182 



♦ KNOW^LEDGE 



[April 1, 1886. 



For I must admit thiit it seamed to me as though our 

 astronomers had been wilfully increasing the difficulty of 

 the problem by the perverse way in which they had chosen 

 to regard it. It was well titled to the noble genius of Sir 

 Wm. Herschel to take a wide view of the sidereirl scheme. 

 Indeed, standing where he did when he first attacked the 

 problem, he had no choice but to select the more obvious 

 and general features of the stollar scheme for his considera- 

 tion. But as he progi-essed with his work he gi-adually 

 be<^in to modify many of the views which he had formed 

 when the work was commencing. Or rather I should 

 perhaps say that he Ijegan to test the genei-al principles on 

 which he had been compelled to base his inquiries. And 

 there are few more interesting subjects of study than the 

 gradual progress by which our great astronomer made his 

 way from one point to another, until towards the end of his 

 life he seemed preparing to lay before the world views 

 which, while the dh-ect fruits of his earlier hypotheses, 

 were yet altogether opposed to them. 



The work which the elder Herschel has thus carried so 

 nearly to its completion fell into no unworthy hands. Sir 

 John Hei-schel, inheriting his father's grand powers of 

 generalisation almost undiminished, possessing also a 

 capacity for laboi-ious and far-sighted observation altogether 

 equal to his father's, and a more thorough acquaintance 

 with mathematical modes of rea.soning, seemed aipable 

 of pushing the theories of the universe to that point 

 which I bjlieve they would most cerUiiuly have attained 

 had Sir Wm. Herschel lived a few yeare longer. 



But there was, I think, a difficulty in the way. The feel- 

 ing I have when I rise from the perusal of any of those 

 noble passages in which the younger Hei-schel presents or 

 discusses the views formed by his father is, that he hiis been 

 at times prevented from prosecuting inquiries which seem 

 opposed to the general direction of his father's researches, 

 by a feeling — very natural and amiable — of respect for his 

 father's work and fame. I could point to many passages 

 which seem to me to force this view upon us, but I will 

 content myself with noticing two singula)' illustrations. 



Sir John Herschel is describing the configuration of the 

 Milky \\"ay in tiie southern heavens. He lias occiision to 

 speak of the striking brightness of the galaxy in the 

 southern skies. Now it need hardly be remarked that on 

 Sir W. Hei-schel's theory of the galaxy this great brightness 

 is very difficult of explanation. That this is so, in fact, is 

 proved by this, that, whereas Sir John Herschel felt that we 

 could only explain the phenomenon naturally by supposing 

 our sun to be nearer this part of the jNIilky Way, Professor 

 Grant points out (very justly) that on Sir William Herschel's 

 theory the phenomenon requires that the sun should be nearer 

 to the opposite part of the Milky Way, for on this supposi- 

 tion alone would the number of stare towards the south be 

 greatest. Sir- John Herschel gives the obvious explanation, 

 however ; and he seems to feel how strongly it is opposed 

 to his father's theory, for he adds that the galaxy " on this 

 view of the subject would come to he considered as a flat 

 ring of immense and irregular breadth and thickness, within 

 which we are eccentrically situated nearer to the southern 

 than to the northern part of its ciiTuit." Yet he nowhere 

 adopts this view. I feel certain that had the disc theory of 

 the galaxy been due to any but Sir W. Herschel, the ob- 

 servation would have led the younger Herschel to adopt at 

 once and finally the ring theory, though I believe he would 

 soon have seen reason to modify his opinion of the ring's 

 shape and figure. 



Again, Sir- John Hei-schel is discussing the Magellanic 

 clouds. He is impressed with the evidence they seem to 

 afford of the fact that, within very moderate limits of dis- 

 tance, the faintest telescopic stars and nebulae of all degrees 



of irresolvability may be mixed up with stars of the eighth 

 and ninth magnitude. Nay, he points out in his own lucid 

 manner that, according to all the laws of probability, we 

 must look on this fact as established beyond dispute. He 

 sees also, as in the preceding instance, that this view is 

 altogether opposed to accepted views respecting the universe. 

 Yet he closes the discussion of the overwhelming evidence 

 thus afforded against one of the most striking of his father's 

 views with the simple remark that, '• It might lead us to look 

 \vith some doubt on conclusions which in former pages of 

 this work have been somewhat positively insisted upon." 

 A certain fact is proved beyond all question, yet in the 

 remaining pages of the " Outlines of Astronomy " that fact 

 is completely ignored. 



Even as it is. Sir John Herschel's views respecting the 

 galaxy are marked by a certain advance upon his father's. 

 Although not definitely adopted, we must look on the ring 

 theory of the Milky Way as that which the younger Herschel 

 held in preference to the disc theory. 



Now, it will be noticed that wherever Sir John Herschel 

 has occasion to refer either to the narrower jjortions of the 

 galaxy or to the branches which appear to extend from it, 

 he always exhibits a preference for the view that these 

 narrow star-beds ai-e in reality the side \'iews of widely 

 extended star-strata. He says, indeed, in one place, speak- 

 ing of a region where several branches of this sort are 

 visible, " it is obviously more reasonable to suppose that 

 these are sheets of stars viewed edgewise, than to imagine 

 they are real columnar excrescences, bristling up from the 

 general level." 



I think we must recogniss in this peculiarity the influence 

 of the preconceived opinion that not merely our sidereal 

 system but all the parts of it exhibit a certain tendency to 

 lateral extension, so that the existence of a columnar star- 

 group, or of what I should prefer to call a star-stream, is 

 improbable a priori. Otherwise, I confess I am unable to 

 conceive how his intimate acquaintance with the principles 

 of probabilities could have failed to enforce upon Sir John 

 Herschel the feeling that the many long and narrow streams 

 which he saw extending from various pai-ts of the galaxy 

 must in most instances, if not in all, be columnar. Nay, 

 even with preconceived views rendering the estimated chance 

 of the existence of galactic star-streams only -j-Jff, yet the 

 existence of two such streams would have balanced that 

 a priori improbabiUty, since we can hardl}' estimate at more 

 than -jJ^ the chance of a sheet of stars being seen edgewise, 

 and therefoie the chance of two being so seen would be onlj' 

 Y^\o. Now, Sir John Herschel Siiw many such excrescent 

 streams. 



It will be seen at once that the existence of small streams 

 extending from the galaxy goes far to prove the stream- 

 formation of the galaxy itself, ^^'hen this evidence is added 

 to that which I adduced in my former paper, the conclusion 

 seems to me to be altogether obvious that the apparent 

 stream of milky light which we term the galaxy is in reality 

 a stream of small stii-s, surrounding us on all sides. 



But I would go further, and as.sert that the naked-eye 

 appearance of the Milky Way is sufficient evidence on which 

 to ground the belief that there is a distinct ring of matter 

 out yonder in space, and that this ring is not flattened, as 

 Sir John Herechel thought, but is (roughly speaking) of 

 nearly circular section throughout its length. I conceive 

 that nothing save the perverse way in which astronomers 

 have chosen to deal with the phenomenon would ever have 

 led them to forget the evidence of their senses in this matter. 

 Of course, if we insist on taking the average number of stai-s 

 visible on a certain space of the heavens as indicating the 

 density of the stiirs over that space, although it is perfectly 

 obvious to the eye that there is a distinct and systematic 



