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 | Remarks. | | | iron-spectrum. | | +--------------+--------------------+----------------------+ | | { 630·08 } | Moderately bright. | | 629·7 | { 629·85 } | | | | | | | | { 539·60 } | | | | { 539·92 } | Mostly very faint. | | 539·0 | { 539·05 } | | | | { 538·85 } | | | | | | | | { 523·43 | Very faint. | | 523·3 | { 523·21 | Moderately bright. | | | { 522·90 | Very faint. | | | | | | | { 519·79 | ” | | | { 519·40 | ” | | 518·9 | { 519·16 | Moderately bright. | | | { 519·06 | ” ” | | | { 518·51 | Very faint. | | | | | | | { 500·65 } | | | | { 500·52 } | | | 500·4 | { 500·49 } | Very faint. | | | { 500·30 } | | | | { 500·20 } | |