The Necker cube is an ambiguous image or visual illusion. As you look at the cube first one corner and then another appears to be closer to you.
Used in Chap. 7: page 102
Links:
illusionsindex.org: The Illusions Index: Necker Cube- Wikipedia: Necker cube
Necker cube by BenFrantzDale - Own work, CC BY-SA 3.0, Wikimedia Commons