PRINCIPAL

, in Arithmetic, or in Commerce, is the sum lent upon interest, either simple or compound.

Principal Point, in Perspective, is a point in the perspective plane, upon which falls the principal ray, or line from the eye perpendicular to the plane. This point is in the intersection of the horizontal and vertical planes; and is also called the point of sight, and point of the eye, or centre of the picture, or again the point of concurrence.

Principal Ray, in Perspective, is that which passes from the spectator's eye perpendicular to the picture or perspective plane, and so meeting it in the principal point.