ON THE NUMBER OF SPIRAL NEBULA. 



By HEBER D. CURTIS. 



(Read April 20, 19 18.) 



The probable total number of the spiral nebulse is a matter of 

 considerable importance in all theories bearing on the constitution 

 of these objects and their place in the sidereal plan. Prior to the 

 introduction of photographic methods fewer than ten thousand 

 nebulae were known. One of the first results deduced by Director 

 Keeler from the program of nebular photography, which he inaugu- 

 rated with the Crossley Reflector, was that there exist many thou- 

 sand very small, uncatalogued nebulse, the great majority of which 

 are undoubtedly spirals. Early in the course of this program, and 

 before photographs of many regions were available, he estimated 

 that there were at least 120,000 small spirals, and regarded this esti- 

 mate as a very conservative one.^ On completing the original 

 Keeler program Perrine^ came to the conclusion, from counts of 

 small nebulse made on fifty-seven of the one hundred and four 

 regions of that program, that at least 500,000 small nebulse were 

 within reach of the Crossley reflector, and deemed it not improbable 

 that the total might ultimately exceed 1,000,000. Later, Fath,^ 

 working with the 60-inch Reflector at Mt. Wilson, took a series of 

 139 plates at the centers of the Kapteyn areas, on which 1,031 

 nebulse were recorded, and estimated that the number within reach 

 of the 60-inch Reflector with exposures of one hour on Lumiere 

 Sigma plates (an approximate equivalent to the exposures of the 

 Crossley program) was 162,000. 



The great numbers of small spirals found on nearly all plates 

 of regions distant from the Milky Way long since led me to the 

 belief that Perrine's estimate of half a million was likely to be under, 



"^Ap. Jour., II, 325, 1900, and Pttbl. Lick Obs., 8. 



2 Lick Obs. Bull., 3, 47, 1904. 



3 Astr. Jour., 28, 75, 1913. 



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