The spectrometer comprises a symmetrical ruled diffraction grating with the collimated light beam under examination being incident thereupon diffracting in the right and left diffraction orders, into two light rays of a preset wavelength. One of these light rays returns to the diffraction grating, being reflected from a scanning mirror common to both light rays. The other light ray returns to the same diffraction grating after being reflected from an additional mirror, whose reflecting surface is parallel to the diffraction grating rulings, then from the scanning mirror, common to both light rays, and again from the additional mirror. Upon returning both light rays diffract in the same direction, interfere with each other and are registered. The method of examining the spectral composition of light realized through the use of the above-mentioned spectrometer, consists in using two twice diffracted light rays which have been reflected (2n+1) and (2n+3) times from two reflecting surfaces, where n is a natural number.