Upper Limits to the Intensity of Background Radiation atλ=1.32, 0.559, AND 0.359mm

From observations of interstellar optical absorption lines of CN, CH, and C${\mathrm{H}}^{+}$ we are able to impose upper limits, at three wavelengths, on the intensity of background radiation in the interstellar medium. These limits are consistent with the existence of thermal background radiation at a temperature of ∼3°K, but can be reconciled with the results of a recent rocket observation by Shivanandan, Houck, and Harwit only if the intense flux which they report is concentrated into a sharp line which happens to avoid the molecular resonances, or if this flux is of local origin.