aches 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 of an approaching
locomotive gives one tone; it passes, and we instantly detect
another. Let two trains, running at a speed of thirty-six yards a
second, approach each other. Let the whistle of one sound the note
E, three hundred and twenty-three vibrations per second. It will be
heard on the other as the note G, three hundred and eighty-eight
vibrations per second; for the speed of each train crowds the
vibrations into one-tenth less room, adding 32+ vibrations per
second, making three hundred and eighty-eight in all. The trains
pass. The vibrations are put into one-tenth more space by the
whistle making them, and the other train allows only nine-tenths of
what there are to overtake the ear. Each subtracts 32+ vibrations
from three hundred and twenty-three, leaving only two hundred and
fifty-eight, which is the note C. Yet the note E was constantly
uttered.

[Illustration: 1. Solar Spectrum. 2. Spectrum of Potassium. 3.
Spectrum of Sodium. 4. Spectrum of Strontium. 5. Spectrum of Calcium.
6. Spectrum of Barium.]

If a source of light approach or depart, it will have a similar
effect on the light waves. How shall we detect it? If a star approach
us, it puts a greater number of waves into an inch, and shortens their
length. If it recedes, it increases the length of the wave--puts
a less number into an inch. If a body giving only the number of
vibrations we call green were to approach sufficiently fast, it
would crowd in vibrations enough to appear what we call blue, indigo,
or even violet, according