ion." It arises from a combination of the two
movements of the earth round the sun and of the light-waves through the
ether. If the earth stood still, or if light spent no time on the road
from the stars, such an effect would not exist. Its amount represents
the proportion between the velocities with which the earth and the
light-rays pursue their respective journeys. This proportion is,
roughly, one to ten thousand. So that here again, if we knew the rate
per second of luminous transmission, we should also know the rate per
second of the earth's movement, consequently the size of its orbit and
the distance of the sun.

But, until lately, instead of finding the distance of the sun from the
velocity of light, there has been no means of ascertaining the velocity
of light except through the imperfect knowledge possessed as to the
distance of the sun. The first successful terrestrial experiments on the
point date from 1849; and it is certainly no slight triumph of human
ingenuity to have taken rigorous account of the delay of a sunbeam in
flashing from one mirror to another. Fizeau led the way,[760] and he was
succeeded, after a few months, by Leon Foucault,[761] who, in 1862, had
so far perfected Wheatstone's method of revolving mirrors, as to be able
to announce with authority that light travelled slower, and that the sun
was in consequence nearer than had been supposed.[762] Thus a third line
of separate research was found to converge to the same point with the
two others.

Such a conspiracy of proof was not to be resisted, and at the
anniversary meeting of the Royal Astronomical Society in February, 1864,
the correction of the solar distance took the foremost place in the
annals of the year. Lest, however, a sudden bound of four million miles
nearer to the centre of our system should shake public faith in
astronomical accuracy, it was explained that the change in the solar
parallax corresponding to that huge leap, amounted to no more than the
breadth of a human hair 125 feet from the eye![763] The Nautical Almanac
gave from 1870 the altered value of 8.95", for which Newcomb's result of
8.85", adopted in 1869 in the Berlin Ephemeris, was substituted some ten
years later. In astronomical literature the change was initiated by Sir
Edmund Beckett in the first edition (1865) of his _Astronomy without
Mathematics_.

If any doubt remained as to the misleading character of Encke's
deduction, so long implicitly trusted in, it