518 



CURTIS— NUMBER OF SPIRAL NEBULA. 



the effective area used in my counts is believed to be less than 0.75 

 square degrees) this would mean, if the two reflectors were of the 

 same focal length, that the images on one half of the angular area 

 of the large plates used by Fath were worse than those in the re- 

 jected edge strips of the Crossley plates. But the greater focal 



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qO O 





o 

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o o 



Fig. 2. Region of many small nebulae at 12" 55"" + 28° 30'. 249 small nebulae 



in an area 38' X 39'. 



ratio of the 60-inch would increase this disadvantage, even allowing 

 for certain advantages which might partially counterbalance this 

 due to the greater linear scale of the plates. The following com- 



