Measurements between two stations 0.4 km apart of the intensity of heat rays transmitted through fog showed that, although 7&mgr; rays were slightly less absorbed by fog than visible light, their penetration into fog was too slight to be of practical interest. For a fog of visibility 0.6 km the distances to reduce the light to 10−2were 910, 970, 980 and 1140 meters for wavelengths 0.6, 1, 3 and 7&mgr;, respectively.