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.
In this case the conditions of pressure on these air-strata are themselves so different that, within certain limits, each will yield its own peculiar spectrum; but we shall see the sum of collective spectra, so to speak, spread out behind each other; and therefore the impossibility of attaining a perfect agreement between the Aurora-spectrum and the artificially exhibited spectra of mixed gases is evident.
A comparison of the Aurora-spectrum with the spectra of inorganic substances may be easily worked out by the help of the above-quoted wave-lengths of the single lines of the former, with due regard to probable errors, and with the aid of Ångström’s Atlas of the Solar Spectrum. Here the perfect harmony of the brightest Aurora-line (which was fixed with an exactitude of about one seventh of the separation of the sodium-lines) with the lines of the iron-spectrum is especially striking. The wave-lengths in the above-cited observations of the bright Aurora-line vary between 556·9 and 557·3, whilst, according to Ångström, two lines of the iron-spectrum are situated at 556·85 and 557·17.
Iron-lines corresponding to the other Aurora-lines, within certain limits of accuracy, are also to be found, as will be seen from the following comparison:—
| Aurora-lines. | Lines of the iron-spectrum. | Remarks. | ||
|---|---|---|---|---|
| 629·7 | 630·08 | Moderately bright. | ||
| 629·85 | ||||
| 539·0 | 539·60 | Mostly very faint. | ||
| 539·92 | ||||
| 539·05 | ||||
| 538·85 | ||||
| 523·3 | 523·43 | Very faint. | ||
| 523·21 | Moderately bright. | |||
| 522·90 | Very faint. | |||
| 518·9 | 519·79 | ” | ||
| 519·40 | ” | |||
| 519·16 | Moderately bright. | |||
| 519·06 | ” ” | |||
| 518·51 | Very faint. | |||
| 500·4 | 500·65 | Very faint. | ||
| 500·52 | ||||
| 500·49 | ||||
| 500·30 | ||||
| 500·20 | ||||
| From | 469·4 | 3 stronger and 4 very faint iron-lines. | ||
| to | 462·9 | |||
Yet this agreement, though remarkable, can only be considered as complete proof of the presence of iron-vapour in the atmosphere when we shall have succeeded in showing by observation analogous modifications of the relative conditions of brilliancy in the iron-spectrum by alterations of temperature and density; and in this way explain the appearance of relatively very faint iron-lines in the Aurora-spectrum, or, on the other hand, the absence of the most intense lines.
It will meanwhile remain far more in accordance with probability to regard the Aurora-spectrum as a modification of the air-spectrum; since we are already aware, in the case of gases, of the alteration of the spectra by conditions of temperature and pressure; and an agreement, at any rate, quite as certain between the spectrum in question and the spectra of atmospheric gases has been proved above.