ASTRONOMY: H. N. RUSSELL 
411 
counteracts the attraction of the nucleus. He suggests light-pressure — ■ 
which would fit in well with views of the origin of the luminosity such 
as are suggested above. In such a case we should anticipate that most 
of the light of the nebula would come from the nucleus, and this appears 
to be usually, though not always, the case. 
id) Measures of the radial velocities of nebulae have already shown that 
the planetary nebulae, as a class, are moving in space much more rapidly 
than the stars that there exist internal motions within them, usually 
of a rotational character, but sometimes more complicated and that, 
in order to keep the moving material from flying away into space, the 
total masses of the nebulae must be very considerable, and probably a 
good deal larger than those of the stars. Much remains to be done 
in the investigation of these motions, and in their interpretation. The 
proper motions of planetary nebulae, and perhaps in some cases the 
internal motions of the nebular material, can be determined by compari- 
son of suitable photographs, and it is probable that in a decade or two 
we shall obtain in this way a fair idea of the distances and real dimensions 
of these bodies. Observations for parallax on some of the larger and 
presumably nearer planetary nebulae are also desirable. 
The extended gaseous nebulae should be examined spectrographically 
to see whether turbulent motions exist in others, as they do in the great 
nebulae of Orion and it would be worth while to compare photographs 
of some of those which show sharp details, in the hope of detecting proper 
motion, either of the whole or of parts. 
Investigations of the distribution within the gaseous nebulae of the 
substances which give the different spectral lines may be made by 
photography either with absorbing screens or with slitless spectro- 
scopes, and promise information regarding the conditions prevailing in 
the nebulae, and the mutual relations of the lines of unknown origin. 
14. {a) The spiral nebulae have been shown by recent investigations to 
be the most extraordinary objects in the heavens. Their enormous radial 
velocities — first detected by Slipher^^ — and the almost equally rapid 
internal motions within them,^^ put them in a class by themselves. 
Further measures of these motions are needed; and, when the radial 
velocities of a sufficient number of spirals, well distributed over the 
heavens, are known, it may be possible to determine definitely the 
direction and rate of the motion of the Sun (and presumably of the 
whole galactic system) with respect to the system of nebulae. The 
provisional determination by Young and Harper, from very scanty 
data, indicates for the motion of our system the enormous velocity of 
600 kilometers per second. 
