Abstract
The classic conception of duction tests is that they determine the amount of prism base out, in, up or down which the eyes can overcome in order to maintain single binocular vision. They are for this reason also referred to as breadth of fusion tests. This conception is purely quantitative and ignores several qualitative elements involved in an analysis of the meaning of duction tests and findings. There are three qualitative phases in a duction test, which can be best exemplified by an illustration. Suppose one makes a near-adduction test, using a tiny spot light at 35 cm. as a target. To begin with, one must make sure that the patient has single binocular vision, and that he sees the light single with both eyes. One then inserts prisms base out of increasing power or uses rotary prisms until the light doubles up. Usually the patient sees the prism break
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