We describe the digitization and reduction of the daily solar chromospheric spectroheliograms obtained in the K-line of Ca II between 1915--1984 at Mt. Wilson Observatory. Our results provide the first reliable information on the behavior of solar magnetic plage area prior to 1947, thus extending by almost 70% the length of the primary data base needed to model past total, UV and EUV solar irradiances. Comparison with other solar activity indices confirms the remarkably linear relation between plage areas, and sunspot number and area, found in the post-1947 data. Our analysis also shows a markedly non-linear relation between the areas of chromospheric plage, and of photospheric white-light faculae. We provide an explanation of this surprising nonlinearity between measurements of two magnetic structures that are known to be closely related in the sun's atmosphere. ¿ American Geophysical Union 1996