THE SPECTROSCOPIC DEPARTMENT
Another department was set on foot by Airy at the same time as
the Heliographic Department, and in connection with it; and it is
the department which has the greatest of interest for the general
public. This deals with astronomical physics, or astrophysics, as
it is sometimes more shortly called; the astronomy, that is, which
treats of the constitution and condition of the heavenly bodies, not
with their movements.
The older astronomy, on the other hand, confined itself to the
movements of the heavens so entirely that Bessel, the man whose
practical genius revolutionized the science of observation,
and whose influence may be traced throughout in Airy's great
reconstitution of Greenwich Observatory, denied that anything but
the study of the celestial movements had a right to the title of
astronomy at all. Hardly more than sixty years ago he wrote:
'What astronomy is expected to accomplish is evidently at all
times the same. It may lay down rules by which the movements
of the celestial bodies, as they appear to us upon the earth,
can be computed. All else which we may learn respecting these
bodies, as, for example, their appearance, and the character of
their surfaces, is, indeed, not undeserving of attention, but
possesses no proper astronomical interest. Whether the mountains
of the moon are arranged in this way or in that is no further
an object of interest to astronomers than is a knowledge of the
mountains of the earth to others. Whether Jupiter appears with
dark stripes upon its surface, or is uniformly illuminated,
pertains as little to the inquiries of the astronomer; and its
four moons are interesting to him only for the motions they
have. To learn so perfectly the motions of the celestial bodies
that for any specified time an accurate computation of these can
be given--that was, and is, the problem which astronomy has to
solve.'
There is a curious irony of progress which seems to delight in
falsifying the predictions of even master minds as to the limits
beyond which it cannot advance. Bessel laid down his dictum as to
the true subjects of astronomical inquiry, Comte declared that
we could never learn what were the elements of which the stars
were composed, at the very time that the first steps were being
taken towards the creation of a research which should begin by
demonstrating the existence in the heavenly bodies of the elements
with which we are familiar on the earth, and should go on to prove
itself a true astronomy, even in Bessel's restricted sense, by
supplying the means for determining motion in a direction which he
would have thought impossible--that is to say, directly to or from
us.
The years that followed Kirchhoff's application of the spectroscope
to the study of the sun, and his demonstration that sodium and iron
existed in the solar atmosphere, were crowded with a succession
of brilliant discoveries in the same field. Kirchhoff, Bunsen,
Angström, Thalèn, added element after element to the list of those
recognized in the sun. Huggins and Miller carried the same research
into a far more difficult field, and showed us the same elements
in the stars. Rutherfurd and Secchi grouped the stars according to
the types of their spectra, and so laid the foundations of what
may be termed stellar comparative anatomy. Huggins discovered true
gaseous nebulæ, and so revived the nebular theory, which had been
supposed crushed when the great telescope of Lord Rosse appeared to
have resolved several portions of the Orion nebula into separate
stars. The great riddle of 'new stars'--which still remains a
riddle--was at least attacked, and glowing hydrogen was seen to be a
feature in their constitution. Glowing hydrogen, again, was, in the
observation of total eclipses, seen to be a principal constituent
of those surroundings of our own sun which we now call prominences
and chromosphere. Then the method was discovered of observing the
prominences without an eclipse, and they were found to wax and wane
in more or less sympathy with the solar spots. Sun-spots, planets,
comets, meteors, variable stars, all were studied with the new
instrument, and all yielded to it fresh and valuable, and often
unexpected, information.
[Illustration: THE GREAT NEBULA IN ORION.
(_From a photograph taken at the Royal Observatory Greenwich,
December 1, 1899, with an exposure of 2-1/4 hours._)]
In this activity Greenwich Observatory practically took no part.
Airy, ever mindful of the original purpose of the Observatory, and
deeply imbued with views similar to those which we have quoted from
Bessel, considered that the new science lay outside the scope of his
duties, until in Mr., now Sir William, Huggins's skilful hands
the spectroscope showed itself not only as a means for determining
the condition and constitution of the stars, but also their
movements--until, in short, it had shown itself as an astronomical
instrument even within Bessel's narrow definition.
The principle of this inquiry is as follows: If a source of light
is approaching us very rapidly, then the waves of light coming from
it necessarily appear a little shorter than they really are, or, in
other words, that light appears to be slightly more blue--the blue
waves being shorter than the red--than it really is. A similar thing
with regard to the waves of sound is often noticed in connection
with a railway train. If an express train, the whistle of which is
blowing the whole time, dashes past us at full speed, there is a
perceptible drop in the note of the whistle after it has gone by.
The sound waves as it was coming were a little shortened, and the
whistle therefore appeared to have a sharper note than it had in
reality. And in the same way, when it had gone by, the sound waves
were a little lengthened, making the note of the whistle appear a
very little flatter.
Such a change of colour in a star could never have been detected
without the spectroscope; but since when light passes through a
prism the shorter waves are refracted more strongly, that is to
say, are more turned out of their course than the longer, the
spectroscope affords us the means of detecting and measuring this
change. Let us suppose that the lines of hydrogen are recognized
in a given star. If we compare the spectrum of this star with the
spectrum of a tube containing hydrogen and through which the
electric spark is passing, we shall be able to see whether any
particular hydrogen line occupies the same place as shown by the two
spectra. If the line from the star is a little to the red of the
line from the tube, the star must be receding from us; if to the
blue, approaching us. The amount of displacement may be measured by
a delicate micrometer, and the rate of motion concluded from it.
[Illustration: THE HALF-PRISM SPECTROSCOPE ON THE SOUTH-EAST
EQUATORIAL.]
The principle is clear enough. The actual working out of the
observation was one of very great difficulty. The movements of the
stars towards us, or away from us, are, in general, extremely slow
as compared with the speed of light itself; and hence the apparent
shift in the position of a line is only perceptible when a very
powerful spectroscope is used. This means that the feeble light of
a star has to be spread out into a great length of spectrum, and
a very powerful telescope is necessary. The work of observing the
motions of stars in the line of sight was started at Greenwich in
1875, the 'Great Equatorial' being devoted to it. This telescope, of
12-3/4 inches aperture, was not powerful enough to do much more than
afford a general indication of the direction in which the principal
stars were moving, and to confirm in a general way the inference
which various astronomers had found, from discussing the proper
motions of stars, that the sun and the solar system were moving
towards that part of the heavens where the constellations Hercules
and Lyra are placed. In 1891, therefore, the work was discontinued,
and as already mentioned, the 12-3/4 telescope by Merz was removed
to make room for the present much larger instrument by Sir Howard
Grubb, upon the same mounting. The new telescope being much larger
than the one for which mounting and observing room were originally
built, it was not possible to put the spectroscope in the usual
position, in the same straight line as the great telescope. It was
therefore mounted under it, and parallel to it, and the light of
the star was brought into it after two reflections. The observer
therefore stood with his back to the object and looked down into
the spectroscope. It had, however, become apparent by this time
that this most delicate field of work was one for which photography
possessed several advantages, and as Sir Henry Thompson had made
the munificent gift to the Observatory of a great photographic
equatorial, it was resolved to devote the 28-inch telescope chiefly
to double-star work, and to transfer the spectroscope to the 'New
Building.'
The 'New Observatory' in the south ground is crowned indeed with
the dome devoted to the great Thompson photographic refractor, but
this is not its chief purpose. Its principal floor contains four
fine rooms which are used as 'computing rooms'--for the office
work, that is to say, of the Observatory. Of these the principal
is in the north wing, where the main entrance is placed, and is
occupied by the Astronomer Royal and the two chief assistants. The
basement contains the libraries and the workshops of the mechanics
and carpenters. The upper floor will eventually be used for the
storage of photographs and manuscripts, and the terrace roofs of
the four wings will be exceedingly convenient for occasional
observations, as, for example, of meteor showers. The central dome,
which rises high above the level of the terraces, is the only room
in the building devoted to telescopic work. As in the New Altazimuth
building, a ring of circular lights just below the coping of the
wall recalls the portholes of a ship, and again reminds us of the
connection of the Observatory with navigation.
[Illustration: THE WORKSHOP.]
Here the spectroscope is now placed, but not, as it happens, on
the Thompson refractor. The equatorial mounting in this new dome
is a modification of what is usually called the 'German' form of
mounting--that is to say, there is but one pier to support the
telescope, and the telescope rides on one side of the pier and a
counterpoise balances it on the other The 'Great Equatorial,' on
the other hand, is an example of the English mounting, and has two
piers, one north and the other south, whilst the telescope swings
in a frame between them. In the new dome three telescopes are found
rigidly connected with each other on one side of the pier, the
telescopes being (1) the great Thompson photographic telescope,
double the aperture and double the focal length of the standard
astrographic telescope used for the International Photographic
Survey; (2) the 12-3/4 telescope by Merz, that used to be in the
great South-East dome, but which is now rigidly connected with the
Thompson refractor as a guide telescope; and (3) a photographic
telescope of 9 inches aperture, already described as the 'Thompson'
photo-heliograph, and used for photographing the sun or in eclipse
expeditions. The counterpoise to this collection of instruments is
not a mere mass of lead, but a powerful reflector of 30 inches'
aperture, and it is to this telescope that the spectroscope is now
attached. At the present time, however (August, 1900), regular work
has not been commenced with it.
[Illustration: THE 30-INCH REFLECTOR WITH THE NEW SPECTROSCOPE
ATTACHED.]
Beside this attempt to determine the motions of the stars as they
approach us or retreat from us, on rare occasions the spectroscope
has been turned on the planets. As these shine by reflected light,
their spectra are normally the same as that of the sun. Mars
appeared to the writer, as to Huggins and others, to show some
slight indication of the presence of water vapour in its atmosphere.
Jupiter and Saturn show that their atmospheres contain some
absorbing vapour unknown to ours. And Uranus and Neptune, faint and
distant as they are, not only show the same dark band given by the
two nearer planets, but several others. More attractive has been
the examination of the spectra of the brighter comets that have
visited us. The years 1881 and 1882 were especially rich in these.
The two principal comets of 1881 were called after their respective
discoverers, Tebbutt's and Schaeberle's. They were not bright
enough to attract popular attention, though they could be seen with
the naked eye, and both gave clear indications of the presence of
carbon, their spectra closely resembling that of the blue part of a
gas or candle flame. There was nothing particularly novel in these
observations, since comets usually show this carbon spectrum, though
why they should is still a matter for inquiry; but the two comets of
the following year were much more interesting. Both comets came very
near indeed to the sun. The earlier one, called from its discoverer
Comet Wells, as it drew near to the sun, began to grow more and more
yellow, until in the first week of June it looked as full an orange
as even the so-called red planet, Mars. The spectroscope showed the
reason of this at a glance. The comet had been rich in sodium. So
long as it was far from the sun the sodium made no sign, but as it
came close to it the sodium was turned into glowing vapour under the
fierce solar heat. And as the writer saw it in the early dawn of
June 7, the comet itself was a disc of much the same colour as Mars,
whilst its spectrum resembled that of a spirit lamp that has been
plentifully fed with carbonate of soda or common salt. The 'Great
Comet' of the autumn of the same year, and which was so brilliant
an object in the early morning, came yet nearer to the sun, and the
heating process went on further. The sodium lines blazed up as they
had done with Comet Wells, but under the fiercer stress of heat to
which the Great Comet was subjected, the lines of iron also flashed
out, a significant indication of the tremendous temperature to which
it was exposed.
There are two other departments of spectroscopic work which it was
attempted for a time to carry on as part of the Greenwich routine.
These were the daily mapping of the prominences round the sun, and
the detailed examination of the spectra of sun-spots. Both are
almost necessary complements of the work done in the heliographic
department--that is to say, the work of photographing the appearance
of the sun day by day, and of measuring the positions and areas of
the spots. For the spots afford but one index out of several, of
the changes in the sun's activity. The prominences afford another,
nor can we at the present moment say authoritatively which is the
more significant. Then again, with regard to the spots themselves,
it is not certain that either their extent or their changes of
appearance are the features which it is most important for us to
study. We want, if possible, to get down to the soul of the spot,
to find out what makes one spot differ from another; and here the
spectroscope can help us. Great sun-spots are often connected with
violent agitation of the magnetic needles, and with displays of
auroræ. But they are not always so, and the inquiry, 'What makes
them to differ?' has been made again and again, without as yet
receiving any unmistakable answer. The great spot of November, 1882,
which was connected with so remarkable an aurora and so violent a
magnetic storm, was as singular in its spectrum as in its earthly
effects. The sun was only seen through much fog, and the spectrum
was therefore very faint, but shooting up from almost every part of
its area, except the very darkest, were great masses of intensely
brilliant hydrogen, evidently under great pressure. The sodium
lines were extremely broadened, and on November 20 a broad bright
flame of hydrogen was seen shooting up at an immense speed from one
edge of the nucleus. A similar effect--an outburst of intensely
luminous hydrogen--has often been observed in spots which have
been accompanied by great magnetic storms; and it may even be that
it is this violent eruption of intensely heated gas which has the
directest connection with the magnetic and auroral disturbances here
upon earth.
This sun-spot work was not carried on for very long, as only one
assistant could be spared for the entire solar work of whatever
character. Yet in that time an interesting discovery was made by the
writer--namely, that in the green part of the spectrum of certain
spots a number of broad diffused lines or narrow bands made their
appearance from time to time, and especially when sun-spots were
increasing in number, or were at their greatest development.
The prominence work had also to be dropped, partly for the same
reason, but chiefly because the atmospheric conditions at Greenwich
are not suitable for these delicate astrophysical researches. When
the Observatory was founded 'in the golden days' of Charles II.,
Greenwich was a little country town far enough removed from the
great capital, and no interference from its smoke and dust had to be
feared or was dreamt of. Now the 'great wen,' as Cobbett called it,
has spread far around and beyond it, and the days when the sky is
sufficiently pure round the sun for successful spectrum work on the
spots or prominences are few indeed.
Whether in the future it will be thought advisable for the Royal
Observatory to enter into serious competition in inquiries of
this description with the great 'astrophysical' observatories of
the Continent and of America--Potsdam, Meudon, the Lick, and the
Yerkes--we cannot say. That would involve a very considerable
departure from its original programme, and probably also a departure
from its original site. For the conditions at Greenwich tend to
become steadily less favourable for such work, and it would most
probably be found that full efficiency could only be secured by
setting up a branch or branches far from the monster town.
With the older work it is otherwise. So long as Greenwich Park
and Blackheath are kept--as it is to be hoped they always will
be--sacred from the invasion of the builder; so long as no
new railways burrow their tunnels in the neighbourhood of the
Observatory, so long the fundamental duties laid upon Flamsteed,
'of Rectifying the Tables of the Motions of the Heavens and the
Places of the Fixed Stars,' will be carried out by his successors on
Flamsteed Hill.