barely one-fifth of a mile per second. Most of the fainter

lines, on the contrary, indicate ascending currents over the solar

surface at an average speed of about a third of a mile a second.[50]

Motion-displacements, besides, due to the earth’s rotation and the

eccentricity of its orbit, can be detected, and have been allowed for.

These latter minute corrections naturally apply to all the solar lines

without distinction.

Enough has been said to give an idea of the manifold considerations

which have to be taken into account in estimating the _true_

wave-lengths of the solar absorption rays. They are changed, according

to a special and complex law, by the sun’s rotation; they are changed by

the movements of approach or recession of the earth as a whole, as well

as of each particular spot on the earth; they undergo alteration through

the solar atmospheric circulation; they are affected by pressure,

perhaps by other undetected influences, and each of these modifying

causes acts variably, either in time, or according to locality on the

solar surface. Happily, most of them act only to an infinitesimal

extent; but their unquestionable, although slight effectiveness

illustrates very strikingly the subtlety which every increase in

accuracy necessitates in the methods of science.

An embarrassing peculiarity of the Fraunhofer lines is their virtually

uniform intensity all over the sun’s disc. Just as the telluric bands

develop with the sinking of the sun, they ought to become strengthened

near the limb; yet they remain sensibly the same, notwithstanding the

greatly augmented depth of the absorbing strata traversed by the light

before reaching the eye. This is really a glaring anomaly, and one

almost forgotten through sheer hopelessness of getting rid of it.

In concluding this brief chapter we would once more draw attention to

the curious individualities of the Fraunhofer lines. They are

constructed, in many cases, at successive levels; they are modified by

various influences. Some are of hair-like fineness; others, emanating

from an identical substance, have nebulous edges. Moreover, the sharp

and the diffuse lines respond differently to pressure, so that their

characteristic aspects are significant of profound distinctions in their

mode of origin. The more closely, in fact, these mysterious rulings are

examined, the less trivial or casual their slightest diversities appear.

They are charged with meaning, transcending, in part, our actual powers

of interpretation, but challenging efforts towards that end, which

cannot fail to breach, if they do not wholly raze, the ramparts of

ignorance.

CHAPTER IV.

THE REVERSING LAYER.

During the eclipse of 22nd December 1870 a new phenomenon came into

view. Professor Young of Princeton, New Jersey, was the fortunate

observer. With the slit of his spectroscope tangential to the sun’s limb

and perpendicular to the moon’s advance, he awaited the moment of second

contact. The thin solar crescent narrowed second by second; at last it

vanished; then “all at once, as suddenly as a bursting rocket shoots out

its stars,” the ordinary Fraunhofer spectrum previously visible was

replaced by a serried array of bright lines on a dark background. They

seemed a complete reversal of the familiar absorption-rays, and the

impression was also conveyed to Mr. Pye, a member of the same party, of

“all the dark lines being converted into bright ones.”[51] The “flash”

at the edge of the eclipsed sun was not unlooked for. Something of the

kind had been anticipated as the due accompaniment of the beginning and

end of totality. For Kirchhoff’s explanation of the Fraunhofer lines

implied the interposition, between the eye and the sun, of a screen of

glowing vapours, which should be separately, if only instantaneously,

visible on the withdrawal of daylight glare—visible, that is to say,

spectroscopically; with the telescope nothing more distinctive than a

silvery shimmer[52] corresponds to the dazzling variotinted fireworks

disclosed by the prism.

But their disclosure was not enough; they demanded close investigation.

The question is fundamental in solar physical theory whether the flash

is the true reversal of the Fraunhofer spectrum, and no conclusive

answer could be given to it except by photographic means, visual reports

as to the details of so intricate and evanescent an apparition counting

for very little. Twenty-six years, however, elapsed before a permanent

record of it was secured. The result ensued from a skilfully-timed

snapshot by Mr. Shackleton at Novaya Zemlya during the Arctic eclipse of

9th August 1896. He gave an exposure of half a second with a “prismatic

camera”—a simple form of spectrograph, destitute both of slit and

collimating lens, the employment of which in eclipse work has been

vigorously promoted by Sir Norman Lockyer. An impression was thus caught

of singular interest and value. We may quote Professor Young’s

description of it. “The photograph,” he writes,[53] “shows a long range

of several hundred bright curved images, of which there are nearly 250

in the blue portion of the spectrum between F and H. About 25 are much

more extensive and conspicuous than the others, and are images of the

chromosphere and prominences. They are due to hydrogen, calcium, helium,

strontium, and one or two other elements which often appear in the

chromosphere. The rest are simply reversals of the Fraunhofer lines, as

Mr. Shackleton has shown by developing the flash spectrum into a

bright-line spectrum of the usual form (which is easily done by a simple

mechanical contrivance), and comparing it with an ordinary dark-line

solar spectrum photographed with the same camera and prisms, but with

the addition of a collimator and slit. The agreement is practically

complete, although there are two or three somewhat conspicuous

Fraunhofer lines which are missing in the flash spectrum, probably

because they originate, not above the surface of the photosphere, but in

its depths, as probably also do the wide, hazy shadings that accompany

the H and K lines and some others; but this is a matter for further

investigation.”

The solitary success of 1896 was manifolded a year and a half later.

“Reversing-layer” photography stood in the forefront of the programme of

work for the Indian eclipse of 22nd January 1898, and the documents

collected during its hundred seconds of obscurity showed that a complete

mastery of the art had been attained. Sir Norman Lockyer and Mr. Fowler,

Captain Hills, Mr. Evershed, and Professor Naegamvala secured

photographs of the flash, not only in its full development, but also

when incipient and vanishing, so that the phenomenon could be traced at

leisure throughout its brief phases. Two specimens are reproduced in

Plate III., both taken instantaneously by Mr. Evershed with a prismatic

camera of 2¼ inches aperture and 36 inches focus.

The conventional straight appearance of the lines results from the

employment of a cylindrical lens to give breadth to narrow slices of the

curvilinear originals. The upper section represents the spectrum of the

last thread of sunlight just as the accustomed dark lines were fading

out before the incoming of their bright correlatives. The range is from

below H and K—the strong pair to the left—to λ 3350 in the

ultra-violet.[54] The lower section corresponds to a moment twenty

seconds later, when the continuous light was gone, and vivid rays

dominated the field. Amid the throng, twenty-seven members of the

hydrogen series are recognisable, and three titanium lines rival them in

importance.

Ample materials were provided during the eclipses of 1898, 1900, and

1901 for at least a preliminary discussion as to the true character of

the reversing layer, although the difficulties still remaining to be

encountered are neither few nor trivial.

[Illustration:

PLATE III.

Flash and Cusp Spectra compared. Ultra-Violet Region λ 4100 to λ 3350.

]

To begin with, the fact has been ascertained that a shell of mixed

incandescent vapours, five or six hundred miles thick, encloses the

photosphere on every side. We see it, however, in projection. The line

of sight penetrates it tangentially at the edge of the sun, and to an

extreme depth near the base of about forty thousand miles. Between the

reversing layer and the chromosphere there is no solution of continuity;

to some extent, undoubtedly, the lower merges into the upper formation;

yet they are essentially distinct. Each has its own spectrum apart,

notwithstanding a certain amount of community, apparent, casual, or

partial. Thus since the reversing layer is visually accessible only

through the enveloping chromosphere, the spectrographic prints taken at

sun-and-moon contacts are inevitably composite. They include the

chromospheric together with the “flash” rays. Discrimination is,

however, facilitated by the notably greater length of the arcs

representative of the former corresponding with the higher extension of

the substances emitting them. The class of discrepancies between the

flash and the Fraunhofer spectrum thus accounted for are discrepancies

by excess. The flash includes helium rays; the Fraunhofer spectrum has

none. The flash exhibits the complete hydrogen series up to its extreme

limit in the ultra-violet; the Fraunhofer spectrum reverses only its

visible members. Titanium lines strong in the flash are feeble in the

Fraunhofer spectrum; besides other analogous dissimilarities. Yet they

do not affect the claim of the reversing layer to be, speaking broadly,

the locus of solar absorption. Rather they bring us face to face with

the totally different question, Why do the chromospheric gases exercise

no appreciable arresting effect upon the light transmitted through them?

Later on we shall attempt to answer it; here we need only remark that

the rays from the chromosphere cannot be excluded from photographs of

the flash. They necessarily appear in them, and it was known beforehand

that they had no counterparts in the Fraunhofer spectrum.

There are besides discrepancies by defect. Many solar absorption lines

do not show bright at the first and last instants of totality. But this

is easily understood. Some are doubtless too faint to assert their

presence photographically. Others may be supposed, with the utmost

probability, to originate out of sight at the base of the reversing

stratum. The shadings of H and K certainly do, for the central lines

start out clear, though strong, in the flash, and their hazy appendages

are indubitable products of augmented pressure. Nor do the denser

vapours rise high enough to make any perceptible display. Mr. Evershed

tells us[55] that, while nearly all the metals with atomic weights less

than 60 are represented in the sun’s marginal spectrum, none, of which

the ultimate particles are heavier than 92, make any assured

contributions to it. A formation at least five hundred miles in vertical

extent must vary widely between top and bottom both in composition and

density. So at least we should reasonably anticipate. In point of fact

the changes indicated are surprisingly slight. One criterion is

available by which chemical differences can be correlated with

differences of depth. This consists in the various lengths of the curved

lines representing the emissions of the sundry constituents of the

“layer.”[56] Substances attaining high altitudes, like the chromospheric

gases, give long arcs because their visibility extends over a wide

section of the sun’s circumference, while low-lying materials,

illuminating a narrow verge, are characterised by short arcs. They are,

moreover, the most difficult to catch as the moon goes by. Now the great

majority of the flash lines are of a nearly equal length, corresponding

to an arc of about 40° on the solar limb, and this equality implies a

considerable approach to uniformity of constitution throughout the

greater part of the momentarily exposed layer. But its basal stratum,

perhaps not more than a few miles in thickness, should be that most

effective in absorption,[57] and it forms a crescent much too fine to be

directly seen. Here below, then, down near the photosphere, missing

Fraunhofer lines with no apparent corresponding radiations may be

produced; nay, _must_, since there is absolutely no evidence of the

corresponding light-stoppage taking place in, or above the

chromosphere.[58] Mr. Evershed’s conclusion is indeed fully warranted,

that “the flash spectrum as a whole appears to represent the upper more

extensively diffused portion of a stratum of gas, which, by its

absorption, gives the Fraunhofer spectrum.”[59] The appellation

“reversing layer” would then seem to be no misnomer, but to indicate

correctly the seat of the linear absorption which serves as our alphabet

for spelling out the secrets of solar chemistry.

The density of this vaporous envelope is measurable by the “pressure

shifts” of the Fraunhofer lines. It would seem to be nowhere _less_ than

that of our atmosphere at sea-level; otherwise displacements towards the

_blue_ end should occur, and none such are perceptible. Nor, on the

other hand, is there proof of its exceeding, even in the lowest depths,

three or four times the standard value. So that the increase of pressure

downward is exceedingly slow—a fact to be carefully noted. The

distinction (already adverted to) between the effects of total and of

partial pressure is also most important. Through the former the

positions of spectral rays are modified, through the latter their

characters. In other words, the shifts of the Fraunhofer lines

correspond to the sum of incumbent vapours, while the quantity of each

separately present determines their width and diffuseness. Most are

associated, by their fineness and sharpness, with _individually_ tenuous

substances. The hydrogen lines, for instance, represent, according to

Mr. Maunder,[60] a pressure of only one-hundredth of an atmosphere. But

the indications in this respect, as in others, vary greatly for the

different vapours.

The most refractory substances, such as titanium and vanadium, are

volatilised in the reversing layer. It is, then, enormously heated. But

it is cooler than the photosphere, since its rays show dark against the

vivid background they are projected upon. Now the temperature of the

photosphere, by the most authentic recent determination, is about 6600°

centigrade,[61] and this marks an upper limit for the temperature of the

reversing layer. A lower limit is fixed by the temperature of the

electric arc, estimated at 3500°. The much higher grade of the spark is

almost certainly not attained. The inverse behaviour of two magnesium

lines, first commented upon by Professors Liveing and Dewar,[62] led Dr.

Schemer to this conclusion.[63] One at λ 4352 is prominent in the sun

and strong in the arc, but fades out in the spark; the other, at λ 4482,

of which a mere trace is perceptible in the sun, is a characteristic

spark-product. It must, however, be borne in mind that comparative

temperatures are subject to great uncertainty where electricity is the

exciting agent. Dissentients are even to be found from the broad

proposition that the spark is hotter than the arc; nor is it one capable

of direct demonstration. Qualifying circumstances come in, and their

separate effects are not easily unravelled.

M. Deslandres made some curious experiments at Paris in 1894 in

photographing the sun by means of the dusky rays in its spectrum.[64]

For their isolation he used a “double slit”; and since they are only

comparatively dark, no difficulty was encountered through want of

actinic power in the rays dealt with. He thus succeeded in obtaining

with each a monochromatic picture of the sun delineated exclusively with

emissions from some particular ingredient of the reversing layer. The

uniformity of elemental distribution was, by this ingenious device, put

to the test. Photographs of the disc, for instance, taken on an iron

line might be expected to show different features from those taken on

calcium or magnesium lines if local accumulations of those vapours were

present; but no divergences of the kind became perceptible. The

composition of the absorbing envelope did not seem to vary _regionally_.

The investigation, however, was not carried far, and would be worth

prosecuting.

The _fact_ of the existence of a true reversing layer may now be looked

upon as established; yet the _mode_ of its existence remains in several

ways perplexing. The slightness of its absorptive action needs

explanation at the outset. One notes with amazement that the miniature

atmosphere surrounding an electric arc is equally effective for

light-stoppage with this ocean of vapours. Then there is the singular,

and perhaps related circumstance that the spectrum from the limb is not

more deeply grooved than the spectrum from the central parts of the

disc. The results upon light of being sifted through six hundred and

through twenty thousand miles of the mixed materials glowing near the

sun are virtually the same. Their comparative tranquillity, too, is

unexpected. The reversing layer lies between two agitated structures.

Beneath are the photospheric clouds, rent and whirling under the stress

of cyclonic disturbances; above, the chromospheric flames, driven hither

and thither by influences of fantastic violence. Yet a region of peace

seems to intervene. The Fraunhofer lines indicate a steady vertical

circulation, but scarcely ever a temporary commotion. By a rare

exception, Father Fényi observed at Kalocsa, 27th July 1887, the dark C

in the neighbourhood of a spot-group, displaced alternately towards the

blue and the red, indicating, he supposed, a powerful disturbance of the

reversing stratum by an irruption of hot hydrogen.[65] Such invasions of

its precincts, however, are under the ban of some prohibitive decree, or

they encounter unknown difficulties. They occur, at any rate, with

remarkable infrequency.

The reversing layer is heated from below, and gravitates downward.

Thermal equilibrium is doubtless maintained by the convective transport

of material, but the due effects of superincumbent weight are

unapparent. Evidence is not indeed wanting of _some_ increase of density

with descent, but of an increase relatively insignificant. Gravity at

the sun’s surface possesses nearly twenty-eight times its terrestrial

power; hence a true solar atmosphere should double its density with each

furlong of approach to the sun’s surface,[66] and the total increase of

pressure in an envelope five or six hundred miles deep would be

“inexpressible by numbers that have name.” Actually there is, at the

most, a quintupling of pressure. This formidable discrepancy is

altogether unexplained. We are debarred by it from considering the

reversing layer to be in statical equilibrium. Its successive strata do

not rest one upon the other under the sole dominion of gravity. Some

counteracting influence is brought to bear. This problem of _levity_—so

to call it—is one that perpetually recurs in studying the solar

surroundings.

CHAPTER V.

HYDROGEN, HELIUM, AND CORONIUM.

Three tenuous gases—hydrogen, helium, and coronium—are of essential

importance in solar physics. The first plays also a leading part in

terrestrial and vital economy. The second exists on the earth merely as

a chemical curiosity. The third must for the present be classed as an

exclusively solar product.

Solar hydrogen was discovered by Ångström in 1862. He recognised it by

the identity of its three least refrangible rays with the Fraunhofer

lines C, F, and G[67] (now designated Ηα, Hβ, and Hγ), to which, in

1865, he associated the indigo line _h_ (Ηδ). A fifth line (Hε),

photographed by H. W. Vogel in 1879, is situated quite close to the

calcium H—so close that, like Teucer behind the shield of Ajax, it

lies concealed, in the sun, under covert of its neighbour’s broad

shadow, if indeed it be present in the Fraunhofer spectrum at all; for

it is so effectually hidden that the point remains uncertain. Shortly

afterwards, Sir William Huggins’s spectrographic investigations of

Sirian stars gave the key to the true character of the hydrogen

emissions. Nine ultra-violet lines came out on his plates, and their

rhythmical arrangement at intervals continually lessening upward left

no doubt of their forming a connected series. That this included the

visible lines was manifest at sight. Its law was stated by Balmer in

1885.[68] The relations expressed by his formula are not those of

wave-_lengths_, but of their reciprocals, wave-_frequencies_. These

quantities obviously bear to each other an inverted proportion. Deep

crimson light, for instance, consists of undulations about twice as

long as those of violet light; only half as many of them, accordingly,

enter the eye in a given time. A doubled length corresponds to a

halved frequency, a tripled length to a frequency of one-third, and so

on. Now oscillation-frequencies are, for several reasons, more

important natural constants than wave-lengths; hence until they were

made the basis of investigation, no real progress was effected in the

detection of spectral series.

Balmer’s law has the following form: N = N_{0} − (4N_{0})/(_m_^2), where

N is the wave-number ((1)/(λ)), N_{0} is a constant to be determined by

trial, and _m_ is any integer greater than 2. By assigning to N_{0} the

empirical value 27418·75, the places in the spectrum of each individual

ray emitted by hydrogen may be calculated with approximate accuracy.

That of C (Hα) corresponds to _m_ = 3, and the series has been

photographed up to _m_ = 34, its constituent lines growing fainter and

more crowded as the scale is ascended. They approach, in fact, with the

increase of _m_, indefinitely near to a definite limit, marked by the

constant N_{0} minus 0 (the second term having disappeared). This limit,

known as the “convergence frequency,” is a distinctive feature of

spectral series.

Many have contributed to their elucidation. Johnstone Stoney,[69]

Alexander Herschel,[70] Hartley,[71] and Cornu[72] prepared the ground,

and the subject was treated, in its larger bearings, and with more

definite results, by Liveing and Dewar,[73] Schuster,[74] Rydberg of

Lund,[75] Runge and Paschen[76] of Hannover, Kayser[77] of Bonn, and

Ames[78] of Baltimore. Their labours have been unexpectedly successful

in educing partial order out of all but total emissive confusion.

Harmonic series of identical type were marshalled from promiscuous

throngs of rays, and their association into sets of three, or even into

double sets of six, simultaneously given forth by a single element,

proves the extraordinary complexity of the molecular systems through the

movements of which they originate. That some of these movements are of

an orbital nature is strongly indicated, and they not improbably show

perturbative effects analogous to those manifested in lunar and

planetary revolutions. “The final impression,” M. Balmer writes,[79]

“which our mind involuntarily receives in contemplating these

fundamental relations is that of a wonderful mechanism of nature, the

functions of which are performed with never-failing certainty, though

the mind can follow them only with difficulty, and with a humiliating

sense of the incompleteness of its perception.”

Until 1897 the spectrum of hydrogen was thought to be of unique

simplicity. It apparently consisted of one individual series resembling

that formed by a musical note and its overtones. No outstanding lines

interrupted the perfect regularity of the progression. All this,

however, was changed by Professor Pickering’s discovery, in a few

peculiar stars, of a second hydrogen series.[80] It is associated with

the first in such a manner as to indicate that both are subordinate to a

principal series, the three together forming a triple group on the

normal pattern. Of the principal series, one member has been probably

identified as a blue band in certain “bright-line” stars,[81] the rest

being placed inaccessibly high up in the ultra-violet. They would be cut

off by atmospheric absorption. None of the new hydrogen rays occur in

the sun, and none have, so far, been rendered visible in the laboratory,

possibly because the temperatures available are inadequate for their

production. This indeed is a matter of conjecture; what is certain is

that hydrogen affords the only known example of a spectral series

capable of isolation from its fellows. Here evidently we have a clue to

some specialty of intimate structure, the guidance of which may lead to

surprising disclosures.

Hydrogen has other singularities. In some respects it is solitary among

the elements. The “periodic law,” by which their properties are

connected with their atomic weights, does not apply to it. Chemically

and electrically it behaves as a metal; reduced to the liquid state,

however, it definitely ranges itself with non-metals. Its condensation

is effected with the utmost difficulty, physical and mechanical agencies

being only just competent to vanquish the elasticity of this lightest of

terrestrial substances. But what force can barely compel, affinity

readily obtains. United to oxygen under the form of water, it can exist

as a liquid up to a temperature of 100° C., and it is of all gases the

most readily “occluded.” Imprisoned thus in metallic masses, it remains

inert for unlimited periods, but recovers freedom and activity by heat.

Meteoric irons bring to the earth no inconsiderable supply of occluded

hydrogen, and palladium can take it up to the extent of six hundred

times its own volume. In this quasi-combination it is, by a curious

anomaly, six times denser than when liquefied by sheer cold.[82]

The volatility of hydrogen perhaps transcends the earth’s power of

control. By a necessary consequence of the kinetic theory, adverted to

by Dr. Johnstone Stoney in 1870,[83] light gases in a free state can be

permanently retained only by massive globes. For atmospheric particles

no sooner attain a speed just overbalancing the holding power of gravity

than they irrevocably fly off into space, and the process being

continued unintermittently, eventuates in the total dissipation of the

envelope they once constituted. It is, however, a matter of some

delicacy to discriminate between the gases that may escape from any

individual planet and those that must remain. According to a recent

calculation,[84] the earth could now maintain a hydrogen atmosphere

virtually without waste; but in former ages, when the agility of the

gaseous molecules was quickened by heat, the strength of its grasp upon

them must have been insufficient for their lasting retention. This was

nevertheless effected by their reduction to the liquid state in the form

of water. The presence of an excess of oxygen hence saved terrestrial

hydrogen.

Only the four lowest members of the hydrogen series show dark in the

sun.[85] The absence of the higher rays is enigmatical. All are ablaze

in the chromosphere; but the chromospheric gases emit sensibly as much

light as they stop. In the reversing layer it would then seem that

hydrogen glows so imperfectly as to emit vibrations of no more than four

or five qualities, the upper “notes” being somehow quenched. It might be

supposed that the temperature there is too low for their production,

were it not that they have been photographed from vacuum tubes held, on

good grounds, to be cool relatively to the electric arc. The true

explanation is probably to be found in the heterogeneous composition of

the stratum in question. Intermixed particles of different kinds of

matter mutually check each other’s oscillations, and those of shortest

periods are the most susceptible to this adverse influence. Its nature

and the laws of its action remain obscure, but much may be learned about

them by careful experimental inquiry.

A similar anomaly is more markedly visible in the case of helium. This

gas exists near the sun in scarcely less profusion than hydrogen, yet

the Fraunhofer spectrum includes no trace of its action. Absorptive

nullity is not a quality inherent in the substance, as we shall see

presently; hence it probably depends, like the partial inertness of

hydrogen, upon conditions present in the reversing layer.

Until March 1895 helium was known only as a chromospheric element. A

bright yellow ray at λ 5876, distinguished as “D_{3},” because it forms

a trio with the sodium pair D_{1} and D_{2}, was noticed in the

prominences uncovered during the eclipse of 18th August 1868, and can

always be observed spectroscopically at the edge of the sun. But the

substance emitting the yellow ray lay outside the range of our

acquaintanceship, and seemed unlikely to be brought within it. That

contingency, nevertheless, came to pass. In the course of a search for

compounds of argon, Professor Ramsay, at the suggestion of Professor

Miers, fortunately examined the reputed nitrogen occluded by the

Scandinavian mineral “clevite.”[86] This velvety-black stone, remarked

as peculiar by Nordenskiöld and analysed by Cleve, is a kind of

pitch-blende, composed of uranate of lead mixed with rare earths. The

gas evolved from it at University College gave a brilliant spectrum, in

which the prominence-line D_{3} shone conspicuous. Helium was indeed

captured! A beautiful confirmation of the identity was soon afterwards

afforded. The golden line seen in the laboratory was perceived by Runge

to have a faint close companion, and he declared that, unless the solar

D_{3} were also double, clevite-gas should be regarded as different from

helium.[87] The challenge was taken up on both sides of the Atlantic.

Professor Hale on 20th June, and Sir William Huggins independently on

10th July, succeeded in resolving the prominence-ray into a delicate,

unequal pair, and our possession of helium as a truly indigenous element

was rendered incontrovertible.

[Illustration:

FIG. 5.—Diagram of the Helium Spectrum.

]

Meantime sundry other leading chromospheric rays—four especially,

coloured deep red, green, blue-green, and intense violet[88]—had been

recognised in the complex spectrum of clevite gas.[89] The task,

however, of reducing its tangled rays to harmonic order seemed desperate

until it was performed. Without exception, they ranged themselves, at

the bidding of MM. Runge and Paschen, into six related series (see Fig.

5). These form two sets, each consisting of a subordinate pair drawing

together towards a common limit in the ultra-violet, with a principal

series “leaping over the other two in large bounds,” and ending in the

more refrangible part of the spectrum.[90]

Not only did Runge and Paschen’s formula (which may be regarded as a

modification of Balmer’s law for hydrogen) include all the perceptible

emissions of helium, but it intimated the presence of others beyond the

reach of ordinary observation. Each of the principal series, it was

inferred, should own a “leader line” far down among the heat rays, and

with the aid of the bolometer the prediction was strictly verified. Thus

“the actual spectra” (as Mr. Maunder remarked) “corresponded to the

theoretical, and were complete from their rise far in the obscure

regions of the infra-red till they died away in the darkness which lies

on the other side of the visible spectrum.”

Their number, however, suggested a twofold origin, since there was then

no precedent for assigning more than three series to a single substance.

Clevite gas was accordingly regarded as a mixture of two solar elements,

distinguished as “helium” and “parhelium,” the rays of the former, like

D_{3}, being all double, those of the latter single. Each set of three

series was, in fact, “analogous to the complete spectrum of a distinct

element.” Yet parhelium has failed to make good its footing in either

cosmical or terrestrial chemistry. Attempts to isolate it have entirely

failed, and the spectroscopic argument for its existence collapsed with

the discovery that oxygen, no less than clevite gas, claims six series,

which are certainly inseparable, and represent in combination the

vibrations of perfectly similar, highly intricate molecular systems.

“Parhelium” may then safely be treated as fictitious. Clevite gas, or

mineral helium, is the identical undiluted material of prominences. The

hypothetical companion-stuff exists neither in the sun nor upon the

earth.

The qualities of helium are most unusual. Like argon, it is monatomic;

its ultimate chemical units are the same as its ultimate mass-units.

This inference is based upon the heat-relations of the substance. Its

vapour-density is hence only half its atomic weight; for the molecule of

helium possesses nearly double the mass of the hydrogen molecule; and it

is, by hypothesis, indivisible, while that of hydrogen includes two

combining atoms. But helium has no “atoms,” or rather its molecules

_are_ its atoms. Its chemical equivalent is accordingly 3·96 on the

hydrogen scale. A value indeed rather nominal than real, since helium is

devoid of sensible affinities. It enters into no combinations. It again

resembles argon in being a “rogue” element. To both equally, one of the

ordinary properties of matter is wanting. They form with three other

inert gases a class apart as “non-valent” substances.[91] In choosing

its mineral cloisters, helium showed, nevertheless, some original

preferences. The heavy metal uranium has a special attraction for it,

and it is constantly associated with rare earths. Once released,

however, it can scarcely be re-incarcerated. Mr. Tilden’s experiments

led him to conclude that helium-yielding rocks must have been

primitively charged under a pressure of several hundred atmospheres.[92]

The earth may in those early days have possessed a vast helium-envelope,

since dissipated in space. Opinions differ on the subject,[93] and data

for precise calculations are wanting. But the probability is strong that

the helium now sparsely lurking on our globe is a mere remnant of a far

ampler store, which terrestrial potencies, whether gravitational or

chemical, were incompetent to hold.

Helium scarcely retards the passage of light. Its refractive index,

which is the smallest known, is expressed by the fraction 0·146, those

of air and hydrogen being respectively 1·0 and 0·5. Still more

unexpected than its low refractivity, is its high conductive power for

electricity. Professor Ramsay ascertained that the “sparking distance”

through helium at atmospheric pressure is nearly 300 millimetres, while

the same current is stopped in hydrogen by a gap of 40, in oxygen by a

gap of just 24 millimetres.[94] Another surprising property of this gas

is its abnormal faculty of diffusion. It has a rate of self-dispersion

ten times that of hydrogen, or fifteen times what, by Graham’s law, it

ought to be. On the other hand, it has the lowest solubility on

record;[95] water absorbs it in evanescent quantities. This led to the

anticipation, amply justified by experience, that helium would prove to

be one of the most obstinately gaseous bodies in existence. It has not

indeed yet (October 1902) surrendered to the compulsion brought to bear

by Professor Dewar in his memorable researches at low temperatures.

Still, the cooling efficacy of liquid hydrogen evaporating under

exhaustion may be expected finally to overcome its all but invincible

recalcitrance, and the “salamander gas” of the chromosphere will assume

the guise of a frigid fluid boiling five or six degrees above absolute

zero.

The reversing layer—properly so-called—emits apparently no helium rays.

A reason for their absence has been already suggested, and is tolerably

obvious. They are extremely sensitive to damping influences. Foreign

admixtures readily occasion their suppression. Thus 10 per cent of

helium just shows spectroscopically in hydrogen, and that only if the

pressure in the tube is very low; while one part of hydrogen in 100,000

of helium glows manifestly when the current is made to pass.[96]

Nitrogen has a similar adverse effect upon helium-radiation, which

would, however, gain relatively in strength with diminution of pressure

in ascending through the reversing-layer into the chromosphere.

Of the chief coronal ingredient no terrestrial trace has yet been found.

A bright green ray observed during total eclipses is its only assured

badge, for eight or nine other more refrangible associated rays may

quite possibly emanate from different substances. As the leading gaseous

constituent of a structure on the borderland of nothingness, coronium

must be an unimaginably subtle form of matter. It exists in prodigious

volumes near the sun, rising to heights altogether inaccessible to

hydrogen or helium, yet under conditions differing from those of an

ordinary atmosphere. Successive coronal strata are not mutually

superincumbent. There is no sign that their density increases downwards.

The characteristic green line is no less fine and sharp given out by the

inner than by the outer corona. No reversal of it has ever been

detected. There is no corresponding Fraunhofer line. Thus the substance

most plentifully present about the sun is, by a strange anomaly,

absolutely passive as regards its light. This is most probably a result,

not of any specific incapacity, but of the peculiar state in which it

subsists. The real qualities of coronium, however, and its entire

spectrum can only be ascertained by laboratory investigations. That

these will ever become practicable it would be rash to assume, but it is

permissible to hope. There seems, at any rate, no valid reason why

coronium should not be added to the number of unearthed or frozen-out

recondite gases. The former possession by our planet of a

coronium-atmosphere may be plausibly surmised. But it most likely

vanished still earlier than that of helium. Had its material been

endowed with chemical affinities, some compound or compounds should have

preserved it more or less abundantly. It would have been detained, as

hydrogen was detained in water, and kept available for our late

acquaintanceship. Since no compound of the kind appears to exist,

coronium presumably resembles helium in being “non-valent.”

CHAPTER VI.

THE PHOTOSPHERE AND ITS DUSKY VEIL.

The sun is virtually bounded by a spherical envelope of intense lustre.

What lies outside is negligible in mass and function. What is hidden

within has its energies concentrated, so to speak, on the maintenance of

the “photosphere” at the highest point of radiative efficiency. This

implies enormous internal activity, the slackening of which would be the

prelude to speedy extinction. The materials of a self-renewing stratum

of concentrated emission are necessarily in a state of flux. Each

constituent particle, as it delivers up its store of light and heat,

becomes instantaneously effete, and is replaced by another. Charging and

discharging processes pursue a ceaseless round, ceaseless, that is to

say, until the growth of viscosity fatally impedes them. When that time

comes, convection-currents cease to flow, superficial cooling advances

rapidly, and the sun-like stage terminates. The epoch of inertness must

in fact arrive when, for a circulatory, a fixed surface is substituted.

During some long antecedent period, again, the same body was presumably

too rare to be definitely limited, and might fitly be designated a

nebula. So that a “sun” is definable as a gaseous mass clothed with a

pellicle of dazzling luminosity, and organised for long continuance in

the capacity of a distributor of light and heat.

The shining pellicle of our sun is, to all appearance, of a cloud-like

nature. It is a locus of condensation, where uprushing gases, chilled by

expansion, momentarily change their state, and thus acquire the power of

suddenly parting with their stored energy. The “mottlings” of the

photosphere mark visibly, perhaps, this rapid course of interchange,

brilliant floccules denoting regions of _arterial_ ascent, dusky tracts

those of corresponding _venous_ descent. That it is accompanied by

violent turmoil, the evidence of the camera shows conclusively. The

reticulated areas are highly evanescent. Ridges and brilliant cumuli,

some hundreds of miles in extent, form only to be swept away. “When we

come to study the minute details of the granulations,” Professor Young

writes, “we find movements at the rate of a thousand miles an hour to be

the rule rather than the exception.”[97]

Since photospheric light is purely continuous, photospheric chemistry

remains a _terra incognita_. Only conjectures are possible regarding the

kind of matter present in the solar condensations. The idea that they

may be formed of carbon, started by Dr. Johnstone Stoney in 1867,[98] is

still very generally entertained. It is indeed hampered by difficulties

at present insurmountable; but the same may be said of every other

hypothesis on the subject. Carbon was recommended for the position

assigned to it by its refractoriness to heat and by its great radiative

power. Lampblack, we need hardly say, is, in this latter respect, the

standard substance. An unfavourable peculiarity, on the other hand, is

its inability to exist as a liquid under conditions at all likely to be

realised in the sun. Carbon has no fusing-point in the ordinary sense.

At a temperature of about 3500° C. it sublimes without melting.

Preparatory to crystallising as diamond it perhaps liquefies through the

incomparable stress of molecular forces, but the process is transitory

and obscure. It has never been observed; it is only reasonably supposed

to take place. Moreover, at or near the photosphere, pressure of the

required intensity certainly does not exist. The cumuli forming it

should then consist, not of carbon droplets, but of carbon dust, and the

analogy with terrestrial clouds would disappear. A still more serious

objection is that carbon volatilises at a temperature far below that of

the photosphere. Nor are we acquainted with any kind of matter the

condensation of which might be thought of as possible under the

conditions there prevailing.

The question of temperature is fundamental in solar physics. Everything

that regards the nature, structure, and innate activity of the solar

globe depends upon the answer furnished to it. And of late the answers

have become much more plausible than those discordant to the extent of

some millions of degrees arrived at thirty years ago. The main cause of

this wide uncertainty lay, not in the actual measurements, which can be

made sufficiently precise, but in the failure to establish on secure

grounds some definite relation between temperature and radiation. There

is no doubt that thermal outflows increase far more rapidly than the

accompanying thermometric rise—that the heat received at a distance

corresponds, in an augmented proportion, to a gain of heat at the

source; but the correspondence has, until lately, been expressed only by

empirical rules, not implicitly or unconditionally to be trusted.

Boltzmann,[99] however, supplied an _ex post facto_ theoretical basis

for a law published by Stefan of Vienna in 1879, according to which

radiation grows as the fourth power of temperature. Its agreement with

facts, so far as they are available, is besides tolerably close. Yet the

security is precarious that it continues to match them in regions of

cosmic heat, unattainable by experiment. It was, however, employed by

Messrs. Wilson and Gray, with some modification and with excellent

results, in their authoritative determination of the sun’s

temperature.[100]

They adopted a method of direct observation, involving the fewest

possible uncertainties of principle. Sun-heat, allowed to fall upon a

“radio-micrometer”—an instrument of extreme sensitiveness invented by

Professor Boys—was measured by the “balancing” of its effects against

those of a strip of platinum heated to a known pitch. This gave the

means, by the aid of Stefan’s law, of translating them into terms of

temperature. Allowance had then to be made for a double absorption,

first in the sun’s, again in the earth’s atmosphere. That only a

fraction of the heat emanating from the solar condensations reached the

apparatus in the West Meath observatory was unmistakable; how large a

fraction was less easy to decide. Langley finds that the intensity of

radiation at the centre of the disc is reduced near the limb by

one-half,[101] and the total loss is estimated by Wilson and Rambaut at

one-third of the whole.[102] Hence the sun’s thermal power would be one

and a half times greater than it is if the emitting surface were

stripped of its absorbent covering, and the correction of temperature

demanded by its action amounts to at least 1000° C.

The despoiling effect of our own air has next to be considered. It is

very large, and so are the discrepancies in its valuation. Rosetti of

Padua, who in 1879 determined the temperature of the sun to be

20,000°,[103] concluded for a zenithal heat-stoppage of 29 per cent;

Langley estimated it at 41; Knut Ångström[104] in 1890, laying stress

for the first time upon the thermal opacity of the carbonic acid

ingredient of the atmosphere, obtained 64 per cent as the ratio of

absorption. This seemingly authentic result, namely, that only 36 per

cent of the heat rays striking the earth vertically are transmitted to

its surface, was provisionally admitted by Wilson and Gray, and after

having made careful allowance for various kinds of possible error, they

arrived in 1894 at an effective solar temperature of 8700° C.

Substituting Langley’s value for terrestrial atmospheric absorption, and

working up fresh experimental data, Mr. Wilson in 1901 reduced this

figure to 6590°,[105] which probably underestimates the truth. At some

such inconceivable degree of heat the undimmed photospheric clouds glow.

This is not all. The value just given belongs to an ideal stratum in the

sun. It stands for the “effective,” not the actual temperature—the

temperature, that is to say, which should be attributed to a surface of

standard radiative capacity sending out the measured quantity of heat.

Now it is certain that the photosphere falls very far short, in emissive

power, of its imaginary substitute. There is no such thing in nature as

a “perfectly black body,” or its correlative, a perfect radiator, the

efficiency even of lampblack being only six-tenths of what it is assumed

to be for purposes of calculation. And the sun is unlikely to be as good

a radiator as lampblack. It must then be hotter in proportion to its

inferiority, but to what extent it falls short of the ideal standard

remains undetermined. It must also be very unequally hot. The brilliant

granules giving its flocculent appearance to the photosphere radiate

much more intensely than the gray interspaces. Hence computed

temperatures represent an average higher than prevails in some

formations, lower than is assignable to others. It is noticeable that

several corrections based upon recent improvements in experimental data

tend to enhance our conception of the tremendous energy of solar heat.

Le Chatelier’s method[106] of employing the intensities of selected rays

in various light-sources as a criterion of temperature gave 7600° C. for

that of the sun (uncorrected for solar absorption). It is, however, of

doubtful validity. A parallel line of research was opened by Langley’s

establishment of the principle that temperature is connected by a

definite relation with the wave-length of maximum energy in the spectrum

of a radiating body. Divergent views, nevertheless, prevail as to the

_form_ of the relation. Michelson[107] and Rubens[108] agree that the

wave-length of most powerful emission varies in length inversely as the

square root of the temperature, while Paschen and Wien[109] maintain

that the simple inverse ratio tallies more closely with facts. The

outcome in determinations of the sun’s heat differs of course vastly

with the law chosen. From Michelson’s, H. Ebert deduced in 1894 a

temperature of 40,000° C., but added the qualifying remark, “The parts

of the sun to which this value applies belong to the more interior

regions; they are at any rate deep under the reversing layer, and

therefore probably below the photosphere.”[110] Now subphotospheric heat

may be of almost any intensity; hence the result, although not very

informing, is safe to be in some sense correct. Paschen, on the other

hand, obtained the low value of 5130°. It might be added that the law

upon which he relied is suspiciously simple, “in view of the known

complexity in the radiation of a solid body, and the various rates of

increment with temperature attaching to different rays.”[111] It seems

to be one of those formulæ which cannot be trusted far out of sight.

They are _not true enough_ to bear extension into regions beyond

experience. Useful over a moderate compass, they prove treacherous

adjuncts to investigation. Difficulties, indeed, all but insuperable

hamper attempts to infer the solar temperature from comparisons of

spectral energy-curves. Unexpected peculiarities are found to

characterise the modes of emission of solid bodies. Even continuous

spectra are to some extent distinctive. Thus the same quantity of energy

is very differently distributed in the rays sent out respectively by

polished and lampblacked platinum, by carbon filaments, copper, and iron

oxides; while with an equal increase of energy, the distribution becomes

diversely modified for each substance. For each, that is to say, the

maximum ordinate of the energy-curve creeps upward at a different rate.

In the absence, then, of precise knowledge as to the composition and

condition of the photosphere, inquiries as to its temperature, based on

this principle, are futile. We should first need to be acquainted, in

Professor Very’s words, with “the selective radiating power of the solar

photosphere.”[112] Generalisations are here eminently unsafe, since laws

of radiation derived from the experiments with one kind of material are

by no means certain to prove applicable to others. Besides, the

“absolute solar spectrum” (as Langley calls it) cannot be directly

observed, and the shape of its representative curve is most materially

altered by the effects of absorption in the solar atmosphere.

On the whole, the straightforward plan of attack on the problem of the

sun’s heat seems the most promising. Messrs. Wilson and Gray’s practical

operations left little room for improvement, and the uncertainties

affecting their final result will gradually diminish with the progress

of other kinds of research. As higher temperatures, for instance, are

brought under command, the range allowed to perilous processes of

“extrapolation” can be restricted. And improvements, sure ere long to be

realised, in the value assignable to telluric atmospheric absorption,

will effectually reduce the marginal errors attached to present

estimates of the _primitive_ heat-power of the sun.

Fluctuations in the sun’s heat-power must be regarded as possible, and

they might be either irregular or periodical. Indeed, the superposition

of both kinds of change would perhaps be more likely than the occurrence

of either separately. Their detection would, in any case, be extremely

difficult, although it is not, in Messrs. Wilson and Gray’s opinion, to

be regarded as hopeless. The required measures would be simply

differential; and differential measures escape many of the snares that

hamper the execution of absolute measures. But comparisons in this

matter are rendered almost nugatory by inconstancy of weather.

Variations in the “solar constant,”[113] even if real, would probably be

masked by local and temporary changes in the diathermancy of the air.

Professor Very holds that “under these circumstances refinements in

actinometry are of small avail,”[114] and he suggests “that the problem

will have to be solved entirely by meteorological methods.”[115] “If

temperature and humidity observations could be collated from the logs of

vessels crossing the torrid zone, estimates of oceanic evaporation from

day to day, combined with rainfall measures, might lead,” he believes,

“to the detection of the variation of solar radiation.” But the chance

of their doing so appears, all things considered, to be incalculably

small. The elements of disturbance are too numerous and too strong to

permit the emergence of the slight residual effects looked for. Far

preferable appears Piazzi Smyth’s plan of earthing thermometers deeply

enough to be inaccessible to superficial vicissitudes of temperature.

And it can scarcely be without significance that the readings of those

buried on the Calton Hill showed oscillations coincident in period with

the sun-spot cycle.

Absorption in the sun’s atmosphere may also prove to be variable. And

here again differential observations should suffice to test the

question. They were undertaken by Wilson and Rambaut in 1892, but

relinquished after one series had been made. The method employed was to

pass an image of the sun across the radio-micrometer, while the motion

of a spot of limelight, reflected from the mirror of the instrument,

recorded the changing amounts of heat received from the different parts

of the disc. The intention was to obtain such “curves of absorption”

frequently throughout an eleven-year cycle, and thus determine the

question of concurrent fluctuations in depth of the absorbing envelope.

“If we find,” the authors wrote, “that such changes are taking place, as

would be shown by the alteration in the ratio of the heat from the limb

and centre of the disc, we think it will be quite possible, by an

investigation of the co-ordinates of these curves, to determine the

change in the value of the solar constant.”[116]

This theoretical possibility, nevertheless, is still a long way from

realisation. Divers indications lead almost irresistibly to the

conclusion that the sun is hotter at certain times than at others; and

Professor Young counts it as “one of the most important and difficult

problems of solar physics now pending to determine the actual amount of

these variations and ascertain the laws that govern them.” But they are,

as we have partly seen, disguised by manifold complications.

The one clear upshot of inquiries into the temperature of the sun is to

show that it stands high above the boiling points of the most refractory

among the chemical elements. The fact is embarrassing, but cannot be

evaded. Apart from its consideration, no theory as to the nature of the

photosphere is of the slightest value. And it is no easy task to frame

one bringing it into harmony with other circumstances equally well

assured, and equally rigid in their consequences. Three alternative

hypothesis may be said to exhaust the possibilities of the subject. They

are as follows:—

1. The photosphere is a surface of condensation for unknown materials

capable of maintaining the liquid or solid state at a transcendent

degree of heat.

2. It is a surface of condensation for known materials under unknown

conditions.

3. It is no true surface of condensation, the substances composing it

being, although viscous, still vaporous.

Now each of these explanations is largely an appeal to ignorance, and so

far scarcely deserves to be ranked as an explanation at all. Yet one of

them must be fundamentally true. The first may be dismissed as

contradictory of a strong consensus of evidence. The third involves

glaring incongruities, both with what can be seen and with what must be

inferred. There remains only the second. We seem bound to adopt the view

that the sun is veritably clothed in a kind of cocoon—a web of

incandescent filaments. It is perhaps of mixed composition. The surface

is irregular. It comprises “fleece-like floors” at apparently different

levels. Possibly they represent the successive condensations of various

substances—silicon, carbon, titanium, vanadium, platinum, to mention a

few of those most resistant to heat. The diversity of their emissive

powers might contribute to produce the _tonings_ brought out in

photographs of the disc, and the arrangement would be analogous to the

surmised replacement in our upper air of aqueous by carbonic acid

cloud-fields. But the postulated “unknown conditions” needed to enforce

condensation at the enormous temperature of the photosphere may long

continue to baffle the scientific imagination.

A darkening of the sun’s disc towards the limb is obvious

telescopically, and conspicuous photographically. Its amount, measured

by Bouguer in 1729, formed the basis of Laplace’s calculation that the

arrest of light indicated was no less than eleven-twelfths of the

entire. The data were correct, but the result, owing to certain mistaken

assumptions, was greatly in error. Modern authorities, nevertheless, are

far from being unanimous on the subject. Pickering finds that the

intrinsic lustre of the sun exceeds its apparent lustre four and

two-third times; the disparity, according to Vogel, is about twofold.

There are, however, distinctions to be made. The absorption in the

solar, as in the terrestrial atmosphere, is markedly selective. The

brunt of its attack falls upon the most refrangible rays. Father Secchi

noticed in 1870, and Professor Langley again in 1875, that the light

from the limb is, in consequence, tinged with chocolate brown, while

that from the central parts of the disc seems bluish by comparison. This

general indication was, in 1877, analysed by Dr. Vogel,[117] who, by

detailed measurements with a spectral photometer constructed on the

polarising principle, ascertained that 30 per cent of the red, but only

13 per cent of the violet marginal rays penetrate the solar atmosphere.

Hence an alteration in tint corresponding in its mode of origin to the

ruddy suffusion of the setting sun. Now Seeliger has pointed out that

selective absorption implies a medium of high refractive power; but

equivalent conditions might, according to E. von Oppolzer,[118] be

supplied by “a rare atmosphere in which flying particles are suspended.”

It is, beyond doubt, an exceedingly shallow one. This was inferred by

Vogel from the rapid degradation of light towards the edge of the disc,

and it is rendered patent to sense by the brilliancy of facular summits,

which, rising above the absorptive strata, shine unveiled against the

dusky limb. Obviously, then, the darkening effect is produced in the

immediate neighbourhood of the photosphere. It cannot be due to _cool_

gases, and _hot_ gases stop light distinctively in isolated beams. An

alternative hypothesis was suggested some time ago by Professor Hastings

of New Haven. The sun’s so-called “atmosphere” is, in this view, nothing

more than a smoke-laden stratum.[119] Minute solid particles of carbon

or silicon, carried upward from the photospheric clouds, are the agents

of obscuration. The assumption of a solar analogue to a London fog is

certainly a daring expedient, yet none more satisfactory is at present

available.

The sun’s “veil” is indeed particularly difficult to fit in with the

rest of its economy. It manifestly exists, and the position seemingly

prescribed for it is between the photosphere and the reversing layer,

although Dr. Scheiner prefers to place it in chromospheric regions.[120]

In some torrid locality, at any rate, it exercises a kind of action

characteristic of cool substances. Its composition out of refrigerated

materials is strongly indicated. The refrigeration, however, may be

excessively transient as regards each individual particle, although

permanent in their aggregate. The general effect is to diminish the

sun’s heat by one-third or one-half, and its light by fully two-thirds,

with an attendant change to pale primrose of its original glacier-green

tint.

Among the many enigmas of solar physics there is none more curious or

more evasive than that which confronts us in this intimate appurtenance

of the photosphere. Even the lines of approach to it are very few. Yet

some are practicable, and almost untried. Researches of a special kind

into the spectra of sun-spots should help towards its elucidation; still

more, perhaps, careful spectroscopic comparisons with the obscurer

interstitial spaces of the brilliant granules strewing the solar

surface. If these are relatively dark through a mere lowering of

temperature, then little can be learned from them in this connection.

But if they are dark through increased absorption, it will be important

to determine whether the absorption corresponds to that produced by the

problematical “veil.” Are they, in other words, sinks for solar _soot_?

Do they mark the lines of subsidence of the same refuse materials which

by their interposition dim and tarnish the shining face of the sun? A

definite reply to the question would bring us preceptibly nearer to the

goal of our inquiries into the nature of the sun’s “smoke” envelope.

What cannot be doubted is the importance of its function as a regulator

of the sun’s output of energy. This has for the first time been

adequately discussed by Dr. J. Halm in a paper incorporated with the

_Annals of the Royal Observatory, Edinburgh_.[121] His views on the

subject demand careful consideration.

CHAPTER VII.

STRUCTURE AND MOVEMENTS OF SUN-SPOTS.

A normal sun-spot consists of a round black “umbra,” garnished with a

circumferential “penumbra.” The chief member of the group shown in Fig.

6 is a good example. The ground should be almost white, with the

granular texture delicately indicated. The details of such objects,

however, are seen much better than they can be photographed even by the

consummate art of M. Janssen.

[Illustration:

FIG. 6.—Sun-spot photographed by Janssen, April 1, 1894 (from

_Knowledge_, vol. xviii. p. 108).

]

One of their characteristic features is the definite separation of their

parts. The umbra does not merge into the penumbra, nor the penumbra into

the photosphere; the lines of demarcation are as sharp as the edge of a

cascade. Their darkness is indeed accentuated by the enhanced brilliancy

of the regions invaded by them. An immense area of disturbance usually

surrounds an active spot, and this area of disturbance is also an area

of actual elevation. A series of micrometrical measures carried out by

M. Sykora of the Charkow observatory in 1895 showed that, as a very

general rule, the sun’s diameter is lengthened in the direction of a

spot on the limb.[122] The fact is most significant, for it indicates

relief of pressure as perhaps the cause, and certainly as an

accompaniment of solar outbreaks, and thereby associates them with

volcanic explosions. Now a region lifted is, on the sun, a region

brightened, the effects of absorption diminishing with the rise of

level. Hence the exceptional vividness of the photosphere in a spotted

neighbourhood.

Faculæ gain higher altitudes, and are consequently still more lustrous.

They are like the summit-ranges of a tableland. Their connection with

spots is intimate, but not inseparable. Every spot, it is true, claims a

retinue of faculæ; but faculæ exist abundantly where there are no spots.

It is a moot point whether spots can come into being apart from

preceding facular disturbance. “Which is the forerunner of the

other?”[123] is a question not to be answered off-hand. As a rule, the

embryo spot has apparent priority. The rule, however, is not invariable,

and the priority recorded may often be illusory, faculæ being extremely

evasive of observation. Their survival, on the other hand, after the

openings enwreathed by them have closed, is obvious and constant. The

complex relations of the two kinds of phenomenon must be unravelled

before the nature of either can be thoroughly understood.

Photospheric structure is very curiously modified in the penumbræ of

spots. The roundish granules of the unbroken surface seem as if drawn

out into threads, which lie side by side, pointing radially inward, and

overhang the umbra with ragged edges, compared by Dawes to those of an

untrimmed straw-thatch. And the eaves of this luminous thatch are its

brightest part, possibly because of the crowding together of materials

forced into a narrow circular space. The whole effect suggests the

subjection of viscid masses to a pulling action emanating from the

centre of disturbance, by which they are stretched and _carded_ like

wool-flocks.

The umbra of a sun-spot shows a cloudy texture, markedly unlike the

streaky aspect of the penumbra. This can be perceived, however, only

when the seeing is exceptionally good. The ordinary impression is of

uniform and very profound darkness. Contrast, indeed, greatly heightens

this effect. The obscurity is only comparative. Mr. Evershed estimates

the light-emissions from umbræ as varying from about one-twentieth to

one-hundredth those of the dazzling photosphere,[124] and when

intersected by the black advancing moon during the progress of a total

eclipse they seem dully bright.[125] Yet with differences. They are

rarely of the same tint throughout. Dawes perceived in 1852 a “black

opening” in the umbra to be a characteristic of all well-developed

spots. It can only, however, be discerned visually, and that by the aid

of special precautions; the sensitive plate takes no notice of this

deeper depth of shadow, which has, accordingly, received somewhat less

attention than it deserves. Fortunately eye-and-hand portrayals still

continue to be made, and they not infrequently afford valuable records

of the Dawes phenomenon. M. de Pereira, a Portuguese observer at the

Azores, wrote as follows of a group which came into view 20th April

1895: “The sense of a cavity in this spot is unmistakable, as though the

crust of the sun were torn and scratched, and the black, or rather dark,

under-skin were visible beneath. Definition on this day was the best I

have ever seen, enabling me to make the smallest detail reliable. On the

24th, this same spot showed a conspicuous black hole in what I may call

the centre of gravity, a dazzling white bridge crossing it from south to

north-east, and a smaller one lying on the northern edge of the abyss,

the brims being full of curiously intertwined points of photospheric

matter.”[126] The chief member of the splendid group visible in the

sun’s southern hemisphere during the last half of February 1894 had also

an inner nucleus,[127] and the same feature has been studied in numerous

examples by Father Cortie, Mr. Maw, and others. It is commonly

associated with the presence of “bridges,” and both belong

characteristically to the final stages of active spots. Bridge-building

is preliminary to the indraught of luminous matter by which photospheric

breaches are closed; it might be compared to the trickle under the dyke

that preludes the rush of inundating waters. The process is a remarkable

one. From abutments (so to call them) at opposite sides of the umbra,

segments of light protrude; then at a given moment they unite with a

leap or a flash, and the arch stands complete. A beautiful photograph by

Janssen of a spot doubly spanned is reproduced in Fig. 7. But the ground

is _altogether too dark_. The surface near the spot was dazzling,

likewise the facular masses crossing the nuclei.

[Illustration:

FIG. 7.—Photograph of a Bridged Sun-spot, by Janssen (from

_Knowledge_, vol. xiii. p. 74).

]

“On the negative,” Mr. Ranyard wrote in describing it, “the brilliant

bridge which stretches across the great spot is seen to break up into a

number of distinct elongated masses,” and these are evidently the

“rice-grains” of the photosphere laid end to end in single file for

suspension above the abyss. This kind of structure is probably always

present, although often imperceptible. Exceptional facilities are needed

to bring out the finer details in spots. Thus Professor Young tells us

that, on the rare occasions when powers of six hundred and upward could

be profitably used with the twenty-three-inch Princeton refractor, he

succeeded in resolving “the apparently club-like, almost bulbous ends of

the penumbral filaments” into “fine sharp-pointed hooks, reminding one

of the curling tips of flames, or grass-blades bending over.

Ordinarily,” he adds, “they are seen as club-like simply because of

their brightness and the irradiation and diffraction effects of

moderate-sized object-glasses.”[128]

The connection of “bridges” with “black holes” was tentatively explained

in a valuable paper presented by Father Cortie to the Royal Astronomical

Society, 11th May 1900. He considers that the latter may be the portions

of the umbra left uncovered by “faculous veils,” which, extending from

the penumbra, not unfrequently lighten up certain regions of nuclear

gloom, leaving others more profoundly dark by contrast. Now the

relationship between “veils” and “bridges” is obviously quite close.

Both represent luminous invasions, although differently organised and

conditioned, and both are heralds of decay. Their kinship is on

occasions emphasised by the development of one from the other. Twice at

least, in 1865 and 1866, the transformation into “roseate veils” of

brilliant arches spanning the umbræ of spots was observed by Father

Secchi at Rome.[129] He was quite positive about the colour of these

“veils,” which seems to intimate for them a gaseous nature, assimilating

them to prominences rather than to faculæ.[130]

Sun-spots are rarely solitary. They ordinarily appear in clusters or

processions, consisting of one or two dominating members and many

satellites, down to mere umbral dots and penumbral scraps. Individual

spots show endless varieties of conformation. The nuclei are often

multiple; as many as nine umbræ have been seen within the compass of a

single penumbra. Again, they become pear-shaped, or spiral, or caudate,

as if through the action of stresses or twisting forces of an unknown

character. The penumbra is equally subject to irregularities. It is

sometimes a mere torn strip of fringe; half the umbra may be duly

furnished with its _valance_, while the other half remains bare; or the

umbra and penumbra may be disjoined by intruding photospheric matter.

Fig. 8 shows a “fimbriated” spot from a drawing by M. de Pereira, the

partially veiled and bridged umbra of which included two conspicuous

black holes.

[Illustration:

FIG. 8.—Sun-spot drawn by J. de M. Pereira, 18th June 1894.

]

[Illustration:

FIG. 9.—Group of Sun-spots drawn by Miss E. Brown, 15th August 1894.

]

In Fig. 9 three “confluent” spots are represented, drawn by the late

Miss E. Brown, 15th August 1894. She described the group as covering “a

vast extent of surface with a mass of nuclei and penumbra partially

connected and very variable in form.” A few hours later the largest

nucleus had assumed a helical form, and seemed to be “throwing out

feelers like a jelly-fish.”[131] Strong hydrogen incandescence was

spectroscopically perceived to be an accompaniment of these rapid

changes. The great spot of September 1898 had in its declining stage a

nucleus divided by intersecting bridges into three lobes; it assumed on

3rd October the shape of an ace of clubs.[132] Mr. Maw perceived in the

same object on 11th September delicate veins of comparative brightness,

termed by him “submerged bridges.”[133] Indeed he believes this to be a

constant feature of large spots, the umbræ of which, viewed with a

suitable eye-piece, appear no longer uniformly dark, but marked with

fine traceries in chiaroscuro. These cannot at present be photographed,

and their visual detection accentuates the indispensable co-operative

functions of the eye and the sensitive plate.

The level of sun-spots has once more become a subject of active debate.

Yet it was believed to have been determined once for all in the

eighteenth century by the geometrical reasonings of Dr. Wilson. The

characteristic perspective effects of depression below the surface were

noted by him in a well-developed spot as it circuited the sun’s globe in

November 1769, and the saucer-like conformation of all such objects was

universally admitted for a hundred years and upwards, notwithstanding

frequent failures to verify the due optical consequences of their

changing situations. At last, however, this “venerable theory” (as

Professor Frost calls it) has been uprooted from the soil of conviction.

It is denied by many on geometrical grounds alone; by some on physical

grounds as well. Not that it has been finally discarded, but its credit

is gravely impaired. Certainly _all_ spots do not follow its

prescriptions; probably very few strictly comply with them.

Irregularities of form account for a good many of these deviations, but

others cannot be so readily explained away. Mr. F. Howlett, in offering

to the Royal Astronomical Society, 14th December 1894, three volumes of

drawings representing the fruits of thirty-five years of solar scrutiny,

declared uncompromisingly that the Wilsonian view must be

abandoned.[134] Father Cortie’s examination of them convinced him too

that “the phenomena presented by many spots are directly contrary” to

the current hypothesis, a mountainous rather than a cavernous structure

being, in certain cases, indicated for the umbra.[135] Nor does the

umbra usually vanish near the sun’s edge, as it should if it were simply

an excavation with sloping sides. It remains, on the contrary,

persistently visible, although foreshortened into a black line. This, to

be sure, might be a simple consequence of refraction by vapours

congested within the cavity. The explanation is tempting, since it would

avail to get rid of many anomalies; still it must not be adopted

unreservedly. Originally suggested by Proctor,[136] it has been taken

into fuller consideration by Mr. East[137] as a means of exit from the

difficulties that hamper attempts to conceive rationally of the build of

sun-spots.

There is, indeed, pressing need to conciliate opposing evidence. Thus M.

Riccò,[138] at Catania, from eleven years’ study of spots in their

geometrical aspect, derived results strongly in favour of the Wilsonian

hypothesis, computing for twenty-three especially symmetrical formations

an average umbral depth of rather more than a thousand kilometers. Yet

the discussion of the long series of Stonyhurst drawings led Father

Sidgreaves[139] to ascribe to most spots a convex rather than a concave

shape. Professor Hale[140] allowed small weight to testimony so

contradictory as that regarding the apparent width of the penumbra at

various distances from the sun’s limb, but was inclined to consider the

advocates of the Wilsonian doctrine as having rather the better of the

argument. “In any case,” he added, “they will hardly be ready to admit

that the umbra is at a higher level than the penumbra, for it cannot be

doubted that the penumbral filaments overlie the umbra, and frequently

unite to form bridges extending completely across it.”

One of the most singular details of spot-phenomena is the occasional

appearance of a large umbra as a notch on the limb. This implies its

projection in a dark mass against the sky, the encroachment upon the

bright disc being perhaps only an effect of irradiation. Its

inconsistency with a depressed form was pointed out both by Mr. Howlett

and by Father Sidgreaves.

Where, then, is truth to be found in this remarkable controversy? How

can the jostling facts be reconciled? Compromises have been resorted to.

Spots, it is averred, are cloud-like at certain stages of their growth,

crateriform at others. Or individual spots belong to one or the other

type, according to the circumstances of their origin. But these are

subterfuges; let us take a broader view. The concavity, at any rate, of

bridged spots is indisputable. The attribution to them of a

“mountainous” character would throw the arrangement of their parts into

utter confusion. Moreover, De la Rue obtained in 1861, by stereoscopic

means, ocular proof of depression in one such object. The experiment

might easily and usefully be renewed. A pair of photographs, taken at an

interval of twenty-six minutes, gives, through the sun’s rotation, just

the right amount of difference in aspect for combination into one

picture in relief. The moot question, “concave or convex,” might thus

receive a direct answer. Then if, in a long succession of instances, the

answer preserved a uniform tenour, it might safely be concluded that

anomalous appearances of lifted umbræ in spots seen obliquely are

illusory and of purely optical production.

But we cannot even so escape from the entanglements of the subject. It

has different bearings, which have all to be taken into account. Spots

are very hot relatively to their light, and their thermal radiations are

peculiarly conditioned. Professor Frost’s determinations,[141] carried

out at Potsdam in 1892, showed that absorption does not take increasing

effect upon them with approach to the limb to anything like the extent

that it does upon the corresponding radiations from the photosphere.

“The reasonable inference from this,” he wrote, “is that the spots are

at a higher level than the photosphere, and hence less subject to the

absorption of the sun’s atmosphere.” Mr. W. E. Wilson’s[142] more

delicate series of observations in 1893–4 showed likewise that “the

radiation from the umbræ of spots does not suffer absorption when near

the limb in the same manner as a point on the photosphere.” Nevertheless

Professor Langley[143] had found in 1874–5 “the decrement of heat in

approaching the limb” to be very nearly in the same ratio for

photosphere and spots. This flagrant contradiction between results

equally authoritative may not be without meaning, since they were

obtained at nearly opposite phases of solar activity: Langley’s, three

years after a maximum, when it was verging towards stagnation; Frost’s

and Wilson’s, during a period of culminating disturbance. The two latter

recommended systematic observations throughout an eleven-year cycle for

the purpose of investigating the nature of the relationship, and they

respectively threw out the alternative suggestions that, during its

course, the thermal condition of spots, or the level at which they are

formed, may undergo progressive changes.

A more promising explanation was offered by Egon von Oppolzer.[144]

Spot-umbræ, he reminded perplexed solar physicists, are surmounted by

abnormally hot chromospheric layers, certainly exempt from absorption.

Hence the indiscriminate sum of their radiations and those of the

underlying spots gains by comparison with those from the simple

photosphere, at and near the marginal parts of the disc. Now the

flame-stratum develops chiefly above spots of an active type; and spots

of an active type predominate at epochs of maximum. This consideration

at once removes the discrepancy between Langley’s results on one side,

and Frost’s and Wilson’s on the other. In the first series, absorption

produced its full and due effects, because quiet spots being presumably

in question, no appreciable overlying source of heat was present. In the

second and third, the overlying source was so strong as in great measure

to efface the gradations of heat-stoppage suffered by the object

beneath. The disproportionate thermal power of spots may be similarly

accounted for. Our instruments measure, not only their direct

radiations, but also those sent out by ignited materials, to some extent

enveloping them.

The solar rotation is a subject much too important to be disposed of in

a paragraph; it need here only be said that a by-product of its detailed

study has been to throw further doubt upon orthodox opinions as to the

location of spots. The rates of axial movement deduced by Stratonoff and

Wolfer from their progression round the sun appear, at least _primâ

facie_, to compel the inference that they are veritably situated at a

level higher than that of the photosphere.[145] Yet here again some

fallacy is likely to be involved. In view of all these complications it

is scarcely to be wondered at that the _Where?_ has almost superseded

the _What?_ in recent discussions about solar maculæ. The upshot, so

far, seems to be that they are essentially depressions, although

depressions very shallow relatively to their superficial extent. Their

abnormal geometrical behaviour is due, in part, as Mr. Maunder has

suggested, to the cavities being _over-filled_, and the umbræ

consequently dome-shaped;[146] in part to the optical elevation into

view of bottoms which should, but for refractive action, be concealed by

shelving sides. Their radiative irregularities, again, are explicable by

the influence of their coronas of hot flames. As to their rotational

anomalies, they must stand over for future consideration, with the

remark that, to deduce the position of spots from the degree of their

conformity to a supposed law of solar rotation, is to attempt the

solution of one enigma by proposing another still more arduous.

The movements of sun-spots are of three kinds. There are first those

that belong to them collectively, as objects attached to a rotating

globe. With these we are not at present concerned. Next, they have

individual “proper motions” of transport over that globe. Finally, they

show internal movements variously connected with the processes of their

development and decay. The last are mostly spiral or circular, and they

evidently ensue upon inrushes of photospheric matter. They are sometimes

performed round “black nuclei” as centres; and black nuclei are

probably, as we have seen, interspaces between obscurely luminous umbral

effusions. But the whirling of spots is not systematic or innate; it

does not characterise them essentially; it occurs incidentally, and as a

result of disturbance. No fixed rules prescribe its mode or direction.

Opposite gyrations have been simultaneously observed in different

members of the same group of umbræ, and even successively in a single

spot. They are executed in other cases intermittently by fits and

starts. A revolution is not often completed; the description of large

angles is exceptional. Spots cannot then be described, in any true

sense, as “solar cyclones”; the vorticose motions occasionally exhibited

by them spring from temporary impulses, and cease when the force of

these is exhausted.

The _proper_ motions of spots are indicative of much more than has yet

been learned from them. Three kinds of influence seem to be effective in

producing them; namely, mutual action, action from without, and action

from within. After segmentation, in the first place, umbræ repulse one

another; they separate with great velocities. They behave like similarly

electrified masses, but whether they really are such or not is an open

question. In the second place, growing spots in general move rapidly

forward. They share the common drift, but with an acceleration often

amounting to three or four hundred miles an hour. It seems as if cooled

materials, pouring down upon them from above, drove them forward with

the added speed due to a wider circle of rotation. If this were actually

the fact, however, macular increase and macular advance should always go

together; and they are not uncommonly disunited. Processes of extension

in spots may even be accompanied by retrogression over the sun’s

surface.[147] Indeed, the conduct of these strange objects is governed

by no invariable rules. Strong tendencies visibly influence it; yet none

that are irresistible. They can be annulled or reversed by

countervailing circumstances. Hence the special need for guarded

inferences in treating of this subject.

The movements of sun-spots in latitude are not visibly related to their

drift in longitude. They are highly irregular, and not often

conspicuous. Carrington, however, perceived in them a kind of inchoate

method. Spots, according to his generalisation, situated within the

solar tropics (so to speak) tend to approach the equator; spots outside

the north and south limit of twenty degrees, to depart farther from it.

But the exceptions observed are so numerous as sometimes to go near

disproving the rule. We have said that spot-movements in longitude bear

some marks of being communicated by exterior agencies. Those in

latitude, on the contrary, suggest interior action. They are connected,

most likely, with the hidden system of circulation prevailing in the

body of the sun, and reflect its local perturbations.

A remarkable feature of photospheric commotions was referred to by

Father Cortie at the meeting of the Royal Astronomical Society, 11th May

1900.[148] He termed it “alternation” in disturbance. A group of spots

generally includes two chief members, posted respectively in the van and

rear of the array. These _take it in turns_ to develop. We are reminded

of the reciprocal flickerings of the fragments of Biela’s comet.

Analogous pulsations, but on a larger scale, manifest themselves in

responsive disturbances north and south of the solar equator. Mr.

Maunder had already pointed out in 1894 that an “active train” of spots

is often “accompanied by a feebler copy of itself a few degrees north or

south. An outbreak of the first magnitude,” he continued, “will indeed

reproduce itself in several directions.”[149] Somewhat similar

correspondences are noticeable between volcanic foci on the earth; yet

the inferences they suggest might prove misleading.

The occultation of a spot 107,500 miles across was observed during the

solar eclipse of 15th March 1858. Its vast dimensions were, however,

exceeded in the same year by those of an object with the “record”

diameter of 143,500 miles. An enormous double spot, which appeared in

June 1883, covered an area of 2500 million square miles; and the great

spot of February 1892, with its dependants and outliers, spread still

further afield. But such gigantic formations are rarely stable. Their

history is one of tumults and vicissitudes. Comparatively small circular

spots possess individually a much more lasting character, although great

outbreaks are the longest lived in their successive modifications. The

maximum duration so far registered was for a “composite disturbance,”

consisting of four very large spot-groups, and thirteen others of

smaller dimensions, which appeared seriatim, and in obvious association,

on a restricted region of the solar surface.[150] The manifestation

continued for 527 days, from 25th September 1891 to 5th March 1893,

while the sun completed nearly twenty-one rotations. About two

rotation-periods represent, according to Father Cortie, the average life

of a spot.

Most of what we know about sun-spots has been learned by a statistical

method of inquiry. Nor can such methods be dispensed with in the future.

But they do not alone suffice. They must be supplemented and reinforced

by _individualisation_. Each notable spot should be studied in itself

and in all its relations, singly, specifically, and generically. Efforts

should be made to determine its nature, as though it were a solitary

specimen. Can it, without doing violence to plain facts, be regarded as

an excavation in the photosphere? Or, if apparent inconsistencies with

this view be present, are they such as might be due to refraction? The

hypothesis can only be tested by confronting it with particular cases,

and trying definitely how far it avails to meet their exigencies.

Refractive possibilities in the sun have been, until lately, almost

ignored; they are now in some quarters vastly exaggerated. Still

allowance has to be made for their realisation, in ways perhaps

corroborative, rather than subversive of received theories.

There are many other doubts to be set at rest besides those regarding

the interpretation of perspective effects. The record, indeed, of no

significant structural detail should be omitted; and what detail of

these enigmatical objects can be called insignificant? Above all,

variations in their parts and features, whether simultaneous,

successive, or alternative, claim the closest attention; since the

establishment of a course of correlated changes comes very near to the

detection of the underlying causal nexus.

The comparison of one spot with another is a natural sequel to the

investigation of each spot in itself. Do their peculiarities, it may be

asked, depend in any way upon heliographical position? Do they vary

periodically? Can certain traits in sun-spots be classed together as

inevitably associated, certain others as mutually exclusive? Wider

questions, too, suggest themselves as to the place of spots in the

general solar economy, and as to the nature of their connection with

faculæ, prominences, coronal streamers, and the totality of solar

phenomena. Attempts have been made, both by speculative and practical

means, to throw light on these obscure topics, but with results not as

yet wholly satisfactory. Meantime, additional facts are needed—facts

systematically collected, methodically sifted and compared. Isolated

observations are rarely of any considerable value in such complex

matters. Meaning accrues to them just in proportion as they can be

allied to others made in correspondence with them, but under modified

conditions. “Correlate and compare” should be the watchword of

astrophysicists.

CHAPTER VIII.

THE SPECTRUM OF SUN-SPOTS.

Sun-spots give a remarkably compounded spectrum. It appears to sum up

five different sets of effects. That is to say, the obscure longitudinal

stripe corresponding to the umbra owns a quintuple origin. Each of its

elements might be made the subject of a somewhat prolonged discussion.

For the sake of clearness we will briefly enumerate them. They consist

of: (1) A bright background of ordinary photospheric light; (2) a nearly

continuous band of dense absorption, extending from the infra-red to the

ultra-violet; (3) a select array of widened Fraunhofer lines; (4) nearly

all the Fraunhofer-lines under a normal aspect; (5) a restricted number

of bright lines.

Dunér of Upsala ascertained in 1891[151] that the _fundamental_ radiance

of spots is indistinguishable from that of the general surface of the

sun. Their darkness is then due to increased absorption, not to

diminished radiation. This fact decisively negatives some current

theories, and thus limits the field of speculation as to the nature of

spots. They are shown by it unmistakably to be regions where cooled

materials of some kind accumulate. Of what kind those materials are, we

can learn something—although not by any means all that could be

wished—from their peculiar modes of arresting light.

A section of a spot-spectrum in the yellow-green is portrayed in Fig. 10

from a photograph by Professor Young. The belt of strong absorption

which is its leading feature seems, but is not really, continuous. Young

himself discovered in 1883 that it is made up of innumerable fine dark

lines, set very closely together, or even actually overlapping. Each

individual in the multitude is, he tells us, “spindle-shaped—_i.e._

thicker in the middle where the spectrum is darkest—and tapers to a fine

hair-like mark at each end; most of them can be traced across the

penumbra-spectrum, and even out upon the general surface of the

sun.”[152] These observations, which require a high resolving power in

the apparatus employed, were amply confirmed by Dunér. He perceived

further that the lines are collected into groups, leaving chinks of

undimmed photospheric brilliancy between.[153] Within the limits of the

“_b_-group” alone, no less than 300 of these dusky _fibres_ were

counted; above F, however, they become merged together by crowding, and

below E by diffusion. Their separation and arrangement are most evident

in quiescent round spots with intensely black nuclei—in spots, that is

to say, commonly regarded as of the minimum type. Now such a spectrum as

they constitute cannot be produced by liquid or solid matter, however

minutely subdivided; it decisively claims a gaseous origin. Hence the

darkening in spots is not merely an intensification of the “smoky”

absorption veiling the entire disc; it is special and peculiar. So much

can be safely asserted.

[Illustration:

FIG. 10.—Portion of Sun-spot Spectrum, photographed by Professor Young

in 1893.

(From Young’s _General Astronomy_. By permission of Messrs. Ginn and

Co.)

]

Perhaps the most distinctive part of the spot-spectrum is the

collection, included in it, of accentuated Fraunhofer lines.[154] They

are picked out to be widened and darkened on some recondite principle of

selection, which varies from spot to spot, and from epoch to epoch. This

was early noticed by Sir Norman Lockyer, and he pursued the inquiry with

striking results. The discussion in 1886 of observations upon the

spectra of seven hundred sun-spots, made at South Kensington on a fixed

plan during six years, led him to the following conclusions:—[155]

(1) “The most widened lines in sun-spots change with the sun-spot

period.”

(2) “At, and slightly after the minimum, the lines are chiefly known

lines of the various metals.”

(3) “At, and slightly after the maximum, the lines are chiefly of

unknown origin.”

In other words,[156] “As we pass from minimum to maximum, the lines of

the chemical elements gradually disappear from among those widened,

their places being taken by lines of which we have at present no

terrestrial representatives.” “Dissociation,” in short, was the _mot de

l’énigme_. As the sun’s temperature increased with the growth of

disturbance, substances in a terrestrial sense “elementary” were

supposed to split up into exotic constituents, giving spectral lines

strange to laboratory experience.

The evidence for the progressive change thus interpreted was indeed

slight, except as regarded iron; and iron alone was taken account of in

the confirmatory Stonyhurst observations. So far as they went, however,

they were decisive, and all the more so that they covered a different

spectral range (B to D) from that (D to F) examined at South Kensington.

They showed demonstratively that, throughout the disturbed interval

between January 1884 and October 1886, iron lines were all but

completely replaced by “unknown lines” in the list of those affected in

spots, while they duly reappeared upon the restoration of photospheric

tranquillity. In connection with their behaviour, nevertheless, Father

Cortie established an important distinction. Their presence or absence

he found to be determined, not by the general flow of solar commotion,

but by the nature of individual spots. In those of rent and ragged

aspect and tumultuous proclivities, iron lines are ousted by

unidentified faint rays; but in tranquil spots the iron spectrum is at

all times prominent. And since the former sort prevail at maximum, the

latter at minimum, the statistical outcome is that the spectral

variations appear to depend simply upon the great cyclical pulsation of

the solar globe. Only on special examination they prove to be determined

more locally and particularly than this would imply. In some unquiet

spots, for instance, which developed near the minimum of 1889, the

effacement of iron lines was as complete as if the epoch had been one of

maximum. In fact (as the Stonyhurst astronomer remarked),[157] the

widening of unknown lines is common to all stages of solar activity,

provided spots of an appropriate character be at hand. This is not a

distinction without a difference. It cuts the ground from under the

assumption of periodical vicissitudes in the general chemistry of the

sun. Iron is not everywhere, and inevitably reduced there to its

elements as temperature and disturbance culminate together, but—if at

all—only as a special effect in the hottest spot-craters. And this again

brings up difficulties connected with relative temperature—difficulties

which, in one form or another, perpetually recur in the study of

astrophysics.

But there is more to be said. Further inquiries have materially altered

the aspect of the case, for they have led to the transference from the

“unknown” to the “known” class of so many spot-lines that the completion

of the process may be confidently anticipated. Rowland’s photographic

comparisons have contributed most effectually to its advance. Young and

Cortie have traced a crowd of sun-spot rays to vanadium; titanium claims

as many, or more; and others perhaps originate from allied “rare”

metals. This singular line of identification is very strongly traced.

Thus _all_ the vanadium lines, twenty-eight in number, between C and D

are by turns broadened in spot spectra, although of evanescent faintness

in the photosphere; nor does the conjecture seem unwarranted that the

high temperature compounds with nitrogen and oxygen, both of this metal

and titanium, may yet be recognised in umbral chemistry. The distension

in a spot of two vanadium lines, at λ 5728 and λ 5731 respectively, is

well shown in Fig. 10. They are of quite minor importance in the

Fraunhofer spectrum.

Father Cortie surmises that the vapours absorbing in spots may be

associated by their approximate conformity to a certain standard of

density. “The level of sun-spots,” he suggests, “is possibly the level

of the faint lines of such metals as have an atomic weight about

50.”[158] Iron, nickel, titanium, and vanadium, all assiduous

frequenters of umbral cavities, belong to this category. But the rule is

compromised by exceptions and incongruities.

The actual state of the case is this. There is no evidence of elemental

dissociation in sun-spots, but spectral diversities are obvious and

persistent. They indicate the disappearance of iron from tumultuous

formations, and the emergence in them of titanium and vanadium. There

are doubtless concomitant changes, but they await ascertainment and

particularisation. A _caveat_, however, has to be entered. The principle

upon which these inquiries have been conducted is imperfectly assured.

It is commonly taken for granted that the widened lines constitute the

spot-spectra; that they, and they alone, represent the emanations of the

constipated vapours blotching the lustrous disc. But this is a somewhat

arbitrary assumption. The theory of line-expansion by pressure is very

imperfectly understood. The phenomenon does not occur uniformly and

invariably. Lines of different substances are differently affected by

it; lines even of the same substance are unlike in their susceptibility

to its influence. The inferential building up then, of spot-spectra out

of widened lines is subject to many qualifications. These do not lessen

the importance of the observed relation, but they importantly modify it.

The iron lines intensified in spots, presumably by the specific action

of their nuclear vapours, are often unsymmetrically broadened. They are

usually diffuse towards the violet side, sharp to the red. This may

point to the presence of chemical compounds;[159] since the

flame-spectra of metals and of their oxides seem to be differentiated

just by the development, in the latter, of these peculiar shadings. The

possibility must accordingly be admitted that iron-oxides exist in the

sun. Yet the implied temperature is improbably low, since they can be

broken up here on the earth by the simplest metallurgical processes. But

they might perhaps form transiently (so to speak) in spots, as a result

of the local chilling of swiftly circulating material. It may be added

that the iron lines distinctive of spot-spectra are so-called

“low-temperature lines.” They are brilliant in the electric arc, but

tend to be outshone by others in the higher excitement of the spark.

Further complexity was imparted to spot-absorption by the appearance of

certain dusky bands, of which nine, situated below D, were observed

early in 1885 at Stonyhurst,[160] and no fewer than seventeen more

refrangible in 1880–3 at Greenwich. One proved identical with a fluting

drawn by Young in 1872,[161] and all were resolvable into densely packed

lines. Nothing is known, or can even be conjectured, as to their origin;

but they are clearly symptoms of disturbance, since, with the sudden

advent of a solar calm in October 1886, they at once utterly vanished.

Mr. Evershed considers the majority of the unaffected Fraunhofer lines

in spot-spectra to be possibly spurious;[162] they may, he thinks, be

inherent, not in umbral light, but in the photospheric glare diffused

equally over spots and the surrounding sky. Some of the ordinary dark

solar lines, however, thin out in crossing umbræ, and a few show traces

of partial brightening. Moreover, the radiations from spots cannot

escape transmission through the reversing layer, and are hence subject

to precisely the same absorption exerted upon sunlight in general, so

that the Fraunhofer spectrum in its integrity truly belongs to spots,

notwithstanding the reinforcement of some of its components and the

enfeeblement of others through influences special to them.

One of the most remarkable features of spot-absorption has still to be

noticed. Helium, as we have seen, makes no show in the Fraunhofer

spectrum. Yet a helium-envelope surrounds the sun to a depth of five

thousand miles. Every ray of sunshine sent abroad into space has been

sifted through this huge volume of gas, which, by its anomalous

inertness, bids defiance to “Kirchhoff’s law.” Emitting complex ranges

of vibrations, it nevertheless exacts no corresponding toll of

absorption. Its transparency seems absolute. Either it is so hot that it

replaces the light arrested, or its arresting power is nullified by

rarefaction. The former alternative is excluded by the consideration

that an excess of temperature should be notified by the presence of

bright helium lines in the general spectrum of the sun, and they are no

more visible in it than dark ones. Hence the absorptive incapacity of

chromospheric helium may provisionally be attributed to extreme

attenuation.[163] This view has gained plausibility through the

discovery that helium in or near spots acts at times normally upon

light, for the vapours and gases producing umbral obscurity are

assuredly, on any theory of spot-formation, denser within than outside

the apparent cavity. The shading then at D_{3}, like the fusiform shape

of the sodium pair below it (see Fig. 11), results from increased

pressure. It is usually significant of vehement disturbance. In twenty

or thirty spots with flaming appendages—and mostly in their penumbral

regions—Professor Young has seen the yellow helium ray reversed;[164] it

was similarly visible to Professor Naegamvala in the huge vortex of

February 1892,[165] and to Mr. A. A. Buss of Manchester, on 17th March

1899,[166] in a spot the incessant activity of which was the more

remarkable on account of its occurrence near an imminent minimum.

Absorption by the deep red helium ray at λ 6678 was three times observed

in spot-spectra by Father Perry during 1883,[167] but its chemical

meaning was then unsuspected. Now that the helium-spectrum has been

unravelled, further particulars might easily be learned as to the

associates of D_{3} in spots. Its isolated occurrence is improbable.

[Illustration:

FIG. 11.— Reversal of the D-Lines in the Spectrum of a Sun-spot

(Young).

]

Spot-spectra are crowned and completed by the frequent superposition

upon them of vivid rays. These originate from the gaseous effusions

often accompanying the formation and transformation of spots. They are

readily identifiable. Hydrogen lines and the H and K of calcium are the

most frequently brightened; D_{3} is sometimes bright over the umbra,

dark in the penumbra, of the same spot; and “double reversals” of the

sodium “D pair” are quite commonly observed. The phenomenon is

illustrated in Fig. 11. The brilliant ray shining at the core of the

fuzzy spindle, into which each of the coupled lines is broadened,

evidently proceeds from an overlying hotter and rarer stratum of

sodium-vapour. The magnesium group “_b_” is occasionally affected in the

same way. The “rosy veils” in spot umbræ give out, as might be expected,

hydrogen rays, and “bridges” are also loci of emission. The ultra violet

members of the hydrogen series are never present, bright or dark, in

spots; and the fifth line (Hε), which falls just within the region of

visibility, has often been looked for in vain. In some of Professor

Hale’s spectrographs, however, of the giant spot of February 1892 it

showed faintly bright beside the more conspicuous H of calcium.[168] So

far the record stands alone. It has a particular interest from the

ambiguous position occupied by this ray in the solar spectrum.

The agitation prevailing in spots is often betrayed by line-distortions,

telling of the swift recession or approach of vapours congregated in

them. The condition of the C-line, as sketched by Professor Hale, 13th

February 1892, in the same spot, is shown in Plate VI., Fig. 2. The

brilliant patch over the umbra is of normal wave-length; it was derived

from a flame radially immobile; but the hook-like appurtenance of the

dark line testifies to an extraordinary outrush of cooler gas from the

lower part of the formation. We can, to a certain extent, trace its

course. It started outward with a uniform velocity away from the earth

of about 120 miles a second. This slackened unequally, as can be seen by

the breadth of the “hook” at its junction with the line; and the whole

mass of hydrogen came to rest at a distance of thirty to forty thousand

miles from the point of issue, which (it is worth noting) was at the

very middle of the nucleus. The nature of the force raising this brief

but tremendous storm cannot readily be imagined. Its abrupt development

marked an acute crisis of disturbance, to which the earth responded with

magnetic twitches and auroral illuminations.

Motions in spots seem to be limited and local. They can be inferred to

prevail with great violence at certain levels, while complete

tranquillity reigns at others. This, at least, is the only explanation

of the chemical peculiarities of solar hurricanes. Nothing, for

instance, is commoner than the raging of hydrogen-storms amid profound

calcium-calms. Nay, lines belonging to the same substance may indicate

for it simultaneously rest and motion. Thus a few iron lines are at

times observed to be displaced or twisted through the effects of rapid

approach or recession, while the remainder maintain their usual

positions and aspect. The anomaly is most striking, and challenges

persistent attention. Sir Norman Lockyer meets it with the

dissociation-hypothesis; but this raises more difficulties than it

removes.

Enough has been said to show that numerous and most curious problems

await solution by students of sun-spot spectra. The subject is wide

enough to occupy a band of specialists, and its remoter implications can

still be only surmised. Nevertheless, definite conclusions are not

wholly out of reach. First, as to the cause of nuclear darkness. It is

certainly to be found in augmented, and (so to speak) reiterated

absorption. Spots are not simply rents in a shining veil, exposing an

obscure substratum. They are not super-heated regions, where processes

of condensation are suspended. The photosphere is screened, not

perforated, by them. Moreover, the screening is by interposed vapours.

Umbral absorption is mainly, if not altogether, of the gaseous kind. It

is essentially linear and banded. No part of it can be safely attributed

to the action of a foggy precipitate such as modifies elsewhere the

“surpassing glory” of the disc. They probably differ in this respect

from “pores” and “veiled spots,” but specific inquiries on the point

have yet to be made.

There are strong indications that spot-spectra originate under

conditions of increased pressure and diminished temperature. Still the

coolest umbræ must be hotter than the reversing layer, for otherwise the

Fraunhofer lines would show bright against them, and, as we know, they

cross them in dusky array. This circumstance is fundamental in solar

thermal relations, yet has been generally overlooked. The ordering

aright of such relations is a prime desideratum in solar physics, and

should serve as an indispensable guide to the interpretation of spectral

diversities.

[Illustration:

PLATE VI.

1. The Corona of 1900. Drawn from Photographs by L. E. Jewell.

2. Reversal and Distortion of the C-Line in Sun-spot (Hale).

]

CHAPTER IX.

FACULÆ AND PROMINENCES.

Faculæ are outgrowths from the photosphere. This is known by direct

observation. They have been seen and photographed jutting from the limb

as rotation brought them into, or carried them out of view. It is also

inferred from their superior brilliancy, which might serve to measure

the opacity of the veil spread over the surface they surmount. On the

other hand, they escape none of the Fraunhofer absorption; the whole

range of solar dark lines is invariably present in their analysed light.

Hence their position can be defined as intermediate between the “smoke

veil” and the reversing layer. This consideration affords a safe

holding-ground for reasonings about the status of these remarkable

objects. They testify to internal commotions of the same nature as those

giving rise to spots, but exempt from their heliographic limitations.

Faculæ are not confined to the spot-zones; they develop all over the

solar globe. Since, however, spots invariably claim their attendance,

they are most numerous in the latitudes frequented by such disturbances,

while showing independent maxima much nearer to the poles.[169] But

their imperfect visibility upon the disc greatly restricted their

observation, until Professor Hale and M. Deslandres almost

simultaneously invented a method for spectrographically recording

them.[170]

It depends essentially upon the use of a double slit—Janssen’s valuable

invention for isolating spectral rays. Celestial objects can in this way

be photographed in monochromatic light—that is to say, their forms in

each separate chemical element can be distinctively recorded. The

importance of the fresh start thus made is difficult to exaggerate. With

a double slit and a sensitive plate, the comparative distribution of

glowing vapours in the sun can be satisfactorily investigated, and

anomalies connected with their distribution, if not removed, at least

fully recognised and defined. All spectral rays, nevertheless, are not

equally available for all purposes. In chromospheric photography, for

instance, the calcium H and K offer immense advantages, not only because

of their actinic efficiency, but still more on account of the broad

bands of shadow rendering them conspicuous as Fraunhofer lines. These

serve to protect against atmospheric glare the bright lines superposed

upon them at the edge of the sun; and glare is the worst foe of daylight

photography. In systematic work, moreover, K is, for more than one

reason, always preferred to H. And so came to be established, in two

continents at once, a new branch of astronomical art—the art of

picturing the sun and its surroundings in calcium-light of a single

quality.

The first and immediate object in view was the day-by-day photography of

prominences; but it was very soon found practicable to extend the work

from the limb to the disc. One slit was caused to travel across the

sun’s image, which had a diameter of two inches in the Chicago

twelve-inch refractor, while the motion of the second was adjusted so

that it exactly kept pace with the K-line, admitting it alone, through a

chink just two thousandths of an inch in width, to impress the sensitive

plate. At Chicago, the first experiment of the kind was made 28th

December 1891, and a similar mode of procedure was described by M.

Deslandres before the Paris Academy of Sciences, 8th February 1892.[171]

The upshot in each case was the discovery that K is doubly reversed over

extensive tracts of the photosphere. Plate IV., Fig. 1, shows the sun

self-portrayed in calcium light, 11th April 1894. The lines crossing the

photograph are accidental imperfections, one set originating from

dust-particles in the jaws of the slit, the other from irregularities in

the movement of the siderostat. The regions of reversal are not confined

to the neighbourhood of spots, but spread irregularly over the sun’s

surface. The individual bright forms they include are often bent into

spirals or doubly curved. They correspond very closely, both in aspect

and position, with faculæ directly seen, and were at once, by Professor

Hale, identified with them. M. Deslandres, however, classing them as a

species of hybrid between faculæ and prominences, bestowed upon them the

compound name, expressive of this mixed quality, of “facular

flames.”[172] Hale’s view, in other words, was that the novelty

disclosed by his “spectroheliograph” consisted in the emission by faculæ

of bright “H and K” by way of supplement to their regular photospheric

spectrum, while Deslandres considered that the new investigation applied

to a distinct kind of objects, neither prominences nor faculæ, although

partaking of the nature of both. The question raised is difficult; let

us briefly examine its bearings.

The calcium-flames photographed at Paris and Chicago are certainly not

prominences. Their positions do not coincide with those of the

chromospheric offshoots. They appear where prominences are not, and

_vice versâ_. Nor are prominences bright enough—unless by a rare

exception—to show in relief against the sun; they are strictly objects

for marginal spectroscopic study. Finally, motion-displacements are

conspicuous in them, and are absent from the sinuous shapes on the disc.

The objections to identifying these with faculæ are less obvious, yet

seem equally insuperable.

[Illustration:

PLATE IV.

1. The Sun portrayed in Calcium Light. From a Photograph by M.

Deslandres.

2. Photograph of the Chromosphere and Prominences taken by M.

Deslandres at 5 h. 40 m. P.M., 31st May 1894.

]

Faculæ belong no less unmistakably to the photosphere than the Himalayas

do to the earth’s crust. They are flung upward from it; they subside

back into it. Their light is of the same brilliantly continuous quality;

it is marked by the same kind and amount of linear absorption. It has

then been sifted through all the vaporous strata amassed above the

surface of the sun. This gives the key to the position; for it compels

the inference that while faculæ are situated beneath the reversing

layer, the allied gaseous forms must be located above it. The argument

for their comparative elevation is cogent. The reversing layer, as we

know, is rich in calcium vapour cooler than the photosphere, and, _à

fortiori_, cooler than the flames detached by their superior lustre from

the photosphere. Their calcium-rays should, accordingly, be effectually

stopped by the reversing layer; they could not by any possibility be

transmitted through it. Hence the bright “K” in which the solar disc can

be depicted must originate above the region where Fraunhofer absorption

takes place. The wreathing forms emitting it belong to a different

locality from that of true faculæ. They are chromospheric, not

photospheric, developments. Yet, as just stated, they cannot be

assimilated to prominences. They probably lie at the base of the

chromosphere, and they follow the distribution of faculæ with close,

though not rigid exactitude. Hydrogen is at times represented in their

spectra by a crimson glimmer of C, so they are not composed of calcium

solely; but their real nature and place in the sun’s economy remain

admittedly enigmatical. We are only sure that they spring up in close

relation with faculæ as symptoms of the same disturbances. That their

relation to them is one of actual identity seemed indeed at first sight

scarcely open to doubt; yet for the reasons just assigned they must be

regarded as separate phenomena.

The connection of faculæ with spots is not, as we have seen, of the

“mutual” kind. Their reciprocity—as a typical Irishman might say—is

one-sided. Outside the spot-zones, where solar commotions have a less

vehement character than nearer to the equator, faculæ lie “extended many

a rood” in sluggish inactivity during intervals long enough for the

working out of manifold changes in the brilliant circumvallations of

spots. These are very often crowned with prominences, which, however,

develop somewhat tardily. Nascent umbræ are rarely accompanied by them,

while moribund umbræ offer a favoured site for their growth. Thus a

remarkably brilliant prominence, agitated by violent motion, towered

above the eastern limb as it was passed, on 3rd March 1892, by the

shrunken remnant of the vast spot-group which had slipped out of sight

round the western limb a fortnight previously.[173]

Prominences are related to the chromosphere in very much the same way

that faculæ are related to the photosphere. They arise from it by

effluence or eruption. Now the chromosphere itself has a markedly

eruptive aspect. It presents no billowy ocean surface, but resembles

rather a Tartarean meadow planted with stalks and grass blades of fire,

waving under some unimaginable furnace-blast. With clear definition, a

filiform texture is everywhere apparent. The “straw-thatch” effect of

penumbræ denotes a similar peculiarity in photospheric materials, and

that it is shared by prominences can be inferred from their frequent

construction, as if out of the untwisted strands of a rope.[174] The red

rim of the eclipsed sun is conspicuously jagged, and its saw-like

outline suggested the name _Sierra_, originally bestowed upon it by Airy

in 1851. It does not, then, represent a fluid envelope in a state of

equilibrium. There is another reason why this is impossible. The

chromosphere does not preserve a uniform depth. Averaging about 5000

miles, it is subject to irregular and temporary variations, as yet

imperfectly observed and entirely unexplained. General subsidences are

even affirmed to take place. One such was noted with surprise by

Trouvelot, June to August 1875.[175] He had frequently observed

extensive local depressions, but never before a universal shoaling. He

attributed to the sharper views of the photosphere afforded by this

removal of part of the interposed medium, his discovery of “veiled

spots,” imperfect umbral formations, owing their abortive character,

perhaps, to their unfavourable situation near the poles. In 1887, the

height of the chromosphere, measured thirty-two times by Dr. Fényi,

S.J.,[176] was found to diminish above the spot-zones, and sensibly to

increase outside their limits. It would be of interest to determine

whether this condition prevails commonly, or whether it is restricted to

epochs of minimum.

Prominences are of two varieties—eruptive and quiescent. These differ

chemically, visually, and heliographically. Eruptive prominences are of

a more mixed constitution than the quiescent sort; they include many

more ingredients; they give an intense carmine light, have jet-like or

upspringing shapes, and are mostly confined to the spot-zones. They are

genuine fire-fountains, while quiescent prominences frequently resemble

cloud-banks. The former are individual outbreaks; the latter unite into

communities, counterfeiting banyan groves, ranges of jungle, fields of

cirrus. Usually connected with the chromosphere by stems or trunks, they

in some cases, not only float free in isolated masses, but, still more

remarkably, are generated at great elevations above it, as if by the

spontaneous illumination of pre-existing material. The analogy with

terrestrial condensations in an azure sky is obvious, but may be quite

misleading.

Quiescent prominences develop on an enormous scale, nor always

tranquilly. Formations, at least, which are in some respects cognate

with them, become, on occasions, the scenes of striking explosive

accidents. So that their generic title must be understood in a

restricted sense. Thus the cloud-like character of a huge object

photographed by Deslandres,[177] 31st May 1894, seemed vouched for by

its vicinity to the south pole, and by the large extent of the solar

limb garnished by it. At 2 P.M., when it was first pictured—as usual, in

calcium light—its height, apart from foreshortening, was 2′ 20″, or

63,000 English miles. At 4^h 27^m it had sprung up to 135,000, and 73^m

later still to 270,000 miles (see Plate IV., Fig. 2), an elevation far

beyond any recorded for objects of the kind situated more than

sixty-five degrees from the solar equator, while this wonderful

structure spread from the seventieth to the eighty-first southern

parallel. Perceptibly filamentous, it seemed to grow by the elongation

of its component threads or ribands, and included within its vast bulk

probably the minimum conceivable quantity of matter. On the same plates,

a group of smaller but intensely active prominences registered

themselves at the solar antipodes.

[Illustration:

FIG. 12.—Prominence observed at the Haynald Observatory, 19th Sept.

1893.

The spectroscopic line-displacements due to motion are illustrated in

the upper sections of the figure.

]

An example of a transient apparition, difficult to classify, is given in

Fig. 12. It simulates a portentous conflagration. From bottom to summit

the red pillar of hydrogen measured 166,000 miles, according to Dr.

Fényi’s observation at 2^h 20^m, 19th September 1893. But it wholly

lacked any interior principle of stability. Opposite, and excessively

swift movements in line of sight of the base and shaft betrayed the

progress of destructive change. The catastrophe was not long delayed.

Within less than half an hour all was over. A rush upward set in at the

rate of 132 miles a second, carrying the frail edifice to a height of

224,000 miles. A minute and a half later, it had faded and dissolved,

“like the baseless fabric of a vision,” leaving just “a rack behind” in

the shape of an insignificant protuberance hedged in with faculæ.

“Throughout the course of its appearance,” Dr. Fényi wrote,[178] “the

entire object consisted simply of very bright luminous bands or strips

scattered one after another in ragged forms, and apparently lying nearly

at right angles to the limb of the sun. They were strikingly bright even

in the highest parts of the prominence. The form as a whole was also

like a band or stripe, which had no pronounced inclination, but stood

erect nearly in the direction of the sun’s radius.”

This amazing outburst was repeated with enhancements, nineteen hours

later, at a point on the sun’s limb almost diametrically opposite to its

predecessor. The height in this case attained was 300,000 miles, the

mean velocity of ascent being 214 miles a second, besides which, retreat

from the earth was indicated for the entire mass at the rate of close

upon 160 miles a second. A convulsion at least equally violent was

witnessed by the same observer on 24th December 1894,[179] when a

brilliant and wide-spreading, yet tolerably tranquil prominence, 56,000

miles high, suddenly began to mount, and attained in thirty-five minutes

the towering stature of 300,000 miles. Speedy disorganisation ensued.

Less than two hours after the explosion its scene was vacant. The body

affected by it had been shattered out of existence by its destructive

violence.

The Kalocsa observations were made with a visual spectroscope.

Simultaneous photographs would have been of especial value for

comparison with them, but leisure was not afforded for combining

methods. The three objects they referred to were palpably identical in

character. All showed precisely the same kind of structure. They were

made up of glowing vertical bands, collected into loose sheaves, or

scattered in outlying detachments. Each enormous aggregation, too, stood

erect during the tumultuous processes of expansion and collapse. They

were related to spot-groups, if at all, only by _diametrical

opposition_. The singular counterbalancing tendency of solar

disturbances is frequently conspicuous, and perhaps rarely or never

absent. Two gigantic, although ephemeral apparitions, thus coupled,

were, for instance, observed by Trouvelot, 26th June 1885;[180] and on

16th August of the same year he noted the apparent connection of a

prodigious chromospheric outburst with a spotted condition of the limb

180° distant. A relief of pressure may be concerned in these phenomena,

as in volcanic eruptions; if so, the lift, or diminution of gravity,

must act right across the solar globe, quickening convection-currents

and facilitating the antipodal delivery of extra supplies of heat. In

point of fact, the usual premonitory symptom of the explosive

development of prominences is a disengagement of light.[181] Exceptional

brilliancy is the forerunner of abnormal activity. No satisfactory

cause, however, can be assigned for the imagined relief of pressure,

since tidal influences are fairly out of the question. Only one

undeniable inference can be derived from the contrary symmetry of solar

commotions. It is that they are extremely deep-seated. They have their

roots in the hidden profundities of the great globe they agitate

exteriorly. They are not then mere local accidents; they make an

intimate part of the solar economy. The recognition of this

characteristic is interesting and important.

The tallest prominences are ordinarily the least coherent in structure.

An example is shown in Fig. 13, from a drawing made by M. Fényi, 3rd

October 1892. The object it portrays was almost unique in its fantastic

and colossal form. Overarching thirty degrees of the limb, it rose above

it to a height of nearly a quarter of a million of miles. Its

fragmentary composition was patent.[182] Under the eyes of the

delineating artist it was “in the act of being blown to shreds.”

Photographs happily secured at Chicago seven hours later, exhibited it

as then fallen to less than one-quarter of its high estate, while of

augmented lateral spread. These represented, of course, its calcium

aspect, the Kalocsa drawing its shape in red hydrogen; but there was no

evidence of the two vapours being differently distributed throughout

this bubble edifice. Facular patches marked, on the sensitive plates,

the points both of rise and re-descent of the materials constituting it.

Next morning only some insignificant wreckage strewed its place; virtual

annihilation had overtaken it.

[Illustration:

FIG. 13.—Prominence observed at the Haynald Observatory, 3rd October

1892. Height, 8′ 51″.

]

The tremendous velocities observed in prominences constitute a

formidable problem in solar physics. They not infrequently exceed the

critical rate of 383 miles per second; that is to say, they transcend

the limit of the sun’s gravitational controlling power, and, apart from

possible resistance by a medium, should carry the substances animated by

them finally away into space. The shapes and motions of prominences,

however, clearly indicate some retardative action,[183] although its

excessive feebleness, at least in coronal regions, can be inferred from

the unimpeded circulation of comets passing within a hundred thousand

miles of the sun’s surface. Hence it is only a surmise, not a certainty,

that chromospheric outbursts are attended by irrevocable loss of matter.

The swiftest so far recorded took place, 17th June 1891, in connection

with a spot just disappearing through rotation. A concentration of vivid

luminosity gave the signal that something unusual was impending, “and

after six P.M., Kalocsa mean time,” M. Fényi wrote,[184] “the point at

281° shone with so great a brilliancy that its reddish light seemed to

become white; an enormous displacement of the spectrum towards the blue,

at a medium height above the sun’s limb, indicated at the same time an

approach of the hydrogen in our direction with a prodigious velocity.”

The entire object, which was composed of glowing filaments, lay, in

fact, on the more refrangible side of the C-line, as the result of

approaching speed up to 550 miles a second. A vertical ascent at the

rate of 300 miles being meanwhile directly visible, a total velocity

(neglecting an uncertain third component) of about 680 miles a second

must have been attained in this amazing explosion. Trouvelot’s

observation a few hours earlier of a brilliant and peculiar facular

blaze at the place where it occurred[185] was significant of the intense

energy pent up in the spot-group, waiting release by the figurative

trigger-touch.

This prominence easily wins the prize for rapidity of movement; yet the

competition has been keen. Velocities of the same order, arising under

similar circumstances, have been frequently determined. They appear,

indeed, improbable, and have been stigmatised as “fabulous.” No actual

transport of matter, it is alleged, can be in question, but merely a

swift transference, through gases previously obscure, of a luminous

condition.[186] But this does not account for the conspicuous shiftings

of spectral lines, which could not ensue from the progress of

incandescence through stationary matter. They demand the strict

application of Doppler’s principle; real velocities must correspond to

them. Nor, even if there could be deception about the movements of

prominences _in_ the line of sight, is illusion possible about those

_across_ it. And both kinds are of the same order of speed; they are

complementary; they represent different aspects of identical

disturbances. This is not all. Prominences are often visibly twisted;

they are composed of spirally mounting flames; while to this helical

conformation correspond gyratory movements, at times disclosed by the

spectroscope. Here, at any rate, we have to do with bodily

transportations of matter; “luminescence” cannot be propagated

vortically. Besides, the measured speeds are as difficult to explain on

one hypothesis as on the other. Chemical action does not spread

instantaneously. Through the tenuous gases of the chromosphere, a

maximum rate of one mile a second might be assigned to its progress—a

rate, that is to say, some hundreds of times slower than the velocities

to be explained. That they are somehow of electrical production is a

safe assertion, likely to be true, if not in an immediate, then in a

remote sense. Yet we are little the wiser for the admission. The

“floating of an idea” in the mind does not constitute knowledge; and a

speculation is only valuable when it offers a definite starting-point

for practical research.

Total eclipses have ceased to be indispensable for the prosecution of

chromospheric studies. Day by day the red rim of the sun, with the

strange forms protruding from it, can be viewed spectroscopically; and

day by day the same objects vestured in violet can be photographed under

the broad shelter of the Fraunhofer K-line. Nevertheless, noontide

darkness, when it comes, brings very appreciable help. Differences are

noticeable between what can be seen in and out of eclipse. According to

the late Professor Tacchini,[187] the chromosphere always appears deeper

under cover of the interposing moon, because it is surmounted by a

pink-white margin, giving continuous light, and therefore

spectroscopically invisible in daylight. Some prominences are probably

of analogous composition. Only their skeleton-forms come out in the

crimson radiance of hydrogen; they are compacted and clothed with white

materials, the shining of which is effaced by the glare of common day. A

spectroscopic survey of the chromosphere and its appendages should hence

always be made immediately before and after every eclipse, for

comparison with the direct photographic records obtained during the

corresponding totalities. For the present the information acquired by

daylight work at the edge of the sun remains under a partial slur of

incompleteness.

The objects called “white prominences” belong indeed wholly to the

pageantry of eclipses. First noticed by Tacchini at Caroline Island, 6th

May 1883, they showed as lucid jets about a hundred thousand miles high,

with a surface like granulated silver. Attempts made, after the return

of daylight, to view them prismatically proved fruitless; they gave

forth no hydrogen or helium rays. Again at Grenada, 29th August 1886, a

gigantic helical structure, described by Mr. Maunder as “of the

intensest silver whiteness,”[188] towered three hundred thousand miles

above the limb of the moon. Its spectrum, photographed by W. H.

Pickering, included bright H and K, and Professor Hale accordingly

entertained the hope that such objects might come within the range of

his spectroheliograph; but so far no trace of them has been caught

outside total eclipses.

Dark chromospheric forms are not unknown. A “black protuberance,”

observed by Trouvelot at Meudon, 7th October 1892,[189] might have been

only a negative impression, like “black” flashes of lightning; yet it in

a measure falls into line with eclipse-records certainly not due to

illusion. In the great “anvil protuberance” disclosed 7th August 1869,

Dr. Lewis Swift saw “many black lines crossing in different directions,

and inasmuch,” he added, “as they must have been, at least, fifty

thousand miles long and a thousand miles broad, it would appear to be

important to understand the cause of this phenomenon, and (to ascertain)

if these markings are always present.”[190] Corroborative observations

were made by Alvan G. Clark and Professor George Davidson. With them may

be usefully compared M. Liais’s description of a black-edged but

colourless prominence watched at Paranagua, in Brazil, during the

eclipse of 7th September 1858.[191] The composite effect was not due to

contrast, since the obscure summit stood out alone when the bright lower

portion had disappeared behind the advancing moon. These singular

phenomena excited little comment, and lapsed into oblivion, so that

dusky ramifications, connecting and defining prominences in the

eclipse-photographs of 9th August 1896,[192] seemed entirely novel

features. They were explained by M. Hansky, a member of the Russian

party on the Amur, as outflows of hydrogen cooled by expansion;[193] but

inadequately. Hydrogen below the temperature of luminosity is

transparent, and the photographed veinings were densely opaque. They

afford a hint, that will certainly not be disregarded, of the workings

of unknown activities in connection with prominence-development.

CHAPTER X.

THE CHROMOSPHERIC SPECTRUM.

The spectrum of the chromosphere is almost purely discontinuous. It is

composed of detached bright lines. Some of these are always present, but

the majority come and go. We will consider first the permanent

radiations.

The chief of these are readily identified; they belong to hydrogen,

helium, and calcium. Thirty hydrogen lines have been seen or

photographed in dispersed chromospheric light, all members of the

original, or “Huggins series,” which attained nearly to its theoretical

limit on plates exposed by Mr. Evershed during the Indian eclipse of

1898. The fundamental C (Hα) is the brightest. To its intensity is due

the crimson glow of chromosphere and prominences; and prominences viewed

spectroscopically on this line appear larger than when imaged in any of

the other qualities of hydrogen-light. This is generally explained by

the influence of temperature, a higher degree of heat being needed to

give complete development to forms of shorter wave-length.[194] A

comparatively moderate chromospheric temperature would thus be

indicated. But there may be other influences in question. Professor J.

J. Thomson made the significant observation in 1895[195] that the red

and green lines of hydrogen show marked differences of intensity at the

two electrodes of vacuum-tubes, the red predominating on the positive,

the green on the negative side of a partition. The hint must indeed be

reserved for future use. We are not yet in a position to apply it

profitably.

The helium-spectrum of the chromosphere also gives rise to some

interesting considerations. For its various constituent series are not

represented indiscriminately at the edge of the sun. These series, we

may remind our readers, are six in number, and they are distributed with

beautiful precision into two corresponding triple systems. They may be

distinguished for convenience as “yellow” and “green,” D_{3} giving the

_tone_ to the former, several vivid green lines characterising the

latter set. In the laboratory, as we have seen, they are inseparable;

one set cannot, by any artifice so far devised, be procured apart from

the companion set. But it is otherwise in the sun. Yellow helium is

always present in the chromosphere and prominences; green helium only

about one-fourth as often as it is looked for. The permanent

chromospheric lines are four, namely, one far down in the red at λ 7065,

the familiar D_{3}, a deep blue ray at λ 4472 (formerly known as “_f_”),

and the ultra-violet “leader line” of the principal series at λ 3889.

These have probably many associates of still shorter wave-lengths; but

photographic data are too scanty as yet for purposes of discrimination

between the constant and the occasional elements of the spectrum. They

do not proceed, it must be borne clearly in mind, from two substances,

but from one indivisible form of matter, differently conditioned. In

what way, it is not easy to imagine. Laboratory experiments show that

the green set of lines gains relatively in brightness with rarefaction,

although the yellow set persists as well to the limit of practicable

exhaustion. This indication, however, does not open a way out of the

chromospheric difficulty. The gases near the sun are of inordinate

subtlety. Helium ought there, if this alone were the determining

quality, to be in the _green_ state. Its most fundamental emanation,

nevertheless, is D_{3}. Nor is its dominant position compromised in the

highest prominences. On the contrary, the green rays nearly always

proceed from lower lying, and therefore from denser strata than the

yellow.[196] Supplementary influences are then active—temperature, mode

of illumination, admixture of foreign materials. To this latter cause of

spectral modification, helium, we know, is abnormally sensitive.[197]

And it is quite possible that some of the series emitted by it may be

more liable to suppression than others, in which case the emergence of

the yellow without the green set would be an effect of damping, not of

density. It must, nevertheless, be admitted that what little relevant

experimental evidence there is, scarcely countenances this surmise. The

complex spectra derived by Professors Liveing and Dewar[198] from the

volatile residuum which survived the freezing-out of the main

constituents of atmospheric air, included rays taken impartially from

all the helium-series. No quantitative analysis of the contents of their

tubes was, however, possible; and the fact has been otherwise learned

that the helium-ingredient of a blend must be predominant to become

spectroscopically conspicuous. Every volume of hydrogen present in the

chromosphere (neglecting the effect of metallic vapours) should thus

probably be diluted with two volumes of helium; constituting the solar

appendage largely a helium envelope. This important piece of information

was brought within reach only by terrestrial observation of the new

element captured from clevite.

The significance of calcium in its chromospheric relations has only of

late been fully recognised. And this for an obvious reason. It is

represented by only one pair of lines—bright H and K—and these are so

near the limit of visibility that they could be effectively studied only

by the aid of photography. They were indeed registered as leading

features of the chromospheric spectrum by Professor Young in 1872; but

he was entirely incredulous as to their calcium origin, holding it

impossible that a substance with a vapour density forty times that of

hydrogen should mount to at least equal elevations above the sun’s

surface. Sir Norman Lockyer,[199] on the other hand, maintained them to

be characteristic of a subtle dissociation-product of calcium, alleging

in support of his view the progressive enfeeblement of the “blue line”

of calcium (λ 4227), concurrently with the enhancement of H and K, as

the substance was more and more completely decomposed at each addition

of intensity to the electric current transmitted through the vacuum

tubes. The case for dissociation appeared strong; it has, nevertheless,

broken down. Sir William and Lady Huggins in 1897 successfully reduced

calcium to the “two-line” condition by attenuation alone.[200] Thus the

enigmatical prominence-spectrum of calcium was at last artificially

produced, and no escape was left from the identification with true

metallic calcium of the form of matter encompassing the sun with violet

radiance.

A formidable problem, however, remained. Calcium near the sun seems to

possess a _levitating_ faculty altogether inexplicable. It floats as

high up as hydrogen, or even overtops it. H and K are the most diffusive

of all the prominence-rays; they are derived from the summits of the

tallest flames, and from every fibre of their texture. This anomalous

agility in a comparatively heavy metal must, according to the late

Professor Keeler,[201] be the index to some remarkable property

unrecognised by ordinary chemical methods. Unless, indeed, something

analogous to electrolytic action be in question. There is much to be

said in favour of M. Deslandres’s opinion that the chromosphere is

electrically luminous;[202] and if so, then “ions,” not molecules or

atoms, are presumably its constituting particles. But ions are on a

Lilliputian scale of magnitude, and they may be of nearly the same mass

for all the chemical elements. Here, however, we trench upon a region of

pure speculation. A region, nevertheless, that is likely ere long to be

annexed, in part at least, to surveyed territory, since pioneers are

actively engaged there. Much, in the interpretation of solar phenomena,

depends upon the results of their work; for here, as in every department

of astrophysics, the experimental decisions of terrestrial physics must

be awaited, not anticipated.

The three ingredients of the chromosphere so far spoken of—hydrogen,

helium, and calcium—are found as well in its eruptive outgrowths.

Gaseous prominences of all sorts and sizes are thus triply compounded.

But we have now to consider a form of matter permanently present in the

chromosphere, though rarely projected to any considerable altitude above

it. Its badge is a single green ray, which has a curious history. It was

momentarily identified with an auroral line; it was long erroneously

identified with the distinctive corona-line. It is apparently reversed

in the sun—that is to say, a Fraunhofer line falls just in its place.

This is Kirchhoff’s “1474” (known as 1474 K), which has proved, under

close scrutiny, to be triple. It results from absorption by iron, by

cobalt, and by an unrecognised substance. Now the chromospheric ray

agrees in position with the iron line, which is one of secondary

importance; yet it cannot at present be asserted confidently that it

really emanates from glowing iron-vapour. If it did, it should be

ordinarily associated with other iron-lines, and none have been

ascertained to make part of the fundamental chromospheric spectrum. The

vapour giving out “1474 K,” however, is never absent from the solar

envelope,[203] although it perhaps subsides at times into its lower

strata.[204] On the other hand, it occasionally rises in metallic

prominences to a height of about fifteen thousand miles. Mr. Lord, at

the Lick Observatory, observed the enigmatical line (λ 5316·8) as

shining vividly at the base of a violently disturbed prominence on 4th

August 1892,[205] and Mr. Evershed records similar experiences. Dr.

Fényi caught a still rarer effect on 19th February 1892 in a prominence

attending the great spot-group then visible.[206] The object was

peculiar, though not unique, in showing complete forms built up of the

various metallic and other substances injected into it from below. Fig.

14 reproduces an instructive drawing made on the spot. The bottom sketch

was taken on the C-line when the eruption was at its height. It reached

an elevation of 56,000 miles. The overarching of three filaments towards

a point at some distance from the base is noteworthy. The second drawing

in Fig. 14 is of a date twenty-four minutes later than the first. It

depicts the flame in “parhelium” light. The image viewed was constructed

on the _red_ ray of “green” helium. This was much smaller than the

simultaneous hydrogen-image, and that obtained on the “1474” line had

shrunken still further. But it was measured at half-past ten, an hour

after the drawing on “C” had been made, when the first vehemence of the

outburst had subsided. The line 1474 K is singularly exempt from

displacement effects through motion. It remains erect and undeviated in

the midst of solar storms. Nor does it widen perceptibly with increase

of pressure downward. Its invariable fineness contrasts remarkably with

the wedged shape near the photosphere of C, H, and K. Unusual agitation

is betokened by its emergence in prominences.

[Illustration:

FIG. 14.—Forms of a Prominence in Hydrogen, Helium, and

Pseudo-Coronium (Fényi).

]

Eclipse-spectrographs do not include it, while they have afforded some

other quite unexpected results. Of special consequence is Mr. Evershed’s

detection of titanium as an unfailing chromospheric element.[207] Plates

exposed by him in India, 22nd January 1898, proved to be crowded with

ultra-violet lines belonging to this metal. Some among them had indeed

been already recognised by Mr. Jewell in Professor Hale’s daylight

photographs of prominence-spectra.[208] The height to which they

extended indicates a diffusiveness for titanium-vapour equal to that of

hydrogen and helium, although inferior to that of calcium. Its atomic

weight on the hydrogen scale is 48; it is just as much lighter than iron

as it is heavier than calcium. But comparative vapour-densities are, so

to speak, impotent for the regulation of elemental distribution near the

sun. Another surprise afforded by the Indian eclipse was the conspicuous

presence in the chromosphere of scandium as well as of manganese and

chromium; while Mr. Hartley[209] has identified as another of its

constituents the rare metal gallium from two of its characteristic lines

(λ 4172 and λ 4033) recorded during the eclipse of 1893.

On 29th September 1897, Professor Hale, using a grating spectroscope on

the forty-inch Yerkes refractor, discerned the green carbon fluting

bright at the edge of the sun.[210] Two years later, he found the

corresponding yellow band, although the third in the blue remained

imperceptible. He inferred the permanent existence near the sun of a

shallow layer of carbon vapour.[211] Its rays do not come near the

surface; they have to be _dredged for_. Hence the extreme delicacy of

detective observations. The anomaly of this behaviour on the part of

carbon is glaring. Its specific lightness ought to carry it to altitudes

far beyond those attained by titanium and calcium. Yet it lies sunken

almost out of sight, while they float manifestly aloft. Some other

sorting-out influence besides that of gravity palpably comes into play

in the chromosphere and prominences. The possession by the sun of a

carbon-envelope, which, although thin visually, must really be at least

500, and may well be 1000 miles in depth, has an important bearing upon

the vexed question of photospheric constitution, and tends to strengthen

a barely indicated analogy between the sun and a peculiar class of red

stars.

The chromospheric spectrum, when its adventitious elements are reckoned

in, is highly complex. During a few weeks of 1872, Professor Young, by

vigilant watching, determined 273 lines seen, in the clear air of the

Rocky Mountains, to flash out intermittently, one by one, or in

companies together. And this first systematic enumeration was

subsequently greatly extended by its author, while crowds of

ultra-violet lines have been added by the photographic investigations of

Hale, Deslandres, Evershed, and others. For the most part the rays

brightened in eruptions are reversals of Fraunhofer lines, picked out

largely at haphazard, yet with an obvious preference, expressly noted by

Sir Norman Lockyer, for such as are vivified when the higher excitement

of the spark is substituted for the arc in laboratory experiments. This

has been taken to imply that the chromospheric is essentially a

high-temperature spectrum; but the facts may be differently interpreted.

Among the elements most apt to shine evanescently in metallic

prominences are sodium, magnesium, “green” helium, barium, iron,

vanadium, and strontium. Gigantic ragged forms, especially when they

appear in high latitudes, are of relatively simple chemical composition;

or it may be that their condition favours the visibility of only the

most persistent radiations. Small compact prominences yield, at any

rate, much richer harvests to gleaners of spectroscopic novelties. A

specimen of this class was observed by Father Sidgreaves, 10th September

1891.[212] It presented the aspect of “four blow-pipe jets intensely

bright at the bends,” was 16,000 miles high, and lasted only an hour.

Twenty-six brilliant lines were counted in the visual part of its

spectrum, the invisible part, in the absence of suitable appliances,

remaining unrecorded. The shape of this flame was equally well defined

in _both_ the red rays of helium, and appeared nearly the same in sodium

and magnesium. The spectral peculiarities of such objects, however, are

most marked in the ultra-violet. A photograph taken at the Kenwood

Observatory, 15th October 1892, registered, from an undistinguished

prominence, no less than seventy-four bright lines between the

wave-lengths 3970 and 3630,[213] most of them being of unknown origin.

But since then it has become possible to identify a dozen and upwards

with rays measured in the spectra of krypton and xenon by Professors

Liveing and Dewar;[214] and the circumstance seems to associate those

scarce atmospheric gases with helium as chromospheric constituents.

Occasionally, under circumstances not yet defined or understood,

prominences emit continuous light. The sheeny white objects sometimes

seen under cover of the moon’s shadow owe their peculiarity to this

cause. The whole gamut of prismatic radiance is derived from them, with

the addition of bright H and K and a suspicion of hydrogen lines. Thus

they are essentially calcium-forms interpenetrated with glowing _dust_.

Their light is probably original. If it were reflected, traces of

Fraunhofer-absorption which seem to be missing should be perceptible.

Distinctively “white” prominences are not common; none were observed

during the totality of 28th May 1900. But “red” prominences differ

considerably in colour-intensity, all the ruddy shades, from deep ruby

to pale pink, being represented in them. Many perhaps consist of a

crimson core veiled in almost colourless material. Certainly all are not

equally well seen in and out of eclipse. From a comparison of drawings

made during the totality of 1870 with his own simultaneous daylight

observations, Tacchini inferred that the spectroscope disclosed only the

cores of flame in such objects;[215] and the experience was repeated on

the occasion of the Egyptian eclipse of 17th May 1882. The four

prominences then measured were of a rosy tint, lightening towards the

margins, which looked as if edged with a lustrous fillet. “These

results,” Professor Hale writes, “may be accepted as establishing an

important difference between the spectroscopic and eclipse-images of

prominences.” Nevertheless the difference is not constantly present.

Some chromospheric forms are identical, viewed prismatically at the edge

of the sun, or directly in the dark beside the occulting moon. These

are, of course, purely gaseous; the others presumably give a mixture of

continuous and discontinuous light.

A very curious feature of the prominence-spectrum was ascertained by Mr.

Evershed during the eclipse of 1898.[216] It _becomes_ continuous in the

extreme ultra-violet. The range of unbroken radiance begins abruptly

just where the hydrogen series ends (near λ 3668), and extends to the

limit of the plate’s sensitiveness. Not even a guess can be hazarded at

the physical condition underlying this radiative vagary. A different

cause must be ascribed to certain local outbursts of white light in

eruptive prominences.[217] This symptom of disturbance has been

interpreted by Professor Hale as follows. “Objects of the kind,” he

says,[218] “are closely related to faculæ, and probably rise from them.

It thus occasionally happens that a violent eruption carries some of the

white-hot particles to a considerable distance above the photosphere. In

such a case the prominence gives a continuous spectrum in addition to

its bright lines.” The explanation may pass muster, since no better has

been offered.

M. Deslandres succeeded in showing, early in 1892,[219] that the sun

may, in a restricted sense, be designated a “bright line star”—that is

to say, he elicited from the aggregate of its light evidence of gaseous

emissions. Treating the sun as a star by admitting into his

spectrographic apparatus the whole of its rays simultaneously, he

obtained vivid reversals of the violet calcium lines. But this is only

possible when the calcium flames crowning faculæ are widely and strongly

developed. Ordinarily their emissions are drowned in the surging flood

of continuous radiance. But facular maxima recur, coincidently with

spot-maxima, once in about eleven years; so that the periodicity of the

sun might conceivably be determined by this one feature at distances

obliterative of all other signs of disturbance. Not that the sun viewed,

say, from Sirius, could with our actual appliances be detected, even at

culminating epochs of agitation, as a bright-line star. Some of his

fellow-stars, however, may be in a greatly enhanced stage of his

condition, and we may learn to follow their vicissitudes by

spectrographic observations of the alternate glimmering and fading of

fine rays projected upon the deep shadow of their calcium-absorption.

Thus the means may be afforded of ascertaining the flow of change in

remote and gigantic orbs; and we shall perhaps in the future be better

acquainted with the cyclical peculiarities of Capella or Arcturus than

with those of our own “particular star.”

CHAPTER XI.

THE CORONA.

The corona is exclusively an eclipse phenomenon. No sooner is totality

established than it is there. It seems to have emerged from nothingness,

to have arrived from nowhere. It starts into view with the abruptness,

the inexplicableness, of an apparition. “The sun,” Professor Langley

says, recording his impressions of the eclipse of 1869, “went out as

suddenly as a blown-out gas-jet, and I became as suddenly aware that all

around where it had been, there had been growing into vision a kind of

ghostly radiance, composed of separate pearly beams, looking distinct

each from each, as though the black circle where the sun once was

bristled with pale streamers stretching far away from it in a sort of

crown.”[220]

The corona presents various aspects, but it may always be described as

composed of extended streamers springing from a much more intensely

luminous ring, the so-called “inner corona.” There is no real

separation; the entire appendage is evidently framed on the same

constructive principle; yet the distinction is obvious visually, and

convenient descriptively. “I do not know,” Mr. Francis Galton wrote of

the corona visible 18th July 1860, “to what I can justly compare that

magnificent meteor. It differed from other objects in the remarkable

whiteness and purity of its light, and also in the definition of its

shape as combined with a peculiar tenderness of outline.”[221] Both he

and Winnecke noticed the curvature of some “long arms of light”

protruding from the ring, while other rays took “a more or less

tangential direction.” Mädler was struck with the _determinateness_ of

the formation. What he saw was no vague light-effusion, but a congeries

of sharply terminated beamy sheaves. This is a radical characteristic.

The solar corona is a texture of significant pattern. There is indeed

much difficulty in laying bare the original design. A spherical

agglomeration projected on a plane gives rise to intricate effects of

perspective, from which the true relations in solid space of the objects

originating them can be deduced only by careful and systematic

interpretation on strict geometrical principles. The problem was

attacked by Professor Schaeberle[222] with the help of an ingeniously

contrived model, photographs of which showed divergent rods inserted

over the surface of a globe as apparently intercrossing and interlacing

in the flat picture imprinted on the plates. It is, however, well to

remember that, while curved rays may be projected so as to seem

straight, straight rays can never appear curved. Beams that show flexure

are inflected. Nor can a semblance of double curvature be given by

perspective to those bent simply in one direction; moreover, rays that

are actually normal to the sun’s surface must, from all points of view,

appear radial to the limb.[223]

Coronal structure is of immense variety. It is intrinsically of a

radiated character, and the fact is of primary importance. There are no

signs of concentric arrangement;[224] coronal materials do not form

shells or envelopes, such as surround the heads of comets; they are, on

the contrary, drawn out into fibres by forces acting upon them in minute

detail. Comparisons to spun glass and to silken filaments indicate the

delicate nature of the shining tissue spread round the obscured sun.

There are indeed differences. The fibres sometimes, as in the corona of

22nd December 1870, look to have been “combed out,” but more or less of

derangement is usually prevalent. Tangled hanks of thread are often

suggested, or “masses of luminous hair in disorder.”[225] These

contorted forms, although their complexity is doubtless augmented by the

superposition of sundry groups of twisted rays presented at different

angles, afford remarkable evidence of disturbance within the corona

itself. Their photographic registration dates from the Sumatra eclipse

of 18th May 1901. On plates there taken with the Lick forty-foot

telescope, and by Mrs. Maunder at Mauritius, a tumultuous area, agitated

as if by the effects of an explosion, was strikingly depicted. “A long

thread-like prominence appeared,” it was stated, “to emanate from the

same source.”[226]

Fleecy coronal tracts are at times intermixed with regions of striation.

The corona of 1868 was perceived at Masulipatam to be “slightly mottled”

near the sun, and the “curdled” aspect of the great nebula in Orion has

often been recalled to telescopic observers. Sir Norman Lockyer, at

Baikul, 12th December 1871, was struck with an “exquisite” and

“strongly-developed structure.” “I at once,” he continued, “exclaimed

‘Like Orion!’ Thousands of interlacing filaments varying in intensity

were visible; in fact, I saw an extension of the prominence-structure in

cooler material.”[227] It may be remarked that nebular tufts, no less

than prominence-jets, are resolvable into fibres under the best

conditions of seeing. The aureola of 1871 was of such incomparable

beauty that M. Janssen could scarcely rouse himself from its delighted

contemplation to carry out his programme of work. Numerous coral red

prominences were relieved against the “velvet white” of the corona, the

exterior shape of which was rudely quadrilateral. Interiorly the

streamers leaned together in pairs so as to imitate flower petals, the

general effect resembling that of a gigantic lucid dahlia, with the

black moon for its heart.

Again and again, in descriptions of successive coronas, the

Orion-similitude recurs. In the “density, brightness, and species of its

light,” that of 1st January 1889 strongly reminded a Nevada State

observer of the nebula, and its slightly greenish tinge of colour

completed the likeness.[228] Again, in examining the coronal photographs

of 9th August 1896, Mr. W. H. Wesley detected an area “broken up by dark

channels into flocculent-looking masses, giving to it somewhat of the

_curdled_ appearance of some parts of the nebula in Orion.”[229] The

similarity is not merely superficial. Laborious photographic comparisons

by Mr. Ranyard (assisted by Mr. Wesley) emphasised the organic analogy

between the great nebula and the solar corona.[230] Synclinal forms (as

the petal-shaped structures are called) emerge in both, and the

branching effusions round the trapezium seem to mimic details legible in

many eclipse-pictures.

A chain of “pearly cones” furrowed spirally, 200,000 to 300,000 miles in

height, and rising above a long bank of red prominences, were perceived

by Professor Cleveland Abbe in the corona of 1869. And in 1893 the sun

appears to have been fringed in this manner nearly all round, the

individual peaks being projected together into such close array as to be

in many places undiscernable apart. “Systems of (approximately)

concentric arches” were also distinguished by Professor Schaeberle in

his large-scale photographs of the same eclipse.[231]

Inverted shapes are also, though less commonly, met with. Paraboloids,

convex towards the limb, now and then replace arcs and cones based upon

it. A curious instance was afforded by Schaeberle’s “coronal comet” of

16th April 1893.[232] This object seemed as if _spitted_ upon a slender,

solitary, nearly radial streamer, from which it had evidently developed.

It was not the only specimen of its class. A well-known drawing by Liais

of the corona of 7th September 1858 shows an immense double paraboloid

lying behind and partly hidden by a “dahlia petal.” The vertex seemed to

be just at the limb. A somewhat similar dusky arc was seen by Winnecke

in the corona of 1860.[233] It looked, he said, like a tracing in sepia.

Again, during the eclipse of 1868, a bright parabolic outline, “with its

vertex towards the sun,” was noted by Weiss. Finally, Homer Lane

observed at Des Moines, Iowa, 7th August 1869, two condensations of

light which “might well be compared to small telescopic comets, with

tails of some length, but without a head, and with no distinct

indication of a head at one end rather than the other. They were not far

from radial in direction relatively to the sun’s centre, and had their

origin above the limb of the moon.”[234] He estimated the length of each

at about 130″. These appearances are full of meaning. They plainly

assert the subjection of coronal matter to a dual repulsion, such as

acts upon the “crystal tresses” of comets. A local centre of

condensation throws off a filmy envelope, the constituent particles of

which, as they approach the sun, are swept backward into a train by a

counter influence proceeding from him. The only genuine “eclipse comet”

so far captured was that seen and photographed at Sohag, 17th May 1882.

It was sharply characterised as such, the effects of swift motion being

unmistakably impressed upon its curved plumage.

An eclipse, visible in the Western States of North America, 29th July

1878, disclosed a surprising spectacle. In lieu of the ordinary radiated

corona there were seen “bristles” of light at the sun’s poles, enormous

“wings” at each side of the equator. Professor Langley observed the

phenomenon from the summit of Pike’s Peak in Colorado, at an elevation

of 14,000 feet in a stainless sky. Thus favourably circumstanced, he was

able to trace one wide beam to a distance of about five millions of

miles from the sun, the other fully twice as far.[235] The direction in

which they lay proved, when carefully measured, to agree closely with

that of the zodiacal light, and “a faint central rib” emphasised the

coincidence. “With the telescope,” he says, “the whole of the bright

inner light close to the sun was found to be made up of filaments, more

definite even than those seen in sun-spots,” and apparently exempt from

the effects of spherical projection; they “fringed the sun’s edge in

definite outline, as though it were really but a disc.”

At the time of this eclipse, the sun was in a state of exceptional

tranquillity, and a search through the solar archives brought out the

notable fact that a similar apparition had, eleven years previously,

spots then too being nearly extinct, been described and depicted by

Grosch of Santiago. He inferred from it the possession by the sun of

“strong magnetic polarity.” And indeed the divergent light-fibres at the

poles, in 1867 no less than in 1878, seemed to trace precisely the lines

of force in a magnetic field. The concurrence of these phenomena with

critical epochs in the sun’s activity started the idea, due, in the

first instance, to Mr. Ranyard,[236] of varying coronal types. It was

amply borne out by subsequent experience. From eclipse to eclipse,

throughout the eleven-year cycle, the corona exhibits changes of form in

marked conformity to spot-vicissitudes. In the accompanying plate, the

original of which is by M. Hansky, the coronas of 1860, 1870, 1883, and

1893, all of pronounced maximum type, are represented in the first

column; those of 1867, 1878, and 1889 in the third. The second and

fourth show coronas of intermediate forms. The last figure in the third

column has a prophetic character. It shows M. Hansky’s anticipation of

the kind of halo due, on the theory of recurring types, in 1900. What

was actually photographed is given for comparison in Plate VI., Fig. 1.

The correspondence leaps to the eye. A definite law of variation indeed

quite obviously regulates the shape of the effluence about the sun. At

spot-maxima its component streamers issue from all latitudes

indiscriminately; they pay little or no regard to heliographic

co-ordinates. Then, as disturbance relaxes, they gradually draw away

from the poles, and tend to form “synclinals” above the spot-zones,

giving to the whole appendage the “form of a four-rayed star, the points

of which are inclined 45° to the sun’s axis.”[237] In the polar regions

abandoned by them, “magnetic” filaments meanwhile become noticeable, and

what may be called the intermediate type is constituted. It is subject,

however, to indefinite variations of detail. A good example is shown in

Plate VII., Fig. 1, from photographs taken on the Amur, 9th August 1896,

by a Russian party under the leadership of M. Bélopolsky.

One ray, it will be noticed, is greatly longer than the others, and the

same peculiarity distinguished the corona of 1898. Only when the tide of

solar agitation is dead out, is the finished type of minimum aureola

realised. We have then a symmetrical arrangement of crested poles and

equatorial extensions, with this one singular qualification to its

symmetry, that the wings are not a pair. One is formed of convergent,

the other of parallel, or even divergent rays. And they seem to be

reversed east and west at alternate epochs. Both are radically double;

they are formed by the closing down upon the equator, as spot-activity

becomes exhausted, of the synclinal groups previously visible in middle

latitudes. It is difficult to realise that these “wings” are merely the

profile-shapes of a vast luminous disc completely encompassing the sun.

Hence an extreme intricacy of structural details most baffling to

efforts towards interpretation.

[Illustration:

PLATE V.

Types of the Corona 1860–96, with an Anticipatory Sketch of the Corona

of 1900. Drawn by M. Hansky.

(From the _Observatory_, February 1898.)

]

Coronal modifications are not so entirely isolated as might at first

sight appear. Looked at more closely, they are perceived to correspond

unmistakably with the cyclical changes in distribution of surface

disturbances. This was insisted upon by M. Bélopolsky in 1897.[238]

Spots descend into lower latitudes with the approach of each minimum.

One after the other, the eleven-year waves of commotion attain their

acme in medium zones, and die out near the equator. Coronal development

pursues the same course. Its most intimate relations, however, are with

chromospheric eruptions. Tacchini[239] was the first to notice that

coronal outflows emanate from regions frequented by prominences, which

at times of maximum spread all over the sun, but near minimum withdraw

from the extensive polar tracts simultaneously denuded of far-spreading

streamers. Particular agreement frequently accentuates this general

correspondence. Thus the springing of a coronal arch has usually a

prominence for its motive. Each pearly pavilion is erected over a red

flame. Coincidences of the kind are of perpetual occurrence.

Chromospheric jets seemed (and doubtless were) appropriated individually

to the “striated cones” observed by Cleveland Abbe in 1869. In

Schaeberle’s fine photographs of the eclipse visible in South America

16th April 1893, one-sixth of the sun’s circumference came out clear of

prominences; and just over the same segment there is a gap in the

elsewhere unbroken range of coronal arches.[240] In some cases arches

are buttressed upon prominences; in others they are symmetrical as

regards them; coronal streamers appear to be vaulted into domes, or bent

together into ogives, through effects of eruptive action in the

chromosphere. These relations were especially marked in the corona of

1896. The wide polar rifts were devoid of prominences, but a prominence

lay at the root of each great streamer, and a prominence was enclosed by

each synclinal structure. These M. Hansky inferred, from their interior

darkness, to be hollow,[241] like the tails of certain comets; and he

noticed curious effects of coronal transparency, a few beams showing

traceably through the substance of those in front of them. The

correlation of prominences with coronal forms was somewhat less

conspicuous in 1898 than in 1896, and was barely perceptible in 1900.

The Lick photographs of 1901, however, showed the envelopment of a

prominence by a “series of coronal hoods,” besides other symptoms of

community in disturbance between the chromosphere and the silvery

aureola.

The more closely the spectrum of the corona is studied, the more

interesting and enigmatical it becomes. It has a triple origin.

Continuous reflected light is mixed up in it with continuous original

light, and these again with bright-line emissions. The three elements

are not easily separated and the proportions of them present vary from

time to time. The gaseous spectrum is feeble, especially near spot

minima. Its leading constituent is a green line at λ 5303, long

confounded with the chromospheric λ 5317. Their disconnection first

became apparent in Mr. Fowler’s eclipse photographs of 22nd January

1898, and was announced by Sir Norman Lockyer[242] as one result of

their examination. The green line characterises the unknown substance

designated “coronium,” the distribution of which round the sun can be

investigated by its means. Photographs taken during totalities with the

“prismatic camera” give separate images of the solar appendages in each

quality of discontinuous light emitted by them, and the “green” coronal

image proves to be approximately ring-shaped. The gas it is derived from

seems to spread through the “inner corona” to an average height of from

160,000 to 200,000 miles, but not to extend into the sheaves and

streamers beyond. There are indeed irregularities. The annulus of

coronium is far from being uniform or homogeneous. It is wider, perhaps

also more condensed, in some places than in others, and spectrograms

taken with a slit by Professor Campbell at Jeur suggested, by the

distortions of the characteristic ray impressed upon them, the progress

of radial movements, such as might well be deemed inevitable in an

aerial envelope obviously not in a state of equilibrium.[243]

Ten or a dozen coronal lines besides the green ray have been

photographed. The wave-lengths of the most authentically recorded are as

follows: 4232, 4086, 3987, 3801, 3643, 3456, 3388, and 3381. No success

has hitherto attended efforts to arrange them in a series; nor is it by

any means certain that all claim the same chemical origin. On the

contrary, the fundamental green line, together with the most refrangible

of those above enumerated, appears capable of segregation from the

violet ray λ 3987 and the first three of its ultra-visible

associates.[244] Two strange gases then, at any rate, are indicated as

co-existing in the corona. And they are unmixed with any familiar

substance. Evidence collected during recent eclipses testifies strongly

to the absence of all the chromospheric materials. Not even the

pervasive trio, hydrogen, helium, and calcium, extend into the vast

solar halo. Some of their radiations, notably H and K, have _seemed_ to

be derived from it, but only through the effects of atmospheric

scattering. They come as well from the black disc of the moon. Probably

only some peculiar forms of matter, or forms of matter in a peculiar

state, constitute the gaseous corona. So far coronium has not been

recognised elsewhere.

The continuous light from the interior halo is mainly original. It

emanates from incandescent solid or liquid particles. But their

incandescence appears to be of an unusual kind. Bolometric experiments,

carried out by Messrs. Abbot and Mendenhall of the Smithsonian

Observatory during the eclipse of 1900,[245] showed the coronal beams to

be almost wholly wanting in thermal power. Compared with them moonlight

is a potent source of heat. They include, according to the results in

question, next to no infra-red waves, and are surmised to be comparable

in quality to the glow of phosphorescent or luminescent substances.

Novel inquiries in the laboratory will be needed to ratify these

significant conjectures; while it is to be hoped that the eclipse of

1904 will afford some positive data as to the distribution of energy in

the coronal spectrum, which may serve as a basis both for photographic

investigations and for theoretical conclusions.

Photospheric light _must_ be reflected from the pulverulent materials of

the corona, and a reflected ingredient is, accordingly, contained in its

radiance. It is, however, small in quantity. Thus the dark Fraunhofer

lines are barely distinguishable in it. They were detected by Janssen in

1871; fifteen were photographed by Pluvinel at Senegal in 1893.[246] On

the same occasion, nevertheless, Deslandres could obtain no trace of

them, and Campbell was equally unsuccessful in 1898.[247] The truth is

that where the original emissions are strong they get _drowned out_.

They show, and that with difficulty, only in the less luminous sections

of the appendage. This was made perfectly obvious by Mr. Perrine’s

discussion of the plates taken in Sumatra, upon which thirty-five

Fraunhofer-lines impressed themselves, but only in regions remote from

the limb. Polarisation-effects give accordant testimony.[248] They are

slight but unmistakable, and plainly indicate the action of scattering

particles right up to the limb. Some hints as to the manner of their

distribution were obtained by Professor Turner in 1898, and again in

1900, through the ingenious device of photographing the corona across a

plate of Iceland spar, and so obtaining two pictures in oppositely

polarised light.[249]

Much remains to be learned about the nature of coronal radiance, and the

opportunities for its investigation are restricted and unsatisfactory.

Yet upon their outcome success in coronal portraiture largely depends.

Eclipse-photography is an art in itself, and one beset by subtle

difficulties. To ensure the best results, the plates employed should

have a curve of sensitiveness as nearly as possible coincident with the

energy-curve of the coronal radiations, and the form of the latter is

entirely unknown. Many questions too arise regarding the quality of the

plates, and the development proper to be given them, regarding the best

kind of instrument for exposing them with, and, above all, the length of

time that should be allotted to the process. And here the obstacle is

encountered that no single exposure is suitable to the entire aureola.

One long enough to bring out the streamers is too long for the delicate

details of the brilliant interior. The choice has to be made between

solarisation and incompleteness of representation. Two remedies have

been tried. The first is by piecemeal delineation. From photographs of

varied exposures, a picture showing the special features rendered by

each is compounded by a skilled draughtsman. But it has no longer an

autographic value; the forms embodied in it have been deliberately

selected and unconsciously emphasised. “To a certain extent,” it has

been well said,[250] “the same personality enters into the examination

of a photograph that is known to exist in naked-eye observations of the

corona.” M. Morin’s drawing of the corona of 1896 (see Plate VII. Fig.

1) is a fine example of what can be done by combining multiplied

photographic impressions. The two best of those availed of were taken

with an ordinary camera in one and two seconds respectively; the rest

with a photoheliograph, getting exposures up to thirty seconds.[251]

The alternative method is purely automatic. It was invented and

successfully applied by Professor C. Burckhalter of the Chabot

Observatory, California, at the eclipse of 28th May 1900.[252] In his

apparatus a system of revolving diaphragms is arranged so as to give

exposures graduated to correspond with distance from the sun. The image

being progressively covered at successive short intervals, time is

allowed for the imprinting of faint coronal extensions, while the bright

parts already effectively portrayed are shielded against further

prejudicial action. The photograph “controlled” in this way was a

striking record, and conveyed some curious intimations of perspective

effects in the mutual overlaying of tufts and beams.

The outlying branches of the corona usually baffle the perception of the

sensitive plate; for they merge into a dimly illuminated sky-ground,

from which they are, unless by special precautions, chemically

indistinguishable. The most conspicuous performance in this direction

was by Mrs. Maunder in 1898.[253] With a lens of only 1½ inches in

aperture, giving small but intensely bright images, exposures were

experimentally made of inordinate length, proportionately to the

shortness of the focus. The unprecedented result was achieved of

photographing rays to a distance of nearly seven solar diameters from

the limb. Mr. Wesley’s drawing from the original negatives is reproduced

in Plate VII. Fig. 2. Four long narrow divergent rays are visible in it,

each starting outward from a synclinal structure. Mr. and Mrs. Maunder

hold it probable that such structures have always rod-like extensions,

needing only protracted photographic exposures to bring them into

view;[254] but this is more than doubtful.

[Illustration:

FIG. 15.—Diagram of Markings in the Corona of 1871. Drawn by W. H.

Wesley (_Knowledge_, vol. xxiii. p. 225).

]

[Illustration:

FIG. 16.—Diagram of Markings in the Corona of 1896. Drawn by W. H.

Wesley (_Knowledge_, vol. xxiii. p. 226).

]

A very perplexing appearance is that of dark markings in the corona.

They are not mere interspaces between brilliant rays. Mr. Wesley, who is

an expert in the scrutiny and interpretation of celestial photographs,

vouches for their reality. Figs. 15 and 16 copy his diagrams of obscure

streaks and veinings in the coronas of 1871 and 1896. In the first case,

they cut right across the lustrous branches of the halo; in the second,

they are in obvious connection with prominences. Indeed, black coronal

and black chromospheric forms belong undeniably to the same order of

effect, and cannot be separated causally. They took another shape in the

corona of 1900. Mr. Wesley’s beautiful drawing from Mr. Maunder’s

photographs (see Plate VIII.) shows rifts apparently darker than the

general background of the sky, and hence of a _positive_ character.

Their substantial presence was confirmed[255] by negatives taken at

Wadesborough, U.S.A., by Miss Gertrude Bacon; but the difficulty of

accounting for them is at present insurmountable. That they are due to

the interposition of opaque bodies can scarcely be admitted. The

objections are prohibitive. Yet the phenomenon is none the less genuine

for being incomprehensible. We must wait and compare.

[Illustration:

PLATE VII.

1. The Corona of 1896. Drawn by M. Morin from Photographs taken on the

Amur.

2. The Corona of 1898. Drawn by W. H. Wesley from Photographs taken by

Mrs. Maunder. (_Knowledge_, vol. xxi. p. 108.)

_N.B._—The sun’s axis meant to be indicated is a _vertical_ line

bisecting the disc, with which the north-and-south line makes a

small angle.

]

The embarrassments attending coronal photography are enormously enhanced

by the effulgence of daylight. Success here is more earnestly desired

the less it can be hopefully anticipated; for the prospect is dim of

realising Sir William Huggins’s scheme of 1882, or any modification of

it. It is true that, during the partial phases of the last couple of

eclipses, sensitive plates were impressed by the inner corona, but it

only showed as a vague glow throwing into relief the small segment of

the moon outside the sun some forty seconds before and after

totality.[256] Still, even this scanty measure of success was welcome as

an earnest of what the incalculable future might bring. Everything

depends upon catching differential effects—upon obtaining plates capable

of _feeling_ the delicate gradation between daylight pure and simple and

daylight plus corona. And this would be greatly facilitated by

acquaintance with the law of intensity in the coronal spectrum. It ought

to be stronger in the upper reaches than the ordinary solar spectrum,

since the corona escapes the heavy toll of blue absorption exacted from

the photosphere by the “smoke-veil”; the question is, can this

presumable superiority be rendered predominant enough for the ends of

portraiture? The use of coloured screens, letting through the more

refrangible rays, and barring out those lower ones in which mere glare

has the advantage, has proved ineffectual; and the “double-slit method,”

so splendidly helpful in other departments of solar physics, has also

been tried in vain.[257] In coronal photography, no bright line can

serve the end in view because the gaseous spectrum belongs only to the

inner corona, and the record aspired after is more especially of the

outer corona, with its characteristic plumes, streamers, and aigrettes.

Until the changes these undergo can be followed day by day, little will

be satisfactorily known of their intimate relations with the different

orders of solar phenomena, and still less of the underlying cause of

variation.

It has yet to be determined whether the corona rotates with the sun.

Opposite motion-displacements above the east and west limbs, of the

green line or one of its companions, would settle the point; but they

show, if at all, most evasively. M. Deslandres first attempted such

measurements at Fundium in West Africa, 16th April 1893.[258] He chose,

however, as the object of his attack the K-line of calcium, since

ascertained to be non-coronal, so that his results were null and void.

Yet they marked a starting-point, for they sufficed to introduce the

research definitively into the eclipse-programme; nor will it be dropped

out of it, we may hope, until a substantial increase of knowledge has

been gained. Subsequent experiments, although legitimately conducted,

have been indecisive. Mr. Newall, in those tried by him at Pulgaon, 22nd

January 1898, went too far afield for their materials.[259] He directed

his spectroscope to points 8′—corresponding to upwards of 200,000

miles—from each limb, whence no bright line could be obtained, since

they lay outside the limits of the gaseous corona. Professor

Campbell,[260] on the other hand, found in spectrograms taken east and

west of the sun during the same eclipse, a difference in position of the

green ray giving an ostensible radial velocity of 3·1 kilometers per

second, suggesting rotation at half that speed; but he regarded its

genuineness as open to grave doubt.

[Illustration:

PLATE VIII.

The Corona of 1900. Drawn by W. H. Wesley from Photographs taken by E.

W. Maunder. (_Knowledge_, vol. xxiii. p. 227.)

]

More hopeful than the method of simple displacements is perhaps the

method of _inclinations_ recommended by Deslandres.[261] A small,

intensely luminous image of the corona being thrown upon the slit of a

powerful spectroscope, the varied deviations of the bright lines derived

from its different parts should tell something as to the mode of motion

prevalent throughout the appendage. If it rotates like a solid body, all

in one piece, the velocity increases outward, and the lines would lie

aslant on the plate in corresponding directions. If, however, the

regimen be analogous to that governing Saturn’s ring-system, in which

every component particle revolves as an independent satellite, then the

speed of coronal matter slackens with increase of distance from the sun,

and the spectral rays emitted by it should be deflected the opposite

way. The criterion, if it prove applicable, will be highly

discriminative as regards rival hypotheses.

These may be classified as ejective, meteoric, and electro-magnetic.

Professor Schaeberle analyses the phenomenon into “streams of matter

ejected from the lower latitudes of the sun.”[262] To the materials of

the longest rays he ascribes initial velocities up to 400 miles a

second, and excursions outward to the remote neighbourhood of Jupiter or

even Saturn; ogives and arches being due to eruptions of less violence;

while the interplay of innumerable curving jets, foreshortened in all

possible ways, explains the complex aspect of the sun’s lucent crown.

Mr. and Mrs. Maunder, again, consider the prominences to “represent

centres of strong eruptive action, and that in consequence of such

action coronal matter is driven upward from the sun over a very wide

area in domelike forms.”[263] A succession of arches results, “the outer

being less definite and complete than the inner ones. Outside all we

find the curves defining the boundaries of the synclinal group.”

There is much plausibility in these inferences. Prominences are visibly

spouted or flung upward, and the lustrous filagree vaults often rising

above them can scarcely have a totally dissimilar origin. But the whole

secret is not thus laid bare. The rationale by eruptive action is no

sooner sought to be made exclusive than it meets contradictory facts. It

offers no admissible explanation of varying coronal types; it ignores

the mysterious coronium envelope; it seems to be negatived by the

bolometric observations of Abbot in 1900; for obviously the light of the

supposed bombs, projectiles, or pulverulent ejected streams should

contain the ordinary solar proportion of heat-rays, which are

nevertheless markedly deficient.

The corona, according to Sir William Huggins, must consist of

“incandescent fog.” And Professor Newcomb,[264] following a similar

train of ideas, avers it to be made up of detached particles, wholly or

imperfectly vaporised. They might be most sparsely distributed. Intense

radiance would, he informs his readers, result from the occurrence of a

single fragment of dust in every cubic mile of space about the sun. But

how is the dust (if dust there be) supplied? Does it come from within or

from without? Here the upholders of the meteoric theory join issue with

the eruptionists. There must be rings and streams of meteors revolving

quite close to the sun in orbits of all possible inclinations and

considerably varied eccentricities. These furnish, we are told, the

materials of the corona, which—as Dr. Scheiner has recently

shown[265]—are raised to a temperature of incandescence by direct solar

radiation. This view, nevertheless, like the eruption-hypothesis, is

scarcely tenable in view of the non-thermal quality of coronal light.

There remains the electrical theory. Formally enounced by Sir William

Huggins in 1885,[266] it remains unverified indeed, yet unrefuted.

Coronal streamers are regarded by it as analogous to comets’ tails; they

issue forth under the influence of a repulsive force emanating from the

sun; they are illuminated by electrical discharges due probably to

differences of potential at their bases and extremities.[267] The

magnetic relations of the phenomenon, vividly indicated by the minimum

forms exemplified at the two eclipses of 1889, were ably discussed by

Professor Bigelow.[268] He analysed coronal structure by spherical

harmonics, on the supposition of its dependence upon some mode of action

similar to that of free electricity, “the rays being lines of force, and

the coronal matter being discharged from the body of the sun, or

arranged and controlled” by a power proceeding from it. He further

showed that the power was of a repulsive nature and varied inversely as

the square of the distance;[269] but he avoided speaking of it as

“electrical” out of “deference to the doubt that free electricity can

exist at such high temperatures as prevail on the sun’s surface,”

content to have proved “that some force is present acting on the corona

according to the laws of electric potential.”

The diagrammatic halo laid down on these principles was indeed the very

“twin Dromio” of the corona of 1889; but they stood ill the test of

prediction. The corona of 1893 failed to exhibit the special features

anticipated for it by Professor Bigelow. Plainly the assertion that an

illuminated magnetic field surrounds the sun, although scarcely

deniable, does not comprise the whole truth. The same may be said of

Hermann Ebert’s “electro-magnetic theory.”[270] He defines the corona as

“the visible reaction of the finely-divided matter in the vicinity of

the sun upon the dielectric polarisation proceeding from the different

parts of the sun.” Luminosity is evoked by Hertzian oscillations

propagated outward with the velocity of light, and its filamentous

texture corresponds to differences in dielectric stress connected with

the distribution of electricity on the sun’s surface.

The auroral aspect of the corona has often been commented upon. M. Ebert

remarks[271] that the magnetic lines of force near the earth are not

more definitely traced out by the play of polar lights than are those

about the sun by the disposition of coronal rays. The truth of this,

however, is patent only as regards aureolas of the minimum type. In

those visible at maximum it is at any rate disguised. For, as the sun’s

internal activity augments, beamy outflows predominate over tufted

effluences, although both kinds of radiance may be simultaneously

present. Professor Holden considered the sun to be _hairy_ all over, and

not merely at the poles. “There is no latitude,” he wrote, “at which we

can say that here the polar rays end and a new species—equatorial

rays—begins.”[272] This was also recognised by M. Hansky in his study of

the corona of 1896;[273] but the superposed streamers and arches can

with difficulty be included in any magnetic theory. On the other hand,

Mr. Pupin of Columbia College obtained in 1892 striking imitations of

the maximum type of corona by means of electrical discharges through

partially exhausted bulbs.[274] Effects of polarity being, however,

wholly absent, the reproduction failed to convey one fundamental

characteristic of the real phenomenon. But the deficiency was supplied,

three years later, by Ebert’s experiment of subjecting “coronoidal”

tubes to the action of a powerful magnetic field. The organising effect

upon the light-effluence was just what was needed, according to

Bigelow’s contention, to give nature’s own imprimatur to his “magnetic

theory of the solar corona.”[275]

A fact of high import in this connection is that the coronal bright

lines are not reversed in the Fraunhofer spectrum. Coronium, and the

gases associated with it, exercise no perceptible absorption upon the

light transmitted through them. Now this kind of inertness, according to

M. Cantor’s experiments,[276] is distinctive of substances glowing by

electrical stimulation, so that we have here—as the late Professor

Fitzgerald pointed out—a confirmation, absolutely _sui generis_, of the

conjecture that coronal emissions are analogous to those of an

illuminated vacuum-tube.

Yet none of the views propounded on the subject are completely

satisfactory. They have points of contact with truth, but they do not

closely embrace it. This, indeed, could hardly be expected at so

comparatively early a stage of coronal research. For the questions

involved are beyond measure baffling and intricate. We may re-enumerate

them.

One that is fundamental relates to coronal heat. Its virtual absence,

attested bolometrically at Wadesborough in 1900, must be confirmed

during future eclipses before Deslandres’s contrary inference[277] as to

the copious presence of long waves in coronal light can be finally

dismissed.

The coronium-envelope offers a problem which stands almost apart from

that of the stellate appendage surrounding it. And it is one that can

scarcely yet be grappled with. No familiar substance enters into its

composition. Its ingredients are altogether exotic. They do not diffuse

into the chromosphere, while those of the chromosphere are as strictly

excluded from the corona. The cause of this extraordinary circumstance

will perhaps long remain obscure. Meanwhile, through the attempted

apportionment of the coronal rays photographed in totalities between

sundry hypothetical substances we are led to regard coronium as only one

of a group of gases foreign to terrestrial experience.

The rotation of the corona can be measured only by the most refined

methods; but there is little doubt that they will be successfully

applied. Movements of other kinds may also be spectroscopically

determined, since they are likely, in many cases, to take directions

oblique to the limb, and therefore to have large components along the

line of sight to the earth. Notwithstanding the lasting, and, in some

respects, the growing importance of securing the best possible

picture-photographs of the corona during totalities, novel revelations

are scarcely to be expected from them. Their leading interest just now

centres, first in the structural relationship of coronal arches with

prominences, next, in the information they may afford about dark

markings in the solar appendages. For the rest, we must look to daylight

photography. When the great desideratum is attained of getting behind

(as it were) the veil of atmospheric glare, we shall be able to trace

the progress of coronal change, to follow the unbuilding and rebuilding

of the typical aureolas, to witness, perhaps, sudden coronal

developments in sympathy with chromospheric outbursts. From the

vantage-ground thus gained, in short, the true function of the corona in

the solar economy can be systematically investigated. During the few

crowded moments of eclipse this is not possible.

CHAPTER XII.

THE SUN’S ROTATION.

The mode of the sun’s rotation is perhaps the most significant feature

of his constitution. A thorough understanding of it would doubtless

bring with it an explanation of many other outstanding difficulties. But

it seems, unfortunately, a long way off. No more has been attained as

yet than the representation of the observed facts by empirical formulæ.

That is to say, a law of order has been discerned in them although their

cause remains obscure.

No single period of rotation can be ascribed to the sun. Each element of

the photosphere, probably each layer of the chromosphere, moves round

the axis in a fashion of its own. The fundamental rate, if such there

be, is so masked by local drifts as to be unrecognisable. The most

hopeful road to its eventual detection seems to be by the gradual

disentanglement of the solar influences affecting terrestrial magnetic

phenomena. The immediate task in hand, however, consists in extending

and giving precision to knowledge on the whole subject, in bringing

varied methods to bear upon it, and in linking into some kind of

sequence the circumstances ascertained.

Until spectroscopic and photographic means became fully available, the

solar rotation could be determined only by timing the circuits of spots.

And spots were very soon found to have “proper motions” precluding them

from discharging the function of points of reference. Hence there could

be no unanimity as to the sun’s rotation, the period arrived at by each

observer depending upon his choice of spots. At length Carrington’s

scrutiny during the years 1853 to 1861 showed the systematic nature of

these baffling diversities. They proved to vary with heliocentric

latitude, and the great solar swirl was brought into evidence. Once in

about twenty-five days the visible surface of the sun sweeps round at

the equator; but as the distance from it increases, the time lengthens

progressively. The rate of retardation is given mathematically in the

following expressions, which, being artificially adjusted to correspond

with observation, might be multiplied and modified indefinitely:—

FORMULÆ OF SOLAR ROTATION.

X = 865′ − 165′ sin ⁷⁄₄ _l_ (Carrington).

X = 1011′ − 203′ sin (41° 13′+ _l_) (Spoerer).

X = 862′ − 186′ sin^2 _l_ (Faye).

X = 858′ − 157′ sin^2 _l_ (Tisserand),

X signifying the daily angular motion, _l_ the solar latitude.

Formulæ of the kind, moreover, are of restricted application. Based

exclusively upon spot-measurements, they can scarcely be trusted outside

of the spot-zones—that is, beyond 38° north and south of the equator.

Carrington’s gives a period of 25^d 9^h 53^m where _l_ = 14°, of 26^d

9^h 9^m for _l_ = 30°; and he adopted as the standard period 25^d 9^h

0^m, conformed to in latitude 13½°. But what this average speed of

transport actually represents, is the mean rate of motion of the

multitude of sun-spots,[278] not the rotation of the body of the sun.

Professor Bigelow argues from meteorological analogies that this

prevails without disguise at the equator, while retrogressive currents

of a “trade-wind” character lengthen the periods derived from

observations in the spot-zones.[279] But such-like comparisons are

plainly inadmissible. More plausible, although far from decisive, is his

contention that the equatorial period must be the true one because it

lends itself to the correlation of terrestrial phenomena—auroræ,

magnetic storms, wind and weather changes—with solar outbreaks.

Solar rotational studies entered upon a new phase with the application

to them of the spectroscope. The particular importance of the innovation

lay in the change of venue which it involved. A new court, so to speak,

was constituted, a fresh set of witnesses called. These were the

Fraunhofer lines; and while requiring more delicate treatment, they

seemed likely to prove more trustworthy than their predecessors. Spots

do not float inertly on the photospheric tide. They are subject to

individual hurryings and laggings that to some extent invalidate the

record of their axial progress. Fraunhofer lines, on the contrary, are

eminently steadfast. They fall short, it is true, of the absolute fixity

formerly ascribed to them; yet after every deduction has been made,

their reputation as natural constants remains substantially intact. It

must, however, be borne in mind that what the Fraunhofer lines tell

about the sun’s rotation is not strictly comparable with the information

derived from spots. For they proceed from a different level, and may

therefore obey a different law of revolution. They indicate the

velocity, not of the sun itself, but of the absorbent strata in which

they originate.

The eastern limb of the sun advances, the western limb recedes at the

rate of 1·2 miles a second. The consequent line-displacements amount to

just ¹⁄₁₅₀ the little gap between the D-lines of sodium. In 1871 their

simple detection by Vogel was a feat of some moment.[280] Five years

later, Young was enabled, by his early possession of a grating or

diffraction spectroscope, to fix their range with approximate accuracy.

Then Langley showed how, by their means, to distinguish at a glance

lines of solar and telluric production. For in juxtaposed spectra from

opposite ends of the equator, the solar lines, being affected by

rotation, are manifestly “notched,” while atmospheric rays run straight

on without a break. They stand self-announced as of domestic production.

The differential plan of measurement thus suggested obviates many forms

of otherwise inevitable error. It was adopted by Dunér in his classic

work on the sun’s rotation, presented to the Royal Society of Upsala

14th February 1891. He selected two iron lines in the red (λλ 6301·72,

6302·72) for comparison with a pair of adjacent oxygen lines from the

“Alpha” band, known to be terrestrial, and therefore exempt from

motion-shiftings.[281] They were then safely treated as fiducial; and

the intervals between them and the solar lines differed, east and west

of the equator, to an extent corresponding with rotation in a period of

twenty-five and a half days. By the use of the utmost refinements, the

observations were carried up to within fifteen degrees of either pole,

where the period was found to be protracted to thirty-eight and a half

days, the intervening zones showing intermediate velocities. Direct

acquaintance with the sun’s axial movement was thus extended far beyond

the regions of spot-occurrence; and only direct acquaintance is, in this

matter, of any avail, since the formulæ devised to suit low latitudes

break down nearer the poles, disclosing their unsoundness by a total

want of agreement in the periods calculated from them.

“Carrington’s law” might have been true of spots only; it might have

denoted some peculiarity in their mode of production, causing a

systematic increase of backward drift, north and south of the equator.

The Upsala measures, however, proved it to apply, irrespectively of

spots, to the solar globe generally. They told, indeed, something more

than this. The period deduced from them was longer than that given by

spots. The difference amounted to about half a day, and it persisted in

all latitudes—that is to say, the vapour of iron surmounting the

photosphere gyrates more slowly than the spots _in_ the photosphere. A

variation of angular speed with elevation above the sun’s surface was

for the first time indicated. Confirmatory evidence was soon

forthcoming.

The photographic investigation of facular movements was attempted by Dr.

Wilsing at Potsdam in 1888.[282] From 1012 measurements executed upon

108 plates, he obtained a constant angular velocity of 14°·27 per diem,

equivalent to a period of 25·23 days, which is just that of spots

situated ten degrees from the equator. But the conclusion that faculæ in

all parts of the sun conform to it was certainly fallacious. M.

Bélopolsky made this apparent in 1892,[283] and M. Stratonoff of

Taschkent still more decisively in 1894–96.[284] His research included

the determination of 2158 positions of 997 faculæ on 316 Pulkowa plates,

and furnished the clearest evidence of their poleward retardation. The

Stonyhurst drawings, discussed by Fathers Sidgreaves and Cortie,[285]

yielded similar results. Concomitant increase of period and latitude is,

in fact, a rule without exception on the sun.

Faculæ, however, have a mean rate of their own. In the same parallels

they are transported more rapidly than either spots or absorbent

vapours. Their period of rotation is the shortest attributable to any

solar formations. The main facts of the case can be taken in by a glance

at the following little table:—

PERIODS IN DAYS

┌────────────┬───────┬──────┬─────────┐

│Heliographic│Faculæ.│Spots.│Reversing│

│ Latitude. │ │ │ Layer. │

├────────────┼───────┼──────┼─────────┤

│ 0°│ 24·66│ 25·09│ 25·46│

│ 15°│ 25·26│ 25·44│ 27·49│

│ 80°│ 25·48│ 25·81│ 31·83│

└────────────┴───────┴──────┴─────────┘

The equatorial facular period, it is worth noting, is nearly identical

with that ascribed by Hornstein, on the ground of magnetic observations,

to the mass of the sun.

The diversities exhibited above are as perplexing as they were

unexpected. They do not even fall readily into any satisfactory order of

progression. The slowest movement belongs to the reversing layer, or at

least to the slice of it stopping out Dunér’s iron lines; for there is

no certainty that the entire stratum rotates unanimously. It is,

however, certain that it covers both spots and faculæ. The level of

absorption is higher than the level of the photosphere with all its

immediate appendages. This fact has been already insisted upon;[286] it

may perhaps usefully be reasserted in the present connection.

Fraunhofer-absorption is stamped in the prismatic rays of faculæ and

sun-spots precisely as in those of the photosphere. They have then been

demonstrably sifted through the same screen of incandescent vapours. We

have yet to learn that they escape any minutest part of the absorptive

effects produced in ordinary sunlight. They must accordingly be

submerged beneath the whole series of strata occasioning them. At the

same time, it has to be borne in mind that Dunér’s deductions rest upon

a narrow basis. He measured only a single pair of lines, and we lack the

specific assurance that those individual lines occur in the facular and

spot spectra. Presumably they do; no reason is apparent why they should

behave exceptionally; but a direct record of their actual presence would

be satisfactory. They should also be looked for in the “flash” at the

edge of the eclipsed sun. Their detection as brilliant lines would

confirm and settle their status.

But here we encounter an anomaly. Since the reversing layer rotates more

slowly, and lies higher than the photospheric formations, a law of

retardation with altitude might be assumed. Its prevalence is,

nevertheless, contradicted by the relations of spots and faculæ. Faculæ

undeniably rise above spots, yet they wheel more rapidly; they give a

shorter period. This seems to be established by Wolfer’s[287]

confirmation of Stratonoff’s results. Thus while retardation outward is

indicated by the reversing layer, acceleration outward is forcibly

suggested by faculæ. And there is a general consensus of opinion that

this rule applies generally, so that the comparative tardiness of the

absorptive region stands over as an unexplained discrepancy. Even within

the region itself, Mr. Lewis E. Jewell has found indications of

diminishing angular velocity towards the photosphere, where also the

equatorial quickening is small compared to its value higher up.[288]

Moreover, Bélopolsky’s speculations as to the nature of the corona,

which probably adumbrate, if they do not convey truths, require that it

should rotate much more swiftly than the sun itself.[289] The forecast

will doubtless be tested ere long by eclipse-spectrograms. Indeed,

further evidence is much needed on a number of crucial points connected

with the sun’s rotation.

Dunér confessed his inability to imagine a “Why” for the singular

rotatory régime of which he had clearly expounded the “How.” “It

constitutes,” he remarked, “one of the most difficult problems in

astrophysics.” One theory after another has been proposed, and sunk out

of sight, overweighted by manifest inconsistencies. A machine “going”

like the sun the wit of man has not yet been able to devise.

Nevertheless, a means of evading the difficulty has been found. The

possibility has been perceived, and perhaps too readily admitted, of

regarding it as a legacy from chaos to cosmos. The embarrassments

surrounding the subject may be relegated to a far-distant age, and to a

state of things for which the investigators of to-day disclaim

responsibility. The anomaly, on this view, is a survival of nebulous

conditions, which, so far from having a present sustaining cause, is,

and has always been, subject to the destructive agency of friction. Why

then, it may be asked, has it persisted throughout the æons of the sun’s

growth? Should it not have been quite early abolished, if it be nothing

more than a residual inequality, begun to be smoothed away in the dim

foretime when the solar globe took shape? Mathematicians reply in the

negative. Wilczynski of Berlin gave in 1896 an apparent demonstration

that internal resistance cannot change the diurnal arc described by any

point on the sun to the extent of two minutes in twenty-seven million

years.[290] But Harzer showed it to be applicable only to cases

non-existent in nature.[291] It would be valid under ideal

circumstances; things being as they actually are, it falls to the

ground.

Starting from less questionable premisses, however, Wilsing of Potsdam

in 1891,[292] and Sampson of Durham in 1894,[293] reached practically

the same conclusion. They agreed that millions of years must elapse

before the sun comes to rotate “all of a piece,” like a solid body.

There is, indeed, one flaw in their reasoning. Both limit convective

circulation within the solar globe to a relatively thin shell of

material. They assign to radiation an essentially superficial character.

In the attempt to prove that the rotational currents flow without

impulsion, they sacrifice the functional efficiency of our great

light-giver. For assuredly its immense output of radiant energy can only

be supplied from interior stores, rendered available by powerful and

_deeply rooted_ vertical currents. But the continued activity of such a

system should speedily efface inequalities of surface drift, unless some

countervailing force maintained them.

Dr. Wilsing asserts that, in the comparatively late “phase of celestial

evolution represented by the fixed stars, radial currents are beginning

to disappear.” It might more plausibly be argued, considering the

intense brilliancy of such bodies, that radial currents are in them at a

maximum of strength and volume. “We are therefore relieved,” the author

continues, “from the difficulty of accounting for ‘Carrington’s law of

rotation’ on mechanical and physical principles, since it appears as the

result of earlier conditions of motion.”[294]

We leave a good deal to the coming time; why should we not shift an

occasional perplexity back to the broad shoulders of antiquity? Much

that is inconceivable in a sun might be possible in a spiral nebula. No

such expedient, however, will answer the present purpose. The

originating cause of the sun’s rotational anomalies, whatever it may

have been, continues to act. In a globe so profoundly disturbed, they

could not possibly have survived of themselves.

Professor Young finds the necessary driving power in falls of cooled

materials, bringing with them to the photosphere the swifter motion

appertaining to a wider circumference. The consequent accelerative

impulse would be greatest at the equator, and would diminish to nothing

at the poles, according, so far, with the observed facts. Spots,

moreover, by onrushes at epochs of reconstruction or recrudescence,

formally acknowledge the receipt of supplies from above. But

Carrington’s “law” governs all the solar formations, and the rationale

by continuous downward precipitations seems to be of very partial

applicability. Faye tried to solve the problem on an inverse principle.

Instead of descents from without, he postulated ascents from within, the

equatorial retardation being less than that in high latitudes because

the rising matter comes from nearer the surface. This, however, was a

purely artificial arrangement, with no voucher for its reality. The

explanation needed to be explained. Speculation here, as elsewhere, must

await the progress of direct inquiry.

CHAPTER XIII.

THE SOLAR CYCLE.

Solar periodicity is a most complex phenomenon. The more it is studied,

the less it seems to be understood. Its effects branch out into endless

entangled and obscure _fibres_ of fact, to trace all of which back to

their root-source would be an almost superhuman task. At present they

can only be dealt with in groups and tentatively. Their subtle, and

often disguised relationships need much patience for unravelment. They

need, above all, a free mind. Prepossessions are sure to compromise

truth.

The sun is subject to a rhythmical tide of disturbance, ebbing and

flowing in about eleven years. But the flow is irregular and

spasmodic. Both the intensity of the crises and the intervals at which

they recur vary largely and unaccountably. Probably the eleven-year

cycle is involved in others. One, there is reason to believe, brings

about alternate accentuations and partial effacements of change

comprised within a term of some sixty-five years.[295] And minor

pulsations—wavelets on the great rollers—are besides evident.

Prediction, nevertheless, remains at fault. Spot-maxima are delayed or

anticipated, they are languid or energetic, as the outcome of modes of

action defying calculation. Not even the loose fetters of an arbitrary

formula have ever been forged for them. The attempt would indeed be

hopeless, since the laws governing them, besides being highly

intricate in themselves, are plainly disturbed in their working.

Circumstances intervene which we must call “accidental.” Could we

describe them in detail the science of solar physics would lie before

us as an open book.

The “error” of the spot-period may amount to nearly half its normal

length. Thus sixteen years elapsed between the maximum of 1788 and the

next certainly ensuing, and only 7·3 years separated the culminating

points in 1829·9 and 1837·2. A characteristic feature of the

representative curve is that it mounts more rapidly than it descends.

Maxima succeed minima, on an average, after 4·5 years, while the

corresponding minima are only reached after 6·1 years. Substantially, a

disparity of this kind is probably always present, although now and

again masked by the prominence of a secondary maximum. These

peculiarities deserve the most careful attention, as sufficing in

themselves to place the sun in the category of variable stars. His

spot-curve might almost be said to be modelled on the light-curves of

such objects; and the analogy is eminently instructive. We learn from

it, for instance, that a spotted condition in the sun matches a phase of

strong luminosity in the stars; and are hence led to infer that the sun

radiates most powerfully when his disc is most maculated. The extreme

difficulty of obtaining direct proof of this relation lends especial

value to the side-wind of evidence thus brought to bear.

[Illustration:

FIG. 17.—Spoerer’s Curves of Sun-spot Latitude (from _The Sun_, by C.

A. Young).

]

Now an increase of radiation involves a quickening of the sun’s internal

circulation; and the process, when hurried, is likely to become

tumultuous. So that a connection is easily traceable between heightened

brilliancy and photospheric laceration. Spots, however, are only the

most conspicuous symptoms of an agitated state. Faculæ and prominences

follow suit. They are indeed too intimately bound up with spot-economy

to do otherwise. The corona, by a less obvious necessity, sympathises,

and the periodicity of all these formations has a double aspect. They

fluctuate in mode of distribution as well as in vigour of development.

Spots, faculæ, prominences, and corona, all vary similarly and

simultaneously in heliographic latitude as the waves of disturbance rise

and fall. The spot-zones are not stationary. They shift over the body of

the sun according to a definite law enounced by Carrington in 1859, and

confirmed by Spoerer in 1861.[296] At maxima they occupy mean positions

in about fifteen degrees of north and south latitude; then, as the cycle

advances, they close together, and the commotion finally dies out near

the equator. Meanwhile, the start of a new series in high latitudes has

anticipated the termination of the old. Feeble at the commencement, it

gains strength as it departs more and more from its native regions.

“Mobilitate viget, viresque acquirit eundo.” Fig. 17, copied from a

diagram of Spoerer’s, illustrates the nature of this progression. The

overlapping of the curves at minimum brings before us the remarkable

circumstance that, as a consequence of successive disturbances breaking

out before those antecedent to them have expired, the full duration of

each is, not eleven, but twelve to fourteen years. Moreover,

spot-production at minima, however inactive, has a twofold nidus; two

widely separated zones in each hemisphere are appropriated to it. A

further characteristic of the cyclical decline in latitude is that it

proceeds in “waves.” “Every fourth or fifth rotation,” Mr. Maunder tells

us, “there will be an effort to reach a higher level, a lift of one or

two degrees, and then a gradual slipping back until a fresh effort

brings another small lift, but a weaker one than the last. And so the

cycle goes on; the decline is continual on the whole, but is broken and

interrupted by these frequent little struggles to get back to a higher

plane.”[297] The growth in spotted area accompanying the descent of the

zones is similarly rhythmical. So closely connected, indeed, are these

two modes of periodicity that irregularities in the cyclical progression

frequently show under the double aspect of abnormal outbreaks of spots,

and abnormal movements in latitude. Theories are accordingly valueless

that fail to rationalise simultaneously both kinds of facts. Several,

indeed, profess to do so, but by constrained expedients.

The zonal law applies, with qualifications, to faculæ and prominences.

Eruptive prominences are strictly governed by it. They frequent the

spot-belts, it may be said, exclusively. The quiescent kind, on the

contrary, avoid them,[298] and have their main gathering-grounds within

fifteen degrees of either pole.[299] They may even occur, near

spot-maxima, right up to 90° of latitude. As activity decreases,

however, they too move downward, and crowd more or less closely towards

the equator, although maintaining at all times a wider range than spots.

Faculæ show a divided allegiance. They attend on spots, and their

principal maxima are hence displaced with the spot-zones, while their

affinity with quiet prominences is evinced by the occurrence of

secondary maxima in high latitudes. Not that their local arrangement is

the same with that of prominences.[300] Discrepancies are frequently

noted; and they are important as indications that the two varieties of

outgrowth do not originate under identical conditions.

The succession of coronal types is in clear accord with the law of

zones. Streamers and prominences march, on the whole, closely together.

They unanimously quit the poles after each maximum; they linger in

company over middle latitudes, where “synclinals” overarch red flames at

epochs of medium activity; finally, they descend towards the equator,

the white wings of the minimum corona meeting and spreading above the

last members of each decadent eruptive series.

The spectral periodicity of the sun is less marked than might have been

expected. It is, indeed, almost confined to spots. The ordinary

spot-spectrum at minimum (to repeat what has been already stated)

includes many broadened iron lines, replaced at maximum by vanadium and

scandium absorption. The nature of the individual spot, however (as we

have seen), not the stage of the cycle, is really the determining cause

of this diversity, which recurs periodically, simply because

minimum-formations are usually of the tranquil sort. The chromospheric

spectrum has quite other relations. It does not vary fundamentally; but

the metallic rays temporarily added to it become fewer as metallic

injections fall off. Nor is the quality of coronal light subject to

radical change. Only the relative strength of its constituents slightly

fluctuates. In the “winged” type the gaseous emissions are feebler than

in the “radiated” type; yet they are always present and always the same.

The virtual invariability of the Fraunhofer spectrum is more surprising,

since the reversing strata are in immediate contact with the

periodically agitated photosphere. They preserve, nevertheless, an

almost inviolable tranquillity, and their composition remains unaltered

from one cycle’s end to the next. A single recurrent modification is,

however, just traceable. It is that produced by the emergence, at

maximum, of the facular bright lines H and K. _Pro tanto_ and _pro

tempore_, the symptom constitutes the sun a “bright line star.”

Analogous detections in stellar spectra would afford a possibility of

determining the spot-periods of globes in the solar condition; but they

are, for the present, scarcely to be hoped for. The observation is

difficult in the sun; in a star, unless facilitated by extraordinary

facular development, it would be impossible.

The throbbings of solar agitation affect his entire system. In how many

ways, and by what hidden means, we can but vaguely surmise. Terrestrial

meteorology, as a whole, is certainly embraced in the great cycle,

although the details of its conformity baffle, by their intricacy, the

most painstaking pursuit. Only in the magnetic department there is no

room for doubt. A thoroughly satisfactory discussion of the subject was

completed in 1898–99 by Mr. William Ellis,[301] who for long years

controlled this branch of work at Greenwich. Comparing the observations

of the diurnal range of magnetic declination and horizontal force made

at the Royal Observatory during the years 1841 to 1896, with the

sun-spot numbers for the same interval determined by Wolf of Zürich, he

found between the two orders of phenomenon, not only a general

parallelism, but a correspondence in irregularities of period and

secondary variations of intensity. This is strikingly evident in Fig.

18, copied by kind permission from his paper read before the Royal

Society, 10th March 1898.

[Illustration:

FIG. 18.—Curves of Sun-spot Frequency and Magnetic Agitation (Ellis).

]

The sympathetic relation extends to auroræ. They even obey a “law of

zones” similar to that regulating the distribution of sun-spots. The

earth is circled—presumably in each hemisphere—by an auroral belt, which

advances into temperate latitudes at epochs of cosmic disturbance, but

retires towards the pole as it quiets down. The reality of the

connection was singularly affirmed by the simultaneous dearth of

sun-spots and auroræ during the seventeenth century. A prolonged solar

calm appears to have set in about 1643. Galileo and Scheiner had been at

no loss for objects of study; but the diligence of their successors,

although unrelaxed, went mostly unrequited. To Cassini, Flamsteed,

Hooke, De la Hire, the occurrence of a spot was an event of rare

interest, which rewarded perhaps a decade of fruitless watching. Yet, as

Mr. Maunder says,[302] the cycle was “submerged” rather than actually

abolished, “the crests of a sunken spot curve” being marked by the

solitary spots perceived in 1660, 1671, 1684, 1695, and 1705.

Definitively, the protracted minimum came to an end in 1716, and there

was a normal maximum in 1718. Meantime auroræ too were in abeyance.[303]

None were seen in England from 1575 to 1706, when a glimmer of polar

lights heralded the magnificent display witnessed by Halley, 17th March

1716. That there was concomitant magnetic quiescence need not be

doubted; but Gauss of Göttingen was still in the distant future, and

nothing could be known on the subject.

Individual outbreaks on the sun are often unmistakably associated with

commotions of the terrestrial magnetic system. These so-called “storms”

are world-wide in their nature, abrupt in their origin, and bear witness

to some sudden _vital_ spasm attacking the globe as a whole, and at

once. Auroras and earth-currents make part of these mysterious

affections, which commonly reach their height when a large spot-group is

nearly central on the disc—that is to say, when it is broadside on to

the earth. Instances abound. On 17th November 1882, the photosphere was,

to the naked eye, conspicuously rent. The coincident aurora and magnetic

storm were said to “beggar description.”[304] A spindle-shaped beam,

which darted that night across the sky, was indeed a unique phenomenon,

and, on its farthing-candle scale, recalled the amazing solar flambeau

of 1st September 1859. Nor can the contemporaneous twitchings of the Kew

magnets on this latter occasion be regarded as accidental, any more than

the sudden small disturbance of all three magnetic elements which

accompanied an outburst of faculous light on 17th June 1891.[305] The

magnetic turmoil raised by the transit of the enormous spot of February

1892 was exceptionally violent. Earth-currents seriously interfered with

telephonic and telegraphic communication in all parts of our busy

world;[306] the needles at Greenwich went completely off the prepared

track of photographic registration;[307] and an auroral pageant

completed the programme of response. Similar concurrences were observed

in February 1894, September 1896, and March 1898, to mention a few out

of a multitude of cases. Yet the sympathetic connection is not

invariably manifest. A hole in the sun may evoke no earth-trouble. Mr.

Maunder hence concludes that “though sun-spots are the particular solar

phenomenon most easily observed, we must not therefore infer that their

number and extent afford the truest indication of the changes in the

solar activity which produce the perturbations we remark in our magnetic

needles.”[308]

Not the spot itself, but the connascent agitation thrills the

terrestrial organism. Quiet formations pass unheeded; crises of growth

or reconstruction meet with instant rejoinders. Tacchini[309]

accordingly holds that _chromospheric_, not _photospheric_ phenomena

are, in this respect, truly influential, and that it is to the fierce

flame-rushes above spots that the magnetic nerve-system is sensitive.

The view is favoured with some hesitancy by Professor Hale.[310]

Professor Bigelow, as the result of much suggestive inquiry, affirms

that “from the sun to the earth come two great supplies of energy, both

types of radiation through the ether”—one propagated by plane waves, the

other by vortical rotation—“and possessing very different properties,

the one visible to the eye, the other visible to magnetic perceivers.”

And he computes, from various indications, a value for the normal

magnetism of the sun about one-fifth the maximum of steel, which may, at

epochs of extraordinary disturbance, be augmented fifty-fold.[311]

Variations in the sun’s electrical state assuredly accompany his more

obvious cyclical changes. “May not,” Dr. Schuster asks,[312] “the

periodicity of sun-spots and the connection between two such dissimilar

phenomena as spots on the sun and magnetic disturbances on the earth, be

due to a periodically recurring increase in the electric conductivity of

the parts of space surrounding the sun?”

The surmised alteration may be a consequence, it can hardly be the cause

of solar periodicity. Its actual occurrence, however, is far from

unlikely. Certain cometary phenomena lend it a qualified support. The

grouping together near sun-spot maxima and minima respectively, of the

bright and faint apparitions of Encke’s comet during a century

(1786–1885), forced itself upon M. Berberich’s attention in 1888.[313]

Correspondences of this unexpected form were even traceable with

displaced epochs of activity, such as the retarded maximum of 1788, and

the premature maximum of 1837. Some indications were besides gathered

that comet-discoveries become more numerous as the tide of solar energy

rises, many that would otherwise pass unseen being lifted into

visibility by accesses of transmitted excitement. But this relation, M.

Berberich admitted, might be more apparent than real.

One still more recondite and unaccountable has been lately adverted to

by Mr. J. Halm.[314] He contends that the physical condition of the sun

reacts perceptibly on the motion of the earth. Variations in its orbital

elements, which have hitherto baffled attempts at explanation on

gravitational principles, are comprised, he finds, within the “great”

spot period of about sixty-five years. The obliquity of the ecliptic,

for instance, instead of decreasing uniformly with the time, shows

subordinate fluctuations synchronising with the long waves of solar

activity. Its shorter waves, on the other hand, prove to be influential

upon the variation of latitude. Mr. Halm considers that the deviations

of the terrestrial pole conform unmistakably to the eleven-year cycle,

with, however, a lag of about one and a half years in the corresponding

epochs. “It may,” he writes, “be taken to be clearly established that

the radius of the circle described by the pole of instantaneous rotation

is greatest at times of sun-spot minima, and smallest at times of

maximum displays of solar spots.” This “holds true,” he adds, “for the

whole interval of about sixty years now covered by Dr. Chandler’s

investigations.” His explanatory hypothesis is both simple and

ingenious. It depends upon the large inclination of the earth’s magnetic

axis to its axis of figure, combined with alterations, due to solar

influence, in the total magnetism of our planet. Molecular strains along

the magnetic axis would—it is plausibly assumed—occasion bodily

distortions of the globe, whence should result displacements of the axis

of figure relative to the axis of rotation. “The outcome of this

hypothesis would” then “be that changes in the state of solar activity,

since they produce a measurable effect on the terrestrial magnetic

forces, should also be accompanied by corresponding changes in the

motion of the earth’s axis.”

The machinery by which electro-magnetic impulses are propagated from the

sun to the earth, completely evades scrutiny. Sundry conjectures on the

subject have been hazarded, but none of them rest on any sure basis.

What we know about modes of communication is chiefly negative. Thus,

Hertzian vibrations are not transmitted to the earth with sunlight. They

do not, at least, reach its surface. Wilsing and Scheiner tried for them

in vain with an electric “bridge” and a galvanometer.[315] They might,

indeed, as the experimenters noted, be atmospherically arrested. All

that seems certain is that direct magnetic action is concerned in

producing the observed perturbations of the terrestrial magnetic system,

which are not explicable like ordinary meteorological phenomena as

effects of thermal vicissitudes, or convective air currents.[316] They

stand apart, and imply special conditions which cannot, without

detriment to science, be ignored.

Little progress has been made towards ascertaining the cause of solar

periodicity. We are only assured that it is not imposed from without,

but arises from within; it resembles a “free,” rather than a “forced

vibration.” This conclusion, it is true, tends to relegate the matter to

obscurity; for the interior of the sun is a _terra incognita_, and seems

likely to remain so. His cyclical changes may belong to his original

constitution; they may date from nebular times, and be as inherent as

the tone of a bell. Or they may simply characterise a stage of growth,

and prove liable to modification and effacement. The study of variable

stars will perhaps help to guide our ideas as to the probabilities of

the case.

Its full bearings, meanwhile, can only be conjectured. The scope of the

disturbance needs to be defined. There are still many open questions

connected with it. Does the “smoke-veil” absorption vary with the

abundance of spots? Are the temperature of the photosphere, and the

depth of the chromosphere affected by it? Can periodical changes of

pressure in the reversing layer be detected? These are among the

problems of the immediate future. They are already within reach of

attack.

CHAPTER XIV.

THE SUN AS A WHOLE.

Modern science contemplates in the sun a huge sphere 867,000 miles in

diameter, bounded by a dazzling cloud-shell, and composed of materials

1·4 times heavier than water, yet heated so far above the “critical

temperature” of any terrestrial substance, that they must be regarded as

in the gaseous state. The imperious demands of radiative emission can be

met only by rapid exchanges, implying the unceasing activity of a system

of profound vertical currents; while rotational surface-drifts of a

peculiar kind notify complexities of internal movement defying

speculation or research. Incidental to them, doubtless, are the dark

spots marring, at times, the brightness of the disc, and giving

evidence, by their more or less copious occurrence, of that far-reaching

periodicity which may be called the central fact in solar physics. Spots

are garlanded with faculæ, which represent photospheric upheavals; and

above them, to a height of some hundreds of miles, rise the metallic

vapours producing, by their absorption, the Fraunhofer lines. The

reversing strata seem to be continuous with the chromosphere, although

the nearly perfect transparency of the latter establishes a noteworthy

distinction between the two formations; but the corona is a thing by

itself, sharply separated, physically and spectroscopically, from every

other appendage of the sun. It has, of course, relations with them, just

as our air has with the ocean it surmounts—relations, however, that do

not even verge towards a confusion of identity.

The two fundamental problems connected with the nature of the sun are

its rotation and its periodicity. They may be quite closely allied, and

in regard to both, “counsels of despair” have begun to prevail. The

spot-cycle, like “Carrington’s law,” is set down as a congenital

peculiarity, and the mists of the past are invoked to cover the

perplexities of the present. There seems little immediate prospect of

their being removed. Early modes of investigating the subject have had

no striking success, and fresh ones await development. So operations

have come to a pause, yet by no means to a dead-lock. The difficulty of

learning how the phenomena are occasioned should only stimulate

diligence in unmasking and tabulating them. The ramifications of the

period can be followed out, even if we cannot get at its roots, and

there are signs that they will be found to take unlooked-for directions.

Similarly as regards the sun’s rotation. Explanatory hypotheses avail

little, but the sifting of facts avails much. The case has not yet been

fully stated. Lacunæ need to be filled up, anomalies to be smoothed

away, errors to be corrected. In the doing of all this, a clue to the

labyrinth may present itself.

The nature of sun-spots must long be under discussion. Questions of

extreme interest are involved, some of them being visibly _answerable_,

since criteria are at hand to determine which way the truth lies.

Discriminative, rather than numerous observations will serve the

purpose. The relations of faculæ with prominences offer another

promising topic of inquiry, as well as those of prominences with coronal

streamers. But this last is an eclipse-problem, and so, unluckily, is at

present the entire subject of the corona. No pains, however, will be

spared in order to bring to bear upon it the full resources of daylight

investigation. As to whether they will be rewarded or no, the balance of

forecast swings pretty even. The unfolding of some condition, now

hidden, may incline it either way.

Solar spectroscopy presents a variety of aspects. It can be studied from

the chemical, thermal, electrical point of view; pressure, density,

motion, possibly magnetic stress, are concerned in it. Nothing in this

branch, however, is more instructive than the spectral diversities of

the various solar formations, or even of different parts of the same

formation. Thus each section, some few miles thick, of the reversing

layer is probably distinguished by emissive modifications; and the

Fraunhofer spectrum integrates the absorption of them all. The

spot-spectrum is a more pronounced variety; the chromospheric spectrum

is largely, the coronal spectrum, wholly peculiar. Now the task of

interpreting these several scripts is not—could not be—easy; yet solar

chemistry is, in the main, less unfamiliar than might have been

expected. It includes only one element—coronium—that can be clearly

distinguished as terrestrially unknown. For unidentified lines are not

necessarily of exotic origin, as is proved by the rapid progress of

their recognition in the Fraunhofer spectrum _pari passu_ with advances

in the photographic registry of metallic spectra. Nor is it probable

that spot-cavities harbour strange forms of matter. The vaporisation in

them of rare metals sufficiently explains what long appeared enigmatical

in their absorptive action.

With the substitution of known for unknown substances in the sun, a

leading argument for dissociation vanished. In this view, the

supramundane species giving lines experimentally unrecorded were

chemical fragments of our elements broken up by enormous heat. But if no

supramundane species exist, the elements presumably remain intact.

Absolute stability it would indeed be extremely rash to ascribe to them;

yet it is certain that their individuality survives fierce ordeals.

Evidence of other kinds tends towards the same conclusion. Mr. Jewell’s

critical examination of the Fraunhofer rays indicated for the shadings

attached to certain iron and calcium lines an origin quite close to the

photosphere.[317] This goes far towards demonstrating the integrity of

extremely complex molecules at the highest temperature prevailing near

the sun. Outbursts of heat from the interior there may be, reaching a

still more exalted pitch; but they must be transitory, since cooling by

expansion should promptly and potently affect them. Further, the thermal

relations of the terrestrial elements are distinctive and uniform. If

these bodies are not really simple, they are at least compounds of a

different order from those known to be such, and artificially

producible. Finally, the extensive detection of spectral series

invalidates the “one line, one element” principle which underlies most

of the arguments for dissociation. Unity of origin is emphatically

claimed, not only by each series, but by each connected set of series.

Researches into the essential nature of matter have, however, entered

upon a new phase through the aid of what we may call electrical

analysis, in which “ions” play the part of atoms in chemical analysis;

and their outcome may perhaps aid in the solution of many intricate

solar problems.

There is a strong temptation to transfer to the sun the conditions

prevailing on the earth, and to model solar upon terrestrial

meteorology. This was the line taken by M. Egon von Oppolzer in 1893. He

admitted, indeed, that it traversed dangerous ground, in view of the

wide divergences of opinion as to the nature of atmospheric processes

belonging to immediate and everyday experience. Still he judged it safe

to apply the kinetic theory of gases and the dynamical theory of heat to

the determination of the state of equilibrium in the sun’s aerial

appendages. The results were scarcely encouraging. At elevations of one

second of arc, or 450 miles, above the photosphere, the prevalent

temperature was found to be 14,000° C. lower than at its surface, which

should hence be extravagantly hot. And although this inconvenience was

abated by forced assumptions, it could not, on the principles adopted,

be removed. They involved, for example, a rise of temperature in

downward currents to the extent of at least 5000° for each descent of

450 miles. Spots were regarded as “places of extreme alternations of

temperature,”[318] where abnormally hot layers cover anticyclonic

regions of increased pressure and reduced thermal excitement. They are

“produced indirectly through a sinking down of masses upon the

photosphere, and directly through extraordinary radiation, brought about

by transparency of the overlying region.” To counterbalance descending

movements in the spot-zones, a continuous uprising of heated matter was

supposed to progress at the poles, constituted so far the analogues of

our equatorial belt of calms. Two great permanent cyclones were thus

centred on the axis of the sun. They had important functions assigned to

them. Upon their regulative power was made to depend the working of the

entire machine, and solar periodicity itself was referred to the

alternate relaxation and enhancement of their activity. They were,

nevertheless, a purely arbitrary creation. Not so much as a tortoise in

mid-air was provided for the earth-bearing elephant to stand upon. The

author claimed, it is true, only the merit of simplification; yet the

contrast of conditions between the earth and the sun largely vitiated

his reasonings. Atmospheric circulation on the earth is maintained by

external heat; agitations on the sun by internal heat; they depend

absolutely upon processes of cooling. Hence the impossibility of

assimilation. Trade winds and cyclones lack in the sun the driving power

by which they are kept going on the earth. They are characteristic of a

planetary body—of a globe _vitalised_ from without.

Eruptive hypotheses of the solar constitution have been proposed under

various forms by a succession of writers—by Secchi, Faye, Lockyer,

Schaeberle,[319] Young,[320] Sidgreaves.[321] Here, at any rate, we are

in touch with reality. Volcanic forces are powerful in the sun, which

might, in a sense, be described as organised upon a volcanic basis. The

conditions of upheaval are everywhere at hand; only some casual relief

of pressure, or access of heat is needed to provoke an actual explosion.

Settling down on the photosphere in a cooled, though still gaseous

state, the products of eruption would then give rise to spot-phenomena.

Dark patches would mark the effects of their general and special

absorption of light, while faculæ and flames attested the vehemence both

of the original outbursts, and of those reactively started by their

partial subsidence. Procedures of this kind on the sun appear

inevitable; they at least count for something, if they do not explain

everything.

A curiously subversive theory in solar physics was propounded in 1891 by

Dr. August Schmidt.[322] Its interest, however, is largely academic. Yet

it is no illusory speculation. It rests upon a sound foundation, and

emphasises an undeniable truth. The mode of action brought into

prominence by it necessarily plays a part in modifying the aspect of the

heavenly bodies; the only question open is whether the part is

conspicuous or insignificant.

The laws of atmospheric refraction were traced to their ultimate

consequences by Kummer in 1860.[323] Some of them are remarkable.

The visual lifting of extra-terrestrial objects results, as is

familiarly known, from the inflected character of the paths pursued by

light-rays through our air. A beam reaching a spectator at sea-level

from a horizontal direction is really bent upward with a curvature

one-seventh that of the earth itself. But on a globe of seven times the

earth’s radius—other things remaining the same—the refracted ray would

possess identically the curvature of its surface, to which it would

accordingly run parallel for ever, or until extinguished by absorption.

It would never reach the eye of a spectator. “Circular refraction” would

take effect upon it. A similar fate would befall a ray starting

horizontally from the surface towards outer space, instead of attaining

to which, it should follow an unending round within the air. On Jupiter,

this critical stage must be considerably overpassed. From a Jovian

atmosphere of the proportionate mass of the earth’s, light could only

escape at angles of elevation exceeding 3° 22′.[324]

Applying his formulæ to the sun, Schmidt found that circular refraction

would there be produced in a hydrogen atmosphere at a temperature of

10,000°C., and of one-ninth the standard density of air. The upshot was

to “explain away” most of the solar appendages. The photosphere he

showed to be an optical illusion, arising at the surface where circular

refraction just comes into operation in an incandescent gaseous globe

diffusing uniformly outward. The difficulty was thus overcome of

accounting for the visibly sharp separation between the dense body of

the sun and its tenuous surroundings; while a phantasmagoria of

granules, spots, flames, and faculæ was easily evoked on the supposition

of irregular refractions within the seeming disc.[325] To this dubious

locality, the reversing layer, too, was transferred by Dr. Knopf of

Jena, who hailed the speculation as the dawn of a new era in solar

physics. Hence, opposite displacements of the Fraunhofer lines cannot be

a rotational effect, and the singular accordance of what they should be

on that supposition with the measures actually obtained, is the outcome

of pure accident. But this is extravagant and unthinkable—a _reductio ad

absurdum_ of the optically composed sun. Plainly, the whole theory is in

the air. Its reasonings, to be sure, are mathematically valid; but they

refer, as Dr. Seeliger has pointed out,[326] to an ideally transparent

globe, from which the absorptive effects, prominent in the sun, are

absent. The solar spectrum, besides, fails to be accounted for by them

“on the accepted principles of physics.”[327] Nor can practised

observers of the solar surface readily be persuaded that they have

watched, not realities, but mirage-effects. In dismissing the hypothesis

as untenable, the important inference was, however, retained by

Professor Frost “that refraction within and on the sun itself may modify

in some considerable degree” observed phenomena.

An ingenious corollary has been added to Schmidt’s theory by M. Julius

of Amsterdam.[328] It was suggested by certain anomalies in the

dispersive action of sodium-vapour upon light, noticed by Becquerel, and

confirmed by himself. They led him far. He was conducted by them to

optical explanations of the “flash” spectrum, of the broadening of lines

in sun-spot spectra, and of line-displacements, usually interpreted on

Doppler’s principle. They are, however, given under reserve, and can be

tested by comparing the lines affected in the sun with those in the

vicinity of which special refraction is exerted. This is a laboratory

task, which M. Julius himself is well qualified to discharge. Should the

provisionallyassumed correspondences be found to subsist, the arguments

for the phantasmic character of solar appurtenances will have gained

imposing strength. But such a result cannot reasonably be anticipated.

Refraction on the sun is likely, owing to the extreme rarity of the

medium in which it should take place, to be of evanescent effect. Only

in spot-cavities, it may perceptibly affect appearances. The entire

matter, nevertheless, deserves to be thoroughly sifted, and cannot in

future be forgotten or ignored.

Here again a fresh prospect is opening into view, and many others invite

exploration by difficult and devious routes. For solar science does not

become less arduous as it advances. One of the surest marks of progress

in any branch is, indeed, the development of new and unforeseen

problems. And the sun, as we have learned by degrees to recognise, is a

body organised in too complex a method for easy apprehension. We

perceive that its energies are specially directed; the purpose of the

machine is obvious, and it is admirably fulfilled. In part we can see

how, but much remains mysterious. Right to the core of the mystery we

may never penetrate; nature is virtually invincible by man; but in

urging our way forward, we shall gain continually larger and more lucid

views.

PART II

PROBLEMS IN SIDEREAL PHYSICS.

CHAPTER I.

PROGRESS OF SIDEREAL PHYSICS.

Sidereal physics includes stellar and nebular physics; the two branches

cannot be separated. They have interlacing offshoots, and progress

during the last hundred years has tended more and more to unite them in

one main stem. Objects closely akin are dealt with in both, and they are

dealt with by methods substantially the same. Stars and nebulæ are not

only related as fellow-members of the grand galactic system, but they

coexist in numerous sky regions, and often in such close connection that

it is difficult to define them apart. A nebula with a stellar nucleus

can scarcely be distinguished from a nebulous star, and nebulæ

altogether devoid of star-like condensations are perhaps non-existent.

Chemical affinity ratifies visual conjunction. Spectroscopic

classification proceeds from stars to nebulæ with hardly a break. The

establishment of an exceedingly low standard of density for certain

varieties of stars forges an additional link between the two sidereal

orders. For it brings to our acquaintance bodies in a transition-stage

from nebular diffuseness to solar condensation—bodies attracting feebly

while radiating powerfully. Variability in light is another quality, the

common possession of which by stars and nebulæ has recently been placed

beyond doubt; and strong evidence is forthcoming that binary systems

occasionally consist of a stellar and a nebular member, united by origin

and inseparable to all time.

The methods of solar and sidereal physics do not differ fundamentally;

all alike depend in the main upon light-analysis and chemical

delineation, and resort to direct telescopic observation only as a

subsidiary expedient. The aims of sidereal science are, however,

profoundly modified by the remoteness of the objects it is concerned

with. Many kinds of inquiry, successfully prosecuted in regard to the

sun, are impracticable for application to suns deprived by distance of

sensible dimensions. Surface-phenomena are in them wholly out of reach,

no less than the paraphernalia that lend their glory to total eclipses.

No star sends us more than a single pencil of light, collected

indiscriminately from a wide hemispherical area to the obliteration of

its local peculiarities. Spots, flames, faculæ (if such there be) mix

their rays inextricably together; for stars have no parts. But while

their distance narrows in some directions the scope of possible

inquiries concerning them, their multitude immensely widens it in

others. They are not all similar, and their differences supply grounds

for a classification, the import of which deepens with every advance in

physical knowledge. Delicate spectral traits are found to be indexes to

conditions of temperature, density, magnetic strain, or electrical

excitation, in part imitable in the laboratory, in part transcending,

and hence contributing to enlarge terrestrial experience.

Classification, moreover, is dominated by the idea of development.

Comparative sidereal study leads inevitably to far-reaching speculations

on cosmical growth.

The sun is solitary; he exercises a “sole dominion.” But it is not so

with all his compeers. Nor are the mutual relations of those linked

together expressed solely in terms of motion. They do not fall within

the exclusive competence of the mathematical astronomer. They involve

constitutional modifications of profound import. Coupled stars,

clustered stars, stars immersed in or attached to nebulæ, are probably

subject to influences, the nature and modes of action of which remain

largely obscure. Their investigation has, however, been tentatively set

on foot, and may give results of peculiar interest. Thus sidereal

science extends and supplements solar science. It assigns to the sun its

status in the universe; it provides objects with which it can be

compared or contrasted. The two modes of knowledge mutually act and

react; what one acquires the other assimilates. The progress of each is,

by this comparative action, quickened and assured.

The population of the heavens is so dense that general conclusions as to

its characteristics can be attained only by statistical methods. And to

employ these effectively, the command of vast masses of information is

required. Data must accordingly be secured wholesale, and the necessity

has been met by the creation of a world-wide international organisation.

But its mills grind slowly, and individual enterprise will not be

stayed. Sir David Gill has already completed, in the _Cape

Durchmusterung_, a preliminary work designed purely in the interests of

geometrical astronomy, but fraught with importance to cosmical physics.

Professor Kapteyn of Leiden, who undertook the examination and

measurement of the plates, detected relationships between the kind of

spectrum given by the stars imprinted upon them and the mode of their

scattering, which promise to affect more and more profoundly all future

conceptions of the universe. Hints of their prevalence had, it is true,

been already gathered. Father Secchi noticed long ago that spectral

types are not indifferently distributed over the sky, a conspicuous

example of local preference being afforded by the constellation Orion,

which might be described as a colossal group of helium stars. The work

of Pickering, McClean, and others has also brought out the facts that

the Milky Way is a distinctive spectroscopic region, and that the

stellar tribes in general show aggregative tendencies not to be

mistaken. Astrophysical considerations then enter into discussions of

celestial structure; they are not wholly alien to questions as to how

the heavens move. The monumental “Draper Catalogue” of stellar spectra

served as the foundation of most of these extensive researches; and the

entire bulk of photographic and spectrographic data collected at Harvard

College with astonishing persistence and skill during the last score of

years, has incalculably promoted the larger interests of sidereal

astronomy.

Physical and descriptive catalogues of nebulæ are still a desideratum.

The materials for their compilation are, indeed, lacking. Comparatively

few such objects have been examined with the requisite care. They are

not easily dealt with. Those of a gaseous nature are strongly

characterised by rays high up in the ultra-violet, where absorption by

glass is formidably effective. Their due photographic registration is

then feasible only by means of reflecting telescopes combined with

prismatic apparatus of crystal and rock-salt, or some other materials

transparent to the shortest wave-lengths. But since these special

arrangements are rarely made, nebular spectrography makes slow progress.

Its conditions, in the case of “white” nebulæ, are still more

embarrassing. Nor are they much alleviated by substituting direct vision

for the camera. All nebulæ are intrinsically faint, and most give

continuous spectra. A scant supply of light makes, however, a much

better show, as can readily be imagined, when concentrated in a few

bright lines, than when dispersed uninterruptedly along the

colour-scale. To the eye, the resulting variegated streak is dim and

featureless; the sensitive plate takes cognisance of it only under the

compulsion of prolonged exposures, and then imperfectly. The task of

overcoming these obstacles is arduous, yet far from hopeless. It has

been taken in hand, and on the success attending its prosecution the

future of nebular physics essentially depends.

The discovery of helium as a terrestrial element marked a fresh point of

departure in the chemistry of the heavenly bodies. Its leading

chromospheric ray, D_{3}, had already been noted as an emission-line in

the Orion nebula by Dr. Copeland, as an absorption-line in Rigel by

Professor Keeler; but this was only preliminary to what was to follow

when recognition-marks were multiplied by complete experimental

acquaintance with the associates of the yellow beam. One of these, a

line in the blue (λ 4472), is much more conspicuous in stellar spectra

than D_{3}, and it was established by Vogel in 1895, with other members

of the conjoined series, as distinctive of a large class of “helium

stars.”[329] Some insight was thus gained into the extraordinary

profusion with which this strange gas, so sparingly occluded by the

earth, is dispensed to the suns in space. In order to show

spectroscopically, it must, as already stated, be voluminously present

in a glowing atmosphere. Its rays scarcely endure competition. Our sun,

for example, although surrounded by huge volumes of helium, is not a

“helium star.” Pretty sure indications, on the other hand, can be

gathered that helium is one of the principal components of all gaseous

nebulæ; and it blazes where stellar incandescence is strong—in “new”

stars, in bright-line stars, and in some “long-period” variables. The

disclosure of its great cosmical rôle is among the most important

consequences of the modern alliance between astronomy and terrestrial

physics.

The identification of oxygen and nitrogen as stellar constituents by Mr.

McClean and Sir William and Lady Huggins respectively, and the detection

of the “Pickering series” of hydrogen in certain stars, are advances of

scarcely less moment. These unusual kinds of absorption emerge, as a

rule, in stars showing helium as well, and generally assumed to be at an

early stage of growth. They apparently tend to supersede metallic

action, which becomes imperceptible when the inchoate stage is

approached. Thus the nebular spectrum includes no lines of known metals,

and they are likewise apparently missing from “Wolf-Rayet” stars.

Acquaintance with these remarkable objects has profoundly altered the

views of stellar physicists. It has introduced them to a borderland

where the prevalent conditions defy forecast. Who, for instance, could

have anticipated Campbell’s observations of mixed bright and dark

spectral series, derived from the same element, in the same object?

Nothing could well be more perplexing; but perplexities are often the

raw material of discoveries.

The application of photography to the examination of “blaze stars” began

with the apparition of Nova Aurigæ in 1892. Opportunities for its

continuance have since been frequently afforded, which would probably

have slipped by unused but for the automatic watch kept on the changes

of the heavens at Harvard College and its southern dependency. Two

generalisations have thus been authorised. One is that the spectra of

Novæ are mainly composed of bright and dark lines in pairs, emanating

from the same substances, but pushed asunder as if by the effect of

swift opposite motions. The second is that fading Novæ put on a nebular

light-vesture. Spectroscopically, they simulate minute “planetaries.”

These surprising facts are seeds of future knowledge; they need time to

germinate and yield fruit.

Stellar variability no longer remains outside the pale of successful

research. The mystery surrounding it has not, to be sure, been

dissipated; but some of its attendant circumstances have become

manifest. Eclipsing stars are now fully open to investigation; and

although not physically variable, they have indisputable connections

with stars that are. Fluctuations executed quite punctually in periods

of a few days or hours indicate a compound nature in the objects

undergoing them, even if they cannot be explained as occultation-phases.

Short-period variables, in short, are non-eclipsing spectroscopic

binaries. Yet spectroscopic binaries, indistinguishable from them as to

their orbital conditions, shine with a perfectly steady lustre. And

these are the majority. It remains to be discovered what are the special

attributes of the differentiated classes. Nor is variability found only

in conjoined objects. Stars apparently single are subject to extreme,

although more or less irregular vicissitudes. The only clue to the

nature of these vicissitudes yet found is the fact that increase of

light is ordinarily attended by the spectroscopic flashing out of

hydrogen-rays. Moreover, the resemblance of stellar light-curves to

sun-spot tracings gives a strong hint that the solar analogy should be

made a starting-point for inquiries into “long-period” variability.

The branch of stellar astronomy concerned with double and multiple

systems gains extension and importance year by year. Their evolution

under the influence of tidal friction has been studied by Dr. See. Sir

William Huggins’s device of a slit with reflective jaws having

facilitated spectroscopic observations of close stars, the analysis of

their light has at last entered upon a stadium of progress; apart from

which perennial obscurity must have hung over sidereal chromatics, and

enveloped theories of sidereal growth. Spectroscopic binaries,

meanwhile, are multiplying on our hands, and their varieties offer a

brilliant field for investigation. Their periods range from less than

one day up to two years; some revolve in subordination to larger

combinations; their orbits are variously inclined, and of various

degrees of ellipticity; and one of the circulating bodies is, more often

than not, sensibly devoid of light. The function and place in creation

of “dark stars” have thus come into the foreground of inquiry. They

occur also in telescopic systems, where their disturbing power produces

a visible _swaying_ in the movements of their bright companions; but

less commonly, it would seem, than as members of spectroscopic couples.

These are, so to speak, just out of the shell; hence dark stars can

hardly be effete suns, although suns presumably lapse with age into

obscurity. The distinction is a delicate one to draw, but should not be

lost sight of.

The scope of sidereal research is limitless—limitless because its

objects are indefinitely numerous. The difficulty is to lay hold of

them. Inferences based on partial surveys are felt to be unsatisfactory.

The star-depths beyond continually invite farther and farther advances.

Astronomical curiosity is only temporarily appeased by learning, for

instance, the radial movements of a few score of stars; they are wanted

by the hundred, and when at hand in hundreds, they will be in demand by

the thousand. Exhaustive inquiries, while remaining unattainable, must,

by the nature of the case, be perpetually aimed at. And what is

primarily needed for them is a plentiful supply of light. Ambition in

telescope-building is then justifiably insatiate, nor has it, so far,

overreached itself. Each addition to instrumental capacity has, on the

contrary, notably widened the horizon of feasible research. The erection

of the great Lick refractor made it possible to obtain legible

spectrographs of the Wolf-Rayet stars, and set the nebulæ in motion by

enabling Professor Keeler to determine their radial velocities. With the

Crossley reflector, mounted in the same superb situation, nebular

photography made a fresh start, and the law of spirality, as a

structural principle, was confirmed and generalised. The Yerkes

telescope has given access to several closed fields. By its means,

variable stars are followed through their semi-extinct phases. Professor

Barnard has performed the unique task of verifying visually the rapid

light-changes of the minute components of globular clusters; and

Professor Hale has examined spectrographically a large number of

“carbon” stars, too faint as well as too red for satisfactory treatment

under average circumstances with the camera. The completion, at the same

observatory, of a five-foot reflector will shortly afford even better

opportunities for prosecuting this work. It may be added that a special

function in nebular investigations is reserved for instruments of

abnormally short focal length, such as the Meudon reflector, in which

the rays collected by a mirror thirty-nine inches across form an image

at only thrice that distance from it.

Centuries, however, of invention and contrivance must elapse before the

_minuta plebs_ of the sky can be individualised by the peculiarities of

their spectra. Inducements to keep up this strain of toil are not

wanting. “A star’s a star for a’ that,” even though it lie beyond reach

of human questioning. To bring it virtually nearer, is the object of

perpetual efforts. And they cannot fail to be rewarded. Novelties in the

heavens are likely to prove endless and surprising. The enticements they

offer to progress are irresistible. No “anchor dropt at eve or morn” can

stay the ideal voyage. “When a man hath done, then shall he begin” his

scrutiny into the works of the Most High.