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SCIENCE 



[N. S. Vol. XLV. No. i169 



Pig. 11. Distribution of Pliuietaiy and Irregular Bright-line Nebulas. 

 Eight Ascen. 12" to 24". Eight Aseen. 0" to 12". 



e = Planetary nebulee. O = Irregular bright-line nebulae. 



The irregular lines enclose the brighter galactic structure. 



The positions of the Greater and Lesser Magellanic Clouds are sho"wn above and to the 

 right of the south pole of the right-hand chart. 



sliiniug l)y their own light. A very large 

 proportion of them are in or adjacent to 

 the structure of the Milky Way, or in the 

 Magellanic Clouds (see Fig. 11). There are 

 a few exceptions, but the exceptions almost 

 certainly find their explanation in the rela- 

 tive nearness of these few to us, so that, 

 being a little to one side of the central 

 plane of the stellar system, they are seen 

 in projection at some distance to one side 

 or the other of the galactic structure. 



The large irregular uebute whose spectra 

 are known to consist of bright lines are 

 charted as open circles. Such nebulse at a 

 considerable distance from the Galaxy are 

 very scarce indeed. 



The motions of approach and recession of 

 the bright-line nebulas have been observed 

 with spectrographs at the Lick and D. 0. 

 Mills observatories. The large and form- 

 less bright-line nebulas, such as the Trifid 

 and Orion nebulas, are almost at rest 

 amongst the stars; their individual veloc- 



ities are small, and the mean velocity of the 

 group, with reference to our stellar system, 

 is zero. These nebufe, considered as a sys- 

 tem, are not moving through the stellar 

 system. They are in it and a part of it. 

 Many of the planetary nebulse (see Figs. 

 1 and 10) have high velocities, as individ- 

 uals, but when we consider them collectively 

 their motion with reference to the stellar 

 system is in effect zero. They too are of 

 our system. 



There are in or not very far from the 

 Milky Way many irregular nebulee, of 

 a great variety of sizes, whose types of spec- 

 tra are for the most part unknown. Thej' 

 are intrinsically faint, and their investiga- 

 tion is a most promising problem of the 

 immediate future. Two of these nebulas, 

 according to Slipher, have spectra identical 

 with the brilliant stars which seem to be 

 immersed in them; that is, continuous 

 spectra, except as the absorption lines of 

 helium and hydrogen are present in both 



