[Sidenote: Ångström’s flask-experiment described.] Professor Ångström then details the examination of an exhausted dry air-flask filled with a discharge analogous to the glow of the negative pole of a vacuum air-tube. [Sidenote: Flask-spectrum compared with Aurora-spectrum.] The experiment is described as follows:—“Into a flask, the bottom of which is covered with a layer of phosphoric anhydride, the platinum wires are introduced, and the air is pumped out to a tension of only a few millimetres. If the inductive current of a Ruhmkorff coil be sent through the flask, the whole flask will be filled, as it were, with a violet light, which otherwise only proceeds from the negative pole, and from both electrodes a spectrum is obtained composed chiefly of shaded violet bands.” The comparison of the spectrum of this violet glow with that of the Aurora gives, according to Ångström, the following results:— Aurora-lines, wave-lengths 4286 4703 5226 Violet light, wave-lengths 4272 4707 5227 Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also compared with other lines in the violet light 4654 and 4601; and the Professor then concludes that it may be in general assumed that the feeble bands of the Aurora-spectrum belong to the spectrum of the negative pole, possibly changed more or less by additions from the banded or the line air-spectrum. [Sidenote: Bright line is due to fluorescence or phosphorescence.] 4thly. That the only probable explanation of the bright line is, that it owes its origin to fluorescence or phosphorescence. The Professor remarks on this point that “an electric discharge may easily be imagined which, though in itself of feeble light, may be rich in ultra-violet light, and therefore in a condition to cause a sufficiently strong fluorescence.” He notes also that oxygen and some of its compounds are fluorescent. [Sidenote: No need of Dr. Vogel’s theory of variability.] 5thly. That there is no need, in order to account for the spectrum of the Aurora, to have recourse to the “very great variability of gas-spectra according to the varying circumstances of pressure and temperature” (Dr. Vogel’s theory). Professor Ångström does not admit such variability, and does not admit that the way a gas may be brought to glow or burn can alter the nature of the spectrum. [Sidenote: Professor Ångström’s conclusions tested.] In order to test some of the Professor’s conclusions in an experimental way, I examined some tube and other spectra not only for line-positions, but also for general resemblance to an Aurora-spectrum. These experiments are detailed in the last Chapter, and the results are comprised in Plates XIV., XV., and XVI., in which the spectra obtained are represented in black for white. [Sidenote: Result of examination of the Professor’s propositions.] The result of the examination of Professor Ångström’s principal propositions seems to be this:—