The optical cell is fabricated from a 75-mm length of 1-inch diameter PTFE round stock, see Fig. . A 5-mm hole is drilled through the centre of the rod and each end is tapped with an M8 thread of length 12.5 mm, resulting in an interior (nonthreaded) path length of 5 cm. The cell is sealed using two 7.5-mm diameter glass or quartz windows that sit at the bottom of the threads. The windows are retained using two flat-bottomed M8 screws that have been centrally drilled with a 3-mm hole for optical access. Liquid enters and exits the cell through ¼" UNF threaded ports in the cylindrical block that connect to the central channel via radially orientated 1-mm diameter drilled holes. The optical cell is mounted in two centring rings (similar to those used for the laser), and the central channel is aligned with the cage axis by adjusting the thumbscrews.