THEORY OF POLYCONIC PROJECTIONS. 



51 



The length of Fp' is equal to the length of Ejp' muius the 

 length of EF; hence the length of Ff' =a sec a. The 

 center for the arc that is the projection of the meridian 

 of longitude X lies on the line Bit' at the point Xm = — a 

 sec q: cot X. With ^' as a center and with any convenient 

 radius construct a circle: divide this circiumerence into 



Fig. 13.— Construction of meridians on stereographic horizon projection. 



equal arcs for whatever interval it is desired to construct 

 the meridians, the initial point of the subdivision being 

 the point where this circle mtersects the central meridian. 

 In the figure we have 



but 



BF= Ff tan Z BfF] 

 Ff' = a sec a. 



