516 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



the Milkj^ Wax. We see the Milky Way, or 

 Galaxy, as a bright band encircling the sky 

 because, looking toward the Galaxy, we are 

 looking out through the greatest depth of 

 stars. There remains considerable uncer- 

 tainty as to the dimensions of the system, 

 chiefly for two reasons : first, the stars near 

 the surface of the ellipsoid are everywhere 

 too far away to let us measure their dis- 

 tances ; and, secondly, the system may be 

 considerably larger than it seems because 

 of possible obstruction of starlight in its 

 passage through space. Newcomb has sug- 

 gested that the shorter radius of the sys- 

 tem, at right angles to the plane of the 

 Galaxy, may be taken as of the order of 

 three thousand light-years. The long radii, 

 those in the plane of the Milkj^ Way, may 

 be at least ten times as great ; that is, thirtj' 

 thousand light-years, or more. 



Our solar system is believed to be some- 

 where near the center of the stellar sj'stem : 

 the counts of stars in all parts of the sky do 

 not indicate that any one section of the 

 Milky Way structure is appreciably closer 

 to us, so to speak, than the other sections of 

 it. It should be said that Easton's studies 

 of the Galaxy place its probable center in 

 the rich region of the constellation Cj^gnus. 



These conceptions of the stellar universe 

 and of the Milkj^ Way agree in all impor- 

 tant particulars with the ideas of Immanuel 

 Kant published in the .year 1755. However, 

 it was the star counts by the two Herschels, 

 father and son, which put this conception 

 of the stellar system upon the basis of con- 

 fidence. Sir William Herschel, using an 

 eighteen-inch reflecting telescope in the 

 northern hemisphere, and Sir John Her- 

 schel, using the same telescope in the 

 southern hemisphere, counted the stars 

 visible in the same ej^e-piece in 7,300 regions 

 distributed rather uniformly over the en- 

 tire sky. They found that the number of 

 stars decreased rapidly as they passed 

 from the central plane of the Galaxy 



toward the north and south poles of the 

 Galaxy. Here is a table deduced by Struve 

 from the Herschels' counts: 



Aver.ige Number of Stars 

 Diameter 



The average number of stars in the Milky 

 Way zone 30° wide, that is, between galac- 

 tic latitudes.-)- 15° and — 15°, visible in the 

 eyepiece of the telescope, was flftj^-six,^ 

 whereas in the regions surrounding the 

 north and south galactic poles (latitudes 

 between 75° and 90°) the average visible in 

 the same eyepiece was but five.^ The great 

 condensation in the Milky Way is not fully 

 evident from the table. The stars are much 

 more numerous near the central 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 pre- 



- The galactic latitude of a stai- is its angular dis- 

 tance from the nearest point of the central line of 

 the Gala:xy, in the same way that the terrestrial 

 latitude of a city is the city's angular distance 

 from the nearest point of the earth's equator. 



3 A recent study of Mr. Franklin Adams 's photo- 

 graphs of the sky, by Chapman and Melotte, shows 

 a considerably smaller ratio than the 56 : 5 found 

 by the Herschels. Seares has recently determined 

 from Mount Wilson photographs that the number 

 of stars per square degree along the central line of 

 the Milky Way is more than twenty times as great 

 as the number per square degree near the galactic 

 poles; a result in remarkably close confirmation of 

 the Herschels' counts. The source of the discord- 

 ance between Chapman and Melotte 's results and 

 Seares 's results remains unexplained. 



