"... in terms of modern solid state physics, what does “transparent” mean? It means that, in the energy spectrum of the electrons in the material, there is a gap of forbidden energies between the occupied states and the empty states ; light quanta corresponding to a visible wave-length do not have the energy needed to make electrons jump across it. This gap is quite a sophisticated concept, entirely dependent on quantum mechanics, and introduced for solids in the 1930’s by the pioneering work of Bloch, Peierls and A. H. Wilson. The theory was based on the assumption that the material was crystalline. ... my coworkers and I ... asked the question "how can glass be transparent?."

“The hypothesis that the electron has a magnetic moment was, as is well known, first introduced to account for the duplexity phenomena of atomic spectra. More recently, however, Dirac has succeeded in accounting for these same phenomena by the introduction of a modified wave equation, which conforms both to the principle of relativity and to the general transformation theory. Formally, at least, on the new theory also, the electron has a magnetic moment of εh/mc, but when the electron is in an atom we cannot observe this magnetic moment directly; we can only observe the moment of the whole atom, or, of course, the splitting of the spectral lines, which we may say is “caused” by this moment.”

“In many materials the electrical behavior changes from metallic to nonmetallic when the pressure, temperature or magnetic field is varied or when the composition is varied, and the theoretical description of these processes is quite complicated. The interest of the problem lies perhaps mainly in our imperfect understanding of the nature of a metal. In the days before quantum mechanics, when I first attended undergraduate lectures on the electron theory of solids, it was taught that in metals one or more atoms from each electron were free, whereas in nonmetals they were somehow fixed to the atoms or ions or to the chemical bonds. The long mean free paths of electrons in metals extending over hundreds or thousands of atomic spacings were not understood, and neither was the absence of any large contribution from the electrons to the specific heat.”

“Any scientist, myself or another, can become so enamoured of his brain child that he resents criticism.”