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The spectrum of hydrogen has probably received more experimental study than that of any other element, and yet in spite of the extended series of lines exhibited by solar hydrogen (a Balmer series of 32 lines), the vacuum tube has steadfastly refused to exhibit more than the first 12 members. The solution of the problem looked rather hopeless at the start, for every possible method of electrical excitation, combined with variable gas pressure, various tube diameters, presence or absence of foreign gases and vapours has been tried over and over again. It will be remembered that by employing absorption as a method of locating the lines, I was able to extend the Balmer series of sodium from the eight members previously known in the emission spectrum, almost indefinitely; 48 in the first investigation, and 56 in a subsequent one made in collaboration with R. Fortrat with the very powerful quartz spectrograph of Prof. Weiss at Zurich, a further extension being merely a matter of increased resolving power and length of the vapour column. This method is impracticable, however, in the case of hydrogen, since this element shows absorption only when subjected to electrical excitation, which of course introduces the same difficulties which occur in the case of the study of the emission spectrum. The first thing to determine, in attempting to add more lines to a series, is the factor which has heretofore fixed the limit of the series.

An extension of the Balmer series of hydrogen and spectroscopic phenomena of very long vacuum tubes