telescope
gradually revealed the fact that many of these are made up of stars,
and so people began to think that all faint shining patches of nebulous
light were really star clusters, which would be resolved into stars if
only we had better telescopes. Since the invention of the spectroscope,
however, fresh light has been thrown on the matter, for the spectrum
which is shown by some of the nebulous patches is not the same as that
shown by stars, and we know that many of these strange appearances are
not made up of infinitely distant stars.

We are talking here quite freely about nebulae because we have met one
long ago when we discussed the gradual evolution of our own system, and
we know quite well that a nebula is composed of luminous faintly-glowing
gas of extreme fineness and thinness. We see in the sky at the present
time what we may take to be object-lessons in our own history, for we
see nebulae of all sorts and sizes, and in some stars are mixed up, and
in others stars are but dimly seen, so that it does not require a great
stretch of the imagination to picture these stars as being born,
emerging from the swaddling bands of filmy webs that have enwrapped
them; and other nebulae seem to be gas only, thin and glowing, with no
stars at all to be found in it. We still know very little about these
mysterious appearances, but the work of classifying and resolving them
is going on apace. Nebulae are divided into several classes, but the
easiest distinction to remember is that between white nebulae and green
nebulae. This is not to say that we can see some coloured green, but that
green appears in the spectrum of some of the nebulae, while the spectrum
of a white nebula is more like that of a star.

It is fortunate for us that in the sky we can see without a telescope
one instance of each of the several objects of interest that we have
referred to.

We have been able to see one very vivid example of a variable star; we
have seen one very beautiful example of a star cluster; and it remains
to look for one very good example of a white nebula.

Just as in finding Algol you were doing a little bit of practical work,
proving something of which you had read, so by seeing this nebula you
will remember more about nebulae in general than by reading many chapters
on the subject. This particular nebula is in Andromeda, and is not far
from Algol; and it is not difficult to find. It is the only one that can
be well seen without