meter would
not be sufficient. We never have seen any sun or stars; we have
only seen the light that left them fifty minutes or years ago, more
or less. Light is the aerial sprite that carries our measuring-rods
across the infinite [Page 22] spaces; light spreads out the history
of that far-off beginning; brings us the measure of stars a thousand
times brighter than our sun; takes up into itself evidences of the
very constitutional elements of the far-off suns, and spreads them
at our feet. It is of such capacity that the Divine nature, looking
for an expression of its own omnipotence, omniscience, and power of
revelation, was content to say, "God is Light." We shall need all
our delicacy of analysis and measurement when we seek to determine
the activities of matter so fine and near to spirit as light.

[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, an