The X-ray M spectra of some of the rare earths.

Two now X-ray curved crystal vacuum spectrometers are described. With the first of these the LX1,2 lines of Lanthnaum were remeasured, and with the second a study of the Mx,b emissons and the absorptions in the expected region of the and Miv and MV edges was undertaken, in addition the and Miv and Mv edges of Barium were remeasured. The Miv and Mv edges for the rare earth are not found and the absorption detected is of a line character and of the same Wavelength as the emission lines Mx and My. A discrepancy between previous measurements loading to the energy difference between the Miv and Mv levels of Lanthanum has been resolved. Assuggestion is made to account in part for the apparent absence of the normal absorption edges. As a direct result of the investigation of the Lanthanum emission and absorptions, it has been possible to give an explanation which accounts for the presence of the Mx and My lines in Lanthanum and also for the reciprocal nature of the absorption and emission processes, processes. Also described in a section dealing with ancillary equipment are a tapo-coupled, variable ratio micro-densitometer of new design, and an optical micrometer for the measurement of the microdensitometer traces.