Perimeter

{per - im’- eter) [irepi, around; fiETpov. measure]. An instrument for measuring the extent of the field of vision. It consists ordinarily of a flat, narrow metal plate, bent in a semicircle, graduated in degrees, and fixed to an upright at its center by a pivot, on which it is movable. Variously colored discs are moved along the metal plate and the point noted at which the person, looking directly in front of him, distinguishes the color. « 488