IAGO
O, beware, my lord, of jealousy!
It is the green-eyed monster which doth mock
The meat it feeds on; (3.3.195-197)
Iago's pretty good at manipulating Othello, don't you think? Here, he pretends to warn Othello not to be a jealous man, pointing out that jealousy ends up destroying the heart of the man who falls prey to it.
Quote 5
IAGO
Trifles light as air
Are to the jealous confirmations strong
As proofs of holy writ. This may do something. (3.3.370-372)
Iago realizes that real proof of Desdemona's supposed infidelity is not necessary because mere suspicion is enough to feed Othello's jealousy. In the case of Othello, Iago will use the handkerchief Othello gave Desdemona in order to convince Othello that Desdemona's been cheating. (Remember, when Desdemona dropped her handkerchief by accident, Emilia picked it up and gave it to Iago. Iago says he's going to drop it for Cassio to find.) Even though the handkerchief is a mere "trifle, light as air," once Othello sees it in another man's possession, he'll think he has solid proof that Desdemona is unfaithful. When Iago says the handkerchief will be "as proofs of holy writ" to Othello, he means that Othello will see the handkerchief as the gospel truth that Desdemona's a cheater.
Quote 6
IAGO
The Moor already changes with my poison;
Dangerous conceits are in their natures poisons,
Which at the first are scarce found to distaste,
But with a little act upon the blood
Burn like the mines of Sulphur.
Enter Othello.
I did say so.
Look, where he comes. Not poppy, nor mandragora
Nor all the drowsy syrups of the world
Shall ever medicine thee to that sweet sleep
Which thou owedst yesterday. (3.3.373-382)
Iago realizes the unbelievable power of jealousy. Here, he claims that he has poisoned Othello's mind by suggesting Desdemona may be up to something naughty. Because Iago has succeeded in making Othello suspicious, Othello will never, ever have a good night of sleep again, not even if he used the best sleeping medicine in the world.