| Summary: | An X-ray curved crystal vacuum spectrograph was made available for the investigation. This and the constructional alterations to the X-ray tube are described. The 'thin' and 'thick' target M emission spectra f and the Miv, v absorption spectra of the elements Europium, Gadolinium and Terbium (in the form of the oxide) have been photographed. Attempts to record the M i, ii, iii absorption edges of Gadolinium and Terbium are also described. The Miv, v absorption spectra of Europium and Terbium (in the form of the fluoride) and Samarium, Gadolinium, Terbium and Ytterbium (in the form of the metals) have been photographed. The observed duality of the Miv, v absorption spectrum of Europium has been explained. Prom the results obtained in the present investigation, it has been possible to complete observations on the M spectra of the rare earth elements. The correspondence between the spectral components of the M emission and absorption spectra has been demonstrated for the middle elements of the series though there are indications that additional transitions are taking place in the emission spectra. It has also been possible to establish the extent to which the Miv, v absorption spectra of these elements is due to transitions into the incomplete 4f shells. Finally, the wavelengths of the spectral components of the M emission and absorption spectra of the middle rare earth elements are quoted and their significance discussed.
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