June 15, 1883.] 



♦ KNOWLEDGE ♦ 



351 



PlAINLYlfQRDED -EXACTLYDESCRIBED 



LONDON: FRIDAY, JUNE 15, 1883. 



OONTBNTS OP No. 85. 



Star-Clouas ii 



R. A. Proct^ 

 The Birth »nd Growth of Myth. 



IX. Bv Edw.rcl Clodd 352 



Oar Chemistrr Cluran : Sea Salt. 



Bt W. Jat'o; F.C.S 353 



XheFisheri.-, Eshibilion. (lllu: 



tr„Ud.) HvJ. E. Ady 351 



A Great Telemope 355 



Pirsl-Cousjn .Marriaeea 357 



Pleajant Hours with the Uicro- 



scope. Bv U. J. Slack. F.G.S., 



F.R.M.S.' (llluilrattd.) 358 



at the Brighto 



359 



The Dolphi. 

 Aqaarium .. 

 KeTiews :- 



.«i«ence aa an Aid to Faith 36) 



The Ueavonly Bodies 360 



The Commercial Frodocts of the 



Sea 360 



Editorial Gossip 362 



Correspondence 383 



Our Mathematical Column 'i&\ 



Our Chess Column : 

 The International Tournament ... 365 



STAR CLOUDS IN OUR GALAXY. 



By Richard A. Proctok. 



IT is singular how persistently even simple errors, if 

 supported by great names, are repeated by the many 

 who care only to think as others tell them to think. One 

 of the nioit striking; illustrations of this is to be found in 

 the theory of nebuhu or star-clouds, quoted again and again 

 in our books of astronomy, as if it were based on observa- 

 tion and direct evidence, instead of being originally but a 

 guess, and a guess which will not bear investigation. 



In the later portion of his observing career, the great 

 astronomer. Sir W. Herschel, who had earlier regarded all 

 nebulie as aggregations of stars, adopted and enforced the 

 theory that many among them are great masses of self- 

 luminous gas. Yet he still retained the idea that large 

 numbers of these objects are great aggregations of suns 

 akin to our own galaxy. According to this view, the 

 nebulaj of the star-clustering kind are galaxies e.xternal 

 to our system of stars ; they are " island universes " 

 as Huniboldt poetically said. Sir John Herschel even 

 iudicated among them one which he regarded as not 

 only a galaxy resembling our own in general charac- 

 teristics, but as one in which the chief details of its structure 

 can be recognised. Arago and other astronomers deservedly 

 eminent have adopted this theory of external star galaxies. 

 Popular writers have accepted the doctrine, and dwelt, not 

 without reason, on the magnificence of the conception that, 

 marvellous as are the glories, mysterious the profundities 

 of our stellar system, they are repeated, with variations 

 perhaps in detail, but resemblance in general character — 

 among thousands of star systems within telescopic range, 

 and doubtless among millions beyond the reach of the 

 most powerful telescopes man has ever made. 



Yet as I showed sixteen years ago (though Mr. Herbert 

 Spencer, clearest of thinkers, had been beforehand with 

 me by many years) there are reasons so strong for rejecting 

 this theory that they need only to be mentioned to be 

 found convincing. 



Thus, towards the end of his observing career. Sir W. 

 Herschel found that even with his mo.st powerful telescopes 

 he could not resolve all parts of the Milky Way. Parts of 



our own system of stars are too remote, then, it seems, to 

 be resolved into stars. Of course, in these parts, the stars 

 may be much smaller and more closely set than elsewhere, 

 but to admit this — which is probable, nay, I believe, un- 

 questionably true — is to adopt a view more essentially in- 

 consistent with Herschel's original ideas respecting the 

 galaxy, than is the one we are dealing with. If, then, the 

 most powerful telescopes yet made by man cannot resolve 

 the remoter parts of the galaxy to which our sun belongs, 

 which cannot lie so much as a diameter of the galaxy from 

 us, how can any such telescope possibly resolve galaxies so 

 remote as to be reduced to mere specks, lying, in fact — 

 as their apparent diameters show — at a di.stance 

 exceeding many hundred times their real diameters? Mr. 

 Herbert Spencer has admirably shown the inherent 

 absurdity of the notion to which many (especially many 

 professional astronomers, who appear to imagine that 

 measuring star-places must give them an insight into astro- 

 nomical truths) cling as if it were a demonstrated truth. 

 " Take a homely example," he says : " Suppose a man, 

 surrounded by a swarm of bees extending, as they some- 

 times do, so high in the air as to be individually almost 

 invisible, were to declare that a certain spot on the horizon 

 was a swarm of bees, and that he knew it because he could 

 see the bees as separate specks. Astounding as the asser- 

 tion would be, it would not exceed in incredibility this 

 which we are criticising." 



Consider again the way in which nebuhe are arranged 

 on the star-sphere. Whether we consider the broader 

 features of arrangement or specific details, we find the 

 clearest evidence that the nebuLa! belong to our galaxy, 

 and not to outside space. The regions richly strewn with 

 stars are almost entirely free from nebul.T, excepting two 

 very special regions, presently to be considered. Is it likely 

 to be an accident which has thus left those parts of the star- 

 sphere where nebula; most do congregate less richly sup- 

 plied with stars than the rest ? Then consider details. 

 Not only is the peculiarity true of the star-sphere regarded 

 in its broader features, but it is true of its smaller portions. 

 The spaces near nebulas and groups of nebulw contain but 

 few stars. Arago dwells on this, not noting its signifi- 

 cance. " Herschel," he says, " found the rule to be in- 

 variable. Thus, every time that, during a short interval, 

 no star approached, in virtue of the diurnal motion, to 

 place itself in the field of view of his motionless telescope, 

 he was accustomed to say to the secretary who assisted 

 him" (his sister, to wit — Miss Caroline Herschel), " ' Pre- 

 pare to write ; nebul;e are about to arrive ! ' " 



Mr. Spencer has admirably pointed out the significance 

 of this "invariable rule." "Shall we believe that, in thou- 

 sands of cases, the far-removed galaxies happen to agree in 

 their visible positions with the thin places of our own 

 galaxy ? Such a belief is next to impossible." Combined 

 with what we have learned about the general arrangement 

 of nebula, the evidence is absolutely decisive. The nebulas 

 must of necessity form part and parcel of the system with 

 whose features, broad as well as more detailed, they thus 

 closely correspond. 



But there is even simpler evidence (we cannot say more 

 striking or more conclusive evidence, for none could be 

 that), proving that nebulas are what they look like, cluster- 

 ing aggregations of small stars, and not (what they might 

 have been found to be) remote congeries of suns like our 

 own galaxy of such orbs. In the Magellanic clouds, who.se 

 figure shows them to be really round — that is, nearly 

 globular systems in space — nebulas of all kinds are found 

 in great numbers in company with many stars, the spaces 

 around the nubecuhe being free alike from nebulas and dis- 

 crete stars. Now, from the observed apparent size of the 



