A study of geometrical illusions

Examines the Muller-Lyer (ML) and Poggendorff (PG) illusions. Overestimation within the ML figure is accompanied by underestimation of the space outside it, and vice versa. Distances on the acute-angle side are underestimated, and on the obtuse-angle side are overestimated. Evidence shows that the PG illusion is not due to a false estimation of angles. It disappears when the intercepted line of the figure is horizontal or vertical. The movement hypothesis states that with increasing intensity of the sensation of movement, estimation of distance increases. Disappearance of the PG illusion at different positions of the figure reveals that lines produce illusions only when they affect the direction of attention. Judgment of the length of the angle's sides, and the distance between the sides at a certain distance from the vertex, are important in angle-size estimation.