May 25, 1917] 



SCIENCE 



515 



ment of one nebula into our sun, its planets 

 aiid their moons. 



It is necessary for the satisfactory pres- 

 entation of our subject that we grasp the 

 principal features of our stellar system, 

 and we shall devote a few sentences to its 

 description. 



The universe of stars — our stellar system 

 — is believed by astronomers to occupy a 

 limited volume of space that is somewhat 

 the shape of a very flat pocket watch ; more 

 strictljr, a much flattened ellipsoid or 



spheroid. It is not intended to convey the 

 impression that the boundaries of the 

 stellar system are sharply defined, nor that 

 the stars are uniformly distributed through- 

 out the ellipsoid (see Figs. 2, 18 and 

 others), but only that the stars are more or 

 less irregularly distributed throughout a 

 volume of space roughly ellipsoidal in form. 

 The thinning out of stars near the confines 

 of the system may be both gradual and ir- 

 regular. The equatorial plane of the ellip- 

 soid is coincident with the central plane of 



Fig. 2. Milky Way around the star Gamma Cygni, photographed by Barnard with the 10-inch Bruce 

 telescope of the Yerkes Observatory. 



