ame 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.

CHAPTER XII

THE SUN

The sun is the chief member of our system. It controls the motions of
the planets by its immense gravitative power. Besides this it is the
most important body in the entire universe, so far as we are concerned;
for it pours out continually that flood of light and heat, without which
life, as we know it, would quickly become extinct upon our globe.

Light and heat, though not precisely the same thing, may be regarded,
however, as next-door neighbours. The light rays are those which
directly affect the eye and are comprised in the visible spectrum. We
_feel_ the heat rays, the chief of which are beyond the red portion of
the spectrum. They may be investigated with the _bolometer_, an
instrument invented by the late Professor Langley. Chemical rays--for
instance, those radiations which affect the photographic plate--are for
the most part also outside the visible spectrum. They are, however, at
the other end of it, namely, beyond the violet.

Such a scale of radiations may be compared to the keyboard of an
imaginary piano