ASTRONOMY: CAMPBELL AND MOORE 497 
Before proceeding to a discussion of the results, those for 12 nebulae 
situated in the Magellanic Clouds are set aside for special consideration, 
since, as will be shown later, they are attended by conditions which 
are not representative of nebulae in other portions of the sky. There 
remain 80 observed objects, of which 7 are extended or irregular in form, 
and 73 have the forms characteristic of planetary and ring nebulae. 
When all the observed velocities have been freed from the effects of the 
solar motion, the average velocities of approach and recession of the 
various groups are found to be: 
Of 7 extended nebulae 10 km. /sec. 
Of 73 plane taries or regular form 39 km./sec. 
Of 34 'stellar' (less than 5" diameter) 50 km./sec. 
Of 39 non-stellar (disks and rings) 29 km./sec. 
The velocities of the few extended nebulae thus far observed are 
low and of the order of the average velocity of stars of Class B, a result 
which is not surprising in view of the intimate relationship known 
to exist between these two classes of objects. The average velocity 
of the 73 planetaries is, on the other hand, more than six times that 
of the Class B stars. That planetary nebulae w^ill eventually become 
helium stars can scarcely be questioned, but the old hypothesis that 
helium stars have in general evolved from planetary nebulae hardly 
appears tenable. 
A division of the 73 planetaries into two groups according to their 
apparent size brings out a further relation of great interest The 34 
nebulae which are described in the catalogues as 'stellar,' or those 
whose apparent diameter is less than 5'', are found to be traveling 
almost twice as fast as the 39 nebulae of greater apparent diameter whose 
forms are those of hazy disks or of concentric or superimposed rings. If 
these stellar nebulae are small objects their higher velocities may be in 
harmony with recent indications concerning stellar motions to the effect 
that the stars of small mass are traveling more rapidly than those of 
great mass. On the other hand, if they appear smaller on account of 
their greater distance, an analogy is suggested to the recent results ob- 
tained by Adams, that the more distant stars of certain spectral classes 
are traveling more rapidly than those which are nearer to us. 
Attention was called in the earlier paper to the fact that the nebular 
velocities are distributed with more or less equal frequency for speeds 
of all observed magnitudes, a circumstance in marked contrast with 
the helium stars, whose peculiar motions follow pretty closely the 
'probabiHty curve.' Similar conclusions even more strongly marked 
may be drawn for the distribution of the velocities of the 34 stellar 
nebulae. 
