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Millikan's Photons. It seemed natural to Dr. Millikan that the cosmic rays were light rays or photons of enormously high frequency and short wave length. He concluded that they were the by-products of atom-building in interstellar space; that when light-weight atoms suddenly combined to form heavier ones, a slight excess of matter—like bricklayer's mortar scraped from the wall—was turned into high-frequency light according to the Einstein equation. To him this was a peculiarly satisfying interpretation as it bespoke "the Creator still on the job."
Nevertheless photons are electrically neutral, are not swerved by magnetic fields. If the cosmic rays were Millikan photons, they should not tend to cluster about Earth's strongly magnetic poles, to avoid the weakly magnetic Equator. Yet a Dutchman named Clay, traveling from Holland to Java, found a drop in cosmic ray intensity at the Equator. Kolhorster took this to mean that at least some of the rays were not photons of light, but electrically charged particles of matter.
At this point Arthur Holly Compton, already a crowned king of terrestrial radiation, leaped into the cosmic quest. He had an impatient desire to collect a mass of far-flung recordings with the greatest possible speed. Eight cooperating expeditions were to measure the rays in Greenland, Denmark, India, Ceylon, Java, Tibet, South Africa, Eritrea, Spitsbergen, Switzerland. One man was to make records from Peru around the Cape of Good Hope to the U. S. Two Compton men were killed trying to scale Mt. McKinley in Alaska. Dr. Compton himself, with his wife and elder son, set out on a cosmic search that took him to a volcano brim in Hawaii, Mt. Cook in New Zealand, Panama, Peru, covered 50,000 miles. He made an airplane flight within 350 miles of the North Magnetic Pole. When all the data was in his hands, he found overwhelming evidence for variation by latitude, ranging up to 20%, concluded that most of the rays were electric corpuscles affected by the varying magnetic pulls of Earth. Dr. Millikan clung to his photons, alleging that the corpuscles were secondary rays knocked out of air atoms by the primary photons.
Retreat. At this stage the problem reached the midwinter convention of the A. A. A. S. at Atlantic City in 1932, which both Nobel Laureates attended. For a time the two seemed on the verge of noisy dispute (TIME, Jan. 9, 1933). Dr. Compton had found charged particles with energies of 30,000,000,000 volts, which could hardly be secondary rays dislodged from the air. Dr. Millikan said that anyone recording voltages over a billion must have muffed the technique.