3. _Comparison of the Aurora-Spectrum with the Spectra of Atmospheric Gases and of Inorganic Substances._ In the next place, I turn to the comparison of the observed spectra of different gases and of the air with the spectrum of the Aurora. The first band of light in the red part of the Aurora-spectrum most probably coincides with the first system of lines in the spectrum of nitrogen (_a_). Probably only the bright part of this group of lines is perceptible, on account of the extreme faintness of the Aurora; and as in nitrogen the increase of the brilliancy of the spectrum takes place towards the violet end, the absence of the intermediate spectrum towards this direction would find its explanation. The most intense line of the Aurora-spectrum at 7·12 is to be also found in the spectrum of nitrogen (_a_)—as a very faint line, however. That this line appears in the Aurora by itself, and with intensity relatively great, need not appear strange, considering the great alteration of the gas-spectra under different conditions of pressure and temperature. The third line of the Aurora-spectrum, very vaguely defined on account of its great faintness, coincides in the same way with a nitrogen-line. The line at 8·71 is met with in the nitrogen-spectrum (_c_), as well as in the air-spectrum (_b_). The third line of the oxygen-spectrum at 8·95, which _seems to appear under very different conditions_, is found again, as the fifth line in the spectrum of the Aurora. _Moreover, the sixth line in the Aurora at 10·06 coincides very exactly_ with the known nitrogen-line appearing in the spectra of some of the nebulæ. Lastly, as to the broad band of light in the Aurora-spectrum from 12·33 to 12·88, several lines are found in this place in the spectrum of nitrogen as well as the air-spectrum (_a, b_); so that here, too, a coincidence between the spectra may be regarded as probable. The observations show with some certainty that at least one line at 10·06 of the Aurora-spectrum coincides with the maximum brilliancy of the air-spectrum, and that the other lines appear with great probability in the spectra of atmospheric gases. In the very great difference of the gas-spectra under varying conditions of pressure and temperature, it would indeed be difficult to succeed in producing artificially a spectrum which should resemble that of the Aurora in all parts. Moreover, it must be admitted, under the hypothesis that the Auroræ are electric discharges in rarefied air-strata, that these strata, qualified for the transmitting of electricity, will have a very considerable thickness.