Her article in thepeerage is here.
From the Google Art Project vis Wikimedia.
Lady Frances, probably at around the time of her 1652 marriage, poses en deshabillé for Mary Beale.
Keywords: Beale, Frances Pierrepont, Frances Cavendish, Duchess of Newcastle, Pierrepont family, Cavendish family, Duchess, British, curly coiffure, chemise, scoop neckline, ruffled modesty piece, full under-sleeves, bodice, vee neckline. quarter length tabbed over-sleeves, wrap, clasps