I THINK that all those who have worked on the spectrum of the aurora will congratulate Prof. Vegard on the notable success he has achieved in photographing so distinctly the line or band about λ5238 as recorded in his letter in NATURE of Mar. 5, p. 349. The technical difficulties which he had to overcome were formidable.The present letter is not written in an unsympathetic spirit, but it seems worth while to point out that an interpretation alternative to that of Prof. Vegard is possible. He identifies the band with one which he has observed in the phosphorescent spectrum of frozen nitrogen. But there is a band in about the same position in the negative band spectrum of gaseous nitrogen. Angstrom and Thalen (quoted by Kayser in " Spectroscopie ") gave the wave-length as x5227-5. The stronger bands of the same series come out with great intensity on Prof. Vegard's photograph as on other photographs of the auroral spectrum: so that it is probable, indeed nearly certain, that a long enough exposure would bring out this band. If, as would appear from Prof. Vegard's letter, precise wave-length comparisons are not feasible, the criterion of intensity distribution remains. I hope Prof. Vegard may think it worth while to photograph this negative nitrogen band with the same instrument, for direct comparison with the auroral spectrum. It is not unlikely that this would help a decision