In a conversation with Othello, Iago says that Cassio has confessed to sex with Desdemona. This revelation is too much for Othello, who becomes incoherent and faints. When Cassio enters, Iago claims that Othello has epilepsy and has had seizures before. Rather than revive him, they must let the fit take its course. Iago sends Cassio away, telling him to come back later. Othello, regaining consciousness, talks of himself as one among many cuckolds, but Iago tells him to hide and observe Cassio, who is returning. Iago says he will draw Cassio out to tell of his amorous adventures with Desdemona.
Othello withdraws, too emotionally involved to understand that Iago is manipulating him, and Iago talks with Cassio about Bianca. Othello sees his smiles and laughter but cannot hear the details and believes he is joking about how much Desdemona loves him. Then Bianca herself enters, with Desdemona's handkerchief, which she throws back at Cassio. Seeing his wife's handkerchief in the hands of Cassio's mistress is, for Othello, the "ocular proof" he sought. He is now convinced of Desdemona's infidelity and knows he must kill both Cassio and Desdemona that very night.
