With the patient’s attention again directed to the large letter “E,” or the Greek cross, this inward rotation should be continued until diplopia or double vision occurs. Like the former, this test should be repeated several times, the refractionist continuing to strive for the highest prism power which the eye will accept. This will indicate abduction of the right eye and should be so recorded as designated in Fig. 24. The amount of abduction ranges from 3 to 10 prism diopters. The normal average is 8. The ratio of adduction to abduction is normally rated at about three to one. In other words, it is conceded that the power of the eye to converge is normally three times as great as its power to diverge, the usual measurements being eight to twenty-four respectively. While applicable in most instances, this may vary in different cases. SUPERDUCTION Superduction, sometimes termed sursumduction, is the relative power of the superior recti to turn the eyes upward. Prism power base down and apex up is employed. To test superduction, the rotary prism should be placed in position with the two cyphers lying horizontally, with the handle pointed vertically (Fig. 23). The patient’s attention should again be directed to the large letter “E”, and the indicator or red line should be rotated downward from zero. The highest prism power that the patient will accept before the object appears to double in the vertical plane will indicate the degree of right superduction. This should be recorded accordingly. Conditions of this kind do not usually exceed two or three degrees. The test, however, should be repeated several times before the final result is recorded, as indicated in Fig. 24. The amount of superduction ranges from 1 to 4 prism diopters. The normal average is 2. [Illustration: Fig. 23—To test superduction, base down is required. Rotary prism’s line or indicator should be rotated downward from zero. To test subduction, base up is required. Indicator should be rotated upward from zero.] SUBDUCTION Subduction, sometimes termed infraduction or deorsumduction, is the relative power of the inferior recti to turn the eyes downward. Prism power base up and apex down is employed. To test subduction, the rotary prism should be operated with zero graduations placed horizontally, as in the superduction test (Fig. 23), but the indicator should be slowly rotated in the reverse direction, or upward from zero. With the patient’s attention again directed to the large letter “E,” or the Greek cross, the strongest degree prism thus secured without diplopia will indicate the right subduction. The amount of subduction ranges from 1 to 4 prism diopters. The normal average is 2. Any difference between superduction and subduction, usually denoting the existence of hyperphoria, should be given careful consideration. PROCEDURE FOR MONOCULAR MUSCLE TESTING