NUMBER 01' TFIH FIXKI) STAU8. 105 



Herschel's twenty-feet telescope, including the stellar light, 

 " which is supposed to require 2000 years to reach our 

 earth ?"* 



The numerical data which I here publish in reference to 

 this subject arc chiefly obtained from the final results of my 

 esteemed friend Argelander, director of the Observatory at 

 Bonn. I have requested the author of the Durc]Lmusteru7ig 

 des 7wrdlichen Himmels (Survey of the Northeryi Ileav- 

 e?is) to submit the previous results of star catalogues to a 

 new and careful examination. In the lowest class of stars 

 visible to the naked eye, much uncertainty arises from or- 

 ganic diflerence in individual observations ; stars between 

 the sixth and seventh magnitude being frequently confoimd- 

 ed with those strictly belonging to the former class. We 

 obtaui, by numerous combinations, from 5000 to 5800 as the 

 mean number of the stars throughout the whole heavens vis- 

 ible to the unaided eye. Argelanderf determines the distri- 



* On the space-penetrating power of telescopes, see Sir John Her- 

 schel, Outlines of Adr., § 803. 



t I can not attempt to include in a note all the grounds on which 

 Argelander's views are based. It will suffice if I extract the following 

 remarks from his own letters to me: "Some years since (1843) you 

 recommended Captain Schwink to estimate from his Mappa Coelestis 

 the total number of stars from the first to the seventh magnitude in- 

 clusive, which the heavens appeared to contain; his calculations give 

 12,148 stars for the space between 30° south and 90° north declination; 

 and consequently, if we conjecture that the proportion of stars is the 

 same from 30° S. D. to the South Pole, we should have 16,200 stars of 

 the above-named magnitudes throughout the whole firmament. This 

 estimate seems to me to approximate very nearly to the truth. It is 

 well known that, on considering the whole mass, we find each class 

 contains about three times as many stars as the one preceding. (Struve, 

 Catalogus Stellarum duplicium, p. xxxiv, ; Argelander, Bonner Zonen, 

 s. xxvi.) I have given in my Uranometria 1441 stars of the sixth mag- 

 nitude north of the equator, whence we should obtain about 3000 for 

 the whole heavens; this estimate does not, however, include the stars 

 of the 6*7 mag., which would be reckoned among those of the sixth, if 

 only entire classes were admitted into the calculation. I think the 

 number of the last-named stars might be assumed at 1000, according 

 to the above rule, which would give 4000 stars for the sixth, and 12 000 

 for the seventh, or 18,000 for the first to the seventh inclusive. From 

 other considerations on the number of tlie stars of the seventh magni- 

 tude, as given in my zones — namely, 2257 (p. xxvi.), and allowing for 

 those which have been twice or oftener observed, and for those which 

 have probably been overlooked, I approximated somewhat more nearly 

 to the truth. By this metliod I found 2340 stars of the seventh magni- 

 tude between 45° and 80° N. D., and, therefore, nearly 17,000 for the 

 whole heavens. Struve, in his Descriptio7i de V Observatoire de Poul- 

 kova, p. 268, gives 13,400 for the number of stars down to the seventh 

 magnitude in the region of the heavens explored by him (from — 15^ 



E 2 



