Astronomical seven-color atmospheric extinction coefficients obtained at the European Southern Observatory, La Silla, Chile, over the period 1978--1997 are analyzed to search for secular trends that might affect the global-dimming issue. The direct, clear-sky measurements indicate about a 1% increase in extinction from the years immediately preceding the El Chichon volcanic eruption in 1982 to the quiescent periods 1985--1990 and 1996--1997. A wavelength dependence study of the extinction change suggests that it is consistent with an increase in tropospheric aerosols of meteorological origin. Both the El Chichon and Pinatubo eruptions produced significant perturbations on atmospheric transmission as seen from La Silla. In combination with solar transmission measurements reported from Mauna Loa, the La Silla data and photometric data from other observatories suggest that there has been no large change in the direct, clear-sky, transmission component of solar or stellar radiation, apart from the effects of volcanic eruptions. It is suggested that the extinction effects of volcanic eruptions may be an important factor in the interpretation of some of the global-dimming data.