ELECTRONS (+ and-), PROTONS, PHOTONS, NEUTRONS, MESOTRONS, AND COSMIC RAYS. originally published in 1934. PREFACE: In 1917 the University pf Chicago Press published a small volume entitled The Electron which was intended to be a rather simple presentation of some of the newer de velopments in physics with which my own work had been closely associated. In 1924 a revised edition of this work appeared. A few years later I had the honor of giving The...