Summary. 



1. For studj'ing the correlation between spectral type and colour-index a 

 graduation of the spectral attribute of a star was made with the help of the spectral 

 indices defined above. 



2. The correlation between these spectral-indices and the color-indices was 

 found to be very strong, the coefficient of correlation having the liigh value 



r = + 0.93 . 



3. The lines of regression giving, respectively, the mean colour-index (c) 

 corresponding to a given spectral-index (s) and the mean spectral index corresponding 

 to a given color-index have the ecjuations 



c = 0.46 s 



s = 2.00 c -]- 0.1 . 



4. These ecjuations represent with a sufficient accuracy the principal series 

 of observations hitherto made on color-indices, namely those of King, Parkhurst 

 and of Schwärzschild. 



5. There seems to be still some uncertainty fegarding the definition of some 

 spectral classes. It is suggested that this uncertainty may be relieved through 

 making an empirical definition of the spectral classes with the help of the color- 

 indices. 



6. The color-index is here defined as tlie difference between the photographic 

 and the visual magnitude of a star. Thereby is to observe that 



a) the 2ero point of the visual scale is taken according to the Harvard system ; 



b) the ^ero point of the photographie scale is chosen in such a way that the mean 

 color index corresponding to the spectral index 0 (= ^0) has the value 0. 



c) The visual magnitudes (excepting a constant defined through the zero point) 

 are taken from the Potsdam photometric system, there being reason to 

 accept a color equation in the Harvard observations (giving rise to nume- 

 rically too small color-indices). 



