EVOLUTION OF THE STABS 215 



that is somewhat the shape of a very flat pocket-watch ; more strictly, a 

 much flattened ellipsoid or spheroid. However, it is not intended to 

 convey the impression that the boundaries of the stellar system are 

 sharply defined, or that the stars are uniformly distributed throughout 

 the spheroid, and all at once, at the surface of the spheroid, cease to 

 exist; but only that the stars are more or less irregularly distributed 

 throughout a volume of space roughly spheroidal in form, and that the 

 thinning out of stars near the confines of the system may be quite 

 gradual and irregular. The equatorial plane of the spheroid is coinci- 

 dent with the central plane of the Milky Way. We see the Milky Way 

 as a bright band encircling the sky, because in looking toward the 

 Milky Way we are looking out through the greatest depth of stars. 

 There is considerable uncertainty as to the dimensions of the system, 

 chiefly for two reasons: first, the stars near the surface of the spheroid 

 are everywhere too far away to let us measure their distances directly, 

 and, in fact, so far away that we have not been able to measure their 

 transverse motions — their proper motions — and thus to gain indirectly 

 an idea of their distances; and secondly, the spheroid may be consid- 

 erably larger than it seems because of possible, and even probable, ab- 

 sorption or obstruction of star-light in its passage through space. 

 Newcomb has suggested that the shorter radius of the spheroid, at right 

 angles to the plane of the Milky Way, may be taken as of the order of 

 3,000 light-years. The long radii of the spheroid, that is, the radii in 

 the plane of the Milky Way, may be at least 10 times as great; that is, 

 30,000 light-years or more. 



The solar system is believed to he somewhere near the center of the 

 stellar system: the counts of stars in all parts of the sky indicate that 

 the Milky Way structure is not much closer to us, .so to speak, in one 

 direction than in other directions; there are about as many stars on 

 one side of a plane through the central line of the Milky Way as there 

 are on the other. Wilhelm Struve's statistical studies of stellar distri- 

 bution led him to conclude that the effective central line of the Milky 

 Way is not a " great circle," but a " small circle," lying at a distance of 

 92° from the north pole of the- galaxy and 88° from the south pole of 

 the galaxy. Interpreted, this means that the solar system lies a short 

 distance north of the central plane of the stellar system. 



This conception of the stellar universe and Milky Way agrees in all 

 important particulars with Immanuel Kant's ideas and description pub- 

 lished in the year 1755 — a remarkable contribution, based essentially 

 on naked-eve observations, without the advantage of accurate observa- 

 tions laboriously made with telescopes. 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 confidence. Sir William Her- 

 schel, using an 18-inch reflecting telescope in the northern hemisphere, 

 and Sir John Herschel. using the same telescope in the southern 



