Heliostat, c 1752.

A heliostat keeps the rays of the Sun in one direction during solar observation. This instrument was attached to the front of a telescope. The Sun's rays were reflected from the lower mirror to the upper fixed mirror from where they were reflected down the telescope tube. The angle of the lower mirror was set for the latitude, and was rotated by hand to compensate for the movement of the Sun. This instrument was made by Jeremiah Sison, and is asociated with Dr Stephen Demainbray (1710-1782), the famous 18th century lecturer.