Commenting on lines 21-28 in Shelley at Work, Neville Rogers makes the following remarks:
The Divinity here robed in the Shelleyan habiliments of Veil, Moon, Star ('most excellent pilot') is more than beauty itself, more than Emilia, more than the conjunction of the two. She is also Veiled Maid of Alastor, she is Cynthia, Asia, and the Witch of Atlas on the ideal plane and, at the same time, on the earthly plane, she has in her something of Harriet and Mary Shelley, Claire Clairmont, even, perhaps, spiritually, something of the poor Brown Demon. (128)
Rogers's statements bring to my mind a prevalent theme in much of Shelley's work: the indescribability of the ideal form. Language can only create signifiers. It is the old Platonic problem that Socrates/Plato uses as a justification for claiming that poets do not belong in the Republic (argument made in Book 10):
Then must we not infer that all these poetical individuals, beginning with Homer, are only imitators; they copy images of virtue and the like, but the truth they never reach? The poet is like a painter who, as we have already observed, will make a likeness of a cobbler though he understands nothing of cobbling; and his picture is good enough for those who know no more than he does, and judge only by colours and figures. (Book 10, par. 50)
Rogers, Neville. Shelley at Work: A Critical Inquiry. 2nd ed. Oxford: Clarendon P, 1967.