Although the formal official reports of the Eclipse Expedition are not
yet published, and may not be for some weeks or months, we are able
from the letters of Lockyer, Jannsen, Respighi, Maclear, etc., to form
some idea of the general results. We may already regard two or three
important questions as fairly answered. The reversal of the dark solar
lines of the spectrum which was first announced by the great Roman
observer, Father Secchi, and seen by him without an eclipse, may now
be considered as established. It is true that all the observers of
1871 did not witness this. Some were doubtful, but others observed it
positively and distinctly.
In such a case negative results do not refute the positive observations
of qualified men, especially when several of such observations have
been made independently; the phenomenon is but instantaneous, a mere
flash of bright stripes in place of dark lines across the colored
riband of the spectroscope, which happens just at the moment before and
after totality, and is presented only when the instrument is accurately
directed to the delicate curved vanishing thread of light which is the
last visible fragment of the solar outline, and that which makes the
first flash of his re-appearance.
A little explanation is necessary to render the significance of this
“reversal” intelligible to those who have not specially studied the
subject.
1st. When the spectroscope is directed to a luminous solid a simple
rainbow-band or “continuous spectrum” is seen. When, on the other hand,
the object is a luminous gas or vapor of moderate density, the spectrum
is not a continuous band with its colors actually blending; it consists
only of certain luminous stripes with blank spaces between them, each
particular gas or vapor showing its own particular set of stripes of
certain colors, and always appearing at exactly the same place, so
invariably and certainly, that, by means of such luminous stripes, the
composition of the gas or vapor may be determined. If, however, the gas
be much compressed, the stripes widen as the condensation proceeds;
they may even spread out sufficiently to meet and form a continuous
spectrum like that from a solid. Liquids also produce continuous
spectra.
2d. When a luminous solid or liquid, or very dense gas, capable of
producing a continuous spectrum, is viewed through an intervening body
of other gas or vapor of moderate or small density, fine _dark lines_
cross the spectrum in precisely the same places as the bright stripes
would appear if this intervening gas or vapor were luminous and seen by
itself.
When the spectroscope is directed to the face of the sun under ordinary
circumstances, it presents a brilliant continuous spectrum, striped
with a multitude of the dark lines. From this it has been inferred
that the luminous face of the sun is that of an incandescent solid
or liquid, and that it is surrounded by the gases and vapors whose
bright stripes, when artificially produced, occupy precisely the same
places as the dark lines of the solar spectrum. This was the theory
of Kirchoff and others in the early days of spectrum analysis, when
it was only known that solids and liquids were capable of producing a
continuous spectrum. The important discovery that gases and vapors,
if sufficiently condensed, will also produce a continuous spectrum,
opened another speculation, far more consistent with the other known
facts concerning the constitution of the sun, viz., that the sun may be
a great gaseous orb, blazing at its surface and gradually increasing in
density from the surface towards the centre.
According to this, the metals sodium, calcium, barium, magnesium,
iron, chromium, nickel, copper, zinc, strontium, cobalt, manganese,
aluminium, and titanium, whose vapors, with those of some few other
substances, give the dark lines that cross the solar spectrum, should
exist neither as solids nor liquids on the solar surface, but as
blazing gases. But such blazing gases, according to what I have stated
above, should give us bright stripes instead of dark lines. Why, then,
are not such bright stripes seen under ordinary circumstances?
This is easily answered. These blazing gases must, as we proceed from
the surface of the sun downwards, become so condensed by the pressure
of their own superincumbent strata, as to produce a continuous spectrum
of great brilliancy. With such a background the bright stripes would
be confounded and lost to sight. Besides this, the outer film of
cooler vapor through which our vision must necessarily penetrate
before reaching the luminous solar surface, will produce the dark
lines exactly where the bright stripes should be, and thus effectually
obliterate them; or, in other words, the intervening non-luminous
vapors are opaque to the particular rays of light which the bright
vapors of the same substance emits.
Therefore, according to this theory, if we could sweep away these
outside darkening vapors, and screen off the inner layers of denser
blazing matter which produces the continuous background, we should have
a spectrum displaying a multitude of bright stripes exactly where the
black lines of the ordinary solar spectrum appear.
Secchi announced that these bright lines were to be seen under
favorable circumstances, when, by skillful management, the rays from
the edge of the sun were so caught by the slit of the spectroscope as
to exhibit only the spectrum of the superficial layer of the sun’s
bright surface. This was disputed at the time by Mr. Lockyer, who, I
suspect, omitted to consider the atmospheric difficulties under which
English astronomers work, and the fact that the atmosphere of Italy is
exceptionally favorable for delicate astronomical observation.
If he had fairly considered this I think he would agree with me in
concluding that an observation of this kind, avowedly made with great
difficulty and questionable distinctness by so skillful a spectroscopic
observer as Father Secchi, could not possibly be seen by any human eyes
through a London atmosphere.
Subsequently Professor Young startled the astronomical world by the
announcement that, at the moment when the thinnest perceptible thread
of the sun’s edge was alone displayed during the eclipse which he
observed, the whole of the dark lines of the solar spectrum flashed
out as bright stripes in a most unmistakable manner. This observation
is now fully confirmed. The first telegrams from Mr. Pogson, the
Government astronomer of Madras, and from Colonel Tennant, both
announce this most positively, Colonel Tennant’s words being, “the
reversion of the lines fully confirmed.” A similar result was obtained
by some, but not by all, of the Ceylon observers.
To understand this clearly, we must consider the fact that what appears
to us as the outline of a flat disc is really that part of the sun
which we see by looking horizontally athwart his rotundity, just as
we look at the ocean surface of our own earth when we stand upon the
shore and see its horizon outline. When the moon obscures all but the
last film of this solar edge, we see only the surface of the supposed
gaseous orb, just that portion of the blazing gases which are not
greatly compressed by those above them, and which accordingly should,
if they consist of the vapors or the gases above named, display a
bright-striped spectrum, provided the intervening non-luminous vapors
of the same metals are not sufficiently abundant to obscure them—at
this particular moment, when only the absolute horizon-line is seen,
and the body of the moon cuts off all the intervening solar surface,
and the lower or denser portion of the intervening super-solar vapors,
though, of course, these are not so entirely cut off as the continuous
background.
The reversion of the dark lines therefore reveals to us the stupendous
fact that the surface of the mighty sun, which is as big as a million
and a quarter of our worlds, consists of a flaming ocean of hydrogen
and of the metals above-named in a gaseous condition, similar to that
of the hydrogen itself.
This fact, coupled with the other revelations of the spectroscope,
which, without the help of an eclipse, reveals the surface outline of
the sun, the “sierra” and the “prominences” tell us that this flaming
ocean is in a state of perpetual tempest, heaving up its billows and
flame-Alps hundreds and thousands of miles in height, and belching
forth above all these still taller pillars of fire that even reach an
elevation of more than a hundred thousand miles, and then burst out
into mighty clouds of flame and vapor, bigger than five hundred worlds.
What does the last eclipse teach us in reference to the corona? Firstly
and clearly, that Lockyer’s explanation which attributed it to an
illumination of the upper regions of the earth’s atmosphere must be
now forever abandoned. This theory has died hard, but, in spite of Mr.
Lockyer’s proclamation of “victory all along the line,” it is now past
galvanizing. There can be no further hesitation in pronouncing that the
corona actually belongs to the sun itself, that it is a marvelous solar
appendage extending from the sun in all directions, but by no means
regularly.
The immensity of this appendage will be best understood by the fact
that the space included within the outer limits of the visible corona
is at least twenty times as great as the bulk of the sun itself, that
above twenty-five millions of our worlds would be required to fill it.
Jannsen says: “I believe the question whether the corona is due to the
terrestrial atmosphere is settled, and we have before us the prospect
of the study of the extra-solar regions, which will be very interesting
and fertile.”
The spectroscope, the polariscope, and ordinary vision all concur
in supporting the explanation that the corona is composed of solid
particles and gaseous matter intermingled. It fulfils exactly all
the requirements of the hypothesis which attributes it to the same
materials as those which in a gaseous state cause the reversion of the
dark lines above described, but which have been ejected with the great
eruptions forming the solar prominences, and have become condensed into
glowing metallic hailstones as their distance from the central heat has
increased. These must necessarily be accompanied by the vapors of the
more volatile materials, and should give out some of the lighter gases,
such as hydrogen, which, under greater pressure, would be occluded
within them, just as the hydrogen gas occluded within the substance of
the Lenarto meteor (a mass of iron which fell from the sky upon the
earth) was extracted by the late Master of the Mint by means of his
mercurial air-pump.
The rifts or gaps between the radial streamers, which have been so
often described and figured, but were regarded by some as optical
illusions, are now established as unquestionable facts. Mr. Lockyer,
the last to be convinced, is now compelled to admit this, which
overthrows the supposition that this solar appendage is a luminous
solar atmosphere of any kind. If it were gaseous or true vapor, it must
obey the law of gaseous diffusion, and could not present the phenomena
of bright radial streamers, with dark spaces between them, unless it
were in the course of very rapid radial motion either to or from the
sun.
The photographs have not yet been published. When they have all
arrived, and can be compared, we shall learn something that I
anticipate will be extremely interesting respecting the changes of the
corona, as they have been taken at the different stations at different
times. I alluded to this subject before, when it was only a matter of
possibility that such a succession of pictures might have been taken.
We now have the assurance that such pictures have been obtained. There
can be no question about optical illusion in these; they are original
affidavits made by the corona itself, signed, sealed, and delivered as
its own act and deed.