1. Laboratoire Léon Brillouin CEA-CNRS, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France, Fakultät für Physik, Universität Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany, Polymers and Colloids Group, Cavendish Laboratory, Madingley Road, University of Cambridge, Cambridge CB3 0HE, U.K., and Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, U.K.