FOUCAULT, LÉON. (JEAN BERNARD LEON). - THE VELOCITY OF LIGHT DETERMINED.

Détermination expérimentale de la vitesse de la lumière; parallaxe du Soleil. (Séance du Lundi 22 septembre 1862). (+) Détermination expérimentale de la vitesse de la lumière; description des appareils. (Séance du Lundi 24 Novembre 1862). (2 papers).

Paris, Mallet-Bachelier, 1862. 4to. No wrappers. In: "Comptes rendus hebdomadaires des séances de l’Académie des sciences", Vol. 55, No 12 a. 21. Pp. 481--519 a. pp. 781-803. (Entire issues offered). With title-page to vol. 55. Foucault's papers: pp. 501-503 a. pp. 792-796. Clean and fine.

First printing of Foucault's famous experiments on the velocity of light with the description of his improved equipment, the rotating mirror. Foucault's method was later developed by Michelson and Morley in their famous experiment in 1887.

"Foucault’s first experiment, carried out in 1850 and written up in full in his doctoral thesis of 1853, was purely comparative; he announced no numerical values until 1862. Then, with an improved apparatus, he was able to measure precisely the velocity of light in air. This result, significantly smaller than Fizeau’s of 1849, changed the accepted value of solar parallax and vindicated the higher value which Le Verrier had calculated from astronomical data. Foucault’s turning-mirror apparatus was the basis for the later determinations of the velocity of light by A. A. Michelson and Simon Newcomb."(DSB).

Leon Foucault, used a similar method to Fizeau. He shone a light to a rotating mirror, then it bounced back to a remote fixed mirror and then back to the first rotating mirror. But because the first mirror was rotating, the light from the rotating mirror finally bounced back at an angle slightly different from the angle it initially hit the mirror with. By measuring this angle, it was possible to measure the speed of the light. Foucault continually increased the accuracy of this method over the years. His final measurement in 1862 determined that light traveled at 299,796 Km/s.

Magee "A Source Book in Physics", p. 342 ff. and "Source Book in Astronomy", p. 282 ff.

Order-nr.: 49458