_SOLAR SPECTROSCOPY_
The new way struck out by Janssen and Lockyer was at once and eagerly
followed. In every part of Europe, as well as in North America,
observers devoted themselves to the daily study of the chromosphere and
prominences. Foremost among these were Lockyer in England, Zöllner at
Leipzig, Spörer at Anclam, Young at Hanover, New Hampshire, Secchi and
Respighi at Rome. There were many others, but these names stood out
conspicuously.
The first point to be cleared up was that of chemical composition.
Leisurely measurements verified the presence above the sun's surface of
hydrogen in prodigious volumes, but showed that sodium had nothing to do
with the orange-yellow ray identified with it in the haste of the
eclipse. From its vicinity to the D-pair (than which it is slightly more
refrangible), the prominence-line was, however, designated D_3, and the
unknown substance emitting it was named by Lockyer "helium." Its
terrestrial discovery ensued after twenty-six years. In March, 1895,
Professor Ramsay obtained from the rare mineral clevite a volatile gas,
the spectrum of which was found to include the yellow prominence-ray.
Helium was actually at hand, and available for examination. The
identification cleared up many obscurities in chromospheric chemistry.
Several bright lines, persistently seen at the edge of the sun, and
early suspected by Young[596] to emanate from the same source as D_3,
were now derived from helium in the laboratory; and all the complex
emissions of that exotic substance ranged themselves into six sets or
series, the members of which are mutually connected by numerical
relations of a definite and simple kind. Helium is of rather more than
twice the density of hydrogen, and has no chemical affinities. In almost
evanescent quantities it lurks in the earth's crust, and is diffused
through the earth's atmosphere.
The importance of the part played in the prominence-spectrum by the
violet line of calcium was noticed by Professor Young in 1872, but since
H and K lie near the limit of the visible spectrum, photography was
needed for a thorough investigation of their appearances. Aided by its
resources, Professor George E. Hale, then at the beginning of his
career, detected in 1889 their unfailing and conspicuous presence.[597]
The substance emitting them not only constitutes a fundamental
ingredient of the chromosphere, but rises, in the fantastic jets thence
issuing, to greater heights than hydrogen itself. The isolation of H and
K in solar prominences from any other of the lines usually distinctive
of calcium was experimentally proved by Sir William and Lady Huggins in
1897 to be due to the extreme tenuity of the emitting vapour.[598]
Hydrogen, helium, and calcium form, then, the chief and unvarying
materials of the solar sierra and its peaks; but a number of metallic
elements make their appearance spasmodically under the influence of
disturbances in the layers beneath. In September, 1871, Young[599] drew
up at Dartmouth College a list of 103 lines significant of injections
into the chromosphere of iron, titanium, chromium, magnesium, and many
other substances. During two months' observation in the pure air of
Mount Sherman (8,335 feet high) in the summer of 1872, these tell-tale
lines mounted up to 273;[600] and he believes their number might still
be doubled by steady watching. Indeed, both Young and Lockyer have more
than once seen the whole field of the spectroscope momentarily inundated
with bright rays, as if the "reversing layer" had been suddenly thrust
upwards into the chromosphere, and as quickly allowed to drop back
again. The opinion would thus appear to be well-grounded that the two
form one continuous region, of which the lower parts are habitually
occupied by the heaviest vapours, but where orderly arrangement is
continually overturned by violent eruptive disturbances.
The study of the _forms_ of prominences practically began with Huggins's
observation of one through an "open slit" February 13, 1869.[601] At
first it had been thought possible to examine them only in
sections--that is, by admitting mere narrow strips or "lines" of their
various kinds of light; while the actual shape of the objects emitting
those lines had been arrived at by such imperfect devices as that of
giving to the slit of the spectroscope a vibratory moment rapid enough
to enable the eye to retain the impression of one part while others were
successively presented to it. It was an immense gain to find that their
rays had strength to bear so much of dilution with ordinary light as was
involved in opening the spectroscopic shutter wide enough to exhibit the
tree-like, or horn-like, or flame-shaped bodies rising over the sun's
rim in their undivided proportions. Several diversely-coloured images of
them are formed in the spectroscope; each may be seen under a crimson, a
yellow, a green, and a deep blue aspect. The crimson, however (built up
out of the C-line of hydrogen), is the most intense, and is commonly
used for purposes of observation and illustration.
Friedrich Zöllner was, by a few days, beforehand with Huggins in
describing the open-slit method, but was somewhat less prompt in
applying it. His first survey of a complete prominence, pictured in, and
not simply intersected by, the slit of his spectroscope, was obtained
July 1, 1869.[602] Shortly afterwards the plan was successfully adopted
by the whole band of investigators.
A difference in kind was very soon perceived to separate these objects
into two well-marked classes. Its natural and obvious character was
shown by its having struck several observers independently. The
distinction of "cloud-prominences" from "flame-prominences" was
announced by Lockyer, April 27; by Zöllner, June 2; and by Respighi,
December 4, 1870.
The first description are tranquil and relatively permanent, sometimes
enduring without striking change for many days. Certain of the included
species mimic terrestrial cloud-scenery--now appearing like fleecy
cirrus transpenetrated with the red glow of sunset--now like prodigious
masses of cumulo-stratus hanging heavily above the horizon. The solar
clouds, however, have the peculiarity of possessing _stems_. Slender
columns can ordinarily be seen to connect the surface of the
chromosphere with its outlying portions. Hence the fantastic likeness to
forest scenery presented by the long ranges of fiery trunks and foliage
occasionally seeming to fringe the sun's limb. But while this mode of
structure suggests an actual outpouring of incandescent material,
certain facts require a different interpretation. At a distance, and
quite apart from the chromosphere, prominences have been perceived, both
by Secchi and Young, to _form_, just as clouds form in a clear sky,
condensation being replaced by ignition. Filaments were then thrown out
downward towards the chromosphere, and finally the usual appearance of a
"stemmed prominence" was assumed. Still more remarkable was an
observation made by Trouvelot at Harvard College Observatory, June 26,
1874.[603] A gigantic comma-shaped prominence, 82,000 miles high,
vanished from before his eyes by a withdrawal of light as sudden as the
passage of a flash of lightning. The same observer has frequently
witnessed a gradual illumination or gradual extinction of such objects,
testifying to changes in the thermal or electrical condition of matter
already _in situ_.
The first photograph of a prominence, as shown by the spectroscope in
daylight, was taken by Professor Young in 1870.[604] But neither his
method, nor that described by Dr. Braun in 1872,[605] had any practical
success. This was reserved to reward the efforts towards the same end of
Professor Hale. Begun at Harvard College in 1889,[606] they were
prosecuted soon afterwards at the Kenwood Observatory, Chicago. The
great difficulty was to extricate the coloured image of the gaseous
structure, spectroscopically visible at the sun's limb, from the
encompassing glare, a very little of which goes a long way in _fogging_
sensitive plates. To counteract its mischievous effects, a second
slit,[607] besides the usual narrow one in front of the collimator, was
placed on guard, as it were, behind the dispersing apparatus, so as to
shut out from the sensitised surface all light save that of the required
quality. The sun's image being then allowed to drift across the outer
slit, while the plate holder was kept moving at the same rate, the
successive sectional impressions thus rapidly obtained finally "built
up" a complete picture of the prominence. Another expedient was soon
afterwards contrived.[608] The H and K rays of calcium are always, as we
have seen, bright in the spectrum of prominences. They are besides fine
and sharp, while the corresponding absorption-lines in the ordinary
solar spectrum are wide and diffuse. Hence, prominences formed by the
spectroscope out of these particular qualities of violet light, can be
photographed entire and at once, for the simple reason that they are
projected upon a naturally darkened background. Atmospheric glare is
abolished by local absorption. This beautiful method was first realised
by Professor Hale in June, 1891.
A "spectroheliograph," consisting of a spectroscopic and a photographic
apparatus of special type, attached to the eye-end of an equatoreal
twelve inches in aperture, was erected at Kenwood in March, 1891; and
with its aid, Professor Hale entered upon original researches of high
promise for the advancement of solar physics. Noteworthy above all is
his achievement of photographing both prominences and faculæ on the very
face of the sun. The latter had, until then, been very imperfectly
observed. They were only visible, in fact, when relieved by their
brilliancy against the dusky edge of the solar disc. Their convenient
emission of calcium light, however, makes it possible to photograph them
in all positions, and emphasises their close relationship to
prominences. The simultaneous picturing, moreover, of the entire
chromospheric ring, with whatever trees or fountains of fire chance to
be at the moment issuing from it, has been accomplished by a very simple
device. The disc of the sun itself having been screened with a circular
metallic diaphragm, it is only necessary to cause the slit to traverse
the virtually eclipsed luminary, in order to get an impression of the
whole round of its fringing appendages. And the record can be extended
to the disc by removing the screen, and carrying the slit back at a
quicker rate, when an "image of the sun's surface, with the faculæ and
spots, is formed on the plate exactly within the image of the
chromosphere formed during the first exposure. The whole operation,"
Professor Hale continues, "is completed in less than a minute, and the
resulting photographs give the first true pictures of the sun, showing
all of the various phenomena at its surface."[609] Most of these novel
researches were, by a remarkable coincidence, pursued independently and
contemporaneously by M. Deslandres, of the Paris Observatory.[610]
The ultra-violet prominence spectrum was photographed for the first time
from an uneclipsed sun, in June, 1891, at Chicago. Besides H and K, four
members of the Huggins-series of hydrogen-lines imprinted themselves on
the plate.[611] Meanwhile M. Deslandres was enabled, by fitting quartz
lenses to his spectroscope, and substituting a reflecting for a
refracting telescope, to get rid of the obstructive action of glass upon
the shorter light-waves, and thus to widen the scope of his inquiry into
the peculiarities of those derived from prominences.[612] As the result,
not only all the nine white-star lines were photographed from a
brilliant sun-flame, but five additional ones were found to continue the
series upward. The wave-lengths of these last had, moreover, been
calculated beforehand with singular exactness, from a simple formula
known as "Balmer's Law."[613] The new lines, accordingly, filled places
in a manner already prepared for them, and were thus unmistakably
associated with the hydrogen-spectrum. This is now known to be
represented in prominences by twenty-seven lines,[614] forming a kind of
harmonic progression, only four of which are visibly darkened in the
Fraunhofer spectrum of the sun.
PLATE I.
[Illustration: Photographs of the Solar Chromosphere and Prominences.
Taken with the Spectroheliograph of the Kenwood Observatory, Chicago, by
Professor George E. Hale.]
The chemistry of "cloud-prominences" is simple. Hydrogen, helium, and
calcium are their chief constituents. "Flame-prominences," on the other
hand, show, in addition, the characteristic rays of a number of metals,
among which iron, titanium, barium, strontium, sodium, and magnesium are
conspicuous. They are intensely brilliant; sharply defined in their
varying forms of jets, spikes, fountains, waterspouts; of rapid
formation and speedy dissolution, seldom attaining to the vast
dimensions of the more tranquil kind. Eruptive or explosive by origin,
they occur in close connection with spots; whether causally, the
materials ejected as "flames" cooling and settling down as dark,
depressed patches of increased absorption;[615] or consequentially, as a
reactive effect of falls of solidified substances from great heights in
the solar atmosphere.[616] The two classes of phenomena, at any rate,
stand in a most intimate relation; they obey the same law of
periodicity, and are confined to the same portions of the sun's surface,
while quiescent prominences may be found right up to the poles and close
to the equator.
The general distribution of prominences, including both genera, follows
that of faculæ much more closely than that of spots. From Father
Secchi's and Professor Respighi's observations, 1869-71, were derived
the first clear ideas on the subject, which have been supplemented and
modified by the later researches of Professors Tacchini and Riccò at
Rome and Palermo. The results are somewhat complicated, but may be
stated broadly as follows. The district of greatest prominence-frequency
covers and overlaps by several degrees that of the greatest
spot-frequency. That is to say, it extends to about 40° north and south
of the equator.[617] There is a visible tendency to a second pair of
maxima nearer the poles. The poles themselves, as well as the equator,
are regions of minimum occurrence. Distribution in time is governed by
the spot-cycle, but the maximum lasts longer for prominences than for
spots.
The structure of the chromosphere was investigated in 1869 and
subsequent years by Professor Respighi, director of the Capitoline
Observatory, as well as by Spörer, and Brédikhine of the Moscow
Observatory. They found this supposed solar envelope to be of the same
eruptive nature as the vast protrusions from it, and to be made up of a
congeries of minute flames[618] set close together like blades of grass.
"The appearance," Professor Young writes,[619] "which probably indicates
a fact, is as if countless jets of heated gas were issuing through vents
and spiracles over the whole surface, thus clothing it with flame which
heaves and tosses like the blaze of a conflagration."
The summits of these filaments of fire are commonly inclined, as if by a
wind sweeping over them, when the sun's activity is near its height, but
erect during his phase of tranquillity. Spörer, in 1871, inferred the
influence of permanent polar currents,[620] but Tacchini showed in 1876
that the deflections upon which this inference was based ceased to be
visible as the spot-minimum drew near.[621]
Another peculiarity of the chromosphere, denoting the remoteness of its
character from that of a true atmosphere,[622] is the irregularity of
its distribution over the sun's surface. There are no signs of its
bulging out at the equator, as the laws of fluid equilibrium in a
rotating mass would require; but there are some that the fluctuations in
its depth are connected with the phases of solar agitation. At times of
minimum it seems to accumulate and concentrate its activity at the
poles; while maxima probably bring a more equable general distribution,
with local depressions at the base of great prominences and above spots.
A low-lying stratum of carbon-vapour was, in 1897, detected in the
chromosphere by Professor Hale with a grating-spectroscope attached to
the 40-inch Yerkes refractor.[623] The eclipse-photographs of 1893
disclosed to Hartley's examination the presence there of gallium;[624]
and those taken by Evershed in 1898 were found by Jewell[625] to be
crowded with ultra-violet lines of the equally rare metal scandium. The
general rule had been laid down by Sir Norman Lockyer that the metallic
radiations from the chromosphere are those "enhanced" in the electric
spark.[626] Hence, the comparative study of conditions prevalent in the
arc and the spark has acquired great importance in solar physics.
The reality of the appearance of violent disturbance presented by the
"flaming" kind of prominence can be tested in a very remarkable manner.
Christian Doppler,[627] professor of mathematics at Prague, enounced in
1842 the theorm that the colour of a luminous body, like the pitch of a
sonorous body, must be changed by movements of approach or recession.
The reason is this. Both colour and pitch are physiological effects,
depending, not upon absolute wave-length, but upon the number of waves
entering the eye or ear in a given interval of time. And this number, it
is easy to see, must be increased if the source of light or sound is
diminishing its distance, and diminished if it is decreasing it. In the
one case, the vibrating body _pursues_ and crowds together the waves
emanating from it; in the other, it _retreats_ from them, and so
lengthens out the space covered by an identical number. The principle
may be thus illustrated. Suppose shots to be fired at a target at fixed
intervals of time. If the marksman advances, say twenty paces between
each discharge of his rifle, it is evident that the shots will fall
faster on the target than if he stood still; if, on the contrary, he
retires by the same amount, they will strike at correspondingly longer
intervals. The result will of course be the same whether the target or
the marksman be in movement.
So far Doppler was altogether right. As regards sound, anyone can
convince himself that the effect he predicted is a real one, by
listening to the alternate shrilling and sinking of the steam-whistle
when an express train rushes through a station. But in applying this
principle to the colours of stars he went widely astray; for he omitted
from consideration the double range of invisible vibrations which
partake of, and to the eye exactly compensate, changes of refrangibility
in the visible rays. There is, then, no possibility of finding a
criterion of velocity in the hue of bodies shining, like the sun and
stars, with continuous light. The entire spectrum is slightly shifted up
or down in the scale of refrangibility; certain rays normally visible
become exalted or degraded (as the case may be) into invisibility, and
certain other rays at the opposite end undergo the converse process; but
the sum total of impressions on the retina continues the same.
We are not, however, without the means of measuring this sub-sensible
transportation of the light-gamut. Once more the wonderful Fraunhofer
lines came to the rescue. They were called by the earlier physicists
"fixed lines;" but it is just because they are _not_ fixed that, in this
instance, we find them useful. They share, and in sharing betray, the
general shift of the spectrum. This aspect of Doppler's principle was
adverted to by Fizeau in 1848,[628] and the first tangible results in
the estimation of movements of approach and recession between the earth
and the stars, were communicated by Sir William Huggins to the Royal
Society, April 23, 1868. Eighteen months later, Zöllner devised his
"reversion-spectroscope"[629] for doubling the measurable effects of
line-displacements; aided by which ingenious instrument, and following a
suggestion of its inventor, Professor H. C. Vogel succeeded at Bothkamp,
June 9, 1871,[630] in detecting effects of that nature due to the solar
rotation. This application constitutes at once the test and the triumph
of the method.[631]
The eastern edge of the sun is continually moving towards us with an
equatorial speed of about a mile and a quarter per second, the western
edge retreating at the same rate. The displacements--towards the violet
on the east, towards the red on the west--corresponding to this velocity
are very small; so small that it seems hardly credible that they should
have been laid bare to perception. They amount to but 1/150th part of
the interval between the two constituents of the D-line of sodium; and
the D-line of sodium itself can be separated into a pair only by a
powerful spectroscope. Nevertheless, Professor Young[632] was able to
show quite satisfactorily, in 1876, not only deviations in the solar
lines from their proper places indicating a velocity of rotation (1·42
miles per second) slightly in excess of that given by observations of
spots, but the exemption of terrestrial lines (those produced by
absorption in the earth's atmosphere) from the general push upwards or
downwards. Shortly afterwards, Professor Langley, then director of the
Allegheny Observatory, having devised a means of comparing with great
accuracy light from different portions of the sun's disc, found that
while the obscure rays in two juxtaposed spectra derived from the solar
poles were absolutely continuous, no sooner was the instrument rotated
through 90°, so as to bring its luminous supplies from opposite
extremities of the equator, than the same rays became perceptibly
"notched." The telluric lines, meanwhile, remained unaffected, so as to
be "virtually mapped" by the process.[633] This rapid and unfailing mode
of distinction was used by Cornu with perfect ease during his
investigation of atmospheric absorption near Loiret in August and
September, 1883.[634]
A beautiful experiment of the same kind was performed by M. Thollon, of
M. Bischoffsheim's observatory at Nice, in the summer of 1880.[635] He
confined his attention to one delicately defined group of four lines in
the orange, of which the inner pair are solar (iron) and the outer
terrestrial. At the centre of the sun the intervals separating them were
sensibly equal; but when the light was taken alternately from the right
and left limbs, a relative shift in alternate directions of the solar,
towards and from the stationary telluric rays became apparent. A
parallel observation was made at Dunecht, December 14, 1883, when it was
noticed that a strong iron-line in the yellow part of the solar spectrum
is permanently double on the sun's eastern, but single on his western
limb;[636] opposite motion-displacements bringing about this curious
effect of coincidence with, and separation from, an adjacent stationary
line of our own atmosphere's production, according as the spectrum is
derived from the retreating or advancing margin of the solar globe.
Statements of fact so precise and authoritative amount to a
demonstration that results of this kind are worthy of confidence; and
they already occupy an important place among astronomical data.
The subtle method of which they served to assure the validity was
employed in 1887-9 by M. Dunér to test and extend Carrington's and
Spörer's conclusions as to the anomalous nature of the sun's axial
movement.[637] His observations for the purpose, made with a fine
diffraction-spectroscope, just then mounted at the observatory of
Upsala, were published in 1891.[638] Their upshot was to confirm and
widen the law of retardation with increasing latitude derived from the
progressive motions of spots. Determinations made within 15° of the
pole, consequently far beyond the region of spots, gave a
rotation-period of 38-1/2, that of the equatorial belt being of 25-1/2
days. Spots near the equator indeed complete their rounds in a period
shorter by at least half a day; and proportionate differences were found
to exist elsewhere in corresponding latitudes; but Dunér's observations,
it must be remembered, apply to a distinct part of the complex solar
machine from the disturbed photospheric surface. It is amply possible
that the absorptive strata producing the Fraunhofer lines, significant,
by their varying displacements at either limb, of the inferred varying
rates of rotation, may gyrate more slowly than the spot-generating
level. Moreover, faculæ appear to move at a quicker pace than
either;[639] so that we have, for three solar formations, three
different periods of average rotation, the shortest of which belongs to
the faculæ, one of intermediate length to the spots, and the most
protracted to the reversing layer. All, however, agree in lengthening
progressively from the equator towards the poles. Professor Holden aptly
compared the sun to "a vast whirlpool where the velocities of rotation
depend not only on the situation of the rotating masses as to latitude,
but also as to depth beneath the exterior surface."[640]
Sir Norman Lockyer[641] promptly perceived the applicability of the
surprising discovery of line-shiftings through end-on motion to the
study of prominences, the discontinuous light of which affords precisely
the same means of detecting movement without seeming change of place, as
do lines of absorption in a continuous spectrum. Indeed, his
observations at the sun's edge almost compelled recourse to an
explanation made available just when the need of it began to be felt. He
saw bright lines, not merely pushed aside from their normal places by a
barely perceptible amount, but bent, torn, broken, as if by the stress
of some tremendous violence. These remarkable appearances were quite
simply interpreted as the effects of movements varying in amount and
direction in the different parts of the extensive mass of incandescent
vapours falling within a single field of view. Very commonly they are of
a cyclonic character. The opposite distortions of the same coloured rays
betray the fury of "counter-gales" rushing along at the rate of 120
miles a second; while their undisturbed sections prove the persistence
of a "heart of peace" in the midst of that unimaginable fiery whirlwind.
Velocities up to 250 _miles a second_, or 15,000 times that of an
express train at the top of its speed, were thus observed by Young
during his trip to Mount Sherman, August 2, 1872; and these were
actually doubled in an extraordinary outburst observed by Father Jules
Fényi, on June 17, 1891, at the Haynald Observatory in Hungary, as well
as by M. Trouvelot at Meudon.[642]
Motions ascertainable in this way near the limb are, of course,
horizontal as regards the sun's surface; the analogies they present
might, accordingly, be styled _meteorological_ rather than _volcanic_.
But vertical displacements on a scale no less stupendous can also be
shown to exist. Observations of the spectra of spots centrally situated
(where motions in the line of sight are vertical) disclose the progress
of violent uprushes and downrushes of ignited gases, for the most part
in the penumbral or outlying districts. They appear to be occasioned by
fitful and irregular disturbances, and have none of the systematic
quality which would be required for the elucidation of sun-spot
theories. Indeed, they almost certainly take place at a great height
above the actual openings in the photosphere.
As to vertical motions above the limb, on the other hand, we have direct
visual evidence of a truly amazing kind. The projected glowing matter
has, by the aid of the spectroscope, been watched in its ascent. On
September 7, 1871, Young examined at noon a vast hydrogen cloud 100,000
miles long, as it showed to the eye, and 54,000 high. It floated
tranquilly above the chromosphere at an elevation of some 15,000 miles,
and was connected with it by three or four upright columns, presenting
the not uncommon aspect compared by Lockyer to that of a grove of
banyans. Called away for a few minutes at 12.30, on returning at 12.55
the observer found--
"That in the meantime the whole thing had been literally blown to shreds
by some inconceivable uprush from beneath. In place of the quiet cloud I
had left, the air, if I may use the expression, was filled with flying
débris--a mass of detached, vertical, fusiform filaments, each from 10"
to 30" long by 2" or 3" wide,[643] brighter and closer together where
the pillars had formerly stood, and rapidly ascending. They rose, with a
velocity estimated at 166 miles a second, to fully 200,000 miles above
the sun's surface, then gradually faded away like a dissolving cloud,
and at 1.15 only a few filmy wisps, with some brighter streamers low
down near the photosphere, remained to mark the place."[644]
A velocity of projection of _at least_ 500 miles per second was, by
Proctor's[645] calculation, required to account for this extraordinary
display, to which the earth immediately responded by a magnetic
disturbance, and a fine aurora. It has proved by no means an isolated
occurrence. Young saw its main features repeated, October 7, 1881,[646]
on a still vaster scale; for the exploded prominence attained, this
time, an altitude of 350,000 miles--the highest yet chronicled. Lockyer,
moreover, has seen a prominence 40,000 miles high shattered in ten
minutes; while uprushes have been witnessed by Respighi, of which the
initial velocities were judged by him to be 400 or 500 miles a second.
When it is remembered that a body starting from the sun's surface at the
rate of 383 miles a second would, if it encountered no resistance,
escape for ever from his control, it is obvious that we have, in the
enormous forces of eruption or repulsion manifested in the outbursts
just described, the means of accounting for the vast diffusion of matter
in the solar neighbourhood. Nor is it possible to explain them away, as
Cornu,[647] Faye,[648] and others have sought to do, by substituting for
the rush of matter in motion, progressive illumination through electric
discharges, chemical processes,[649] or even through the mere reheating
of gases cooled by expansion.[650] All the appearances are against such
evasions of the difficulty presented by velocities stigmatised as
"fabulous" and "improbable," but which, there is the strongest reason to
believe, really exist.
On the 12th of December, 1878, Sir Norman Lockyer formally expounded
before the Royal Society his hypothesis of the compound nature of the
"chemical elements."[651] An hypothesis, it is true, over and over again
propounded from the simply terrestrial point of view. What was novel was
the supra-terrestrial evidence adduced in its support; and even this had
been, in a general and speculative way, anticipated by Professor F. W.
Clarke of Washington.[652] Lockyer had been led to his conclusion along
several converging lines of research. In a letter to M. Dumas, dated
December 3, 1873, he had sketched out the successive stages of
"celestial dissociation" which he conceived to be represented in the sun
and stars. The absence from the solar spectrum of metalloidal absorption
he explained by the separation, in the fierce solar furnace, of such
substances as oxygen, nitrogen, sulphur, and chlorine, into simpler
constituents possessing unknown spectra; while metals were at that time
still admitted to be capable of existing there in a state of integrity.
Three years later he shifted his position onward. He announced, as the
result of a comparative study of the Fraunhofer and electric-arc spectra
of calcium, that the "molecular grouping" of that metal, which at low
temperatures gives a spectrum with its chief line in the blue, is nearly
broken up in the sun into another or others with lines in the
violet.[653] This came to be regarded by him as "a truly typical
case."[654]
During four years (1875-78 inclusive) this diligent observer was engaged
in mapping a section of the more refrangible part of the solar spectrum
(wave-lengths 3,800-4,000) on a scale of magnitude such that, if
completed down to the infra-red, its length would have been about _half
a furlong_. The attendant laborious investigation, by the aid of
photography, of metallic spectra, seemed to indicate the existence of
what he called "basic lines." These held their ground persistently in
the spectra of two or more metals after all possible "impurities" had
been eliminated, and were therefore held to attest the presence of a
common substratum of matter in a simpler state of aggregation than any
with which we are ordinarily acquainted.
Later inquiries have shown, however, that between the spectral lines of
different substances there are probably no _absolute_ coincidences.
"Basic" lines are really formed of doublets or triplets merged together
by insufficient dispersion. Of Thalèn's original list of seventy rays
common to several spectra,[655] very few resisted Thollon's and Young's
powerful spectroscopes; and the process of resolution was completed by
Rowland. Thus the argument from community of lines to community of
substance has virtually collapsed. It was replaced by one founded on
certain periodical changes on the spectra of sun-spots. They emerged
from a series of observations begun at South Kensington under Sir Norman
Lockyer's direction in 1879, and continued for fifteen years.[656]
The principle of the method employed is this. The whole range of
Fraunhofer lines is visible when the light from a spot is examined with
the spectroscope; but relatively few are widened. Now these widened
lines alone constitute (presumably) the true spot-spectrum; they, and
they alone, tell what kinds of vapour are thrust down into the strange
dusky pit of the nucleus, the unaffected lines taking their accustomed
origin from the over-lying strata of the normal solar atmosphere. Here
then we have the criterion that was wanted--the means of distinguishing,
spectroscopically and chemically, between the cavity and the absorbing
layers piled up above it. By its persistent employment some marked
peculiarities have been brought out, such as the unfamiliar character of
numerous lines in spot-spectra, especially at epochs of disturbance; and
the strange _individuality_ in the behaviour of every one of these
darkened and distended rays. Each seems to act on its own account; it
comports itself as if it were the sole representative of the substance
emitting it; its appearance is unconditioned by that of any of its
terrestrial companions in the same spectrum.
The most curious fact, however, elicited by these inquiries was that of
the attendance of chemical vicissitudes upon the advance of the sun-spot
period. As the maximum approached, unknown replaced known components of
the spot-spectra in a most pronounced and unmistakable way.[657] It
seemed as if the vapours emitting lines of iron, titanium, nickel, etc.,
had ceased to exist as such, and their room been taken by others, total
strangers in terrestrial laboratories. These were held by Lockyer to be
simply the finer constituents of their predecessors, dissociation having
been effected by the higher temperature ensuing upon increased solar
activity. But Father Cortie's supplementary investigations at
Stonyhurst[658] modified, while they in the main substantiated, the
South Kensington results. They showed that the substitution of unknown
for known lines characterizes disturbed spots, at all stages of the
solar cycle, so that no systematic course of chemical change can be said
to affect the sun as a whole. They showed further[659]--from evidence
independent of that obtained by Young in 1892[660]--the remarkable
conspicuousness in spot-spectra of vanadium lines excessively faint in
the Fraunhofer spectrum. Lockyer's "unknown lines" may probably thus be
accounted for. They represent absorption, not by new, but by scarce
elements, especially, Father Cortie thinks, those with atomic weights of
about 50. The circumstance of their development in solar commotions,
largely to the exclusion of iron, is none the less curious; but it
cannot be explained by any process of dissociation.
The theory has, however, to be considered under still another aspect. It
frequently happens that the contortions or displacements due to motion
are seen to affect a single line belonging to a particular substance,
while the other lines of _that same substance_ remain imperturbable.
Now, how is this most singular fact, which seems at first sight to imply
that a body may be at rest and in motion at one and the same instant, to
be accounted for? It is accounted for, on the present hypothesis, easily
enough, by supposing that the rays thus discrepant in their testimony,
do _not_ belong to one kind of matter, but to several, combined at
ordinary temperatures to form a body in appearance "elementary." Of
these different vapours, one or more may of course be rushing rapidly
towards or from the observer, while the others remain still; and since
the line of sight across the average prominence-region penetrates, at
the sun's edge, a depth of about 300,000 miles,[661] all the
incandescent materials separately occurring along which line are
projected into a single "flame" or "cloud," it will be perceived that
there is ample room for diversities of behaviour.
The alternative mode of escape from the perplexity consists in assuming
that the vapour in motion is rendered luminous under conditions which
reduce its spectrum to a few rays, the unaffected lines being derived
from a totally distinct mass of the same substance shining with its
ordinary emissions.[662] Thus, calcium can be rendered virtually
monochromatic by attenuation, and analogous cases are not rare.
Sir Norman Lockyer only asks us to believe that effects which follow
certain causes on the earth are carried a stage further in the sun,
where the same causes must be vastly intensified. We find that the
bodies we call "compound" split asunder at fixed degrees of heat
_within_ the range of our resources. Why should we hesitate to admit
that the bodies we call "simple" do likewise at degrees of heat
_without_ the range of our resources? The term "element" simply
expresses terrestrial incapability of reduction. That, in celestial
laboratories, the means and their effect here absent should be present,
would be an inference challenging, in itself, no expression of
incredulity.
There are indeed theoretical objections to it which, though probably not
insuperable, are unquestionably grave. Our seventy chemical "elements,"
for instance, are placed by the law of specific heats on a separate
footing from their known compounds. We are not, it is true, compelled by
it to believe their atoms to be really and absolutely such--to contain,
that is, the "irreducible minimum" of material substance; but we do
certainly gather from it that they are composed on a different principle
from the salts and oxides made and unmade at pleasure by chemists. Then
the multiplication of the species of matter with which Lockyer's results
menace us, is at first sight startling. They may lead, we are told, to
eventual unification, but the prospect appears remote. Their only
obvious outcome is the disruption into several constituents of each
terrestrial "element." The components of iron alone should be counted by
the dozen. And there are other metals, such as cerium, which, giving a
still more complex spectrum, would doubtless be still more numerously
resolved. Sir Norman Lockyer interprets the observed phenomena as
indicating the successive combinations, in varying proportions, of a
very few original ingredients;[663] but no definite sign of their
existence is perceptible; "protyle" seems likely long to evade
recognition; and the only intelligible underlying principle for the
reasonings employed--that of "one line, one element"--implies a throng
beyond counting of formative material units.
Thus, added complexity is substituted for that fundamental unity of
matter which has long formed the dream of speculators. And it is
extremely remarkable that Sir William Crookes, working along totally
different lines, has been led to analogous conclusions. To take only one
example. As the outcome of extremely delicate operations of sifting and
testing carried on for years, he finds that the metal yttrium splits up
into five, if not eight constituents.[664] Evidently, old notions are
doomed, nor are any preconceived ones likely to take their place. It
would seem, on the contrary, as if their complete reconstruction were at
hand. Subversive facts are steadily accumulating; the revolutionary
ideas springing from them tend, if we interpret them aright, towards the
substitution of electrical for chemical theories of matter. Dissociation
by the brute force of heat is already nearly superseded, in the thoughts
of physicists, by the more delicate process of "ionisation." Precisely
what this implies and involves we do not know; but the symptoms of its
occurrence are probably altogether different from those gathered by Sir
Norman Lockyer from the collation of celestial spectra.
A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate
founder, so to speak, of solar spectroscopy. His great map of the
"normal" solar spectrum[665] was published in 1868, two years before he
died. Robert Thalèn was his coadjutor in its execution, and the immense
labour which it cost was amply repaid by its eminent and lasting
usefulness. For more than a score of years it held its ground as the
universal standard of reference in all spectroscopic inquiries within
the range of the _visible_ emanations. Those that are invisible by
reason of the quickness of their vibrations were mapped by Dr. Henry
Draper, of New York, in 1873, and with superior accuracy by M. Cornu in
1881. The infra-red part of the spectrum, investigated by Langley,
Abney, and Knut Ångström, reaches perhaps no definite end. The
radiations oscillating too slowly to affect the eye as light may pass by
insensible gradations into the long Hertzian waves of electricity.[666]
Professor Rowland's photographic map of the solar spectrum, published in
1886, and in a second enlarged edition in 1889, opened fresh
possibilities for its study, from far down in the red to high up in the
ultra-violet, and the accompanying scale of absolute wave-lengths[667]
has been, with trifling modifications, universally adopted. His new
table of standard solar lines was published in 1893.[668] Through his
work, indeed, knowledge of the solar spectrum so far outstripped
knowledge of terrestrial spectra, that the recognition of their common
constituents was hampered by intolerable uncertainties. Thousands of the
solar lines charted with minute precision remained unidentified for want
of a corresponding precision in the registration of metallic lines.
Rowland himself, however, undertook to provide a remedy. Aided by Lewis
E. Jewell, he redetermined, at the Johns Hopkins University, the
wave-lengths of about 16,000 solar lines,[669] photographing for
comparison with them the spectra of all the known chemical elements
except gallium, of which he could procure no specimen. The labour of
collation was well advanced when he died at the age of fifty-two, April
16, 1901. Investigations of metallic arc-spectra have also been carried
out with signal success by Hasselberg,[670] Kayser and Runge, O.
Lohse,[671] and others.
Another condition _sine quâ non_ of progress in this department is the
separation of true solar lines from those produced by absorption in our
own atmosphere. And here little remains to be done. Thollon's great
Atlas[672] was designed for this purpose of discrimination. Each of its
thirty-three maps exhibits in quadruplicate a subdivision of the solar
spectrum under varied conditions of weather and zenith-distance.
Telluric effects are thus made easily legible, and they account wholly
for 866, partly for 246, out of a total of 3,200 lines. But the death of
the artist, April 8, 1887, unfortunately interrupted the half-finished
task of the last seven years of his life. A most satisfactory record,
meanwhile, of selective atmospheric action has been supplied by the
experiments and determinations of Janssen, Cornu and Egoroff, by Dr.
Becker's drawings,[673] and Mr. McClean's photographs of the analysed
light of the sun at high, low, and medium altitudes; and the autographic
pictures obtained by Mr. George Higgs, of Liverpool, of certain
rhythmical groups in the red, emerging with surprising strength near
sunset, excite general and well-deserved admiration.[674] The main
interest, however, of all these documents resides in the information
afforded by them regarding the chemistry of the sun.
The discovery that hydrogen exists in the atmosphere of the sun was made
by Ångström in 1862. His list of solar elements published in that
year,[675] the result of an investigation separate from, though
conducted on the same principle as Kirchhoff's, included the substance
which we now know to be predominant among them. Dr. Plücker of Bonn had
identified in 1859 the Fraunhofer line F with the green ray of hydrogen,
but drew no inference from his observation. The agreement was verified
by Ångström; two further coincidences were established; and in 1866 a
fourth hydrogen line in the extreme violet (named _h_) was detected in
the solar spectrum. With Thalèn, he besides added manganese, titanium,
and cobalt to the constituents of the sun enumerated by Kirchhoff, and
raised the number of identical rays in the solar and terrestrial spectra
of iron to no less than 460.[676]
Thus, when Sir Norman Lockyer entered on that branch of inquiry in 1872,
fourteen substances were recognised as common to the earth and sun.
Early in 1878 he was able to increase the list provisionally to
thirty-three,[677] all except hydrogen metals. This rapid success was
due to his adoption of the test of _length_ in lieu of that of
_strength_ in the comparison of lines. He measured their relative
significance, in other words, rather by their persistence through a wide
range of temperature, than by their brilliancy at any one temperature.
The distinction was easily drawn. Photographs of the electric arc, in
which any given metal had been volatilised, showed some of the rays
emitted by it stretching across the axis of the light to a considerable
distance on either side, while many others clung more or less closely to
its central hottest core. The former "long lines," regarded as certainly
representative, were those primarily sought in the solar spectrum; while
the attendant "short lines," often, in point of fact, due to foreign
admixtures, were set aside as likely to be misleading.[678] The
criterion is a valuable one, and its employment has greatly helped to
quicken the progress of solar chemistry.
Carbon was the first non-metallic element discovered in the sun. Messrs.
Trowbridge and Hutchins of Harvard College concluded in 1887,[679] on
the ground of certain spectral coincidences, that this protean substance
is vaporised in the solar atmosphere at a temperature approximately that
of the voltaic arc. Partial evidence to the same effect had earlier been
alleged by Lockyer, as well as by Liveing and Dewar; and the case was
rendered tolerably complete by photographs taken by Kayser and Runge in
1889.[680] It was by Professor Rowland shown to be irresistible. Two
hundred carbon-lines were, through his comparisons, sifted out from
sunlight, and it contains others significant of the presence of
silicon--a related substance, and one as important to rock-building on
the earth, as carbon is to the maintenance of life. The general result
of Rowland's labours was the establishment among solar materials, not
only of these two out of the fourteen metalloids, or non-metallic
substances, but of thirty-three metals, including silver and tin. Gold,
mercury, bismuth, antimony, and arsenic were discarded from the
catalogue; platinum and uranium, with six other metals, remained
doubtful; while iron was recorded as crowding the spectrum with over two
thousand obscure rays.[681] Gallium-absorption was detected in it by
Hartley and Ramage in 1889.[682]
Dr. Henry Draper[683] announced, in 1877, his imagined discovery, in the
solar spectrum, of eighteen especially brilliant spaces corresponding to
oxygen-emissions. But the agreement proved, when put to the test of very
high dispersion, to be wholly illusory.[684] Nor has it yet been found
possible to identify, in analysed sunlight, any significant _bright_
beams.[685]
The book of solar chemistry must be read in characters exclusively of
absorption. Nevertheless, the whole truth is unlikely to be written
there. That a substance displays none of its distinctive beams in the
spectrum of the sun or of a star, affords scarcely a presumption against
its presence. For it may be situated below the level where absorption
occurs, or under a pressure such as to efface lines by widening and
weakening them; it may be at a temperature so high that it gives out
more light than it takes up, and yet its incandescence may be masked by
the absorption of other bodies; finally, it may just balance absorption
by emission, with the result of complete spectral neutrality. An
instructive example is that of the chromospheric element helium. Father
Secchi remarked in 1868[686] that there is no dark line in the solar
spectrum matching its light; and his observation has been fully
confirmed.[687] Helium-absorption is, however, occasionally noticed in
the penumbræ of spots.[688]
Our terrestrial vital element might then easily subsist unrecognisably
in the sun. The inner organisation of the oxygen molecule is a
considerably _plastic_ one. It is readily modified by heat, and these
modifications are reflected in its varying modes of radiating light. Dr.
Schuster enumerated in 1879[689] four distinct oxygen spectra,
corresponding to various stages of temperature, or phases of electrical
excitement; and a fifth has been added by M. Egoroff's discovery in
1883[690] that certain well-known groups of dark lines in the red end of
the solar spectrum (Fraunhofer's A and B) are due to absorption by the
cool oxygen of our air. These persist down to the lowest temperatures,
and even survive a change of state. They are produced essentially the
same by liquid, as by aërial oxygen.[691]
It seemed, however, possible to M. Janssen that these bands owned a
joint solar and terrestrial origin. Oxygen in a fit condition to produce
them might, he considered, exist in the outer atmosphere of the sun; and
he resolved to decide the point. No one could bring more skill and
experience to bear upon it than he.[692] By observations on the summit
of the Faulhorn, as well as by direct experiment, he demonstrated,
nearly thirty years ago, the leading part played by water-vapour in
generating the atmospheric spectrum; and he had recourse to similar
means for appraising the share in it assignable to oxygen. An electric
beam, transmitted from the Eiffel Tower to Meudon in the summer of 1888,
having passed through a weight of oxygen about equal to that piled above
the surface of the earth, showed the groups A and B just as they appear
in the high-sun spectrum.[693] Atmospheric action is then adequate to
produce them. But M. Janssen desired to prove, in addition, that they
diminish proportionately to its amount. His ascent of Mont Blanc[694] in
1890 was undertaken with this object. It was perfectly successful. In
the solar spectrum, examined from that eminence, oxygen-absorption was
so much enfeebled as to leave no possible doubt of its purely telluric
origin. Under another form, nevertheless, it has been detected as
indubitably solar. A triplet of dark lines low down in the red,
photographed from the sun by Higgs and McClean, was clearly identified
by Runge and Paschen in 1896[695] with the fundamental group of an
oxygen series, first seen by Piazzi Smyth in the spectrum of a
vacuum-tube in 1883.[696] The _pabulum vitæ_ of our earth is then to
some slight extent effective in arresting transmitted sunlight, and
oxygen must be classed as a solar element.
The rays of the sun, besides being stopped selectively in our
atmosphere, suffer also a marked general absorption. This tells chiefly
upon the shortest wave-lengths; the ultra-violet spectrum is in fact
closed, as if by the interposition of an opaque screen. Nor does the
screen appear very sensibly less opaque from an elevation of 10,000
feet. Dr. Simony's spectral photographs, taken on the Peak of
Teneriffe,[697] extended but slightly further up than M. Cornu's, taken
in the valley of the Loire. Could the veil be withdrawn, some
indications as to the originating temperature of the solar spectrum
might be gathered from its range, since the proportion of quick
vibrations given out by a glowing body grows with the intensity of its
incandescence. And this brings us to the subject of our next Chapter.
FOOTNOTES:
[Footnote 596: _Phil. Mag._, vol. xlii., p. 380, 1871.]
[Footnote 597: _Astr. Nach._, No. 3,053, _Amer. Jour._, vol. xlii., p.
162; Deslandres, _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 598: _Proc. Roy. Society_, vol. lxi., p. 433.]
[Footnote 599: _Phil. Mag._, vol. xlii., p. 377.]
[Footnote 600: Frost-Scheiner, _Astr. Spectroscopy_, pp. 184, 423.]
[Footnote 601: _Proc. Roy. Soc._, vol. xvii., p. 302.]
[Footnote 602: _Astr. Nach._, No. 1,769.]
[Footnote 603: _Am. Jour. of Science_, vol. xv., p. 85.]
[Footnote 604: _Journ. Franklin Institute_, vol. xl., p. 232_a_.]
[Footnote 605: _Pogg. Annalen_, Bd. cxlvi., p. 475; _Astr. Nach._, No.
3,014.]
[Footnote 606: _Astr. Nach._, Nos. 3,006, 3,037.]
[Footnote 607: This device was suggested by Janssen in 1869.]
[Footnote 608: _Astr. and Astrophysics_, vol. xi., pp. 70, 407.]
[Footnote 609: _Astr. and Astrophysics_, vol. xi., p. 604.]
[Footnote 610: _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 611: _Astr. and Astrophysics_, vol. xi., p. 50.]
[Footnote 612: _Ibid._, pp. 60, 314.]
[Footnote 613: Wiedemann's _Annalen der Physik_, Bd. xxv., p. 80.]
[Footnote 614: Evershed, _Knowledge_, vol. xxi., p. 133.]
[Footnote 615: Secchi, _Le Soleil_, t. ii., p. 294.]
[Footnote 616: Lockyer, _Chemistry of the Sun_, p. 418.]
[Footnote 617: _L'Astronomie_, August, 1884, p. 292 (Riccò); see also
Evershed, _Jour. British Astr. Ass._, vol. ii., p. 174.]
[Footnote 618: Averaging about 100 miles across and 300 high. _Le
Soleil_, t. ii., p. 35.]
[Footnote 619: _The Sun_, p. 192.]
[Footnote 620: _Astr. Nach._, No. 1,854.]
[Footnote 621: _Mem. degli Spettroscopisti Italiani_, t. v., p. 4;
Secchi, _ibid._, t. vi., p. 56.]
[Footnote 622: Its non-atmospheric character was early defined by
Proctor, _Month. Not._, vol. xxxi., p. 196.]
[Footnote 623: _Astroph. Jour._, vol. vi., p. 412.]
[Footnote 624: _Ibid._, vol. xi., p. 165.]
[Footnote 625: _Ibid._, p. 243.]
[Footnote 626: _Sun's Place in Nature_, pp. 111, 288.]
[Footnote 627: _Abh. d. Kön. Böhm Ges. d. Wiss._, Bd. ii., 1841-42, p.
467.]
[Footnote 628: In a paper read before the Société Philomathique de
Paris, December 23, 1848, and first published _in extenso_ in _Ann. de
Chim. et de Phys._, t. xix., p. 211 (1870). Hippolyte Fizeau died in
September, 1896.]
[Footnote 629: _Astr. Nach._, No. 1,772.]
[Footnote 630: _Ibid._, No. 1,864.]
[Footnote 631: A. Cornu, _Sur la Méthode Doppler-Fizeau_, p. D. 23.]
[Footnote 632: _Am. Jour. of Sc._, vol. xii., p. 321.]
[Footnote 633: _Ibid._, vol. xiv., p. 140.]
[Footnote 634: _Bull. Astronom._, February, 1884, p. 77.]
[Footnote 635: _Comptes Rendus_, t. xci., p. 368.]
[Footnote 636: _Month. Not._, vol. xliv., p. 170.]
[Footnote 637: See _ante_, p. 147.]
[Footnote 638: _Recherches sur la Rotation du Soleil_, Upsal, 1891.]
[Footnote 639: Harzer, _Astr. Nach._, No. 3,026; Stratonoff, _Ibid._,
No. 3,344.]
[Footnote 640: _Publ. Astr. Pacific Soc._, vol. ii., p. 193.]
[Footnote 641: _Proc. Roy. Society_, vols. xvii., p. 415; xviii., p.
120.]
[Footnote 642: _Comptes Rendus_, t. cxii., p. 1421; t. cxiii., p. 310.]
[Footnote 643: At the sun's distance, one second of arc represents about
450 miles.]
[Footnote 644: _Amer. Jour. of Sc._, vol. ii., p. 468, 1871.]
[Footnote 645: _Month. Not._, vol. xxxii., p. 51.]
[Footnote 646: _Nature_, vol. xxiii., p. 281.]
[Footnote 647: _Comptes Rendus_, t. lxxxvii., p. 532.]
[Footnote 648: _Ibid._, t. xcvi., p. 359.]
[Footnote 649: A. Brester, _Théorie du Soleil_, p. 66.]
[Footnote 650: Such prominences as have been seen to grow by the spread
of incandescence are of the quiescent kind, and present no deceptive
appearance of violent motion.]
[Footnote 651: _Proc. Roy. Soc._, vol. xxviii., p. 157.]
[Footnote 652: "Evolution and the Spectroscope," _Pop. Science Monthly_,
January, 1873.]
[Footnote 653: _Proc. Roy. Soc._, vol. xxiv., p. 353. These are the H
and K of prominences. H. W. Vogel discovered in 1879 a hydrogen-line
nearly coincident with H (_Monatsb. Preuss. Ak._, February, 1879, p.
118).]
[Footnote 654: _Proc. Roy. Soc._, vol. xxviii., p. 444.]
[Footnote 655: Many of these were referred by Lockyer himself, who first
sifted the matter, to traces of the metals concerned.]
[Footnote 656: _Chemistry of the Sun_, p. 312; _Proc. Roy. Society_,
vol. lvii., p. 199.]
[Footnote 657: _Lockyer's Chemistry of the Sun_, p. 324.]
[Footnote 658: _Month. Not._, vol. li., p. 76.]
[Footnote 659: _Ibid._, vol. lviii., p. 370.]
[Footnote 660: _Astr. and Astrophysics_, vol. xi., p. 615.]
[Footnote 661: Thollon's estimate (_Comptes Rendus_, t. xcvii., p. 902)
of 300,000 _kilometres_, seems considerably too low. Limiting the
"average prominence region" to a shell 54,000 miles deep (2' of arc as
seen from the earth), the visual line will, at mid-height (27,000 miles
from the sun's surface), travel through (in round numbers) 320,000 miles
of that region.]
[Footnote 662: Liveing and Dewar, _Phil. Mag._, vol. xvi. (5th ser.), p.
407.]
[Footnote 663: _Chemistry of the Sun_, p. 260.]
[Footnote 664: _Nature_, October 14, 1886.]
[Footnote 665: The normal spectrum is that depending exclusively upon
wave-length--the fundamental constant given by nature as regards light.
It is obtained by the interference of rays, in the manner first
exemplified by Fraunhofer, and affords the only unvarying standard for
measurement. In the refraction spectrum (upon which Kirchhoff's map was
founded), the relative positions of the lines vary with the material of
the prisms.]
[Footnote 666: Scheiner, _Die Spectralanalyse der Gestirne_, p. 168.]
[Footnote 667: _Phil. Mag._, vol. xxvii., p. 479.]
[Footnote 668: _Astr. and Astrophysics_, vol. xii., p. 321;
Frost-Scheiner, _Astr. Spectr._, p. 363.]
[Footnote 669: Published in _Astroph. Jour._, vols. i. to vi.]
[Footnote 670: _Astr. and Astrophysics_, vol. xi., p. 793.]
[Footnote 671: _Astroph. Jour._, vol. vi., p. 95.]
[Footnote 672: _Annales de l'Observatoire de Nice_, t. iii., 1890.]
[Footnote 673: _Trans. Royal Society of Edinburgh_, vol. xxxvi., p. 99.]
[Footnote 674: Rev. A. L. Cortie, _Astr. and Astrophysics_, vol. xi., p.
401. Specimens of his photographs were given by Ranyard in _Knowledge_,
vol. xiii., p. 212.]
[Footnote 675: _Ann. d. Phys._, Bd. cxvii., p. 296.]
[Footnote 676: _Comptes Rendus_, t. lxiii., p. 647.]
[Footnote 677: _Ibid._, t. lxxxvi., p. 317. Some half dozen of these
identifications have proved fallacious.]
[Footnote 678: _Chemistry of the Sun_, p. 143.]
[Footnote 679: _Amer. Jour. of Science_, vol. xxxiv., p. 348.]
[Footnote 680: _Berlin Abhandlungen_, 1889.]
[Footnote 681: _Amer. Jour. of Science_, vol. xli., p. 243. See
Appendix, Table II.]
[Footnote 682: _Astrophy. Jour._, vol. ix., p. 219; Fowler, _Knowledge_,
vol. xxiii., p. 11.]
[Footnote 683: _Amer. Jour, of Science_, vol. xiv., p. 89; _Nature_,
vol. xvi., p. 364; _Month. Not._, vol. xxxix., p. 440.]
[Footnote 684: _Month. Not._, vol. xxxviii., p. 473; Trowbridge and
Hutchins, _Amer. Jour. of Science_, vol. xxxiv., p. 263.]
[Footnote 685: Scheiner, _Die Spectralanalyse_, p. 180.]
[Footnote 686: _Comptes Rendus_, t. lxvii., p. 1123.]
[Footnote 687: Rev. A. L. Cortie, _Month. Not._, vol. li., p. 18.]
[Footnote 688: Young, _The Sun_, p. 135; Hale, _Astr. and Astrophysics_,
vol. xi., p. 312 Buss, _Jour. Brit. Astr. Ass._, vol. ix., p. 253.]
[Footnote 689: _Phil. Trans._, vol. clxx., p. 46.]
[Footnote 690: _Comptes Rendus_, t. xcvii., p. 555; t. ci., p. 1145.]
[Footnote 691: Liveing and Dewar, _Astr. and Astrophysics_, vol. xi., p.
705.]
[Footnote 692: _Comptes Rendus_, t. lx., p. 213; t. lxiii., p. 289.]
[Footnote 693: _Ibid._, t. cviii., p. 1035.]
[Footnote 694: _Ibid._, t. cxi., p. 431.]
[Footnote 695: _Astroph. Jour._, vols. iv., p. 317; vi., p. 426.]
[Footnote 696: _Trans. Roy. Soc. Edin._, vol. xxxii., p. 452.]
[Footnote 697: _Comptes Rendus_, t. cxi., p. 941; Huggins, _Proc. Roy.
Soc._, vol. xlvi., p. 168.]