The results obtained earlier [1, 2] for a scalar model are now applied to a real problem of wave propagation in uniaxial nematic. It is shown that the extinction coefficient $\sigma$ in the detumescence law $I\sim\exp(-\sigma r)$ of the intensity of the extraordinary mode because of infrared divergencies is logarithmically dependent upon the distance: $\sigma=m(a\ln(mr)+b)$ where $r$ is the ray path length in the medium, $m$ is the inverse wave length of the light. The dimensionless coefficients $a$ and $b$ depend upon the angle between the anisotropy direction and the ray vector and are calculated explicitely in terms of the asymptotics of the first self-energy diagram in the neighbourhood of the mass shell. The results are illustrated by calculations for three nematics, MBBA, BMOAB and H-106 at the neon-argon laser frequency.