It is not always advisable, however, to allow the patient to wear the same degree of prism for any length of time. Many authorities suggest a constant change with the idea that a prism is nothing more than a crutch. Should the same degree be constantly worn, even though it afforded temporary relief, the eye would become accustomed to it and the purpose of the prism entirely lost. Prisms should be prescribed with extreme care, their use being identical with that of dumb-bells, where weight is first increased to maximum and subsequently reduced, viz.: PRISM REDUCTION METHOD Where prisms are prescribed, it is considered good practice to make a binocular muscle test and the duction test (Fig. 24) at the end of each three months’ period, employing the phorometer, Maddox rod, and rotary prisms, as already explained. If the condition shows any decrease, the prism degree should be proportionately decreased. For example, in the case originally showing 6 degrees of exophoria, one-quarter degree prism for each degree of imbalance was prescribed, or 1½ degree for each eye. If the same case subsequently indicated 4 degrees, only one degree for each eye should be prescribed—and so on, a gradual reduction of prism value being constantly sought. Except in rare cases, prisms should not be prescribed with the base or apex at oblique angles, as the eye is rarely at rest with such a correction. An imbalance may be caused by a false condition in one rectus and a true imbalance in the other, giving one the impression that cyclophoria exists, as explained in a following chapter. Having now employed the three methods, the refractionist can readily understand that a marked percentage of muscular imbalance cases may be directly benefited through the aid of the Ski-optometer. If these three methods of procedure fail, there is nothing left but the fourth and last method—that of operative procedure. CHAPTER XII A CONDENSATION OF PREVIOUS CHAPTERS ON THE PROCEDURE FOR MUSCLE TESTING WITH THE SKI-OPTOMETER The present chapter, intended for those desiring a synopsis or condensed summary of muscular imbalance work, should prove of the utmost assistance to the busy refractionist. Muscular imbalance work can be successfully conducted if the following routine is studied and memorized, with the Ski-optometer _constantly before the reader_. The chapters containing the corresponding figures and diagrams or illustrations will then be readily comprehended. It is also important to carefully note the captions under each diagram. 1. Without any testing lenses before patient’s eyes, direct attention to a 20-foot distant muscle testing spot of light (Fig. 9). 2. Place phorometer handle vertically (Fig. 16). Place red Maddox rod vertically (Fig. 15). Patient should see a white spot of light, and a red horizontal streak (Fig. 17).