The nose is the minimal flaw that proves the book's thesis, and the mirror is the place where Vitangelo Moscarda tries, without success, to see what others see in him. The one is the trigger, the other the instrument of the experiment.
The nose leans to the right, says Dida, calmly and with a "dear." The flaw is small, and what shakes him is having lived for years with it without knowing and needing a wife to find out. He asks a friend whether he is looking at his nose, discovers that the friend already knew, and concludes that he was never, for anyone, the Moscarda with the straight nose he supposed himself to be. The whole city, infected, begins to examine its own face in shop windows.
The mirror comes next. Before it, spontaneity ends, every gesture seems feigned, and Moscarda says he cannot watch himself live. He catches sight of himself in a street mirror and does not recognize himself: he sees a stranger passing by, talking. He makes faces before all the mirrors at home, without ever catching a real emotion, because it would cease the moment he saw it. Finally, with his eyes closed, he goes to the wardrobe mirror and sees a body without name or owner, sneezing from a draft of air.
The lesson of the mirror comes down to a sentence that Moscarda will later say to Anna Rosa: one cannot live before a mirror. At the end of the novel, at the asylum, he never looks at himself again.