Apparent length of the arms of acute and obtuse angles, and the components of the muller-lyer illusion

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Abstract
Muller‐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 Experiments 1 and 2 in which the angle between a horizontal 50‐mm test line and an attached inducing line was varied between 0 and 180 in 15 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 with the magnitude of overestimation exceeding that of underestimation. In Experiments 3 and 4 the contribution of the angle effect in more complex figures was investigated using figures consisting of one, two and four angles, the last being the conventional Müller‐Lyer figures. In acute angle figures (Experiment 3) the illusion appeared to be due to two 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 Müller‐Lyer illusion are discussed.