( 728 ) 
present already in the sector), and finally each of the leaves with , 
curves of the first kind (see fig. 5). 
The sectors of the second category we shall call positive (resp. ,* 
negative ) parabolic sectors. 
In special cases the whole inner domain of c can reduce itself to f 
a single positive (resp. negative) parabolic sector. A point zero where | 
this occurs we shall call a source point resp. vanishing point. 
The structure of the field in the vicinity of an isolated 
singular point. Second principal case. 
In this case any vicinity of P contains a simple closed tangent 
curve inside which P lies. We can then construct a fundamental ! 
series c, c' t c", .. .. of simple closed tangent curves converging to P ,j 
of which each following one lies inside each preceding one, and we 
can fill in the following way the inner domain of c with tangent 
curves not crossing each other. 
In each annular domain between two curves c(") and efa+i) we 
choose a point having from the boundary of that domain a distance ; 
as great as possible and we lay through it a tangent curve situated 
in the annular domain. According to § 1 it is either closed or 
it gives rise to two closed curves, situated in the annular domain 
with its boundary, into which it terminates or to which it converges 5 
spirally, and which we draw likewise. (These closed tangent curves can 
entirely or partially coincide j 
with c£”) or cC*+0). So the 
annular domain is either made 
singly connected or it is divided 
into two or three (annular or 
singly connected) new domains. 
In each of these we again 
choose a point having from the 
boundary a distance as great 
as possible and we lay through 
it again a tangent curve. A 
singly connected domain is 
certainly divided by it into 
two singly connected domains; 
on an annular domain it has 
the effect just now mentioned. 
We repeat this process inde¬ 
finitely. For each dpmain it can 
6. Rotation point. 
