In the Vol. I, 27-7 of Feynman's Lectures on Physics, he derives a formula for the resolving power of an optical system from the general rule t2 - t1 > 1/nu (nu is frequency) as D > lambda/n*sin(teta). It appears, however, from the corresponding Fig. in the book that the latter inequality should have a form D > lambda/2*n*sin(teta), i.e. the right side is two times smaller. The approximate equality I derived from the Fig. is (P'R - P'S)*2*(PS) = 4*(PS)*D*sin(teta). Since P'R - P'S = c*(t2 - t1) and c/n*(nu) = lambda we get formula with 2 in the denominator. Is that the mistype in the book or am I missing something?