What proof does Iago give Othello about Desdemona's guilt?

Desdemona drops the handkerchief that Othello gave her on their honeymoon; Emilia knew that her husband had wanted it for something, so she doesn't feel too guilty about taking it. Emilia gives it to Iago, who decides to use the handkerchief for his own devices. Othello re-enters, and tells Iago that he now doubts his wife; Othello demands "ocular proof" of Desdemona's dishonesty, so Iago sets about making stories up about Cassio talking in his sleep, and says that Cassio has the handkerchief that Othello gave to Desdemona.