As previously explained, after a duction test of each of the four muscles of the right eye, the rotary prism before that eye should be placed out of position and the procedure for adduction, abduction, superduction and subduction repeated by means of the rotary prism before the left eye. In case of an existing imbalance, after testing the muscle of both right and left eyes, the refractionist can quickly determine which muscle or muscles may be lacking in strength (Fig. 24). In practically every instance muscle exercises or correcting prisms may then be prescribed with definite knowledge of requirements, as further described in the following paragraphs. A binocular muscle test made with the phorometer, Maddox rod and distant muscle-testing point of light might quickly indicate six degrees of exophoria, both before and after the optical correction is made. While this would doubtless be the correct amount of the manifest imbalance, it would be a difficult matter to ascertain which muscles caused the disturbance. To determine this important question, the monocular or duction test should be invariably employed. DIAGNOSING A SPECIFIC MUSCLE CASE Assuming, for example, a specific case where six degrees of exophoria was determined in the binocular test that the muscle findings in the duction test show right adduction of twenty-four degrees, with an accompanying abduction of eight degrees; likewise a superduction and subduction of two degrees for each eye. With the aid of a chart or diagram—which should be made in every case—a comparison of these figures would indicate an exophoria of approximately six degrees, with a corresponding weak left internus (Fig. 24). This not only shows the muscle pull of each eye individually, but a comparison of the two eyes as indicated by the dotted lines. Thus the relationship of the two eyes, and their corresponding muscles is quickly indicated. [Illustration: Fig. 24—Duction chart should be made in every case. Above readily shows existence of muscular imbalance and proves subduction and superduction for both eyes are equal; otherwise hyperphoria would be disclosed. Also note abduction for both right and left eye are equal, otherwise esophoria would be disclosed. Also note adduction for right eye is 24° while left is but 18°, proving a case of 6° of exophoria with a left weak internus.] A glance at the above diagram discloses the following three important facts, all of which should be known to the refractionist before a single thought can be devoted to the correcting of the case: 1. 6° exophoria is the amount of the insufficiency. 2. 18° adduction (which should be 24°). 3. Left weak internus. As previously stated, the power to converge is normally rated 3 to 1, or 8 to 24, as shown above, while the power of the eye to look upward, is equal to the power to look downward. The diagram accordingly proves that the muscles of the right eye are in perfect balance, having equal muscular energy.