Smart, Edward Howard · London : Macmillian and Co. · leaf 252
havea common point. .*. they are in perspective, andthe joins of qb, q^b; qc, q^c ; qd,q^d are concurrent. But three tangents to a coniccannot be concurrent (Chap. VIII.§2). .*. no such lines as q-^ can exist. … Fig. 114a. Ex. Find the locus of centres of conies circumscribing a givenquadrangle. By Chap. VIII. § 12 each side of the quadrangle is parallel to thediameter conjugate to the diameter which bisects it. But, if C bethe centre and P, Q, R, S the middle points of the sides, C{ PQRS}=:thecross-ratio…
Geometry, Projective
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