Spectral and pyrheliometric measurements of atmospheric transmission were made by staff members of the Astrophysical Observatory of the Smithsonian Institution at Washington, D. C., and at Mount Wilson, California, during the years 1901--1920. These valuable data are analyzed here with the help of a new inversion method to derive the effective, or area-weighted, radii of stratospheric aerosols formed after three very large volcanic eruptions in this period. After the great eruptions of Katmai (1912) and Santa Maria (1902), reff remained close to 0.3 μm for at least two years. This near constancy of reff has been duplicated in modern times by the aerosols from El Chich¿n (1982). Following Ksudach's (1907) eruption, reff grew from 0.2--0.3 μm to 0.4--0.5 μm in about 1 year. Pinatubo's (1991) aerosols grew similarly. Generally speaking, the pyrheliometric optical depth perturbation, as measured for the three early eruptions, is not equal to the visual optical depth perturbation, but in fact the latter is larger by a factor of about 1.6, at least for reff=0.25--0.45 μm. Wider physical implications of the present results are discussed.¿ 1997 American Geophysical Union