Part i

., p. 257. Sir John Herschel, 'Astron.', 616. ("The 'nebulous' region of the heavens forms 'a nebulous Milky Way', composed of distinct nebulae, as the other of stars." The same observation was made in a letter he addressed to me in March, 1829.)

The Milky Way composed of nebulae does not belong to our starry stratum, but surrounds it at a great distance without being physically connected with it, passing almost in the form of a large cross through the dense nebulae of Virgo, especially in the northern wing, through Comae Berenicis, Ursa Major, Andromeda's girdle, and Pisces Boreales. It probably intersects the stellar Milky Way in Cassiopeia, and connects its dreary poles (rendered starless from the attractive forces by which stellar bodies are made to agglomerate into groups) in the least dense portion of the starry stratum.

We see from these considerations that our starry cluster, which bears traces in its projecting branches of having been subject in the course of time to various metamorphoses, and evinces a tendency to dissolve and separate, owing to secondary centers 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 tenth to the eleventh degree of magnitude,* but appearing, when considered individually, of very different magnitudes, while isolated starry clusters (starry swarms) almost always exhibit throughout a character of great uniformity in magnitude and brilliancy.

[footnote] *Sir John Herschel, 'Astron.', 585.

In whatever part the vault of heaven has been pierced by powerful and far-penetrating telescopic instruments, stars or luminous nebulae are every where discoverable, the former, in p 152 some cases, not exceeding the twentieth or twenty-fourth degree of telescopic magnitude. A portion of the nebulous vapor would probably be found resolvable into stars by more powerful optical instruments. As the retina retains a less vivid impression of separate than of infinitely near luminous points, less strongly marked photometric relations are excited in the latter case, as Arago has recently shown.*

[footnote] *Arago, in the 'Annuaire', 1842, p. 282-285, 409-411, and 439-442.

The definite or amorphous cosmical vapor so universally diffused, and which generates heat through condensation, probably modifies the transparency of the universal atmosphere, and diminishes that uniform intensity 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.*

[footnote] *Olbers, on the transparency of celestial space, in Bode's 'Jahrb.', 1826, s. 110-121.

The assumption of such a distribution in space is, however, at variance with observation, which shows us large starless regions of space, 'openings' in the heavens, as William Herschel terms them -- one, four degrees in width, in Scorpio, and another in Serpentarius. In the vicinity of both, near their margin, we find unresolvable nebulae, of which that on the western edge of the opening 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 and agglomerative forcesof the marginal groups.*

[footnote] *"An opening in the heavens," William Herschel, in the 'Phil. Trans.' for 1785, vol. lxxv.,