From the Christie's Web site. Finding this identified as Dorothy, Lady Dormer in October 2015 caused me to re-check Christie’s and verify that they had listed it as Lady Denman. Here are Christie’s lot notes: "This portrait is recorded by Roger Warner as having originally borne an indistinct inscription in the background above the sitter's right shoulder which Roger Warner read as '[?]Lady Denmar[...]' but which also seems to have been read alternatively as 'Lady Denman'. The portrait's early provenance is unfortunately unknown and the identity of the sitter remains uncertain. The only possible sitters with similar names at that time were the wives of various Denhams who had been knighted, as there were no peers named 'Denman', 'Denham' or any obvious variants of that name between 1570 and 1640. Of these the most likely possibility is that the portrait may show Cicely, first wife of Sir John Denham (1559-1639), who was knighted in 1609. She was the widow of Richard Kellefet of Egham, Chief Groom in Queen Elizabeth I's 'removing gardrobe of beddes', and died in 1612.
On the basis of the sitter's dress, the portrait is datable to the end of Queen Elizabeth I's reign or the early years of the reign of her successor King James I, circa 1595 to 1605. The sitter's dress and remarkable jewellery indicate that she is likely to have been of considerable social standing. She wears a heart-shaped cap, standing collar, farthingale and sleeves that were fashionable at the end of the sixteenth century and opening years of seventeenth century, and is also in the Queen's colours of black and white. The sitter's elaborate bodice is embroidered with armillary spheres and coiled serpents. These emblems suggest that the sitter was linked to the court of Queen Elizabeth I. The emblem of the armillary sphere was used extensively by Queen Elizabeth I herself, who is shown wearing such a sphere as an earring in the celebrated full-length of her by Gheeraerts of circa 1592 which originally hung at Ditchley in Oxfordshire (London, National Portrait Gallery). The armillery sphere represented the earth and the motion of the celestial bodies around it, and is thought to have been symbolic of constancy and of (specifically) Protestant religious fidelity, as well as being an attribute of Urania, the muse of astronomy and the liberal arts and the goddess of celestial love. Such a sphere also appears embroidered on the sleeves of Antonis Mor's portrait (National Portrait Gallery, London) of the courtier Sir Henry Lee of Ditchley, who may have been the inventor of the Queen's persona as Urania. The emblem of the serpent, usually symbolic of Wisdom and Prudence, was also associated with Queen Elizabeth I, and a bejewelled coiled serpent appears prominently on the Queen's sleeve together with an armillary sphere in the celebrated 'Rainbow' portrait of the Queen of circa 1600-3 (Hatfield House). Roy Strong cited Frances Yates in suggesting that the likely source for the combination of these two symbols was Cesare Ripa's emblem Intelligenza, from his iconographic manual Iconologia (1593). Ripa explained the emblem by noting that in order to understand high and celestial matters, one must go first to the ground like a serpent, and from thence rise heavenwards (R. Strong, Gloriana, The Portraits of Queen Elizabeth I, London, 1987, pp. 159-60). The reappearance of this very specific combination of emblems in the present picture links the costume closely to the intellectual world of the Elizabethan court. The sitter also wears a cameo of a female figure in profile with a gold and enamel border and a suspended pearl drop, as well as a remarkably unusual series of small jewelled cameos pinned to the shoulders of her dress, each illustrating a different scene. There is no other known example of the latter jewelled ensemble in English portraiture or documented in other primary sources.
Warner records having purchased this picture 'Privately at the suggestion of the then owner Leonard Huskinson (one time Art Lecturer at the Ruskin Art School, Ashmolean Museum, Oxford) at time of his retirement & move from Faringdon, Berks, previously he had been a tenant of The Old Parsonage at Stanton Court'. Always attentive to even the most anecdotal leads that might serve to help shed light on a picture's history, Warner adds, 'When acquired I was told the sitter had been a lady in waiting at the Court of Elizabeth I ... This story is not confirmed’."
I cannot locate a John Denham in thepeerage from the appropriate time period.