Definition

A system of wires, hairs, threa­ds, etched lines, or the like, placed normal to the AXIS of a TELESCOPE at its principal FOCUS, by means of which the TELESCOPE is sighted on a STAR, SIGNAL, or TARGET, or by means of which appropriate readings are made on some SCALE, such as a LEVELLING or STADIA ROD. See also CROSS WIRES, GRATICULE, DIA­PHRAGM. In PHOTOGRAMMETRY, a mark such as a cross or system of lines lying in the image plane of a viewing apparatus and used singly as a refer­ence mark in certain types of monocular instruments or as one of a pair to form a FLOATING MARK as in certain types of STEREOSCOPES.