Consists of scientific correspondence from the period Shapley was at Mount Wilson Observatory. Also included are some items from the early years at Harvard that were not filed with the director's records. Topics include galaxies, globular clusters, variable stars, pulsation hypothesis, eclipsing binaries, stellar and nebular photometry, spectroscopy, and cosmology. Correspondents include Robert G. Aitken, Solon I. Bailey, Robert H. Baker, Edward E. Barnard, William W. Campbell, Julian L. Coolidge, Heber O. Curtis, Arthur S. Eddington, Henry G. Gale, George Ellery Hale, Ejnar Hertzsprung, Jacobus Cornelius Kapteyn, Oliver D. Kellogg, Adrian van Maanen, Edward C. Pickering, D. S. Richardson, Henry N. Russell, Frank Schlesinger, Frederick H. Seares, Vesto M. Slipher, Joel Stebbins, Robert Trumpler, William M. Wheeler, Edwin B. Wilson, and Robert M. Yerkes.