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observed for two modifying influences, viz. the influence of the 

 brightness and that of the altitude above the horizon. Quantitatively 

 nothing is known about the values of these influences; experiments 

 of OsTHOFF himself to determine the influence of the brightness 

 have as yet yielded few results. Therefore we must derive them 

 here from the material of stellar colours themselves, which serve 

 for our investigation ; this may be done in the very probable assump- 

 tion that the real colour within each spectral class is an almost 

 constant value and is independent of brightness. 



^ 2. The stars of Osthoff's list which occur in the spectral cata- 

 logue of Maury, were arranged according to their classes and then 

 (excluding those which are marked c, ac, C, P or L, as was always 

 done in this investigation) always taking together some classes, we 

 classified them according to their brightness and combined their 

 magnitudes and classes to mean values. These mean values must 

 show the influence of the brightness on the colour; they are given 

 in the following tables: 



