absorption frequency and each
absorption frequency should obviously, according to Bohr, correspond exactly to the frequency
of an emission line in the
X-ray spectrum, i.e.,
This so-called Kossel relation is of course applicable to all X-ray and optical spectra. Indeed, in the latter field it appeared before the Bohr theory under the name of the “Ritz combination principle.” It has been one of the most important keys to the unlocking of the meaning of spectra and the revealing of atomic structure.