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Published online by Cambridge University Press: 20 January 2009
The anharmonic ratio of four points A, B, C, D depends on the order in which they are taken. The A.I.G.T. “Syllabus,” following Cremona, defines the symbol (ABCD) to stand for , when A is conjugate to B and C to D. Thus, taking λ to denote the value of this double-ratio, we have