Stare at the eye of the red parrot while you count slowly to 20, then look immediately at one spot in the empty bird cage. The faint, ghostly image of the bluegreen bird will appear in the cage.

Try the same thing with the green cardinal. A faint magenta bird will appear in the cage.

The ghostly birds you see here are called after images. An after image is an image that stays with you even after you have stopped looking at the object. The back of your eye is lined with light sensitive cells, called cones, which are only sensitive to certain colors of light. When you stare at the red bird, your red-sensitive cells start to grow tired and lose their sensitivity. So, when you shift your gaze suddenly to the white background of the bird cage, you see white minus red where the red-sensitive cells have become fatigued. White light minus red light is blue-green light. That's why the after image you see is blue-green-and in the shape of a parrot. The same thing happens when you stare at the green bird, but this time it's the green-sensitive sensors that get tired. White,minus green light is magenta light, so you see the after image as a magenta cardinal.