Assuming that the patient finds that the streak _cuts through_ the point of light, the refractionist instantly notes the absence of hyperphoria. Should the point of light and the red streak _not_ bisect, prism power must be added by rotating the phorometer’s handle to a position that will cause the streak to cut through the light (Fig. 17). While testing for hyperphoria, the red scale should alone be employed, the white scale being totally ignored. [Illustration: Fig. 18—The Maddox rods placed horizontally test esophoria or exophoria, causing a vertical streak to be seen by the patient.] ESOPHORIA AND EXOPHORIA The next step is to set the white lines of the red Maddox rod either at white zero, or 180° line, with the rods in a horizontal position (Fig. 18) and the phorometer on the white neutral line, with handle horizontal (Fig. 19), thus making the test for esophoria or exophoria, technically known as lateral deviations. The red streak will now be seen in a vertical position. Should it bisect the spot of light, it would show that no lateral imbalance exists. Should it not bisect, the existence of either esophoria or exophoria is proven, necessitating the turning of the phorometer handle. Should the refractionist rotate the handle in a direction opposing that of the existing imbalance, the light will be taken further away from the streak, indicating that the rotation of the prisms should be reversed. [Illustration: Fig. 19—The phorometer handle placed horizontally denotes horizontal muscles are undergoing test for esophoria or exophoria indicated by “Es.” or “Ex.”] At the point of bisection (Fig. 20), the phorometer will indicate on the white scale whether the case is esophoria or exophoria and to what amount. In testing esophoria (ES) or exophoria (EX), the white scale is alone employed, no attention being given to the red scale. [Illustration: Fig. 20—The vertical streak bisecting muscle testing spot-light for horizontal imbalance, as patient should see it.] MAKING MUSCLE TEST BEFORE AND AFTER OPTICAL CORRECTION It is considered best to make the binocular test _before_ regular refraction is made, making note of the findings; and again repeating the test after the full optical correction has been placed before the patient’s eye. This enables the refractionist to definitely determine whether the correction has benefited or aggravated the muscles. Furthermore, by making the muscle test before and after the optical correction, a starting point in an examination is frequently attained. For example, where the phorometer indicates esophoria it is usually associated with hyperopia, whereas exophoria is usually associated with myopia, thus serving as a clue for the optical correction.