132 



KNOWLEDGE. 



[June 1, 1894. 



stellar ecliptic plane in which the sun and the majority of 

 the stars are moving. If we were looking through a vista 

 of stars which extended to a great depth in every part of 

 the Milky Way, such differences of constitution in diflerent 

 directions would indicate a radial distribution of the 

 diflerent types of structure about the sun's place, and 

 would involve the assumption that the sun occupies the 

 centre of the system, and that the material of the universe 

 is arranged symmetrically in sectors about it, an assump- 

 tion which is abhorrent to modern ideas, and irreconcilable 

 with what we know of the proper motions of the sun and 

 stars. 



The assumption that the Milky Way corresponds to a 

 stratum of stars also involves other difficulties, for we must 

 account for the fact that the galactic stream of nebulosity 

 and thickly clustered stars is split into two streams through 

 half of its course round the heavens, and also for the fact 

 that the main streams are again di%'ided and branched. If 

 we hold to the assumption that these streams and branches 

 correspond to a stratum of considerable thickness in a 

 direction radial to the sun's place, the form of the 

 galactic system becomes very complicated. It is evidently 

 altogether simpler to assume that the streams have a 

 thickness which does not difl'er greatly from their apparent 

 breadth. 



The photographic evidence which we now possess does 

 not indicate that the streams have the definitely defined 

 boundaries which they were thought to possess by Mr. 

 Proctor and the old school of map makers, who had to rely 

 chiefly on drawings made with the naked eye. If the Milky 

 Way pictures which have been published in Knowledge are 

 examined, they will be found to show novery definite outlines 

 of the galactic streams, and no clearly recognizable central 

 core, dark lanes or streams of stars running parallel with 

 the axis of the galactic stream. In our second plate, repro- 

 duced from a photograph of the region about the ry Argiis 

 nebula taken by Mr. Russell with a large camera, the 

 general axis of the Milky Way stream runs horrizontally 

 across the page, but there is no marked condensation of 

 stars or nebulosity at the centre of the stream, and the 



curiously con- 

 torted stream 

 lines of stars 

 which are recog- 

 nizable seem to 

 be inclined at all 

 angles to the axis 

 of the great 

 stream of the 

 Milky Way. 



There can be no 

 doubt about the 

 general parallel- 

 ism of arrange- 

 ment over the 

 heavens of certain 

 classes of objects with respect to the Milky Way ; thus the 

 smaller nebulas are grouped around the galactic poles, and 

 there is a marked absence of them in the neighbourhood of 

 the Milky Way. The larger nebulse and clusters, on the other 

 hand, are intimately associated with the distribution of the 

 lucid matter and stars of the Milky Way. The aggrega- 

 tion of star clusters upon the Milky Way, especially along 

 its central region, is very strikingly shown in the map 

 puWished by Mr. Proctor and Mr. Waters in the Monthly 

 Notices of the Pvoyal Astronomical Society for August, 1873. 

 The stars exhibiting a bright line spectrum are also dis- 

 tributed along the central axis of the Milky Way ; and it 

 has long been known that the brighter stars all over the 



J'IG. 1. — Cluster with radiating streams of 

 Stars on the left or following side of the 

 X Argus Nebula. 



heavens show a general arrangement with respect to the 

 Milky Way, and a general tendency to cluster towards it, 

 so that they, or a large proportion of them, must be in 

 some way connected with the Milky Way system.''- It is 

 evident, therefore, that a large proportion of the matter 

 which our sense of sight reveals to us is controlled by, or 

 at all events is symmetrically arranged with respect to, 

 matter distributed along the axis of the Milky Way. 



In the photograph of the region we have before us, we 

 have an opportunity of studying clusters and streams of 

 stars which probably are actually as well as apparently in 

 close proximity to the axis of symmetry around which the 

 visible imiverse, or a large part of it, is symmetrically 

 arranged, and we naturally look for evidence of condensa- 

 tion along the central axis, or for some evidence indicating 

 motion around the central axis, but we look in vain. 



In the star clusters which we are able to study in various 

 parts of the heavens, a central condensation is not always 

 to be detected. Thus the Pleiades and Hyades hardly 

 show any evidence of condensation towards a centre ; the 

 clusters in the sword hand of Perseus may be taken as an 

 intermediate type in which there is partial condensation or 



Fig. 2.— The i; Argus Nebula with radiating Star Streams, the 

 position of which has been roughlj' indicated by scratches 

 on the block. 



crowding towards a centre, and at the other end of the 

 scale we have clusters like that shown in Fig. 1, or like 

 the still more centralized cluster in Hercules, photographs 

 of which were given in the May and June numbers of 

 Knowledge for last year (1893.) 



In the cluster shown in Fig. 1 and in the Hercules 

 cluster, as well as in the photographs of all similarly 

 condensed clusters which I have had an opportunity of 

 examining, the central mass or group of stars is surrounded 

 by radiating streams of stars which branch in a direction 

 away from the centre of the cluster, and the central mass 

 IS associated with dark structures or light-absorbing masses 

 evidently situated between the eye and the nebulous back- 

 ground or haze about the central stars. 



The presence of an attracting mass at the centre of a clus- 

 ter is evidenced not by curves of stars concentric with the 

 centre, but by streams of stars which appear to spring from 

 the centre and break into branches in a direction away from 

 the centre. Similarly we should expect to find the pre- 

 sence of an attracting mass, or attracting masses, arranged 

 along the centre of the Milky Way stream evidenced by 

 chains of stars associated with dark branching structures. 

 If the streams of stars and dark structures were arranged 

 radially with respect to the axis of the Milky Way, we 

 should expect to find the best evidence of the radiating 

 structure at the edges of the stream and not at its centre, 

 where we should look down upon such radiating streams 

 vertically. 



* Our own sun, and other swiftly moving stars, may not be so 

 connected. 



