CURTIS— NUMBER OF SPIRAL NEBULA. 



515 



about the two galactic poles, we shall find that 217 regions, or 49.4 

 per cent., are located in the first, or galactic area, and 222 regions, 

 or 50.6 per cent., in the polar areas. The balance in the distribution 

 of the regions in the galactic and extra-galactic zones is, then, 

 nearly perfect. 



On the other hand, bearing in mind the well-known concentration 

 of the spiral nebulae in the vicinity of the north galactic pole, and 



24- 



\t M 



Fig. I. Chart showing distribution of regions on which small nebulae were 



counted. 



the fact that the present program deals primarily with the regions 

 containing spirals, it is necessary to investigate further any possible 

 efifect which a concentration of regions near the north galactic pole 

 might have upon the resulting estimate. This objection was urged 

 by Fath (loc. cit.) in explanation of the difference between his esti- 

 mate of 162,000 and the 500,000 of Perrine. He found, on plotting 

 Perrine's regions, that 33 per cent, of these were within 45° of the 

 north galactic pole, while less than 20 per cent, of the Kapteyn areas 

 were within this distance. In the present program I find that 117 

 regions, or 26 per cent, of the regions which I have used, are situ- 

 ated wuthin 45° of the north galactic pole, in an area amounting to 

 but 14.6 per cent, of the sky, and these 117 regions contain 2,997 

 spirals, or about 48 per cent, of the whole. 



To avoid the efifect of this concentration in the north polar galac- 

 tic area, it will be advisable to subdivide the material available. We 



