We see the whole case now, through his mind, in absolute quiet.

He has been on his terrace to look at the stars, and their solemn peace is with him.

He feels that he is now alone with God and his old age.

And being alone, he is not concise, but garrulous and discursive.
Browning makes him so on purpose.

But discursive as his mind is, his judgment is clear, his sentence determined.