You say 'tis hard to copy well,
Where Nature does herself excel.
Allow'd -- yet still let me advise:
Near as you can, to Nature rise;
Nor Time, nor Colours will be lost;
The Draught will more than pay the Cost.
Then dare to draw that Angel Face;
The Pencil may the Features trace;
And, should her Air thy Art defeat,
Add Wings, the Piece will be complete.

Mary Barber

http://www.poemhunter.com/poem/to-mrs-newans/