They subsequently used these emulsions to make two of the most significant discoveries in physics of the 20th century. First, in 1947 Cecil Powell, César Lattes, Giuseppe Occhialini and Hugh Muirhead (University of Bristol), using plates exposed to cosmic rays at the Pic du Midi Observatory in the Pyrenees and scanned by Irene Roberts and Marietta Kurz, discovered the charged Pi-meson.