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C ARL D. AN D E R S O N The production and properties of positrons Nobel Lecture, December 12, 1936 Information of fundamental importance to the general problem of atomic structure has resulted from systematic studies of the cosmic radiation carried out by the Wilson cloud-chamber method. After Skobelzyn in 1927 had first shown photographs of tracks of cosmic- ray particles, Professor R. A. Millikan and the writer in the spring of 1930 planned a cloud-chamber apparatus suitable for cosmic-ray studies, in par- ticular to measure the energies of cosmic-ray particles by means of their curvatures in a strong magnetic field. The chamber, of dimensions 17 x 17 x 3 cm, was arranged with its long dimension vertical, and incorporated into a powerful electromagnet capable of maintaining a uniform magnetic field up to 24,000 gauss strength. In the summer of 1931 the first results were obtained with this technique. The direct measurement of the energies of atomic particles was extended from about 15 million electron-volts, the highest energy measured before that time, to 5 billion electron-volts. In the spring of 1932 a preliminary paper on the energies of cosmic-ray particles was published in which energies over I billion electron-volts were reported. It was here shown that particles of positive charge occurred about as abundantly as did those of negative charge, and in many cases several positive and negative particles were found to be projected simultaneously from a single center. The presence of posi- tively charged particles and the occurrence of showers of several particles showed clearly that the absorption of cosmic rays in material substances is