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Page 9
[Illustration: Fig. 4.--Velocity of Light measured by Eclipses of
Jupiter's Moons.]
We first seek the velocity of light. In Fig. 4 the earth is 92,500,000
miles from the sun at E; Jupiter is 480,000,000 miles from the sun
at J. It has four moons: the inner one goes around the central
body in forty-two hours, and is eclipsed at every revolution. The
light that went out from the sun to M ceases to be reflected back
to the earth by the intervention of the planet Jupiter. We know
to a second when these eclipses take place, and they can be seen
with a small telescope. But when the earth is on the opposite side
of the sun [Page 23] from Jupiter, at E', these eclipses at J' take
place sixteen and a half minutes too late. What is the reason? Is
the celestial chronometry getting deranged? No, indeed; these great
worlds swing never an inch out of place, nor a second out of time.
By going to the other side of the sun the earth is 184,000,000 miles
farther from Jupiter, and the light that brings the intelligence of
that eclipse consumes the extra time in going over the extra
distance. Divide one by the other and we get the velocity, 185,000
miles per second. That is probably correct to within a thousand
miles. Methods of measurement by the toothed wheel of Fizeau confirm
this result. Suppose the wheel, Fig. 5, to have one thousand teeth,
making five revolutions to the second. Five thousand flashes of
light each second will dart out. Let each flash travel nine miles to
a mirror and return. If it goes that distance in 1/10000 of a
second, or at the rate of 180,000 miles a second, the next tooth
will have arrived before the eye, and each returning ray be cut off.
Hasten the revolutions a little, and the next notch will then admit
the ray, on its return, that went out of each previous notch: the
eighteen miles having been traversed meanwhile. The method of
measuring by means of a revolving mirror, used by Faucault, is held
to be even more accurate.
[Illustration: Fig. 5.--Measuring the Velocity of Light.]
When we take instantaneous photographs by the exposure [Page 24] of
the sensitive plate 1/20000 part of a second, a stream of light nine
miles long dashes in upon the plate in that very brief period of
time.
The highest velocity we can give a rifle-ball is 2000 feet a second,
the next second it is only 1500 feet, and soon it comes to rest.
We cannot compact force enough behind a bit of lead to keep it
flying. But light flies unweariedly and without diminution of speed.
When it has come from the sun in eight minutes, Alpha Centauri
in three years, Polaris in forty-five years, other stars in one
thousand, its wings are in nowise fatigued, nor is the rapidity
of its flight slackened in the least.
It is not the transactions of to-day that we read in the heavens,
but it is history, some of it older than the time of Adam. Those
stars may have been smitten out of existence decades of centuries
ago, but their poured-out light is yet flooding the heavens.
It goes both ways at once in the same place, without interference.
We see the light reflected from the new moon to the earth; reflected
back from the house-tops, fields, and waters of earth, to the moon
again, and from the moon to us once more--three times in opposite
directions, in the same place, without interference, and thus we
see "the old moon in the arms of the new."
_Constitution of Light._
[Illustration: Fig. 6.--White Light resolved into Colors.]
Light was once supposed to be corpuscular, or consisting of transmitted
particles. It is now known to be the result of undulations in ether.
Reference has been made to the minuteness of these undulations.
Their velocity is equally wonderful. Put a prism of glass into
a ray of light coming into a dark room, and it is [Page 25]
instantly turned out of its course, some parts more and some less,
according to the number of vibrations, and appears as the seven
colors on different parts of the screen. Fig. 6 shows the
arrangement of colors, and the number of millions of millions of
vibrations per second of each. But the different divisions we call
colors are not colors in themselves at all, but simply a different
number of vibrations. Color is all in the eye. Violet has in
different places from 716 to 765,000,000,000,000 of vibrations per
second; red has, in different places, from 396 to
470,000,000,000,000 vibrations per second. None of these in any
sense are color, but affect the eye differently, and we call these
different effects color. They are simply various velocities of
vibration. An object, like one kind of stripe in our flag, which
absorbs all kinds of vibrations except those between 396 and
470,000,000,000,000, and reflects those, appears red to us. The
field for the stars absorbs and destroys all but those vibrations
numbering about 653,000,000,000,000 of [Page 25] vibrations per
second. A color is a constant creation. Light makes momentary color
in the flag. Drake might have written, in the continuous present as
well as in the past,
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