Spectra of the night sky, the zodiacal light, the aurora, and the cosmic radiations of the sky

All who have thoughtfully looked upon the clear moonless night‐sky are aware that it is not quite dark but that, besides the stars, planets, and Milky Way, it is everywhere faintly luminous. Believing valuable information would be gained regarding the nature and origin of this weak, diffuse sky‐light by spectrographic investigation, I made, in 1915, at the Lowell Observatory, the first attempt to photograph its spectrum. Since then I have continued the study as opportunity has afforded, and with rather remarkable results.