This distinguished physicist has submitted the Velocity of Light
to terrestrial measurement by means of an ingeniously constructed
apparatus, in which artificial light (resembling stellar light),
generated from oxygen and hydrogen, is made to pass back, by means of
a mirror, over a distance of 28,321 feet to the same point from which
it emanated. A disc, having 720 teeth, which made 12·6 rotations in a
second, alternately obscured the ray of light and allowed it to be seen
between the teeth on the margin. It was supposed, from the marking of
a counter, that the artificial light traversed 56,642 feet, or the
distance to and from the stations, in 1/1800th part of a second, whence
we obtain a velocity of 191,460 miles in a second.[12] This result
approximates most closely to Delambre’s (which was 189,173 miles), as
obtained from Jupiter’s satellites.
The invention of the rotating mirror is due to Wheatstone, who made
an experiment with it to determine the velocity of the propagation
of the discharge of a Leyden battery. The most striking application
of the idea was made by Fizeau and Foucault, in 1853, in carrying
out a proposition made by Arago, soon after the invention of the
mirror: we have here determined in a distance of twelve feet no
less than the velocity with which light is propagated, which is
known to be nearly 200,000 miles a second; the distance mentioned
corresponds therefore to the 77-millionth part of a second. The
object of these measurements was to compare the velocity of light
in air with its velocity in water; which, when the length is
greater, is not sufficiently transparent. The most complete optical
and mechanical aids are here necessary: the mirror of Foucault
made from 600 to 800 revolutions in a second, while that of Fizeau
performed 1200 to 1500 in the same time.--_Prof. Helmholtz on the
Methods of Measuring very small Portions of Time._