The first considerable list of lines due to singly-ionised fluorine (72 in number, extending from λ 4934 to λ 3145) was given in 1912 by Porlezza,* who obtained the spectrum by passing discharges through vacuum tubes containing silicon tetrafluoride. At that time it was not possible to assign the lines to the ionised atom, and Porlezza made no attempt to analyse the spectrum. About the same time, Exner and Haschek recorded a list of lines, obtained from a spark between electrodes of carbon impregnated with potassium fluoride, which they attributed to fluorine. The list included a few of Porlezza’s lines and a considerable number of others whose identity remains obscure. Excluding a few minor investigations, recorded in Kayser’s “Handbuch der Spectroscopie,” vol. 7, pp. 401-404, the next contribution of note towards our knowledge of this spectrum was made by Gale and Monk, who obtained 24 of the lines given by Porlezza, as well as 6 others, from a spark discharge in fluorine gas. Although Gale and Monk did not attribute these lines to the ionised atom, they distinguished them from lines produced in vacuum tube discharges in fluorine which have since been shown to belong to the neutral atom. The first record of a spectrum definitely assigned to F + was made by de Bruin,§ about half of whose list of 71 lines between λ 4447 and λ 3057 were included in Porlezza’s list. De Bruin discussed the possibility of analysis of the spectrum, but his data, as it now appears, were insufficient to permit of any satisfactory results. Up to now, therefore, the spectrum has been very imperfectly known, and the present investigation (which forms part of a general research on the successive spectra of fluorine, of which some of the results have already been published||) was undertaken in the hope of obtaining more definite knowledge. 469 lines have been obtained, of which nearly 200 have been classified. The most prominent lines are those given by Porlezza, Gale and Monk, and de Bruin, and de Bruin’s assignment of the spectrum to the singly ionised atom has been confirmed. The large addition to the lists of those authors is due partly to the inclusion of fainter lines and the extension of the spectrum beyond the limits of their work, but also in a considerable degree to the use of high resolving power, by which several lines previously recorded as single have been found to be multiple. For example, λ 4103 has been resolved into five components, λλ 4447, 4207 into three each, and the “triplets” at λ 3500 and λ 3900 into nine and five, respectively. It should be mentioned also that some of Porlezza’s and de Bruin’s lines have not been confirmed. While some of the fainter lines now recorded might well be due to undetected impurities, there is no doubt that the great majority belong to F II.