182 HISTORY OF ASTRONOMY. 



observations of Sir J. Herschel. If we imagine the celes- 

 tial sphere to be divided into a series of zones, each meas- 

 uring 15 in breadth, and bounded by parallels to the 

 galactic circle, the average number of stars included 

 within a circle 15' in diameter will be that which is 

 given in the second column of the following table. 



Galactic Latitude. Average number of stars in a 



circle 15' in diameter. 



90 75 North. . . ' \ li ^ . 4.32 

 75o_ 60 o tt ...... 5.42 



60 45 " 8.21 



45o_ 30 o . . m - m f 13 61 



30 15 u ...... 24.09 



15o_ QQ . 53 43 



Galactic Circle 122.00 



15 South 59.06 



150300 " ..... 26.29 



30 450 ..... 13.49 



450 60 " . . . '. . . " 9.08 



6075 ..... 6.62 



75090 6i0 5 



It appears, therefore, that the variation of density of 

 the visible stars, in proceeding from the galactic poles, is 

 subject to almost exactly the same law of decrease in 

 either hemisphere; the density being, however, some- 

 what greater in the southern than in the northern hemi- 

 sphere. 



M. Struve has attempted to form an estimate of the rel- 

 ative distances of stars of the different magnitudes as de- 

 duced from their number, supposing the stars to be dis- 



