Dr. Schuster was kind enough to examine the spectra I mapped out, and which are shown in Plate XVIII. fig. 15, with the following results:—The lines Oα, Oβ, Oγ are those he has referred to under that designation in his communications to ‘Nature,’ and undoubtedly belong to oxygen. The bands A, B, and C are the bands characteristic of the negative pole. He finds A divided into two parts by a dark space. The spectrum of the negative pole, under good exhaustion, stretches into the capillary part; hence B appears in the capillary as a faint band. A similar thing happens with nitrogen. I., II., III., and possibly 8 and 9, he thinks, are due to the spark-spectrum of oxygen, obtained when the jar and a break are interposed, the brighter lines of the line-spectrum being always present at the negative pole. These last-mentioned lines I have already referred to, as having been found by me in a tube showing phosphorescence after the spark has passed. (Compare Plate XVIII. fig. 15, O violet pole, with Plate XV. spectrum 5.) Nos. 1 and 2, he thinks, are due to some foreign matter, as they are not in all his tubes. Dr. Schuster often finds that a spectrum due to the aluminium electrodes is seen in tubes under great exhaustion; and this he considers is the spectrum of aluminium oxide. A drawing of this spectrum is found in Watts’s ‘Index of Spectra,’ plate iii., “Aluminium first Spectrum.” To this, he thinks, are also due the bands, or sets of lines in my aluminium-arc spectrum (‘Photographed Spectra,’ plate ii.), and he believes lines 3, 4, 5, 6, and 7 in the mapped-out spectra are due to it. It would thus appear that the lines due to O are few in number, and do not well compare with the Aurora-spectrum. PART III. MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA. INTRODUCTION. [Sidenote: Object of experiments. Description of apparatus employed. Electro-magnet. Battery.]