Fig. 21—Turning rotary prism’s pinioned handle gives prism value from zero to 30° as indicated by prism’s red line indicator.
It is obviously essential to know where the base of the rotary prism is located. Therefore if prism in or out is desired, the zero graduations should be placed vertically and the red line or indicator set at the upper zero ([Fig. 21]).
A rotation inward to 10 would give a prism equivalent of ten degrees, base in. A rotation from zero to 10 outward would give a prism equivalent of ten degrees, base out, etc. With zero graduations horizontal and the red line or indicator set therewith, a rotation upward to ten on the scale would give a prism equivalent of ten degrees, base up. A rotation from zero downward to 10 would give a prism equivalent of ten degrees, base down.
An understanding of the foregoing will show that a rotation of the red line, or indicator, will give prism value from zero to 30, with base up, down, in or out.
Use of the Rotary Prism in
Binocular Muscle Tests
Should a case be one of esophoria, exceeding the ten degree range of the phorometer, the rotary prism should be brought into operative position with cypher (0) graduations vertical ([Fig. 21]), while the red line or indicator should be set at 10 on the outer or temporal scale. The phorometer’s indicator should again be set on the center or neutral line on the white scale. The rotary prism will then add ten degrees to the esophoria reading indicated on the phorometer.
Should the case be one of exophoria, exceeding ten degrees, the indicator should be set at ten degrees upon the inner or nasal scale and the indicator of the phorometer should then be set at the white center or neutral line, as in the previous test. Should prism power ever be required to supplement the phorometer in hyperphoria, the rotary prism should be employed with zero graduations horizontal, and the red line or indicator set at ten degrees on upper or lower scale, as required.