Figure 3.19 | The Müller-Lyer Illusion | (a and b) The two vertical line segments are equal in length. However, the one with the arrow feather endings on the right appears longer than the one with the arrowhead endings on the left. (c and d) Like the illusions in Figure 3.18, this illusion is created by the brain misapplying the geometry relating the retinal image size of objects and their distance from us. Based on its experiences with receding and projecting corners as illustrated here, our brain mistakenly thinks that the line with arrow feather endings is a receding corner and thus farther from us than the line with arrowhead endings, which our brain thinks is a projecting corner. Because the retinal images of the two lines are identical, the brain incorrectly lengthens the arrow feather line in our conscious perception.

(Data from The Intelligent Eye, by R. L. Gregory, 1970, Orion Publishing Group Ltd., p. 80.)