A photographic spectrum of the aurora of May 13-15, 1921, and laboratory studies in connection with it

On May 13, 1921, and the following days, there was a magnetic storm of almost unprecedented violence, connected without doubt with a large and highly eruptive spot near the centre of the sun’s disc. At the time I was systematically photographing the spectrum of the diffused light of the sky every night for the investigation of the faint aurora line ordinarily present, The instrument used was the spectrograph No. 1 having a Rutherford prism, and a cinematograph lens of 3 inches focus, working at F/1·9. An orthoehromatic plate was used. The direction of view was 45° up to the north.