After the case in question has been reduced to 30 degrees, having no further use for the rotary prism, it may be removed from before the right eye and the same exercising procedure continued as before with the remaining left side rotary prism by reducing its power, until it is likewise down to zero. Having reduced both prisms to zero, each prism should again be placed in position with zero graduations vertical and the prism indicator on upper zero. Both prisms should then be turned simultaneously about four degrees toward the nasal side of the patient, thus tending to jointly force corresponding muscles of both eyes. HOME TREATMENT FOR MUSCULAR EXERCISE—SQUARE PRISM SET USED IN CONJUNCTION WITH THE SKI-OPTOMETER Where a patient is unable to call each day for this muscular treatment or exercise, the work will be greatly facilitated by employing a specially designed set of square prisms ranging in strength from ½ to 20 degrees for home treatment. As in the case previously cited, it is necessary to carefully instruct the patient that the interni muscles _must_ be developed, hence prism base _out_ with apex in must be employed. Attention should then be directed to a candle light, serving as a muscle testing spot of light and stationed in a semi-dark room at an approximate distance of twenty feet. Having determined through the Ski-optometer the strength of the prism required after each office treatment, its equivalent should then be placed in a special square prism trial-frame which permits rotation of the prism, although the patient is frequently taught to twirl the lens before the eye. This exercise may be continued for about five minutes each day. The patient should also be instructed to call at the end of each week, when the work may be checked by means of the Ski-optometer’s rotary prisms, making the duction test as previously explained and outlined in Fig. 24. It is then possible to determine whether or not satisfactory results are being obtained. Otherwise the exercise should be abandoned. Should the second method employed in the work of muscular imbalance not prove effective, the third method requiring the use of prisms would be next in routine. CHAPTER XI THIRD METHOD OF TREATMENT—PRISM LENSES 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.