Jean-Bernard-Léon Foucault performed two of the most important experiments in nineteenth-century physics: the accurate determination of the velocity of light on Earth and the demonstration of Earth's rotation, using his famous pendulum. He also made important improvements to the telescope, carried out significant experiments in optics, and invented several widely used instruments. Persistently in poor health, Foucault lived a quiet life devoted to scientific experimentation and his numerous discoveries and inventions brought about major advances in several scientific fields.

Born in Paris in 1819, Foucault was the son of a book seller and publisher. A frail child, Foucault was educated at home. Planning to become a surgeon, he entered the École de Médecine. However, Foucault couldn't stand the blood and abandoned his studies. Instead, he became an assistant in clinical microscopy to Alfred Donné and coauthored a textbook with him. In 1845, Foucault succeeded...