142 COSMOS. 



cording to difference of magnitude, down to the 9th, in about 

 the following proportion, 



1st Mag. 2nd Mag. 3rd Mag. 4th Mag. 5th Mag. 



20 65 190 425 1100 



6th Mag. 7th Mag. 8th Mag. 9th Mag. 



3200 13000 40000 142000 



which have probably been overlooked, I approximated some- 

 what more nearly to the truth. By this method, I found 2340 

 stars of the 7th magnitude, between 45 and 80 N. D. ; and 

 therefore, nearly 17000 for the whole heavens. Struve, in 

 his Description de I' Observatoire de Poulkova, p. 268, gives 

 13400 for the number of stars down to the 7th magnitude, in 

 the region of the heavens explored by him (from 15 to + 

 90), whence we should obtain 21300 for the whole firma- 

 ment. According to the Introduction to Weisse's Catal. e 

 Zonis Regiomontanis, ded. p. xxxii. Struve found in the zone 

 extending from 15 to -j- 15 by the calculus of probabili- 

 ties, 3903 stars from the 1st to the 7th, and therefore 15050 

 for the entire heavens. This number is lower than mine, 

 because Bessel estimated the brighter stars nearly half a mag- 

 nitude lower than I did. We can here only arrive at a mean 

 result, which would be about 18000 from the 1st to the 7th 

 magnitudes inclusive. Sir John Herschel, in the passage of the 

 Outlines of Astronomy, p. 521, to which you allude, speaks 

 only of " the whole number of stars already registered, down to 

 the seventh magnitude inclusive, amounting to from 12000 to 

 15000." As regards the fainter stars, Struve finds within the 

 above-named zone, (from 15 to + 15) for the faint stars 

 of the 8th magnitude, 10557, for those of the 9th. 37739. 

 and consequently, 40800 stars of the 8th, and 145800 of the 

 9th magnitude for the whole heavens. Hence, according to 

 Struve, w r e have from the 1st to the 9th magnitude inclusive, 

 15100 + 40800 + 145800 = 201700 stars. He obtained 

 these numbers by a careful comparison of those zones or parts 

 of zones, which comprise the same regions of the heavens, 

 deducing by the calculus of probabilities the number of stars 

 actually present from the numbers of those common to, <>- 

 different, in each zone. As the calculation was made from 

 ti very large number of stars, it is Reserving of great 



