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Page 17
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 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 to its speed. If it receded sufficiently
fast, it would leave behind it only vibrations enough to fill up
[Page 53] the space with what we call yellow, orange, or red,
according to its speed; yet it would be green, and green only, all
the time. But how detect the change? If red waves are shortened they
become orange in color; and from below the red other rays, too far
apart to be seen by the eye, being shortened, become visible as red,
and we cannot know that anything has taken place. So, if a star
recedes fast enough, violet vibrations being lengthened become
indigo; and from above the violet other rays, too short to be seen,
become lengthened into visible violet, and we can detect no movement
of the colors. The dark lines of the spectrum are the cutting out of
rays of definite wave-lengths. If the color spectrum moves away,
they move with it, and away from their proper place in the ordinary
spectrum. If, then, we find them toward the red end, the star is
receding; if toward the violet end, it is approaching. Turn the
instrument on the centre of the sun. The dark lines take their
appropriate place, and are recognized on the ruled scale. Turn it on
one edge, that is approaching us one and a quarter miles a second by
the revolution of the sun on its axis, the spectral lines move
toward the violet end; turn the spectroscope toward the other edge
of the sun, it is receding from us one and a quarter miles a second
by reason of the axial revolution, and the spectral lines move
toward the red end. Turn it near the spots, and it reveals the
mighty up-rush in one place and the down-rush in another of one
hundred miles a second. We speak of it as an easy matter, but it is
a problem of the greatest delicacy, almost defying the mind of man
to read the movements of matter.
It should be recognized that Professor Young, of [Page 54]
Princeton, is the most successful operator in this recent realm of
science. He already proposes to correct the former estimate of the
sun's axial revolutions, derived from observing its spots, by the
surer process of observing accelerated and retarded light.
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