The correlation of spectroscopic and chemical investigations in recent years has been highly beneficial of many reasons. Around 1950, no valid explanation was available of the colours of compounds of the five tran sition groups. Later, it was possible to identify the excited levels with those expected for an electron configuration with adefinite number of electrons in the partly filled shell. I t is not generally recognized that this is equivalent to determining spectroscopic oxidation states related to the preponderant electron configuration and not to estimates of the fractional atomic...