An optical instrument in which the phenomenon of interference produced by multiple reflection between two precisely parallel glass plates is exploited to achieve a high degree of resolution in the study of spectra. The pair of plates, with an adjustable air space between them, is called an etalon. In transmission, the illuminated field of view appears as a series of bright concentric rings on a dark background, the angular diameters of which depend on the spacing of the etalon and the wavelength of the light. Inserted in front of the grating or prism in a spectrograph, it provides a means of revealing fine structure in a spectrum that could not be detected by the use of a diffraction grating alone.