front of the slit we shall get a brilliant continuous spectrum
not crossed by any lines, either bright or dark. Insert now the
alcohol-sodium-thallium lamp between the lime light and the
slit, and the observer will see the two sodium lines and one
thallium line in the same places as before, but as dark lines
on a background of bright continuous spectrum, as: illustrated
in the lower of the two spectra. Let us insert a screen between
the lamp and the lime light so as to cut out the latter, and we
shall see the bright lines of sodium and thallium reappear as
in the upper of the two spectra. These simple facts illustrate
Kirchhoff's immortal discovery of certain fundamental
principles of spectroscopy, in 1859. The gases and vapors in
the lamp flame are at a lower temperature than the lime source.
The cooler vapors of sodium and thallium have the power of
absorbing exactly those rays from the hotter lime or other
similar source which the vapors by themselves would emit to
form bright lines.

When we apply the spectroscope to celestial objects we find
apparently an endless variety of spectra. We shall illustrate
some of the leading characteristics of these spectra as in
Figs. 13 to 18, inclusive, and Figs. 21, 22, 23 and 24. The
spectra of some nebulae consist almost exclusively of isolated
bright lines, indicating that these bodies consist of luminous
gases, as Huggins determined in 1864; but a very faint
continuous band of light frequently forms a background for the
brilliant bright lines. Many of the nebular lines are due to
hydrogen, others are due to helium; but the majority, including
the two on the extreme right in Fig. 13, which we attribute to
the hypothetical element nebulium, and the close pair on the