1. D. M. Dacey and D. K. Stafford, Department of Biological Structure, Box 357420, University of Washington, Seattle, WA 98195-7420, USA.
2. B. B. Lee, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
3. J. Pokorny and V. C. Smith, Visual Sciences Center, University of Chicago, Chicago, IL 60637, USA.