An Assumed Case
We will assume a case where 42 degrees is required to enable the patient to first see the red streak as produced by the Maddox rod to the extreme left. Through a continued gradual reduction of 4 degrees (or to 38 degrees), we next learn that the streak was carried over until it bisected the white spot of light, giving single binocular vision and producing a position of rest.
Fig. 28—Simplified chart showing the prism action employed in developing a weak ocular muscle through alternating prism exercise. Either side of 38° in excess of 4° causing diplopia.
The patient has now established the limitation of the exercise, which is four degrees, this limitation being determined by the difference between the point where the streak was first seen to the extreme side and where it bisected the spot. The same amount of four degrees should then be used for the opposite side, thus reducing the prism strength to 34 degrees.
This again produces diplopia, because of the lesser amount of prism power employed to give single binocular vision. The refractionist should then return to 38 degrees, where single binocular vision had originally been determined ([Fig. 28]), alternating back to 42, returning to 38, over to 34, back to 38, and so on. This procedure should be employed once a day just after meals for about five minutes, and repeated ten times, constantly striving for a slight reduction of prism power from day to day.