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This is a device of high resolving power. In its simplest form it consists of a pair of fixed mirrors, a certain distance apart that can be adjusted for parallelism.
If the distance can be altered by actually moving one of the mirrors then the device can be termed as an interferometer.

The Fabry-Perot Etalon works by interference.

Light is reflected off both mirrors and the transmission from them is collected by a lens. The difference in distance the light travels from each mirror to the lens causes interference.

If a dark spot is observed, this equates to destructive interference and a light spot to constructive interference. The separation of each light spot, is equal to one free spectral range (fsr) which is c/2t where t is the mirror separation thus the frequency of unknown spots can be then be calculated in fractional terms of 1 fsr.

It was designed by French Physicists Charles Fabry and Alfred Perot in 1896 when they both worked at Marseilles University. Modern versions are still used today in optics, spectroscopy and astronomy.

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