In 1864 Maxwell was able to deduce from his equations the existence of electromagnetic waves with a velocity of propagation:

.
And in the course of forty years afterwards the measurement of the velocity v (the electromagnetic measurement of the velocity of electromagnetic waves) had a central role in research activity on electromagnetism.
In 1900, following numerous measurements of the velocity v, the hypothesis advanced by Maxwell on the nature of light could without any doubt be regarded as experimentally founded on the basis of the essential identity of the order of magnitude
of the kinematic velocity of light and of the electromagnetic velocity

( v=

In 1905 Einstein formulated the hypothesis (the second postulate of the Theory of Special Relativity) according to which the two velocities not only had the same order of magnitude but were identical.
It was an hypothesis that, on the basis of available data, could not in any way be considered experimentally founded; especially taking into account that the Earth's motion gives a physical reason for a discrepancy between and v [1].
It could be regarded, at most, to quote Maxwell, "not contradicted by the comparison of these results such as they are".
But Einstein's hypothesis was not followed by an experimental check as accurate as that following Maxwell's hypothesis, since from 1905 until the present electromagnetic measurements of the speed of light have not been carried out.
I consider it timely, with this paper, to retrace this history.