On the structure of the line λ = 6708 A. of the isotopes of lithium

From considerations based on known similarities between the series spectra of the alkali elements, it was to be expected that the members of the series v = (1·5, s) — (m, p) for lithium should consist of close doublets. In 1913 Zeeman investigated the absorption of the wave-length λ = 6708 A., the first member of the above series for this element, and found by the use of a concave grating of high resolving power that it consisted of a close doublet, with an estimated separation of 0·144 A. In the following year, Takamine and Yamada also investigated the structure of this line, and found that it could be obtained in the form of a comparatively sharp doublet, with a separation of 0·165 A. In their experiments the source of radiation consisted of a brush discharge between iron terminals moistened with a 10 per cent, solution of LiCl. The spectral separation was measured with a 35-plate échelon grating.