Scientists have observed that an eye which has been stressed while looking at a certain color will generate the complementary color when relaxed. This is known as an afterimage. The part of the eye responsible for this color production is called the cone.
2 3x5" lined white index cards
yellow, black, and navy blue markers
(A) For the teacher: Put one index card aside. With the other you are going to make a version of the American flag. Holding the card horizontally, mark where the star field will be. Make a square in the upper left corner of the card. The dimensions should be 2" wide by 1 5/8" long. (1 5/8" is approximately the distance from the top of the card down to the sixth blue line.)
Since there are only 12 lines -- or stripes -- on the card, divide the top space lengthwise to make the 13th stripe needed for the flag.
Use the yellow marker to fill in the background of the star field. Use the blue marker to color every other stripe, starting with the top one and ending with the bottom. Use the black marker to draw the 50 stars (alternating rows of 6 then 5 stars) and also to fill in the remaining stripes.
(B) For the teacher and students: In good light, stare at the lower right hand corner of the star field for at least 30 seconds (have someone else keep time). Quickly place the blank side of the other index card over the flag. Stare in the same spot as before. What do you see? (The American flag in red, white, and blue.)