Another thing you can try when doing this Snack is to close your left eye and stare at the bright image with your right eye. Then close your right eye and look at the wall with your left eye. You will not see an afterimage.
Negative afterimages do not transfer from one eye to the other. This indicates that they are produced on the retina and not in the visual cortex of the brain, where the signals would have been fused together.
For up to 30 minutes after you walk into a dark room, your eyes are adapting—after that time, your eyes may be up to 10,000 times more sensitive to light than they were when you entered the room. We call this improved ability to see night vision. It’s caused by the chemical rhodopsin in the rods of your retina. Rhodopsin, popularly called "visual purple," is a light-sensitive chemical composed of retinal (a derivative of vitamin A) and the protein opsin.
You can use the increased presence of rhodopsin to take “afterimage photographs” of the world. Here’s how:
Cover your eyes to allow them to adapt to the dark. Be careful that you do not press on your eyeballs. It will take at least 10 minutes to store up enough visual purple to take a “snapshot.” When enough time has elapsed, uncover your eyes. Open your eyes and look at a well-lit scene for half a second (just long enough to focus on the scene), then close and cover your eyes again. You should see a detailed picture of the scene in purple and black. After a while, the image will reverse to black and purple. You can take several snapshots after each 10-minute adaptation period.
The phenomenon of afterimages may also help explain a common illusion you might have noticed. The full moon often appears larger when it is on the horizon than when it is overhead. The disk of the moon is the exact same size in both cases, and its image on your retina is also the same size. So why does the moon look bigger in one position than in the other?
One explanation suggests that you perceive the horizon as farther away than the sky overhead. This perception might lead you to see the moon as being larger when it’s near the horizon (just as the afterimage appeared larger when you thought it was on a distant wall), and smaller when it’s overhead (just as the afterimage appeared smaller when you thought it was in the palm of your hand).