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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