44 



C. V. L. Charlier 



Let US design as spectral-index a number expressing the spectral character of 

 a star. Tlien we may conveniently detine this conception in the following way. 

 Let ^0 correspond to the spectral-index 0.0, FQ to -|- l.o, GO to + 2.0, KO to + 3.0, 

 MO to +4.0 and BO to — l.o. Let furthermore the subclasses A\, A2, A'd etc. have 

 the spectral-indices +0.1, +0.2, +03 etc. and in like manner the other inter- 

 mediate subclasses. Whether the introduction of spectral-indices is convenient or 

 not depends on the resulting correlation between spectral-indices and color-indices. 



There are 631 stars in the catalogue of Parkhurst, which may be used in 

 the discussion. For the remaining stars either the spectral-class or the photographic 

 magnitude is wanting. Ordering the material into classes, each class having a 

 breadth of 0.2 colour-indices and 0.5 spectral-indices we get the following corre- 

 lation table. 



TABLE 7. 



Correlation between color-index and spectral-index. 



ectral- 1 

 type ' 



ectral- 

 ndex 









C 



0 1 



0 r - 



I n 



d e 



X 









Sum 



dean 

 ir-index 



a, 





-0.40 



- 0.20 



0.00 



+ 0.20 



+ 0.40 



+ 0.60 + 0.80 



+ loo 



+ 1.20 + 1.40 + 1.60 



+ 1.80 



"o 



O 



Bb 



- 0.5 



4 



3 























7 



— 0.81 



AO 



0.0 



4 



30 



73 



34 



1 

















142 



0.00 



Ab 



+ 0.5 





3 



10 



57 



17 



2 















89 



+ 0.21 



FO 



+ 1.0 







1 



14 



44 



10 















69 



+ 0.38 



Fb 



+ 1.5 







1 



3 



9 



26 



3 



1 











43 



+ 0.54 



GO 



+ 2.0 











1 



11 



27 



58 



7 



1 







105 



+ 0.92 



Gb 



+ 2.5 















3 



24 



28 



8 



1 





64 



+ 1.14 



KO 



+ 3.0 

















5 



19 



24 



11 



1 



60 



+ 1.35 



Kb 



+ 3.5 





















7 



23 



14 



44 



+ 1.63 



MO 



+ 4.0 





















1 



4 



3 



8 



+ 1.65 



Sum 



8 



36 



85 



108 



72 



49 



33 



88 



54 



41 



39 



18 



631 





Mean 

 Spectral-index 



- 0.02 



0.00 



+ 0.07 



+ 0.44 



+ 0.91 



+ 1.47 



+ 2.00 



+ 2.19 



+ 2.61 



+ 2.99 



+ 3.38 



+ 3.56 







From this correlation table we get 



Mean color-index + 0.63.5 



Mean spectral-index + L443 



Dispersion in the color-indices .... + 0.573 



» » » spectral-indices . . -|- 1.196 



For the coefficient of correlation we get the value 

 (1) ?• = + 0.958 ± 0.011 , 



an exceptionally high value. 



