n, a smallpractical difficulty arises from the fact that a lens, as drawn, usually hasan appreciable thickness. Thus let CAB, fig. 89, be a lens, PR a rayincident parallel to the axis FAF, A the optical centre, F the principalfocus, and, F a point on the axis to the right of A such that AF is… … Fig. 89. single refraction. But since the lens is to be treated as very thin, R andS are very close together, we may without serious error suppose themcoincident, and treat RF as the emergent ray, the two lines RF and SF 120 LIGHT. [CH. VI are so near together that no appreciable error is…
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