Nonlinear variations of signal intensity with time during cyclical scanning of a spectrum can be largely corrected by a special kind of linear interpolation. To calculate the ratio of intensities of components A and B in the spectrum, measured at times t_a and t_b from the beginning of each scan of duration tau , pairs of measurements A₁, A₂ and B₁, B₂ should be interpolated on to a time ¹/₂( tau +t_a+t_b) from the beginning of two successive scans. Second-degree variations in intensity are corrected exactly by this procedure.