FIZEAU, H. (ARMAND HIPPOLYTE). - THE FIRST TERRESTRIAL MEASUREMENT OF THE VELOCITY OF LIGHT.

Sur une expérience relative à la vitesse de propagation de la lumière. (Séance du Lundi 23 Juillet 1849).

Paris, Bachelier, 1849. 4to. No wrappers. In "Comptes rendus hebdomadaires des séances de l’Académie des sciences", Vol. 29, No 4. With htitle a. titlepage to vol. 29. Pp. 65-96 (entire issue offered). Fizeau's paper: pp. 90-92. Stamps on title-page. Clean and fine.

First printing of the paper in which Fizeau describes how he produced the first terrestrial measurement of the speed of light, using a rapidly rotating toothed wheel to break a light signal into continuous pulses whose speed could then be estimated. That was the most accurate measurement to-date of the speed of light. This method was later used by Albert Michelson.

"Fizeau was not satisfied merely with determining the relative velocities of light. He wanted to measure with some precision the absolute velocity. In 1849 he had conceived an ingenious mechanism that would enable him to achieve his goal: a large toothed wheel was spun rapidly about its axis, and a beam of light sent through the spaces between the teeth was reflected back to its source by a fixed mirror. When the wheel was rotated rapidly enough, the intermittent light rays returning from the mirror intersected the path of the teeth and thus became invisible to the observer stationed behind the wheel. As the mechanism was turned faster and faster, the light reappeared and disappeared alternately. The time required for the light to travel through the carefully measured distance was a simple function of the angular displacement of the wheel.
In 1849 Fizeau made a trial of his new method between his father’s house at Suresnes and Montmartre. The figure he obtained for the speed of light (about 315,000 kilometers per second) was not quite as accurate as the results of astronomical calculations, but the practicability of the method was established and became the basis of the more precise determinations made by Alfred Cornu in the 1870’s." (DSB).

Parkinson "Breakthroughs" 1849 P.

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