act _4 scene 1
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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.
Othello is incensed by Cassio, still believing that he was speaking of Desdemona, rather than Bianca. Now, Othello is resolved to kill Desdemona himself, and charges Iago with murdering Cassio.