The sodium-line reversal method, as previously described, using a photomultiplier and oscillograph, has been modified. Two light beams are now employed, and interference filters are used in front of the photomultipliers instead of a spectrograph. In one beam the background source is viewed directly, through the shock tube, and in the other beam the background source is viewed through the shock tube by a mirror system with a neutral filter interposed to reduce its effective brightness temperature. With a suitably chosen temperature for the background, one oscillograph trace indicates absorption and the other indicates emission of the sodium lines. It is thus possible, from the records of a single shock, to determine the temperature history behind the shock wave to about $\pm $ 20 degrees C. Nitrogen and oxygen again show relaxation effects near the front. Temperatures in argon tend to come low, owing to radiative disequilibrium; excitation processes in argon are discussed. With this system it is possible to determine temperatures rather higher than that of the background source. Some work has also been done, with a single-beam method, using a carbon arc as background and following reversal of the indium blue line. Temperatures up to 3600 degrees K have been measured in shocks through nitrogen, but the time resolution is not so good.