Abstract
Birefringence properties of incommensurate [N(CH3)4]2. FeCl4 crystal are studied using by Senarmont's method (λ = 633 nm). The specific effects observed in these crystals proceed under the influence of the some generalized force. The force which influences on the solitons and enforces “viscous” interaction effect is caused by excess concentration of movable impurities arising around the soliton network.