Jamieson, Fausset and Brown on Song of Solomon 5:12

12. as the eyes of doves--rather, "as doves" (Psa 68:13); bathing in "the rivers"; so combining in their "silver" feathers the whiteness of milk with the sparkling brightness of the water trickling over them (Mt 3:16). The "milk" may allude to the white around the pupil of the eye. The "waters" refer to the eye as the fountain of tears of sympathy (Eze 16:5, 6; Lu 19:41). Vivacity, purity, and love, are the three features typified.

fitly set--as a gem in a ring; as the precious stones in the high priest's breastplate. Rather, translate as Vulgate (the doves), sitting at the fulness of the stream; by the full stream; or, as Maurer (the eyes) set in fulness, not sunk in their sockets (Re 5:6), ("seven," expressing full perfection), (Zec 3:9; 4:10).