The resonant-scattering spectrometer, developed in Birmingham and used to study the radial velocity of the solar surface, is described as it was during 1976. Possible instrumental sources of error are analysed and evaluated. It is shown that angular changes in the light path through the electro-optic modulator can give rise to an oscillatory error of up to 1 m/s, and that no other instrumental effects give rise to errors of this order.