What! are men mad? Hath nature given them eyes"Twinn'd" might not mean what you think it is (more on that in a moment). There is also a textual crux to note: "th'unnumbered beach" or "the number'd" beach? (See Note 1).
To see this vaulted arch, and the rich crop
Of sea and land, which can distinguish 'twixt
The fiery orbs above, and the twinn'd stones
Upon the numbered beach, and can we not
Partition make with spectacles so precious
'Twixt foul, and fair?
A plausible enough reading would be something like, "Wow, how is it that certain men (not naming any names or anything) can distinguish really fine details, e.g. distinguish which star is which, or distinguish between two grains of sands that look identical from a distance ... and yet can fail to tell the difference between a foul woman and a fair one?"
Nosworthy's Arden Cymbeline seems a bit confused in this scene overall. Like most editors, Nosworth interprets "distinguish" as "distinguish orb from orb, and stone from stone." (See Note 2). The more recent Wayne Arden edition is way better and hugely useful, but it also goes with "the countless stones that the human eye can differentiate from one another."
But there's another strong reading here. It's not altogether different: the two readings are as different as two grains of sand, maybe?
