Analysis of As imperceptibly as Grief

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—
The Dusk drew earlier in—
The Morning foreign shone—
A courteous, yet harrowing Grace,
As Guest, that would be gone—
And thus, without a Wing
Or service of a Keel
Our Summer made her light escape
Into the Beautiful.