To determine this, place both of the Ski-optometer’s rotary prisms in position with the handle of each pointing outward horizontally. The red line or indicator of each prism should then be placed at 30° of the outer scale (Fig. 26). The red Maddox rod should be horizontally positioned before the eye, the white line on indicator pointing to 180° of the scale (Fig. 27). The strength of the rotary prism before the right eye should thereupon be reduced by rotating the prism indicator or red line toward the upper zero (0) to a point where the patient first sees the red streak—assuming that the red line appears at 42 degrees, that is 30 degrees before the left eye and 12 degrees before the right. [Illustration: Fig. 26 (A and B)—First position of rotary prisms to determine amount of prism exercise to be employed for building up the weak muscle.] The prism should then be still further reduced until the vertical streak produced by the Maddox rod directly bisects the muscle testing spot of light. Assuming that this point be thirty-eight degrees, which is four degrees less, _single binocular vision is produced_. [Illustration: Fig. 27—Position of red Maddox rod used in conjunction with Fig. 26 for prism exercising.] For example, sixty degrees of prism power (the combined power of the two rotary prisms) will usually cause complete distortion. Therefore, as outlined in Figure 28, the patient, seeing only out of the _right_ eye, will detect nothing but a white light. By gradually reducing the strength of the prism before the right, which is the good eye, the patient will eventually see a red streak off to the left. A continued and gradual reduction to a point where the red streak bisects the white light, will determine how much prism power is required for the patient to obtain single binocular vision, thus establishing the same image at the same time on each fovea or retina (Fig. 20). This has taught the patient to do that which he has never before accomplished. Therefore, after having been taught how to make the two eyes work in relation to each other, the natural tendency thereafter will be to strive for the same relationship of vision with both eyes. The refractionist should then aim to reduce the excessive amount of prism required to give binocular vision, which can be accomplished by muscular exercise. It must always be remembered before the refractionist is ready to employ the muscular exercise or second method, that the degree of prism required to give the patient single binocular vision must be determined with the optical correction in place. The exercise must be practised daily in routine, a daily record being essential. AN ASSUMED CASE