Heliostat, Jacobus van der Cloesen, Leiden, c. 1730
Inv.no. V09233

For optical experiments eighteenth-century physicists had to rely on sunlight; strong lamps did not yet exist. One great inconvenience was the movement of the sun across the sky, so that the direction and angle of the light falling into the room was constantly changing. The Leiden professor Willem Jacob 's Gravesande (1688-1742) therefore designed this heliostat or 'sun stiller'. This instrument consists of a diagonally mounted mirror, which caught the sunlight. The mirror was linked to a clockwork mechanism in such a way that it revolved exactly in time with the sun. In this way 's Gravesande obtained a shaft of light that remained stationary for a long period, with which he could experiment without interruption.