| N. | VN. | VN. | | | -------------+--------+------+------+------+------+------+--------+------ * No results in the examined spectra; but see Plate XIII. fig. 2. ** Too uncertain in position for comparison (see Plate XIII. fig. 1). Tested by coincidence, or close proximity of lines to those of the Aurora, we arrange the spectra in the following order:—(1) iron, (2) air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) carbon and oxygen. The air-tube spectrum might perhaps stand higher in the scale but for its broad bands, which make comparison doubtful. Lines of oxygen possibly escape detection in the Aurora from the faint character of its spectrum. The phosphorus and iron spectra are especially interesting in connexion with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546). [Sidenote: Additional Table of compared spectra.] As an addendum to the foregoing, on Plate IX. fig. 1 will be found a Table I have prepared, in which a type Aurora and also Vogel’s and Barker’s Auroræ are compared with eight other spectra, viz.:— S. Solar spectrum. N. Nitrogen (air): Watts. O. Oxygen (air): Watts. C.H. Carburetted-hydrogen vacuum-tube: Watts. C.I. Carburetted-hydrogen flame: Watts. C.C. Blue base of candle-flame: Capron. O.P. Oxygen vacuum-tube: Procter. I. Iron: Watts. The divisions and vertical lines will guide the eye in making comparison of the spectra. CHAPTER XII. SOME NOTES ON PROFESSOR ÅNGSTRÖM’S THEORY OF THE AURORA-SPECTRUM. [The substance of these appeared in the ‘Philosophical Magazine’ for April 1875, in conjunction with the “Comparison of the Tube and other Spectra” (Chapter XI.), but they are now, for the sake of convenience, made a separate article.] [Sidenote: Professor Ångström’s propositions.] In a contribution by the late Professor Ångström to a solution of the problem of the Aurora-spectrum (an abstract of which appeared in ‘Nature’ of July 16, 1874), the Professor is stated, amongst other things, to have laid down certain propositions in substance as follows:— [Sidenote: That the Aurora has two spectra.] 1st. That the Aurora has two different spectra—the one comprising the one bright line in the yellow-green only, and the other the remaining fainter lines. [Sidenote: That bright line does not coincide with HC₂.] 2ndly. That the bright line falls within a group of hydrocarbon lines, but does not actually coincide with any prominent line of such group, and that Dr. Vogel’s finding this line to coincide with a not well-marked band in the air-spectrum must be regarded as a case of accidental coincidence. [Sidenote: That moisture is _nil_ in Aurora region.] 3rdly. That moisture in the region of the Aurora must be regarded as _nil_, and that oxygen and hydrogen must alone there act as conductors of electricity.