44 



♦ KNOWLEDGE ♦ 



[Dec. 1, 1885. 





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eighteenth century Messier pub- 

 lished a list of 103. Then began 

 the work of William Herschel, -who 

 swept the northern heavens with 

 his great gauging telescope, sending 

 in to the Hoyal Society two lists of 

 a thousand each, and then a supple 

 mentary list of 500. John Herschel 

 followed in the same work. Fi 

 surveying the northern hemisphere 

 to accustom himself to the methods 

 of observation followed by his father, 

 he then went to the Cape, and there 

 completed the survey of the whole 

 star-sphere. Over 4,400 star-clouds 

 were discovered by these two great 

 astronomer.5, — the total number now 

 known being about 5,000. 



The grand idea was thrown out, 

 first, I believe, by Wright, of 

 Durham, that these star-clouds are 

 external galaxies like the system 

 of stars of which our sun is a 

 member. Sir William Her.5chel 

 early adopted this view. But he 

 also early modified it in a way 

 which few readers of his paper.s 

 seem to have noticed. For he 

 recognised many of the star-clouds 

 as parts of our own galaxj-, broken 

 up in long-past a>on.s. Then later, 

 he noted a distinction between the 

 various orders of nebula;, which led 

 him to separate one large class as 

 probably not star-clouds at all, but 

 rather great masses of self-luminous 

 vapour. Still he retained even to 

 the last the belief that among the 

 nebula; are some which really 

 are external galaxies. Sir John 

 Herschel, while recognising strong 

 evidence against thisbelief, definitely 

 pointing out indeed the most strikinw 

 fact of all those which oppose the doctiine, retained it 

 as a possible hypothesis, and even noted those among 

 the star-clouds '\>hich seem to resemble our own o-alaxv 

 most closely. 



Mr. Herbert Spencer was the first to show that the 

 doctrine of external galaxies — at any rate, as that doctrine 

 had been propounded by Arago, Humboldt, and some 

 others— is inconsistent with observed facts. In par- 

 ticular, he called attention to two very strikinsr objec- 

 tions, one indeed fatal, the other presenting very strong 

 probabilities against the theory that the" nebul.T are 

 external to our system. 



Sir W. Herschel found, late in his career as an observer, 

 that even his most powerful telescopes would not fathom 

 the prof and ities of some parts of the Milky Way. With 

 each increase of telescopic power more and more stars 

 came into view ; but also new areas of nebulous light 

 appeared, which only higher powers could resolve into 

 stars. Of those depths, Herschel said that they were 

 (for his telescopes at any rate) " altogether unfathomable. " 

 If then regions within the bounds of our stellar system, 

 are unfathomable, so that the stars composing them can- 

 not be individually seen, how utterly impossible must it 

 be for the same telescope to resolve" into stars, or even 

 bring into view, galaxies lying far beyond our own ? If 

 for instance the nebula in Andromeda, which just now 





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Section from my Chart of 324,198 Stars, showing where the New Star appeared. 

 (For KeVj see Opposite Page.) 



attracts so milch attention, is an external galaxy, then 

 its apparent size shows that it must lie at a distance 

 about three hundred times greater than its own greatest 

 diameter. The unfathomable parts of our galaxy lie less 

 than half its diameter from us. If then the Andromeda 

 nebula is of the same size as our galaxy, it lies about 200 

 times farther away than those regions which Herschel's 

 most powerful telescope refused to resolve, and failed in 

 part even to see. This of coiirsj leads to an obvious 

 absurdity. But if, on the other hand, the Andromeda 

 nebula is no farther off than the parts of our galaxy to 

 which Herschel's telescope reached, then it is necessarilj- 

 an object very different from our galaxy, having less than 

 the 200th part of its diameter, and less than the 

 8,000,000th part of its spatial extent : moreover lying 

 at that small distance it must be rejrarded as belonging 

 to the same region of space, and therefore to the same 

 system. 



The other argument, rightly understood, is almost 

 equally convincing. Whenever Sir W. Herschel found 

 he was approaching regions where stars were few, — 

 poverty-stricken regions — he used to call to his sister, 

 Caroline Herschel, who acted as his assistant, " Prepare 

 to write : nebulse are approaching." He recognised that 

 nebulfe occupy spaces where stars are wanting. This 

 agreement can be no mere chance. I^ecessarUy then the 



