light coming from the prominences
of the sun which was not given by any substance known on earth, and
attributed this to an unknown gas which he called helium, from the Greek
_helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
minerals the same gas identified by the spectroscope._ We can say,
therefore, that this gas was discovered in the sun nearly thirty years
before it was found on earth; this discovery of the long-lost heir is as
thrilling a chapter in the detective story of science as any in the
sensational stories of the day, and makes us feel quite certain that our
methods really tell us of what elements sun and stars are built up. The
light from the corona of the sun, as we have mentioned indicates a gas
still unknown on earth, which has been christened Coronium.

Measuring the Speed of Light

But this is not all; soon a new use was found for the spectroscope. We
found that we could measure with it the most difficult of all speeds
to measure, speed in the line of sight. Movement at right angles to the
direction in which one is looking is, if there is sufficient of it, easy
to detect, and, if the distance of the moving body is known, easy to
measure. But movement in the line of vision is both difficult to detect
and difficult to measure. Yet, even at the enormous distances with which
astronomers have to deal, the spectroscope can detect such movement and
furnish data for its measurement. If a luminous body containing, say,
sodium is moving rapidly towards the spectroscope, it will be found that
the sodium lines in the spectrum have moved slightly from their usual
definite positions towards the violet end of the spectrum, the amount of
the change of position increasing with the speed of the luminous body.
If the body is moving away from the spectroscope the shifting of the
spectral lines will be in the opposite direction, towards the red end of
the spectrum. In this way we have discovered and measured movements that
otherwise would probably not have revealed themselves unmistakably to us
for thousands of years. In the same way we have watched, and measured
the speed of, tremendous movements on the sun, and so gained proof that
the vast disturbances we should expect there actually do occur.

[Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES

This pictorial diagram illustrates the principal of Spectrum Analysis,
show