Using the scheme first described by Furnas and Harker, for the collection of three-dimensional intensity data from a single crystal in which the motion of the counter tube is restricted to a horizontal plane, a comparatively simple instrument has been designed. Particular attention has been given to the design of simple bearings which are free from backlash and wear, and which permit the recording of diffracted beams up to an angle of 2θ of 165°. The instrument has provision for scanning and recording integrated intensities over an arc of 4°. Some details of performance are given.