Solvay Conference, 1921 (third conference) Group photograph of the attendees of the third Solvay Conference on Physics, taken outside the Institute of Physiology in Leopold Park, Brussels, Belgium. Back row (left to right): W. L. Bragg, E. van Aubel, W. J, de Haas, E. Herzen, C. G. Barkla, P. Ehrenfest, M. Siegbahn, J. E. Verschaffelt, L. Brillouin. Middle row (left to right): M. Knudsen, J. Perrin, P. Langevin, O. W. Richardson, J. Larmor, H. Kamerlingh Onnes, P. Zeeman, M. de Broglie. Front row (left to right): A. A. Michelson, P. Weiss, M. Brillouin, E. Solvay, H. A. Lorentz, E. Rutherford, R. A. Millikan, M. Skłodowska-Curie. 1921