I do not notice that the Professor touches upon the external features of the Aurora in respect of this question. October 20, 1870.—I noted the grand Auroral display of that evening, including “streamers of opaque-white _phosphorescent_ cloud very different from the more common transparent Auroral diverging streams of light.” February 4, 1872.—A fine display. The first signs were (in dull daylight) “a lurid tinge upon the clouds, which suggested the reflection of a distant fire, while, scattered among these, torn and broken masses of white vapour, _having a phosphorescent appearance_, reminded me of a similar appearance in October 1870.” (Other instances of this effect will be found in the section Aurora and Phosphorescence.) Day Auroræ, too, we might suppose could hardly be seen without the presence of some phosphorescent glow. [Sidenote: Other confirmatory circumstances. Conclusion in favour of the theory.] Having regard to the near proximity of the phosphoretted-hydrogen band to the bright Aurora-line, to the circumstance of this band brightening by reduction of temperature (a phenomenon probably connected with ozone), to the peculiar brightening of one line in the green in the “Aurora” and “phosphorescent” tubes (the phosphorescent tubes probably containing O), and to the observed circumstance that the electric discharge has a phosphorescent or fluorescent after-glow (isolated, I believe, by Faraday), I feel there is strong evidence in favour of such an origin to the principal Aurora-line, if not to the red line as well. [Sidenote: Invariability of gas-spectra questioned.] 5th. Professor Ångström opens a wide door to discussion in his proposition of the invariability of gas-spectra, and I do not now attempt to follow in detail this interesting part of the present subject. Suffice it to say, that if the Professor lays down this proposition in its strictest sense (I can hardly suppose he so meant it), there is, so far as I am aware, no one spectrum that can at all claim comparison with the Aurora-spectrum. Giving greater latitude to the Professor’s words, I reply, upon competent authority, that lines vary in number and brilliancy with temperature, and in breadth with pressure. Kirchhoff, too, in speaking of vapour-films as increasing the intensity of lines, states “it may happen that the spectrum appears to be totally changed when the mass of vapour is altered.” We may, too, now add magnetism as capable of effecting a change in certain spectra, not only as to brilliancy, but even as to position of lines. (Chautard’s Researches, ‘Philosophical Magazine,’ 4th series, vol. 1. p. 77, and experiments detailed in Chap. III. of this work.) CHAPTER XIII. THE OXYGEN-SPECTRUM IN RELATION TO THE AURORA (PROCTER AND SCHUSTER). [Sidenote: Procter’s oxygen-spectrum.]