Between April 1977 and April 1981, four balloon flights of a solar ultraviolet spectrometer were made to altitudes near 40 km in the stratosphere. Analysis of these measurements shows that the solar irradiance in the wavelength range 2350--2870 ¿ has been constant within ¿7% over this period. The central emission cores of the Mg II resonance lines increased in intensity by about 20% over the same time period.