When and How Employed
As stated in the preceding chapter, on ascertaining the failure of the second muscular treatment or method, prisms are employed for constant wear. When prism lenses are used, whether the case is exophoria or esophoria, or right or left hyperphoria, it is always safe to prescribe one-quarter degree prism for each degree of prism imbalance for each eye. For example, in a case of 6 degrees of esophoria, a prism of 1½ degree base out should be prescribed for each eye; or in 6 degrees of exophoria, employ the same amount of prism, but base in. In right hyperphoria, place the prism base down before the right eye and up before the left, and vice versa for left hyperphoria.
It is not always advisable, however, to allow the patient to wear the same degree of prism for any length of time. Many authorities suggest a constant change with the idea that a prism is nothing more than a crutch. Should the same degree be constantly worn, even though it afforded temporary relief, the eye would become accustomed to it and the purpose of the prism entirely lost. Prisms should be prescribed with extreme care, their use being identical with that of dumb-bells, where weight is first increased to maximum and subsequently reduced, viz.: