Photography of the Infra-red Solar Spectrum

Abstract

NEARLY fifty years ago, Sir William Abney (Phil. Trans., Part II., p. 653, 1880, and Part II., p. 457, 1886) photographed and measured fine detail in the solar spectrum out to λ9867. He also recorded, with low resolving power, a few broad absorption bands of greater wave-length, but he evidently observed no individual absorption lines having wavelengths exceeding that of the line mentioned. It is remarkable that, in spite of some subsequent improvements in equipment, no one has measured lines in the solar spectrum out to the limit reached by Abney. The nearest approach of which I am aware is that of Brackett (Astro-physical Journal, 53, 121; 1921), who measured λ9849 and could see a few more faint lines beyond.