Practical plane and solid geometry for advanced students, including graphic statics, adapted to the requirements of the South Kensington syllabus (1899)
Harrison, Joseph, Baxandall, George Albert · London Macmillan · leaf 88
bola, ellipse, andhyperbola. The point F is called a focus, the inscribed sphere afocal sphere, the line DN is a directrix, and the ratioPF: /Wis called the eccentricity of the conic section.Thus we obtain the following definition :—■Definition.—A conic section is the carve described by apoint… … Referring again to the figure, the triangle PLM may-be supposed to rotate about PM until its plane coincideswith that of the triangle PMN, when LAIN becomes onestraight line. The triangles will then appear as showndetached at PQLQM0N. The left-hand diagram of tiiefigure is an elevation on a plane…
