in Geometry, denotes a plain surface, or one that lies evenly between its bounding lines; and as a right line is the shortest extension from one point to another, so a plane surface is the shortest extension from one line to another. In astronomy, conic sections, &c., the term plane is frequently used for an imaginary surface, supposed to cut and pass through solid bodies; and on this foundation is built the whole doctrine of conic sections. In mechanics, planes are either horizontal,—that is, parallel to the horizon,—or inclined to it. In optics the planes of reflection and refraction are those drawn through the incident and reflected or refracted rays. In perspective we meet with the perspective plane, which is supposed to be pellucid, and perpendicular to the horizon; the horizontal plane, supposed to pass through the spectator's eye, parallel to the horizon; and the geometrical plane, likewise parallel to the horizon, in which the object to be represented is supposed to be placed. The plane of projection in the stereographic projection of the sphere, is that on which the projection is made, corresponding to the perspective plane.