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In this paper an account is given (i) of new tests of stellar radiometers, (2) of new measurements of the total radiation of stars in the region of o hrs. to 12 hrs. in right ascension, not previously measured, and (3) of tests of a method of determining the spectral energy distribution of stars by means of transmission screens, which, either singly or in combination, are placed in front of a vacuum thermocouple used as a radiometer. The results obtained verify previous observations showing that red stars emit three to four times as much total radiation as blue stars of the same visual magnitude. By means of a series of transmission screens (of yellow and red glass, of water, and of a thick plate of quartz) wide spectral regions were isolated, and the radiation intensities in the spectrum from 0.3M to 0.43/1, 0.43^ to 0.6/u, o.6j« to 1.4M, i.4J» to 4.1^, and 4.1/i to jofi were determined. In this manner the energy distribution in the complete spectrum of 16 stars was determined. The observed spectral radiation components were compared with similar data computed for a black body at various temperatures. In this manner it was found that the temperature which a complete radiator or so-called black body would have to attain in order to emit a spectral energy distribution similar to that observed varies from 3000° K for red, class M, stars, to 10 ooo° K, or even higher, for blue, class B, stars. The water-cell transmission of binary stars, having companions of low luminosity, is low, indicating that the companion stars emit considerable infra-red radiation. CONTENTS Page I. Introductory statement 726 II. Apparatus and methods 727 1. Subsidiary investigations in stellar radiometry 728 2. Recent tests of thermocouple material 728 3. Description of vacuum thermocouples used in the present work. . . 731 4. Description of the transmission screens 733 5. The reflecting telescope 736 III. Measurement of the total radiation from stars 737 IV. Measurement of the spectral radiation of 16 stars 739

Tests of stellar radiometers and measurements of the energy distribution in the spectra of 16 stars