Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final cause in electrical discharges in the upper strata of the atmosphere, and that these, whether disruptional or continuous, take place sometimes on the outer boundary of the atmosphere, and sometimes near the surface of the earth.
(2) That the Aurora has two different spectra.
(3) That the green line is due to fluorescence or phosphorescence, and that there is no need to resort to Dr. Vogel’s variability of gas-spectra according to circumstances of pressure and temperature.
(4) That an agreement exists between the lines of the Aurora (except the red and green before mentioned) and the lines or bands of the violet light which proceed from the negative pole in dry air.
Zöllner’s remark as to temperature of Aurora and character of spectrum.
Zöllner has pointed out that the temperature of the incandescent gas of the Aurora must be exceedingly low, comparatively, and concludes that the spectrum does not correspond with any known spectrum of the atmospheric gases—only because, though a spectrum of our atmosphere, it is one of another order, and one which we cannot produce artificially.
CHAPTER XI.
THE COMPARISON OF SOME TUBE AND OTHER SPECTRA WITH THE SPECTRUM OF THE AURORA.
[In part from an Article in the ‘Philosophical Magazine’ for April 1875.]