112 



This plane system of points is therefore called a net-plane (fig. 101) ; 

 and if in fig. TOO we had started with any other point of the motif 



9 



*r 



. 



< 



<j> 



O 



-*- 



Bj 



> ' &. 



*j 

 - 



o 



- 



Fig. 100. 



F, for instance with <2, or 5, or F, we should in the same way 

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



a. 



Fig. 101. 



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. 



