Foncault siderostat, Paris Observatory, France, 1876.
© Science Museum / Science & Society Picture Library
Photograph of a siderostat, designed by the French physicist Jean Bernard Leon Foucault (1819-1868), from the Paris Observatory. The instrument was used to direct starlight into an ancillary instrument, such as a spectroscope, for analysis. It uses a flat mirror, driven by a clockwork mechanism, to follow the stars as they move acros the sky. This siderostat was displayed at the 1876 Loan exhibition that was held on the site of today's Science Museum at South Kensington, London.