insensible, its movement round the planet is
almost rigorously circular and uniform. The
result is a great regularity in the successive
reproduction of the eclipses, so much so that
Jupiter and his satellites have been compared to
a clock hung up in the sky. Their motions are
easily observed from the earth by making use of
telescopes of no great power.

The eclipses of Jupiter's first satellite having
been exactly calculated, it was found that, at
certain times, he came out of the shadow sooner,
and at other times later than he ought. By
comparing those times one with the other,
Roëmer found that the satellite emerged later
precisely when the earth, in consequence of her
annual movement, was running away from
Jupiter; which led him to form the happy
conjecture that light might employ a certain time
to travel from one point to another. That
granted, if the satellite appeared to emerge
later in proportion to our increased distance
from his planet, it was not that he really
emerged later, but that his light required a
longer time to traverse a wider interval of space.
On the other hand, he ought to emerge sooner,
in proportion as the earth approached nearer to
Jupiter. Roëmer made calculations and
predictions, which fully bore out his supposition.
The motion of light was made known to the
public in December, 1676.

Nevertheless, Roëmer's conclusions, though
generally admitted by learned men, were not
entirely free from objection and cavil. They
were drawn from an isolated instance, and
required further confirmation. The confirmation
came in a remarkable way. It was supplied by
the accidental discovery of Aberration, made fifty
years afterwards (when he was not looking for it)
by our celebrated astronomer, James Bradley.

Aberration consists in an apparant displacement
experienced by all the fixed stars and the
planets, in consequence of the complication
of the earth's velocity with the velocity of
light. It may be illustrated by several familiar
examples. When a sportsman fires at a
swift-flying bird, he takes aim, not exactly at the bird
itself, but a little in front of it, allowing for the
bird's progress in the air during the time the
shot takes to reach it. The shot, therefore, hits
the bird at a point of its course which it had
not yet reached at the actual moment when the
gun was fired.

Substitute, in imagination, the light of a star
for the shot, and the earth rolling along her
orbit for the bird flying through the air, and it
is clear that the light which hits the earth at
any moment will appear to proceed from a point
different to that from which it really issues.

Again: Suppose that we are in a railway
carriage on a rainy day, with no wind, so that
the drops of rain fall perfectly perpendicularly.
While the carriage stops, we see the drops of
rain fall, exactly as they do fall, vertically.
But as soon as the carriage is in motion, things
present a different aspect. The rain-drops
appear to fall obliquely, as if the carriage were
running against a wind, which compelled the
straight lines described by the falling rain
to slope in the direction of its action.

Similar circumstances present themselves,
when the rain, instead of falling vertically, falls
obliquely in consequence of the action of a gale.
If the wind drive the rain against the carriage,
that is to say, contrary to the direction in which
the carriage is moving, that motion causes the
obliquity of the rain to appear greater than it
really is. If the wind drive it the same way the
carriage is travelling, the inclination with which
it really falls, appears diminished. If the
carriage be running round a circular railroad
(like the earth in her orbit), the influence of its
motion on the apparent direction of the falling
rain-drops changes continually and progressively;
so that the rain appears to be successively
driven from different points of the sky,
which points are situated around the point
whence it actually comes.

Substitute the light of the stars for the rain,
and the earth in her orbit for the circular railway,
and the Aberration discovered by Bradley
is not difficult to comprehend. He was puzzled
by changes in the places of the stars.
Referring them to the real cause, he examined in
detail what ought to be the result, on the
apparent position of the stars, of the earth's
velocity combined with the velocity of light;
and he found a perfect agreement between what
is and what ought to be. The successive
transmission of light in space became an
incontestable fact.

Hitherto, the motion of light had only been
proved by its traversing enormous distances,
such as those from the earth to the sun, from
the earth to Jupiter at opposite points of their
respective orbits, and from the sun to the fixed
stars. Arago—following up a hint of Wheatstone's
who had conceived the notion of
employing a rapidly revolving mirror, in order
to render sensible excessively small intervals
of time—imagined a method of making it
evident by experiments carried out in a limited
space. He proposed, by means of a revolving
mirror, through mathematical speculations too
complicated to quote here, to decide not only
the speed of light, but the grand question
whether light is a body or an undulation. He
felt sure beforehand that the latter would prove
to be the case.

Afterwards, M. Fizeau invented a plan for
measuring the speed of light by the help of a
lamp behind which the observer should be
posted, a distant mirror to reflect it, and a
circular disk with a toothed edge, which could
be made to revolve with increasing rapidity.
The teeth on the edge of the disk were for the
purpose of alternately stopping and allowing to
pass, the light reflected from the mirror. Again,
the principles on which the experiment was
founded are too complex for citation in this
paper, but are fully stated in the Annuaire.
M. Fizeau, repeating his experiment, assigned to
light, a speed differing but very slightly from
that previously deduced from astronomical
phenomena. The average of twenty-eight