A quartz wedge with refracting angle O = 3.5° is inserted between two crossed Polaroids. The optical axis of the wedge is parallel to its edge and forms an angle of 45° with the principal directions of the Polaroids. On transmission of light with wavelength λ = 550 nm through this system, an interference fringe pattern is formed. The width of each fringe is ∆x = 1.0 mm. Find the difference of refractive indices of quartz for ordinary and extraordinary rays at the wavelength indicated above.