This is a stylized picture of a generic pretty lady of the mid 1700's,
created in the mid 1800's to illustrate a sentimental poem in a book collection of such for the amusement of genteel young ladies. It is not a portrait of any particular person.
The original of this image, captioned "Lady of the Court of Louis XVth", appears in Fisher's ...
Possibly Émilie du Châtelet who was certainly of the right historical period (1706-1740, compared with Louis XV's reign of 1715-1774).
Her best remembered achievement was a translation of Newton's Principia which (according to Wikipedia) is still considered to be the standard French translation.