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Published online by Cambridge University Press: 24 October 2008
When a triangle ABC is in perspective with A′B′C′ and with B′C′A′, so is it also in perspective with C′A′B′, and the two triangles may then be said to be in cyclic perspective. The centres of perspective then form a third triangle XYZ, such that, of the three triangles, every two are in cyclic perspective with the vertices of the third for centres. The figure thus arising is the figure of Pappus' Theorem with nine points and nine lines incident by threes.
* Math. Ann., vol. xvii, 1880, p. 21.Google Scholar
* A point and a line are said to be pole and polar with respect to a triangle when the projections of the point from the vertices on the opposite sides are the harmonic conjugates in regard to the vertices on the side considered of the intersection of that side with the line. Cf. Baker, , Principles of Geometry, vol. ii, p. 38.Google Scholar