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This paper reports on the observation of patterns induced on the transverse section of a probe beam traversing a sodium cell when a stronger pump beam, of smaller diametre, is propagated in the opposite direction. The setup consists of a 10 cm long, 2 cm diametre spectroscopic cell (Hellma), containing sodium vapour at variable density values (adjusted through the temperature of a cold finger), in the absence of a buffer gas. A longitudinal, variable magnetic field (up to 10 Gauss) is applied, while no compensation for the Earth's magnetic field (forming an angle of ≈ 60° with the optical axis) is introduced. Results show, however, that the magnetic field has no substantial influence on the nature of the resulting patterns.