On comparing the velocities of solar, stellar, and terrestrial light,
which are all equally refracted in the prism, with the velocity of the
light of frictional electricity, we are disposed, in accordance with
Wheatstone’s ingeniously-conducted experiments, to regard the lowest
ratio in which the latter excels the former as 3:2. According to the
lowest results of Wheatstone’s apparatus, electric light traverses
288,000 miles in a second. If we reckon 189,938 miles for stellar
light, according to Struve, we obtain the difference of 95,776 miles as
the greater velocity of electricity in one second.
From the experiment described in Wheatstone’s paper (_Philosophical
Transactions_ for 1834), it would appear that the human eye is capable
of perceiving phenomena of light whose duration is limited to the
millionth part of a second.
In Professor Airy’s experiments with the electric telegraph to
determine the difference of longitude between Greenwich and Brussels,
the time spent by the electric current in passing from one observatory
to the other (270 miles) was found to be 0·109″ or rather more than
_the ninth part of a second_; and this determination rests on 2616
observations: a speed which would “girdle the globe” in ten seconds.