_RECENT SOLAR ECLIPSES_
By observations made during a series of five remarkable eclipses,
comprised within a period of eleven years, knowledge of the solar
surroundings was advanced nearly to its present stage. Each of these
events brought with it a fresh disclosure of a definite and unmistakable
character. We will now briefly review this orderly sequence of
discovery.
Photography was first systematically applied to solve the problems
presented by the eclipsed sun, July 18, 1860. It is true that a
daguerreotype,[512] taken by Berkowski with the Königsberg heliometer
during the eclipse of 1851, is still valuable as a record of the corona
of that year; and some subsequent attempts were made to register partial
phases of solar occultation, notably by Professor Bartlett at West Point
in 1854;[513] but the ground remained practically unbroken until 1860.
In that year the track of totality crossed Spain, and thither,
accordingly, Warren de la Rue transported his photo-heliograph, and
Father Secchi his six-inch Cauchoix refractor. The question then
primarily at issue was that relating to the nature of the red
protuberances. Although, as already stated, the evidence collected in
1851 gave a reasonable certainty of their connection with the sun,
objectors were not silenced; and when the side of incredulity was
supported by so considerable an authority as M. Faye, it was impossible
to treat it with contempt. Two crucial tests were available. If it could
be shown that the fantastic shapes suspended above the edge of the dark
moon were seen under an identical aspect from two distant stations, that
fact alone would annihilate the theory of optical illusion or "mirage";
while the certainty that they were progressively concealed by the
advancing moon on one side, and uncovered on the other, would
effectually detach them from dependence on our satellite, and establish
them as solar appendages.
Now both these tests were eminently capable of being applied by
photography. But the difficulty arose that nothing was known as to the
chemical power of the rosy prominence-light, while everything depended
on its right estimation. A shot had to be fired, as it were, in the
dark. It was a matter of some surprise, and of no small congratulation,
that, in both cases, the shot took effect.
De la Rue occupied a station at Rivabellosa, in the Upper Ebro valley;
Secchi set up his instrument at Desierto de las Palmas, about 250 miles
to the south-east, overlooking the Mediterranean. From the totally
eclipsed sun, with its strange garland of flames, each observer derived
several perfectly successful impressions, which were found, on
comparison, to agree in the most minute details. This at once settled
the fundamental question as to the substantial reality of these objects;
while their solar character was demonstrated by the passage of the moon
_in front_ of them, indisputably attested by pictures taken at
successive stages of the eclipse. That forms seeming to defy all laws of
equilibrium were, nevertheless, not wholly evanescent, appeared from
their identity at an interval of seven minutes, during which the lunar
shadow was in transit from one station to the other; and the singular
energy of their actinic rays was shown by the record on the sensitive
plates of some prominences invisible in the telescope. Moreover,
photographic evidence strongly confirmed the inference--previously drawn
by Grant and others, and now with fuller assurance by Secchi--that an
uninterrupted stratum of prominence-matter encompasses the sun on all
sides, forming a reservoir from which gigantic jets issue, and into
which they subside.
Thus, first-fruits of accurate knowledge regarding the solar
surroundings were gathered, while the value of the brief moments of
eclipse gained indefinite increase, by supplementing transient visual
impressions with the faithful and lasting records of the camera.
In the year 1868 the history of eclipse spectroscopy virtually began, as
that of eclipse photography in 1860; that is to say, the respective
methods then first gave definite results. On the 18th of August, 1868,
the Indian and Malayan peninsulas were traversed by a lunar shadow
producing total obscuration during five minutes and thirty-eight
seconds. Two English and two French expeditions were despatched to the
distant regions favoured by an event so propitious to the advance of
knowledge, chiefly to obtain the verdict of the prism as to the
composition of prominences. Nor were they despatched in vain. An
identical discovery was made by nearly all the observers. At Jamkandi,
in the Western Ghauts, where Lieutenant (now Colonel) Herschel was
posted, unremitting bad weather threatened to baffle his eager
expectations; but during the lapse of the critical five and a half
minutes the clouds broke, and across the driving wrack a "long,
finger-like projection" jutted out over the margin of the dark lunar
globe. In another moment the spectroscope was pointed towards it; three
bright lines--red, orange, and blue--flashed out, and the problem was
solved.[514] The problem was solved in this general sense, that the
composition out of glowing vapours of the objects infelicitously termed
"protuberances" or "prominences" was no longer doubtful; although
further inquiry was needed for the determination of the particular
species to which those vapours belonged.
Similar, but more complete observations were made, with less atmospheric
hindrance, by Tennant and Janssen at Guntoor, by Pogson at Masulipatam,
and by Rayet at Wha-Tonne, on the coast of the Malay peninsula, the last
observer counting as many as nine bright lines.[515] Among them it was
not difficult to recognise the characteristic light of hydrogen; and it
was generally, though over-hastily, assumed that the orange ray matched
the luminous emissions of sodium. But fuller opportunities were at hand.
The eclipse of 1868 is chiefly memorable for having taught astronomers
to do without eclipses, so far, at least, as one particular branch of
solar inquiry is concerned. Inspired by the beauty and brilliancy of the
variously tinted prominence-lines revealed to him by the spectroscope,
Janssen exclaimed to those about him, "Je verrai ces lignes-là en dehors
des éclipses!" On the following morning he carried into execution the
plan which formed itself in his brain while the phenomenon which
suggested it was still before his eyes. It rests upon an easily
intelligible principle.
The glare of our own atmosphere alone hides the appendages of the sun
from our daily view. To a spectator on an airless planet, the central
globe would appear attended by all its splendid retinue of crimson
prominences, silvery corona, and far-spreading zodiacal light projected
on the star-spangled black background of an absolutely unilluminated
sky. Now the spectroscope offers the means of indefinitely weakening
atmospheric glare by diffusing a constant amount of it over an area
widened _ad libitum_. But monochromatic or "bright-line" light is, by
its nature, incapable of being so diffused. It can, of course, be
_deviated_ by refraction to any extent desired; but it always remains
equally concentrated, in whatever direction it may be thrown. Hence,
when it is mixed up with continuous light--as in the case of the solar
flames shining through our atmosphere--it derives a _relative_ gain in
intensity from every addition to the dispersive power of the
spectroscope with which the heterogeneous mass of beams is analysed.
Employ prisms enough, and eventually the undiminished rays of persistent
colour will stand out from the continually fading rainbow-tinted band,
by which they were at first effectually veiled.
This Janssen saw by a flash of intuition while the eclipse was in
progress; and this he realised at 10 A.M. next morning, August 19,
1868--the date of the beginning of spectroscopic work at the margin of
the unobscured sun. During the whole of that day and many subsequent
ones, he enjoyed, as he said, the advantage of a prolonged eclipse. The
intense interest with which he surveyed the region suddenly laid bare to
his scrutiny was heightened by evidences of rapid and violent change. On
the 18th of August, during the eclipse, a vast spiral structure, _at
least_ 89,000 miles high, was perceived, planted in surprising splendour
on the rim of the interposed moon. If was formed as General Tennant
judged from its appearance in his photographs, by the encounter of two
mounting torrents of flame, and was distinguished as the "Great Horn."
Next day it was in ruins; hardly a trace remained to show where it had
been.[516] Janssen's spectroscope furnished him besides with the
strongest confirmation of what had already been reported by the
telescope and the camera as to the continuous nature of the scarlet
"sierra" lying at the base of the prominences. Everywhere at the sun's
edge the same bright lines appeared.
It was not until the 19th of September that Janssen thought fit to send
news of his discovery to Europe. It seemed little likely to be
anticipated; yet a few minutes before his despatch was handed to the
Secretary of the Paris Academy of Sciences, a communication similar in
purport had been received from Sir Norman Lockyer. There is no need to
discuss the narrow and wearisome question of priority; each of the
competitors deserves, and has obtained, full credit for his invention.
With noteworthy and confident prescience, Lockyer, in 1866, before
anything was yet known regarding the constitution of the "red flames,"
ordered a strongly dispersive spectroscope for the express purpose of
viewing, apart from eclipses, the bright-line spectrum which he expected
them to give. Various delays, however, supervened, and the instrument
was not in his hands until October 16, 1868. On the 20th he picked up
the vivid rays, of which the presence and (approximately) the positions
had in the interim become known. But there is little doubt that, even
without that previous knowledge, they would have been found; and that
the eclipse of August 18 only accelerated a discovery already assured.
Sir William Huggins, meanwhile, had been tending towards the same goal
during two and a half years in his observatory at Tulse Hill. The
principle of the spectroscopic visibility of prominence-lines at the
edge of an uneclipsed sun was quite explicitly stated by him in
February, 1868,[517] and he devised various apparatus for bringing them
into actual view; but not until he knew where to look did he succeed in
seeing them.
Astronomers, thus liberated, by the acquisition of power to survey them
at any time, from the necessity of studying prominences during eclipses,
were able to concentrate the whole of their attention on the corona. The
first thing to be done was to ascertain the character of its spectrum.
This was seen in 1868 only as a faintly continuous one; for Rayet, who
seems to have perceived its distinctive bright line far above the
summits of the flames, connected it, nevertheless, with those objects.
On the other hand, Lieutenant Campbell ascertained on the same occasion
the polarisation of the coronal light in planes passing through the
sun's centre,[518] thereby showing that light to be, in whole or in
part, reflected sunshine. But if reflected sunshine, it was objected,
the chief at least of the dark Fraunhofer lines should be visible in it,
as they are visible in moonbeams, sky illumination, and all other
sun-derived light. The objection was well founded, but was prematurely
urged, as we shall see.
On the 7th of August, 1869, a track of total eclipse crossed the
continent of North America diagonally, entering at Behring's Straits,
and issuing on the coast of North Carolina. It was beset with observers;
but the most effective work was done in Iowa. At Des Moines, Professor
Harkness of the Naval Observatory, Washington, obtained from the corona
an "absolutely continuous spectrum," slightly less bright than that of
the full moon, but traversed by a single green ray.[519] The same green
ray was seen at Burlington and its position measured by Professor Young
of Dartmouth College.[520] It appeared to coincide with that of a dark
line of iron in the solar spectrum, numbered 1,474 on Kirchhoff's scale.
But in 1876 Young was able, by the use of greatly increased dispersion,
to resolve the Fraunhofer line "1474" into a pair, the more refrangible
member of which he considered to be the reversal of the green coronal
ray.[521] Scarcely called in question for over twenty years, the
identification nevertheless broke down through the testimony of the
eclipse-photographs of 1898. Sir Norman Lockyer derived from them a
position for the line in question notably higher up in the spectrum than
that previously assigned to it. Instead of 5,317, its true wave-length
proved to be 5,303 ten millionths of a millimetre;[522] nor does it make
any show by absorption in dispersed sunlight. The originating substance,
designated "coronium," of which nothing is known to terrestrial
chemistry, continues luminous[523] at least 300,000 miles above the
sun's surface, and is hence presumably much lighter even than hydrogen.
A further trophy was carried off by American skill[524] sixteen months
after the determination due to it of the distinctive spectrum of the
corona. The eclipse of December 22, 1870, though lasting only two
minutes and ten seconds, drew observers from the New, as well as from
the Old World to the shores of the Mediterranean. Janssen issued from
beleaguered Paris in a balloon, carrying with him the _vital parts_ of a
reflector specially constructed to collect evidence about the corona.
But he reached Oran only to find himself shut behind a cloud-curtain
more impervious than the Prussian lines. Everywhere the sky was more or
less overcast. Lockyer's journey from England to Sicily, and shipwreck
in the _Psyche_, were recompensed with a glimpse of the solar aureola
during _one second and a half_! Three parties stationed at various
heights on Mount Etna saw absolutely nothing. Nevertheless important
information was snatched in despite of the elements.
The prominent event was Young's discovery of the "reversing layer." As
the surviving solar crescent narrowed before the encroaching moon, "the
dark lines of the spectrum," he tells us, "and the spectrum itself,
gradually faded away, until all at once, as suddenly as a bursting
rocket shoots out its stars, the whole field of view was filled with
bright lines more numerous than one could count. The phenomenon was so
sudden, so unexpected, and so wonderfully beautiful, as to force an
involuntary exclamation."[525] Its duration was about two seconds, and
the impression produced was that of a complete reversal of the
Fraunhofer spectrum--that is, the substitution of a bright for every
dark line.
Now something of the kind was theoretically necessary to account for the
dusky rays in sunlight which have taught us so much, and have yet much
more to teach us; so that, although surprising from its transitory
splendour, the appearance could not strictly be called "unexpected."
Moreover, its premonitory symptom in the fading out of these rays had
been actually described by Secchi in 1868,[526] and looked for by Young
as the moon covered the sun in August 1869. But with the slit of his
spectroscope placed _normally_ to the sun's limb, the bright lines gave
a flash too thin to catch the eye. In 1870 the position of the slit was
_tangential_--it ran along the shallow bed of incandescent vapours,
instead of cutting across it: hence his success.
The same observation was made at Xerez de la Frontera by Mr. Pye, a
member of Young's party; and, although an exceedingly delicate one, has
since frequently been repeated. The whole Fraunhofer series appeared
bright (omitting other instances) to Maclear, Herschel, and Fyers in
1871, at the beginning or end of totality; to Pogson, at the break-up of
an annual eclipse, June 6, 1872; to Stone at Klipfontein, April 16,
1874, when he saw "the field full of bright lines."[527] But between the
picture presented by the "véritable pluie de lignes brilliantes,"[528]
which descended into M. Trépied's spectroscope for three seconds after
the disappearance of the sun, May 17, 1882, and the familiar one of the
dark-line solar spectrum, certain differences were perceiving, showing
their relation to be not simply that of a positive to a negative
impression.
A "reversing layer," or stratum of mixed vapours, glowing, but at a
lower temperature than that of the actual solar surface, was an integral
part of Kirchhoff's theory of the production of the Fraunhofer lines.
Here it was assumed that the missing rays were stopped, and here also it
was assumed that the missing rays would be seen bright, could they be
isolated from the overpowering splendour of their background. This
isolation is effected by eclipses, with the result--beautifully
confirmatory of theory--of _reversing_, or turning from dark to bright,
the Fraunhofer spectrum. The completeness and precision of the reversal,
however, could not be visually attested; and a quarter of a century
elapsed before a successful "snap-shot" provided photographic evidence
on the subject. It was taken at Novaya Zemlya by Mr. Shackleton, a
member of the late Sir George Baden-Powell's expedition to observe the
eclipse of August 9, 1896;[529] and similar records in abundance were
secured during the Indian eclipse of January 22, 1898,[530] and the
Spanish-American eclipse of May 28, 1900.[531] The result of their
leisurely examination has been to verify the existence of a
"reversing-layer," in the literal sense of the term. It is true that no
single "flash" photograph is an inverted transcript of the Fraunhofer
spectrum. The lines are, indeed, in each case--speaking broadly--the
same; but their relative intensities are widely different. Yet this need
occasion no surprise when we remember that the Fraunhofer spectrum
integrates the absorption of multitudinous strata, various in density
and composition, while only the upper section of the formation comes
within view of the sensitive plates exposed at totalities, the low-lying
vaporous beds being necessarily covered by the moon. The total depth of
this glowing envelope may be estimated at 500 to 600 miles, and its
normal state seems to be one of profound tranquillity, judging from the
imperturbable aspect of the array of dark lines due to its sifting
action upon light.
The last of the five eclipses which we have grouped together for
separate consideration was visible in Southern India and Australia,
December 12, 1871. Some splendid photographs were secured by the English
parties on the Malabar coast, showing, for the first time, the
remarkable branching forms of the coronal emanations; but the most
conspicuous result was Janssen's detection of some of the dark
Fraunhofer lines, long vainly sought in the continuous spectrum of the
corona. Chief among these was the D-line of sodium, the original index,
it might be said, to solar chemistry. No proof could be afforded more
decisive that this faint _echoing back_ of the distinctive notes of the
Fraunhofer spectrum, that the polariscope had spoken the truth in
asserting a large part of the coronal radiance to be reflected sunlight.
But it is usually so drenched in original luminosity, that its special
features are almost obliterated. Janssen's success in seizing them was
due in part to the extreme purity of the air at Sholoor, in the
Neilgherries, where he was stationed; in part to the use of an
instrument adapted by its large aperture and short focus to give an
image of the utmost brilliancy. His observation, repeated during the
Caroline Island eclipse of 1883, was photographically verified ten years
later by M. de la Baume Pluvinel in Senegal.[532]
An instrument of great value for particular purposes was introduced into
eclipse-work in 1871. The "slitless spectroscope" consists simply of a
prism placed outside the object-glass of a telescope or the lens of a
camera, whereby the radiance encompassing the eclipsed sun is separated
into as many differently tinted rings as it contains different kinds of
light. These tinted rings were simultaneously viewed by Respighi at
Poodacottah, and by Lockyer at Baikul. Their photographic registration
by the latter in 1875 initiated the transformation of the slitless
spectroscope into the prismatic camera.[533] Meanwhile, the use of an
ordinary spectroscope by Herschel and Tennant at Dodabetta showed the
green ray of coronium to be just as bright in a rift as in the adjacent
streamer. The visible structure of the corona was thus seen to be
independent of the distribution of the gases which enter into its
composition.
By means, then, of the five great eclipses of 1860-71 it was
ascertained: first, that the prominences, and at least the lower part of
the corona, are genuine solar appurtenances; secondly, that the
prominences are composed of hydrogen and other gases in a state of
incandescence, and rise, as irregular outliers, from a continuous
envelope of the same materials, some thousands of miles in thickness;
thirdly, that the corona is of a highly complex constitution, being made
up in part of glowing vapours, in part of matter capable of reflecting
sunlight. We may now proceed to consider the results of subsequent
eclipses.
These have raised, and have helped to solve, some very curious
questions. Indeed, every carefully watched total eclipse of the sun
stimulates as well as appeases curiosity, and leaves a legacy of
outstanding doubt, continually, as time and inquiry go on, removed, but
continually replaced. It cannot be denied that the corona is a
perplexing phenomenon, and that it does not become less perplexing as we
know more about it. It presented itself under quite a new and strange
aspect on the occasion of the eclipse which visited the Western States
of North America, July 29, 1878. The conditions of observation were
peculiarly favourable. The weather was superb; above the Rocky Mountains
the sky was of such purity as to permit the detection of Jupiter's
satellites with the naked eye on several successive nights. The
opportunity for advancing knowledge was made the most of. Nearly a
hundred astronomers, including several Englishmen, occupied twelve
separate posts, and prepared for an attack in force.
The question had often suggested itself, and was a natural one to ask,
whether the corona sympathises with the general condition of the sun?
whether, either in shape or brilliancy, it varies with the progress of
the sun-spot period? A more propitious moment for getting this question
answered could hardly have been chosen than that at which the eclipse
occurred. Solar disturbance was just then at its lowest ebb. The
development of spots for the month of July, 1878, was represented on
Wolf's system of "relative numbers" by the fraction 0·1, as against
135·4 for December, 1870, an epoch of maximum activity. The
"chromosphere"[534] was, for the most part, shallow and quiescent; its
depth, above the spot zones, had sunk from about 6,000 to 2,000
miles;[535] prominences were few and faint. Obviously, if a type of
corona corresponding to a minimum of sun-spots existed, it should be
seen then or never. It _was_ seen; but while, in some respects, it
agreed with anticipation, in others it completely set it at naught.
The corona of 1878, as compared with those of 1869, 1870, and 1871, was
generally admitted to be shrunken in its main outlines and much reduced
in brilliancy. Lockyer pronounced it ten times fainter than in 1871;
Harkness estimated its light at less than one-seventh that derived from
the mist-blotted aureola of 1870.[536] In shape, too, it was markedly
different. When sun-spots are numerous, the corona appears to be most
fully developed above the spot-zones, thus offering to our eyes a rudely
quadrilateral contour. The four great luminous sheaves forming the
corners of the square are made up of rays curving together from each
side into "synclinal" or ogival groups, each of which may be compared to
the petal of a flower. To Janssen, in 1871, the eclipsing moon seemed
like the dark heart of a gigantic dahlia, painted in light on the sky;
and the similitude to the ornament on a compass-card, used by Airy in
1851, well conveys the decorative effect of the beamy, radiated kind of
aureola, never, it would appear, absent when solar activity is at a
tolerably high pitch. In his splendid volume on eclipses,[537] with
which the systematic study of coronal structure may be said to have
begun, Mr. Ranyard first generalised the synclinal peculiarity by a
comparison of records; but the symmetry of the arrangement, though
frequently striking, is liable to be confused by secondary formations.
He further pointed out, with the help of careful drawings from the
photographs of 1871 made by Mr. Wesley, the curved and branching shapes
assumed by the component filaments of massive bundles of rays. Nothing
of all this, however, was visible in 1878. Instead, there was seen, as
the groundwork of the corona, a ring of pearly light, nebulous to the
eye, but shown by telescopes and in photographs to have a fibrous
texture, as if made up of tufts of fine hairs. North and south, a series
of short, vivid, electrical-looking flame-brushes diverged with
conspicuous regularity from each of the solar poles. Their direction was
not towards the centre of the sun, but towards each summit of his axis,
so that the farther rays on either side started almost tangentially to
the surface.
But the leading, and a truly amazing, characteristic of the phenomenon
was formed by two vast, faintly-luminous _wings_ of light, expanded on
either side of the sun in the direction of the ecliptic. These were
missed by very few careful onlookers; but the extent assigned to them
varied with skill in, and facilities for seeing. By far the most
striking observations were made by Newcomb at Separation (Wyoming), by
Cleveland Abbe from the shoulder of Pike's Peak, and by Langley at its
summit, an elevation of 14,100 feet above the sea. Never before had an
eclipse been viewed from anything approaching that altitude, or under so
translucent a sky. A proof of the great reduction in atmospheric glare
was afforded by the perceptibility of the corona four minutes after
totality was over. For the 165 seconds of its duration, the remarkable
streamers above alluded to continued "persistently visible," stretching
away right and left of the sun to a distance of at least ten million
miles! One branch was traced over an apparent extent of fully twelve
lunar diameters, without sign of a definite termination having been
reached; and there were no grounds for supposing the other more
restricted.
The resemblance to the zodiacal light was striking; and a community of
origin between that enigmatical member of our system and the corona was
irresistibly suggested. We should, indeed, expect to see, under such
exceptionally favourable atmospheric conditions as Professor Langley
enjoyed on Pike's Peak, the _roots_ of the zodiacal light presenting
near the sun just such an appearance as he witnessed; but we can imagine
no reason why their visibility should be associated with a low state of
solar activity. Nevertheless this seems to be the case with the
streamers which astonished astronomers in 1878. For in August, 1867,
when similar equatorial emanations, accompanied by similar symptoms of
polar excitement, were described and depicted by Grosch[538] of the
Santiago Observatory, sun-spots were at a minimum; while the corona of
1715, which appears from the record of it by Roger Cotes[539] to have
been of the same type, preceded by three years the ensuing maximum. The
eclipsed sun was seen by him at Cambridge, May 2, 1715, encompassed with
a ring of light about one-sixth of the moon's diameter in breadth, upon
which was superposed a luminous cross formed of long bright branches
lying very nearly in the plane of the ecliptic, and shorter polar arms
so faint as to be only intermittently visible. The resemblance between
his sketch and Cleveland Abbe's drawing of the corona of 1878 is
extremely striking. It should, nevertheless, be noted that some
conspicuous spots were visible on the sun's disc at the time of Cotes's
eclipse, and that the preceding minimum (according to Wolf) occurred in
1712. Thus, the coincidence of epochs is imperfect.
Professor Cleveland Abbe was fully persuaded that the long rays
carefully observed by him from Pike's Peak were nothing else than
streams of meteorites rushing towards or from perihelion; and it is
quite certain that the solar neighbourhood must be crowded with such
bodies. But the peculiar structure at the base of the streamers
displayed in the photographs, the curved rays meeting in pointed arches
like Gothic windows, the visible upspringing tendency, the filamentous
texture,[540] speak unmistakably of the action of forces proceeding
_from_ the sun, not of extraneous matter circling round him.
A further proof of sympathetic change in the corona is afforded by the
analysis of its light. In 1878 the bright line so conspicuous in the
coronal spectrum in 1870 and 1871 had faded to the very limit of
visibility. Several skilled observers failed to see it at all; but Young
and Eastman succeeded in tracing the green "coronium" ray all round the
sun, to a height estimated at 340,000 miles. The substance emitting it
was thus present, though in a low state of incandescence. The continuous
spectrum was relatively strong; faint traces of the Fraunhofer lines
attested for it an origin, in part by reflection; and polarisation was
undoubted, increasing towards the limb, whereas in 1870 it reached a
maximum at a considerable distance from it. Experiments with Edison's
tasimeter seemed to show that the corona radiates a sensible amount of
heat.
The next promising eclipse occurred May 17, 1882. The concourse of
astronomers which has become usual on such occasions assembled this time
at Sohag, in Upper Egypt. Rarely have seventy-four seconds been turned
to such account. To each observer a special task was assigned, and the
advantages of a strict division of labour were visible in the variety
and amount of the information gained.
The year 1882 was one of numerous sun-spots. On the eve of the eclipse
twenty-three separate maculæ were counted. If there were any truth in
the theory which connected coronal forms with fluctuations in solar
activity, it might be anticipated that the vast equatorial expansions
and polar "brushes" of 1878 would be found replaced by the star-like
structure of 1871. This expectation was literally fulfilled. No lateral
streamers were to be seen. The universal failure to perceive them, after
express search in a sky of the most transparent purity, justifies the
emphatic assertion that _they were not there_. Instead, the type of
corona observed in India eleven years earlier, was reproduced with its
shining aigrettes, complex texture and brilliant radiated aspect.
Concordant testimony was given by the spectroscope. The reflected light
derived from the corona was weaker than in 1878, while its original
emissions were proportionately intensified. Nevertheless, most of the
bright lines recorded as coronal[541] were really due, there can be no
doubt, to diffused chromospheric light. On this occasion, the first
successful attempt was made to photograph the coronal spectrum procured
in the ordinary way with a slit and prisms, while the prismatic camera
was also profitably employed. It served to bring out at least one
important fact--that of the uncommon strength in chromospheric regions
of the twin violet beams of calcium, designated "H" and "K"; and
prominence-photography signalised its improvement by the registration,
in the spectrum of one such object, of twenty-nine rays, including many
of the ultra-violet hydrogen series discovered by Sir William Huggins in
the emission of white stars.[542]
Dr. Schuster's photographs of the corona itself were the most extensive,
as well as the most detailed, of any yet secured. One rift imprinted
itself on the plates to a distance of nearly a diameter and a half from
the limb; and the transparency of the streamers was shown by the
delineation through them of the delicate tracery beyond. The singular
and picturesque feature was added of a bright comet, self-depicted in
all the exquisite grace of swift movement betrayed by the fine curve of
its tail, hurrying away from one of its rare visits to our sun, and
rendered momentarily visible by the withdrawal of the splendour in which
it had been, and was again quickly veiled.
From a careful study of these valuable records Sir William Huggins
derived the idea of a possible mode of photographing the corona _without
an eclipse_.[543] As already stated, its ordinary invisibility is
entirely due to the "glare" or reflected light diffused through our
atmosphere. But Huggins found, on examining Schuster's negatives, that a
large proportion of the light in the coronal spectrum, both continuous
and interrupted, is collected in the violet region between the
Fraunhofer lines G and H. There, then, he hoped that, all other rays
being excluded, it might prove strong enough to vanquish inimical glare,
and stamp on prepared plates, through _local_ superiority in
illuminative power, the forms of the appendage by which it is emitted.
His experiments were begun towards the end of May, 1882, and by
September 28 he had obtained a fair earnest of success. The exclusion of
all other qualities of light save that with which he desired to operate,
was accomplished by using chloride of silver as his sensitive material,
that substance being chemically inert to all other but those precise
rays in which the corona has the advantage.[544] Plates thus sensitised
received impressions which it was hardly possible to regard as spurious.
"Not only the general features," Captain Abney affirmed,[545] "are the
same, but details, such as rifts and streamers, have the same position
and form." It was found, moreover, that the corona photographed during
the total eclipse of May 6, 1883, was intermediate in shape between the
coronas photographed by Sir William Huggins before and after that event,
each picture taking its proper place in a series of progressive
modifications highly interesting in themselves, and full of promise for
the value of the method employed to record them.[546] But experiments on
the subject were singularly interrupted. The volcanic explosion in the
Straits of Sunda in August, 1883, brought to astronomers a peculiarly
unwelcome addition to their difficulties. The magnificent sunglows due
to the diffractive effects on light of the vapours and fine dust flung
in vast volumes into the air, and rapidly diffused all round the globe,
betokened an atmospheric condition of all others the most prejudicial to
delicate researches in the solar vicinity. The filmy coronal forms,
accordingly, which had been hopefully traced on the Tulse Hill plates
ceased to appear there; nor were any substantially better results
obtained by Mr. C. Ray Woods, in the purer air either of the Riffel or
the Cape of Good Hope, during the three ensuing years. Moreover,
attempts to obtain coronal photographs during the partial phases of the
eclipse of August 29, 1886, completely failed. No part of the lunar
globe became visible in relief against circumfluous solar radiance on
any of the plates exposed at Grenada; and what vestiges of "structure"
there were, came out almost better _upon_ the moon than _beside_ her,
thus stamping themselves at once as of atmospheric origin.
That the effect sought is a perfectly possible one is proved by the
distinct appearance of the moon projected on the corona, in photographs
of the partially eclipsed sun in 1858, 1889, and 1890, and very notably
in 1898 and 1900.[547]
In the spring of 1893, Professor Hale[548] attacked the problem of
coronal daylight photography, employing the "double-slit" method so
eminently serviceable for the delineation of prominences.[549] But
neither at Kenwood nor at the summit of Pike's Peak, whither, in the
course of the summer, he removed his apparatus, was any action of the
desired kind secured. Similar ill success attended his and Professor
Riccò's employment, on Mount Etna in July, 1894, of a specially designed
coronagraph. Yet discouragement did not induce despair. The end in view
is indeed too important to be readily abandoned; but it can be reached
only when a more particular acquaintance with the nature of coronal
light than we now possess indicates the appropriate device for giving it
a preferential advantage in self-portraiture. Moreover, the
effectiveness of this device may not improbably be enhanced, through
changes in the coronal spectrum at epochs of sun-spot maximum.
The prosperous result of the Sohag observations stimulated the desire to
repeat them on the first favourable opportunity. This offered itself one
year later, May 6, 1883, yet not without the drawbacks incident to
terrestrial conditions. The eclipse promised was of rare length, giving
no less than five minutes and twenty-three seconds of total obscurity,
but its path was almost exclusively a "water-track." It touched land
only on the outskirts of the Marquesas group in the Southern Pacific,
and presented, as the one available foothold for observers, a coral reef
named Caroline Island, seven and a half miles long by one and a half
wide, unknown previously to 1874, and visited only for the sake of its
stores of guano. Seldom has a more striking proof been given of the
vividness of human curiosity as to the condition of the worlds outside
our own, than in the assemblage of a group of distinguished men from the
chief centres of civilisation, on a barren ridge, isolated in a vast and
tempestuous ocean, at a distance, in many cases, of 11,000 miles and
upwards from the ordinary scene of their labours. And all these
sacrifices--the cost and care of preparation, the transport and
readjustment of delicate instruments, the contrivance of new and more
subtle means of investigating phenomena--on the precarious chance of a
clear sky during one particular five minutes! The event, though
fortunate, emphasised the hazard of the venture. The observation of the
eclipse was made possible only by the happy accident of a serene
interval between two storms.
The American expedition was led by Professor Edward S. Holden, and to it
were courteously permitted to be attached Messrs. Lawrance and Woods,
photographers, sent out by the Royal Society of London. M. Janssen was
chief of the French Academy mission; he was accompanied from Meudon by
Trouvelot, and joined from Vienna by Palisa, and from Rome by Tacchini.
A large share of the work done was directed to assuring or negativing
previous results. The circumstances of an eclipse favour illusion. A
single observation by a single observer, made under unfamiliar
conditions, and at a moment of peculiar excitement, can scarcely be
regarded as offering more than a suggestion for future inquiry. But
incredulity may be carried too far. Janssen, for instance, felt
compelled, by the survival of unwise doubts, to devote some of the
precious minutes of obscurity at Caroline Island to confirming what, in
his own persuasion, needed no confirmation--that is, the presence of
reflected Fraunhofer lines in the spectrum of the corona. Trouvelot and
Palisa, on the other hand, instituted an exhaustive, but fruitless
search for the spurious "intramercurian" planets announced by Swift and
Watson in 1878.
New information, however, was not deficient. The corona proved identical
in type with that of 1882,[550] agreeably to what was expected at an
epoch of protracted solar activity. The characteristic aigrettes were of
even greater brilliancy than in the preceding year, and the chemical
effects of the coronal light proved unusually intense. Janssen's
photographs, owing to the considerable apertures (six and eight inches)
of his object-glasses, and the long exposures permitted by the duration
of totality, were singularly perfect; they gave a greater extension to
the coronal than could be traced with the telescope,[551] and showed its
forms as absolutely fixed and of remarkable complexity.
The English pictures, taken with exposures up to sixty seconds, were
likewise of great value. They exhibited details of structure from the
limb to the tips of the streamers, which terminated definitely, and as
it seemed actually, where the impressions on the plates ceased. The
coronal spectrum was also successfully photographed, and although the
reversing layer in its entirety evaded record, a print was caught of
some of its more prominent rays just before and after totality. The use
of the prismatic camera was baffled by the anomalous scarcity of
prominences.
Using an ingenious apparatus for viewing simultaneously the spectrum
from both sides of the sun, Professor Hastings noticed at Caroline
Island alternations, with the advance of the moon, in the respective
heights above the right and left solar limbs of the coronal green line,
which were thought to imply that the corona, with its rifts and sheaves
and "tangled hanks" of rays, is, after all, merely an illusive
appearance produced by the diffraction of sunlight at the moon's
edge.[552] But the observation was assuredly misleading or
misinterpreted. Atmospheric _diffusion_ may indeed, under favouring
circumstances, be effective in deceptively enlarging solar appendages;
but always to a very limited extent.
The controversy is an old one as to the part played by our air in
producing the radiance visible round the eclipsed sun. In its original
form, it is true, it came to an end when Professor Harkness, in
1869,[553] pointed out that the shadow of the moon falls equally over
the air and on the earth, and that if the sun had no luminous
appendages, a circular space of almost absolute darkness would
consequently surround the apparent places of the superposed sun and
moon. Mr. Proctor,[554] with his usual ability, impressed this
mathematically certain truth upon public attention; and Sir John
Herschel calculated that the diameter of the "negative halo" thus
produced would be, in general, no less than 23°.
But about the same time a noteworthy circumstance relating to the state
of things in the solar vicinity was brought into view. On February 11,
1869, Messrs. Frankland and Lockyer communicated to the Royal Society a
series of experiments on gaseous spectra under varying conditions of
heat and density, leading them to the conclusion that the higher solar
prominences exist in a medium of excessive tenuity, and that even at the
base of the chromosphere the pressure is far below that at the earth's
surface.[555] This inference was fully borne out by the researches of
Wüllner; and Janssen expressed the opinion that the chromospheric gases
are rarefied almost to the degree of an air-pump vacuum.[556] Hence was
derived a general and fully justified conviction that there could be
outside, and incumbent upon the chromosphere, no such vast atmosphere as
the corona appeared to represent. Upon the strength of which conviction
the "glare" theory entered, chiefly under the auspices of Sir Norman
Lockyer, upon the second stage of its existence.
The genuineness of the "inner corona" to the height of 5' or 6' from the
limb was admitted; but it was supposed that by the detailed reflection
of its light in our air the far more extensive "outer corona" was
optically created, the irregularities of the moon's edge being called in
to account for the rays and rifts by which its structure was varied.
This view received some countenance from Admiral Maclear's observation,
during the eclipse of 1870, of bright lines "everywhere"--even at the
centre of the lunar disc. Here, indeed, was an undoubted case of
atmospheric diffusion; but here, also, was a safe index to the extent of
its occurrence. Light scatters equally in all directions; so that when
the moon's face at the time of an eclipse shows (as is the common case)
a blank in the spectroscope, it is quite certain that the corona is not
noticeably enlarged by atmospheric causes. A sky drifted over with thin
cirrus clouds and air changed with aqueous vapour amply accounted for
the abnormal amount of scattering in 1870.
But even in 1870 positive evidence was obtained of the substantial
reality of the radiated outer corona, in the appearance on the
photographic plates exposed by Willard in Spain and by Brothers in
Sicily of identical dark rifts. The truth is, that far from being
developed by misty air, it is peculiarly liable to be effaced by it. The
purer the sky, the more extensive, brilliant, and intricate in the
details of its structure the corona appears. Take as an example General
Myer's description of the eclipse of 1869, as seen from the summit of
White Top Mountain, Virginia, at an elevation above the sea of 5,523
feet, in an atmosphere of peculiar clearness.
"To the unaided eye," he wrote,[557] "the eclipse presented, during the
total obscuration, a vision magnificent beyond description. As a centre
stood the full and intensely black disc of the moon, surrounded by the
aureola of a soft bright light, through which shot out, as if from the
circumference of the moon, straight, massive, silvery rays, seeming
distinct and separate from each other, to a distance of two or three
diameters of the solar disc; the whole spectacle showing as on a
background of diffused rose-coloured light."
On the same day, at Des Moines, Newcomb could perceive, through somewhat
hazy air, no long rays, and the four-pointed outline of the corona
reached at its farthest only a _single semidiameter_ of the moon from
the limb. The plain fact, that our atmosphere acts rather as a veil to
hide the coronal radiance than as the medium through which it is
visually formed, emerges from further innumerable records.
No observations of importance were made during the eclipse of September
9, 1885. The path of total obscurity touched land only on the shores of
New Zealand, and two minutes was the outside limit of available time.
Hence local observers had the phenomenon to themselves; nor were they
even favoured by the weather in their efforts to make the most of it.
One striking appearance was, however, disclosed. It was that of two
"white" prominences of unusual brilliancy, shining like a pair of
electric lamps hung one at each end of a solar diameter, right above the
places of two large spots.[558] This coincidence of diametrically
opposite disturbances is of too frequent occurrence to be accidental. M.
Trouvelot observed at Meudon, June 26, 1885, two active and evanescent
prominences thus situated, each rising to the enormous height of 300,000
miles; and on August 16, one scarcely less remarkable, balanced by an
antipodal spot-group.[559] It towered upward, as if by a process of
_unrolling_, to a quarter of a million of miles; after which, in two
minutes, the light died out of it; it had become completely extinct. The
development, again from the ends of a diameter, of a pair of similar
objects was watched, September 19 and 20, 1893, by Father Fényi,
Director of the Kalocsa Observatory; and the phenomenon has been too
often repeated to be accidental.
The eclipse of August 29, 1886, was total during about four minutes over
tropical Atlantic regions; and an English expedition, led by Sir Norman
Lockyer, was accordingly despatched to Grenada in the West Indies, for
the purpose of using the opportunity it offered. But the rainy season
was just then at its height: clouds and squalls were the order of the
day; and the elaborately planned programme of observation could only in
part be carried through. Some good work, none the less, was done.
Professor Tacchini, who had been invited to accompany the party,
ascertained besides some significant facts about prominences. From a
comparison of their forms and sizes during and after the eclipse, it
appeared that only the growing vaporous cores of these objects are shown
by the spectroscope under ordinary circumstances; their upper sections,
giving a faint continuous spectrum, and composed of presumably cooler
materials, can only be seen when the veil of scattered light usually
drawn over them is removed by an eclipse. Thus all modestly tall
prominences have silvery summits; but all do not appear to possess the
"red heart of flame," by which alone they can be rendered perceptible to
daylight observation. Some prove to be ordinarily invisible, because
silvery throughout--"sheeted ghosts," as it were, met only in the dark.
Specimens of the class had been noted as far back as 1842, but Tacchini
first drew particular attention to them. The one observed by him in 1886
rose in a branching form to a height of 150,000 miles, and gave a
brilliantly continuous spectrum, with bright lines at H and K, but no
hydrogen-lines.[560] Hence the total invisibility of the object before
and after the eclipse. During the eclipse, it was seen framed, as it
were, in a pointed arch of coronal light, the symmetrical arrangement of
which with regard to it was obviously significant. Both its unspringing
shape, and the violet rays of calcium strongly emitted by it,
contradicted the supposition that "white prominences" represent a
downrush of refrigerated materials.
The corona of 1886, as photographed by Dr. Schuster and Mr. Maunder,
showed neither the petals and plumes of 1871, nor the streamers of 1878.
It might be called of a transition type.[561] Wide polar rifts were
filled in with tufted radiations, and bounded on either side by
irregularly disposed, compound luminous masses. In the south-western
quadrant, a triangular ray, conspicuous to the naked eye, represented,
Mr. W. H. Pickering thought, the projection of a huge, hollow cone.[562]
Branched and recurving jets were curiously associated with it. The
intrinsic photographic brightness of the corona proved, from Pickering's
measures, to be about 1/54 that of the average surface of the full moon.
The Russian eclipse of August 19, 1887, can only be remembered as a
disastrous failure. Much was expected of it. The shadow-path ran
overland from Leipsic to the Japanese sea, so that the solar
appurtenances would, it was hoped, be disclosed to observers echeloned
along a line of 6,000 miles. But the incalculable element of weather
rendered all forecasts nugatory. The clouds never parted, during the
critical three minutes, over Central Russia, where many parties were
stationed, and Professor D. P. Todd was equally unfortunate in Japan.
Some good photographs were, nevertheless, secured by Professor Arai,
Director of the Tokio Observatory, as well as by MM. Bélopolsky and
Glasenapp at Petrovsk and Jurjevitch respectively. They showed a corona
of simpler form than that of the year before, but not yet of the
pronounced type first associated by Mr. Ranyard with the lowest stage of
solar activity.
The genuineness of the association was ratified by the duplicate
spectacle of the next-ensuing minimum year. Two total eclipses of the
sun distinguished 1889. The first took place on New Year's Day, when a
narrow shadow-path crossed California, allowing less than two minutes
for the numerous experiments prompted by the varied nature of modern
methods of research. American astronomers availed themselves of the
occasion to the full. The heavens were propitious. Photographic records
were obtained in unprecedented abundance, and of unusual excellence.
Their comparison and study placed it beyond reasonable doubt that the
radiated corona belonging to periods of maximum sun-spots gives place,
at periods of minimum, to the "winged" type of 1878. Professor Holden
perceived further that the equatorial extensions characterising the
latter tend to assume a "trumpet-shape."[563] Their extremities diverge,
as if mutually repellent, instead of flowing together along a medial
plane. The maximum actinic brilliancy of the corona of January 1, 1889,
was determined at Lick to be twenty-one times less than that of the full
moon.[564] Its colour was described as "of an intense luminous silver,
with a bluish tinge, similar to the light of an electric arc."[565] Its
spectrum was comparatively simple. Very few bright lines besides those
of hydrogen and coronium, and apparently no dark ones, stood out from
the prismatic background.
"The marked structural features of the corona, as presented by the
negatives" taken by Professors Nipher and Charroppin, were the filaments
and the streamers. The filaments issued from polar calottes of 20°
radius.
"The impression conveyed to the eye," Professor Pritchett wrote,[566]
"is that the equatorial stream of denser coronal matter extends across
and through the filaments, simply obscuring them by its greater
brightness. The effect is just as if the equatorial belt were superposed
upon, or passed through, the filamentary structure. There is nothing in
the photographs to prove that the filaments do not exist all round the
sun.[567] The testimony from negatives of different lengths of exposure
goes to show that the equatorial streamers are made up of numerous
interlacing parts inclined at varying angles to the sun's equator."
The coronal extensions, perceptible with the naked eye to a distance of
more than 3° from the sun, appeared barely one-third of that length on
the best negatives. Little more could be seen of them either in
Barnard's exquisite miniature pictures, or in the photographs obtained
by W. H. Pickering with a thirteen-inch refractor--the largest
instrument so far used in eclipse-photography.
The total eclipse of December 22, 1889, held out a prospect,
unfortunately not realized, of removing some of the doubts and
difficulties that impeded the progress of coronal photography.[568]
Messrs. Burnham and Schaeberle secured at Cayenne some excellent
impressions, showing enough of the corona to prove its identical
character with that depicted in the beginning of the year, but not
enough to convey additional information about its terminal forms or
innermost structure. Any better result was indeed impossible, the
moisture-laden air having cut down the actinic power of the coronal
light to one-fourth its previous value.
Two English expeditions organized by the Royal Astronomical Society
fared still worse. Mr. Taylor was stationed on the West Coast of Africa,
one hundred miles south of Loanda; Father Perry chose as the scene of
his operations the Salut Islands, off French Guiana. Each was supplied
with a reflector constructed by Dr. Common, endowed, by its extremely
short focal length of forty-five, combined with an aperture of twenty
inches, with a light-concentrating force capable, it was hoped, of
compelling the very filmiest coronal branches to self-registration. Had
things gone well two sets of coronal pictures, absolutely comparable in
every respect, and taken at an interval of two hours and a half, would
have been at the disposal of astronomers. But things went very far from
well. Clouds altogether obscured the sun in Africa; they only separated
to allow of his shining through a saturated atmosphere in South America.
Father Perry's observations were the last heroic effort of a dying man.
Stricken with malaria, he crawled to the hospital as soon as the eclipse
was over, and expired five days later, at sea, on board the _Comus_. He
was buried at Barbados. And the sacrifice of his life had, after all,
purchased no decisive success. Most of the plates exposed by him
suffered deterioration from the climate, or from an inevitably delayed
development. A drawing from the best of them by Miss Violet Common[569]
represented a corona differing from its predecessor of January 1,
chiefly through the oppositely unsymmetrical relations of its parts.
Then the western wing had been broader at its base than the eastern; now
the inequality was conspicuously the other way.[570]
The next opportunity for retrieving the mischances of the past was
offered April 16, 1893. The line of totality charted for that day ran
from Chili to Senegambia. American parties appropriated the Andes; both
shores of the Atlantic were in English occupation; French expeditions,
led by Deslandres and Bigourdan, took up posts south of Cape Verde. A
long totality of more than four minutes was favoured by serene skies;
hence an ample store of photographic data was obtained. Professor
Schaeberle, of the Lick Observatory, took, almost without assistance, at
Mina Bronces, a mining station 6,600 feet above the Pacific, fifty-two
negatives, eight of them with a forty-foot telescope, on a scale of four
and a half inches to the solar diameter. Not only the inner corona, but
the array of prominences then conspicuous, appeared in them to be
composed of fibrous jets and arches, held to be sections of elliptic
orbits described by luminous particles about the sun's centre.[571] One
plate received the impression of a curious object,[572] entangled amidst
coronal streamers, and the belief in its cometary nature was ratified by
the bestowal of a comet-medal in recognition of the discovery. Similiar
paraboloidal forms had, nevertheless, occasionally been seen to make an
integral part of earlier coronas; and it remains extremely doubtful
whether Schaeberle's "eclipse-comet" was justly entitled to the
character claimed for it.
The eclipse of 1893 disclosed a radiated corona such as a year of
spot-maximum was sure to bring. An unexpected fact about it was,
however, ascertained. The coronal has been believed to have much in
common with the chromospheric spectrum; it proved, on investigation with
a large prismatic camera, employed under Sir Norman Lockyer's directions
by Mr. Fowler at Fundium, to be absolutely distinct from it. The
fundamental green ray had, on the West African plates, seven more
refrangible associates;[573] but all alike are of unknown origin. They
may be due to many substances, or to one; future research will perhaps
decide; we can at present only say that the gaseous emission of the
corona include none from hydrogen, helium, calcium, or any other
recognisable terrestrial element. Deslandres' attempt to determine the
rotation of the corona through opposite displacements, east and west of
the interposed moon, of the violet calcium-lines supposed to make part
of the coronal spectrum, was thus rendered nugatory. Yet it gave an
earnest of success, by definitely introducing the subject into the
constantly lengthened programme of eclipse-work. There is, however,
little prospect of its being treated effectively until the green line is
vivified by a fresh access of solar activity.
The flight of the moon's shadow was, on August 9, 1896, dogged by
atrocious weather. It traversed, besides, some of the most inhospitable
regions on the earth's surface, and afforded, at the best, but a brief
interval of obscurity. At Novaya Zemlya, however, of all places, the
conditions were tolerably favourable, and, as we have seen, the trophy
of a "flash-spectrograph" was carried off. Some coronal photographs,
moreover, taken by the late Sir George Baden-Powell[574] and by M.
Hansky, a member of a Russian party, were marked by features of
considerable interest. They made apparent a close connection between
coronal outflows and chromospheric jets, cone-shaped beams serving as
the sheaths, or envelopes, of prominences. M. Hansky,[575] indeed,
thought that every streamer had a chromospheric eruption at its base.
Further, dark veinings of singular shapes unmistakably interrupted the
coronal light, and bordered brilliant prominences,[576] reminding us of
certain "black lines" traced by Swift across the "anvil protuberance"
August 7, 1869.[577] In type the corona of 1896 reproduced that of 1886,
as befitted its intermediate position in the solar cycle.
The eclipse-track on January 22, 1898, crossed the Indian peninsula from
Viziadrug, on the Malabar coast, to Mount Everest in the Himalayas. Not
a cloud obstructed the view anywhere, and an unprecedented harvest of
photographic records was garnered. The flash-spectrum, in its successive
phases, appeared on plates taken by Sir Norman Lockyer, Mr. Evershed,
Professor Campbell,[578] and others; Professor Turner[579] set on foot a
novel mode of research by picturing the corona in the polarised
ingredient of its light; Mrs. Maunder[580] practically solved the
problem of photographing the faint coronal extensions, one ray on her
plates running out to nearly six diameters from the moon's limb. Yet she
used a Dallmeyer lens of only one and a half inches aperture. Her
success accorded perfectly with Professor Wadsworth's conclusion that
effectiveness in delineation by slight contrasts of luminosity varies
inversely with aperture. Triple-coated plates, and a comparatively long
exposure of twenty seconds, contributed to a result unlikely, for some
time, to be surpassed. The corona of 1898 presented a mixed aspect. The
polar plumes due at minimum were combined in it with the quadrilateral
ogives belonging to spot-maxima. A slow course of transformation, in
fact, seemed in progress; and it was found to be completed in 1900, when
the eclipse of May 28 revealed the typical halo of a quiescent sun.
The obscurity on this occasion was short--less than 100 seconds--but was
well observed east and west of the Atlantic. No striking gain in
knowledge, however, resulted. Important experiments were indeed made on
the heat of the corona with Langley's bolometer, but their upshot can
scarcely be admitted as decisive. They indicated a marked deficiency of
thermal radiations, implying for coronal light, in Professor Langley's
opinion,[581] an origin analogous to that of the electric
glow-discharge, which, at low pressures, was found by K. Ångström in
1893 to have no invisible heat-spectrum.[582] The corona was
photographed by Professor Barnard, at Wadesborough, North Carolina, with
a 61-1/2-foot horizontal "coelostat." In this instrument, of a type now
much employed in eclipse operations and first recommended by Professor
Turner, a six-inch photographic objective preserved an invariable
position, while a silvered plane mirror, revolving by clockwork once in
forty-eight hours (since the angle of movement is doubled by
reflection), supplied the light it brought to a focus. A temporary
wooden tube connected the lens with the photographic house where the
plates were exposed. Pictures thus obtained with exposures of from one
to fourteen seconds, were described as "remarkably sharp and perfectly
defined, showing the prominences and inner corona very beautifully. The
polar fans came out magnificently."[583]
The great Sumatra eclipse left behind it manifold memories of foiled
expectations. A totality of above six minutes drew observers to the Far
East from several continents, each cherishing a plan of inquiry which
few were destined to execute. All along the line of shadow, which, on
May 18, 1901, crossed Réunion and Mauritius, and again met land at
Sumatra and Borneo, the meteorological forecast was dubious, and the
meteorological actuality in the main deplorable. Nevertheless, the
corona was seen, and fairly well photographed through drifting clouds,
and proved to resemble in essentials the appendage viewed a year
previously. Negatives taken by members of the Lick Observatory
expedition led by Mr. Perrine[584] disclosed the unique phenomenon of a
violent coronal disturbance, with a small compact prominence as its
apparent focus. Tumbling masses and irregular streamers radiating from a
point subsequently shown by the Greenwich photographs to be the seat of
a conspicuous spot, suggested the recent occurrence of an explosion, the
far-reaching effects of which might be traced in the confused floccular
luminosity of a vast surrounding region. Again, photographs in polarised
light attested the radiance of the outer corona to be in large measure
reflected, while that of the inner ring was original; and the inference
was confirmed by spectrographs, recording many Fraunhofer lines when the
slit lay far from the sun's limb, but none in its immediate vicinity. On
plates exposed by Mr. Dyson and Dr. Humphrys with special apparatus, the
coronal spectrum, continuous and linear, impressed itself more
extensively in the ultra-violet than on any previous occasion; and Dr.
Mitchell succeeded in photographing the reversing layer by means of a
grating spectroscope. Finally, Mrs. Maunder, at Mauritius, despite
mischievous atmospheric tremors, obtained with the Newbegin telescope an
excellent series of coronal pictures.[585]
The principles of explanation applied to the corona may be briefly
described as eruptive and electrical. The first was adopted by Professor
Schaeberle in his "Mechanical Theory," advanced in 1890.[586] According
to this view, the eclipse-halo consists of streams of matter shot out
with great velocity from the spot-zones by forces acting perpendicularly
to the sun's surface. The component particles return to the sun after
describing sections of extremely elongated ellipses, unless their
initial speed happen to equal or exceed the critical rate of 383 miles a
second, in which case they are finally driven off into space. The
perspective overlapping and interlacing of these incandescent outflows
was supposed to occasion the intricacies of texture visible in the
corona; and it should be recorded that a virtually identical conclusion
was reached by Mr. Perrine in 1901,[587] by a different train of
reasoning, based upon a distinct set of facts. A theory on very much the
same lines was, moreover, worked out by M. Bélopolsky in 1897.[588]
Schaeberle, however, had the merit of making the first adequate effort
to deduce the real shape of the corona, as it exists in three
dimensions, from its projection upon the surface of the sphere. He
failed, indeed, to account for the variation in coronal types by the
changes in our situation with regard to the sun's equator. It is only
necessary to remark that, if this were so, they should be subject to an
annual periodicity, of which no trace can be discerned.
Electro-magnetic theories have the charm, and the drawback, of dealing
largely with the unknown. But they are gradually losing the vague and
intangible character which long clung to them; and the improved
definition of their outlines has not, so far, brought them into
disaccord with truth. The most promising hypothesis of the kind is due
to Professor Bigelow of Washington. His able discussion of the eclipse
photographs of January 1, 1889,[589] showed a striking agreement between
the observed coronal forms and the calculated effects of a repulsive
influence obeying the laws of electric potential, also postulated by
Huggins in 1885.[590] Finely subdivided matter, expelled from the sun
along lines of force emanating from the neighbourhood of his poles, thus
tends to accumulate at "equipotential surfaces." In deference, however,
to a doubt more strongly felt then than now, whether the presence of
free electricity is compatible with the solar temperature, he avoided
any express assertion that the coronal structure is an electrical
phenomenon, merely pointing out that, if it were, its details would be
just what they are.
Later, in 1892, Pupin in America,[591] and Ebert in Germany,[592]
imitated the coronal streamers by means of electrical discharges in low
vacua between small conducting bodies and strips of tinfoil placed on
the outside of the containing glass receptacles. Finally, a critical
experiment made by Ebert in 1895 served, as Bigelow justly said, "to
clear up the entire subject, and put the theory on a working basis."
Having obtained coronoidal effects in the manner described, he proceeded
to subject them to the action of a strong magnetic field, with the
result of marshalling the scattered rays into a methodical and highly
suggestive array. They followed the direction of the magnetic lines of
force, and, forsaking the polar collar of the magnetised sphere,
surrounded it like a ruffle. The obvious analogy with the aurora polaris
and the solar corona was insisted upon by Ebert himself, and has been
further developed by Bigelow.[593] According to a recent modification of
his hypothesis, the latter appendage is controlled by two opposing
systems of forces; the magnetic causing the rays to diverge from the
poles towards the equator, and the electrostatic urging their spread,
through the mutual repulsion of the particles accumulated in the
"wings," from the equator towards either pole. The cyclical change in
the corona, he adds, is probably due to a variation in the balance of
power thus established, the magnetic polar influence dominating at
minima, the electrostatic at maxima. And he may well feel encouraged by
the fortunate combination of many experimental details into one
explanatory whole, no less than by the hopeful prospect of further
developments, both practical and theoretical, along the same lines.
What we really know about the corona can be summed up in a few words. It
is certainly _not_ a solar atmosphere. It does not gravitate upon the
sun's surface and share his rotation, as our air gravitates upon and
shares the rotation of the earth; and this for the simple reason that
there is no visible growth of pressure downwards (of which the
spectroscope would infallibly give notice) in its gaseous constituents;
whereas under the sole influence of the sun's attractive power, their
density should be multiplied many million times in the descent through a
mere fraction of their actual depth.[594]
They are apparently in a perpetual state of efflux from, and influx to
our great luminary, under the stress of opposing forces. It is not
unlikely that some part, at least, of the coronal materials are provided
by eruptions from the body of the sun;[595] it is almost certain that
they are organized and arranged round it through electro-magnetic
action. This, however, would seem to be influential only upon their
white-hot or reflective ingredients, out of which the streamers and
aigrettes are composed; since the coronal gases appear, from
observations during eclipses, to form a shapeless envelope, with
condensations above the spot-zones, or at the bases of equatorial
extensions. The corona is undoubtedly affected both in shape and
constitution by the periodic ebb and flow of solar activity, its
low-tide form being winged, its high-tide form stellate; while the rays
emitted by the gases contained in it fade, and the continuous spectrum
brightens, at times of minimum sun-spots. The appendage, as a whole,
must be of inconceivable tenuity, since comets cut their way through it
without experiencing sensible retardation. Not even Sir William
Crookes's vacua can give an idea of the rarefaction which this fact
implies. Yet the observed luminous effects may not in reality bear
witness contradictory of it. One solitary molecule in each cubic inch of
space might, in Professor Young's opinion, produce them; while in the
same volume of ordinary air at the sea-level, the molecules number
(according to Dr. Johnstone Stoney) 20,000 trillions!
The most important lesson, however, derived from eclipses is that of
partial independence of them. Some of its fruits in the daily study of
prominences the next chapter will collect; and the harvest has been
rendered more abundant, as well as more valuable, since it has been
found possible to enlist, in this department too, the versatile aid of
the camera.
FOOTNOTES:
[Footnote 512: _Vierteljahrsschrift Astr. Ges._, Jahrg. xxvi., p. 274.]
[Footnote 513: _Astr. Jour._, vol. iv., p. 33.]
[Footnote 514: _Proc. Roy. Soc._, vol. xvii., p. 116.]
[Footnote 515: _Comptes Rendus_, t. lxvii., p. 757.]
[Footnote 516: _Comptes Rendus_, t. lxvii., p. 839.]
[Footnote 517: _Month. Not._, vol. xxvii., p. 88.]
[Footnote 518: _Proc. Roy. Soc._, vol. xvii., p. 123.]
[Footnote 519: _Washington Observations_, 1867, App. ii., Harkness's
Report, p. 60.]
[Footnote 520: _Am. Jour._, vol. xlviii. (2nd series), p. 377.]
[Footnote 521: _Am. Jour._, vol. xi. (3rd series), p. 429.]
[Footnote 522: Campbell, _Astroph. Jour._, vol. x., p. 186.]
[Footnote 523: Keeler, _Reports on Eclipse of January 1, 1889_, p. 47.]
[Footnote 524: Everything in such observations depends upon the proper
manipulation of the slit of the spectroscope.]
[Footnote 525: _Mem. R. A. S._, vol. xli., p. 435.]
[Footnote 526: _Comptes Rendus_, t. lxvii., p. 1019.]
[Footnote 527: _Mem. R. A. S._, vol. xli., p. 43.]
[Footnote 528: _Comptes Rendus_, t. xciv., p. 1640.]
[Footnote 529: Young, _Pop. Astr._, Oct., 1897, p. 333.]
[Footnote 530: J. Evershed, _Indian Eclipse_, 1898, p. 65; _Month.
Not._, vol. lviii., p. 298; _Proc. Roy. Soc._, Jan. 17, 1901.]
[Footnote 531: Frost, _Astroph. Jour._, vol. xii., p. 85; Lord, _Ibid._,
vol. xiii., p. 149.]
[Footnote 532: _Comptes Rendus_, t. cxvii., No. 1; _Jour. Brit. Astr.
Ass._, vol. iii., p. 532.]
[Footnote 533: Lockyer, _Phil. Trans._, vol. clvii., p. 551.]
[Footnote 534: The rosy envelope of prominence-matter was so named by
Lockyer in 1868 (_Phil. Trans._, vol. clix., p. 430).]
[Footnote 535: According to Trouvelot (_Wash. Obs._, 1876, App. iii., p.
80), the subtracted matter was, at least to some extent, accumulated in
the polar regions.]
[Footnote 536: _Bull. Phil. Soc. Washington_, vol. iii., p. 118.]
[Footnote 537: _Mem. R. A. S._, vol. xli., 1879.]
[Footnote 538: _Astr. Nach._, No. 1,737.]
[Footnote 539: _Correspondence with Newton_, pp. 181-184; Ranyard, _Mem.
Astr. Soc._, vol. xli., p. 501.]
[Footnote 540: S. P. Langley, _Wash. Obs._, 1876, App. iii., p. 209;
_Nature_, vol. lxi., p. 443.]
[Footnote 541: Schuster (_Proc. Roy. Soc._, vol. xxxv., p. 154) measured
and photographed about thirty.]
[Footnote 542: Abney, _Phil. Trans._, vol. clxxv., p. 267.]
[Footnote 543: _Proc. Roy. Soc._, vol. xxxiv., p. 409. Experiments
directed to the same end had been made by Dr. O. Lohse at Potsdam,
1878-80. _Astr. Nach._, No. 2,486.]
[Footnote 544: The sensitiveness of chloride of silver extends from _h_
to H; that is, over the upper or more refrangible half of the space in
which the main part of the coronal light is concentrated.]
[Footnote 545: _Proc. Roy. Soc._, vol. xxxiv., p. 414.]
[Footnote 546: _Report Brit. Assoc._, 1883, p. 351.]
[Footnote 547: Maunder, _Indian Eclipse_, p. 125; _Eclipse of 1900_, p.
143.]
[Footnote 548: _Astr. and Astrophysics_, vol. xiii., p. 662.]
[Footnote 549: See _infra_, p. 197.]
[Footnote 550: Abney, _Phil. Trans._, vol. clxxx., p. 119.]
[Footnote 551: _Comptes Rendus_, t. xcvii., p. 592.]
[Footnote 552: _Memoirs National Ac. of Sciences_, vol. ii., p. 102.]
[Footnote 553: _Wash. Obs._, 1867, App. ii., p. 64.]
[Footnote 554: _The Sun_, p. 357.]
[Footnote 555: _Proc. Roy. Soc._, vol. xvii., p. 289.]
[Footnote 556: _Comptes Rendus_, t. lxxiii., p. 434.]
[Footnote 557: _Wash. Obs._, 1867, App. ii., p. 195.]
[Footnote 558: Stokes, Anniversary Address, _Nature_, vol. xxxv., p.
114.]
[Footnote 559: _Comptes Rendus_, t. ci., p. 50.]
[Footnote 560: _Harvard Annals_, vol. xviii., p. 99.]
[Footnote 561: Wesley, _Phil. Trans._, vol. clxxx., p. 350.]
[Footnote 562: _Harvard Annals_, vol. xviii, p. 108.]
[Footnote 563: _Lick Report_, p. 20.]
[Footnote 564: _Ibid._, p. 14.]
[Footnote 565: _Ibid._, p. 155.]
[Footnote 566: _Pub. Astr. Soc. of the Pacific_, vol. iii., p. 158.]
[Footnote 567: Professor Holden concluded, with less qualification,
"that so-called 'polar' rays exist at all latitudes on the sun's
surface." _Lick Report_, p. 19.]
[Footnote 568: Holden, _Report on Eclipse of December, 1889_, p. 18;
Charroppin, _Pub. Astr. Soc. of the Pacific_, vol. iii., p. 26.]
[Footnote 569: Published as the Frontispiece to the _Observatory_, No.
160.]
[Footnote 570: Wesley, _Ibid._, p. 107.]
[Footnote 571: _Lick Observatory Contributions_, No. 4, p. 108.]
[Footnote 572: _Astr. and Astrophysics_, vol. xiii. p. 307.]
[Footnote 573: Lockyer, _Phil. Trans._, vol. clxxxvii., p. 592.]
[Footnote 574: He died in London, November 20, 1898.]
[Footnote 575: _Bull. Acad. St. Pétersbourg_, t. vi., p. 253.]
[Footnote 576: W. H. Wesley, _Phil. Trans._, vol. cxc, p. 204.]
[Footnote 577: _Lick Reports on Eclipse of January 1, 1889_, p. 204.]
[Footnote 578: _Astroph. Jour._, vol. xi., p. 226.]
[Footnote 579: _Observatory_, vol. xxi., p. 157.]
[Footnote 580: _The Indian Eclipse_, 1898, p. 114.]
[Footnote 581: _Science_, June 22, 1900; _Astroph. Jour._, vol. xii., p.
370.]
[Footnote 582: _Ann. der Physik_, Bd. xlviii., p. 528. See also Wood,
_Physical Review_, vol. iv., p. 191, 1896.]
[Footnote 583: _Science_, August 3, 1900.]
[Footnote 584: _Lick Observatory Bulletin_, No. 9.]
[Footnote 585: _Observatory_, vol. xxiv., pp. 321, 375.]
[Footnote 586: _Lick Report on Eclipse of December 22, 1889_, p. 47;
_Month. Not._, vol. l., p. 372.]
[Footnote 587: _Lick Obs. Bull._, No. 9.]
[Footnote 588: _Bull. de l'Acad. St. Pétersbourg_, t. iv., p. 289.]
[Footnote 589: _The Solar Corona discussed by Spherical Harmonics_,
Smithsonian Institution, 1889.]
[Footnote 590: Bakerian Lecture, _Proc. Roy. Soc._, vol. xxxix.]
[Footnote 591: _Astr. and Astrophysics_, vol. xi., p. 483.]
[Footnote 592: _Ibid._, vol. xii., p. 804.]
[Footnote 593: _Am. Journ. of Science_, vol. xi., p. 253, 1901.]
[Footnote 594: See Huggins, _Proc. Roy. Soc._, vol. xxxix., p. 108;
Young, _North Am. Review_, February, 1885, p. 179.]
[Footnote 595: Professor W. A. Norton, of Yale College, appears to have
been the earliest formal advocate of the Expulsion Theory of the solar
surroundings, in the second (1845) and later editions of his _Treatise
on Astronomy_.]