60 Clusters of Stars and Nebulai. 



bable, nothing is, nothing can be, proved as to the relative 

 position of nebulae in space, without a determination of parallax, 

 which the want of precision in their boundaries renders pecu- 

 liarly difficult, if it should ever be possible. 



Few of the nebulae, or clusters, which an astronomer would 

 designate by that title, are visible to the naked eye. The 

 Coma Berenices and the Pleiades are undoubtedly groups which, 

 removed to increasing distances, would gradually put on the 

 appearance of telescopic clusters, resolvable, and at length 

 milky nebulas. Several other groups or clusters were classed 

 by the ancients as " nebulous stars." Ptolemy, who wrote 

 about a.d. 140, mentions five — the grand cluster in the hand of 

 Perseus; Prassepe (see Double Star list, No. 5), "the manger," 

 lying between two small stars, called in those days " Aselli," 

 the little asses — incongruous appellations, the general accepta- 

 tion of which it is not easy to understand; a group near 

 Scorpio ; another in the eye of Sagittarius ; and one in the head 

 of Orion (Double Star list, No. 99). But objects of this kind 

 would give way at once upon the first application even of pri- 

 mitive telescopes, and thus Galileo readily perceived and 

 delineated twenty-one stars in the head of Orion, and thirty-six 

 in PraBsepe. In addition to these, and two or three lying too 

 far south to be visible in our latitudes, we may enumerate the 

 great nebulas in the sword of Orion, and in the girdle of An- 

 dromeda — neither of them, singularly enough, referred to by 

 the ancients, or even by Galileo — a minute speck in Hercules, 

 and another in Gemini, and we shall have pretty fairly ex- 

 hausted the list as far as the unaided eye is concerned ; though 

 it is not improbable that, under very favourable circumstances, a 

 few more of the larger clusters might be made out, as spots of 

 evanescent faintness. The telescope, of course, would imme- 

 diately enlarge the catalogue ; yet this did not take place as 

 rapidly as might have been expected. Halley picked up a few ; 

 li.'i Caille added to the number; but the French observer, 

 Messier, who was called the "Comet-ferret," from his un- 

 paralleled diligence in searching for those visitants, and was 

 thus naturally led to notice every object of a similar appearance, 

 fi irmeda much more extensive list, comprising, with those already 

 known, one hundred and three nebulas. This was published in 

 1783, to be superseded* three years afterwards, by the labours of 

 S r \\ r . Bersohel. He bad already attacked the subject in 1774, 

 anil the extraordinary and previously unattained " space-pene- 

 trating " power — to use his own word — of his great reflectors, 

 bher with the unwearied diligence with which he swept the 

 heavens, enabled him to produce, in 178G, a catalogue of one 

 thousand clutters and aeonles; in 1789, a second quite as 

 numerous j and in 1802, another comprising Jive hundred more. 



