That an accurate notion of the vertical line may be obtained, the plane of the picture must be supposed to be perpendicular to the horizontal plane. If a straight line be drawn from the spectator's eye, perpendicular to this plane of the picture, that line will fall upon the plane at a point in the horizontal line directly opposite to the eye. This point, C, is called the centre of the picture, or centre of view. In reference to the eye of the spectator, every straight line perpendicular to the plane of the picture appears to converge towards this point or centre. The line which, drawn from the eye of the spectator, determines this centre C, is called the vertical line. It is a straight line through S, perpendicular to the horizontal line, and the base of the picture; and is represented by the line EC.