Find your blind spot

Each retina has a place where the optic nerve leaves the eye. All the nerve fibres from the retina gather there and exit through a small round area called the optic disc. That area has no rods or cones, so light that lands on it is not detected. This is your blind spot (the medical name is the physiological scotoma).

The optic disc sits about 15 degrees away from the centre of your gaze, on the side towards your ear (the temporal side). Its width is roughly 5 to 7 degrees of visual angle. That is large: roughly ten full Moons side by side (the Moon is about half a degree wide) could disappear in it.

You can find it with a simple geometry problem. Draw a small "+" and a dot on a page, a distance s apart, with the dot to the right of the "+". Close your left eye and look at the "+" with your right eye. The dot lands on the blind spot when the angle between them is about 15 degrees, which happens at a viewing distance

d = s / tan(15°) ≈ 3.7 × s

If s = 6 cm, then d ≈ 22 cm. If s = 8 cm, then d ≈ 30 cm. Slowly move your head towards the page and back, keeping your eye on the "+", and the dot vanishes around that distance. Use the card-size ruler in the scene to set the scale, so that "6 cm" on screen really is 6 cm.

So why does your world not have two black holes? Two reasons.

1. Your two eyes cover for each other. The blind spot of the right eye sits on the right side of the visual field, and the blind spot of the left eye sits on the left side. Whatever one eye misses, the other one sees.
2. Even with one eye closed, the brain fills the gap. You do not see a dark patch; you see the background continuing. Lesson 3 is about that.

Common myth: "A blind spot is an illness." It is a normal part of the eye's design. The nerve has to leave through the retina somewhere.