His commentary on Cicero's De Inventione (in Halm's Rhetores Latini Minores, 1863) is very diffuse, and is itself in need of commentary.
Halm subsequently showed that if N may differ in different cases, the equation is a considerable improvement on Rydberg's.
If r =1, we obtain Pickering's equation, which is the one advocated by Halm.
Halm,' to whom we owe a careful comparison of the above equation with the observed frequencies in a great number of spectra, attached perhaps too much weight to the fact that it is capable of representing both line and band spectra.