514 CURTIS— NUMBER OF SPIRAL NEBULA. 



rather than in excess of, the truth. The extent of the existing 

 photographic material now makes possible and useful a new deter- 

 mination of the number of the spiral nebulae. Descriptions of 762 

 nebulse and clusters are being published in a University of Califor- 

 nia semi-centennial volume, and, in preparing this paper, I have 

 made counts of the numbers of small nebulae occurring on all the 

 available regions of the complete Crossley program covered by the 

 above list, extending from 1898, when systematic work was com- 

 menced at Mt. Hamilton with this instrument, to February i, 1918. 



In all, 439 regions were available for these counts, giving a total 

 of 5,698 small, uncatalogued nebulae. To these must be added the 

 513 spirals described in the list, making the total number of all 

 nebulae found in all the regions 6,211. In the belief that practically 

 all these small nebulae are spirals, I have designedly omitted the dif- 

 fuse nebulosities and the planetaries. 



The exposed area of a Crossley plate is about nine tenths of a 

 square degree; the edges and corners of this area are, however, so 

 poorly defined, owing to the distance from the optical axis, that 

 only the brighter of the small nebulae can be picked up in these outer 

 parts of the plate. The greater proportion of the small nebula are 

 found in the more central parts having an area of six tenths of a 

 square degree, or less. I have assumed 0.75 of a square degree as 

 the average effective area on which the counts were made ; this is 

 certainly somewhat in excess, but is an error on the conservative side. 



We shall first assume that these 439 regions are sufficiently uni- 

 form in their distribution to be taken as a fair representation of the 

 whole sky; their area will be 329.25 square degrees. If the propor- 

 tion shown by these regions holds over the entire sky, we should ex- 

 pect the number of the spiral nebulae to be 778,000. A plot of the 

 regions shows that their distribution may well be regarded as an 

 approximately uniform one. There is, it is true, a marked concen- 

 tration of regions in the vicinity of the north galactic pole, but this 

 would appear to be balanced by a similar preponderance of regions 

 in the Milky Way between 17 and 20 hours of right ascension (see 

 Fig. i). If we divide the celestial sphere into two equal areas, one 

 consisting of a zone 60° wide extending for 30° on each side of the 

 galactic plane, and the other comprising the two zones of 60° radius 



