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