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., Part i., p. 256. Le Francais Lalande, in the 'Connaiss. des Tems pour l'An.' VIII., p. 383. Arago, in the 'Annuaire', 1842, p. 425.
"They are parts of our starry stratum," says he, with his usual graceful animation of style, "that have experienced great devastation from time." If we picture to ourselves the telescopic stars lying behind one another as a starry canopy spread over the vault of heaven, these starless regions in Scorpio and Serpentarius may, I think, be regarded as tubes through which we may look into the remotest depths of space. Other stars may certainly lie in those parts where the strata forming the canopy are interrupted, but these are unattainable by our instruments. The aspect of fiery meteors had led the ancients likewise to the idea of clefts or openings ('chasmata') in the vault of heaven. These openings were, however, only regarded as transient, while the reason of their being luminous and fiery, instead of obscure, was supposed to be owing to the p 153 translucent illuminated ether which lay beyond them.*
[footnote] *Aristot., 'Meteor.', ii.,, 5, 1. Seneca, 'Natur. Quaest.', i., 14, 2. "Coelum discessisse," in Cic., 'de Divin.', i., 43.
Derham, and even Huygens, did not appear disinclined to explain in a similar manner the mild radiance of the nebulae.*
[footnote] *Arago, in the 'Annuaire', 1842, p. 429.
When we compare the stars of the first magnitude, which, on an average, are certainly the nearest to us, with the non-nebulous telescopic stars, and further, when we compare the nebulous stars with unresolvable nebulae, for instance, with the nebula in Andromeda, or even with the so-called planetary nebulous vapor, a fact is made manifest to us by the consideration of the varying distances and the boundlessness of space, which shows the world of phenomena, and that which constitutes its causal reality, to be dependent upon the 'propagation of light'. The velocity of this propagation is according to Struve's most recent investigations, 166,072 geographical miles in a second, consequently almost a million of times greater than the velocity of sound. According to the measurements of Maclear, Bessel, and Struve, of the parallaxes and distances of three fixed stars of very unequal magnitudes ('a' Centauri, 16 Cygni, and 'a' Lyrae), a ray of light requires respectively 3, 9 1/4, and 12 years to reach us from these three bodies. In the short but memorable period between 1572 and 1604, from the time of Cornelius Gemma and Tycho Brahe to that of Kepler, three new stars suddenly appeared in Cassiopeia and Cygnus, and in the foot of Serpentarius. A similar phenomenon exhibited itself at intervals in 1670, in the constellation Vulpis. In recent times, even since 1837, Sir John Herschel has observed, at the Cape of Good Hope, the brilliant star [Greek symbol] in Argo increase in splendor from the second to the first magnitude.*
[footnote] *In December, 1837, Sir John Herschel saw the star [Greek symbol] Argo, which till that time appeared as of the second magnitude, and liable to no change, rapidly increase till it became of the first magnitude. In January, 1838, the intensity of its light was equal to that of 'a' Centauri. According to our latest information, Maclear in March, 1843, found it as bright as Canopus; and even 'a' Crucis looked faint by [Greek symbol] Argo.