152 COSMOS. 



Borne cases, not exceeding the twentieth or twenty-fourth do 

 gree of telescopic magnitude. A portion of the nebulous vapoi 

 would probably be found resolvable into stars by more power 

 ful optical instruments. As the retina retains a less vivid im- 

 pression of separate than of infinitely near luminous points, 

 less strongly marked photometric relations are excited in tho 

 latter case, as Arago has recently shown.* The definite 01 

 amorphous cosmical vapor so universally diffused, and which 

 generates heat through condensation, probably modifies tht 

 transparency of the universal atmosphere, and diminishes that 

 uniform intensity of light which, according to Halley and Ol- 

 bers, should arise, if every point throughout the depths of space 

 were filled by an infinite series of stars. f The assumption of 

 such a distribution in space is, however, at variance with ob- 

 servation, which shows us large starless regions of space, open- 

 ings in the heavens, as William Herschel terms them one, 

 four degrees in width, in Scorpio, and another in Serpentari- 

 us. In the vicinity of both, near their margin, we find un- 

 resolvable nebulae, 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 at- 

 tractive and agglomerative forces of the marginal groups. $ 

 " 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 Scor- 

 pio 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 (cliasmata) 

 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 



* Arago, in the Annuaire, 1842, p. 282-285, 409-411, and 439-4*2. 



t Olbers, on the transparency of celestial space, in Bode's Jahrb., 

 1826, B. 110-121. 



t " An opening in the heavens," William Herschel.in the Phil. Trans 

 for 1783, vol. Ixxv., Tart i., p. 25G. Le Fran<jais Lalande, in the Con 

 tiaiss. det Terns pour V An. VIII., p. 383. Arago, in the Annimire, 

 1842, p. 423 



