Ellen Wallace Sharples (March 4, 1769-March 14, 1849) was an English painter who specialized in portraits in pastel and watercolor miniatures on ivory.
Biography on Wikipedia: https://en.wikipedia.org/wiki/Ellen_Sharples
Joseph Priestley by Ellen Sharples
c.1797 / Pastels / 9 1/2″x7 1/2″ / National Portrait Gallery, London, UK
Ellen Sharples on Artnet: http://www.artnet.com/artists/ellen-wallace-sharples/
This article about Priestley includes a short bio of Ellen Sharples, on pages 10-11 of the pdf file: