Assuming for example that the binocular muscle test shows six degrees of esophoria without the optical correction, and with it but four degrees, it is readily seen that the imbalance has been benefited by the optical correction. Under such conditions it is safe to believe that the optical correction will continue to benefit as the patient advances in years, tending to overcome muscular defect. WHEN TO CONSIDER CORRECTION OF MUSCULAR IMBALANCE In correcting an imbalance, it is also a good plan to adhere to the following rule: In case of hyperphoria, either right or left, consider for further correction only those cases that show one degree or more. In exophoria, those showing three degrees or more. In esophoria, correct those showing five degrees or more, except in children, where correction should be made in cases showing an excess of 3° of esophoria. These rules are naturally subject to variation according to the patient’s refraction and age, but they are generally accepted as safe. FOUR METHODS FOR CORRECTION OF MUSCULAR IMBALANCE There are four distinct methods for correcting muscular imbalance, each of which should be carried out in the following routine: 1. _Optical correction_ made with spheres or cylinders, or a combination of both. 2. _Muscular exercising_ or “ocular gymnastics.” This is accomplished on the same principle as the employment of other forms of exercises, or calisthenics. 3. _The use of Prisms_: When the second method fails, prisms are supplied, with base of prism before the weak muscle, for rest only. 4. _Operation_: If the above three methods, as outlined in the following chapters, have been carefully investigated, nothing remains but a tetonomy or advancement, or other operative means for relief and satisfaction to the patient. THE ROTARY PRISM The rotary prism of the Ski-optometer, (Fig. 21) consists of a prism unit, having a total equivalent of thirty degrees. It is composed of two fifteen-degree prisms, back to back, so that the turn of its pinion or handle causes each of its lenses to revolve, one on the other. When its bases are opposite, they neutralize; when directly together, they give a total value of thirty degrees. While revolving from zero to maximum strength, they give prism values which are indicated on the scale of measurements, the red line denoting the total prism equivalent. [Illustration: Fig. 21—Turning rotary prism’s pinioned handle gives prism value from zero to 30° as indicated by prism’s red line indicator.] It is obviously essential to know where the base of the rotary prism is located. Therefore if prism in or out is desired, the zero graduations should be placed vertically and the red line or indicator set at the upper zero (Fig. 21).