As sometimes in a dead man's face, To those that watch it more and
more, A likeness, hardly seen before,
Comes out—to some one of his race:
So, dearest, now thy brows are cold, I see thee what thou art, and know Thy likeness to the wise below,
Thy kindred with the great of old.
But there is more than I can see, And what I see I leave unsaid, Nor speak it, knowing Death has
His darkness beautiful with thee.
You know how, when you stare at someone's face for a long time (or your own reflection in a mirror), the person (or you) starts to look weird and maybe even like another person?
Well, that's what the speaker is getting at in this first stanza here. And this totally goes along with what was happening back in Canto 70, when Tennyson wasn't able to remember what Arthur looked like.
This time, though, he's starting to see something or someone else there when he thinks of Arthur's face. But wait—he's seeing something of "the wise below" in his friend.
Okay, we're in figurative territory here. He's seeing similarities between Arthur and the great, wise dudes who have lived in the past.