Figure 3.18 | The Ponzo and Terror Subterra Illusions | (a) The two red bars in the Ponzo illusion are identical in size. However, the top bar appears to be larger. (b) The two monsters in the Terror Subterra illusion are identical in size, but the pursuing monster (the top one) appears to be larger. To convince yourself that the two bars and the two monsters are identical, trace one of the bars (monsters) on a thin sheet of paper and then slide the tracing over the other bar (monster). Both illusions seem to be caused by the brain’s misinterpretation of the relative distance from us of the two bars in (a) and the two monsters in (b).
((a) Sorapop Udomsri/Shutterstock; (b) Mind Sights: Original Visual Illusions, Ambiguities, and Other Anomalies, by R. N. Shepard, 1990, New York: W.H. Freeman/Henry Holt.)