To correct a case of muscular imbalance, where six degrees of esophoria has been determined, the first rule of making the test for optical correction with the Ski-optometer’s spherical and cylindrical lenses, would be in the line of routine. The binocular test made with the phorometer and combined use of the red Maddox rod would have determined the six degrees of esophoria. The reason for making the binocular muscle test before and after the optical correction is because an imbalance is often aggravated or benefited by the correcting lenses. The optical correction frequently eliminates the need for further muscular treatment. For example, we will assume that the optical correction tends to decrease the degree of esophoria from six degrees to four degrees. According to the previously mentioned rule for correcting cases exceeding one degree in hyperphoria, three degrees in exophoria and five degrees in esophoria, the condition would indicate that of being “left alone.” Just what is taking place should be fully understood—its cause as well as its effect. [Illustration: Fig. 25—Comparative diagram showing how a decentered lens before a centered eye has the same effect as a centered lens before a decentered eye.] When not otherwise specified, accurately centered lenses are of primary importance. The pupil of the eye should be directly behind the center of each lens (Fig. 25). Figure “A” of the latter sketch illustrates a perfectly centered lens—its center indicated by a cross, the circle representing an eye directly behind it. Figure “B” illustrates a perfectly centered pupil behind a prism, with its center designated by a cross. To ascertain how the centered spherical lens takes the place of a prism, Figure “C” should be compared with Figure “B”; this will show that the eye is decentered, while the lens is centered. A further comparison will prove that the results in Figures “B” and “C” are identical, the correcting lenses having practically the same effect through the decentration of the eye as if a prism were prescribed, nature supplying its own decentration. TREATMENT FOR CORRECTING ESOPHORIA IN CHILDREN In case of esophoria, regardless of amount, slightly increased spherical power is frequently prescribed for children. This will naturally blur or fog the patient’s vision, but in their effort to overcome the blur, accommodation is relaxed, usually tending to correct the muscular defect. In such cases, as a rule, a quarter diopter increased spherical strength may frequently be added for each degree of esophoria as determined before the optical correction was made. In a case of 6 degrees of esophoria, the refractionist may prescribe +1.50 diopter spherical added to the optical correction, which, let us assume, is +1.00 sph. = -1.00 cyl. ax. 180°, so that the treatment glasses would be +2.50 sph. = -1.00 ax. 180° (See Procedure on Page 74).