An angular resolution of about 10 seconds of arc may be obtained with a double calcite crystal spectrometer for CuK&agr;‐radiation. However, the shape of the rocking curve at large angles is such as to make the detection of radiation from weakly scattering materials difficult. Of the order of 106counts per second are detected with a stationary, water‐cooled copper target and parallel crystals. Wing intensities, even at large angles, do not fall much below 102counts per second. Wing heights may be considerably reduced by the addition of a third calcite crystal. With this modification the crystal spectrometer is a useful instrument for most scattering experiments in which extremely high resolution is desired. It is inferior to a slit system and Geiger counter arrangement when poorer resolution may be tolerated but high sensitivity is required. A slit instrument with an angular resolution of 4 or 5 minutes of arc provides an effective incident flux of more than 106counts per second while wing intensities may be as low as 1 count per second.