A rotation inward to 10 would give a prism equivalent of ten degrees, base _in_. A rotation from zero to 10 outward would give a prism equivalent of ten degrees, base _out_, etc. With zero graduations horizontal and the red line or indicator set therewith, a rotation upward to ten on the scale would give a prism equivalent of ten degrees, base _up_. A rotation from zero downward to 10 would give a prism equivalent of ten degrees, base _down_. An understanding of the foregoing will show that a rotation of the red line, or indicator, will give prism value from zero to 30, with base up, down, in or out. USE OF THE ROTARY PRISM IN BINOCULAR MUSCLE TESTS Should a case be one of esophoria, exceeding the ten degree range of the phorometer, the rotary prism should be brought into operative position with cypher (0) graduations vertical (Fig. 21), while the red line or indicator should be set at 10 on the outer or temporal scale. The phorometer’s indicator should again be set on the center or neutral line on the white scale. The rotary prism will then add ten degrees to the esophoria reading indicated on the phorometer. Should the case be one of exophoria, exceeding ten degrees, the indicator should be set at ten degrees upon the inner or nasal scale and the indicator of the phorometer should then be set at the white center or neutral line, as in the previous test. Should prism power ever be required to supplement the phorometer in hyperphoria, the rotary prism should be employed with zero graduations horizontal, and the red line or indicator set at ten degrees on upper or lower scale, as required. CHAPTER VIII THE MONOCULAR DUCTION MUSCLE TEST MADE WITH BOTH ROTARY PRISMS While the previously described binocular muscle test made with the phorometer and Maddox rod, only determines the existence and amount of esophoria, exophoria, and hyperphoria, neither the faulty nor the deviating muscle is located, hence a _monocular muscle test_ is essential in order to determine whether the muscles of the right or left eye are faulty. Furthermore, an imbalance may possibly be due to either a faulty muscular poise, or lack of nerve force in one or both eyes. A “duction test” should accordingly be made of each muscle of each eye separately, followed by a comparison of the muscular pull of both eyes collectively. These tests are commonly termed adduction, abduction, superduction and subduction, and are defined in the order named. They include tests of the vertical and horizontal muscles of each eye, made individually by means of the rotary prisms, each being placed before the eye undergoing the test. LOCATING THE FAULTY MUSCLE