142 COSMOS. 



subject in the course of time to various metamorphoses, and 

 evinces a tendency to dissolve and separate, owing to secon- 

 dary centres of attraction is surrounded by two rings, one 

 of which, the nebulous zone, is very remote, while the 

 other is nearer, and composed of stars alone. The latter, which 

 we generally term the Milky Way, is composed of nebulous 

 stars, averaging from the 10th to the llth degree of magni- 

 tude,* but appearing, when considered individually, of very 

 different magnitudes ; whilst isolated starry clusters (starry 

 swarms) almost always exhibit throughout a character of great 

 uniformity in magnitude and brilliancy. 



In whatever part the vault of heaven has been pierced by 

 powerful and far- penetrating telescopic instruments, stars or 

 luminous nebulae are everywhere discoverable, the former in 

 some cases not exceeding the 20th or 24th degree of telescopic 

 magnitude. A portion of the nebulous vapour would probably 

 be found resolvable into stars by more powerful optical instru- 

 ments. As the retina retains a less vivid impression of sepa- 

 rate than of infinitely near luminous points, less strongly 

 marked photometric relations are excited in the latter case, 

 as Arago has recently shown.f The definite or amorphous 

 cosmical vapour so universally diffused, and which generates 

 heat through condensation, probably modifies the transparency 

 of the universal atmosphere, and diminishes that uniform in- 

 tensity of light which, according to Halley and Olbers, should 

 arise, if every point throughout the depths of space were filled 

 by an infinite series of stars. J The assumption of such a dis- 

 tribution in space is, however, at variance with observation ; 

 which shows us large starless regions of space, openings in the 

 heavens, as William Herschel terms themone, four degrees 

 in width, in Scorpio, and another in Serpentarius. In the 

 vicinity of both, near their margin, we find unresolvable ne- 

 bulae, of which that on the western edge of the opening in 

 Scorpio is one of the most richly thronged of the clusters of 

 small stars by which the firmament is adorned. Herschel 

 ascribes these openings or starless regions to the attractive 



* Sir John Herschel, Astron., 585. 



f Arago, in the Annuaire, 1842, pp. 282-285, 409-411, and 

 439-442. 



J Olbers, on the transuarency of celestial space, in Bode's Jahrlf., 1826, 

 B. 110-121. 



