THE connexion between superconductivity and lattice vibrations seems now well established through the discovery of the dependence of the transition temperature on isotope mass1; and through the investigations of Frohlich and of Bardeen2 and their followers—preceded by the pioneer work of F. London—the explanation of this puzzling phenomenon by means of the principles of quantum theory seems well under way. There are still difficulties, however, and recently Drell3 has concluded that the interaction between electrons and phonons is not sufficient to produce the energy gap or small electron mass at the Fermi surface predicted by the calculations of Frohlich and Bardeen. On the other hand, the strong degeneracy of the electron–phonon system with corresponding resonances gives rise to such mathematical complication that Drell's conclusion seems also open to doubt4. Therefore, some independent check of the main features of the theory would seem desirable.