Directrix

Definition and stakes

Portrait of Derrick Norman Lehmer Derrick Norman Lehmer,  An Elementary Course in Synthetic Projective Geometry

“ The axis which contains the foci is called the principal axis, and the intersection of the axis with the curve is called the vertex of the curve. The directrix is at right angles to the principal axis. In a parabola the vertex is equally distant from the focus and the directrix, these three points and the point at infinity on the axis being four harmonic points. In the ellipse the vertex is nearer to the focus than it is to the directrix, for the same reason, and in the hyperbola it is farther from the focus than it is from the directrix. ”
Source: Gutenberg

1911 Encyclopædia Britannica, Volume 6… (1911)

“ The definition of conics as sections of a cone was employed by the Greek geometers as the fundamental principle of their researches in this subject; but the subsequent development of geometrical methods has brought to light many other means for defining these curves. One definition, which is of especial value in the geometrical treatment of the conic sections (ellipse, parabola and hyperbola) in plano, is that a conic is the locus of a point whose distances from a fixed point (termed the focus) and a fixed line (the directrix) are in constant ratio. ”
Source: Wikisource

Portrait of Charles Dickens Charles Dickens,  Little Dorrit — Book 2, Chapter 15 (1857)

“ Mr Dorrit, seeing the whole Marshalsea and all its testimonials in the pause, instantly flew at him with, ‘How dare you, sir? What do you mean?’
‘I beg your pardon, sir,’ pleaded Mr Tinkler, ‘I was wishful to know——’
‘You wished to know nothing, sir,’ cried Mr Dorrit, highly flushed.
‘Don’t tell me you did. Ha. You didn’t. You are guilty of mockery, sir.’
‘I assure you, sir——’ Mr Tinkler began.
‘Don’t assure me!’ said Mr Dorrit. ‘I will not be assured by a domestic. You are guilty of mockery. You shall leave me—hum—the whole establishment shall leave me.
”
Source: Wikisource

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