.ill 





Thedemoi j ttat given allows alao thai the parabola is a 



ing ease of an. 



Parabola the limit of aa tlUpet,* or of an hyperbola. If a vertex 

 ami the corr|"'i,.i::.^ focus of an ellipse remain fixed in poaition 

 the center moves further and further away, the major axis becoming 



itely long. form of the ellipeo approaches mora ao 



nearl;. t a parabola having the aame vertex and fooua. 



Thw w easily shown aa follows : 



The - : .M. referred to iu major axia and the tangent 



left-hand vertex, aa coordinate axe*, ia (Part I, Art. 1 1 .') 



. may be written in the form 



If now the fixed distance OF be represented by />, then 

 p = OP = OC - FC a o -ViTTe^ 



tli-r-i.-r.- = 4 , and 



. 



Thia fact b of Importance in astronomy in 



(HA :., L: .-- r 



