Measurements of the magnitude and the sign of certain quadratic electro-optic coefficients of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) were made with an actively stabilized Michelson interferometer. The results obtained for these coefficients are, in units of 10 Ϫ20 m 2 V Ϫ2 (as opposed to literature values of order 10 Ϫ18 m 2 V Ϫ2 ), as follows: (KDP)g xxxx ϭ Ϫ3.4 Ϯ 0.5, g yyxx ϭ Ϫ0.2 Ϯ 0.4, and g zzxx ϭ Ϫ0.7 Ϯ 0.4; (ADP)g xxxx ϭ Ϫ7.4 Ϯ 1.0, g yyxx ϭ Ϫ1.7 Ϯ 0.9, and g zzxx ϭ Ϫ1.4 Ϯ 0.9. The quadratic Faust-Henry coefficient describing the lattice and the electronic contributions to the quadratic electro-optic effect in KDP and ADP is estimated from our results. These show that the nonlinear susceptibility responsible for the quadratic electro-optic effect in these crystals is due mainly to nonlinear interactions of the lowfrequency electric field with the crystal lattice.

The inter-dependence between refractive index and density of inorganic solids has been investigated. It is found that the available experimental data on refractive index n and density d may be expressed by the relations (n-1)/d=CM-u and (n2-1)/(n2+2)d=KM-v, where C and K are constants, the exponents u and v are approximately equal to 0.4, and M is the averaged atomic mass of a substance. For a large number of substances C=0.65 and K=0.375, but alkali halides are exceptions. These results are discussed in terms of the classical theory of dielectrics and the Phillips-Van Vechten description of the dielectric constant.

Measurements of the temperature dependence of the quadratic electrooptic coefficient |g 1111 -g 2211 | of ADP were made. The coefficient is found to decrease with an increase in temperature. The results show that the intrinsic quadratic electrooptic coefficient |M 1111 -M 2211 | defined in terms of polarization also dependes on temperature. Employing the temperature dependence of |M 1111 -M 2211 | estimation of antipolarization in lowtemperature antiferroelectric phase of ADP is made.

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The Jones matrix calculus is applied to an electro-optic crystal with uniaxial symmetry when the light beam is incident nearly normally on the crystal face. The approach allows one to treat refracted waves and rays that diverge in the crystal and are modulated by an external low-frequency field. The effect of partial interference of overlapping refracted beams is allowed for and calculated for the case of uniform intensity of the beam over its cross section. The method is employed to analyze optical systems containing an imprecisely cut and aligned electro-optic crystal plate.

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The isotopic effect on the magnitude of quadratic electrooptic coefficients in KDP is investigated. By employing both the Miller rule and assumption of a rough constancy of intrinsic electrooptic coefficients, one can expect these quadratic electrooptic coefficients which are related to the electric field applied in the [100] direction, i.e. the coefficients g1111, g2211 and g3311 to be in DKDP almost twice larger than that for KDP. Our measurements of the relative quadratic coefficient n03g1111‐ne3g3311 in DKDP are in contradiction with the prediction. We found g1111, g2211 and g3311 to be weakly dependent on the deuteration.