APPARENT LENGTH OF ARMS OF ACUTE AND OBTUSE ANGLES, AND COMPONENTS OF MULLER-LYER-ILLUSION

Mueller-Lyer first drew attention to differences between the apparent length of lines bounding angles of different sizes. This simple form of his illusion was investigated in 2 experiments with human subjects in which the angle between a horizontal 50 mm test line and an attached inducing line was varied between 0 and 180.degree. in 15.degree. steps. With acute angles the test line was underestimated relative to a plain line and with obtuse angles overestimated. Maximum effects were found for angles of about 30 and 150.degree. with the magnitude of overestimation exceeding that of underestimation. In 2 later experiments the contribution of the angle effect in more complex figures was investigated using figures consisting of 1, 2 and 4 angles, the last being the conventional Mueller-Lyer figures. In acute angle figures the illusion appeared to be due to 2 components, that due to angle and that due to the longitudinal space enclosed between the obliques but in obtuse angle figures it appeared to derive from the additive effect of angles. The implications of these findings for an explanation of the Mueller-Lyer illusion were discussed.