rmediate
grade to the violet.

[Illustration: Fig. 18.--Dispersion of Light by the Prism.]

We have in the prism the means of decomposing the light from the sun, or
the light from any other source, into its component parts. The
examination of the quality of the light when analysed enables us to
learn something of the constitution of the body from which this light
has emanated. Indeed, in some simple cases the mere colour of a light
will be sufficient to indicate the source from which it has come. There
is, for instance, a splendid red light sometimes seen in displays of
fireworks, due to the metal strontium. The eye can identify the element
by the mere colour of the flame. There is also a characteristic yellow
light produced by the flame of common salt burned with spirits of wine.
Sodium is the important constituent of salt, so here we recognise
another substance merely by the colour it emits when burning. We may
also mention a third substance, magnesium, which burns with a brilliant
white light, eminently characteristic of the metal.

[Illustration: PLATE XIII.

SPECTRA OF THE SUN AND STARS.

I. SUN.
II. SIRIUS.
III. ALDEBARAN.
IV. BETELGEUZE.]

The three metals, strontium, sodium, and magnesium, may thus be
identified by the colours they produce when incandescent. In this simple
observation lies the germ of the modern method of research known as
spectrum analysis. We may now examine with the prism the colours of the
sun and the colours of the stars, and from this examination we can learn
something of the materials which enter into their composition. We are
not restricted to the use of merely a single prism, but we may arrange
that the light which it is desired to analyse shall pass through several
prisms in succession in order to increase the _dispersion_ or the
spreading out of the different colours. To enter the spectroscope the
light first passes through a narrow slit, and the rays are then rendered
parallel by passing through a lens; these parallel rays next pass
through one or more prisms, and are finally viewed through a small
telescope, or they may be intercepted by a photographic plate on which a
picture will then be made. If the beam of light passing through the slit
has radiated from an incandescent solid or liquid body, or from a gas
under high pressure, the coloured band or _spectrum_ is found to contain
all the colours indicated on Plate XIII., without any interruption
between the colours. This