A flat-crystal spectrometer is described that is fully evacuated to permit analysis of light elements. A uniselector in conjunction with a programing unit allows the crystal to be moved to any series of angles with stopping intervals as close as 0.01°. Detectors are a scintillation counter and a flow proportional counter driven at twice the angular velocity of the crystal to maintain them in correct position, A further detector is used to monitor the primary X-ray beam. Programing is achieved by means of a patchboard, and each board allows up to 20 elements to be measured at one time and in a sequence as desired, Pinal results appear in tabular forrn on an electric typewriter. Provisions are also provided for continuous scanning, the X-ray spectrum being plotted on a strip chart recorder. The sample is brought into position with a slide arranged so that the spectrometer may be kept continually evacuated and only a small pocket of air need be pumped. Alternately, helium may be used with resultant low consumption per sample.