A pp′ Group in the Arc Spectrum of Zinc

ONE of us (R. A. S.) in a study of pp′ groups in atomic spectra, which will be published in full later, has observed that in many two-valence-system spectra the frequency of the first pp′ group is nearly a mean between the frequency of the first line of the principal series of singlets in the spectrum and the first line of the principal series of doublets of the once more ionised atom.