Emorfily Potsidis preps Jack Hogan, 13, for an eye exam at Mass. Eye and Ear on Monday.

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BOSTON — The machine looked like a giant eyeball. There was a hole where the pupil should have been, and the technician told Jack Hogan to stick his head inside. As the white dome began to flash with light, electrical messages began zinging up from his retina to his brain — and every flicker of voltage was picked up by the electrode that had been stuck onto his cornea.

“It hurts,” Jack said, his voice echoing around the Giant Eyeball Machine.