Foreword

SOME sixty active investigators of cosmic rays, including representatives from most of the world's leading research groups engaged in this study, attended a congress held under the auspices of the University of Chicago from June 27 to 30, 1939. The attendance of those interested in the subject numbered about 300.

The papers presented at the symposium, either in full or in abstract, together with a part of the unusually active discussion, are published in this number of the Reviews of Modern Physics.

An editorial problem has arisen with regard to the designation of the particle of mass intermediate between the electron and the proton. In the original papers and discussion no less than six different names were used. A vote indicated about equal choice between meson and mesotron with no considerable support for mesoton, barytron, yukon or heavy electron. Except where the authors have indicated a distinct preference to the contrary, we have chosen to use the term mesotron.