Absorption and Resonance Radiation in Excited Helium and the Structure of the 3889 Line.

In a recent issue of Nature' a brief letter was published on the results of some experiments on the optical properties of excited helium. A detailed account of the experimental work and plates representing the results obtained are being sent to the Philosophical Magazinte for publication. The following is a summary of those results, with the addition of some discussion of their significance. The method used differed from the usual photometric. method in that the light, after passing through the absorbing column, was resolved by means of an echelon grating and examined for a weakening of the central portion of the line, which would be affected by the absorption. It was found possible to completely reverse several lines of the helium spectrum by an absorbing column 80 cm. in length. The lines reversed included not only the principal series of both the singlet and doublet systems, but also the diffuse subordinate series of both systems, not, however, the lines of the sharp subordinate series. Table 1 gives a list of the lines which were reversed.