Apparatus to perform correlation spectroscopy utilizes a zero dispersion monochromator having entrance, intermediate and exit slits. A ruled grating is located in the beam path between entrance and exit slits and is operable to disperse a beam of radiation incident thereon both prior and subsequent to radiation passage through the intermediate slit. A first axis is parallel to the grating rulings, the entrance and exit slits symmetrically disposed at opposite sides of a plane bisecting and normal to a line extending between the entrance and exit slits, the plane also passing through the intermediate slit. That axis is located to have intersection with and to extend at an angle .PSI. relative to a normal to the plane that passes through said intersection thereby to substantially eliminate spectral overlap. Further, an array of sets of slits is provided adjacent the intermediate slit position of the monochromator, the sets of slits in the array being successively movable into the focal plane at the intermediate slit position.