At the end of each three months’ period, the patient should be requested to return, when the binocular and the duction test should again be made, comparing results with the work previously accomplished. An improvement tending to build up the left weak externus will possibly permit of a decrease of the excessive spherical power, so that excessive spherical power is reduced until completely removed, in all probability overcoming the muscular defect. Esophoria is almost invariably a false condition and frequently is outgrown under this treatment as the child advances in years. On the other hand, esophoria uncared for in the child may tend to produce exophoria in the adult. HOW OPTICAL CORRECTION TENDS TO DECREASE 6° ESOPHORIA IN A CHILD [Illustration: Assume binocular muscle test made before optical correction shows 6° Esophoria. +1. Sph. = -1. Cyl. Ax. 180.] Next, locate faulty muscle by making a duction test, which shows how abduction of left eye is made to equal that of right eye, change being made quarterly with treatment lenses in accordance with following rule. Note as abduction is increased, esophoria is reduced. Rule—prescribe a quarter diopter increased sphere for each degree of imbalance or 0.25 × 6 equals: +1.50 added to optical correction. 1/1/19 (assumed date) prescribed treatment lenses equal: +2.50 = -1. × 180°. 4/1/19 (3 months later) assuming abduction has increased from 2° to 3° showing difference of 5 Es. or 0.25 × 5. equals +1.25 added to optical correction, prescribed treatment lenses equal: +2.25 = -1. × 180. 7/1/19 (3 months later), assuming abduction has increased from 3° to 4° showing difference of 4° Es. or 0.25 × 4 equals +1.00 which added to optical correction would make prescribed treatment lenses equal: +2.00 = -1. × 180. And so on, every three months treatment lenses are prescribed until both right and left eye show 8° of abduction. In this way the treatment lenses are reduced to original correction of +1.00 = -100 × 180. This would have required six changes of lenses, three months apart—thus consuming 18 months time. CHAPTER X SECOND METHOD OF TREATMENT—MUSCULAR EXERCISE MADE WITH TWO ROTARY PRISMS AND RED MADDOX ROD EXOPHORIA If a case is one of exophoria of six degrees, where the second method of treatment or muscular exercise is in line of routine, it is essential to first determine through a duction test and the preparation of the diagram exactly which one of the four muscles are faulty (Fig. 24). Having determined, with the aid of the diagram, first, the existence of 6 degrees of exophoria; second, 18 degrees of adduction; third, a weak left internus—the next procedure is to determine what degree of prism will enable the patient to obtain single binocular vision, with both eyes looking “straight.”