A very lengthy page from friesian.com discussing Bohr's theory of electronic energy levels and the explanation of commonly observed atomic emission line spectra. The concept of a photon and Einstein's observation of the photoelectric effect are briefly discussed and related to subsequent theories in history. The various electronic energy transitions (Lyman, Balmer, Paschen series) are described and explained. Equations relating the series transitions to the observed wavelengths are discussed. The quantum mechanical model of the atom is discussed, explained and illustrated. Subsequent portions of the page describe units of measurement, the electromagnetic spectrum, physical constants, and duodecimal counting. An informative, well-illustrated but very technical piece of writing.