The speed of light had been measured for the first time by Hippolyte Fizeau (1819-1896) in 1849, but his measurement lacked precision. Foucault remeasured it using his rotating mirror in the large meridian hall (today the Salle Cassini) and found a value of 298,500 kilometres per second, close to the currently accepted 299,782.458 km/s (186,000 miles per second).

caption : Rotating Mirror - credits : Observatoire de Paris