NEBULA. 323 



bable) it is simply illumined by the central Sun. It was 

 the opinion of Derham, and to some extent also of Lacaille, 

 who discovered many nebulous stars at the Cape of Good 

 Hope, that the stars were situated far from the nebula? on 

 which they were projected. Mairan appears (1731) first to 

 have expressed the view that nebulous stars are surrounded by 

 an atmosphere of light appertaining to them. 54 We even 

 find that some of the larger stars (of the 7th magnitude, 

 for instance, as No. 675 of the Catalogue of 1833), have a 

 photosphere, whose diameter measures from 2' to S'. 6 * 



The large nebulous masses of irregular configuration com- 

 pose a class of nebulae differing entirely from those we have 

 described as regular, and w r hich are, at all events, faintly 

 defined. They are characterized by the most variously un- 

 Bymmetrical forms, having indefinite and confused outlines. 

 These bodies, which constitute mysterious phenomena sut 

 generis, have mainly given occasion to the opinions advanced 

 in reference to the existence of cosmical clouds and self- 

 luminous nebula, supposed to be distributed through the 

 regions of space, and to resemble the substratum of the 

 zodiacal light. These irregular nebula?, which cover a por- 

 tion of the firmament several square degrees in extent, 

 present a striking contrast with the smallest of all the 

 regular isolated and oval nebulous discs, w r hich is equal 

 in luminous intensity to a telescopic star of the 14th 

 magnitude, and is situated between the constellations Ara 



14 Mairan, Traite de VAurore lorcale, p. 263; Arago, in 

 the Annuaire pour 1842, pp. 403-413. 



86 In other instances these nebulous 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.) 



VOL. IV. I) 



