ABSTRACT
This chapter presents a series of illusions in which the apparent shape or orientation of stimuli systematically differs from what would be expected on the basis of actual physical measurements. Illusions of direction and location are quite striking when encountered. Illusions of direction involve distortions in which the direction or orientation of a contour or figure is altered. In 1860 Zoellner noticed an interesting illusion of direction. If a line is intersected by a number of other lines that form acute angles with it, the main line seems to be tilted in the direction opposite to the direction of these intersecting elements. One of the oldest and the most familiar is the Wundt–Hering illusion in which two horizontal lines are drawn straight and parallel, yet they appear to be curved, such that the lines bow out at the center. Subjective contour illusions involve the perception of contours that are not physically present in the array.
