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The double refraction of 32 liquid crystalline substances and their dependence on temperature and wavelength has been studied. The measurements were made on uniaxially oriented liquid crystalline layers using the principle of AbbBs double prism. Such optically uniaxial layers are obtained from the nematic states and the smectic modifications A and B. The smectic modification D proved to be optically isotropic. The smectic modification C could not be obtained uniaxially. The double refraction at the phase transition between two liquid crystalline modifications changes discontinuously and the several liquid crystalline modifications show a characteristic temperature dependence of the refractive indices.The dispersion of ne, no, ni is normal and increases in the sequence no, ni, n,. The dispersion of tte and no changes in a characteristic manner with the temperature. Within the homologous series an alternating decrease of ne, no and (ne-no) is observed. An attempt is made to explain these regularities on the basis of the structure and the order state in the liquid crystalline modifications.* The dispersion of the prism (m = 1.9270) and the temperature coefficients of n at different wavelengths are known.

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