SPECTRUM ANALYSIS
If white light (that of the sun, for instance) be passed through a glass
prism, namely, a piece of glass of triangular shape, it will issue from
it in rainbow-tinted colours. It is a common experience with any of us
to notice this when the sunlight shines through cut-glass, as in the
pendant of a chandelier, or in the stopper of a wine-decanter.
The same effect may be produced when light passes through water. The
Rainbow, which we all know so well, is merely the result of the sunlight
passing through drops of falling rain.
White light is composed of rays of various colours. Red, orange, yellow,
green, blue, indigo, and violet, taken all together, go, in fact, to
make up that effect which we call white.
It is in the course of the _refraction_, or bending of a beam of light,
when it passes in certain conditions through a transparent and denser
medium, such as glass or water, that the constituent rays are sorted out
and spread in a row according to their various colours. This production
of colour takes place usually near the edges of a lens; and, as will be
recollected, proved very obnoxious to the users of the old form of
refracting telescope.
It is, indeed, a strange irony of fate that this very same production
of colour, which so hindered astronomy in the past, should have aided it
in recent years to a remarkable degree. If sunlight, for instance, be
admitted through a narrow slit before it falls upon a glass prism, it
will issue from the latter in the form of a band of variegated colour,
each colour blending insensibly with the next. The colours arrange
themselves always in the order which we have mentioned. This seeming
band is, in reality, an array of countless coloured images of the
original slit ranged side by side; the colour of each image being the
slightest possible shade different from that next to it. This strip of
colour when produced by sunlight is called the "Solar Spectrum" (see
Fig. 9, p. 123). A similar strip, or _spectrum_, will be produced by any
other light; but the appearance of the strip, with regard to
preponderance of particular colours, will depend upon the character of
that light. Electric light and gas light yield spectra not unlike that
of sunlight; but that of gas is less rich in blue and violet than that
of the sun.
The Spectroscope, an instrument devised for the examination of spectra,
is, in its simplest form, composed of a small tube with a narrow slit
and prism at one end, and an eye-piece at the other. If we drop ordinary
table salt into the flame of a gas light, the flame becomes strongly
yellow. If, then, we observe this yellow flame with the spectroscope, we
find that its spectrum consists almost entirely of two bright yellow
transverse lines. Chemically considered ordinary table salt is sodium
chloride; that is to say, a compound of the metal sodium and the gas
chlorine. Now if other compounds of sodium be experimented with in the
same manner, it will soon be found that these two yellow lines are
characteristic of sodium when turned into vapour by great heat. In the
same manner it can be ascertained that every element, when heated to a
condition of vapour, gives as its spectrum a set of lines peculiar to
itself. Thus the spectroscope enables us to find out the composition of
substances when they are reduced to vapour in the laboratory.
[Illustration: FIG. 9.--The Solar Spectrum.]
In order to increase the power of a spectroscope, it is necessary to
add to the number of prisms. Each extra prism has the effect of
lengthening the coloured strip still more, so that lines, which at first
appeared to be single merely through being crowded together, are
eventually drawn apart and become separately distinguishable.
On this principle it has gradually been determined that the sun is
composed of elements similar to those which go to make up our earth.
Further, the composition of the stars can be ascertained in the same
manner; and we find them formed on a like pattern, though with certain
elements in greater or less proportion as the case may be. It is in
consequence of our thus definitely ascertaining that the stars are
self-luminous, and of a sun-like character, that we are enabled to speak
of them as _suns_, or to call the sun a _star_.
In endeavouring to discover the elements of which the planets and
satellites of our system are composed, we, however, find ourselves
baffled, for the simple reason that these bodies emit no real light of
their own. The light which reaches us from them, being merely reflected
sunlight, gives only the ordinary solar spectrum when examined with the
spectroscope. But in certain cases we find that the solar spectrum thus
viewed shows traces of being weakened, or rather of suffering
absorption; and it is concluded that this may be due to the sunlight
having had to pass through an atmosphere on its way to and from the
surface of the planet from which it is reflected to us.
Since the sun is found to be composed of elements similar to those which
go to make up our earth, we need not be disheartened at this failure of
the spectroscope to inform us of the composition of the planets and
satellites. We are justified, indeed, in assuming that more or less the
same constituents run through our solar system; and that the elements of
which these bodies are composed are similar to those which are found
upon our earth and in the sun.
The spectroscope supplies us with even more information. It tells us,
indeed, whether the sun-like body which we are observing is moving away
from us or towards us. A certain slight shifting of the lines towards
the red or violet end of the spectrum respectively, is found to follow
such movement. This method of observation is known by the name of
_Doppler's Method_,[9] and by it we are enabled to confirm the evidence
which the sunspots give us of the rotation of the sun; for we find thus
that one edge of that body is continually approaching us, and the other
edge is continually receding from us. Also, we can ascertain in the same
manner that certain of the stars are moving towards us, and certain of
them away from us.
[9] The idea, initiated by Christian Doppler at Prague in 1842, was
originally applied to sound. The approach or recession of a source from
which sound is coming is invariably accompanied by alterations of pitch,
as the reader has no doubt noticed when a whistling railway-engine has
approached him or receded from him. It is to Sir William Huggins,
however, that we are indebted for the application of the principle to
spectroscopy. This he gave experimental proof of in the year 1868.