d 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