since without some clear ideas on the subject no
proper understanding of recent astronomical progress is possible--to
take a cursory view of the elementary principles of spectrum analysis.
To many of our readers they are doubtless already familiar; but it is
better to appear trite to some than obscure even to a few.

The spectrum, then, of a body is simply the light proceeding from it
_spread out_ by refraction[399] into a brilliant variegated band,
passing from brownish-red through crimson, orange, yellow, green, and
azure into dusky violet. The reason of this spreading-out or
"dispersion" is that the various colours have different wave-lengths,
and consequently meet with different degrees of retardation in
traversing the denser medium of the prism. The shortest and quickest
vibrations (producing the sensation we call "violet") are thrown
farthest away from their original path--in other words, suffer the
widest "deviation;" the longest and slowest (the red) travel much nearer
to it. Thus the sheaf of rays which would otherwise combine into a patch
of white light are separated through the divergence of their tracks
after refraction by a prism, so as to form a tinted riband. This
_visible_ spectrum is prolonged _invisibly_ at both ends by a long range
of vibrations, either too rapid or too sluggish to affect the eye as
light, but recognisable through their chemical and heating effects.

Now all incandescent solid or liquid substances, and even gases ignited
under great pressure, give what is called a "continuous spectrum;" that
is to say, the light derived from them is of every conceivable hue.
Sorted out with the prism, its tints merge imperceptibly one into the
other, uninterrupted by any dark spaces. No colours, in short, are
missing. But gases and vapours rendered luminous by heat emit rays of
only a few tints, which accordingly form an interrupted spectrum,
usually designated as one of lines or bands. And since these rays are
perfectly definite and characteristic--not being the same for any two
substances--it is easy to tell what kind of matter is concerned in
producing them. We may suppose that the inconceivably minute particles
which by their rapid thrilling agitate the ethereal medium so as to
produce light, are free to give out their peculiar tone of vibration
only when floating apart from each other in gaseous form; but when
crowded together into a condensed mass, the clear ring of the
distinctive note is drow