Illusion of length in acute- and obtuse-angle figures.

The apparent expansion and contraction of the length of the arms of an obtuse (160 degrees) and an acute (20 degrees) angle were measured as a function of the relative lengths of the arms. 20 subjects reproduced the apparent length of the fixed-length (50-mm) horizontal lower arm of the angle. The length of the oblique arm was varied from 10 to 90 mm in 20-mm steps. The expansion of the obtuse-angle horizontal arm and the contraction of the acute-angle horizontal arm decreased as a function of the length of the oblique arm. The acute-angle illusion did not reverse from apparent contraction to apparent expansion for the longer oblique arms. These results are relevant to assimilation theories of visual-length distortions; they suggest that the assimilatory processes mediating the angle-length illusion are confined mainly to spatial regions near the vertex of the angle.