CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 225 



DISTRIBUTION OF SPIRALS 



There is still left one outstanding and unexplained problem 

 in the island universe theory or any other theory of the spiral 

 nebulae. Neither theory, as outHned, offers any satisfactory 

 explanation of the remarkable distribution of the spirals. On 

 the older theory, if a feature of our galaxy, what has driven them 

 out to the points most remote from the regions of greatest star 

 density? If, on the other hand, the spirals are island universes, 

 it is against all probability that our own universe should have 

 chanced to be situated about half way between two great groups 

 of island universes, and that not a single object of the class 

 happens to be located in the plane of our Milky Way. 



There is one very common characteristic of the spirals which 

 may be tentatively advanced as an explanation of the peculiar 

 grouping of the spirals. 



A very considerable proportion of the spirals show indubitable 

 evidence of occulating matter, lying in the plane of the greatest 

 extension of the spiral, generally outside the whorls, but occas- 

 ionally between the whorls as well. This outer ring of occulting 

 matter is most easily seen when the spiral is so oriented in space 

 as to turn its edge toward us. But the phenomenon is also seen 

 in spirals whose planes make a small, but appreciable angle 

 with our line of sight, manifesting itself in such appearances 

 as "lanes" more prominent on one side of the major axis of the 

 elongated elUptical projection, in a greater brightness of the 

 nebular matter on one side of this major axis, in a fan-shaped 

 nuclear portion, or in various combinations of these effects. 

 The phenomenon is a very common one. Illustrations of seventy- 

 eight spirals showing evidences of occulting matter in their peri- 

 pheral equatorial regions, with a more detailed discussion of 

 the forms observed, are now being pubUshed,^ and additional 

 examples of the phenomenon are constantly being found. 



While we have as yet no definite proof of the existence of 

 such a ring of occulting matter lying in our galactic plane and 

 outside of the great mass of the stars of our galaxy, there is a 



* Curtis, H. D. Occulting effects of spiral nebulae. Univ. Calif. Semi-Cent. 

 Publ. (in press). 



