A system of very sharp dark fringes is observed in the spectra formed by both of the first-order reflections occurring within the quartz plates of a Fabry-Perot etalon. In the second-order reflections, two intersecting systems of fringes appear, the fringe separation in one being half of that in the other. The first-order fringes are complementary to the fringes observed many years ago by Perot and Fabry in the light passing through two nonparallel etalons of equal thickness. They arise by passage of the light through the etalon and subsequent reflection from it in the reverse direction. The two systems of fringes in the second-order reflections arise from one passage through the etalon, followed or preceded by two successive reflections from it.
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