216 THE POPULAR SCIENCE MONTHLY 



hemisphere, counted the stars visible in the eyepiece, 15 minutes of 

 arc in diameter, in 7,300 regions distributed rather uniformly over 

 the entire sky. They found that the number of stars decreased rapidly 

 as they passed from the central plane of the Milky Way toward the 

 north and south poles of the galaxy. Here is a table deduced by Struve 

 from the Herschels' counts. 



Tbe average number of stars in the Milky Way zone 30° wide, that 

 is, in galactic latitude +15 to — 15, visible in the eyepiece of the 

 telescope, was 56, whereas in the region surrounding the north and 

 south galactic poles the average number visible in the same eyepiece 

 was but 5. The great condensation in the Milky Way is not fully evi- 

 dent from the table. The stars are much more numerous near the cen- 

 tral line of the Milky Way than they are near its borders. The average 

 number along the central line, found by Sir William Herschel, was 122. 

 There is no reason to doubt that the preponderance of stars visible in 

 the direction of the Milky Way is due to the greater extension of the 

 stellar system in that direction than in the direction of the galactic 

 poles. 



It has been noted by several observers that the faintest stars visible 

 in telescopes of moderate size, that is, stars of the 14th, 15th and 16th 

 magnitudes, are plentiful in the Milky Way and very scarce at a dis- 

 tance from the Milky Way. The contrast between Milky Way and 

 non-Milky Way regions is scarcely noticeable in the naked eye stars, but 

 it becomes stronger and stronger as we. pass to the fainter stars. 2 



If there is an absorption of light in its passage through space, such 

 that the very distant stars are appreciably reduced in brightness, then 

 the stars of average size and physical condition must be invisible to us 

 when they are farther away than a certain limiting distance, and in 

 that case the extent of tbe universe in the direction of the Milky Way 



2 A recent study of Mr. Franklin Adams's excellent photographs of the sky, 

 by Messrs. Chapman and Melotte, shows a considerably smaller disparity in the 

 numbers of faint stars in the galactic and non-galactic regions than the Her- 

 schels and others found. 



