Fig 15. A patient who suffered from trauma to the right side of his head and orbit developed a constant right hypertropia which increased on down-gaze. The prismatic correction necessary to eliminate the diplopia in the primary direction of gaze was insufficient to correct down-gaze. In order to eliminate diplopia in the primary direction of gaze, it was necessary to prescribe 4Δ of base down prism before the right eye. An additional 8Δ of base down prism was required to eliminate the diplopia for down-gaze. This was applied in the form of a Fresnel membrane prism over the lower half of this lens.