Act 4, Scene 1

Othello is trying, even after swearing that Desdemona was unfaithful, not to condemn her too harshly. He is talking with Iago about the handkerchief still, and its significance in being found; but, soon, Iago whips Othello into an even greater fury through mere insinuation, and Othello takes the bait. Othello falls into a trance of rage, and Iago decides to hammer home his false ideas about his wife. Iago calls Cassio in, while Othello hides; Iago speaks to Cassio of Bianca, but Othello, in his disturbed state, believes that Cassio is talking of Desdemona, which is the last "proof" he needs before declaring his wife guilty.