The spectroscope is an instrument for analysing light. So important is
it in the revelations it has given us that it will be best to describe
it fully. Every substance to be examined must first be made to glow,
made luminous; and as nearly everything in the heavens _is_ luminous the
instrument has a great range in Astronomy. And when we speak of
analysing light, we mean that the light may be broken up into waves of
different lengths. What we call light is a series of minute waves in
ether, and these waves are--measuring them from crest to crest, so to
say--of various lengths. Each wave-length corresponds to a colour of the
rainbow. The shortest waves give us a sensation of violet colour, and
the largest waves cause a sensation of red. The rainbow, in fact, is a
sort of natural spectrum. (The meaning of the rainbow is that the
moisture-laden air has sorted out these waves, in the sun's light,
according to their length.) Now the simplest form of spectroscope is a
glass prism--a triangular-shaped piece of glass. If white light
(sunlight, for example) passes through a glass prism, we see a series of
rainbow-tinted colours. Anyone can notice this effect when sunlight is
shining through any kind of cut glass--the stopper of a wine decanter,
for instance. If, instead of catching with the eye the coloured lights
as they emerge from the glass prism, we allow them to fall on a screen,
we shall find that they pass, by continuous gradations, from red at the
one end of the screen, through orange, yellow, green, blue, and indigo,
to violet at the other end. _In other words, what we call white light is
composed of rays of these several colours. They go to make up the effect
which we call white._ And now just as water can be split up into its two
elements, oxygen and hydrogen, so sunlight can be broken up into its
primary colours, which are those we have just mentioned.
This range of colours, produced by the spectroscope, we call the solar
spectrum, and these are, from the spectroscopic point of view, primary
colours. Each shade of colour has its definite position in the spectrum.
That is to say, the light of each shade of colour (corresponding to its
wave-length) is reflected through a certain fixed angle on passing
through the glass prism. Every possible kind of light has its definite
position, and is denoted by a number which gives the wave-length of the
vibrations constituting that particular kind of light.
Now, other kinds of light besides sunlight can be analysed. Light
from any substance which has been made incandescent may be observed with
the spectroscope in the same way, and each element can be thus
separated. It is found that each substance (in the same conditions of
pressure, etc.) gives a constant spectrum of its own. _Each metal
displays its own distinctive colour. It is obvious, therefore, that the
spectrum provides the means for identifying a particular substance._ It
was by this method that we discovered in the sun the presence of such
well-known elements as sodium, iron, copper, zinc, and magnesium.
[Illustration: _Yerkes Observatory._