Identity:
Lady Ross has not been identified but appears to have been a collector of JW's pastels. It is possible that she was Rebecca Sophia Barnes, Lady Ross, the widow of Sir Charles William Augustus Ross (1812-1883), 8th Baron Ross. However the address given in the letter (#00866) does not correlate with that given in Debrett's.
Bibliography:
Burke, E., The Peerage and Baronetage of Great Britain and Ireland, London, 1876; Debrett's Illustrated Peerage, Baronetage and Knightage, London, 1896.