Darken the room and direct the patient’s attention to the usual muscle-testing spot of light, located approximately twenty feet away and on a direct plane with the patient’s eye. The optical correction, if one is required, should always be left in place—just as in making other previously described muscle tests. The rotary prism should then be brought before the patient’s right eye with the handle-pointing upward and with zero graduations horizontal. The indicator or red line should then be rotated upward from zero to eight upon the prism scale, creating the equivalent of a prism of 8 diopters with base up (Fig. 29). This normally caused diplopia, although in some cases it may be necessary to place the prism at 10 or 12 degrees before diplopia is produced. [Illustration: Fig. 30—(A. and B.)—First position of both Maddox rods used in conjunction with Fig. 29 for determining cyclophoria.] The red Maddox rod should then be brought into operative position before the patient’s left eye (Fig. 30a) and the white Maddox rod before the patient’s right eye, (Fig. 30b) setting each one so that the rods lie in a vertical position with their white line on the large red zero (0). The patient should now see two separate and distinct streaks of light, one appearing below the other. [Illustration: _DETERMINING CYCLOPHORIA_ _RIGHT EYE_ _LEFT EYE_ _Fig. 31_ _Fig. 34_ _NO CYCLOPHORIA_ _NO CYCLOPHORIA_] [Illustration: _Fig. 32_ _Fig. 35_ _+ CYCLOPHORIA_ _+ CYCLOPHORIA_] [Illustration: _Fig. 33_ _Fig. 36_ _- CYCLOPHORIA_ _- CYCLOPHORIA_ Figs. 31-36—Diagram showing how streaks appear to patient, as produced by the Maddox rods in testing for cyclophoria.] Should there be no cyclophoria of the right eye, the streaks will appear in a horizontal plane parallel to each other (Fig. 31). Should the red streak appear horizontally to the left eye, and the white streak seen by the right eye appear at an angle therewith, cyclophoria of the right eye would be indicated (Fig. 32). In brief, should the white streak dip towards the patient’s left side, the case would be one of right _plus_ cyclophoria (Fig. 32); whereas right _minus_ cyclophoria would be indicated should the white streak dip to the patient’s _right_ side (Fig. 33). Next, setting the rotary prism of 8 degrees, placed base up before the patient’s left eye, the _red_ streak should appear below the _white_ one. Should the two streaks appear horizontally, parallel with each other, there would be _no_ cyclophoria of the left eye (Fig. 34). If, however, the upper or white streak should appear horizontal, and the lower or red streak at an angle therewith dipping toward the patient’s _right_ side, the left eye would be cyclophoric and the case would be one of left _plus_ cyclophoria, as the chart indicates (Fig. 35). Should the red streak dip in toward the patient’s _left_ side, left _minus_ cyclophoria would be designated (Fig. 36).