360 
The Great Nebulae 
[ch. XIII 
The only result that seems to emerge with some clearness is that the spiral 
arms are permanent features of the nebulae. They appear to have been formed 
in the process of shrinkage, two convolutions or thereabouts being formed 
by each nebula, and to have been perpetuated in static form ever since. Their 
further interpretation forms one of the most puzzling, as well as disconcerting, 
problems of cosmogony. 
Not only so, but until the spiral arms have been satisfactorily explained, 
it is impossible to feel confidence in any conjectures or hypotheses in con 
nection with other features of the nebulae which seem more amenable to 
treatment. Each failure to explain the spiral arms makes it more and more 
difficult to resist a suspicion that the spiral nebulae are the seat of types of 
forces entirely unknown to us, forces which may possibly express novel and 
unsuspected metric properties of space. The type of conjecture which presents 
itself, somewhat insistently, is that the centres of the nebulae are of the 
nature of “ singular points,” at which matter is poured into our universe from 
some other, and entirely extraneous, spatial dimension, so that, to a denizen 
of our universe, they appear as points at which matter is being continually 
created.