for a thousand years with an
unabated speed of a mile a minute, the journey would certainly not
have been completed. Nor do I venture to say what ages must elapse ere
the terminus at the other side of the ring nebula would be reached.

A cluster of stars viewed in a small telescope will often seem like a
nebula, for the rays of the stars become blended. A powerful telescope
will, however, dispel the illusion and reveal the separate stars. It
was, therefore, thought that all the nebulae might be merely clusters
so exceedingly remote that our mightiest instruments failed to resolve
them into stars. But this is now known not to be the case. Many of
these objects are really masses of glowing gas; such are, for
instance, the ring nebulae, of which I have just spoken, and the form
of which I can simulate by a pretty experiment.

We take a large box with a round hole cut in one face, and a canvas
back at the opposite side. I first fill this box with smoke, and there
are different ways of doing so. Burning brown paper does not answer
well, because the supply of smoke is too irregular and the paper
itself is apt to blaze. A little bit of phosphorus set on fire yields
copious smoke, but it would be apt to make people cough, and, besides,
phosphorus is a dangerous thing to handle incautiously, and I do not
want to suggest anything which might be productive of disaster if the
experiment was repeated at home. A little wisp of hay, slightly damped
and lighted, will safely yield a sufficient supply, and you need not
have an elaborate box like this; any kind of old packing-case, or even
a bandbox with a duster stretched across its open top and a round hole
cut in the bottom, will answer capitally. While I have been speaking,
my assistant has kindly filled this box with smoke, and in order to
have a sufficient supply, and one which shall be as little
disagreeable as possible, he has mixed together the fumes of
hydrochloric acid and ammonia from two retorts shown in Fig. 7. A
still simpler way of doing the same thing is to put a little common
salt in a saucer and pour over it a little oil of vitriol; this is put
into the box, and over the floor of the box common smelling-salts is
to be scattered. You see there are dense volumes of white smoke
escaping from every corner of the box. I uncover the opening and give
a push to the canvas, and you see a beautiful ring flying across the
room; another ring and another follows. If you were near