After Banquo, Fleance and the servants have exited the scene, Macbeth is left alone onstage to converse with the audience. He asks, "Is this a dagger I see before me/The handle toward my hand?" His vision is of a dagger, which seems to be offering itself to him rather than threatening him, since "the handle" is pointed "towards" his hand.
Macbeth goes on in this soliloquy to describe how he clutches for it, but cannot grasp it. However, there is question about whether an actual image of a dagger appears during this soliloquy. Macbeth asks:
. . .art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
So, Macbeth is acknowledging that he might, in fact, be tormented by an image from his own mind -- his conscience? -- rather than observing an actual dagger floating in the air before him.
Either way, he definitely makes the image of a dagger real in his next line.
I see thee yet, in form as palpable
As this which now I draw.
And with the drawing of his own actual weapon, Macbeth seals his decision to go through with the murder, invoking the dark spirits of night to aid him in his treason.
For more on this scene, please follow the links below.
Shakespeare, in alluding to Hecate, Queen of the Night,and Tarquin,raper of Lucrece, keeps murder firmly in the mind of the listener,as he steels himself to do the deed. An earlier reference to daggers,both real or imaginery, simply clears the mind of extraneous matter, leaving the listener clear headed,and aware of dreadful murder!
Macbeth, creeping nearer and nearer to Duncan's bedchamber, "Moves like a ghost", telling the earth to not hear his steps, and asking time to alter his place in it,so as not to be responsible for the deed he has to do.
A bell rings, Macbeth now knows his destiny.(It may have been the bell his wife rung to let him know his 'drink' is ready!)
"I go, and it is done. The bell invites me". A death knell for Duncan.