<P> Fizeau--Foucault apparatus is a term sometimes used to refer to two types of instrument historically used to measure the speed of light . The conflation of the two instrument types arises in part because Hippolyte Fizeau and Léon Foucault had originally been friends and collaborators . They worked together on such projects as using the Daguerreotype process to take images of the Sun between 1843 and 1845 and characterizing absorption bands in the infrared spectrum of sunlight in 1847 . </P> <P> In 1834, Charles Wheatstone developed a method of using a rapidly rotating mirror to study transient phenomena, and applied this method to measure the velocity of electricity in a wire and the duration of an electric spark . He communicated to François Arago the idea that his method could be adapted to a study of the speed of light . Arago expanded upon Wheatstone's concept in an 1838 publication, emphasizing the possibility that a test of the relative speed of light in air versus water could be used to distinguish between the particle and wave theories of light . </P>

Which instrument is used to measure the speed of light