Act 4, Scene 1

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. Bianca comes in, and gives the handkerchief back to Cassio, since she swears she will have nothing to do with it.