Unfortunately, I cannot find a larger version of this image. From hoydensandfirebrands.blogspot.com/2010/10/irrepressible-katherine-sedley-countess.
Catherine's bodice has an unusual neckline that passes through by arm pits to join the notched sleeves. It may be a turned over panel or a bertha panel because the gap in her split bodice can be seen below it. The gap between this neckline and the customary neckline is filled in by her chemise. Her dress probably has a vee waistline obscured in shadow.
Keywords: 1685, Kneller, Catherine Sedley, Katherine Sedley, Countess, British, mistress, James II, James VII, long curly coiffure, chemise, modesty piece, off shoulder vee décolletage, elbow length full notched under-sleeves, cuffs, split bodice, quarter length full over-sleeves, full skirt