Size of the Mueller-Lyer illusion as a function of its dimensions: Theory and data

In tests with naive subjects, it was found that: the apparent length of a test shaft (1) increases with angle of wing to shaft to a maximum of about 160° (wings out), and (2) is an inverted-U function of wing length, with maximum when wing length equals shaft length; (3) all effects are approximately proportional to shaft length; and (4) the angle exerts its effect within a small region near the tip of the shaft, the region extending about one-seventh the shaft length. The effects apparently do not result from simple distortion of the retinal image, but some may result from assimilation or confusion in coding image into lengths, and some effects apparently arise as lengths are combined into judgments. The new data are shown to agree in quantitative detail with results collected from 1896 to the present.