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also for other schemes. In our calculation the transfer functions of the spectrometer and the detector are not considered, because usually the contributions to the lineshape and intensity of these parts can be safely neglected or are easily taken into account. Our results can be applied to uniaxial crystals and also to orthorhombic crystals as long as the scattering plane coincides with a plane of symmetry of the indicatrix. In the last part of the paper the calculated results are compared with experimental results from trigonal KBr03, where significant changes of width, height and asymmetry of the polariton lines have been observed. 2 The experimental arrangement The set up of Claus (1971), shown in figure 1 in a horizontal projection, allows a precise control of the mean scattering Abstract The instrumental lineshapes of polaritons observed in near forward scattering experiments with the arrangement suggested by Claus have been calculated by means of geometrical optics. The width of the exciting laser beam, the finite apertures of blinds and the acceptance angles of lenses have been considered. The results are compared with experimental lineshapes of a phonon polariton in uniaxial potassium bromate.