Each eye has a hole in its vision, and the brain hides it. This episode explains the physiological blind spot, the punctum caecum, which sits where the optic nerve exits through the optic disc, an area with no photoreceptors at all. It lies about 12 to 15 degrees toward the temple and 1.5 degrees below the line of sight, measures about 7.5 by 5.5 degrees, roughly the size of a fist at arm’s length, and is joined by angioscotomas, the shadows cast by retinal blood vessels. The visual cortex covers the gap using the other eye’s view and, with one eye closed, by interpolating patterns such as brick walls from the surrounding area.

Octopus and squid eyes evolved separately and route their nerve fibers behind the retina, so they have no blind spot, while the vertebrate layout traces back to the retina forming as an outgrowth of the brain. Anatomists long assumed the optic nerve’s entry point was the most sensitive part of the retina until French scientist Edme Mariotte showed around 1668 that it detects no light, demonstrating it to King Louis XIV and the Royal Society of London. The episode ends with the central night blind spot, caused by a fovea packed with cones and nearly empty of rods.

• The R and L test that makes a letter vanish when viewed with one eye
• Why a jagged black void would distract more than an invented patch of background
• Convergent evolution of camera-style eyes in vertebrates and cephalopods
• Dark-adapted rods becoming an estimated 10,000 times more sensitive than cones
• Averted vision, looking 5 to 10 degrees off center to catch faint lights, in aviation and astronomy