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We follow the trajectory of John H. Van Vleck from his 1926 Bulletin for theNational Research Council (NRC) on the old quantum theory to his 1932 book, TheTheory of Electric and Magnetic Susceptibilities. We highlight the continuityof formalism and technique in the transition from dealing with spectra in theold quantum theory to dealing with susceptibilities in the new quantummechanics. Our main focus is on the checkered history of a numerical factor inthe Langevin-Debye formula for the electric susceptibility of gases. Classicaltheory predicts that this factor is equal to 1/3. The old quantum theorypredicted values up to 14 times higher. Van Vleck showed that quantum mechanicsdoes away with this "wonderful nonsense" (as Van Vleck called it) and restoresthe classical value 1/3. The Langevin-Debye formula thus provides aninstructive example of a Kuhn loss in one paradigm shift that was regained inthe next. In accordance with Kuhn's expectation that textbooks sweep Kuhnlosses under the rug, Van Vleck did not mention this particular Kuhn lossanywhere in his 1926 NRC Bulletin (though he prominently did flag a Kuhn lossin dispersion theory that had recently been regained). Contrary to Kuhn'sexpectations, however, he put the regained Kuhn loss in susceptibility theoryto good pedagogical use in his 1932 book. Kuhn claimed that textbooks mustsuppress, truncate, and/or distort the prehistory of their subject matter ifthey are to inculcate the exemplars of the new paradigm in their readers. Thisclaim is not borne out in this case. Because of the continuity of formalism andtechnique that we draw attention to that Van Vleck could achieve hispedagogical objectives in his 1932 book even though he devoted about a third ofit to the treatment of susceptibilities in classical theory and the old quantumtheory in a way that matches the historical record reasonably well.

Kuhn Losses Regained: Van Vleck from Spectra to Susceptibilities