NOTES. xlvii 



1st to the 7th magnitude in the zone from 15 to + 15 ; whence, in the 

 whole heavens, 15050 such stars. This number is less than mine, because 

 Bessel estimated the brighter stars about half a magnitude lower than 1 did. 

 It is only a mean number which we can look for here ; and this we might 

 take, therefore, at 18000 stars from the 1st to the 7th magnitude inclusive. 

 In the passage of the Outlines of Astronomy, p. 521, of which you remind 

 me, Sir John Herschel speaks only of the stars already registered : ' The 

 whole number of stars already registered, down to the 7th magnitude 

 inclusive, amounting to from 12000 to 15000.' As respects the fainter stars 

 of the 8th and 9th magnitudes, Struve finds, in the above-mentioned zone of 

 -15 to +15, of stars of the 8th magnitude, 10557; and of stars of the 

 9th magnitude, 37739 : consequently, for the whole heavens, 40800 stars of 

 the 8th, and 145800 stars of the 9th magnitude. Thus we should have, 

 according to Struve, from the 1st to the 9th magnitude inclusive, 

 15100 + 40800+145800=201700 stars. These numbers were found by 

 Struve by carefully comparing those zones or parts of zones which included 

 the same parts of the heavens; and from the number of stars common to 

 them, and the number of those which were different, concluding, by the 

 calculus of probabilities, the number of stars actually existing. This calcu ] a- 

 tion deserves very great confidence, as a large number of stars have contributed 

 to form its basis. Bessel has entered in the whole of his zones between 

 -15 and +45, about 61000 different stars from the 1st to the 9th magni- 

 tude inclusive (after deducting stars observed more than once, and stars 

 of 9'10 magnitude) ; whence, taking into account those which have 

 been probably overlooked, there would follow, for the part of the heavens 

 which has been mentioned, about 101500 of the magnitudes in question. My 

 zones between +45 and + 80 contain about 22000 different stars (Durch- 

 musterung des nordl. Himmels, S. xxv.) : from this number we must deduct 

 about 3000 of the 9'10 magnitude; leaving 19000. My zones are 

 somewhat richer than Be&sel's, and I do not think I can assume more than 

 28500 as the number of actually existing stars within their limits (between 

 + 45 and +80) : so that we should have 130000, to the 9th magnitude 

 inclusive, between 15 and +80. This is about 0'62181 of the entire 

 heavens ; so that, assuming a generally equable distribution, we should have 

 over the entire firmament 209000 stars, or thus again nearly the same number 

 as that assigned by Struve : perhaps even it may be one not inconsiderably 

 greater, as Struve has reckoned the stars of 9'10 magnitude as belonging to the 

 9th magnitude. The numbers which, according to my views, I should say 



