The spectra of the comets

The Introduction deals with the essential information on the nucleus, the coma, and the tail of comets, which may be obtained by direct photography, photometry, and polarization techniques; the need for "monochromatic" observations is emphasized. It is followed by a description of the emission spectrum of the coma (from the OH emission near 3100 to the red CN band near 8100) and by that of the spectrum of the tail in the ultraviolet and photographic regions, with emphasis on the recently discovered bands. The complex profile of the emission bands is shown to be due to a resonance fluorescence excitation, account being taken of the absorption lines of the exciting solar radiation; other possible excitation mechanisms are briefly mentioned. The probable assignment of the 4050 group to the tricarbon molecule and its consequences are discussed. The paper ends with physical considerations on the still unidentified emissions, on desirable physical investigations, on the parent molecules, and the production and evolution of cometary atmospheres, and on the densities and the extent of molecules in the cometary head.