IN a letter to NATURE of February 11, Prof. J. W. Nicholson discusses the recent interesting experiments of Mr. Evans on the spectrum emitted from a vacuum tube containing highly purified helium and subject to a heavy discharge. Evans found that the 4686 series and the Pickering series can be obtained in a helium spectrum showing no trace of the ordinary hydrogen lines. These series were observed a few years ago by Prof. Fowler by sending a heavy discharge through a mixture of hydrogen and helium; previously they had been observed only in star spectra. In addition, Evans observed that under the same conditions as the 4686 series and the Pickering series a new series of lines appeared, which, with regard to position and intensity, could be united with the Pickering series into a single series of the same type as the 4686 series. The lines of the new series have wave-lengths very close to the hydrogen lines of the Balmer series. In his letter Prof. Nicholson expresses the opinion that Evans's results cannot be used to discriminate between the different theoretical interpretations of the spectra in question, since the new series as well as the Pickering series can be deduced from the 4686 series on the general principle of combination of spectral lines. I cannot agree with this view, and should like here to state my reasons.