34 R. E. Peierls take electron-electron forces into account. This pessimism has since proved un justified. An important episode for my understanding of conduction problems arose from a paper by Kretschmann, 5) who attacked the then accepted theory of conductivity and claimed that the basis of the papers by Bloch and others was quite wrong He had a number of objections which were mostly not very well conceived, but he claimed, in particular, that in the usual derivation of the Boltzmann equation one had made unjustified use of perturbation theory. In trying to defend the theory I therefore set out to prove that perturbation theory was in order, and to my amaze- ment I found that this was very questionable, if not exactly for the reasons given by Kretschmann. It appeared that the usual application of fermis 'golden rule depended on the inequality here T is the collision time. This was not satisfied for many metals. For most the wo quantities were of the same order of magnitude. Indeed Landaus dimensional analysis made them comparable, (16) This created a dilemma, in which Landau again came to the rescue. He produced transfer in collisions was negligible, i. e. both at high temperatures, abore ts in ingenious argument showing that, at least in situations in which the energ Debye 0, and also where impurity scattering was dominant, it is sufficient that o /T<er which is much less restrictive, and holds for all normal metals. For intermediate temperatures, and for semiconductors, the point is still obscure This argument of Landau'sam is still essential today. It is now somewhat easier to visualize in terms of Kubos formula, and it is expressed in more highbrow ways in more recent transport theories, unless the problem is simply ignored, which is also not uncommon In the 1930s we were strongly influenced by the impressive successes of quantum mechanics in clearing up the basic problems of solid state physics, and this was probably responsible for a tendency to concentrate on general points of principle rather than on specific cases. This was certainly the attitude of Pauli, who had in a sense, opened up the field with his paper on paramagnetism, and who maintained an interest in the field to the extent of attending conferences devoted to solid state theory( e. g in Geneva in 1934). But as the work became more detailed, and required more ad hoc assumptions, it seemed to him 'dirty physics Perhaps it is typical therefore that, when during the Zurich period, I became interested in the optical properties of solids, 8)I looked only at the spectra of pure substances, using the tight-binding model, so that the theory applied strictly only to the rare-earth salts, or to crystals made of large organic molecules in which the optically active atoms were well shielded from each other. I was, of course, aware of the work by Pohl and others on the origin of luminescence and other optical34 R. E. Peierls take electron-electron forces into account. This pessimism has since proved un￾justified. An important episode for my understanding of conduction problems arose from a paper by Kretschmann,(15) who attacked the then accepted theory of conductivity and claimed that the basis of the papers by Bloch and others was quite wrong. He had a number of objections which were mostly not very well conceived, but he claimed, in particular, that in the usual derivation of the Boltzmann equation one had made unjustified use of perturbation theory. In trying to defend the theory I therefore set out to prove that perturbation theory was in order, and to my amaze￾ment I found that this was very questionable, if not exactly for the reasons given by Kretschmann. It appeared that the usual application of Fermi's 'golden rule' depended on the inequality niT ~ kT, where T is the collision time. This was not satisfied for many metals. For most the two quantities were of the same order of magnitude. Indeed Landau's dimensional analysis made them comparable.(16) This created a dilemma, in which Landau again came to the rescue. He produced an ingenious argument showing that, at least in situations in which the energy transfer in collisions was negligible, i.e. both at high temperatures, above the Debye 0, and also where impurity scattering was dominant, it is sufficient that which is much less restrictive, and holds for all normal metals. For intermediate temperatures, and for semiconductors, the point is still obscure. This argument of Landau's(17) is still essential today. It is now somewhat easier to visualize in terms of Kubo's formula, and it is expressed in more highbrow ways in more recent transport theories, unless the problem is simply ignored, which is also not uncommon. In the 1930s we were strongly influenced by the impressive successes of quantum mechanics in clearing up the basic problems of solid state physics, and this was probably responsible for a tendency to concentrate on general points of principle rather than on specific cases. This was certainly the attitude of Pauli, who had, in a sense, opened up the field with his paper on paramagnetism, and who maintained an interest in the field to the extent of attending conferences devoted to solid state theory (e.g. in Geneva in 1934). But as the work became more detailed, and required more ad hoc assumptions, it seemed to him 'dirty physics'. Perhaps it is typical therefore that, when during the Zurich period, I became interested in the optical properties of solids, (18) I looked only at the spectra of pure substances, using the tight-binding model, so that the theory applied strictly only to the rare-earth salts, or to crystals made of large organic molecules in which the optically active atoms were well shielded from each other. I was, of course, aware of the work by Pohl and others on the origin of luminescence and other optical