Annals of the Astrophysical Observatory of the Smithsonian Institution (1900)
Washington : Government printing office · leaf 165
ANNALS Of THE ASTROPHYSICAL OBSERVATORY VOL. 7, PL. 7 Top: Radiation instruments on platform at Camp Lee, Va. Bottom: Smith¬ sonian thermoelements with shutters open, Camp Lee.
Infrared spectra · Solar radiation
Internet Archive · Smithsonian Libraries · Public domain (published before 1931). Free to use for anything.
