San Pietro risana gli infermi con la sua ombra

The miracle in this scene is a shadow. According to the Book of Acts, the sick were laid out in the street so that Saint Peter's shadow might fall across them and heal them as he passed. Masaccio took that literally and painted the shadows, the figures and the buildings casting them consistently, as if one real sun lit the whole street. Around 1425 in Florence that was close to new. Painters were only just learning to build convincing space and light, the same ideas the architect Brunelleschi was working out in those very years. Masaccio walks Peter down a Florentine street lined with faces that look like real, worried men. He would be dead within about three years, in his late twenties, yet young painters came to this chapel to study him for generations afterward, Michelangelo among them.