ABSTRACT
Electron and nuclear spin leave distinctive signatures on spectra. Electron spin came first, 1 so that, to a degree, the effects of nuclear spin could be treated by analogy, but the nucleus was a complex entity compared to an electron and raised new questions. A solution called for insights from quantum mechanics, molecular orbital theory, quantum statistics, group theory, and thermodynamics, all except the last being recent additions to the armory of physical theory. The consequences of electron and nuclear spin were worked out in principle in five years, between 1926 and 1930, though the task of unraveling the practical implications for optical spectra, or cataloging the spin of all nuclei, isotopes included, went on much longer.
