Numerous experiments have been made in order to find out some admitted connexion between the spectrum of the Aurora and the spectra of the principal gases composing the atmosphere. I limit myself to noticing some of the often-repeated observations in Plücker’s tubes, which contained oxygen, hydrogen, and nitrogen, as well as the observations of the spectrum of the air under different conditions. The experiments were made with a small inductive apparatus, in which the length of the spark between platinum points in ordinary air was 15 millims. at the most. As Zöllner (in the pamphlet mentioned) comes to the conclusion, that if the development of the light in the Aurora, according to the analogy of gases brought to glow in rarefied spaces, is of an electric nature, it must belong to very low temperature—in order to bring the gases enclosed in the tubes to glow at the lowest possible temperature, I have always employed such weak currents that the gas was only just steadily alight. The following observations have been repeated often and at various times. The figures are averages of the indications of the micrometer-screw, so that the uncertainty of the figures will, in the rarest cases, amount to no more than 0·015 turn of the screw, and must be reckoned somewhat more highly only in the case of completely faint misty lines. The spectrum apparatus was that described above, and the slit was nearly the same in every experiment, and so narrow that the sodium-lines could be seen separated. The measurements, for the most part, extend only to the Fraunhofer line G, as I feared lest, through further turning the telescope by means of the micrometer-screw, too great a pressure might be exercised on the worms of the latter. I. _Oxygen._ _a._ In the narrow part of the Plücker tube. +------+------------+--------------------------------------------------+ |Screw.|Wave-length.| Remarks. | +------+------------+--------------------------------------------------+ | 3·97 | 6562 |Moderately bright. | | 5·04 | 6146 |Very bright. | | 6·98 | 5603 |Very bright, misty towards the violet. | | 8·19 | 5332 |Faint. | | 8·95 | 5189 |Moderately bright. | |10·97 | 4870 | ” ” | |11·02 | 4863 |Faint. | |11·26 | 4829 |Bright; misty towards the red end of the spectrum.| |13·30 | 4583 |Very faint. | |14·05 | 4506 |Moderately bright. | |15·55 | 4372 | ” ” | +------+------------+--------------------------------------------------+ _b._ In the wide part of the Plücker tube.