NEBULA. 3 



(^ 



of the Catalogue of 1833) have a photosphere, whostj diam- 

 eter measures from 2' to 3'.=^ 



The large nebulous masses of h'regular configuration com« 

 pose a class of nebuiss differing entirely from tliose we have 

 described as regular, and which are, at all events, faintly de- 

 fined. They are characterized by the most variously un sym- 

 metrical forms, having indefinite and confused outlines. These 

 bodies, which constitute mysterious phenomena sui generis, 

 have mainly given occasion to the opinions advanced in ref 

 erence to the existence of cosmical clouds and self-luminous 

 nebulcB, supposed to be distributed through the. regions of 

 space, and to resemble the substratum of the zodiacal light. 

 These irregular nebulae, which cover a portion of the firma- 

 ment several square degrees in extent, present a striking con- 

 trast with the smallest of all the regular isolated and oval 

 nebulous disks, which is equal in luminous intensity to a tel- 

 escopic star of the 14th magnitude, and is situated between 

 the constellations Ara and Apus, in the southern hemisphere.! 

 No two of the unsymmetrical, difiused nebulous masses re- 

 semble one another \X but, adds Sir John Herschel, from the 

 experience of many years' observation, one thing observed in 

 reference to them, and which gives them a peculiar charac- 

 ter, is, that all are situated within or very near to the mar- 

 gins of the Milky Way, and may be regarded as offshoots from 

 it. On the contrary, the regularly shaped and well-defined 

 small nebulous spots are partly scattered over the whole heav- 

 ens, and partly compressed together in special regions, far 

 from the Milky Way, as, for instance, in the northern hemi- 

 sphere, in the regions of Virgo and Pisces. Although the large 

 irregular nebulous mass in the sword of Orion is certainly sit- 

 uated at a considerable distance from the visible margin of 



* In other instances these nebnlons stars are only of the eighth to the 

 ninth magnitude ; as Nos. 311 and 450 of the Catalogue of 1833, fig 31 

 having photospheres of 1' 30". {Outlines, $ 879.) 



t Observations at the Cape, p. 117, No. 3727, pi. vi., fig. 16. 



X We meet with remarkable forms of irregular nebulae, as, for in- 

 stance, the omega-shaped {Observations at the Cape, pi. ii., fig. 1, No. 

 2008), which has been investigated and described by Lament, and by 

 a mei'itorious North American astronomer, Mr. Mason, wliose early loss 

 is much to be lamented {Mem. of the Amer. Philos. Society, vol. vii., p 

 117) ; a nebula havmg frum 6 to 8 nuclei {Observations at the Cape, p 

 i9, pi. iii., fig. 4) ; the cometary tuft-like form in which the nebulous 

 rays seem occasionally to expand, as from a star of the ninth magni- 

 tude (pi. vi., fig. 18, Nos. 2534 and 3688) ; a silhouette profile, or bust- 

 like outline (pi. iv., fig. 4, No. 3075) ; a lissure-like opening, inclosing 

 a filiform nebula (No 3501, pi. iv., fig. 2 ; Outlines, $ 883 ; O'scrvatiom 

 Hi the Cape, <5 12".^. 



