It is scarcely possible so to strain the imagination as to conceive
the Velocity with which Light travels. “What mere assertion will make
any man believe,” asks Sir John Herschel, “that in one second of time,
in one beat of the pendulum of a clock, a ray of light travels over
192,000 miles; and would therefore perform the tour of the world in
about the same time that it requires to wink with our eyelids, and in
much less time than a swift runner occupies in taking a single stride?”
Were a cannon-ball shot directly towards the sun, and were it to
maintain its full speed, it would be twenty years in reaching it; and
yet light travels through this space in seven or eight minutes.
The result given in the _Annuaire_ for 1842 for the velocity of light
in a second is 77,000 leagues, which corresponds to 215,834 miles;
while that obtained at the Pulkowa Observatory is 189,746 miles.
William Richardson gives as the result of the passage of light from the
sun to the earth 8´ 19″·28, from which we obtain a velocity of 215,392
miles in a second.--_Memoirs of the Astronomical Society_, vol. iv.
In other words, light travels a distance equal to eight times the
circumference of the earth between two beats of a clock. This is a
prodigious velocity; but the measure of it is very certain.--_Professor
Airy._
The navigator who has measured the earth’s circuit by his hourly
progress, or the astronomer who has paced a degree of the meridian, can
alone form a clear idea of velocity, when we tell him that light moves
through a space equal to the circumference of the earth in _the eighth
part of a second_--in the twinkling of an eye.
Could an observer, placed in the centre of the earth, see this
moving light, as it describes the earth’s circumference, it would
appear a luminous ring; that is, the impression of the light at the
commencement of its journey would continue on the retina till the
light had completed its circuit. Nay, since the impression of light
continues longer than the _fourth_ part of a second, _two_ luminous
rings would be seen, provided the light made _two_ rounds of the
earth, and in paths not coincident.