Chains of these devices are used as multiplexers and demultiplexers in WDM systems. In one type of these devices, the quantity “pi times the square root of R all over one minus R” defines the finesse. Gyroscopes such as RLGs and FOGs are examples of these devices that operate using the Sagnac (“sahn-YAHK”) effect. A type of these devices called an etalon consisting of a cavity that filters out waves not in resonance is named for Fabry and Pérot. (10[2])One of these optical devices consisting of a stone slab in a pool of mercury failed to detect a 0.04 fringe shift induced by the Earth’s motion. For 10 points, name these devices that split a beam of light and later recombine them to form patterns, which include one used to disprove the luminiferous aether in the Michelson–Morley experiment. ■END■