A wedge cell of the wedge angle of order of few milliradians was used to measure the phase shift between ordinary and extraordinary rays of transmitted light, for the Freedericksz transition induced by a magnetic or an electric,field. A nematic liquid crystal, PCB, filling the cell, was of the planar alignment enforced by the treatment of the flat boundary plates. A polyimide, MK8-poly (amic acid), was used as aligning substance. A system of interference fringes appeared in the cell placed in normally incident light between the analyser and the polariser crossed. In, the neighbourhood of each fringe the cell can. be considered as a flat-parallel one and hence it is equivalent to a system of flat cells of different precisely determined thickness. The dependence of the phase shift on applied external fields was measured for several interference fringes. The nematic liquid crystal material parameters (the splay and bend elastic constants, the anisotropy of diamagnetic susceptibility and the boundary tilt angle as a function of the torque) were determined by fitting the calculated characteristics to the measured ones.