Experience in using the first list which we drew up of band heads arranged in order of wavelength showed that in extending this list to include a large number of molecules it was desirable to modify the system. The Tables for the Identification of Molecular Spectra are therefore divided into two sections. The first section consists of a list of the most prominent heads of the more persistent and better-known band systems of each molecule. These heads are listed in order of wavelength, with abbreviated information about the direction of degradation of the bands and their appearance, and, of course, the molecule responsible. In earlier editions we made an attempt to include estimates of relative intensity of the heads within the system, listing these intensities under the sources in which the head was likely to be observed; however, since only the most prominent heads of each system were included, most of the intensities were '10' and in many eases information was not really adequate. This information about intensities has therefore been omitted from the first section in this Fourth Edition. This has enabled the table to be set more compactly.