Have you ever stood between two parallel mirrors and seen your reflection repeated in both directions, becoming smaller each time? The first image reflects you at your normal size; the second is reflected from the other mirror and is smaller.
Leonardo described an eight-sided room with eight flat mirrors as the walls. He noted that if a person stood inside, they "will be able to see every part (of themselves) endless times". Leonardo may have used a mirror room like this in his studio, looking through the hole with his right eye to the subject positioned inside, allowing him to freely paint with his left hand.