She Walks in Beauty Principles Quotes Page 1

The sound of this line, with its repeated s sound (a.k.a. "sibilance"), reflects the meaning of the line as well: the line sounds as soothing and "serene" as her thoughts supposedly are.

The speaker assumes that the calm and "serene" expression on the woman's face reflects the "pur[ity]" of her mind.

The speaker also assumes that the woman's sweet expression "tell[s]" the story of her past goodness – that's a lot to read just from someone's facial expression.