Mirror optics with grazing incidence reflection

In an improved infrared (IR) microscope for a Fourier transform (FT) IR spectrometer comprising a Cassegrain mirror-lens having an optical axis, the lens comprising a convex mirror and a concave mirror which are both rotationally symmetric to the optical axis, the lens being arranged for focusing, at an angle of incidence .beta.<60.degree. relative to the axis of the lens and after travelling through an optical path length x from the concave lens, an incident IR beam onto a first point-shaped region on the surface of a sample, the point-shaped region simultaneously constituting the point of intersection of the optical axis of the lens through a sample surface, the improvement comprising

• a first planar mirror and a second planar mirror arranged between the concave mirror and the surface of the sample wherein the beam reflected from the concave mirror in the direction of the sample is focused, following an optical path length x.sub.1, onto the first planar mirror and, following an optical path length x.sub.2, impinges on the second planar mirror, and after an optical path length x.sub.3, is incident on a second point-shaped region on the surface of a sample, the second pointed-shaped region also lying on the axis of the lens but closer to the lens than the first point-shaped region, whereby the sum of the optical path lengths x.sub.1 +x.sub.2 +x.sub.3 is equal to the optical path length x, and an angle of incidence .beta.' between the beam incident on the sample surface and the axis of the lens is larger than 60.degree. and further comprising a third and a fourth planar mirror arranged, relative to the optical axis, symmetrically with respect to the second and first planar mirrors respectively, the third and fourth mirrors being arranged for guiding the light reflected from the second point-shaped region back through the IR microscope.