After the witches show Macbeth three apparitions, which he mistakenly takes for good omens, he asks them:
Yet my heart
Throbs to know one thing: tell me, if your art
Can tell so much, shall Banquo's issue ever
Reign in this kingdom?
They answer with this:
Show his eyes, and grieve his heart;
Come like shadows, so depart!
A show of eight Kings, and Banquo last with a glass in his hand.
Thou are too like the spirit of Banquo. Down!
Thy crown does sear mine eyeballs. And thy hair,
Thou other gold-bound brow, is like the first.
A third is like the former. Filthy hags!
Why do you show me this? A fourth! Start, eyes!
What, will the line stretch out to the crack of doom?
Another yet! A seventh! I'll see no more:
And yet the eighth appears, who bears a glass
Which shows me many more; and some I see
That twofold balls and treble sceptres carry:
Horrible sight! Now I see ’tis true;
For the blood-bolter'd Banquo smiles upon me,
And points at them for his. What, is this so?
Eight kings appear, and they all look like Banquo. The last apparition/king holds a mirror which shows even more kings. So more than eight generations of Banquo's children will be kings, stretching, as legend has it, all the way to the King to whom Shakespeare dedicated Macbeth: King James I.