on the surface of still water after throwing in a
stone; with the sole difference that the luminous
waves are spherical, instead of circular, and
have the luminous body for their centre, instead
of the stone. Imagine a straight line proceeding
from the luminous body indefinitely into
space (like a wire stretched tight from the sun
to the earth), each luminous wave will reach
successively different points along this straight line.
The Speed of Light is the distance along this line
traversed by the wave in a given unity of time.

But that light has a rate of speed, we may
understand, without adopting any hypothesis as
to the way in which photological phenomena
are produced. If a candle, for instance, be
suddenly lighted or extinguished, that instantaneous
phenomenon may not be perceived at
the same instant at all the points of space where
it is possible to observe it. If you be quite
close to the spot, you will perceive it immediately;
if you be a long way off, a certain time
may elapse between the moment of the
phenomenon's taking place and the moment of its
perception by the eye; and that time would be
longer in proportion to the increase of the
distance between the eye and the source of light.
The photological phenomenon, therefore, in that
case, takes a certain time to traverse the distance
between the spot where it is produced and the
observer's eye. We may naturally admit that
it travels at an uniform rate; that is, traverses
equal distances in equal times. The Speed of
Light, then, would be the distance at which
an observer's eye must be from the spot at
which a luminous phenomenon is produced, in
order that a given unity of time—say a second
—may elapse between the moment of its
production and the instant of its perception.

After what has been stated, nothing seems
easier than to imagine a mode of measuring the
quickness of light. All that is required is, to
do as we do when we measure the quickness of
any other object moving at a regular pace.
Suppose you want to measure the speed of a
train running, at a steady rate, along a railway.
You take your seat in a carriage, and with a
watch in your hand which marks the seconds,
you note how many seconds it takes to travel
from one milestone to another. Say it takes
two hundred and twenty seconds to do a
mile; you divide the distance traversed, one
thousand seven hundred and sixty yards, by two
hundred and twenty. The quotient, eight,
indicates that the train travels eight yards per
second. Generally, the time employed by a body
in motion to travel a known distance, is noted;
the distance is divided by the given unity of
time; and the quotient of the division is the
speed required to be ascertained.

Experiments have been tried, by two
observers, with two lamps placed at a distance of
several miles. If the light of a lamp be
suddenly cut off, its extinction will not be perceptible
to the opposite observer until after the
interval of time required by light to traverse
that distance. But however great may be the
distance between lamps so placed on the surface
of the earth, the time which elapses between
the extinction of the light and its perception by
the observer is always found to be absolutely
inappreciable. The result is as if the speed of
light, were infinitely great, or as if light were
transmitted instantaneously. It is this experiment
which led Galileo to that conclusion.

But the reason of the negative result is, that
the speed of light, without, being infinite, is
actually enormously great. Light travels, in one
second, a distance equal to seven and a half times
the earth's circumference. For the lamp
experiment it therefore employs only so small a
fraction of a second as to be utterly immeasurable
by ordinary modes of observation.

Everybody must have remarked that sound
takes time to travel to a distance. The varying
interval between a flash of lightning and the
thunder which follows it, is a familiar example.
Watch a woodman felling a tree. Unless you
be quite near him, you will see the blow given
by the axe before you hear the stroke. In
consequence of the enormous swiftness of light, we
may assume that you see the blow at the very
instant it is given. The interval, therefore,
between seeing and hearing the blow, is the time
which sound takes to travel from the tree to the
spot where you are; and this very appreciable
interval becomes longer the further you are
removed from the working woodman. Retire to
a spot where the interval of time shall be exactly
a second, measure the distance from that spot to
the tree, and you have the speed of sound, or the
number of yards which sound travels in a second.

A very strong analogy exists between the
progressive transmission of distant photological
phenomena and the progressive transmission of
sound through the atmosphere; only light travels
incomparably quicker. And in order to understand
the transmission of light, we shall do well
to bear in mind the circumstances connected with
the transmission of sound.

The discovery of the speed of light was made
by Roëmer, a Danish astronomer, while observing
the eclipses of Jupiter's first satellite (the
one nearest to the planet). Jupiter, the largest
of the planets which, like the earth, revolve
round the sun, is accompanied by four moons or
satellites. Planets and their satellites have no
proper light; we see them, only because the sun
shines on them. If any obstacle prevent the
sunlight from falling on any of these heavenly
bodies, it becomes invisible, or, in other words,
is eclipsed. This happens frequently with each
of Jupiter's satellites. The satellite, revolving
round its planet, gets behind it, relatively to the
sun, from time to time. The planet, then, by
intercepting the sun's rays, eclipses the satellite
during a certain time. The phenomenon is
exactly similar to the eclipses of the moon which
we occasionally witness.

The eclipses of Jupiter's satellite are much
more frequent than those of the others, in
consequence of the rapidity with which it completes
its orbit. They occur at intervals of about
forty-two hours and a half. Moreover, the
eccentricity of this satellite's orbit being