CONDITIONS OF CONCENTRATION. 257 



situated. But we are not dependent merely on reasoning 

 for this belief. Observation shows us that the less con- 

 centrated external parts of nebulae, are left behind by the 

 more concentrated, internal parts. Examined through high 

 powers, all nebulae, even when they have assumed regular 

 forms, are seen to be surrounded by luminous streaks, of 

 which the directions show that they are being drawn into 

 the general mass. Still higher powers bring into view still 

 smaller, fainter, and more widely-dispersed streaks. And 

 it cannot be doubted that the minute fragments which no 

 telescopic aid makes visible, are yet more numerous and 

 widely dispersed. Thus far, then, inference and observa- 

 tion are at one. 



Granting that the great majority of these outlying por- 

 tions of nebulous matter will be drawn into the central 

 mass long before it reaches a definite form, the presump- 

 tion is that some of the very small, far-removed portions 

 will not be so ; but that before they arrive near it, the cen- 

 tral mass will have contracted into a comparatively moder- 

 ate bulk. What now will be the characters of these late- 

 arriving portions ? 



In the first place, they will have extremely eccentric 

 orbits. Lefl behind at a time when they were moving to- 

 wards the centre of gravity in slightly-deflected lines, and 

 therefore having but very small angular velocities, they 

 will approach the central mass in greatly elongated ellipses; 

 and rushing round it will go off" again into space. That is, 

 they will behave just as we see comets do ; whose orbits 

 are usually so eccentric as to be indistinguishable from 

 parabolas. 



In the second place, they will come fi-om all parts of 

 the heavens. Our supposition implies that they were left 

 oehind at a time when the nebulous mass was of irregu- 

 lar shape, and had not acquired a definite rotary motion ; 

 and as the separation of them would not be from any 



