How your eye views itselfS

What is the sound of one hand clapping? More importantly, what is the sight of the eye seeing itself? Well, Purkinje Shadows allow your eyeball to dissect...your eyeball.

Just like the auditory portion of the conundrum, this can be resolved by a technicality. Contrary to popular belief, the eyes are not windows to the soul. That would imply that they are a clear pathway to an object, with nothing in the way. Actually, they are complex structures, filled with various kinds of goop which comes shooting out whenever someone gets stabbed in the eye during horror movies. To maintain that goop, blood vessels crisscross the eyeball. Though they are delicate and small, they still block the light meant to reach the retina and the optic nerve at the back of the eye, which means we should see their shadows.

We also shouldn't see them. The body already has a way to tell us that something is interfering with blood vessels in sensitive areas, and it's called the nervous system. More specifically it's called pain, and it works really well. The eyeball, contrary to most philosophical screeds, shouldn't be looking within. It should be looking without, to alert us to things that might eat us, knock us over a cliff or ask us for money. Because of that, the eye has evolved to look around the stationary, permanent and meaningless shadows that it always sees. It's done so so effectively that it wasn't until the eighteen hundreds that Jan Evangelista Purkinje documented the phenomenon of people seeing the inside of their own eyes.

Feel like giving yourself an optical check-up? Some people (like me) see it if they stare up at a diffuse light source and rapidly blink, or close their eyes and then look back and forth quickly. The internet-approved way of seeing the inner working of your own eyeball is to put a pinhole in an index card and either shimmy the card side to side or rotate it quickly. You should be able to see the squiggly lines of blood vessels over a white background. (The blinking or eyes-closed-but-moving method produces what I think is a cooler looking black vessels against an orangey-red background.) Ninety-nine percent of people with decent vision can see it.

So there we have it. The mysteries of the ages. One hand clapping makes a muted ‘thhhhhwap' sound. An eye seeing itself looks like a road map. Next week? What to say when the void stares back. (It's, "Quit staring at my chest, pervert.")

[Via How Stuff Works, QI and NCBI.]