CHAP. III., 2.] 



ASTRONOMY. SIR WILLIAM HERSCHEL. 



45 



(196.) 

 Herschel's 

 opinions 

 on the dis- 

 tance of 

 nebulae ; 



(197.) 

 and their 

 changes of 

 form. 



198.) 

 Theory of 

 condensa- 

 tion. 



of diffuse celestial matter, such as the Milky Way ex- 

 hibits, and Tycho alleged that the new star was sur- 

 rounded by an obscure space, half as large as the 

 moon's disk. The history of Science scarcely pre- 

 sents a more curious anticipation. 



One practical difference between the earlier and 

 later views of Herschel on the nature of nebulse, was 

 as to their distance from our system. For if the milky 

 aspect be due to a confusion of small stars, it is 

 assumed that they must be almost incredibly distant. 

 In his memoir of 1785, he estimates this distance at 

 not less than 6000 or 8000 times that of Sirius. 

 If, on the other hand, the nebulous appearance be 

 due to a diffusion of starry matter, the distance may 

 be that of any order of fixed stars ; and this opinion 

 derives weight from the evident connection of some 

 nebulse with stars round which they seem to cluster 

 or hang almost like a drapery. Thus in 1811 he 

 supposed that the nebula in Orion need not be more 

 distant than a star of the 7th or 8th magnitude. 



It appears from his paper of 1802 (page 500), 

 that his ideas about nebulae were then wavering. Be- 

 tween that time and 1811 he elaborated the nebular 

 hypothesis, so far as it is due to him. He seems to 

 suppose that starry matter was once in a state of in- 

 definite diffusion. That during " an eternity of past 

 duration" (Ph. Tr., 1811, p. 287) it has been " break- 

 ing up" by condensation toward centres more or less 

 remote. That the Milky Way or at least the nebu- 

 lous parts which it contained and the dispersed ne- 

 bulae, are the relics of this former state of things. 

 That where condensation has gone on more energeti- 

 cally, we have nebulse with a gradually or rapidly 

 increasing brightness towards the centre ; if still 

 more energetic, a nucleus, or it may be a planetary 

 nebula ; next a nebulous star, which he supposes 

 our sun to be, and the zodiacal light a relic of its 

 nebula (p. 311); finally, the completely formed 

 stars may be assumed to be merely consolidated ne- 

 bulaa. (See pp. 284, 285, 299, 310.) This conden- 

 sation, he believes, must be accompanied by rotation 

 due to the originally irregular distribution of the 

 gravitating particles (p. 312-319). 



The proofs on which the whole of this cosmogony 

 rests are, \st, the gradation of appearances above 

 described, to be collected from distinct objects in the 

 heavens ; and, 2dly, supposed changes observed by 

 him in the nebula of Orion and others, during thirty 

 years, and even during intervals of a few years. As to 

 the last argument, it is admitted, we believe, by the 



which we cannot afford space. Its idea is essentially 

 derived from the Natural History sciences, and I can- 

 not help thinking that Herschel must have derived it 

 from some one more conversant with these than with 

 mechanical physics. This opinion is confirmed by a 

 curious illustration which he uses in the same paper 

 of 1811. " There is perhaps not so much difference 

 between them" (viz., the whole group of nebular and 

 quasi-nebular phenomena), " if I may use the com- 

 parison, as there would be in an annual description 

 of the human figure, were it given from the birth of 

 a child until he became a man in his prime" (p. 271). 



A theory or hypothesis in some respects similar (199.) 

 had been previously expounded by Laplace in the Laplace's 

 concluding chapter of his Systeme du Monde. At neb ^ la . r ^ 

 least it so far resembles it, that he puts forth Tycho's connected 

 notion of the star of 1572 being a condensation of a with Her- 

 widely-spread stellar atmosphere (without, however, 

 naming Tycho), and supposes that our solar atmo- 

 sphere might also once have extended to the limit of 

 the system, and that the planets were thrown off suc- 

 cessively in the form of nebulous rings (subsequently 

 condensed into spheres) from the equatoreal parts of 

 this vast revolving mass during its contraction. He 

 ascribes to Saturn's ring a like origin. It required all 

 the authority of a name like that of Laplace to circu- 

 late a theory so bold, if not extravagant. He adhered, 

 however (at that time), to the old idea of the consti- 

 tution of nebulse, which he considered to be composed 

 entirely of stars, and to be at once the most distant 

 and the most massive aggregations of matter in the 

 universe. Herschel could hardly fail of being ac- 

 quainted in 1811 with Laplace's views, and they pro- 

 bably in some degree influenced his own, particularly 

 as to the cause of the rotation of his condensing suns. 

 But he prudently reserved his hypothesis for sidereal 

 objects, and did not deduce from it a planetary cos- 

 mogony. In the later editions of the Systeme du 

 Monde, Laplace quotes Herschel's observations in 

 confirmation of his views, adopts the notion of a dif- 

 fuse nebulous matter, and considers that an hypothesis 

 arrived at by a " remarkable coincidence" in oppo- 

 site directions, is thereby invested with a great pro- 

 bability. 



Even Herschel's more limited conclusions have (200.) 

 been very dubiously received, and notwithstanding Doubts re- 

 the weighty adhesion of Humboldt and Arago to g ardin S i4 - 

 them, it may be affirmed that natural philosophers 

 generally are content to leave the solution of this 

 cosmical problem to that distant posterity which 

 alone can hope to witness unequivocal evidence of pro- 



best authorities, to be without weight. The changes 



of aspect of such curiously faint and graduated ob- gressive change in those wonderful objects, 

 jects, even to the same eye using the same telescope, 

 are numberless, and may be due to the slightest at- 

 mospheric and physiological influences. This test 

 (as applied hitherto) is therefore generally rejected. 

 Against the first proof much might also be urged, for 



III. Of the Grouping of the Stars generally^ 

 Space, and the 



(201.) 



earlier astronomers, and particularly Halley, had gtarry hea . 



the idea that the nebulae which abound so remark- vens,andof 



the Milky 



1 The most intelligible account of the nebular hypothesis, in its least objectionable form, will be found in Sir John Herschel's Way in par- 

 Outlines of Astronomy, Articles 870 to 872. ticular. 



