Complex sentence: He entered Harvard College, from which he graduated in 1939, and then Columbia University, where he earned his master's, and later his doctorate, writing his 1942 doctoral thesis on "Beta-Ray Spectra of Arsenic, Rubidium and Krypton".
Split into simpler sentences: He entered Harvard College, from which he graduated in 1939. He then earned master's and doctoral degrees from Columbia University, writing his 1942 doctoral thesis on "Beta-Ray Spectra of Arsenic, Rubidium and Krypton".