516 



CURTIS— NUMBER OF SPIRAL NEBULA. 



may divide the Celestial sphere into four areas, two of which will be 

 the zones of 45° radius about the two galactic poles; a third area 

 will comprise the two zones, each 15° in width, extending from 

 — 30° to — 45° and from +30° to +45° galactic latitude, and the 

 fourth will be the zone 60° wide extending from — 30° to +30° 

 galactic latitude. The results are indicated in the following short 

 table : 



Galactic Latitude, 



+ 45° to +90° 



-45° to -90° 



± 30° to + 45° 



-30° to +30° 



Totals 



No. 

 Regions. 



Sq. 

 Deg. 



No. 

 Spirals. 



No. per 

 Sq. Deg. 



No. in 

 Area. 



117 

 43 

 62 



217 



88.50 



32.25 



46.50 



162.75 



2,997 



918 



1,117 



1. 179 



34 

 28 

 24 



7 



205,000 

 169,000 

 204,000 

 144,000 



439 



6,211 



722,000 



It will be seen from the above that there is a concentration of 

 the smaller nebulae in the vicinity of the north galactic pole similar 

 to that which obtains among the larger, visually discovered, spirals, 

 and that the density about the south galactic pole is somewhat less 

 marked. The data given in the third line of the table are of special 

 interest, showing that the small spirals persist to a distance of at 

 least 60° from the galactic poles, with only a slight diminution in 

 the degree of density which obtains in the polar areas. 



As this revised estimate, 722,000, is equal to that of Fath plus 

 that of Perrine, with several thousand to spare, a discussion of the 

 possible reasons for the discrepancy becomes imperative. The fol- 

 lowing points may be considered : 



A. It may be urged that my count has not been sufficiently con- 

 servative, and that I have possibly included many spurious objects. 

 The detection of the faintest and smallest nebulas is very largely a 

 matter of experience, and all who have worked with photographic 

 plates soon learn, by hard necessity, to recognize the average flaw 

 at a glance. A very large proportion of the objects are unmistak- 

 ably nebulae. As to the faintest nebulae, it is astonishing how faint 

 and small are the nebulae which two " clean " duplicate plates will 

 reproduce. For a large proportion of my regions no duplicate plates 

 exist, and I have been necessarily guided by the experience derived 



