The principles on which spectrum analysis rests can be stated so
concisely that I shall set them forth for the special use of such
readers as may not be entirely familiar with the subject. Every one
knows that when the rays of the sun pass through a triangular prism
of glass or other transparent substance they are unequally refracted,
and thus separated into rays of different colors. These colors are not
distinct, but each runs into the other by insensible gradations, from
deep red through orange, yellow, green and blue to a faint violet.
This result is due to the fact that the light of the sun is composed of
rays of an infinite number of wave-lengths, or, as we might express it,
of an infinite number of shades of color, since to every wave-length
corresponds a definite shade. Such a spreading out of elementary
colors in the form of a visible sheet is called a spectrum. By the
spectrum of an incandescent object is meant the spectrum formed by the
light emitted by the object when passed through a refracting prism,
or otherwise separated into its elementary colors. The interest and
value which attach to the study of spectra arise from the fact that
different bodies give different kinds of spectra, according to their
constitution, their temperature and the substances of which they are
composed. In this manner it is possible, by a study of the spectrum of
a body, to reach certain inferences respecting its constitution.
In order that such a study should lead to a definite conclusion, we
must recall that to each special shade of color corresponds a definite
position in the spectrum. That is to say, there is a special kind of
light having a certain wave-length and therefore a certain shade which
will be refracted through a certain fixed angle, and will therefore
fall into a definite position in the spectrum. This position is, for
every possible kind of light, expressed by a number indicating its
wave-length.
If we form a spectrum with the light emitted by an ordinary
incandescent body, a gaslight for example, we shall find the series of
colors to be unbroken from one end of the spectrum to the other. That
is to say, there will be light in every part of the spectrum. Such a
spectrum is said to be continuous. But if we form the spectrum by means
of sunlight, we shall find the spectrum to be crossed by a great number
of more or less dark lines. This shows that in the spectrum of the sun
light of certain definite wave-lengths is wholly or partly wanting.
This fact has been observed for more than a century, but its true
significance was not seen until a comparatively recent time.
If, instead of using the light of the sun, we form a spectrum with
the light emitted by an incandescent gas, say hydrogen made luminous
by the electric spark, we shall find that the spectrum consists only
of a limited number of separate bright lines, of various colors. This
shows that such a gas, instead of emitting light of all wave-lengths,
as an incandescent solid body does, principally emits light of certain
definite wave-lengths.
It is also found that if we pass the light of a luminous solid through
a sufficiently large mass of gas, cooler than the body, the spectrum,
instead of being entirely continuous, will be crossed with dark lines
like that of the sun. This shows that light of certain wave-lengths is
absorbed by the gas. A comparison of these dark lines with the bright
lines emitted by an incandescent gas led Kirchhoff to the discovery of
the following fundamental principle:
Every gas, when cold, absorbs the same rays of light which it emits
when incandescent.
An immediate inference from this law is that the dark lines seen in
the spectrum of the sun are caused by the passage of the light through
gases either existing on the sun or forming the atmosphere of the
earth. A second inference is that we can determine what these gases are
by comparing the position of the dark lines with that of the bright
lines produced by different gases when they are made incandescent.
Hence arose the possibility of spectrum analysis, a method which has
been applied with such success to the study of the heavenly bodies.
So far as the general constitution of bodies is concerned, the canons
of spectrum analysis are these:
Firstly, when a spectrum is formed of distinct bright lines, the light
which forms it is emitted by a transparent mass of glowing gas.
Secondly, when a spectrum is entirely continuous the light emanates
from an incandescent solid, from a body composed of solid particles,
which may be ever so small, or from a mass of incandescent gas so large
and dense as not to be transparent through and through.
Thirdly, when the spectrum is continuous, except that it is crossed
by fine dark lines, the body emitting the light is surrounded by a
gas cooler than itself. The chemical constitution of this gas can be
determined by the position of the lines.
Fourthly, if, as is frequently the case, a spectrum is composed of an
irregular row of bright and shaded portions, the body is a compound
one, partly gaseous and partly solid.
It will be seen from the preceding statement that, in reality, a mass
of gas so large as not to be transparent cannot be distinguished from
a solid. It is therefore not strictly correct to say, as is sometimes
done, that an incandescent gas always gives a spectrum of bright lines.
It will give such a spectrum only when it is transparent through and
through.[C]
[C] As this principle is not universally understood, it may be
well to remark that it results immediately from Kirchoff’s
law of the proportionality between the radiating and
absorbing powers of all bodies for light of each separate
wave-length. When a body, even if gaseous in form, is of
such great size and density that light of no color can
pass entirely through it, then the consequent absorption
by the body of light of all colors shows that throughout
the region where the absorption occurs there must be an
emission of light of these same colors. Thus light from all
parts of the spectrum will be emitted by the entire mass.
A gaseous mass, so large as to be opaque, would, if it were of the
same temperature inside and out, give a continuous spectrum, without
any dark lines. But the laws of temperature in such a mass show that
it will be cooler at the surface than in the interior. This cooler
envelope will absorb the rays emanating from the interior as in the
case when the latter is solid. We conclude, therefore, that the fact
that the great majority of stars show a spectrum like that of the
sun, namely, a continuous one crossed by dark lines, does not throw
any light on the question whether the matter composing the body of
the star is in a solid, liquid or gaseous state. The fact is that
the most plausible theories of the constitution of the sun lead to
the conclusion that its interior mass is really gaseous. Only the
photosphere may be to a greater or less extent solid or liquid. The
dark lines that we see in the solar spectrum are produced by the
absorption of a comparatively thin and cool layer of gas resting upon
the photosphere. Analogy as well as the general similarity of the
spectra lead us to believe that the constitution of most of the stars
is similar to that of the sun.