SIR JOHN FREDERICK WILLIAM HERSCHEL. 131 



In the course of that stupendous work which has already been pointed 

 out — the work of surveying those regions of space too distant to be seen 

 by the naked eye — it would be a greater marvel than all their united dis- 

 coveries had the Herschels never found occasion to change their views 

 and remodel their theories. They did this, both father and son, once 

 and again. " If it should be remarked," wrote Sir William Herschel in 

 1811, " that in this new arrangement I am not entirely consistent with 

 what I have already in former papers said on the nature of some objects 

 that have come under mj-^ observation, I must freely confess that, by 

 continuing my sweeps of the heavens, my opinion of the arrangement 

 of stars and their magnitudes, and of some other particulars, has un- 

 dergone a gradual change ; and, indeed, when the novelty of the sulyect 

 is considered, we cannot be surprised that many things, formerly taken 

 for granted, should on examination prove to be different from what they 

 were generally but incautiously supposed to be. For instance, an e(iual 

 scattering of the stars maybe admitted in certain calculations; but 

 when we examine the milky way, or the closely compressed clusters of 

 stars, this supposed equality of scattering must be given up. We may 

 also have surmised nebuhe to be no other than clusters of stars dis- 

 guised by their very great distance, but a longer experience and better 

 acquaintance with the nature of nebuhie will not allow a general admis- 

 sion of such a principle, although undoubtedlj- a cluster of stars may 

 assume a nebulous appearance when it is too remote for us to discern 

 the stars of which it is composed." In fact, M. Aragx)'s memoir of Sir 

 William Uerschel, as well as the numerous papers of himself aud Sir 

 John Jlerschel, which appeare<I from time to time, during more than 

 three-quarters of a century, in the Transactions of the Royal Society 

 and the Astronomical Society, show not only that the former modified his 

 theories, gradually, indeed, but not infrequently, in accordance with 

 newly-discovered facts, but also that Sir John Herschel's discoveries, 

 though considerably in advance of the points reached by his father, but 

 lying, nevertheless, strictly in the direction along which the elder had 

 been progressing, led to the same result. Sir William uu)ditied his views 

 about unequal double stars, concluding that the fainter orb is physically 

 associated with the brighter one, instead of being far beyond it. He 

 modified his views as to star- groups, regarding at last the masses of the 

 milky way as aggregations of stars instead of depths extending into 

 space. He had come to regard many star-clusters as part and parcel of 

 the milky way; large numbers of nebulie as vai)orous luniinousmasses; 

 and galaxies external to our system, as he once believed, a portion of 

 the heavens with which he was familiar. Neither father nor son ever 

 regretted to see hypotheses, though never so dearly cherished, pass 

 beyond the iield of controversy into the domain of tiie known. 



Let us now turn to another consideration of Sir John Herschel — still 

 necessarily but less closely, perhaps, connecting him with his father — 

 the consideration of his character as a theorist in astronomy. As au 



