The narrator from Edgar Allan Tell-Tale Heart is obsessed with what about the old man?

The narrator in Edgar Allan Poe's "The Tell-Tale Heart" is obsessed with the old man's "vulture eye." He describes it as a "pale blue eye, with a film over it." This eye, he claims, drives him to madness and ultimately leads him to murder the old man.

The narrator's obsession with the eye is a powerful symbol of his psychological instability and the descent into madness. He fixates on this small detail, magnifying it into a monstrous, disturbing feature that consumes his thoughts and ultimately leads to a terrible act.