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/

Further reading:
https://emuseum.mountvernon.org/people/793/ellen-sharples/objects
https://www.artbiogs.co.uk/2/societies/royal-west-england-academy
https://www.lindahall.org/joseph-priestley-2/

This article about Priestley includes a short bio of Ellen Sharples, on pages 10-11 of the pdf file:
http://acshist.scs.illinois.edu/bulletin_open_access/v30-2/v30-2%20p77-90.pdf