115 



This plane system of points is, therefore, called a net-plane (fig. 



and if in fig. 102 we had started with any other point of the repeat 



Fig. 102. 



F, for instance with (), or S, or F, we should in the same way 

 have found a number of other endless point-systems QQ'Q"Q"' 



Pn 



a b. 



Fig. 103. 



SS'S"S'" , etc., which are all completely congruent with the first 



system PP'P"P'", and only with respect to the latter shifted along 

 the directions PQ, PS, PV , etc. 



