What is the formula for prescribing Rx using Percival's criterion?

• A. 1/3 (greater vergence) - 2/3 (lesser vergence)
• B. 2/3 (greater vergence) - 1/3 (lesser vergence)
• C. 1/3 (lesser vergence) - 2/3 (greater vergence)
• D. 1/2 (greater vergence) - 1/2 (lesser vergence)

Answer: (A)

Percival's criterion guides how to distribute prism power between the two vergence demands that a patient must meet to maintain comfortable binocular single vision. The idea is to provide more support to the vergence component that has the smaller, less capable fusional reserve. In practical terms, this means the prism should be allocated so that the lesser vergence receives the larger share, while the greater vergence gets the smaller share. Put numerically, the ratio is 1/3 for the greater vergence and 2/3 for the lesser vergence. This setup helps alleviate strain on the weaker vergence system and improves the likelihood of comfortable fusion without overburdening the stronger, more adaptable vergence mechanism. The alternative allocations would place too much corrective weight on the stronger system, making the prism less effective for relieving symptoms.