a is at such a stupendous distance that any light which the sun
could send out there would be just as feeble as the light we receive
from a fixed star. Should we be able to show our smoke-rings, for
instance, if, instead of having the electric light, I merely cut a
hole in the ceiling and allowed the feeble twinkle of a star in the
Great Bear to shine through? In a similar way the sunbeams would be
utterly powerless to effect any illumination of objects in these
stellar distances. If the sun were to be extinguished altogether, the
calamity would no doubt be a very dire one so far as we are concerned,
but the effect on the other celestial bodies (moon and planets
excepted) would be of the slightest possible description. All the
stars of heaven would continue to shine as before. Not a point in one
of the constellations wrould be altered, not a variation in the
brightness, not a change in the hue of any star could be noticed. The
thousands of nebulae and clusters would be absolutely unaltered; in
fact, the total extinction of the sun would be hardly remarked in the
newspapers published in the Pleiades or in Orion. There might possibly
be a little line somewhere in an odd corner to the effect "Mr.
So-and-So, our well-known astronomer, has noticed that a tiny star,
inconspicuous to the eye, and absolutely of no importance whatever,
has now become invisible."

If, therefore, it be not the sun which lights up this nebula, where
else can be the source of its illumination? There can be no other star
in the neighborhood adequate to the purpose, for, of course, such an
object would be brilliant to us if it were large enough and bright
enough to impart sufficient illumination to the nebula. It would be
absurd to say that you could see a man's face by the light of a candle
while the candle itself was too faint or too distant to be visible.
The actual facts are, of course, the other way; the candle might be
visible, when it was impossible to discern the face which it lighted.

Hence we learn that the ring nebula must shine by some light of its
own, and now we have to consider how it can be possible for such
material to be self-luminous. The light of a nebula does not seem to
be like flame; it can, perhaps, be better represented by the pretty
electrical experiment with Geissler's tubes. These are glass vessels
of various shapes, and they are all very nearly empty, as you will
understand when I tell you the way in which they have been