[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.
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