Colours of lines are functions of wave-length, subject, however, to the observation that in a weak spectrum the colours lose their intensity. The red line in the Aurora has sometimes been found brighter than the green. It has been suggested that the red and green may be independent spectra; but the variations of tint observed in the capillary of hydrogen and other tubes according to resistance of the current, demonstrate that the varying colours of the Aurora may be connected with the lighting-up of particular parts of the spectrum, and do not necessarily indicate that different gases and spectra are excited.
Absorption may also play an important part in the nature of the Aurora-spectrum (Zöllner’s theory that the lines are really spaces between absorption bands). Most gases will give a continuous spectrum under certain circumstances, even at a low pressure.
The question of cosmic dust is inviting, but the facts collated hardly warrant at present its probable connexion with the Aurora.
If Auroræ were composed of incandescent glowing meteors, it would be reasonable to expect to find in the spectrum the lines of iron, a metal constituting so prominently the composition of meteorites. No connexion between the iron and the Aurora-spectra is, however, proved; though it may be suspected. The iron-spectrum, as remarked elsewhere, contains so many lines that some may, as a mere accidental circumstance, closely agree with the Aurora-lines.
The iron-lines are, it may be remarked, as a rule, sharper and finer than the Auroral lines, though it is possible that these characteristics might vary if the spectrum were obtained in a rarefied medium. Tubes with iron terminals are said to evolve a compound gas of H and Fe. I have not had an opportunity to verify this.
It may be added that the comparative faintness of the more refrangible lines of the Aurora-spectrum suggests a feeble resistance to the exciting current, and a low temperature inconsistent with a meteoric theory; and this is not contradicted by the brightness of the red and green lines, if these are due to a phosphorescent origin. Expansion of a line is recognized to be dependent on pressure, and consequently the breadth of the green or red lines might indicate the height of the Aurora; while their brightness or otherwise might also give some idea as to its density. No observations in this direction have, as far as I am aware, been recorded.
As the general result of spectrum work on the Aurora up to the present time, we seem to have quite failed in finding any spectrum which, as to position, intensity, and general character of lines, well coincides with that of the Aurora. Indeed, we may say we do not find any spectrum so nearly allied to portions even of the Aurora-spectrum, as to lead us to conclude that we have discovered the true nature of one spectrum of the Aurora (supposing it to comprise, as some consider, two or more). The whole subject may be characterized as still a scientific mystery—which, however, we may hope some future observers, armed with spectroscopes of large aperture and low dispersion, but with sufficient means of measurement of line positions, and possibly aided by photography, may help to solve. The singular absence of Auroræ has, for some time past, given no opportunity in that direction. May some of my readers be more fortunate in obtaining opportunities of viewing the glorious sky-fires, and assist to unravel so interesting a paradox!