Fillets of Soles with Ravigote Sauce

Place some fillets of soles in a saucepan with a lump of butter, the juice of half
a lemon and a little pepper and salt, cook over a slow fire, but do not brown them.
Pour two and one-half teacupfuls of white sauce into a saucepan, with one and onehalf
teacupfuls of white broth, and boil for three or four minutes, keeping it well
stirred. Mix a little chopped chervil, parsley and tarragon with two ounces of fresh
butter and stir it into the sauce, with two teaspoonfuls of tarragon vinegar, seasoning with
salt and pepper and stirring it over the fire for a minute longer. Place the fillets on
a hot dish, pour the sauce over and serve.