Meet in her aspect and her eyes:
Thus mellow'd to that tender light
Which heaven to gaudy day denies.
One shade the more, one ray the less,
Had half impair'd the nameless grace
Which waves in every raven tress,
Or softly lightens o'er her face;
Where thoughts serenely sweet express,
How pure, how dear their dwelling-place.
And on that cheek, and o'er that brow,
So soft, so calm, yet eloquent,
The smiles that win, the tints that glow,,
But tell of days in goodness spent,
A mind at peace with all below,
A heart whose love is innocent!
He begins the line “but” to suggest that a typical girl of this beauty might well be less than virtuous. She, by contrast, has moral beauty to match that of her face. 

She Walks in Beauty