Out of a total of 146 auroral lines, with wave-lengths longer than 3684 tenth-metres, Westman identifies 82 with oxygen or nitrogen lines at the negative pole in vacuum discharges.
Very elaborate observations have been made during several Arctic expeditions of the azimuths of the summits of auroral arcs.
During some displays, auroral light appears in irregular areas or patches, which sometimes bear a very close resemblance to illuminated detached clouds.
Amongst the lines thus identified are the two principal auroral lines having wave-lengths 4276.4 and 3913.5.
The association of high auroral and sun-spot frequencies shown in Table V.
Rubenson (14), from whom Tromholt derives his data for Sweden, seems to accept this view, assigning the apparent increase in auroral frequency since 1860 to the institution by the state of meteorological stations in 1859, and to the increased interest taken in the subject since 1865 by the university of Upsala.
Upsala, however, is a place where the auroral spectrum can often be observed in the sky, even when no aurora is visible, and it has generally been believed that what Angstrom really saw was an auroral and not a zodiacal spectrum.
In addition, he assigns wave-lengths for 156 other auroral lines between wave-lengths 5205 and 3513.
To my imagination it retained throughout the day more or less of this auroral character, reminding me of a certain house on a mountain which I had visited a year before.
A bright aurora visible over a large part of Europe seems always accompanied by a magnetic storm and earth currents, and the largest magnetic storms and the most conspicuous auroral displays have occurred simultaneously.