has been suggested that the seeming gaps, or many of them, are not
openings at all, but opaque screens cutting off the light from stars
behind them. That this is quite possible in some cases is shown by
Barnard's later photographs, particularly those of the singular region
around the star Rho Ophiuchi. Here are to be seen somber lanes and
patches, apparently forming a connected system which covers an immense
space, and which their discoverer thinks may constitute a ``dark
nebula.'' This seems at first a startling suggestion; but, after all,
why should their not be dark nebulæ as well as visible ones? In truth,
it has troubled some astronomers to explain the luminosity of the
bright nebulæ, since it is not to be supposed that matter in so
diffuse a state can be incandescent through heat, and phosphorescent
light is in itself a mystery. The supposition is also in accord with
what we know of the existence of dark solid bodies in space. Many
bright stars are accompanied by obscure companions, sometimes as
massive as themselves; the planets are non-luminous; the same is true
of meteors before they plunge into the atmosphere and become heated by
friction; and many plausible reasons have been found for believing
that space contains as many obscure as shining bodies of great size.
It is not so difficult, after all, then, to believe that there are
immense collections of shadowy gases and meteoric dust whose presence
is only manifested when they intercept the light coming from shining
bodies behind them.

This would account for the apparent extinguishment of light in open
space, which is indicated by the falling off in relative number of
telescopic stars below the tenth magnitude. Even as things are, the
amount of light coming to us from stars too faint to be seen with the
naked eye is so great that the statement of it generally surprises
persons who are unfamiliar with the inner facts of astronomy. It has