The following figure gives a general idea of the reseau: 

 S l¥ m 2S 



55' 



n 



«- 



■7 



/ 



/ 



:r 



/ 



/ 



/ 



^/ 



\ 



\ 



\ 



\ 



\ 



3/- 



/^ m 



X 



K 



\ 



\ 



\ 



:^ 



/ 



/ 



/ 



/ 



/c 



/ 



/ 



26 



.!Ï5 



-n 



■*3 



■31 



The lines in the figure to wliicli numbers are affixed are the extreme 

 lines investigated; those marked m and 11 represent any pair of lines selected 

 at randoni. To avoid any ambiguity in designating the lines, we shall adopt 

 the convention that any line produced will pass through its number. Thus 

 in the figure the niiddle vertical line is called line 14. 



We shall designate by the symbol 



3i[14L 



the distance, measured along the 14-line, from its intersection with line 31 

 to its intersection with line 55. And in general we shall use the symbol 



43 



[»»],; 



to designate the distance measured along line m, from its intersection with 

 line 43 to its intersection with line )i. The distance from the point 2,43 to 



