The interferometer consists of two accurately flat glass plates semi silvered and mounted parallel to one another. This arrangement is called an ‘etalon’. The distance d between the plates can be varied continuously, while keeping the surfaces parallel, by means of a precision micrometer acting through a lever. When the light is incident on the interferometer, sharp fringes are formed by the mutual interference of the directly transmitted beam with beams that have suffered a number of reflections at the silvered surfaces. The type of instrument is therefore referred to as a ‘Multiple Beam Interferometer” in contrast to the Michelson interferometer which is Two-beam interferometer. The basic property that distinguishes the Fabry-Perot from other spectroscopic devices is that, for a given resolving power, it is the most luminous instrument available. When multiple interfering beams are involved (i.e where we consider all possible multiple reflected emerging beams at a given angle to right not just the bottom 2), the fringes observed obey similar rules to those above, but become much sharper. The phase parameter is essentially a measure of the angle theta, so each successive peak represents the intensity of a circular fringe or ring at a given angle. The increasing phase angle represents decreasing angular size.