This phase is known as the old quantum theory. Never complete or self-consistent, the old quantum theory was rather a set of heuristic corrections to classical mechanics. The theory is now understood as a semi-classical approximation to modern quantum mechanics. Notable results from this period include, in addition to the work of Planck, Einstein and Bohr mentioned above, Einstein and Peter Debye 's work on the specific heat of solids, Bohr and Hendrika Johanna van Leeuwen 's proof that classical physics cannot account for diamagnetism, and Arnold Sommerfeld 's extension of the Bohr model to include special-relativistic effects.