Visual IllusionsA Distance-Size IllusionLook carefully at the image below. Which of the two people is taller?
Most people think the woman on the left looks considerably taller, yet if you click on the image you'll see that parallel lines can connect the tops and bottoms of the two figures, revealing that they're exactly the same size. This illusion illustrates the fact that the actual size an image projects onto your retina isn't the only thing that affects perceived size. The other crucial factor is distance. Objects that are farther away project smaller images onto your retina, so when you know something is far away, your visual system compensates to create a sense of perceived size that tends to correspond more closely to the actual size of things in the environment. In this case, the background of the path through Harvard Yard provides depth cues about where the two people are with respect to each other. Since the woman to the left looks farther away, she looks bigger even though the two figures are in fact the same size in the 2-D image. There are many versions of this illusion, but a particularly compelling one comes from Roger Shepard. It's hard to believe that the two monsters below are the same size, but measure them and you'll see... 
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