A double crystal spectrometer control has been built to precisely measure semiautomatically both the small angles (20″) of refraction suffered by a thermal neutron beam in passing through a prism and, by substitution of a compatible second goniometer element, the large angles due to Bragg reflection of the monochromatic neutron beam. The construction, calibration, and sensitivity are described. With it a value of 4.147(2) fm was found for the neutron scattering amplitude of the silicon nuclear potential.