The invention of the multiple-beam interferometer may be understood as having proceeded from the fortunate convergence of two independent developments. On the one hand, during two centuries of near-misses, the multiple-beam interference phenomenon was consistently observed (and even computed by Poisson and Airy) but its specific and pregnant feature, the fringe sharpening was not understood before the 1892 Thesis of Charles Fabry. Throughout, the motivation had been purely intellectual search. On the other hand, a long succession of technicians, most of them unknown, strove to produce better mirrors for wholly commercial purposes; the end result was the semi-transparent silver film, essential for multiple-beam interferometry. The corresponding fringes, first observed by Boulouch, were immediately put to good use by Fabry and Perot in 1896. Through a remarkable coincidence, essentially the same phenomena were simultaneously discovered in the Hertz laboratory with Hertzian waves. One more convergence, with Einsteinian stimulated emission, has since given us coherent light.