related to the magnetic
phenomena of the earth and therefore is connected with the electrical
influence of the sun.]

It is from the study of sun-spots that we have learned that the sun's
surface does not appear to rotate all at the same speed. The
"equatorial" regions are rotating quicker than regions farther north or
south. A point forty-five degrees from the equator seems to take about
two and a half days longer to complete one rotation than a point on the
equator. This, of course, confirms our belief that the sun cannot be a
solid body.

What is its composition? We know that there are present, in a gaseous
state, such well-known elements as sodium, iron, copper, zinc, and
magnesium; indeed, we know that there is practically every element in
the sun that we know to be in the earth. How do we know?

It is from the photosphere, as has been said, that we have won most of
our knowledge of the sun. The instrument used for this purpose is the
spectroscope; and before proceeding to deal further with the sun and the
source of its energy it will be better to describe this instrument.

A WONDERFUL INSTRUMENT AND WHAT IT REVEALS

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 colou