As printed on the page, is the figure on the right larger than the figure on the left?
Take a few of seconds to come up with you own answer before reading further.
The correct answer is that the figure on the right and the one on the left have exactly the same size! The corridor in which the figures are seen is drawn in perspective and appears to go into the depth plane. Your brain and perceptual system automatically interprets the picture as a three-dimensional scene, not as an image printed on a flat paper surface. In the 3-D interpretation, the person on the right is both much farther away and much larger than the person on the left. For most of us, this impression of 3-D size is overwhelming. Only visual artists and experienced photographers have developed the skill of seeing the drawing as an object on the page. For the rest of us, substitution occurs: the dominant impression of 3-D size dictates the judgment of 2-D size. The illusion is due to a 3-D heuristic.