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[[image - mathematical diagram of light refracted through a curved lens; with points labeled A, a, B, b, C, D, E, F, G, H, I, K, L, M (point M also labeled as Focus) and broken lines labeled a, b]]

[[Put?]]
GA = Q
CB = R
CD = [[V?]]
FM = F
FB = [[Y?]]
LN = Y


Maskelyne's Theorem at p. 77 of my M.S. 
See philosophical Tranactions 1763 No. 31: p. 173.