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photometric differences among samples, light sources or light detectors (including the human eye). Thus, in the case of two brown panels as shown in Fig. 3, Curves A and B, a pair of Filters C and D could be used so as to cause B to JOURNAL OF THE OPTICAL SOCIETY OF AMERICA flicker while A (the standard) remains entirely constant in hue and brightness. We are much indebted to Mr. James M. Moran for handling all mechanical designing of the flickering filter described above.

From the known amounts of the various gases of the atmosphere from sea level to about 20 km, from the observed light absorption coefficients of the gases and from the albedo of the earth's surface the temperature of the atmosphere in radiative equilibrium is calculated on the assumption that the sunlight is the only source of energy. The calculation is perhaps more rigorous than has hitherto been attempted, although it contains a number of approximations. The sea level temperature comes out to be about 19° above the observed world-wide average value 287°K, and the temperature above about 3 km falls many degrees below the observed temperatures. The temperature gradient in levels from 3 to 6 km is greater than that of convective equilibrium and hence the atmosphere would not be dynamically stable if radiation equilibrium prevailed. Therefore air currents take place to bring about convective equilibrium. Continuing the calculation it is found that only when the convective region extends to about 12 km (as is observed), with radiative equilibrium above 12 km (as is observed), does the atmosphere satisfy the conditions of dynamic stability and thermal equilibrium with the received solar energy. For this case the calculated sea level temperature is 290°K in good agreement with the observed value 287°K.