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 approaches or recedes, yet it will dash by
in a minute. How can the movements of the stars be comprehended
when they are at such an immeasurable distance?

It can best be illustrated by music. The note C of the G clef is
made by two hundred and fifty-seven vibrations of air per second.
Twice as many vibrations per second would give us the note C an octave
above. Sound travels at the rate of three hundred and sixty-four
yards per second. If the source of these two hundred and fifty-seven