262 



CARNEGIE INSTITUTION OF WASHINGTON. 



Professor Hertzsprung with the 60-inch reflector. The relation of the 

 zero-point of the Aktinometrie to that of the Mount Wilson system 

 and the color equation of the instrument are known. A comparison 

 can therefore be made with the Mount Wilson system as transferred 

 to the Pleiades by Mr. Shapley's observations. For the well-deter- 

 mined interval of the scale the differences, in hundredths of a mag- 

 nitude, are in the accompanying table. 



The mean difference for the photographic magnitudes is -1-0.02 mag. 



The correction for zero-point difference is —0.06, and for reduction 

 of Hertzsprung's results to the color system of the reflector, -|-0.04. 

 Hence, the final difference for the photographic scale is 0.00 mag. 

 In other words. Professor Hertzsprung's measurement of the interval 

 between the sixth and thirteenth magnitudes is the same as that made 

 at Mount Wilson. The small systematic difference of 0.05 mag. in 

 the color-indices is within the uncertainty of the determmation. The 

 same difference reappears in the photovisual magnitudes with the 

 reversed sign, since Hertzsprung's results for these were obtained by 

 combining his photographic magnitudes and color-indices. 



A third test, which concerns the Mount Wilson color-indices, is an 

 amplification of one described in Proceedings of the National Academy 

 of Sciences, vol. 3, 29, 1917. The method of detennining colors by expo- 

 sure ratios, which is entirely independent of scales of magnitudes, was 

 calibrated on the bright stars among the Polar Standards and then used 

 to determine the color indices of the standards fainter than the tenth 

 magnitude. A comparison of these results with the values of the color 

 derived from the photographic and photovisual magnitudes gave no 

 evidence of any systematic difference as far as magnitude 15.5. The 

 comparison has now been greatly strengthened and extended to a 

 little below the sixteenth magnitude, with results that are equally 

 accordant. 



Color of the Brighter Stars in the Pleiades. 



Mr. Seares has reduced a series of 15 exposure-ratio photographs 

 of the Pleiades for the determination of the colors of the bright stars 

 and of some of the fainter objects as far as the twelfth magnitude, 

 mainly as a test of the method of reduction. Color-indices were ob- 



