However some observed phenomena such as the anomalous Zeeman effect remain unexplained.
2.
Heisenberg's paper on the anomalous Zeeman effect was accepted as his " Habilitationsschrift " ( Habilitation thesis ) under Max Born at G�ttingen.
3.
Land?s Frankfurt investigations ( December 1920 until April 1921 ) ended with the discovery of the well-known Land?g-formula and an explanation for the anomalous Zeeman Effect.
4.
These two successes, and the continuing failure of the Bohr-Sommerfeld model to explain the outstanding problem of the anomalous Zeeman effect, led Heisenberg to use the virtual oscillator model to try to calculate spectral frequencies.