[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 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 vibrations could approach us at three hundred and sixty-four yards per second, it is obvious that twice as many waves would be put into a given space, and we should hear the upper C when only waves enough were made for the lower C. The same [Page 52] result would appear if we carried our ear toward the sound fast enough to take up twice as many valves as though we stood still.  This is apparent to every observer in a railway train.  The whistle