MOUNT WILSON OBSERVATORY. 271 



being indicated by circles. The nature of the giant and dwarf 

 division is shown clearly, as well as the marked grouping of the stars 

 with the Cepheid characteristics of spectrum. The extraordinary 

 condensation of stars around Kl to K5 is due in part to the Har- 

 vard system of classification, which requires modification to include 

 spectral types between K5 and Ma. 



An investigation by Mr. Lindblad of the influence of the effect of 

 absolute magnitude on the spectrum of stars of types B and A has led 

 to the discovery that the portion of the continuous spectrum between 

 X3895 and X3907, when compared with the adjoining region toward 

 longer wave-lengths, is less intense in stars of low luminosity than in the 

 brighter stars. The effect appears to be due to a widening of the wings 

 of H^ in the fainter stars and to increased width and intensity of the 

 arc lines of iron and the silicon line at X3906. The method of compari- 

 son is to determine the exposure-ratio necessary to secure equal 

 photographic impressions for the two spectral regions X3895-X3907 

 and X3907-X3925. Between absolute magnitudes and +3 the rate 

 of change in exposure ratio is found to be 0.15 mag. for one unit 

 in absolute magnitude, with a probable error of about 0.10 mag. 

 for a single plate having from 3 to 8 measured images. The effect 

 appears to be most prominent in stars of types B8 to A5,and to become 

 less for stars with types on either side in the spectral sequence. 



The principal applications of the method have been to stars in mov- 

 ing clusters such as the Ursa Major group, the Hyades, Pleiades, 

 and Praesepe, and to the B-type stars of the Orion and Scorpius- 

 Centaurus groups. In all these cases the parallaxes as determined 

 by this spectroscopic method are in good agreement with those derived 

 from group motion or by other means. 



Mr. Lindblad has also investigated the cause of the sharp falling off 

 in intensity of the continuous spectrum beyond X3889, which from his 

 own work at Upsala and that of Kapteyn is known to be correlated 

 with absolute magnitude. The effect is found to be due to increased 

 absorption of the "cyanogen" band X3883 in the more luminous stars. 

 The same result, but in less degree, exists for the bands at X4216 and 

 X3950. The most sensitive point in the band at X3883 appears to lie 

 between the two heads at X3883 and X3871, where the effect seems to 

 be marked even among the giant stars of the same spectral type. 



The exposure-ratio method already referred to has been used for 

 measuring the amount of the change of intensity between the two 

 regions to be compared, as, for example, X4144-X4184 and X4227-X4272. 

 The amount of the effect appears to be greatest for stars of tji^es 

 G5-K2 and decreases considerably toward GO and Ma. Slitless 

 spectrograms of stars as faint as apparent magnitude 13.5 indicate 

 that the method can be applied with success to distinguish between 

 giant and dwarf stars of this brightness. 



