A method and an interferometric device for spectroscopically or spectrometrically examining a sample, comprising: a) generating a laser beam having a wavelength, b) splitting the laser beam into a measurement beam and a reference beam, c) interacting the sample with the measurement beam, d) interacting a reference with the reference beam, e) overlaying the measurement beam and the reference beam, f) detecting a first output beam, g) detecting a second output beam, h) forming a differential signal between the first output signal and the second output signal, i) controlling the differential signal to a predefined target value, j) determining a refractive index of the sample from the adjustment of the phase difference between the measurement beam and the reference beam, k) repeating steps a) to j) for additional wavelengths of the laser beam.