ody we first mark out
a certain distance, and then measure the time which the body requires to
traverse that distance. We determine the velocity of a railway train by
the time it takes to pass from one mile-post to the next. We learn the
speed of a rifle bullet by an ingenious contrivance really founded on
the same principle. The greater the velocity, the more desirable is it
that the distance traversed during the experiment shall be as large as
possible. In dealing with the measurement of the velocity of light, we
therefore choose for our measured distance the greatest length that may
be convenient. It is, however, necessary that the two ends of the line
shall be visible from each other. A hill a mile or two away will form a
suitable site for the distant station, and the distance of the selected
point on the hill from the observer must be carefully measured.

The problem is now easily stated. A ray of light is to be sent from the
observer to the distant station, and the time occupied by that ray in
the journey is to be measured. We may suppose that the observer, by a
suitable contrivance, has arranged a lantern from which a thin ray of
light issues. Let us assume that this travels all the way to the distant
station, and there falls upon the surface of a reflecting mirror.
Instantly it will be diverted by reflection into a new direction
depending upon the inclination of the mirror. By suitable adjustment the
latter can be so placed that the light shall fall perpendicularly upon
it, in which case the ray will of course return along the direction in
which it came. Let the mirror be fixed in this position throughout the
course of the experiments. It follows that a ray of light starting from
the lantern will be returned to the lantern after it has made the
journey to the distant station and back again. Imagine, then, a little
shutter placed in front of the lantern. We open the shutter, the ray
streams forth to the remote reflector, and back again through the
opening. But now, after having allowed the ray to pass through the
shutter, suppose we try and close it before the ray has had time to get
back again. What fingers could be nimble enough to do this? Even if the
distant station were ten miles away, so that the light had a journey of
ten miles in going to the mirror and ten miles in coming back, yet the
whole course would be accomplished in about the nine-thousandth part of
a second--a period so short that even were it a t