t the star which seems to
be in the midst of it resolves itself under the telescope into not one
but six, of various sizes.

Nebulae are in most cases too enormously remote from the earth for us to
have any possible means of computing the distance; but we may take it
that light must journey at least a thousand years to reach us from them,
and in many cases much more. Therefore, if at the time of the Norman
Conquest a nebula had begun to grow dim and fade away, it would, for all
intents and purposes, still be there for us, and for those that come
after us for several generations, though all that existed of it in
reality would be its pale image fleeting onward through space in all
directions in ever-widening circles.

That nebulae do sometimes change we have evidence: there are cases in
which some have grown indisputably brighter during the years they have
been under observation, and some nebulae that have been recorded by
careful observers seem to have vanished. When we consider that these
strange bodies fill many, many times the area of our whole solar system
to the outermost bounds of Neptune's orbit, it is difficult to imagine
what force it is that acts on them to revive or quench their light. That
that light is not the direct result of heat has long been known; it is
probably some form of electric excitement causing luminosity, very much
as it is caused in the comets. Indeed, many people have been tempted to
think of the nebulae as the comets of the universe, and in some points
there are, no doubt, strong resemblances between the two. Both shine in
the same way, both are so faint and thin that stars can be seen through
them; but the spectroscope shows us that to carry the idea too far would
be wrong, as there are many differences in constitution.

We have seen that there are dark stars as well as light stars; if so,
may there not be dark nebulae as well as light ones? It may very well be
so. We have seen that there are reasons for supposing our own system to
have been at first a cool dark nebula rotating slowly. The heavens may
be full of such bodies, but we could not discern them. Their thinness
would prevent their hiding any stars that happened to be behind them. No
evidence of their existence could possibly be brought to us by any
channel that we know.

It is true that, besides the dark rifts in the bright nebulae, which may
themselves be caused by a darker and non-luminous gas, there are also
strange rifts in