X. Bakerian Lecture .–On the spectrum of hydrogen

Modern theoretical investigations have met with signal success in providing an explana­tion for the production of the primary spectrum of Hydrogen, generally known as the Balmer series5 but the many-lined or secondary spectrum has hitherto proved to be a much more formidable problem. There can be little doubt that a completely satis­factory explanation of its genesis will mark an important step in our knowledge or the origin of spectra. The detection of regularities amongst such a vast number of lines is in itself an exceedingly difficult task, and one for which it is essential that the data relating to wave-lengths should be accurate and complete. Although some of the early investigators were of the opinion that the secondary spectrum was to be referred to impurities in the discharge tube, it is now generally agreed that both the Balmer series and the secondary spectrum are to be attributed to Hydrogen. The relative intensities of the two spectra vary in a surprising manner. Such traces of Hydrogen as are necessary to exhibit the earlier members of the Balmer series are indeed difficult to eliminate from luminous sources, but the secondary spectrum only appears in sources in which considerable quantities of Hydrogen are present, and its intensity relative to that of the Balmer series is greatly enhanced by the complete removal of all impurities.