Doubly refracting prisms are used to convert an ordinary Ebert monochromator into a duochromator adapted to measuring small wavelength shifts in a spectral line. The instrument has the advantages of simplicity, good time resolution, high luminosity, and geometrically identical channels. An example of its use is given.
Joseph G. Hirschberg, "Simple Duochromator for the Measurement of Small Displacements of a Spectral Line," Appl. Opt. 4, 243-245 (1965)