a lens twenty-six inches in diameter, and bends all the light that
passes through it to a focus, then magnifies the image and takes
it into his eye. Or he takes a mirror, six feet in diameter, so
hollowed in the middle as to reflect all the rays falling upon it
to one point, and makes this larger eye fill his own with light.
By this larger light-gathering he discerns things for which the
light falling on his pupil one-fifth of an inch in diameter 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 aërial 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