Clusters of Stars and Nebulce. 61 



The worthy inheritor of his name and talent observed, between 

 1825 and 1833, no less than two thousand five hundred, of 

 which five hundred were discovered by himself; and his cata- 

 logue, published in the Philosophical Transactions for the latter 

 year, is still the standard authority upon the general subject ; 

 the subsequent most memorable researches of the Earl of Rosse, 

 with some important observations by Secchi and others, relat- 

 ing only to select objects. Very recently, however, the obser- 

 vatory of Copenhagen having been provided with a magnificent 

 achromatic by Merz, of about eleven English inches aperture 

 and sixteen feet focus, Professor D' Arrest, the director, com- 

 menced, in 1861, a general revision of all the nebulae visible in 

 that latitude — a noble enterprise, to which every lover of astro- 

 nomy will cordially wish eminent success. The rank of his 

 telescope he estimates exactly at a medium between Sir J. 

 HerscheFs of 18| inches, and LasselPs of 24 inches; it shows 

 all the nebulae of the former, and had even detected, during 

 8 months, 100, out of 776, decidedly unknown before. He finds 

 that the nebulas of the two Herschels, where their observations 

 were sufficiently detailed and repeated, are nearly all un- 

 changed; and he thinks we shall never discover more than a 

 few variable nebulae : the difference of brightness now dis- 

 cernible in a large proportion of Sir W. HerschePs objects 

 being referred by him to an insensible alteration in the 

 estimate formed by that great observer. The material upon 

 his hands, he says, grows beyond all expectation ; but the study 

 is beset with much difficulty and uncertainty, and he thinks it 

 not much more advanced, even now, than the knowledge of 

 insulated stars was in the days of Tycho, or at the commence- 

 ment of Flamsteed's labours. His observations on double 

 nebulas, including in that term those within a limit of 5' of arc, 

 are most interesting. He had, up to May, 1862, recorded 50, 

 and thinks their whole number may be about 300 among the 

 3000 of the general list. Their physical connection as binary 

 systems, already suggested by Sir J. Herschel, he considers 

 unquestionable : the mere aspect proves it abundantly, espe- 

 cially when some unusual form characterizes each object ; and 

 in future times the computation of the orbits of such systems 

 will doubtless become matter of inquiry. He points out, but 

 as a mere indication, a double nebula, whose distance was 

 given by Sir W. Herschel in 1 785 = 60"; by Iris son in 1827 

 = 45", with a position-angle = 45°; and by D'Arrest in 1862 

 = 28" and 56* 32'. Like all other observers, except those at Poul- 

 kowa, he testifies to the entire disappearance of Hind's wonderful 

 nebula in Taurus (see Intellectual Observer, iii. 244) ; but ho 

 considers doubtful a similar occurrence stated by Sir J. Herschel 

 to have taken place in Coma Berenices, on account of the great 



