May 25, 1917] 



SCIENCE 



519 



whose principal planes are distril)utecl in 

 direction at random. Many of them are 

 approximately circles, as if they are spirals 

 seen flatwise (Fig. 5) ; others appear as 



Fig. 6. Spiral Nebula, edgewise, N.G.C. 42-14, 

 photographed by Keeler with the Crossley Eeflector 

 of the Lick Observatory. 



spindles, or as spirals seen edgewise (Fig. 

 6) ; and the great majority have intermedi- 

 ate elliptic forms (Fig. 7). The plain 

 inference is that the very faint nebulte are 

 for the most part, and perhaps almost en- 

 tirely, spirals. 



We have mentioned Sir John Herschel's 

 charting of the nebulee with reference to 

 their distribution over the sky. Proctor 

 charted all the nebulae and star clusters 

 known up to 1869, as in the illustration 

 (Fig. S). The positions of the nebulaj are 

 indicated on the charts by the small dots. 

 Their peculiar relationship to the Galaxy 

 is apparent. The star clusters are plotted 

 on the charts as crosses. It is clear that 

 the clusters are found prevailingly in the 

 galactic regions and in the two Magellanic 

 Clouds (see Figs. 22 and 23), the Greater 

 and the Lesser, which are about 20° 



from the south pole of the sky and are far 

 to one side of the Galaxy. We sliould per- 

 haps explain that the two ilagellanic 

 Clouds are great clusters of stars and 

 nebulfB, covering many square degrees each, 

 which have the same general appearance 

 as many of the cloud forms of the Galaxy 

 itself. We shall later find reason for be- 

 lieving that these Clouds are isolated stellar 

 systems, separate from and independent of 

 our stellar system. 



It is the small star clusters, the clusters 

 of rather widely separated stars,, and the 

 clusters of irregular form which show the 

 strongest preference for the galactic re- 

 gions; and there is no reason to doubt that 

 all of such clusters belong to our system. 

 The globular clusters, rich in stars and sym- 

 metrical in form, of which only 83 are 

 known in the whole sky, likewise show a 

 strong preference for the Milky Way, and 

 it is probable that most of them, in fact 

 nearly all of them, are within our stellar 

 system ; but the greatest of these clusters. 



Fig. 7. Spiral Nebula N.G.C. 2.53, photographed 

 by Keeler with the Crossley Reflector of the Lick 

 Observatory. 



such as the cluster in Hercules and two 

 clusters even more prominent in the south- 

 ern sky, are seen at considerable angular 

 distances from the Milky Way structure. 



