Following the success of the 1911 Solvay Conference and the rapid dissemination of the proceedings, the emphasis of research shifted towards the understanding of the spectra of atoms and molecules. With the availability of precision spectroscopic techniques, the bewildering variety of spectral features of atoms and molecules became apparent. The efforts described in Sect. 1.6 indicate how regularities were found in the patterns of spectral lines, the culmination of these investigations being the discovery of the formula for the Balmer series and the various formulae to account for the principal, diffuse and sharp series of the lines in the spectra of sodium, potassium, magnesium, calcium and zinc. As Planck remarked in 1902,

The Zeeman effect: Lorentz and Larmor's interpretations

In 1862, Faraday attempted to measure the change in wavelength of spectral lines when the source of the lines was placed in a strong magnetic field, but failed to observe any positive effect (Jones, 1870). Inspired by this negative result, Pieter Zeeman repeated the experiment and discovered the broadening of the D lines of sodium when a sodium flame was placed between the poles of a strong electromagnet (Zeeman, 1896a).