Abstract: The optical configuration described previously (Ibid., 1984, 56, 2336) is potentially useful for detection in liquid chromatography as well as for the cited applications. A 4-mW randomly polarized beam at 632.8 nm from a He - Ne laser is split by a polarizing beam splitter (Oriel model 2630) into a 2-mW pump beam, which is modulated, usually at 70 Hz, and a probe beam, which remains unchopped. Both beams are focused, and crossed at the centre of a 0.1-cm-pathlength flow-cell; under these conditions, the entire length of the thermal lens created by the pump radiation inside the cell defocuses the probe beam. The system has been used to detect bromophenol blue in a flow injection stream of ethanol. Its advantages over other configurations are discussed.