Color Reversal shows some of the interesting effects that occur when color receptors in the eye become fatigued. The opening in a rotating wheel reveals a picture of a red tree with a green background. If the wheel spins in one direction, the picture is followed by a white card; if the wheel spins in the opposite direction, it is followed by a black card. If white follows the picture quickly enough, you will see a green tree because the red color receptors in your eyes become fatigued (white minus red equals green). If black follows, the tree will remain red, since black absorbs all color.