Cardboard Tube Syllabus I Hole in Your Hand (5 minutes or more) Take one of the tubes that you made from a full sheet of paper in your right hand. Hold it up to your right eye and look through the tube, keeping both eyes open. Now put your left hand, fingers up, palm toward your face, up against the left side of the tube, about two-thirds of the way down. Notice that you see a hole in your hand. One eye sees a hole, the other sees a hand. Your eyes and brain add the two images together, creating a hand with a hole in it! Back to main page On to "Overlapping Spots" Cardboard Tube Syllabus II Overlapping Spots (5 minutes or more) Take two round tubes that you made from full sheets of paper. Put the tubes up to your eyes and look through them at the white screen, wall, or sheet of paper. First close one eye, and then open it and close the other. Does the brightness of the spot appear the same for each eye? Move the tubes to overlap the two spots. Notice that there is a brighter area where the two spots overlap. Overlap the spots completely. Does the combined spot look brighter than either spot alone? Find out by closing one eye. When you partly overlap the two spots, your open eye and brain conclude that the sum of the two spots of light should be brighter than one spot alone. If the spots overlap completely, the brain seems to ignore one of them. Back to "Hole In Your Hand" Back to main page On to "Circles or Ovals" Cardboard Tube Syllabus III Circles or Ovals (5 minutes or more) Hold one of the round tubes up to one eye and the tube that you flattened up to the other eye. Look through the tubes at the white screen, wall, or paper. Overlap the spots. Do you see the circle or the oval? Switch the tubes and repeat. If you saw only the circle before, you may see the oval now. Your eyes and brain have trouble merging the different shapes. Most people have a dominant eye. The brain will choose to see the image that is coming from the dominant eye. Some people do not have a dominant eye, and therefore see the two shapes overlapped. The best baseball hitters do not have a dominant eye. Back to "Overlapping Spots" Back to main page On to "Lateral Inhibition" Cardboard Tube Syllabus IV Lateral Inhibition (5 minutes or more) With both eyes open, look at the white screen, wall, or paper through one of the tubes you made from a full sheet of paper. Notice that the spot of light that you see through the tube appears brighter than the wall of the tube. Do the same thing using the tube that you made from a narrow strip of paper. Notice that the spot appears darker than the wall of the tube. When light receptors in your eyes receive light, they send a signal to your brain. A receptor receiving light also sends signals to neighboring receptors, telling them to turn down their own sensitivity to light. When you look at the white wall without a tube, you see a uniform field of brightness because all the receptors are equally inhibited. When you look through the tube that you made from a full sheet of paper, the spot of light is surrounded by the dark ring of the tube. The spot appears brighter because the receptors in the center of your retina are not inhibited by signals from the surrounding dark ring. In contrast, light shines through the walls of the tube that you made from a strip of paper. When you look through this thinwalled tube, the spot appears darker because light comes through the wall of the tube, causing the receptors at the center of your retina to be inhibited. This is known as lateral inhibition. Back to "Circles or Ovals" Back to main page