To study the new elementary particles, Bridge and Martin Annis operated a large rectangular multi plate cloud chamber at Echo Lake. This investigation provided the basis for a 1951 PhD thesis by Annis, supervised by Rossi. The next year, these authors, with another student of Rossi's, Stanislaw Olbert, showed how to derive information on particle energies from measurements of their multiple scattering. This added another way to use cloud chambers to measure the properties of elementary particles. In early 1953, with Bridge, Richard Safford and Charles Peyrou, Rossi published results of a comprehensive cloud chamber study of the elementary particles that became known as kaons. Peyrou was a visitor from at the École Polytechnique, where he had obtained an accurate value of the muon mass in 1947, and Safford was Rossi's student.