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.

The subject becomes clearer by a study of the chromolithic plate.
No. 1 represents the solar spectrum, with a few of its lines on an
accurately graduated scale. [Page 51] No.3 shows the bright line of
glowing sodium, and, corresponding to a dark line in the solar
spectrum, shows the presence of salt in that body. No. 2 shows that
potassium has some violet rays, but not all; and there being no dark
line to correspond in the solar spectrum, we infer its absence from
the sun. No.6 shows the numerous lines and bands of barium--several
red, orange, yellow, and four are very bright green ones. The lines
given by any volatilized substances are always in the same place on
the scale.

A patient study of these signs of substances reveals, richer results
than a study of the cuniform characters engraved on Assyrian slabs;
for one is the handwriting of men, the other the handwriting of
God.

One of the most difficult and delicate problems solved by the
spectroscope is the approach or departure of a light-giving body
in the line of sight. Stand before a locomotive a mile away, you
cannot tell whether it appro