42 



Among these 3 have long periods. Moreover the magnitudes are 

 given for 25 comparison-stars. 



In Shilow's catalogue the magnitudes of i 006 stars are mentioned. 

 The interval of magnitudes is small and the magnitudes are inaccurate. 

 Nor did we succeed in reducing them to a more exact scale by 

 means of Bailey's magnitudes. We tind the results rr = 0".0002 and 

 TT =z 0".0009 ; consequently as average value : 



.T = 0".0005^ ± 0".0002. 



According to Shapley the average photogr. magnitude of the 

 variable stars is 15'". 25 and we found 5 log. .t^ — 16.3; therefore 

 ;1/^15'«.25 — 11'". 3^4'". 0. So we get for the mean absolute magnitude 

 of the variable cluster stars 4.0. 



If we determine tiie parallax from the variable stars with a 

 known period, we find, when making use of Uektzsprung's numbers : 



jt = 0".0002. 

 Messier 13. 



N.G.C. 6205; «,,„„=16^38'».l, ff,,„„=36°39', />== -h40°. /= 26°; 

 class : C 3. 



J. ScHEiNEK. Der grosze Sternhaufen ini Hercules Messier J 3, 

 Abhandl. Kgl. Akad. Berlin 1892. 



The catalogue contains 823 stars. The magnitudes are uncertain. 



H. LuDENDORFF. Der grosze Sternhaufen im Hercules Messier 13. 

 Pnbl. Astroph. Observ. Potsdam, Bd. XV, N». 50, 1905. 



This catalogue contains 1118 stars. The brightness is not expressed in 

 magnitudes; but the diameters are estimated in J 6 "Helligkeitsstufen". 



H. Shapley. Studies etc. Second Paper: Thirteen hundre(J stars in 

 the Hercules Cluster (Messier 13). Contrib. Mt. Wilson Observ. 

 N". 116, 1915. 



The photogr. and photovis. magnitudes of 1300 stars have been 

 determined; but of only 650 stars they have been published. For 

 the statistical investigation 1049 magnitudes and colour indices 

 were used. 



We make use of Ludendorff's catalogue and we availed ourselves 

 of Shapley's results in reducing the "Helligkeitsstufen" to magnitudes. 

 First we can express the "Stufen" in photographic magnitudes by 

 means of a table in Shapley's work (p. 25, Table VIII) and these 

 may be reduced to photovisual ones by means of the Tables XIV 

 and XVI. No correction is wanted for the difference between the 

 scales of Harvard and Mount Wilson, because the visual Harvard 



