Stephen Arnold is director of the Microparticle Photophysics Laboratory and Thomas Potts Professor of Physics at the Polytechnic University in New York. Arnold has also held visiting appointments at Caltech, the University of Tokyo and the École normale supérieure in Paris. He is a Fellow of the American Physical Society and the Optical Society of America. Address: Department of Physics, Polytechnic University, 6 Metrotech Center, Brooklyn, NY 11201. Internet: arnold@photon.poly.edu