An interferometer comprising a grating and first and second mirrors of which at least one is semitransparent which grating and mirrors are so disposed that a light beam orthogonally incident on said grating originates a first reflected part as constituent part of the interferometer reflected beam and one or more diffracted parts of which one, of a given order, orthogonally incides on said first mirror and, being reflected thereby, originates a beam which reincides on said grating where it splits into one or more diffracted beams of which one, of said given order, is constituent part of the interferometer reflected beam and into a second reflected part which by orthogonally inciding on said semitransparent mirror originates a transmitted part as constituent part of the interferometer transmitted beam and a third reflected part which, by inciding on the grating, splits into one or more diffracted parts of which one, of the above given order, is constituent part of the interferometer reflected beam and into a fourth reflected part which, orthogonally inciding on said first mirror, reflects on it whereby said part last cited begins again the path previously traveled.