Nebula in Lyra. 



404 Mr. A. S. Davis on a Theory of Nebulas, and Comets. 



mass. At each revolution it would graze the 

 surface of the larger mass, and the elliptic orbit 

 would gradually be changed into a circular one. 

 It would at length come to revolve about the 

 larger mass in such a manner as continually to 

 graze the surface, and an annular nebula would 

 be the result. 



Those nebulae, like the large one in Andromeda, in which 

 streaks of darkness cross the bright parts, 

 offer the greatest difficulty in their expla- 

 nation. 



We might imagine that a mass of rare 

 ga3 has become entangled between twomasses 

 of dense gas, and that the masses of dense 

 gas, being attracted to one another, crush out 

 the rare gas into a thin sheet, which, looked 

 at edgewise, presents the appearance of a dark 

 streak. 



Some of the nebulae of this kind have 

 nuclei ; and it may be remarked that the po- 

 sition of the nucleus, which will be the centre 

 of gravity of the nebula, is in the position in 

 which we should expect to see it if the appa- 

 rently vacuous streaks were in reality filled 

 with matter. Lassell, pi. 5. fig. 21 A. 



Lassell, pi. 5. fig. 21. 



Many of the nebulae are very much diffused, and in shape 

 very irregular ; but even these show certain characteristics which 

 seem to indicate their mode of formation. The extreme faint- 

 ness of their light favours the hypothesis that they are not lu- 

 minous throughout, but that their luminosity occurs only here 

 and there in comparatively thin shells. 



Again, if these nebulae were luminous throughout, the inten- 

 sity of their light should be greatest where the thickness of mat- 

 ter looked through is greatest, and should fade away at the 

 boundaries, whereas, if the visible parts occurred in shells, the 

 boundaries should appear brighter than other parts ; for at the 

 boundaries the line of sight would be very oblique to the shell, 

 and a greater thickness of luminous matter would be looked 

 through. Now if we examine any of the irregular nebulae, we 

 see some parts of it ending abruptly with a clearly defined out- 



