"counting of number of passes between stationary points", as stated by Bohr originally as, . See references above for more detail. While it would be later shown that this assumption is not entirely correct, it in fact ends up being rather close to the correct expression for the orbital angular momentum operator's (eigenvalue) quantum number for large values of the quantum number , and indeed this was part of Bohr's own assumption. Regard the consequence of Bohr's assumption , and compare it with the correct version known today as . Clearly for large , there is little difference, just as well as for , the equivalence is exact. Without going into further historical detail, it suffices to stop here and regard this era of the history of quantization to be the "old quantum theory", meaning a period in the history of physics where the corpuscular nature of subatomic particles began to play an increasingly important role in understanding the results of physical experiments, whose mandatory conclusion was the discretization of key physical observable quantities. However, unlike the era below described as the era of first quantization, this era was based solely on purely classical arguments such as Wien's displacement law, thermodynamics, statistical mechanics, and the electromagnetic theory. In fact, the observation of the Balmer series of hydrogen in the history of spectroscopy dates as far back as 1885. Nonetheless, the watershed events, which would come to denote the era of first quantization, took place in the vital years spanning 1925-1928. Simultaneously the authors Born and Jordan in December of 1925, together with Dirac also in December of 1925, then Schrodinger in January 1926, following that, Born, Heisenberg and Jordan in August 1926, and finally Dirac in 1928. The results of these publications were 3 theoretical formalisms 2 of which proved to be equivalent, that of Born, Heisenberg and Jordan was equivalent to that of Schrodinger, while Dirac's 1928 theory came to be regarded as the relativistic version of the prior two. Lastly, it is worth mentioning the publication of Heisenberg and Pauli in 1929, which can be regarded as the first attempt at "second quantization", a term used verbatim by Pauli in a 1943 publication of the American Physical Society. For purposes of clarification and understanding of the terminology as it evolved over history, it suffices to end with the major publication that helped recognize the equivalence of the matrix