When we add that in the southern skies the Milky Way presents the most fantastic configuration, here expanding into fan-shaped masses, there winding about in a multitude of strange convolutions, here suddenly narrowing into a bright neck or isthmus, there exhibiting a nearly circular vacancy, it becomes clear that the galaxy cannot have the figure assigned to it by Sir W. Herschel. It must consist of streams and sprays of stars at different distances. Such streams by their fantastic convolutions serve to explain all the peculiarities of the galaxy's structure.
Sir William Herschel noticed that there are places where the nebulae are much more densely crowded than elsewhere, and he was disposed to suspect that precisely as the stars by their aggregation form the zone of the Milky Way, so there is a zone of nebulae. But when Sir John Herschel had completed the survey of the heavens it was found that a very different law of distribution made its appearance. Instead of being collected in a zone or band around the heavens, the nebulae are arranged in two distinct hut irregular clusters, separated by a well-marked zone almost entirely free from nebulae. And this zone coincides almost exactly with the Milky Way.
What are we to understand by so special an arrangement as this? A modern astronomer says it clearly proves that the nebulae do not belong to the star-world; but I can see no escape from an exactly opposite view. A simple illustration will serve to exhibit the nature of the case. Suppose a person found a space of ground on which gravel was arranged in the form of a ring, and that rough stones were thickly spread over the whole space except the gravel ring, would he conclude that there was no association between the arrangement of the gravel and the arrangement of the stones, because few stones were to be found on or near the gravel? Would he not rather find in this peculiarity distinct evidence that there was some association? He would, we think, argue that the gravel had been collected into one place and the stones into another, in pursuance of some one particular scheme. The corresponding conclusion in the case of the stars and nebulae would clearly be that the stars had been drawn together in one direction and the nebulae in another, out of a common world of cosmical matter.. In other words we should look on the nebulae as members of the same system or scheme that the stars belong to.
And here it may be asked how the conclusion thus deduced from the arrangement of stars and nebulae can be said to tend to enlarge our views of the world of stars. On the contrary, it might he urged, the views which had prevailed before, presented us with nobler conceptions of the universe. For we were able to recognise in the thousands of nebulae which fleck the dark background of the sky, sidereal systems as noble as that of which our sun is a member; and in the existence of countless star-systems we had a spectacle to contemplate before which the human intellect was compelled to bow in its utter powerlessness and insignificance : whereas it seems as though the new views would reduce the scope of our vision to a single galaxy of stars, unless some few members of the nebular system may still be looked on as outer star-schemes.
But on a closer inspection of the views I have been maintaining, it will appear that they largely enhance our conceptions of the scale on which the world of stars is constructed. Until now it has been held that the telescopes which man has been able to construct enabled us to scan the limits of our sidereal system, and to pass so readily beyond those limits as to become sensible of the existence of thousands of other schemes as noble as our own or nobler. But if the new views should be established, we should be compelled to recognise in the world of stars a system which our most powerful instruments are not fully able to gauge. The clusters of stars, whose splendour has so worthily excited the admiration of the Herschels, the Rosses, the Struves, and the Bonds, must be looked upon as among the glories of our own system, and indicative of the multiplied forms of structure or of aggregation to be found within its boundaries. As of late our conceptions of the wealth of the solar system have been enhanced by the discovery of numberless new objects and new forms of matter existing within its range, and co-ordinating themselves in regular relations with the earlier known members of the system, so we seem now called on to recognise in the stellar world an unsuspected wealth of material, a hitherto unrecognised variety of cosmical forms, and an extension into regions of space to which our most powerful telescopes have not yet been able to penetrate.
But now I would call attention to a peculiarity of the southern skies which, while apparently affording conclusive testimony in favour of the new views, has unaccountably (in my opinion) been urged as an argument tending in quite another direction. There are to be seen in those skies two mysterious clouds of light, which were called by the first Europeans who sailed the southern seas the Magellanic clouds, and are now commonly spoken of by astronomers as the Nubeculae. Examined by the powerful telescope of Sir John Her-schel, these objects have been found to consist of small fixed stars and nebulae, grouped together without any evidence of special arrangement, but still obviously intermixed, - not merely seen projected on the same field of view.
These strange objects have given rise to many speculations; and among the definite views put forward respecting them is one recently expressed in a most valuable communication to the Royal Astronomical Society from the pen of Mr. Cleveland Abbe, an astronomer who has laboured in the sound school of the Poulkowa Observatory. Having recognised in the peculiar arrangement of stars and nebulae above referred to, an argument that the nebulae lie beyond our system, Mr. Abbe suggests that the Magellanic clouds are two of the nearest of the nebular systems, which thus exhibit larger dimensions than their fellow-schemes.
The converse of this, which may be termed the positive theory of the Nubeculae, is the hypothesis which may be termed the negative theory. Whatever these objects may be, astronomers have said, they are quite distinct from the sidereal system, nor are the nebulae seen within them to be looked upon as fellows of the other nebulae. For in the Nubeculae we see what we recognise nowhere else, the combination namely of clustering groups of stars and freely scattered nebulae. It is the characteristic (still I am quoting the theory) of the sidereal system that where its splendours are greatest nebulae are wanting; it is the characteristic of nebular aggregation that it withdraws itself in appearance from the neighbourhood of clustering star groups. But in the Magellanic clouds neither of these characteristics is to be recognised; therefore these objects are distinct from either system.
Nor has another argument been wanting to indicate the distinction that exists between the Magellanic clouds and the other splendours of the celestial vault. Sir
John Herschel, sweeping over their neighbourhood with his 18-inch reflector, was struck with the singular barrenness of the skies around them. With that expressive verbiage which gives so great a charm to his astronomical descriptions, he forces on our attention, again and again, the poverty of the regions which lie around the Nubeculae. 'Oppressively barren' he describes them in one place; 'the access to the Nubeculae on all sides is through a desert,' he says in another. And this peculiarity, thus established by the certain evidence of an observer so able and trustworthy, has been held by many to imply in the clearest and most distinct manner that there is no connection between the Nubeculae and the stellar system.
To me the evidence afforded by the barrenness of the regions round the Magellanic clouds points irresistibly in the opposite direction. Why should some outer system, free as is assumed of all association with our own, occupy that peculiarly barren space which so attracted the attention of Sir. John Herschel ? But if we look on the coincidence as striking in the case of one, how much more remarkable will it appear when the only two outer systems of the sort, thus brought within our ken, are associated in this way with the most singularly barren region in the whole heavens! Surely the more natural conclusion to be drawn from the phenomenon is that the richness of the Magellanic clouds and the poverty of the surrounding districts stand to each other in the most intimate correlation. Is there not reason for concluding that those districts are poor because of the action of the same process of aggregation which has attracted within the Nubeculae a larger share than usual of stellar and nebular glories? l It need hardly be mentioned that the former argument, on which the distinction between the Nubeculae and other celestial objects has been founded, is disposed of at once if we recognise the stellar and nebular systems as in reality forming but a single scheme. Not only so, but the Nubeculae afford a striking argument in favour of the latter view. To return to the somewhat homely illustration made use of above. Our conceptions of the original association between the stones and the gravel arranged in the manner indicated would certainly be strengthened, or would even be changed into absolute certainty, if we perceived in a part of the ground two heaps in which stones and gravel were intermixed. When I add that there are two distinctly marked nebular streams leading towards the Nubeculae, as well as several well-marked star-streams tending in the same direction, the evidence of association seems greatly strengthened.
If these views be accepted, we shall have to look upon the world of stars as made up of all classes of clustering aggregations, besides strange wisps and sprays extending throughout space in the most fantastic convolutions. Then also, while dismissing the idea that the nebulas as
1 Sir William Herschel has recorded a peculiarity respecting nebulae which is worthy of mention in connection with the facts above considered. 'I have found,' he says, 'that the spaces preceding nebulae were generally quite deprived of stars, so as often to afford many fields without a single star.' a class are external systems, we may accept as highly probable the conclusion that some of the spiral or whirlpool nebulae really lie far beyond the confines of our system. For we see in these objects the very picture of what the new views show our sidereal system to be. There are the spiral whorls corresponding to the double ring of the Milky Way; there, are faint outlying streamers corresponding to the phantom star-streams traced by Sir John Herschel; there also, are bright single stars and miniature clusters,-nay, there also, may even be recognised large knots or lobes of clustering stars, forming no inapt analogue of the Magellanic clouds.
Fraser's Magazine for July 1869.