4, 5] Survey of the Problem 7 



Lalande 1966 and 242 kms a sec. for Cordoba Z. 5. 243), but regarding the 

 problem as a whole, it is clear that they approach nearer to the velocities of 

 the spiral nebulae than to those of ordinary stars. 



J With the possible exception of special nebulae such as these, last two, it is 

 clear that we may, with good reason, suppose that the irregular and planetary 

 nebulae form a part of our system, and are moving with it, while the spiral 

 nebulae must be supposed to be systems independent of, and outside of, our 

 own system. 



Further evidence of this essential difference between the spiral and plane- 

 tary nebulae is afforded by a study of their positions in the sky. The spiral 

 nebulae are found to be concentrated towards the poles of the milky wa^, 

 while the planetary nebulae are sparse near the poles of the milky way and 

 shew a very pronounced tendency to collect in the galactic plane. Now there 

 is every reason to believe that our system is of the shape of a coin or watch, 

 our sun being near the middle, and the remote edges being represented by 

 the milky way. Thus the most obvious, although perhaps not the only, 

 explanation of the observed differences of concentration of the spiral and 

 planetary nebulae is this: The planetary nebulae appear to favour the milky 

 way because, being inside our system and intermingled with the other stars 

 of the system, we see most of them in the directions in which we look into 

 the deepest layer of stars, namely directions in the galactic plane. The 

 spirals on the other hand appear to shun the milky way because the absorbing 

 matter of our system blots out or partially obscures such of them as lie in 

 directions near the galactic plane. In confirmation of this view R. F. Sanford* 

 has recently shewn that spirals near the milky way are on the average less 

 bright than those in other parts of the sky. F. G. Brown f has also shewn 

 that the spiral nebulae of larger angular size are in general the brighter, but 

 this is not true of spiral nebulae near the milky way where the visible 

 nebulae are large but faint. All evidence is consistent with the view that 

 the spiral nebulae are uniformly scattered in the sky but are quite outside 

 our system, so that of those which lie in the direction of the galactic plane, 

 the brighter ones are partially, and the fainter ones wholly, obscured by 

 obstructing matter in our own system. 



Campbell and Moore % have recently found that quite a large proportion 

 of planetary nebulae give spectroscopic evidence of internal motion. Of 33 

 examined, 16 gave definite evidence of internal motion, 12 gave no indi- 

 cations and the remaining 5 were doubtful. In a previous investigation! 

 internal motions had been found in the two nebulae N.G.C. 7009 and N.G.C. 

 6543. The motions are believed to consist in most cases of rotations about 



* Lick Obs. Bull. No. 297. 



t Monthly Notices R.A.S. 72 (1912), pp. 195 and 718. 



J Nat. Acad. Sci. 2 (1916), p. 566. Lick Obs. Bull. 9 (1916), No. 278. 



