stellar clusters 47 

 TABLE 8. 



Harvard color-indices from PG-magnitudes and Harvard spectral types. 





Spectral- 

 type 



Color- 

 index 



N 





s 



Spectral- 

 type 



C(il(ir- 

 iiidex 



N 



-1.0 



BO 



— 0,247 



1 





+ 1.0 



FO 



+ 0 368 



4 



r\ n 

 — U.y 



7^ 1 

 J5 1 



— U.0<2 



z 





+ 1.5 



r 0 



+ 0.441 



D 



— 0.8 



B2 



— 0.414 



H 





+ 1.8 



Få 



-f ().r,30 



(5 



-0.7 



B3 



— 0.257 



5 





+ 2.0 



GO 



4- 0.779 



5 



— 0.5 



Bb 



— 0.122 



4 





+ 2.5 



Cri) 



+ l.ico 



C. 



— 0.2 



B8 



— 0.105 



6 ■ 





+ 3.0 



KO 



-f 1.175 



u 



-0.1 



B'd 



+ 0.203 



8 





+ 3.2 



K-2 



+ 1.633 



4 



0.0 



AO 



- O.045 



IG 





+ 3.5 



Kb 



+ 1,7S1 



b 



+ 0.2 



.12 



+ 0.113 



7 





+ 4.0 



Ma 



+ l.ecô 



4 



+ 0.3 



A3 



+- 0.231 



4 









— 0.ÜS9 



4 



+ 0.5 



Ah 



+ 0.160 



7 













The first column gives the spectral-index, the second the spectral-type, the 

 third the mean color-index and the fourth the number of stars. Using the method 

 of least squares we get the following relation between the spectral-index and the 

 corresponding mean color-index 



(3) c = + 0.436 .<^. 



I have compared the Harvard observations with the fornjula (2) (instead of 

 with (3)) in figure 4. 



The agreement is not very close, but not worse than could be expected from 

 so few observations. Another source of error is perhaps the tendency of the 

 Harvard observers to favour the first subclasses of each type. 



The relation (3) is nearly the same as that previously deduced from the Yerkes 

 observations. (Compare formula (2)). 



25. Proceeding next to the Yerkes observations I first determined the zero- 

 point and found that the difference between the Yerkes photographic magnitudes 

 and the P. G. visual magnitudes had to be augmented by + 0.298 for being reduced 

 to the international scale. We then get the following values of the color-index 

 for the different spectral-types. 



TABLE 9. 



Yerkes color-indices from PG-magnitudes and Harvard spectral types. 





Spectral- 

 tvpe 



Color- 

 index 



N 





s 



Spectral- 

 type 



Color- 

 index 



A' 



-0.7 



B3 



— 0.506 



1 





+ 1.0 



FO 



+ 0.455 



27 



-0.2 



BS 



— 0.096 



3 





+ 1.2 



F2 



4- 0.542 



!) 



— 0.1 



BQ 



- - 0.062 



5 





+ 1-5 



Fb 



-f 0.410 



8 



0.0 



AO 



+ 0.092 



Ü5 





+ 1.8 



F8 



-4- 0.560 



9 



+ 0.2 



A2 



+ 0.114 



21 





+ 2.0 



ao 



+ 0.72!:! 



11 



+ 0.3 



A3 



-f 0.198 



b 





+ 2.2 



G2 



-f 1 409 



2 



+ 0.5 



Ab 



+ 0.278 



5 





+ 2.5 



Gb 



+ 1 .102 



21 











+ 3.0 



KO 



+ 1 357 



80 













4-3.2 



K2 



4- 1.538 



9 













+ 3.5 



Kb 



4- 1.700 



8 













+ 4.0 



Ma 



-f 1.824 



7 



