 Although Schrodinger's equation went a lot further than Bohr and De Broglie, there were still a couple of things about the atom that were not completely explained. One was, when examined very closely, many spectral lines showed up as pairs instead of single lines as called out by Schrodinger's equation, and two, the splitting of spectral lines by magnetic fields was not accounted for. This is known as the Zeeman effect. This effect is now used to measure the strength of magnetic fields around distant stars. And three, it was not understood why all the electrons did not move to the innermost lowest energy orbital. In order to deal with these issues, Wolfgang Pauli proposed a fourth quantum number and his exclusion principle. In classical physics, the exclusion principle states that no two objects can occupy the same space at the same time. Pauli's exclusion principle stated that no two particles could occupy the same quantum state at the same time. But Pauli could find no explanation for the fourth quantum number.