As imperceptibly as Grief
The Summer lapsed away —
Too imperceptible, at last
To seem like Perfidy —
A Quietness distilled
As Twilight long begun
Or Nature spending with herself
Sequestered Afternoon —
Stanzas two and three depict  personified  Nature in late summer/early autumn, a time for  quiet contemplation — hence “A Quietness distilled” —  with the  Twilight lasting longer as the nights draw in. There is a cosiness about Nature indulging herself in the solitude of a “Sequestered” — meaning secretive and isolated — afternoon. 
 Note again the capitalised nouns, giving the meaning of the poem universality.
As Imperceptibly as Grief