50 



C. V. L. Charlier 



memoir of Schwarzschild. In this diagram the color-indices are represented for 

 each spectral-class as given in H. 50 and in H. 56, the latter containing »Classi- 

 fication of 1477 stars by means of their photographic spectra » by Miss Annie J. 

 Cannon. It is immediately evident from the diagram that Miss Cannon defines 

 the subclasses GO and Gb in another manner than is made in H. 50. 



To advocate the preference of the one definition over the other may be a 

 delicate task. At any rate we find that the definition followed in H. 50 gives a 

 simpler relation between color-indices and spectral indices than that iuaugmented 

 in H. 56*. 



Starting from H. 56 Schwaezschild has indeed found it necessary to introduce 

 spectral indices which assign to the interval between GO and KO a value twice as 

 great as the interval between FO and GO and to the interval between KO and Ma 

 a value trice as great as the named interval. Such a complexity in the graduation 

 of the spectral indices is by no means necessary if the spectral classes are defined 

 as in H. 50. Some uncertainty may thus subsist in the definition of the spectral- 

 class GO and of the adjacent classes, but a rather slight modification suffices for 

 bringing both scales into concordance. It seems that this question as well as similar, 

 though more insignificant, discrepancies regarding other spectral classes could be 

 most simply settled through studying the spectra corresponding to each individual 

 color-index and thus deriving an empirical definition of each spectral class. 



28. In conclusion I give a small table for converting color-indices into 

 spectral indices and vice versa. They are computed with the help of the formulae 



c - 0.46 s 

 5 = 2c + 0.1 



which are the formulae deduced from the Yerkes observations. 



TABLE 10 giving tlie mean color-index corresponding to a given 

 spectra! class or spectral index. 



Spectral 



Color- 

 index 



class 



index 



-BO 



— 1.0 



— 0.46 



BÔ 



- 0.5 



-0.23 



AO 



00 



0.00 



Ab 



+ 0.5 



+ 0.23 



FO 



+ 1.0 



+ 0.46 



Fb 



+ 1.5 



+ 0 09 



Spectral 



Color- 

 index 



class 



index 



GO 



+ 2.0 



+ 0.92 



GÖ 



4-2.5 



-f 1-15 



KO 



+ 3.0 



+ 1.38 



Kb 



+ 3.5 



+ 1.61 



MO 



+ 4.0 



+ 1 84 



* I do not know which memoir is earlier that in H. 56 or H. 50. But I suppose that H. 50 

 represents the official Harvard classificaton. 



