The method of producing and isolating the nitrogen afterglow adopted in the observations dealt with in the present paper was that described by one of us in the recent Bakerian lecture. A continuous stream of nitrogen was passed at low pressure through the exciting discharge tube, from which it was drawn into a side tube which was viewed end-on through a quartz window. The source of nitrogen was the commercial compressed gas, but it was purified to the required degree by passage through a tube containing chips of phosphorus and other tubes containing caustic potash and phosphorus pentoxide. The purer the nitrogen the more brilliant was the luminosity of the afterglow; a small amount of oxygen in the nitrogen sufficed to destroy the deep yellow glow, and only a bluish-white luminosity was then visible. Three spectrographs have been employed in the investigation; (1) a small quartz spectrograph, giving a linear dispersion of 5·5 cm. from λ 7000 to λ 2200; (2) a larger quartz spectrograph giving 11·5 cm. for the same extent of spectrum; (3) a Littrow spectrograph, with a prism of light flint glass, giving 16 cm. for the region λ 7000 to λ 3800. For the determination of wave-lengths, comparison spectra of copper (with impurities of lead, etc.) were chiefly used.
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Fowler et al. (1911) studied this question.