The remainder of this thesis considers the longitudinal coherence of the copper vapour laser, and describes the design and initial performance of a narrow bandwidth dye laser for use with a CVL. Careful construction of a grazing-incidence grating arrangement similar to that of Littman has resulted in a laser which is particularly simple to align and which promises ease of tuning over a wide range. Operation confined to a single longitudinal mode has been demonstrated, corresponding to single shot bandwidths below IGHz, but instabilities caused by vibration and uneven dye flow broaden the time-averaged width to around 3GHz. Suggested improvements have not yet been implemented.