is known as a continuous spectrum. But if we
examine light from a gas under low pressure, as can be done by placing a
small quantity of the gas in a glass tube and making it glow by an
electric current, we find that it does not emit rays of all colours, but
only rays of certain distinct colours which are different for different
gases. The spectrum of a gas, therefore, consists of a number of
detached luminous lines.

When we study the sunlight through the prism, it is found that the
spectrum does not extend quite continuously from one end to the other,
but is shaded over by a multitude of dark lines, only a few of which are
shown in the adjoining plate. (Plate XIII.) These lines are a permanent
feature in the solar spectrum. They are as characteristic of the
sunlight as the prismatic colours themselves, and are full of interest
and information with regard to the sun. These lines are the characters
in which the history and the nature of the sun are written. Viewed
through an instrument of adequate power, dark lines are to be found
crossing the solar spectrum in hundreds and in thousands. They are of
every variety of strength and faintness; their distribution seems guided
by no simple law. At some parts of the spectrum there are but few lines;
in other regions they are crowded so closely together that it is
difficult to separate them. They are in some places exquisitely fine and
delicate, and they never fail to excite the admiration of every one who
looks at this interesting spectacle in a good instrument.

There can be no better method of expounding the rather difficult subject
of spectrum analysis than by actually following the steps of the
original discovery which first gave a clear demonstration of the
significance of the dark "Fraunhofer" lines. Let us concentrate our
attention specially upon that line of the solar spectrum marked D. This,
when seen in the spectroscope, is found to consist of two lines, very
delicately separated by a minute interval, one of these lines being
slightly thicker than the other. Suppose that while the attention is
concentrated on these lines the flame of an ordinary spirit-lamp
coloured by common salt be held in front of the instrument, so that the
ray of direct solar light passes through the flame before entering the
spectroscope. The observer sees at once the two lines known as D flash
out with a greatly increased blackness and vividness, while there is no
other perceptible effect on t