ter the predicted time. Once this
was proved, the great discovery was quickly made by Roemer, a Danish
astronomer, in 1675. When the satellite enters the shadow, its light
gradually decreases until it disappears. It is the last ray of light
from the eclipsed satellite that gives the time of the eclipse; but that
ray of light has to travel from the satellite to the earth, and enter
our telescope, before we can note the occurrence. It used to be thought
that light travelled instantaneously, so that the moment the eclipse
occurred was assumed to be the moment when the eclipse was seen in the
telescope. This was now perceived to be incorrect. It was found that
light took time to travel. When the earth was comparatively near Jupiter
the light had only a short journey, the intelligence of the eclipse
arrived quickly, and the eclipse was seen sooner than the calculations
indicated. When the earth occupied a position far from Jupiter, the
light had a longer journey, and took more than the average time, so that
the eclipse was later than the prediction. This simple explanation
removed the difficulty attending the predictions of the eclipses of the
satellites. But the discovery had a significance far more momentous. We
learned from it that light had a measurable velocity, which, according
to recent researches, amounts to 186,300 miles per second.

One of the most celebrated attempts to ascertain the distance of the sun
is derived from a combination of experiments on the velocity of light
with astronomical measurements. This is a method of considerable
refinement and interest, and although it does not so fulfil all the
necessary conditions as to make it perfectly satisfactory, yet it is
impossible to avoid some reference to it here. Notwithstanding that the
velocity of light is so stupendous, it has been found possible to
measure that velocity by actual trial. This is one of the most delicate
experimental researches that have ever been undertaken. If it be
difficult to measure the speed of a rifle bullet, what shall we say of
the speed of a ray of light, which is nearly a million times as great?
How shall we devise an apparatus subtle enough to determine the velocity
which would girdle the earth at the equator no less than seven times in
a single second of time? Ordinary contrivances for measurement are here
futile; we have to devise an instrument of a wholly different character.

In the attempt to discover the speed of a moving b