perpendiculars 



perpendicular to the straight line FF. l>ra S'L 



Then //- makes a constant angle (*Z F7>.< ) with the plane M\ 

 P7, is parallel to FF], anO, if p repraeenU the distance from the 

 ; /Mo the plane J/.V, 



pPinA . . (1) 



\!*o PE t bein^ *" element of the oone, make* a eonstant angle 

 with the plane .l/.Y, and 



. . ( ) 



Again, since tangenU from an external point to a sphere are equal, 



(3) 



tX. 0*0*. S 



