On the common stimulus condition and explanation of the Müller‐Lyer,Poggendorff and Zöllner illusions: The basis for a class of geometrical illusions

Although the Müller‐Lyer, Poggendorff and Zöllner figures and the illusions with which they are associated– those of length, misalignment and non‐parallelism, respectively – are quite different, all three are attributable to the same basic effect, that of the difference in the apparent length of equal lines forming acute and obtuse angles. The role of this basic affect, reported originally by Müller‐Lyer but overlooked by him and others as a possible cause of the three illusions, is identified and discussed. It is suggested that the demonstration of a basic stimulus condition (acute and obtuse angles) and its associated illusion (difference in apparent length) generating a class of “higher order” geometric illusions has implications not only for a closer understanding of the three illusions under consideration but for other classes of illusory phenomena that might also share a common basis.