This leads up to the notable fact, first established by Professor
Bailey a few years ago, that such clusters are populous with variable
stars. Omega Centauri and the Hercules cluster are especially
remarkable in this respect. The variables found in them are all of
short period and the changes of light show a noteworthy tendency to
uniformity. The first thought is that these phenomena must be due to
collisions among the crowded stars, but, if so, the encounters cannot
be between the stars themselves, but probably between stars and meteor
swarms revolving around them. Such periodic collisions might go on for
ages without the meteors being exhausted by incorporation with the
stars. This explanation appears all the more probable because one
would naturally expect that flocks of meteors would abound in a close
aggregation of stars. It is also consistent with Perrine's discovery
-- that the globular star clusters are powdered with minute stars
strewn thickly among the brighter ones.

In speaking of Professor Comstock's extraordinary theory of the Milky
Way, the fact was mentioned that, broadly speaking, the nebulÊ are
less numerous in the galactic belt than in the comparatively open
spaces on either side of it, but that they are, nevertheless, abundant
in the broader half of the Milky Way which he designates as the front
of the gigantic ``plough'' supposed to be forcing its way through the
enveloping chaos. In and around the Sagittarius region the
intermingling of nebulÊ and galactic star clouds and clusters is
particularly remarkable. That there is a causal connection no
thoughtful person can doubt. We are unable to get away from the
evidence that a nebula is like a seed-ground from which stars spring
forth; or we may say that nebulÊ resemble clouds in whose bosom
raindrops are forming. The wonderful aspect of the admixtures of
nebulÊ and star-clusters in Sagittarius has been described in Chapter
1. We now come to a still more extraordinary phenomenon of this kind
-- the Pleiades nebulÊ.

The group of the Pleiades, although lying outside the main course of
the Galaxy, is connected with it by a faint loop, and is the scene of
the most remarkable association of stars and nebulous matter known in
the visible universe. The naked eye is unaware of the existence of
nebulÊ in the Pleiades, or, at the best, merely suspects that there is
something of the kind there; and even the most powerful telescopes are
far from revealing the full wonder of the spectacle; but in
photographs which have been exposed for many hours consecutively, in
order to accumulate the impression of the actinic rays, the revelation
is stunning. The principle stars are seen surrounded by, and, as it
were, drowned in, dense nebulous clouds of an unparalleled kind. The
forms assumed by these clouds seem at first sight inexplicable. They
look like fleeces, or perhaps more like splashes and daubs of luminous
paint dashed carelessly from a brush. But closer inspection shows that
they are, to a largë extent, woven out of innumerable threads of filmy
texture, and there are many indications of spiral tendencies. Each of
the bright stars of the group -- Alcyone, Merope, Maia, Electra,
Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
remember, alike to the naked eye and to the telescope), and these
particular stars are veiled from sight behind the strange mists.

Running in all directions across the relatively open spaces are
nebulous wisps and streaks of the most curious forms. On some of the
nebular lines, which are either straight throughout, or if they change
direction do so at an angle, little stars are strung like beads. In
one case seven or eight stars are thus aligned, and, as if to
emphasize their dependence upon the chain which connects them, when it
makes a slight bend the file of stars turns the same way. Many other
star rows in the group suggest by their arrangement that they, too,
were once strung upon similar threads which have now disappeared,
leaving the stars spaced along their ancient tracks. We seem forced to
the conclusion that there was a time when the Pleiades were embedded
in a vast nebula resembling that of Orion, and that the cloud has now
become so rare by gradual condensation into stars that the merest
trace of it remains, and this would probably have escaped detection
but for the remarkable actinic power of the radiant matter of which it
consists. The richness of many of these faint nebulous masses in
ultra-violet radiations, which are those that specifically affect the
photographic plate, is the cause of the marvelous revelatory power of
celestial photography. So the veritable unseen universe, as
distinguished from the ``unseen universe'' of metaphysical
speculation, is shown to us.