Smart, Edward Howard · London : Macmillian and Co. · leaf 62
have merely to reverse the steps of thepreceding argument, and the verification is left as an exercise. § 5. By far the most important property of the harmonicrange is contained in the following fundamental proposition : If a pencil be formed by connecting the four points of aharmonic range to any… … Let a pencil be formed by joining these points to O. Let a straight line ECF be drawn through C parallel to theray through the conjugate point A, to meet the rays throughthe other pair of conjugates in E and F. HARMONIC FORMS 36 rr., . AB OA 1 AD OA Then — = — and —= - . BC CF CD EC But by…
Geometry, Projective
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