Shapes that aren’t there
Your blind spot
Close your left eye and look at the top cross with your right eye. Move slowly toward the screen and back. What happens to the dot?
At one distance the dot vanishes. Then look at the lower cross: the gap in the bar closes and the bar looks whole.
Why? Where the nerve from your eye leaves it, there are no cells to see with: a blind spot in each eye. You never notice it, because your brain fills it in with what’s around it: plain white, or more bar.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)