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 l , and indeed this was part of Bohr's own assumption. Regard the consequence of Bohr's assumption L 2 = l 2 ℏ ℏ 2 , and compare it with the correct version known today as L 2 = l ( l + 1 ) ℏ ℏ 2 . Clearly for large l , there is little difference, just as well as for l = 0 , 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.