erwise have
illuminated those lines, and hence they became darker with the sodium
flame than without it.

We are thus conducted to a remarkable principle, which has led to the
interpretation of the dark lines in the spectrum of the sun. We find
that when the sodium vapour is heated, it gives out light of a very
particular type, which, viewed through the prism, is concentrated in two
lines. But the sodium vapour possesses also this property, that light
from the sun can pass through it without any perceptible absorption,
except of those particular rays which are of the same characters as the
two lines in question. In other words, we say that if the heated vapour
of a substance gives a spectrum of bright lines, corresponding to lights
of various kinds, this same vapour will act as an opaque screen to
lights of those special kinds, while remaining transparent to light of
every other description.

This principle is of such importance in the theory of spectrum analysis
that we add a further example. Let us take the element iron, which in a
very striking degree illustrates the law in question. In the solar
spectrum some hundreds of the dark lines are known to correspond with
the spectrum of iron. This correspondence is exhibited in a vivid manner
when, by a suitable contrivance, the light of an electric spark from
poles of iron is examined in the spectroscope side by side with the
solar spectrum. The iron lines in the sun are identical in position with
the lines in the spectrum of glowing iron vapour. But the spectrum of
iron, as here described, consists of bright lines; while those with
which it is compared in the sun are dark on a bright background. They
can be completely understood if we suppose the vapour arising from
intensely heated iron to be present in the atmosphere which surrounds
the luminous strata on the sun. This vapour would absorb or stop
precisely the same rays as it emits when incandescent, and hence we
learn the important fact that iron, no less than sodium, must, in one
form or another, be a constituent of the sun.

Such is, in brief outline, the celebrated discovery of modern times
which has given an interpretation to the dark lines of the solar
spectrum. The spectra of a large number of terrestrial substances have
been examined in comparison with the solar spectrum, and thus it has
been established that many of the elements known on the earth are
present in the sun. We may mention calcium, iron, hydrog