For obvious reasons, breasts are a classic symbol of female beauty. We'll keep this G-rated, but you can see why talking about this woman's breasts forces us to think about how we define an ideal woman, and what seems beautiful about her.
- Line 3: Here the speaker avoids a direct simile. He just gives us the strong image of sparkling white snow, and lays it next to the equally strong image of dun (grayish-brown) breasts. He's playing with our associations with these colors. White is a long-standing symbol of purity, cleanliness, virginity, and all that Next to that squeaky clean image, the mistress's breasts seem dirty and polluted. But the speaker will go on to show us that these old stereotypes might not really mean much at all.