emerge unchanged in character. Different portions of the ray are
differently bent, so that when they emerge from the prism they no
longer travel side by side as before. The violet part of the light
is bent most, the red least; the various colors from violet through
blue, green, and yellow, to red being bent gradually less and less.
The prism then _sorts_, or _sifts_, the light-waves.
But we want the means of sifting the light-waves more thoroughly. The
reader must bear with me while I describe, as exactly as possible in
the brief space available to me, the way in which the first rough
work of the prism has been modified into the delicate and significant
work of the spectroscope. It is well worth while to form clear views
on this point, because so many of the wonders of modern science are
associated with spectroscopic analysis.
If, through a small round hole in a shutter, light is admitted into
a darkened room, and a prism be placed with its refracting angle
downward and horizontal, a vertical spectrum, having its violet end
uppermost, will be formed on a screen suitably placed to receive it.
But now let us consider what this spectrum really is. If we take the
light-waves corresponding to any particular color, we know, from
optical considerations, that these waves emerge from the prism in a
pencil exactly resembling in shape the pencil of white light which
falls on the prism. They therefore form a small circular or oval
image on their own proper part of the spectrum. Hence the spectrum is
in reality formed of a multitude of overlapping images, varying in
color from violet to red. It thus appears as a rainbow-tinted streak,
presenting every gradation of color between the utmost limits of
visibility at the violet and red extremities.
If we had a square aperture to admit the light, we should get a
similar result. If the aperture were oblong, there would still be
overlapping images; but if the length of the oblong were horizontal,
then, since each image would also be a horizontally placed oblong,
the overlapping would be less than when the images were square.
Suppose we diminish the overlapping as much as possible? in other
words, suppose we make the oblong slit as narrow as possible? Then,
unless there were in reality an infinite number of images distributed
all along the spectrum from top to bottom, the images might be so
narrowed as not to overlap; in which case, of course, there would
be horizontal dark spaces or gaps in our spectrum. Or, again, if we
failed in finding gaps of this sort by simply narrowing the aperture,
we might lengthen the spectrum by increasing the refracting angle of
the prism, or by using several prisms, and so on.
The first great discovery in solar physics, by means of the analysis
of the prism (though the discovery had little meaning at the time),
consisted in the recognition of the fact that, by means of such
devices as the above, dark gaps or cross-lines _can_ be seen in the
solar spectrum. In other words, light-waves of the various gradations
corresponding to all the tints of the spectrum from violet to red
do _not_ travel to us from the great central luminary of our system.
Remembering that the effect we call color is due to the length of
the light-waves, the effect of red corresponding to light-waves of
the greatest length, while the effect of violet corresponds to the
shortest light-waves, we see that in effect the sun sends forth to
the worlds which circle around him light-waves of many different
lengths, but not of all. Of so complex and interesting a nature is
ordinary daylight.
But spectroscopists sought to interpret these dark lines in the solar
spectrum, and it was in carrying out this inquiry—which even to
themselves seemed almost hopeless, and to many would appear an utter
waste of time—that they lighted upon the noblest method of research
yet revealed to man.
They examined the spectra of the light from incandescent substances
(white-hot metals and the like), and found that in these spectra
there are no dark lines.
They examined the spectra of the light from the stars, and found that
these spectra are crossed by dark lines resembling those in the solar
spectrum, but differently arranged.
They tried the spectra of glowing vapors, and they obtained a
perplexing result. Instead of a number of dark lines across a
rainbow-tinted streak, they found bright lines of various colors.
Some gases would give a few such lines, others many, some only one or
two.
Then they tried the spectrum of the electric spark, and they found
here also a series of bright lines, but not always the same series.
The spectrum varied according to the substances between which the
spark was taken and the medium through which it passed.
Lastly, they found that the light from an incandescent solid or
liquid, when shining through various vapors, no longer gives a
spectrum without dark lines, but that the dark lines which then
appear vary in position, according to the nature of the vapor through
which the light has passed.
Here were a number of strange facts, seemingly too discordant and too
perplexing to admit of being interpreted. Yet one discovery only was
wanting to bring them all into unison.
In 1859, Kirchhoff, while engaged in observing the solar spectrum,
lighted on the discovery that a certain double dark line, which had
already been found to correspond exactly in position with the double
bright line forming the spectrum of the glowing vapor of sodium, was
intensified when the light of the sun was allowed to pass through
that vapor. This at once suggested the idea that the presence of
this dark line (or, rather, pair of dark lines) in the spectrum of
the sun is due to the existence of the vapor of sodium in the solar
atmosphere, and that this vapor has the power of absorbing the same
order of light-waves as it emits. It would of course follow from
this that the other dark lines in the solar spectrum are due to the
presence of other absorbent vapors in its atmosphere, and that the
identity of these would admit of being established in the same way,
supposing this general law to hold, that a vapor emits the same
light-waves that it is capable of absorbing.
Kirchhoff was soon able to confirm his views by a variety of
experiments. The general principles to which his researches led—in
other words, the principles which form the basis of spectrum
analysis—are as follows:
1. An incandescent solid or liquid gives a continuous spectrum.
2. A glowing vapor gives a spectrum of white lines, each vapor having
its own set of bright lines, so that, from the appearance of a
bright-line spectrum, one can tell the nature of the vapor or vapors
whose light forms the spectrum.
3. An incandescent solid or liquid shining through absorbent vapors
gives a rainbow-tinted spectrum crossed by dark lines, these dark
lines having the same position as the bright lines belonging to the
spectra of the vapors; so that, from the arrangement of the dark
lines in such a spectrum, one can tell the nature of the vapor or
vapors which surround the source of light.[24]
The application of the new method of research to the study of
the solar spectrum quickly led to a number of most interesting
discoveries. It was found that, besides sodium, the sun’s atmosphere
contains the vapors of iron, calcium, magnesium, chromium, and
other metals. The dark lines corresponding to these elements appear
unmistakably in the solar spectrum. There are other metals, such as
copper and zinc, which seem to exist in the sun, though some of the
corresponding dark lines have not yet been recognized. As yet it
has not been proved that gold, silver, mercury, tin, lead, arsenic,
antimony, or aluminium exist in the sun—though we can by no means
conclude, nor indeed is it at all probable, that they are absent from
his substance. The dark lines belonging to hydrogen are very well
marked indeed in solar spectrum, and, as we shall see presently,
the study of these lines has afforded most interesting information
respecting the physical constitution of the sun.
Now we notice at once how importantly these researches into the
sun’s structure bear upon the subject of this treatise. It would be
indeed interesting to consider the actual condition of the central
orb of the planetary scheme, to picture in imagination the metallic
oceans which exist upon his surface, the continual evaporation from
those oceans, the formation of metallic clouds, and the downpour of
metallic showers upon the surface of the sun. But apart from such
considerations, and viewing Kirchhoff’s discoveries simply in their
relation to the subject of other worlds, we have enough to occupy our
attention.
If it could have been shown that, in all probability, the substance
of the sun consists of materials wholly different from those which
exist in this earth, the conclusion obviously to be drawn from such
a discovery would be that the other planets also are differently
constituted. We could not find any just reason for believing that
in Jupiter or Mars there exist the elements with which we are
acquainted, when we found that even the central orb of the planetary
system exhibits no such feature of resemblance to the earth. But now
that we know, quite certainly, that the familiar elements, iron,
sodium, and calcium, exist in the sun’s substance, while we are led
to believe, with almost perfect assurance, that all the elements we
are acquainted with also exist there, we see at once that, in all
probability, the other planets are constituted in the same way. There
may of course be special differences: in one planet the proportionate
distribution of the elements may differ, and even differ very
markedly, from that which prevails in some other planet. But the
general conclusion remains, that the planets are formed of the
elements which have so long been known as terrestrial; for we can not
recognize any reason for believing that our earth alone, of all the
orbs which circle around the sun, resembles that great central orb in
general constitution.
Now, we have in this general law a means of passing beyond the
bounds of the Solar System, and forming no indistinct conceptions
as to the existence and character of worlds circling around other
suns. For these orbs, like our sun, contain in their substance many
of the so-called terrestrial elements, while it may not unsafely
be asserted that all, or nearly all, those elements, and few or no
elements unknown to us, exist in the substance of every single star
that shines upon us from the celestial concave. Hence we conclude
that round those suns also there circle orbs constituted like
themselves, and therefore containing the elements with which we are
familiar. And the mind is immediately led to speculate on the uses
which those elements are intended to subserve. If iron, for example,
is present in some noble orb circling around Sirius, we speculate
not unreasonably respecting the existence on that orb—either now or
in the past, or at some future time—of beings capable of applying
that metal to the useful purposes which man makes it subserve.
The imagination suggests immediately the existence of arts and
sciences, trades and manufactures, on that distant world. We know
how intimately the use of iron has been associated with the progress
of human civilization, and though we must ever remain in ignorance
of the actual condition of intelligent beings in other worlds, we
are yet led, by the mere presence of an element which is so closely
related to the wants of man, to believe, with a new confidence, that
for such beings those worlds must in truth have been fashioned.
I would fain dwell longer on the thoughts suggested by the researches
of Kirchhoff. Gladly too would I enter at length on an account of
those interesting discoveries which have been made in connection with
the total eclipses of the sun. One point, however, remains which is
too intimately connected with my subject to be passed over.
I refer to the sun’s corona.
It has been proved that the solar prominences consist of glowing
vapors, hydrogen being their chief constituent. It has been
found also, by comparing Mr. Lockyer’s observations of the
prominence-spectra with Dr. Frankland’s elaborate researches
into the peculiarities presented by the spectrum of hydrogen at
different pressures, that even in the very neighborhood of the solar
photosphere these vapors probably exist at a pressure so moderate as
to indicate that the limits of the sun’s vaporous envelope can not
lie very far (relatively) from the outer solar cloud-layer.
Now, the solar corona has been seen, during total eclipses of the
sun, to extend to a distance at least equal to the sun’s diameter
from the eclipsed orb. So that, assuming the corona to be a solar
atmosphere, it would have a depth of about eight hundred and fifty
thousand miles, and being also drawn toward the sun by his enormous
attractive energy (exceeding more than twenty-seven times that of
the earth), it could not fail to exert a pressure on his surface
exceeding many thousand-fold that of our air upon the earth. In
fact, such an atmosphere, let its outermost layers be as rare as we
can conceive, would yet have its lower layers absolutely liquefied,
if not solidified, by the enormous pressure to which they would
be subjected. We can not, then, believe this corona to be a solar
atmosphere.
[Illustration: Fig. 33.—Tychonic System]
Yet it is quite impossible to dissociate the corona, either wholly
or in part, from the sun. I am aware that physicists of eminence
have attempted to do this, and not only so, but to make of the
zodiacal light a terrestrial phenomenon. But they have overlooked
considerations which oppose themselves irresistibly to such a
conclusion.
In the first place, the mere fact that, during a total eclipse, the
moon looks black, in the very heart of the corona, affords, when
properly understood, the most conclusive evidence that the light of
the corona comes from behind the moon. If the glare of our atmosphere
could by any possibility account for the corona (which is not the
case), then that glare should appear over the moon’s disk also.
That this is so is proved by the fact that, when the glare really
does cover the moon, as while the sun is but slightly eclipsed,
the moon is not projected as a black disk on the background of the
_sky_, though, where her outline crosses the sun, it appears black,
by contrast with the intensity of his light.[25] The point seems,
however, too obvious to need discussion.
And, secondly, as Mr. Baxendell has pointed out, during totality the
part of the earth’s atmosphere between the eye and the corona is not
illuminated by the sun. Over a wide space all round the sun we are
looking through an atmosphere which is completely dark. In fact, if
the earth’s atmosphere alone were in question, we ought to see a dark
or negative corona around the sun, the illuminated atmosphere only
beginning to be faintly visible at a considerable angular distance
from the sun. This argument, rightly understood, is altogether
decisive of the question.[26]
But the spectroscope has given certain very perplexing evidence
respecting the light of the corona, and it remains that we should
endeavor to see how that evidence bears on the interesting problem
which the corona presents to our consideration.
During the total eclipse of 1868 the American observers found that
the spectrum of the corona is continuous, but crossed by certain
bright lines. If we accept the absence of dark lines as established
by the evidence (which is doubtful), this result seems at first
sight very difficult to explain. Referring to the principles of
spectroscopic analysis stated on pp. 338-339, it will be seen that
we should be led to infer that the corona consists of incandescent
matter surrounded by certain glowing gases. It is difficult to
suppose that this is the real explanation of the phenomenon.
Mr. Lockyer suggests that, if the corona shone by reflecting the
solar light, the continuous spectrum might be accounted for by
supposing the light from the glowing vapors around the sun to supply
the part wanting where the solar dark lines are, and that some of
these vapors shining yet more brightly would exhibit their bright
lines upon the continuous background of the spectrum. This view, as
applied by Mr. Lockyer to the theory that the corona is a terrestrial
phenomenon, is untenable, for the reasons already adduced. But,
independently of those reasons, there are others which render such a
solution of the difficulty unavailable.
Now, remembering that we have two established facts for our
guidance—(1) the fact that the corona can not be a solar atmosphere,
and (2) the fact that it must be a solar appendage—I think a way may
be found toward a satisfactory explanation.
Let it be premised that the bright lines of the coronal spectrum
correspond in position to those seen in the spectrum of the aurora,
and that the same lines are seen in the spectrum of the Zodiacal
Light, and in that of the phosphorescent light occasionally seen over
the heavens at night.
Since we have every reason to believe that the light of the aurora is
due to electrical discharges taking place in the upper regions of the
air, we are invited to the belief that the coronal light may be due
to similar discharges taking place between the particles (of whatever
nature) constituting the corona.
Now, though the appearance of an aurora is due to some special
terrestrial action (however excited), yet the material substances
between which the discharges take place must be assumed to be at all
times present in the upper regions of air. In all probability, they
are the particles of those meteors which the earth is continually
encountering. And since we know that meteor-systems must be
aggregated in far greater numbers near the sun than near the earth,
we may regard the coronal light as due to electrical discharges
excited by the sun’s action, and taking place between the members of
such systems. Besides this light, however, there must necessarily be
a large proportion of light reflected from these meteoric bodies.
In this way the peculiar character of the coronal spectrum may be
readily accounted for. We know, from the auroral spectrum, that the
principal bright lines due to the electrical discharges would be
precisely where we see bright lines in the coronal spectrum. But,
besides these, there would be fainter bright lines corresponding
to the various elements which exist in the meteoric masses. These
elements, we know, are the same as those in the substance of the sun.
Thus the bright lines would correspond in position with the dark
lines of the solar spectrum. Hence, as light reflected by the meteors
would give the ordinary solar spectrum, there would result from the
combination a continuous spectrum, on which the bright lines first
mentioned would be seen, as during the American eclipse.
What the polariscope has told us respecting the corona is in
accordance with this view.
In the same way the quality of the Zodiacal Light admits of being
perfectly accounted for, without resorting to the hypothesis that
this phenomenon is a terrestrial one.
The explanation thus put forward has at least the advantage of being
founded on well-established relations. We know that the auroral light
is associated with the earth’s magnetism, and that meteoric bodies
are continually falling upon the earth’s atmosphere. We know, also,
that the sun exerts magnetic influences a thousand-fold more intense
than those of the earth, and that in his neighborhood there must be
many million times more meteoric systems.
But we have other and independent reasons, which must not be
overlooked, for considering the corona to be of some such nature as I
have suggested. Leverrier has shown that there probably exists in the
neighborhood of the sun a family of bodies whose united mass suffices
appreciably to affect the motions of the planet Mercury. It would not
be safe to neglect considerations thus vouched for.
Mr. Baxendell also has shown that certain periodic variations in the
earth’s magnetism point to the existence of such a family of bodies;
and he has been able to assign to them a position according well with
that determined by Leverrier.
Now, whatever opinion we form as to the exact character of the
system of bodies pointed to by the researches of Leverrier and
Baxendell—whether we suppose that system to form a zone around
the sun, or that (as I believe) the system is merely due to the
aggregation of meteoric perihelia in the sun’s neighborhood—we may
be quite certain of this, that during a total solar eclipse the
system could not fail to become visible. Hence there is a double
objection to the view put forward by Mr. Lockyer and others. In the
first place, it fails to account for the appearance presented by
the corona; in the second place, it fails to render an account of
the implied non-appearance of the system which, according to the
researches of Leverrier and Baxendell, circles around the sun.
[Illustration: Fig. 34.—Scale of Planets
_Jupiter and Saturn are shown in their true axial positions, Uranus
and Neptune in the axial positions inferred from the motions of their
satellites_]
We know that the sun is the sole source whence light and heat are
plentifully supplied to the worlds which circle around him. The
question immediately suggests itself—Whence does the sun derive
those amazing stores of force from whence he is continually supplying
his dependent worlds? We know that, were the sun a mass of burning
matter, he would be consumed in a few thousand years. We know that,
were he simply a heated body, radiating light and heat continually
into space, he would in like manner have exhausted all his energies
in a few thousand years—a mere day in the history of his system.
Whence, then, comes the enormous supply of force which he has
afforded for millions on millions of years, and which also our reason
tells us he will continue to afford while the worlds which circle
around him have need of it—in other words, for countless ages to come?
Now, there are two ways in which the solar energies might be
maintained. The mere contraction of the solar substance, Helmholtz
tells us, would suffice to supply such enormous quantities of heat
that, if the heat actually given out by the sun were due to this
cause alone, there would not, in many thousands of years, be any
perceptible diminution of the sun’s diameter. But, secondly, the
continual downfall of meteors upon the sun would cause an emission
of heat in quantities vast enough for the wants of all the worlds
circling round him; while his increase of mass from this cause would
not be rendered perceptible in thousands of years, either by any
change in his apparent size or by changes in the motions of his
family of worlds.
It seems far from unlikely that both these processes are in operation
at the same time. Certainly the latter is, for we know, from the
motions of the meteoric bodies which reach the earth, that myriads
of these bodies must continually fall upon the sun. And if the corona
and Zodiacal Light really be due to the existence of flights of
meteoric systems circling around the sun, or to the existence in his
neighborhood of the perihelia of many meteoric systems, then there
must be a supply of light and heat from this source very nearly if
not quite sufficient to account for the whole solar emission.