May 25, 191.7] 



SCIENCE 



521 



Fig. 10. Planetary Nebulas, (a) and (6) are composite drawings frum iihotographs, 

 and (c) is a direct copy of a photograph, made by Curtis with the Orossley Reiiector of 

 the Lick Observatory. 



(a) N.G.C.i 351. (?;) N.G.C. 6818. (c) N.G.C. 7293. 



1 inch = 8'.5. 



Herschel and Proetor had establislied, they 

 cluster more densely in the neighborhood 

 of the pole of the Galaxy. In the southern 

 hemisphere they show the same tendency, 

 but not so strongly marked.* Two or three 

 hundred of the brighter nebulfe have long 

 been known to exist in and near the Milky 

 Way, but faint nebulse, such as those scores 

 of thousands which Keeler showed are still 

 awaiting discovery, are practically non- 

 existent in the galactic region. 



Let us examine more carefully the distri- 

 bution of the nebulse with reference to the 

 Galaxy, and with reference to the physical 

 conditions which seem to exist within them. 



Out of approximately fifteen thousand 

 nebulae thus far discovered, fewer than 150 

 are planetaries. They exist in a variety of 



< It is not intended to convey the impression that 

 the nebular distribution is merely a function of the 

 galactic latitude; the observed nebula are more 

 numerous in certain galactic longitudes than in 

 others. 



sizes, from a few that are only two or three 

 seconds of arc up to others a quarter of a 

 degree in diameter. The difference in size 

 is due, at least in large part, as Curtis has 

 recently made clear, to a difference in the 

 distances of the bodies from us. A con- 

 siderable number of them appear to be more 

 and more condensed as we approach their 

 centers, but the ring-form planetaries are 

 the prevailing type (see Pig. 10). These 

 rings of nebulosity are apparently not true 

 rings, existing chiefly in two dimensions, 

 but ellipsoidal shells of matter seen as rings 

 in projection on the background of the sky. 

 If they were true rings, we should see some 

 of them as extremely elongated ellipses, 

 and others ought to be long and slender as 

 a result of seeing them edgewise. Those 

 forms are wholly unknown. Now all of the 

 planetaries give spectra consisting chiefly 

 of isolated bright lines (see Fig. 32) ; that 

 is, they are gaseous in constitution, and are 



