The sun is the cheapest and most intense source of white light available and, starting from the 18th century, sunlight was widely use in physical experiments and continued to be used even when gas lamps, oxygen and hydrogen lamps and the electric arc were widespread.
A heliostat is a device that was used to project into a dark room, through a large hole in a window, a beam of sunlight reflected by a rectangular mirror that could be turned around two orthogonal axes.
The exemplar presented here consists of a thick square brass plate (25 x 25 cm) with a central, circular aperture (diameter 9.5 cm) around which is a metal ring painted black holding two arms, they too painted black, orthogonal to the plane of the plate, which hold a flat rectangular mirror (12 x 27.5 cm) at the centre of the two long sides.
The mirror can turn on an axis perpendicular to the two arms passing through the two pins and, together with the entire ring, around an axis parallel to the two arms that support it. ...continues