Louis Herrman was a partner in the firm of artists' suppliers Gilby and Herrman. His wife, Isabel, was born about 1840 in Lewisham.
Hermann appears to have left Gilby by 1878 when he had an art gallery at 60 Great Russell Street, Bloomsbury (#08878). Around this time Herman lent JW some money, a loan that was negotiated through Charles Augustus Howell. However, JW was unable to keep up with repayments (#02778). He was one of the creditors listed in the records of the London Bankruptcy Court on 7 May 1879 (#11711). Indeed, in 1881 he was listed in the Post Office Directory as a 'picture dealer'. However, in the City of London Directory for 1900 he was listed as a partner in Gilbey and Herrmann, printing material manufacturers, at 7 Plum Tree Court.
UK census 1881 from http://www.familysearch.org (accessed 2004); MacDonald, Margaret F., James McNeill Whistler. Drawings, Pastels and Watercolours. A Catalogue Raisonné, New Haven and London, 1995; The Owl in the clouds (M.676).