CURTIS: MODERN THEORIES OI^ SPIRAL NEBULAE 219 



whose light is apparently a reflection phenomenon from bright 

 stars involved within the nebulae, both these types are of 

 gaseous constitution, showing a characteristic bright-Une spec- 

 trum. 



Differing radically from the galactic gaseous nebulae in form, 

 and distribution, we find a very large number of nebulae pre- 

 dominantly spiral in structure. The following saUent points 

 must be taken into account in any adequate theory of the spiral 

 nebulae. 



1. In apparent size the spirals range from minute flecks, 

 just distinguishable on the photographic plate, to enormous 

 spirals like Messier 33 and the Great Nebula in Andromeda, 

 the latter of which covers an area four times greater than that 

 subtended by the full moon. 



2. Prior to the application of photographic methods, fewer 

 than ten thousand nebulae of all classes had been observed visu- 

 ally. One of the first results deduced by Director Keeler from 

 the program of nebular photography which he inaugurated with 

 the Crossley Reflector at Lick Observatory, was the fact that 

 great numbers of small spirals are within reach of modern power- 

 ful reflecting telescopes. He estimated their total number as 

 120,000 early in the course of this program, and before plates 

 of many regions were available. I have recently made a count 

 of the small nebulae on all available regions taken at the Lick 

 Observatory during the past twenty years- and from these counts 

 estimate that there are at least 700,000 spiral nebulae accessible 

 with large reflectors. 



3. The most anomalous and inexplicable feature of the spiral 

 nebulae is found in their peculiar distribution. They show an 

 apparent abhorrence for our galaxy of stars, being found in 

 greatest numbers around the poles of our galaxy. In my counts 

 I found an approximate density of distribution as follows: 



Galactic Latitude + 45 ° to + 90 ° 34 per square degree. 



Galactic Latitude —45 to —90° 28 per square degree. . 



Galactic Latitude +30° to -|-45° and— 30° to — 45° 24 per square degree. 



Galactic Latitude — 30° to +30° 7 per square degree. 



* CtTRTis, H. D. On the number of spiral nebulae, Proc. Amer. Phil. Soc. 57: 

 313. 1918. 



