Two very different faces are set side by side. First, the speaker remembers a black girl whose dark skin seemed like winter’s night. When she smiled, her parted lips revealed teeth so bright they looked like the Milky Way pouring across her face; when her lips closed, the darkness returned. Then he looks at a fair maiden, bright as a summer’s day, and sees the same effect: her smile is so dazzling that the daylight around her seems black by comparison. When she stops smiling, that inner light vanishes.
The repetition gives the poem a quiet, wondering tone. The speaker is not judging either woman’s beauty; he is struck by how a smile can transform a face, making light and darkness feel sudden and dramatic. The mood is tender and a little wistful, as if beauty itself is fleeting — there one moment, gone the next.