Figure 3.16 The Ponzo and Corridor Illusions (a) The two red bars in the Ponzo illusion are identical in size. However, the top bar appears to be larger. (b) The three silhouettes appear to increase in size from the one at the bottom of the figure to the one at the top, but all three are identical in size. To convince yourself that the two bars and the three silhouettes are identical, trace one of the bars (silhouettes) on a thin sheet of paper and then slide the tracing over the other bar (two silhouettes). 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 three silhouettes in (b).

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