bove and below the axis. If PQ is to be treated aspositive we ought to call pq negative and then 5-4 The result can be obtained from figure 72 and the formula for we have P?L=l=r_^L. PQ OQ u-r Now --. v r r u r-v u-r G. L. 98 LIGHT. [CH. V Thus m = -. u The formulae for direct reflexion from a… … Fig. 73. the angle QRq. Therefore by Euclid vi. 3, the segments of the base ofthe triangle RqQ are proportional to the sides. OQ QR Hence —- = 2- . Oq qR But when the incidence is direct R is very close to A, QR is verynearly equal to QA and qR to qA. Hence in this case we have 2£*fi4 Oq qA £H- 112…
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