a small beam of light. In order to have the colors thoroughly
dispersed, the best instruments pass the beam of light through a
series of prisms called a battery, each one spreading farther the
colors which the previous ones had spread. In Fig. 17 the ray is
seen entering through the telescope A, which renders the rays
parallel, and passing [Page 50] through the prisms out to telescope
B, where the spectrum can be examined on the retina of the eye for a
screen. In order to still farther disperse the rays, some batteries
receive the ray from the last prism at O upon an oblique mirror,
send it up a little to another, which delivers it again to the prism
to make its journey back again through them all, and come out to be
examined just above where it entered the first prism.

[Illustration: Fig. 17.--Spectroscope, with Battery of Prisms.]

Attached to the examining telescope is a diamond-ruled scale of glass,
enabling us to fix the position of any line with great exactness.

[Illustration: Fig. 18.--Spectra of glowing Hydrogen and the Sun.]

In Fig. 18 is seen, in the lower part, a spectrum of the sun, with
about a score of its thousands of lines made evident. In the upper
part is seen the spectrum of bright lines given by glowing hydrogen
gas. These lines are given by no other known gas; they are its
autograph. It is readily observed that they precisely correspond
with certain dark lines in the solar spectrum. Hence we easily
know that a glowing gas gives the same bright lines that it absorbs
from the light of another source passing through it--that is, glowing
gas gives out the same rays of light that it absorbs when it is
not glowing.