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, then we will begin to see a green tree because the color receptors in our eyes become fatigued from seeing at red (white minus red equals green). If black follows, the tree remains red, as black absorbs all color.