Le Retour de Pappus

In my 1993 paper, "Pappus's Theorem and the Modular Group", I explained how the iteration of Pappus's Theorem gives rise to a $2$-parameter family of representations of the modular group into the group of projective automorphisms. In this paper we realize these representations as isometry groups of patterns of geodesics in the symmetric space $X=SL_3(\R)/SO(3)$. The patterns have the same asymptotic structure as the geodesics in the Farey triangulation, so our construction gives a $2$ parameter family of deformations of the Farey triangulation inside $X$. We also describe a bending phenomenon associated to these patterns.