Romney, expert infallibly to trace
On chart of canvas, not the form alone
And semblance, but, however faintly shown,
The mind's impression too on every face;
With strokes that time ought never to cease
Thou hast so pencilled mine, that though I own
The subject worthless, I have never known
The artist shining with superior grace.
But this I mark, -- that symptoms none of woe
In thy incomparable work appear.
Well -- I am satisfied it should be so,
Since, on maturer thought, the cause is clear
For in my looks what sorrow couldst thou see,
When I was Hayley's guest, and sat to thee?

William Cowper

http://www.poemhunter.com/poem/sonnet-to-george-romney-esq-on-his-picture-of-me-in-crayons/