Percival’s criterion

Percival’s criterion is a rule for judging whether a horizontal heterophoria is likely to be comfortable. For comfort, the working fixation point should lie in the middle third of the total fusional amplitude, found by adding the divergent to the convergent blur points.¹ Put another way, the opposing fusional reserves should be balanced so that neither is less than half the other.¹

Percival’s criterion ignores the size of the phoria. Sheard’s criterion, by contrast, relates the phoria to its opposing blur point.¹ Percival’s criterion seems appropriate for near vision only, because normal distance values do not follow the middle-third rule.¹

Percival also developed the criterion as a guideline for prescribing prism. The clinician calculates the prism needed to place the patient in the middle third of the vergence range, and a result of zero or less means no prism is required.² In graphical analysis, the prescribed prism places the Donders line in the middle third of the graph.³ Little clinical research supports the criterion, although studies suggest it is most effective with esophoria.²