The velocity of light was experimentally found, in 1862, by Fizeau and Foucault, each using an independent method.
I shall endeavor to make the principle of Fizeau's apparatus clear to you.
You know Wheatstone's rotating mirror, Fizeau's wheel, Plateau's perforated rotating disks, etc.
In 1845 Foucault and Fizeau took a daguerreotype photograph of the sun.
Fizeau's method will then permit us to arrive at a close approximation.
A measurement of the kind referred to was executed in 1849 by Fizeau (born at Paris in 1819).
Wheatstone's device of the revolving mirror was afterwards employed by Foucault and Fizeau to measure the velocity of light.
The only positive result ever supposed to be attained was in a very difficult polarisation observation by Fizeau in 1859.
Fizeau was unable to observe interference when the difference of path amounted to 50,000 wave lengths.
If we examine Fizeau's apparatus closely, we shall recognise in it an old acquaintance: the arrangement of Galileo's experiment.