“Few visual moments are as strange as the scene at the beginning of act two, in which Macbeth sees a dagger floating in the air, apparently leading him to Duncan’s bedchamber. This hallucination provokes one of Shakespeare’s most famous speeches: ‘Is this a dagger which I see before me?’ … For this strangest of plays, the paradox is fitting: its best-known prop is almost certainly invisible.” Andrew Dickson looks at the ways some of the great actors and directors have handled the scene.