Geometry of the problem. At each point P of the medium, the magnetic field vector B is specified by the angles θ_B and χ_B, defined in the fixed reference system Σ ≡ (xyz). The polarization unit vector e_a(Ω) specifies the reference direction for positive Q of the radiation flowing through P in the direction Ω, specified by the angles θ and χ in the fixed reference system.