The expressions for the spontaneous polar contribution delta n (i) (s) to the principal values of the refractive index due to the quadratic electro-optic effect in ferroelectrics have been considered within the phenomenological approach taking into account the polarization fluctuations. A method has been proposed for calculating the magnitude and temperature dependence of the root-mean-square fluctuations of the polarization (short-range local polar order) P (sh) = aEuroP (fl) (2) aEuro parts per thousand(1/2) below the ferroelectric transition temperature T (c) from temperature changes in the spontaneous polar contribution delta n (i) (s) (T) if the average spontaneous polarization P (s) = aEuroPaEuro parts per thousand characterizing the long-range order is determined from independent measurements (for example, from dielectric hysteresis loops). For the case of isotropic fluctuations, the proposed method has made it possible to calculate P (sh) and P (s) only from refractometric measurements. It has been shown that, upon interferometric measurements, the method developed in this work allows calculating P (sh) and P (s) directly from the measured temperature and electric-field changes in the relative optical path (the specific optical retardation) of the light.