I 



I 



19 365 



III"' order. The measures of the spectra of the P' order are divided into two groups 

 according to the description given above. For each image the number of the plate 

 is followed by the diameter d or equivalent diameter d' of the central image given 

 to hundredths of a millimeter. Then follows the measured distance between the 

 two spectra corrected to normal intensity (denoted by d or rf') of the image. Finally 

 the weight of the effective wavelength is given, unit weight corresponding to a 

 mean error of ±1" in the equivalent colour index /;. Half the normal weight was 

 given to images of inferior qualit}'. This star here chosen as an example is, of 

 course, one of those where the number of measured effective wavelengths is largest. 



13. Other Determinations of Colour Equivalents of Stars 



in the Pleiades. 



In Table 13 I have compared the effective wavelengths, obtained from the 

 Mount Wilson plates, with colour equivalents from other sources all reduced to the 

 scale of Ix used in the present paper. 



The first part of Table 13 contains the 300 Gaultier stars, which comprise 

 the stars brighter than about 13"''5 photographically within 55''4 of the central star 

 Alcyone. A few bright stars outside this field are given in the second part of 

 Table 13. 



The different sources of supplementary colour equivalents used in forming 

 mean values of Ii are: 



1. Effective wavelengths for 59 stars from plates taken with a 81 mm refractor 

 (/■= 1236 mm) at the Urania observatory, Copenhagen (Potsdam Publ. 63, Table 13). 



2. Colour indices for 19 bright Pleiades using the photographic magnitude given 

 in the Göttingen Actinometry (Teil B, Abh. der Ges. der Wiss. zu Göttingen, Math.- 

 phys. Klasse, Neue Folge, Bd. 8, Nro. 4, 14; 1912) in combination with the visual 

 Harvard magnitudes. 



3. Colourindices for 92 stars, using the Potsdam UV Zeisstriplet photographic 

 magnitudes combined with the visual magnitudes of Müller and Kempf (A. N. 3587, 

 Bd. 150, 193; 1899). 



4. Colour indices for 234 stars estimated photographically by Tikhoff (Publ. de 

 rObs. Centr. Nicolas, Sér. 2, Vol. 17) comparing on an arbitrary magnitude scale 

 the differences between the wavelengths 540 and i2b fifi. 



14. Relation between Brightness and Colour of the Physical 

 Members and Hypothetical Parallaxes of the Pleiades. 



A complete separation of the phj'sical members of a cluster from other stars 

 projected on the field investigated is not possible, because it may always happen, 

 that stars not belonging to the group have proper motions (and radial velocities) 

 agreeing within the errors of observation with the motion of the cluster. It is there- 



48* 



