wholly or in part. The following comparison is a fair one: the
ordinary point image of a star is as if all the books in the
university library were thrown together in a disorderly but
compact pile in the center of the reading room: we could say
little concerning the contents and characteristics of that
library; whether it is strong in certain fields of human
endeavor, or weak in other fields. The spectrum of a star is as
the same library when the books are arranged on the shelves in
complete perfection and simplicity, so that he who looks may
appraise its contents at any or all points. Let us consider the
fundamental principles of spectroscopy.

1. When a solid body, a liquid, or a highly-condensed gas is
heated to incandescence, its light when passed through a
spectroscope forms a continuous spectrum: that is, a band of
light, red at one end and violet at the other, uninterrupted by
either dark or bright lines.

2. The light from the incandescent gas or vapor of a chemical
element, passed through a spectroscope, forms a bright-line
spectrum; that is, one consisting entirely of isolated bright
lines, distributed differently throughout the spectrum for the
different elements, or of bright lines superimposed upon a
relatively faint continuous spectrum.

3. If radiations from a continuous-spectrum source pass through
cooler gases or vapors before entering the spectroscope, a
dark-line spectrum results: that is, the positions which the
bright lines in the spectra of the vapors and gases would have
are occupied by dark or absorption lines. These are frequently