A flow through cell (100) for use in a spectrophotometer for analysis of dissolved chemical substances in a flowing liquid stream is made up of at least an intermediate body member (114) located between two other body members (112, 116). The body members are of regular shape, for example rectangular parallelepiped and are clamped together, for example by machine screws (134, 136). Thus the cell is relatively easily manufacturable. The clamped together body members define a flow through passage which includes a hole (118) through the intermediate body member (114) and a liquid inlet region at one end of the hole and a liquid outlet region at the other end of the hole, which regions may be provided by galleries (150, 152) in scaling gaskets (130, 132) between the body members. The two other body members each include an optically transparent window (142, 146) at an end of the hole (118) thereby providing an optical pathway (A, A') through part of the flow through passage. The liquid inlet and outlet regions at the ends of hole (118) are such that the liquid flow on entering and leaving the optical pathway (A, A') occurs adjacent the optical windows transversely of the flow along the optical pathway. Thus the liquid flow sweeps across the optical windows (142, 146) and flushes the hole (118) of the optical pathway by successive increments of the liquid with minimal mixing.