Foucault, Jean Bernard Léon (1819–1868)
Jean Foucault was a French physicist and physician who was the first to show how a pendulum can track Earth's rotation (see Foucault pendulum). He also invented the gyroscope (1852), showed that the speed of light is less in water than in air (1850), and improved the mirrors of reflecting telescopes. More important even than his perfection of the silver-on-glass process was his development of a simple technique, now known as the Foucault knife-edge test, for determining the exact figure of a mirror. With Armand Fizeau, he took the first clear photograph of the Sun. He also noted the occurrence of eddy currents which became known as Foucault currents.