Day R H, Bellamy S, Norman A
J Exp Psychol Hum Percept Perform. 1983 Feb;9(1):113-25. doi: 10.1037//0096-1523.9.1.113.
The Morinaga illusion is the apparent misalignment of the aligned apexes of oppositely facing angles. It is also called the Morinaga paradox because its direction is opposite to that intuitively expected from the Müller-Lyer illusion. Six experiments are reported. The first showed that the illusion is greater when the apexes are aligned obliquely than when aligned vertically or horizontally (the oblique effect); the second showed that the illusion is undiminished when the two outer angles are replaced with single lines coincident with the arms of the angles; and the third showed that the illusion is undiminished when the central apex is replaced by a dot but diminished by about half when the two outer apexes are each replaced by dots. The fourth experiment showed that the illusion also occurs with the aligned ends of parallel lines and edges of squares but not with the aligned tangential points of circles of about the same size. Experiments 5 and 6 showed that the effect is markedly greater with small, widely spaced elements. Explanations in terms of perceptual normalization to a line and perceptual assimilation have been considered. The latter explanation is the more plausible of the two, although it is conceivable that both processes contribute to the effect.
