Here's a quick fun demonstration of this. Close your left eye, then stare at some spot. If you move a finger around the right side of that spot (arm half extend

Here's a quick fun demonstration of this. Close your left eye, then stare at some spot. If you move a finger around the right side of that spot (arm half extended, maybe ~20-30 degrees to the right). If you manage to not move your eye while paying attention to the finger, you'll find that it doesn't look quite right when you put it over the blind spot. With your finger you know it should be there, so something finger-like fills the blind spot. If instead of a finger you try with something like a black dot, you'll see it disappear completely.