chemical constitution of the sun has made rapid progress, and new facts
are continually being observed, which serve to furnish more and more
definite views. Mr. Lockyer considers that, extending to a great
distance around the sun is an atmosphere of comparatively cooler
hydrogen, or perhaps of some still lighter substance which is unknown to
us. It is this which forms what is termed the _corona_, or circle of
light which is seen surrounding the sun in a total eclipse. Immersed in
this, and extending to a much smaller distance from the nucleus of the
sun, is another envelope, termed the _chromosphere_, consisting of
incandescent hydrogen and some glowing vapours of magnesium and calcium.
The brightest part of this envelope, which lies nearest the sun, is that
which gives off the red rays by which the prominences may be observed
without an eclipse. These prominences have been shown to be tremendous
outbursts of glowing hydrogen, belched up with sometimes an enormous
velocity from below, since they have been observed to spring up 90,000
miles in a few minutes. Beneath the chromosphere, and nearer to the body
of the sun, are enormous quantities of the vapours of the different
elements—sodium, iron, &c.—to which the dark lines of the solar spectrum
are due. This stratum Mr. Lockyer calls the _reversing layer_, because
it reverses (turns to dark) the lines which would otherwise have
appeared bright, just as Kirchhoff’s sodium vapour did in the experiment
described on page 437. Beneath the reversing layer is the _photosphere_,
from which emanates the light that is absorbed in part by the reversing
layer, and which there is good reason to believe is either intensely
heated solid or liquid matter.
In 1861 Dr. Huggins devoted himself, with an ardour which has since
known no remission, to the extension of prismatic analysis to the other
heavenly bodies. The difficulties of the investigations were great.
There was first the small quantity of light which a star sends to the
spectator; this was obviated by the use of a telescope of large
aperture, which admitted and brought to a focus many more rays from the
star, and therefore the brightness of the image was proportionately
increased. Not so the size of the image: the case of the fixed stars for
this always remains a mere point. It was, of course, necessary to drive
the telescope by clockwork, so that the light of the star might be
stationary on the field of the spectroscope. As the spectrum of the
image of the star formed by the object-glass would be a mere line,
without sufficient breadth for an observation of the dark or light lines
by which it might be crossed, it is necessary to spread out the image so
that the whole of the light may be drawn out into a very narrow line,
having a length no greater than will produce a spectrum broad enough for
the eye to distinguish the lines in it. This is accomplished by means of
a cylindrical lens placed in the focus of the object-glass, and
immediately in front of the slit. Covering one-half of the slit is a
right-angled prism by which the light to be compared with that of the
star is reflected into the slit. The light is usually that produced by
taking electric sparks between wires of the metal in the manner already
described. The dispersive power of the spectroscope was furnished by two
prisms of very dense glass, and the spectrum was viewed through a
telescope of short focal length. Dr. Huggins’s observations lead him to
the conclusion that the planets Mars, Jupiter, and Saturn possess
atmospheres, as does also the beautiful ring by which Saturn is
surrounded; for he noticed in the spectrum of each different dark lines
not belonging to the solar spectrum.
[Illustration:
FIG. 229.—_The Planet Saturn._
]
Passing to the results obtained in the case of the fixed stars, we may
remind the reader of the enormous distance of the bodies which are
submitted to the new method of analysis. Sir John Herschel gives the
following illustration of the remoteness of Sirius—supposed to be one of
the nearest of the fixed stars: Take a globe, 2 ft. in diameter, to
represent the sun, and at a distance of 215 ft. place a pea, to give the
proportionate size and distance of the earth. If you wish to represent
the distance of Sirius _on the same scale_, you must suppose something
placed _forty thousand miles_ away from the little models of sun and
earth. But not only do we know with certainty some of the substances
contained in Sirius, but the star spectroscope has taught us a great
deal about orbs so remote, that their distance is absolutely
unmeasurable. About Aldebaran we know that there are hydrogen gas and
vapours of magnesium, iron, calcium, sodium, and some four or five other
elements. Generally the lines indicate the presence of hydrogen in these
distant suns; but there is, at least, one remarkable exception in α
_Orionis_, the spectrum of which yields no trace of the hydrogen lines,
although it is evident that magnesium, sodium, calcium, &c., are
present. The spectra of celestial bodies are of several kinds. Many of
the stars have, like our sun, a continuous spectrum crossed by dark
lines. Such is that of Sirius, No. 10, Plate XVII. Others have, however,
both dark and bright lines, and some are marked by only three bright
spaces. Of the spectra of the nebulæ some have three bright lines (see
No. 11, Plate XVII.), and the bodies producing them are, therefore, to
be considered as masses of incandescent gas, while some give continuous
spectra. One of the bright lines in the spectra of the nebulæ coincides
with one of the hydrogen lines, and another—the brightest of the
three—with one of the brightest nitrogen lines; but the third does not
agree with any with which it has as yet been compared. The inference
from these appearances is that the nebulæ contain hydrogen and nitrogen,
but the absence of the other lines of these substances has not been
fully explained; although the observation of Dr. Huggins, that when the
light of incandescent nitrogen and hydrogen is gradually obscured by
interposing layers of neutral tinted glass, the lines corresponding with
those in the nebular spectra are the last to disappear, seems to suggest
a probable solution of the difficulty.
[Illustration:
FIG. 230.—_Solar Prominences, No. 1._
]
There is another very interesting line of spectroscopic research in the
power the prism gives us of estimating the velocity with which the
distances of the stars from our system are increasing or diminishing. On
closely examining the hydrogen lines of Sirius, and comparing them with
the bright lines of hydrogen rendered incandescent by electric
discharges in a Geissler tube, the spectrum of which his instrument
enabled him to place side by side with that of the star, Mr. Huggins was
surprised to find that the lines in the latter did not exactly coincide
in position with those of the former, but appeared slightly nearer the
red end of the spectrum. This indicated a longer wave-length, or
increased period of vibration, according to the theory of light, which
would be accounted for by a receding motion between Sirius and the
earth, just as the crest of successive waves of the sea would overtake a
boat going in the same direction at longer intervals of time than those
at which they would pass a fixed point, while, if the boat were meeting
the waves, these intervals would, on the other hand, be shorter. Hence
if the position of the lines in the spectrum depends on the periods of
vibration, that position will be shifted towards the red end when the
luminous body is receding from the earth with a velocity comparable to
that of light, and towards the violet end when the motion is one of
approach. The change in refrangibility observed by Mr. Huggins
corresponded with a receding velocity of 41·4 miles per second, and when
from this was subtracted the known speed with which the earth’s motion
round the sun was carrying us from the star at the time, the remainder
expressed a motion of recession amounting to about twenty miles a
second, which motion, there is reason to believe, is chiefly due to a
proper movement of Sirius. These deductions from prismatic observations
are of the highest value astronomically, since they will eventually
enable the real motions of the stars to be determined, for ordinary
observation could only show us that component of the motion which is at
right angles to the visual ray, while this gives the component along the
visual ray. In the same manner, it is inferred that Arcturus, a bright
star in the constellation _Boötes_, is approaching us with a velocity of
fifty-five miles per second.
[Illustration:
FIG. 231.—_Solar Prominences, No. 2._
]
When the solar spots are examined with the spectroscope, the dark image
of the slit produced by the hydrogen line, F, is observed to show a
strange crookedness when it is formed by rays from different parts of
the spot. This distortion is due to the same cause as the displacement
of the stellar lines, namely, motions of approach or recession of the
masses of glowing hydrogen. Mr. Norman Lockyer, to whom we are indebted
for the most elaborate investigations of the solar surface, has
calculated, from the position of the lines, the velocities with which
masses of heated hydrogen are seen bursting upwards, and those which
belong to the down-rushes of cooler gas. Velocities as great as 100
miles per second were, in this way, inferred to occur in some of the
storms which agitate the solar surface. Two drawings of a solar storm,
given by Mr. Lockyer, are shown in Figs. 230 and 231. These are
representations of one of the so-called red prominences, the first
giving its appearance at five minutes past eleven on the morning of
March 14th, 1869, and the last showing the same _ten minutes
afterwards_. The enormous velocity which these rapid changes imply will
be understood when it is stated that this prominence was 27,000 miles
high. “This will give you some idea,” says Mr. Lockyer, “of the
indications which the spectroscope reveals to us, of the enormous forces
at work in the sun, merely as representing the stars, for everything we
have to say about the sun the prism tells us—and it was the first to
tell us—we must assume to be said about the stars. I have little doubt
that, as time rolls on, the spectroscope will become, in fact, almost
the pocket companion of every one amongst us; and it is utterly
impossible to foresee what depths of space will not in time be gauged
and completely investigated by this new method of research.”
The light of comets has also been examined by the spectroscope, and many
interesting results arrived at. Our limits do not, however, permit us to
enter into a discussion of these interesting subjects.
Fig. 232 is a section of another of Mr. Browning’s popular instruments,
which is named by him the “Amateur’s Star Spectroscope.” It exhibits
very distinctly the different spectra of the various stars, nebulæ,
comets, &c.
[Illustration:
FIG. 232.—_Section of Amateur’s Star Spectroscope._
]
The reader who is desirous of learning more of this fascinating subject
is referred to Dr. Roscoe’s elegant volume, entitled, “Lectures on
Spectrum Analysis.” This work, which is embellished with handsome
engravings and illustrated by coloured maps and spectra, gives a clear
and full account of every department of the subject, and in the form of
appendices, abstracts of the more important original papers are
supplied, while a complete list is given of all the memoirs and
publications relating to the spectroscope which have been published.
This brief account of the spectroscope and its revelations, which is all
that our space permits us to give, will not fail to awaken new thoughts
in the mind of a reader who has obtained even a glimpse of the nature of
the subject, especially in relation to that branch of which we have last
treated, for in every age and in every region the stars have attracted
the gaze and excited the imagination of men. The belief in their
influence over human affairs was profound, universal, and enduring; for
it survived the dawn of rising science, being among the last shades of
the long night of superstition which melted away in the morning of true
knowledge. Even Francis Bacon, the father of the inductive philosophy,
and old Sir Thomas Browne, the exposer of “Vulgar Errors,” believed in
the influences of the stars; for while recognizing the impostures
practised by its professors, they still regarded astrology as a science
not altogether vain. It was reserved for the mighty genius of Newton to
prove that in very truth there are invisible ties connecting our earth
with those remote and brilliant bodies—ties more potent than ever
astrology divined; for he showed that even the most distant orb is bound
to its companions and to our planet by the same power that draws the
projected stone to the ground. And now the spectroscope is revealing
other lines of connection, and showing that not gravitation alone is the
sympathetic bond which unites our globe to the celestial orbs, but that
there exists the closer tie of a common constitution, for they are all
made of the same matter, obeying the same physical and chemical laws
which belong to it on the earth. We learn that hydrogen, and magnesium,
and iron, and other familiar substances, exist in these inconceivably
distant suns, and there exhibit the identical properties which
characterize them here. We confirm, by the spectroscope, the fact
partially revealed by other lines of research, that the stars which
appear so fixed, are, in reality, careering through space, each with its
proper motion. We learn also that the stars are the theatres of vast
chemical and physical changes and transformations, the rapidity and
extent of which we can hardly conceive. There is, for example, the case
of that wonderful star in the constellation of the Crown, which, in
1866, suddenly blazed out, from a scarcely discernible telescopic star,
to become one of the most conspicuous in the heavens, and the bright
lines its beams produced in the spectroscope revealed the fact that this
abrupt splendour was due to masses—who can imagine how vast?—of
incandescent hydrogen. This brightness soon waned, and τ _Coronæ
Borealis_ reverted once more to all but telescopic invisibility. The
seeming fixity of the stars is an illusion of the same nature as that
which prevents a casual observer from recognizing their apparent diurnal
motion, and now we have also ample evidence that permanence of physical
condition, even in the stars, is impossible. Everywhere in the universe
there is motion and change; there is no pause, no rest, but a continual
unfolding, an endless progression.
“Know the stars yonder,
The stars everlasting,
Are fugitive also,
And emulate, vaulted,
The lambent heat-lightning
And fire-fly’s flight.”
ROENTGEN’S X RAYS.
[Illustration:
FIG. 232_a_.—_Living Hand. Exposure, 4 Minutes._
]
On page 507 reference will be made to certain remarkable effects
observed by Mr. Crookes when the electric discharges from an induction
coil are passed through very highly exhausted tubes. These
phosphorescent and mechanical effects Mr. Crookes attributed to streams
of “radiant matter” shot off from the _negative_ pole with immense
velocities—the matter not being that of the electrode itself, but
particles of the extremely rarefied residual gas, which, being
comparatively few, could mostly traverse the tube in straight lines
without coming into collision with their fellows, and thus a class of
phenomena, different from the striated discharges in the ordinary and
less highly exhausted Geissler tubes, comes into view. The emanations
from the negative pole, or _cathode_, in highly rarefied gases became
known as the “cathode rays,” and they began to be further examined by
other observers, and more particularly in 1894 by Hittorf, and by M.
Lenard, a Hungarian physicist, who found that they pass through thin
plates of metal, and through wood and other substances not transparent
to ordinary light. It was also observed by Lenard, and also previously
by Hertz, that there are several kinds of cathode rays, which differ
from each other as regards their powers of exciting phosphorescence,
capability of being deflected by a magnet, and the degrees in which they
are absorbed by various media. But universal attention was drawn to this
subject by the announcement, at the end of 1895, of certain discoveries
made by Dr. W. K. Roentgen, a professor of physics at Wurzburg. He
covered a highly-exhausted Crookes’ tube with black cardboard, and found
that when the discharge of a large induction coil was passed through the
tube in a dark room, a piece of paper coated on one side with
platino-cyanide of barium, and held near the covered tube, glowed with a
brilliant fluorescence, no matter which side of the paper was turned
towards the tube; and even at a distance of two yards some fluorescence
was still visible. On experimenting with various bodies interposed
between the covered tube and the fluorescent screen, it was found that
the emanations passed through nearly every substance with more or less
facility. The screen lit up when placed behind a book of a thousand
pages, also behind two packs of cards. A single layer of tin-foil
scarcely threw a shadow, and several thicknesses were required to
produce a distinct effect. Deal boards, an inch thick, offered little
resistance. A very thick plate of aluminium (6/10 inch) reduced the
fluorescence, but still allowed some rays to pass. The hand held before
the fluorescent screen showed a dark shadow of the bones only, with but
a faint outline of their fleshy investment. Copper, silver, gold,
platinum, and lead, in comparatively small thicknesses, intercept these
rays. Thus a plate of lead only five hundredths of an inch thick almost
stops them. Increase of thickness increases the resistance to their
passage in all cases; but the comparative transparency of a body cannot
be deduced from its thickness and density. Many other bodies besides
platino-cyanide of barium become fluorescent under the influence of
these rays, such as certain kinds of glass, Iceland spar, rock-salt,
etc. Dr. Roentgen is convinced that these rays are not the cathode rays
or any part of them; but as the theoretical nature of the new rays has
not yet been explained, he has preferred to provisionally call them the
X rays, a denomination doubtless suggested by the use of the symbol _x_
in algebra to represent unknown quantities.
The source of the X rays, Roentgen states, is at the place where the
cathode rays strike the walls of the exhausted tube, and produce the
most brilliant phosphorescence; but they cannot be cathode rays which
have merely passed through the glass, for, contrary to what has been
observed with respect to the latter, they cannot be deflected by a
magnet. Nor is glass the only substance in which they can be generated,
for they were obtained from an apparatus in which the cathode rays were
made to impinge upon a plate of aluminium nearly one-tenth of an inch
thick. Photographic dry plates are also sensitive to the X rays, and
their power to pass through wood, ebonite, etc., makes the experiments
of testing the opacity, or otherwise, of various objects for them quite
easy. It is necessary merely to place the object on the closed cover of
the dark slide, and place the whole under the vacuum tube; all the
exposure, which is somewhat prolonged in most cases, may be made in
ordinary light. But the light-tight boxes, in which photographic plates
are packed, cannot, of course, be brought near the apparatus, as they
are completely permeable to the X rays, and their whole contents may be
rendered useless. The impression obtained on the photographic plate is
not so much a photograph as a _shadow_ of the interposed object—a shadow
more or less dense in the positive print according to the permeability
of the object, and the length of the exposure. These photographic
results have sometimes been called “shadowgrams,” “radiograms,”
“radiographs,” “skiagraphs,” etc. The word _skiagraph_ appears the most
appropriate designation. That the emanations from the phosphorescing
substance on which the cathode rays impinge are entitled to be also
called rays, appears from the regularity of the shadows thrown on the
fluorescent screen or photographic plate; and the fact of their
propagation in straight lines was proved by Dr. Roentgen obtaining a
_pin-hole_ photograph of the phosphorescing part of the vacuum tube,
when the latter was enveloped in black paper. Why a _pin-hole_ and not a
lens was used for taking this photograph will presently appear.
[Illustration:
FIG. 232_b_.—_Skiagraph of a Hand, by Dr. Roentgen. The Third Finger
has a Ring on it._
]
One of Dr. Roentgen’s experiments excited the attention and interest of
the general public, as well as of the scientific world, in the most
extraordinary degree, and though its announcement was received in some
quarters with incredulity, experimenters in all parts of the world
immediately set themselves at work to test the truth of the alleged
discovery. Electrical apparatus of different kinds, with various
adjustments, were employed, with results that were in some cases
failures, in others confirmations of the German professor’s statements,
and not unfrequently the variations in the conditions gave rise to
increased knowledge of the phenomena generally. The experiment just
alluded was one in which a dry photographic plate contained in one of
the camera dark slides, now so familiar to every one, was placed (with
the slide still closed by its wooden cover of nearly one quarter of an
inch thick) a few inches below the Crookes’ tube, and the hand of a
living-person being extended on the outside of the cover, a shadow of
the bones of the hand, as if seen through the surrounding tissues, was
obtained. Much popular misconception as to the powers of the “new
photography” arose from want of knowledge of the process by which these
strange pictures were obtained, the common notion being that these
photographs were produced by some method of using a camera, and that
outlines of people’s bodies and skeletons could be taken
instantaneously, not only through their clothes, but through doors and
walls. Much nearer the mark was the allusion of a scientific writer as
to the possibility of the new process realising Dickens’ description of
Marley’s ghost: “His body was transparent, so that Scrooge, observing
him, and looking through his waistcoat, could see the two buttons on his
coat behind.” The value of the new discovery for medical and surgical
purposes was immediately recognised, and very soon its application was
successfully practised.
Dr. Roentgen found that the X rays are incapable of refraction, in this
respect differing from ordinary light (see pages 397 and following), and
among the experiments which most impressed and astonished his auditors
when he was lecturing at Potsdam on his new discovery before the
Imperial Court of Germany, was one in which he showed the X rays passing
in a straight line through water without undergoing refraction. The rays
pass without interruption equally through the substances, whether these
be coherent or in a layer of fine powder of the same thickness, and this
again shows that there can be no regular reflection or refraction.
Prisms and lenses, whether of glass, ebonite, or aluminium, fail to
afford evidence of refractive action, hence the X rays cannot be focused
like those of ordinary light, and that is why the photograph of the
vacuum tube had to be taken by a pin-hole. Again, glass lenses could not
be used, because this substance, so transparent to light, is
particularly opaque to the X rays, and would in a great degree intercept
them, while lenses of ebonite and of aluminium, which were tried, were
inoperative on account of the irrefrangibility of the rays.