130 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
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 outlined, 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 halfway 
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 occulting matter, lying in the plane of the greatest ex- 
tension of the spiral, generally outside the whorls, but occasionally 
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 elliptical 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 
ehiese effects. The phenomenon is a very common one. [Illustrations 
of 78 spirals showing evidences of occulting matter in their peripheral 
equatorial regions, with a more detailed discussion of the forms ob- 
served, are now being published,’ 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 great deal of 
evidence for such occulting matter in smaller areas in our galaxy. 
Many such dark areas are observed around certain of the diffuse 
nebulosities, or seen in projection on the background furnished by 
such nebulosities or the denser portions of the Milky Way; these ap- 
pearances seem to be actual “dark nebulae.”® The curious “rifts” 
5 Curtis, H. D., Occulting effects in spiral nebulae, Univ. Calif. Semi-Cent. Publ. (in 
press). 
6 Barnard, E. E., On the dark markings of the sky, with a catalogue of 182 such 
objects, Astrophys. Journ. 49:1, 1919; Curtis, H. D., Dark nebulae, Publ. Astron. Soc. 
Pacific 30:65, 1918, 
