xlvi 



NOTES. 



( 167 ) p. 86. Kosmos, Bd. iii. S. 49 and 57, Anm. 32 and 33 (English 

 edition, p. 39, and Notes 78 and 79). 



( 168 ) p. 87. Kosmos, Bd. i. S. 185 and 428, Anm. 14 (English edition, 

 p. 168, Note 144). 



( 169 ) p. 89. On the Space- penetrating Power of Telescopes, in Sir John 

 Herschel's Outl. of Astr. 803. 



( 17 ) p. 89. I cannot attempt to compress within the limits of a note all 

 the reasons upon which Argelander's views are founded. It will be sufficient 

 for me to insert the following extracts from some of his letters to myself: 

 " A few years ago (] 843) you requested Captain Schwink to estimate for you 

 the number of stars visible on the whole celestial vault, from the 1st to the 

 7th magnitude inclusive, according to the proportion of those entered in his 

 Mappa coelestis. He finds 12148 stars between -30 and +90 Decl.; 

 consequently, assuming the same frequency of stars from 30 Decl. to the 

 South Pole, there would be in the entire firmament 16200 stars of the above- 

 named magnitudes. This estimation seems to me also to come very near the 

 truth. We know that, if we only consider the general mass, each successive 

 class or magnitude contains about three times as many stars as the preceding 

 one (Struve, Catalogus Stellarum duplicium, p. xxxiv. ; Argelander, Bonner 

 Zonen, S. xxvi.) Now in my Uranometrie I have 1441 stars of the 6th 

 magnitude North of the Equator, whence there would follow, for the entire 

 heavens, about 3000; but this does not include stars of the 6*7 mag- 

 nitude, which yet, if whole classes only were counted, would be reckoned 

 as belonging to the 6th class. I think we might take these at 1000 : so that 

 we should have 4000 stars of the 6th magnitude ; and thus, according to the 

 above-mentioned rule, 12000 stars of the 7th magnitude, or 18000 stars 

 from the 1st to the 7th magnitude inclusive. I arrive at a rather more 

 exact conclusion, by means of other considerations respecting the number of 

 stars of the 7th magnitude which I have marked in my zones, viz. 2251 

 (pag. xxvi.) ; having regard to stars which have been observed more than 

 once, and to those which have probably been overlooked. In this way I find, 

 between 45 and 80 North Decl., 2340 stars of the 7th magnitude, and thence, 

 over the whole heavens, about 17000 stars. Struve, in the Description de 

 1'Observatoire de Poulkova, p. 268, gives the number of stars down to the 7th 

 magnitude, in the region of the heavens examined by him (i. e. 15 to 

 4- 90), 13400 ; whence there would follow, for the entire heavens, 21300. 

 According to the Introduction to "Weisse's Catal. e Zonis Regiomontanis ded. 

 p. xxxii., Struve finds, by the calculus of probabilities, 3903 stars from the 



