530 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



November 12 and 13', 1901. January 31 and February 2, 1902. 



EiG. 19. Apparent movement of nebulous matter about Nova Persei. Photographed by Perrine with 

 the Crossley Eeflector of the Lick Observatory. 



[The motion is best shown by the bright mass above and to the right of the center, in comparison with 

 the surrounding stars.] 



not over 400,^ are found in the galactic 

 zone. That is, not more than one fortietli 

 or one fiftieth of the known nebulae are in 

 that one fourth of the sky which contains 

 the Milky Way; and these possible 400 

 galactic nebula include nearly all of the 

 planetary nebulae, nearly all of the large 

 gaseous nebula, nearly all of the regions 

 where large absorbing or obstructing neb- 

 ula are seen to be effective ; in other words, 

 as I have endeavored to make clear, nearly 

 all of the nebula that are really within our 

 stellar system. The other three quarters of 

 the sky contain, on the contrary, nearly 

 15,000 recorded nebula. The nebula to the 

 south of the galaxy have not been so well 

 observed as those to the north, and we shall 

 here consider the northern galactic hem- 

 isphere alone. The one quarter of the 

 northern hemisphere immediately around 

 the pole of the Galaxy contains three 

 fourths of all the recorded nebula in the 



corded with a short exposure may be seen to be but 

 parts of one great nebula when the exposure is 

 longer. 



whole hemisphere, and the quarter of the 

 hemisphere adjoining the central line of 

 the Galaxy contains about one fiftieth of all 

 the recorded nebula in the hemisphere. 

 The density of nebular distribution in the 

 Galaxy is only one fortieth that in the 

 quarter-area farthest away from the Gal- 

 axy. A still more interesting fact concern- 

 ing nebular distribution is this : thousands 

 of spiral nebulce are known to exist, hut not 

 a single spiral neiula has been found within 

 the galactic structure. Some spirals have 

 been found in regions adjoining the Galaxy, 

 but they are relatively few. The spirals 

 in particular abhor the Milky Way. As we 

 said above, the very avoidance of the Milky 

 Way seems at first sight to show that they 

 are arranged with reference to it ; that they 

 hold some relation to it. Is this relation- 

 ship real, or only apparent ? Are the spiral 

 nebula in or attached to our system, or are 

 they outside of our system, at tremendous 

 distances from us? This question is a live 

 one in the astronomj^ of to-day. The old 

 hypothesis that the unresolved nebula are 



