An improved method of fitting resultant prism in treatment of two-axis strabismus.

A new chart determines resultant prism power and angle when correction of combined horizontal and vertical deviation with a single prism, set at an angle intermediate between horizontal and vertical, is desired. The chart is based on a derivation so weighted as never to permit a vertical error greater than one tenth as large as the associated horizontal error, with use of commercially available sizes of plastic Fresnel-type prisms. This weighting takes into account differences in horizontal and vertical fusional reserves. A compass rose facilitates proper alignment of the prisms.