The incisions are usually made near the top of the eye, so the affected area usually cannot be seen afterward because it is behind the upper eyelid.

"I'm watching where the tips of the scissors are," he says with the same silky calm, "to make sure I'm not tearing anything." If you want to practice making this flap on your own, he says, try doing it on the surface of a lime. The pithy tissue under the citrus skin, he counsels, is very similar in texture and feel to the eye's tissue.

I had not expected to be able to watch this, but I stare at the screen for a full hour, fascinated as Brandt helps Jackie tie one suture and then another before they roll the eye gently but firmly back under the lid. Absurdly, I think of a billiard ball being rolled into a pocket.

Is it hard to get used to watching this stuff? I ask one of the volunteer medicos on board.

Dr Rory Allar, a fellow in glaucoma at the University of California at Davis, where Brandt is a professor of surgery, smiles at the question.

"I don't have a problem with that—most of us don't," he says, referring to his fellow eye surgeons. "But the truth is, there's something that everybody thinks is gross to handle. For some it's eyes, some it's feet. Others can't cope with vomit or excrement. For me, it's respiratory expectorant."

Many people are squeamish about eyes, Allar says. "As sensitive as we all know the eye is, though, it's also very tough and resilient, especially the white part, which acts like a casing, like protective fat.

"Furthermore, in your face the eye sits in a bone pocket like a pearl in an oyster, and it's spongy," he says. "So while the structure within the eye is delicate, I am always amazed at how much trauma the eye can take."