GEOLOGY. 313 



when such spheres are not deformed by the encroachment of the spheres 

 of the abundant neighboring stars of that region. Larger and freer 

 deployment is there permitted, as also a longer life. The centroidal 

 tract should, to this extent, be richer in nebular development than the 

 zone of maximum gravitation that lies outside it, and this seems to 

 accord with observation. But even here the limitations are severe and 

 very large nebulae of any symmetrical or systematic type are practically 

 inhibited, chiefly by the encroaching individual spheres of control that 

 traverse the region. Irregular nebulae, especially those of incongruous 

 distributions of matter, together with vague nebulosities that scarcely 

 show any organization at all, are consistent with the dynamic condi- 

 tions that prevail in this centroidal region. 



3. Outside the zone of maximum galactic gravity, the individual 

 spheres of control increase gradually in volume to an indeterminate 

 extent. Theory requires that an occasional star — very rarely indeed — 

 shall be shot away from the galaxy at a velocity beyond its control. 

 The individual sphere of control of this ''runaway" star would then 

 grow in dimensions indefinitely as it passed away, if it did not encounter 

 some unknown sphere of control outside our galactic system. It is 

 important here only to recognize that, in the outer part of this sparse 

 zone, the spheres of control of stars would almost certainly be large 

 enough for the deployment of even giant spiral nebulae, under the view 

 that they are star deployments or deployments of sub-clusters of the 

 galactic system. It is to be noted further that here systematic nebular 

 developments would find their greatest freedom from distortion. And so, 

 also, the endurance of nebulae in this outer zone should reach an order of 

 value far beyond that assignable to any form of nebula generated under 

 the conflicting conditions that dominate the denser part of the galaxy. 



In the main, the orbits of stars in this outermost zone should, theo- 

 retically, be either the outermost loops of very extensive elUpses whose 

 inner loops traverse the denser part of the galaxy, or else broad envel- 

 oping ellipses that encircle the whole galaxy. In either case the 

 inherited velocities are habitually low, sub-parallelism abounds, and 

 mutual attraction has a relatively free field. Very close approaches 

 and very wide typical deployments are favored. The sparseness of 

 stars in this region renders close approaches due simply to inherited 

 movements very rare, but the effectiveness of mutual gravity should 

 render the ratio of close approaches comparatively high for such degree 

 of sparseness and the nebulae should be relatively large and lasting. 

 This is the zone to which giant spiral nebulae are assigned. 



Spiral nebulae formed in the interior of the galaxy are, as already 

 noted, necessarily small and evanescent as observable spirals. Their 

 secondary forms are to be sought among the small nebulae that give 

 e\ddence of some advance in evolution. Certain small nebulae give 

 hints of spirality or of knotty structures, perhaps assignable to a spiral 

 ancestry. Of the small nebulae in the denser part of the galaxy, com- 



