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232 ASTRONOMY: F. H. SEARES diction has been found between the spectroscopic and the trigonometric results. This is the more remarkable since we should expect occasional idiosyncracies in the spectra of stars subject to exceptional physical conditions, and the rareness of such cases is interesting evidence for the uniformity of the development of stellar spectra in general. RELATION OF COLOR TO INTRINSIC LUMINOSITY IN STARS OF THE SAME SPECTRAL TYPE BY FREDERICK H. SEARES MOUNT WILSON OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON Communicated by G. E. Hale. Read before the Academy, April 28, 1919 It has long been known that the color of a star is intimately related to its spectral type. More recently it has been found that color is also correlated with intrinsic brightness.1 Thus, two stars having different luminosities, but the same spectrum so far as the usual criteria are concerned, may differ in color-index by more than half a magnitude.2 The phenomenon is perhaps to be accounted for as follows: Similarity in spectrum implies at least approximate equality in surface brightness. Difference in luminosity is therefore mainly a matter of size, the brighter object being the larger. But inequality.of dimensions doubtless entails differences in atmospheric constitution, and hence also in the selective absorption occurring in the atmospheres. Since color is determined by the distribution of intensity in the continuous spectrum, while type relates more particularly to the characteristics of the spectral lines, one star may thus appear redder or bluer than another, although both are of the same type. But whatever the explanation, an extension of the results of Adams, van Rhijn, Monk, and others to include a wide range of luminosity for each spectral type is immediately desirable, because of their bearing upon the difficult problem of stellar constitution. Further, any phenomenon corr