This Mrs. Isham exists in art, but is not traceable. Jacques d'Agar was a great painter, but was a Protestant so he left France in 1682.
The curls over the forehead date this d'Agar Ice Queen portrait to turn of the century.
Keywords: d'Agar, Isham family, British, long straight coiffure, chemise, scoop neckline, criss-cross bodice, wrap