Abstract: A process and device for measuring the isotope ratio, specifically of stable isotopes, of chemical substances in a gas to be examined, the process including alternately charging a sample cell with a reference gas and a measuring gas containing the gas to be examined; adding a metrologically neutral gas to the gas to be examined; varying the mixing ratio of this measuring gas by changing the share of neutral gas being added; and calculation of the isotope ratio V.sub.p of the substance according to the equation: ##EQU1## The device includes a White cell having mirrors on each end of the longitudinal length of the cell, a pair of first beam openings located on a longitudinal end, a pair of second beam openings in spaced parallelism on the longitudinal sides of the White cell and a gas inlet and gas outlet located near each mirror.

Abstract: A monochromator apparatus comprises a monochromator (9) and a reversing device (2) which, with the aid of four reversing mirrors (10, 11, 17, 18) shifts the exit ray (13), of a monochromator (9) introduced in an intermediate focus of an external path of rays (4, 5), in the external path of rays (5) so that the external path of rays (4, 5) will not be influenced by the presence or absence of the monochromator apparatus (2, 9).