optical square

A reflecting instrument used by surveyors and others for laying off lines at right angles to each other. It consists of a box containing two plane mirrors, set at an angle of 45° with each other, so that the image of an object reflected from one mirror to the other will form an angle of 90° with its true position, indicating the correct direction in which a perpendicular offset to the main line shall be measured. IMG:content-0328.png:[blocks in formation] IMG:content-0329.png:[blocks in formation] or optics.