195 with the variation of total energy, or an expansion and contraction of the star. The variations in diameter necessary to explain the observations are from O'/OßS, nearly at maximum light, to O'/OSS, at about one-third phase after maximum light. The radial velocities which would result from these variations in diameter have a maximum of +4 km/sec. throughout minimum phase and -10 km/sec. about one-tenth phase after maximum light. To make them comparable with those obtained by the

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... , these radial velocities have been computed by means of a parallax of 0'/017 and an average velocity over the stellar disk equal to three-fourths the velocity of expansion and contraction. Although the amplitudes of the variations in the computed and the spectroscopic radial velocities are of the same order of magnitude, they are nearly opposite in phase. The object of this investigation was to determine the intensity of sun-spots and faculae at different distances from the center of the Sun relative to the intensity of the photosphere surrounding them. Such observations give information, about sun-spots and faculae which any adequate theory must satisfy. The data for this work were obtained from the direct photographs of the Sun taken at the Mount Wilson Observatory during the last seven years. Two photographs of the Sun are taken on every clear day, and it was therefore possible to select from the large number of plates available only those of the finest quality. The diameter of the solar image on these plates is about 17 cm (6y 2 in.). Three types of plates, sensitive to different spectral regions, were chosen :