pure and simple,
but yet that beam is composed of all sorts of colors mingled together,
in such proportions as to form white light. I take a wedge-shaped
piece of glass called a prism, and when I introduce it into the course
of the beam, you see the transformation that has taken place (Fig. 4).
Instead of the white light you have now all the colors of the
rainbow--red, orange, yellow, green, blue, indigo, violet, marked by
their initial letters in the figure. These colors are very beautiful,
but they are transient, for the moment we take away the prism they
all unite again to form white light. You see what the prism has done;
it has bent all the light in passing through it; but it is more
effective in bending the blue than the red, and consequently the blue
is carried away much further than the red. Such is the way in which we
study the composition of a heavenly body. We take a beam of its light,
we pass it through a prism, and immediately it is separated into its
components; then we compare what we find with the lights given by the
different elements, and thus we are enabled to discover the substances
which exist in the distant object whose light we have examined. I do
not mean to say that the method is a simple one; all I am endeavoring
to show is a general outline of the way in which we have discovered
the materials present in the stars. The instrument that is employed
for this purpose is called the spectroscope. And perhaps you may
remember that name by these lines, which I have heard from an
astronomical friend:--

"Twinkle, twinkle, little star,
Now we find out what you are,
When unto the midnight sky,
We the spectroscope apply."

[Illustration: FIG. 4. HOW A RAY OF LIGHT IS SPLIT UP.]

I am sure it will interest everybody to know that the elements which
the stars contain are not altogether different from those of which the
earth is made. It is true there may be substances in the stars of
which we know nothing here; but it is certain that many of the most
common elements on the earth are present in the most distant bodies. I
shall only mention one, the metal iron. That useful substance has been
found in some of the stars which lie at almost incalculable distances
from the earth.

The Nebulae.

In drawing towards the close of these lectures I must say a few words
about some dim and mysterious objects to which we have not yet
alluded. They are what are called nebulae, or little clouds; and in
one