Abstract: A mechanism for selective translation of an object along first and second axes comprises a frame, a carriage mounted on the frame for sliding movement with respect to the frame along the first axis, a holder for said object and an intermediate mounting means coupled to the holder and the carriage for mounting the holder on the carriage for sliding movement with respect to the carriage along the second axis. A motor is provided for selectively driving a first shaft and a drive/brake assembly provides a second shaft. In a drive mode of the assembly the second shaft is allowed to be driven synchronously with the first shaft and, in a brake mode of the assembly, is held non-driven while the first shaft is being driven. First and second endless belts are coupled to the first and second shafts, respectively, for receiving selective rotational movement therefrom.