Risley prism system is a pair of matched glass wedges in series， to Maddox Phoria Measure and Anisophorias (the change in the phoria away from the primary position). This tests for anisophorias, vertical and horizontal, in all positions of gaze in real space instead of behind of phoropter, also handy with small kids. The prism is approximately 1" diameter and allows variable prism demand up to 20PD.