Mc Cavity's not there
Wrap the dowel with aluminum foil.
Unscrew the back of the mini maglite and remove one AA battery, replace it with the aluminum foil wrapped dowel. Replace the back.
Unscrew and remove the front of the mini maglite.
The light will come on as a dim point source.
To Do and Notice
Turn the roomlights on, or open the shades.
Close your right eye.
Hold the dim pointsource of the mini maglite in front of your nose at arms length.
Move it slowly to the left. Keep your left eye looking straight ahead, do not look at the light. At some point (within a handspan of straight ahead with the light at at arms length) the light will go out as it enters your blindspot. If you move your eye to look at the light, it will once again be visible.
Repeat the experiment with your right eye open and your left eye closed.
Once you find your blindspot map its shape by moving the light around and finding the edges of the region in which it is invisible.
Notice that even though the light disappears in your blindspot, you do not perceive a hole in the background at this location.
What's Going On?
The nerves which carry signals from your retina and the blood supply for your retina are in front of your retina. Light goes through them before it is detected. Both the optic nerve and the retinal blood supply pass through the retina at a place where there are no light detectors, your blindspot. Your eye and brain fill-in the hole in your vision.
You perceive your blindspot to be outside of center of your field of view.
Your fovea is located at the center of your field of view.
Your brain inverts images top-to-bottom and right-to-left.
This means that the optic nerve connects to your retina inside of the center. That is, on the nose side of the fovea.
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22 May 2000