568 
ASTRONOMY: CAMPBELL AND MOORE 
7. About half of the nebulae in which relative motion has been de- 
tected give spectral lines of forms and degrees of inclinations which are 
satisfactorily interpreted as due to fairly rapid rotations of the central 
parts of the nebulae about axes approximately perpendicular to the 
longer axes of figure, and slower to rotations of the outer parts of the 
nebulae. These objects belong in general to the class of smaller plane- 
taries whose diameters are of the order of 10 seconds of arc. 
8. Five nebulae, most of which are ring nebulae, give lines which 
suggest the presence of other form or forms of relative internal motion 
in addition to those interpretable as rotational. 
9. The spectra of 3 of the larger ring nebulae, among them the well- 
known ring nebula in Lyra, while definitely showing internal motions 
of considerable magnitude, are apparently not interpretable on a simple 
rotation hypothesis. If rotation effects exist, they appear to be modi- 
fied or concealed by some other type of motion whose nature we have 
not yet determined. 
10. The central sections of the lines in the spectra of the ring nebula 
in Lyra and a few other nebulae, corresponding to the central areas of 
the nebular images, are of bowed form, convex to the red, and are 
broadened; the central section of the lines in the spectrum of N. G. C. 
7662 (R. A. 23 h. 21 m.) is doubled, with one bowed form convex to 
the red and the other convex to the violet; and the central section of 
the lines in the spectra of several nebulae are broadened toward the 
red. These phenomena, if interpreted as Doppler-Fizeau effectS; sug- 
gest motion of nebular materials toward the nebular nuclei at the cen- 
ters of the objects, but the high values of the corresponding velocities 
make the acceptance of such an idea difficult. However, the hypothesis 
should at least be given careful consideration. 
11. The observations indicate that in the nebulae which are more 
and more condensed as the center is approached we seem to be dealing 
with simple cases of rotating bodies whose outer strata rotate more 
slowly both linearly and angularly than the strata nearer the center. 
In other nebulae, of the ring form with central nuclei, rotation effects 
seem to be combined with motions of other nature. 
12. Measures of the rotational velocity of a nebula enable us to 
draw some interesting conclusions concerning its probable mass. As- 
suming that a particle in a nebula at a given angular distance from 
the center is moving in obedience to the gravitational force of the cen- 
tral mass, then, if the central mass is assumed to be equal to the mass 
of our Sun, the maximum distances of 9 nebulae whose relative motions 
seem to be interpretable as rotations lie between 250 light years and 5 
