MOUNT WILSON OBSERVATORY. 263 



tained for 60 stars. The agreement with Professor Hertzsprung's 

 results referred to above is excellent, only two of the differences for 

 individual stars exceeding a tenth of a magnitude. The mean syste- 

 matic difference ranges from —0.02 mag. for the brightest stars to 

 +0.02 mag. for those near the twelfth magnitude. This close agree- 

 ment over an interval of 8 magnitudes is good evidence that the dis- 

 turbing influence of gradation has been well eliminated. 



The measures confirm two anomalous results which had been sus- 

 pected from other evidence. The color-indices of the 14 AO stars 

 which are members of the cluster increase with increasing magnitude, 

 the change amounting to about 0.2 mag. in an interval of two magni- 

 tudes. Further, the color-indices of the F and G stars are about 

 one-third the values usually found for these types. For the A stars, 

 at least, the result is probably to be attributed to systematic errors 

 in the spectral classification. 



Galactic Clouds and Obscured Regions. 



The colors of stars in rich galactic fields in Perseus and Taurus have 

 been compared with those in several closely adjacent areas within and 

 on the boundaries of regions showing great obscuration. In addition, 

 measures of color have been made in a number of Selected Areas, 

 especially Nos. 63, 64, 88, 109, 110, 111, and in several rich fields in the 

 Scorpius-Sagittarius region. Area 110 is on the equator, between the 

 two branches of the Milky Way, and is notable because of the low-star 

 density, which is comparable with that in high galactic latitudes. As 

 far as the reductions are complete, they indicate the following results : 



The galactic fields show a correlation of color with apparent magni- 

 tude which is the inverse of that previously found for extra-galactic 

 regions, but similar to that occurring among the most luminous stars 

 in globular clusters and the Cepheids and pseudo-Cepheids of our own 

 system. On the average, the stars in the galactic clouds become bluer 

 with increasing magnitude. There are exceptions, as might be ex- 

 pected, for although there is reason to believe that the correlation 

 exists among the more luminous stars in a galactic aggregation which 

 are at the same distance from the observer, there are two factors which 

 tend to prevent its detection : the presence of any considerable number 

 of apparently bright blue stars, and, second, exceptional thickness of 

 the cloud in the line of sight. These will decrease the average color 

 of the apparently brighter stars and increase that of the fainter ob- 

 jects. The very distant red giants, say of absolute magnitude —4, 

 in the remote regions of the cloud, will have the same apparent magni- 

 tude as nearer and intrinsically fainter blue stars. The result will be 

 the presence of stars of all colors in every interval of apparent magni- 

 tude, and an average color that changes little with decreasing bright- 

 ness. The nearby dwarfs, which are all reddish, can have little in- 

 fluence because of their small number. 



