Annals of the Astrophysical Observatory of the Smithsonian Institution (1900)
Washington : Government printing office · leaf 163
VOL. 7, PL. 5 ANNALS OF THE ASTROPHYSlCAL OBSERVATORY Top: Modified Angstrom pyrheliometer in motor driven mounting. Bottom: 1 wo views of the Harvard photometer.
Infrared spectra · Solar radiation
Internet Archive · Smithsonian Libraries · Public domain (published before 1931). Free to use for anything.
