Plate from 18th century encyclopedia showing various optical instruments. The Main illustration shows a heliostat (from helios, the Greek word for sun, and stat, as in stationary) A heliostat includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space. Other illustrations show devices for grinding and honing reflector mirrors, and the horopter. In studies of binocular vision the horopter is the locus of points in space that yield single vision. Also included are diagrams that study virtual focus of the eye and the halo.