124 



KNOWLEDGE 



[July 1, 1891. 



jected into it, anil so help to increase the impression of its 

 beiuK globular. The obvious inference is that we are 

 unable to decide oft'-liand whether any single chister is 

 really a cluster or a drift of stars. 



But such a theory with regard to the many clusters 

 lying on the Milky Way would involve the existence of 

 many straight streamers radiating towards our sun. If 

 the Milky Way is a ring, and the sun occupies a central 

 position, this might be possible, and the theory is supported 

 to some slight extent by the behaviour of those apparent 

 clusterings in the field of the Milky Way, whose brightness 

 increases not towards their centres but from one side to 

 the other, the magnitudes of the larger stars upon them 

 following the same order. This is exactly what should 

 happen in the case of a spicular projection diijhth/ inclined 

 to the line of sight. Other instances having the same 

 tendency will readily occur to the student. But if any of 

 the apparent clusterings are streaming appendages seen in 

 perspective, we are met at the outset by the ditliculty that 

 none of them are so bright as we should expect; reasoning 

 merely from first principles, their brightness is evidence 

 enough that their length (if length they have) is small in 

 comparison with the great arm stretching from Cygnus to 

 Ophiuchus. The nebulous clouds in Aquila would cer- 

 tainly be darkness itself contrasted with the condensed 

 brilliancy which would be exhibited by the spur in Scorpio 

 to an observer situated on its produced major axis. In- 

 deed the conspicuous cluster in the sword handle of 

 Perseus offers the only possible comparable example, and 

 not altogether a good one either. 



It is necessary, then, to give up the assumption that any 

 great galactic drifts lie within the space enclosed by the 

 Milky Way, and pointing towards us. This may tend to 

 shake our faith in a ring-form theory of the Milky Way — 

 indeed in any of the present theories of its structure based 

 on its streamy nature, such as Proctor's spiral theory ; 

 furthermore, according to ths same reasoning, it seems 

 probable that the great branch reaching to Ophiuchus, and 

 the meanderings in Scorpio, are what they give the im- 

 pression of being, namely, at approximately the same 

 distance from us, and therefore parallel to the main course 

 through Aquila and Sagittarius. We shall, however, have 

 presently to consider this matter in another light when 

 speaking on the probable position in space of the great 

 appendages in Perseus and Cepheus. And incidentally 

 we shall have reason to point out that the Milky Way is 

 by no means obviously a stream of stars nf all si-e.s. 



Any theory which could account for the extraordinary 

 evenness of outline of the Milky Way would have much in 

 its favour, and this a ring theory has, as well as the spiral 

 stream theory. 



It is curious that the assertiveness of the Milky Way 

 among the stars has always been tacitly recognized as a 

 sign of its importance. Thomas Wright thought that its 

 brilliancy represented the greater depth of the universe in 

 its plane ; and the two Herschels followed suit with 

 different degrees of scepticism notwithstanding John 

 Michell and his mathematical formul*. Proctor thought 

 that the Milky Way was a stream of stars of all sizes, all 

 those scattered over the rest of the sky being in a sense 

 sporadic, and not of any particular moment in modifying 

 what he thought to be the architecture of the universe ; 

 and it must be admitted that his reproduction in one 

 photograph of Argelander's forty charts might well seem 

 to be very substantial grounds for the idea. 



But although it is true that the Milky Way is richer in 

 lucid stars than any area of equal size outside it, it is not 

 true that it is the richest ijieat-cinnlnr hdt of the sky. 

 That distinction is claimed by a belt arranged about a 



gi'eat circle through Cygnus, Perseus, Taurus, Orion, 

 Crux, and Scorpio. Besides, the stars lying within it 

 offer the clearest example of a star-stream it is possible to 

 conceive. That it is a real star-stream is suggested first 

 of all by its undeviating direction. A person standing 

 under Orion will have overhead a most imposing arch of 

 stars, springing with perfect symmetry from the horizon 

 on either hand in t'rux and Perseus without anything 

 that can be called a break. Moreover, it passes across an 

 exceedingly dark part of the sky. Below the horizon on 

 the one hand it can be distinctly traced as far as £ and !; 

 Ophiuchi, where it seems to end, on the border of an empty 

 space, sending out, meanwhile, a small arm from Crux, 

 along the main course of the Milky Way as far as u 

 Sagittarii. In the same way, and as easily, we can trace 

 the other continuation of the arch through Perseus, 

 Cassiopeia, Cepheus, Cygnus, and Lyra almost or quite to 

 a Ophiuchi on the other side of the empty space mentioned 

 above. This end also sends out an arm in the direction 

 of Aquila apparently as a feeler for the star. In one 

 respect the great star-belt offers a curious analogy to the 

 Milky Way : both are cut completely across, one in 

 Ophiuchus, the other in Avgo ; and both spread out fan- 

 wise on either side of the respective gaps, and to complete 

 the resemblance, just as the nebulous magellanic clouds 

 lie off, though having no defined connection with the gap 

 in Argo, so do the apparently free star-groups of Ursa 

 Major and Hercules lie off' the gap in Ophiuchus. 



Still these facts only suggest, and do nothing to prove, 

 that the great star-belt actually marks the course of a 

 real ring of stars in space. But there is a class of facts, 

 well worth examination, which seems to place the matter 

 beyond doubt. 



First, then, if we trace the course of the Milky Way 

 from Auriga through Monoceros we shall find it the 

 most tame and unexciting object imaginable. It is as 

 monotonous as an unvarying brightness can make it. 

 With the solitary exception of /3 Tauri, no star of any 

 magnitude occurs until we come to Argo, in which 

 constellation the Milky Way and the great star-belt 

 intersect at a very acute angle, so that the two are prac- 

 tically in company for a considerable space. Here there 

 is a sudden metamorphosis : from being regular and 

 unbroken the former becomes torn up into indescribable 

 confusion — torn into ribbons so to speak, and it is pretty- 

 clear that the stars are the agents effecting the disrup- 

 tion, as we have shown elsewhere. For here we get all 

 the associations referred to in the first two paragraphs of 

 this article. Parting company with the stars (in Sagit- 

 tarius) the Galaxy resumes very nearly its even outline 

 and untroubled aspect until it intersects the star-belt 

 again in Cygnus. Here the same phenomena of disrup- 

 tion recur. 



Furthermore, there is not a galactic off"-set of any size 

 worth mention, whose shape, direction, and aspect are not 

 determined by the stars in the great star-belt. The 

 streamer in Perseus lies directly along, and that in 

 Cepheus, across it. Those in Ophiuchus and Scorpio bend 

 equally towards it. Indeed, the Milky ^^'ay itself, between 

 Cams Jlajor and Sagittarius, seems to have a decided 

 double -warp of the same nature : curving round from 

 either constellation perchance to come sooner into the 

 plane of the star-belt. The spreading finger-shaped pro- 

 jections, facing each other across the gap in Argo, 

 illustrate very forcibly the predominance of the latter in 

 this respect. 



Lastly, the area covered by the Milky Way is rich in 

 lucid stars, because it crosses the richest parts of the star- 

 belt. The conclusion seems to be emphatically forced 



