1622-1691

French astronomer and physicist who, with Jean Picard, fashioned and made systematic observations with an improved micrometer (1666). Consisting of two parallel hairs whose separation could be varied by a precision screw, their instrument allowed measurements of image sizes at a telescope's focal point. They also contributed to the systematic use of telescopic sights (1667-71). By forming one vacuum inside another, Auzout also demonstrated air pressure is responsible for the rise of mercury in barometers (1647).