It is well known that the spectrum of the aurora is characterised by two outstanding features, the first of which is a set of bands with heads at or near λ = 3914 A, λ = 4278 A, and λ = 4708 A. The second is a strong narrow sharply-defined line close to λ = 5577 A. As to the bands, Lord Rayleigh, Dr. Slipher, Prof. Vegard and others have shown them to be identical with the so-called “negative” bands obtained with molecular nitrogen in the singlyionised state. Nitrogen in this state must, therefore, be one of the main constituents of that portion of the upper atmosphere in which auroral displays occur. As to the line λ = 5577 A, it is the strongest constituent of the spectrum of the aurora. Lord Rayleigh, Dr. Slipher, Dr. Babcock and others have shown that it can be obtained as well in the spectrum of the light of the night sky. In 1923 Dr. Babcock made an accurate determination of its wave-length with a Fabry and Perot interferometer and found it to be 5577·350 ± 0·005 I. A.
A Strong Infrared Radiation from Molecular Nitrogen in the Night Sky.