A moderate amount of anisometropia is frequent and usually does not require any compensation in the lens prescription to insure the patient's comfort. Larger degrees of anisometropia are not uncommon, and when fusion is not suspended compensating prisms are often required to obviate the hyperphoria developed when the visual axes are moved from the primary to the reading or working position. The problem, although frequently overlooked, is familiar to any one doing refraction. It has been well covered from the optical standpoint by Obrig.1 However, the following facts will bear repetition.
If a 1 D. lens is decentered 1 cm., there is an image displacement of 1 prism diopter. The same phenomenon occurs in spectacle lenses when the visual axes are moved from the primary to the reading position, the power of the prism produced depending on the angular displacement of the visual axes and the power and type