The paper is largely a commentary on previous work by the author and collaborators. The spectra studied are shown to be capable of classification into:—(1) A benzene type, characterized by bands at frequency intervals of 102 and 15·9 waves/mm. (2) A benzene aliphatic-derivative type with band intervals of 40 or 80 in the ultra-violet. These substances under the Tesla discharge also usually give a green glow consisting of two bands at an interval of 160. (3) “The Blue Bands”—a spectrum produced by the Tesla discharge through a number of substances of which benzaldehyde is an outstanding example, characterized by three very strong bands at intervals of 172 units. (4) A condensed nuclei type, perhaps not so well established. Anthracene and derivatives show three or four characteristic bands near 4000 Å.U. at intervals of 142 units, whilst naphthalene has a series of bands at one-third of this interval. Many substances do not give any emission, or only one of a continuous character, and so cannot be classified. It is shown that in the infra-red, bands of absolute frequency equal to the above ultra-violet frequency differences are in each case well marked and characteristic.