account for its existence by a supposition like the above, the black
gaps would remain unexplained, unless one could make a further draft
on the imagination and suggest that the stars had been thrown into a
vast eddy, or system of eddies, whose vortices appear as dark holes.
Only a maelstrom-like motion could keep such a funnel open, for
without regard to the impulse derived from the projectile, the proper
motions of the stars themselves would tend to fill it. Perhaps some
other cause of the whirling motion may be found. As we shall see when
we come to the spiral nebulæ, gyratory movements are exceedingly
prevalent throughout the universe, and the structure of the Milky Way
is everywhere suggestive of them. But this is hazardous sport even for
the imagination -- to play with suns as if they were but thistle-down
in the wind or corks in a mill-race.

Another question arises: What is the thickness of the hedge of stars
through which the holes penetrate? Is the depth of the openings
proportionate to their width? In other words, is the Milky Way round
in section like a rope, or flat and thin like a ribbon? The answer is
not obvious, for we have little or no information concerning the
relative distances of the faint galactic stars. It would be easier,
certainly, to conceive of openings in a thin belt than in a massive
ring, for in the first case they would resemble mere rifts and breaks,
while in the second they would be like wells or bore-holes. Then, too,
the fact that the Milky Way is not a continuous body but is made up of
stars whose actual distances apart is great, offers another quandary;
persistent and sharply bordered apertures in such an assemblage are a
priori as improbable, if not impossible, as straight, narrow holes
running through a swarm of bees.

The difficulty of these questions indicates one of the reasons why it