As the clouds that are so light,
Beautiful, swift, and bright,
Cast shadows on field and park
Of the earth that is so dark,

And even so now, light one!
Beautiful, swift and bright one!
You let fall on a heart that was dark,
Unillumined, a deeper mark.

But clouds would have, without earth
To shadow, far less worth:
Away from your shadow on me
Your beauty less would be,

And if it still be treasured
An age hence, it shall be measured
By this small dark spot
Without which it were not.

Edward Thomas

http://www.poemhunter.com/poem/as-the-clouds-that-are-so-light/