Cover your left eye and stare at the cross below. A dot sits to its right. Slide it outward and, at one particular spot, it does not just blur. It disappears, and the background closes over the gap as if nothing had been there.

1. Close your left eye. Keep the other one on the cross and do not let it wander.
2. Set the distance slider to roughly how far you are from the screen.
3. Slide the shape toward the spot the readout suggests, then nudge it until it vanishes.

Screens differ, so the readout is a guide, not a promise. If the shape will not vanish, lean in or back a little. On a phone, hold it nearer, around 15–20 cm.

A hole where the nerve leaves the eye

The back of each eye is lined with the retina, a sheet of light-sensing cells. The signals from those cells have to leave the eye somehow, so they gather into a cable, the optic nerve, which exits through a small round patch called the optic disc. That patch has no photoreceptors at all. Light that lands on it is simply not detected.

The result is a blind spot in every eye. In humans it sits about 12–15 degrees to the outer side of where you are looking, a little below the horizontal, and it measures roughly 5.5 degrees wide and 7.5 degrees high. For scale, the full Moon is about half a degree across, so the hole could hide around ten of them side by side.

Why you never notice it

There are two reasons. First, your two eyes cover for each other. Each eye's blind spot falls on a different part of the scene, so anything one eye misses, the other sees. You only expose the hole by closing one eye, which is exactly what the demo asks of you.

Second, even with one eye shut you do not see a black patch. The brain fills the gap using the surroundings. Try the second shape above: a line broken by a gap, with the blind spot sitting on the gap. The line appears whole. Try the stripes and the pattern continues straight through. The brain is not remembering what was there. It is guessing from the edges, and the guess is so convincing that you see it as real.

An old discovery

The first documented observation of the blind spot comes from the French physicist Edme Mariotte, around 1668. He found that an object can vanish from view when its image falls where the optic nerve enters the eye, and he reported it to the Royal Society in London and in France. It overturned a tidy assumption of the time, that the point where the nerve enters the eye should be the most sensitive part of the retina, not a dead one.

Wired backwards

The blind spot is a side effect of how vertebrate eyes are built. Our photoreceptors sit behind the wiring, so the nerve fibres have to run across the retina's inner surface and pass through it to reach the brain. Every vertebrate has this spot. Octopus and squid eyes, which evolved separately, are wired the other way round, with the nerve fibres behind the receptors, and they have no blind spot at all.

So the next time you feel you are seeing the whole world in one clear, continuous picture, remember the hole. Part of what you see is your brain's best guess, painted in without telling you.