This paper discusses the further work in experimental and theoretical spectroscopy still urgently needed for the understanding of many astronomical phenomena. Most lines or bands in astronomical spectra have been identified, thanks to the laboratory and analysis work performed during the last twenty five years; nevertheless, there remain many unidentified astrophysical lines or bands. Their assignment proceeds at a rather slow pace, owing probably to the fact that pure laboratory spectroscopy has lost some of the glamor it used to have. Also, the interests of astrophysicists are not always identical with those of laboratory spectroscopists. The first part of this paper discusses the spectral sequence, pointing out characteristic cases of unidentified emission or absorption lines. In the second part are indicated some of the essential needs of the astrophysicists. Except in a few rare cases reference is made to only astrophysical investigations of the last twelve years. A complete compilation of all the literature on unidentified lines or bands is not included in the present paper; but rather, typical examples have been indicated. Single identification may occasionally lead to important developments: for example, the assignment of the single interstellar line 4300.3 A to CH [ P. Swings and L. Rosenfeld, Ap. J. 86, 483 ( 1937)], or of the Wolf-Rayet emission of WN-stars 5806 A to C IV (see reference 19), had interesting consequences.
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