Geometric illusions in reading graphs

Abstract

The reading of a point on a graph is influenced by the slope of a line passing from the point in the direction of the calibrated edge against which the point is read. The sloping line is perceived to be too nearly at right angles to the calibrated edge. The direction of the error corresponds to the direction of the slope of the line; the size of the error increases with the distance of the point from the calibrated edge. A number of sloping lines lying between the point and the correct reading on the calibrated edge will increase the size of the illusion. The illusion is of about half the size of the conventional Poggendorff illusion. © 1985 Psychonomic Society, Inc.