Optics, etc. †right shadow: the figure thrown by an opaque body upon a horizontal plane to which it is perpendicular. †contrary shadow, †versed shadow…

A reflecting surface (originally that of a fluid at rest) used in taking elevations, and now usually forming part of a sextant.

a small mirror of plate-glass fixed on the frame of a quadrant or sextant, having one half unsilvered so that the horizon-line or other object can…

A filter of darkened glass fitted to a sextant, telescope, etc., to reduce the intensity of the sun's light during observation. Now chiefly historical…

a level reflecting surface, usually of mercury, used in taking altitudes; also called artificial horizon.