Recollections similarity transformation that made an exact connection between the behaviour at high temperatures and that at low. Thanks to this relation, Kramers Wannier were able to deduce a rigorous expression for the Curie temperature in terms of the exchange integral, for whose validity it was necessary only to assume that there was a single singularity at the Curie temperature. Several years later, wannier visited me at my home in New York, where I was engaged in war work. Onsager's exact solution for the two-dimensional ferromagnet had just been published, and Wannier was very depressed. He was carrying a copy of Van der Waerden, s Modern Algebra and announced that he would now have to learn a whole new field of mathe matics before he could understand Onsager's paper and be back in the swim of things. By the time he joined the staff at Bell Laboratories in the late 1940s, he had indeed mastered the new approach, and he and Phil Anderson kept the chalk flying in the tea room with ideas for further extensions 5. MISCELLANEOUS AREAS When Wannier arrived in Princeton-I think it was in the autumn of 1936-as a new postdoctoral from Switzerland, he was just in the course of changing his field from quantum field theory to solid state. His first major research project was a formulation of exciton theory for general energy bands. His desire to make the treatment analogous to the local picture of excitons in terms of localized atomic states led him to the concept of what are now called Wannier functions. The concept eceived practically simultaneous application in Slater's paper of the same year on magnetic exchange and ferromagnons, referred above Despite early work by Bloch, Stoner and others, and despite the subsequent development of the itinerant model of magnetism into a whole school of thought by Stoner, most magneticians for many years considered such models to be much less useful and less realistic than models based on localized atomic moments. In this period some of the most important efforts to describe ferromagnetic phenomena n terms of realistic energy bands, rather than the idealized models used by Stoner and his school, were made by Slater. Just before his 1937 paper just mentioned he had written two papers on the ferromagnetism of nickel, which attempted semiquantitative description in terms of an almost filled d band. However, these papers made very inadequate allowance for correlation effects. A little later, some of Van Vleck's students took up itinerant-electron ferromagnetism: Brooks showed how the theory of ferromagnetic anisotropy could be formulated for a band model Hurwitz showed that even a very large correlation energy could be taken inte account in what he called a" minimum-polarity model'. After the distractions of World War II had cleared away, Van Vleck returned to the question of correlation energy, and showed that if the quantum mechanical fluctuations in the ground state of metallic nickel allowed two d holes to be on the same nickel atom even a small proportion of the time, the Hund, s-rule coupling of these holes could suffice to account for the ferromagnetism of nickel. I had some long conversations with