sunbeams are refracted and reflected from
tiny globes of water in the clouds; these convey to us the sunlight, and
in doing so decompose the white beams into the seven primary hues--red,
orange, yellow, green, blue, indigo, and violet.

[Illustration: PLATE A.

THE SUN.

_Royal Observatory, Greenwich, July 8, 1892._]

[Illustration: Fig. 17.--The Prism.]

The bow set in the cloud is typical of that great department of modern
science of which we shall now set forth the principles. The globes of
water decompose the solar beams; and we follow the course suggested by
the rainbow, and analyse the sunlight into its constituents. We are
enabled to do this with scientific accuracy when we employ that
remarkable key to Nature's secrets known as the spectroscope. The beams
of white sunlight consist of innumerable beams of every hue in intimate
association. Every shade of red, of yellow, of blue, and of green, can
be found in a sunbeam. The magician's wand, with which we strike the
sunbeam and sort the tangled skein into perfect order, is the simple
instrument known as the glass prism. We have represented this instrument
in its simplest form in the adjoining figure (Fig. 17). It is a piece of
pure and homogeneous glass in the shape of a wedge. When a ray of light
from the sun or from any source falls upon the prism, it passes through
the transparent glass and emerges on the other side; a remarkable change
is, however, impressed on the ray by the influence of the glass. It is
bent by refraction from the path it originally pursued, and is compelled
to follow a different path. If, however, the prism bent all rays of
light equally, then it would be of no service in the analysis of light;
but it fortunately happens that the prism acts with varying efficiency
on the rays of different hues. A red ray is not refracted so much as a
yellow ray; a yellow ray is not refracted so much as a blue one. It
consequently happens that when the composite beam of sunlight, in which
all the different rays are blended, passes through the prism, they
emerge in the manner shown in the annexed figure (Fig. 18). Here then we
have the source of the analysing power of the prism; it bends the
different hues unequally and consequently the beam of composite
sunlight, after passing through the prism, no longer shows mere white
light, but is expanded into a coloured band of light, with hues like the
rainbow, passing from deep red at one end through every inte