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Methods of observing.-The vacuum thermocouple attached to the 100-inch telescope was used as in measuring stellar radiation. A microscope cover-glass was used to separate the reflected light from the planetary heat. A fluorite screen and water-cell were used for special purposes. Reduction of the measurements-Comparison stars, whose radiometric magnitudes we have already determined, were used to reduce the galvanometer deflections to abso- lute intensities. These are expressed in terms of the radiometric magnitude of the planetary heat in a solid angle of i sq. sec. of arc. The relation of radiometric magnitude to absolute temperature is given by the equatio
Pettit et al. (Sat,) studied this question.