A. H. Wilson concerned the answer probably is that they were much more interested in genera theory than in specific applications, But Fowler and Pauli were interested in appli cations but missed the main one. I knew Fowler well, but I only met Pauli once in Copenhagen in April 1931. When I brought the subject up with Fowler, he said I had the thing right under my nose but I couldn't see it was there. I kick myself whenever I think of it. Pauli was more explicit. He had been engaged over many years in dealing with various magnetic problems by means of the old quantum theory, with varying success. Some problems could be solved satisfactorily, others yielded to a mixture of sound theory and currently unfounded conjectures, while others were quite intractable. One of the problems of the third kind was the weak paramagnetism of the alkali metals. But he had left this somewhat narrow field behind him for the more exciting developments which led to the birth of the new quantum mechanics. However, when the papers of Fermi and Dirac appeared, it occurred to Pauli in a flash that here was the solution to a minor problem which had long been troubling him. But once he had written his paper, solid state magnetic problems were to him a completed chapter, and it never occurred to him that there might be another more exciting chapter on a related theme. His main interest was to establish his theory of the spinning electron To revert to Sommerfeld, he took over Lorentz's theory in its entirety, but with the free electrons obeying the Fermi-Dirac statistics instead of the classical, Maxwellian, statistics. It was therefore essentially a phenomenological theory depending upon two parameters, n, the number of free electrons per unit volume, and 4, the mean free path of the electrons. Since the specific heat of the electrons temperatures, n and I could be deduced purely from the conduction phenomen 4 was negligible compared with that of the lattice vibrations, except at very lo e. The theory of the Hall effect showed that n must be of the same order of magnitude the number of atoms per unit volume, and, to obtain the correct value of the conductivity, for example for silver at room temperature, the mean free path l had to be of the order of 100 interatomic distances and be proportional to 1/T, This behaviour of the mean free path was inexplicable on any classical collision theory, and the correct explanation was given by F. Bloch in 1928 by a thoroughgoing application of quantum theory, on the assumption that a single-electron theory was adequate for this purpose It was shown by G. Floquet in 1883 that the fundamental solutions of any linear differential equation L[f]=0, with one independent variable a, whose coefficients are periodic functions of a with period 2T, are of the form f(a)=e/tu(a), where the exponent u is either complex or purely imaginary and where a(er)=u(a + 2T). Now the potential energy of an electron moving in a crystal lattice must have the same periodicity characteristics as those of the lattice, and Bloch generalized Floquet's theorem to show that the wavefunction of such an electron must be of the form y(r)=e ru (r), where u(r) has the periodicity of the lattice In other words, provided that k is real, the wave function of an electron in a crystal lattice s a modulated plane wave spread over the whole crystal, and a conduction electron42 A. H. Wilson concerned the answer probably is that they were much more interested in genera.l 白白'y由an in specific applica.t.ions. But Fowler and Pauli were inte sted in appli cations bu missed the main one. 1 knew Fowler well, but 1 only met Pauli once - in Copenhagen in April t931. When 1 brough the subject up with Fowler, he said '1 ha.d the hing right under my no bu也 I uldn' see there. 1 kick myself wheneve I 也hink of 'Pau li more exp1icit . He ha.d been engaged Qver ma.ny years in dealing wi various magnetic problems by means of the old quantum eory Wl varying succ s. Some problems could be solved satisfactorily, others yielded to a mixture of sou nd eory and currently unfounded conjectures, while 。也hers were qui ractable. One of the problems of the third kind WM the weak paramagnetism of he a.lkali metals. Bu he had left this somewha narrow field behind him for the more exciting developments which 时也 the bir也h of the new quantum mechanics. However, when he papers of Fermi and Dirac a. ppear时，她 occurred to Pauli in a. flash hat here was the solution to a minor problem which had long been roubling him. Butonce he had wri ten his paper, solid state a.gn ie problems were him a completed chap and never occurred him that there might be ano her more exciting chapter on a. re ated heme. His main interest was to establish his th ryof 'p in electron To revert to Sommerfeld, he took over Lorentz's theory in its entire bu with the free electrons obeying the Fermi- Dirac sta.tistics in.stead of the sical Ma.xwel1ian, statistics. It 础也herefore entially a phenomenological theory depending upon two para. mete阻，饨，也he number of free ec rons per um volume a.nd 1, the mea.n free pa.th of electrons. Since the specific he of the electrons was negligible mpa. red w比 that of the la.ttice vibrations, except very low temperatures, n a.nd 1 could be deduced purely from the conduction phenomena The theory of Hal! effec showed atnm stbeof he sa.me oroer of magnitude as the number of atoms per unit volume, and ，也 obta.in the correct value of the conductivity, for ex nple for silver at room temperature ，也 he mea.n free pa.th l ha.d to be of the order of 100 interatomic distances and be proportional to 1fT. This beha.viour of the mean fr path inexplicable on any cl sica ∞IIi sion theory, and heωη阳也 exp anation was given by F. Bloch in 1928 by a thoroughgoing application of quantum 怕回ry on the assumption that a single ee也，on heory was adequa.te for 也hi purpo was shown by G. Floquet in 883 a. the fund a.mental solutions of a.ny linea.r differential equation L[f] = 0, wi one independen a. riable x , wh