THE STELLAR UNIVERSE. 



Nevertheless, these nebulae may he strata of stars, of which the 

 thickness is small compared with their other dimensions ; and 

 supposing their real outline to be circular, they will appear ellip- 

 tical if the plane of the stratum be inclined to the visual line, and 

 more or less excentrically elliptical, according as the angle of 

 inclination is more or less acute. In cases in which the brightness 

 does not increase in a striking degree from the edges inwards, this 

 form is more probable than the globular or the spheroidal. 



Nebulae may be conveniently classed according to their apparent 

 form and structure ; but whatever arrangement may be adopted, 

 these objects exhibit such varieties, assume such capricious and 

 irregular forms, and undergo such strange and unexpected 

 changes of appearance according to the increasing power of the 

 telescope with which they are viewed, that it will always be 

 found that great numbers of them will remain unavoidably 

 unclassified. 



82. Like individual stars, nebula are found to be combined in 

 pairs too frequently to be compatible with the supposition, that 

 such combinations arise from the fortuitous results of the small 

 obliquity of the visual rays, which causes mere optical juxta- 

 position. 



In figs. 21, 22, 23, and 24, four double nebulae of this class are 

 represented. 



In fig. 21, the visual line passes between them without touching 



Fig. 21. 



either, and they are consequently seen completely separated. 

 They are in this ease equal in magnitude. 



In fig. 22 they are also equal in magnitude, but the distance 

 between their centres being less than their diameter, they partially 

 overlay each other. 

 26 



