C. herring of the thesis dealt with energy coincidences between different bands, i. e for n+n, that were not required by symmetry, and noted the interesting fact that some such coincidences are indestructable, in the sense that they cannot be made o disappear(though they can be moved about in k-space) by any infinitesimal change in the lattice potential. I found this work intellectually fascinating, but had grave misgivings about its applicability to real electronic energy bands, because I felt that correlation effects(see below)must make the picturing of metal electrons in one-electron terms a very poor approximation. Fortunately, I realized that the results would have a much more clearly valid applicability trum of phonons, so I did not feel that i was being awarded a degree under entirely In the following academic year(1937), John Slater visited Princeton(Institute for Advanced Study for a sabbatical semester. His methods of reasoning and working were very different from Wigner's, or from my own, and did not at first appeal to me, but I was very impressed by his productivity: in that one semester, by simply barging ahead with calculations in areas that seemed to him to contain pay dirt,, he succeeded in completing four papers, two of which have emerged in historical perspective as classics. (These were the invention of the APw method and the formulation of the theory of ferromagnetic exchange coupling for an insu lator in band-theoretical terms, respectively. )In the hope that some of this facility might rub off on me, I applied to the National Research Council for a fellowship to do postdoctoral research with him at m.I.T. The fellowship was awarded, so I spent the years 1937-9 at M I T. However, my strongest intellectual contacts were not with Slater, but with John Bardeen, who had gone from Princeton to Harvard a year earlier to be a Junior Fellow there, and with some of the younger people, mostly experimental, at M.I. T. I was particularly impressed, in view of my develop ing interest in band calculations(see below), by Bardeen's improvements to the Wigner-Seitz approach, especially in the direction of accurate calculation of the effective mass at the bottom of a band My original intention in these postdoctoral years was to develop more realistic models of lattice vibrations, particularly in metals. Since I was convinced that interatomic force constants could not be adequately described in terms of mere nearest neighbour forces or in terms of simple pair potentials, I decided to try to determine the force constants from first principles by suitably generalizing the Wigner-Seitz method of calculating lattice energies. Though such calculations were eventually made in the late 1950s by Toya, and have since become quite common- place thanks to the concept of pseudopotentials, I found the task too difficult for me at the time, and decided that one would need first to develop a new and more tractable formalism for the calculation of band structures in general. For the simple metals it seemed that one ought to be able to capitalize on the great similarity of their wavefunctions to plane waves; yet it was already known that the strong