The phorometer and the Maddox rod should be removed from operative position, discontinuing the use of the muscle-testing spot-light, employed in the previously described binocular test. The optical correction, if one is required, should be left in place, while the patient’s attention should be directed, with both eyes open, to the largest letter on the distant test chart; or if preferable, the Greek cross in the Woolf ophthalmic, chimney may be used. Either one, however, should be located on a plane with the patient’s head. As a guide for the operator, it might be well to remember that when the handle of the rotary prism is in a horizontal position, the lateral or horizontal muscles are being tested. On the other hand, when the handle is in a vertical position, the vertical muscles are undergoing the test. ADDUCTION Adduction, or relative convergence, is the power of the internal muscles to turn the eyes inward; prism power base out and apex in, is employed. [Illustration: Fig. 22—To test adduction, base out is required. Rotary prism’s line or indicator should be rotated from zero outwardly. To test abduction, base in is required. Indicator should be rotated inwardly from zero.] To test adduction of the patient’s right eye, the rotary prism should be placed in position before the right eye, the red line or prism indicator being registered at zero upon the prism upper scale. The two cyphers (0) should be placed in a vertical position with the handle pointed horizontally (Fig. 21). The rotary prism should then be rotated so that its red line or indicator is rotated outward from zero until the large letter—preferably the largest letter, which is usually “E”—on the distance test-type or the Greek cross previously referred to, first appears to double in the horizontal plane. The reading on the scale of measurements should accordingly be noted. This test should be repeated several times, constantly striving for the highest prism power that the patient will accept without producing diplopia. The prism equivalent thus obtained will indicate the right adduction and should be so recorded, as designated in Fig. 24. The amount of adduction ranges from 6 to 28, prism diopters, the normal average being 24. ABDUCTION Abduction is the relative power of the external muscles to turn the eyes outward. Prism power base in and apex out is employed. To determine abduction, or the amount of divergence of the external rectus muscle of the right eye, prism power with base in or toward the nasal side should be employed. The rotary prism will therefore remain in the same relative position as in making the adduction test (Fig. 22), with the two cyphers (0) or zero graduations vertical, but the indicator or red line should be rotated _inward_ from zero, or towards the patient’s nose.