Parting is such sweet Sorrow

Take this kiss upon the brow!
And, in parting from you now,
Thus much let me avow-
You are not wrong, who deem
That my days have been a dream;
Yet if hope has flown away
In a night, or in a day,
In a vision, or in none,
Is it therefore the less gone?
All that we see or seem
Is but a dream within a dream.