Wrap the dowel in aluminum foil.
Unscrew the back of the Mini MagLite and remove one AA battery. Replace the battery with the aluminum foil-wrapped dowel.
Unscrew and remove the front of the Mini MagLite.
The light will come on as a dim point source.
Darken the room- turn off the lights and close the shades.
Don't poke yourself in the eye!
It is best to wear eye protection and hold the light just in front of it.
Hold the Mini MagLite about 1 cm in front of, and slightly below the center of, the pupil of one of your eyes.
Look at the sheet of black construction paper. The black paper should fill your field of view.
Move the light slowly from side to side a short distance (0.5 cm).
Do not follow the motion of the light with your eye.
Keep doing this for 20 seconds.
Notice the network that appears. It will look like the branches of a tree or the branching of a river viewed from high above.
The network is the pattern of arteries and veins that supplies blood to your retina. It spreads out from the dark blob of your blind spot.
In human eyes, the blood supply of the retina is in front of the retina. That is, light passes through the blood supply on its way to the retinal detectors. You do not see the retinal blood supply because it never changes, and your eye ignores 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.