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, Thal·n, 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.
THE GREAT NEBULA IN ORION.
(From a photograph taken at the Royal Observatory Greenwich, December 1, 1899, with an exposure of 21/4 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.
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 123/4 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 123/4 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.
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 123/4 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.
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.