A Mini Maglite or a penlight with an incandescent bulb (CAUTION: do not try this with an LED Mini Maglite; the bulb is too bright to hold so close to your eye)
3/8-inch dowel, cut to the length of an AA battery
A room that you can darken
A sheet of black construction paper
Wrap the dowel completely in aluminum foil (click to enlarge image).
Unscrew the back of the Mini Maglite and remove one AA battery. Replace the battery with the aluminum foil–wrapped dowel (click to enlarge diagram).
Unscrew and remove the front cover of the Mini Maglite. The light will come on as a dim point source.
Darken the room by turning off the lights and closing the shades.
To Do and Notice
Hold the Mini Maglite about 1 cm in front of, and slightly below, the center of one of your pupils. (Don't poke yourself in the eye! It's best to wear eye protection and hold the light just in front of it.)
Look at the sheet of black construction paper (click to enlarge the diagram below). The black paper should fill your field of view.
While looking straight ahead, move the light slowly from side to side a short distance, about 0.5 cm in each direction. Do not follow the motion of the light with your eye—keep looking straight ahead as you move the light back and forth for 20 seconds.
Notice what appears in your field of vision. It will look like the branches of a tree, or the branching of a river viewed from high above.
What’s Going On?
What you see is the pattern of arteries and veins that supply blood to your retina. The network of blood vessels spreads out from the blob of your blind spot.
In human eyes, the blood supply of the retina is located in front of the retina. That is, light passes through the blood supply on its way to the retinal detectors. We don't see the retinal blood supply because it never changes, and our eyes ignore unchanging images.
The point source of light casts a shadow of the retinal blood supply on your retina. When you move the point of light from side to side, the shadow moves. You can then see the changing shadow.
Glaucoma is a disease of the eye in which pressure builds up inside the eyeball. The pressure squeezes on the retinal blood supply network, reducing blood flow to the periphery of the eye and resulting in the death of the retina, starting at the periphery and working in toward the center. One of the symptoms of glaucoma is tunnel vision.