We will assume a case where 42 degrees is required to enable the patient to first see the red streak as produced by the Maddox rod to the extreme left. Through a continued gradual reduction of 4 degrees (or to 38 degrees), we next learn that the streak was carried over until it bisected the white spot of light, giving single binocular vision and producing a position of rest. [Illustration: Fig. 28—Simplified chart showing the prism action employed in developing a weak ocular muscle through alternating prism exercise. Either side of 38° in excess of 4° causing diplopia.] The patient has now established the limitation of the exercise, which is four degrees, this limitation being determined by the difference between the point where the streak was first seen to the extreme side and where it bisected the spot. The same amount of four degrees should then be used for the opposite side, thus reducing the prism strength to 34 degrees. This again produces diplopia, because of the lesser amount of prism power employed to give single binocular vision. The refractionist should then return to 38 degrees, where single binocular vision had originally been determined (Fig. 28), alternating back to 42, returning to 38, over to 34, back to 38, and so on. This procedure should be employed once a day just after meals for about five minutes, and repeated ten times, constantly striving for a slight reduction of prism power from day to day. EFFECT OF MUSCULAR EXERCISE This muscular treatment, or constructive exercising, should enable the patient to overcome his amount of four degrees in either direction in about a week. Hence in the case showing 38 degrees for single binocular vision, results may be looked for in about nine weeks—four degrees divided into 38 degrees. While the patient is undergoing the treatment, which is nothing more than the strengthening of the interni muscles or developing adduction, it is natural to believe that the amount of imbalance is likewise being conquered. This, however, is readily determined from time to time by making the binocular muscle test with the phorometer and Maddox rod, as well as the duction chart test (Fig. 24), as previously outlined. To fully appreciate the effect of this muscular treatment, the reader need only hold his head in a stationary position, casting his eyes several times from the extreme right to the extreme left, not failing to note the apparent muscular strain. On the other hand, with the aid of the Ski-optometer’s rotating prisms, the refractionist not only has complete control of the patient’s muscles at all times, but scientifically accomplishes muscular exercise without any tiresome strain, overcoming all possible exertion.