The book gives the theoretical background of the principles governing the structure of line spectra, but not an account of all observed spectra, experimental data being introduced only as illustrative examples (as in Appendix III). A detailed account of the structure of spectra of different types with mention of all those known in 1927 is given in the excellent book by F. Hund, "Linienspektren und periodisches System der Elemente," J. Springer, Berlin, 1927.