spoken of as Fraunhofer lines.

To illustrate: the gases and vapors forming the outer strata of
the Sun's atmosphere would in themselves produce bright-line
spectra of the elements involved. If these gases and vapors
could in effect be removed, without changing underlying
conditions, the remaining condensed body of the Sun should have
a continuous spectrum. The cooler overlying gases and vapors
absorb those radiations from the deeper and hotter sources
which the gases and vapors would themselves emit, and thus form
the dark-line spectrum of the Sun. The stretches of spectrum
between the dark lines are of course continuous-spectrum
radiations.

These principles are illustrated in Fig. 12. The essential
parts of a spectroscope are the slit--an opening perhaps
1/100th of an inch wide and 1/10th of an inch long--to admit
the light properly; a lens to render the light rays parallel
before they fall upon the prism or grating; a prism or grating;
a lens to receive the rays after they have been dispersed by
the prism or grating and to form an image of the spectrum a
short distance in front of the eye, where the eye will see the
spectrum or a sensitive dry-plate will photograph it. If we
place an alcohol lamp immediately in front of the slit and
sprinkle some common salt in the flame the two orange bright
lines of sodium will be seen in the eyepiece, close together,
as in the upper of the two spectra in the illustration. If we
sprinkle thallium salt in the flame the green line of that
element will be visible in the spectrum. If we take the lamp
away and place a lime light or a piece of white-hot iron in