The measurements reported were made during a flight to an altitude of 40 km, on Mar. 13, 1974. A polarizing Michelson interferometer cooled, by a liquid helium bath, to a temperature of 1.4 K was used in the measurements. One hour of useful time was achieved at float altitude. The fitted spectrum obtained is shown in a graph. The measured spectrum is a factor less than 2 higher than that of a 2.7 K background at wavelengths above 0.1 cm.