Wave-lengths of additional lines in the many-lined spectrum of hydrogen

During the investigation of the secondary spectrum lines of hydrogen, in which Prof. Richardson and the present author have been recently engaged, we found many weak lines which are not recorded in any published tables. Most of them were found on the two spectrum plates taken by Prof. Merton and lent to us, as well as on our plates taken under quite different conditions from the above. Because our investigation was partly to find the regularity of the spectrum, it was thought highly desirable to know the accurate wave-lengths of these weak lines. On the above-mentioned plates of Prof. Merton, which were taken with an Anderson concave grating, the spectral lines were much better resolved than on our plates. They were also rather over-exposed, and therefore quite suitable to the present purpose. For these reasons the following measurements were entirely made with those plates. It should be added that these two plates were taken by Prof. Merton with a vacuum tube having a narrow capillary, and the current density must have been at least twenty-five times as great as in the tubes which Merton and Barratt used for measuring the lines given in their tables. There is little doubt that this and perhaps other differences in the conditions of excitation is the reason why the additional lines which I have measured were not observed by Merton and Barratt on the plates they used for measurement.