Equatorially mounted reflecting telescope built by Leon Foucault, a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation. Foucault showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye. An equatorial mount allows a telescope to follow the rotation of the sky (celestial sphere) by having one rotational axis parallel to the Earth's axis of rotation. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion by driving one axis at a constant speed. Coloured illustration from 'The Heavens' by Amedee Guillemin, Publ. Richard Bentley, 1878.