Exploratorium: Science Snacks: Fading Dot

(5 minutes or less)Use the blue paper to make a 1 inch (2.5 cm) dot, and place the dot in the center of the pink paper. Cover the paper with a sheet of waxed paper. Look through the waxed paper at the colored papers below. Lift the waxed paper from the pink paper until you see very faint blue color in a field of pale pink.

(15 minutes or more)Stare at a point next to the fuzzy dot for a while without moving your eyes or your head. The blue will gradually fade into the field of pink. As soon as you move your head or eyes, notice that the dot reappears. Experiment with other color combinations.

Even though you are not aware of it, your eyes are always making tiny jittering movements. Each time your eyes move, they receive new information and send it to your brain. You need this constant new information to see images.

Your eyes also jitter when you look at this dot, but the color changes at the edge of the dot (as seen fuzzily through the waxed paper) are so gradual that your eyes can't tell the difference between one point on the dot and a point right next to it. Your eyes receive no new information, and the image seems to fade away. If the dot had a distinct border, your eyes would immediately detect the change when they jittered, and you would continue to see the dot.

You may have noticed that, although the dot fades, just about everything else in your field of vision remains clear. That's because everything else you see has distinct edges.

For more information, we suggest you read the sections on lateral inhibition and chromatic lateral inhibition in Seeing the Light, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).