DR. ANDERSON'S discovery of the positive electron or positron arose also through the study of cosmic radiation. During an investigation of the properties of the rays by means of a cloud chamber in a strong magnetic field, Anderson found certain photographs which revealed the tracks of particles with about the same ionization, and so about the same mass, as electrons, but which were curved by the magnetic field in the direction corresponding to particles with a positive charge. This exceedingly important result was published in 1932 as a short communication to Science, in which Dr. Anderson wrote: “For the interpretation of these effects it seems necessary to call upon a positively charged particle having a mass comparable with that of an electron.” Thus a new member was added to the select list of fundamental particles. Subsequent work has shown, not only that about half the cosmic ray particles are positrons, but that they are also often produced as one of the partners of pairs of positive and negative electrons, when gamma-rays of high energy are absorbed by matter. These positrons do not live long, as they readily combine with other negative electrons to form more radiation. The discovery of the positron provided the final proof of the validity of Dirac's theory of the electron, for this theory requires the existence of just such a particle.