How does Iago further influence Othello as they discuss Cassio's and Desdemona's murders?

Iago continues his insinuations when speaking to Othello; he provides more "proofs" that are anything but ocular, though Othello has calmed, and seems more troubled and less angry. In the last act Othello was trying to act as Desdemona's defender, and Iago was the accuser; ironically, they seem to have switched places here, and Iago seems to be defending Desdemona, all while producing more "evidence" to condemn her. The handkerchief, however, is as important a symbol as ever; "it comes o'er my memory as a raven doth over an infected house," Othello says. The metaphor highlights how crucial this object is to him, as Othello burdens that single object with more and more significance.