in Optics, a privation or diminution of light, by the interposition of an opaque body; or a plane where the light is either altogether obstructed, or greatly weakened, by the interposition of some opaque body between it and the luminary.

in Perspective. The appearance of an opaque body, and a luminous one whose rays diverge, being given; to find the just appearance of the shadow, according to the laws of perspective. The method is this. From the luminous body, which is here considered as a point, let fall a perpendicular to the perspective plane or table, that is, find the appearance of a point upon which a perpendicular, drawn from the middle of the luminary, falls on the perspective plane; and from the several angles, or raised points of the body, let fall perpendiculars to the plane. These points, on which the perpendiculars fall, connect by right lines with the point upon which the perpendicular fell from the luminary falls, and continue the lines to the side opposite to the luminary. Lastly, through the raised points draw lines through the centre of the luminary, intersecting the former; the points of intersection are the terms or bounds of the shadow.